Programmable Shape-Morphing in Homogeneous Hydrogels via Interfacial Confinement.

Programmable shape-morphing in hydrogels typically requires complex chemical gradients or oriented fillers, which strongly limits its scalability. Here, we design a geometric strategy to induce anisotropic deformation in homogeneous hygroscopic hydrogels through interfacial confinement. By mechanically interlocking the hydrogel with a microporous substrate, we suppress lateral expansion at the base and redirect isotropic volume gain into directional displacement. Systematic variation of sample thickness reveals a morphological transition: thin films undergo quasi-uniaxial vertical swelling, whereas thick films develop mushroom-like architectures featuring coexisting interfacial anisotropy and distal isotropy. Meanwhile, the ionic osmotic pressure (∼107 Pa) overwhelmingly exceeds the elastic penalty imposed by the constraint (∼105 Pa), which makes the equilibrium water uptake of the corresponding hydrogels irrespective of boundary geometry. Poroelastic simulations quantitatively reproduce the experimentally observed deformation profiles, and cross-sectional electron microscopy confirms a stress-attenuation-driven microstructural gradient consistent with Saint-Venant-type decay. These findings establish interfacial confinement as a scalable paradigm for programming hydrogel actuation without requiring compositional heterogeneity, with implications for soft robotics and adaptive devices.