2020 3rd IEEE International Conference on Soft Robotics (RoboSoft)
Yale University, USA

Shape Memory Silicone Using Phase-Changing Inclusions
Trevor L. Buckner1 , Michelle C. Yuen1 , Rebecca Kramer-Bottiglio1

Abstract— Functional, responsive materials are attractive foruse as key components in soft robots as they can replace
otherwise rigid or bulky parts. In this work, we present afunctional silicone with shape memory properties that, due
to the retention of elasticity and flexibility, can be seamlessly
integrated into the body of a soft robot. By dispersing particles
of low-melting-point metal alloy (Field’s metal) into a silicone
matrix, the resulting composite can be “frozen” into various
shapes by sequentially heating to melt the Field’s metal particles, stretching the composite, and cooling to solidify the Field’s
metal particles in the deformed configuration. The ramificationsof this operational capability include both stiffness control and3D shape reconfiguration. In this paper, we characterize thethermomechanical behavior and shape memory performanceof the Field’s metal/silicone composite. We then highlight applications of the material to impedance and trajectory control,
and topography recording.

I. I NTRODUCTION

Fig. 1. Field’s metal particles are embedded in silicone for shape memory
applications. Here, Field’s metal/silicone composite (gray) is bonded tonative silicone (blue). After melting the Field’s metal, the composite is
stretched. Upon solidification of the Field’s metal, the composite retains
the applied elongation, resulting in a curled bilayer structure as the native
silicone contracts to its original length. Width of the specimen is 1cm.
Microscope images of the composite show the morphologies of the Field’s
metal particles in the unstrained (bottom left) and strained (top right). Scalebar is 200µm.

Much of soft robots’ flexible nature is possible through the
adoption of actuators, sensors, and stiffness control mechanisms that leverage the functionality of responsive materials.
Such materials can often be made into flexible, monolithic
analogs of rigid, multi-part components, and when combinedwith other materials in composite, may become responsive tomore than one stimulus. In many cases, these materials canbe used simultaneously as the body of the robot itself. This
capacity to perform multiple functions with the same material
can allow for drastically lowered part counts, simplified
manufacture, and mass reduction. In the field of soft robotics,
responsive materials have been widely used for actuation(NiTi shape memory alloys, hydrogels, liquid crystal elastomers [1], [2]), sensing (piezoresistive [3], temperature [4]),and stiffness control (thermoplastics [5], low-melting-point
alloys [6]). However, challenges remain in integrating manyof these responsive materials seamlessly with soft robotic
structures due to material property mismatch. Soft robots aremost commonly constructed from silicone due to its highflexibility, yield strain and elastic cyclability. Responsive
silicones in particular, then, become an ideal material foruse in soft robotic components.
Multifunctional silicone elastomers generally take theform of a composite, with the additional functionality arisingfrom the filler material. Most commonly, silicone elastomers
have been loaded with electrically conductive fillers including carbon-based nanomaterials [7], expanded intercalated
graphite [8], metal nanowires [9], and liquid metal [10] tocreate stretchable conductors or sensors. Similarly, enhanced

and anisotropic thermal conductivity can be achieved byshear mixing liquid metal into elastomers [11]. To achieve
actuation using a silicone-based composite, magnetic fillers
have been mixed into elastomers and programmed to respond to external magnetic fields [12], [13], and ethanol
has been mixed into silicone to serve as a thermallyresponsive volumetrically-expanding actuator [14]. For stiffness change, functional inclusions include iron microparticles
and magneto-rheological fluid which have been used tocreate magnetoactive elastomers that stiffen in the presenceof a magnetic field [15], [16], thermoplastics that undergo
glass transition [17] [18], and low-melting-point alloys that
undergo solid-liquid phase change [19]–[21].
The ability to change the stiffness of silicone has close
ties to shape memory capabilities. Shape memory broadly
refers to the ability to trigger a return to a programmed
state after being manipulated [22]. Shape memory in elastomers has been demonstrated previously using liquid crystal
elastomers [23] and semi-crystalline elastomers [24], [25].
Other approaches involve embedding percolating networksof cellulose nanofibers in an elastomer matrix [26] andpreparing a polymeric gel network filled with a supercooledsalt solution [27]. Shape memory behavior has been used toreversibly program 3D shapes into 2D planar materials [28]

