Post on 28-Jul-2020
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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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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.