Adhesive Hydrogel Inks with Boronic Acid-Cis-Diol Complexation for On-Muscle Printing

For addressing the challenges regarding muscle injuries, 3D printing has been a promising technique to fabricate patient-specific scaffolds and effectively guide myotube alignment. Although methacrylate-conjugated gelatin (GelMA) is widely used as an ink material for 3D printing because of its facile photo-crosslinking and cell-adhesive properties, its intrinsic low viscosity and weak mechanics require high concentrations of the polymer for 3D printing and matching with tissue-like modulus, while its limited tissue adhesion further restricts its applicability in on-muscle printing. In this study, we propose a printable and bioadhesive hydrogel ink (PBAink) with low polymer concentration of alginate tethered with phenylborate and methacrylate (AlMABA), which exhibits a storage modulus similar to that of muscle tissues and undergoes rapid crosslinking within 90 s under blue light irradiation, making it suitable for 3D printing. Additionally, it exhibits low swelling under physiological conditions and good biocompatibility, owing to its excellent hydrophilic properties imparted by phenylborate groups, making it a suitable material for direct on-muscle printing. Notably, because dynamic bonds between cis-diols and phenylborate groups are formed in phosphate-buffered saline environments, we optimized the salt concentration of the buffer solution mixed with AlMABA to enhance cohesion; this led to the development of PBAink, which enhanced fidelity of printing at low concentrations, while the methacrylate groups ensured structural stability via photo-crosslinking. Moreover, PBAink exhibits tissue-adhesive properties compared with methacrylate-conjugated alginate and GelMA, supporting the direct on-muscle printing and conformal integration of the printed hydrogel with the muscle tissue. The PBAink exhibited intriguing cell-adhesive properties, inducing C2C12 clustering, while also promoting cell spreading. Finally, these features contributed to increased cell density and enhanced F-actin coverage on 3D-printed PBAink scaffolds, thereby highlighting its potential as an effective alternative to conventional GelMA-based inks.