Showcasing research from Prof. Rebecca Kramer-Bottiglio’s
laboratory, School of Engineering & Applied Science,
Yale University.

As featured in:Volume 16
Number 25
7 July 2020
Pages 5789–5992

Uniform conductivity in stretchable silicones via multiphaseinclusions

Soft Matterrsc.li/soft-matter-journal

A multiphase silicone composite, containing solid graphite
and liquid metal, experiences structural microcracking
when stretched. The microcracking causes liquid metal
coalescence to percolate through the silicone, enabling
uniform electrical conductivity.
Themed issue: Liquid composites
ISSN 1744-6848

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Justin D. Glover and Jonathan T. Pham
Capillary-driven indentation of a microparticle into a soft,
oil-coated substrate

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Uniform conductivity in stretchable silicones via
multiphase inclusions†
R. Adam Bilodeau, ‡ab Amir Mohammadi Nasab,
Rebecca Kramer-Bottiglio *a‡a Dylan S. ShahaandMany soft robotic components require highly stretchable, electrically conductive materials for proper
operation. Often these conductive materials are used as sensors or as heaters for thermally responsive
materials. However, there is a scarcity of stretchable materials that can withstand the high strains
typically experienced by soft robots, while maintaining the electrical properties necessary for Joule
heating (e.g., uniform conductivity). In this work, we present a silicone composite containing both liquidand solid inclusions that can maintain a uniform conductivity while experiencing 200% linear strains. This
composite can be cast in thin sheets enabling it to be wrapped around thermally responsive soft
Received 3rd March 2020,
Accepted 19th April 2020
DOI: 10.1039/d0sm00383b

materials that increase their volume or stretchability when heated. We show how this material opens up
possibilities for electrically controllable shape changing soft robotic actuators, as well as all-silicone
actuation systems powered only by electrical stimulus. Additionally, we show that this stretchable
composite can be used as an electrode material in other applications, including a strain sensor with arsc.li/soft-matter-journallinear response up to 200% strain and near-zero signal noise.

1 Introduction
Soft robotic technologies have achieved increasing success in awide range of actuation domains including robotic grippers,1,2
wearable assistive devices,3–5 and robot arms.6 Simultaneously,
stretchable electronics have shown success in a variety of use
cases, including as biocompatible devices,7,8 wearable technologies,5,9 and, of course, soft robotic electronics.10,11 Though
there are many ways to create mechanically compliant stretchable electronics (patterning, printing, etc.),12,13 within the context of soft robots it is often architecturally simpler to embed
conductivity into a bulk stretchable material that matches thematerial compliance of the other soft robot components.14
Recently, researchers have integrated stretchable, electricallyconductive materials into soft robots to add sensing,15 heating,16,17
or signal and power lines,18 while reducing manufacturing
complexity of the overall system.19
To create high-strain, bulk conductive composites, manygroups have embedded rigid conductive inclusions into stretchable materials. Only a few of these types of composites canhandle linear strains of up to 100%,14,20 and those compositesaDepartment of Mechanical Engineering and Materials Science, School ofEngineering and Applied Science, Yale University, 9 Hillhouse Ave., New Haven,
CT 06511, USA. E-mail: rebecca.kramer@yale.edubSchool of Mechanical Engineering, Purdue University, 585 Purdue Mall,West Lafayette, IN 47907, USA† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0sm00383b‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2020

often require specialized composite filler materials withcomplex manufacturing techniques21,22 or require specializedrubbers.23 Even so, conductive composites can be used forelectrical signaling and sensing, but when their conductivity
is not spatially uniform under strain, they experience unevenenergy dissipation while Joule heating, creating ‘‘hot spots’’ inthe material. This non-uniform conductivity prevents themfrom being an embedded heat source for thermally triggered
soft robotic components, such as volumetrically expanding
silicones,24,25 stiﬀness-varying stretchable silicones,26 andliquid crystal elastomer actuators.27 In a recent work, we
showed that adding expanded intercalated graphite (EIG)28 intoa silicone matrix in high enough concentrations (47 vol%)creates a composite with uniform electrical conductivity capableof Joule heating up to 150 1C.29 However, due to the high quantityof stiﬀ EIG inclusions, the composite could not be stretchedhigher than 40% strain without cracking internally, creating
thermal hot spots for Joule heating (although the material
retained its electrical conductivity).
To improve material performance beyond the current limitsof rigid inclusions, we draw on principles from a new class ofliquid metal embedded elastomers (LMEEs). LMEEs are madeby mechanically mixing a room temperature liquid metal(eutectic gallium–indium, or eGaIn) into uncured silicone athigh volumetric ratios (430%), which creates a highly-stretchable
thermally-conductive composite that maintains the elastomer’s
mechanical properties, while often improving other characteristics, such as dielectric strength and thermal conductivity.30–32

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Although non-conductive as cast, LMEEs easily integrate withother elastomers to improve current soft robotic actuator
technologies.33,34
LMEEs can be activated using mechanical ‘‘sintering’’ techniques which cause the liquid metal inclusions to flow together
and create channels with high electrical conductivity.18 Sinteringhas been achieved by applying localized pressure on thecomposite18,27 and freezing the system to simultaneously activate
the entire composite.35 Hybrid composites exist in which liquid
metal and a solid conductor are blended in the same silicone tocreate a composite that is conductive as-cast.36–38 So far this has
been primarily to use the liquid metal to enhance the functionality of the solid conductive filler, as evidenced by low volume
fractions of liquid metal. Wang et al., recently reported a highstrain elastomer composite (4700%) using rigid particles
and liquid metal inclusions,39 but the composite requires over1000 cycles to stabilize its resistance, and exhibited relatively
low conductivity compared with other liquid metal-silicone
composites.40,41
In this paper, we show that the addition of a small amountof rigid EIG filler enables highly stretchable LMEEs to selfactivate, becoming highly conductive and Joule heating-capablecomposites. This novel multiphase composite (MPC) selfactivates under high linear strains (4200%), which drops
its electrical resistivity by up to five orders of magnitude too4  105 O m. The MPC maintains a uniform, low resistivity
while undergoing 200% linear strain, even when cycled 10 000 times.
MPC silicone can be used in a wide range of applications, including
Joule heating (Fig. 1) and strain sensing. We show that, becauseof its high deformation capability, MPC silicone is highly compatible with other new soft robotic silicone composites andenables the creation of solid-state soft actuators that are robust
to damage, and multifunctional thin film pneumatic bladders
that can dynamically change their trajectory while providing
sensory feedback. With its high conductivity and high strain
capacity, MPC silicone is a step towards versatile soft robotic

Fig. 1 An activated 4  16 cm sample of the MPC being simultaneously
twisted and stretched, while maintaining a consistent, uniform Joule
heating profile. The sample is fully encapsulated in silicone.

