Frontal polymerization of acrylamide/GelMA/gelatin hydrogels with controlled mechanical properties and inherent self-recovery

Low mechanical resistance represents one of the significant problems of hydrogels, limiting their applicability in many fields. One approach to overcome this issue is synthesizing interpenetrating polymeric networks. In this work, the frontal polymerization technique was used to synthesize two series of novel hydrogels: (i) poly(acrylamide) (PAAm)-based hydrogels copolymerized/crosslinked with methacrylate gelatin (GelMA) (AAm-GelMA copolymer networks), and (ii) semi-IPN made of AAm-GelMA copolymer networks and a physically crosslinked gelatin network. With the final objective of improving the rheological, mechanical, morphological, thermal, and swelling properties of PAAm hydrogels, GelMA with two different degrees of methacrylation (30 and 75 mol%) was used. Interactions between GelMA chains, which give rise to physical network formation (i.e., GelMA-GelMA interactions), resulted in very efficient crosslinking for PAAm-based hydrogels, requiring a significantly lower methacrylic group concentration (0.04 mol%) for hydrogel formation compared to N,N′-methylene-bis-acrylamide (1 mol%), which is the agent typically used as a crosslinker for PAAm. Furthermore, the degree of GelMA methacrylation markedly affected the properties of the hydrogels. For example, regarding the swelling degree, hydrogels containing 22 wt% of GELMA30 had an SR% of 2870, while those containing the same amount of GELMA75 swelled much less (870 %). The introduction of gelatin as a secondary network in semi-IPNs influenced the rheological and mechanical properties, resulting in increased hydrogel modulus and stiffness attributed to enhanced physical interactions within the network. Finally, dynamic rheological shear strain and cyclic loading compression tests demonstrated exceptional recovery capabilities in all hydrogel formulations: samples subjected to alternating low (0.1 %) and high (300 % or 10 %) shear strain demonstrated a complete and prompt recovery of G′ and G″ values.