Distributed Electric Field Induces Orientations of Nanosheets to Prepare Hydrogels with Elaborate Ordered Structures and Programmed Deformations

Living organisms use musculatures with spatially distributed anisotropic structures to actuate deformations and locomotion with fascinating functions. Replicating such structural features in artificial materials is of great significance yet remains a big challenge. Here, a facile strategy is reported to fabricate hydrogels with elaborate ordered structures of nanosheets (NSs) oriented under a distributed electric field. Multiple electrodes are distributed with various arrangements in the precursor solution containing NSs and gold nanoparticles. A complex electric field induces sophisticated orientations of the NSs that are permanently inscribed by subsequent photo‐polymerization. The resultant anisotropic nanocomposite poly(N‐isopropylacrylamide) hydrogels exhibit rapid deformation upon heating or photoirradiation, owing to the fast switching of permittivity of the media and electric repulsion between the NSs. The complex alignments of NSs and anisotropic shape change of discrete regions result in programmed deformation of the hydrogels into various configurations. Furthermore, locomotion is realized by a spatiotemporal light stimulation that locally triggers time‐variant shape change of the composite hydrogel with complex anisotropic structures. Such a strategy on the basis of the distributed electric‐field‐generated ordered structures should be applicable to gels, elastomers, and thermosets loaded with other anisotropic particles or liquid crystals, for the design of biomimetic/bioinspired materials with specific functionalities.

hydrogels mainly relies on molecular self-assembly, [9] mechanical strain/shear, [10] external electric/magnetic fields, [11] and 3D printing [1e,5b,6e] to orient the molecules or nanoparticles before, during, or after the polymerization process. For example, electric or magnetic fields have been used to orient nanoparticles to prepare monodomain hydrogels. [3d,5a,11a] However, it is a major challenge to program the distribution of external fields to form elaborate alignments of nanoparticles in one step. Besides, 3D printing technology is utilized to align nanofillers by mechanical shearing, and the resultant anisotropic hydrogels exhibit biomimetic shape change to form 3D morphologies. [5b] However, this strategy is time-consuming and limited to few specific systems. A facile and general approach is sought after for the design of hydrogels with elaborate ordered structures allowing for programmable deformation and locomotion.
Here, we demonstrate a simple and efficient strategy to fabricate patterned hydrogels with complex ordered structures by generating an intricate yet programmable electric field to induce orientations of highly charged nanosheets (NSs) before the polymerization process. Multiple electrodes are distributed in the precursor solution to complete the intricate orientations in one step, where the NSs align along the electric field. [12] Gold nanoparticles (AuNPs) with high photothermal conversion efficiency are incorporated to afford the anisotropic poly(N-isopropylacrylamide) (PNIPAm) hydrogels with photoresponsibility. [13] The resultant composite hydrogels with intricate ordered structures exhibit fast and isochoric deformations to form various 3D configurations upon heating or light irradiation. Locomotion of the patterned composite hydrogel is realized by using a moving light beam to spatiotemporally trigger the localized deformation. Such a strategy should be applicable to other nanofillers or liquid crystals for the development of soft active materials with biomimetic structures, deformations, and locomotion.
The fluorohectorite [Na 0.5 ][Li 0.5 Mg 2.5 ][Si 4 ]O 10 F 2 NSs used in this work have a high aspect ratio of ≈20 000 and a high charge density of 1.1 nm −2 . Delamination into single lamellae is achieved by repulsive osmotic swelling in water, producing a nematic phase even at very low content of NS applied here (≈0.3 wt%). [3d,14] As shown in Figure 1, the NSs with anisotropic permittivity align along the electric field with a pair of point electrodes, forming a macroscopically ordered spindle-like structure. Before the application of electric field, the nematic suspension of NSs without long-range alignment shows weak birefringence in polarizing optical microscopy (POM). After the high-frequency AC electric field is applied to the suspension for 10 min ( Figure S1 and Movie S1, Supporting Information), strong birefringence appears. We should note that the polarity of the electrodes distributed in the precursor solution is frequently and synchronously switched under the application of AC field. According to the birefringence colors, we can identify the localized alignments of the optically positive NSs, which show yellow and blue birefringence colors when oriented in the northwest-southeast and northeast-southwest directions, respectively ( Figure S2, Supporting Information). In other words, NSs orient along the electric field to form the spindle-like structure. Such complex ordered structure is well maintained for several minutes after the electric field is switched off, and gradually destroyed over a prolonged period of time due to thermal relaxation ( Figure S3, Supporting Information). The electrically oriented structure of NSs can be set permanently by subsequent polymerization of the precursor solution containing N-isopropylacrylamide (NIPAm), chemical crosslinker, and photoinitiator that is completed in 1 min. Systematic experiments are performed to optimize the synthesis parameters for the electrical orientation ( Figure S4, Supporting Information). In the following section, we set the electric-field strength of 4 V mm −1 , frequency of 10 kHz, and action time of www.advmat.de www.advancedsciencenews.com 60 min for the electrical orientation of NSs before the synthesis of anisotropic hydrogels.
