Eliminating adhesion errors in nanoindentation of compliant polymers and hydrogels

Nanoindentation is a valuable tool for characterization of biomaterials due to its ability to measure local properties in heterogeneous, small or irregularly shaped samples. However, applying nanoindentation to compliant, hydrated biomaterials leads to many challenges including adhesion between the nanoindenter tip and the sample. Although adhesion leads to overestimation of the modulus of compliant samples when analyzing nanoindentation data using traditional analysis techniques, most studies of biomaterials have ignored its effects. This paper demonstrates two methods for managing adhesion in nanoindentation analysis, the nano-JKR force curve method and the surfactant method, through application to two biomedically-relevant compliant materials, poly(dimethyl siloxane) (PDMS) elastomers and poly(ethylene glycol) (PEG) hydrogels. The nano-JKR force curve method accounts for adhesion during data analysis using equations based on the Johnson-Kendall-Roberts (JKR) adhesion model, while the surfactant method eliminates adhesion during data collection, allowing data analysis using traditional techniques. In this study, indents performed in air or water resulted in adhesion between the tip and the sample, while testing the same materials submerged in Optifree Express(®) contact lens solution eliminated tip-sample adhesion in most samples. Modulus values from the two methods were within 7% of each other, despite different hydration conditions and evidence of adhesion. Using surfactant also did not significantly alter the properties of the tested material, allowed accurate modulus measurements using commercial software, and facilitated nanoindentation testing in fluids. This technique shows promise for more accurate and faster determination of modulus values from nanoindentation of compliant, hydrated biological samples.