Full Paper
www.afm-journal.deEnhanced Variable Stiffness and Variable Stretchability
Enabled by Phase-Changing Particulate Additives
Trevor L. Buckner, Michelle C. Yuen, Sang Yup Kim, and Rebecca Kramer-Bottiglio*
structure holds a particular orientation in
space or rigidly supports a load, and later
becomes deformable on demand to allow
shape change or repositioning, as often
used in medical endoscope design.[3] Bothuse-cases of variable-stiffness can be madepossible through a variety of methods,
including the use of opposing actuators
to “lock” a joint in a specific position,[4,5]granular and layer jamming to hold anappendage in shape (often by applying
vacuum),[6–9] inflating a bladder to stiffen
a joint,[10,11] and manipulating magnetorheological fluid that stiffens in the presence of a magnetic field.[12–15]
Thermally responsive materials thatsoften with exposure to heat can also be
used as the basis of a variable-stiffness
structure.[16–20] After softening, the material can be plastically deformed into a new
configuration which will be held passively(i.e., without external forces) when cooled
and stiffened. An effective thermally
responsive variable-stiffness material will
exhibit a large change in stiffness over a
working temperature range so as to minimize energy expenditure, transition time,
and thermal damage to other system components. Thermallyinduced material softening occurs primarily under two phenomena: melting and glass transition. Melting a material naturally results in a very large change in stiffness as the materialchanges from a solid to a liquid, but necessitates a containing
medium to prevent leakage.[21] Continuous, encapsulated blocks
or networks of melting polymers,[22] waxes,[23] and metals[24–26]
have all been used in variable-stiffness systems, reaching
stiffness change ratios of 100–9000×, depending on the encapsulating material and method. However, melting can also resultin discontinuous material segments as it flows apart, requiring
an external force to rejoin them.[27] In contrast, materials undergoing a glass transition remain continuous, but experience acomparatively limited stiffness change, generally 5–100×.[16–19]
Upon heating through the glass transition temperature (Tg),polymer chains increase in energy, becoming more mobile, andwill thus readily slide past one another when subjected to force,
resulting in a rubber-like behavior, while remaining solid.
In pursuing the desired properties of a stiffness-changing
material, one might turn to composite material design. A composite material benefits from the combined properties of two or
more component materials. The choice of components determines the behavior of the composite, and the wide variety of

A novel phase-changing particulate that amplifies a composite’s modulus
change in response to thermal stimulus is introduced. This particulate
additive consists of a low melting point alloy (Field’s metal; FM) formed
into microparticles using a facile fabrication method, which enables its
incorporation into polymer matrices using simple composite manufacturing
processes. The effect of the solid–liquid phase change of the FM particles
is demonstrated in two host materials: a thermally responsive epoxy anda silicone elastomer. In the epoxy matrix, this thermal response manifests
as an amplified change in flexural modulus when heated, which is highly
desirable for stiffness-changing move-and-hold applications. In the silicone
matrix, the stretchability can be switched depending on the phase of the
FM particles. This phenomenon allows the silicone to stretch and holda strained configuration, and gives rise to mechanically programmableanisotropy through reshaping of the FM inclusions. FM particles present
many opportunities where on-demand tunable modulus is required, and isparticularly relevant to soft robotics. Because the melting temperature of FM
is near room temperature, triggering the phase change requires low power
consumption. The utility of FM particle-containing composites as variable
stiffness and variable stretchability elements for soft robotic applications isdemonstrated.

1. Introduction
Mechanical structures that demonstrate a change in stiffness are sought after in a variety of applications.[1] Such astructure might simply change its mechanical impedance
to influence system dynamics, as seen in the human bodywhen co-contraction of antagonistic muscles can increase a
joint’s resistance to motion.[2] More frequently in the fieldof soft robotics, the use-case of variable-stiffness involves“move-and-hold” operations for which a variable-stiffness
T. L. Buckner, M. C. Yuen, Dr. S. Y. Kim, Prof. R. Kramer-Bottiglio
School of Engineering and Applied Science
Yale University
New Haven, CT 06520, USA
E-mail: rebecca.kramer@yale.edu
Dr. M. C. Yuen
School of Mechanical Engineering
Purdue University
West Lafayette, IN 47907, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201903368.

