Foaming photopolymers as a high-resolution biomimetic printing platform

Nature creates a diverse range of foam-like materials, from bones to banana peels, through a rich orchestration of physical and biochemical processes across hierarchical length scales¹. Here, we introduce deep-foam photolithography, a spatially controlled, light-induced foaming process in which a photosensitized polymer film is exposed to deeply penetrating UV-A light, which initiates the production of both scission and crosslinked polymers throughout the depth of the film. Hypotonically driven case II permeation² of a weak solvent triggers pore nucleation, expansion and a controllable viscoelastic collapse to create foams through a localized solubilization of polymer fragments within a crosslinked network. The resemblance to amorphous structural whites in nature enables inkless printing in white and greyscale; the resolution is high enough (about 20,000 dots per inch) to generate diffractive colour. This uniquely allows the printing of mechanically expanded and collapsed foams with high aspect ratios (20×); controlled collapse creates hierarchically rough surfaces with lotus-like super-hydrophobicity. Deep-foam photolithography is applied to both films and fibres, using a range of amorphous and semi-crystalline polymeric materials. Patterning μm-level spatial differences in foamability, wettability, optical properties and local thickness enables high-resolution printing, microfluidics and picolitre capture of liquids and colloidal matter.