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Stimuli-Responsive Materials

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Addressable, Stretchable Heating Silicone Sheets
R. Adam Bilodeau, Michelle C. Yuen, and Rebecca Kramer-Bottiglio*
utility through integration with thermally
responsive soft materials.
Many thermally responsive materials
are utilized by the soft robotics community. Thermally softening polymers(such as thermoplastics and thermosets) are passively stiff, actively soft
variable stiffness materials[6,11] that havebeen used in robotic systems for moveand-hold operations[12,13] as well as inconformable and on-demand structural
systems.[7,14] Additionally, thermally
responsive shape memory polymers andalloys have been employed as both stiffening agents and actuators.[6,8] Finally,
thermally responsive silicone-based
actuators have been recently developed,
which undergo bulk expansion whentrapped solvent is vaporized.[10] All ofthese previous examples required somethermal control, and so researchers
have sought out ways to embed a closecontact heater where the compliance ofthe heater was properly matched with its thermally responsive material pair.[12,13,15,16]
In an effort to fill the need for compliance-matched thermal
control, flexible and stretchable heaters are an active area of
research, as are addressable heating structures and systems.
Stretchable heating composite materials have been successfully
developed with either conductive fillers or microstructures[17–21]
and have demonstrated linear strains of up to 100%.[22,23] Proposed applications have included wearable technology andactive therapy,[22,24–29] surface-based temperature control of
nonplanar objects,[30] and thermo-chromatic flexible, and active
microdisplays.[31–33] These examples, however, require highcomplexity patterning techniques (e.g., lithographic masking)and often only heat up moderately. To facilitate thermal controllability across a 2D plane, researchers have begun to implement
thermal addressability onto stretchable or flexible surfaces, inwhich only localized regions of the surface are heated.[34–38]
These pioneering works have successfully implemented theconcept, but they also require lithographic masking, complex
assembly processes, high operational voltages (>100 V), or a
fixed framework around only a few small pockets of thermally
responsive material. Prior work has demonstrated stretchability
and addressability in geometrically soft material systems (i.e.,silicon-based electronics made stretchable through patterning
crystalline materials onto prestretched silicone),[24,25] but toour knowledge, there has not yet been a combination of these
two functions in a system which uses intrinsically soft materials (i.e., silicones and liquids). With our work, we combine
the manufacturing simplicity gained from using a bulk silicone

Research into new active materials for soft robots has generated promising
new thermally responsive materials for functions such as variable stiffness,on-demand shape change, and actuation. These new thermally responsive
materials require a form of addressable thermal control that is compliancematched to its host system. In response to this need, this work presents
stretchable, addressable heating silicone sheets that can control soft,
thermally responsive materials. The sheets are created using layer-by-layer
deposition of a bulk conductive elastomer that can be Joule heated, withembedded liquid–metal microchannels used as electrodes. This combination allows the bulk, addressed material to be stretched and twisted while
in operation. Local, addressable heating is demonstrated in a silicone-based
composite that is capable of cyclic strains of up to 40%, while still selfheating to over 100 °C. The heating sheets are demonstrated as a thermal
control platform in both color changing and stretch-and-hold operations
using all silicone-based composites. Additionally, this platform is bonded toa variable-stiffness polymer that, with its selective heating capability, enables
folding at targeted locations.

1. Introduction
Soft robotic systems are of great interest due to their robustness to impacts and vibrations, conformability to surfaces andobjects, and potential to undergo large deformations due to lowmaterial stiffness. These advantages allow soft robots to fulfill
functions such as grasping diverse and delicate objects, maneuvering around obstacles, or traveling over difficult terrain.[1–5]
Research into new active materials for soft, highly deformablesystems has yielded promising new branches of study, such as
thermally responsive variable stiffness materials[6–8] and actuators.[9,10] Yet, these new thermally responsive materials require
a form of thermal control that is compatible with the hostmaterials. In this work, we present a stretchable, addressable
silicone sheet that enables thermal control, and demonstrate its

R. A. Bilodeau
Purdue University
School of Mechanical Engineering
585 Purdue Mall, West Lafayette, IN 47907, USA
R. A. Bilodeau, Dr. M. C. Yuen, Prof. R. Kramer-Bottiglio
Yale University
School of Engineering and Applied Science
10 Hillhouse Avenue, New Haven, CT 06520, USA
E-mail: rebecca.kramer@yale.edu
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/admt.201900276.

