Bovine skin derived gelatin

Gelatin scaffolds were fabricated using previously reported methods [24 (link), 25 (link)]. Briefly, to synthesize the methacrylated gelatin (GelMA), bovine skin-derived gelatin (Sigma-Aldrich, St. Louis, MO) was dissolved in deionized H₂O and then reacted with methacrylic anhydride (Sigma-Aldrich) overnight at 37°C. The reaction product was dialyzed against distilled water using a dialysis cassette (3.5K molecular weight cut-off membrane, ThermoFisher, Waltham, MA) for 5 days to remove the salts and methacrylic acid. The solution was then lyophilized, and the dried sponge was stored in a desiccator. To make the monomer solution, GelMA was dissolved into Hanks’ Balanced Salt Solution (HBSS, with Ca²⁺ and Mg²⁺; ThermoFisher) at 15% (w/v), and the photoinitiator, lithium phenyl-2,4,6-trimethyl-benzoyl phosphinate (LAP, Sigma-Aldrich, St. Louis, MO), was added at 0.15% (w/v).
To generate the hydrogel scaffolds, the GelMA/LAP solution was first poured into silicon molds, which had a cylindrical void space (2 mm height × 5 mm diameter). Then, polymerization was induced using a flashlight with a wavelength at 395 nm for 8 s or 2 min to generate soft or stiff scaffolds.