SOFT ROBOTICS
Volume 00, Number 00, 2023
Mary Ann Liebert, Inc.
DOI: 10.1089/soro.2023.0004

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Characterization of Temperature and Humidity Dependence
in Soft Elastomer Behavior
Elze Porte,1–3 Sophia Eristoff,1 Anjali Agrawala,1 and Rebecca Kramer-Bottiglio1

Abstract

Soft robots are predicted to operate well in unstructured environments due to their resilience to impacts,
embodied intelligence, and potential ability to adapt to uncertain circumstances. Soft robots are of further
interest for space and extraterrestrial missions, owing to their lightweight and compressible construction. Most
soft robots in the literature to-date are made of elastomer bodies. However, limited data are available on thematerial characteristics of commonly used elastomers in extreme environments. In this study, we characterize
four commonly used elastomers in the soft robotics literature—EcoFlex 00-30, Dragon Skin 10, Smooth-Sil
950, and Sylgard 184—in a temperature range of -40C to 80C and humidity range of 5–95% RH. We performpull-to-failure, stiffness, and stress-relaxation tests. Furthermore, we perform a case study on soft elastomers used instretchable capacitive sensors to evaluate the implications of the constituent material behavior on component
performance. We find that all elastomers show temperature-dependent behavior, with typical stiffening of thematerial and a lower strain at failure with increasing temperature. The stress-relaxation response to temperaturedepends on the type of elastomer. Limited material effects are observed in response to different humidity conditions.
The mechanical properties of the capacitive sensors are only dependent on temperature, but the measured capacitance shows changes related to both humidity and temperature changes, indicating that component-specificproperties need to be considered in tandem with the mechanical design. This study provides essential insights into
elastomer behavior for the design and successful operation of soft robots in varied environmental conditions.
Keywords: soft robotics, sensors, space, environmental conditions, tensile test, electromechanical test
robots can offer additional advantages because of theirpotential to adapt to uncertain environments1–5 and to packinto tight spaces.6 Real environments pose different conditions to the robots than the laboratory environments in which
they are developed and tested. For example, temperatures on

Introduction

R

obots are used to explore dangerous and difficult-toaccess environments, such as the Mars Rover in space
or autonomous underwater vehicles in the deep-sea. Soft
1
2
3

Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut, USA.
Department of Mechanical Engineering, University College London, London, United Kingdom.
Department of Civil, Environmental & Geomatic Engineering, University College London, London, United Kingdom.

 Elze Porte et al., 2023; Published by Mary Ann Liebert, Inc. This Open Access article is distributed under the terms of the Creative
Commons License [CC-BY] (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction inany medium, provided the original work is properly cited.

12

the Moon’s equator can vary between 123C during the
day and -170C at night, and the temperature can drop
below -230C at the poles.7,8 Extreme low humidity conditions can be experienced on the Earth toward the poles andextreme high humidity in hot climates toward the equator,
since the ability to hold water vapor is temperature dependent.9 Successfully operating soft robots in any of theseenvironments requires a detailed understanding of the response to the environmental conditions of their constituent
materials.
The base materials in soft robots’ comprising structures,
actuators,10–13 and sensors14–17 are often commercial silicones. These materials are attractive to use because they canoften stretch to very large strains (>50%) and are easy toprocess. The material characteristics provided by the manufacturer, such as Shore A hardness and tensile strength, are
not always sufficient to accurately predict the behavior of
the robot, which is why more detailed characterizations havebeen carried out in recent studies to advance soft robot
design.18–20 Such studies highlight the nonlinear and viscoelastic behavior of the silicones, which is not capturedby the manufacturer’s data. The detailed characterizations
can help in soft robot design, since the nonlinear material
parameters can be used in finite element (FE) models andhelp better predict the robot’s response across a range of
motions.
Successful operation of soft robots in various environments will require a further understanding of the constituent
materials in appropriate conditions. To date, limited information is available on the response of commonly used elastomers in soft robots to environmental conditions, including
temperature and humidity. Although the curing temperature of polydimethylsiloxanes (PDMS) is known to be an
important factor for the properties of the material,21,22 thesestudies often do not include the effect of the operating temperature. A study on EcoFlex 00-30 found an increasein stiffness with increasing temperature of the environment
for a temperature range of -40C to 140C,23 indicating thatenvironmental temperature should be considered in theoperational model of the robots.
Elastomers are known to swell when exposed to water orsolvents,24 but research mostly focuses on achieving large
volumetric expansion in elastomeric seals25,26 and provides
little insight into the effects of humidity on the broader
mechanical behavior of the elastomers required for soft robot
operation.
Recent work has focused on building a database with themechanical behavior of commercially available silicones toprovide the soft robotics community with modeling parameters to run FE simulations and to choose appropriate materials for their applications.19 In line with this effort, we arecharacterizing four common commercial elastomers (EcoFlex 00-30, Dragon Skin 10, Smooth-Sil 950, and Sylgard
184) for a range of temperature and humidity conditions. We
provide a case study on the performance of elastomer-based
capacitive sensors to showcase environmental effects on asoft robotic component. The aim of this work is to provide
insight into the effect of environmental conditions on thematerial characteristics of elastomers to aid the design of soft
robots for both terrestrial and extraterrestrial exploratory
missions, with a special focus on working toward lunar
missions.

