Voltage-Induced Wrinkle Performance in a Hydrogel by Dielectric Elastomer Actuation - MDPI
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polymers
Article
Voltage-Induced Wrinkle Performance in a Hydrogel
by Dielectric Elastomer Actuation
Chao Tang 1 , Bo Li 1,2, *, Chenbang Zou 1 , Lei Liu 1,3 and Hualing Chen 1,2,4
1 School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China; wstcsues@163.com (C.T.);
zouchenbang@stu.xjtu.edu.cn (C.Z.); aist456@163.com (L.L.); hlchen@xjtu.edu.cn (H.C.)
2 Shaanxi Key Laboratory for Intelligent Robot, Xi’an Jiaotong University, Xi’an 710049, China
3 State Key Laboratory of Fluid Power and Mechanic Systems, Zhejiang University, Hangzhou 310027, China
4 State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi’an Jiaotong University,
Xi’an 710049, China
* Correspondence: liboxjtu@xjtu.edu.cn; Tel.: +86-29-8266-0487
Received: 8 May 2018; Accepted: 19 June 2018; Published: 22 June 2018
Abstract: Hydrogel is a type of soft smart material and is responsive to stimuli. In the development
of actuation in hydrogel, electrical actuation features a fast and universal strategy which is favored
in the engineering system. Due to the difficulty in direct electrical actuation in hydrogel, in this
study, an indirect actuation was attained via a dielectric elastomer. An aligned wrinkle pattern was
obtained in the hydrogel upon a direct-current voltage, and it is reversible. The morphology and
nonlinear mechanics of the electro-wrinkling deformation was characterized and analyzed. The
optical property of the wrinkle in hydrogel was investigated, demonstrating a tunable blurring effect
in optics. The electro-wrinkling performance offers a potential application with soft and tunable
optical property in hydrogel-based actuators.
Keywords: hydrogel; wrinkle; actuation; dielectric elastomer; optics
1. Introduction
Hydrogel is a type of soft materials, consisting of long-chain polymer networks and diverse
liquid solvents. Owing to its large liquid component, a piece of hydrogel can deform largely, in terms
of volume change by 50 times after swelling [1] or the attainable maximum stretch of 22 times in
pure shear configuration [2]. This unique mechanical property offers promising applications in tissue
engineering [3], soft robots [4] and biomimetic systems [5].
Hydrogel can be responsive if subjected to different stimuli, including pH value [6],
temperature [7], humidity [8], light intensity [9], and pneumatic pressure [10]. Then, the hydrogels may
form crease patterns, if constrained by a mechanical boundary layer [11], which can be developed as
an active surface [12]; or hydrogel can generate a bending locomotion as a worm-like robot [13], as well
as a non-invasive gripper [14]. However, these actuation strategies are sensitive to the environment
and are only applicable in selected application cases. Pneumatic actuation is direct and easy to control,
but requires complex air chamber design with an extra air pumping source. Thus, a more universal
strategy for hydrogel in actuation is expected.
Electrical voltage or current is well-recognized and is the most adapted physical quantity in the
mechatronic system, requiring a cable only for fast, clean and long distance transmission. Yet, an
electrically activated hydrogel is more favorable. Owing to the ions in hydrogel, directly applying
voltage on hydrogel may cause a local chemical instability or even breakdown [15]. So in this
study, we present a new actuation route for the hydrogel via the electromechanical deformation
of a dielectric elastomer.
Polymers 2018, 10, 697; doi:10.3390/polym10070697 www.mdpi.com/journal/polymersPolymers 2018, 10, x FOR PEER REVIEW 2 of 12
we present a new actuation route for the hydrogel via the electromechanical deformation of a
Polymers 2018, 10, 697 2 of 12
dielectric elastomer.
Dielectric elastomer is able to generate large mechanical stretch under voltage. The voltage-
triggered deformation
Dielectric in dielectric
elastomer is ableelastomer
to generatehas been
largeexplored in various
mechanical actuators
stretch under for direct stroke
voltage. The
[16] or motion output
voltage-triggered [17]. Recently,
deformation dielectricelastomer
in dielectric elastomerhas is abeen
candidate for reversible
explored in variousand electrical-
actuators for
stretching platform on the micro-scale. Under this spotlight, a millimeter-wave phase shifter
direct stroke [16] or motion output [17]. Recently, dielectric elastomer is a candidate for reversible and has been
assembled with twoplatform
electrical-stretching dielectric
on elastomers in coordination
the micro-scale. [18]. Another
Under this spotlight, example is phase
a millimeter-wave a dielectric
shifter
elastomer based soft with
has been assembled manipulation for inelastomers
two dielectric vivo cell stretching [19]. These
in coordination reports inspired
[18]. Another example us is atodielectric
use the
dielectric
elastomerelastomer
based softtomanipulation
stretch a hydrogel electrically.
