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Robotic flexible electronics with self-bendable films

Hunpyo Ju1, Jinmo Jeong1, Pyo Kwak1, Minjeong Kwon1 & Jongho Lee1,2*

Keywords: soft robotics, flexible electronics, actively bendable flexible electronics, self-

bendable film.

1School of Mechanical Engineering, Gwangju Institute of Science and Technology (GIST),

Gwangju 61005, Republic of Korea

2Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and

Technology (GIST), Gwangju 61005, Republic of Korea

*Correspondence should be address to J.L. (jong@gist.ac.kr)

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Abstract

Mechanical flexibility introduced in functional electronic devices has allowed electronics to

avoid mechanical breakage, conform to non-planar surfaces, or attach to deformable surfaces,

leading to greatly expanded applications, and some research efforts have already led to

commercialization. However, most of these devices are passively bendable by external driving

forces. Actively bendable flexible thin-film devices can be applied to new fields with new

functionalities. Here, we report robotic flexible electronics with actively self-bendable flexible

films that can serve as a platform for flexible electronics and other applications with the

capability of reversible bending and unbending by electrical control. Experimental studies

along with mechanical modeling enables the predictable and reversible transformation into

different structures by adjusting the design parameters. Demonstrations for self-bendable

flexible displays and soft robotic hands prove the feasibility of the concept.

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Introduction

Over the past decade, active research on flexible electronics has remarkably broadened

potential uses of electronics to include diverse applications, such as flexible displays1-3, flexible

photovoltaics4,5, wearable6,7 and bio-integrated8,9 devices and others10-16, by designing flexible

films that serve as substrates to accommodate mechanical strain in active devices when the

flexible films are passively bent by external mechanical driving forces. Although passively

bendable flexible films can successfully absorb large passive bending, actively self-bendable

films with smooth curvature can provide new opportunities with new functionalities in the

fields of flexible electronics, soft robotics, and other applications. Although soft robots17-21

designed for soft grippers22, manipulators23, locomotion devices24, and others25,26 are capable

of bending and unbending by pneumatic control, they are not film-type devices, i.e.,

thicknesses on the millimeter to centimeter scale. Advanced material technologies, including

shape memory polymers27-29 and hydrogels22,30,31 have successfully demonstrated smooth,

active changes in the shape of their structures. However, mechanically actuating functional

devices realized with these materials may be inconvenient or challenging because the actuation

occurs in certain controlled surrounding environments by changing the temperature or light

conditions or by submersion in water. In addition, the slower deformation speed may further

restrict their practical operation. Other types of actuators, such as shape memory alloys, offer

one-way actuation with relatively high contraction speed, as demonstrated in robotic

applications32-34 using folding hinges connecting relatively large rigid plates. Other approach

that uses shape memory alloys on both sides of rigid links demonstrated reversible actuation,

similar with human fingers35. Recent study introduced soft tube and wrist-like actuators by

embedding shape memory alloys in elastomer36. While these studies using shape memory

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alloys are very attractive, they are not for film-type actuators. Film-type actuators can reduce

mechanical strains when integrated with flexible electronics since the maximum mechanical

strain of flexible electronics in bending depends on the thickness of the devices37. Designing

and integrating self-bendable flexible films that can reversibly bend with a smooth curvature

at a reasonable speed without controlling the environmental temperature or submerging in

water can provide greater opportunities for new applications. Here, we present a concept of

robotic flexible electronics with self-bendable flexible films that can serve as reversibly

bendable substrates or actuators by embedding established wire actuators in the composites of

elastomeric films and structural features that control smooth curvatures. The resulting self-

bendable films in a relatively thin form factor are electrically controllable. We also present

experimental and theoretical studies that capture the mechanics to provide the design

parameters. Demonstrations using a self-bendable flexible light emitting diode (LED) display

and soft robotic hands validate the concepts.

Materials and Methods

Fabrication of the self-bendable films. First, carrier substrates were prepared by cleaning

and depositing a parylene layer (~1 µm, poly-para-xylylene-C, parylene-C) on a glass slide (76

mm × 52 mm) using low-pressure chemical vapor deposition (PDS 2010, Specialty Coating

Systems) to facilitate the clean removal of the self-bendable films at the end of the process. An

elastomeric layer (thickness: 150 µm – 450 µm) was formed by spinning a poly-

dimethylsiloxane (PDMS, base: curing agent = 10:1, Sylgard 184, Dow Corning) on the carrier

substrate and curing at 150 °C in a vacuum oven for 2 h. After depositing another layer of

parylene (~1 µm) on the elastomer, the bases (SU-8, thickness: ~13 µm, width: 2.9 mm – 36

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mm) of holding blocks were defined to align the wire actuators (shape memory alloy, SMA,

Nitinol, d = 38 µm, Dynalloy Inc.) in the raised configuration from the parylene surface, using

a custom-built wire guide (Supplementary Fig. 1). While the actuators were temporarily held

with adhesive tape, we phototopographically formed the top sections (SU-8, ~80 µm) of the

holding blocks to embed the actuators in the holding blocks. More details with illustrations are

in the Supporting Information. The exposed actuators are loose at room temperature because

they shrink at raised temperature (95 °C) when baking the SU8 layer to form the holding blocks.

