A UV-free and cytocompatible crosslinking strategy for methacrylated chitosan hydrogels via bisulfite-initiated polymerization for embedded 3D bioprinting

Methacrylated chitosan (ChiMA) is a versatile polymer ink for fabricating 3D tissue-like structures due to its biocompatibility and processability. However, traditional UV-based crosslinking methods offer limited precision control of light intensity and pose risks for cell-laden hydrogels. This study introduces a novel, UV-free crosslinking strategy based on bisulfite-initiated radical polymerization for ChiMA hydrogels. By integrating bisulfite-initiated radical polymerization into an embedded printing process, ChiMA ink was successfully printed within a sodium bisulfite (SBS)-containing support bath to fabricate complex-shaped 3D structures. ChiMA hydrogels crosslinked with SBS were characterized for structural, mechanical, rheological, morphological, swelling, degradation, and cytocompatibility properties. Their mechanical properties increased notably with higher SBS content, as Young's modulus ranged from 20.5 to 151.3 kPa and compressive strength from 14 to 80 kPa. Gelation of the ChiMA solution, occurring within seconds at neutral pH, was delayed to 4–23 min in the slightly acidic environment. Swelling ratios decreased from 36 % to 29 %, and average pore size from 220 μm to 105 μm with increasing SBS. The hydrogels with 0.05 % SBS degraded by ∼20 %, while those with higher SBS (0.25–0.5 %) degraded only ∼5 % over 28 days. Cell viability remained consistently high (>97 %) throughout the 7-day culture period in all bioprinted samples.