Engineering Layered Magnetic Hydrogels for Cell Placement via Shear and Magnetic Field-Induced Assembly.

The design of hydrogel-based artificial tissues capable of reversible, programmed, and complex motions requires both stimuli-responsiveness and structural anisotropy. In this work, non-unidirectional anisotropies are generated in biocompatible hydrogels by structuring magnetic particle suspensions into lamellar architectures through two distinct routes: the application of an unsteady magnetic field to a quiescent sample, and the superposition of a steady magnetic field with shear flow. In both approaches, magnetic particles undergo directed self-assembly within a polymer matrix that subsequently gels, thereby preserving the formed structures. We analyze the assembly kinetics, characterize the resulting lamellar patterns, and construct phase diagrams for each method. The morphology and periodicity of the lamellae are shown to depend strongly on geometric confinement, enabling tunable interlamellar spacing from tens to hundreds of microns. Crucially, it is demonstrated that the resulting layered hydrogels can confine human fibroblasts between adjacent particle-rich lamellae, maintain cell viability above 95% over 7 days of culture, and promote preferential cell alignment parallel to the layered structures. These findings establish magnetic field-directed lamellar structuring as a versatile route to anisotropic hydrogels with programmable internal architecture, opening new opportunities in tissue engineering, bioactuation, and soft robotics.