1 School of Engineering & Applied Science, Yale University, New Haven,
CT, USA. email: {trevor.buckner, michelle.yuen,

rebecca.kramer}@yale.edu
259

a minimum of 2h after which the films were sliced into
rectangular coupons (100mm x 8mm). Microscope images
taken of the cross-section showed that the Field’s metal
particles settled to the bottom into a 474.8 ± 10.7µm layer,
leaving a less dense “supernatant” 156.6 ± 6.8µm thicklayer on top. Fabric reinforcement tabs (20mm x 8mm)
were adhered to both ends of each specimen, resulting ina stretchable region of 60mm x 8mm.
Additional compositions of the FMSi can also be used.
Changing the volume fraction of FM inclusions affects
the stiffness ratio achieved, as discussed in [32]. However,at overly high volume fractions of FM, the films have atendency to leak when deformed while the FM particles are
liquid. Changing the matrix material can affect the stiffnesses
in both the hot and cold states, and the curing time, and thus
the distribution of FM particles and/or void spaces in thecomposite. Further work can explore tuning the compositionof the FMSi to better suit application requirements.

Fig. 2. Thermomechanical behavior of FMSi under pull-to-failure tests.
Stress vs. strain plots of FMSi in cold (FM solid) and hot (FM liquid) statesshow drastic differences in stiffness and failure strain.

notably via thermally-responsive [29] and humidity-sensitive
materials [30], or stress-driven assembly [31].
We build upon our previous work with Field’s
metal/silicone (FMSi) composites, where we showed thatthe modulus and stretchability can be switched through thesolid-liquid phase change of the Field’s metal [32]. The solidliquid phase change, in combination with the stretchabilityof the silicone matrix enabled “stretch-and-hold” operations
where the composite could be heated, stretched, and thencooled in place to maintain a strained configuration. The
work presented here more directly explores this stiffnesschanging material as a source of shape memory behavior(Fig. 1). By adjusting the degree and direction of strain
during the shape-fixing process, we can thermomechanicallyprogram different in-plane moduli and achieve anisotropic
behavior. We also quantify the ability of FMSi to hold andrecover shape, and measure recovery forces which show
potential as a stored-energy actuator. Finally, we demonstrate
FMSi as an on-demand modulus and shape-changing material with potential for soft robotics applications.

B. Hot vs. Cold
The thermal effect on tensile behavior of the FMSi was
first investigated (Fig. 2). In previous work, we focusedon mechanical properties under small bending deformations [32]. Here, we investigate the behavior under muchhigher strains. Specimens were clamped into a materials
testing system (Instron 3345) and pulled to failure at 200%
strain/min (120mm/min). FMSi and unmodified silicone
samples were tested at room temperature (Cold) and heated
to between 80-100◦ C (Hot). The pure silicone samples
showed negligible change in stiffness between the hot andcold states. However, the heated silicone samples failed earlier ( 300% vs. 800% strain). The stiffness of the hot FMSi
sample (172kPa) was similar to that of the pure silicone(186kPa). The cold FMSi sample was initially much stiffer(976kPa), with a noticeable elbow in the stress-strain curve
occurring at approximately 70% strain, followed by a lower
stiffness region (59kPa). We hypothesize that this lower
stiffness region corresponds to the debonding regime, where
the silicone matrix has separated from the Field’s metal
particles. When not pulling to failure, preliminary results(not shown here) suggest that the cyclic strain behaviorof the FMSi exhibits little change in stress-strain response
between cycles, similar to that of neat silicone. However,
we did observe that at higher strains beyond the debonding
limit, there was a high degree of hysteresis on the initial
cycle of strain, exceeding the characteristic Mullens effectthat was observed in neat silicone, followed by negligible
hysteresis in subsequent cycles. We attribute this behavior to
the separation of the silicone matrix from the FM particles.

