2017 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)
September 24–28, 2017, Vancouver, BC, Canada

A Move-and-Hold Pneumatic Actuator Enabled by Self-Softening
Variable Stiffness Materials
Trevor L. Buckner, Edward L. White, Michelle C. Yuen, R. Adam Bilodeau, and Rebecca K. Kramer
Abstract— Materials exhibiting variable stiffness properties
have great potential for use in the growing field of soft robotics.
Soft structural materials allow a robot to fit into enclosed spaces,
resist shock and vibration, or even reconfigure its geometry
and adapt to various environments. Rigid structural materials
on the other hand allow environmental interactions throughapplication of force and load-bearing capabilities. Materials
that can be selectively switched between these two extremes
could greatly expand the functionality of a robot that requires
the properties of both. In this paper, we introduce a conductive
epoxy composite that is self-softening through Joule heating via
direct application of electrical current. The polymer can thenbecome load-bearing and rigid again after being formed into a
new shape. We demonstrate the capabilities of this material byattaching a pneumatic actuator and showing that the resulting
variable stiffness device can be softened from its initial rigid
state, reshape itself using the pneumatic actuator, then become
rigid again and hold this new position without additional power
being supplied to the actuator.

I. INTRODUCTION
The relatively young field of soft robotics aims to introduce extended functionality to traditional mechanical andelectronic systems by allowing highly flexible, extensible,
or reconfigurable structures and components to take theplace of traditional rigid, heavy, and bulky robotic parts.
This allows the development of machines which are able tonavigate unpredictable obstacles with adaptive geometries,
fit inside confined spaces, and withstand violent forces orvibrations. These properties however, become disadvantageouswhen attempting to apply forces to other bodies or directly
manipulate the environment in which the robot operates.
Where traditional rigid robots can utilize their weight and stiff
structure to impart forces on objects and support loads, soft
robots instead tend to flow around the target object, collapse,
or slip because they lack sufficient blocking force. In order toovercome this challenge, soft robots need a way to selectively
adjust their stiffness to create rigid load paths on demand.
A brute-force solution to manipulation in soft systems is tocreate systems with large free displacements and low stiffness.
This approach can generate the required blocked force, but atthe cost of bulky actuators. Further, in the case of pneumatic
actuators, the pressures required to achieve large blocked
forces can create unworkably large stresses in non-contacting
sections of the actuator. Finally, in many cases actuators mustbe left on to hold a blocked force, requiring the expenditureof energy to produce no work. An alternative approach is touse materials with variable stiffness to tune the mechanical
All authors are with the School of Mechanical Engineering Purdue University, West Lafayette, IN, USA {tbuckne, white335, yuenm,

rbilodea, rebeccakramer}@purdue.edu
978-1-5386-2682-5/17/$31.00 ©2017 IEEE

Fig. 1. Left: Softened variable stiffness pneumatic device inflated andsupporting a 200g mass. Right: Supporting the same mass in a rigid material
state with no power input. Weight is supported by a platform secured to theend of the finger.

response of a robotic system. This allows the robot to soften,
actuate to the desired position, and then become rigid again
without the need for continuous power input.
In this work, we introduce a robotic system comprisedof an elastomer-based pneumatic actuator bonded to anovel, variable-stiffness, self-softening material. This system
can be made to hold loads when in the rigid state, yetbecome flexible as desired to be reshaped into a new rigid
configuration, shown in Figure 1. The material consists ofa matrix of thermally-responsive shape memory polymer(SMP) with a conductive phase of expanded intercalated
graphite (EIG). This composite material takes advantage ofthe low glass-transition temperature of the SMP thermoset,
thus providing both a rigid and soft material state within areasonable temperature range. The glass transition temperature
can be reached through Joule heating by applying electric
current directly to the conductive material. By combining
the pneumatic actuator and variable stiffness material, we
demonstrate that we are able to passively hold a deformed
state under load and adjust component stiffness to achieve
deformations at reasonable pneumatic pressures.
II. PREVIOUS WORK
There have been many research efforts exploring theactuation of soft robots. Materials have been reported thatchange shape in response to light, pH levels, heat, or otherstimuli [1], such as shape memory alloys [2] and shapememory polymers [3]. Other solutions utilize fluidics [4]or electric motors coupled to cables [5]. However, theseapproaches rely on continuous input stimuli to maintain themodified shape or position, resulting in only intermittent
functionality or large energy expenditures to maintain state
under load. In addition to adjusting the shape of the robot,

