The growing impact of

photonics in consumer

electronics and automotive

markets Markus Bilger

Viavi OSP

E-mail: markus.bilger@viavisolutions.com

Abstract

Photonics is experiencing a period of high

growth. The drive to ever smarter and more

automated devices results in the need for these

devices to sense their environment as well as or

preferably better than humans. Optical thin film

coatings are a key ingredient in elevating

sensory performance. The ability to see at

wavelengths and speeds beyond the human eye

is key to a span of things ranging from 3D

sensing in smartphones to Lidar systems in

automotive.

Packaging based on bioORMOCER®

Sabine Amberg-Schwab, Katharina Emmert Fraunhofer Institute for Silicate Research ISC, Neunerplatz 2, 97082 Würzburg, Germany

Non-degradable plastic packaging causes pollution, onshore and offshore. Biodegradable packaging may well be the solution. Biodegradable plastics offer a potential for creating a more sustainable society and solving global environmental waste problems.

However, the properties of the state-of-the art bio-based and biodegradable plastics often do not meet all the necessary requirements and are still inferior to those of established non-biodegradable plastic products. Therefore, biopolymers have only had limited use as high-quality packaging materials for food up to now. This is because they do not provide sufficient protection of food against penetrating oxygen and water vapor and so cannot guarantee the required shelf life.

Our idea was to improve these properties by a biodegradable coating. We have been developing high-barrier coatings based on a class of materials with glass-like structural units, so called ORMOCER® (inorganic-organic hybrid polymers) for many years. This material class exhibits excellent barrier properties against gases and vapors and thus is used as a coating for food packaging materials. However, the state-of-the-art ORMOCER® is not biodegradable. Therefore, the aim of our work was to modify these ORMOCER® to be biodegradable, and at the same time, preserve

the barrier and functional properties to provide new potential barrier materials for functional biodegradable packaging.

To realize the new biodegradable coatings, several suitable bio-based / biodegradable compounds have been identified and chemically functionalized. In this way, new bio-precursors were developed that could be used for the synthesis of functional biodegradable coatings. This was achieved by nano-chemical incorporation and chemical linkage of the chemically modified biopolymers into the ORMOCER® network (s. Fig. 1).

Both bio-based and biodegradable natural materials (chitosan, cellulose derivatives) and petroleum-based biodegradable educts (e.g. polycaprolactone-triol (PCL-T)) were used for bioORMOCER® development. In order to guarantee incorporation of the biodegradable components into the hybrid polymer network, some of these components were subjected to chemical modification. For example, the polycaprolactone derivative was functionalized with triethoxysilane groups in order to subsequently allow attachment of these biodegradable components to the oxidic inorganic network via hydrolysis and condensation reactions. The cellulose was treated similarly. In this case, attachment of biodegradable precursors to the organic network of the hybrid material was achieved by functionalization with epoxy groups. The reactive epoxy groups subsequently participated in the polymerization reactions for formation of the organic network. In contrast, chitosan required no modification because it can be linked to the organic network via some of its intrinsic amino groups.

Fig. 1. From conventional hybrid (ORMOCER®) to biodegradable hybrid materials (bioORMOCER®).

Fig. 2. Improvement of the oxygen transmission rate (OTR) through bioORMOCER®.

The resulting state-of-the art bioORMOCER®

coatings are transparent. They provide very good barriers to oxygen and water vapor and can furthermore be used as a planarization layer for substrates with rough surfaces. Figure 2 shows the results regarding the oxygen transmission rates of various bioORMOCER®

coatings. All systems lead to barriers sufficient for usage in food packaging.

The bioORMOCER® based coating materials represent a new material class. The coatings have good adhesion properties on different polymer film surfaces. The coatings are abrasion resistant and robust. The kinetics of biodegradability is temporally adaptable [1, 2].

Further investigations into the promising new material class of bioORMOCER® are currently being done within the EU-project HyperBioCoat [3]. The aim of this project is to develop new formulations of bioORMOCER®

derived from food processing by-products, which can provide the high levels of protection required for the demanding areas of food, cosmetic and medical device packaging. In this project we will first focus on the chemical modification of commercially available biopolymers, which are then covalently bound to the ORMOCER®. In theory it should be possible to apply those modifications also to extracted biopolymers from fruit, vegetable or paper pulp residues. The covalent modification of the ORMOCER® is realized either by binding of the functionalized biodegradable biopolymers to the inorganic network or to the organic network (s. Fig. 3).

In the scope of the HyperBioCoat project two different inorganic modification reactions were carried out in order to introduce triethoxysilane groups to the biopolymers. Those triethoxysilane groups then provided the means to covalently attach the biodegradable components to the oxidic inorganic

ORMOCER® network via hydrolysis and condensation reactions.

Furthermore, the organic modification of the biopolymers with amino groups allowed the bonding to the organic network of the hybrid material via reaction with ORMOCER® epoxy groups. Also, the modification with amino groups leads to products which are readily soluble in the aqueous-alcoholic ORMOCER®

solvent system.

All modification methods were first tested on the commercially available cellulose and then the results were transferred onto different hemicelluloses. The successful modification of the biopolymers was confirmed by spectroscopic methods (FT-IR, Raman, NMR), elemental analysis and/or titration in each case. Different incorporation methods of the functionalized biopolymers into the ORMOCER® system were investigated (cf. Fig. 4):

Method 1: Inorganically functionalized (hemi)celluloses were used as bio-precursors in the lacquer synthesis in exchange for part of the conventional inorganic precursors

Method 2: A basic, acidic or neutral Amino-(hemi)cellulose solution was directly added into ORMOCER® lacquers

Method 3: The aqueous Amino-(hemi)cellulose solution was utilized as a part of the ORMOCER® synthesis (hydrolysis).

Fig. 3. bioORMOCER® structure: type I (pink), type II (red, M = metal), type III (blue) and type IV (green, biodegradable).

Fig. 43. Modification reactions and incorporation of biopolymers into ORMOCER®

as part of the EU-project HyperBioCoat

The resulting coatings were evaluated regarding optics, adhesion, barrier properties and biodegradability. The best results so far were achieved with the amino-functionalized biopolymers, which were incorporated as basic additive. The films containing amino-biopolymers were generally clear and transparent, the adhesion on all tested substrates was high and first signs of biodegradation were visible after a few weeks in a test-compost. Furthermore, the barrier performances of the newly

developed bioORMOCER® are comparable to the barrier properties of the state-of-the-art ORMOCER®.

The new bioORMOCER® can provide biodegradable coatings from ecologically friendly resources and can improve biodegradable polymer films, which could in the future lead to the use of these biopolymer systems for food, cosmetic and pharmaceutical packaging.

bioORMOCER® can play a decisive part in the future design and development of innovative, more environmentally friendly food packaging materials.

Reference

[1] S. Amberg-Schwab, et al., Protection for Bioplastics, European Coatings Journal 12 (2015) , 32-36.

[2] S. Amberg-Schwab, Functional Barrier Coatings on the Basis of Hybrid Polymers, in: Handbook of Sol-Gel Science and Technology; L. Klein, M. Aparicio, Andrei Jitianu (Eds.), Springer, Berlin, 2016.

[3] http://hyperbiocoat.eu, No 720736.

Maturity of moisture barriers

for large volume production Luca Gautero1, Ewout van Vugt1, Dana

Borsa1, Edward Clerkx1

1Meyer Burger (Netherlands) B.V.

Abstract

Marketability of wearable or flexible electronics, for example organic light emitting device (OLED) displays, demands inexpensive thin film encapsulation (TFE) against moisture. Large volume manufacturing tools able to

create a reliable and cost compatible moisture

barrier are not trivial. The scope of this work is

to present large volume manufacturing of TFE

based on stacks of Microwave Plasma

enhanced chemical vapour deposition (MW-

PECVD) or spatial atomic layer deposition (s-

ALD) inorganics, and jettable or dispensable

organics.

Keywords: Encapsulation, moisture, barrier,

WVTR, flexible electronics, OLEDs

1 Introduction

Barriers, with water vapour transmission rates

(WVTR) in the range of 1E-6 g/day/m2, are

manufactured at low temperatures with contact

less techniques on laboratory scale systems

(Salem et al. 2016). Throughput and yield

might not be representative for large

production.

In Europe, within the Innovation program

Horizon 2020, effort has been addressed to the

improvement of barriers by sponsoring

multiyear projects. One concrete result of this

approach is a pilot scale production of state of

the art flexible OLED for lighting (“European

Commission : CORDIS : Projects and Results :

Bringing Flexible Organic Electronics to Pilot

Innovation Scale” n.d.).

The U.S. department of energy proposed to

foster the large volume production of solid

state lighting. Part of this program addresses

OLED lighting and therefore its encapsulation

needs (Department of Energy 2016). The cost

and performances have been given aggressive

roadmaps. Companies, like OLEDWorks and

LG Display, two of the major OLED lighting

producers by volume, are meant to live by this

roadmap in order to give clear expectations on

the competition front.

Following its mission and technological

competence, Meyer Burger (Netherlands) B.V.

has developed both sheet to sheet (S2S) and

roll to roll (R2R) equipment for the fabrication

of TFE as stacks of inorganic and organic

layers. The individual equipment tools gather

their maturity from hundreds of installations

targeting several applications (photovoltaic,

display, plastic electronics and PCB

applications) at both scientific institution and

manufacturing industry. Inorganic thin film

deposition and organic layer coating are

therefore combined into single, fully automated

cluster tools with industrial manufacturing

capabilities dedicated to TFE (“Meyer Burger

Ships an Inkjet+PECVD OLED Encapsulation

System to an Asian Customer | OLED-Info”

n.d.).

The technologic transfer of the Holst processes

technology towards a large volume

manufacturing environment resulted in two

applications: S2S with PECVD and IJP

technology and R2R with s-ALD and slot dye

coating. The transfer promoted improvements

to decrease the cost of ownership of the

technologies. The goal is to exceed throughput

above several square meter/hour.

The large volume cluster combines processes

of thin film (vacuum for PECVD and

atmospheric for s-ALD) deposition of dielectric

layers as implemented on the FLEx family

tools (both R2R and S2S) together with an

inkjet printing process as implemented on the

Pixdro family tools for the organic layers into a

single fully automated cluster.

For the case of S2S, novelties in the deposition

of the layers and their characteristics will be

presented together with a discussion on their

direct implications.

2 Experimental

One important task of the technology transfer

from laboratory to industrial scale is to identify

bottlenecks of production. These were put in

light during the design phase of the S2S TFE

cluster tool. The printing process of the organic

layer can be scaled without affecting

significantly the design or the final layer

properties. Instead, the inorganic layer, an

hydrogenated amorphous silicon nitride film

(SiNx) deposited by PECVD, needs to be

carefully designed to allow a functional process

with high throughput.

Two key requirements on the inorganic layer

have the highest priority for the integration with

a TFE production cluster: High deposition rate

and accurate control of the substrate

temperature during the whole deposition

process.

Increasing the deposition rate is

straightforward on almost any plasma based

deposition tool: increasing precursor gases

rate and pressure would guarantee more

radicals to reach the substrate per amount of

time. However, the layer quality varies

dramatically (Anders 2010).

For the TFE application the quality of the

inorganic layer is key for the overall barrier

functionality and reliability over time.

Therefore, the process window is smaller.

Inorganic layers are deposited at low

temperatures (<80 ºC) by PECVD SiNx. Its

deposition rate can be as high as 12nm/s. In

Table 1 the list of the parameters of

importance for layers deposited with deposition

rates higher than 10nm/s are shown.

Table 1: Characterization of inorganic layers

Inorganic layers are characterized by Ca-test

(Nisato et al. 2014) at accelerated conditions

(60 ºC/90 %rH, a factor of 40 is used to

translate the number to 20 ºC/50 %rH

conditions) to reveal their WVTR. Microscope

glass strips are coated to map stress. Foils are

coated to measure the film failure strain.

Silicon and glass substrates are coated with

SiNx and exposed to harsh conditions

(85ºC/85%rH) while being monitored for any

possible shift in thickness, refractive index and

haze.

3 Result and Discussion

The WVTR value guarantees good performances over a long span of time.

However, the measurement of WVTR is not affected by the known oxidizing effect of harsh condition on the surface of low temperature

SiNx layers (Chiang, Ghanayem, and Hess

1989). Therefore, we observed it indirectly and

optimized the layers to extend their lifetime.

The mechanical properties reported in this

study can be used to simulate the behaviour of

a stacked configuration. From these

calculation, it turns out that the thickness of the

flexible substrate has a large role in cracking

nucleation. It influences strongly the neutral

line, which can be far from the deposited SiNx

and therefore affect it imposing a great strain.

Layer: SiNx Value Unit Relevant Condition description

WVTR 3.5±0.17 (E-6 g/day/m2) @300nm

Stress 70±20 (MPa) @300nm

Strain <0.8 (%) @150nm

Hydrolyses rate <0.1 (nm/h) @85ºC/85%rH

Haze <0.5 - @85ºC/85%rH

4 Conclusions

Well established laboratory scale processes

able to realize highly performing TFE have

been transferred to large volume production

cluster tools thanks to adaptation and

extensions of existing tool capabilities.

Increased production speed is not affecting the

quality of the production

Acknoledgements

The authors would like to thank Pavel

Kudlacek for his support in the verification of

high speed barriers.

References

[1] Anders, André. 2010. “A Structure Zone

Diagram Including Plasma-Based

Deposition and Ion Etching.” Thin Solid

Films 518 (15): 4087–90.

https://doi.org/10/b3vv5v.

[2] Chiang, J. N., S. G. Ghanayem, and D.

W. Hess. 1989. “Low-Temperature

Hydrolysis (Oxidation) of Plasma-

Deposited Silicon Nitride Films.”

Chemistry of Materials 1 (2): 194–98.

https://doi.org/10.1021/CM00002A006.

[3] Department of Energy. 2016. “Solid-State

Lighting 2016 R&D Plan.” Solid-State

Lighting (SSL) R&D Plan.

https://www.energy.gov/sites/prod/files/20

16/06/f32/ssl_rd-plan_%20jun2016_2.pdf.

[4] “European Commission : CORDIS :

Projects and Results : Bringing Flexible

Organic Electronics to Pilot Innovation

Scale.” n.d. Accessed April 23, 2018.

https://cordis.europa.eu/project/rcn/19917

5_en.html.

[5] “Meyer Burger Ships an Inkjet+PECVD

OLED Encapsulation System to an Asian

Customer | OLED-Info.” n.d. Accessed

April 23, 2018. https://www.oled-

info.com/meyer-burger-ships-inkjetpecvd-

oled-encapsulation-system-asian-

customer.

[6] Nisato, Giovanni, Hannes Klumbies, John

Fahlteich, Lars Müller-Meskamp, Peter

van de Weijer, Piet Bouten, Christine

Boeffel, et al. 2014. “Experimental

Comparison of High-Performance Water

Vapor Permeation Measurement

Methods.” Organic Electronics 15 (12):

3746–55. https://doi.org/10/gcrnj3.

