Screen Printed Large Area Sensors for Pressure Distribution...

Screen Printed Large Area Sensors for

Pressure Distribution Monitoring in

Wheelchairs

Jawad Ahmad

Main supervisor: Associate Professor Johan Sidén

Co-supervisor: Dr. Henrik Andersson

Department of Electronics Design

Thesis for Licentiate Degree in Electronics Design

Mid Sweden University

Holmgatan 10

851 70 Sundsvall

Akademisk avhandling som med tillstånd av Mittuniversitetet i

Sundsvall fram-läggs till offentlig granskning för avläggande av

licentiatexamen torsdag, den 04 Juni 2019, kl. 09:30, sal M108,

Mittuniversitetet Sundsvall. Seminariet kommer att hållas på engelska.

Screen Printed Large Area Sensors for Pressure Distribution

Monitoring in Wheelchairs

© Jawad Ahmad

Printed by Mid Sweden University, Sundsvall, Sweden, 2019

ISSN: 1652-8948

ISBN: 978-91-88947-05-5

Department of Electronics Design

Faculty of Science, Technology and Media

Mid Sweden University, Holmgatan 10, SE-851 70, Sundsvall, Sweden

Phone: +46 (0)10 142 80 00

Mid Sweden University Licentiate Thesis 159

ii

Abstract

A sedentary lifestyle can induce health related problems including pres-

sure ulcers. Prolonged sitting inadequacies constitute a risk for pressure

ulcer to many individuals, in particular people with disabilities and re-

duced mobility. The measurement of distributed pressure and detection

of irregular sitting postures are essential in prevention of the risk of devel-

oping pressure ulcers.

In this thesis, a screen-printed pressure sensor for a large area is pre-

sented, with the objective of measuring the distributed pressure of a

seated person in a wheelchair. The conductors and interdigital patterns

are printed with silver-based ink. A blend of a non-conductive and a low

resistive ink is used for customized resistance for an optimal sensing range

of the pressure sensor. The effect of moisture and temperature are realized

in an environment chamber. For characterization, other key performance

tests such as repeatability, drift and flexibility are carried out. The surface

morphology is carried out for structural analysis of printed samples. The

sensor data is acquired and processed using an 8-bit ATmega-2560 micro-

controller and wirelessly transmitted to a PC for post-processing, storage

and analysis. For real-time data presentation of distributed pressure

points, a GUI has been developed to display the values obtained from the

large area sensor. The detection of four sitting postures; forward leaning,

backward leaning, left leaning and right leaning along with a normal sit-

ting posture is attained. An analysis for stretchable printed tracks has been

conducted to investigate the changes in electrical resistance using elonga-

tion tests, surface morphology and EDS. The optimal curing time and tem-

perature were investigated to manufacture stretchable conductive tracks.

In summary, the contributions in this thesis provides an effective ap-

proach regarding pressure distribution measurement and recognizing ir-

regular sitting postures for wheelchair users.

iv

Sammanfattning

En stillasittande livsstil kan framkalla hälsorelaterade problem, inklusive

trycksår. Långvariga sittande brister utgör en risk för trycksår för många

individer, särskilt personer med funktionsnedsättning och nedsatt

rörlighet. Mätningen av distribuerat tryck och detektering av

oregelbundna sittställningar är avgörande för att förebygga risken för att

utveckla trycksår.

I denna avhandling presenteras en screentryckt tryckgivare för stora

areor för att mäta det tryckfördelningen hos en person i en rullstol.

Ledarna och interdigitala mönstren är tryckta med silverbaserat bläck. En

blandning av ett ickeledande och ett lågresistivt bläck används för

anpassad resistans för optimalt avkänningsområde för tryckgivaren.

Effekten av fukt och temperatur uppnås i en miljökammare. För

karakterisering utförs andra prestandatester såsom reproducerbarhet,

drift och flexibilitet. Ytmorfologi utförs för strukturell analys av tryckta

prover. Sensordata inhämtas och bearbetas med en 8-bitars ATmega-2560

mikrokontroller och överförs trådlöst till en dator för processning, lagring

och analys. För datapresentation av distribuerade tryckpunkter i realtid

har ett GUI utvecklats för att detektera sittställningen för en person utifrån

mätning av de distribuerade tryckpunkterna. Detektering av fyra

sittställningar; framåtlutande, bakåtlutande, vänsterlutande och

högerlutande tillsammans med normal sittställning är uppnått. En analys

för sträckbara tryckta ledare har gjorts för att undersöka förändringar i

elektrisk resistans med hjälp av töjningstester, ytmorfologi och EDS. Den

optimala härdningstiden och temperature undersöktes för framställning

av töjbara ledande spår.

Sammanfattningsvis ger bidragen i denna avhandling ett effektivt

tillvägagångssätt när det gäller mätning av tryckfördelning och

erkännande av oregelbundna sittställningar för rullstolsburna.

vi

Acknowledgements

First and foremost, I am thankful to Almighty Allah, the most merciful

and the most beneficent for the countless blessings. Secondly, I am very

grateful to my supervisor Assoc. Prof Johan Sidén for his kind advices,

guidance and personal interest throughout this thesis work. His encour-

agement and support provided me with an opportunity to work freely on

my thesis.

Thanks to my co-supervisor Dr. Henrik Andersson for his support, con-

structive comments that helped me in starting and continuously improv-

ing this work. Thanks to Dr. David Krapohl for his internal review. Special

thanks to my family for their endless love, countless prayers and extraor-

dinary support.

I am especially thankful to Xiaotian Li, Farhan Alam, Muhammad Abu

Bakar, Seyed Morteza Ebadi, Muhammad Nazar ul Islam, Qaiser Anwar,

An Siwen, Cristian Vilar, Irida Shallari, Hiba Alqaysi and Waqas Ahmad

for extremely valuable support and encouragement. I would acknowledge

all colleagues whose names are not mentioned here for their support and

kindness, during my entire study programme. I am thankful to all non-

academic friends who supported and encouraged me.

viii

Contents

Abstract ..................................................................................................................... ii

Sammanfattning ..................................................................................................... iv

Acknowledgements ................................................................................................ vi

Terminologies ........................................................................................................ xii

List of Figures ....................................................................................................... xiv

List of Tables ......................................................................................................... xvi

List of Papers ...................................................................................................... xviii

1 Introduction ........................................................................................................... 1

1.1 General overview ...........................................................................................1

1.2 Problem motivation .......................................................................................1

1.3 Research goals ................................................................................................4

1.4 Contributions ..................................................................................................5

1.5 Thesis outline ..................................................................................................7

2 Background information and functional materials ........................................... 9

2.1 Printed electronics ..........................................................................................9

2.2 Printing technologies for printed electronics .............................................9

2.2.1 Screen-printing and fabrication of conductive interconnects ...........9

2.2.2 Inkjet printing of conductive interconnects ....................................... 10

2.2.3 Transfer printing for printed electronics............................................ 11

2.2.4 Non-conventional electronics with methods other than printing .. 12

2.3 Functional materials .................................................................................... 12

2.3.1 Conductive printable inks .................................................................... 12

ix

2.3.2 Nanoparticle inks based on Ag ........................................................... 12

2.3.3 Nanoparticle inks based on Carbon ................................................... 13

2.3.4 Nanoparticle inks based on Au ........................................................... 13

2.3.5 Ink based on conductive polymers ..................................................... 14

2.4 Substrates ...................................................................................................... 14

2.4.1 Polyethylene terephthalate (PET) ....................................................... 14

2.4.2 Thermoplastic polyurethane (TPU) .................................................... 14

2.4.3 Polydimethylsiloxane (PDMS) ............................................................ 15

2.4.4 Related work .......................................................................................... 15

3 Large area pressure sensor ............................................................................. 17

3.1 Design of a large area pressure sensing system ....................................... 17

3.1.1 Design of large area pressure sensor matrix ..................................... 17

3.1.2 Fabrication of printed sensors and interconnects ............................. 19

3.1.3 Design of read-out electronics ............................................................. 19

3.1.4 Sitting posture recognition .................................................................. 21

4 Results ................................................................................................................. 23

