Extreme Mechanics Letters 21 (2018) 35–43

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Extreme Mechanics Lettersjournal homepage: www.elsevier.com/locate/eml

Stimuli-induced bi-directional hydrogel unimorph actuators
Shanliangzi Liu a,b , Elisa Boatti c , Katia Bertoldi c,d , Rebecca Kramer-Bottiglio a,b, *aSchool of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA
Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511, USA
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USAd
Kavli Institute, Harvard University, Cambridge, MA 02138, USAbcarticleinfo

Article history:
Received 26 January 2018
Received in revised form 14 March 2018
Accepted 14 March 2018
Available online 22 March 2018
Keywords:
Hydrogel-elastomer
Humidity-sensitive
Interfacial bond
Bi-directionala b s t r a c t
Stimuli responsive hydrogels may be promising materials for shape transforming structures due to theirability to deform significantly in response to an environmental stimulus without the need for external
power. It is well-known that by laminating swelling hydrogels and non-swelling flexible materials,
such as inert elastomers, bending actuation can be achieved. In this work, we report an improvedhydrogel-elastomer bonding process and demonstrate the utility of the process with a stimuli-induced bidirectional hydrogel unimorph actuator. The actuator is comprised of polyacrylamide (PAAm) hydrogel,a humidity-sensitive gel that can swell to 17 times its original volume, bonded to polydimethylsiloxane(PDMS) elastomer, which is humidity insensitive. The unimorph actuator bends in one direction duringswelling in an aqueous environment and in the opposite direction when drying in air, without any
observed delamination between layers. We achieve the strong bonding between PAAm hydrogel and
PDMS elastomer through both chemical and mechanical means, and quantify an order-of-magnitudeimprovement in interfacial bond strength relative to bonding through chemical means alone. We observe
the bi-directional bending behavior of hydrogel unimorph actuators as a function of environmental
humidity and prove that the structure’s motion can be tuned by manipulating the thickness of the PDMS
elastomer layer, as well as predicted using a simplified analytical approach. Lastly, we expand upon asimple rectangular unimorph structure to demonstrate more complex bending, wrinkling, and surface
texture behaviors.
© 2018 Elsevier Ltd. All rights reserved.

1. Introduction
Soft, intricate multifunctional structures seen in nature [1–4]have inspired the design of shape transforming materials capableof reconfiguration in response to environmental stimuli [5]. Various kinds of materials have been shown to exhibit shape changing behaviors, including shape memory alloys [6,7], electroactive
polymers [8–10], shape memory polymers [11,12] and polymer
gels [13,14]. Such materials have applications in microfluidics[15–17], biomedical devices [18–20], soft robotics [21–23], opticaldevices [24,25], microfabrication [26–28] and stimuli-responsive
surfaces [29–31]. As a class of stimuli-responsive materials, hydrogels are able to change volume with reversible swelling anddeswelling in response to environmental stimuli such as temperature [32–39], pH [32,40–43], light [44–46], enzymes [47],ion strength and solvent composition [48,49], electric and magnetic fields [14,21,50,51], elastic instabilities and surface oscillation [52,53]. The earliest published results on hydrogel actuation
author.
* Corresponding
E-mail address: rebecca.kramer@yale.edu (R. Kramer-Bottiglio).
https://doi.org/10.1016/j.eml.2018.03.001
2352-4316/© 2018 Elsevier Ltd. All rights reserved.

used the material for liquid flow control in microfluidic devices[17,38,39,54,55]. In these systems, hydrogels served as valves andunderwent homogeneous expansion and shrinkage to control fluid
flow. However, homogeneous swelling and deswelling only result in volumetric material changes. Shape change induced byinhomogeneous deformations, such as bending [11,21,34,49,56],
twisting [11,49,56] and buckling [57,58] require more complex
designs of hydrogel composites. A number of approaches havebeen developed to make reversible and controllable 3D shapechanging systems by integrating multiple active layers with different soft material networks that respond differently to the same
stimulus. For instance, electro-actuated hydrogel walkers havebeen achieved by incorporating two opposite charge hydrogels:acrylamide (AAm)/sodium acrylate (NaAc) copolymer and acrylamide/quaternized dimethylaminoethyl methacrylate (DMAEM
A-Q) copolymer that deform in opposite directions in responseto electric fields in aqueous solutions [21]. Also, 3D printed reversible shape changing components have been demonstrated byintegrating hydrogels with shape memory polymers (SMPs), which
are regulated by both the swelling properties of hydrogels and thetemperature sensitivities of SMPs [11].

