Prepolymerization Enables On‐Demand Viscosity Tuning for Xolography of Soft Hydrogels

Xolography enables contactless volumetric additive manufacturing by suspending objects in a viscous resin. This avoids the build platforms and shear forces that distort soft materials in layer-by-layer printing. However, low-viscosity resins such as hydrogels allow printed parts to sediment, and conventional thickeners can compromise the final material. Here, we report an in situ prepolymerization strategy that raises viscosity without altering the base resin composition. A room-temperature redox reaction converts a fraction of the monomer into high-molecular-weight linear chains, tuning viscosity by more than four orders of magnitude (4.37 to >90000 mPa s) through reaction time alone. Across six formulations, longer prepolymerization suppresses sedimentation and lowers the critical light dose by up to 40%, enabling high-fidelity printing of complex hydrogels within 4 min. The strategy extends to a solvent-free organic resin, achieving 80 µm resolution. Sequential overprinting yields multi-material objects with mechanically isolated rigid components embedded in a hydrogel matrix, a geometry difficult to produce by platform-based printing. The resulting hydrogels retain 99.3% of their peak stress over 5000 compression cycles.