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Soft Mattercomponents which can change their shape and sense theirdeformations, using the same material.

2 Principles of operation
MPC silicone has three main components: eGaIn as the liquid
metal (LM) alloy; EIG as the rigid conductive filler; and silicone
elastomer (SE) as the composite matrix. In brief, it is manufactured by shear mixing 60% LM with 40% SE by volume, andthen adding 1% vol EIG filler to the mixture (additional details
in Materials and methods). As cast, the EIG fillers bridge theLM particles in the silicone matrix. This means the material
is electrically conductive while also being stretchable up to
B100% strain, although it has a high electrical resistivity(1.28  0.57 O m).
To activate the composite and increase its electrical conductivity, the material is strained to such a degree that thesilicone matrix tears locally, allowing the LM inclusions toflow and coalesce into a low resistance network percolating
throughout the composite (see Videos S1 and S2, ESI†). This
strain-based activation requires the MPC to be supported by anextensible backing layer that adheres to the MPC to prevent
catastrophic failure, by absorbing some of the energy released
when the matrix tears. The supportive backing layer can be
another silicone layer or even a stretchable adhesive tape (3M
VHB tape) which is removable after activation (Fig. S1, ESI†).
Most of the samples in this paper use some form of silicone
backing layer, to improve mechanical robustness and facilitate
the addition of electrodes.
Thrasher et al., recently demonstrated a similar strainactivated mechanism in so-called ‘‘polymerized liquid metal
networks’’.42 Acrylate ligands were bound to the surface ofliquid metal particles, then cast and polymerized together,
forming a 99% LM particle film on a substrate. Similar to themechanism shown here, strain caused the initially isolated LM
particles to rupture, allowing the liquid metal cores to flow andcoalesce into a percolating conductive network. Our work
diﬀers from this in that we use a lower concentration of liquid
metal, and our silicone matrix is typically more compatible withother traditional materials used in soft robots.43
To demonstrate MPC silicone’s activation mechanism,
we compared the activation (i.e., first-strain behavior) of a
LM–SE–EIG composite with that of LM–SE and SE–EIG composites (Fig. 2). When 60% LM and 40% SE by volume are shearmixed and cured (Fig. 2a–c, left column), the material is neither
electrically conductive as cast, nor after being stretched, evenwith the high volume fraction of conductive LM. With nointernal stress concentrators (i.e., rigid particles) to incite
microscopic failures in the silicone matrix, the liquid metalcannot coalesce into a conductive network. When 90% SE ismixed with 10% EIG by volume (Fig. 2a–c, middle column), thematerial is electrically conductive as cast, but also comparatively brittle (failure at 40% strain) due to the rigid inclusions.
When the two composites are combined to create LM–SE–EIG
composite, the MPC silicone develops a uniform distribution ofThis journal is © The Royal Society of Chemistry 2020

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Fig. 2 Principles of operation of the MPC. (a) Schematic compares preand post-stretch structures of three composite materials of LM–SE,30,44
SE–EIG,29 and LM–SE–EIG (left to right). In the left column, although somemicrocracks appear on the surface of the LM–SE composite while being
stretched, a conductive pathway through the whole structure does notform. In the middle column, the majority of conductive pathways in the
SE–EIG composite are lost due to macroscopic cracks forming in thematerial. In the right column, the EIG facilitates microcracks in the LM–SE–
EIG composite during stretching, which creates conductive pathways
throughout the structure (activation). (b) mm- and mm-scale images show
the surface features of the three composites as cast, while stretched, andrelaxed post-stretch. Light reflections from the microscope are visible on
the LM beads expelled from the LM–SE–EIG surface. (c) Optical andthermal images of samples of the three aforementioned composites Jouleheated with electrical current at rest and stretched states, after a one-time‘activation’ event. Temperature variance is used to view the resistanceuniformity, as higher resistance areas generate more heat when a current
is passed through the system.

Paper
microcracks both internally and visibly on the surface after
stretch (Fig. 2a–c, right column). The LM particles coalescethrough these pathways creating a continuous conductive network(seen as reflective pools forming at the microcrack sites, visible inVideo S2, ESI†), uniformly lowering the resistivity by several ordersof magnitude. Additional evidence of a continuous internal liquid
metal network is presented in Fig. S2 (ESI†), where liquid metal
drains out the bottom of a vertically-oriented MPC sample over a
period of 40 minutes.
Fig. 2b presents optical images of the surface of the threecomposite structures pre- and post-stretch. In the LM–SE
composite (left column), LM particles maintain their boundaries, even at 200% strain. Some minor microcracks form on
the surface of the LM–SE composite, but these are insufficient
to create a conductive pathway through the whole structure(Fig. 2c, left). This same microscale robustness holds when the
LM–SE composite is compressed, a technique used in otherworks to create conductive pathways using silicones with lower
yield strains.18 In contrast, the SE–EIG composite exhibits brittle
behavior from macrocracks on the surface when stretched to only
40% strain (Fig. 2b, middle column). These cracks cause a rapid
rise in the electrical resistance of the structure and a non-uniform
electrical conductivity (Fig. 2c, middle). Neither of these composite mixtures generate the desired high stretchability with uniform
conductivity that is shown in our final MPC mixture.
Combining the fillers for the two non-functioning composites,
microcracks are formed in the LM–SE–EIG silicone matrix whenstretched (Fig. 2, right column), resulting in a low, uniform
electrical resistivity (Fig. 2c, right). When released, some LM
inclusions are displaced from inside the system and LM beads
form on the surface of the MPC (Video S2, ESI†), a phenomenon
previously observed in other strain-activated liquid metalcomposites.42 We hypothesize that the displaced LM is due tothe plastic deformation of the MPC (see Fig. S1, ESI†) relative tothe elastic deformation of its backing layer during strain, which
induces buckles and wrinkles throughout the MPC whenrelaxed.45 Such wrinkling is visible in Fig. 2b and Video S2 (ESI†),and explains the displaced volume of LM during strain activation.
To aid in transporting or attaching the MPC to another material
system, we simply remove the excess LM beads by gently wiping
the surface of the material with a cloth infused with Simple Green(Sunshine Makers, Inc) or another acidic or basic cleaning agent.
Fig. 2c shows 10  20 mm samples of three composites atrest and in stretched states while they were Joule heated. MPC
silicone shows a uniform temperature distribution on its surface
as an indication of uniform electrical conductivity. Note that coolspots on the thermal image are not actually lower in temperature.
They are the LM surface beads causing the IR camera to misreporttheir temperature as cooler, since LM has a lower emissivity thansilicone.