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Several studies have focused on aqueous-induced bi-directional
reversible 3D shape changing structures by integrating hydrogels
with other soft materials [11,21,42,56,59–61]. In these previous
works, the interfacial bond strength between active and inactive
materials has often not been evaluated, with only a few observable cycles reported [36,42]. Soft material systems that can attainmany cycles of bidirectional bending triggered by swelling anddeswelling with controllable interfacial strength and structures
have yet to be achieved. Such systems could be embedded into
soft mechanical assemblies and enable functionality in an ordinary
aqueous environment with only a single stimulus.
In this work, we describe a humidity-sensitive bi-directionalhydrogel unimorph actuator enabled by coupled chemical andmechanical interfacial bond processes. The bending actuator iscomprised of an active hydrogel layer (polyacrylamide (PAAm))and an inactive elastomer layer (Sylgard 184 polydimethylsiloxane(PDMS)), where the interfacial bond strength between the twomaterials is controllable by the adjusting contact area via surface
texture of the elastomer.
Leveraging the high swelling ratio of polyacrylamide hydrogel,
we achieve large shape changes using differential swelling in theunimorph structure without any delamination between layers. We
further analytically predict the curvature of rectangular bi-layered
unimorph structures as a function of elastomer layer thicknesses.
Finally, we extend the robust bonding technique to more complex structures to demonstrate a blooming and un-blooming soft
flower, as well as changes in surface wrinkling and surface texture
induced by humidity.
2. Results and discussion
2.1. Hydrogel unimorph actuator fabrication
Tough bonding between a hydrogel and an elastomer is required to achieve remarkable bending behavior using hydrogels
with high swelling ratios. We selected acrylamide as the monomer
to form polyacrylamide hydrogel with a swelling ratio of ∼17
when fully swollen. The swelling ratio of polyacrylamide hydrogel
as a function of time is shown in Figure S1 (Supporting Information). A schematic of the hydrogel unimorph actuator fabrication
process is shown in Fig. 1. First, the surface of an acrylic mold
was textured using a laser engraving system (Universal Laser Systems VLS 2.30, fitted with a 30 W CO2 laser). The laser engravedlines in perpendicular directions on the surface, which resulted
in micro-scale texture (Fig. 1(a)–(b)). We note here that surface
texture can be easily tuned using this approach by adjusting
power and speed of the laser, as detailed in the Experimental
Section. Next, a micro-textured PDMS elastomer sheet was madeby pouring liquid elastomer after mixing and defoaming into thelaser-textured acrylic mold, followed by a degassing step in air
for an hour to remove extra air bubbles and a cure step at 60o
C for 1.5 h (Fig. 1(c)). Once cured, the elastomer was removed
from the mold and flipped (Fig. 1(d)). Then, the micro-textured
PDMS elastomer surface was modified by exposure to oxygen
plasma to produce hydroxyl groups on the surface. Exposed surfacehydroxyl groups enabled grafting of 3-(Trimethoxysilyl)propyl
methacrylate (TMSPMA), which generated methacrylate groups
on the surface and served as a chemical adhesive in the subsequent step (Fig. 1(e)). Next, PAAm pre-gel solution was poured
into a smooth acrylic mold and the TMSPMA-functionalized microtextured PDMS elastomer was placed over the solution (Fig. 1(f)).
The exposed methacrylate groups on the PDMS elastomer surface formed linkage with acrylate groups in acrylamide under
UV photo-polymerization (Fig. 1(g)), generating PAAm hydrogel
chemically bonded onto PDMS elastomer. Finally, after fabricationof the bilayer sheet, hydrogel unimorph actuators (30 mm × 4 mm)