for in vivo Herein,
cell stretching theThese
[19]. actuation stretch
reports of the
inspired us hydrogel
to use the
with dielectric
dielectric elastomer
elastomer as awell
to stretch as the
hydrogel wrinkling
electrically. morphology
Herein, is demonstrated.
the actuation The optical
stretch of the hydrogel with
performances in transmission are characterized, illustrating a new application in
dielectric elastomer as well as the wrinkling morphology is demonstrated. The optical performances soft and stretchable
optics.
in transmission are characterized, illustrating a new application in soft and stretchable optics.
This
Thismanuscript
manuscriptisisorganized
organizedasas follows.
follows.Section
Section2 introduces
2 introduces the the
deformation
deformationmechanism
mechanism of theof
dielectric elastomer. Section 3 describes the experiments and the result. An
the dielectric elastomer. Section 3 describes the experiments and the result. An electromechanical electromechanical
theoretical
theoreticalanalysis
analysisisisdeveloped
developedininSection
Section4,4,with
withcalculation
calculationresult. Section
result. 5 extends
Section 5 extendsthethemodel
modelto
study thethe
to study optical
opticalproperties, accompanied
properties, accompanied bybyananillustration
illustrationininelectrical
electricalblurring
blurringeffect.
effect.Section
Section66
concludes the results with discussion and prospects.
concludes the results with discussion and prospects.
2.2.Deformation
DeformationMechanism
Mechanismof
ofDielectric
DielectricElastomer
Elastomer
AAdielectric
dielectricelastomer
elastomerisisananinsulating
insulatingmaterial
materialininaamembrane
membraneshape.
shape.When
Whenthe
thevoltage
voltagein inthe
the
order
order of
of kV
kV is applied, positive
positive andandnegative
negativecharges
chargesare
areaccumulated
accumulated separately
separately onon each
each surface
surface of
of the
the dielectric
dielectric elastomer.
elastomer. Due Due
to thetoattraction
the attraction of charges,
of charges, the electrostatic
the electrostatic force
force will will compress
compress the
the dielectric
dielectric
elastomer, elastomer, and and
and this soft this elastic
soft and elastic deforms
material material in
deforms in thinning
terms of terms of in
thinning in thickness,
thickness, as shownas in
shown
Figure in Figure
1. The 1. factor
main The main
which factor which
caused caused the deformation
the deformation of elastomer
of the dielectric the dielectric elastomer
is Maxwell is
stress.
Maxwell stress.Maxwell
The principal The principal Maxwell
stress is definedstress
as is defined as
2
V 2
PMaxwell ==ε ε0ε rε V
0 r h
PMaxwell (1)(1)
h
where, ε 0 is the permittivity of free space, ε r is the relative permittivity, V is the applied voltage,
where, ε 0 is the permittivity of free space, ε r is the relative permittivity, V is the applied voltage, and h
is theh real
and is the real membrane
membrane thickness.
thickness.
From
Fromthe theEquation
Equation(1),
(1),we
wecan
cansee
seethat
thatan
anincrease
increasein inthe
theactuation
actuationvoltage
voltageor
oraadecrease
decreaseof
ofthe
the
initial thickness of the dielectric elastomer is an effective method to improve the actuation
initial thickness of the dielectric elastomer is an effective method to improve the actuation of the of the
dielectric elastomeractuator.
dielectric elastomer actuator. Therefore,
Therefore, the dielectric
the dielectric elastomer
elastomer membrane
membrane usually
usually needs needs a
a mechanical
mechanical
pre-stretch pre-stretch to obtain alarge
to obtain a stabilized stabilized large[20,21].
actuation actuation [20,21].
(a) (b)
Dielectric elastomer Thickness contraction
+Q
Compliant electrodes
Area expansion
Figure
Figure1.1.Schematic
Schematicofofan
anactuated
actuateddielectric
dielectricelastomer
elastomer(a)
(a)Reference
Referencestate;
state;(b)
(b)Actuation
Actuationstate.
state.