3 Material characterization
As previously noted, the primary MPC mixture we use is initiallymixed to 60% LM and 40% SE by volume, with subsequentThis journal is © The Royal Society of Chemistry 2020

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addition of 1% EIG by volume (suspended in cyclohexane as acarrier solvent). Although this makes a composite that is59.4%–39.6%–1%, we use a shorthand convention to simplify
notation, as 60–40–1. This notation is used for all other mix
ratios tested.

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3.1

Electrical and mechanical properties

Volume fractions of LM and EIG were varied in the MPC
compositions to investigate their role in the electrical conductivity of MPC silicone (Fig. 3). For the tests evaluating theelectrical resistivity, the MPC samples were supported with apure silicone backing layer. Fig. 3a shows the electrical resistivity of the activated (post-stretch) samples of MPC silicone forvarious volume fraction of LM and EIG. The electrical resistivityr, was calculated by measuring the electrical resistance R,
thickness t, length l, and width w of MPC silicone samples
using r = R(tw)/l.
As volume fractions of LM or EIG increased, the electricalresistivity of the MPC decreased. We found that there is a lowerlimit to filler concentrations that, if the combination of LM and
EIG is too low, the material does not activate (black pyramids in
Fig. 3a). If the material is filled with 60% LM volume fraction,even adding just 0.5% EIG will make the MPC electricallySoft Matter
conductive. This concentration is close to the theoretical upper
limit based on the maximum packing fraction in a randomly
dispersed 3D mixture for fillers with aspect ratio close to one(about 64%46). Since the LM does not have an aspect ratio ofexactly one, it might be possible to increase the concentration
beyond 60% LM, but we discovered that any more LM made thematerials very diﬃcult to mix together uniformly.
Fig. 3a shows that for compositions with LM volume fractionof 50%, increasing the EIG content from 1% to 2% decreased
the electrical resistivity of the MPC after activation by 66%.
However, for MPC compositions with LM volume fraction of
60%, that same increase in EIG content only decreased theelectrical resistivity of the MPC by 22%. This indicates that
the EIG, as a conductive filler, contributes relatively little to thematerial’s activated conductivity for compositions with a high(450%) LM volume fraction.
Fig. 3b and c is a before and after comparison of theelectrical resistivity of MPC silicone for various material compositions. Varying the volume fraction of EIG in the MPC, theresistance drop after activating the material is 5, 4, and 3 ordersof magnitude for 60–40–0.5, 60–40–1, and 60–40–2 compositions, respectively (Fig. 3b). Notably, the electrical resistivityof the activated 60–40–1 composition is about one order ofFig. 3 Electrical and mechanical characterization of MPC silicone. (a) Electrical resistivity of the MPC after stretch as a function of LM and EIG volume
fractions. Truncated pyramids show the compositions with electrical conductivity and full pyramids show the non-conductive compositions. (b and c)Electrical resistivity of MPC silicone pre- and post-activation. (b) LM 60%–SE 40% composition, varying the EIG volume fraction. (c) EIG 1% composition,
varying the LM–SE volume fraction. (d) Mechanical testing of unactivated MPC (LM 60%–SE 40%, no backing layer), with 0.5%, 1.0%, and 2.0% volume
fractions of EIG showing the elongation at break. (e) Cyclic loading of MPC silicone (LM 60%–SE 40%) with 1.0% volume fraction of EIG. The material
displays a Mullins effect at the first loading cycle at each strain. The stress is significantly smaller in the next cycles at the same strain level. (f) Activation
process of the MPC at two strain rates of 5% s1 and 25% s1. Five samples at each speed are stretched up to 250% strain for the first time and theresistivity change is recorded. The material begins activating at lower strains when stretched with a higher strain rate. A representative plot of resistancedrop for a sample is shown in Fig. S3 (ESI†). Error bars in (b and c), and shaded regions in (d and f) represent 1 standard deviation.

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Soft Matter
magnitude lower than that of the activated 60–40–0.5 composition. This notable drop in electrical resistivity is likely due to anincrease of microcrack nucleation sites in the composite matrix
from the increased EIG content (rather than a direct result of
the EIG being conductive) enabling an improved LM network in
the final, activated composite. The pre- and post-activationelectrical resistivity of different MPC compositions with varying
LM concentrations and 1% EIG is shown in Fig. 3c, with a more
gradual downward trend in the resistivity as LM concentration
increases.
It has been shown that solvents trapped in silicones caninfluence mechanical properties.47 Cyclohexane is a relatively
volatile solvent and it evaporates during the process of mixing
the material and casting it into thin sheets. However, a small
amount of solvent still evaporates after the MPC is cured
leaving some voids behind in the structure of the composite.
To investigate the eﬀect of the EIG carrier solvent (cyclohexane)
volume fraction on the mechanical properties of the MPCs,
evaluations were performed for two mixture ‘recipes’: (1) the
ratio of EIG to cyclohexane kept constant while the EIG content
is varied in the mixture, and (2) the volume fraction of cyclohexane kept constant in the mixture disregarding variations in
the EIG content, which resulted in a higher fraction of cyclohexane to silicone in the uncured mixture. Fig. 3d shows
the tensile mechanical behavior of unactivated MPC silicone
samples (with no backing layer) for compositions 60–40–0.5,
60–40–1, and 60–40–2 as samples were stretched to failure.
Increasing the EIG volume fraction in the composition
increases the elastic modulus and decreases elongation atbreak. In the first recipe, MPC silicone with 0.5%, 1.0%, and
2.0% EIG volume fractions break at 216%, 120%, and 31%
strain, respectively. For the second recipe, the elongation atbreak is only slightly lower at each composition, although theelastic modulus is reduced, as expected.47 These observations
support our hypothesis that elongation at break is strongly
dependent on EIG content after curing and only weakly dependent on the solvent content during mixing. Additionally, theultimate strains recorded here provide minimum strainsrequired to begin activation of each material composition suchthat there will be internal microcracking, and demonstrate theneed for the backing layer (so the system can stretch to 250%)
during activation.
MPC silicone displays a Mullins eﬀect at the first loading
cycle to an elevated strain value (Fig. 3e). The stress is significantly smaller in the next cycles at the same strain level. This
response to the initial loading is expected in liquid-composite
elastomers,30 as it is observed in most elastomers. If the MPC is
stretched to the same strain after the first loading, the Mullins
eﬀect disappears, but returns if the MPC is stretched more thanits previous maximum strain.
MPC silicone also exhibits viscoelastic properties (i.e., strain
rate dependencies), as would be expected by a silicone elastomer
system. This viscoelasticity results in higher stresses in thecomposite when the material is strained at higher rates, triggering
the nucleation of microcracks at lower strain levels. Therefore the
MPC activates at a lower strain level when stretched at a higher