The self-bendable film is naturally bent due to the contraction of the elastomer at room

temperature. For a higher curvature, the self-bendable films were further stretched (8%) using

a motorized stage, released, and baked (110 °C, 10 s) to cause plastic deformation in the

parylene thin film.

Measurements and actuations. All the curvatures and bending angles of the self-bendable

film were obtained from the images taken from the side view together with the reference scales

at room temperature (25 °C). For fair comparison, all the fresh self-bendable films went

through stretching (8%) and relaxing at 100 µm/s using a motorized stage, heating (110 °C) for

10 s and cooling (25 °C) before conducting measurements unless mentioned otherwise. A

conventional power supply (E3634A, Agilent Technologies) was used to electrically control

the curvature of the self-bendable films by applying a current (0 – 70 mA) to the actuators. For

the curvature measurements that depend on the applied currents, we averaged 10 measurements

after applying a specific current for 40 s. For the durability tests, we maintained an applied

current (60 mA) for 30 s and off (0 mA) for 30 s to give enough time for steady states for one

cycle.

Fabrication of the robotic self-bendable flexible displays. Ti (20 nm) and Au (260 nm) were

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sputtered to form thin metal layers on a polyimide (PI, 12.5 µm) film that was temporarily

attached on a PDMS-coated (cured at 80 °C for 1h) glass (76 mm × 52 mm) substrate. Metal

interconnects on the PI film were defined by photolithography with positive photoresist (PR)

masks (AZ5214, AZ Electronic Materials) and by a wet chemical etching process (Ti etchant

TFT, Au etchant TFA, Transene Company). After spin-coating and curing another PI (3 µm)

layer on the metal patterns, a single reactive ion etching process (RIE, O2, 50 sccm, 100 W for

160 min) created through holes over metal interconnects and, at the same time, removed

unnecessary regions all the way through the PI films whose lateral shapes are designed to

accommodate strain while bending. The flexible multilayer film with metal interconnects was

released from the glass substrate using a water-soluble tape (3M) and integrated onto the self-

bendable film aided by liquid PDMS as an adhesive. Finally, inorganic LEDs (SML-P1, 1.0

mm × 0.6 mm × 0.2 mm, Rohm Semiconductor) were bonded on the self-bendable film using

a conductive adhesive (Epoxy Technology Inc.).

Results

Designs and actuation principles of thin self-bendable films. Figure 1a shows schematic

illustrations of the design and exposed and assembled views of the self-bendable film. The

fabrication process starts with spin-casting and curing the elastomer layer (poly-

dimethylsiloxane, PDMS, thickness: 150 µm – 450 µm) on a carrier substrate (glass slide: 76

mm × 52 mm) at 150 °C, followed by depositing the thin film layer (~1 µm, poly-para-

xylylene-C, parylene-C) on the elastomer with low-pressure chemical vapor deposition (PDS

2010, Specialty Coating Systems). The first layer of the holding blocks (SU-8, thickness: ~13

µm, width: 2.9 mm – 36 mm) formed by photolithography provides clearance for the wire

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actuators (shape memory alloy, SMA, Nitinol, d = 38 µm) from the base (thin film: ~1 µm,

poly-para-xylylene-C, parylene-C), serving as one design parameter in determing bending

angles. The other layer of the holding blocks (SU-8, ~42 µm) simply holds the periodic regions

of the actuators that are aligned across the lower holding blocks with the aid of a custom wiring

rack (Supplementary Fig. 1). At room temperature, the exposed actuators are loose, as shown

with the lower illustration and optical microscope image in Fig. 1a because the actuators are

embedded in a shrunken state when curing the top holding blocks at an increased temperature

(95 °C). More details of the fabrication process are available in the Experimental section and

Supplementary Information (Supplementary Fig. 2). If a higher curvature is required after

being separated from the carrier substrate, applying strain with a motorized microstage,

followed by relaxing and curing (110 °C, 10 s, Supplementary Fig. 3) provides additional

curvature of the fabricated films. For consistency, unless otherwise noted, we used a strain of

8%, which is no more than the maximum recovery strain (8%) of the actuators, when preparing

most samples.

