Magnetic imprinting of gelation pathways enables external programming of hydrogel stiffness

Magnetic hydrogels provide powerful opportunities for engineering anisotropic soft materials, yet their mechanical response is governed by two coupled but distinct processes: (i) magnetic particle self-assembly and (ii) hydrogel network formation. Here we demonstrate that the timing of magnetic field application, rather than the field geometry alone, is a critical determinant of mechanical reinforcement. Using uni-, bi- and tri-axial magnetic fields, we systematically probe how particle structuration interacts with gelation kinetics in an oxidized laminarin-gelatin dynamic covalent hydrogel. Suspensions of particles at the same concentrations show negligible viscoelasticity compared to the hydrogel, confirming that particle networks do not directly reinforce the matrix. Instead, we show that field-induced particle structures modulate the crosslinking pathway and gelation timescale, thereby altering the final mechanical properties. Stable anisotropic reinforcement is achieved only when percolation of particle structures occurs within a temporal window in which the polymer network is sufficiently developed to immobilize them. These findings reveal temporal synchronization between particle assembly and gelation as a previously overlooked design parameter, enabling external control of hydrogel mechanics and anisotropy.