Multiscale bilayer hydrogels enabled by macrophase separation

Bilayer hydrogels with intelligent functions have been considered as the ideal candidates for mimicking soft tissues and robots to adapt their shapes or motions in response to external stimuli. However, conventional fabrication strategies often require multiple complicated steps to create an anisotropic bilayer structure with poor structural defect tolerance due to chemo-mechanical mismatch between two layers. Herein, we report a new working principle and fabrication strategy to construct a family of bilayer hydrogels with a seamless interface and programmable layer thicknesses by controlling spontaneous macrophase separation. The macrophase separation mainly stems from different heat-induced reversible phase transitions of both polymer layers, allowing the decoupling of two gelation processes for the first polymer layer via a typical sol-to-gel transition and the second layer via a spontaneous hydrophilic-to-hydrophobic separation. This work demonstrates a new working principle for seamless integration of thermal-responsive polymers with macrophase separation to create hydrogels with multi-scale hierarchical structures.