II. M ATERIAL C HARACTERIZATION
A. Fabrication
The FMSi composite consisted of 30vol% Field’s metal
particles and 70vol% silicone (Dragon Skin 10 Medium,
Smooth On). The Field’s metal particles were fabricatedby shear-mixing bulk Field’s metal (RotoMetals) in hot
water, using the process discussed in [32]. The Field’s metal
particles and silicone were hand-mixed for approximately
1min to distribute the particles evenly throughout the silicone
matrix. The mixture was then cast into a film by bladecoating with a 1mm gap, resulting in a film thickness ofapproximately 631.4 ± 10.9µm (mean ± 1 stdev). With
the largest FM particles having a diameter of 355µm after
sieving, the thickness of the FMSi film ensured that there
would be a sufficient amount of silicone for a continuous
matrix, preventing immature rupturing of the film arounda single large particle. The film was allowed to cure for

C. Pre-strain
After characterizing the effect of the FM’s phase on the
FMSi’s mechanical properties, we then held the FMSi material under strain as the FM cooled from a liquid to a solidstate. When the FMSi composite is heated and then strained,
any melted, spherical Field’s metal microparticles are also
stretched along with the surrounding silicone matrix into an
260

rods, and held in place using a fixture. After cooling understrain on the fixture to create the elongated FM “needles”,
rectangular coupons were cut from the film at the appropriate
angles and fabric reinforcements were adhered to the endsof each coupon. Five specimens of each treatment were
mounted in the materials testing system and pulled to failure
at 200% strain/min.
The stress-strain curves from these tests are plotted inFig. 3A. The no pre-strain samples from Fig. 2 are plottedhere as well for comparison. It is apparent that the angleof pre-strain had a significant impact on the initial stiffnessof the composite, with the 0◦ samples being much stiffer
than all others, followed by the 45◦ samples, and then by
the 90◦ samples (Fig. 3B). Interestingly, the 45◦ samples
and the un-pre-strained samples had similar stiffnesses inboth the intact and debonded states. All samples appeared tocommence their debonded behavior at approximately 70%
strain. Notably, the debonding event occurred at approximately the same amount of strain irrespective of the amountor angle of pre-strain, suggesting that this debonding isdetermined by the matrix-Field’s metal interaction, rather
than the particulate shape. Following debonding of the matrix
from the Field’s metal particles, which resulted in a reduction
in stiffness by approximately 15x, the differences in stiffness
across samples were smaller. Additionally, in the 0◦ case, anincreased amount of pre-strain corresponded to an increasein stiffness, presumably because more of the Field’s metal
was aligned with the direction of applied strain.
The stiffness values shown in Fig. 3B show that theanisotropic stiffness of FMSi film can be controlled by heating and stretching the film. In the most extreme case shown
here (0◦ -90◦ , 50% strain), the stiffness difference between
the two orthogonal directions was 2.15x. Higher amounts
of pre-strain should yield even more skewed differencesin stiffness between the axial and transverse directions ofapplied strain.

Fig. 3. Mechanical behavior of pre-strained FMSi under tensile pull-tofailure tests. (A) Stress vs. strain plots of FMSi programmed with prestrains of 25% and 50%, at three angles (0◦ , 45◦ , 90◦ ) with respect to theloading direction. Specimens were pulled to failure at 200% strain/min. (B)Stiffnesses of the FMSi samples in the intact (0-35% strain) and debonded(100-135% strain) strain regimes.

D. Shape memory
The shape memory performance of a material can befurther characterized by a variety of measures that gauge theeffectiveness of the shape change. Noting that some of thesevalues will tend to degrade over time, we repeated these measurements over the course of several days to determine thelongevity of the FMSi shape memory effect. The following
data was collected from several sheets of FMSi which were
heated in boiling water and axially stretched to 100% strain,
then cooled in place on a fixture as in the previous section.
Before removal from the fixture, several coupons of material
140mm in length and 5mm wide were cut from these sheets
and used as the specimens in the tests below.
The primary measure of a shape memory is known as
Shape Fixity, which indicates the extent to which the temporary shape is retained. Shape fixity is quantified by theratio Rf = ε/εload · 100%, for the current strain ε, and theapplied strain εload . Upon being cut out from the fixture,
the material specimens experienced some amount of immediate strain relaxation to a length of approximately 120mm,