PORTE ET AL.
Materials and Methods
Elastomer sample preparation

Samples were prepared according to ASTM D412 using
dumbbell shape C and cast in acrylic molds with a thicknessof 2 mm. Figure 1a shows an example of a Smooth-Sil
950 sample. For the failure tests, the size of the dumbbell
shape was reduced by 50% to ensure tests could be performed within the boundaries of the environmental chamber(38 · 46 · 61 cm). This sample size is not part of the ASTM
D412, but was an unavoidable alteration due to the setup
restrictions. Dragon Skin 10 Medium (Smooth-on, Inc.) and
EcoFlex 00-30 (Smooth-on, Inc.) were prepared by mixing
parts A and B in a 1:1 ratio. Smooth-Sil 950 (Smooth-on,Inc.) and Sylgard 184 (Dow) were prepared by mixing parts
A and B in a 10:1 ratio.
All materials were mixed in a centrifugal mixer (Thinky
ARE-310) at 2000 rpm for 30 s and degassed at 2200 rpm for
30 s. The elastomers were poured in the molds and put in a
vacuum chamber (-80 to -100 kPa) to remove air bubbles
from the mixtures. Excess elastomer was removed with a flat
metal scraper to level the samples. All samples were cured atroom temperature. Three equally spaced dots [3% wt. black
Silc Pig, 97% EcoFlex 00-10 (1:1, A:B), Smooth-on, Inc.]
were painted on the narrow middle to image the real strain.
The width of the samples was assumed to be the casted width(6 mm). The thickness of the samples was found by measuring the thickness with a micrometer at three different
locations on the sample and taking an average.
Sensor manufacturing

Two sets of sensors were prepared: one with Dragon Skin
10 and one with Smooth-Sil 950 as base material. Capacitive
sensors were manufactured similar to the method described inan earlier work.14,17 In short, expanded intercalated graphite (EIG) was prepared as conductive filler for the sensor
electrodes. Expandable graphite (5 g; Sigma Aldrich) was
expanded at 800C and soaked in cyclohexane (0.5 L; Thermo Fisher Scientific). The EIG/cyclohexane mixture wassonicated for 1 h (70%; Q500 1/2’’ tip, Qsonica) and sieved
through a 212 lm sieve. After settling of the particles (1 h),
excess cyclohexane was decanted and the mixture was boiled
down to *3% wt. EIG. The EIG mixture was manuallystirred into the elastomers at *79% wt. to achieve 10% wt.
EIG in the electrodes after evaporation of the cyclohexane
during curing. Elastomers were prepared using the same
mixing ratios as for the elastomer sample preparation.
The sensors consist of five layers; three electrode layers
and two dielectric layers. The layers of the sensors were
coated using a film applicator (SH0340; TQC Sheen). The
applicator was set to 500 lm for all electrode layers and to
1200 lm for the Dragon Skin dielectric and 800 lm for the
Smooth-Sil 950 dielectric layers. The final thickness of thecured layers was slightly lower than the coated thickness due
to spreading of the elastomers before curing.
The initial electrode and first dielectric layer were coatedwith a width of 110 mm. The second electrode layer wascoated on top of the dielectric layer to a width of *100 mm,
by covering one edge with a paper mask. The second dielectric layer was coated to a width of *100 mm by covering
the other edge with a paper mask. The final electrode layer