[7] Salem, A., H. B. Akkerman, P. van de

Weijer, P. C. P. Bouten, J. Shen, S. H. P.

M. de Winter, P. Kudlacek, et al. 2016.

“Thin-Film Flexible Barriers for PV

Applications and OLED Lighting.” In 2016

IEEE 43rd Photovoltaic Specialists

Conference (PVSC), 1661–63.

https://doi.org/10.1109/PVSC.2016.77499

05.

Innovative Coatings for

Advanced Applications Pavel Bartovský1, Adolfo Benedito Borrás1,

José María García Pérez1, Belén Monje

Martínez1, Amador García Sancho1

AIMPLAS, Synthesis Department, Paterna

(Valencia), Spain

Abstract

Ice accumulation is a serious concern in many

applications such as aviation, shipping,

communications, and in power generation and

transmission. There is no coating available that

prevents the build-up of ice for all cases.

However, several approaches have been

employed to reduce ice formation and

adhesion on treated metal, plastic, composite,

ceramic and glass surfaces.

Superhydrophobic coatings can increase the

contact angle of water-droplets with treated

surface and therefore reduce the accumulation

of water which can lead to ice formation.

Hereby, we present results on the

development of superhydrophobic coatings

based on polymers modified with different

types of silica nanoparticles. These

dramatically increase the contact angle

between treated surfaces and water droplets

and therefore can be used for several

applications such as naval construction, wind

turbines or antifouling coatings.

When dealing with icing, the superhydrophobic

materials are not always the best solution as

they are usually not able the prevent ice

formation and accumulation. Therefore,

increasing interest is observed in the

development of materials for anti-icing

passive and active coatings that could reduce

the icing of aircrafts, ships, rail vehicles, wind

turbines and other objects and structures

exposed to dynamic extreme environmental

conditions. To reach the objectives we have

used a combination of modified polymers with

a variety of carbon-based nanoparticles which

allowed us to control the nano-rugosity of the

treated surfaces and thus to reduce the ice

adhesion and accumulation in the case of

passive prevention and to melt the already

accumulated ice using self-heated coatings as

an active anti-icing strategy.

Keywords: Coatings, superhydrophobicity, anti-

icing, nanoparticles, polymers, composites

1 Introduction

Icing is an important problem in several sectors

from aeronautics to off-shore structures,

including power lines, wind turbines, steam

power plants and even for trucks and rail-

vehicles among others. Its effects can have

catastrophic consequences which could result

in large socioeconomical losses and fatal

accidents with loss of human lives. Icing is

particularly critical for aircrafts so several anti-

icing systems (preventing ice formation) have

been developed to avoid or reduce it. Most of

these technologies require continuous supply

of hot air, chemical products, electrical power

or breaking up ice formations, usually by

inflatable boots placed on the wing leading

edge. However, modifying the structure of the

surface to reduce or eliminate icing is a much

more attractive solution. For instance, the use

of ice-phobic coatings on top of the exposed

surfaces constitutes a strategy that has

generated promising results. On the other

hand, some of the icing affected components

can also suffer in flight particle erosion causing

coating wear and losing their anti-icing

function.

SUPERHYDROPHOBICITY and

ICEPHOBICITY (passive anti-icing)

Superhydrophobic surfaces are highly

hydrophobic and thus extremely difficult to wet.

The contact angles of water droplet exceed

150º. This is also referred to as the lotus effect,

inspired by the superhydrophobic leaves of the

lotus plant. A droplet impacting on this kind of

surfaces should fully rebound. These solutions

are useful in situations when water

accumulation can cause problems such as on

the central and rear parts of wings and helices

of wind turbines, marine platforms and many

others. However, superhydrophobic surfaces

are not the best protection against icing as we

have demonstrated in this work.

The reason is the different behaviour of water

droplets and ice crystals, particularly in a

dynamic environment. The contact angle

although critical for hydrophobicity lacks

relevance when dealing with ice crystals where

different physical models are taking place as

resumed in the figure 1:

Fig 1: Hydrophobicity and icephobicity evaluation tests

The methods of protection based on

icephobicity of the composites is a very

attractive research area and these methods

are usually referred to as PASSIVE

METHODS.

To reach the main objective, the passive

protection should meet the following criteria:

1. Reduction of ice adhesion

2. Reduction of icing/nucleation

phenomena

3. Polymer matrix need to be transformed

in icephobic material

An additional challenge is to maintain the

adhesion and mechanical properties of the

polymer.

In this work, we present results on

development on novel composite coatings

comprising a superhydrophobic matrix

reinforced with hard particles in order to protect

treated surfaces from icing. For the matrix,

already known materials such as functionalized

silicones or teflons, to which nano- or micro-

particles such as oxides, carbides, carbon

nanotubes, quasicristalline materials, etc. were

added. Low cost and appropriate deposition

techniques for large surfaces were employed

in a way so that industrial application can

become a reality.

DEVELOPED PASSIVE METHODS

• Combination of nanoparticles and low

energy polymers

• PU, polyacrylates

• Specific silica modification

• Applied as coating

• Spray techniques or paint brush, roller

application

A big challenge has been poor adhesion and

high viscosity of some composites.

COATINGS WITH THERMAL PROPERTIES –

based on Joule effect (active anti-icing)

Another complementary strategy is the use of

thermal systems based on the Joule effect. In

such cases, the polymer “non-conductive”

matrix needs to be transformed in a material

able to generate heat due to passage of

electric current through the coating and thus to

reduce ice formation and/or eliminate

accumulated ice on the treated surface.

This solution can be applied on polyurethanes,

polyesters, polyacrylates and epoxy- coatings

which can be combined with passive methods

of antiícing protection such as the use of

fluorinated polymers. The specific task of heat

generation requiers the use of highly

conductive materials such as carbon

nanotubes (CNT). For this purpose, specific

techniques of CNTs modification and

dispersion are required. For a real-life

application it is necessary to develop

formulations that can be applied using

common techniques used in coating such as

spray techniques and brush or roller

application. In this point a big chalenge is to

enhance the polymer conductivity without the

loss of coating adhesion and without increase

of the coating viscosity.

2 Experimental

2.1 Superhydrophobic coatings and

passive anti-icing coatings

2.1.1 Synthesis of silica nanoparticles with

controlled size

The synthesis of the nano-silicas was

performed in size-controlled manner using well

described reactivity of tetraethyl orthosilicate

(TEOS) which undergoes reaction with water

that can be resumed by the following

equations:

This hydrolysis reaction is an example of the

SOL-GEL process. The reaction proceeds via

a series of condensation reactions that convert

the TEOS molecule into a mineral-like solid via

the formation of Si-O-Si linkages. Rates of this

conversion are sensitive to the presence of

acids and bases, both of which serve as

catalyst. Figure 2 shows different processes

employed to synthesis monodispersed nano-

silicas in a size-controlled manner:

Fig 2: Synthesis of controlled size silica

nanoparticles – schematic representation

Employing the sol-gel method described above

we prepared different monodisperse silica

spheres with diameter ranging from 100 to

1200 nm in a controlled manner and with

reaction yields between 95-99%. Two

representative examples are shown in the

table 1.

Table 1:. Some typical examples of prepared silica nanoparticles

Material NH3

Ratio

TEOS/EtOH

Reaction

time (h)Shape

Yield

(%)

Diameter

(nm)

PRO13-0333-

03-07-b2 83 1/0,88 2 Sphere 99 1100-1200

PRO13-0333-

03-23-b2 94+surfactant 1/8 1,5 Sphere 95 180-200

Figure 3 represents an example of SEM

analysis of the prepared silicas.

Fig 3:Example of SEM images of prepared silica

nanoparticles

From the industrial point of view it is worth to

mention that the synthesis has been optimized

and scale-up has been made up to the

kilograms quantity of monodispersed

nanoparticles so it allows real-life application of

the developed process.

2.1.2 Functionalization of prepared silica

nanoparticles

To improve the hydrophobicity even more and

to modify the nano-rugosity of the coatings and

thus to enhance the passive protection against

ice formation we developed nano-silicas with

controlled size functionalized with various

silanes with different chain types following the

procedure shown below:

Some of the silanes employed for the silica

functionalization were chlorotrimethylsilane

(ClTMS), dichloromethylsilane (DCMS) and

alkylchloromethylsilane (ALKCMS). The

reaction conditions and yield are shown in

table 2.

Table 2: Functionalization of silica nanoparticles with silanes

Material Silane Reaction time (h) T (ºC) Yield (%) Hydrophobicity

PRO13-0333-03-23 70% ClTMS 24 60 72 OK

PRO13-0333-03-23 70% DCMS 24 60 85 OK

PRO13-0333-03-2370%

ALKCMS24 60 83 OK

2.1.3 Development of novel passive anti-icing

formulations

The prepared silica nanoparticles and

functionalized silicas were typically employed

in matrices based on polyurethanes and

polyacrylates. Most of them were commercially

available paintings such as the following

example whose composition has been

analysed with results shown below:

Polyurethane (commercial sample):

A component (100 g):

• 30% Butyl acetate

• 40% Charges (nano-SiO2 and nano-

TiO2

• 30% Aromatic diol (phthalic) 500 g/mol

B component (33 g):

• 100% Isocyanate bi/trifunctional NCO

19%

PU coatings were modified by dispersion of a)

silica nanoparticles prepared vía sol-gel

synthesis; b) silica nanoparticles functionalized

with alkylsilanes; c) functionalized fluorinated

polymers (3 types of polyethers), d)

perfluoroalkyl silanes, e) perfluorinated diols

among others.

In some formulations NaCl, CsF, CaCl2 and

cationic polymers were added as anti-nucleic

agents.

2.2 Active anti-icing coatings

The main problem is the functionalization of a

nonconductive polymer matrix to convert it in to

an electrical and thermal conductor. Our

approach is to focus on the modification of

polyurethanes polyacrylates and

fluoropolymers with electrical and thermal

superconductors. For this purpose, different

carbon nanotubes were employed.

Specific functionalization to synthesize a

copolymer or grafted polymer was used with

the aim to create strong F-F interactions. The

results were patented (ES 2398274 (A1) –

2013-03-15).

To quantify the heating capacity of the

developed coatings several assays have been

performed. First, the resistance was measured

and from the resistivity (Ω/cm) the values of

conductivity in S/cm were calculated to confirm

whether the coatings reach the values of

conductivity which correspond to the Joule

effect as shown in the figure 4.

Fig 4: Electric conductivity scale – Joule heating

range.

Beside the theoretical evaluation, practical

tests of the heating capacity have been

performed.

Samples covered with CNTs modified coatings

were connected to a power source maintaining

constant geometry (distance between the two

electrodes and sample size) and constant

electric current. We measured the temperature

using an IR camera as shown in the figure 5.

Table 3 shows some of the formulations tested

as possible active coatings.

Fig 5. Heating capacity of a CNT modified coating –

IR camera

Table 3: Families of active coatings based on CNTs

modified

Commercial

Material

CNTs

%

Joule

Con-

ductivity

Heat-

able

Polyacrylate

(coatings) 4%

Polyacrylate

(coatings)

4%

mod

Polyurethane

(coating) 2%

Polyurethane

(coating)

2%

mod

Capstone

(fluoropolymer) 6%

Capstone

(fluoropolymer)

6%

mod

3 Results and Discussion

3.1 Superhydrophobic and passive anti-icing coatings

From the comparison of hydrophobic and ice-

phobic properties of the developed coatings it

can be observed that the superhydrophobicity

is not the optimal solution for prevention of ice

formation and accumulation. Coatings with

high contact angle present poor ice reduction

capacity as can be seen in the table 4. On the

other hand, most of the coatings with contact

angles between 110 and 120ºC were those

with better capacity to reduce ice formation on

treated surfaces.

Table 4: Hydrophobicity vs. ice-phobic capacity

These results show that more factors should

be taken in to account while predicting anti-

icing capability. It suggests that other factors

play important role along with hydrophobicity.

Measuring contact angle is not the most

relevant method of quantification in this case.

Instead sliding angle (roll-off) should be used.

However, even this approach is only a

simplified approximation because the physio-

mechanical behaviour of an ice cube or crystal

differs significantly from that of a water droplet.

For this reason, we performed ice-adhesion

tests and ice accretion/formation tests under

controlled climatic conditions which simulate

the real dynamic environment.

The most important internal factors that

influence the ice-formation were found to be

surface roughness, nucleation elements and

surface temperature besides the environmental

factors that are all shown in figure 6.

Fig 6: Environmental factors and coating properties that influence ice formation

3.2 Anti-icing active coatings

The developed active anti-icing coatings were

tested on aluminium surfaces

in simulated real environmental conditions (air

flow, humidity, temperature). The results were

very encouraging with a temperature increase

of up to 90ºC in 10 seconds. The assay is

schematically represented in figure 7 (video

available).

Fig 7: Anti-icing coating applied treated on surface

4 Conclusion

Passive anti-icing coatings were developed

based on modified hydrophobic polymers. The

use of silica nano and microparticles resulted

in enhanced hydrophobicity and nano-rugosity

modification. Combination of silicas, low

superficial energy polymers and antinucleant

agents gave as result an improvement of anti-

icing capacity of the coatings. Improvement of

our systems requires a deep knowledge in

icing formation. Ice adhesion force, ice

nucleation, roughness and sliding angle are

powerful tools in order to predict the anti-icing

behaviour. Reduction of roughness is one of

the most promising tools to improve the

passive anti-icing properties.

Increasing the thermal conductivity of the

coatings will help us to improve the efficiency

of active systems reducing energy

consumption. Anti-icing solutions require an

overall approach including surface chemistry

and roughness and environmental conditions,

e.g. temperature, speed, and water droplets.

Acknowledgments

The authors wish to kindly thank for financial

support to the Spanish Ministry of Economy

(MINECO) and to European Commission for

the financial support.

