Where: k1 is the rate constant for photolysis [A]I is the initial concentration of the crosslinker Vsample is the total volume of the sample irradiated P is the power of the incoming light, measured in Watts (J sec -1 ) Eph is the energy of the photon, given by /1 2 , where h is Plank's constant, c is the speed of light in a vacuum, and l is the wavelength of incident light NA is Avogadro's number Hydrogel formation. Hydrogels were formed via SPAAC by mixing of PEG-tetraBCN (final concentration of 3 mM) with the diazide crosslinkers (final concentration of 6 mM). For hydrogels used in cell culture, an azide-modified RGD peptide (N3-GRGDSG-NH2) was included (final concentration of 1 mM).