Vitamin C‐Induced Photo‐Redox Threshold Enables High‐Fidelity Volumetric Printing of Collagen

Tomographic volumetric printing (TVP) enables rapid fabrication of complex, centimeter‐scale 3D architectures. TVP of native proteins such as collagen is attractive because it preserves native bioactive motifs that regulate cell–matrix signaling. However, direct TVP of collagen remains challenging because dityrosine crosslinking, driven by visible‐light‐activated Ru(II)bpy 3 2+ /sodium persulfate (SPS), lacks an effective inhibitory mechanism. This results in near‐immediate crosslinking upon light exposure, leading to an insufficient nonlinear threshold response that fails to suppress background curing. Here, we introduce vitamin C (L‐ascorbic acid) as a biocompatible redox regulator to overcome this limitation. We demonstrated that vitamin C established a photo‐redox threshold by suppressing Ru(III) accumulation and scavenging persulfate radicals within the Ru/SPS system, thereby delaying dityrosine crosslinking until vitamin C depletion. This threshold generated the nonlinear response required for TVP, enabling high‐fidelity volumetric printing of collagen. Post‐printing construct densification (∼53% shrinkage) further improved feature resolution (80 µm positive; 120 µm negative) and yielded mechanically stable and highly stretchable hydrogels (up to 180% strain). Collagen resin with vitamin C supported both cell seeding post‐printing and cell‐laden printing with high cell density and viability, enabling the rapid biofabrication of cell‐instructive 3D microenvironments.