Cell-Compatible, Site-Specific Covalent Modification of Hydrogel Scaffolds Enables User-Defined Control over Cell–Material Interactions

SpyCatcher, a 15 kDa protein domain that spontaneously forms a site-specific covalent bond with the 13 amino acid peptide SpyTag, was used to covalently link a model recombinant protein containing a SpyCatcher domain and the adhesive ligand Arg-Gly-Asp-Ser (RGDS) (RGDS-SC) into SpyTag-containing poly­(ethylene glycol) (PEG) hydrogels. This new strategy for covalent immobilization of proteins or peptides provides an easy and gentle mechanism for biochemical modification of hydrogels. Labeling efficiency was approximately 100% when soluble RGDS-SC was applied to SpyTag-containing hydrogels at a 1:1 molar ratio. RGDS-SC remained stably bound throughout the 5 days of rinsing, and 3T3 fibroblasts were able to adhere to PEG gels presenting RGDS-SC, but did not adhere when the scrambled amino acid sequence RDGS was presented instead. Fibroblasts encapsulated within 3D cell-degradable PEG hydrogels containing SpyTag did not spread until RGDS-SC was added to the gels, at which point cell spreading was induced. This cell-friendly site-specific ligation strategy could have great utility in driving specific cellular outcomes using biochemically dynamic hydrogels.