3.3.Experimental
Experimentaland
andResults
Results
3.1.Hydrogel
3.1. HydrogelSynthesis
Synthesis
The hydrogel
The hydrogel was
wassynthesized, as shown
synthesized, in Figure
as shown 2. With
in Figure 2. reference to the research
With reference to the ofresearch
conductive
of
gel [22], acrylamide
conductive (AAm, A8887,
gel [22], acrylamide Sigma-Aldrich,
(AAm, Saint Louis, Saint
A8887, Sigma-Aldrich, MO, USA)
Louis,was
MO, used as the
USA) wasmonomer,
used as
N,N-methylenebisacrylamide (MBAA, M7279, Sigma-Aldrich, Saint Louis, MO, USA) was thePolymers
Polymers 2018,
2018, 10,
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FOR PEER REVIEW 33of
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the monomer, N,N-methylenebisacrylamide (MBAA, M7279, Sigma-Aldrich, Saint Louis, MO, USA)
crosslinkers,
was N,N,N 0 ,N
the crosslinkers,
0 -tetramethylethylenediamine (TEMED, T7024, Sigma-Aldrich, Saint Louis,
N,N,N′,N′-tetramethylethylenediamine (TEMED, T7024, Sigma-Aldrich, Saint
MO, USA)
Louis, MO,was
USA) thewas
crosslinking accelerator,
the crosslinking and LiCland
accelerator, (V900067, Sigma-Aldrich,
LiCl (V900067, Saint Louis,
Sigma-Aldrich, MO,Louis,
Saint USA)
was the
MO, electrolyte.
USA) AAm and AAm
was the electrolyte. LiCl were dissolved
and LiCl in deionized
were dissolved water at the
in deionized concentrations
water of 2.2 and
at the concentrations
3.3 M, respectively. MBAA was then added as 0.0006 wt % the weight of AAm.
of 2.2 and 3.3 M, respectively. MBAA was then added as 0.0006 wt % the weight of AAm. After After degassing in a
degassing in a vacuum chamber, TEMED 0.0025 to the weight of AAm was added. The solutionsa
vacuum chamber, TEMED 0.0025 to the weight of AAm was added. The solutions were poured into
× 150.0 into
150.0 poured × 0.1a mm 3
were 150.0 glass mold.
× 150.0 × 0.1The
mmgels were
3 glass cured
mold. at gels
The roomwere
temperature
cured at for
room90 temperature
min and cut intofor
the designed shape using a laser cutter.
90 min and cut into the designed shape using a laser cutter.
AAm
LiCl
AP
TEMED DI
MBAA water
Figure
Figure 2.
2. The
Thesynthesis
synthesisof
ofthe
thehydrogel.
hydrogel.
3.2. Hydrogel and Dielectric Elastomer in Actuation
3.2. Hydrogel and Dielectric Elastomer in Actuation
The VHB membrane was stretched to four times their initial length and width and was fixed to
The VHB membrane was stretched to four times their initial length and width and was fixed to a
a square-shaped acrylic frame. The inner side length of the square acrylic frame is 60 mm. Two pieces
square-shaped acrylic frame. The inner side length of the square acrylic frame is 60 mm. Two pieces of
of hydrogel were attached to the pre-stretched dielectric elastomer (VHB 4910, 3M company, Sao
hydrogel were attached to the pre-stretched dielectric elastomer (VHB 4910, 3M company, Sao Paulo,
Paulo, MN, USA), consisting of fully transparent tri-layer laminates, shown in Figure 3. The length
MN, USA), consisting of fully transparent tri-layer laminates, shown in Figure 3. The length and width
and width of the hydrogel is 60 mm × 10 mm. Two copper electrodes which placed on the frame
of the hydrogel is 60 mm × 10 mm. Two copper electrodes which placed on the frame linked the
linked the hydrogel electrodes and the voltage amplifier. Hydrogel also serves as a compliant
hydrogel electrodes and the voltage amplifier. Hydrogel also serves as a compliant electrode, owing to
electrode, owing to LiCl electrolyte solution. Thus, the area in the dielectric elastomer covered by
LiCl electrolyte solution. Thus, the area in the dielectric elastomer covered by hydrogel can deform
hydrogel can deform with the voltage on. So it is defined as electro-active; while the rest of the
with the voltage on. So it is defined as electro-active; while the rest of the dielectric elastomer cannot
dielectric elastomer cannot be electrically actuated, so it is defined as the passive part providing
be electrically actuated, so it is defined as the passive part providing mechanical boundary force.
mechanical boundary force. A signal generator (No. TMDG4062, RIGOL AgilentTM, Santa Clara, CA, USA)
A signal generator (No. DG4062, RIGOL Agilent , Santa Clara, CA, USA) and a voltage amplifier
and a voltage amplifierTM (No. 1010B-HS, TrekTM, Amplifier, New York, NY, USA) generate an
(No. 1010B-HS, Trek , Amplifier, New York, NY, USA) generate an incremental voltage with a step
incremental voltage with a step of 20 V to the dielectric elastomer until the electrical breakdown
of 20 V to the dielectric elastomer until the electrical breakdown occurs. The experimental setup is
occurs. The experimental setup is illustrated in the Supplementary materials (SM).
illustrated in the Supplementary materials (SM).
