Time-Dependent Nanomechanics of Cartilage

In this study, atomic force microscopy-based dynamic oscillatory and force-relaxation indentation was employed to quantify the time-dependent nanomechanics of native (untreated) and proteoglycan (PG)-depleted cartilage disks, including indentation modulus E-ind, force-relaxation time constant tau, magnitude of dynamic complex modulus vertical bar E*vertical bar, phase angle 6 between force and indentation depth, storage modulus E', and loss modulus E ''. At similar to 2 nm dynamic deformation amplitude, vertical bar E*vertical bar increased significantly with frequency from 0.22 +/- 0.02 MPa (1 Hz) to 0.77 +/- 0.10 MPa (316 Hz), accompanied by an increase in delta (energy dissipation). At this length scale, the energy dissipation mechanisms were deconvoluted: the dynamic frequency dependence was primarily governed by the fluid-flow-induced poroelasticity, whereas the long-time force relaxation reflected flow-independent viscoelasticity. After PG depletion, the change in the frequency response of vertical bar E*vertical bar and delta was consistent with an increase in cartilage local hydraulic permeability. Although untreated disks showed only slight dynamic amplitude-dependent behavior, PG-depleted disks showed great amplitude-enhanced energy dissipation, possibly due to additional viscoelastic mechanisms. Hence, in addition to functioning as a primary determinant of cartilage compressive stiffness and hydraulic permeability, the presence of aggrecan minimized the amplitude dependence of vertical bar E*vertical bar at nanometer-scale deformation.