were cut using the laser engraving system (Fig. 1(h)). We notehere that a variety of tools and methods may be used to define
the cut shape of the unimorph, and that more intricate 2D shapes
would result in more complex 3D structures upon swelling anddeswelling.
2.2. Hydrogel-elastomer interfacial bond strength
The primary novelty of the enhanced hydrogel-elastomer bonding process is the introduction of continuous texture to the bonded
surfaces, which increases the area of interaction between thosesurfaces (Fig. 2(a)). Our approach was inspired by Zhang et al. [62],who employed micro-pillar surface structures fabricated using
photolithography to enhance bonding. Here, we employ a continuous surface texture produced by laser rastering to further reduce
the hydrophobicity of the surface (relative to a non-continuous
texture) while maintaining increased contact area. Texture createdby laser rastering is also a highly scalable, fast and easy process.
We evaluated the strength of the interfacial bond between theelastomer and the hydrogel in the flat state (right after UV curing).
To measure the interfacial bond strength of hydrogel-elastomer
bonding, 90o peel strength tests were performed using an Instronmaterials testing system. As illustrated in Fig. 2(b), a stiff polyethylene terephthalate (PET) film was attached to the top surface of thehydrogel with cyanoacrylate to avoid hydrogel stretching during
peeling. The bottom surface of the elastomer was adhered to a
3D printed adapter fixed to the Instron using elastomer as theadhesive. The adhesive elastomer was poured into trapezoidal
channels in the adapter plate and the cured hydrogel-elastomer
bilayer was placed on top. When cured, this provides a strong
mechanical bond to the adapter plate below, and a strong chemicalbond to the elastomer layer above. The samples were sealed ina pan filled with water until taken out for peel tests to prevent
drying. We determined that dehydration over the duration of thetests was negligible since the tests only required a few minutes.
The dehydration ratio is plotted as a function of dehydration time
as shown in Figure S2 (Supporting Information). All the peel tests
were performed with a constant peeling rate of 20 mm/min. Theinterfacial bond strength is calculated as the peeling force in thesteady state per unit width and measured in units of N/m [63–65].
To measure the effect of surface texturing, we evaluated hydrogel-elastomer interfacial bond strength as a function of surface
roughness. Different surface roughnesses were achieved by adjusting the laser power and speed settings. We analyzed the lasertextured surface using an optical profiler (Zeta Instruments) andmeasured the root mean square roughness (Srms ; root mean square
average of the roughness data points) and surface area ratio (Sdr ;the actual interfacial surface area relative to the geometric projected surface area). As shown in Fig. 2(c), the interfacial bondstrength initially improves with increasing surface roughness due
to increasing contact area at the material interface. At a surface
roughness of Srms = 28.85 µm Sdr = 2.30, the bond strength
peaks (∼500 N/m) and monotonically decreases with increasing
surface roughness thereafter. We attribute this to wetting mechanics between the liquid hydrogel solution and the rough elastomer
surface. As surface roughness initially increases, wetting is characterized as a Wenzel state, where homogeneous wetting means that
increasing the elastomer surface roughness increases the contact
area between the liquid hydrogel and elastomer. However, above
a critical surface roughness threshold, wetting is characterized as a
Cassie state, where the liquid cannot fully penetrate the roughness
and contact area at the liquid–solid interface decreases. In our
experiments, this decreasing contact area manifests as decreasing
interfacial bond strength, and the peak indicated in Fig. 2(c) marks
the Wenzel–Cassie transition where interfacial bond strength isoptimized.

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Fig. 1. Schematic of hydrogel unimorph actuators fabrication process. (a) Laser engraving on acrylic sheet along y direction. (b) Rotating the acrylic sheet by 90o to engrave
along x direction. (c) Pouring uncured PDMS into laser-textured acrylic mold. (d) Peeling the cured PDMS from the mold and flipping. The inset shows 3D image of the microtextured PDMS surface and a z profile of the line section. (e) Chemically modifying micro-textured PDMS surface (oxygen plasma followed by 3-(Trimethoxysilyl)propyl
methacrylate (TMSPMA) grafting). (f) Pouring the pre-gel solution onto the micro-textured, chemically treated PDMS and then (g) photo-polymerizing under UV. (h) Laser
cutting unimorph actuators from the bilayer. The inset shows the exposed methacrylate groups on the PDMS surface formed linkage with acrylate groups in acrylamide.

Using the optimized surface roughness parameters, we measured the contact angle between water and the chemically unmodified laser-textured elastomer surface to be approximately
121.6o , and the same static contact angle using our PAAm hydrogel
solution in place of water was measured as approximately 110.5o .
Furthermore, this contact angle reduces to approximately 25o after
chemical surface modification, which indicates strong wetting of
the hydrogel to the elastomer surface, resulting in increased bonding sites and a stronger interfacial bond. The dramatic reduction incontact angle in response to chemical surface modification highlights the importance of the chemical contribution to the overallbonding process.
Hydrogel was also bonded to non-textured elastomer using
the same chemical treatment as the micro-textured elastomer
surfaces. Without surface texturing, the hydrogel layer was easily peeled off the PDMS with no surface deformations observed(Fig. 2(d)). In contrast, as shown in Fig. 2(e), hydrogel peelingfrom the micro-textured elastomer surface occurred in two phases.
First, stretching at the interface was observed as the stress on
the hydrogel layer was increased. Second, once the critical tearforce was reached, mechanical failure initiated at the edge of thehydrogel and propagated down the length of the film. We further
note that after the peel tests, hydrogel remnants remained on
the PDMS surfaces for all the samples, which indicates that thehydrogel-elastomer bond strength exceeds the tensile strength of
the hydrogel.
Representative data from the peel tests are shown in Fig. 2(f).
The interfacial bond strength between chemically modified (nontextured) elastomer surfaces and hydrogel is found to be ∼14 N/m.
As a comparison, Yuk et al. assembled polyacrylamide commonhydrogels and elastomers into hybrids by modifying elastomer

