Micromechanical Properties of Polyacrylamide Hydrogels Measured by Spherical Nanoindentation

Abstract

Hydrogels are very compliant materials suitable for tissue engineering in various areas of biological and clinical research. Appropriate and effective application of hydrogels for specific cellular regeneration often requires precise knowledge of their mechanical properties. The present work focuses on measurements of mechanical deformation and creep properties of polyacrylamide hydrogels using a novel indentation system. Four concentrations of polyacrylamide gel were tested under four different loading rates to study the mechanical response of the material to various loading rates. A spherical indenter with large radius was used in the experiments and all indentations were done with the sample completely immersed in water. The results show that higher acrylamide concentration in the gel leads to higher elastic modulus and decrease of creep. Similarly, faster loading rates lead to higher elastic modulus and larger creep during the hold period. The data were analyzed using both Hertzian fit to the loading part and Oliver-Pharr approach to the unloading part. The discrepancy between these two approaches and significant creep behavior are related to the viscoelasticity of the tested materials. This work contributes to understanding the results of instrumented indentation of extremely compliant materials with respect to their viscoelastic properties.