ENVIRONMENTAL STUDY OF SOFT ELASTOMERS

3

FIG. 1. Overview of the testing setup: (a) example of Smooth-Sil 950 sample; (b) picture of the environmental chamber
setup; (c) schematic of the environmental chamber and material testing system.
was coated on top of the second dielectric layer to approximately the same width, again using a paper mask.
Sensors were cut to size (110 · 10 mm) with a laser cutter(VLS 2.30; Universal Laser Systems). The first and third
electrode were connected at the short edge by painting theedge with a conductive glue made from SilPoxy (Smooth-On,
Inc.) and EIG with the same mixing ratio as the electrodes.
Copper strips were attached to the first and second electrodeusing the same conductive glue. The entire sensor was encapsulated in a thin layer of elastomer to prevent the electrodes from shorting in the humidity tests. A schematic of thedifferent sensor layers and a picture of a sensor sample areprovided in Supplementary Figure S1.
Material testing and environmental conditioning

The mechanical testing system comprises a material testing system (Instron 3345) fitted with a custom-built environmental chamber (Model 5500-8485; ETS). An image andschematic of the setup are provided in Figure 1b and c,
respectively. The environmental chamber has a temperature
range of -40C to 80C and a humidity range of <5% to>95% at room temperature. For the temperature tests, samples were left in the environmental chamber for at least
10 min before testing, as prescribed by ASTM D412. For any
humidity tests, samples were left in the environmental
chamber overnight (16–24 h).
Three different characterization tests were performed:
pull-to-failure tests, stiffness tests, and relaxation tests. The
temperature dependence was determined between -40C and80C at increments of 20C, except for the relaxation tests,
which were only conducted at -40C, 20C, and 80C. The
humidity dependence on material properties was determinedby conducting tests at 95% RH, 40–60% RH (room conditions), and 5% RH.
An Instron 2530—1 kN load cell was used for the pull-tofailure and stiffness tests. An Instron 2519—50 N load cell
was used for the relaxation tests because of its much lower
creep specification (–0.1% of force capacity over a period of
20 min, compared with –0.1% of force capacity over a periodof 3 min for the 2530—1 kN). Both load cells have a linearity
and repeatability of –0.25% of the reading (from 0.2/0.5 to100% of force capacity for the 2530—1 kN/2519—50 N, respectively).
For the stiffness tests, EcoFlex and Dragon Skin samples
were strained to *300% and the Smooth-Sil and Sylgard
samples to *100% at a rate of 500 mm/min. Sylgard samples
under certain conditions (i.e., change in humidity, higher
temperature conditions) exhibited failure before the preset
100% strain. In this case, the maximum strain was reduced to
80% to prevent failure. Before a stiffness test, samples were
subjected to a preload to remove slack in the sample and aprestretch cycle at 1000 mm/min to remove the Mullin’s effect.27 A faster rate was chosen here than for the recorded tests
to increase the testing efficiency. The preloads were 0.04, 0.1,
0.4, and 0.4 N for EcoFlex, Dragon Skin, Smooth-Sil, andSylgard, respectively, and the maximum strains during theprestretch cycle were *350% for the EcoFlex and Dragon Skinsamples, and 120% for the Smooth-Sil and Sylgard samples.
For the failure tests, samples were stretched to failure at a
rate of 500 mm/min. A small preload of 0.02, 0.1, 0.2, and
0.2 N was applied to the EcoFlex, Dragon Skin, Smooth-Sil,and Sylgard samples, respectively, to remove slack in thesamples before testing.
In the stress-relaxation tests, EcoFlex and Dragon Skinsamples were stretched to *90–100% strain and Smooth-Sil

4and Sylgard to *40–50% strain at a strain rate of1000 mm/min, and held in the stretched position for 30 min.
The stress-relaxation procedure was based on earlier work18
and follows the guidance of ASTM E328-21, which describes
a general procedure for stress-relaxation tests. The rate was
set to the maximum rate of the testing system (1000 mm/min)to increase testing efficiency and to create a high viscoelastic
response. The relaxation percentage was determined bycomparing the stress at the beginning of the holding periodwith the final stress at the end of the test.
The beginning of the holding period was defined as thepoint at which the final displacement was first reached toeliminate stress fluctuations due to the displacement controlof the testing system. The final stress was calculated as theaverage over the final *3.5 min of the test to account forthermal fluctuations caused by the heating/cooling control ofthe environmental chamber.
Images of the samples were captured with an imaging
system (Grasshopper 3; Point Grey Research) every second toanalyze the real strain in the gauge area using the painted dots.
The relationship between the measured displacement from theInstron and the measured strain from the images was obtained
through a cubic fit of the data, with typical R2 values >0.99.
A cubic fit was chosen because this was the lowest order fit thatyielded a fit through the origin. The strain measurements
showed a deviation of –2%, which includes any variations
caused by sample loading. Most images taken at subzero
temperatures were not usable due to the liquid nitrogen mist in
the chamber, except for the failure tests at 0C for Dragon

PORTE ET AL.