The design with the elastomer layer and embedded actuators enables reversible self-bending

and unbending. When Joule heating is off, the bi-layers of the thin film (including blocks) and

elastomer formed at the elevated temperature (150 °C) causes bending at room temperature

because the elastomer tends to contract. The elastomer cured at high temperature (150 °C) tends

to contract at room temperature, but contraction of the top surface is restricted due to the thin

film and holding blocks on top, resulting in downward bending, as shown in the optical image

in Fig. 1b and c. In contrast, at high temperature (110 °C), the neighboring holding blocks are

pulling each other due to shortening of the exposed actuators, resulting in unbending of the

self-bendable films, as shown in Fig. 1d and e.

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The bending curvature can be controlled by the local Joule heating effect of the actuators

using the electrical current. Local Joule heating is comparably fast and convenient38 as it can

avoid controlling the temperature of the entire environment although local temperature rise

may cause local deformation of the surrounding polymer layers. Figure 1f shows the sideview

images of the deformed self-bendable film in different curvatures with respect to different input

currents (0 mA, 30 mA, 50 mA and 70 mA) at room temperature (25 °C). Initially, the self-

bendable film is bent without an applied current because the bottom of the elastomer layer

contracts at room temperature. As the applied current increases, the curvature, defined as an

inverse of the bending radius, decreases. Figure 1g summarizes the experimental results of the

curvatures with respect to applied currents, acquired by applying fixed specific currents (0 –

70 mA, with an increment of 10 mA) for 40 s. For each current, the measurements were

repeated 10 times. The experimental results of the curvatures with respect to temperature are

also included in Supplementary Fig. 7. Robustness of the self-bendable film was accessed by

mechanical cyclic bending and unbending tests through repetitively applying electrical current

on (60 mA) and off (0 mA) up to 3,000 cycles to as in Fig. 1h. The results indicate no significant

variation (on: 0.0031 – 0.0020 mm-1, off: 0.13 – 0.12 mm-1) in the curvatures of the self-

bendable film.

Experimental results and theoretical models of self-bendable films by various design factors.

The experimental results reveal quantitative bending mechanics that allow theoretical models

to predict the mechanical behavior and provide guidance in designing the self-bendable films

for target applications. Figure 2a provides illustrations of the unit angle (qunit: bending angle in

one recessed region) and total angle (qtotal: bending angle in the total recessed regions) with the

design parameters, such as recess width (wr), block width (wb), elastomer thickness (te), thin-

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film thickness (tf) and elevation of the actuators (ea), of the self-bendable films. Measurements

of the unit angle (qunit, Fig. 2b) of the self-bendable films (n = 5 for each data point, total 60

samples) that have various elastomer thicknesses (te: 150 µm to 460 µm) and thin-film

thicknesses (tf: 1 µm, 3 µm and 6 µm) with other design parameters fixed (ea = 13 µm, wb = 1

mm, wr = 1 mm), indicate that the qunit of the self-bendable films with a thinner elastomer (150

µm) is higher (23° for tf = 3 µm) and becomes lower (12.4° for tf = 3 µm) as the elastomer

thickness increases (450 µm). The optical images in Fig. 2b show the side view of the self-

bendable films (tf = 3 µm) with different bending angles depending on the elastomer thickness.

The results demonstrate that the bending angle of a film depends on the difference in thickness

between the top and bottom surfaces due to lateral contraction. The elastomer layer that

contracts at room temperature causes bending because the top surface is restricted from

contracting laterally by the thin film and holding blocks, but the bottom surface is not restricted.

For the thinner elastomers, the distance between the top and bottom surface is shorter, thus

resulting in higher bending. It should be noted that although the thinner elastomer can provide

a higher bending angle, a blocking force39 by the thinner elastomer will be lowered. The thin-

film thickness (tf: 1 µm, 3 µm, 6 µm) is less significant than the elastomer thickness in

determining the bending angles. The black and white regions in the inset illustration in Fig. 2b-

e show the top view of the holding blocks and recesses of the self-bendable films, respectively.

The other design parameters, such as the elevation of the actuators from the base, recess

width and density affect the bending angles. Experiments (n = 8 for each point, total 96 samples)

using various elevations (ea: 13 µm, 42 µm and 61 µm) of the actuators from the base and

recess width (wr: 100 µm, 200 µm, 400 µm and 600 µm), as shown in Fig. 2c, indicate that

lower elevation (e.g., ea = 13 µm, blue solid circle) of the actuators provides a higher unit

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bending angle (qunit = 12.8°) when other parameters are fixed (e.g., wr = 400 µm) because the

actuators located closer to the neutral plane can cause more bending for the same applied strain

in the actuator. A wider recess also provides more bendable regions and thus higher unit

bending angles (e.g., qunit = 16.1° for wr = 600 µm), as shown in Fig. 2c. Fig. 2d also indicates

that, when the recess widths are equal (wr = 500 µm), the unit angles (red dots) are also similar