DOI: 10.1002/admt.201900276

Adv. Mater. Technol. 2019, 4, 1900276

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www.advmattechnol.decomposite with the utility of a thermally addressed heating platform to gain the strengths from both sets of prior research.
Here we present a planar, stretchable, silicone-based Joule
heating composite material strengthened by a pure-silicone
support structure, with fluidic electrical interfaces, creating arobust “heater silicone” capable of stretching up to 40% while
maintaining heater functionality. The support structure canbe embedded with liquid metal wiring to enable an addressable
electrical connection for localized Joule heating, which we use
to demonstrate a mechanically robust, 2D thermal “pixel” display. This addressable heater sheet can be coupled with thermally responsive materials, such as variable stiffness and phasechanging materials. In this work we show coupling of theheating composite to a variable-stiffness thermoplastic (poly­lactic acid, or PLA) or a thermally triggered, stiffness-changing
silicone (wax–silicone composite), ensuring uniform heatingof the adjacent material and enabling both reversible shapechanges and stretch-and-hold operations. Finally, we demonstrate example systems comprising the addressable, stretchable
heating sheet coupled to a uniform PLA sheet (with no pre-set
mechanical hinges) reshaping in both two and three dimensions, showing how this platform opens up possibilities of ondemand origami folding in the future.

2. Manufacturing and Characterization of Heater
Silicone Sheets
We first studied the effect of various manufacturing para­
meters of the bulk heater silicone material on its performance(i.e., electrical resistance as a function of applied strain). We
then developed a compliant interfacing technique that enables
heating while undergoing large deformations and demonstrate
its utility and robustness by performing heating tasks while
undergoing significant deformation.

Figure 1. Manufacturing steps and electrical characterization data fromthe strain tests performed on high aspect ratio samples of the heater
silicone. a) Multiple layers of EIG composite silicone are deposited ontoa neat silicone backbone to create the heater silicone sheet, and samples
are cut out of the sheet with a laser cutter for electrical testing. b,i) An
example of a cyclic test dataset (sample design parameters: 10 cm × 1 cm,
35 wt% EIG, n = 2 layers). b,ii–b,iii) cyclic tests focusing on 10% and
15% strain (10 cm × 1 cm, 15 wt% EIG, n = 2 layers) with the samey-scale and color distinguishing test cycles (1st cycle always omitted).
c,i) Representative resistance-strain curves from heater silicone samples(10 cm × 1 cm, n = 3 layers) with different concentrations of EIG. Curves
show the final 10 repetitions of a 50-cycle test (steady-state response).
c,ii) Sheet resistance of unstrained heater silicone with different concentrations of filler and increasing numbers of layers.

2.1. Conductive Composite Electrical Characterization
The heater silicone is an electrically conductive composite madefrom expanded intercalated graphite (EIG) conductive filler dispersed in a silicone matrix, supported by a pure-silicone backbone, creating a monolithic, electrically conductive material
sheet. EIG is an inexpensive filler formed by bulk exfoliation ofgraphene sheets from graphite flakes,[39,40] and has been used tocreate conductive composites in rigid-polymer composites,[41,42]
including self-heating variable stiffness composites.[12] It has
been used as a low loading (<15 wt% EIG) filler for stretchable,
conductive elastomer composites and sensors,[43–45] and for ourheater silicone we simply increased the loading (≥15 wt% EIG)
to improve material conductivity. We adapted the rod-coating
technique demonstrated by White et al.[45] to manufacture ourcomposite (see Figure 1a), but coated multiple layers of conductive silicone on top of each other to increase the thickness of
our conductive layer, reducing the overall sheet resistance inour samples. To create the heater silicone in a stretchable, sheet
form, we encased the initial conductive composite in a pure silicone backbone (a strengthening support that bonds through thecomposite), increasing the cyclic stretchability of the composite

Adv. Mater. Technol. 2019, 4, 1900276(embrittled from the high filler loading) with minimal impact
on conductivity (see Figure S1, Supporting Information).
We performed a detailed characterization of the electricalresistance of the heater silicone under cyclic strain, whilevarying several manufacturing and design parameters. In
Figure 1, we highlight some of the results from varying two key
manufacturing parameters: the weight percent of the expanded
intercalated graphite (EIG) filler (15, 25, and 35 wt%), and thenumber of layers of the heater silicone (n = 1, 2, or 3, based on
the number of times we rod coated the heater silicone onto thesheet). The thickness of the conductive layer varied between 52
and 300 µm and was dependent on both the filler ratio and thenumber of layers (Figure S1, Supporting Information). We also
varied the design of the heater silicone sheets by including asilicone backbone reinforcement on one-side or both sides of
the conductive composite, and those results and comparisons
can be found in Figure S1 in the Supporting Information. We
tested three samples at each level of each design parameter,
for a total of 54 samples tested. Figure 1b,i is an example of adataset collected for a single sample with the following para­
meters: 35 wt% EIG heater silicone concentration, two layers of

