Figure 2. Photodegradable hydrogel synthesis and patterning.

(a) Photodegradable hydrogels are formed by redox initiated, free radical (R*) chain polymerization of PEGdiPDA crosslinking molecules, rapidly forming an insoluble hydrogel. The resultant network structure is comprised of polyacrylate kinetic chains (green) connected by PEG-based crosslinks (black) with photocleavable o-nitrobenzylether moieties (blue) in the backbone. (b) Chrome masks (black and white layer) can be placed at the surface of photodegradable gels (orange) to selectively occlude incident light (purple). With sufficient irradiation time (t) features are formed at the surface of PEGdiPDA gels, where the depth of feature formation depends on the duration of exposure. This photolithographic degradation can be used to generate features in the surface of a gel with minutes of irradiation time. (c) The focal point of a laser can be used to pattern features within PEGdiPDA hydrogels. Here, a laser scanning microscope (LSM) with a 405 nm laser is used to draw regions of interest (ROI) in the material. The laser focal point is rastered through the ROI and into the z-dimension to completely erode 3D features within the hydrogel in seconds. Both of these patterning strategies can be translated for spatiotemporally controlled presentation of biochemical ligands with gels containing the photoreleasable tether. (d) An overview of irradiation parameters for degradation and complete erosion of photodegradable hydrogels. Ultimately, these patterning approaches allow the rapid, in situ creation of micron-scale features in the presence or absence of cells in 2D and 3D.