Influence of Material Surfaces on Cell MotilityTimo Grunemann1,2, Patrick Witzel1,2, Martin

Emmert1,2, Franz L. Edel1,2, Doris Heinrich3

1 Faculty for Chemistry und Pharmacy,

Julius-Maximilians-Universität Würzburg,

Röntgenring 11, 97070 Würzburg, Germany 2 Fraunhofer Institute for Silicate Research ISC,

Neunerplatz 2, 97082 Würzburg, Germany 3 Leiden University, LION Leiden Institute of

Physics, Niels Bohrweg 2, 2333 CA Leiden,

The Netherlands * Corresponding author: Doris Heinrich,

Heinrich@physics.leidenuniv.nl

Abstract

The extracellular matrix plays an important role for essential cell functions like adhesion, migration, proliferation, and differentiation. A clear understanding of biophysical interaction mechanisms between a cell and its environment provides a solid base for a cell-type specific design of three dimensional (3D) scaffolds in terms of surface topography and chemical surface functionalization. In this work, we analyze motion patterns of D. discoideum cells on three distinct surfaces: plain glass, nano-structured silica fiber and polydimethylsiloxane (PDMS) surfaces, utilizing a global mean-squared displacement (MSD) as well as time-resolved, local mean-squared displacement (LMSD) techniques. Global MSD analysis reveals a significantly higher cell migration activity on PDMS and nano-structured fiber surfaces compared to plain glass, yet the cell movement patterns on PDMS and silica fiber cannot be further distinguished. Further evaluation based on a LMSD approach yields a more in-depth picture of the specific surface influence on cell migration as this method enables differentiation between quasi-random and directed migration phases. We find that the nanostructure of the silica fibers induces significantly more directed migration phases compared to the glass and PDMS surfaces. Furthermore, the hydrophobic PDMS surface leads to greatly increased directed migration velocities. This analysis shows the advantage of a time-resolved LMSD analysis to reveal substantial differences in cell-material interaction. These insights will facilitate development of new biomedical materials with intrinsic properties designed to control cell migration leading to novel diagnostic and therapeutic concepts.

Keywords: Cell Motility, Material Surface, Cell-Material Interaction, Mean-squared Displacement, Live-cell Imaging

1 Introduction

Cell migration plays a vital role in a wide range of processes in the human body including wound healing [1], immune response, and embryogenesis [2,3], as well as for aberrations like cancer metastasis. As such, the investigation of cell migration behavior should be taken into account for the development of novel materials designed for direct contact with human tissue. Additional control of migration patterns could enable new applications in tissue engineering ranging from drug testing in vitro, reducing expensive clinical studies, to implants minimizing engraftment times.

In this work, we demonstrate the immense influence of different surfaces on cell migration behavior utilizing global mean-squared displacement (MSD) as well as time-resolved local mean-squared displacement (LMSD) analysis [4,5]. It is well known that cell motility is affected by several cues of the cellular surrounding, called the extracellular matrix within an organism, including surface chemistry [6], Young’s modulus [7], and topography [4,8]. It is, however, still unclear which of these external cues are dominant and overwrite other influences on cell migration behavior. To get a deeper insight, we obtained and analyzed migration data of Dictyostelium discoideum (D. discoideum) cells on three distinct surfaces. D. discoideum is a social amoeba exhibiting amoeboid migration modes comparable to stem cells or immune cells [9,10]. Strains of these cell type are often used as a model organism for chemotaxis [11], phagocytosis, and human diseases because of their completely sequenced genome and the relatively simple cell culture and handling [12]. The investigated surfaces comprised glass, as a standard and widely utilized biomedical reference, silica fibers with a nano-rough surface structure, used as wound healing inserts, and polydimethylsiloxane (PDMS) as an example for the polymeric material class and commonly used in experimental microfluidic and cellular assays.

2 Experimental

2.1 Preparation and Characterization of Silica Fibers Silica fibers were fabricated by pressure extrusion on cover glass substrates according to [8,13]. The resulting silica gel fibers’ composition

was determined to be [Si(OH)0.2O1.9]n by thermal analysis in a previous work [13] similar to glass. The fibers exhibited a dog-bone-like shape with measured fiber widths of 60 ± 20 μm and fiber heights of 24 ± 8 μm. Freshly fabricated fibers exhibited a smooth surface with no distinct topography. After contact with aqueous media nano-sized surface patterns were observed through biodegradation [13]. SEM investigations on fibers that have been stored in solution for several days revealed pits and elevations on the fiber surface with feature sizes below 200 nm [8].

2.2 PDMS Preparation The polydimethylsiloxane (PDMS) is processed as a two-component material. The precursor of Sylgard 184 (Dow Corning, USA) is mixed with the curing agent at weight ratio of 9:1. For air removal the solution was degassed at 160 mbar in a desiccator for 30 min. The resin was flood coated on a silicon wafer passivated by perfluorodecyltrichlorsilane (Sigma-Aldrich, St. Louis, Missouri, USA) and again degassed at 6×10-2 mbar with a temperature progression from room temperature to 50 °C over 30 min. The PDMS was cured at 65 °C for 6 h. Previous to the migration experiments the surface of the PDMS was activated by ozone plasma for 15 min at a power level of 70 % (Diener electronic GmbH + Co. KG, Ebhausen, Germany).

2.3 Cell Culture For the migration experiments cells of the D. discoideum strain AX2 expressing homogenously distributed green fluorescent protein (GFP) in the cytoplasm were grown under 40% confluency at 19–21 °C in petri dishes at ambient air. For cell culture, HL-5C medium with glucose (HLC0102, ForMediumTM, UK) including 20 μg mL−1 gentamycin (G-418-Biochrom A2912, Biochrom AG) was used.

2.4 Preparation and Migration Experiments Previous to the migration experiments, the cells were washed four times with phosphate buffered saline (PBS) with subsequent centrifugation at 2000 rpm for 4 minutes (Eppendorf MiniSpin®

Plus, Eppendorf AG) and resuspension of the cell pellet in PBS. Afterwards the solutions’ cell density was adjusted to achieve a surface cell density of 400,000 cells per cm2. The cells were kept 30 mins at rest after seeding to the surface. Image acquisition was performed with a Nikon Eclipse Ti fluorescence microscope (Nikon, Germany) with an Intensilight (Nikon, Germany) light source and a 20x objective (Nikon, Germany). To enable GFP imaging, an

excitation filter (F36-525 HC, AHF, Germany) ensured excitation wavelengths of 457–487 nm and detection wavelengths of 500–540 nm. The signals were detected with an EM-CCD camera (Hamamatsu, Japan). Images were acquired every 7 seconds for the glass and silica fiber experiments and every 8 seconds for the measurements on PDMS for at least 45 minutes to exclude short-time effects.

2.5 Migration analysis For migration analysis, the cell’s time dependent center-of-mass coordinates were determined by single-particle tracking based on brightness clustering. The images were edited and post-processed with the software ImageJ (National Institutes of Health, USA). The tracking was performed utilizing the ImageJ plugin “Cell Evaluator” [14]. The plugin tracks the center-of-mass of each cell yielding time dependent cell trajectories. The resulting trajectory data was analyzed by a global mean-squared displacement:

MSD: ⟨ΔR()⟩ = ⟨R (t + ) − R (t)

⟩ (1)

with R (t) as the center-of-mass position of the cell at time t and the lag time .

The analysis of the time-resolved, local mean-squared displacement (LMSD) was calculated with a MATLAB® algorithm [4,8,15,16]. This algorithm is able to reliably distinguish between two modes of amoeboid cell migration: directed cell runs and phases of quasi-random migration. For this purpose, a local mean-squared displacement (LMSD) for different lag times is calculated for every time point over a rolling window of 16 frames (corresponding to a duration of = 112 s, respectively 128 s):

LMSD: (2)

⟨ΔR(τ)⟩ = ⟨R(t + τ) − R(t)

⟩

Subsequently the LMSD data is fitted by a power law for every time point by

⟨ΔR()⟩

= A

(3)

with two chosen reference values ( = 1 μm and = 1 s), so the prefactor carries no physical dimension [17]. The exponent α characterizes the migration mode of a cell. Thus, a time point is assigned to a directed run phase if α ≈ 2. In the other case of α < 2, the time point is classified as quasi-random motion and a generalized, also called efficient, diffusion coefficient is retrieved from the prefactor A of the power law fit by

=⟨()⟩

2

=

2

(4)

with d as the number of spatial dimensions (in this work 2). is a general indicator of cell motility during random migration phases.

3 Results and Discussion

The migration behavior of D. discoideum cells is investigated on three distinctly different surfaces: glass, nano-rough silica fibers and polydimethylsiloxane (PDMS). A typical example of a D. discoideum cell migrating on the PDMS surface is shown in figure 1A. A first approach on characterizing motion behavior in general and widely used for cell migration analysis utilizes the mean-squared displacement [18–22]. The value of the MSD at a given lag time describes the motility of a cell. The lag time dependence of the MSD can be described by a power law ⟨ΔR()⟩ ∝ . The exponent characterizes the type of motion. For a linearly rising MSD ( = 1) the motion is diffusive, whereas a ballistic motion yields = 2.

Cell motion analysis by MSD reveals the lowest cell motility on glass, while the data on silica fibers and PDMS samples yield nearly identical cell motion behavior, see figure 1B. Furthermore, a lag time dependent change in the MSD behavior is observed, where the motion pattern changes from superdiffusive behavior with an exponent of = 1.75, for small lag times up to 30 seconds, to nearly diffusive motion with exponents of = 0.9 on glass and = 1.1 for the silica fiber and the PDMS surface, respectively. This transition has already been observed and is well known [18,19]. Yet the MSD analysis is quite limited in terms of revealing the underlying motion patterns on those two surface types.

Fig. 1: (A) Composite of a brightfield (greyscale) and fluorescence (green) microscopy image of a Dictyostelium discoideum cell expressing GFP-protein within the cytosol, migrating on a flat PDMS surface. The cell is shown at the start and the end point, after 2,000 s, of the cell center-of-mass migration trajectory (solid white line). (B) Global mean-squared displacement (MSD) of cell motility over lag time for the different material surfaces: glass (black), silica fiber (blue), and PDMS (red). Dashed grey lines indicate power law progressions for three α exponents: 1.75, 1.1 and 0.9.

A much deeper insight is gained by analyzing the mean-squared displacement with a local, respectively time-resolved MSD approach (LMSD). This enables the differentiation of two amoeboid cell migration modes: phases of quasi-random motion and directed runs. In figure 2A, the same cell trajectory, as displayed in figure 1A, is now distinguished into the two different motion types by color code using red and blue. The corresponding time course of the LMSD function is shown in figure 2B, revealing phases of directed runs for higher α values and quasi-random motion at lower α values.

Fig. 2: (A) Composite of a brightfield (greyscale) and fluorescence (green) microscopy image of a Dictyostelium discoideum cell expressing GFP-protein within the cytosol migrating on a flat PDMS surface. The cell is shown at the start and the end point, after 2,000 s, of the cell center-of-mass migration trajectory (see figure 1A) subdivided into parts of directed cell runs in red and phases of quasi-random cell motion in blue. (B) Plot of the local mean-squared displacement (LMSD) over experimental time and lag time . The exponent α of the power law fit for each time point is shown in colorcode.

The distributions of motion parameters from the statistical LMSD analysis of the cellular motion behavior on the different substrates – also separated into directed and random motion phases – are presented in figure 3. The general LMSD motion pattern is accurately described by the distribution of the α exponent shown in figure 3A. Here, glass and PDMS exhibit similar distributions with mean values of ⟨⟩ =1.45 and ⟨⟩ =1.47, respectively. In comparison, the mean value for migration on the silica fiber surface ⟨⟩ =1.61 is significantly higher, revealing an overall increase of directed cell motion activity. This trend is also reflected in the distributions of the state durations (figure 3B). While the second peak of the directed state

durations is stable for all surfaces, revealing an inherent underlying migration pattern, the amount of directed cell migration states is strongly surface dependent. Here, the highest amount of directed runs is found for migration on silica fiber samples (36.0 %) followed by PDMS (21.6 %) and glass (16.1 %). The nano-rough topography of the fiber induces more directed migration phases of the cells compared to the chemically similar plain surface of the glass.

The measured mean cell migration velocities (see figure 3C, black) on these three substrates confirm the trend set by the state durations with the fastest cell migration velocities on the silica fiber sample with a mean value of ⟨⟩ = 0.070 µm s-1, followed directly by PDMS with a mean velocity of ⟨⟩ = 0.066 µm s-1. The slowest migration speed is observed on the glass samples with a mean velocity of ⟨⟩ = 0.041 µm s-1. A slightly different picture is drawn by the velocities of the directed migration phases

(Figure 3C, red). The highest migration velocities in directed states are observed on the PDMS surface, 12 % faster than the migration on the fiber sample and 47 % faster than on glass. Here, the polarity of the substrates is a possible explanation. The hydrophobic surface of the PDMS compared to the very hydrophilic surface of both the silica fibers and the glass samples leads to a lower amount of cell adhesion sites and a reduced bonding strength between the cells and the substrate. This, in return, enhances the cell migration velocity [23] and leads to the observed distribution of directed cell migration velocities. The value of the effective diffusion coefficient characterizes the motility of the cells in their quasi-random motion phases. The values of these diffusion coefficients (see figure 3D) are consistent with the previously discussed influences of the surface polarity and topography on the migration behavior. The highest diffusion coefficients are observed on PDMS with ⟨⟩ =6.02 µm2 s-1 succeeded by ⟨⟩ = 5.22 µm2 s-1

obtained for the silica fiber surface. Cell migration on glass exhibits the lowest diffusion coefficients with a mean of ⟨⟩ = 2.53 µm2 s-1

confirming the trend of the directed cell migration velocities.

Fig. 3: Cell motion parameter distributions for the three distinct material surfaces: glass, silica fiber, and PDMS (A) Distribution of α exponents derived from the power law fits of the local mean-squared displacement analysis. Black: overall distribution, blue: quasi-random states, red: directed cell motion, dashed line: mean values (B) Distribution of the state durations of directed cell migrations runs, dashed line indicates mean values. The amount of directed runs compared to the overall experimental time is given in percent. (C) Instantaneous velocity distributions of the cell migration. Black: overall distribution, blue: quasi-random states, red: directed cell motion, dashed lines: mean values. (D) Distribution of the effective diffusion coefficient in the quasi-random states, dashed line indicates mean values.

Table 1: Obtained cell migration parameters from LMSD analysis of D. discoideum on three different surfaces.

Glass Fiber PDMS

Mean alpha α 1.45 1.61 1.47

% directed phases of total time 16.1 36.0 21.6

Mean velocity (µm s-1) ⟨⟩ 0.041 0.070 0.066

Mean directed velocity (µm s-1) ⟨⟩ 0.058 0.097 0.111

Mean diffusion coefficient (µm2 s-1) 2.53 5.22 6.02

Number of cells Ncells 45 25 369

Number of data points N 23.975 12.421 151.439

4 Conclusion

In this work, we elucidated cell migration behavior on three different surfaces: glass, nano-rough, biomedically used silica fibers and polymeric PDMS. Migration data was obtained utilizing Dictyostelium discoideum cells, a commonly used model organism for amoeboid migration of stem and immune cells. A time-resolved, local mean-squared displacement (LMSD) approach enabled distinguishing between different phases of cell motility: directed runs and quasi-random motion phases typical for amoeboid cell migration. We found that the nanoscale surface structured silica fibers lead to more directed phases in the cell’s migration pattern compared to both plain glass and PDMS substrates. Regarding directed motion states, we discovered that the hydrophobic PDMS yielded the highest mean cell migration speed compared to the hydrophilic substrates.