4.1 Characterization ........................................................................................... 23

4.1.1 Drift ......................................................................................................... 24

4.1.2 Environmental chamber testing .......................................................... 24

4.1.3 Bending testing ...................................................................................... 25

4.1.4 Stretchable conductive tracks .............................................................. 26

4.1.5 Surface morphology of printed samples ............................................ 27

x

5 Discussions ......................................................................................................... 29

5.1 Validity of results ......................................................................................... 29

5.2 Limitations and possible improvements of proposed large area

pressure sensor ................................................................................................... 29

5.3 Ethics and impact on society ...................................................................... 30

5.3.1 Ethics ....................................................................................................... 30

5.3.2 General considerations ......................................................................... 30

5.3.3 Impact on society .................................................................................. 30

6 Summary ............................................................................................................. 31

6.1 Conclusion..................................................................................................... 31

6.2 Future work .................................................................................................. 32

Bibliography............................................................................................................ 33

Appendix I ............................................................................................................... 39

xii

Terminologies

Abbreviations and Acronyms

Ag ---------------- Silver

ASTM ---------------- American society for testing and materials

Au ---------------- Gold

C ---------------- Carbon

CNP ---------------- Carbon nanopa rticles

CNT ---------------- Carbon nanotubes

Cr ---------------- Chromium

kPa ---------------- kilo Pascal

KOH ---------------- Potassium hydroxide

LCD ---------------- Liquid crystal display

NP ---------------- Nanoparticle

OLED ---------------- Organic light emitting diode

PEDOT ---------------- Poly (3, 4-ethylenedioxythiophene)

PDMS ---------------- Polydimethylsiloxane

PI ---------------- Polyimide

PU ---------------- Polyurethane

RFID ---------------- Radio frequency identification

TPU ---------------- Thermoplastic polyurethane

xiv

List of Figures Figure 1.1: Research work flow……………………………………………5

Figure 1.2: Map of contributions to goals of research studies……….…7

Figure 2.1: Graphical representation of transfer printing process…….11

Figure 2.2: Stretchable electrodes using PDMS……………………...….15

Figure 3.1: Figure 3.1: Block diagram of large area pressure

monitoring system………………………………...…….…....17

Figure 3.2: Placement of sensing elements in large area

Pressure sensor…………………………...…………….…......18

Figure 3.3: PCB design of read-out electronics

(Front-side and Backside) ……….………………………...... 20

Figure 3.4: Response time for normal actuation

of a sensing element………………………….………….…....20

Figure 3.5: Screenshots of pressure intensities acquired

at about 35° leaning angle. (a) Forward leaning

(b) Backward leaning (c) Right leaning

(d) Left leaning………………………………………….…….21

Figure 4.1: Resistance graph as a function of force ….………………...23

Figure 4.2: Temperature and humidity effects…………………………25

Figure 4.3: Elongation setup for stretchable interconnects…………….26

Figure 4.4: Morphology of printed samples.

(a) Nanometre sized carbon particles

(b) Micrometre sized Ag flake

(c) Cross section of printed carbon

(d) Cross section of printed Ag flake.………………….…....27

Figure 4.5: (a) SEM micrograph (SE) of a sample cured

at 150 °C for 30 minutes (b) SEM micrograph (in-beam SE)

of a sample cured at 110 °C for 3 minutes

(c) SEM micrograph of a sample cured at

110 °C for 10 minutes, having damaged

conductivity (overstretched)……………………………...…28

xvi

List of Tables

Table 1.1: Authors’ Contributions……………………………………………6

Table 4.1: Sensor Drift (%) ………………………………………………….24

Table 4.2: Bending Test…………………………………………………...…25

Table 4.3: Elemental Analysis ...……………………………………………28

xviii

List of Papers

This thesis is mainly based on the following papers, herein referred to by

their roman numerals:

Paper I ………………………………………………………………………... 41

Sitting Posture Recognition using Screen Printed Large Area

Pressure Sensors

Jawad Ahmad, Henrik Andersson and Johan Sidén

In Proceedings of IEEE Sensors. (IEEE Sensors). pp. 232-234.

Paper II ………………………………………………………………………. 47

Screen-Printed Piezoresistive Sensors for Monitoring Pressure

Distribution in Wheelchair

Jawad Ahmad, Henrik Andersson and Johan Sidén

IEEE Sensors Journal, vol. 19: 6, pp. 2055-2063.

Paper III ………………………………………………….………….….……. 59

An Analysis of Screen-Printed Stretchable Conductive Tracks

on Thermoplastic Polyurethane

Jawad Ahmad, Xiaotian Li, Johan Sidén and Henrik Andersson

Accepted in IEEE FLEPS, July 7-10, 2019.

1

1 Introduction 1.1 General overview The current consumer electronics market is shifting towards thin, com-

pact and flexible electronics. In general, a considerable number of re-

search studies are being carried out to find alternatives to copper con-

ductor [1]. A variety of cheap, thin and flexible sensing devices are being

made that can be used as an advancement or alternative to existing de-

vices. Nonconventional electronics have the potential to be utilized as

sensing devices that can be bent, folded and even stretched. For health

monitoring, user-interactive materials with unique sensing capabilities

are being used to detect vital signs of the human body [2].

This thesis aims at design and implementation of large area pressure

sensors for measurement of distributed pressure of wheelchair-bound in-

dividuals. The unobtrusive pressure measurement method a uses large

area printed pressure sensor that is placed inside a wheelchair cushion.

It is flexible and suitable for long-term operation. The sensing measure-

ments are not noticeable and may not pose any discomfort to the wheel-

chair users. The large area sensing system assists the health monitoring

of disabled patients. A runtime pressure distribution mapping has been

developed that demonstrates intensity of pressure over distributed pres-

sure. The developed system is capable of recognizing four sitting pos-

tures; forward-leaning, backward-leaning, right-leaning and left-leaning

with an overall recognition accuracy of over 80 %.

1.2 Problem motivation Pressure ulcers can affect the quality of life of wheelchair users [5], [7].

The data about the pressure distribution of a wheelchair user is very im-

portant in order to prevent pressure sores from developing in the first

place [6]. Some environmental factors such as humidity, temperature and

cleanliness influences the skin and underlying tissues. The effect of pres-

sure is proportional to the weight of the person, and can affect the skin if

the wheelchair user is constantly sitting in a specific position [50]. The

vessels get less blood and less oxygen, which adversely affects the skin

tissues.

2

The problem is severe in those individuals who have lost sensation in

a body part due to spinal cord injuries, partial paralysis or any other phys-

ical impairment [8]. This group of people cannot get a feeling of move-

ment due to a lack of sensation and are therefore unable to change their

sitting or lying position. This category of patients possesses a higher risk

of developing pressure sores.

The National Pressure Ulcer Advisory Panel (NPUAP) has catego-

rized the pressure ulcer in four stages [9].

1) In stage one, the skin is intact and pressurized areas develop light or

dark pigmentation.

2) In stage two, partial loss of skin occurs and an exposed wound be-

comes visible in the form of a raptured blister.

3) In stage three, the skin becomes deeply damaged and the wounds

reach the inner layers of skin.

4) In stage four, the skin flap disappears while muscles and bones under

the skin become visible.