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there are a number of methods used to adjust the stiffness to
structural parts [6]. These include pressure differentials [7],granular jamming [8], and changing shape to make use ofgeometric stiffening [9].
The method we have selected to achieve variable stiffness is the use of thermally responsive materials. Previous
temperature-based approaches have demonstrated stiffnesschange through heating low-melting metallic alloys [10], [11],[12] or thermoplastics [13], [14], [15], [16] to achieve a statetransition. Often methods involve an interface between two
or more materials, with one acting as a heating element,
and another as the variable-stiffness material. We havepreviously developed variable-stiffness fibers by encapsulating
nickel titanium shape memory alloy wire in thermoplastic
polymers which were softened by resistively heating thewire [17]. However, the approach of employing a discrete
heating component inherently suffers from uneven thermal
gradients, risk of component delamination, and complicatedmanufacturing steps due to the need for embedded elements
and materials properly interfacing with each other.
There has been much research into adding conductive
materials to various polymers to impart desirable electricalcharacteristics. These have been used to develop stretchable or printable electronics using conductive inks [18]or conductive 3D printer filament [19] and have been
primarily accomplished with carbon black, graphene particles,
carbon nanotubes, and metallic powders [20], [21], [22],[23]. However, these generally require high additive loading
percentages to reach a sufficient level of conductivity, which
in turn leads to processing difficulties [19]. Material properties
are also affected by processing techniques; polymers require alow viscosity to effectively introduce additive particles, which
often requires processing at high temperatures beyond thematerial melting point [24]. This causes difficulties especiallywhen working with polymer blends of differing thermal
characteristics [25]. Alternatively, our own experiments haveshown that processing thermoplastics in a solvent lends itself
to high porosity in the material as the solvent is evaporated,which in turn causes mechanical brittleness and poor electricalconnectivity through the structure. The solution is often
additional hot processing under compression to reach the final
material shape and density [19]. Finally, previous attempts atusing conductive polymers as variable stiffness materials havebeen successful, but require high voltages and exhibit limited
stiffness on par with leather when in the rigid state, thus
limiting the possibility for load-bearing applications [26].
Our novel composite material overcomes the issues mentioned above. We elected to use a thermally responsive
epoxy as a base, which means a robotic device using thismaterial in its structure would only need to expend energy
during the softening and reshaping period; otherwise, thematerial remains rigid and load-bearing without expending
energy. The composite is also intrinsically conductive yetretains mechanical robustness. This not only simplifies thedevice by reducing the number of distinct materials needed
to function as a heater, but also eliminates the problems
generated by interfacing with a discrete heating element. In

Fig. 2. Top: Variable stiffness sample in initial flat configuration. Bottom:
Variable stiffness sample bent into a curve demonstrates passive holding ofa deformed shape. The sample is approximately 10cm from end to end, notincluding the conductive fabric electrodes.

the rigid state, the stiffness of the material is comparable
to acrylic, yet decreases by more than a full order ofmagnitude when softened. Finally, the material can be mixed
at room temperature which facilitates processing and attaining
homogeneous material properties.
III. MATERIALS AND MANUFACTURE
Samples of self-softening variable-stiffness material (VSM)
were made by mixing together two liquid shape memory
polymer (SMP) epoxy components with graphene particles,
followed by curing for 12hr at 60◦ C in a silicone rubber
mold. Sections of silver-coated nylon fabric (Adafruit) wereembedded in either end to serve as electrical terminals. An
SMP epoxy was selected partially for its ease of processing
at room temperature while still in the liquid state. The
SMP epoxy was created by combining an epoxy curing
agent, Jeffamine D400 (Huntsman International, LLC), witha standard epoxy resin, EPON 828 (Momentive Performance
Materials Inc.) at a ratio of 4:10 by weight [27].
The graphene particles were created by expanding intercalated graphite particles (Sigma-Aldrich) in an oven at 800◦ C
for 5 minutes, and then sonicating in a 1wt% solution of
cyclohexane for two hours at an amplitude of 36 µm (QSonica
Q700 Ultrasonicator). This produces small graphene particles
on the order of 50 nanometers in diameter. The solvent wasthen allowed to fully evaporate, leaving behind a dry aggregateof graphene in the form of a spongy solid. The grapheneaggregate was then measured by weight and vigorously mixed
into the uncured SMP epoxy by hand for several minutes
until a uniform pasty consistency was reached. Samples werecreated at 5.0wt%, 7.5wt%, 10.0wt%, 12.5wt%, and 15.0wt%
weight percent loading of graphene. When attempting tocreate mixtures of loadings higher than 15.0wt%, the resulting
material became too dry to effectively incorporate all of theadditive material; even after heavy mixing by hand, small
pockets of loose graphene were left behind.
After mixing, the viscosity of the liquid epoxy increased,
and the resulting composite had to be pressed into a silicone
mold. In these experiments, a 100mm × 25mm × 2.8mm
rectangular mold was used. A strip of conductive fabric wasset into each end of the mold and coated over with composite

3729

Fig. 3.