The anisotropic structure of NSs in the hydrogel ( Figure S5, Supporting Information) is further characterized by smallangle X-ray scattering (SAXS). As shown in Figure 2a,b, when the X-ray beam is irradiated from the normal (n) and parallel (//) directions, the composite hydrogel exhibits an elliptical diffusive pattern with the longer axis perpendicular to the direction of electric field. In contrast, isotropic scattering is observed when the X-rays are irradiated from the perpendicular (⊥) direction of the hydrogel. This result confirms that NSs align along the electric field. Accordingly, the azimuth plots of 2D SAXS patterns measured from the n and // directions have two peaks at azimuth angle of 90º and 270º (Figure 2c), corresponding to orientation order parameters of 0.90 and 0.75. www.advmat.de www.advancedsciencenews.com The anisotropic structure affords the hydrogel with anisotropic mechanical properties. Rectangular samples are cut from the central region of the hydrogel along or perpendicular to the connection line of the electrodes (Figure 2a). The hydrogels exhibit a higher Young's modulus (E) in the direction parallel to the alignment of NSs, ≈2 times of that perpendicular to the alignment of NSs (Figure 2d,e). The AuNP-containing hydrogel also shows anisotropic responses to temperature and photo irradiation. After being transferred into hot water (37 ºC), the rectangular hydrogel sheet readily undergoes anisotropic, isochoric deformation with expansion perpendicular to the alignment of NSs and contraction parallel to alignment of NSs (Figure 2f). The isochoric deformation of the anisotropic hydrogel is related to the dehydration of PNIPAm chains when the temperature is above the low critical solution temperature (LCST, ≈32 ºC). The release of water molecules previously bonded to the polymer chains results in a sudden increase in electrostatic permittivity of the media, leading to an electrostatic repulsion among NSs. [11a] Cyclic immersion of the gel in 25 and 37 ºC water bath leads to reversible dimension change ( Figure 2g).
The presence of AuNPs endows the hydrogel with a characteristic absorption peak at 520 nm ( Figure S6, Supporting Information). Under irradiation with 520 nm green light at intensity of 0.8 W cm −2 , the local temperature of the composite hydrogel quickly rises from 25 to 50 ºC within 2 s (Figure 2i). Consequently, the hydrogel lengthens by a factor of 1.26 in the direction perpendicular to the alignment of NSs and contracts by a factor of 0.88 in the direction parallel to the alignment of NSs (Figure 2h,k). When the light is switched off, the local temperature returns to 25 ºC, accompanied by a recovery of the gel's dimensions to the original ones within 13 s. This photoresponsive behavior is fully reversible (Figure 2j,k; Movie S2, Supporting Information).
The fast anisotropic deformation of hydrogels, especially under the photo irradiation, is beneficial for the programmed deformation and locomotion. [1c,13g] In the following, several hydrogels are designed to demonstrate the control of distributed alignments of NSs by manipulating the electric fields.
As shown in Figure 3a, a hydrogel disc with radial alignment of NSs is prepared by using a circular electrode and a point electrode at the center with opposite polarity. Upon heating or light irradiation, the disc gel with radial alignment of NSs buckles into a saddle-shaped configuration, because the gel expands along the azimuthal direction and contracts along the radial direction that results in the built up of compressive hoop stresses. Replacing the circular electrode with a triangular one leads to similar radial alignment of NSs at the central region of the hydrogel (Figure 3b), while the alignments of NSs at the corners are weakened due to the diminishing electric field. This triangular hydrogel also deforms into a saddle-shaped configuration. Simply by varying the shape and arrangement of the electrodes, various composite hydrogels with programmable alignments can be fabricated. As shown in Figure 3c,d, the hydrogels with two or three regions of radial alignments of NSs are obtained, and they deform into sophisticated 3D configurations upon heating or light irradiation.