elongated needle-like formation, as seen in the microscope
images in Fig. 1. If the strained composite is held in thestrained state and cooled, the deformed FM particles willsolidify in place and resist the recovery strain of the matrix
upon release from the fixture. This ability to “stretch andhold” a new configuration is referred to as “shape memory”
behavior.
We characterized the effect of pre-strain angle (0◦ , 45◦ ,90◦ ) and amount of pre-strain (25% and 50%) on thepull-to-failure behavior of FMSi. A pre-strain angle of 0◦
corresponded to the FM “needles” being aligned with thedirection of tension; 90◦ corresponded to the FM “needles”
being perpendicular to the direction of tension. To create
samples for these tests, first, films of FMSi were cast. Fabricstrips were adhered to the ends, leaving a loop throughwhich rods were threaded. The films were heated in boilingwater, stretched to the appropriate amount of strain using the261

ratio Rr = εload − εrecovered · 100% where εrecoveredis the remaining strain in the specimen after the recovery
transition has occurred. Given the initial nominal strainof 100% and coupon length of 140mm, we calculate thatthe recovered specimen length should be approximately
70mm. After holding the strained shape for approximately
15 minutes, some samples were heated and allowed to relax
freely toward their initial shape. In this case, specimens were
measured to have recovered 100.7% of their strain, meaning
specimens measured slightly shorter than expected (average
length of 69.49mm). Again, the longer the wait time before
triggering shape recovery, the less complete that recovery
became (Fig. 4B). After about 30 days, this recovery loss
accelerates to some degree, possibly indicating permanent
creep of the silicone polymer.
A third, interesting feature of shape memory is that of the
Recovery Stress, which is a measure of the force with whicha material is able to recover its shape. Recovery stress canbe measured by holding a specimen in an isostrain fixture
and measuring the strain recovery forces as the material ismade to transition. This test was performed in a dynamic
mechanical analyzer (DMA 850, TA Instruments) in a film
tension clamp. Specimens were clamped in the fixture witha torque of 1in-lbs, and heated from 25◦ C to 55◦ C at a
rate of 5◦ C/min, at 3◦ C/min between 55◦ C and 65◦ C, andagain at 5◦ C/min to a maximum temperature of 100◦ C. The
resulting blocked forces, after normalization by specimen
cross-section into recovery stresses, also appear to follow a
logarithmic trend, plateauing after the 30 day mark (Fig. 4C).
III. A PPLICATIONS
We demonstrated the shape memory behavior of FMSi material in several applications. All applications have associatedvideo clips (single Supplemental Video file).
A. Variable impedance control of linkage
Noting the change in elastic modulus that can be achieved
both by temperature change and by material prestrain, we
demonstrate the use of a bar of FMSi as a variable ratespring. The spring is attached to the arm of a pinned linkage,thereby creating a joint with variable impedance which could
be used to constrain its motion or manipulate its dynamic
response in response to changing loads. We demonstrate
three unique cases: an extension spring, a contracting storedenergy actuator, and a combination of the two.
First, the spring is sized to a nominal length of
L0 = 60mm. When attached to the linkage, the weightof the arm will stretch the spring to a length of ≈65mm,
causing it to hang horizontally and the spring to be oriented
at a 45◦ angle. A 50g weight is loaded onto the arm, causinga small deflection as the spring extends further. The springis then heated, which allows the arm to drop significantly
further under the load. Allowing the spring to cool in thisposition locks in a new spring constant that will deflect to thesame position when loaded with the same weight (Fig. 5A).
Second, the spring is reheated and stretched to
L0 = 120mm, a strain of 100%, and cooled in place. After