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strain rate, as shown in Fig. 3f. Fig. S3 (ESI†) shows the representative resistance–strain activation curve for a single sample
strained up to 250% strain.
Among all the compositions evaluated in Fig. 3, 60–40–1 and
60–40–2 MPC silicone compositions always result in uniform
electrical conductivity when strained to 250% strain. We suspect
the reason to be that the increased volume fraction of LM
increases the statistical uniformity of the distribution of LM
inclusions, as well as their local proximity to each other. This
uniform distribution of inclusions, combined with slightly elevated quantities of EIG, likely ensures that there is simultaneous
microcracking throughout the composite during activation.
We note that other factors influence the uniform conductivity of
the MPC as well. Such factors include constant sample thickness,
activation strain value (mature vs. premature activation), anduniformity of the activation strain field, as shown in Fig. S4(ESI†). We chose a 60–40–1 MPC silicone composition for theapplications shown in the paper, to keep uniform electricalconductivity after activation.
3.2

Thermal characteristics and cyclic robustness

Conductive silicone composites have been previously used as
Joule heaters, trading oﬀ between high temperature-change
capabilities48 or improved stretchability.16,17,29,49 In order tobe easily integrated into soft robots, desired properties include
high temperature change capability, stretchability, and uniform
resistivity (leading to uniform heating capabilities). Here we aim
to quantify the suitability of the MPC as a heater material, while
identifying potential limitations and advantages of our material.
Specifically, we used 60–40–1 MPC silicone as a heater, andevaluated its thermal response to different electrical inputs.
Fig. 4a shows the surface temperature of a 40  10  0.4 mm
MPC sample supported with a 1 mm thick silicone backing
layer. The initial resistance of the sample at 0% strain was 1 O.
The sample was stretched to 100 and 200% strain and still
demonstrated a uniform heat distribution over its surface,
though the resistance increase caused the maximum achievabletemperature at a fixed electrical current input (2.5 A) todecrease. Reducing the thickness of the backing layer allows
the material to heat up faster, since there is less mass to absorb
the thermal energy. An example is shown in Fig. S5 (ESI†), in
which an activated MPC silicone and its silicone backing layer
have a total thickness of less than 250 mm.
To further characterize MPC silicone’s robustness as aheater, we coated neat silicone on both sides of 30  10 
0.4 mm MPC samples and cycled them 10 000 times to 200%
strain. In Fig. 4b we show electrical resistance over the wholetest on one sample, with an initial settling in the resistance for
300 cycles followed by a plateau in the sample’s resistance.
Mechanical strength after 10 000 cycles was observed to be
similar to the original material, consistent with prior work inwhich robust cyclic mechanical testings have been performedon similar materials.28,33 IR images of the sample Joule heated
at 0 and 200% strain with 2.2 A electrical current is displayed inFig. 4c, showing negligible diﬀerence in heating performanceof the sample before and after the cyclic test.

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Fig. 4 Thermal characteristics and cyclic robustness. (a) Temperature of a40  10  0.4 mm activated MPC silicone sample supported with a 1 mm
thick backing layer while being Joule heated with 0.5 A, 1.5 A, and 2.5 A for
four minutes at 0% strain. The sample was then allowed to cool for six
minutes at room temperature. Additionally overlaid is the same sample
heated with 2.5 A at 100% and 200% strain. (b) Resistance change of a MPC
silicone sample (30  10  0.4 mm) cycled 10 000 times to 200% strain.
Inset shows 30 s of cycling. (c) IR images of the sample heating up withelectrical current of 2.2 A before and after the cyclic test, showing novisible decrease in uniformity inside the main body.

We performed additional cyclic tests on samples that were
coated with only a single backing layer of neat silicone (i.e.,‘one-sided’ backings). These samples could only achieve 10 000
cycles if the strain was reduced to 150% (Fig. S6, ESI†). If they
were strained cyclically to 200%, the one-sided samples would
fail between 3000 and 5000 cycles. We therefore conclude thatthe highest cyclic performance is enabled by coating neatsilicone on both sides of the MPC, which agrees with moredetailed investigations done in prior work.29

Soft Matter
deployed in numerous applications ranging from measuring
human body motion,50,51 reconfigurable robotic skins,52 robot
grippers,19 and numerous other applications.53,54 The classic
3-layer design (used in this work) has stretchable planar electrodeson the top and bottom of a stretchable dielectric material.
Capacitive sensors provide a more consistent response thanresistive sensors, where comparisons between the capacitanceand resistance of sensors of the same material are available.28,50
Measuring the capacitance of such sensors (typically on the orderof 1–100 pF) is a challenging task however, and several methods
have been employed.55 Each proposed method has benefits anddrawbacks, but generally low electrode resistance and high capacitance are desirable, giving our new MPC silicone an opportunity
to improve current sensor systems.
One drawback to capacitive sensing methods is that they are
relatively susceptible to external noise. Noise can come frommany sources, including an unstable ground or supply line.
Additionally, the sensor will pick up external electromagnetic
noise in the form of an unstable electrode voltage and correspondingly noisy capacitance measurements. Filtering can beused to mitigate the eﬀects of such noise, for example using
analog low-pass filters,56 or using digital signal processing
methods (as is done on the MPR121, manufactured by NXP
Semiconductor). Here, we found that our sensor paired with the
MPR121 results in a stable system, with resting noise standard
deviation s of only B0.05% of the full scale range of thesensors’ response over 200% strain, or s = 0.1% (further
analysis presented in ESI† as Table S1).
To investigate the advantages and disadvantages of various
combinations of sensors and sensing circuits, we compared the
MPC sensors to SE–EIG sensors28 (i.e., sensors made from theconductive graphite composite without the added liquid inclusions) using two sensing methods: a commercial charge integration circuit (MPR121), and an in-house designed charge
integration circuit described in prior works56 (Fig. 5). This second