The dielectric elastomer that is covered with hydrogel gel is defined as the electro active part.
When(a) subjectedDielectric elastomer
to voltage, Hydrogel
the electro active area expands. Upon the expanding, it meets the rigid
boundaries of the (prestretched)
acrylic frame, the hydrogel with the dielectric elastomer wrinkles, V tuning its
transparency and showing a blurring effect in optics, as Assemble
shown in Figure 3. Figure 4 collects the
snap-shots of the process in the actuation using a microscope (No. SZX16, OLYPUMTM , Tokyo, Japan).
+
After the voltage is turned on, the dielectric elastomer expands and drives the hydrogel to expand as
Copper
well. Then upon a critical voltage, the hydrogel wrinkles by forming a regular patterned morphology.
electrode
As the voltage ramps up further, the dielectric elastomer thins down consequently;
Voltage on so that it withstands
an even
(b) higher electrical field which will finally causes the electrical breakdown failure. So as long as
the voltage is below the breakdown criteria, the hydrogel is able to quickly change its transparency by
forming wrinkles and can be used in optical applications.
10mm
5mm 5mmhydrogel can deform with the voltage on. So it is defined as electro-active; while the rest of the
dielectric elastomer cannot be electrically actuated, so it is defined as the passive part providing
mechanical boundary force. A signal generator (No. DG4062, RIGOL AgilentTM, Santa Clara, CA, USA)
and a voltage amplifier (No. 1010B-HS, TrekTM, Amplifier, New York, NY, USA) generate an
incremental voltage with a step of 20 V to the dielectric elastomer until the electrical breakdown
Polymers 2018, 10, 697 4 of 12
occurs. The experimental setup is illustrated in the Supplementary materials (SM).
(a) Dielectric elastomer Hydrogel
(prestretched) V
Assemble
+
Copper
Polymers 2018, 10, x FOR PEER REVIEW electrode
4 of 12
Voltage on
Figure 3. Hydrogels are attached to pre-stretched dielectric elastomer, consisting of a soft and fully
(b)
transparent tri-layer laminate. After applying voltage, the hydrogel deforms and wrinkles. (a)
Fabrication process of the actuator. (b) Blurring effect in optics. The white dash box showing the area
which is covered with hydrogel.
The dielectric elastomer that is covered with hydrogel gel is defined as the electro active part.
When subjected to voltage, the electro active area expands. Upon the expanding, it meets the rigid
boundaries of the acrylic frame, the hydrogel with the dielectric elastomer wrinkles, tuning its
transparency and showing 10mma blurring effect in optics, as shown in Figure 3. Figure 4 collects the snap-
shots of the process in the actuation using a microscope (No. 5mm SZX16, OLYPUMTM, Tokyo, 5mm Japan).
After the voltage is turned on, the dielectric elastomer expands and drives the hydrogel to expand as
well. Figure
Then upon a critical voltage,
3. Hydrogels the hydrogel
are attached wrinkles
to pre-stretched by forming
dielectric a regular
elastomer, patterned
consisting of amorphology.
soft and
As the voltage
fully ramps
transparent up further,
tri-layer theAfter
laminate. dielectric elastomer
applying voltage, thins down deforms
the hydrogel consequently; so that it
and wrinkles.
withstands an evenprocess
(a) Fabrication higherofelectrical field(b)which
the actuator. willeffect
Blurring finally causesThe
in optics. thewhite
electrical breakdown
dash box failure.
showing the
So asarea
longwhich is voltage
as the covered with hydrogel.
is below the breakdown criteria, the hydrogel is able to quickly change its
transparency by forming wrinkles and can be used in optical applications.
(a) Voltage off (b) Expanding
Hydrogel
Dielectric elastomer 2mm 2mm
(c) Wrinkling (d) Breakdown
2mm 2mm
Figure 4. Actuation and wrinkling of the hydrogel via a dielectric elastomer. (a) Without a voltage, it
Figure 4. Actuation and wrinkling of the hydrogel via a dielectric elastomer. (a) Without a voltage, it is
is flat.
flat. (b)(b) With
With a low
a low voltage,
voltage, theand
the gel gelelastomer
and elastomer
expand.expand. (c) aUpon
(c) Upon criticala voltage,
critical voltage, the gel
the gel wrinkles.
wrinkles. (d) As the voltage ramps, the actuator fails
(d) As the voltage ramps, the actuator fails at breakdown. at breakdown.