A constant pressure over a period of time causes impediment in blood

flow, limiting oxygen supply to skin tissues and leads to localized and

deep tissue pressure injury (DTPI) [10]. The tolerance of pressure varies

for individuals; however, it is suggested that a pressure exceeding 32 mm

Hg may cause occlusion in blood vessels [4]. In wheelchair users, a pres-

sure ulcer most commonly appears around ischial tuberosities, calcaneus

and the back of the knee [11], but can also occur in other areas of skin due

to lack of mobility, moisture, friction and shear. Most pressure ulcer pa-

tients suffer not only from pain, but also require a costly treatment. It is

then suggested that early stage diagnosis of pressure sores helps in their

treatment and recovery, otherwise late diagnosis results in a complicated

process of treatment. Pressure ulcer treatment is extremely difficult, ex-

pensive [12], [13], takes a lot of time and patience, and sometimes requires

plastic surgery and a skin flap [14]. It is therefore suggested to monitor

the pressure distribution for the wheelchair user for a specific time so as

to develop prevention and rehab plans [15], [16]. In order to obtain an

overview of a patient’s preliminary sitting habits and to get the pressure

distribution data, pressure sensors are required to measure the amount of

pressure over a certain area. For this purpose, a light and thin large area

pressure sensor is proposed that can measure the pressure over distrib-

3

uted points of the whole sitting area, recognize movements and deter-

mine centre of balance. The data can be processed to show a real-time

pressure distribution intensity in the form of a pressure map along with

sitting posture recognition. The data is stored as a log to be analysed by a

medical doctor or therapist for the development of prevention measures

or recovery strategies.

Manufacturing of such measurement sensors and devices would be

possible on larger areas by using cost effective printing processes such as

screen-printing, gravure printing, flexographic printing and inkjet print-

ing [3]. Printed electronics possesses the advantage of fast and large vol-

ume manufacturing. Moreover, the materials such as plastic and paper

being used in the printed electronics industry are cost-competitive. The

screen-printing method is being used for high throughput large area

printed electronics. A screen print resolution of 100 µm and higher can

easily be achieved using commercially available conductive inks on com-

mon screen-printing setup [4]. Pressure is force over unit area and math-

ematically it is described as:

𝑃 =𝐹

𝐴 (1)

For the human body, the pressure is related to body weight over skin

contact surface area. The human skin contains three layers: epidermis,

dermis and subcutis. All three layers are composed of different skin tis-

sues and muscles. The measurement of pressure at a specific point is re-

lated to body weight and the stiffness and composition of skin tissues. A

constant pressure over a longer period of time causes restriction in blood

flow in the skin capillaries (ischemia) that can induce a pressure ulcer.

Subcutis tissues and muscles are more susceptible to pressure sores and

the risk of an internal wound is higher than what appears on the skin.

Pressure measurement and sitting posture recognition is very important

for wheelchair-bound patients to encounter pressure ulcer. A reposition-

ing is required every 2 hours [51], and the caregiver treats this manually.

A smart powered wheelchair has the capability of moving the seat to a

particular extent while maintaining the balance. A powered wheelchair

equipped with large area pressure sensor can analyse the continuous po-

sition of a seated person for a specific time period, and can then lean au-

tomatically to a different position using an electromechanical setup.

4

1.3 Research goals The findings can have a strong implication for the development of cost-

effective pressure sensors, especially within medical and health care ap-

plications and sitting posture monitoring.

The clear objectives and verifiable goals are:

• Utilization of a cost-effective manufacturing technique and non-

conventional thin and flexible electronics for pressure sensor fab-

rication.

• Distributed pressure determination/measurement and sitting

posture recognition using wirelessly received data, processing

and logging.

• Characterization of the fabricated sensors in normal and harsh

situations (environmental testing) to inspect the functionalities

and robustness.

• Development of stretchable interconnects that will lead to fabri-

cation of printed stretchable pressure sensors.

A combination of theoretical and experimental research has been

used to identify appropriate design parameters, to support available ex-

perimental methods, and to fabricate and characterize large area pressure

sensor. The methodology is primarily based on three key research ques-

tions which are stated as follows:

RQ 1: Investigation of a feasible, suitable and cost-effective

method to fabricate thin and flexible pressure sensors that can be

used for larger areas such as wheelchair cushions.

RQ 2: What would be the efficiency, robustness and practical im-

plementation scenarios of a fabricated large area pressure sensor?

RQ 3: Investigation on new materials to further enhance the ca-

pabilities of pressure sensors to incorporate features such as

stretchability.

5

Literature Review

Research problem

identification

Research approach

definition

State of the art

investigation

Research question

formulation

Proposed sensor

designCharacterization

Result / analysisFeedback

Figure 1.1: Research work flow

A review has been conducted for research objective verification on the

basis of the research questions formulated and with a view to future work

directions.

1.4 Contributions The work in this thesis is summarized in the following papers, which are

included at the end of this work:

• Paper I aim to answer RQ1 by introducing the core idea of em-

ploying a large area pressure sensor for use in a wheelchair. Sen-

sor design and read-out electronics design are presented. A sitting

posture recognition algorithm determines four sitting positions.

The posture recognition algorithm is tested on five able-bodied

volunteers and has an accuracy of over 80 %.

• Paper II is an extension of Paper I, and aims at design improve-

ments and a detailed approach in which a characterization is car-

ried out for the printed sensors to determine the performance un-

der different environmental conditions. An improved and com-

pact read-out electronics is also developed for fully wireless inte-

gration. This work aims to address RQ2.

• Paper III proposes a gateway towards the fabrication of multi-

layered stretchable pressure sensors. The first step is to determine

6

the performance of stretchable substrates and stretchable inks un-

der elongation in order to obtain characterization of the conduc-

tion parameters of stretchable printed interconnects. The pro-

posed work aims to address RQ3.

Table 1.1 Authors’ Contributions

Paper JA 1 JS 2 HA 3 XL 4 Contribution

I M C C

JA: main author, design, model-

ling and implementation

JS: co-author, main supervision,

and idea

HA: co-author, supervision and

revision

II M C C

JA: main author, design, imple-

mentation and characterization

JS: co-author, supervision and re-

vision


revision

III M C C C

JA: main author, design, model-

ling, implementation,

JS: co-author, supervision and re-

vision


revision

XL: co-author, EDS analysis and

comments

1 Jawad Ahmad M = Main contributor

2 Johan Sidén C = Co-author

3 Henrik Andersson

4 Xiaotian Li

7

1.5 Thesis outline The flowchart in Fig 1.2 describes the general goal, sub-goals and contri-

butions.

Printed sensors for pressure ulcer prevention

Flexible pressure sensors

Sitting posture recognition and characterization

Stretchable interconnects

Paper I Paper II Paper III

Contribution:Design and

fabrication of large area pressure sensors

Contribution:Characterization of large area pressure

sensors

Contribution:Stretchable

interconnects and characterization

RQ1 RQ2 RQ3

Figure 1.2: Map of contributions to goals of research studies

8

The thesis is divided into six chapters and is organized as follows:

1. Introduction The chapter features the thesis over-

view, problem motivation, objectives

and research goals.

2. Background information

and functional materials

This chapter contains background

reading about printing techniques

and related work. It also includes

state of the art for conductive inks,

flexible and stretchable substrates

3. Large area pressure sen-

sor

This chapter describes the design and

fabrication of a large area pressure

sensing system.

4. Results This chapter describes general and

characterization results

5. Discussions This chapter contains discussions

about limitations, improvements and

ethical aspects

6. Summary The final chapter presents the pri-

mary conclusion of the thesis and

suggestions for future work.

9

2 Background information and functional materials

2.1 Printed electronics Printed electronics relate to the process of manufacturing electronics

products that employ graphical printing techniques. This type of electron-

ics is sometimes called non-conventional electronics. The wide range of

functional materials and substrates enables the products to feature ad-

vantages and functionalities that are unrivalled by other manufacturing

methods. Printed electronics are usually manufactured at low cost and

comprise high-speed processes such as screen-printing, roll-to-roll pro-

cess, gravure, flexography etc.

