Achieving Swollen yet Strengthened Hydrogels by Reorganizing Multiphase Network Structure

Many living tissues, such as muscle, become mechanically stronger with growth. Yet, synthetic hydrogels usually exhibit an opposite size-mechanical property relation, that is, swelling-weakening behavior. Herein, a series of swollen yet strengthened polyampholyte (PA) hydrogels are developed via a simple metal-ion solution soaking strategy. In this strategy, a dynamic PA hydrogel (with ionic bonds) is dialyzed in ZrOCl2 solutions (Step-I) and deionized water (Step-II) successively to obtain equilibrated hydrogels. Due to the specific Zr4+ ions and PA network structure, Step-I takes several months with sample size and mechanical performance increasing continuously, while Step-II only needs several days. Through this strategy, the resultant hydrogel networks are reorganized and eventually constructed by ionic and metal-ligand bonds, enabling the swelling yet strengthening behavior. A systematic study confirms that dialysis time in Step-I and corresponding ZrOCl2 concentration can significantly affect the multiphase microstructures of the hydrogels, resulting in different mechanical enhancements. The optimized hydrogel possesses 39.2 MPa of Young's modulus and 3.7 MPa of tensile strength, which are 302 and 5.5 times these of the original PA gel, respectively. Despite distinct swelling, these hydrogels still mechanically surpass many existing high-performance hydrogels. This study opens a novel pathway for fabricating swollen yet strengthened hydrogels.