Thermoresponsive Wrinkles on Hydrogels for Soft Actuators

Prof. Y. Tokudome, H. Kuniwaki, K. Suzuki, Prof. M. Takahashi Department of Materials Science Graduate School of Engineering Osaka Prefecture University Sakai , Osaka 599-8531 , Japan E-mail: tokudome@photomater.com; masa@photomater.com Prof. Y. Tokudome, Dr. D. Carboni, Prof. M. Takahashi International Institute for Nano/Meso Materials Science Osaka Prefecture University Sakai , Osaka 599-8531 , Japan Dr. D. Carboni Laboratorio di Scienza dei Materiali e Nanotecnologie D.A.D.U. Università di Sassari CR-INSTM Palazzo Pou Salit Piazza Duomo 6, 07041 Alghero , SS , Italy Dr. G. Poologasundarampillai School of Materials The University of Manchester Oxford Road , Manchester M13 9PL , UK

and micro valves are crucial components for achieving these advanced platforms. [ 3 ] Thin-fi lm piezoelectric actuators promise reliable operation and high throughput, however complex motions and fl exible deformations, observed in natural systems, are hard to achieve. For example, aortic valves, one of the most frequently diseased tissues, are repaired by transplanting a synthetic mechanical device or an allograft tissue alternative. These artifi cial ones are far from the original organs whose specialized motions are achieved by fl exible hierarchically structured tissues with complex architectures. [ 4 ] On the other hand, hydrogel-based soft actuators are promising [ 5 ] due to the large extent to which they can be deformed in aqueous media, thereby conforming to an architecture that mimics natural systems. Large complex 3D architectures can be fabricated with ease, if spontaneous chemical reactions are involved in their fabrication method. Moreover, 3D inkjet printing is reportedly applicable to the production of these polymer-based actuation systems. [ 1 ] We here demonstrate adaptive microarchitectures that exhibit a special peristaltic motion by employing hydrogel actuation on bilayered materials. The surface architectures were designed by mimicking wrinkles in nature. The organ, skin, is a good example of wrinkles with an adaptive response, where wrinkles appear and disappear depending on the movement of the muscles underneath. Here, greatly simplifying, these surface wrinkles form on a bilayered material of a crust top layer (skin) and an elastic bottom layer (muscles). As the bottom layer contracts (or the top layer swells), stress-driven instabilities are generated at the interface of the layers. The accumulated stress is then released through the formation of wrinkles on the crust layer. Such stimuli-responsive wrinkles were reported on artifi cial bilayered materials prepared from elastomers or resins. [ 6 ] Nevertheless, artifi cial wrinkles, which exhibit large deformations and undergo reversible adaptive response in liquids, are still challenging and are highly required to exploit applications in biological systems. In the present study, we designed and fabricated a bilayered structure made of PNIPAM and hybrid silica ( Figure 1 a). 3-(Methacryloxy)propyl triethoxysilane was used as a silica source to allow copolymerization with NIPAM and a crosslinker ( N,N′ -methylenebis(acrylamide), MBA) at the interface of the heterogeneous two layers. The thickness of the hybrid silica layer was tuned from 680 nm to 1.5 µm by using Pluronic F127 as a viscosity improver. This approach allows the strong adhesion at the interface and, most importantly, the complex actuation in liquid phase. PNIPAM undergoes shape change at the lower critical solution temperature (LCST), [ 7 ] and thereby wrinkles of submillimeter dimensions appear and disappear as a function of the changing environmental temperature. Signifi cantly large deformations (>40%), due to the fl exibility of NIPAM hydrogel, could be attained to achieve biomimicking microstructures, Soft microactuators with stimuli-responsive tunable chemistries are highly demanded in complex multifunctional and biocompatible microdevice platforms including microelectromechanical systems and lab-on-a-chip. Developing multifunctional hydrogel actuators is especially of signifi cance for the nano/micro artifi cial anatomies, which perform functions such as transportation, selection, recognition, and others. Here we demonstrate adaptive surface wrinkles, which exhibit a peristaltic motion and a size selective transportation of micro-objects on bilayered soft materials under thermal stimulus. The bilayered materials made of thermoresponsive poly(N-iso-propylacrylamide) (PNIPAM) and hybrid silica form responsive wrinkles of submillimeter dimensions with temperature variation. This hydrogel-based actuation works at a quasi-physiological environment, which meets the critical requirement for artifi cial alternatives in bioapplications. Rational design of autonomous actuators is possible through the present strategy because the formation of microarchitectures is driven through a versatile physical process, buckling.