surfaces with benzophenone [63]. The measured interfacial bondstrength between the non-textured elastomer (chemically treated)
and the polyacrylamide hydrogel layer achieved here is comparable to their results (∼25 N/m), which indicates that the surface
treatment we use is on-par with state-of-the-art techniques. By
creating surface microtextures, the value is increased by an orderof-magnitude (∼500 N/m).
Since the bilayer structures are to be actuated in an aqueous
environment, cyclic testing was conducted to determine if thebonding between the two materials is sufficient to resist the deformation of the hydrogel layer caused by water intake. As illustrated
in Fig. 2(g), one end of the unimorph was fixed onto a 3D printed
stage. The unimorph then went through cycles of hydration and dehydration with different color-coded water representing different
cycles. As of this writing, samples have undergone more than 50
swelling–deswelling cycles without any delamination observed.
When unwanted external forces were applied during operation,
the textured actuators remained operative, as observed from theunimorph samples of other shapes shown in Figure S3 (Supporting
Information). No delamination occurred during bending, squeezing, twisting or stretching as shown in Video S1 (Supporting Information).
2.3. Quantifying curvature of hydrogel unimorph actuator
We sought to show that unimorph actuation may be manipulated by tuning the thickness of the elastomer layer. The hydrogelelastomer bilayer strips (30 mm × 4 mm) were cut by laser froma large bilayer sheet and fully dehydrated in an incubator at 60 o C.
Next, with one end fixed, the unimorph strips were immersed in awater bath and observed as a function of time. Hydrogel hydration

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Fig. 2. Experimental results of peel strength tests, surface roughness optimization and cyclic tests. (a) SEM images of non-textured and micro-textured PDMS surfaces. The
scale bars represent 100 µm. (b) Experimental setup for peel strength tests. (c) Plot of interfacial bond strength versus surface roughness of elastomer surfaces. (d) Peelingprocess of PAAm hydrogel bonded on non-textured, chemical treated PDMS in the flat state. (e) Peeling process of PAAm hydrogel bonded on micro-textured, chemicaltreated PDMS in the flat state (right after UV curing). (f) Plot of measured interfacial bond strength versus displacement for PAAm hydrogel bonded on chemically treated
non-textured and micro-textured PDMS, and a referenced result from published work [63]. The y axis is in logarithmic scale. (g) Hydrogel unimorph actuators in fully dried,
half-way dried and fully swollen states. The scale bars represent 7.5 mm.

during the experiment setup was assumed to be negligible since
the air exposure time was minimized (a few minutes). We tested
two groups of samples, both with a hydrogel layer thickness of∼200 µm in the flat state (right after UV curing), but differingin PDMS layer thickness. We denote a ‘‘thick actuator’’ groupwith a PDMS layer thickness of ∼1000 µm and a ‘‘thin actuator’’group with a PDMS thickness of ∼600 µm Fig. 3(a)–(b) shows
representative thin and thick unimorph actuators swelling in an
aqueous environment over 30 min and 7 h, respectively. The PDMS
layers are in pink color, and the PAAm layers are transparent. The
curvature changes over time of the two samples are shown in Video
S2 (Supporting Information). The thin actuator (Fig. 3(a)) and thethick actuator (Fig. 3(b)) have similar initial bending curvatureand comparable shape configurations, and they reach the same
position when they are fully actuated. However, the actuation time
for the thin actuator is 13 times less than the thick actuator.
The bending behavior of the hydrogel unimorph actuators sh
own in Fig. 3(a)–(b) is attributed to inhomogeneous 1D constrainedswelling of the hydrogel layer in water. There are two primaryeffects which cause this behavior. First, the stress state within thehydrogel is different along the thickness direction. When stressed,
the hydrogel cannot absorb additional water, hence swelling isconstrained near the interface between the hydrogel and elastomer layers. Second, water absorption is limited by diffusion