4 Applications
In the previous experiments, we have shown that MPC silicone
can be used to make planar, skin-like sheets with high stretchability and uniform conductivity. We now use the material as a
key component in three distinct systems, achieving multiple
functions while reducing system complexity. The first application leverages the high conductivity of the MPC to enable highstrain sensory feedback with low noise. We then apply the MPC
as a ‘‘skin’’ around a solid silicone composite that increases involume when heated, producing a solid-state, all-soft actuator.
Finally, we integrate it into the walls of a pneumatically inflating
bladder with thermally controlled stretchability, allowing a singlebladder pneumatic actuator to attain multiple motion trajectories
via electrical stimulus, all while using the MPC to simultaneously
measure the actuator’s state.
4.1

High deformation sensing

We used activated 60–40–1 MPC silicone to create capacitive
strain sensors which exhibit a repeatable linear response overstrains as high as 200%. Capacitive strain sensors have been

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Fig. 5 Response of MPC strain sensors. (a) Comparison to SE–EIG capacitive sensors,28 as measured using our charge-integration capacitancemeasuring circuit over 10 cycles. Normalized response is highly stable
across all eight MPC sensors, while each of the three graphite sensors
behaved similar over strains lower than B60% but had large inter- andintra-specimen variation at high strains. (b) MPC sensors stretched to 200%
for 1000 cycles. Three sensors are plotted individually, showing highlinearity and that they mainly differ in their intercept and slope. Dashedlines represent the mean over 10 cycles. Shaded regions in both plots
represent 1 standard deviation.

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Soft Mattercircuit was designed to work well with the relatively high resistance of the graphite sensors (B1–10 kO initially, and increasing
to B100–1000 kO at 200% strain). The low resistance of MPC
silicone was predicted to result in more stable sensor response,
which was confirmed experimentally in Fig. 5a and Fig. S7 (ESI†).
The low, stable resistance over large strains resulted in stablecapacitance readings when MPC silicone was used as the electrode material. In contrast, the graphite sensors experienced
cracking of their electrodes after around 60% strain, making
them difficult to use as a strain sensor. MPC sensor readings S
S  S0were normalized as S ¼
, where Smax and Smin are the
Smax  Smin
maximum and minimum reading over 10 cycles, respectively.
Once this was done, the data from all eight MPC sensors that
we tested collapsed into a single, linear response curve. Graphite
S  S0sensors were normalized as S ¼
, and plotted individually
10 000due to the high inter-sensor and intra-sensor variability at strains
greater than B60%. Raw data for each sensor are plotted individually in Fig. S7 (ESI†).
In addition to yielding stable readings, low electrode resistance makes it possible to utilize commercial oﬀ-the-shelfcomponents. Here we used the commercial MPR121 circuit tomeasure capacitance over 1000 cycles to 200% strain, for three
sensors separately (Fig. 5b). The relatively high resistance of thegraphite sensors made them impossible to reliably read using
the MPR121, giving an essentially flat curve after an inconsistent initial increase (Fig. S7, ESI†).
4.2

Solid state, damage resilient actuation

Recent developments in solid-state soft actuators have been
boosted by the development of other liquid-inclusion silicone
composites, such as the ethanol silicone blend developedby Miriyev et al.24,57 This composite operates on a simple
principle: heating the ethanol above its boiling temperaturecauses expansion via the liquid–gas transition. This in turncauses the silicone to increase in volume, up to 900%.24 It is
possible to harness this volumetric expansion inside of a
McKibben-like mesh sleeve to control the direction of motion
and the output forces, resulting in similar behavior to pneumatic
actuators. The authors of the original work note that although thesystem can be rejuvenated by soaking in an ethanol bath for aperiod of time,58 Joule heating the system with a Nichrome wire
often results in localized burning of the silicone and eventual
degradation of the silicone system beyond recovery.
Joule heating ethanol silicone with a soft, stretchable heater
is diﬃcult due to the large expansion of the system. One work
has embedded kirigami-cut conductive fabrics inside of theethanol silicone to distribute heat throughout the core.59
We previously used our conductive SE–EIG composite as a Joule
heating core inside these solid-state actuators to generatehigh contraction forces (4200 N),25 but due to the low strain
capacity of the initial heating composite (Fig. 2), we could only
show contraction actuation.
Because the ethanol silicone composite is a closed-cell foamlike composite with millimeter sized ethanol capsules,24 it isThis journal is © The Royal Society of Chemistry 2020