DOI: 10.1002/adfm.201903368

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www.afm-journal.deFigure 1. Field’s metal/polymer composites. Field’s metal is made into particles which can be mixed into polymers for variable modulus materials.
A) Mixing FM particles into epoxy (right column) yields a material that exhibits an extended range of stiffness (as compared to neat epoxy, left column)
when heated. At elevated temperatures, both specimens are able to be deformed to similar curvatures (top). Yet, at ambient temperatures, the FM/
epoxy composite is stiffer than the neat epoxy and thus deflects less than the neat epoxy specimen (bottom). B) Mixing FM particles into silicone yields
a “variable stretchability” material that is able to hold a deformed (stretched or compressed) state. Sequence starts in top left panel; the red arrows
indicate heating phases; the blue arrow indicates a cooling phase.

matrix and filler materials available allow for tuning of specificmaterial properties. Briefly, a filler additive might be chosen
to increase mechanical strength, raise the elastic modulus,
enhance thermal conductivity, or simply reduce product costby using an inexpensive extender filler.[28–30] A filler material might also add a new property to an inert host material,
such as endowing rubbers with electrical conductivity[31,32] orthermal conductivity,[33] or adding self-healing functionality toa polymer.[34] However, while composite filler materials can be
specially chosen to alter a wide range of material properties,
the ability to tune the range of stiffness change that a material
experiences when subjected to heat is not a typical applicationof conventional fillers.
In this paper, we showcase a particulate additive whichconstitutes a new class of composite filler that is designed toundergo a repeatable, solid–liquid phase change during operation. To achieve this behavior, we selected microscale Field’s
metal (FM) particles as the additive material. Field’s metal isa eutectic, fusible alloy of bismuth, indium, and tin with thefollowing percentages by weight: 32.5% Bi, 51% In, 16.5% Sn.
Field’s metal melts at ≈62.5 °C, and can thus be melted easily
in hot water. With a flexural modulus of ≈10 GPa (Figure S1,
Supporting Information), FM is capable of a drastic, rapidchange in stiffness at fairly low temperatures via the solid–
liquid transition. We suspect that this class of phase-changing
additive could be expanded in the future to include other similarmaterials such as other fusible metallic alloys (Cerrolow 117,
Wood’s metal) or possibly waxes and other polymers.[22–26,35]
Given the ease of rapid phase change, Field’s metal particles
act as a unique composite additive that substantially expand therange of modulus change of the composite material, althoughAdv. Funct. Mater. 2019, 29, 1903368

this behavior manifests differently depending on the matrix
material. To demonstrate this effect, we present a series of composites that make use of microscale FM particles and explore
their influence on material behavior, namely the change in stiffness by applying heat (Figure 1). We demonstrate how mixing
FM particles in an epoxy matrix creates a material that exhibits
variable stiffness behavior, as it transitions from a rigid to acompliant state through both the glass transition of the polymeric matrix and the melting of FM particles. We then demonstrate how the use of a stretchable silicone as the matrix
yields a composite that is stretchable at room temperature,
yet becomes much more stretchable when the FM particles
are melted, a behavior that we will refer to as variable stretchability. FM is also highly electrically conductive, which allows
it to be used as an electronic pathway or even as a solder.[36]We briefly characterize this property and its synergy with thevariable-stiffness feature.

2. Results
The FM particles were fabricated by vigorously disrupting
liquid-phase bulk Field’s metal in hot water. A homogenizer
was used to shear the bulk metal into smaller particles while
simultaneously, an overhead mixer created a larger vortex toensure all contents passed through the homogenizer. Thismethod is similar to previously presented work for creating
microparticles of low-melting-point materials[35,37] but allows
processing of much larger volumes of material (60 g FM per
batch vs 1.1 g[37]). The resulting particles were approximately
spherical in shape and ranged in size from 10 to 800 µm in

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www.afm-journal.deFigure 2. Morphological characteristics of field’s metal particles. (Top)SEM image of Field’s metal particles. (Bottom) Number and volume fraction distribution of Field’s metal particles.

diameter (Figure 2; Figure S2, Supporting Information). Dueto the high degree of nonuniformity in particle morphology inlarger particles, only particles with diameter less than 355 µm
were used in composite manufacture. The particle sizes had
an approximately lognormal distribution with most (≈90%)
particles by count being smaller than 100 µm in diameter.
However, the volume of these smaller particles was negligible;
larger diameter particles (>100 µm) comprised ≈96% of thetotal volume.
We mixed the FM particles at a range of volume fractions
into two matrix materials to create an epoxy-based composite(FMEpoxy) and a silicone elastomer-based composite (FMSi)(Figure 1A,B, respectively). In order to gauge the modulus
change offered by the FM particles, we characterized the effectof FM volume fraction on the stiffness of the composite overa temperature sweep using a dynamic mechanical analyzer(DMA).