Skin, Smooth-Sil, and Sylgard. An average fit of all the data
above 0C was used when the images were not usable.
The strains at which most EcoFlex and Dragon Skinsamples broke were too large to be captured within the fieldof view of the camera. Six additional samples per material
were tested with a sufficiently large field of view. This setup
was, however, not feasible to perform all tests because of amuch more labor-intensive image postprocessing and restrictions on laboratory space. The resulting average fit ofthese large field of view tests was used to fit all EcoFlex andDragon Skin data.
Sensor tests

Dragon Skin sensors were stretched to *100% strain and
Smooth-Sil sensors to *50% strain at a rate of 5 mm/s using
the material testing system. The average strain was calculated
from the initial length of the sensor at the start of the test. The
capacitance of the sensors was recorded with an LCR meter(E4980AL; Keysight Technologies) at an excitation frequency of 200 Hz. Sensors were acclimatized in the same
way as the elastomer samples.
Results and Discussion
Stiffness response to environmental conditions

Figure 2 shows the stress–strain curves of all materials
under different thermal conditions. For clarity purposes, only
the data at -40C, -20C, 20C, and 80C are shown. The

FIG. 2. Stress–strain curves at different temperature conditions for (a) EcoFlex 00-30; (b) Dragon Skin 10; (c) Smooth-Sil
950; and (d) Sylgard 184. Shaded areas represent standard deviations (n = 6).

ENVIRONMENTAL STUDY OF SOFT ELASTOMERS

data at these temperatures are representative of the observed
trends. The additional data at temperatures 0C, 40C, and60C are available in Supplementary Figure S2. Sylgard was
only tested to 80% strain at 80C due to early failure of thematerial. The graphs show that, in general, the higher thetemperature the stiffer the material. Although the EcoFlex
00-30 measurements show relatively large variation, thedifference between low-temperature (-20C) and hightemperature (80C) measurements is sufficiently large toconclude there is a temperature effect.
An increasing stiffness with temperature is reported forvarious unfilled elastomers23,28,29 and can be attributed toentropic-related energy.28,30,31 The configuration entropy ofelastomers increases with temperature, enabling the molecules to return to a less ordered state,31 leading to an overall
contraction of the material and higher stresses.28,30,31 This
stiffening effect can potentially be offset or reversed by thepresence of reinforcing fillers in elastomers, reducing thecontribution of the entropic-related elastomer response.32 We
further note that temperature effects may present differently
in the first stretch cycle compared with the subsequent stretch
cycles,23 which is not captured in our data because we used aprestretch cycle to remove the Mullins effect from the measurements.
The exception to the general trend of increasing stiffness
with increasing temperature is the measurements at -40C
for Smooth-Sil, Dragon Skin, and EcoFlex. These materials
exhibit higher stiffness at -40C than at -20C (Fig. 2a–c).
The distinction between the EcoFlex measurements is lessclear because of the relatively large variation in the EcoFlex

5

tests, resulting in overlapping confidence intervals between
the -40C and -20C tests.
A potential explanation for an increase in stiffness withdecreasing temperature is a transition to a glassy state of theelastomer.29 Although the glass transition temperature is notreported in the technical data supplied from the manufacturer,
EcoFlex is known to have a glass transition temperature around-30C.33,34 The technical data from the manufacturer list thesame useful temperature range for Dragon Skin and Smooth-Sil
as for EcoFlex (-65F to 450F, -54C to 232C), and so, it islikely their glass transition temperatures are similar too.
Changes in the mechanical properties due to glass transition can
gradually take place over a temperature range of 50C,30 which
may explain the relatively small changes in stiffness. No
stiffening effect was observed for the Sylgard 184, which has amuch lower glass transition temperature around -120C.35,36
The effect of humidity on respective material properties is
exhibited in Figure 3. No clear trend is observed between the
stiffness of the materials and the humidity conditions. While
Dragon Skin and Smooth-Sil appear to show marginallyhigher stiffnesses when samples are subject to either low(5%) or high humidity (95%), the differences are not as
substantial as those found in temperature changes. The differences in stiffness in EcoFlex, Smooth-Sil, and Sylgard are
too small compared with the relatively large standard deviations to attribute to changes in humidity. Both Dragon Skinand Smooth-Sil show the lowest stiffness at ambient room
humidity. A decreasing stiffness with increasing humidity
was observed for a VHB elastomer,37 which means thehumidity response likely depends on the type of elastomer.

