Voltage-Induced Wrinkle Performance in a Hydrogel by Dielectric Elastomer Actuation - MDPI

 
Voltage-Induced Wrinkle Performance in a Hydrogel by Dielectric Elastomer Actuation - MDPI
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/polymers
Polymers 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 the
Polymers
Polymers 2018,
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               10, x697
                     FOR PEER REVIEW                                                                          33of
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                                                                                                                   12

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                              5mm
hydrogel 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 is
Polymers 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
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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;
         2018,therefore,
               10, x FOR the
                         PEER  REVIEW
                             wrinkle propagates.                                                               6 of 12

                                                  (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
 gel and2018, 10, 697 through complex reflection and refraction, suppressing the transmission.
          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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