2.1. Epoxy-Based Composite (FMEpoxy)
The epoxy we selected is a thermoset polymer that transitions
from a stiff glassy state to a flexible rubbery state at Tg ≈ 35 °C,
making it useful for applications that require on-demandchanges in stiffness. By adding increasing amounts of FM particles, the variable stiffness nature of the composite materialAdv. Funct. Mater. 2019, 29, 1903368

is amplified further. As the composite material is heated,
two distinct transitions are apparent: the glass transition of
the epoxy, followed by the melting of the Field’s metal inclusions (Figure 3A; Figures S3 and S4, Supporting Information). At high particle-loading (55 vol%), the stiffness of theepoxy in its glassy state (T < Tg) is amplified more than two-fold,relative to the neat (unmodified) epoxy (Figure 3B,D). Whilethe composite is in the rubbery state but prior to melting of
the Field’s metal (Tg < T < Tm), increased Field’s metal loading
yields a higher stiffness. After melting (T > Tm), the stiffnessof the composite is consistently low across all loading percentages, dropping slightly lower with larger volumes of the melted
liquid metal. This result shows that incorporation of Field’s
metal simultaneously increases the rigid (cold) stiffness of thecomposite and reduces the lower (hot) stiffness bound whenmelted, thereby extending the total range of stiffness traversedby the composite material through a temperature sweep. This
represents a stark increase in the rigid-to-soft modulus ratio
from ≈400× to over ≈1500× (Figure 3C,D), and we compare
this performance to the representative values of prominent
examples of different types of variable stiffness mechanisms(Figure 4). Our results are in agreement with the theoreticalresults expected from the Halpin–Tsai composite model,[38]except at very high additive loading percentages, where the
manufacturing process becomes inconsistent, as detailed further in Figure S5 (Supporting Information).
Often, it is desirable to Joule heat thermally responsive
variable-stiffness materials via direct application of electricalcurrent. This can be accomplished by making the material
electrically conductive through the introduction of conductive
particles, such as graphite,[39] carbon black,[40] or solid metal
powder.[31] However, these additives also have the side effectof increasing both the upper and lower stiffness bounds of thecomposite (Figure 3E). Alternatively, conductive liquid metal
can be used,[18] although this has the opposite effect of limiting
the upper stiffness bound due to the presence of small pockets
of liquid throughout the composite.
To overcome the tradeoff between increased electrical conductivity and a narrowed modulus window, we investigatedas a secondary characteristic the effect of the FM additive onelectrical conductivity of the epoxy composite (Figure 5). On
its own, Field’s metal is highly conductive, with a resistivity of
5.2 × 10−7 Ω m, or about 31 × that of copper.[41] However, until
a certain volume fraction of conductive filler has been added,
known as the percolation threshold, no bulk conductivity will
be observed as the chances of individual particles coming incontact to form a continuous circuit are rare. The size andgeometry of the particles greatly influence what this threshold
might be. Spherical particles contribute to a very high percolation threshold, since all of the particulate mass is concentrated into isolated spheres of low aspect ratio. For this reason,even at up to 60% by volume, the FMEpoxy composite was notconductive. However, upon adding small amounts of carbon
black (CB), which is also conductive and has a high surface
area and branching, amorphous geometry, the small particles
of Field’s metal can be linked together, resulting in a conductive composite material that can be softened via Joule heating.
By fixing the amount of carbon black at 3 vol% and gradually
increasing the concentration of Field’s metal, we observed

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www.afm-journal.deFigure 3. Stiffness measurements of Field’s metal/epoxy composite material (FMEpoxy). A) Flexural modulus of the epoxy-based composites as atemperature sweep is applied by the DMA, passing through the epoxy glass transition (Tg) and Field’s metal melting point (Tm). Composites containvarying volume percentages of Field’s metal and carbon black. B) Flexural modulus of the epoxy-based composites at 23 °C (i.e., initial/cold stiffness)
as a function of FM and CB volume fraction. C) Hot ( 72 °C) − cold ( 23 °C) flexural modulus ratio of the epoxy-based composites as a function of
FM and CB volume fraction. D–F) The data from (B) and (C) reproduced as 2D scatterplots. D) As larger volumes of Field’s metal are added to thecomposite, both the cold stiffness and ratio of stiffness change increase. E) With the inclusion of some amount of carbon black, increasing volumesof Field’s metal continue to raise the cold stiffness. However, the stiffness ratio does not increase as dramatically due to the stiffening effect of carbon
black even when heated. F) Further increasing the volume of carbon black does not seem to improve stiffness-change properties in any way. The plotsshow the mean response across three specimens of each composition; error bars indicate one standard deviation. Full datasets can be found in the
Supporting Information.