through the hydrogel. Thus, water must be absorbed first by the
exposed hydrogel surface, then diffuse through the hydrogel layer
to the inner material. After swelling is complete, the hydrogel
layer has compressive stress at the interface, and tensile stressat the surface. When in the dehydrated state, the reverse is true.
Physically, this manifests as the bilayer strip bending towards the
inextensible elastomer layer in the swollen state and the opposite
direction in the dehydrated state. The bilayer material with thicker
elastomer is stiffer, thus requiring more stress and swelling timeto achieve the same curvature. Additionally, the stress generated
by the swelling of the hydrogel is biaxial. In order to constrain thebending to a single direction, the aspect ratio of laser-cut parts is
set to 7.5 (30 mm × 4 mm), which causes the bending in the longaxis to dominate bending in the short axis.
To further analyze the bending behavior of the unimorph actuators, the bending curvature of the hydrogel-elastomer bilayers corresponding to the samples in Fig. 3(a)–(b) is plotted as afunction of time as shown in Fig. 3(c)–(d). Both the thick andthin actuator exhibit a three-phase hydration sequence: (1) anincrease in curvature in the direction of the dehydrated state, (2)an abrupt transition and reversal of the curvature, and (3) increasing curvature in the direction of the hydrated state. We initiallyhypothesized two reasons for the initial curvature change in the
direction of dehydrated state: water diffusion into PDMS elastomer

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Fig. 3. Bending behavior of unimorph actuators in aqueous environment. (a) Overlaid photo sequence of the thin actuator curvature change over a duration of 30 min. (b)Overlaid photo sequence of the thick actuator curvature change over a duration of 7 h. The scale bars represent 5 mm. (c) Experimentally measured bending curvature of
the thin actuator corresponding to (a). (d) Experimentally measured bending curvature of the thick actuator corresponding to (b).

and thermal effects. According to Lee et al. [66] and as well as ourown simple experiments (detailed in the Experimental Section),
PDMS elastomer is not able to absorb enough water to increase
the bending curvature at first. A more reasonable explanation isthermal contraction of the hydrogel. During the curvature tests,
the unimorph was immersed in 20 o C water only minutes after
removal from an incubator at 60 o C, which caused immediate
thermal contraction. By increasing the water bath temperature to
60 o C to match the temperature of the incubator, we found thatthe initial curvature increase was reduced from ∼15% to ∼5%. In
addition, it took only 2 s to reverse the curvature direction in the
60 o C water bath, while this reversal took 5 s in the 20 o C bath.

does not require a priori knowledge of material parameters otherthan the Young’s moduli of the two layers and can predict witha satisfactory degree of accuracy the motion of unimorphs withidentical PAAm thickness, but different PDMS thicknesses, by using
the data extracted from a single experiment.
We started by using one experimental test to determine theevolution of the swelling strain of the hydrogel layer as shown in
Fig. 4(a). The swelling strain was easily inferred from the experimentally obtained curvature-time relationship, by using for eachtime instant the Timoshenko formula,2.4. Hydrogel actuator bending behavior prediction

where is εPAAm (t) is the swelling strain for the hydrogel layer
at time t, which is assumed to be constant in the material [73],κ (t) is the experimentally measured curvature at time t , m =hPDMS /hPAAm is the ratio of the thickness of the two layers in the flatstate, n = EPDMS /EPAAm is the ratio of the Young’s moduli of the twomaterials that are shown in Figure S4 (Supporting Information) andh = hPDMS + hPAAm is the total thickness of the unimorph. In Eq. (1),
we assumed that εPDMS = 0, since PDMS elastomer does not swellin water. In Fig. 4(a) we report the evolution of the swelling strain
obtained for a sample characterized by and hPDMS = 1000 µm andhPAAm = 200 µm. In this analysis, we considered only the swelling
process that starts when the experimentally measured curvature
begins to decrease and we assumed that at ∆t = 0 the sample isfully dried.
Next, we used the obtained swelling strain curve and Eq. (1)to predict the curvature evolution for other samples characterized
by hPAAm , but different values of hPDMS . In Fig. 4(b) we report theanalytically predicted evolutions of the curvature during swellingfor a range of hPDMS , while holding hPAAm = 200 µm constant.
Despite the simplicity of the approach, for hPDMS = 600 µm
agreement is found between analytical and numerical results asseen from the unimorph configurations in Fig. 4(c), suggestingWith the aim to facilitate the design of hydrogel actuators capable of achieving targeted responses, we adopted a simple strategy
that allows to predict the evolution of their curvature as a functionof the swelling time. Although it has been recently shown that thetransient response of gels can be captured by treating the material
as hyperelastic and adopting the nonlinear field theory of coupled
diffusion and deformation [67–69], such simulation procedures
are complex and normally require a high number of parameters
and a relatively high computational effort. Therefore, we took asimplified approach that builds on the Timoshenko formula [70].
This formula was originally developed to capture the bending
deformation of a uniformly heated bilayer strip with different
expansion coefficients for each layer, but it has been shown thatis also appropriate to describe the behavior of bilayers including aswollen soft material [71,72].
Our simplified analytical approach is based on the following assumptions: (1) negligible curvature along the width of the sample,(2) uniform, isotropic and linear elastic material properties, and(3) negligible solvent diffusion and uniform swelling throughout
the hydrogel layer at each time instant. Importantly, our approach