Fig. 4.
Shape memory behavior of FMSi and the change over time.
Specimens were heated, stretched to 100% strain, cooled in place, andthen set aside for the indicated number of days. Error bars show one
standard deviation. (A) Shape fixity behavior. 100% shape fixity indicatesa specimen remained at its programmed strain without any relaxation. (B)Shape recovery behavior. 100% shape recovery indicates a specimen fully
relaxed into its initial unstrained length after heating. (C) Recovery stressof FMSi specimens when heated to 100◦ C in an isostrain clamp.

resulting in a nominal shape fixity of Rf = 85.27%. We
hypothesize that this immediate strain relaxation arises fromcontraction of silicone. Over time, the material did continue
to experience a small additional amount of shape loss in aroughly logarithmic manner, which eventually plateaued toapproximately Rf = 81.98% after about 30 days (Fig. 4A).
Shape Recovery is a measure of how fully the material
returns to its original shape after holding the temporary
shape for some time. Shape Recovery is defined by the262

Fig. 6. The inflation trajectory of a bladder with opposing FMSi and neatsilicone faces can be controlled by varying the prestrain in the FMSi face.
The inflation pressure is 10kPa in all cases. (A) When the FMSi face holds
a high prestrain, the strain mismatch causes the bladder to curl. Inflation
favors curvature toward the neat silicone layer. (B) A lower prestrain in the
FMSi allows the bladder to inflate to a straight shape where the stress is evenbetween the two faces. (C) Zero prestrain in the FMSi causes the bladder
to inflate with curvature toward the FMSi face, as it acts as a strain-limiting
layer.
Fig. 5. FMSi used as a variable-impedance spring on a linkage joint. L0 is
the nominal spring length before attachment to the linkage. (A) Acting as anextension spring, the FMSi will change stiffness when heated. Under a 50g
load, the linkage will deflect to different points depending on the state of the
FMSi. (B) Acting as a contracting actuator, the prestrained FMSi is able tolift a 10g load when heated and allowed to relax toward its unstrained shape.
(C) The prestrained FMSi is shortened, allowing it to function as a stiffer
extension spring under a 50g load. However, when heated and unloaded,
the FMSi will relax and actuate.

B. Soft body trajectory control
FMSi’s ability to memorize a strained position can also be
used to for soft body trajectory control. Inspired by publishedworks which utilize strain-limiting lamina to direct motion[33], we use an inflatable bladder formed from a sheet ofneat Dragonskin 10 silicone and a sheet of FMSi.
In the first case, we heat the FMSi face and apply a large(≈100%) strain to the bladder which is then cooled in place.
Upon release, the strain mismatch between the two faces
causes the bladder to curl up into a tight roll. When inflated,the bladder unrolls and achieves a curved shape, bending
toward the neat silicone face (Fig. 6A).
If the bladder is reheated while remaining inflated, the
FMSi will relax further until the stress between the two faces
is equal, and the bladder will straighten. When the FMSi iscooled at this strain, the bladder can be deflated and reinflated
to the same straightened position (Fig. 6B).
When the bladder is deflated and reheated a final time,
the FMSi will fully relax, and both faces will be unstrained.
When cooled again, the FMSi will act as a strain limiting
layer due to its higher stiffness compared to neat silicone.
Inflating will now cause the bladder to curve in the opposite
direction, toward the FMSi face (Fig. 6C).

attaching to the linkage, a 10g weight is loaded onto thearm, causing a small deflection. The spring is then heated,but rather than deflect further, the FMSi acts as a contracting
spring and is able to lift the weight. When cooled in thisposition, the spring contracts further after removal of theweight (Fig. 5B).
Third, the spring is again reheated and stretched to a strainof 100% and cooled in place. The clips holding the spring are
adjusted such that the spring is once again the original length(L0 = 60mm). When attached to the linkage, the higher
spring rate caused by the prestrain prevents the arm fromhanging horizontally under its own weight. A 50g weight isloaded onto the arm, causing a small deflection as the spring
extends. When heated, the spring experiences negligible
deflection, as it is already highly strained. Removing the loadcauses the beam to lift as the spring contracts (Fig. 5C).
263