While the analysis solely based on a global MSD suggested similar migration behavior on silica fibers and PDMS, the time-resolved LMSD analysis revealed striking differences between migration on these substrates. Our results recommend a time-resolved analysis for comprehensive cell migration studies to gain a deeper understanding of the cell-material interaction. Based on these approaches, future studies will focus on cell migration, proliferation, and morphology in more complex shaped 3D environments, fabricated from novel and smart materials, mimicking real physiological environments of the human body to enhance drug testing assays and enable personalized medicine.

Acknowledgements

We acknowledge Prof. Dr. Günther Gerisch (Max-Planck Institute for Biochemistry, Germany) for providing the Dictyostelium discoideum strains and the funding from the Deutsche Forschungsgemeinschaft (grant HE5958-2-1), the Volkswagen-Foundation (grant I85100) and the Fraunhofer Attract Program for the grant “3DNanoZell”.

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Influence of Material Surfaces on Cell Motility Timo Grunemann1,2, Patrick Witzel1,2, Martin Emmert1,2, Franz L. Edel1,2, Doris Heinrich3

1 Faculty for Chemistry und Pharmacy, Julius-Maximilians-Universität Würzburg, Röntgenring 11, 97070 Würzburg, Germany 2 Fraunhofer Institute for Silicate Research ISC, Neunerplatz 2, 97082 Würzburg, Germany 3 Leiden University, LION Leiden Institute of Physics, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands * Corresponding author: Doris Heinrich, Heinrich@physics.leidenuniv.nl

Abstract The extracellular matrix plays an important role for essential cell functions like adhesion, migration, proliferation, and differentiation. A clear understanding of biophysical interaction mechanisms between a cell and its environment provides a solid base for a cell-type specific design of three dimensional (3D) scaffolds in terms of surface topography and chemical surface functionalization. In this work, we analyze motion patterns of D. discoideum cells on three distinct surfaces: plain glass, nano-structured silica fiber and polydimethylsiloxane (PDMS) surfaces, utilizing a global mean-squared displacement (MSD) as well as time-resolved, local mean-squared displacement (LMSD) techniques. Global MSD analysis reveals a significantly higher cell migration activity on PDMS and nano-structured fiber surfaces compared to plain glass, yet the cell movement patterns on PDMS and silica fiber cannot be further distinguished. Further evaluation based on a LMSD approach yields a more in-depth picture of the specific surface influence on cell migration as this method enables differentiation between quasi-random and directed migration phases. We find that the nanostructure of the silica fibers induces significantly more directed migration phases compared to the glass and PDMS surfaces. Furthermore, the hydrophobic PDMS surface leads to greatly increased directed migration velocities. This analysis shows the advantage of a time-resolved LMSD analysis to reveal substantial differences in cell-material interaction. These insights will facilitate development of new biomedical materials with intrinsic properties designed to control cell migration leading to novel diagnostic and therapeutic concepts.

Keywords: Cell Motility, Material Surface, Cell-Material Interaction, Mean-squared Displacement, Live-cell Imaging

1 Introduction

Cell migration plays a vital role in a wide range of processes in the human body including wound healing [1], immune response, and embryogenesis [2,3], as well as for aberrations like cancer metastasis. As such, the investigation of cell migration behavior should be taken into account for the development of novel materials designed for direct contact with human tissue. Additional control of migration patterns could enable new applications in tissue engineering ranging from drug testing in vitro, reducing expensive clinical studies, to implants minimizing engraftment times.

In this work, we demonstrate the immense influence of different surfaces on cell migration behavior utilizing global mean-squared displacement (MSD) as well as time-resolved local mean-squared displacement (LMSD) analysis [4,5]. It is well known that cell motility is affected by several cues of the cellular surrounding, called the extracellular matrix within an organism, including surface chemistry [6], Young’s modulus [7], and topography [4,8]. It is, however, still unclear which of these external cues are dominant and overwrite other influences on cell migration behavior. To get a deeper insight, we obtained and analyzed migration data of Dictyostelium discoideum (D. discoideum) cells on three distinct surfaces. D. discoideum is a social amoeba exhibiting amoeboid migration modes comparable to stem cells or immune cells [9,10]. Strains of these cell type are often used as a model organism for chemotaxis [11], phagocytosis, and human diseases because of their completely sequenced genome and the relatively simple cell culture and handling [12]. The investigated surfaces comprised glass, as a standard and widely utilized biomedical reference, silica fibers with a nano-rough surface structure, used as wound healing inserts, and polydimethylsiloxane (PDMS) as an example for the polymeric material class and commonly used in experimental microfluidic and cellular assays.

2 Experimental 2.1 Preparation and Characterization of Silica Fibers Silica fibers were fabricated by pressure extrusion on cover glass substrates according to [8,13]. The resulting silica gel fibers’ composition

was determined to be [Si(OH)0.2O1.9]n by thermal analysis in a previous work [13] similar to glass. The fibers exhibited a dog-bone-like shape with measured fiber widths of 60 ± 20 μm and fiber heights of 24 ± 8 μm. Freshly fabricated fibers exhibited a smooth surface with no distinct topography. After contact with aqueous media nano-sized surface patterns were observed through biodegradation [13]. SEM investigations on fibers that have been stored in solution for several days revealed pits and elevations on the fiber surface with feature sizes below 200 nm [8].

2.2 PDMS Preparation The polydimethylsiloxane (PDMS) is processed as a two-component material. The precursor of Sylgard 184 (Dow Corning, USA) is mixed with the curing agent at weight ratio of 9:1. For air removal the solution was degassed at 160 mbar in a desiccator for 30 min. The resin was flood coated on a silicon wafer passivated by perfluorodecyltrichlorsilane (Sigma-Aldrich, St. Louis, Missouri, USA) and again degassed at 6×10-2 mbar with a temperature progression from room temperature to 50 °C over 30 min. The PDMS was cured at 65 °C for 6 h. Previous to the migration experiments the surface of the PDMS was activated by ozone plasma for 15 min at a power level of 70 % (Diener electronic GmbH + Co. KG, Ebhausen, Germany).

2.3 Cell Culture For the migration experiments cells of the D. discoideum strain AX2 expressing homogenously distributed green fluorescent protein (GFP) in the cytoplasm were grown under 40% confluency at 19–21 °C in petri dishes at ambient air. For cell culture, HL-5C medium with glucose (HLC0102, ForMediumTM, UK) including 20 μg mL−1 gentamycin (G-418-Biochrom A2912, Biochrom AG) was used.

2.4 Preparation and Migration Experiments Previous to the migration experiments, the cells were washed four times with phosphate buffered saline (PBS) with subsequent centrifugation at 2000 rpm for 4 minutes (Eppendorf MiniSpin® Plus, Eppendorf AG) and resuspension of the cell pellet in PBS. Afterwards the solutions’ cell density was adjusted to achieve a surface cell density of 400,000 cells per cm2. The cells were kept 30 mins at rest after seeding to the surface. Image acquisition was performed with a Nikon Eclipse Ti fluorescence microscope (Nikon, Germany) with an Intensilight (Nikon, Germany) light source and a 20x objective (Nikon, Germany). To enable GFP imaging, an

excitation filter (F36-525 HC, AHF, Germany) ensured excitation wavelengths of 457–487 nm and detection wavelengths of 500–540 nm. The signals were detected with an EM-CCD camera (Hamamatsu, Japan). Images were acquired every 7 seconds for the glass and silica fiber experiments and every 8 seconds for the measurements on PDMS for at least 45 minutes to exclude short-time effects.

2.5 Migration analysis For migration analysis, the cell’s time dependent center-of-mass coordinates were determined by single-particle tracking based on brightness clustering. The images were edited and post-processed with the software ImageJ (National Institutes of Health, USA). The tracking was performed utilizing the ImageJ plugin “Cell Evaluator” [14]. The plugin tracks the center-of-mass of each cell yielding time dependent cell trajectories. The resulting trajectory data was analyzed by a global mean-squared displacement:

MSD: ⟨ΔR2(𝜏)⟩ = ⟨R (t + 𝜏) − R (t)2⟩𝑡 (1)

with R (t) as the center-of-mass position of the cell at time t and the lag time 𝜏.

The analysis of the time-resolved, local mean-squared displacement (LMSD) was calculated with a MATLAB® algorithm [4,8,15,16]. This algorithm is able to reliably distinguish between two modes of amoeboid cell migration: directed cell runs and phases of quasi-random migration. For this purpose, a local mean-squared displacement (LMSD) for different lag times 𝜏 is calculated for every time point 𝑡𝑖 over a rolling window of 16 frames (corresponding to a duration of 𝑇 = 112 s, respectively 128 s):

LMSD: (2)

⟨ΔR2(τ)⟩i = ⟨R(ti + τ) − R(ti)2⟩ti−

T2 < ti < ti+

T2

Subsequently the LMSD data is fitted by a power law for every time point 𝑡𝑖 by

⟨ΔR2(𝜏)⟩𝑖 𝑙2

= A𝑖 𝜏𝜏0𝛼𝑖

(3)

with two chosen reference values (𝑙 = 1 μm and 𝜏0 = 1 s), so the prefactor 𝐴𝑖 carries no physical dimension [17]. The exponent α characterizes the migration mode of a cell. Thus, a time point is assigned to a directed run phase if α ≈ 2. In the other case of α < 2, the time point is classified as quasi-random motion and a generalized, also called efficient, diffusion coefficient 𝐷eff is retrieved from the prefactor A of the power law fit by

𝐷 =⟨𝛥𝛥2(𝜏)⟩

2𝑑𝜏0=

𝐴𝑙2

2𝑑𝜏0 (4)

with d as the number of spatial dimensions (in this work 2). 𝐷eff is a general indicator of cell motility during random migration phases.

3 Results and Discussion

The migration behavior of D. discoideum cells is investigated on three distinctly different surfaces: glass, nano-rough silica fibers and polydimethylsiloxane (PDMS). A typical example of a D. discoideum cell migrating on the PDMS surface is shown in figure 1A. A first approach on characterizing motion behavior in general and widely used for cell migration analysis utilizes the mean-squared displacement [18–22]. The value of the MSD at a given lag time describes the motility of a cell. The lag time dependence of the MSD can be described by a power law ⟨ΔR2(𝜏)⟩ ∝ 𝜏𝛼. The exponent 𝛼 characterizes the type of motion. For a linearly rising MSD (𝛼 = 1) the motion is diffusive, whereas a ballistic motion yields 𝛼 = 2.

Cell motion analysis by MSD reveals the lowest cell motility on glass, while the data on silica fibers and PDMS samples yield nearly identical cell motion behavior, see figure 1B. Furthermore, a lag time dependent change in the MSD behavior is observed, where the motion pattern changes from superdiffusive behavior with an exponent of 𝛼 = 1.75, for small lag times up to 30 seconds, to nearly diffusive motion with exponents of 𝛼 = 0.9 on glass and 𝛼 = 1.1 for the silica fiber and the PDMS surface, respectively. This transition has already been observed and is well known [18,19]. Yet the MSD analysis is quite limited in terms of revealing the underlying motion patterns on those two surface types.

Fig. 1: (A) Composite of a brightfield (greyscale) and fluorescence (green) microscopy image of a Dictyostelium discoideum cell expressing GFP-protein within the cytosol, migrating on a flat PDMS surface. The cell is shown at the start and the end point, after 2,000 s, of the cell center-of-mass migration trajectory (solid white line). (B) Global mean-squared displacement (MSD) of cell motility over lag time 𝜏 for the different material surfaces: glass (black), silica fiber (blue), and PDMS (red). Dashed grey lines indicate power law progressions for three α exponents: 1.75, 1.1 and 0.9.

A much deeper insight is gained by analyzing the mean-squared displacement with a local, respectively time-resolved MSD approach (LMSD). This enables the differentiation of two amoeboid cell migration modes: phases of quasi-random motion and directed runs. In figure 2A, the same cell trajectory, as displayed in figure 1A, is now distinguished into the two different motion types by color code using red and blue. The corresponding time course of the LMSD function is shown in figure 2B, revealing phases of directed runs for higher α values and quasi-random motion at lower α values.

Fig. 2: (A) Composite of a brightfield (greyscale) and fluorescence (green) microscopy image of a Dictyostelium discoideum cell expressing GFP-protein within the cytosol migrating on a flat PDMS surface. The cell is shown at the start and the end point, after 2,000 s, of the cell center-of-mass migration trajectory (see figure 1A) subdivided into parts of directed cell runs in red and phases of quasi-random cell motion in blue. (B) Plot of the local mean-squared displacement (LMSD) over experimental time 𝑡 and lag time 𝜏. The exponent α of the power law fit for each time point is shown in colorcode.

The distributions of motion parameters from the statistical LMSD analysis of the cellular motion behavior on the different substrates – also separated into directed and random motion phases – are presented in figure 3. The general LMSD motion pattern is accurately described by the distribution of the α exponent shown in figure 3A. Here, glass and PDMS exhibit similar 𝛼 distributions with mean values of ⟨𝛼⟩ =1.45 and ⟨𝛼⟩ =1.47, respectively. In comparison, the mean value for migration on the silica fiber surface ⟨𝛼⟩ =1.61 is significantly higher, revealing an overall increase of directed cell motion activity. This trend is also reflected in the distributions of the state durations (figure 3B). While the second peak of the directed state

durations is stable for all surfaces, revealing an inherent underlying migration pattern, the amount of directed cell migration states is strongly surface dependent. Here, the highest amount of directed runs is found for migration on silica fiber samples (36.0 %) followed by PDMS (21.6 %) and glass (16.1 %). The nano-rough topography of the fiber induces more directed migration phases of the cells compared to the chemically similar plain surface of the glass.

The measured mean cell migration velocities (see figure 3C, black) on these three substrates confirm the trend set by the state durations with the fastest cell migration velocities on the silica fiber sample with a mean value of ⟨𝑣⟩ = 0.070 µm s-1, followed directly by PDMS with a mean velocity of ⟨𝑣⟩ = 0.066 µm s-1. The slowest migration speed is observed on the glass samples with a mean velocity of ⟨𝑣⟩ = 0.041 µm s-1. A slightly different picture is drawn by the velocities of the directed migration phases 𝑣dir (Figure 3C, red). The highest migration velocities in directed states are observed on the PDMS surface, 12 % faster than the migration on the fiber sample and 47 % faster than on glass. Here, the polarity of the substrates is a possible explanation. The hydrophobic surface of the PDMS compared to the very hydrophilic surface of both the silica fibers and the glass samples leads to a lower amount of cell adhesion sites and a reduced bonding strength between the cells and the substrate. This, in return, enhances the cell migration velocity [23] and leads to the observed distribution of directed cell migration velocities. The value of the effective diffusion coefficient 𝐷eff characterizes the motility of the cells in their quasi-random motion phases. The values of these diffusion coefficients (see figure 3D) are consistent with the previously discussed influences of the surface polarity and topography on the migration behavior. The highest diffusion coefficients are observed on PDMS with ⟨𝐷eff⟩ = 6.02 µm2 s-1 succeeded by ⟨𝐷eff⟩ = 5.22 µm2 s-1 obtained for the silica fiber surface. Cell migration on glass exhibits the lowest diffusion coefficients with a mean of ⟨𝐷eff⟩ = 2.53 µm2 s-1 confirming the trend of the directed cell migration velocities.