Complex multifunctional microdevice platforms are required for several areas of engineering and sciences, for example to provide biological insights into cells and tissues. [ 1,2 ] Soft microactuators that drive and regulate micropumps The copyright line of this paper was updated 29 July 2016 after initial publication.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. such as folding (different from creasing that takes place on soft materials without hard skins [ 8 ] ) and nested wrinkling structures (Figure 1 a). Anisotropic ordering of the responsive wrinkles (Figure 1 b) is possible by applying a simple patterning process to the hybrid silica coating. Figure 1 c and Video S1 (Supporting Information) show nested wrinkles, aligned wrinkles of the periodicity of 350 µm, and dimple-shaped wrinkles of ≈50 µm size, fabricated on a patterned hybrid silica crust layer. The aligned wrinkles were found to show a peristaltic motion on cooling. The motion can be observed transiently on cooling from 36 °C to room temperature. On the other hand, the dimple-shaped wrinkles stay at their original position. In order to assess the capability of the hierarchical wrinkled structure in transporting micro-objects with a size-dependent selectivity, we have deposited microparticles of different sizes onto that structure. The aligned wrinkles convey the microparticle whose dimension is comparable to the periodicity of the wrinkles (red particle in Figure 1 c), whereas the smaller particle (yellow particle in Figure 1 c) stays at its original position. In short, the wrinkle recognizes and selects particles by size and transfers them from one place to another. The present approach, in principle, can provide stimuli-responsive architectures covering a wide range of sizes, over fi ve orders of magnitude in length scales, and spanning from a few nanometers to a few millimeters. [ 9 ] The size diversity and the solution-/environment-mediated responsiveness will open up versatile nano/micro hydrogel actuation at physiological conditions.
The hybrid silica/PNIPAM bilayered hydrogel was prepared via a solution process (Supporting Information Scheme S1).
The surface crust layer is composed of a hybrid silica containing polymerizable methacrylate groups. The soft bottom layer is PNIPAM gel network crosslinked with MBA. The bilayered material was found to form various 3D corrugated architectures on its surface depending on the external temperature. Figure 2 a shows the typical surface morphologies of the bilayered hydrogels at three different temperatures. The hydrogel surface is fl at at 34 °C. Upon cooling to ambient temperature, the surface becomes corrugated at 28 °C (periodic length, λ = 207 µm, amplitude of wrinkle, A = 133 µm), and then folded at 20 °C ( λ = 253 µm, A = 159 µm). Cross-sectional observation also confi rms the formation of both wrinkling and folding structures (Supporting Information Figure S1). The transition among these structures occurs reversibly and rapidly on a time scale of seconds to minutes. The responsive wrinkles capture particles on folding (see Supporting Information Figure S2) and release them on unfolding. The result demonstrated here is the fi rst example of taking advantage of a reversible wrinkling-to-folding transition, different from the irreversible transition reported on a bilayered photocurable resin. [ 10 ] The present adaptive and foldable material has higher throughput of capturing target objects than conventional self-folding capsules, [ 11 ] as a result of numerous folds that are able to capture a great number of properly sized objects at one time. This property has also been confi rmed by performing X-ray micro-computed tomography of the sample (Supporting Information Video S2).
The thermal response of the hybrid silica layer (crust) and the PNIPAM gel layer (bottom) was separately examined to elucidate the mechanism of wrinkling and folding. degree of swelling of the two layers on cooling at 20 °C. The thermal response of the hybrid silica layer is explained by the presence of PNIPAM copolymerized with methacrylate groups in the silica networks. [ 12 ] The copolymerization of PNIPAM with the crust hybrid silica layer was evidenced by Fourier Transform Infrared Spectroscopy (FTIR) analysis and the NIPAM concentration in the hybrid silica layer was estimated as 60 mol% (Supporting Information Figure S3). By cooling the fi lm below the LCST of PNIPAM in water (32 °C), [ 7 ] hydrogen bonding between the copolymer and water is strengthen, and water is absorbed into the polymer matrix, resulting in swelling of the copolymer hydrogel. Although the bottom PNIPAM layer can also swell on cooling, the swelling was found to be smaller and slower compared to that of crust hybrid silica layer. The hybrid silica showed 44% of swelling in 0.5 min, whereas the PNIPAM gel layer exhibited 14% of swelling in 180 min. The time constants of swelling are 8.6 s and 4.5 × 10 2 s for hybrid silica and PNIPAM gel layers, respectively. The thermal responses of the two layers were further assessed by the setup shown in Supporting Information Figure S4. A fi lm clamped and submerged in water showed curling in response to the temperature variation. The curling behavior confi rms that the crust hybrid silica layer shows the larger and faster deformation compared to PNIPAM layer; on heating, the hybrid silica layer rapidly shrinks in a few minutes, whereas PNIPAM gel slowly shrinks in more than 1 h. The hybrid silica layer of 1.3 µm (coating at 1000 rpm) is thin enough to rapidly include water into (or exclude water from) the polymer matrix, which allows the faster deformation on cooling (or heating) compared to the bulk PNIPAM gel layer. The formation mechanism of wrinkling and folding is illustrated in Figure 2 c. During cooling, the predominant swelling of crust hybrid silica layer accumulates stress in the out-of-plane direction of the fi lm, thus inducing a stress release by wrinkling and folding on the surface.