Printed electronics requires an in-depth knowledge of material sci-

ence, as there are a considerable number of technological advancements

being made towards exciting new materials that can benefit this technol-

ogy. Many electronic devices rely on printing technology, which are dif-

ficult to manufacture with other processes. Some of devices include flexi-

ble screens, smart posters, intelligent labels etc. It is estimated that by year

2025, the global revenue of the printed electronics industry will reach over

19 billion US dollars [17]. There is a huge existing market for printed elec-

tronics for glucose sensors, OLED displays, RFID antennas and printed

sensors.

2.2 Printing technologies for printed electronics Printing techniques for flexible and stretchable electronics depend on the

nature of the ink and substrate, but also relate to the viscosity of the ink,

curing technique, temperature tolerance of the substrate and resolution of

printed pattern. Screen-printing and inkjet printing techniques are dis-

cussed in the next section.

2.2.1 Screen-printing and fabrication of conductive interconnects

Screen-printing is one of the fast and cost competitive printing techniques

for fabrication of flexible and bendable devices and interconnects. Screen

printing is widely used for large area printing in industrial processes.

Screen printing is mainly a batch process, but advanced rotary machines

exist too. In screen-printing, high viscosity conductive ink is applied to a

10

substrate by forcing it through a fine mesh of synthetic thread with the

help of a squeegee. Silk or any other synthetic fabrics can also be used for

making a mesh that is firmly attached to a metal or wooden frame. The

patterns are made by a photographic technique with stencils while allow-

ing inks to flow through certain parts and blocking other parts. Mesh size

is calculated in threads per inch or threads per centimetre. More threads

result in a better print resolution, but also depends on the viscosity of the

ink.

The current printed electronics technology mostly relies on carbon

and silver conductive inks, which are made by mixing flakes or nanopar-

ticles in temperature or ultraviolet curable /dryable solvents and adhe-

sives. Screen-printing is therefore used to print thicker inks of relatively

high viscosity. Thick layers (> 10 µm) are possible via this method, but the

print resolution is limited to about 100 µm. This method is effective in

printing multiple thick layers to achieve a certain thickness and required

conductance. RFID antennas, car window heaters, membrane keyboards,

are common examples of the devices that uses screen-printed techniques.

Multiple wet-on-wet coatings are applied using screen-printing [18] and

variable print thickness and sheet resistances are attained. Similarly, mul-

tiple dry coatings are possible with screen printing. 6 coatings of a Ag ink

EDAG 725-A is reported in [19], achieving a 7.3 mΩ/sq/42 µm sheet re-

sistance value, while first printing 3 wet-on-wet coatings and curing them

and again printing 3 wet-on-wet coatings aligned to previously cured

coatings.

2.2.2 Inkjet printing of conductive interconnects

Inkjet printing is considered the most promising technique to print out

very fine patterns on different substrates. This modern printing method

uses electronics to control the movement of the nozzles and the number

of droplets of ink. The high-end inkjet printer can “jet” a 5-micron droplet

size and the common notation for drop size is pico-litres. Inkjet printing

is a non-contact printing process that deposits printable composites onto

a variety of substrates in a drop-by-drop method. With the current ad-

vancements, it is now feasible to print prototypes and even large-scale

industrial production at a relatively cheap cost. Thicker multi layered pat-

terns can be deposited by multiple print sequences to allow higher con-

ductance but the robustness of bonding depends on the first layer and

11

substrate surface. The droplets can be generated by different mechanisms;

A continuous inkjet mechanism uses an electrical field to direct the drop-

lets and the remainder are collected and recycled, this is the first ap-

proach. The second mechanism uses the droplet on demand principle

with subcategories thermal inkjet, electrostatic inkjet and piezo inkjet. For

printing electronics circuits and interconnects, nanoparticle-based inks

are commonly used, but any less viscous ink can be filled in the print car-

tridges.

A brief review is reported in [20], which covers the technology as

well as the use of this method along with functional materials in fabrica-

tion of flexible/stretchable electronics. Multilayer inkjet-printed passive

components for printed electronics is reported in [21], using Ag nanopar-

ticle ink and utilizing piezoelectric print heads in Fujifilm’s Dimatix

(DMP) 3000.

2.2.3 Transfer printing for printed electronics

Transfer printing is an emerging technique for fabricating electronic de-

vices within different material classes and functional layouts. This print-

ing method is most useful in the fabrication of flexible and stretchable

electronics. In this printing method, the circuit or conductive interconnect

is first printed onto a flexible substrate (non-textile) that is called a stamp

(donor) and later transferred to other substrates (receivers) such as tex-

tiles via a separate method. The process includes bringing up the stamp

in contact with first substrate (donor), which is printed with inks and re-

moved via chemical etching or dry lifting off. This stamp is brought into

contact with the desired substrate (receiver) with appropriate adhesives

and finally, the removal of the stamp completes the printing process. The

transfer printing process is shown graphically in Fig. 2.1. This is quite a

promising technique for complex flexible and stretchable electronics but

Figure 2.1: Graphical representation of transfer printing process [22].

12

this printing process has a lower yield. Common transfer techniques in-

clude dye sublimation transfer, heat/melt transfer, film release and wet

transfer. Various transform techniques are listed in [22] on different sub-

strates and varying stamps.

2.2.4 Non-conventional electronics with methods other than printing

A variety of methods has been reported to manufacture stretchable elec-

tronics. For example, the construction of a stretchable tri-axis force sensor

using conductive liquid is reported in [23], which uses electroconductive

liquid encapsulated channels/tubes inside an elastomer. The overall

thickness of the constructed device has been reported as 2.1 mm.

Similarly, metallic interconnects are embedded inside an elastomer such

as PDMS, where the metallic wire is kept in the shape of a horseshoe,

sawtooth, zigzag or square wave patterns for easy stretching without

breakage of the metallic wire. CNT electrodes are patterned on PU

substrate in [24] and lithographic technique combined with KOH etching

was used.

2.3 Functional materials 2.3.1 Conductive printable inks

Many commercial inks are based on conductive fillers such as silver and

carbon. Expensive inks that contains Au nanoparticles are available for

research purposes. Graphene and carbon nanoparticle-based inks are be-

ing investigated to achieve higher conductance. Common categories of

commercially available inks include: Ag flake-based inks, carbon nano-

tubes (CNT) filler-based inks, Ag particle-based inks, Ag nanowires-

based inks, graphene-based inks and other conductive polymers and

composites. Most inks are soluble and washable with organic solvents

such as Methyl Ethyl Ketone (MEK).

2.3.2 Nanoparticle inks based on Ag

Ag nanoparticle-based inks are mostly studied as they are inherently

highly conductive yet only moderately expensive. Ag is a metal with su-

perior conductivity at room temperature and is resistant to oxidation and

corrosion. Instead of using bulk metal, a powdered form can be used in

making an ink, which is a multicomponent formula that contains adhe-

sive binders and metal (micrometre/nanometre scaled flakes or particles).

This gives more flexibility of usage/printing on a range of substrates at

13

the cost of an increased resistance of mΩ scale. Flexible Ag inks are most

commonly used in the electronics industry as the alternative to rigid

wires, while more modern stretchable Ag inks are also commercially

available [25]. Adding Ag nanoparticles to currently available flake based

inks can further improve the conductivity [26], [27]. Most of the available

screen printable commercial inks have a sheet resistivity of 10 - 15

mΩ/sq/25 µm and offer temperature curability at 120 °C. Inkjet printing

of silver inks has less abrasion resistance (ASTM 3), but their overall sheet

resistance when the sample is cured at 130 °C remain in the range of ≤ 5

mΩ/sq/25 µm [28]. Ag nanoparticle-based inks have been reported to

have even lower resistances 30 µΩ/cm, while Ag nano-platelets have 7.4

µΩ/cm using novel sintering methods [29].

