Although conventional polymer gels are known as mechanically weak materials, their fracture toughness can be effectively improved by introducing weak and brittle bonds into soft and stretchy polymer networks. This toughening method, denoted as the ‘sacrificial bond principle’, has been recently proposed. In this focus review, I describe some extremely tough gels prepared using this principle, e.g., double- or multiple-network gels with high water content featuring covalent sacrificial bonds, self-healing polyampholyte gels containing ionic sacrificial bonds and PDGI/PAAm gels based on hydrophobic sacrificial bonds exhibiting stress-responsive structural colors.