Fig. 3: Cell motion parameter distributions for the three distinct material surfaces: glass, silica fiber, and PDMS (A) Distribution of α exponents derived from the power law fits of the local mean-squared displacement analysis. Black: overall distribution, blue: quasi-random states, red: directed cell motion, dashed line: mean values (B) Distribution of the state durations of directed cell migrations runs, dashed line indicates mean values. The amount of directed runs compared to the overall experimental time is given in percent. (C) Instantaneous velocity distributions of the cell migration. Black: overall distribution, blue: quasi-random states, red: directed cell motion, dashed lines: mean values. (D) Distribution of the effective diffusion coefficient in the quasi-random states, dashed line indicates mean values.

Table 1: Obtained cell migration parameters from LMSD analysis of D. discoideum on three different surfaces.

Glass Fiber PDMS

Mean alpha α 1.45 1.61 1.47

% directed phases of total time 16.1 36.0 21.6

Mean velocity (µm s-1) ⟨𝑣⟩ 0.041 0.070 0.066

Mean directed velocity (µm s-1) ⟨𝑣dir⟩ 0.058 0.097 0.111

Mean diffusion coefficient (µm2 s-1) 𝐷eff 2.53 5.22 6.02

Number of cells Ncells 45 25 369

Number of data points N 23.975 12.421 151.439

4 Conclusion

In this work, we elucidated cell migration behavior on three different surfaces: glass, nano-rough, biomedically used silica fibers and polymeric PDMS. Migration data was obtained utilizing Dictyostelium discoideum cells, a commonly used model organism for amoeboid migration of stem and immune cells. A time-resolved, local mean-squared displacement (LMSD) approach enabled distinguishing between different phases of cell motility: directed runs and quasi-random motion phases typical for amoeboid cell migration. We found that the nanoscale surface structured silica fibers lead to more directed phases in the cell’s migration pattern compared to both plain glass and PDMS substrates. Regarding directed motion states, we discovered that the hydrophobic PDMS yielded the highest mean cell migration speed compared to the hydrophilic substrates.

While the analysis solely based on a global MSD suggested similar migration behavior on silica fibers and PDMS, the time-resolved LMSD analysis revealed striking differences between migration on these substrates. Our results recommend a time-resolved analysis for comprehensive cell migration studies to gain a deeper understanding of the cell-material interaction. Based on these approaches, future studies will focus on cell migration, proliferation, and morphology in more complex shaped 3D environments, fabricated from novel and smart materials, mimicking real physiological environments of the human body to enhance drug testing assays and enable personalized medicine.

Acknowledgements

We acknowledge Prof. Dr. Günther Gerisch (Max-Planck Institute for Biochemistry, Germany) for providing the Dictyostelium discoideum strains and the funding from the Deutsche Forschungsgemeinschaft (grant HE5958-2-1), the Volkswagen-Foundation (grant I85100) and the Fraunhofer Attract Program for the grant “3DNanoZell”.

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Circulating tumor cells (CTC) filtration of non-fluorescent membrane FilterMasaru Hori1, Naoto Kihara1,2, Kenji Ishikawa1,

Hidefumi Odaka1,2, Daisuke, Onoshima1, and

Yoshinobu Baba1

1 Nagoya University, Furo-cho, Chikusa, Nagoya,

Aichi, 464-8601 Japan2 Asahi Glass Co.,Ltd. Yokohama 221-8755 Japan

Abstract

Rapidly increasing demands for trapping of circulating tumor cells (CTCs) from a whole human blood, it is necessary to develop methods that biological cells are filtrated rapidly and precisely for the medical diagnostics and therapies. The commercial polymer membrane filters are in general opaque and have been fabricated by opening randomly pores for the filtration with their size. To diagnose capturing cells rapidly and precisely, the targeting cells are separated based on the mechanism of size-exclusion and cytoskeletal deformability, and then they are usually stained by fluorescent molecules of the cell-surface expression markers. The membrane filter with through-holes was fabricated by using the photolithographic patterning and dry etching method. The holes were precisely aligned more than 300,000 with a diameter of 1-μm-scale in a typical area of a diameter of 13 mm. Non-fluorescent polymer membrane is used in order for the fluorescent detection of rare CTCs in the human blood. Here, advanced technology of the non-fluorescent filter membranes with precisely aligned through-holes will be shown.

Keywords: Membrane filter; Circulating tumor cells (CTC); Non-fluorescent polymer membrane

1 Introduction

Membrane filters have been commonly used for separation purposes in the analytical biochemistry. The commercial polymer membrane filters are in general opaque and have been fabricated by opening randomly pores for the filtration with their size.

Rapidly increasing demands for trapping of circulating tumor cells (CTCs) from a whole human blood, it is necessary to develop methods that biological cells are filtrated rapidly and precisely for the medical diagnostics and therapies. The membrane filters are candidates

for cell-separation on the mechanism of size-exclusion. To diagnose capturing cells rapidly and precisely, the targeting cells are usually stained by fluorescent molecules of the cell-surface expression markers.[1]

The size-exclusive microfiltration device made of polyethylene terephthalate (PET) for the CTCs were successfully developed by our group.[2] This has been fabricated by using photolithography-patterned metallic masks and plasma etching. The PET filter could isolate cells precisely with size exclusions, however suffers the auto-fluorescence in the microscopy. The materials used for filter are issued on auto-fluorescence of polydimethylsiloxane (PDMS),[3] polycarbonate (PC),[4-6] and the photoresist.[14] The metals such as Ni[7-9] and Pd [10] provide only opaque membranes. In addition, the track etching method fabricates randomly pores, in which connected plural holes unfortunately act to be relatively large holes.[6,11]

Ethylene-tetrafluoroethylene (ETFE) is a copolymer, which indicates the excellent chemical durability for both acid and alkali, and transmittance to the visible light.[12,13] and mediates the auto-fluorescence problem. This is also a high heat-resistant polymer compared to PET and PC. In use of ETFE, the large thermal expansion of ETFE is taken into account for plasma etching. We have optimized a fabrication method and etch-mask material.

Here we report an optically transparent ETFE membrane-filter with several tens of millions of bored-through holes. This has been fabricated by using photolithography-patterned metallic masks and plasma etching. The ETFE membrane filter was fabricated with high etching rate (1.35 μm/min), has been realized with high etching selectivity of Ti mask and precisely aligned 380,000 holes. The holes with each diameter of 7 μm, were precisely fabricated. This filter can be used for the filtration of the CTCs in the whole blood.

2 ETFE membrane filter

2.1 Materials and methods

Figure 1 shows fabrication scheme of ETFE membrane filters. A silicon wafer with a diameter of 150 mm was used as a substrate. To paste an ETFE film (AGC Asahi Glass) with a thickness of 25 μm to the Si wafer, a double-coated polyimide tape with a square of 100 mm, one side of weak adhesive force and the other side of strong

adhesive force was pasted on the Si substrate by a hand roller. Subsequently, the ETFE film was pasted on the front-side face of the double-coated adhesive tape.

A Ti film was deposited with a thickness of 150 nm on the ETFE surface by a DC magnetron sputtering apparatus. During the sputtering, an ultra-high purity Ar gas was introduced and the pressure was kept at 0.7 Pa. metallic Ti has good resistances in thermal crack due to very large coefficients of ETFE for thermal expansion.

Photolithographic patterns were prepared using a positive tone photoresist resin. The photomask patterns were a diameter of 7 μm arrayed with a fixed distance of 20 μm in an area with a diameter of 13 mm. The exposed patterns were then developed. The 0.38 millions of hole patterns were successfully formed. Then, the resist patterns were transferred to the Ti film by plasma etching using Cl2 gas. A commercial inductively coupled plasma (ICP) etcher was used for Ti etching. RF power was applied to an antenna coil. Simultaneously, RF bias power was applied to a wafer stage of the ICP etcher. The pressure was fixed at 1 Pa.

Through-holes etching processes were performed using O2 plasma at 0.5 and 4 Pa. The patterns on the Ti-mask were transferred to the ETFE film by plasma etching using pure O2 gas. A commercial ICP etcher was also used. The temperature of cooling water of the stage was 12 °C. The back side of the Si substrate was filled with a helium gas with a pressure of 1000 Pa for temperature control.

After the ETFE etching, the Ti film was removed by exposure of Cl2 plasma. The bored ETFE film was peeled off by using a tweezer, from the front side of the double-coated adhesive polyimide tape on the Si substrate.

Fig. 1: Fabrication scheme of an ETFE membrane filter.[14,15]

2.2 Results and discussionEtching condition was optimized by these parameters. The antenna power was fixed at 200 W. The bias power, flow rate of, process pressure and etching area were varied 50-250 W,

20-100 sccm, 0.5-4.0 Pa and 1.3-5.3 cm2, respectively.

The etch rates of both ETFE and Ti increased similarly with the increase of the substrate bias power, while the selectivity of ETFE over Ti remained was a constant of 670. The etch rates increased in the ion-enhanced physical sputtering manner.

For the pure Ti in the O2 plasma, the physical sputtering determined the selectivity of ETFE over Ti. Large etching rates enhanced chemically by fluorine. The ETFE-etch byproducts remediated the Ti etch rates. The selectivity for the Ti mask decreased with the increase of processing pressure and the ETFE-exposed etch area and the decrease of O2 flow.

In order to clarify why the Ti etch rate increased, the as-etched Ti surface was analyzed using XPS and fluoride species such as Ti-F and C-F were detected. The result indicated that etch-byproducts of ETFE in O2 plasma reacted with the Ti surface. The fluorides were considered to generate volatiles such as TiF4. In order to remain the Ti mask after etching process, high selectivity or thick Ti mask was required. Since thick Ti mask cause cracks and worse productivity, high selectivity was required. High selectivity was obtained lower pressure and larger O2 flow condition. Lower fluorine partial pressure condition improved selectivity. As a consequence, the 150 nm Ti mask for the ETFE etch processes enabled us to fabricate the through-hole membranes on the 25 μm thick ETFE film (Fig. 2).

Fig. 2: Fabricated ETFE membrane filter. (a) Top view and (b) cross-sectional view.[15]

ETFE etching byproducts generated inside the hole adhere to the sidewall, leading to adhesion of a thick film. As the pressure increased, a gas residence time was 8 times longer 0.70 s at 4 Pa than 0.09 s at 0.5 Pa. The mean free path of oxygen was 21.1 mm for 0.5 Pa and 2.6 mm for 4 Pa. Composition of the sidewall of the film was

analyzed by XPS. The holes were covered by a thin layer comprising CF2 and CH2 which is the composition of ETFE.[16] Under the high pressure condition, the sputtering rate was low because the amount of gas in the chamber was large, and the oxygen ions accelerated by the bias lost energy due to collision with the gas before reaching the Ti surface. The hole-shape on the front side was a circle reflecting the pattern of the photomask regardless of the processing pressure. The hole-shape on the back side was smooth circular like the surface under the high pressure 4 Pa condition. In contrast, at low pressure 0.5 Pa sample, the roughness on the inner wall increased as the etching proceeded.

As a mechanism of occurrence of sidewall roughness of the inner wall is considered: (1) something adheres to the inner wall surface of the ETFE and becomes an etching mask, (2) reflecting roughness of the Ti mask. Ti originated from the Ti mask was detected, caused by metallic fine particles sputtered from the etching chamber walls serve as an etching mask.[17] Ti fine particles adhere to the inner wall of the hole. The amount of Ti adhering to the inner wall (Ti/F or Ti/C) was significantly higher in the low pressure condition than that of in the high pressure condition.

The reason why the shape of the back surface is different while both Ti adhere at low pressure condition 0.4 Pa and high pressure condition 4 Pa is considered as follows. At the low pressure condition of 0.5 Pa, the scattering of incident ions at sidewall is small because the mean free path of oxygen is as long as 21.1 mm, and the shape of the Ti film adhered near the entrance of the hole is magnified and projected. In contrast, in the high pressure condition 4 Pa, the mean free path of oxygen is as short as 2.2 mm and there are many scatterings, so it is presumed that a smooth circular shape was obtained without reflecting the shape of the Ti film adhered near the hole inlet. In addition, the surface roughness generated in the resist is transferred to the Ti mask, which is enlarged and projected to cause sidewall roughness.

Lastly, in the fabricated ETFE membrane filters, the holes were straight shape and precisely aligned 380,000 holes with a diameter of 7 μm covered an area of a diameter of 13mm. Etch selectivity of the Ti mask played important roles for the fabrication of the through-holes. Inappropriate etching condition caused disappearing of Ti mask after processing, and rough ETFE surface was obtained. Although

lower process pressure condition improves the selectivity of Ti mask and suppresses sidewall film deposition and delamination, the shape of the back side hole was rough. Lower process pressure condition in order to prevent the sidewall deposition from becoming contamination at the genetic analysis was chosen in this study.

3 CTC entrapment

The non-small lung cancer cell line NCI-H358 was cultured in RPMI 1640 medium containing 2 mML-glutamine, 10% (v/v) FBS, and 1% (v/v) penicillin/ streptomycin solution for 3−4 days at 37 °C in a humidified atmosphere containing 5% CO2. Immediately before each experiment, confluent cells were trypsinized and resuspended in PBS.

The Normal human blood samples were collected from healthy donors at the Nagoya University. Samples were collected in a collection tube with EDTA to prevent coagulation and used within 12 h.

The fabricated ETFE filter membrane was put in a commercial filter holder, and input syringe was set on upper side of the filter holder. Waste syringe set on commercial syringe pump and lower side of the filter holder were connected by a commercial tube.

Auto-fluorescent and optical absorption of ETFE films are low at wavelength from 340 to 370 nm in the laser excitation wavelength and from 430 to 480 nm in the DAPI region, as compared with PET films.

The blood sample (7.5 mL) was diluted two times with PBS (7.5 mL). Then, H358 cell suspension of 1000 cells was spiked in the blood sample. The blood sample was added to the reservoir. Subsequently, negative pressure was applied to the cell suspension with a syringe pump. The sample was passed through the filter at a flow rate of 1 mL/min for 15 min. To remove blood cells that remained on the filter, PBS was added to the reservoir and passed through the filter at a flow rate of 1 mL/min for 5 min.

Cell staining solution was introduced into the reservoir and passed through the filter with a syringe pump after washing. To identify CTCs and leukocytes, 200 μL of cell staining solution containing 1 μg/mL Hoechst 33342, a cocktail of PE-labeled anti-EpCAM antibodies and an anti-CD45 antibody was passed through the filter at a flow rate of 4 μL/min for 50 min. Finally, the filter was washed with 3 mL of PBS to remove excess dye. Fluorescence images were obtained

with a fluorescence microscope integrated with DAPI, TRITC, Cy5 filter sets.