Hydrogels with periodically ordered structures can also be prepared by using an array of point electrodes to program the distribution of electric field. As aforementioned, NSs orient along the electric field. It is well-known that electric field lines run between the electrodes with opposite polarity rather than those with identical polarity. Therefore, the hydrogel prepared by four electrodes with alternating polarities produces ordered structures similar to concentric alignments (Figure 4a). As expected, the gel deforms into a dome-like configuration. In www.advmat.de www.advancedsciencenews.com contrast, the hydrogel prepared by two parallel-arranged pairs of electrodes possesses an ordered structure similar to a unidirectional alignment (Figure 4b). Upon heating or light irradiation, the composite hydrogel deforms into an arch configuration since the regions between the two electrodes with the same polarity are isotropic, which constrain the anisotropic deformation of other regions. These two kinds of electrode arrangements and corresponding configurations of hydrogels after stimuli serve as the basic building blocks for patterned hydrogels with periodically ordered structures. As shown in Figure 4c, patterned hydrogels composed of five blocks in the cross-arrangement are facilely prepared by controlling the distribution of electrodes with alternating polarities. Under external stimuli, each unit deforms into a dome configuration, and thus the hydrogel deforms into a configuration resembling an opened box. As the number of blocks further increases, the hydrogel composed of a 3 × 3 array of basic units deforms into a 3D configuration with alternating concave-convex structure (Figure 4d). The neighboring units spontaneously buckle toward opposite directions upon heating or light irradiation. This cooperativity results from the interaction between the neighboring units that buckle upward or downward under stimuli; the buckling of neighboring units in opposite directions can minimize the localized curvature of the connection region and thus the total elastic energy. [15] By tuning the distributions of electrodes, other patterned hydrogels capable of forming distinct ordered structures can be obtained. As shown in Figure 4e, the hydrogel prepared by parallelly arranged electrodes possesses a quasiunidirectional alignment of NSs. The isotropic regions between the electrodes of the same polarity constrain the anisotropic deformation of the other regions. Consequently, the gel folds along the connection lines of electrodes with the same polarity and deforms into a rugged arch under external stimuli. When the electrodes are specially arranged as shown in Figure 4f, the overall hydrogel deforms into a dome with the central region buckling downward and forming a concavity. We should note that the shape change of the patterned hydrogels is driven by permittivity-mediated electrostatic repulsion between the NSs, and thus readily completes upon stimuli and is fully reversible, enabling the design of soft robots with fast response.
Two patterned hydrogels are designed to demonstrate the programmed deformation and locomotion under spatiotemporal light irradiation that results in time-variant localized shape change. A ring-shaped hydrogel with radial orientation of NSs is fabricated by using a pair of circular electrodes (Figure 5a). When the overall hydrogel is immersed in hot water or irradiated under green light, it deforms into a saddle-like configuration, similar to the disc gel with radial alignment of NSs. When www.advmat.de www.advancedsciencenews.com the hydrogel is irradiated by using a laser beam (intensity: 0.8 W cm −2 ) to scan along the ring shape (speed: 1.05 rad s −1 ), a traveling buckle is observed, leading to the rocking back and forth and thus rotary and translational motions of the hydrogel (Figure 5b; Movie S3, Supporting Information). The localized irradiation leads to asymmetric deformation and shifts the barycenter of the ring-shaped hydrogel. Repeated light scanning powers and steers the locomotion of the ring-shaped hydrogel. [16] The second proof-of-concept example is the drifting motion of a patterned hydrogel under localized light stimulation, like the gliding of seagulls. A rectangular hydrogel is prepared following a similar protocol as described above; the ordered structures are confirmed by POM observation and are illustrated in the scheme (Figure 5c). When the integrated hydrogel is heated or irradiated by green light, it deforms into a 3D configuration like a flying seagull. Two discrete regions form dome shapes and are linked by an articulate folding. Locomotion is achieved by localized irradiation (spot diameter: 11 mm; intensity: 0.8 W cm −2 ) on the fold (Figure 5d; Movie S4, Supporting Information). Owing to the complex ordered structures, the hydrogel is not flat even when equilibrated in water at room temperature. In the middle region, the NSs orient along the width direction of the rectangular sheet, and the mismatched anisotropic swelling leads to slightly buckling. A short-pulse light irradiation at this region dramatically enhances the buckling downward, and the integrated hydrogel folds upward at a fast speed. After the light irradiation is switched off, the gel is inclined to recover to the original state. Due to the asymmetric hydrodynamic dragging forces, the central region is lifted quickly (≈2 mm within 0.5 s) but settles gradually under gravity (Figure 5d). The sideward drifting by ≈4 mm at settling is due to the geometric asymmetry between the left and the right parts of the composite hydrogel. Such drifting motion is sustainable by repeated light irradiation (Movie S5, Supporting Information). Interestingly, when the hydrogel is turned over with the middle region buckled upward, it quickly bends downward with central part moving upward upon a short-pulse light irradiation, resulting in jump motion of the hydrogel (Movie S6, Supporting Information).
In conclusion, we have developed a facile strategy to prepare hydrogels with elaborate ordered structures by using multiple www.advmat.de www.advancedsciencenews.com electrodes to program the distribution of the electric field that orients the NSs before the polymerization and crosslinking process. These spatially ordered structures afford the hydrogels with controllable deformations and locomotion under external stimuli. Other hydrogels or elastomers with various sophisticated structures can be fabricated by arranging the shape and distribution of electrodes to orient the nanorods/NSs or liquid crystal molecules. These structures can be minimized to micrometer-scale or nanoscale provided the precise control of electrodes; other sophisticated locomotion should be expected by using a structured light for spatiotemporal stimulation. [13g,16b] The resultant programmed deformations and locomotion of the active soft materials, especially those fueled by spatiotemporal light, should find applications as biomedical devices, flexible electronics, soft actuators/robots, and so forth.