(qunit = 16.7°) regardless of the width of the holding blocks (wb: 571.4 µm – 3250 µm). The

total angle (blue dots) is a sum of the angles based on the number of recesses. However, the

total bending angles (blue dots) are higher for the same duty ratio (Swr / (Swr + Swb) = 50%)

of the recesses when the holding blocks are more divided, as shown in Fig. 2e. This can be

explained by introducing the parameter dblock, which is defined as the length of the lateral

contraction of the lower surface of the elastomer under the edges of the holding blocks, as

illustrated in Fig. 2f. If drecess and dfilm are defined as the contraction of the lower surface of

elastomer under the recesses and the permanent elongation of the thin film caused by stretching

during fabrication, respectively, then the lateral difference (~(drecess + dfilm + dblock)) of the top

and bottom surfaces causes bending (Fig. 2g). Finite element model (FEM) analysis (Fig. 2g)

also indicates that the lower surface of the elastomer under the edges of the holding blocks

experiences a relatively higher principal strain than other regions under the holding blocks.

Although drecess and dfilm are proportional to the recess width (wr), e.g., drecess = 0.0255 ´ wr,

dfilm = 0.006 ´ wr, for te = 150 µm and tf = 1 µm, respectively, the experimental results indicate

that dblock is independent of wr (dblock: ~25.5 µm), as shown in Supplementary Fig. 4. In addition,

the experimental results in Fig. 2d also indicate that dblock is independent of the block width

(wb). More details are available in the SI. As a result, the bending angle per unit recess width

of the bendable films can be modeled as the dashed line in Fig. 2h. Although all the studied

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design parameters affect final shapes of the self-bendable films, the recess width can determine

the unit angle in wider range most conveniently after other design parameters are fixed. The

experimental results (green circles, blue squares, red triangles) from various self-bendable

films agree well with the model. The calculations of the model are described in detail in the

Supplementary Information.

Various 3D structures formed from 2D self-bendable films with different layout designs.

Adjusting the design parameters of different planar layouts enables different three-dimensional

(3D) structures by the reversible bending and unbending of the self-bendable films through

actuation. The representative examples in Fig. 3 include a planar layout design (first column),

3D structures through bending (second column), and planar shapes after unbending (the third

column). Relatively simple structures (Fig. 3a-g) were operated by electrical (Joule) heating

with the power supply (E3634A, Agilent Technologies) but relatively complex structures (Fig.

3h-j) by direct thermal heating on a hot plate to avoid complex wiring routes that may require

additional wiring racks. The first three are basic structures enabled by simple planar layouts

with various block widths (wb), recess widths (wr) or holding block tilting angles. Because the

holding blocks (wb) embedding the actuators do not bend, the wide holding blocks (wb = 8 mm)

remain flat but the recesses between the narrow holding blocks (wb = 1 mm) bend, resulting in

the triangular structure in Fig. 3a. Electrical actuation can be used to control the formation of

a triangle structure by bending (middle) or a planar structure by unbending (right image). The

length of the scale bars is 5 mm. The incremental recess widths (wr = 100 – 2100 µm, increasing

by 100 µm) gradually change the curvatures, resulting in the spiral structure, as shown in Fig.

3b. In addition, the tilting angle (30°) of the holding blocks can also enable the helical structure

as in Fig. 3c. The detailed dimensions of the designs are provided in the Supplementary

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Information.

By combining the basic structures and adjusting the design parameters, the bending

structures can become more complex. Figure 3d shows the self-bendable film with a wide root

(wr = 100 µm, curvature (k) = 0.08) connected to two branches with different curvatures (k =

0.16 with wr = 200 µm, k = 0.19 with wr = 1000 µm). The combination of two different tilting

angles (30° and - 30°) provides the symmetric helical structure in Fig. 3e. The combination of

the holding blocks with different block widths (wb = 10 mm and 1.5 mm) and tilting angles

(30°) generate a structure with the flat and bending regions combined with the helical region,

as in Fig. 3f. More complex planar layouts along with tailored design parameters yield

volumetric structures such as the rectangular-like (Fig. 3g), cube-like (Fig. 3h), pyramid-like

(Fig. 3i) and animal-like (claiming elephant-like) structures (Fig. 3j).