2.3.3 Nanoparticle inks based on Carbon

Carbon inks are inexpensive and are used as printed resistors, electrical

attachments, heaters, membrane switches, polymer thick film circuitry

and sometimes as a protective layer of another circuitry. Pure carbon is a

non-metal with inferior conductive properties at room temperature and

resistant to oxidation and corrosion. Different states and isotopes of car-

bon (carbon black, graphite, graphene and nanowires) are used to manu-

facture carbon ink. Newer conductive carbon inks offer a higher level of

conductivity in the ranges of 10 15 Ω/sq/25 µm and are commonly used

as contacts in flexible circuitries. These contacts can have over 100 years

of lifespan with more than 2 million contact cycles [31]. A wide range of

carbon inks is available with varying sheet resistances and some of them

can be mixed together to customize the sheet resistance for different ap-

plications. In most cases, thicker carbon nanoparticles (CNP) based inks

are used in screen-printing while CNT based inks are widely used for

inkjet printing of conductive interconnects. Carbon filler based CNP inks

have been utilized to develop pressure sensors [32], and CNT-based inks

have been reported in [33] for a stretchable strain sensor. Graphene-based

inks have much higher conductivity and other unique features such as

optical transmittance up to 97 %, and these inks are widely used in LCD

touchscreens and flexible displays [34], [35].

2.3.4 Nanoparticle inks based on Au

Gold (Au) inks are extremely expensive as the Au metal itself is precious,

but due to extraordinary conduction capabilities, Au inks are used for

14

making printed interconnects for lead-free packaging. Some sintering

methods can achieve conductivities of up to 70 % of bulk [36], and exhibit

a resistivity of 5µΩ cm. Inkjet printing of interconnects onto glazed paper

in [37], shows a sheet resistance of 10 mΩ/sq. Most of the Au based inks

are available for inkjet printing, where laser sintering methods are ap-

plied for enhancing the conductivity.

2.3.5 Ink based on conductive polymers

Conductive polymer-based inks are mostly organic compounds with a re-

sistivity range of 100 300 Ω-cm. Organic conductive compounds are

widely employed in environmentally degradable bioelectronics and other

bio sensors. Inorganic polymers have a lack of conductivity but due to

their energy storage properties, nano-hybrids (organic polymers and in-

organic polymers) conductive polymers are frequently used in electrical

energy devices [38]. Poly(3,4-ethylenedioxythiophene) (PEDOT) is a com-

mon organic compound used in transparent conductive layers in different

applications.

2.4 Substrates Flexible and stretchable substrates play an important role in the printed

electronics industry in realizing smart sensors and their application in

medical care devices. One of the requirements for body area network sen-

sors is that they must mechanically comply with body movements and

can stretch when the skin stretches.

2.4.1 Polyethylene terephthalate (PET)

Polyethylene terephthalate is a general-purpose thermoplastic polymer

that is considered an economic and efficient substrate for printed elec-

tronics. PET sheets have properties that are biaxially oriented, thermally

stabilized and resistant to cracking. PET sheets are inert material, which

is widely accepted by health authorities.

2.4.2 Thermoplastic polyurethane (TPU)

Thermoplastic Polyurethane or TPU is a very versatile elastomer (poly-

mer with elastic characteristics) and is considered as a bridge between

rubbers and plastics. The material is smooth, durable and highly flexible

having rubber like stretching characteristics. TPU is manufactured with

different compounds known as base materials, and these base materials

determine the mechanical characteristics such as stretching, mechanical

15

durability and moisture absorption. TPU materials are widely used in

electronics to construct electronic skin [39], [40]. TPU sheets [41] are

screen printable with a high tensile strength up to 9000 PSI and an ulti-

mate elongation of up to 450 %. Furthermore, TPU sheets are utilized as

a printing, bonding and adhesion material for stretchable electronics [42].

2.4.3 Polydimethylsiloxane (PDMS)

Polydimethylsiloxane (PDMS) belongs to the silicones group and it is a

frequently used silicon-based organic polymer. Its unique optically

opaque, non-toxic and non-flammable properties make it ideal for a wide

range of applications from contact lenses to elastomers. Physically, it is

viscoelastic and commonly used to mould microfluidic devices.

Semi-transparent stretchable Ag films coated on a wavy patterned

PDMS are reported in [43], for constructing stretchable electrodes for

stretchable and transparent interconnects as depicted in Fig. 2.2. Embed-

ded rigid silicon chips in a PDMS stretchable system [44] show a material

gradient approach for stretchable functional materials between hard sili-

con and soft substrate materials. Structured PDMS is used as a dielectric

to construct a stretchable pressure-sensing device [45]. Pyramid-like

structures are patterned using different methods to manufacture a pres-

sure sensor based on capacitive change method. Moulding methods are

used to produce results for a pressure ranges up to p ≥ 3 kPa.

2.4.4 Related work

Significant work has been done in the literature for pressure ulcer preven-

tion by different pressure measurement mechanisms. An air-filled wheel-

chair cushion for patients with a spinal cord injury is presented in [55].

However, this sensor is slow in data acquisition. A rubber-based air-alter-

nating wheelchair seat is designed and evaluated using resistive pressure

sensors to measure interface pressure [56], but the solution requires an

Figure 2.2: Stretchable electrodes using PDMS [43].

16

extensive control mechanism. Multi-walled carbon nanotubes

(MWCNT)-polydimethylsiloxane (PDMS) screen-printed pressure sensor

is developed in [57], that is used in an insole to detect unhealthy rollover

patterns, with a functionality to measure plantar pressure. A pressure

sensing system using PDMS and carbon nanotubes (CNT) is developed

[58], but the sensor has a hysteresis behaviour in sensing.

Quantum tunnelling composites (QTC) are metal particles in an elas-

tic polymer matrix and functions through the quantum tunnel effect [59].

A sensor array structure based on QTC is presented [60]. The sensor suf-

fers recovery time, to return to its original state after deformation. A force

sensitive resistor using a copper plating process and ‘Velostat’ as the sens-

ing mechanism is presented [61]. However, the repeatability and perfor-

mance are challenging in that approach. An unobtrusive support system

for prevention of dangerous health conditions in wheelchair users is re-

ported [62]. The system uses force sensors and an accelerometer on a bat-

tery powered wheelchair.

17

3 Large area pressure sensor 3.1 Design of a large area pressure sensing system

The proposed large area sensing system comprises three main units as

shown in Fig. 3.1:

• Large area pressure sensor

• Read-out electronics

• Post processing and data presentation

Read-out electronics consist of a microcontroller, Bluetooth module

and a power management unit. In the current proposed design, the post-

processing unit is a computer with data processing software such as

MATLAB. However, it can be any smart device capable of processing and

displaying pressure intensities.

3.1.1 Design of large area pressure sensor matrix

When a piezoresistive material is put under uniaxial stress, a narrowing

of the distance between the conductive filler particles occurs and the con-

ductivity changes in accordance with the applied force [53]. In the pro-

posed design, there are 16 sensing elements in the large area pressure sen-

sor, which has an area of 505 cm2. The horizontal and vertical dimensions

are 23.5 cm × 21.5 cm. The distance between each sensing element in the

matrix is 5.5 cm and 3.0 cm in X and Y directions, respectively. These are

the typical seating dimensions of a person for ischial areas. The sensor is

usually placed inside or underneath the foam cushion. The assembly re-

Figure 3.1: Block diagram of large area pressure monitoring system.

18

quires a very thin and flexible pressure sensor to be able to tolerate me-

chanical deformations due to movements of the wheelchair user. The pro-

posed large area sensor comprises three sheets of PET (top, centre and

bottom), where each sheet is 100 µm thick. The top sheet is printed with

interdigital structures and interconnects with conductive Ag ink and op-

erates as conductive layer. The bottom sheet is printed with carbon filler

based blended inks that has both resistive and piezoresistive properties,

and operates as sensing layer. The centre sheet is being used as a separa-

tor between two layers and also functions as an adhesion/bonding be-

tween the top and bottom sheets. The centre sheet has laser cut openings

and patterns in arrangement with the top and bottom layers. In an idle

state, both layers are isolated by 100 µm (centre sheet thickness) and when

the pressure is applied on the surface of the sensing elements, both layers

make a contact thus allowing the current to flow between the two sides of

the interdigital structures. Further pressure allows more contacts result-

ing in more conducting paths. Hence, the resistance decreases by applica-

tion of force and this resistance is translated into pressure values with cal-

ibration using a force gauge.