that the conductivity does continue to rise, showing that FM
particles do contribute to increased electrical conductivity of acomposite (Figure 5B). To compare, we also held the volume of
Field’s metal constant at 30 vol% and adjusted the percentage
of carbon black (Figure 5C). As expected, an increase in carbon
black also leads to an increased electrical conductivity.
We note that for both FM and carbon black additives, theaddition of large amounts of filler (above 50 vol% FM, or above
2 vol% CB) gives rise to manufacturing difficulties that result
in defects (Figure 3F). In those cases, the composite slurry
becomes thick and paste-like, resulting in trapped air pockets
and an inability for the mixture to flow smoothly into its mold.
We hypothesize that the resulting voids are the reason for theunexpected deviations from predicted behavior at high loading
percentages. Carbon black in particular appears to contributeto significantly higher porosity in specimens even at very low
volume percentages, resulting in a cold state that is softer than
expected, while still raising the stiffness of the warm state dueto the stiffening effect of CB (Figure S4, Supporting Information). These effects partially counteract the stiffness-changeenhancements granted by the FM particles; further research
may reveal a more compatible conductive material that synergistically lowers the percolation threshold for FM particles withfewer negative effects. Additionally, FM particles with a higher
aspect ratio could have more favorable effects on conductivity.
To simulate the deformations expected during useof these materials, we compared the deflection ofAdv. Funct. Mater. 2019, 29, 1903368

FMEpoxy cantilever samples (neat epoxy, 30%FM 0%CB,
30%FM 3%CB) (Figure 6A; Figures S6 and S7, Supporting
Information) of size 80 mm × 10 mm × 3.7 mm. At room
temperature, the neat epoxy clearly deflected the most under a
200 g load, followed by the 30FM 3CB, and lastly the 30FM 0CB
sample when subjected to the same loading conditions. Thoughthe 30FM 3CB is expected to be stiffer than the 30FM 0CB due
to the stiffening effect of carbon black particles, the above-mentioned manufacturing difficulties with CB resulted in a more
flexible composite due to a higher concentration of voids. In
the soft state, all samples collapse under only 50 g, exhibitingnearly the same deflection. This demonstration shows that theaddition of Field’s metal particles clearly enhances the stiffness
when cold without adversely increasing the stiffness in theheated state.
An application of the FMEpoxy in a soft robotic actuator isshown in Figure 6B and in Movie S1 (Supporting Information).
Silicone-based Pneunet actuators are designed to bend when
inflated; removing the inflation pressure allows the stretchedsilicone to relax back into a flat configuration. By affixing a thinlayer of FMEpoxy to the strain-limiting layer, the actuator canperform move-and-hold operations. Softening the FMEpoxy
via Joule heating (or otherwise) allows the actuator to bend
when inflated. After cooling, the new plastically deformed
configuration will be maintained even after the actuator isdeflated. Furthermore, the FMEpoxy in the stiff state resistsbending when the actuator is inflated.

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progressively stiffens the composite while the particles remain
as solid-phase. However, in contrast to the FMEpoxy, the liquidphase FM inclusions also contributed to a minor increase in thecomposites’ stiffness, relative to neat silicone. We hypothesizethat this may be due to a combination of two factors: 1) We suspect that the majority of this stiffness occurs due to regions ofphase-separated metal that fail to melt at or near the eutectic
point.[37] Formation of a thin solid oxide shell on the droplet
surface and oxide fragments that remain solid at elevatedtemperatures may also have played a significant role in thisstiffening behavior.[36] 2) To a lesser extent, small liquid dropletsγ−1D ≤  with sufficient surface tension (γ ≈ 387.17 mN m ,E

Figure 4. Comparison of stiffness-change performance to representative
results in the literature. The FMEpoxy composite shows high stiffnesschange performance when compared to other variable-stiffness methods.
The range of stiffness change is orders of magnitude above most techniques, second only to methods that involve melting continuous blocks ofmetal. Because of this, the FMEpoxy can become softer than natural latex
rubber, which can be deformed even by low-force actuators. On the other
hand, the stiffened composite is similar to rigid acrylic, which can act asa lightweight, yet sturdy structural element able to withstand a great dealof force. The variable stiffness methods in this figure, in order, include
melting techniques: solid blocks of FM in a polydimethylsiloxane (PDMS)
skin,[24] FM in silicone tubing,[27] polycaprolactone (PCL) polymer insilicone tubing;[22] this work: neat SMP epoxy, epoxy with 30 vol% FM
loading, and epoxy with 55 vol% FM loading; glass transition techniques:
Acrylonitrile butadiene styrene (ABS) polymer,[17] polylactic acid (PLA)
polymer,[45] polyethylene (PE) copolymer;[18] jamming techniques: tendon
jamming,[46] layer jamming,[47] particle jamming;[48] and some other types:
flexing of natural skeletal muscle in a living human,[40] high internal pressure in a tube,[49] and magnetorheological elastomers.[50]2.2. Silicone-Based Composite (FMSi)
Because silicone elastomers have a Tg far below our working
temperature range, the FMSi exhibited a single transition—
the melting of the FM particles (Figure 7A; Figure S8, Supporting Information). Predictably, the addition of FM particles

