Rheometric characterization of hydrogel viscoelasticity. Hydrogels were formed on the rheometer (20 µL, working distance = 0.3 mm) and gelation was observed in a timesweep experiment (angular frequency = 6.28 rad s -1 ). Following complete hydrogel formation as indicated by storage modulus plateau, gels were swelled in 0.1 mM NaN3 to cap any remaining BCN groups prior to photodegradation. Hydrogels were exposed to light via a quartz bottom plate coupled to a fiber optic light guide from a multiwavelength LED light source.