In Situ Photopolymerization of Hydrogels in Radical Covalent Organic Frameworks

Molecular initiating agents, particularly those used for photoinitiation, have dominated hydrogel synthesis for decades. However, their small molecular nature leads to several issues in hydrogels: migration out of the polymer matrix, poor photostability, limited functionality, and relatively poor mechanical properties. In this study, we report macromolecular examples of radical covalent organic frameworks (COFs) as next-generation photoinitiating agents for hydrogel synthesis, along with a proposed dual-initiation mechanism. In situ polymerization in radical COFs enables hydrogels with an ultrahigh tensile strength of 4.3 MPa, outstanding stretchability with a fracture strain improved by 1 order of magnitude (13,382%), an exceptional toughness of 262 MJ/m³, and fully recoverable large deformation (up to 1500% strain). This exceptional performance is attributed to the physical nanoconfined effect of the framework and chemical crosslinking between hydrogels and COF walls. Furthermore, the exceptional strength, toughness, and durability of the hydrogel enable its excellent applicability in wearable sensors, delivering multiangle detection with high sensitivity over prolonged operation.