Refractive-index-matched jammed microgel assemblies enable volumetric printing of porous hydrogel architectures

Jammed microgels form porous hydrogel architectures in which packing-derived interstitial voids provide transport-accessible pathways while retaining the processability of soft granular matter. However, the same microgel-continuous phase interfaces scatter and redistribute projected light, complicating volumetric printing. Here, we develop refractive-index-matched jammed microgel assemblies by independently tuning the refractive indices of gelatin/acrylamide microgels and an immiscible silicone-oil continuous phase. Cooling-induced gelatin gelation stabilizes discrete microgels, whereas acrylamide photopolymerization integrates the packed particles within the irradiated regions. Matching the refractive indices of the microgels and surrounding oil confines the projected 405 nm light field and restores design-to-print fidelity to a level comparable to that of a homogeneous bulk-gel control despite the high density of particle interfaces. This optical improvement is achieved without eliminating the particle-assembled architecture. The printed constructs retain packing-derived interstitial porosity and can be transferred from the oil-containing printing state into an aqueous hydrogel state while maintaining their overall geometry and measurable mechanical integration. Across the size-varied formulations, assemblies containing larger microgels exhibit wider interstitial features and greater dye penetration, whereas those containing smaller microgels show more restricted dye penetration and greater rheological and compressive resistance. These findings establish independent refractive-index control of the microgel and continuous phases as a materials design strategy for reconciling the optical requirements of volumetric printing with the structural and functional heterogeneity of particle-assembled soft materials.