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heater silicone coated onto the sample, and a backbone on bothsides of the material.
Each sample was mounted in a materials tester (Instron3345) and clamped using an electrically conductive clamp tomeasure the resistance of the sample in situ. Samples werecyclically strained 200 times to 10% strain and then strained
50 times to incrementally higher strain values (5% strain
increase per increment, making the test 15%, 20%, 25%, etc.).
The reason for the decrease in cycle count at higher strains
is based on the material more rapidly reaching a steady-statestrain response after the 200 cycle “break-in.” Initially, at 10%
strain, the samples would undergo a lengthy settling response(see Figure 1b,ii) and so we cycled the sample to ensure thatthe sample’s resistance had settled before testing at higher
strains. After that initial break-in, each sample settled after
about 40 cycles (see Figure 1b,iii), and so all comparisons ofsample performance were based on the resistances measured
in the last set of 10 cycles (cycles 41–50) at each strain value.
We note that the overall decrease in resistance during cycling
is an interesting characteristic of the composite material.
We suspect that the decrease is due to a combination of theporosity of the material, slow alignment of the disk-like EIG
particles during a (relatively) slow strain of the material, andviscoelastic effects of the silicone undergoing constant strain
cycling.
The last 10 cycles of three samples are plotted in Figure 1c,i.
Each sample shown was manufactured with the same numberof conductive layers but, by increasing the loading, the initial(unstrained) resistance of the samples decreased, as did theresistivity of the sample over the entire strain curve. We also
observed that the backbone allowed us to repeatedly strain samples over 25% while maintaining a good electrical resistancefor Joule heating (for additional information, see Figure S1,
Supporting Information). Although the lowest weight percent(15 wt% EIG) showed a significantly higher resistance (at lower
strains), this did not make it ineligible for use as a Joule-heating
material. Instead, we altered subsequent sample geometries
to take into account the higher initial resistance. Figure 1c,ii
shows that, all other parameters held constant, increasing thenumber of layers resulted in a decrease in the overall “sheet
resistance” of the material. This holds logically, since sheet
resistance does not consider the thickness of the conductive
layer, whereas overall resistance is a volumetric property, and,
therefore, effected by material thickness. The material characterization performed here enabled us to select which manufacturing and design parameters to use, based on the desiredgeometry or stretchability goals, while performing the subsequent experiments.

2.2. Liquid Metal Interfacing and Localized Heating
To create stretchable electrodes that can connect to the heater
silicone, we embedded soft wiring directly into the supporting
backbone using a eutectic gallium–indium–tin alloy (galinstan) as conductive (conductivity = 3.4 × 104 S cm−1)[46]stretchable wiring. Galinstan is liquid at room temperatureallowing it to bend and stretch with the surrounding silicone.[46,47] Additionally, the liquid alloy is stable at the elevatedAdv. Mater. Technol. 2019, 4, 1900276

temperatures used in our heater, preventing electrode burnout
problems.
While experimenting with stretchable liquid metal wiring,
we qualitatively noticed that liquid metals interfaced better withthe heater silicone as compared to copper wire, i.e., the system
had a reduced overall resistance. To confirm our observations,
we performed a simple test to measure the difference in resistance for a sample of material with a copper wire electrode anda liquid metal electrode of the same geometry, as demonstrated
in Figure S2 in the Supporting Information. The liquid metalelectrode dropped the contact resistance by over 10 Ω, whencompared to the uncompressed wire connection (see Figure S2
in the Supporting Information for more details). The numericvalue of this improvement depends heavily on the geometry of
the heater silicone sample and the electrode connection, but inthe subsequent examples we present throughout this work, thereduction of contact resistance greatly improved our ability toevenly heat the planar silicone sheets.
To improve the functionality of the heating silicone, we
wanted to be able to do more than just heat the whole systemfrom end-to-end, but also at select locations in the body. We
used direct filament casting to embed conductive wiring
throughout the heater silicone so that we could create soft,
localized contacts and, therefore, heat targeted areas of the conductive silicone sheet. Li et al. demonstrated a direct filamentcasting method to create high-resolution liquid-metal circuits
in parallel, in an additive-based manufacturing processes.[48] By
placing a filament during the silicone casting process where aliquid-metal channel is desired, hollow channels are created in
the cured silicone once the filaments are removed which canthen be filled with liquid metal. We implemented this method
using copper wire as our filament (28–30 AWG, 0.25–0.3 mmdia) and embedded the liquid metal channels directly into theheater silicone backbone.
In Figure 2 we show the results of casting two types ofliquid-metal channels during the heater manufacture. Since
we used a wire as the mold for the electrode traces, the electrodes could be shaped into nearly any 2D shape we desired(so long as the wire can be removed after casting). For thiswork, we used U-shaped and straight-line electrodes to apply
an electric field in the heater silicone between two parallellines and two points, respectively, which allows us to change
the type of heat addressability between heating a swath ofsilicone, to focusing the heat on a single spot (see Figure 2).
The spot-heat sample in Figure 2 was designed with the liquid
metal line embedded into the conductive composite; therefore,
the tear-drop shape is due to Joule heating along the entire
liquid metal line. This inspired us to move the liquid metallines to be fully embedded in the backbone (except at the spotheat location) to reduce the electrical contact between electrode and heater.

