Magneto-Orientation of Magnetic Double Stacks for Patterned Anisotropic Hydrogels with Multiple Responses and Modulable Motions

Abstract

Reported here is a multi-response anisotropic poly(N-isopropylacrylamide) hydrogel developed by using a rotating magnetic field to align magnetic double stacks (MDSs) that are fixed by polymerization. The magneto-orientation of MDSs originates from the unique structure with γ-Fe₂ O₃ nanoparticles sandwiched by two silicate nanosheets. The resultant gels not only exhibit anisotropic optical and mechanical properties but also show anisotropic responses to temperature and light. Gels with complex ordered structures of MDSs are further devised by multi-step magnetic orientation and photolithographic polymerization. These gels show varied birefringence patterns with potentials as information materials, and can deform into specific configurations upon stimulations. Multi-gait motions are further realized in the patterned gel through dynamic deformation under spatiotemporal light and friction regulation by imposed magnetic force. The magneto-orientation assisted fabrication of hydrogels with anisotropic structures and additional functions should bring opportunities for gel materials in biomedical devices, soft actuators/robots, etc.

Keywords: Anisotropic Hydrogels; Ferronematic Liquid Crystals; Magnetic Orientation; Nanosheets; Soft Robots.

Figure 1

a) Schematic of the sandwich‐like structure of MDSs. b) Magnetic hysteresis loop of MDS powder. c) Photos of a ferronematic suspension of MDS (1 wt%) between a pair of crossed polarized films. A magnet is placed under the cuvette filled with the MDS suspension. Scale bar: 5 mm. d–f) Schematic for the gel synthesis and the alignments of MDSs (top) and POM images of the hydrogel sheets observed from different directions (bottom). The hydrogels were prepared without magnetic field (d), with static magnetic field (e), or with rotating magnetic field (f). A: analyzer, P: polarizer, Z′: slow axis of 530 nm tint plate. Scale bar: 1 mm.

Figure 2

a–c) 2D SAXS patterns (a), scattering intensity–azimuth plots (b), and orientation degree of MDS (c) of the as‐prepared hydrogel. Orientation degree of MDS in the equilibrated gel is also presented in (c). d) Photos of the anisotropic hydrogel before and after the swelling process. e) Tensile stress–strain curves of the isotropic gel and anisotropic gel stretched from // and ⊥ directions. f) Corresponding mechanical parameters of the gels, including Young's modulus E, breaking stress σ _b, and breaking strain ϵ_b. Error bars represent the standard deviation of the mean (n=3).

Figure 3

a) Photos showing the temperature‐mediated anisotropic deformation of the hydrogel. b, c) Variations of the dimensions of the hydrogel after transferring it from 25 to 40 °C in a water bath. The initial change of the dimensions is enlarged in (c). d) Variations of local temperature of the anisotropic gel under irradiation of 520 nm light with different power intensity. e) Variations of the local temperature of the gel under cyclic irradiation of 520 nm light with the intensity of 2.34 W cm⁻². The insets present the anisotropic deformation of the gel under light irradiation.

Figure 4

a) Schematic (i) and corresponding POM images (ii) of a stripe‐patterned anisotropic hydrogel. b) Photos of the stripe‐patterned anisotropic hydrogels before and after the shape changes upon incubation in a 40 °C warm water bath. The thickness of the sample is 0.6 mm. c,d) Schematic (i) and corresponding POM images (ii) of patterned hydrogels with different alignment of MDSs at specific regions by rotating the directions of analyzer, polarizer, and tint plate. The thickness of the samples is 0.4 mm. Scale bars in (a), (c), and (d) are 2 mm; scale bars in (b) are 5 mm.

Figure 5

Snapshots showing the walking gait of the stripe‐patterned anisotropic hydrogel under a scanning laser beam from left to right without magnetic force (a), with moderate magnetic force (b) and large magnetic force (c). Schematics are presented above the snapshots to show the motions of the two “feet”. Gel dimensions, 15 mm×5 mm×0.6 mm; light intensity, 2.34 W cm⁻²; scanning speed, 1 mm s⁻¹. Scale bar: 5 mm.