Comparison of mechanical testing methods for biomaterials: Pipette aspiration, nanoindentation, and macroscale testing.

Characterization of the mechanical properties of biological materials is often complicated by small volume, irregular geometry, fragility, and environmental sensitivity. Pipette aspiration and nanoindentation testing deal well with these limitations and have seen increasing use in biomaterial characterization, but little research has been done to systematically validate these techniques for soft materials. This study compared the results of pipette aspiration, nanoindentation, and bulk uniaxial tension and compression in determining the small-strain elastic moduli of a range of biomedically-relevant materials, a series of silicone elastomers and polyacrylamide hydrogels. A custom apparatus was developed for pipette aspiration testing, a commercial Hysitron instrument with custom spherical tip was used for nanoindentation, and standard commercial machines were used for tension and compression testing. The measured small-strain elastic moduli ranged from 27 to 368 kPa for the silicones and 11 to 44 kPa for the polyacrylamide gels. All methods detected expected trends in material stiffness, except for the results from one inconsistent silicone. Pipette aspiration and nanoindentation measured similar elastic moduli for silicone materials, but pipette aspiration measured consistently larger stiffness in the hydrogels, which may be explained by the gels' resistance to tension. Despite the difference in size scale among the testing methods, size does not appear to influence the results. These results suggest that both pipette aspiration and nanoindentation are suitable for measuring mechanical properties of soft biomaterials and appear to have no more limitations than bulk techniques.