2.3. Stretchable Heating
To corroborate the results of the electrical characterization,
we performed cyclic strain tests on samples of the heater
silicone while Joule heating them to over 100 °C. We choseto test shorter samples than in our electrical tests (with a

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Figure 2. Two different liquid metal channels cast into the heating silicone, and their corresponding effect on the way the material heats up.
a) Top: Two parallel line electrodes create a heat “patch” over an area.
Middle: Liquid metal channels ending at point create a point of heat.
Bottom: photographs showing the copper wires cast into the silicone tocreate the channels. b) The left column has a semi-transparent IR image
overlaid on a visible light image of the heater silicone/liquid metal line
pattern. The right column is a zoomed in view of the IR image.

3:1 aspect ratio), and so we were able to use the 15 wt% EIG
loading to improve the uniformity of the heating at highstrains. We designed a custom straining device that allowed
us to use liquid metal contacts to Joule heat only the portionof the heater silicone experiencing a uniform strain field.
This removed any effects of clamping on the sample’s electrical performance, so we could focus on the effects of linear
strain on the sample’s ability to Joule heat. Preliminary pullto-failure tests revealed that the heater silicone could robustly
support 40% strains while continuing to uniformly heat up to
over 100 °C. We then performed cyclic tests on fresh samples,
cycling them to 40% strain, while continuously Joule heatingthem to over 100 °C. Figure S3 in the Supporting Information
has more details on our strain setup, as well as the preliminary
pull-to-failure tests.
Figure 3 shows the results of the cyclic tests for three samples, as well as IR images of one of the samples tested. Boththe normalized, average resistance and the average power consumed by the samples are shown over the 1000 cycles. The
sample resistance was calculated from the voltage and current
draw (as measured by the programmable power supply) and,unsurprisingly, increased while the samples were strained anddecreased while the strain was being released. The average
resistance of each cycle is presented in Figure 3a, normalized
by the lowest resistance value of the final cycle. As expectedfrom the electrical tests (see Figure 1), the samples each show
a steady decrease in their resistances until they plateau after
100–200 cycles for the remainder of the 1000 cycles. Uniform
power consumption of the samples (Figure 3a), paired with thesample maintaining the same temperature, (Figure 3b) acts asa good initial indicator of the health of each sample throughout
all 1000 cycles (see Figure S3 in the Supporting Information
for more details). Occasional spikes and drops in the calculated
power (due to the power supply) were smoothed out by taking
the average power consumed within each cycle. Figure 3b

Adv. Mater. Technol. 2019, 4, 1900276

Figure 3. Results from cyclic strain tests while Joule heating samples ofheater silicone. a) The normalized, average resistance of the sample ineach cycle, as well as the average power required to Joule heat the sample,
presented against the cycle number. The plateau in the resistance curve
and the flat power consumption indicate good sample health throughout
the tests. b) IR images of one of the samples tested, at 0% strain andthen at the maximum strain (40% strain) over the 1000 cycle period,
showing little-to-no degradation in the sample’s ability to evenly heat.
Sample heating area is ≈3 cm × 1 cm at 0% strain.

shows the IR image from one of the samples that, after
1000 cycles, shows no difference in its heating pattern whencompared to its initial first cycle. Additionally, a quantitative
analysis of the centerline temperature of the sample is plottedin Figure S6 in the Supporting Information for the IR images
in Figure 3b, showing the sample’s consistent temperature profile throughout the entire cycling process. From this test, weconcluded that, when properly manufactured, the heater silicone functions well over 1000 cycles. Additional details are discussed further in Figure S3 in the Supporting Information.

3. Implementation in Silicone-Based Material
Devices
The utility of our heater silicone arises from its stretchability
and addressability, allowing it to be seamlessly embedded into“all-soft” systems. We demonstrate these attributes by integrating thermally responsive silicones with the heater silicone
platform to create both stretch-and-hold operations and color
changing functionality.

3.1. Stretch-and-Hold Operations
To demonstrate the utility of a repeatably stretchable, siliconebased heater, we performed some basic stretch-and-hold experiments using a silicone-based, variable elasticity “wax silicone”(see Figure 4). The wax silicone is a composite of paraffin wax
microparticles embedded into a silicone matrix. At room temperature, the wax is solid and increases the silicone’s elastic
modulus. When the wax melts, the silicone is “softened” andcan stretch like any other closed-cell silicone foam. If the wax
is cooled (and solidified) while the silicone matrix is stretched,
the solidified wax inclusions hold an elongated shape, preventing the silicone matrix from relaxing back to its original

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3.2. Robust, Addressed Spot Heating

Figure 4. “Stretch and hold” shape change. a) Manufacture of the temperature-sensitive wax–silicone composite that leverages the low temperature phase change of wax to hold a new length when cooled while
stretched. b,i) a sample of heater silicone bonded to wax silicone used forthis demonstration, with inextensible tabs to allow for an even strainingof the heater silicone while reinforcing the liquid metal electrical contacts.
b,ii–viii) the sequential steps as the system starts from cooled state, isheated to melt the wax silicone, stretched, cooled to solidify the wax silicone, allowed to relax (with the wax silicone holding its extension), andthen reheated to re-melt the wax silicone and restore it to its unstrained
state.

