Metal-Free Orthogonal Photochemistry Enables Porous Multinetwork Microgels for Tough Printable Hydrogels

Microgels exhibit excellent toughening capabilities for the construction of strong and resilient hydrogels; however, their fabrication often involves complex procedures, sophisticated architectures, or biocompatibility-compromising metal catalysts. To address these limitations, we develop a metal-free orthogonal photochemical strategy that employs organic riboflavin to prepare porous double-network microgels for the fabrication of tough and printable hydrogels. This metal-free system demonstrates photochemical performance comparable to that of conventional ruthenium-based catalysts. Under light irradiation, riboflavin rapidly and orthogonally triggers free-radical polymerization of vinyl monomers and covalent crosslinking via phenolic coupling, while simultaneously enabling controlled Ca 2+ release for ionic crosslinking (within several tens of seconds). The resulting microgels possess a synergistic architecture comprising a double-network structure, which imparts exceptional energy dissipation, and an internal porous structure that enhances entanglements. When incorporated as reinforcing fillers into the hydrogel matrix, these microgels endow the bulk hydrogels with superior mechanical performance, achieving a fracture energy of ∼500 J m –2 and a toughness of ∼380 kJ m –3 . Furthermore, they confer pronounced shear-thinning characteristics and rapid self-recovery to the precursor inks. As a result, high-fidelity 3D printing of intricate architectures with feature sizes down to ∼100 μm is readily achieved, as demonstrated by a customizable pressure sensor array capable of discriminating both applied loads and object shapes. This work offers a biocompatible, efficient, and versatile strategy for preparing advanced microgel tougheners and bulk hydrogels, paving the way toward printable, tough hydrogels for a broad range of applications.