Oxygen-inhibited photopolymerization of droplets containing Acryl-PEG-biotin produces functional hydrogels with surfactant-free surfaces. A. Schematic illustration of hydrogel microparticle fabrication via oxygen-inhibited photopolymerization. B. Micrographs illustrating the process described in A, with NA-Rh fluorescence imaging showing the presence of a radial crosslinking density gradient in the hydrogel network. Droplet content: 30 wt% PEGDA 700, 0.5 wt% LAP, 0.5 wt% Acryl-PEG-Biotin, exposed for 700 ms, 350 µW/cm2.