length. After this, when the wax is re-melted, the silicone’s
restoring elastic force allows the system to return to its original,
unstrained length.
We cast the wax silicone onto our heater silicone to createa material system capable of stretch-and-hold operations. In
Figure 4a, we demonstrate the basic process used to make thisstretch-and-hold composite. We mixed melted wax by hand intoa heated Part A of the two-part silicone pre-cure at a high enoughtemperature to keep the wax melted. The mixture was thencooled by mixing in a room-temperature Part B, creating solid
wax particulates, and cast onto the heater silicone. Once again,
the backbone of our heater silicone provides an additional advantage as it bonds very well to the wax silicone’s matrix. Figure 4b
shows both the final sample used (with inextensible tabs to pullfrom and liquid metal electrodes (not visible)) as well as a fully
reversible stretch-and-hold cycle, described as follows. The heater
silicone melts the wax (resulting in a translucent color change inthe wax silicone), and the composite system is stretched while
cooling the wax silicone. Once cooled and released, the system
pops up out-of-plane to obtain a minimum energy state between
the heater silicone’s restoring elastic compressive energy andthe wax silicone’s elongation. To restore the original state of thesystem, we simply reheat the heater silicone to melt the wax. At
this point, the stretch-and-hold cycle can begin anew. We repeatedthis cycle several times, achieving a pop-up structure each timewith no observable degradation in the heater silicone between
cycles. One cycle is demonstrated in Video S1 in the Supporting
Information.

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Having successfully demonstrated both the stretchability andthe addressability of our heater silicone sheets, we then createda 2D addressed heater with rectangular arrays of spot heating“pixels” (shown in Figure 5a). Each pixel in the array wascapable of heating locally to above 100 °C, and the system could
be stretched and manipulated during operation. To get this 2D
array of localized contact points, the liquid metal wires had tooverlap the active area of the heater silicone. As mentioned
previously, the bottom-up heater silicone sheet manufacturing
allowed us to lift the liquid metal wiring out-of-plane fromthe conductive composite silicone (see Figure 5b). After the
conductive composite cured, copper wires were gently placed
on top and droplets of the composite were deposited at eachpixel, connecting the copper wire at these locations to the bulk
heater underneath. As the silicone backbone was cast on top
to finish the heater silicone sheet, liquid silicone seeped under
the wires, separating the rest of the electrode’s channel fromthe conductive silicone with a thin, nonconductive barrier. Once
the wires were removed and the channels filled with liquid
metal, the only connection between the liquid metal traces
and the planar heater was at the individual points. To control
the activation of each pixel, we designed a high current PCB
which could switch each channel between a high-voltage state,
ground state, or a neutral (“floating”) state (see Section S4,
Supporting Information).
To demonstrate one application of this heating array, we created a mechanically robust digital thermal display. To elicit a
response in the visible-light spectrum from the display, a layerof silicone loaded with thermochromic pigment was added on
top of the heater patterned with a 4 × 5 pixel array in a 2 in × 1.5
in rectangle (5.1 cm × 3.8 cm) (see Figure 5c). We activatedgroups of pixels and showed that we could both heat the wholearea or cause various letters of the alphabet to appear (seeFigure 5d and Video S2, Supporting Information). Since thewhole system was made of silicone composites, we were able
to change the orientation and folding direction of the display
while it was changing letters. Additionally, to test an increased
density of heating pixels, we manufactured a higher density
pixel array with 6 × 6 pixels in the same active rectangularregion as the 4 × 5 pixel display. This increased the pixel
density from 1 to 1.9 pixels cm−2. We used this higher-density
array to demonstrate the robustness of the system to external
perturbations, by both crumpling and stretching the display,while it continued to display different letters (see Figure 5c,vi
and Video S3, Supporting Information). Though a thermally
responsive, color changing display may seem limited in utility,
we see additional applications for digitally controllable, stretchable, color changing silicones in situations similar to those presented by Morin et al., who added visible light camouflage totheir soft actuators by actively changing the color of the silicone
by injecting colored fluids into their actuator.[49]4. On-Demand Shape Change
After demonstrating the robustness of the heater silicone
and its addressable heating functionality while undergoing

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Figure 5. a) A visible light image of our fully soft 2D addressable heater composed of a heater silicone square with liquid metal electrodes. b) Manufacturing the 2D addressable heater pixel interfaces by adding a droplet of heater silicone at the location of interest on the wire molds. The liquidsilicone seeps under the wire separating the rest of the liquid metal channel from the conductive composite. c,i) The 2D addressable heater with atemperature-responsive silicone laid on top (visible and IR images). c,ii) The addressable heater functions even when warped/flexed during operation(visible and IR images). c,iii) a higher-density board with 36 pixels (6 × 6) in the same form-factor as the 4 × 5 board, showing several letters. c,iv) The
6 × 6 board remains active while experiencing a large deformation stress. d) Visible light images of the automated system printing out the letters“HELLOWORLD” consecutively, and IR images of other letters captured, and organized to spell “Faboratory”.

deformations, we desired to implement the platform inreversible shape changing structures. To do this, we used theheater silicone to soften a variable stiffness material (PLA) anddemonstrated several shape-change modes in the variable stiffness sheet. Additionally, we integrated the addressable heating
to selectively soften regions for on-demand shaping andfolding.

