Measuring the mechanical response of swollen hydrogels

A combined experimental and analytical study has been conducted to understand the mechanical response of a swollen poly(vinyl alcohol) (PVA) hydrogel. These PVA hydrogels are formulated using a freeze-thaw crosslinking process, making them suitable for biomedical applications. In order to predict the mechanical response, a finite elastic constitutive theory is used. Two stress-strain relations are derived to describe the response of hydrogels that are either submerged in water or in air. These stress-strain relations motivate the experimental protocols, which are needed to investigate the applicability of the constitutive theory as a model for the PVA hydrogels used in this study. A uniaxial load frame has been created that performs individual or coupled experiments of stretching and swelling. The apparatus uses a CCD camera to measure finite deformation by tracking spheres attached to a small, central region of the hydrogel, while a load-cell measures the applied traction. Deformation of the hydrogel can be used as feedback to control the rate of deformation during stretching tests. The constitutive parameters can be estimated from the results of stretching specimens to failure, while they are out of water and swelling specimens subject to applied loads. After experimental data are obtained from these two experiments, a nonlinear least squares optimization routine estimates material parameters from the governing stress-strain relations. It is found that a modified Ogden model, which incorporates the Flory-Huggins theory, is a good predictor of the mechanical response of PVA hydrogels.