Growth order of stiff and soft domains in gels controls morphology

In biology, neighboring soft and stiff domains can grow at different times, so the growth of one domain influences the subsequent growth of the next. To isolate key factors controlling this complex spatiotemporal behavior, we model gels undergoing biomimetic stepwise growth. The gel's top surface is patterned with a stiff cross, while the underlying non-patterned domains are less crosslinked and softer. At ambient pressure, if growth of the stiff cross occurs before growth of soft layers, the structure displays a concave shape; reversing the growth order yields a gel exhibiting a convex structure. The findings reveal how the shapes and properties of these heterogeneous materials co-evolve as they reach equilibrium morphologies. By increasing the hydrostatic pressure, we also isolate morphologies that remain pressure resistant. Our findings reveal an approach to control a material's geometric patterning and mechanical properties within these patterns and can provide insight into physicochemical factors affecting biological morphogenesis.