Mold Surface Chemistry and Evaporation Kinetics Govern Hydrogel Gelation Behavior

It is well-known that the physicochemical properties of hydrogels are dependent on formulation variables such as polymer type, molecular weight, and cross-linking stoichiometry. Less understood are the "mold effects", wherein prior literature on synthetic hydrogels has demonstrated that mold surface chemistry significantly influences network formation and mechanical properties. These effects remain unquantified for hydrogels fabricated using naturally derived polymers or slow-reacting cross-linking mechanisms. Therefore, this study investigates how mold chemistry (hydrophilic borosilicate glass, GL, vs hydrophobic silicone, SI), surface area-to-volume ratio (SA/V, small vs large), and evaporation rate (low vs high) govern gelation behavior in hyaluronic acid-type I collagen hydrogels cross-linked via Diels-Alder bioorthogonal click chemistry. Our results demonstrated that SI molds under high evaporation promoted 4.4-7.1-fold higher solvent loss compared to GL, resulting in 4.1-9.1-fold increases in mechanical properties. These results were driven by interfacial differences between the mold and the hydrogel surfaces resulting in interfacial imprinting and surface-core heterogeneity. Conversely, low evaporation minimized the differences across mold types, demonstrating the synergistic effects of evaporation-controlled network density. Notably, our results are the direct opposite of prior literature, suggesting that the types of polymers utilized, as well as the cross-linking reaction chemistry, also influence this "mold effect".