High-definition and arbitrary grayscale hydrogel paintings that can appear reversibly with hydration/dehydration are realized through spatially grayscale exposure. Spatio-temporally grayscale images are used to guide the exposure of a hydrogel processor to dictate the gradient cross-linking density spatially, which thereafter results in the heterogeneity of hydrogels in swelling ratio, mechanical properties, and especially visible light transmittance, leading to swelling-induced patterns by gradient and local visible light scattering difference based on tunable mesh size and microphase separation. The resultant grayscale hydrogel patterns, with visible light transmittance adjustable, are reversible during hydration (in 1-2 s) and dehydration and possess the feature size of 70 μm and more pattern information compared with previous hydrogel patterning. Uniquely, the patterns can be realized not only on the outmost surface of hydrogels as usual but inside. Combining the unique grayscale exposure with three-dimensional printing technology, arbitrary hydrogel patterns that have plenty of details and even vary at different layers are fabricated readily, indicating its broad potential in smart anti-counterfeiting of security field, mechanics, engineering, and many others.
Keywords: digitally grayscale drawing; gradient crosslinking density; high definition; hydrogel patterning; reversible appearance.