Paper
relatively agnostic to damage. If the silicone is sliced or pierced,
the material will continue to expand so long as it has a heatsource. This adds an additional layer of complexity to heating
the system: finding a heat source that is both compatible withthe large expansion and also agnostic to damage received while
operating.
Since the MPC is a bulk composite material, it can bedamaged and continue to function. Fig. 6a shows an activated
4  16 cm strip of 60–40–1 MPC silicone (on a silicone backing
layer) Joule heating before and after being cut with a blade.
As long as the electrodes are reinstalled, the system continues
to perform as it did prior to being damaged, with each segment
showing uniform Joule heating.
As shown in Fig. 6b–d (and Video S3, ESI†), we created a
directional-controlled, damage resilient expansion actuator bywrapping an ethanol silicone core in a fiber-reinforced silicone
composite and then our MPC silicone. The fiber-reinforced
silicone is similar to the material found here,60 and preventsradial expansion of the ethanol-silicone tube. Fig. S8 (ESI†)shows that this can enable both extension and rotation actuation,although we focused on the high-strain extension actuation case,activated by the MPC layer wrapped around the outside.
Since the system is solid state (i.e., actuating with no parts
moving relative to each other), and MPC silicone is robust todamage, we added electrodes by simply piercing the tube withpins (see Fig. 6b and c insets). The pins provided a mechanicalanchor for electrical clips, such that there was no concern ofthe clips loosing connection to the heater during operation.
After applying extra liquid metal to reduce contact resistance
between the pins and MPC silicone,29 we Joule heated theexternal MPC to over 140 1C, causing the actuator to extend
over 45% in length.
We demonstrated additional damage resilience by cutting
the actuator in half and reactivating it (Fig. 6d). Since both theheater and the actuator were made of bulk materials, trimmingthem down with scissors only resulted in a shorter actuator.
Adding new electrodes near the cut re-established electricalconductivity and the system could resume extension actuation
via Joule heating.
Electrically interfacing with a highly deformable soft material
is usually diﬃcult (see Materials and methods for more discussion). Being able to run a pin through the entire system is a
simple solution to a traditionally complex interfacing problem.25
The pin becomes a robust connection point for rigid electronics
that bring the power to the actuator (i.e., copper wires or cables).
Even though the system is undergoing large strains, the pins
remain firmly lodged in the actuator. In this way, we gain thebenefits demonstrated in our prior work,25 with improved ability
to interface with the electrical heater and an actuator that can
extend instead of just contract.
4.3

Multi-directional single-channel pneumatic soft actuators

In addition to functioning as the heat source for a liquid-to-gas
phase change (driving the silicone actuator), MPC silicone canalso induce solid-to-liquid phase changes, resulting in silicones
with a variable material modulus. For example, it has beenSoft Matter, 2020, 16, 5827--5839 | 5833

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Fig. 7 MPC silicone applied as a heater on a stretchability-changing
pneumatic bladder. (a) Front view of the pneumatic bladder, along withan IR image showing the MPC increasing the temperature suﬃciently tomelt the metal particles inside the Field’s metal silicone (FMSi) bladder wall.
(b) Cross-sectional diagram of the actuator. (c) Still frames (both IR andvisible light) of the trajectory-changing pneumatic bladder cycling throughdiﬀerent trajectories enabled by selectively heating the bladder’s walls.

Fig. 6 MPC silicone damage resilience. (a) MPC silicone shown to be
conductive before and after being cut into pieces. (b) Cross-sectional
diagram of the actuator system, with photograph inset of the same.
(c) Joule heating the MPC boils the ethanol trapped in the silicone core.
The resulting expansion is controlled by the fiber reinforced silicone wrap,
resulting in a 46% maximum elongation of the actuator. (Inset) The solidstate nature of the system allows for easy electrode creation by piercing
the system with a pin. (d) Both the actuator and the MPC silicone heater
are agnostic to being cut in half, and, once the electrodes are restored, thesystem can continue to produce a 25% elongation.

shown that when Field’s metal is added as a composite filler
into a thin film of silicone, and attached to a pneumatic
bladder, selectively heating the walls allows for a trajectorychange as the pneumatic bladder inflates.26,61 This Field’s
metal silicone composite (FMSi) experiences a change in its
stretchability when the Field’s metal is melted (Tm = 62 1C).
Additionally, it has been shown that this phase transition
allows the FMSi to experience a type of ‘‘shape locking’’.
If the silicone is cooled while stretched, the inclusions will

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freeze and attempt to hold their deformed shape. In both priorexamples though, an external heat source was needed.
To create a self-activating trajectory changing actuator, we
prepared a demonstration similar to that shown by Buckner
et al.,26 with an additional layer of pre-activated 60–40–1 MPC
silicone on the outside in order to allow for on-demand Joule
heating of the FMSi walls. Fig. 7a shows a front view of the
bladder, with one of the MPC heaters clearly visible in a
U-shape on top of the FMSi. We chose this shape to allow forheating of the whole surface while also enabling us to accessboth ends of the heater from the top of the pneumatic bladder.
We attached heaters to both sides of the bladder so that we
could independently control the stretchability of each side
(Fig. 7b). Stiffness control is derived from thermally controlling
the phase of the Field’s metal particles (solid particles below
Tm, liquid inclusions above Tm): melted particles increase thecomposite’s ability to stretch.
By patterning material stiﬀness on the surface of an inflatable actuator, the trajectory of inflation can be changed
on-demand. Fig. 7c shows that selectively softening one sideof the bladder (by applying current to the co-located stretchable
Joule heater MPC sheet) influenced actuator bending in thestiﬀ-material direction. Selectively softening the opposite sideof the bladder influenced bending in the opposite direction.
This demonstration shows that it is possible to achieve at least
three diﬀerent actuation trajectories in a pneumatic bladder(straight, bending left, and bending right) using only a single
air input. The high-strain, highly electrically uniform MPC
silicone on the outside of the bladder enabled Joule heatingof the bladder’s walls, yielding a way to electrically control theactuator’s inflation trajectory.

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4.4 Pneumatic bladder trajectory change with sensor
feedback
Embedding sensors into pneumatic actuators is diﬃcult due tothe noise generated by the pneumatic inflation against thesensor. Often, researchers resort to using flexible (but notstretchable) sensors.19,62,63 Stretchable sensors added to thesystem require careful planning of sensor location so as to reduce
the noise generated by the pneumatic inflation on the sensor’s
response to the curvature in the actuator.64 In this work, our MPC
silicone is capable of withstanding high strains and, when used asa capacitive strain sensor, provides a low-noise signal. In Fig. 7, weshow that the synergy between heating and strain capabilities
allowed the variable-trajectory actuator to sense its motion.
Specifically, we added a second layer of MPC onto the outsideof the bladder (see diagram and cross-sections in Fig. 8a), andobserved the sensor response as the system underwent inflation,
trajectory change, and shape recovery (Fig. 8b, and Video S4, ESI†).
To minimize system complexity, we used the heater MPC
silicone layer as the ground plane of the capacitive sensor(Fig. 8a). This allowed us to reduce the thickness to approximately 3.25 mm per wall. Each layer of MPC silicone on eachwall is relatively thin (120 mm each, for a total of B240 mm),while the remaining thickness came from the glue layer
between the MPC silicone and the FMSi, in addition to thedielectric layer. For this reason, we used a stiﬀer silicone for the
FMSi, so that the glue layer’s thickness would not dominate
the wall’s stretchability (see the Materials and methods section
for more details). Note that the FMSi is not conductive, and so it
could not be used as part of the sensor.