Separation test of CTC model cells spiked into whole blood was performed using our fabricated microfiltration system. Tens of billions of blood cells in 15 mL sample were able to pass through the filter without clogging, and CTC model cells were trapped on the filter. The trapping efficiency of CTC model cells was over 96% (Fig. 3). In contrast, almost all leukocytes were depleted, partly because leukocytes include cells that differ in size and deformability. Our system will have potential as a tool for isolating CTCs from whole blood with high efficiency and selectivity.

Fig. 3: (a) Size-based CTC isolation system. (b) Fluorescent image and (c) trapping efficiency of CTC model cells and Leukocytes.[15]

4 Conclusions

Optically transparent and extremely low auto-fluorescent ETFE membrane filters with several tens of millions of through-holes were developed by using the photolithography and the plasma etching. Etch-byproducts of ETFE in O2

plasma reacted with the Ti surface. At low pressure, lower partial pressure of fluorine with a large O2 flow improved selectivity of etching of the Ti mask. The low pressure condition suppressed sidewall film deposition and delamination. The shape of the back side hole was rough at low pressure condition. Very high trapping efficiency of over 96% was obtained at separation test of CTC model cells spiked into whole blood using our fabricated microfiltration system. The method of fabricating device for size-base capture of rare cells in blood such as CTCs was established in this study.

Acknowledgements

This research was in part supported by the

Center of Innovation Program at Nagoya University (Nagoya University-COI) from the Japan Science and Technology Agency (JST).

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[13] D. W. Smith, S. T. Iacono, and S. S. Iyer, Handbook of Fluoropolymer Science and Technology (Wiley, New York, 2014).

[14] N. Kihara, H. Odaka, D. Kuboyama, D. Onoshima, K. Ishikawa, Y. Baba, and M. Hori, Jpn. J. Appl. Phys.57, 037001 (2018)

[15] N. Kihara, D. Kuboyama, D. Onoshima, K. Ishikawa, R. Koguchi, H. Tanaka, N. Ozawa, T. Hase, H. Yukawa, H. Odaka, Y. Hasegawa, Y. Baba, and M. Hori, Jpn. J. Appl. Phys. (2018) in press.

[16] Y. Yamamoto, S. Higashi, and K. Yamamoto, Surf. Interface Anal. 40, 1631 (2008).

[17] H. Nabesawa, T. Hitobo, S. Wakabayashi, T. Asaji, and T. Abe, Sens. Actuators, B 132, 637 (2008).

Coating technology for locally varying optical function on 2d and 3d elementsD. Gloess*, H. Bartzsch, T. Goschurny, A. Drescher, U. Hartung, P. FrachFraunhofer-Institut für Organische Elektronik, Elektronenstrahl- und Plasmatechnik FEP, Winterbergstraße 28, 01277 Dresden, GERMANY *daniel.gloess@fep.fraunhofer.de

Keywords: pulse magnetron sputtering, optical filters, lateral thickness gradients, freeform

In this paper, a coating technology will be presented that allows achieving a locally varying optical function on 2d and 3d elements. This can be used, for example, to compensate layer thickness errors occurring during the deposition on tilted surfaces of larger 3d optical components. Other applications can be coatings for lateral varying light extraction of large-area waveguides in displays, wavefront correction or variable optical filters for hyper spectral cameras. In the coating plant PreSensLine at FEP (by Von Ardenne GmbH, s. Fig. 1) deposition rates in the range of 20 to 50 nm∙m/min allow efficient fabrication of optical layer systems for use in IR, VIS and UV [1].

Fig. 1: Deposition equipment for large area precision coatings at FEP: PreSensLine.

In this coating plant, various lateral gradient layer systems were fabricated. A special pulse parameter variation with pulse unit UBS-C2 (by FEP in cooperation with dresden elektronik ingenieurtechnik GmbH), adapted to the substrate movement (rotation/translation) with the precision drive (by LSA GmbH) allows to precisely adjust the deposition rate in dependence on the substrate position. On this basis, lateral varying layer thicknesses can be deposited on 2d substrates (Figs. 2-4). Therefore, uniform coatings of 3d substrates can be realized, among other things.

Fig. 2: Layer thickness distribution of a linear gradient of 300 nm across 400 mm substrate width deposited with PreSensLine.

Fig. 3: Sample photo and layer thickness distribution of a non-linear lateral thickness gradient layer of Nb2O5.

Fig. 4: Sample photo and layer thickness distribution of a 2D gradient coating deposited in PreSensLine: saddle shaped layer of Nb2O5.

In case of layer deposition on lenses, instead of an inhomogeneous layer thickness distribution with constant translation speed, a homogeneous distribution can be realized on the curved surface. This requires a superposition of substrate rotation with a precise controlled variation of the translation velocity. With the presented technology, for flat as well as for spheric, aspheric or freeform surfaces, exceptional layer thickness profiles can be realized to adapt layer systems to special requirements.

Acknowledgement

Part of the results within project funded by the European regional development fund (ERDF) and the Free State of Saxony.

References

[1] P. Frach, D. Gloess, T. Goschurny, A. Drescher, U. Hartung, H. Bartzsch, A. Heisig, H. Grune, L. Leischnig, S. Leischnig, C. Bundesmann, "Large area precision optical coatings by pulse magnetron sputtering", Proc. SPIE 10181, Advanced Optics for Defense Applications: UV through LWIR II, 101810K (11 May 2017); doi: 10.1117/12.2262541.

Plastic lens that has the effect of

reducing visible light uniformly at

each wavelength Ryosuke Suzuki1, Hirotoshi Takahashi1

1 TOKAI OPTICAL CO., LTD. R&D Department, Okazaki-city, Aichi, Japan

Abstract

In general, anti-glare spectacle lenses are produced

by dyeing their plastic substrates with some

dyestuffs. Because the dyestuffs have steep

absorption peaks at each specific wavelength, even

the lenses dyed gray also do not reduce visible light

uniformly at each wavelength in strict sense.

Therefore the color of vision with wearing the lenses

seems different from the actual color. On the other

hand, the neutral-density filter that is one of camera

filters has the effect of reducing visible light almost

uniformly.

In this research, we tried to make the anti-glare

spectacle lens that has the effect of reducing visible

light almost uniformly by using the multilayer coating

of the neutral density filter. Unsaturated nickel oxide

NiOx (x=0~1) was used as the absorbent layer. We

controlled uniformity of the transmittance by

controlling oxidized state of the NiOx layer. The

transmittance of visible light was reduced and

uniformed at each wavelength in the lens. The

spectral luminance meter was used for confirming the

change of color with the lens.

Keywords: plastic, lens, neutral density (ND) filter,

spectacle lens, luminance,

1 Introduction

In general, anti-glare spectacle lenses are produced

by dyeing their plastic substrates with some

dyestuffs. The color of the lens is determined by

choosing dyestuffs, and the transmittance of the lens

is determined by the amount of dyestuffs.

Compared to other colors, lenses dyed gray reduce

the color change of vision with wearing the lenses.

However, because the dyestuffs have steep

absorption peaks at each specific wavelength, even

the lenses dyed gray also do not reduce visible light

uniformly at each wavelength in a strict sense.

Therefore, the color of vision with wearing the lenses

seems different from the actual color.

On the other hand, the neutral density (ND) filter that

is one of camera filters has the effect of reducing

visible light almost uniformly. In many ND filters, the

effect is caused by the multilayer coating with optical

absorbent layers. Unsaturated metal oxides are used

as the absorbing layer [1]. They are produced by

depositing the metal with flowing oxygen gas. The

optical absorption property is controlled by the

deposition condition and optimized for the property of

the ND filter. The multilayer coating of the ND filer

also has the effect of the antireflective coating in

visible light [2].

In this research, we tried to make the anti-glare

spectacle lens that has the effect of reducing visible

light almost uniformly using the multilayer coating of

the ND filter.

2 Experimental

2.1 Sample

We prepared the spectacle lens plastic substrate that

refractive index was 1.76. The substrate was first

coated with the buffer layer and second coated with

scratch-resistance layer. These layers were coated

by the dipping method. The buffer layer had the

refractive index of 1.67 and the thickness of about 1.0

μm. The scratch-resistance layer had the refractive

index of 1.60 and the thickness of about 2.5 μm.

The ND multilayer coatings were produced by a

vacuum deposition method. The unsaturated nickel

oxide NiOx (x=0~1) layer was used as the optical

absorbent layer. The NiOx layer was produced by

depositing the nickel with flowing 10 sccm oxygen

gas. The deposition rate of the NiOx layer was 0.3

nm/s. The refractive index and the extinction

coefficient of the NiOx layer are shown in Fig.1. The

multilayer was consisted of NiOx, Al2O3, and SiO2

doped with a small amount of Al2O3. The materials

were heated by the electron beam. The deposition

rate of the Al2O3 layer and the SiO2 layer was 1 nm/s.

The starting pressure of the deposition was 1.0×10-3

Pa. The preset temperature of the process chamber

was 70 °C. The ND multilayer was coated on the

concave surface of the lens substrate. The convex

surface of that was coated with the antireflective

coating consisted of SiO2 and ZrO2. These

multilayers were coated on the scratch resistance

layer.

Layer MaterialThickness

[nm]Material

Thickness

[nm]Material

Thickness

[nm]

1 Al2O3 35 Al2O3 30 Al2O3 30

2 SiO2+Al2O3 30 NiOx 4.2 NiOx 4.5

3 NiOx 3 SiO2+Al2O3 35 SiO2+Al2O3 60

4 SiO2+Al2O3 75 NiOx 4.2 NiOx 5.5

5 - - SiO2+Al2O3 60 SiO2+Al2O3 50

6 - - - - NiOx 5.5

7 - - - - SiO2+Al2O3 70

Air - - - - - -

T=80% T=50% T=25%

Fig. 1: Refractive index and extinction coefficient of the

NiOx layer

Three types of the ND multilayer that transmittance

properties were about 80%, 50%, and 25% were

designed and produced. These multilayers also had

the effect of the antireflective coating in visible light.

Their film constructions are shown in Table 1. The

transmittance properties of the lenses are shown in

Fig. 2. The reflectance properties of the ND

multilayers are shown in Fig. 3.

Table 1: Film constructions of the ND multilayers.

Fig. 2: Transmittance properties of the lenses.

Fig. 3: Reflectance properties of the ND multilayers.

2.2 Evaluation method

2.2.1 Color measurement

A spectral luminance meter was used for confirming

the change of color with the sample lens. In the

darkroom, the plates painted in bright colors was

placed 3 m away from the spectral luminance meter.

The colors of the plate were red, blue, green, and

yellow. The object color of the plate lighted with the

daylight white fluorescent lamp was measured. The

measurement was performed with putting the sample

lens in front of the spectral luminance meter’s lens.

The sample lenses had no power. The measurement

also was performed without putting the sample lens.

The results were plotted to L*a*b* chromaticity

diagram.

2.2.2 Weathering and adherence test

The sample lens was weathered 240 hours by a

sunshine carbon arc weather meter. The adherence

test was carried out every 60 hours of the weathering

test in the following steps. The cross-cut of 100 cells

were made on the coatings with a cutter knife. The

distance of the cut was 1mm. A cellophane tape was

applied and removed rapidly five times. The number

of the peeled cells was counted.

2.2.3 Constant temperature and humidity test

The sample lens was put seven days in an

environment of with constant temperature and

humidity control. The temperature was 60 °C and the

humidity was 95%. The transmittance was measured

before and after the test.

T=80% T=50% T=25%

0h 0 (No Peeling) 0 (No Peeling) 0 (No Peeling)

60h 0 (No Peeling) 0 (No Peeling) 0 (No Peeling)

120h 0 (No Peeling) 0 (No Peeling) 0 (No Peeling)

180h 0 (No Peeling) 0 (No Peeling) 0 (No Peeling)

240h 0 (No Peeling) 0 (No Peeling) 0 (No Peeling)

Color Lens L* a* b* C* h ⊿h

--- 79,72 -5,87 74,83 75,06 94,49 ---

T=80% 74,43 -5,42 71,29 71,50 94,35 -0,14

T=50% 60,79 -4,58 59,25 59,43 94,42 -0,07

T=25% 44,95 -3,36 46,83 46,95 94,11 -0,38

--- 51,40 -27,32 13,01 30,26 154,54 ---

T=80% 47,78 -25,37 12,36 28,22 154,03 -0,51

T=50% 39,00 -21,57 10,66 24,06 153,70 -0,84

T=25% 27,04 -16,78 7,01 18,18 157,33 2,79

--- 51,77 -8,20 -28,75 29,89 254,07 ---

T=80% 47,95 -8,23 -25,95 27,22 252,41 -1,66

T=50% 38,80 -7,07 -21,81 22,93 252,04 -2,03

T=25% 27,01 -4,21 -19,50 19,95 257,81 3,74

--- 40,52 40,84 23,77 47,25 30,20 ---

T=80% 37,72 37,94 22,27 44,00 30,41 0,21

T=50% 29,98 32,15 18,74 37,21 30,24 0,04

T=25% 20,03 26,20 13,68 29,56 27,56 -2,64

Red

Blue

Green

Yellow

3 Results and Discussion

3.1 Color measurement

The results of the measurement are shown in Table 2

and plotted to L*a*b* chromaticity diagram in Fig. 4.

The straight lines in Fig.4 are drawn from the origin to

the each point that is measured without the sample

lenses. There are the points near the line

corresponding to the each color when they are

measured with the sample lenses. It is suggested that

there are little change in the color of vision with

wearing the sample lenses, because hue is denoted

the direction from the origin to the point in the

diagram. Hue is shown as hue angle (h) that is given

as h = tan-1(b*/a*). The absolute values of h change

are lower than five in each color measured with the

lenses.

The point tends to be nearer the origin when the

transmittance is lower. The distance between a point

and the origin denotes Chroma (C*). C* becomes

lower when the transmittance becomes lower.

Lightness (L*) changes similar to C*.

Table 2: Results of the color measurement

Fig. 4: L*a*b* chromaticity diagram plotted the results of

the color measurement

3.2 Weathering and adherence test

The results of the test are shown in Table 3. The

peeling of the coating was not found. It is thought that

the ND multilayers have good adhesion to the lens

substrate.

Table 3: Results of the adherence test

3.3 Constant temperature and humidity test

The transmittances of the T=25% lens before and

after the test are shown in Fig.5. There was little

change in the transmittance after the test. It is

thought that the NiOx layers are not affected in the

environment of the test.

Fig. 5: Transmittance change of the T=25% lens through

the constant temperature and humidity test

4 Conclusion

The neutral density multilayer coating that had the

optical absorbing layer was deposited on the

spectacle lens substrate. The spectral luminance

meter was used for confirming the change of color

with the lens. Because change of the hue angle is

little in each color measured with the lens, it is

suggested the lens have the anti-glare effect and

change the color of vision little. The coatings have the

basic durability applied the spectacle lens.