4.1. Reversible Shape Change
To create reversible shape change (and shape holding), we
bonded the heater silicone to the PLA using a robust epoxy
bond, resulting in a composite heater/variable stiffness material (see Figure 6a, and Figure S5 and S6 in the Supporting
Information for more details).[7,13] Since the bonded heater
silicone can both stretch and compress without loss of bondintegrity or functionality, the system could maintain elevatedtemperatures for reshaping and then be cooled to hold theAdv. Mater. Technol. 2019, 4, 1900276

new shape (see Figure 6b,i). It also allowed us to reheat theshaped PLA to reverse the shape change. Though the heater
silicone could heat the PLA sheet to 150 °C, when the PLA’s
temperature increased above 65 °C we could begin to reshape
the sheet (see also Figure S7, Supporting Information). Various
shapes were imparted onto the sheet including smooth curves(Figure 6b,i), 90° angles (Figure 6b,ii), and, permanent twists(Figure 6b,iii). This last demonstration goes beyond traditional
origami folding, showing that this variable stiffness material iscapable of twist-and-hold as well as bend-and-hold operations(see also Video S4, Supporting Information). We made several of these samples to test the ability of the material system
to undergo several different shape changes, like tighter twists
and being crumpled by hand (Figure 6c). Though these shapechanges could be reversed, the 3D printed PLA did not prove
to be a very good variable-stiffness material in this regard as it
remained permanently deformed from where it was forced tostretch. However, Taylor et al. recently demonstrated a simple
chemical platform that enables the bonding of many types

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Figure 6. Reversible shape change. a) Manufacturing the heater silicone sheets with bonded variable stiffness elements (PLA). b) Heater silicone
bonded to PLA is Joule heated enabling shape changing and holding operations in the PLA. b,i) The bond keeps the silicone connected to the PLA
even when compressed and stretched. b,ii) A 90° fold in the variable stiffness sheet. Inset shows the shape-memory recovery of the material when
Joule heated again. b,iii) The bonded system is given a permanent twist, with IR images of the sample as it is heated and cooled into the twisted
shape. c) A second sample of the material undergoing a much tighter twist & hold (360°). After reheating, the PLA showed permanent warping. The
heated system was then was crumpled to produce additional folds, twists, and bends, demonstrating the continued functionality of the heater silicone.

of commercial polymers (with glass transition temperatures<150 °C) to pure silicone, which could be employed to achievematerial combinations that exhibit better performance.[50]In Figure 6b,ii we also show that, due to both the elasticityof the silicone and the shape memory effect in the PLA variable
stiffness sheet,[51,52] the material is able to recover some of theset shape when reheated after shape fixing. The ≈90° bend flattened itself, recovering 69% of its bend, and the twisted 180°
sample was able to recover 66% of its twist (see also Video S4,
Supporting Information). Though this effect is relatively weak,
it is valuable to note that the bonded heater silicone can instigate this self-reversing shape change.

4.2. Reversible Hinges
To create reversible on-demand hinges in a stiff sheet, we
combined the heater silicone, liquid metal wiring, and silicone-PLA bonding into a 1D, addressable, heating array with
5 liquid metal channels bonded to a variable stiffness sheet,creating 4 addressed variable stiffness “pixels” in a straightline (see Figure 7a,c). Each 1D pixel heating area in the heater
silicone can be activated (i.e., the temperature raised to above
65 °C) independently or concurrently with other active pixels(see Figure 7b). This capability enables folds-on-demand in
high-modulus thermoplastics. In Figure 7d, we show that byAdv. Mater. Technol. 2019, 4, 1900276

selectively engaging an individual pixel, we can bend the systemon-demand at targeted locations while contacting the system
at only a single point (i.e., a single fingertip). Since the rest of
the system is rigid, only the targeted location bends under the
input force (see inset to Figure 7d,ii–iii).
To demonstrate the versatility of a pixelized variable stiffness system, we formed several functional structures. We first
formed the planar sheet into a load-bearing, raised cantilever(Figure 7d,ii). We then improved the load bearing design byadding another bend, turning the whole sheet into a raised
table (see Figure 7d,ii and also Video S5, Supporting Information). Finally, we demonstrated that the reversibility of the pixels,
as enabled by bonding the heater silicone to the variable stiffness sheet, allows the whole system to revert to its original
shape. We did this by activating all the pixels simultaneously,
and then manually flattening the system back into a straightplank (Figure 7e). It should be noted that the PLA sheet does
not return to a pristine condition, as the heating and folding
process naturally causes some warping in the re-flattened sheet,
but regardless of this, the re-flattened plank could still support amass (Figure 7f). This demonstration shows the possibilities foraddressed fold-on-demand systems, as none of the load-bearing
structures were preprogrammed into the material but formed by
selectively heating one (or more) of four pixels. This concept canbe further extended to joints-on-demand which has ramifications
for controlling the active workspace of continuum manipulators.