Paper
Fig. 8b shows the data, as well as the experimental steps
used to test the sensor response. First, the system was inflatedpartially, to engage the sensors. Then, we heated up the rightwall (from the perspective shown throughout Fig. 8), using the
MPC heater. As the Field’s Metal particles melted, the system
subtly shifted the direction it was pointing and both sensors on
the bladder responded to the positional shift in the expectedmanner: as the right wall heated up and softened, the wholewall expanded from the constant air pressure in the bladder.
This caused the corresponding sensor to increase its capacitance. Furthermore, the sensor on the left (the cold wall)
dropped its capacitance as the wall was able to contract slightly
from its initial stretched state as the softer right wall expanded
further.
After initiating trajectory change, we followed a similar
method to that of the demonstration in Fig. 7c. First, we
inflated the actuator the rest of the way, observing that thesensor on the heated wall experienced a much greater change
in capacitance. Then, we cooled the wall down and deflated theactuator, noting that the sensors detected the pressure change.
Once the bladder was deflated, the right wall was holding an‘‘expanded’’ shape, and so the sensor did not return to its
original value. We used the MPC silicone heater to re-melt theField’s Metal particles in that wall and the sensor values
dropped back to their relaxed states. Since the pressure in the
bladder was in equilibrium with atmosphere, the left handsensor did not respond to the heating and restoration of
the original shape, as the left wall was already relaxed. Thissequence showed that it is possible to use MPC silicone to

Fig. 8 MPC silicone applied as a heater and a sensor on a stiﬀness-changing pneumatic bladder. (a) Schematics and photographs of the cross section of
the pneumatic bladder. Inset scalebar is 1 mm. Silicone glue layers are not shown in the schematic unless they serve an additional functional purpose (b)visible light and IR photographs of the sensorized bladder at various times, along with graphs of the raw sensor response as the system undergoes bothtemperature and pressure changes.

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5 Conclusions
We have presented a liquid-composite silicone with a high,spatially uniform conductivity (as demonstrated by Joule heating), which is robust to high deformations (200% strain) at highcycles of use (over 10 000 cycles), and has only a 2 increase inresistance at 200% strain. All these properties together in a thinfilm enable many applications, some of which we demonstrated, including a stretchable Joule heater and a capacitive
sensor responsive to strains up to 200%. We also showed boththe heater and sensor being applied to drive high-strain linear
actuation and stretchability change in high-deformation,
shape-locking silicone composites. Furthermore, we showedthat this material is robust to damage—if the system is cut, onemust simply add a new electrode and it will continue tofunction as normal—a potentially impactful trait for a new soft
robotic material.
In the future, we hope to continue to explore new activation
methods, as well as new rigid filler materials. We do this withthe goal of slightly increasing the overall material resistivity
while maintaining the current manufacturing reliability anduniform conductivity. A higher resistivity would help us toscale-up the system to larger areas (geometrically reducing
the resistance), so that we can Joule heat the MPC silicone
using higher voltages and lower currents. By choosing a diﬀerent
activation stimulus (such as freeze activating35) or non-conductive
rigid fillers, it may be possible to reduce the volume fraction ofliquid metal in the MPC silicone so that the system could haveboth a lower mass and a higher resistance, while still maintaininga highly reliable activation mechanism.

6 Materials and methods
6.1

Materials and material preparation

6.1.1 Silicone. Ecoflex 50 (Smooth-On Inc.) was used as theprimary silicone elastomer for all experiments, as both matrix
for the liquid inclusions and the reinforcing backing layer.
Dragonskin 10 (Smooth-On Inc.) was used as the matrix silicone
for the Field’s metal silicone (FMSi), to ensure that the stiﬀness ofthe FMSi sheets dominated the eﬀects of the glue layers in thecomposite system.
6.1.2 Composite fillers. Expanded intercalated graphite(EIG) was prepared using the techniques detailed in previous
works.28,29 Expandable graphite (Sigma-Aldrich) was roasted at
800 1C, added to a carrier solvent (cyclohexane, BDH1111,
VWR), then sonicated to break the graphite into microscopic,
graphene-like particles. Once the excess solvent was decanted,
the solution was weighed and mixed into silicone precursors.
Eutectic gallium–indium (eGaIn) was prepared in-house by
mixing 1 kg of indium (99.9% pure, RotoMetals) with 3 kg ofgallium (99.99% pure, RotoMetals) in a beaker heated to 150 1C
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Soft Matter
scraped off the top of the liquid alloy, then the alloy wasallowed to cool back down to room temperature. Once cooled,the eutectic components were decanted off from the remaining
residual solids and stored for later use.
Field’s metal alloy (Roto144F Low Melt Fusible Ingot Alloy,
RotoMetals) was reduced into microscopic filler particles (sizerange 100–375 mm) using the same techniques detailed inprevious works.26 The alloy was melted in a beaker of hot water(80 1C), mixed with a homogenizer for 2.5 min to break theliquid alloy into particles. When the cycle finished, cold water
was immediately added to reduce the ambient temperaturearound the particles to less than 62 1C, solidifying the particles
and preventing them from fusing. The particles then sink to thebottom and are sieved out of the water, dried, and stored forlater use.
6.1.3 Composites preparation. LM–SE–EIG multiphasecomposite (MPC) silicone was manufactured in the following
steps: (1) both parts of a two-part silicone elastomer werepremixed, ensuring a proper 1-to-1 ratio of the two components. (2) LM was carefully shear-mixed with silicone elastomer
manually for several minutes, until no visible droplets of LM
remained. (3) The EIG/cyclohexane solution was added to the
LM–SE mixture in sufficient quantity so that the amount of EIG
remaining in the system after the cyclohexane evaporated
would be the desired volumetric ratio. (4) Once all parts weremixed together, if the system’s viscosity was higher thandesired, additional cyclohexane was added. A mask was cut
out of 20 mm thick paper, and placed on a flat acrylic sheet. The
final composite mixture was poured at one end of the mask andcoated over the whole mask using custom made drawbars of
300 mm, 500 mm, and 1 mm thickness. After the mask wasremoved, the composite was left at room temperature to dry
and then cure. At this point, the composite could be lifted
and removed from the acrylic, or the first silicone elastomer
reinforcing backing layer could be draw-coated over the top,
depending on the sample being made.
LM–SE, and SE–EIG were manufactured using the same
steps as the MPC silicone, withholding only the material thatneeded to be omitted.
Ethanol silicone was prepared using the same techniques
detailed in previous works.25 Parts A and B of a two part silicone
and pure ethanol were mixed in a 2–2–1 ratio (by volume)
resulting in a final composite of 20 vol% ethanol. The silicone
was mixed slowly at first by hand to trap the ethanol in small
bubbles internal to the system (approximately 1 minute of
mixing). Once all the liquid ethanol on top of the silicone
was mixed in, the composite was mixed at 2000 rpm in aplanetary centrifugal mixer machine (Thinky Mixer, Thinky,USA) for an additional minute in order to reduce the size of theethanol bubbles. Then, the silicone was cast into its desired
shape (a tube) and sealed to prevent the ethanol from escaping
during the curing process.
The fiber reinforced silicone was manufactured using methods
similar to those recorded in previous works.60 A 0.3 mm diameter
polyester fiber (sewing thread) was wound around a large drum
(6 in [15.24 cm] diameter) with a spacing of approximately 0.8 mm