Adv. Funct. Mater. 2019, 29, 1903368

Figure S9, Supporting Information) in sufficiently soft solids(Young’s modulus, E ≈ 100–200 kPa) are known to behave
more rigidly than predicted by the rule of mixtures and theHalpin–Tsai theory regarding stiffness of composite materials(Figure S5, Supporting Information).[33,38,42] At these small
length scales, the surface tension of the liquid-phase FM inclusions resists the imposed deformation of the host matrix,
effectively increasing the stiffness of the composite. However,from the particle distribution found previously, particles smaller
than this characteristic length scale (D ⩽ 2 µm) comprise only
≈7% of all particles (by count), which contributes only marginally to this phenomenon.
In addition to the stiffness measurements of the as-prepared samples over a temperature sweep, we tested samples
that were subjected to two modes of deformation, compression and tension, each along two axes (Figure 7B, Supporting
Information). The stiffnesses of the samples were measured at
25 °C prior to any deformation, while in the deformed state,
and after recovery to the original shape. To deform the samples,
we heated the FMSi in hot water to melt the FM, deformed thesample (20% compression strain or 50% extension strain), andthen quenched the FMSi in cold water while holding it in its
deformed state until the Field’s metal resolidified. To evaluate
if any permanent damage occurred as a result of deformation,
samples were returned to their original shape by melting the
FM in hot water once again and letting the silicone matrix
pull itself back into its original shape. We found that the
deformation processes had minimal effect on the stiffness ofthe composite; the stretched samples exhibited a slightly lower
stiffness after recovering its original shape.
The samples showed clear changes in stiffness while in the
deformed, cooled state. Compressing the sample narrower (presumably forming vertical FM “pancakes”) and stretching thesample longer (forming FM “needles” along the length, seeMovie S2 in the Supporting Information) resulted in a significant increase in stiffness. These vertical FM “pancakes” and thelengthwise “needles” would be aligned with the compression andtension stresses applied by the DMA during the bending tests
and thus would be able to take more of the load, resulting in ahigher stiffness. Compressing the sample flatter (forming horizontal FM “pancakes”) also resulted in stiffening due to FM
inclusions taking on a similar role to that of the lengthwise “needles,” but to a lesser extent. Lastly, stretching the sample wider(forming FM “needles” parallel to the neutral axis of the crosssection) reduced the vertical height of the FM inclusions andresulted in a slight decrease in stiffness. These results indicate

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www.afm-journal.deFigure 5. Electrical conductivity measurements of Field’s metal/epoxy composite material (FMEpoxy). A) Electrical conductivity of the epoxy-based
composites as volume percentages of Field’s metal and carbon black are varied. On its own, any amount of Field’s metal does not result in a conductive
composite. B,C) The data from A) reproduced as 2D scatterplots. B) With the inclusion of some amount of carbon black, increasing volumes of Field’s
metal continue to lower the electrical resistivity of the composite. C) Further increasing the volume of carbon black continues to lower the resistivity of
the composite. The plots show the mean response across three specimens of each composition; error bars indicate one standard deviation.

that cooling the FMSi composite in a deformed state creates amaterial with anisotropic mechanical properties. Moreover, thisdeformation is fully reversible and the original as-cast shape of
the FMSi can be fully recovered by applying heat to allow theFM inclusions to return to their roughly spherical shape.
To demonstrate the stretch-and-hold capability of the silicone-based composite, we first heated and stretched a specimenby hanging a weight (Figure 1B; Figure S10, Supporting Information). We then let the FM particles solidify and removed theweight, showing that the sample stayed stretched, holding 87%
of its elongation. Finally, we reheated the sample, allowing it
to recover its original shape. Additionally, we found that the
spring stiffness of the silicone-based composite decreased by
5.2× when the FM particles were melted, as compared to thestiffness when the FM particles were solid in an undeformed
state (Figure S10, Supporting Information). The results shown
here closely parallel those shown by van Meerbeek et al.
wherein an open-cell silicone foam was infused with Field’s

metal to perform similar stretch-and-hold operations.[21] However, our FMSi composite remains soft, stretchable, and flexible
throughout its operating temperature range, rather thanswitching between a stiff, rigid state (i.e., a porous metal element) and a soft, compliant state (i.e., a porous silicone foam).
The persistent compliance of the FMSi is especially advantageous for integration into soft robotic applications, as demonstrated below. Additionally, the particulate composite nature of
the FMSi and its ability to hold deformed shapes via deformation of FM particles is similar to the work presented by Changet al., however here, we focus on leveraging the repeatablephase-change transition of the FM particles.[43]We used the FMSi to create an on-demand pop-up tactile
display, a bidirectional pneumatic gripper, and a multitrajectorypneumatic actuator (Figure 8). In the pop-up tactile display, flat
bladders were first patterned into a silicone film, and a film of
the FMSi composite was bonded to the top (Figure 8A; Movie S3,
Supporting Information). The FMSi was heated and the

