Magnetic-field-guided programming of symmetry-defined anisotropy in hydrogels

Hydrogels with anisotropic architectures can direct mechanical responses, guide mass transport, and enable programmable shape morphing. However, scalable strategies for encoding symmetry-defined anisotropy with strong modulation of bulk properties remain limited. Here, we present a simple single-step approach for programming hydrogel anisotropy, based on aligning micron-scale disk-shaped platelets under low-strength magnetic fields during photopolymerization. Rather than relying on the intrinsic magnetic properties of the platelets, we coat superparamagnetic nanoparticles on the surfaces of the platelets to impart sufficient magnetic susceptibility, which enables large magnetic torques and robust alignment. By tailoring the magnetic field configuration to be absent, static, or rotating, we have fabricated isotropic, transversely isotropic, and co-planarly anisotropic hydrogels, respectively. We employ confocal laser scanning microscopy to directly visualize the three-dimensional, magnetic-field-dependent alignment, compression tests to confirm anisotropic reinforcement, and thermal actuation to drive reversible, patterned shape morphing. As a result, the co-planarly anisotropic hydrogels exhibit pronounced mechanical stiffening and accelerated deswelling perpendicular to the platelet normals, which may reflect reduced transport tortuosity associated with platelet alignment. This strategy provides a scalable route to symmetry-defined anisotropic hydrogels with tunable mechanical and deswelling responses, with potential applications in soft robotics, responsive biointerfaces, and programmable fluidics.