All-Soft Material System for Strong Soft Actuators
R. Adam Bilodeau*1,2 , Aslan Miriyev*3 , Hod Lipson3 , Rebecca Kramer-Bottiglio1,2Abstract— In this paper, we present an improvement to a
recently developed thermally responsive silicone-based actuator. The actuator is made of silicone with dispersed solvent
droplets and expands when the droplets undergo a liquidgas phase transition, which is achieved at low voltages using
an embedded Joule heater. In previous work, the embedded
heater was a nickel-chromium spiral-shaped Joule heating wire.
In the present work, we replace the wire with a siliconebased conductive composite to create a fully soft actuator. We
characterize the thermal response of the conductive composite
for Joule heating the actuator and the blocked force of theactuator when implemented as a McKibben-like muscle. From
this, we show that the conductive composite performs as well as
the original wire heater, with improved material compatibility.
Finally, we demonstrate a 20 g silicone actuator embedded withthe conductive composite lifting its 4.3 kg DC power source.

Keywords: all-soft system; soft artificial muscle; soft electrically conductive heater; thermally responsive actuator;
I. I NTRODUCTION
Soft-material robotics aims to provide robots with thecompliance and large strains necessary for performing natural and delicate tasks [1], [2]. Rigid robotic systems haveconsistently had the advantage of high-speed, high-precision,and high-output force actuation that allows them to carryboth their own power supplies and control circuits while
performing autonomous (untethered) tasks. Developing allsoft actuators for replication and amplification of naturalmuscle functionality so that soft robots can compete with
their rigid counterparts is an ongoing challenge for the soft
robotics research community [3].
Recently Miriyev et al. reported a self-contained, stimulusresponsive composite material by simply mixing silicone
rubber and ethanol [4]. This new soft, robust composite canbe used as a high-stress actuator at very low voltages (as low
as 8 V), with output linear strains of up to 140%. However,lacking a soft, robust Joule heater, actuation of this material
was demonstrated using an embedded, nickel-chromium (NiCr) spiral-shaped wire. Using a wire limited the ability togeometrically design an internal heater and the ability tosimultaneously 3D-print the entire heater/actuator system [4].
A recent development of an electrically-conductive elastomer
composite for stretchable sensors [5] possesses a promising*RAB and AM are co-first authors.
1 School of Engineering and Applied Sciences, Yale University, New
Haven, CT, USA
2 SchoolofMechanicalEngineering,PurdueUniversity,
West
Lafayette,IN,USA
rbilodea@purdue.edu,

rebecca.kramer@yale.edu
3 Department of
New York, NY,

Mechanical Engineering, Colombia University,
USA aslan.miriyev@columbia.edu,

hod.lipson@columbia.edu

Fig. 1. All-soft heater/actuator being used as a McKibben-like muscle. The
actuator is lifting its DC power supply, whose mass is greater than 200x
that of the actuator.

opportunity to address the need for a soft, bulk-material
Joule heater. The low-cost conductive composite, a silicone
elastomer matrix with an expanded intercalated graphite(EIG) conductive additive, retains its performance even atlarge strains of up to 250%.
In this work, we fabricate an EIG-silicone conductive
composite for use as a Joule heater and encase it within thethermally responsive ethanol-silicone composite to form anall-soft actuator. We demonstrate that at 15 wt% conductive
filler, the heater core is capable of stretching with theencasing actuator and has the low resistances required forJoule heating at low voltages. We characterize the heater
core’s capabilities with various geometries and demonstrate
its application as a Joule heater in a fully-soft artificial
McKibben-like muscle that compares in force output tothe original variant with a Ni-Cr wire heater. Finally, as
demonstrated in Figure 1, we show that this all-soft actuator
can lift its own power supply (200x heavier than the actuator
itself).
II. BACKGROUND
Many forms of soft actuators have emerged in recent
years as partial solutions to the need for high-force, highdeflection actuators, using a wide range of activation methods. These include fluidic elastomer actuators (FEA) [6]–Fig. 2. All-soft actuator operation. Heating the core causes the externalsilicone to expand as the ethanol droplets trapped inside boil.

