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As a biomaterial, collagen has been used throughout tissue engineering and regenerative medicine. Collagen is native to the body, is highly biocompatible, and naturally promotes cell adhesion and regeneration. However, collagen fibers and the inherent weak mechanical properties of collagen hydrogels interfere with further development of collagen as a bio-ink. Herein, we demonstrate the use of a modified type-I collagen, collagen methacrylamide (CMA), as a fibril-forming bio-ink for free-form fabrication of scaffolds. Like collagen, CMA can self-assemble into a fibrillar hydrogel at physiological conditions. In contrast, CMA is photocrosslinkable and thermoreversible, and photocrosslinking eliminates thermoreversibility. Free-form fabrication of CMA was performed through self-assembly of the CMA hydrogel, photocrosslinking the structure of interest using a photomask, and cooling the entire hydrogel, which results in cold-melting of unphotocrosslinked regions. Printed hydrogels had a resolution on the order of $\sim$350$\mu$m, and can be fabricated with or without cells and maintain viability or be further processed into freeze-dried sponges, all while retaining pattern fidelity. A subcutaneous implant study confirmed the biocompatibility of CMA in comparison to collagen. Free-form fabrication of CMA allows for printing of macroscale, customized scaffolds with good pattern fidelity and can be implemented with relative ease for continued research and development of collagen-based scaffolds in tissue engineering.

Natural methacrylate polysaccharides and proteins represent a versatile group of modified macromolecules. The methacrylic group is capable of undergoing a gelling process through a photo-crosslinking polymerization into the macromolecule backbone. This chapter presents an overview of the methacrylation reaction of biopolymers, such as chitosan, laminarin, hyaluronic acid, dextran, gellan gum, gelatin and platelet lysates, developed by our research group. The biomedical applications, such as tissue engineering, therapeutic delivery and stem cells modulation, are highlighted.