FIG. 3. Stress–strain curves at different humidity conditions for (a) EcoFlex 00-30; (b) Dragon Skin 10; (c) Smooth-Sil
950; and (d) Sylgard 184. Shaded areas represent standard deviations (n = 6).

6
Failure response to environmental conditions

Figure 4 shows the effect of temperature on the failure
tests. In general, as the temperature increases, the failure
stress and strain of the materials decrease. Although thedifferences between consecutive temperature intervals are
not always distinguishable due to the overlapping standard
deviations, the general trend of higher failure stress and strain
at lower temperatures is clearly visible in the graphs. Comparable trends with both a reduced energy input and strain atbreak with increasing temperature are reported for otherelastomers in a similar temperature range.38,39 Early failure,similar to our observation for Sylgard in the stress–strain
tests, was observed in tests at elevated temperatures.23,40
The effect of humidity on failure tests is shown in Figure 5.
The differences in failure stress and strain are very small or
negligible for all elastomers. The early failure observed insome of the Sylgard stiffness tests (Fig. 3d) is therefore likely
unrelated to the humidity conditions and due to variation in
the samples. The most noticeable difference is observed for
the Smooth-Sil samples at 95% RH compared with room
conditions (Fig. 5c). Although the stress at failure is very
similar, the strain at break is slightly higher. A lower modulus
may be expected for swollen elastomers,30 but this is not what
we observed in the stiffness tests for Smooth-Sil.
Previous work18 showed that batch-to-batch differences inthe strain at break for Smooth-Sil are relatively large with a
75% strain difference between batches (300–375% strain).
Although we attempted to minimize batch effects by selecting

PORTE ET AL.

samples from at least two different batches in one test group,
these batches may be different between test groups. Due to therelatively large standard deviation in these tests and the lack ofa similar trend in the other elastomers, the differences may be
due to large differences between batches.
Relaxation response to environmental conditions

Figures 6 and 7 show the average relaxation percentages
after 30 min of relaxation for different temperature andhumidity conditions, respectively. The graphs indicate thatthe relaxation behavior is dependent on temperature, but not
on humidity. A small increase in relaxation at low humidity
is observed for Sylgard (Fig. 7d), but this small change likely
does not require special consideration in the design process ofa soft robot. The relaxation behavior of all four materials ismost sensitive to freezing temperatures (-40C) comparedwith elevated temperatures (80C).
The most notable difference in relaxation behavior basedon temperature is observed between the freezing and room
temperature conditions (Fig. 6). Sylgard has an increased
average relaxation that is about five times as high as therelaxation observed at room temperature (Fig. 6d), whereas
EcoFlex shows stiffening rather than relaxing behavior(Fig. 6a). Stress recovery after stress softening in the initialstrain cycles (Mullins effect) is also a temperature- andtime-dependent process with recovery times decreasing
with increasing temperature.41 The change in relaxation

FIG. 4. Failure tests at different temperature conditions for (a) EcoFlex 00-30; (b) Dragon Skin 10; (c) Smooth-Sil 950;
and (d) Sylgard 184. Representative example curves are shown for -40C, 0C, and 80C to guide the reader. The icons
represent average stress and strain at failure, and the error bars represent standard deviations (n = 6).

ENVIRONMENTAL STUDY OF SOFT ELASTOMERS

7

FIG. 5. Failure tests at different humidity conditions for (a) EcoFlex 00-30; (b) Dragon Skin 10; (c) Smooth-Sil 950; and(d) Sylgard 184. Representative example curves are shown to guide the reader. The icons represent average stress and strain
at failure and the error bars represent standard deviations (n = 6).
between 20C and 80C is relatively small for all elastomers.
The standard deviation of the tests performed at -40C for
Dragon Skin and Smooth-Sil is too big to attribute the larger
average relaxation to the temperature effect with certainty(Fig. 6b, c).

