Gelatin hydrogels formed by orthogonal thiol–norbornene photochemistry for cell encapsulation
Covalently cross-linked gelatin hydrogels have received considerable attention in biomedical fields due to the inherent bioactivity of gelatin and the stability of covalent bonds linking the gelatin chains. Derivatives of gelatin, such as gelatin-methacrylamide (GelMA), can be cross-linked into covalent hydrogels through radical-mediated chain-growth photopolymerization. However, accumulating evidence suggests that chain-growth polymerized hydrogels may not be ideal for the encapsulation of cells and proteins prone to radical-mediated damage. The formation of heterogeneous kinetic chains following chain-growth polymerization of (meth)acrylates or (meth)acrylamides may also hinder molecular transport or alter cell–cell/cell–material interactions. This study presents a new synthesis route for preparing norbornene-functionalized gelatin (GelNB) that could be used to form orthogonally cross-linked gelatin-based hydrogels via a thiol–ene photo-click reaction. GelNB was synthesized through reacting gelatin with carbic anhydride in aqueous buffered solution, and the degree of norbornene substitution was controlled by adjusting the reaction time and the solution pH value. GelNB hydrogels were prepared by step-growth thiol–ene photopolymerization using multifunctional thiols as gel cross-linkers and the degree of GelNB hydrogel cross-linking was tuned by adjusting the thiol concentration, GelNB content, or cross-linker functionality. The cytocompatibility of orthogonally cross-linked GelNB hydrogels were demonstrated by in situ photo-encapsulation of human mesenchymal stem cells (hMSCs). When compared with the chain-growth GelMA hydrogels, the orthogonally cross-linked GelNB hydrogel promoted a faster and higher degree of cell spreading.