Robotic self-bendable flexible display and hand. The self-bendable films can serve as a

platform for applications that require flexibility and actuation. One example includes robotic

self-bendable flexible electronics, which are distinguished from flexible electronics that can

bend passively by external forces. Figure 4a shows the concept of robotic self-bendable flexible

displays. Integrating LEDs (ROHM Semiconductor) onto electrodes (Ti / Au = 20 / 260 nm)

that are encapsulated with polyimide films (upper: 3 µm, lower: 12.5 µm) forms a flexible LED

array. Serpentine interconnectivity can accommodate strains in the recess regions (Fig. 4b)

when bending the flexible display (radius of curvature ~10 mm), as shown in Fig. 4c. The

robotic self-bendable flexible display can continuously and reversibly change the curvature

from round (left) to flat (right) by controlling the input current (70 mA), as shown with the

time-lapse (0 s, 11 s, 22 s) optical images in Fig. 4d. This example application may lead to a

display that can adjust the curvature automatically depending on distances from viewers. The

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lower images were taken with the ambient light off. The concept of embedding thin actuators

in films also enables the design of soft robotic hands, as shown in Fig. 4e. Because the structure

is deformable, external forces can entangle the fingers without breaking (Fig. 4f). Actuation of

the soft robotic hands by unfolding and folding the fingers one by one (Fig. 4g) through

controlling the applied current can untangle the fingers (Fig. 4h).

Conclusion

In conclusion, the self-bendable films designed with holding blocks on a bi-layer of thin-

film and elastomeric substrates enable reversible bending and unbending with a smooth

curvature for robotic flexible electronics by electrical control in a thin form factor.

Experimental and theoretical studies provide guidance toward designing various planar layouts

combined with different design parameters for the reversible transformation of planar films to

various predictable 3D structures. The concepts of robotic flexible electronics presented here

should be useful for developing new types of flexible electronics, as demonstrated by the

robotic self-bendable flexible displays that can transform into real 3D displays, and these

concepts can serve as a platform for many other applications, including soft robotics and bio-

medical and wearable devices.

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References

1 Kim T-H, Cho K-S, Lee EK, Lee SJ, Chae J, Kim JW, et al. Full-colour quantum dot

displays fabricated by transfer printing. Nat Photonics 2011;5:176–182.

2 Kim S, Kwon H-J, Lee S, Shim H, Chun Y, Choi W, et al. Low-Power Flexible

Organic Light-Emitting Diode Display Device. Adv Mater 2011;23:3511–3516.

3 Biswas S, Schöberl A, Mozafari M, Pezoldt J, Stauden T, Jacobs HO. Deformable

printed circuit boards that enable metamorphic electronics. NPG Asia Mater

2016;8:e336.

4 Lee J, Wu J, Shi M, Yoon J, Park S Il, Li M, et al. Stretchable GaAs photovoltaics

with designs that enable high areal coverage. Adv Mater 2011;23:986–991.

5 Na S-I, Kim S-S, Jo J, Kim D-Y. Efficient and Flexible ITO-Free Organic Solar Cells

Using Highly Conductive Polymer Anodes. Adv Mater 2008;20:4061–4067.

6 Lipomi DJ, Vosgueritchian M, Tee BC-K, Hellstrom SL, Lee JA, Fox CH, et al. Skin-

like pressure and strain sensors based on transparent elastic films of carbon nanotubes.

Nat Nanotechnol 2011;6:788–792.

7 Kim R-H, Kim D-H, Xiao J, Kim BH, Park S-I, Panilaitis B, et al. Waterproof

AlInGaP optoelectronics on stretchable substrates with applications in biomedicine

and robotics. Nat Mater 2010;9:929–937.

8 Son D, Lee J, Qiao S, Ghaffari R, Kim J, Lee JE, et al. Multifunctional wearable

devices for diagnosis and therapy of movement disorders. Nat Nanotechnol

2014;9:397–404.

15

9 Kim D-H, Viventi J, Amsden JJ, Xiao J, Vigeland L, Kim Y-S, et al. Dissolvable films

of silk fibroin for ultrathin conformal bio-integrated electronics. Nat Mater

2010;9:511–517.

10 Choi S, Lee H, Ghaffari R, Hyeon T, Kim DH. Recent Advances in Flexible and

Stretchable Bio-Electronic Devices Integrated with Nanomaterials. Adv Mater

2016;28:4203–4218.

11 Cheng T, Zhang Y, Lai WY, Huang W. Stretchable thin-film electrodes for flexible

electronics with high deformability and stretchability. Adv Mater 2015;27:3349–3376.

12 Sekitani T, Nakajima H, Maeda H, Fukushima T, Aida T, Hata K, et al. Stretchable

active-matrix organic light-emitting diode display using printable elastic conductors.

Nat Mater 2009;8:494–499.

13 Someya T, Kato Y, Sekitani T, Iba S, Noguchi Y, Murase Y, et al. Conformable,

flexible, large-area networks of pressure and thermal sensors with organic transistor

active matrixes. Proc Natl Acad Sci 2005;102:12321–12325.

14 Lee P, Lee J, Lee H, Yeo J, Hong S, Nam KH, et al. Highly stretchable and highly

conductive metal electrode by very long metal nanowire percolation network. Adv

Mater 2012;24:3326–3332.