[11] and pneumatic artificial muscles (PAM) [12]–[14] which
operate off of pressurized fluids, as well as electrically active(and Joule-heated) solutions such as electroactive polymers(EAP) (including dielectric elastomer actuators (DEA)) [15]–[19], ionic polymer-metal composites (IPMC) [20], [21],and shape-memory alloys (SMA) [22], [23]. Each of theseactuator techniques has an advantage-disadvantage trade-off,making them only partial solutions to the development ofautonomous soft robotic systems.
Actuation methods combining high actuation stress alongwith high actuation strain include FEAs, PAMs and DEAs[4]. FEAs and PAMs require compressors and pressure
regulating equipment to inflate and deflate elastomer bladders
or chambers. The required external equipment accompanying
these types of soft actuators is usually rigid, bulky and heavy,
negating their high force output [1]. DEAs operate at >1 kV
to create electromechanical response of an elastomeric membrane between two electrodes, requiring a high voltage converter and complicating the safety of human-interaction withthese devices [24]. Though SMA actuators can achieve highstrains through Joule heating at low voltages when properly
programmed, even small amounts of overheating causes a
rapid degradation of the SMA programming [25]. Alternatively, monopropellant decomposition (and even combustionbased actuation) has been reported as a light-weight fluidpressure source for completely soft pneumatic robots [1],[26]. Designing a soft robot with fuel-decomposition-based
actuators has resulted in moderate output forces and complex
design challenges, since noise, high temperature and toxic
by-products need to be taken into account [27]. Furthermore, despite the high actuation impact of a combustionpropelled robot, controlling such actuators has been a challenge. Finally, actuation methods which show limited strain
but sufficient actuation stress include paraffin-based solidliquid phase transition actuators (only 3% strain) [28] andpiezo-polymers (less than 1% actuation strain). These strain
values are insufficient for most soft robotic applications, andtherefore do not meet current needs.
The silicone-ethanol actuator reproduced in this work has
been shown to function on a basic principle: By mixing thetwo materials together, the ethanol is distributed throughout the silicone matrix in micron-scale pockets that aresuspended in place after curing. Upon heating to 78.4◦ C,
ethanol boils inside the micro-pockets, leading to tremendous

increase in volume of the entire composite (see Figure 2).
Ethanol was chosen for its low toxicity, its good ability to
mix with silicone, and its moderate boiling temperature [4].
Previous work has succeeded in producing volumetric strainsof up to 900% and exerting a force sufficient to lift a weightover 1000 times larger than the composite material.
Prior work with the EIG material has shown that it ispossible to increase the amount of additive in a silicone
matrix, without loosing stretchability [5]. The EIG material
has also already been demonstrated as a conductive filler
enabling internal heating in a rigid epoxy composite [29]. It
enabled the Joule heating of the epoxy, causing the composite
to soften and change its basic shape. This previous successmotivated us to increase the concentration of EIG in thesilicone composite and use it as a soft heater.
III. M ANUFACTURING
We note that, for clarity, the silicone-ethanol actuator
mixture will be referred to as the actuator composite, andthe EIG-silicone mixture will be referred to as the heater
composite. The manufactured silicone composite with aheater core will be referred to as the silicone actuator. Finally,
when referencing the whole system (the silicone actuator in amesh sleeve like a McKibben actuator), the system is referred
to as a muscle.
A. Silicone Actuator Manufacturing
We used Ecoflex 00-50 (Smooth-On) for both the actuator
composite and the heater composite. It is received from the
manufacturer in two parts (part A and B) and we used it a 1:1
ratio. To create the actuator composite, we cast the silicone
ethanol mixture into a 20 mm dia. cylindrical mold, 50 mm
tall with a central shaft (see Figure 3a and 5a). We mixed
both part A and B with ethanol (20 vol% ethanol) by handfor 1 min and then in a planetary centrifugal mixer machine(Thinky Mixer, Thinky USA) for 1 min. Once the silicone
cured, the 3-part mold was removed, leaving an internal
hollow core for later insertion of the heater composite.
For the heater composite, expandable graphite was purchased in a pre-prepared, dry state from Sigma-Aldrich.
It was expanded and sonicated into EIG through a hightemperature oven roasting process (800◦ C) previously detailed by White et al. [5], with one key difference being theuse of ethanol as the sonication solvent. We made this change(from cyclohexane used previously) to improve compatibility
with the actuator, with no noticeable degradation of the
conductivity of the final silicone composite. After preparationof the EIG-ethanol mixture (through sonication and drying),
the mixture is still mostly ethanol (only ∼4 wt% graphite).
Part B of the silicone was mixed with the EIG composite,
and then air was blown across the composite to force theevaporation of the ethanol (see Figure 3b). This dries the
mixture down while still enabling dispersion of the EIG in
the silicone. Once the composite was dried to only 50 wt%
ethanol, Part A was added and the entire composite was
mixed thoroughly to create a ratio of 15 wt% EIG-silicone.
The heater composite was then injected into the hollow coreFig. 4. Macro- and microstructure images of the all-soft material system
for actuators. (a) SEM image of the soft heater composite. (b) SEM imageof a silicone/heater interface cross-section. (c)-(f) macrostructure of actuator
composites after preparation with different mixing modes and rates: (c) handmixing for 4 min, (d) mixer machine mixing for 0.5 min, (e) mixer machine
mixing for 1 min, (f) mixer machine mixing for 3 min (scale bars are
500 µm). All-soft actuator composite before (g) and during (h) handstretching, showing 50% linear strain extension in the core.

