Synthesis of Methacrylated Alginate Biopolymer

Methacrylated alginate (MeALG) was synthesized as described previously [85 (link),94 (link)]. Briefly, 0.5% (w/v) was prepared by dissolving 5 g of medium viscosity alginate (alginic acid sodium salt from brown algae, Sigma-Aldrich Inc., St. Louis, MO, USA) in 1 L of DI water. The solution was kept under magnetic stirring at 1–4 °C. Once the alginate was fully dissolved, 10 mL of methacrylate anhydride (MA, Sigma-Aldrich Inc., St. Louis, MO, USA) was added dropwise into the solution within a span of 1.5–2 h. 2 M NaOH solution (Sigma-Aldrich Inc., St. Louis, MO, USA) was simultaneously added dropwise to adjust the pH of the solution to 8–9. After the addition of the MA, pH of the mixture was maintained by gradually dripping 2 M NaOH solution for 8 h using an automated pH controller. The solution was kept at 4 °C overnight. The reaction was resumed the following day by adding 5 mL of MA while maintaining the pH at 8–9. The material was then dialyzed (Spectra/Por^®1 dialysis membrane, 6–8 kDa, Fisher Scientific, Pittsburgh, PA, USA) against DI water for 5 days and lyophilized using a benchtop freeze dryer (Labconco FreeZone 4.5 L, Fisher Scientific, Pittsburgh, PA, USA). ¹H NMR (Bruker Advance III HD 500 MHz, Bruker Scientific, Billerica, MA, USA) was used to confirm the methacrylate percentage as described previously [85 (link)].