15 Oh MJ, Kim YH, Choi GH, Park AR, Lee YM, Park B, et al. Percolation-Controlled

Metal/Polyelectrolyte Complexed Films for All-Solution-Processable Electrical

Conductors. Adv Funct Mater 2016;26:8726–8734.

16 Lu N, Kim D-H. Flexible and Stretchable Electronics Paving the Way for Soft

Robotics. Soft Robot 2014;1:53–62.

16

17 Mosadegh B, Polygerinos P, Keplinger C, Wennstedt S, Shepherd RF, Gupta U, et al.

Pneumatic networks for soft robotics that actuate rapidly. Adv Funct Mater

2014;24:2163–2170.

18 Marchese AD, Onal CD, Rus D. Autonomous Soft Robotic Fish Capable of Escape

Maneuvers Using Fluidic Elastomer Actuators. Soft Robot 2014;1:75–87.

19 Katzschmann RK, Marchese AD, Rus D. Autonomous Object Manipulation Using a

Soft Planar Grasping Manipulator. Soft Robot 2015;2:155–164.

20 Matia Y, Gat AD. Dynamics of Elastic Beams with Embedded Fluid-Filled Parallel-

Channel Networks. Soft Robot 2015;2:42–47.

21 Majidi C. Soft Robotics: A Perspective—Current Trends and Prospects for the Future.

Soft Robot 2014;1:5–11.

22 Yuk H, Lin S, Ma C, Takaffoli M, Fang NX, Zhao X. Hydraulic hydrogel actuators

and robots optically and sonically camouflaged in water. Nat Commun 2017;8:14230.

23 Hawkes EW, Blumenschein LH, Greer JD, Okamura AM. A soft robot that navigates

its environment through growth. Sci Robot 2017;2:eaan3028.

24 Shepherd RF, Ilievski F, Choi W, Morin SA, Stokes AA, Mazzeo AD, et al. Multigait

soft robot. Proc Natl Acad Sci 2011;108:20400–20403.

25 Ainla A, Verma MS, Yang D, Whitesides GM. Soft, Rotating Pneumatic Actuator.

Soft Robot 2017;4:297-304.

26 Martinez R V., Fish CR, Chen X, Whitesides GM. Elastomeric origami:

Programmable paper-elastomer composites as pneumatic actuators. Adv Funct Mater

17

2012;22:1376–1384.

27 Lendlein A, Schmidt AM, Schroeter M, Langer R. Shape-memory polymer networks

from oligo(ε-caprolactone)dimethacrylates. J Polym Sci Part A Polym Chem

2005;43:1369–1381.

28 Huang WM, Yang B, An L, Li C, Chan YS. Water-driven programmable polyurethane

shape memory polymer: Demonstration and mechanism. Appl Phys Lett 2005;86:1–3.

29 Raviv D, Zhao W, McKnelly C, Papadopoulou A, Kadambi A, Shi B, et al. Active

Printed Materials for Complex Self-Evolving Deformations. Sci Rep 2015;4:7422.

30 Sydney Gladman A, Matsumoto EA, Nuzzo RG, Mahadevan L, Lewis JA. Biomimetic

4D printing. Nat Mater 2016;15:413–418.

31 Morales D, Palleau E, Dickey MD, Velev OD. Electro-actuated hydrogel walkers with

dual responsive legs. Soft Matter 2014;10:1337–1348.

32 Hawkes E, An B, Benbernou NM, Tanaka H, Kim S, Demaine ED, et al.

Programmable matter by folding. Proc Natl Acad Sci 2010;107:12441–12445.

33 Felton S, Tolley M, Demaine E, Rus D, Wood R. A method for building self-folding

machines. Science 2014;345:2–5.

34 She Y, Chen J, Shi H, Su H-J. Modeling and Validation of a Novel Bending Actuator

for Soft Robotics Applications. Soft Robot 2016;3:71–81.

35 Andrianesis K, Tzes A. Development and Control of a Multifunctional Prosthetic

Hand with Shape Memory Alloy Actuators. J Intell Robot Syst 2015; 78(2): 257-289.

36 Rodrigue H, Wei W, Bhandari B, Ahn SH. Fabrication of wrist-like SMA-based

18

actuator by double smart soft composite casting. Smart Mater Struct

2015;24(12):125003.

37 Ko HC, Shin G, Wang S, Stoykovich MP, Lee JW, Kim DH, et al. Curvilinear

electronics formed using silicon membrane circuits and elastomeric transfer elements.

Small. 2009;5(23):2703-2709.

38 Featherstone R, Teh YH. Improving the Speed of Shape Memory Alloy Actuators by

Faster Electrical Heating. Experimental Robotics IX. Berlin, Heidelberg: Springer

Berlin Heidelberg, 2006; 67–76.

