Spontaneous Macrophase Separation Strategy for Bridging Hydrogels from Bilayer to Double-Network Structure

Bilayer hydrogels and double-network (DN) hydrogels represent two distinct classes of soft-wet materials, each characterized by their distinctive network structures, design principles, synthesis methods, and core functions targeted for their specific applications. Bilayer hydrogels are structured in two different layers, each with their anisotropic structure and unique properties. This dual-layer configuration facilitates targeted responses or controlled actuation in response to environmental stimuli, making them ideal for applications requiring responsive material behavior. On the other hand, DN hydrogels consist of two interwoven yet independent networks: one brittle and the other elastic. This dual-network structure, featuring contrasting network properties, allows for substantial energy dissipation and mechanical enhancement, often far exceeding that of traditional single-network hydrogels. Despite the individual strengths and specialized applications of each hydrogel type, a unified fabrication strategy that addresses both types of hydrogels has been conspicuously missing due to their inherent structural differences. This gap in the hydrogel field presents significant challenges but also opens opportunities for innovation in material design and application.

In this Account, we introduce a new macrophase separation strategy that leverages differential polymerization rates and sol-to-gel phase transitions, enabling a bridging of the design and manufacturing gap between bilayer and DN hydrogels. This strategy facilitates the smooth creation of hydrogels with varied structures, from bilayer to DN structures, enabling the precise control of topological networks and multiscale hierarchical architectures. The approach is grounded in the selection of polymer pairs that are compatible with the macrophase separation concept, ensuring that the distinct characteristics of both bilayer and DN hydrogels are effectively realized in terms of their structures, design strategies, synthesis routes, and primary functions. Three distinct macrophase separation strategies are outlined, each demonstrating the concept through the careful selection of compatible polymer pairs. By demonstrating the versatility and functionality of the bilayer and DN hydrogels, the macrophase separation strategy not only achieves rapid and reversible actuation in bilayer hydrogels and outstanding mechanical strength and interfacial adhesion in DN hydrogels but also combines dynamic actuation abilities with robust mechanical integrity within both bilayer and DN hydrogels.

The macrophase separation strategy surpasses conventional fabrication methods such as layer-by-layer 3D/4D printing, self-assembly, and composite integration, due to its straightforward preparation process, exceptional phase separation efficiency, improved control of layer thickness, and faster responsiveness. This strategy acts as a transformative approach for readily integrating multifunctional and stimuli-responsive components into cohesive hydrogel systems, thus paving the path for the development of next-generation materials beyond the traditional scope of smart hydrogel materials and systems.