Fig. 3. Manufacturing the all-soft silicone based actuator. (a) A siliconeethanol mixture is poured into a mold to cure, creating a cylindrical actuatorwith a hollow core. (b) To make the heater core, part B is mixed with an
EIG-ethanol mixture, set under an airflow until nearly dry, and then part A
is mixed in. Before it cures, the silicone-EIG composite is injected into thehollow core of the actuator. (c) The ends of the actuator are sealed withsilicone and liquid metal is injected into the joint at the end of the heater
composite. Finally, a pin (or copper wire) is inserted into the liquid metalbead to create an electrical connection to the heater core.

of the actuator composite using a syringe, and allowed tocure.

B. Muscle Assembly and Electrode Contacts
We used eutectic gallium-indium-tin (galinstan, Sigma
Aldrich), a room-temperature liquid metal, to create a simple
interface connecting the soft core heating composite withrigid wires and electrical components (Figure 3c). We sealed
each end of the silicone actuator with additional (stronger)
silicone (Dragonskin Fast, Smooth-on) and then injected
galinstan into the ends of the actuator using a 25 gauge
syringe needle. On removing the syringe, we sealed thehole with additional silicone and assembled the muscle byplacing the silicone actuator into a mesh sleeve (Polyurethane
terephtatalate mono-filament yarn, Techflex) and tying it off.
In order to electrically connect to the core, we punctured
a needle through the mesh and into the liquid metal bead,allowing us to connect the heater to a power source. In thecase of experiments where free-expansion (or heating) was
required, the mesh sleeve was omitted. We note here thatwhile these electrodes provided an ideal electrical interface,
they were susceptible to leakage under high pressures, which
manifested as electrical noise.

IV. C HARACTERIZATION
A. Manufacturing Implementation
1) Heating Core Resistance: The resistance in the heater
silicone cores varied between 15-65 Ω, comparable to the
15 Ω in the original Ni-Cr wires [4]. Two key factors in ourdesign and manufacturing allowed for such a low, uniform resistance in the composite material: the use of the liquid metalelectrodes and prevention of the ethanol from drying out of
the composite material during preparation. The liquid metal
contacts decrease the contact resistance between copper wire
electrodes and the soft heater composite. Without the liquid
metal, the resistance in the heater core increases by 20-40 Ω,
often doubling the sample resistance and causing uneven
Joule heating due to the additional resistance concentrated
at ends of the actuator. This is why the liquid metal wasused even though it leaked during actuation.
Preventing the ethanol from fully drying out of the heater
composite before it cures is the second key to maintaining
low resistance. If the heater composite dries completely
before mixing in Part A (during the manufacturing, aspreviously explained) the resultant resistance of the core
jumps 1-2 orders of magnitude (from 25-65 Ω to 800-2500 Ω,
measured on cores with a diameter of 5 mm and 7 mm). It
is still possible to mix in Part A and inject the material,
but the high resistance makes the core unusable as a heater.
We suspect that when the composite dries prematurely the
EIG flakes clump, resulting in poor dispersion throughout thecomposite. If the silicone cures in the 50% ethanol mixture,
the EIG flakes remain suspended in the unmoving polymer
after the ethanol dries off.
2) Heater Microstructure: We placed a silicone actuator
into a scanning electron microscope (SEM) in order determine the quality of the heater composite mixture and tovisually observe the joint between the actuator composite