Effect of graphene loading percentage on conductivity.

material until the mold was filled, and excess material was
scraped from the top. The conductive SMP epoxy sample
was then cured for 12hr at 60◦ C. After curing, samples were
easily removed from the flexible mold. Completed samples
in various rigid configurations are shown in Figure 2.
IV. CHARACTERIZATION
A. Electrical Properties
A preliminary study was conducted to determine the usefulrange of graphite loading to produce a heating element.
Initial resistivity measurements were taken of samples atgraphene loading percentages from 0.0wt% to 15.0wt%. It
was found that samples at or below 7.5wt% loading had
negligible conductivity. On the other hand, samples above
10.0wt% loading were found to have useful conductivity.
As conductive additive content increases from 0.0wt%, thecomposite eventually reaches a critical loading percentage
where the randomly-oriented filler particles begin to form
continuous conducting paths. This is seen as a sudden increasein the conductivity of the material, and is known as thepercolation threshold [28]. The percolation threshold canvary for each material depending on additive size, shape, andprocessing technique [25], [28]. This phenomenon becomes
an important point as robotics increasingly makes use of newmaterials, particularly those that require modified electricalor thermal properties while largely retaining the physicalcharacteristics of another material. From Figure 3, it canbe seen that the percolation threshold for this VSM occurs
somewhere between 7.5wt% and 10.0wt% graphene loading.
This low threshold promotes improved mechanical propertiesof the material, as the brittleness imposed by high loading
percentages is minimized. Further testing was only performedon the top three loading percentages (10.0wt%, 12.5wt%, and15.0wt%) since samples of any loading lower than 10wt%
essentially acted as insulators, thus making them unusable asheating elements.
A total of 15 additional samples (5 each of 10.0wt%,
12.5wt%, and 15.0wt% loading) were used for all further
tests. The sets of 5 samples at each loading percentage
were created from the same batch of material. By applying
electrical current through each sample at varying levels of
power, it was observed that the material exhibits a positiveFig. 4. Effect of increasing temperature on the material’s electrical resistance.
Note that lower loading percentages exhibit increased variability in measured
resistance across all temperature levels. See Figure 6 for power-to-temperature
conversion.

temperature coefficient of resistance, meaning the resistanceincreases as temperature rises. Resistance was measured at
approximately 1 watt increments of applied power. At constantcurrent, the rising resistance also caused the voltage to rise.
Thus, the temperature and power were allowed to stabilize forthree minutes before making the measurement. The resulting
data reveals an approximately linear relationship between
resistance and temperature, as shown in Figure 4. This is aninteresting effect with potentially useful application, sincea positive temperature coefficient material can be designed
to predictably reach a maximum temperature for a given
voltage; at some point any further increase in temperature
would be met with greater electrical resistance, resulting inan equilibrium state.
Figure 4 also shows that the positive temperature coefficientis more pronounced at lower loadings of graphite. Also
notable is the drastic influence the additive percentage hason the deviation in resistance, which may be an effect ofthe proximity to the percolation threshold. As the additive
percentage approaches the percolation threshold, the probability of achieving a complete electrical bridge from end to
end becomes less certain, thus making the material more
susceptible to changes in resistance, and higher loading
percentages would seem to grant more predictable electricalproperties. A balance can be struck between consistentand desirable electrical properties (favoring higher loading
percentages) and robust mechanical properties (favoring lower
loading percentages). In this case, however, the 15wt% loading
was found to exhibit no detrimental mechanical properties
within the scope of the experiments performed, as detailed
later in this work.
B. Thermal Properties
A differential scanning calorimetry (DSC) test was performed on the samples at 10.0wt% and 15.0wt% loading.
This test is a thermo-analysis technique that measures thedifference in heat flux between the sample and a reference,usually air, as a function of the temperature. This allows us
to determine the temperature at physical state changes of thematerial. The samples were heated from -20◦ C to 200◦ C at
10◦ C/min, repeating the cycle 5 times per sample. Differences3730

