Competitive (Spatiotemporal) Techniques to Fabricate (Ultra)stiff Polymer Hydrogels and Their Potential Applications

Abstract

Hydrogels are water-swollen, three-dimensionally cross-linked polymeric networks widely recognized for their biological compatibility and immense potential across a broad range of applications. However, their inherently poor mechanical properties pose a significant challenge to their widespread practical use. Although numerous strategies have improved the strength and toughness of hydrogels, achieving high stiffness (in the ≥10 MPa range) in addition to these properties remains challenging. The general approach of increasing cross-linking density to enhance stiffness often results in reduced stretchability and toughness, leading to brittle materials. Designing hydrogels that achieve both high stiffness and toughness is fundamentally challenging, as these parameters are often interdependent and conflicting yet essential for practical applications. Recent advancements have enabled the development of ultrastiff hydrogels with elastic modulus exceeding 10 MPa. Remarkably, many of these hydrogels are viscoelastic in nature and also exhibit significant energy dissipation, thereby preserving the toughness. Unlike elastic hydrogels, which rely on spatial design (governed by the polymer architecture), viscoelastic hydrogels also incorporate temporal structures, where cross-linking kinetics (lifetime) significantly affects their mechanical properties. The respective contributions from the spatial and temporal component toward hydrogel performance depends on the hydrogel fabrication strategy. For instance, in semicrystalline hydrogels featuring long chain alkyl domains, the strength of physical cross-links primarily contributes to hydrogel stiffness, whereas enhancing polymer backbone rigidity by incorporating α-methyl groups may increase hydrogel stiffness even when weak cross-links are employed. Some of these aspects, including examples of synergistic combination of different hydrogel network stiffening strategies, are elaborated in this review. This review highlights breakthroughs, examining the fabrication processes, mechanism behind enhanced stiffness, and the trade-offs addressed in these systems. Finally, we highlight promising applications (in the emergent fields of flexible electronics, soft robotics, and biomedical implants) that underscore the immense potential of these advanced hydrogels.