References

[1] R.Suzuki, H.Takahashi, Japanese Unexamined

Patent Application Publication No.2017-151219

[2] M.Ikeya, M.Sugiura, Japanese Patent

No.5066644

Heterogeneous microoptical

structures with a precision

below 1 µm Sönke Steenhusena, Frank Burmeisterb,

Matteo Großa, Gerhard Domanna, Ruth

Houbertzc, Stefan Nolteb,d

a Fraunhofer ISC, Neunerplatz 2, 97082

Würzburg, Germany b Fraunhofer IOF, 07745 Jena, Germany c Multiphoton Optics GmbH, Friedrich-Bergius-

Ring 15, 97076 Würzburg, Germany d Institute of Applied Physics, Abbe Center of

Photonics, Friedrich-Schiller-Universität Jena,

07743 Jena, Germany

Abstract

We demonstrate the fabrication of microoptical

elements made from hybrid polymers using

two-photon polymerization (2PP). To overcome

the throughput limitations of 2PP we use

galvanometric mirrors and propose a hatching

strategy for rotationally symmetric objects

which allows a significant process acceleration

to tolerable fabrication times while preserving

the surface accuracy of the fabricated

elements. Using the new strategy and

optimized processing parameters we

demonstrate the fabrication of an aspheric

microlens for diffraction limited focusing which

could be fabricated in less than 1.5 minutes.

Thorough topographic characterization with

atomic force microscopy (AFM) and laser

scanning microscopy (LSM) reveal excellent

agreement of the fabricated surfaces with the

theoretical design with a surface accuracy

below 100 nm. Our approach enables the

generation of arrays of custom shaped lenses

and large arrangements of freeform

microoptical elements within a few hours.

1 Introduction

The application of microlenses in e.g. micro-

imaging, beam-shaping and integrated optics is

a crucial factor for increased performance and

further miniaturization of devices and setups

ranging from the laboratory scale to consumer

products. Aspheric and freeform surfaces might

further advance the application of microlenses

in optical systems. However, as conventional

technology platforms for the fabrication of

microlenses are limited to spherical surfaces, a

demand for 3D fabrication technologies arises.

Two-photon polymerization (2PP) has proven

to be a versatile tool for the fabrication of

complex three-dimensional structures in

several photoresists [1–3]. In this technique,

femtosecond laser pulses are tightly focused

into the resist triggering two-photon absorption

in the focal volume of the employed

microscope objective. Thus, the

photopolymerization, i.e. solidification of the

resist, is confined strongly and it is possible to

create almost arbitrary shapes in a single

process step by simply scanning the focus in

3D space. Its flexibility and scalability pave the

way for 2PP in several fields of research:

Alongside applications in biomedicine like drug

delivery [4] or tissue scaffolds [5], 2PP enables

the fabrication of very sophisticated devices

with interesting physical properties like carpet

cloaks [6] and mechanical metamaterials [7,8].

In addition, 2PP allows permanent modification

of the polymers' refractive index which can be

exploited for optical waveguides [9,10]. In

contrast to other state of the art techniques

such as photoresist reflow [11], e-beam

lithography [12], focused ion beam milling [13],

selective chemical etching [14], and gray-tone

lithography [15], 2PP is an additive

manufacturing technology, which is almost free

of any constraints. Consequently, the

generation of microlenses is a very promising

area which can benefit from 2PP. This was

demonstrated impressively by single

microlenses or microlens arrays directly

attached to a substrate [16] and by very

complex freeform lens assemblies on optical

fibers [17–19].

However, despite its 3D capability 2PP is a

comparably slow process as the solidification of

the photopolymer is accomplished in a serial

(or point-to-point) manner with sub-µm volume

pixels (voxels). Employing higher velocities of

the positioning system or using low numerical

aperture (NA) focusing optics to increase the

polymerization rate (polymerized volume per

time) results in a trade-off between fabrication

duration and surface accuracy. Furthermore,

for the creation of functional optical elements

with 2PP, it is necessary to utilize a photoresist,

which reveals superior optical quality as well as

mechanical stability and that must be

polymerizable using femtosecond laser pulses.

In this paper we address these requirements by

utilizing a special ORMOCER®, which was

designed for optical applications and which has

low losses at telecom and datacom

wavelengths. To cope with the trade-off

between fabrication duration and surface

accuracy in 2PP, we developed and optimized

a hatching strategy for rotationally symmetric

microlenses. Combined with a systematic

investigation of the influence of fabrication

parameters on the lens' topography, this

strategy provides a significant acceleration for

the fabrication of highly accurate microlenses

with low surface roughness. This is

demonstrated by the fabrication of an aspheric

microlens. Additionally, we demonstrate the

fabrication of different microoptical elements

using galvanometric mirrors for rapid

positioning of the focal volume.

We believe that our approach with the special

choice of material, structuring hardware and

hatching strategy can spur the use of 2PP

written structures in future microoptical

applications like micro-imaging, beam-shaping

and optical tweezers.

2 Experimental details

2.1 Patterning setup for two-photon

polymerization

The setup for the two-photon fabrication of

microoptical elements is based on a 1030 nm,

10 MHz femtosecond oscillator (Amplitude

Systems t-pulse 200) as a laser source [20,21].

Pulses are frequency doubled to 515 nm and

focused into the sample using a microscope

objective with a NA of 1.4 (Zeiss Plan

Apochromat). Adjustment of pulse energy is

accomplished with a combination of polarizing

beamsplitter and halfwave plate mounted on a

computer controlled rotary stage (Aerotech

ADR-160). In order to monitor the structuring

process in-situ, we use a dichroic mirror, red

light illumination and a CCD camera. The

sample is mounted on a three axis positioning

system (Aerotech ABL1000) providing

velocities up to 30 mm/s with sub-micron

accuracy. The total travel of the axis system is

15 x 15 x 10 cm3 allowing for the patterning of

up to six inch wafers. Rapid beam positioning

inside the field of view of the microscope

objective is accomplished by galvanometric

mirrors (Lightfab GmbH). Fast switching of the

laser by an acousto-optical modulator (AOM) is

necessary to ensure synchronization to the

position of the axis system or the galvanometric

mirrors, respectively. This allows a highly

accurate switching exactly at the position

where the laser is supposed to be "on" or "off"

on a timescale of microseconds. Additionally,

the 2PP system is equipped with an autofocus

system for automatic substrate detection. A

second cw laser with a wavelength of 635 nm

is focused into the sample. Due to its low

intensities the detection laser does not interact

with the polymer. The reflected intensity

detected with a photodiode has a maximum for

the position of the focus at the interface

between substrate and polymer.

2.2 Materials

As a material for the fabrication of microlenses

we use an acrylate ORMOCER® (abbreviated

OC-V) formulated with 3 wt.-% of Irgacure®

OXE02 as the radical photoinitiator [21]. Like

other ORMOCER®s it is synthesized from

alcoxysilane precursors which undergo

hydrolysis and polycondensation reactions and

result in an organically functionalized inorganic

network (still liquid resin). In general, the

inorganic network of the ORMOCER® is

responsible for the material’s glass-like

properties i.e. excellent chemical, thermal and

mechanical stability as well as low shrinkage

and low optical absorption. With respect to

these properties ORMOCER®s predominantly

yield purely organic photoresists, which makes

them ideal candidates for 2PP fabrication with

high surface accuracy. Regarding the organic

functionalization, photochemically or thermally

polymerizable moieties allow for the

solidification and thus patterning of

ORMOCER®s analogously to conventional

(organic) polymers. The properties of

ORMOCER®s can be tailored to the needs of

the target application by the choice of

precursors, synthesis condition and processing

conditions, respectively. Details on the

synthesis and properties of the class of

ORMOCER® materials can be found in the

literature [22,23].

The formulation of OC-V + Irgacure® OXE02

exhibits strong interaction with the employed

515 nm laser pulses, which is a requirement for

patterning with high velocities [20]. At first, this

is a consequence of the larger overlap of the

initiator's linear absorption spectrum with the

employed laser wavelength compared to

800 nm illumination. Two-photon absorption

(TPA) occurring at 515/2 nm = 267.5 nm is

located in a regime of high linear absorption

while TPA at 800/2 nm = 400 nm has very

small overlap as the absorption of most

photoinitiators vanishes towards visible

wavelengths. Secondly, in previous z-scan

studies [24] it was found, that Irgacure®

OXE02 has a TPA cross section twice as high

as Irgacure® 369, which is a conventionally

employed photoinitiator [21].

For 2PP fabrication a droplet of OC-V +

Irgacure® OXE02 is casted in between a

sandwich of two microscope coverslips which

are separated by a 100 µm thick spacer. The

cover slip facing to the focusing optics is the

substrate. After 2PP the sample is rinsed using

a 1:1 solution of isopropyl alcohol and MIBK

(methyl isobutyl ketone).

2.3 Structure characterization

Surface profiles and roughness of the

fabricated elements were characterized using

atomic force microscopy (AFM, WITec Alpha

300) in tapping mode. Additionally, laser

scanning microscopy (LSM, Keyence VK-X210)

was used to obtain surface profiles semi-

automatically and faster than by using AFM.

Using a benchmark microlens, it was verified

that both AFM and LSM deliver equivalent

results.

The focal intensity distributions of the

fabricated microlenses were characterized by

scanning a microscope objective in 3D across

the focal region and monitoring the detected

intensity with a photomultiplier. For this the

microlenses were illuminated by a collimated

laser with a wavelength of 532 nm. Further

characterization was carried out by scanning

electron microscopy (SEM, Zeiss Supra).

3. Results and Discussion

3.1 Optimization of the hatching

strategy

To improve the fabrication times for aspheric

microlenses while maintaining high accuracy a

new hatching strategy for 2PP fabrication is

necessary. We propose a combination of

annular hatching [25] and shell hatching. This

means that only the shell of the structure is

solidified by 2PP using adjacent circular

motions which form the surface of the lens.

This is particularly useful for rotationally

symmetric lenses. After 2PP and solvent wash,

the liquid core of the lens is treated by UV flood

exposure for five minutes. In comparison to full

volume or XYZ hatching (inscribing stacks of

rods), this yields a process acceleration of

approximately three orders of magnitude. To

demonstrate this strategy Figure 1 (a-d) shows

transmission microscopy images of fabricated

microlenses.

Fig.: 1: Optimization of the hatching strategy for

manufacturing rotationally symmetric lenses. (a)

Non-optimized strategy. (b) Acceleration and

deceleration distances. (c) Randomly distributed

starting points. (d) Combination of acceleration and

deceleration distances + randomly distributed

starting points.

In Figure 1 (a) structural quality is affected,

because a line evolves on the surface at the

starting and ending positions of the circular

hatching traces as already observed by

Malinauskas et al. [26]. This is caused by two

effects: First, the limited acceleration of the

positioning system has to be taken into

account. Due to larger dwell times at the

starting and ending positions of the ring

compared to the ring itself, an elevation

evolves at the surface. To compensate this, we

introduced acceleration and deceleration

distances (where the laser is off) to the

hatching traces as depicted in Figure 1 (b). At a

closer look, the surface quality is better but still

not seamless, due to a small overlap of

beginning and ending of a hatching trace.

Since this is difficult to eliminate, we distributed

the starting points randomly over the surface as

depicted in Figure 1 (c). As it can be seen

there, a random distribution itself does not

avoid surface deterioration. Only the

combination of randomly distributed

beginning/ending points with tangentially

orientated acceleration distances, as shown in

Figure 1 (d), enables smooth surface

fabrication. Please note that in the hatching

strategy optimization the hatching distance is

too large and individual circular traces are

visible on the surface of the lens.

3.2 Fabrication of an aspheric lens

To demonstrate the potential of using 2PP with

optimized hatching strategy and fabrication

parameters we defined a target design for an

aspheric lens with a diameter d = 50 µm, a

height

h = 10 µm, and a focal distance f = 50 µm. The

design was carried out using commercial

raytracing Software (Zemax). The aspherical

surface is described by Equation 1, with c

being the curvature (1/r) and k the conic

constant.

() =

1 + 1(1 + )(1)

With fixed parameters for d, h, and f

optimization of the surface function assuming a

refractive index

n = 1.502 (determined by Abbe refractometry

on UV-treated OC-V layers) resulted in c = -

39.811 and

k = -2.257. According to the design this surface

focuses incoming light (λdesign = 532 nm) to a

diffraction limited spot.

For a diffraction-limited focal spot of the

microlens, the surface needs to have a peak-

to-valley error smaller than λ/4 (Rayleigh

criterion) and a RMS error smaller than λ/10

(Marechal criterion). To investigate if these

criteria can be met we fabricated lenses with a

writing velocity v = 500 µm/s and a hatching

distance ∆x = 0.1 µm. As the lens' surface has

a varying slope over the cross section, the

ellipticity of the voxel might influence the

surface formation despite the constant ring

separation (=∆x) along the surface. For this

reason, an array of microlenses according to

the design was fabricated. In this array the

average laser power was varied starting from

P = 1000 µW to P = 1800 µW with an

increment of ∆P = 100 µW. Additionally, the

starting z-position, a, with respect to the

substrate was varied by using the autofocus

system. This was done to compensate axial

offsets of the patterning laser and the detection

laser and to determine the optimum z-position

of the lens on the substrate. The parameter a

was varied starting from a = -2.0 µm to

a = 0.0 µm with an increment of ∆a = 0.25 µm

(negative values correspond to a position

deeper inside the photopolymer).

The topography of all fabricated lenses was

analyzed by performing a two-dimensional

surface fit according to Equation (1). We found

the best fit parameters c = -38.67983 and k = -

2.22479 for the lens fabricated with

P = 1500 µW and a = -0.25 µm. The mean

deviation of both parameters from the design is

only 3.18 %. This deviation from the design

parameters might originate from a slight tilt of

the substrate during the characterization of the

topography or the little polymerization

shrinkage. In addition, the employed increment

with respect to P and a might have been too

large, resulting in a (still) non-optimized

combination of voxel size and

distance to substrate.

Figure 2 depicts a cross-section through the

fabricated lens (upper part; inset: 2D surface

representation).

Fig. 2: Characterization of the fabricated aspheric

microlens: cross-section of the best matching

experimental data (black line) and fitted data

(equation 1) blue line.

The good agreement with the design is

indicated by the fit function according to

Equation (1) (solid blue line). In the lower part

of Figure 2 the deviations of the experimental

data from the design can be seen.

It is obvious, that the experimental data match

the aspheric design with maximal deviations of

200 nm, which are particularly prominent in the

center of the microlens. This might be due to

smaller annular radii in the center, which lead

to higher angular accelerations of the axis

system and thus larger positioning errors.