Figure 3.2: Placement of sensing elements in large area pressure sensor.

19

3.1.2 Fabrication of printed sensors and interconnects

The top layer of the sensor matrix is printed with Ag flakes-based ink,

EDAG 725-A, which has a sheet resistance <15 mΩ/sq/25 µm and it is tem-

perature curable in a convection oven at 120 °C for 20 minutes. A 7 µm

Ag ink layer is manually screen-printed on a 100 µm PET sheet with a

100-40 (100 threads per cm, 40 µm thick) meshed screen. Methyl Ethyl

Ketone (MEK) is used for removing any dust or fat particles from the PET

sheets. After curing, the resistance between the two ends of the longest

printed conductive line (215 mm) is measured as 16 Ω with a single coat-

ing of Ag ink. With two wet-on-wet coatings of the ink, the thickness is 12

µm and resistance is 12 Ω, and with three wet-on-wet coatings, the thick-

ness is 14 µm and resistance is 8 Ω, respectively.

The sensing layer is printed using commercially available Loctite ECI

7004-LR, which is a low resistivity carbon particle-based inks and Loctite

NCI-7002 non-conductive ink from Henkel AG & Co. The inks can be

blended to obtain controllable ranges of electrical resistance. These inks

are blended together with a 30 % and 70 % ratio, respectively to obtain an

approximately 150 kΩ/sq/10 µm customized initial sheet resistance,

which is observed as an optimal value (neither too conductive nor too

resistive) for the scope of the design.

The blending is carried out at 100 rpm for 10 minutes to acquire a ho-

mogenous ink composite for screen-printing. A 10 µm layer of this

blended ink is printed on a 100 µm PET sheet using screen-printing with

a 100-60 (100 threads per cm, 60 µm thick) meshed screen. All the inks

used in fabrication, are temperature curable at 120 °C for 20 minutes and

have an adhesion standard of ISO class 0 (ASTM class: 5B).

3.1.3 Design of read-out electronics

The initial data processing in the read-out electronics is carried out with

an 8-bit Atmel ATmega-2560 microcontroller operating at 16 MHz. The

data acquisition from pressure sensing elements is performed with six-

teen dedicated ADC channels of 10-bit resolution of the microcontroller.

In the design, each sensing element is scanned at a frequency of 10 Hz.

The Bluetooth module (HC-06) is used in the read-out electronics for

wireless data transfer over serial port profile (SPP) for sensor readings.

The read-out electronics, along with the Bluetooth module, is mounted on

a 2 cm × 5 cm PCB shown in Fig. 3.3.

20

The read-out circuitry is operated at 5.0 V and can be powered with a

3000-mAh battery pack for fully wireless implementation. A small boost

converter is utilised, when the read-out circuit is operated with battery.

The small form factor enables the sensor matrix, read-out electronics, and

portable battery pack to easily fit inside the seat cushion. A twenty-pin

flex cable of 0.5 mm pitch connects the sensor matrix to the PCB. A single

sensing element draws a maximum of 0.56 mA current, when connected

in a 5.0 V voltage divider configuration with high precision 10 kΩ resis-

tors. The overall read-out electronics utilizes an average of 66 mA current.

The voltage diver configuration is described by equation (2).

𝑉out = 𝑉in ×𝑅sensor

𝑅10k + 𝑅sensor (2)

Fig. 3.4 presents the response and recovery time of a sensing element,

when loaded and unloaded in terms of rise time and fall time in a 5.0 V

voltage divider configuration with a 10 kΩ resistor of 0.01 % precision.

Figure 3.4: Response time for normal actuation of a sensing element.

Figure 3.3: PCB design of read-out electronics (Front-side and Backside).

21

3.1.4 Sitting posture recognition

For processing of the data and designing the graphical user interface GUI,

a MATLAB software package is used, in which the mapping of each dis-

tributed pressure point is carried out to display pressure intensities in

real-time by continuously updating the graph. A delay of 100 µs is given

at the transmitter side for the reliable transfer of data. The overall screen

refresh rate in the GUI is 200 ms.

The algorithm for sitting postures recognition can identify four un-

balanced sitting postures, forward leaning, backward leaning, right lean-

ing and left leaning, with leaning angles of about 25° and 35° from the

straight (90°) natural sitting position of a seated person. It can also detect

and recognize a balanced sitting position.

The classification of sitting posture is based on the total number of

activated sensors, single sensing element values, the sum of all sensing

element values, and the average of sensing element values, which are pro-

cessed through a decision tree algorithm. This classification scheme is

simple, provides good results and does not require extensive computing

resources compared to support vector machine (SVM) [43] and k-nearest

neighbors (k-NN) [44]. Fig. 3.5 show intensities of pressure over sensing

elements, based on leaning positions.

(a) (b) (c) (d)

Figure 3.5: Screenshots of pressure intensities acquired at about 35° leaning angle.

(a) Forward leaning (b) Backward leaning (c) Right leaning (d) Left leaning

23

4 Results

4.1 Characterization

The graph in Fig. 4.1 exhibits decreasing resistance values when individ-

ual sensing elements receive an actuation from 1.0 N to 100 N. The graph

is nonlinear and resembles an inverse power law. At a higher amount of

pressure i.e. 50-100 N, marginal decrement in resistance steps take place.

Four individual sensing elements are taken from printed large area pres-

sure sensor and characterized to obtain a resistance graph as a function of

force. The force data is obtained by a Lutron FG-6020SD dynamometer

mounted on a translation stage and resistance data by an Agilent 34405A

multimeter.

The result shows a nearly 8 % difference between the output values

of four sensing elements. It is observed that a pressure exceeding 200

N/cm2 can cause irreversible deformation to sensor layers that results in

partial or total failure of sensing element. It is also noticed that when the

sensor matrix is placed under the foam, the observed average pressure

per single sensing element is between 30 N‒50 N for an able-bodied per-

son weighing between 70 Kg‒90 Kg and sitting straight at 90°.

Figure 4.1: Resistance graph as a function of force.

24

4.1.1 Drift

Drift is an unwanted variation that occurs due to mechanical instability

and in some cases, environmental effects such as temperature and humid-

ity. The main reasons for sensor drift are the electrical drift in the inter-

connect system, the non-uniform mechanical stress at the sensor surface

and structural surface tension after exertion of load. The drift is reversible

if it occurs within the functional limits of the sensor. For the drift test, a

constant load of 2.0 N, 5.0 N, 10 N and 25 N is applied on sensing elements

for 60 minutes and up to 24 hours. A rubber-based actuator of 7 mm di-

ameter and 3 mm thickness is used for sensor actuation. The resistance is

logged at one sample per minute. The results for 60 minutes and 24 hours

drift tests are shown in Table 4.1.

4.1.2 Environmental chamber testing

The sensor is also put under test for calculating the change in sensor char-

acteristics due to variations in temperature and humidity. For this pur-

pose, an environmental chamber (TESTEQUITY 1007H) is used and the

temperature is varied while keeping the humidity constant. However, it

is only possible to control the humidity level between temperatures rang-

ing +10 °C and +85 °C. The drift effect is also present as these tests take up

to 40‒50 minutes. The graphs in Fig. 4.2 displays the readings taken at

different humidity and temperature settings for sensing units under 5 N

and 10 N downwards force. The readings are taken about 25 seconds after

the exertion of force. The test is conducted from a colder temperature to a

hotter temperature and from a hotter temperature to a colder temperature

in the environment chamber. The findings indicate a higher variation,

while changing from colder to hotter temperatures due to the increasing

influence of the drift factor. In the hotter to colder test, it displays a small

TABLE 4.1

SENSOR DRIFT (%)

Time / Force 2 N 5 N 10 N 25 N

60 Minutes 3.89 5.01 5.30 1.45

24 Hours 6.84 8.05 8.63 3.78

25

drift and then an increase in electrical resistance due to colder tempera-

tures. The sensor is also tested for a temperature above normal environ-

mental conditions i.e. 90 °C, shown by a blue trend line in the plot.