Relaxation behavior is highly dependent on the typeof polymer,42 but the observed trend of increasing
relaxation with decreasing temperature in the current
work is in line with observations on several other elastomers.29,40

FIG. 6. Relaxation tests at different temperature conditions for (a) EcoFlex 00-30; (b) Dragon Skin 10; (c)Smooth-Sil 950; and (d) Sylgard 184. Error bars represent
standard deviations (n = 6).

FIG. 7. Relaxation tests at different humidity conditionsfor (a) EcoFlex 00-30; (b) Dragon Skin 10; (c) Smooth-Sil
950; and (d) Sylgard 184. Error bars represent standard
deviations (n = 6).

8

The stiffening behavior that is observed for EcoFlex may
be explained by the glass transition temperature around-30C.33,34 Extreme stiffening behavior is observed for oneof the EcoFlex samples at the lowest temperatures of thecooling cycle (-45C) with the maximum stress during
relaxation as high as four times the initial stress. Similar
extreme stiffening behavior is observed for two Dragon Skinsamples (Supplementary Fig. S3). These samples were allfrozen solid when removed from the test setup, and areremoved from the presented data as outliers. Sylgard has
a much lower glass transition temperature (around-120C35,36), and no stiffening effect was observed.
Effect of extreme temperature exposure

Since lunar conditions can exhibit fluctuations in temperature ranging from -230C to 120C, and the environmental
chamber is not equipped to reach these extreme temperatures,
additional tests were performed to investigate the effect of
extreme temperature exposure. The aim is to identify potential long-term changes in material properties that can affectthe usability of the materials in space.
Two additional sets of samples were tested per material;
one set was heated at 150C and the other set was cooled at-85C overnight (16 h). All tests were performed at room
temperature, at least 30 min after the thermal treatment. For
the stiffness tests, all samples were tested before and afterthermal treatment. The effect of extreme temperature exposure on the stress–strain and failure behavior is displayed in

PORTE ET AL.

Figures 8 and 9, respectively. The ‘‘pristine’’ data are repeated from the stress–strain and failure studies (Figs. 2 and
4). Separate data of the same samples before and after thermal treatment are available in Supplementary Figures S4 andS5.
The most substantial changes to mechanical properties areobserved when samples are subject to heat treatment, where
all materials undergo a permanent increase in stiffness(Fig. 8). An increase in stiffness after heat treatment agrees
with previous works,21 as the chemical crosslinking of PDMS
can be aided by an increase in temperature. All the materials tested are silicones that undergo an additional reaction
between vinyl groups and silane functionalized PDMS, catalyzed by organometallic platinum. Heating up this system
accelerates the ability for platinum catalysts to react with therespective vinyl and PDMS groups, enabling a higher degreeof crosslinking and thus higher stiffness. In addition, Sylgard
and Ecoflex samples fail at much lower strains than pristine
samples (Fig. 9).
The effect of a freeze–thaw cycle on all materials is not aslarge compared with the heat treatment, with only smallchanges observed in the stiffness of Smooth Sil and Dragon
Skin. A direct comparison of the pre- and postfreezing
stiffness curves for Dragon Skin (Supplementary Fig. S5b)shows no effect of the freeze–thaw procedure and the small
deviation may have resulted from batch or test differences.
The elevated stiffness after freeze–thaw cycles may relate tocold crystallization,43,44 where molecular crystallization
leads to an increase in crosslinking density. Silicones have

FIG. 8. Stress–strain curves of samples before and after thermal treatment at 150 C and -85C for (a) EcoFlex 00-30;(b) Dragon Skin 10; (c) Smooth-Sil 950; and (d) Sylgard 184. Shaded areas represent standard deviations (n = 6).