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Figure 7. 1D pixelized addressable variable stiffness system. a) Schematic of the system demonstrating the various components allowing for addressable variable stiffness. b) IR images of the PLA side of the functional system with each pixel heated above the glass transition temperature (≈62 °C),showing the capability of a digitally controlled system to heat various pixels simultaneously. c) Visible light photo of the addressable system. d) The
system undergoing shape changes into two load-bearing forms (see ii) and iv)) through heating of individual pixels. Insets in ii) and iii) show that theaddressed pixel is the only heated point, allowing the system to be folded by a single finger. e) All pixels activated allowing the system to relax and be
flattened into its original shape. f) The system, once cool, is again load bearing.

4.3. 2D On-Demand Hinges
Finally, returning to our initial motivation of “blank-slate” foldon-demand, shape-changing structures, we used our 2D heatingarrays to create some origami-like folding structures both adjacent to and bonded to the heating arrays (see Figure 8). In our
first demonstration, we used the 4 × 5 addressable heater tofold a 2D sheet of PLA into a 3D “paper” airplane (see bothFigure 8a,b and Video S6, Supporting Information). This wasmade possible by selectively heating both straight and diagonallines using individual pixels in the heater, while the PLA sheet
was placed on top of the silicone. We did not bond the array tothe variable stiffness sheet, simply to allow for sharper folds.
To further illustrate the possibilities of a system that allows
for folds-on-demand, with reversible folds, we bonded a sheet of
PLA to the 6 × 6 addressable heater. This allowed us to use the2D pixel array to create localized folding lines in the PLA sheet
in multiple directions, as demonstrated in Figure 8c. As visible in the IR images in Figure 8c,ii, we observe that the pixels
heated (≥62 °C) a spot with diameter 0.87 ± 0.11 cm. The spot
size variation was due to the open-loop nature of the heating
controller used. The center-to-center distance between diagonally adjacent pixels was 1.27 cm. Though the pixel density canbe modulated, for our purposes the density of our 6 × 6 pixel

Adv. Mater. Technol. 2019, 4, 1900276

array provided a good balance between the temperature required
to heat each spot and the overlap between each pixel creating
a continuous heated (softened) region. We demonstrate adding
folds to the planar, stiff sheet, removing them to add additional
folds, bi-directionality of the folds (a fold able to bend in both
directions) and adding multiple folds at a time to a single sheet.
For the sake of visibility, we held each fold partially open rather
than having them completely flattened as in Figure 8b. These
are just two demonstrations from the myriad possibilities offold-on-demand sheet shaping via selective Joule heating.

5. Conclusion
We have developed a material-based tool to fill the need of astretchable, addressable heater useful for controlling thermally
responsive soft materials. We demonstrated a conductive heater
silicone composite sheet that can heat an adjacent material to
150 °C and can be cyclically strained 1000 times at 40% strain
while heating, due to reinforcement of the conductive silicone
composite with an all-silicone backbone. This backbone became
the key to successful implementation of localized, addressedheating within the heater silicone sheets, as well as both
extend-and-hold demonstrations and fold-on-demand (shape1900276 (8 of 11)© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 8. a-b) An unprogrammed variable stiffness sheet is heated using the 4 × 5 pixel heater to create artificial folds. a,i) The sheet laying on theheater. a,ii–iv) Top-down IR images showing the heated folds on the sheet. b) Images of the sheet in i) its initial state, ii-iii) intermediate states, andiv) final airplane-like shape after being cooled. c,i) PLA sheet bonded to a 6 × 6 pixel heater silicone sheet. c,ii–iv) The PLA sheet is bent (and flattened)
along lines not previously defined in the polymer block, simply by heating a selection of pixels that make up a line with the heater silicone sheet. In
both ii) and iii) the top view and a side-profile view of two successive bends are presented, along with an IR photo just before the bend occurred with
the PLA sheet outlined. iv) An isometric view showing multiple bends in the same sheet, as well as an isometric view of the initial straight-line bendon the bottom (with its associated IR image). All IR images show temperatures up to 90 °C. All PLA sheets are 4 cm × 5.5 cm.