Figure 6. Applications of Field’s metal/epoxy composite material (FMEpoxy). A) Demonstration of the change in stiffness when the epoxy-basedmaterials are heated from 20 °C (top row) to 70 °C (bottom row). The composites (middle and right columns) show a stiffer (less deflected) cold statecompared to the neat epoxy (left column; outlined for clarity). After heating, the self-weight of the composites causes them to droop more than theneat epoxy. When loaded, it is apparent that the composites are as soft as the neat epoxy. B) Application of the FMEpoxy material in a pneumaticallyactuated bending actuator for a move-and-hold operation. Red arrows indicate heating; blue arrows indicate cooling phases. When the FMEpoxy ishot, the bending actuator is able to inflate and curl (top right vs bottom left). After the FMEpoxy cools while in the bent state, the actuator holds its
bent configuration even with the inflation pressure removed (bottom right).

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www.afm-journal.deFigure 7. Stiffness measurements of Field’s metal/silicone composite material (FMSi). A) Flexural modulus (mean ± standard deviation) of the silicone-based composite as measured in the DMA as a temperature ramp is applied. The plots show the mean response across three specimens of eachcomposition; the error clouds indicate one standard deviation. Full datasets can be found in the Supporting Information. The same single-cantileverloading condition was used across all DMA characterization experiments. B) Cold stiffnesses of the silicone-based composite prior to deformation(initial; gray bar), after deforming and cooling in place (darker colored bar), and after heating to recover its original shape (lighter colored bar). The
schematics show the mode and direction of deformation applied and the resulting morphology (‘pancake’, ‘needle’) of an initially spherical Field’s
metal particle. The direction of deformation is drawn with respect to the same coordinate frame as the load condition schematic in (A).

bladders were inflated, stretching the FMSi. While maintaining
the inflation, the FMSi film was allowed to cool to capture the
pop-up bubble shape. After the air was removed, the FMSi
film retained the inflated shape, creating a potential interactive
display on demand. Upon heating the film again, the display
returned to a fully flat state.
For the second application of the FMSi composite, we fabricated a Pneunet powered soft gripper[44] out of a soft silicone
material with the FMSi as a stiffer layer on the bottom (Figure 8B;
Movie S4, Supporting Information). Upon inflation, the soft
silicone stretched more than the stiffer FMSi, causing thegripper to close downward. While maintaining the inflation,
the gripper was placed in hot water, melting the Field’s metal
particles. This resulted in the bottom FMSi layer becoming
even softer than the silicone material, and for the gripper toflip and close in the other direction with the FMSi on the outerstretched layer. The gripper was then submerged in cold waterto freeze the FM particles in the flipped gripper configuration.
After removing the inflation, the gripper stayed in the closed
configuration because the FMSi was cooled in a stretched state.
Upon reimmersion in hot water, the FM melted, allowing thegripper to reset itself into a flat configuration.
Lastly, we coated a flat pneumatic bladder with FMSi on
either side (Figure 8C). We selectively heated up sides or regionsof the FMSi and then inflated the bladder. When one FMSi sideof the bladder was heated, the actuator displayed a curved configuration because heated regions were able to stretch morethan the cool regions. When both sides were heated, the actuator simply elongated. Finally, when only a portion of the FMSi
was heated, the actuator inflated into a partially curved state.
These simple demonstrations illustrate the potential of using
FMSi as a method to control the motion of soft, inflated structures using targeted heating.