Fig. 5.
CAD diagrams and photographs of the various heater coregeometries. (a) A cut-away of the CAD rendering of the three-part mold.
(b) Models of the four different cores used in the mold. (c) Cross-sectional
photographs of three of the resultant heater core/actuator combinations.

and the heater composite. Figure 4a,b are images taken ofboth the pure heater composite and the heater composite
interfacing with a pure silicone exterior. We also prepareda sample without ethanol in the actuator (to remove incompatibility with the high-vacuum SEM), and cut it witha knife to expose an inner cross-section for imaging. The
heater composite has a textured microstructure topography
on a scale from single microns to tens of microns, caused by
the conductive EIG additive (Figure 4a). Although individual
EIG flakes are too small to be seen at this scale, no EIG
clumps are observable, indicating a good mixture of the EIG
into the silicone. The interface between the heater composite
and pure silicone rubber is characterized by multiple microcracks, and a noticeable difference in topography between
the two materials (Figure 4b).
3) Actuator Macrostructure: To ensure that the ethanol
was distributed evenly throughout the actuator composite
when machine mixed, we tested the effect of mixing theactuator composite by hand or in a mixer machine on the
bubble density of the composite. Four samples of siliconeethanol composite were inspected under a microscope toobserve the density of air-pockets (or ethanol droplets) in themixed composite. Each sample was prepared with different
mixing time/method. The first sample was mixed by handfor four minutes. The other three were mixed by hand forone minute (to fold the low-viscosity ethanol into the highviscosity silicone) and then mixed in the mixer machine for
0.5 min, 1 min, and 3 min respectively. All samples werethen cast into sheets 5 mm thick to allow the microscopeto capture the images of the bubbles through the sample.
Figure 4c-f shows that the largest amount of air bubbles wasachieved after 30 s of mixing in the mixer machine, whereas
mixing for three minutes resulted in the lowest density of
air bubbles. This validates our mixing for 1 min, as theactuator composite has a comparable macrostructure to thehand-mixing method used in previous work [4].
4) Heater Stretchability and Interfacing: Although theinterface in the all-soft material system was shown to havemicrocracks (Figure 4b), we still achieved a high bond
between the heater composite and the actuator composite.
To demonstrate this, we cast a large, flat sample of theFig. 6. IR images of several Joule heating experiments using 9 W of input
power. (a)-(b) Two silicone heater composite cores are compared with a
Ni-Cr wire heater via encapsulation in pure silicone. (a) At 2 min, the Xshaped core is distributing its heat better to the exterior of the silicone, whencompared to the other two systems. (b) The same cross section achieves anexternal temperature of ∼78◦ C two minutes faster than the other systems.
(c) The Ni-Cr wire heater compared with (d) the X-shape core, encapsulated
in the actuator composite in their initial (cool) and final (heated) states. Scalebars are 1 cm.