Fig. 5. Differential scanning calorimetry test of conductive SMP. Glasstransition occurs between approximately 30◦ C and 40◦ C.

between the two loading percentages were negligible. The
glass transition of the material was found to occur from 30◦ C
to 40◦ C. The resulting plot is shown in Figure 5.
We also sought to correlate power input to temperature of
the samples. Using a thermal camera, the maximum local
steady-state temperatures at a range of power input levels(from 0 to 5W) was recorded on one sample from each loading
percentage. During recording, samples were suspended oneach end by a rubber stopper to minimize interference fromthermal conduction. It was found that the final steady-state
temperature formed a linear relationship with the input power.
Results are shown in Figure 6.
C. Mechanical Properties
The 15 samples used in the earlier electrical properties tests(5 each of 10.0wt%, 12.5wt%, and 15.0wt% loading) werereused by testing at varying power input levels in a three-point
bending test to determine the change in bending modulus asincreasing power was applied. Samples were allowed threeminutes to reach a stable temperature at each power level
before running the bending test; samples tended to reach asteady state at approximately two and a half minutes. At
constant current, we observed some voltage drift due to thepositive temperature coefficient of resistance. To account for
this, current was periodically adjusted such that the desired
power level was attained at the final steady-state temperature.
Testing was conducted with an Instron 3345 fitted with a

Fig. 6. Maximum temperature found with a thermal camera at various
electrical power inputs, found to follow a linear trend.

Fig. 7. Decreasing bending modulus with increasing power input at 15.0%
graphite loading by weight. Temperature scale found in Figure 6 is included
along the top axis.
TABLE IAVERAGE BENDING MODULUS PROPERTIES AT THE STIFF ( NO POWER )AND SOFT (5W POWER ) STATES FOR DIFFERENT LOADING WEIGHTS

Loading
10.0wt%12.5wt%15.0wt%Rigid E [GPa]
1.88
2.30
3.61

Soft E [GPa]0.08290.0619
0.132

Ratio
22.7
37.2
27.7

50N load cell. The crosshead was moved at 10mm/min to a
maximum displacement of 10mm; the supports were set 10cm
apart, with the point of load application in the center. After
bending, the sample was heated at 5W for 10 seconds to allow
it to soften and return to the initial flat shape where it washeld and allowed to cool by a fan for two minutes, at which
point the sample was approximately at room temperature
before testing at the next power level. The bending modulus
was calculated from the force-displacement data collected
at each power level. The resulting plot for this data on the
15% loading percentage is shown in Figure 7. Plots for theother two loading percentages follow the same trend, but
are shifted downward slightly, suggesting that higher levelsof graphite additive may have served to increase the overall
stiffness of the material, as previous literature suggests [29].
The average bending modulus for the rigid and soft (5W)
states of all three loading weights are listed in Table I.
The inset in Figure 7 shows a typical change in bending
modulus in response to thermal stimulus for a thermoset
polymer. Bending modulus is unchanged until around around
0.1W, then monotonically decreases until a plateau is reachedat around 1W. This decrease in stiffness is a result of theglass transition, and appears to occur between 28◦ C and 38◦ C,
which closely matches with our results from DSC testing.
Sources of the small 2◦ C discrepancy between the two could
be due to temperature gradients within the material samples,
as well as a temperature lag between the material sample andthe recorded DSC temperatures. In each test however, theentire glass transition occurs over a range of approximately
10◦ C. Increasing the power does cause the material to reachits rubbery state more quickly (1 minute at 1W, but within
10 seconds at 5W), but further changes in stiffness after the3731

Fig. 8. Demonstration of self-softening variable stiffness material in conjunction with a pneumatic actuator. (a),(b) The pneumatic actuator is unable tobend the material when in the rigid state. (c) VSM is heated and softened. (d) Soft VSM can be bent by the actuator. (e) Pneumatic actuator holds position
while the VSM cools. (f) Pressure is released and VSM retains actuated position. (g) Further heating demonstrates the shape memory effect of the material
and elasticity of the pneumatic which pulls the device back to near the initial position.