The actuation deformation of the hydrogel experiences a transition from in-plane expansion to
The actuation deformation of the hydrogel experiences a transition from in-plane expansion to
out-of-plane deflection via dielectric elastomer under a voltage [23]. Without a designed shape of
out-of-plane deflection via dielectric elastomer under a voltage [23]. Without a designed shape of
actuation area, the out-of-plane deflection would be quite irregular and randomly patterned, since
actuation area, the out-of-plane deflection would be quite irregular and randomly patterned, since
the imperfection in a local spot will disorder the wrinkle pattern. Buckled, crumpled, and wrinkled
the imperfection in a local spot will disorder the wrinkle pattern. Buckled, crumpled, and wrinkled
pattern have been identified using a circular-shaped electrode [24,25]. Aligning and tuning the
pattern have been identified using a circular-shaped electrode [24,25]. Aligning and tuning the
wrinkle morphology has great potential in optical application, especially the property of
wrinkle morphology has great potential in optical application, especially the property of transparency.
transparency. Therefore, a rectangular-shaped hydrogel is presented here.
Therefore, a rectangular-shaped hydrogel is presented here.
3.3. Wrinkle Morphology Characterization
The morphology of the wrinkle surface was measured to show its out-of-plane displacement, as
can be observed in the experimental setup in Figure S1 of SM. Figure 5 shows the morphology of the
wrinkled hydrogel under a voltage of 5130 V. The wrinkles are in parallel along the width direction.
The wavelength and depth of the wrinkle are consistent throughout the surface, indicating there isPolymers 2018, 10, 697 5 of 12
3.3. Wrinkle Morphology Characterization
The morphology of the wrinkle surface was measured to show its out-of-plane displacement,
as can be observed in the experimental setup in Figure S1 of SM. Figure 5 shows the morphology
of the wrinkled hydrogel under a voltage of 5130 V. The wrinkles are in parallel along the width
direction. The wavelength and depth of the wrinkle are consistent throughout the surface, indicating
there is no global buckling. Figure 6 shows the growth of the wrinkle amplitude as a function of the
voltage. The hydrogel is flat until a critical voltage, then it wrinkles, and evolves its pattern as the
Polymers2018,
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voltage ramps up. By examining the experiments, the change in wavelength and amplitude were
characterized in statistics. By accounting for all the wrinkles in the whole membrane, the average value
valueof
value ofthe
theamplitude
amplitudeand andthe
thewavelength
wavelengthare arecalculated
calculatedwithwitherror
errorbarbarshowing
showingthethevariance.
variance.TheThe
of the amplitude and the wavelength are calculated with error bar showing the variance. The overall
overall tendency
overall tendency in in the
the waveform
waveform vs. vs. the
the voltage
voltage isis shown
shown inin Figure
Figure 7.7. As
As the
the voltage
voltage increases,
increases,
tendency in the waveform vs. the voltage is shown in Figure 7. As the voltage increases, hydrogel
hydrogelmaintains
hydrogel maintainsits itslevel
levelofofwavelength,
wavelength,but butits
itsamplitude
amplitudedecreases
decreasesslightly.
slightly.Compared
Comparedwith withthethe
maintains its level of wavelength, but its amplitude decreases slightly. Compared with the wrinkle in
wrinkle in
wrinkle in elastomer,
elastomer, different
different wrinkle
wrinkle patterns
patterns can
can be
be switched
switched ifif the
the strain
strain ramps
ramps up
up [26],
[26], but
but inin
elastomer, different wrinkle patterns can be switched if the strain ramps up [26], but in our experiment,
ourexperiment,
our experiment,we weobserved
observedonlyonlyone
onetype
typeofofwrinkle
wrinklepattern
patternwith
withaaslight
slightchange
changeininits
itsamplitude.
amplitude.
we observed only one type of wrinkle pattern with a slight change in its amplitude. If the voltage
IfIfthe
thevoltage
voltageramps
rampsup, up,the
theelectrical
electricalbreakdown
breakdownhas hasoccurred,
occurred,leading
leadingto tothe
thefundamental
fundamentalfailure.
failure.
ramps up, the electrical breakdown has occurred, leading to the fundamental failure.
Figure5.5.
Figure
Figure 5.The
The characterization
Thecharacterization ofthe
characterizationof
of thewrinkled
the wrinkledhydrogel
wrinkled hydrogelsurface.
hydrogel surface.
surface.
5110VV
5110 5080VV
5080
[mm]
Amplitude [mm]
5120VV
5120
Amplitude
5070VV
5070 5060VV
5060
5090VV
5090
Length[mm]
Length [mm]
Figure Therelationship
Figure6.6.The relationshipof
ofwrinkle
wrinklemorphology
morphologyas
asaafunction
function of
of the voltage.
the voltage.
voltage.