Furthermore, the time for the creation of a

single ring decreases the closer the laser spot

is to the center of the element. That is

why chemical interactions of radicals produced

along the rings might be stronger in the center

of the element and influence structure

formation. This is

a typical phenomenon also present in the

literature [26]. Furthermore, deviations are

comparably strong at the margins of the lens.

This might be due to substrate-induced effects

such as surface tension when the fabricated

voxels are close the substrate. Despite both

above mentioned criteria not being met, the

presented result is still excellent compared to

deviations published in the literature with sub 1

µm accuracy [17].

Both main sources of surface deviation from

the target design are also visible in Figure 3 (a)

which shows a color-coded two-dimensional

plot of the deviations.

Fig. 3: Characterization of the fabricated aspheric

microlenses: (a) 2D color representation of surface

deviations from the design. (b) microcope image of

best matching lens (P = 1500 mW, a = -0.25 mm).

(c) SEM image of lens array.

The RMS deviations are less than 76 nm,

which is λ/7 and to our knowledge a significant

improvement for 2PP fabricated microlenses.

However, ring shaped surface distortions which

might stem from laser power instabilities can

clearly be seen in Figure 3 (a) [27]. These

surface distortions might scatter incoming light

and consequently corrupt the imaging quality of

the fabricated lens. An optical image of an

individual lens (v = 500 µm/s, P = 1500 µW,

a = -0.25 µm, ∆x = 0.1 µm) is shown in Figure 3

(b).

Finally, the optimized hatching strategy in

combination with the capability of the axis

system for large area and high velocity

fabrication together with the highly efficient

material combination enable a reduction of the

fabrication time of a single lens to 1.5 minutes

allowing the fabrication of arrays in just a few

hours. This is demonstrated in Figure 3 (c)

where an array of 25 x 25 microlenses was

fabricated.

For preliminary optical characterization the

focus of the fabricated lens was mapped by

taking 128 axial slices with a resolution of 256 x

256 pixels. The resulting geometry which was

scanned had a volume of 10 x 10 x 25 µm³ (XYZ) around the focus.

Fig. 4: Focal slices of the fabricated microlens.

Fig. 5: Results of optical characterization for beam width, w(z), and maximum intensity I0(z) and focal intensity I

(r, z = 0). (a) On-axis intensity and beam width as a function of z. (b) Focal intensity.

Figure 4 depicts a selection of images of these

focal slices starting from index 24 which

corresponds to

-8 µm to index 122 (11.3 µm) with an increment

of 2 slices (positive values correspond to

increasing distance from the lens' surfaces). It

can be seen that the incoming light is focused

to a small spot, which has highest intensity at

slice index 66 close to the center of the

mapped volume. The absolute position of the

focal spot is 50 µm away from the lens apex.

This agrees very well with the design focal

length.

However, it can also be seen, that a second

peak with smaller intensity occurs between

slices 108 and 120. This is a typical indication

for aberrations caused by erroneous surface

shapes. To obtain a more detailed insight into

the formation of the focal volume, we analyzed

the radial intensity distribution in the focal plane

I(r,z = 0) depicted in Figure 5 (a) and the

average focal width (1/e²) and the on-axis

intensity for each of the focal slices shown in

Figure 5 (b).

In both plots dots represent the measured data

and solid lines simulated data. From

Figure 5 (a) a beam width of w0 = 650 nm could

be derived. On the other hand, the simulated

beam width is only 503 nm. The source of this

deviation might be the characterization device

itself: The small focal intensity distribution is

sampled by a high NA microscope objective

which has a comparable focal geometry. Thus,

the detected signal is a convolution of both

focal geometries and might not represent the

microlenses focal distribution as desired. This

can only be accomplished by (theoretically)

employing a Dirac-δ-shaped sampling function.

This effect can be estimated by assuming two

Gaussian intensity distributions. The focal

geometry of the microscope objective can be

described by its focal width according to

0.61 λ / NA = 361 nm. The resulting width of a

convolution of two Gaussian signals is given by

= +

. The resulting signal width

would be 619 nm, which is close to the

measured beam width. The effect of sampling

with a finite NA is also visible in I(z) in

Figure 5 (b) which is significantly broader than

the simulated on-axis intensity distribution. For

this reason, in future the optical properties of

2PP fabricated microlenses will be analyzed

using different methods e.g. by measurement

of the MTP (modulation transfer function).

Despite the described drawbacks of the

characterization method, it is still obvious that

the focusing behavior is not ideal as the second

maximum of I(z) is also visible in Figure 5 (b).

The cause of this maximum needs to be

investigated more detailed in the future.

3.3 Heterogeneous microoptical

elements

As 2PP is a freeform technology, the

fabrication of microoptical elements is not

limited to a single design that is fabricated at

predetermined spots, particularly regular

arrays, on a substrate. On the contrary, 2PP

allows the generation of different shapes and

sizes of elements directly located next to each

other. This is depicted in Figure 6.

Fig. 6: Different microoptical elements fabricated on

the same substrate.

Here, a pyramid, a prism, a hemisphere, and a

cone were placed directly next to each other.

The elements were fabricated by using the

galvanometric mirrors with a scanning velocity

of 10 mm/s and an average laser power of

3 mW. In this case the entire volume filled with

a line distance of 0.1 µm in the lateral (XY)

plane and the slice distance in the axial

direction (Z) was 0.1 µm as well. It can clearly

be seen, that the four designs with a height

between 30 and 40 µm could be fabricated

precisely. The edges of the prism and the

pyramid are sharp, the surfaces are flat without

any grating-like structure and the curved

surface on the hemisphere and the cone are

very smooth.

3.4 Custom arrangements of

microoptical elements

The rapid fabrication of refractive microoptical

elements using galvanometric mirrors and/or

shell hatching enables the fabrication of large

arrangements of these elements on a cm scale.

This is demonstrated in Figure 7 with an

arrangement of 13,600 small total internal

reflection mirrors (TIRs) forming the logo of the

ICCG 12.

Fig.7: Arrangement of microoptical elements. (a)

Design of the total internal reflection mirror. (b)

Microscope image of a small section. (c) Photograph

of the entire arrangement under blue light

illumination.

Figure 7 (a) depicts the computer design with a

size of 25 µm in each direction (XYZ). A

microscope image of a small section is shown

in Figure 7 (b). Finally, in Figure 7 (c) the entire

arrangement can be seen under blue light

illumination. The individual elements were

fabricated with a scanning velocity of 10 mm/s

and an average laser power of 3.5 mW. Axial

and lateral lines distances were 0.25 µm. Such

arrangements or also regular arrays could be

used as light outcoupling layers because they

can redirect incoming light into a direction

determined by the slope of the elements by

means of total internal reflection. In addition to

the significant acceleration of the 2PP

processing, these arrangements can also be

replicated easily. First experiments were

conducted in which a PDMS mold was

fabricated using the 2PP-written pattern as a

master structure. Then this mold was used to

fabricate an exact copy of the arrangement by

UV-assisted nanoimprint lithography. Details on

this process will be published in the future.

4 Conclusion and Outlook

Strategies for fast and accurate fabrication of

complex refractive microoptical elements using

2PP were developed and experimentally

demonstrated. In particular, it was shown that

differently shaped elements can be fabricated

on a single substrate.

By employing a shell hatching strategy in which

only the surface is fabricated by 2PP and the

liquid core is UV-treated after 2PP fabrication,

we could reduce the fabrication duration of an

individual microlens to 1.5 minutes. Despite the

acceleration, a surface accuracy below 100 nm

could be obtained by optimizing the annular

hatching (i.e. randomly distributed starting

points, acceleration distances) and analyzing

the impact of the relevant parameters such as

hatching distance, laser power, and starting z-

position offset. This enables the fabrication of

custom designed microlens-arrays for real

world applications. As a demonstrator a

diffraction limited microlens was designed and

fabricated taking the above mentioned hatching

considerations into account. Thorough

topographical characterization reveals good

agreement between design and fabricated

structure. However, measurements of the focal

intensity distribution of the fabricated

microlenses show differences from simulations,

indicating aberrations caused by surface

deviations. This will be investigated in more

detail in the future.

The advance in process acceleration has to be

accompanied by further material research as

the ORMOCER®'s polymerization rate has to

keep pace with increasing scanning speeds.

Acknowledgements

The authors acknowledge the financial support

by the German Research Foundation (DFG,

grants HO 2475/3-1 and TU 92/19-1). We also

thank our coworkers in Würzburg and Jena for

their support and fruitful discussions.

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Synthesis and Characterization

of Polyurethane Acrylate

Encapsulation Materials for

Organic Photonic Systems Oguzhan Cimen1,2, Canan Varlikli31 Ege University, Solar Energy Institute, Izmir, Turkey 2 Gizem Seramik Frit ve Glazür San. ve Tic. A.Ş, Sakarya,Turkey 3,Izmir Institute of Technology, Department of Photonic, Izmir, Turkey

Abstract

In this study we have designed and synthesized a new polyurethane acrylate derivative to be used in encapsulation of organic photonic devices. Polyurethane acrylate type of polymers have to potential of utilization as UV-curable materials under ambient conditions. Herein, the synthetic steps and structural characterization (FTIR, TGA and 1H NMR, 13C NMR) results of one of the derivatives is presented.

Keywords: polyurethane, acrylate, co-polymer, organic photonics

1 Introduction

Organic photonic systems, such as, organic light

emitting diodes and organic photovoltaic devices

are in high demand due to their lower production

costs, and higher flexibility compared to their

inorganic counterparts. [1]

Organic photonic devices do consist of organic materials and metal electrodes. The oxidation of metal electrodes due to moisture and oxygen directly affects the lifetime of the complete device. For this reason, the primary challenge is to improve the efficient moisture barrier layers which protect the device from moisture and oxygen. [2-3]

Increasing the lifetime of organic photonic systems by preserving moisture and oxygen is a promising industrial R & D topic.

Herein, the synthetic steps and structural characterization (FTIR, TGA and 1H NMR, 13C NMR) results of a polyuretan acrylate co-polymer derivative are presented.

2 Experimental

2.1 Synthesis of model compound

2-(((isocyanatomethyl)3,5,5-trimethylcyclohexyl)carbomylethyl methacrylate (U-AC)

58.57 g of isophorone diisocyanate, 0.4 g DBTL and 0.3 g hydroquinone were added to a three-neck flask with a mechanical stirrer under nitrogen atmosphere. 100 g of 2-hydroxyethyl acrylate were then dropwise added to the system in 1 h. The reaction mixture was stirred at 35 ºC in water bath for 30 min.

2-((3-((3-((3-hydroxy-2-(hydroxymethyl)-2-methylpropanoyl)oxy)-2-(((2-(hydroxymethyl)-2-methylbutanoyl)oxy)methyl)-2-methylpropanoyl)oxy)-2,2-dimethylpropoxy)carbonyl)-2-methylpropane-1,3-diyl bis(3-hydroxy-2-(hydroxymethyl)-2-methylpropanoate) (HBPP)

Bis-MPA (0.6mol, 80.48 g), NPG (0.2 mol, 20.83 g) and p-TSA (0.4 g) were mixed in a three-necked flask equipped with mechanical stirrer. The flask was placed in an oil bath previously heated to 150 ºC. The reaction was left under N2

atmosphere for 2 hs. Then Bis-MPA (0.6 mol, 80.48 g) and p-TSA (0,405 g) were added and the mixture was allowed to stir for 2 h. The reaction was followed via FTIR spectra.

2-(10-(((3-hydroxy-2-(hydroxymethyl)-2-methylpropanoyl)oxy)methyl)-6-(hydroxymethyl)-1-(5-(((2-(methacryloyloxy)ethoxy)carbonyl)amino)-1,3,3-trimethylcyclohexyl)-6,10,14,14-tetramethyl-3,7,11-trioxo-4,8,12,16-tetraoxa-2-azaheptadecan-17-oyl)-2-methylpropane-1,3-diyl bis(3-hydroxy-2-(hydroxymethyl)-2-methyl propanoate (HBPU-AC)

HBPP (111.53 g, 0.1 mol) was dissolved in DMF at 90 ºC. UAC (22.23 g, 0.1 mol) was added to a flask with mechanical stirrer under N2

atmosphere. The flask was placed in an oil bath previously heated to 90 ºC. After that HBPP was added and allowed to mix for 3.5 h. The product was precipitated with water and filtered. The filtrate was separated in ethyl acetate with a separating funnel and the ethyl acetate was removed using a rotary evaporator.

3 Results and Discussion

3.1 Synthesis of HBPU-AC oligomer

HBPU-AC oligomers synthesis via a three step procedure as shown in the following Scheme. First, urethane monoacrylate (IPDI-HEMA) was obtained from the reaction between the secondary cycloaliphatic NCO group of IPDI and the OH group of HEMA using DBTDL as catalyst. Second, The addition polymerization of NPG and MPA was carried out at 150 ° C with p-TSA catalyst. Third, the UAC reacted with HBPP at 90 ºC for 3.5 h, and resulted in HBPU-AC oligomer.

Scheme. Synthesis route for urethane acrylate (HBPU-AC)

Figures 1a and 1b show the FTIR spectra of the UAC and HBPP, resepctively, and the spectra of HBPU-AC are shown in Figure 1c.

Fig. 1. a) FTIR spectrum of UAC, b) FTIR spectrum of HBPP, c) FTIR spectrum of HBPU-AC.

The NCO peak stretching absorption appears at 2250 cm-1 and the NH hydrogen bond peak at 3350 cm-1 for UAC (Fig. 1a). As the reaction continues, the carboxylic acids carbonyl peak disapeared (1696 cm-1), the ester carbonyl peak appeared (1730 cm-1) (Fig. 1b) and finally the NCO peak stretching absorption at 2250 cm-1

disappeared (Fig. 1c).

The 1H-NMR spectrum of HBPU-AC is shown in Fig. 2. The peaks at 7.4 ppm and 7.0 ppm clearlyconfirm the existence of the N-H bond. The peaks at 6.0 ppm and 5.6 ppm can be attributed to the acrylate group in the HBPU-AC. There is a broad peak at about 4.3 ppm, which is an overlap peak associated with the methylene groups bond with oxygen in the ester group. The analysis above confirms the formation of HBPU-AC oligomer, which is in accordance with the results of FTIR.

Fig.2 a) 1H NMR spectra of HBPU-AC, b) 13C NMR spectra of HBPU-AC.

a)

b)

c)

a)

b)

The TGA spectrum of HBPU-AC is shown in Fig 3. The decomposition temperature for the synthesized material is 180 °C. 50% decomposition was observed at 320 °C.

Fig. 3. TGA curve of HBPU-AC.

4 Conclusion

In this work, results related to the synthesis and characterization of an urethane acrylate oligomer for the development of UV curable coatings for organic photonic systems have been reported.

Acknowledgements

Financial support from the Gizem Seramik Frit ve Glazür San. ve Tic. A.Ş R&D Center.

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