actuator composite, with a small, 10 mm wide rectangularcore, and filled the core with the heater silicone composite(Figure 4g). This enabled us to stretch the external silicone
and observe the reactionary stretching of the internal heater.
As Figure 4g-h demonstrates, we could achieve both goodadhesion between the disparate composites and a 50% linear
expansion of the heater composite. The ability to both adhere
the bulk composites together and for both to have high strain
capacities is vital for the success of these all-soft actuators.
B. Heating and Energy Distribution
1) Core Design: We used four different types of internal
shafts in order to compare differences in heater performancebased on the geometry of the core (Figure 5). Three actuators
had circular cores with diameters varying between 5 mm,
7 mm and 9 mm. We also created a fourth core in the shapeof an X, with the same cross-sectional area as the 7 mm circle(38.38 mm2 , see Figure 5c). This shape has the same volumeof heater composite as the 7 mm core, but a larger contact
area with the actuator composite. Ni-Cr wire (Remington
Industries) heaters were coiled in a double-spiral and castdirectly into the actuator composite in order to compare thecurrent all-soft system to the original work employing thissilicone-ethanol actuator technology [4].
2) Core Performance: We used a thermal camera (Fluke
TiX580, Fluke) to characterize the ability of the heater
composite to heat the surrounding actuator composite up to(78.4◦ C), the boiling temperature of ethanol. We started byheating a simple, pure elastomer sheath around the heatercore (to remove the effects of the expanding ethanol). Allfour varieties of heater composite cores (5, 7, 9 mm, Xshape) and a Ni-Cr wire heater were encased in the silicone
rubber and Joule heated with 9 W of power (only two of
which are shown in Figure 6a,b).
After two minutes of heating, the X-shaped heater composite shows better heat distribution than any other composite(Figure 6a). The X-shaped heater also heated the entire
outer surface to 78◦ C two minutes faster than the other
setups, taking only 5.5 min. The X-shaped heater core
proved to be the best Joule heater design when power wascontrolled as it can heat the required volume both quickly
and evenly. Though it has a higher resistance than the 9 mmheater composite rod and Ni-Cr wire spiral (also shown in
Figure 6a,b), those two suffered from localized overheatingnear the heaters and/or peripheral under-heating. The higher
contact area of the X-shaped core with the actuator composite
is more efficient as it nearly eliminates localized overheating.
Note that the slight expansion of the specimen with the Xshaped heater composite core is caused by a small amount of
ethanol trapped in the heater core during the filling process.
The spiral-shaped Ni-Cr wire has the smallest volume ofthe heating cores, and, though rigid, it causes significant
expansion and shape deformation of the silicone actuator due
to the high flexibility of the wire spiral and local overheating(Figure 6c). When heated, the X-shaped heater composite
showed a smaller but more even expansion of the surrounding silicone actuator (Figure 6d). This difference, however,does not prevent the X-shaped heater from producing large
output forces when constrained in a mesh (see next section).
C. Force Output
In Figure 7 and 8, we show the potential of our allsoft material system to be the actuator in a fully soft
McKibben-like artificial muscle. When encased in a braided
mesh sleeve (Figure 7a,b), the silicone actuator is capableof radial expansion in the net, leading to axial contractionof the entire muscle. The muscle is activated by supplying
electrical power to the internal Joule heater. For comparison,
the original work, which also demonstrated the actuator as a
McKibben-like muscle [4], was also recreated (Figure 7).
To determine the maximum pulling force of these muscles,
we performed a series of blocked force tests. We clamped
a new artificial muscle in an Instron 3345 using pneumatic
grips (Figure 7c) and Joule heated the actuator with 30 V
and unregulated DC current. We measured the force output of
the statically-held muscle until it either failed mechanically(Figure 7c) or electrically. As the current fluctuated whileheating the actuators with heater composite cores, we were

Fig. 7. Blocked force setup and test. (a) A Ni-Cr wire heated actuator.
(b) The same actuator inside a mesh sleeve for use as a McKibben-likemuscle. (c) The actuator installed in a blocked force test. Insets show theJoule heated actuator at its maximum force and then after complete failure(due to melting the external mesh).

not able to record a steady value and were therefore unable
to calculate power consumption.
We tested all four configurations of the all-soft muscles
and the original Ni-Cr spiral-heated version in order tocompare their maximum blocked force. The results, shownin Figure 8a, show that most variations of the all-softmuscles can consistently exert more than 200 N of force,corresponding to more than 1000x the weight of the entire
artificial muscle (<0.2 N), as can the Ni-Cr heated version.
On average, the 9 mm heater composite performed best,exerting a blocked force of nearly 300 N, significantly higher
than that produced by the 5 mm cores (barely 60 N).
Both the 9 mm cores and Ni-Cr wire actuators failed
mechanically. The pressure of the expanding silicone actuator
exceeded the strength of the mesh, typically resulting insudden and complete failure and force loss (see Figure 8c
inset). Though the mesh failed for both types of actuators, theNi-Cr averaged significantly less pulling force before failing(nearly 100 N less). The Ni-Cr wires would consistently draw
1.5 A of current, with the 9 mm cores drawing between 12 A during their testing. Although they both were drawing
similar power, the 9 mm cores isolated the incoming energy
as heat in the central core of the actuator, causing only
an expansion of the ethanol. In contrast, the Ni-Cr wire
created localized hot-spots at the exterior of the actuator(see Figure 6) weakening the mesh, and causing them tofail prematurely. This was especially true at the ends of theactuator, where the Ni-Cr wire leads entered the mesh andwhere the wire turns around. Input power regulation (andFig. 8. Blocked force test results for the McKibben-like muscles. (a) The
measured results of the force output of several all-soft muscle designs,
compared with a Ni-Cr wire heater. Error bars represent 95% confidence.
(b) Characteristic curves from the build up of force in the actuator muscle
over time for three examples of different heater types. The 9 mm core andNi-Cr wire, with nearly identical resistances, heat up at nearly the samerate, whereas the 7 mm core (with a higher resistance) takes longer to heatup.

