title: Poroelastic response of articular cartilage by nanoindentation creep tests at different characteristic lengths.
abstract: Nanoindentation is an experimental technique which is attracting increasing interests for the mechanical characterization of articular cartilage. In particular, time dependent mechanical responses due to fluid flow through the porous matrix can be quantitatively investigated by nanoindentation experiments at different penetration depths and/or by using different probe sizes. The aim of this paper is to provide a framework for the quantitative interpretation of the poroelastic response of articular cartilage subjected to creep nanoindentation tests. To this purpose, multiload creep tests using spherical indenters have been carried out on saturated samples of mature bovine articular cartilage achieving two main quantitative results. First, the dependence of indentation modulus in the drained state (at equilibrium) on the tip radius: a value of 500 kPa has been found using the large tip (400 μm radius) and of 1.7 MPa using the smaller one (25 μm). Secon, the permeability at microscopic scale was estimated at values ranging from 4.5×10(-16) m(4)/N s to 0.1×10(-16) m(4)/N s, from low to high equivalent deformation. Consistently with a poroelastic behavior, the size-dependent response of the indenter displacement disappears when characteristic size and permeability are accounted for. For comparison purposes, the same protocol was applied to intrinsically viscoelastic homogeneous samples of polydimethylsiloxane (PDMS): both indentation modulus and time response have been found size-independent.
authors.affiliation: Department of Chemistry, Materials and Chemical Engineering, Laboratory of Biological Structure Mechanics (LaBS), Politecnico di Milano, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Taffetani
authors.affiliation: Ri.MED Foundation, Palermo, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Gottardi
authors.affiliation: Department of Chemistry, Materials and Chemical Engineering, Laboratory of Biological Structure Mechanics (LaBS), Politecnico di Milano, Italy.
authors.forename: D
authors.initials: D
authors.lastname: Gastaldi
authors.affiliation: Department of Informatics, Bioengineering, Robotics and System Engineering, University of Genova, Genova, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Raiteri
authors.affiliation: Department of Chemistry, Materials and Chemical Engineering, Laboratory of Biological Structure Mechanics (LaBS), Politecnico di Milano, Italy; IRCCS, Istituto Ortopedico Galeazzi, Milano, Italy. Electronic address: pasquale.vena@polimi.it.
authors.forename: P
authors.initials: P
authors.lastname: Vena
mesh_terms: D000818:Animals; D002358:Cartilage, Articular / Q000502:physiology* / Q000648:ultrastructure*; D002417:Cattle; D003198:Computer Simulation; D055119:Elastic Modulus / Q000502:physiology; D006244:Hardness / Q000502:physiology; D006245:Hardness Tests / Q000379:methods; D066298:In Vitro Techniques; D008954:Models, Biological; D053758:Nanoparticles / Q000737:chemistry / Q000648:ultrastructure*; D058608:Nanopores / Q000648:ultrastructure; D016062:Porosity; D013718:Tensile Strength / Q000502:physiology; D014783:Viscosity
keywords: Articular cartilage; Nanoindentation; Poroelasticity, Size effect; Time-dependence