Rapid formation of uniformly layered materials by coupling reaction–diffusion processes with mechanical responsiveness

Abstract

Straightforward manufacturing pathways toward large-scale, uniformly layered composites may enable the next generation of materials with advanced optical, thermal, and mechanical properties. Reaction–diffusion systems are attractive candidates to this aim, but while layered composites theoretically could spontaneously arise from reaction–diffusion, in practice randomly oriented patches separated by defects form, yielding nonuniformly patterned materials. A propagating reaction front can prevent such nonuniform patterning, as is the case for Liesegang processes, in which diffusion drives a reaction front to produce layered precipitation patterns. However, while diffusion is crucial to control patterning, it slows down transport of reactants to the front and results in a steady increase of the band spacing as the front advances. Here, we circumvent these diffusive limitations by embedding the Liesegang process in mechanically responsive hydrogels. The coupling between a moving reaction front and hydrogel contraction induces the formation of a self-regulated transport channel that ballistically carries reactants toward the area where patterning occurs. This ensures rapid and uniform patterning. Specifically, large-scale ( > 5-cm) uniform banding patterns are produced with tunable band distance ( d = 60 to 160 µm) of silver dichromate crystals inside responsive gelatin–alginate hydrogels. The generality and applicability of our mechanoreaction–diffusion strategy are demonstrated by forming patterns of precipitates in significantly smaller microscopic banding patterns ( d = 10 to 30 µm) that act as self-organized diffraction gratings. By circumventing the inherent limitations of diffusion, our strategy unlocks the potential of reaction–diffusion processes for the manufacturing of uniformly layered materials.