therefore temperature regulation of the Ni-Cr wire) mightenable the Ni-Cr wire actuators to produce higher forces by
preventing overheating of these ends.
In contrast, the 5 mm dia., 7 mm dia., and X-section heatercores all failed electrically, losing connection between thetwo electrodes. This reduced the power input to near-zero,though only temporarily for the 7 mm dia. and X-section
cores. After cooling briefly, both types would typically regain
conductivity. For the 5 mm diameter cores, however, thisloss-of connection was permanent, probably due to the small
contact area in the thin cores and the heater material burning
around the electrodes. Between their high initial resistanceand the permanent heater-core death, muscles with thesecores performed poorly.
Figure 8b shows three typical curves of blocked force asa function of time. The 9 mm heater composite and the NiCr wire heat almost at the same rate, while the 7 mm coreheats up slower. This may be attributed to the similar initial
electrical resistance of the Ni-Cr wire and the 9 mm coreused to gather these datasets (approximately 15 Ω each),compared to the higher resistance of the 7 mm core (at about25 Ω).
V. R ESULTS AND D ISCUSSION
To demonstrate the potential of these light-weight, all-soft
actuator muscles, we used one to lift its own power supply(Figure 1). The actuator had a 7 mm dia. internal silicone
heater core, and was strong enough to lift the power supply
5 mm (10% the length of the actuator) straight up, eventhough the supply is over 200x the mass of the actuator (seealso Supplemental Video 1). Operating on a 15 V input, theactuator lifted the power supply to its maximum height in
22 s, and held it suspended for well over 1 min before being
shut off.
The novel material combination of a silicone-EIG heatercore with the silicone-ethanol actuator opens up many new

design parameters for exploration. Apart from the entire
system system being composed of elastomer composites(enabling bulk stretchability), a bulk heating material gives
way to numerous possibilities in geometric design choices forthe heater, beyond the few presented here. This can improve
the way in which the heat from the heater core is distributed
throughout the actuator (as demonstrated with the X-shapedcore), potentially preventing over-heating of the ethanol andburning of the silicone immediately surrounding the actuator.
By adding larger volumes of the bulk-heater, it is possible
to both speed up the force generation (as demonstrated by
the 9 mm cores), and increase the maximum blocked force
produced by the muscles. With this bulk system, it will bepossible to optimize both the size and shape of the heatercore in order to improve the speed and efficiency of the allsoft muscles.
There are other advantages of this all-soft system whencompared to its predecessor using the Ni-Cr wire. Although
the Ni-Cr wire can have more coils looped through theactuator to facilitate heating, the system short-circuits andwill not heat if any of the coils touch, a non-issue with abulk silicone resistor. Furthermore, by limiting the heatercore to a system exclusively internal to the actuator, it isless likely that an external mesh sleeve (or other materials)
will heat up and weaken. Finally, because we successfully
removed the Ni-Cr wire from the actuator, both the actuatorand the heater are malleable in their uncured state, allowing
for continued improvement to the design and manufacturingof the system.
VI. C ONCLUSION AND F UTURE W ORK
In this paper, we have demonstrated that we can integrate a
new, soft, stretchable, central core material that removes theneed for a Joule heating wire used in the original system. We
show that actuators powered with this soft composite heater
are capable of producing comparable blocked forces, while
also being able to improve the heat distribution by geometric
design. Finally, we demonstrate that a single one of theselight-weight, all-soft muscles is strong enough to lift over
200x its own weight. This is a big step forward for the soft
robotics field, as the ability for a soft robotic actuator to carryweight (such as batteries and control units) is crucial towards
the development of autonomous soft robotic systems.
This all-soft system is a significant advancement from theoriginal work, as both the soft actuator material and thesoft heater may potentially be patterned using 3D-printing.
This would allow automated fabrication and endless varietiesof actuator (and heater) shapes, while retaining the actuator
expanding performance. 3D-printing of the silicone actuatormaterial was shown in the original work [4], while 3Dprinting of the heater composite will be explored as a partof a future work.
The accelerated Joule heating of the actuators came witha cost, reducing them to one-time use, though the muscles
have been shown to be repeatably actuated at slower heating
rates [4], [30]. We desire to improve both driving speed anddurability by continuing to improve the mix ratios of thecomposite materials in the core, optimizing for stretchability,
robustness, and printability. We will look to improve theinterfacing of the material systems, focusing on both theinterface between the heater composite and the actuator
composite and the interface between the heater and any
electrical leads used for power supply. Furthermore, withdifferent bubble densities caused by different mixing times,
and by using 3D printing technologies, it might be possible
to combine actuator composites with different mixing times
to create asymmetrically expanding actuators.
VII. ACKNOWLEDGMENTS
This work was partially supported by the US Air Force Officeof Scientific Research under award number FA9550-16-1-0267,
by which RAB was funded. Columbia University research was
supported in part by the Israel Ministry of Defense (IMOD) Grant
number 4440729085 for Soft Robotics. AM acknowledges support
from Columbia University funds.