εPAAm (t ) = −κ (t) · h(3(1 + m)2 + (1 + m · n)(m2 + m1·n ))6(1 + m)2

(1)

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that the approach can be used to design unimorphs with desired
responses. However, it should be noted that in the initial minutes
when dynamic swelling is faster, the curvature is more difficult tocapture and predict, thus the experimental and analytical results
do not perfectly overlap. Finally, since the thickness of the hydrogel
layer affects its swelling rate, we note that our approach requires
recalculation of a new swelling evolution curve from experimental data for each different thickness value of the hydrogel layer.
Additional experimental and analytical results for the unimorph
samples are included in Figure S5–6 (Supporting Information).
2.5. 3D shape changing objects
To prove that hydrogel deformations induced by swelling constrained on an elastomer surface can lead to significant shapechange in two directions, and that the bond strength on the interface is strong enough to resist the large deformations, we sought
to demonstrate various 3D shape changing structures including
a blooming and un-blooming soft flower, as well as changes insurface wrinkling and surface texture induced by humidity.
As observed previously, the bilayer structures of different thickness have different rates of curvature change. To illustrate thispoint, we fabricated a single flower structure composed of a thinner unimorph forming the top inner petals and a thicker unimorph
forming the bottom outer petals, as shown in Fig. 5(a). This resultsin fast reactions of the top petals while swelling/deswelling, as
well as larger curvature change when compared to the bottom
petals. The flower structure was designed in a 2D graphic tooland cut directly from the bilayer sheets by a laser cutter. The
top petals were attached to the bottom petals when both were
fully dried and curved towards the hydrogel side, as illustrated inFig. 5(a). To highlight the two layers, the elastomer layer of the top
petals were dyed green and the bottom petals were dyed orange.
When immersed in blue coded water for ∼10 min, both hydrogel
layers swell, forming a blooming flower in the opposite directionof the dried state. After removing the flower from the water bath,the structure starts to dry, and the bending curvature is reduced.
Eventually, it recovers to the fully dried state.
We further sought to show that the strong hydrogel-elastomer
bond could also be applied to stimuli-responsive surfaces in soft
systems. If the elastomer layer of the unimorph structure is thickenough relative to the hydrogel layer, the stress generated duringhydrogel swelling is not large enough to cause any curvaturechange. This is demonstrated in Fig. 5(c) where shapes such as
the hydrogel letters ‘‘FAB’’ and pillars are bonded onto elastomer
substrates. After immersion in water, the hydrogel layer swells,with the elastomer as a stiff supporting substrate. In Fig. 5(b),swelling of the high aspect ratio letters that are bonded to theelastomer induces wrinkling, and the shape recovers when fully
dried. In Fig. 5(c), swelling of the pillars changes the surface texture. The swollen hydrogel pillars are inverse-trapezoidal due toconstrained swelling at the bottom, while the top swells freely inall directions allowing neighboring pillars to connect. When fully
dried, the pillars are separated again. Optical images of a hydrogel
pillar in dried and swollen states and thickness profiles of the line
sections are shown in Figure S7 (Supporting Information). In bothFig. 5(b) and (c), dye molecules are left within the hydrogel matrix
after immersion in color-coded water. Upon re-swelling, these dyemolecules dissipate and take on the color of the new aqueous
medium.
3. Conclusion
In this work, we have demonstrated a stimuli-induced bidirectional hydrogel unimorph actuator that can swell and deform
in aqueous environments without the need for any additional

Fig. 4. Comparison between experimental and analytical results for hydrogel
unimorph actuator. (a) Curvature evolution for the reference sample (hPDMS =1000 µm and hPAAm = 200 µm) and corresponding swelling strain evolutionof the PAAm hydrogel layer εPAAm obtained using Eq. (1). (b) Predicted curvature
evolutions for different PDMS elastomer thicknesses (assuming hPAAm = 200 µm);
experimental data for the case hPDMS = 600 µm are reported as markers. (c)Comparison between experimentally observed and analytically predicted configurations for a bistrip with hPDMS = 600 µm and hPAAm = 200 µm at different time
instants.

