title: Time-dependent nanomechanics of cartilage.
abstract: 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 τ, magnitude of dynamic complex modulus |E(∗)|, phase angle δ between force and indentation depth, storage modulus E', and loss modulus E″. At ∼2 nm dynamic deformation amplitude, |E(∗)| 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 δ (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 |E(∗)| and δ 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 |E(∗)| at nanometer-scale deformation.
authors.affiliation: Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
authors.forename: Lin
authors.initials: L
authors.lastname: Han
authors.forename: Eliot H
authors.initials: EH
authors.lastname: Frank
authors.forename: Jacqueline J
authors.initials: JJ
authors.lastname: Greene
authors.forename: Hsu-Yi
authors.initials: HY
authors.lastname: Lee
authors.forename: Han-Hwa K
authors.initials: HH
authors.lastname: Hung
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Grodzinsky
authors.forename: Christine
authors.initials: C
authors.lastname: Ortiz
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D002356:Cartilage / Q000502:physiology*; D002417:Cattle; D055119:Elastic Modulus; D005109:Extracellular Matrix / Q000378:metabolism; D018625:Microscopy, Atomic Force; D049329:Nanostructures / Q000737:chemistry*; D011509:Proteoglycans / Q000378:metabolism; D013997:Time Factors
chemical_list: D011509:Proteoglycans