Figure 2.

Strategies to control spatial illumination of hydrogels. a,b) Photomasks can be used to selectively block collimated light to create a) patterns or b) gradients in illumination that are uniform in the axial direction. c) A maskless photolithography technique is used to project patterns based on graphical inputs to a digital micromirror device (DMD). Reprinted from Ma et al.¹⁴⁰ Copyright (2016) National Academy of Sciences. d) Alternatively, focused laser light can be used to illuminate regions with 3D resolution. The light illumination is illustrated by photobleaching uniformly bound fluorescein within a hydrogel at a single plane with light of 488 nm (single photon absorption) and 800 nm (two photon absorption). Two-photon absorption events are localized to the imaging plane. Photobleaching and confocal imaging performed with a 1.0 NA objective. Scale bar = 25 μm.