Complex Shear Modulus of Hydrogels Using a Dynamic Nanoindentation Method

The micromechanical properties of soft tissues and materials are of considerable interest for biomedical applications. Nanoindentation is a powerful technique for determining localized material properties of biological tissues and has been used widely for hard tissue and material characterization. However, the technique is much more challenging when utilized for soft tissues due to their compliance as well as due to the limitations of commercial instruments which were originally developed for stiff, engineering materials. This study explores the use of a dynamic indentation method with a cylindrical punch (100 mu m diameter) to characterize gelatin gel and low molecular weight hydrogels. A Keysight Technologies DCM II actuator is used with the Continuous Stiffness Measurement (CSM) to determine the complex shear modulus of these gels. The method overcomes surface detection issues with standard quasi-static nanoindentation as a change in phase angle can be used to accurately detect the sample surface. The data collected in this study are found to be comparable with macroscopic rheology and demonstrates the utility of the method for characterization of hydrogels.