Micropatterned hydrogel stamps carrying etchant solutions are used to microstructure glass and silicon with complex 3D architectures (see figure and inside cover). A two-way reaction-diffusion mechanism initiated from the stamp enables direct printing of entire microdevices (left to right: two-level microfluidic channels, curvilinear microlenses in glass, nanostructured silicon) into solid substrates with resolution down to 300 nm.