ENVIRONMENTAL STUDY OF SOFT ELASTOMERS

9FIG. 9. Failure tests after thermal treatment at 150C and -85C compared with tests without any treatment for(a) EcoFlex 00-30; (b) Dragon Skin 10; (c) Smooth-Sil 950; and (d) Sylgard 184. Representative example curves are shown
to guide the reader. The icons represent average stress and strain at failure, and the error bars represent standard deviations(n = 6).
been shown to exhibit a cold crystallization phase aroundapproximately -70C,43,44 which may explain the increased
stiffness of the Smooth-Sil samples after freezing.
Although these tests do not describe the material behaviors
at the more extreme temperatures (as low as -85C and up to
150C), they show that material integrity is preserved after
exposure and stiffness changes can be expected, which is thefirst step in gaining a full understanding of the material
characteristics in lunar conditions.
Sensor case study

Capacitive sensors can be used to measure strain since
their measured capacitance directly relates to the geometry of
the sensor:
C ¼ 0 r

Lw
t

(1)

where 0 and r are the free space and relative permittivity,
respectively; L and w are the length and width of the electrodearea, and t is the thickness of the dielectric layer. Since themeasured capacitance of elastomer-based sensors can change
based on the environmental conditions,15,45 capacitive sensors were chosen for a case study on the performance of a soft
robotic component under different environmental conditions.
The sensors were made with Dragon Skin or Smooth-Sil asbase material. The Dragon Skin sensors were used for a temperature study and the Smooth-Sil sensors for a humidity study.
The results of the temperature study are captured in
Figure 10. The sensors showed increased stiffness withincreasing temperature between -20C and 80C (Fig. 10a),which was expected based on the elastomer tests. The average stiffness increase between -20C and 80C was, however, not as large (about 15%) as for the neat elastomer (about70%). The graphite in the graphite-filled electrodes could
explain the reduced temperature response, since the stiffnessof the graphite may decrease with temperature in the tested
temperature range.46–48
Figure 10b shows the linear relationship between capacitance and strain at four representative temperatures (all data
are included in Supplementary Fig. S6), and Figure 10cshows the relative capacitance change at 0% strain for all
temperatures. Both graphs show a trend of increasing
capacitance with decreasing temperature between 0C and80C and decreasing capacitance with decreasing temperature between -40C and 0C. We observed a reduced linear
strain region at temperatures below 0C, which is represented
by the shorter capacitance–strain curve at -40C (Fig. 10a).
Assuming the geometry of all sensors is the same at 0%
strain, the change in capacitance is most likely caused by achange in the dielectric constant (C0 / r ). The dielectric
constant of elastomers is known to be temperature dependentand similar trends are reported for other elastomers49–51 andcapacitive sensors.15 The dielectric constant can be related to10

PORTE ET AL.

FIG. 10. Sensor tests at different temperature conditions: (a) average stiffness; (b) average capacitance normalizedagainst the initial capacitance at 20C (C0(20C)); (c) relative difference between C0 and C0(20C); and (d) gauge factor. Error
bars represent standard deviations (n = 6 for all tests, except at -40C [n = 4]).
dipole polarization, which becomes easier with increasing
temperature and thus increases the dielectric constant.49,50
The observed peak in the initial capacitance relates to thetemperature at which increased thermal motion of the molecules reduces the polarization, leading to a subsequent
decrease in dielectric constant.49
The sensor sensitivity, represented by the gauge factor(GF = (C/DC)/n), does not show a clear trend based on temperature (Fig. 10d) with values very close to 1. The average
GFs at -20C and -40C are slightly lower, but have a large
standard deviation. Additional tests were performed tomeasure the Poisson’s ratio of both the sensors and Dragon
Skin at 0C and 20C, since the sensitivity of the sensors isdetermined by the Poisson’s ratio of the dielectric layer.14 No
evidence was found for a relationship between the Poisson’s
ratio and temperature that can explain the change in GF, andso, the small deviations can likely be attributed to normal
deviations between tests.
The overall sensor performance reduces at temperatures
below 0C. At temperatures of 20C and above, all sensors
show a linear capacitance response with strain to at least
100%. Below 0C, the sensors show reduced linearity withstrain. At 0C, one of the sensors shows reduced linearity
after about 80% strain. At -40C, two sensors did not produce
any reliable sensor measurements, and the other sensors had amaximum measurement range between 20% and 70% strain.
We observe an increasing electrode resistance of the sensors with decreasing temperature (Supplementary Fig. S7),
which is in line with observations on other conductive
polymers, although at temperatures above 0C.15 A highelectrode resistance can affect the maximum measurement
range of elastomer capacitance sensors,52,53 and the increasing resistance with temperature explains the reduced
performance of the sensors. The breakdown voltage of dielectric elastomers is also temperature dependent,54 but thisis unlikely to affect the sensor performance since thesebreakdown voltages are of a different order of magnitude(kV) compared with the voltages that are relevant to capacitive sensors (several V).
The results of the humidity study are shown in Figure 11.
No change in the sensor stiffness was observed based onhumidity (Fig. 11a), which is in line with observations fromthe elastomer tests. The measured capacitance normalizedagainst the initial capacitance at room conditions shows smallchanges with humidity (Fig. 11b). The most observable difference is the change of the initial capacitance with humidity(Fig. 11c). The lowest humidity results in the lowest initial
capacitance and the highest humidity results in the highest
initial capacitance. The GF for all humidities are all very
close to 1 (Fig. 11d). The observation of increasing capacitance with increasing humidity agrees with other studies thatshow increased capacitance when capacitive sensors are incontact with water,45,55 due to increased current leakage orspace charge.45,56
The sensor case study shows that the constituent material
characterization can be used in the design of soft robotic
components, but that the composition and function of thecomponent need to be carefully considered. The characterization in this study focuses on the mechanical properties ofthe elastomers. The sensors performed as expected with