stimulus and deform in the opposite direction when fully dried.
To achieve this, we have bonded a large swelling ratio PAAm
hydrogel onto a PDMS elastomer using both chemical modificationand continuous micro-textures. We have quantified an order-ofmagnitude improvement in interfacial bond strength relative tobonding through chemical means alone. Furthermore, we havedemonstrated that the bending behavior of the actuator can betuned by the relative thickness of the two layers and successfully
predicted by a simplified analytical approach. We also have shownS. Liu et al. / Extreme Mechanics Letters 21 (2018) 35–43

41

Fig. 5. Illustration of bilayer 3D shape changing objects. (a) Blooming flower 2D pattern and 3D object when fully dried, fully swollen and half-way dried. (b)–(c) PAAm
hydrogel letters and pillars bonded on PDMS substrates, and the surface texturing and wrinkling induced by humidity. The scale bars represent 4 mm.

that this fabrication process can produce more complex 3D reversible shape transforming structures operated in aqueous environments. The proposed fabrication techniques may be extended
to other types of polymers and used to achieve 3D shapes towards
developing more complex bidirectional structures and stimuli
responsive surfaces. Further optimization of the analytical approach will enable the prediction of more complex shape transforming behaviors.
4. Experimental
4.1. Materials
Acrylamide (AAm; Sigma–Aldrich A8887; ≥ 99%), 2-Hydroxy2-methylpropioph-enone (Irgacure 1173; Sigma–Aldrich 405655;97%), N , N ′ -Methylenebisacrylamide (MBAM; Sigma–Aldrich M7279; ≥ 99.5%), Polydimethylsiloxane (PDMS Elastomer; Dow Corning Sylgard 184 Silicone Elastomer Kit). PDMS elastomers werecolored by mixing with silicone color pigments (Silc Pig; SmoothOn). Hydrogel solutions were dyed with food color (Electric Color
Soft Gel Paste Food Color 12 Pack Kit; Americolor). PET films(thickness 0.025 mm; McMaster–Carr; 8567K12) were used asa stiff backing on the top surface of the hydrogel layer during
the 90o peel strength tests, together with cyanoacrylate (Loctite Instant-Bonding Adhesive 414; McMaster–Carr; 7625A16)
as the adhesive. Both PDMS elastomers and PAAm hydrogels
were molded from optically clear acrylic sheets (thickness 2 mm,
3.175 mm, McMaster–Carr), and 3D printed PLA frames (thickness
1 mm, 0.6 mm, McMaster–Carr) were used as the spacers for theacrylic molds.
4.2. Hydrogel and elastomer polymerization
PAAm Hydrogels were prepared by photo-polymerizing acrylamide monomers using MBAM as a crosslinker and Irgacure 1173
as a photoinitiator. The aqueous pre-gel solutions (65.4 wt% deionized water, 33.4 wt% acrylamide, 0.05 wt% MBAM, 1.2 wt% Irgacure1173) were mixed, degassed and then irradiated under the 10
mW/cm2 ultraviolet light (OAI Model 30 UV Light Source) for 80
s. PDMS elastomers were prepared by mixing the elastomer baseand the curing agent in a 10:1 ratio using a Thinky Mixer (Planetary
Centrifugal Solder Paste Mixer SR-500) for one minute, then mixed
elastomer sat in air for an hour to degas and cured in an incubatorat 60 o C for 1.5 h.