3. Discussion and Conclusions
Field’s metal is a promising phase-changing material for tuning
the modulus of structures because its melting temperature isAdv. Funct. Mater. 2019, 29, 1903368

so close to room temperature. However, processing and patterning bulk Field’s metal blocks or networks can be difficultcompared to processing of polymer-based composites. The
manufacturing method to create Field’s metal particles at
unprecedented volumes thus extends the potential impact
and utilization of Field’s metal because now it can be used asa functional inclusion in a composite material system. We are
motivated by the field of soft robotics and thus are particularly interested in variable stiffness materials. By mixing the
Field’s metal particles into specific polymers, namely epoxy andsilicone, we can leverage each material’s functional properties
and extract even more use from the composite than from theconstituent materials separately.
Field’s metal particles as inclusions in polymer-based
composites yield enhanced stiffness change when coupled withglass-transition polymers and endow elastomers with variable
stretchability properties. Here, we showed that Field’s metal
particles in an epoxy matrix increased the “cold” stiffness of thematerial by up to 3×, and unlike permanently solid inclusions,did not raise the “hot” stiffness, but instead lowered it slightly.
The addition of carbon black particles allowed the electricalconductivity of the FM to contribute to the overall electricalconductivity of the composite, allowing Joule heating to be used
to soften the epoxy and melt the Field’s metal. Silicone elastomers with embedded Field’s metal particles exhibit increased
stiffness compared to neat silicone both when the Field’s metal
is solidified and melted. We demonstrated that the particles
can be leveraged for variable stretchability and for performing
stretch-and-hold operations. Furthermore, the altered morphology of the Field’s metal particles during “stretch-and-hold”
operations results in anisotropic stiffness in the material.

4. Experimental Methods
Field’s Metal Particle Fabrication: Field’s metal particles were createdin by melting the metal in warm water and agitating vigorously. A
block of Field’s metal (Roto144F, RotoMetals) is weighed out to
60 g. 150 mL of deionized water was added to a 500 mL beaker andthe Field’s metal was placed inside. A silicone oil bath was heated on

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www.afm-journal.deFigure 8. Applications of Field’s metal/silicone composite material (FMSi). A) FMSi film (30 vol% FM) used to create pop-up displays and buttons
in a silicone membrane patterned with flat, inflatable pockets. From top to bottom, the bladders are: heated and inflated, resolidified and deflated,heated, fully recovered. B) FMSi film (30 vol% FM) integrated into a pneumatically actuated gripper for bidirectional grasping. Switching between thesolid and liquid FM particles causes the FMSi layer to alter between being stiffer than the silicone body (shown in green) and being more stretchable,
which enables control of the direction of the grasping motion. C) A flat, pneumatic bladder was coated on both sides with a layer of FMSi. Heating up
specific sides or regions of the FMSi allows for control of the bladder trajectory when inflated.
a hot plate with a magnetic stirring rod up to 90 °C, and the 500 mL
beaker was supported inside the silicone oil bath such that the waterlevel was below the surface of the oil. The water was allowed to warmfor ≈15 min until it settled at ≈70 °C and the metal was fully melted.
At this stage, a mechanical homogenizer (VDI25, VWR) with a 20 mm
diameter attachment and a vortex generator (RW16, IKA-werke) witha 42 mm diameter turbine attachment were placed into the water on
opposite sides of the beaker. The beaker was tilted slightly such that theheavy metal will flow downward toward the homogenizer rather thangathering at the edges of the beaker. At this point, the homogenizer
was turned on to a speed of 24 000 rpm and manually swept throughthe mixture for several seconds so as to initially break up the mass ofmetal which is held together by surface tension, dispersing it into thewater. The homogenizer was turned off and then fixed in place. With
both instruments in position, the homogenizer was again turned on toa speed of 24 000 rpm to further break up the melted particles, and thevortex generator was turned on to a speed of 1200 rpm, which keeps themetal in motion and prevents it from settling and agglomerating. Thisagitation was applied for 10 min.
Next, the beaker was quickly raised out of the hot silicone oil bathto cool. The vortex generator remained on to keep particles in motion
until they had cooled solid, but the homogenizer was turned off so as
not to shatter the solidified particles, creating unpredictable particle
geometries and sizes. The mixture was allowed to cool for ≈15 min
until its temperature was well below the freezing point of Field’s metal(≤50 °C), at which point the vortex generator was turned off, and the
mixture was allowed to settle. After several hours, the majority of theparticles settled to the bottom of the beaker, and the supernatant was
extracted via syringe (rather than pouring) so as not to disturb the small
particles. The remaining particle sediment was then stirred and “fluffed”
to minimize any cementing during the drying process. The particles weredried in a vacuum oven at 60 °C to remove as much of the remaining
moisture as possible. The resulting dry material may clump together likesandstone and need to be shaken apart. The resulting material had afine, sand-like appearance.
Carbon Black Preparation: Carbon black pellets (PRINTEX XE2-B,
Orion Engineered Carbons) were broken apart in a commercial blender(Hamilton Beach) by pulsing at maximum speed for 5 min. The
resulting particles are ≈5 µm in diameter as measured in a scanningelectron microscope.