39 Wang QM, Cross LE. Tip deflection and blocking force of soft PZT-based cantilever

RAINBOW actuators. J Am Ceram Soc 1999;82(1):103–110.

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Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded

by the Korean government (MSIP) (No.2016R1A2B4012854) and the GIST-Caltech Research

Collaboration and GIST Research Institute (GRI) Project through a grant provided by GIST.

Author contributions

J.L., H.J., P.K., J.J. proposed the concept. J.L. and H.J. designed the experiments. H.J.

fabricated the devices. H.J. and M.K conducted the experiments. J.L. and H.J. wrote the

manuscript.

Author disclosure statement

No competing financial interests exist.

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Figure 1. Designs and actuation principles of thin self-bendable films. (a) Schematic

illustration of the self-bendable film. The actuators (shape memory alloy (SMA, nitinol) wires,

d = 38 µm) embedded in the holding blocks (SU-8) are raised from the surface of the thin film

(P-xylylene, parylene, ~1 µm) deposited on the elastomer (poly-dimethylsiloxane, PDMS,

~150 µm). At the end of the fabrication process at high temperature (110 °C) on a flat carrier

substrate, the exposed actuators are in the contracted state, and the elastomer is in the expanded

state. (b-c) The self-bendable film separated from the flat substrate bends downward at room

temperature (25 °C). (d-e) At high temperature (110 °C), the self-bendable film returns to the

flat state by contraction of the actuators. (f) Optical images of the self-bendable film with

different applied currents (0 mA, 30 mA, 50 mA, 70 mA) in the actuators at room temperature

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(25 °C). (g) Measured curvatures depending on the applied currents. Error bars are the standard

deviation. (h) Measurements of the curvatures after alternatively applying and removing

current up to ~3000 cycles.

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Figure 2. Experimental results and theoretical models of self-bendable films by various

design factors. (a) Illustrations of the self-bendable film with annotations of the unit (qunit) and

total (qtotal) angles depending on the design parameters such as the block width (wb), recess

width (wr), elevation (ea) of an embedded actuator, and thickness of the thin film (tf) and

elastomer (te). (b) Unit angles depending on the elastomer thicknesses at different thin-film

thicknesses. The optical images show the degree of the bending angle with a thin-film thickness

of 3 µm. (c) Experimental (solid symbols) and computational (dashed lines) results of the unit

23

angles depending on the recess widths (wr) at various elevations (ea). The unit angle is higher

for lower elevation and higher recess width. (d) Unit and total angles for the same recess width

(500 µm). The unit angle is uniform, while the total angle increases linearly for higher number

of recesses. (e) Unit and total angles for the same duty ratio (50%); total recess widths over

combined recess and block widths, depending on recess widths. Although the duty ratio is the

same, the total angle becomes larger for smaller recess widths, i.e., for a larger number of

recesses. All error bars are the standard deviation. (f and g) Schematic illustrations and FEM

analysis of the bending mechanisms of the self-bendable film. Bending occurs by the

combination of the thermal contraction (drecess) of the elastomer under the recess, the permanent

expansion (dfilm) of the thin film, and the thermal contraction (dblock) of the elastomer under the

block. The colorized numbers refer to the maximum principal strains. (h) The bending angle

per unit recess width from separate experimental results with the different design parameters.

The model (dashed line) is a good representation of the experiments results.

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Figure 3. Various 3D structures formed from 2D self-bendable films with different layout

designs. (a-c) Basic layout designs and the corresponding optical images of 2D bendable film

and 3D structure. (a) Design with different block widths. The wide blocks remain flat, enabling

localized bending. (b) Spiral structure from the design with the incremental recess widths and

(c) helical structure from the design with tilted holding blocks. (d-f) Structures with combined

layout designs such as (d) with 3 different recess widths, (e) with 2 different tilted recesses, or

(f) with different block widths and tilted recesses. (g-j) Volume-based structures include (g)

square, (h) cube, (i) pyramid, or (j) animal shapes.

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Figure 4. Robotic self-bendable flexible display and hand. (a-b) Schematic illustration of

the robotic self-bendable flexible display. LED arrays are interconnected on the PI film

(polyimide, ~15.5 µm) in a stretchable form to accommodate bending strains. (c) Optical image

of the robotic self-bendable flexible display with a bending radius of ~10 mm. (d) Time-lapse

images of the robotic self-bendable flexible display with and without ambient light by

controlling the electrical current. (e) Optical images of the soft robotic hand whose fingers are

individually foldable and unfoldable by selectively controlling the input currents to the

actuators.