This isis due
This due to to the
the nonlinearity
nonlinearity inin the
the mechanical
mechanical property
property of of hydrogel.
hydrogel. InIn our
our experiment,
experiment, the the
propagation of
propagation of the
the wrinkle
wrinkle was
was recorded,
recorded, asas shown
shown inin Figure
Figure S2S2 in
in the
the SM.
SM. TheThe hydrogel
hydrogel andand
dielectric elastomer
dielectric elastomer deform
deform with
with phase
phase coexistence:
coexistence: The
The wrinkled
wrinkled part
part and
and flat
flat part
part coexist
coexist at
at the
the
same level
same level of of voltage.
voltage. WithWith the
the increasing
increasing voltage,
voltage, the
the area
area of
of the
the wrinkled
wrinkled part
part expands
expands atat the
the
expenseof
expense ofthe
theflat
flatpart;
part;therefore,
therefore,the
thewrinkle
wrinklepropagates.
propagates.Polymers 2018, 10, 697 6 of 12
This is due to the nonlinearity in the mechanical property of hydrogel. In our experiment, the
propagation of the wrinkle was recorded, as shown in Figure S2 in the SM. The hydrogel and dielectric
elastomer deform with phase coexistence: The wrinkled part and flat part coexist at the same level of
voltage. With the increasing voltage, the area of the wrinkled part expands at the expense of the flat
Polymerspart;
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(a)
Wavelength
Wavelength [mm]
(b)
Amplitude
Amplitude [mm]
Voltage [V]
FigureFigure
7. Voltage tuned
7. Voltage wrinkling
tuned morphology
wrinkling morphology of thehydrogel
of the hydrogelin in
(a)(a) wavelength
wavelength and
and (b) (b) amplitude.
amplitude.
3.4. Optical Transmission
3.4. Optical Measurement
Transmission Measurement
The optical
The optical propertyisischanged
property changed inin thethe
demonstration of the blurring
demonstration of the effect on background
blurring effect onChinese
background
characters, which can be found in Figure 3. The light transmission was measured in the visible light
Chinese characters, which can be found in Figure 3. The light transmission was measured in the
spectrum using a spectrophotometer, as shown in Figure 8. It is interesting that the hydrogel is even
visible light spectrum using a spectrophotometer, as shown in Figure 8. It is interesting that the
clearer, (high transmission and low absorbance) in the actuation before the wrinkled state. According
hydrogel is even
to the clearer,law
Beer-Lambert (high transmission
[27], and low
when the hydrogel absorbance)
expands in the actuation
with the elastomer, before
the thickness the wrinkled
decreases,
state. According
promoting the to transmission.
the Beer-Lambert lawin[27],
However, when the
the wrinkled hydrogel
state, expands
light travels in thewith theelastomer
gel and elastomer, the
thickness decreases,
through complexpromoting
reflection andtherefraction,
transmission. However,
suppressing in the wrinkled state, light travels in the
the transmission.
Using the hydrogel electrode, we proposed a new route
gel and elastomer through complex reflection and refraction, suppressing for visible light manipulation to achieve
the transmission.
a soft and wearable camouflage by voltage tuning. The VIDEO (Video S1) is shown in the SI. In the
video, the applied voltage is of 5200 V amplitude and 1 Hz frequency.visible light spectrum using a spectrophotometer, as shown in Figure 8. It is interesting that the
hydrogel is even clearer, (high transmission and low absorbance) in the actuation before the wrinkled
state. According to the Beer-Lambert law [27], when the hydrogel expands with the elastomer, the
thickness decreases, promoting the transmission. However, in the wrinkled state, light travels in the
Polymers
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elastomer 7 of 12
Figure8.8.The
Figure Theoptical
opticaltransparency
transparencyin in
thethe hydrogel
hydrogel is actuated
is actuated by voltage
by the the voltage
fromfrom
flat toflat to wrinkled
wrinkled state
state and transmission visible spectra are measured in the three actuation
and transmission visible spectra are measured in the three actuation states. states.
4. Theoretical Analysis
4.1. On the Critical Condition
Soft material wrinkles as a result of mechanical instability. Wrinkles have been well characterized
in the study of nonlinear mechanics [28] and are then harnessed as routes in manufacturing tunable
surface adhesion [29], photonics [30], bio-mimetic camouflage [31], and acoustic meta-materials [32].