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between fiber centers. Then, while the drum continued to rotate,
uncured silicone was coated over the fibers using a flat surface toensure an even coating of silicone over all the fibers. Finally, thedrum was allowed to spin gently for several hours as the silicone
cured to ensure a uniform thickness of silicone. Once cured, thesheet was removed from the drum. This resulted in a thin sheet offiber reinforced silicone (0.75 mm thick).
The directional controlled solid-state actuators were madeby cutting the fiber reinforced silicone into appropriate sizes,
wrapping it twice around the ethanol silicone tubes, and gluing
it together with additional uncured silicone.
Field’s metal silicone was manufactured by mixing 30%
Field’s Metal particles by volume into Part A and B of Dragonskin 10 (SMooth-On Inc.). The uncured composite paste wasthen coated on a flat acrylic sheet with a 1 mm drawbar. After
curing for 4 hours, appropriate sized sheets were cut and glued
to each side of the silicone pneumatic bladder using an
additional thin layer of silicone.
6.2

Mechanical and electrical characterization

Elongation at break and cyclic loading of LM–SE–EIG rectangular specimens (40 mm gauge length by 10 mm width andbetween 0.2 to 0.4 mm thickness) were evaluated in Instron
3345 testing machine equipped with a 50 N load cell (Fig. 3dand e). Electrical characterization specimens included active
dimensions of 20 mm  10 mm  0.2–0.4 mm thickness, andwere supported by an additional backing layer of pure silicone
to prevent premature failure. Resistance change in the activation process of MPC silicones (Fig. 3f) was evaluated by measuring the voltage across the specimens while simultaneously
straining them in the Instron testing machine. We also measured the resistance of two samples (1-sided and 2-sided backing layer) with 50–50–2 composition over a period of 90 days
(Fig. S9, ESI†). After an initially sharp rise in resistance over the
first 7 days, both samples settled into a very gradual increase
for the remaining time.
6.3

Electrodes and interfaces

For the simplest demonstrations, a bead of LM was placed on
the surface of the MPC silicone and copper wires gently connected to the beads. The bead of LM reduced contact resistance
between the copper wires and the MPC,29 enabling the demonstration of uniform conductivity through the MPC itself.
For the more detailed demonstrations (including the activation testing done in a vertical testing apparatus (Fig. 3), as wellas the cyclic testing (Fig. 4)), a low-resistance, high stretchable
electrode is a technical challenge. Often, the biggest challenge
is the interface between the soft, high-deformation material
and inextensible copper wires.65 To create most of the electricalinterfaces in this paper, we followed a similar tactic used in ourprevious works29 where a bead of LM was placed directly on
the MPC, and a copper wire (or film) was interfaced into that
bead. Then, the electrode was sealed with silicone and an
inextensible layer (porous muslin fabric tabs) that significantly
reduced the strains in the joint between the liquid metal

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and the copper. This method is also supported by recent
literature.66
The only deviance from these two methods was in the solidstate actuator demonstration, where a pin was simply driven
through the actuator, and a small layer of LM wrapped aroundthe MPC to create an electrode with uniform conductivity along
the entire circumference of the heating skin.
6.4

Capacitive strain sensors

Two types of three-layer sensors (electrode, dielectric, electrode)were manufactured using a conductive SE–EIG composite madewith 10 wt% EIG,28 and the new MPC as electrodes materials.
The active region of both types was 10 cm by 1 cm.
Two capacitance measuring devices were used: MPR121 (byNXP Semiconductor, used as a component in a breakout board
sold by Adafruit, Inc.), and a customized charge-integration
circuit developed by our lab (described in ref. 28, theory
elaborated in ref. 56). This custom circuit charged the sensors’
top electrode for a constant time, then measured the time it
took to discharge through a 10 M resistor to a known voltage
reference. Importantly, the MPR121 is designed for capacitorswith low electrode resistance, while the custom circuit was
designed to be used with capacitive sensors with a wide rangeof resistances. Three test protocols were considered: (1) the
sensors at resting length (e = 0%) with no noise intentionallyinjected; (2) the sensors at resting length with a DC motor
placed 3 cm away and ran at 12 V (resulting in B100 rpm; item
number 2826 from Pololu); (3) the sensors with no noise
intentionally injected, pulled (using an Instron 3345) at
200 mm min1 until 200 mm (e = 200%) for 10 cycles. Data
were collected for all combinations of sensor type, measuring
device, and these three test protocols. Finally (test protocol 4),
three MPC sensors were tested with the MPR121 and pulled at
200 mm min1 until 200 mm for 1000 cycles. Key findings were
presented in the main text, while noise characterization andcomplete raw data are presented in the ESI.†

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
This work was supported by the NSF Emerging Frontiers in
Research and Innovation program (grant no. 1830870). DSS was
supported by a NASA Space Technology Research Fellowship(Grant 80NSSC17K0164). The authors would like to thankDr. Michelle C. Yuen for her contributions to the early development of this composite.

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