Comparison of Spherical and Flat Tips for Indentation of Hydrogels

Although both spherical and flat tips have been used in nanoindentation studies of soft biomaterials, care must be taken in selecting and validating a tip for a specific application. This article compares the moduli measured using spherical nanoindentation, flat tip (specifically, a flattened cone) nanoindentation, and unconfined compression testing of three polyacrylamide gels with nominal moduli between 10 kPa and 50 kPa. Although spherical indentation moduli were consistent with compression testing moduli and were independent of indentation depth, the flat tip results showed a significant increase in modulus with depth when analyzed using a flat punch model. Alternative methods are proposed to analyze the flat tip data to bring the flat tip results into alignment with the moduli measured using the other mechanical testing techniques.