Michelle Oyen, Associate Professor, Biomedical Engineering, Washington University in St. Louis

Nanoindentation techniques originated with thin films and hard, stiff engineering materials. However, this technique is advantageous for characterizing hydrated materials, including biological tissues and hydrated hydrogels. There are unique challenges associated with handling and testing hydrated samples. First, the instrumentation must be adapted for testing materials six or more orders of magnitude more compliant than stiff materials. For hydrated materials, a poroelastic or poroviscoelastic analysis, which explicitly treats the fluid flow through the porous material, is used to extract material properties from experimental data. This approach requires multiple indentation tests with different probe sizes for decoupling phenomena with and without an intrinsic length scale. Some key results from recent works using nanoindentation to evaluate hydrated materials are presented as examples in the context of these challenges. Finally, as these studies represent relatively recent developments in the nanoindentation field, an outlook for the future is presented. It is clear that a consensus is emerging for the quantitative evaluation of hydrated materials via a modified nanoindentation technique.