ENVIRONMENTAL STUDY OF SOFT ELASTOMERS

11

FIG. 11. Sensor tests at different humidity conditions: (a) average stiffness; (b) average capacitance normalized against
the initial capacitance at 40–60% RH (C0(50%RH)); (c) relative difference between C0 and C0(50%RH); and (d) gauge factor.
Error bars represent standard deviations (n = 7 for all tests).
increasing stiffness with increasing temperature and withlimited effects based on humidity. However, the composite
nature of the sensors likely affected the temperature response,
which means further characterization of all constituent
materials is required to accurately predict the behavior ofcomposite structures. The properties specific to sensor design, such as the electrode resistance and dielectric constant,were not captured in the mechanical study. This means thatcomponent-specific properties may need separate characterization to predict their full performance.
Conclusion

This work investigated the mechanical behavior of fourcommonly used elastomers in soft robotics over a range oftemperature and humidity conditions. A case study on elastomer capacitive sensors was performed to showcase theperformance of a soft robotic component in various conditions. In general, this study showed that the operating conditions should be considered in the design and testing ofelastomers in soft robot applications.
Three main conclusions can be drawn. First, we observed
temperature-dependent behavior for all tested elastomers,
which presented as increasing stiffness and decreasing strain
at failure with increasing temperatures. No distinct effects
were observed for changes in humidity. Second, extreme
stiffening behavior was observed at -40C during prolonged
exposure in several stress-relaxation tests. Although thematerials seem to be able to withstand the low temperatures,
active use below the glass transition temperature should be
avoided to prevent early failure.

Third, the elastomer-based sensor performance was dependent on both humidity and temperature conditions. The sensors
showed increasing stiffness with temperature, but this increase
did not precisely match that of the constituent elastomers, indicating that composite materials should be independently
characterized for high-fidelity performance predictions. Furthermore, the sensors showed both a temperature and humidity
dependence due to electric and dielectric impacts, while theconstituent elastomers were relatively humidity-independent,
which speaks to the importance of considering mechanical andelectrical effects in tandem for electromechanical components.
These insights will aid the design for a successful soft robotic
operation outside a laboratory setting.
Acknowledgments

We thank our collaborators at Otherlab, Tim Duggan,
Conor O’Brien, Josh Land, and Jim McBride, for valuable
discussions regarding the applications of this work.
Authors’ Contributions

E.P.: Conceptualization, methodology, validation, formal
analysis, investigation, data curation, writing, review andediting, visualization, supervision, and project administration. S.E.: Methodology, formal analysis, investigation,
writing, review and editing, and visualization. A.A.: Formal
analysis, investigation, and writing. R.K.-B.: Conceptualization, supervision, project administration, review and editing, resources, and funding acquisition.

12

PORTE ET AL.

Author Disclosure Statement

No competing financial interests exist.
Funding Information

This work was supported by a NASA Small BusinessTechnology Transfer grant (80NSSC17C0030). S.E. was
supported by the NASA Space Technology Graduate
Research Fellowship (80NSSC21K1269).
Supplementary Material

Supplementary Figure S1
Supplementary Figure S2
Supplementary Figure S3
Supplementary Figure S4
Supplementary Figure S5
Supplementary Figure S6
Supplementary Figure S7
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Address correspondence to:Elze Porte
Department of Mechanical Engineering
Roberts Engineering Building
University College London
Torrington Place
London WC1E 7JE
United Kingdom
E-mail: e.porte@ucl.ac.uk