Mechanical characterization of hydrogels

Hydrogels are used in wide range of applications from biomedical engineering to the food industry, due to their water-rich composition and biocompatibility. However, most hydrogel materials exhibit poor mechanical properties, which limits their applications considerably. Thus, designing hydrogels with improved mechanical properties is highly desirable yet a challenging task, due to our incomplete understanding of hydrogel mechanics. Mechanical behavior of hydrogels is inherently complex, intimately linked to material chemistry and morphology at different length scales. A hydrogel's response to far-field loading or deformation involves multiple mechanisms including nonlinear elasticity, viscoelasticity, and poroelasticity. In this chapter, an overview of the current understanding of hydrogel properties is presented, as measured through multiscale characterization techniques from macroscale to molecular scale. The discussion begins with a classification of hydrogels based on source, composition, network configuration, and cross-linking method. The mechanical properties of hydrogels as measured by tension, compression, and fracture tests are presented. Time-dependent properties as measured by indentation testing are discussed next, with specific emphasis on the roles of poroelasticity and viscoelasticity in load-relaxation behavior of hydrogels. At the molecular scale, different microscopy-based techniques that are used to measure mass transport and diffusion properties are discussed. Finally, elastic, fracture, and time-dependent properties of a large variety of hydrogel materials are compared to highlight the state of the art of this emerging class of material.