Wrinkle instability in hydrogel has been documented and analyzed when the hydrogel swells in
solution [33]. So, we used a well-established model with a combination of voltage-triggered instability
in dielectric elastomer [34,35]. Figure 9a,b sketches the wrinkled state. Initially, without the voltage,
the hydrogel is of the dimension L × L g , and at the wrinkled state the dimension changes to L × λ a L g
where λ a represents the actuation stretch. The critical voltage Φc on the wrinkle initiation is essential.
Herein, due to the negligible inter-facial polarization, the critical voltage in the wrinkle instability is
expressed as [34,36]
Φc 2 H 2
2
πE
ε 2 ∼ 2
1 + n2 (2)
H 12(1 − E ) Lλ a
where ε is the permittivity, H is the original total thickness of the two hydrogels and the dielectric
elastomer before actuation, E is the elastic modulus, ν is the Poisson ratio, λ a is the actuation stretch
before wrinkling, and n = 1, 2, 3 . . . is the constant.
The value of the parameters is as follows: ε = 4 × 10−11 F/m, H = 0.26 × 10−3 m, E = 150 kPa,
ν = 0.49, L = 0.06 m, λ a = 1.5 (see the experiments in Figures S3 and S4 of the SM for the material
mechanical property). Figure 9c shows the results. Nonlinear mechanical material wrinkles in different
instability modes, displaying different patterns on its surface [28]. Hydrogel is able to wrinkle to
diverse patterns, including ridge, crease, double period, and crater. Here, in our observation, only
the first mode (n = 1) sinusoidal pattern was identified with critical voltage (5100 V), which is in
agreement with the experiment shown in Figure 5. To trigger the higher order mode of wrinkle, higher
voltage is required, which may exceed the electrical breakdown limit in dielectric elastomer, so it is
hardly attainable.Polymers 2018, 10, 697 8 of 12
Polymers 2018, 10, x FOR PEER REVIEW 8 of 12
Figure 9. (a) The initial state of the hydrogel and dielectric elastomer; (b) the wrinkled state in actuation;
Figure 9. (a) The initial state of the hydrogel and dielectric elastomer; (b) the wrinkled state in
(c) the critical voltage at different order mode of wrinkling instability, where n = 1, 2, 3.
actuation; (c) the critical voltage at different order mode of wrinkling instability, where n = 1, 2, 3.
4.2. Optical Properties
The value of the parameters is as follows: ε = 4 ×10 F/m , H = 0.26 ×10 m ,
−11 −3
= 150kPa
E Figure ν = 0.49
10a , shows L = 0.06m
the ,sketch λa = 1.5
of the, optical transmission through theinwrinkled
(see the experiments Figures S3hydrogel
and S4 and
of the
dielectric elastomer. With reference to the experiment, the measurement is illustrated in Figure S5 in
SM for the material mechanical property). Figure 9c shows the results. Nonlinear mechanical material
SM. Assume that the incident light is at the hydrogel surface position x, with an incident angle α, so
wrinkles in different instability modes, displaying different patterns on its surface [28]. Hydrogel is
the refracted angle γ is expressed by [37]:
able to wrinkle to diverse patterns, including ridge, crease, double period, and crater. Here, in our
observation, only the first mode (n = 1) sinusoidal
q pattern was
1 identified
with critical voltage (5100
−1 2 − sin2 α − sin 2α
γ = sin sin α f
V), which is in agreement with the experiment shown in Figure (3)
5. To trigger the higher order mode
2
of wrinkle, higher voltage is required, which may exceed the electrical breakdown limit in dielectric
where f is theso
elastomer, reflected index
it is hardly of the overall hydrogel-elastomer laminate and is taken as 1.2–1.9 [38].
attainable.
The calculated result is plotted in Figure 10b by varying reflected index, the refracted angle changes,
4.2. Optical
within the rangeProperties
from −π/10 to π/10. Thus, the deeply wrinkled surface is able to absorb more
light. This result coincides with a similar blurring actuation in liquid crystal-elastomer hybrid, where
Figure 10a shows the sketch of the optical transmission through the wrinkled hydrogel and
the orientation of the liquid crystal is controlled either by the voltage-induced orientation or by the
dielectric elastomer. With reference to the experiment, the measurement is illustrated in Figure S5 in
stretching force [39].
SM. Assume that the incident light is at the hydrogel surface position x , with an incident angle α ,
so the refracted angle γ is expressed by [37]:
1
γ = sin −1 sin α f 2 − sin 2 α − sin 2α (3)
2 The calculated result is plotted in Figure 10b by varying reflected index, the refracted angle changes,
within the range from −π/10 to π/10. Thus, the deeply wrinkled surface is able to absorb more light.