4.1.3 Bending testing

To conduct the bending test, sensing elements are acquired from the large

area sensor matrix. Six cylinders ranging from 6 cm to 1 cm are 3D-printed

to conduct the test. It is observed that the sensing element maintains its

functionality and stability up to a bending diameter of 2.0 cm. At a bend-

ing diameter of 1.0 cm, the mechanical stress becomes too much and the

100 µm gap between conductive layer and sensing layer reaches a mini-

mum, thus causing a false actuation reading while bending. However,

this is temporary and recovers when the sensing element is released from

the bending cylinder. Table 4.2 shows the percentage variation in re-

sistance readings when a constant 10 N force is applied before, during

and after bending. TABLE 4.2

BENDING TEST

Radius of cylinder 6 cm 5 cm 4 cm 3 cm 2 cm 1 cm

Test Pass Pass Pass Pass Pass Fail

During Bending % 1.37 1.91 2.40 3.63 5.89 -

After Bending % 0.15 0.26 0.43 0.68 0.97 1.25

Figure 4.2: Temperature and humidity effects.

26

4.1.4 Stretchable conductive tracks

Stretchable screen-printed conductive tracks on a flexible substrate can

withstand high mechanical strain while maintaining good electrical con-

ductivity. Straight track samples with a measured thickness of 12 µm and

various trace widths are printed using a stretchable Ag ink ECI-1036. An

88 µm thick TPU stretchable film [41], is employed as a substrate. Four

temperatures: 110 °C, 120 °C, 130 °C and 150 °C are chosen to cure the

stretchable ink, where four curing times: 3, 10, 20 and 30 minutes, are used

for each temperature. In total, 16 curing temperature-time combinations

are used. Cyclic strain tests with 25 % and 50 % elongations are carried

out in order to observe the variation in electrical resistance after being

subjected to mechanical deformation and fatigue. The test setup is shown

in Fig. 4.3. The results from elongation test indicate that a sample cured

at 150 °C for 30 minutes provides a better conductivity even after multiple

elongation cycles. The result of the elongation test to show variation in

printed track resistance before and after 25 % and 50 % elongations are

included in Appendix I. However, a few samples showed a tendency of

stretching nonuniformly mainly due to slight thickness variation of the

TPU substrate.

Figure 4.3: Elongation setup for stretchable interconnects.

27

4.1.5 Surface morphology of printed samples

The surface morphology is carried out to obtain an insight into the inter-

action between two printed layers when they make a contact. The samples

are printed onto PET substrate. Fig 4.4(a) shows an SEM image of conduc-

tive interconnect that is printed with Ag flake-based ink EDAG-725-A. A

cross section analysis is also carried out to determine the quality and uni-

formity of printed layer that is depicted in Fig 4.4(c). The sensing layer is

also examined and Fig 4.4(b) and Fig 4.4(d) show the images for the sur-

face and cross section of printed and cured carbon ink samples, respec-

tively.

To investigate morphological changes within the stretchable printed

samples, SEM images are recorded that are shown in Fig 4.5. The images

show surfaces of samples that are printed with ECM-1036 stretchable ink

onto the TPU substrate. Fig 4.5(a) shows an SEM image of a cured sample

at 150 °C for 30 minutes in convection oven. Fig 4.5(b) shows an SEM im-

age of a cured sample at 110 °C for 3 minutes. Although there is a signifi-

(a) (b)

(c) (d)

Figure 4.4: Morphology of printed samples.

(a) Micrometre sized Ag flake (b) Nanometre sized carbon particles

(c) Cross section of printed Ag flake-based ink (d) Cross section of printed carbon-based ink

28

cant difference in the conductance of screen-printed tracks when meas-

ured by a multimeter, it is nevertheless, difficult to observe a difference

between two samples in SEM images. The outcomes of EDS analysis are

presented in Table 4.3, that reveals a very little change in Ag particles con-

centration between conductive samples. It is interesting to see the close-

up image of a sample that has damaged conductivity due to overstretch-

ing. A sample is prepared by stretching it and placing it on a sample

mount. Fig 4.5(c) reveals that conductive paths are broken between Ag

flakes while elastomeric binding material can also be observed in the im-

age.

(a) (b) (c)

Figure 4.5 (a) SEM micrograph (SE) of a sample cured at 150 °C for 30 minutes

(b) SEM micrograph (in-beam SE) of a sample cured at 110 °C for 3 minutes

(c) SEM micrograph of a sample cured at 110 °C for 10 minutes, having damaged conductivity

(overstretched)

TABLE 4.3

ELEMENTAL ANALYSIS

Sample /Element Ag % C % O % Cl % σ

110 °C for 3 minutes 85.8 8.0 3.7 2.6 0.7

150 °C for 30 minutes 89.3 7.8 2.1 0.8 0.9

Damaged conductivity 69.6 15.8 9.7 3.2 0.4

29

5 Discussions 5.1 Validity of results The main focus of the thesis has been on the fabrication of a large area

pressure sensor, which is thin and flexible. The understanding of the in-

teractions between functional materials (inks and substrates), would pro-

vide a grounding for manufacturing printed electronics-based devices

and sensors. The range and sensitivity of a pressure sensor can be tuned

and optimized by adjusting the sheet resistance of pressure sensitive ink.

The sampling and readings are carried out with calibrated measure-

ment tools. It is found that the large area pressure sensor is providing

satisfactory results within its sensor class [48], [49], [54]. The calibration

of printed pressure sensors using a force gauge may require a second

force gauge from another manufacturer for verification of calibration.

While conducting the elongation test for stretchable interconnects, a slight

nonuniformity in the thickness of TPU substrate can affect elongation

over weaker areas and can therefore, affect the conductivity of overall

printed track after elongations.

5.2 Limitations and possible improvements of pro-posed large area pressure sensor

Limitations and shortcomings of this posture recognition technique in-

clude a relatively small sampling set and limited number of test persons.

This large area pressure sensor is only capable of measuring the pressure

in a downward direction and cannot measure shear. The fabricated sensor

cannot stretch and hence the functionality is limited to flat or semi-flat

surfaces.

The designed prototype is currently using Bluetooth classic, which

has a limited range and character support. It also requires some time to

establish a connection to the host device. The module is going to be re-

placed by BLE v5.0 so as to achieve power efficiency, improve data trans-

fer rates, security and a longer range. It is worthwhile pointing out that,

in order to attain more accurate insight about the functionalities and per-

formance of the proposed large area sensor, test conditions can be broad-

ened by including actual wheelchair users as volunteers and participants.

30

5.3 Ethics and impact on society There are several important ethical and social aspects that need consid-

erations.

5.3.1 Ethics

There are always fundamental ethical aspects to consider when the re-

search deals with a patient’s data. The privacy of the patient’s data is very

important. The researcher is bound to fulfil the privacy requirements as

per regulations. Another ethical aspect is the disposal of waste electronics,

as the waste including plastic and ink residuals that must be disposed

properly at designated disposal stations.

5.3.2 General considerations

The developed sensors use plastic materials as substrate which may not

be quite environmentally friendly material. Moreover, the inks that have

been used to develop conducting and sensing layers might have hazard-

ous effects on the environment. The solvents such as Acetone and Methyl

Ethyl Ketone (MEK) employed for cleaning, must be used in a properly

ventilated room. A warning sign can be placed at the entrance of a shared

lab for possible side effects from these solvents.

