Hydrogel mechanics across application domains: a regime-based framework for rational design

Hydrogel technologies have rapidly evolved from simple hydrated materials to multifunctional systems for soft robotics, drug delivery, tissue engineering and surgical simulation. As their complexity increases, designing hydrogels for a specific application requires identifying which mechanical phenomena govern material behaviour under the relevant operating conditions, rather than optimizing individual properties in isolation. In this Review, we propose a regime-based mechanical perspective to organize hydrogel mechanics around the physical mechanisms that dominate under different functional scenarios. After introducing a minimal mechanical framework, we examine representative applications to show how elasticity, swelling, solvent transport, viscoelasticity, fracture and mechanical instabilities become alternatively dominant depending on geometry, loading conditions and characteristic timescales. This approach provides a physical framework for identifying the mechanical aspects that should guide material design in different applications, offering a unified strategy for understanding hydrogel behaviour and supporting the rational development of future multifunctional systems.