changing) structures. The backbone was critical to successful
implementation of addressed heating pixels as it carries stretchable liquid metal electrode channels out-of-plane, until the electrode is ready for interfacing at specific locations in the heater
silicone. We demonstrated a 6 × 6 array of heating pixels witha density of 1.9 pixels cm−2, with each pixel able to (via openloop control) heat up and soften an ≈0.87 cm diameter spot inadjacent PLA sheets. By introducing closed-loop control on the
power being passed through each addressable pixel (rather thanthe current open-loop state) we envision future work in which
the local temperature can be far more precisely controlled,
enabling uniform, gradient, or targeted heating. Although the
backbone underpins the stretchability of the heater silicone,
manufacturing of the composite layer also had a large impact
on the uniformity of the heating and the stretchability of thesample. In future work, we aim to improve the thermal conductivity of the system so that the heater silicone is able to self-regulate the heat and distribute it more evenly around each point
and swath.
The silicone backbone also allows for strong bonding to botha wax silicone composite and variable-stiffness polymer sheets
to create composite material sheets which can change theirstiffness on-demand. With the wax silicone, we were able toillustrate the potential of an all-silicone, bulk material, planarheater by performing stretch-and-hold operations, which canpotentially open a whole new frontier of planar material reconfiguration in which parts of the plane can be made stretchable.

Adv. Mater. Technol. 2019, 4, 1900276

Future work in this area will involve balancing the force thewax-silicone exerts when in its extended-and-held state with theheater silicone’s restoring force, so that the system can remainboth elongated and flat. We envision the methods presented
here used to create ready-to-use slates that can be stretched,folded, twisted, and formed into complex 3D structures using
pixel-controlled soft “origami” folding in an otherwise blankmaterial.

6. Experimental Section
Materials: Dragonskin 10 (Smooth-On Inc.) was used as the silicone
for all experiments. Variable stiffness sheets were FDM 3D printed
polylactic acid (PLA) sheets. This allowed for control over all three
dimensions (length, width, and height) in the variable stiffness sheets
for the various applications and demonstrations. Paraffin wax (melting
temperature 60–65 °C) was used as the stiffness changing filler in thestretch-and-hold silicone examples. It was mixed into the silicone matrix
in a 1:2 ratio (33.3% wax by weight).
Expanded intercalated graphite composite was prepared by first
exfoliating graphene sheets from graphite flakes and second, mixing
it into the composite via a solvent carrier. 10 g of dry expandable
graphite (Sigma-Aldrich) was heated in an oven at 800 °C, causing theexfoliation of the graphene flakes into noodle-like, ultra-low-density
structures in ≈20 s. These noodles were soaked in cyclohexane(BDH1111, VWR) and sonicated for 4 h (Q700 sonicator fitted with
1/2” tip, QSonica, 20 kHz, 36 µm (30% setting) amplitude) creating

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www.advmattechnol.dea slurry of solvent and individual EIG particles. The slurry was allowed
to settle, and excess solvent was decanted off until the solution had aratio between 3 and 5 wt% EIG, giving it a paste-like consistency. To
finish the composite, equal parts A and B of the silicone elastomer werepremixed and chilled to prevent premature crosslinking. A few gramsof this mixture were added to the EIG, according to the final desiredmix ratio and volume of composite, and the whole solution was mixed
thoroughly. The composite was then rod-coated into sheets, allowing
the remaining cyclohexane solvent to evaporate away. Subsequent
layers were rod-coated after the previous layer had fully cured.
Thermally responsive color-changing silicone (for Figure 3c) was
made by adding a thermochromatic additive powder (Hali Industrial Co.,
Ltd.) to the silicone. The material began to exhibit a visible color change
at ≈30 °C, though higher temperatures caused a more distinct change.
Strain Testing: A heater silicone specimen was cyclically strained inan Instron 3345 materials tester while measuring the resistance acrossthe sample. The active (stretched) area of samples tested was 10 cm
long, 1 cm wide, with a conductive layer <300 µm thick. The strain
test procedure was as follows: samples were cyclically strained to 10%
strain 200 times at 200 mm min−1. The samples were then strained
at incrementally higher strain values for 50 cycles per increment (5%strain increase per increment, making the test 15%, 20%, 25%, etc.) for
50 cycles at each strain value up to 50% strain.
Cyclic Heat Testing: Heater silicone was cyclically strained in a custom
test apparatus, while maintaining an elevated, Joule heated temperature.
The electrically active area (between two liquid metal electrodes) was
3 cm × 1 cm, and was strained at a strain rate of 35 mm min−1.

Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
The authors would like to thank Dr. Edward L. White for his contributions
to preliminary studies leading up to this work, including his help
designing the circuit boards used to power the addressed silicone heaters.
R.A.B. was funded by an AFOSR Young Investigator award (Grant No.
FA9550-16-1-0267) and by a NASA Early Career Faculty award (Grant
No. NNX14AO52G). M.C.Y. was supported under the National Science
Foundation Graduate Research Fellowship program (DGE-1333468). Any
opinions, findings, and conclusions or recommendations expressed in thismaterial are those of the authors and do not necessarily reflect the viewsof the US Air Force Office of Scientific Research, the National Aeronautics
and Space Administration, or the National Science Foundation.

Conflict of Interest
The authors declare no conflict of interest.

Keywords
local thermal control, stretchable conductive composites, stretchable
heaters, variable stiffness systems
Received: April 1, 2019
Revised: June 5, 2019
Published online: August 2, 2019

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