R EFERENCES[1] B. Mosadegh, D. J. Fitzgerald, G. M. Whitesides, J. A. Lewis,
M. Wehner, R. J. Wood, and R. L. Truby, “An integrated design
and fabrication strategy for entirely soft, autonomous robots,” Nature,
vol. 536, p. 451, Aug. 2016.
[2] D. Rus and M. T. Tolley, “Design, fabrication and control of soft
robots,” Nature, vol. 521, pp. 467–475, May 2015.
[3] S. Kim, C. Laschi, and B. Trimmer, “Soft robotics: a bioinspired
evolution in robotics,” Trends in Biotechnology, vol. 31, pp. 287–294,
May 2013.
[4] A. Miriyev, K. Stack, and H. Lipson, “Soft material for soft actuators,”
Nature Communications, vol. 8, p. 596, Sept. 2017.
[5] E. L. White, M. C. Yuen, J. C. Case, and R. K. Kramer, “Low-Cost,
Facile, and Scalable Manufacturing of Capacitive Sensors for Soft
Systems,” Advanced Materials Technologies, vol. 2, pp. n/a–n/a, Sept.
2017.
[6] S. Sridar, C. J. Majeika, P. Schaffer, M. Bowers, S. Ueda, A. J. Barth,
J. L. Sorrells, J. T. Wu, T. R. Hunt, and M. Popovic, “Hydro Muscle-a novel soft fluidic actuator,” in 2016 IEEE International Conferenceon Robotics and Automation (ICRA), pp. 4014–4021, May 2016.
[7] R. F. Shepherd, F. Ilievski, W. Choi, S. A. Morin, A. A. Stokes, A. D.
Mazzeo, X. Chen, M. Wang, and G. M. Whitesides, “Multigait soft
robot,” Proceedings of the National Academy of Sciences, vol. 108,
pp. 20400–20403, Dec. 2011.
[8] M. T. Tolley, R. F. Shepherd, B. Mosadegh, K. C. Galloway,
M. Wehner, M. Karpelson, R. J. Wood, and G. M. Whitesides, “A
Resilient, Untethered Soft Robot,” Soft Robotics, vol. 1, pp. 213–223,
Sept. 2014.
[9] P. Polygerinos, Z. Wang, K. C. Galloway, R. J. Wood, and C. J.
Walsh, “Soft robotic glove for combined assistance and at-homerehabilitation,” Robotics and Autonomous Systems, vol. 73, pp. 135–
143, Nov. 2015.
[10] A. D. Marchese, R. K. Katzschmann, and D. Rus, “A Recipe for Soft
Fluidic Elastomer Robots,” Soft Robotics, vol. 2, pp. 7–25, Mar. 2015.
[11] A. De Greef, P. Lambert, and A. Delchambre, “Towards flexible
medical instruments: Review of flexible fluidic actuators,” Precision
Engineering, vol. 33, pp. 311–321, Oct. 2009.
[12] C.-P. Chou and B. Hannaford, “Measurement and modeling of McKibben pneumatic artificial muscles,” IEEE Transactions on Robotics
and Automation, vol. 12, pp. 90–102, Feb. 1996.

