Poroelasticity of Cartilage at the Nanoscale

Atomic-force-microscopy-based oscillatory loading was used in conjunction with finite element modeling to quantify and predict the frequency-dependent mechanical properties of the superficial zone of young bovine articular cartilage at deformation amplitudes, delta, of similar to 15 nm; i.e., at macromolecular length scales. Using a spherical probe tip (R similar to 12.5 mu m), the magnitude of the dynamic complex indentation modulus, vertical bar E*vertical bar, and phase angle, phi, between the force and tip displacement sinusoids, were measured in the frequency range f similar to 0.2-130 Hz at an offset indentation depth of delta(0) similar to 3 Am. The experimentally measured vertical bar E*vertical bar and phi corresponded well with that predicted by a fibril-reinforced poroelastic model over a three-decade frequency range. The peak frequency of phase angle, f(peak), was observed to scale linearly with the inverse square of the contact distance between probe tip and cartilage, 1/d(2), as predicted by linear poroelasticity theory. The dynamic mechanical properties were observed to be independent of the deformation amplitude in the range delta = 7-50 nm. Hence, these results suggest that poroelasticity was the dominant mechanism underlying the frequency-dependent mechanical behavior observed at these nanoscale deformations. These findings enable ongoing investigations of the nanoscale progression of matrix pathology in tissue-level disease.