Recent Strategies for Strengthening and Stiffening Tough Hydrogels

Almost all components of the human body include hydrogels, which primarily consist of water molecules within polymer networks. Such natural hydrogels (biological tissues) are ionically conductive and adaptable to various morphologies in physiological environments, exhibiting a unique combination of mechanical properties, including variable strength, stiffness (elastic modulus), toughness (fracture energy), and fatigue resistance (fracture threshold). However, because these tissues usually demonstrate a limited ability to self-heal ability after damage, researchers have attempted to develop synthetic hydrogels with compositions resembling natural tissues. Furthermore, in bioelectronic applications, synthetic hydrogels are utilized as soft backbones that bridge rigid electronics and soft tissues. The number of such applications has been gradually increasing since the development of tough hydrogels, such as double-network (DN) and polyampholyte (PA) gels. A typical DN gel is composed of two interpenetrating polymer networks: a rigid first-network serving as a sacrificial skeleton and a stretchable second-network serving as a deformable substance. These networks with contrasting mechanical properties synergistically interact to surpass the inferior mechanical characteristics of single-network hydrogels. In PA gels, the copolymerization of oppositely charged monomers produces supramolecules containing repetitive cationic and anionic groups. The resulting weak bonds are used for reversible sacrificial crosslinking, and strong bonds serve as permanent crosslinking that preserve the overall hydrogel structure. The presence of multiple ionic bonds with different strengths leads to the superior mechanical properties of PA gels. Accordingly, the unique polymeric structures of DN and PA gels are related to their characteristic fracture mechanisms involving effective energy dissipation and fracture resistance, which considerably expand the hydrogel application area. Such synthetic hydrogels have recently started emulating soft and tough tissues such as the neural, muscle, and epithelial tissues whose elastic moduli lie in the range of a few pascals to several kilopascals (Figure 1). Despite these developments, the strength and stiffness of hydrogels remain significantly lower than those of load-bearing connective tissues organizing tendons, ligaments, cartilages, and blood vessels of which elastic moduli lie in the range of MPa-to-GPa, which are substantially stiffer than other tissues (Figure 1). To replace, fill, and support the damaged D. Ji, Prof. J. Kim School of Chemical Engineering Sungkyunkwan University (SKKU) Suwon 16419, Republic of Korea E-mail: kimjaeyun@skku.edu Prof. J. Kim Department of Health Sciences and Technology Samsung Advanced Institute for Health Science and Technology (SAIHST) Sungkyunkwan University (SKKU) Suwon 16419, Republic of Korea Prof. J. Kim Biomedical Institute for Convergence at SKKU (BICS) Sungkyunkwan University (SKKU) Suwon 16419, Republic of Korea Prof. J. Kim Institute of Quantum Biophysics (IQB) Sungkyunkwan University (SKKU) Suwon 16419, Republic of Korea