The periodic length of wrinkles, λ , becomes large with the thickness of the crust layer, t , as shown in Figure 3 a. The relationship between λ and t follows the scaling law reported by Cerda et al., λ ∝ t ( E 1 / E 2 ) 1/3 , where E 1 and E 2 are Young's moduli of crust and substrate layers, respectively. [ 13 ] The increment of λ as a function of t gives a ( E 1 / E 2 ) 1/3 of 1.7 × 10 2 which is in good agreement with the estimated value (( E 1 / E 2 ) 1/3 ≈10 2 ) using reported Young's moduli for hybrid silicas [ 14 ] and PNIPAM, [ 15 ] where E 1 and E 2 are in GPa and kPa orders, respectively. The theoretical description indicates that λ can be controlled by changing t over diverse size range. The ratio of NIPAM/MBA also infl uences the resultant wrinkle structures. PNIPAM was polymerized at NIPAM/MBA = 1000/3 to achieve a highly deformable hydrogel network. At NIPAM/MBA = 1000/3, a nested wrinkle structure which has hierarchical wrinkles in discrete size scales forms on cooling. Figure 3 b shows optical microscope images showing structure evolution on cooling. Typical isotropic wrinkles form in 3 min. Additional structures of smaller wrinkles and folding start to form at 5 min. Importantly, these hierarchical structures reversibly appear/disappear as temperature changes in minutes scale. The present nested wrinkles are suitable as storages/separators of colloidal particles of discrete size due to their adaptive response in solvents. [ 16 ] The shape of wrinkles was reported to depend on over-stress, a physical parameter at the interfaces on buckling. [ 17 ] Accordingly, the wrinkles of a labyrinth shape (1000 rpm of Figure 3 a), a ridge shape (3000 rpm of Figure 3 a), and a nested shape (Figure 3 b) were observed in the present case. The shape diversity of wrinkles is defi nitely important for controlling some properties of the surface, such as friction and wettability. [ 18 ] In summary, the periodicity and the shape of the wrinkles are controllable by tuning the coating condition and the NIPAM/MBA ratio.
The adaptive stimuli-responsive wrinkles appeared on cooling, disappeared on heating, and exactly restored on Adv. Mater. Interfaces 2016, 3, 1500802 www.advmatinterfaces.de www.MaterialsViews.com subsequent cooling (Supporting Information Figure S5). Such shape-memory effect is a characteristic of surface wrinkles on bilayered fi lms. On the other hand, very interestingly, we have found that the present materials are also capable of "sliding" wrinkles and driving the peristaltic motion as shown in Figure 1 c. The peristaltic motion was achieved on a patterned hybrid silica layer (triangle-shaped patterning) placed on the top of PNIPAM (Figure 3 c(1)). Oriented anisotropic wrinkles form on cooling (Figure 3 c(2)), because the patterning can induce anisotropic stress accumulation. [ 19 ] Stress relaxation takes place in the x-direction, and the stress is uniaxially accumulated in the y-direction to form anisotropic parallel wrinkles. On the other hand, continuous large swelling of crust hybrid silica exceeds the critical expansion of the x-direction. The stress which is accumulated in the x-direction is released by forming 2D wrinkles (green part of Figure 3 c(3)). Another domain of hybrid silica (red part of Figure 3 c(3)) keeps the anisotropic geometry of wrinkles, whereas wrinkles move toy direction in the course of stabilization of the accumulated stress (Supporting Information Video S1). The motion size-selectively conveys the particle whose size is comparable with the periodicity of the wrinkles (Figure 1 c). The speed of the peristaltic motion was 5 µm s −1 at the cooling rate of 1.8 °C min −1 . The occurrence of the peristaltic motion and the direction of the propagated wave (anisotropic wrinkles) are simply controlled by the shape of the applied pattern. This leads to the conclusion that by designing bilayered architectures and employing the properties of the PNIPAM-based hydrogel it is possible to successfully generate unique peristaltic motion of the wrinkled surfaces.
In summary, by manipulating the stress accumulation at the interfaces of bilayered materials that deform at different stimuli-responsive rates, hydrogel-based actuation with complex architectures featuring specifi c motion capabilities is achieved. In particular, various wrinkling and folding architectures can be reversibly controlled by mimicking biological systems. The simple patterning of hybrid silica gives rise to motion of the microarchitecture on changing the external temperature. This hydrogel-based actuation works at a near-physiological environment, thus meeting the critical requirement for artifi cial alternatives in bioapplications. Moreover, the working temperature Adv. Mater. Interfaces 2016, 3, 1500802 www.advmatinterfaces.de www.MaterialsViews.com www.advmatinterfaces.de www.MaterialsViews.com at which wrinkles appear/disappear is fi nely tunable (Supporting Information Figure S6). The materials described in this work are bilayered composites whose structures are much simpler than that of peristaltic actuators based on piezoelectric materials. [ 20 ] The phenomenon demonstrated here is expected to be further developed as a component of multistimuli responsive soft devices by coupling them with, for example, optical tweezers. [ 21 ]
Characterization : A laser scanning confocal microscope (SFT-3500, Shimadzu Corp.) was used to evaluate thickness of the fi lms. Optical microscopes (BX51, OLYMPUS Corp. and 2.4 GHz wireless digital microscope, 3R SYSTEMS Corp.) were employed to observe surface wrinkle structures. Periodic length was estimated by optical microscope images. More than ten locations on each sample were measured and averaged.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.