glass transition are greatly reduced, although not negligible;
the stiffness of a sample heated with 5W is approximately
one-fourth the stiffness when heated with 1W. Based on thesetests, an average change in stiffness of more than an order ofmagnitude can be reached using only 2 watts. This significant
change is comparable to a transition from acrylic to a polymer
foam. In the rigid state, the material can be used to support
significant loads with minimal deflection; in the soft state,
the material can be reshaped with small forces. For example,
after attaching a sample of this material to our pneumatic
actuator, we found that a pressure of 15kPa was required tobend it at 1W power. However, when fully cooled and rigid,
the actuator required 117kPa to bend it.
It is also interesting to note that the uncertainty in themodulus increases drastically at lower power levels. Thismay be partially due to differences in the amount of polymer
chain conformation, polymer crosslinking, or manufacturing
defects across samples which become more evident near the
crystalline phase of the material, but begin to diminish as
the epoxy approaches a rubbery state and polymer chainsare able to shift more freely. Due to the very low glasstransition temperature, other sources of discrepancy could be
the effects of fluctuating ambient temperature at low power
levels. A difference of only a couple degrees can create alarge change in stiffness. This can be favorable due to therelatively low operating temperatures and narrow temperature
ranges required to enact stiffness switching. However, thisalso introduces complexity and uncertainty if attempting totune device stiffness to a specific value, especially within theglass transition range.
V. APPLICATIONS
We demonstrate the effectiveness of this material in
conjunction with a silicone pneumatic actuator, shown in
Figure 8. The VSM is initially in the rigid glassy state, andthe pneumatic actuator inflated by hand to 15kPa is unable tobend it at all. However, as 3W of electric power is applied,
the material begins to soften. After one minute, the actuator
can then easily bend the material to a new position, where
it is held for 6min while the VSM cools. After cooling,
the VSM has become rigid once again and holds the newshape. An interesting effect of the SMP is the shape memory

Fig. 9. Pneumatic actuator lifting weights. Scale bars are 2cm. (a) VSM
softened. (b) Actuator is able to lift a 100g weight. (c). VSM stiffened after
cooling for 6 minutes, still holding the 100g weight. (d) Added a 500g
weight to the load. The VSM sags very slightly but still holds the weight.

effect which, in combination with the elastic properties ofthe pneumatic actuator, is able to return the device nearly toits original position. A second actuator on the opposite side
could easily provide the needed force to bend the device in theother direction or return it to the original flat configuration.
In another demonstration (Figure 9), we show that thepneumatic actuator can be used to lift masses with the VSM
in the soft state, and can then hold the masses in position using
only the stiffened VSM. At 15kPa, the pneumatic actuator isonly able to lift 100g to a height of approximately 2cm. With
this mass held by the actuator, the VSM is allowed to coolfor 6 minutes to the rigid state. Interestingly, after releasing
the pneumatic pressure, the VSM is not only able to keep
the 100g mass suspended without detectable sagging, but
is also able to support an added mass, totaling 600g before
substantially deforming (a drop of about 0.6cm is visible in
the figure). In the soft state, the VSM is not contributing
structurally to the support of the mass; the inflated pneumaticis stiffer than the softened VSM and is solely responsible
for the lifting capability. In the stiff state, the opposite istrue; the VSM is much stiffer than the inflated pneumatic,
allowing it to support much more weight than the actuatoron its own.
VI. CONCLUSION AND FUTURE WORK
We have developed a robotic device by combining anelastomer-based pneumatic actuator with a novel material
which exhibits variable-stiffness properties and is activated3732

through simple application of low power electricity. This
material can be processed with a straightforward manufacturing process and molded into any desired shape. The robotic
system is able provide a repeatable order-of-magnitude change
in stiffness allowing move-and-hold operations, supporting
significant mass with minimal deformation and without
continuous energy expenditure. This technology has potential
for use in the growing field of soft robotics, enabling increased
functionality and adaptability for future robotic designs byenhancing the load-bearing capability, actuator effectiveness,and energy-efficiency of soft machines.
Since temperature-dependent systems intrinsically sufferfrom long cooling times, future work will consider bothmaterial and geometry in an effort to shorten the softening andstiffening cycle length. Additional follow-up will investigate
integration of the VSM with other forms of actuation,
including different pneumatic designs, shape-memory alloys,
and cable-driven mechanisms. The highly-viscous nature of
the uncured VSM makes it an ideal candidate for molding
or extruding into arbitrary shapes, embedding into fabricsubstrates, and 3D printing. Using these techniques, newgeometric designs will be tested to allow for movement inthree dimensions and elongation in tension.
ACKNOWLEDGMENT
This work was primarily supported by the Air Force Officeof Scientific Research under award number FA9550-16-10267. ELW and MCY are supported under the National
Science Foundation Graduate Research Fellowship program(DGE-1333468). RAB is supported by an Early Career
Faculty grant from NASAs Space Technology Research
Grants program (NNX14AO52G).
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