Supplementary Data

Finite Element Analysis

Finite element model analysis was conducted with a com-mercial software package (ABAQUS) to calculate the thermalcontraction of each layer of the self-bendable film (Fig. 2g).We modeled the layers of SU-8, polydimethylsiloxane

(PDMS), and parylene with the linear quadrilateral elements(CPS4) for the range of temperature change from 110!C to25!C. The thermal expansion coefficients a [m/m!C] we usedare 300 · 10-6 for PDMS, 52 · 10-6 for SU-8, and 38 · 10-6

for parylene.

SUPPLEMENTARY VIDEO S1. Bending and unbending of the helical structure.

SUPPLEMENTARY VIDEO S2. Bending and unbending of the film with two different tilted recesses.

SUPPLEMENTARY VIDEO S3. Robotic self-bendable flexible display.

SUPPLEMENTARY VIDEO S4. Actuation of the robotic hand.

SUPPLEMENTARY FIG. S1. Custom wiring rack. (a) Optical image of the custom wiring rack built by a 3D printer(UnionTech, RS 6000). The wire actuators (d = 38 lm) can be aligned on the substrate by lining through the racks shownwith (b) the magnified image.

SUPPLEMENTARY FIG. S2. Fabrication process of the self-bendable film. (a) Clean a carrier substrate (glass slide,76 · 52 mm). (b) Deposit a parylene layer (*1 lm) on the carrier substrate using low-pressure chemical vapor depositionfor easy removal of the self-bendable film at the end of the process. (c) Form the elastomer layer (thickness: 150–450 lm)on the parylene layer by spin coating PDMS and curing at 150!C in a vacuum oven for 2 h. (d) Deposit the parylene layer(*1 lm) again on the PDMS layer. (e) Pattern the base holding blocks (SU-8, *13 lm) using photolithography on theparylene layer. (f) Align the wire actuators using the custom wiring rack on the base holding blocks. (g) Coat with anotherSU-8 precursor and bake (95!C) to cause the wire actuators to shrink. (h) Form the top holding blocks (SU-8, *80 lm) byphotolithography. The exposed wire actuators are loose at room temperature. (i) Peel the fabricated self-bendable film fromthe carrier substrate. PDMS, polydimethylsiloxane.

SUPPLEMENTARY FIG. S3. Straining the self-bendable films. (a) Optical image of the motorized stage(resolution: 0.1 lm, speed: 0.1 mm/s), which provides con-sistent strain for additional curvatures. The percentage strainis based on the displacement over the total recess width. (b,c) Optical images of the film (b) before and after (c) thestraining process followed by relaxing and curing (110!C,10 s). (d) Experimental results of the curvatures before andafter the straining process are applied to the bilayers ofthe thin film (parylene, *1 lm) and elastomer (PDMS,*150 lm). (e) Experimental results of the unit angles afterapplying an 8% strain to the self-bendable films with dif-ferent recess widths (400, 600, 800, 1000, and 2000 lm).The patterns used are inserted below each plot.

SUPPLEMENTARY FIG. S4. Curvatures with respectto temperature. Measurements of the curvatures were re-peated five times at each temperature in a convection ovenfor uniform heating, error bars note standard deviations.

SUPPLEMENTARY FIG. S5. Calculation of drecess, dfilm, and dblock. (a) Calculation of drecess. Without the strainingprocess and holding blocks, the curvature (0.1701 mm-1, R1 = 5.8769 mm) of the bilayer of parylene (thickness: *1 lm) andelastomer (thickness: *150 lm) is caused by drecess. (b) Calculation of dfilm. With applied strain (8%), the curvature(0.2063 mm-1, R2 = 4.8466 mm) of the bilayer of parylene and elastomer is caused by drecess and dfilm. (c) Calculation ofdblock. With applied strain and holding blocks, the curvature is caused by drecess, dfilm, and dblock. (d) dblock is calculated fromvarious samples fabricated for different experiments. The results indicate that dblock (average: *25.5 lm) is not verydependent on the recess widths.

SUPPLEMENTARY FIG. S6. Calculation of the model of bending angle per unit recess width. (a) The schematicillustration indicates the parameters of the self-bendable film. (b) Calculation of bending angle per unit recess width (dashedline in Fig. 2h). From the equations of drecess, dfilm, and dblock in the Supplementary Figure S4, the unit angle per unit recesswidth can be represented as a function of wr. Through the values mentioned in Supplementary Figure S4, A = 0.0255,B = 0.0311, and C = 6.6666 and the unit is rad/mm.

SUPPLEMENTARY FIG. S7. Dimensions of the planar designs. Upper illustrations note symbols for the design pa-rameters. (a) Planar dimensions of self-bendable films with (a) different block widths, (b) spiral structure, (c) helicalstructure, (d) three different recesses, (e) two different tilted recesses, (f) different block widths and tilted recesses, (g)square, (h) cube, (i) pyramid, and (j) animal shape.