2.3. Assessing hydrogel degradation mechanism and rate

Non-degradable, hydrolytically-degradable, MMP-degradable, and mixed mode-degradable hydrogels were formed as depicted in Fig. 1. Briefly, solutions of 2 mM norbornene-functionalized 4-arm PEG macromer (PEG-ester-norbornene or PEG-amide-norbornene), 4 mM dithiol crosslinkers (PEGDT or MMP-degradable peptide), and 0.028 wt% of the photoinitiator LAP were prepared. The macromer solutions were pipetted into 1 mL syringes with the tips cut off (30 μL/syringe) and photopolymerized (∼5 mW/cm², 365 nm, 3 min). Polymerized hydrogels were incubated in 1 mL DPBS at 37 °C for 24 h before adding 1 μg/mL Collagenase Type II (Gibco). This concentration was identified via the literature [44,48,49] and preliminary dose-dependent degradation experiments. Compression testing was performed on hydrogels as a measure of temporal degradation. Specifically, using a Q-Test/5 material test frame with a 5 N load cell (MTS), Young's modulus values were computed from resistant force measurements after compressing hydrogels to 95% of their uncompressed heights. Every 48 h, enzyme was replaced (to account for enzyme inactivation) and compression testing was performed.