5.3.3 Impact on society

The research findings do have a great impact on society as they provide a

prevention measure against pressure ulcer development in wheelchair

users. The pressure sensing system can also provide a similar solution for

bedridden people. The pressure sensors developed can be effectively

used to determine the sitting behaviour of a patient by measuring the dis-

tributed pressure points. The large area sensor can, therefore, be used as

a prevention measure and also for developing treatment plans if the pa-

tient has already developed pressure sores [52]. The overall impact would

be a lesser burden on hospitals, reduced nursing hours, medication costs

and more importantly, the avoidance of ineffable pain that a pressure ul-

cer patient will suffer. Hence, the wheelchair users as well as the individ-

uals with longer sitting hours with a sedentary lifestyle and individuals

such as drivers, gamers and office workers may be the primary benefi-

ciary of such system.

31

6 Summary Manufacturing of electronic devices through printing requires an under-

standing of inks, substrates and application methods. The thesis aims to

address important challenges regarding the fabrication and implementa-

tion of large area pressure sensors for wheelchair users. The pressure sen-

sors can be integrated into a powered wheelchair or can function as

standalone pressure monitoring system. In summary, the contribution in

the thesis presents a health care solution for wheelchair users that may

assist in preventing the development of pressure ulcer and hence help re-

duce the financial burden on the healthcare system.

6.1 Conclusion The overall benefits of printed electronics are they are capable of provid-

ing low-cost, easy production, simple integration, a thin form factor and

flexible solutions for fabrication of integrated electronic circuits. The non-

conventional electronics and new materials such as stretchable polymers,

plastics and different fabrics have unlocked new possibilities in the elec-

tronics industry. This research has presented a non-conventional, cost-ef-

fective and flexible sensor design for a large area pressure sensing system,

utilised in a wheelchair seat cushion by using screen-printing. This thesis,

in general, has provided one solution regarding large area pressure sen-

sors to be employed in healthcare for the prevention of pressure ulcers in

wheelchair users.

Different transduction mechanisms are studied along with their com-

plexity, response time, stability and repeatability. As per design parame-

ters for thin and flexible printed sensors, the proposed pressure sensor is

320 µm thick, and each sensing element is capable of sensing a force be-

tween 1 N up to 100 N. The designed sensor is characterized by investi-

gating its parameters in terms of resistance, response time, recovery time,

drift, durability and repeatability. The large area pressure sensor is

slightly sensitive to ambient conditions. An algorithm for sitting posture

recognition has been developed and it has demonstrated an accuracy of

more than 80 %.

Recent advancements in printable and stretchable TPU sheets opened

many doors to stretchable sensors and other electronic devices. As dis-

32

cussed in the section above, stretchable inks printed over stretchable sub-

strates maintain their electrical conductivity parameters even after multi-

ple elongation cycles, if the substrate and curing times are adequate. The

outcome of the research work may open a way to investigate the possibil-

ities of multi-layered stretchable printed pressure sensors.

6.2 Future work Possible future investigations can be considered in order to handle the

limitations associated with the proposed work. The new approach of us-

ing stretchable printed pressure sensors would give more flexibility and

comfort to wheelchair users. This large area pressure sensor can be used

effectively as an insole pressure monitoring system for gait analysis as

well.

The decision tree algorithm using sum values and mean values of sen-

sors gives a fair accuracy of posture detection that is above 80%, but it can

be further enhanced by the implementation of more complex machine

learning algorithms. Moreover, to achieve a more detailed pressure dis-

tribution data, additional areas in the wheelchair can be equipped with

pressure sensors, such as the back rest, head rest, foot rest and arm rest.

The number of sensors can also be increased so as to attain a higher spatial

resolution, with subsequent extended possibilities for extracting infor-

mation about the seated person. This will allow collection and integration

of detailed data, which would be used to improve posture recognition ef-

ficiency. Moisture and temperature sensors may be embedded along with

pressure sensors. The read-out electronics can be made even more effi-

cient using high-end integrated chips and communication modules. Fur-

thermore, smart phone applications could be developed to facilitate

health professionals for monitoring of pressure distribution as well as in

creating alarm features.

33

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Appendix I

RESISTANCE (Ω) READINGS ACQUIRED BEFORE AND AFTER ELONGATIONS

Curing temperature

Time 3 minutes 10 minutes 20 minutes 30 minutes

Line width (cm)

pre stretch

25 % stretch

ed

50 % stretch

ed

pre stretch

25 % stretch

ed

50 % stretch

ed

pre stretch

25 % stretch

ed

50 % stretch

ed

pre stretch

25 % stretch

ed

50 % stretch

ed

110 °C

1 4.63 51.9 174.4 4.08 156.2 206.8 3.96 90.1 201.4 2.43 48.1 179.7

0.5 7.44 65.2 211.6 7.11 226.4 291.7 6.73 119.3 341.1 4.15 64.5 128.6

0.25 11.82 106.5 364.5 13.35 374.1 564.3 11.43 209.1 467.7 6.39 132.3 158.8

0.125 20.84 195.1 537.1 36.01 721.8 1368.0 21.03 230.4 519.8 10.81 172.1 233.9

0.0625 50.09 244.3 563.2 42.07 1450.3 1255.4 37.11 184.2 226.2 16.06 162.2 261.8

0.0312 88.91 362.3 1051.9 60.01 1367.5 1972.4 48.09 NaN NaN 27.49 266.4 NaN

120 °C

1 4.54 145.6 205.5 2.65 23.2 45.6 2.04 18.3 41.7 1.71 16.7 53.4

0.5 7.14 159.2 258.4 4.11 38.6 99.1 3.67 24.4 52.6 2.91 38.3 70.5

0.25 11.53 176.7 566.4 7.73 65.2 168.9 6.89 38.1 76.4 4.63 42.5 91.9

0.125 19.17 217.1 694.1 14.27 97.3 149.7 12.08 71.2 144.2 9.21 56.7 83.2

0.0625 26.99 264.4 613.7 26.19 121.7 198.5 27.52 328.1 745.3 17.27 48.5 73.0

0.0312 55.01 594.3 912.4 53.91 404.4 763.1 67.02 NaN NaN 30.41 92.1 106.6

130 °C

1 2.12 8.5 44.0 1.72 14.9 18.3 1.07 5.0 7.2 0.81 0.9 1.5

0.5 3.69 15.4 76.3 3.14 40.0 79.6 1.82 6.6 11.4 1.22 1.8 2.7

0.25 6.37 34.1 126.1 5.02 60.4 89.2 2.92 9.1 14.2 1.98 3.1 4.9

0.125 11.96 62.2 194.5 8.65 70.7 121.6 6.23 17.9 24.5 4.69 7.3 11.2

0.0625 17.21 101.4 237.4 14.43 137.2 243.1 10.05 26.8 38.2 7.51 14.3 19.3

0.0312 40.75 236.1 440.1 21.39 114.4 295.7 19.11 409.1 582.6 14.59 30.8 44.9

150 °C

1 1.01 7.6 5.6 0.74 2.9 3.1 0.66 1.3 2.1 0.59 1.5 3.3

0.5 1.81 13.5 9.3 1.26 3.8 4.2 1.14 2.2 3.4 1.09 2.6 5.2

0.25 3.18 20.7 15.5 2.32 7.4 9.4 2.03 3.8 6.0 2.02 5.1 10.2

0.125 6.73 41.5 34.5 4.98 13.5 18.4 4.51 8.4 12.6 4.36 11.6 19.4

0.0625 10.45 57.1 46.1 8.02 18.8 30.6 7.17 14.9 19.5 7.08 15.8 30.1

0.0312 17.19 72.2 67.2 15.34 39.8 64.4 12.31 43.1 41.6 10.13 46.1 52.7

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