[13] Y. Sun, Y. S. Song, and J. Paik, “Characterization of silicone rubber
based soft pneumatic actuators,” in 2013 IEEE/RSJ International
Conference on Intelligent Robots and Systems, pp. 4446–4453, Nov.
2013.
[14] B. Audergon, G. Agarwal, J. Paik, and N. Besuchet, “Stretchable Materials for Robust Soft Actuators towards Assistive Wearable Devices,”
Scientific Reports, vol. 6, p. 34224, Sept. 2016.
[15] Y. Bar-Cohen, K. J. Kim, H. R. Choi, and J. D. W. Madden,“Electroactive polymer materials,” Smart Mater. Struct., vol. 16, Apr.
2007.
[16] F. Carpi, ed., Electromechanically Active Polymers. Cham: Springer
International Publishing, 2016. DOI: 10.1007/978-3-319-31530-0.
[17] A. OHalloran, F. OMalley, and P. McHugh, “A review on dielectric
elastomer actuators, technology, applications, and challenges,” Journalof Applied Physics, vol. 104, p. 071101, Oct. 2008.
[18] P. Brochu and Q. Pei, “Advances in Dielectric Elastomers for Actuators and Artificial Muscles,” Macromolecular Rapid Communications,vol. 31, pp. 10–36, Jan. 2010.
[19] F. B. Madsen, A. E. Daugaard, S. Hvilsted, and A. L. Skov, “The
Current State of Silicone-Based Dielectric Elastomer Transducers,”
Macromolecular Rapid Communications, vol. 37, pp. 378–413, Mar.
2016.
[20] K. J. Kim and M. Shahinpoor, “A novel method of manufacturing three-dimensional ionic polymermetal composites (IPMCs)
biomimetic sensors, actuators and artificial muscles,” Polymer, vol. 43,
pp. 797–802, Feb. 2002.
[21] M. Shahinpoor and K. J. Kim, “Ionic polymer-metal composites: I.
Fundamentals,” Smart Materials and Structures, vol. 10, no. 4, p. 819,
2001.
[22] W. Huang, “On the selection of shape memory alloys for actuators,”
Materials & Design, vol. 23, pp. 11–19, Feb. 2002.
[23] D. Ratna and J. Karger-Kocsis, “Recent advances in shape memory
polymers and composites: a review,” Journal of Materials Science,
vol. 43, pp. 254–269, Jan. 2008.
[24] S. Pourazadi, A. Shagerdmootaab, H. Chan, M. Moallem, andC. Menon, “On the electrical safety of dielectric elastomer actuatorsin proximity to the human body,” Smart Materials and Structures,vol. 26, no. 11, p. 115007, 2017.
[25] F. Schiedeck and S. Mojrzisch, “Design of a robust control strategy forthe heating power of shape memory alloy actuators at full contraction
based on electric resistance feedback,” Smart Materials and Structures,vol. 20, p. 045002, Apr. 2011.
[26] N. W. Bartlett, M. T. Tolley, J. T. B. Overvelde, J. C. Weaver,B. Mosadegh, K. Bertoldi, G. M. Whitesides, and R. J. Wood, “A
3d-printed, functionally graded soft robot powered by combustion,”
Science, vol. 349, pp. 161–165, July 2015.
[27] M. Wehner, M. T. Tolley, Y. Meng, Y.-L. Park, A. Mozeika, Y. Ding,
C. Onal, R. F. Shepherd, G. M. Whitesides, and R. J. Wood, “Pneumatic Energy Sources for Autonomous and Wearable Soft Robotics,”
Soft Robotics, vol. 1, pp. 263–274, Oct. 2014.
[28] J. I. Lipton, S. Angle, R. E. Banai, E. Peretz, and H. Lipson, “Electrically Actuated Hydraulic Solids,” Advanced Engineering Materials,
vol. 18, pp. 1710–1715, Oct. 2016.
[29] T. L. Buckner, E. L. White, M. C. Yuen, R. A. Bilodeau, and
R. K. Kramer, “A move-and-hold pneumatic actuator enabled by selfsoftening variable stiffness materials,” in 2017 IEEE/RSJ International
Conference on Intelligent Robots and Systems (IROS), pp. 3728–3733,
Sept. 2017.
[30] A. Miriyev, G. Caires, and H. Lipson, “Functional properties ofsilicone/ethanol soft-actuator composites,” Materials & Design, Feb.
2018.