Poroelastic nanoindentation responses of hydrated bone

Indentation techniques are used for the measurement of mechanical properties of a wide range of materials. Typical elastic analysis for spherical indentation is applicable in the absence of time-dependent deformation, but is inappropriate for materials with time-dependent creep responses active in the experimental time frame. In the current work, a poroelastic analysis-a mechanical theory incorporating fluid motion through a porous elastic network-is used to examine spherical indentation creep responses of hydrated biological materials. Existing analytical and finite element solutions for the poroelastic Hertzian indentation problem are reviewed, and a poroelastic parameter identification scheme is developed. Experimental data from nanoindentation of hydrated bone immersed in water and polar solvents (ethanol, methanol, acetone) are examined within the poroelastic framework. Immersion of bone in polar solvents with decreasing polarity results in increased stiffness, decreased Poisson's ratio, and decreased hydraulic permeability. Nanoindentation poroelastic analysis results are compared with existing literature for bone poroelasticity at larger length scales, and the effective pore size probed in indentation creep experiments was estimated to be 1.6 nm, consistent with the scale of fundamental collagen-apatite interactions. Results for water permeability in bone were compared with studies of water diffusion through fully dense bone.