4.3. Hydrogel unimorph actuator fabrication
The elastomer layer of the unimorph structure was moldedfrom a laser rastered acrylic mold. A piece of 3.175 mm thickness
acrylic sheet was scanned by laser to ablate material from thesurface using a raster-scan mode. In this mode, the laser beamscans back and forth across the surface and pulses on and off
in rapid succession (35% power, 5% speed, total laser power is30 W) as shown in Fig. 1(a). Then, the workpiece was rotated
90o and rastered again with the same laser power and speed asshown in Fig. 1(b)). The engraved lines crossing in perpendicular
directions produced a micro-scale textured acrylic surface. Liquid
PDMS elastomer was poured into the rastered mold, degassed,and cured in an incubator at 60 o C for 1.5 h. The elastomer
layer thickness was controlled by 3D printed spacers (600 µm,
1000 µm) clamped on the acrylic molds. Next, cured elastomers
were taken out of the mold, flipped, cleaned thoroughly with isopropanol, ethanol and deionized water, and completely dried withnitrogen.
An image of the micro-textured elastomer surface (2333 µm ×1750 µm) was taken from an optical profiler (Zeta Instruments) by
scanning the sample over a specified z range (446 µm) as shownin Fig. 1(d) inset). A z profile of a line section taken from theimage is also included in Fig. 1(d). Root mean square roughness
Srms and surface area ratio Sdr were averaged over three boxes(500 µm × 500 µm) randomly picked from the image.
Next, a 200 µm film (100 mm × 65 mm × 3.175 mm) was madeby spin coating liquid PDMS elastomer on an acrylic plate at 200
rpm for 60 s. Then, trimming off a rectangular piece (80 mm × 50
mm × 200 µm) from center of the film created a 200 µm basin to
be filled by PAAm hydrogel solution. Once filled, the basin and the
PDMS elastomer film around it were covered by a micro-textured
PDMS elastomer layer. Next, irradiating the whole assembly underultraviolet light for 80 s produced the desired hydrogel unimorph.
Afterwards, a laser cutter (Universal Laser Systems VLS2.3) wasused in the vector engraving mode to cut the rectangular shapes(30 mm × 4 mm), flower petals or other shapes from the hydrogel
unimorph (50% power, 20% speed).
4.4. Laser settings for surface roughness optimization
Elastomer surfaces with textures of varying roughnesses werecreated. The laser settings and corresponding average surface
roughness parameters in Fig. 2(c) are: 1.18% power, 90% speed,
Srms = 4.48 µm; 2. 25% power, 70% speed, Srms = 12.21 µm; 3.

42

S. Liu et al. / Extreme Mechanics Letters 21 (2018) 35–43

35% power, 70% speed, Srms = 20.74 µm; 4. 35% power, 5% speed,
Srms = 28.85 µm; 5. 25% power, 5% speed, Srms = 34.75 µm; 6.
40% power, 5% speed, Srms = 44.62 µm.
4.5. Hydrogel-elastomer interfacial bond tests
The interfacial bond strength of hydrogel-elastomer interfaces
were measured using 90o peel strength tests with the Instrontesting system. Hydrogel solution was filled in an acrylic mold(100 mm × 20 mm × 3.175 mm) and covered by a chemicallytreated PDMS elastomer with different surface roughnesses (100mm × 65 mm × 2 mm). After UV polymerization, the elastomer
layer with the hydrogel layer bonded on top was adhered to a
PLA custom pull test adapter manufactured by a PrinterBot 3D
printer using liquid PDMS elastomer as the adhesive. The pull test
adapter was designed to be fixed on the Instron test machine.
The values reported in this paper were calculated from triplicate
measurements.
4.6. Hydrogel unimorph actuator curvature tests
The hydrogel unimorph strips (30 mm × 4 mm) cut from thebilayer sheets were put into the incubator at 60 o C until fully
dehydrated, then removed from the incubator and clamped with
one end fixed on the fixture plate. Next, the fixture plate withthe hydrogel unimorph strips were immersed in 20 o C deionizedwater. A Nikon D5200 camera was used to shoot one image every
minute. From all the collections of images, 30 representative configurations of one individual test were picked and stacked into one
image using ImageJ as shown in Fig. 3(a)–(b).
To confirm that PDMS layer is insensitive to humidity hence
has no effect on the initial curvature change of hydrogel unimorphs, we took several PDMS samples (3 mm × 3 mm × 3 mm,
5 mm × 5 mm × 5 mm) and immersed them into a water bath. No
dimension changes were observed over a duration of a few days.
To indicate the impact of thermal effects on the initial curvature
increment in the unimorph curvature tests, water in the acrylic
box was heated up to 60 o C in the incubator and then the box wasplaced on a 60 o C hotplate during the tests. Images were taken only
for the first few minutes until curvature started to increase in the
direction of the hydrated state.
4.7. Material properties tests
Young’s moduli of PAAm hydrogel and PDMS elastomer are
input parameters for the Timoshenko [70] formula. Tensile tests
for both materials were completed using an Instron testing systemequipped with a 10 N load cell and grips cut from polystyrene
sheets. The testing samples for both materials were formed via
acrylic molds into dogbone shapes (gage width 11 mm, gage length
50 mm, total width 17 mm, total length 100 mm). The crosshead
speed was set at 10 mm/min for all the tests.
Acknowledgment
This material is based upon work supported by the National Science Foundation under CAREER Grant No. 1454284. Any opinions,findings, and conclusions or recommendations expressed in thismaterial are those of the authors and do not necessarily reflect theviews of the National Science Foundation.
Appendix A. Supplementary data
Supplementary material related to this article can be foundonline at https://doi.org/10.1016/j.eml.2018.03.001.

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