Fig. 7. Detailed topography recording via vacuum forming around a 3D object. (A) The chosen 3D object is placed on the vacuum forming platform.
(B) A film of FMSi is secured over the object with a frame bolted into the platform. (C) After heating the film, a vacuum is applied. (D) The film forms
tightly about the object, and is allowed to cool in place, memorizing the new shape. (E) The film and object are removed from the platform. The FMSi
film retains its shape without requiring any support from the object. (F) Details on the inner surface of the film can be examined. (G) As heat is once
again applied, the Field’s metal in the film melts. (H) The film returns to its original fully flat shape, read to be reused on a new object. (I-J) The FMSi
film is able to record highly detailed features. The square film has an edge length of 6in (15.24cm).

of molds of objects without expending large volumes ofmaterial which must then be cured. However, the generalprinciple of localized strain memorization opens up possible
uses in highly specialized trajectory control, complex ondemand spring design, or multi-directional stored-energy
actuation in a single material sheet.

C. Topography recording
Beyond memorizing simple unidirectional strains, FMSi
is capable of recording the topography of highly detailed
shapes with precise, localized strains distributed throughout
the material. This can be clearly demonstrated by using FMSi
to assume a complex 3D shape by vacuum forming it arounda chosen object.
First the 3D object to be recorded (in this case, a toy car) is
placed on a vacuum forming platform, where a film of FMSi
is then stretched over and secured in place using an acrylic
frame (Fig. 7A-B). The film is then heated thoroughly. With
the Field’s metal fully melted, a vacuum is pulled to conformthe FMSi film to the surface of the object (Fig. 7C-D). Withthe vacuum pressure maintained, the FMSi is then allowed
to cool thereby “memorizing” the surface contours. Oncecooled, the frame can be separated and the 3D object isremoved from the FMSi. The FMSi film remains highly
flexible and stretchable, but the dense structural detail ofthe memorized FMSi shape allows it to stand under its own
weight (Fig. 7E-F). The film can then be heated again to reset
it to a flat configuration (Fig. 7G-H), where it can be reused
again. The fine features of the toy car were recorded at ahigh fidelity, with intricate details on the order of 0.5-1mm
able to be captured by the FMSi film (Fig. 7I-J).
The particular application of reusable, stretchable, flexible
vacuum forming sheets allows for extremely fast creation

IV. C ONCLUSIONS
Field’s metal particles embedded in silicone yield astretchable composite with shape memory capabilities. The
relatively low-temperature solid-liquid melting transition of
the Field’s metal is used to reversibly record high-strain
deformations. Straining the material to varying levels or indifferent orientations results in the creation of anisotropic
mechanical properties which can then be locked in. The
composite is able to retain a memorized shape and recoverits initial shape with little change in performance evenafter months. By varying the order of heating, applying
loads, and cooling, a single specimen can leverage its shapememory capability to serve a variety of functions. We show
applications where the specimen can act in separate instances
as an extension or a compression spring, and dictate thetrajectory of an inflated soft body. Additionally, the material
can be used to capture topographies, a form of on-demand
shape reconfiguration.
We suspect that the on-demand switching of material behavior has applications in other areas as well, from adjustable264

damping buffers, to programmable inflatable patterns forpneumatic actuation. This material may even be applicableto wearables and rehabilitative equipment, where stretch
and flexibility is paramount, but localized regions of highstiffness can provide needed joint support.
To maximize impact and utility, future work aims tointegrate FMSi with similarly stretchable, onboard heating
elements. Conductive composites using graphite [8] or liquid
metal [10], [34] are candidate materials that could work as
addressable heaters to locally melt Field’s metal particles,
and may even make use of the natural conductivity of FM to
Joule heat FMSi directly. Future work should also address
the cyclability of the material, in terms of strain cycles in an
isothermal state, and in terms of shape memory cyclability.

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ACKNOWLEDGEMENTS
TLB was supported by the US Air Force Office of Scientific Research (FA9550-16-1-0267). MCY was supported by NSF
EFRI award EFMA-1830870. The DMA system was funded by
an AFOSR Defense University Research Instrumentation Program(FA9550-19-1-0221).

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