This result coincides with a similar blurring actuation in liquid crystal-elastomer hybrid, where the
orientation of the liquid crystal is controlled either by the voltage-induced orientation or by the
stretchingPolymers
force2018, 10, 697
[39]. 9 of 12
α α
δ
β δ
β
γ
0.5
1.9
0.4 1.8
1.7
0.3 1.6
1.5
0.2 1.4
1.3
0.1
1.2
γ 0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.5 0 0.5
α
Figure 10. Optical transmission analysis. (a) The refraction optical path through the wrinkled hydrogel.
Figure 10.
(b) Optical transmission
Calculation of optical exciteanalysis. (a) The refraction optical path through the wrinkled
incident angle.
hydrogel. (b) Calculation of optical excite incident angle.
5. Discussion
In this study, the hydrogel was actuated and wrinkled electrically via a dielectric elastomer. In the
study of solid mechanics, when hydrogel has been attached on a thin substrate and wrinkled by
compression, this phenomenon is considered as a global instability. The thickness of the two layers is
also known to affect the wrinkle profile [40]. In a recent study, thicker hydrogel deforms into double
period patterns, which is a surface instability located on the half-infinite substrate [41].
In our experiment, when examining the center part of the hydrogel, we found that with the
increment of voltage, one wrinkle period was separated into two small wrinkles, as highlighted in
Figure 11. This indicates a similarity in the bifurcation instability [42]. An advanced theoretical study
is expected to extend the research of hydrogel actuation and its wrinkle instability.
Reducing the dielectric elastomer actuation voltage is a challenge in application. Current methods
include prestretching the membrane, or the use of a thinner film. It is reported that by prestretcing a
dielectric elastomer into a thickness level of 14 um, one can lower the actuation voltage into 100 V [43].
This may be employed in the future development of dielectric elastomer-based devices with low
energy consumption.is expected to extend the research of hydrogel actuation and its wrinkle instability.
Reducing the dielectric elastomer actuation voltage is a challenge in application. Current
methods include prestretching the membrane, or the use of a thinner film. It is reported that by
prestretcing a dielectric elastomer into a thickness level of 14 um, one can lower the actuation voltage
into2018,
Polymers 10010,
V 697
[43]. This may be employed in the future development of dielectric elastomer-based 10 of 12
devices with low energy consumption.
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
10 12 14 16 18 20
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
10 12 14 16 18 20
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
10 12 14 16 18 20
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
10 12 14 16 18 20
Figure
Figure 11. Local
11. Local growth
growth of aofsmall
a small wrinklewith
wrinkle withthe
thevoltage
voltageat
at(a)
(a) 5170
5170 V
V (b)
(b) 5180
5180VV(c)
(c)5200
5200VV(d)
(d)5220
5220 V.
V.
6. Conclusions
In summary, a hydrogel was synthesized and actuated electrically using a dielectric elastomer
substrate. The hydrogel wrinkles together with the dielectric elastomer, showing an abrupt change in
transparency. The electro-wrinkled surface was characterized experimentally. The wavelength and
amplitude of the wrinkles were identified at different voltage levels, showing the nonlinear relationship
between mechanical deformation and voltage actuation. The change in transparency was measured in
the visible light spectrum, illustrating a potential application as a wrinkle-based soft cloaking by a
voltage actuation.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/7/697/s1.
Figure S1: The experimental setup for the measurement of the wrinkle. (A) The overall system. (B) The
system on the platform. The sample was settled on a manual translation stage. Figure S2: The propagation
of the wrinkle through the surface of the hydrogel. Figure S3: Stress-strain curve of the hydrogel electrode.
Figure S4: Stress–strain curve of the dielectric elastomer VHB 4910. Figure S5: A sketch of measurement by the
spectrophotometer. Figure S6: Blurring effect test (a) a~f were the wrinkled state under the voltage of 4900 V,
5000 V, 5240 V, 5400 V, 5450 V and 5500 V (b) normalized image definition. Video S1: The response of the wrinkling
in hydrogel, with respect to a dynamic voltage, of sinusoidal wave of 5200 V amplitude and 1 Hz frequency.
Author Contributions: C.T., B.L., L.L. and H.C. conceived and designed the experiments; C.T. and C.Z. performed
the experiments and analyzed the data; B.L. wrote the paper.Polymers 2018, 10, 697 11 of 12
Acknowledgments: This work was supported by NSF China (Grant NO. 91748124 and 91648110) and Open
Foundation of the State Key Laboratory of Fluid Power and Mechatronic Systems (GZKF-201508). B. L. thanks
School of Mechanical Engineering for supporting fund of young faculty.
Conflicts of Interest: The authors declare no conflict of interest.
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