Fabrication of PEG Microgels

PEG microgels were fabricated via submerged electrospraying^{79 (link),80 (link)}. Briefly, PEGaNB, KCGPQGIAGQCK, CGRGDS and LAP were dissolved in endotoxin free and sterile PBS. Precursor solutions were mixed together at a 10 wt% working concentration of PEGaNB, 0.75:1 thiol-ene molar ratio, and 1 mM and 10 mM working concentration of CGRGDS and LAP, respectively. Pre-polymer solution was electrosprayed (12 mL/h, 4.0 kV, 22-gauge needle, 16 mm tip-to-ring distance) into light mineral oil (Fisher Scientific) with Span-80 (0.05 wt%) and photopolymerized with a 365 nm light at 60 mW/cm². Microgels were isolated and washed twice in sterile PBS, once in 70% ethanol, and twice in sterile PBS. Microgels were briefly vortexed and centrifuged at 3000 × g for 15 min in between each washing step. Microgel diameter was measured using the line tool on ImageJ.