Adv. Funct. Mater. 2019, 29, 1903368

Epoxy Composite Fabrication: The epoxy composite is a polydisperse
mixture of both Field’s metal and carbon black particles suspended ina two-part polymer epoxy. First, the epoxy was created by combining
an epoxy curing agent, Jeffamine D400 (Huntsman International, LLC),
with a standard epoxy resin, EPON 828 (Momentive Performance
Materials Inc.) at a ratio of 4:10 by weight. This ratio is well below the
stoichiometric ratio of 58:100, resulting in a softer material and a lower
glass transition temperature. This mixing ratio was chosen to move theepoxy glass transition temperature below the melting temperature of
Field’s metal to clearly demonstrate them as two separate transitions.
After stirring lightly by hand, the epoxy mixture was placed in a vacuum
chamber for 15 min to reduce porosity by removing trapped air. At thisstage, the desired amounts of Field’s metal and carbon black particles
were measured out by weight, then poured into a measured portionof epoxy and stirred by hand until thoroughly dispersed. The resulting
composite was then poured or scooped into a mold as required
depending on the thickness of the mixture. The composite was allowed
to reach a gel state over 16 h at room temperature, which further
reduces the presence of voids which may form by heating of air pockets
in the material in the next step. Finally, the gelled composite was placedin an incubator at 60 °C for at least 12 h for final curing.
Silicone Composite Fabrication: The silicone composite is a polydisperse
mixture of Field’s metal particles suspended in a silicone matrix. The
appropriate mass of Field’s metal particles was measured out into a
container. The two parts of the silicone (DragonSkin 10 Slow, Smooth-On)
were then mixed together using a planetary mixer (Thinky ARE-310) andthen measured out into the same container as the Field’s metal particles.
The two constituents were stirred together until the Field’s metal particles
were well dispersed in the silicone. The mixture was then poured into
molds or cast into films. All samples were allowed to degas in ambientconditions, rather than under vacuum. The specimens were then cured
for a minimum of 6 h prior to use.
DMA Testing: Stiffness change was measured using a dynamic
mechanical analyzer (DMA Q800, TA Instruments). Composite
specimen dimensions were determined such that the expected
minimum and maximum stiffness measurements were both within
the measurement range of the equipment. For epoxy samples,
this was 30 mm × 12.5 mm × 5 mm. For silicone samples, this was
30 mm × 15 mm × 5 mm. Specimens were placed in a single-cantileverclamp. Assuming the two ends of the specimen are clamped and do

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not rotate, the specimen flexural modulus Ef can be calculated from themeasured force: Ef = FL3/12δI, where L is the length of the beam, I is thearea moment of inertia of its cross-section, and F is the force needed todeflect the beam a distance δ.
DMA Testing—Epoxy Composite Stiffness Measurement: The epoxy
composite samples were applied a temperature sweep from 20 to 80 °C
at a rate of 1 °C min−1, around the phase transition zones from 30 to
40 °C (epoxy glass transition) and from 60 to 70 °C (metal melting),
then at 5 °C min−1 everywhere else. Specimens were applied a sinusoidal
force on the moving end of cantilever at a rate of 5 Hz, at a strain of
0.01%. Specimens were clamped at a torque of 6 in lbs.
DMA Testing—Silicone Composite Stiffness Measurement: The silicone
composite samples were applied a temperature sweep from 20 to 80 °C
at a rate of 1 °C min−1, around the single phase transition zone from60 to 70 °C (metal melting), then at 5 °C min−1 everywhere else.
Specimens were applied a sinusoidal force on the moving end of
cantilever at a rate of 5 Hz, at a strain of 3.75%. Specimens were clamped
at a torque of 2 in lbs (0.226 N m). For the deformed samples, thespecimens were compressed to 20% strain or stretched to 50% strain.
Upon release of load, the specimens relaxed: CompressNarrow retained
13.3% compressive strain; CompressFlat retained 18.2% compressive
strain; StretchWide retained 42.1% tensile strain; and StretchLong
retained 44.4% tensile strain.

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

Acknowledgements
T.L.B. and M.C.Y. contributed equally to this work. The authors would
like to thank Jennifer C. Case and R. Adam Bilodeau for their helpful
insights. The DMA instrument was supported under the auspices of the
National Science Foundation by Harvard’s Materials Research Scienceand Engineering Center (DMR-1420570). This work was supported by
the US Air Force Office of Scientific Research (FA9550-16-1-0267) andthe National Science Foundation (EFMA-1830870).

Conflict of Interest
The authors declare no conflict of interest.

Keywords
multifunctional materials, soft robotics, variable stiffness, variable
stretchability
Received: April 27, 2019
Revised: July 1, 2019
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