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Technical Briefs

A Large Strain Material Model for Soft Tissues With Functionally Graded Properties

[+] Author and Article Information
Uwe-Jens Görke

Department of Environmental Informatics, Helmholtz Centre for Environmental Research-UFZ, Permoserstrasse 15, D-04318 Leipzig, Germany

Hubert Günther

 AO Research Institute, Clavadelerstrasse, CH-7270 Davos, Switzerland

Thomas Nagel

Trinity Centre for Bioengineering, Mechanical and Manufacturing Engineering, School of Engineering, Trinity College, Dublin 2, Ireland

Markus A. Wimmer1

Department of Orthopedic Surgery, Rush University Medical Center, Amour Academic Facilities, Suite 761 1653 West Congress Parkway, Chicago, IL 60612Markus_A_Wimmer@rush.edu

1

Corresponding author.

J Biomech Eng 132(7), 074502 (Jun 02, 2010) (6 pages) doi:10.1115/1.4001312 History: Received September 14, 2009; Revised January 26, 2010; Posted February 22, 2010; Published June 02, 2010; Online June 02, 2010

The reaction of articular cartilage and other soft tissues to mechanical loads has been characterized by coupled hydraulic (H) and mechanical (M) processes. An enhanced biphasic material model is presented, which may be used to describe the load response of soft tissue. A large-strain numerical approach of HM coupled processes has been applied. Physical and geometrical nonlinearities, as well as anisotropy and intrinsic rate-dependency of the solid skeleton have been realized using a thermodynamically consistent approach. The presented material model has been implemented into the commercially available finite element code MSC MARC . Initial verification of the model has been conducted analytically in tendonlike structures. The poroelastic and intrinsic viscoelastic features of the model were compared with the experimental data of an unconfined compression test of agarose hydrogel. A recent example from the area of cartilage research has been modeled, and the mechanical response was compared with cell viability. All examples showed good agreement between numerical and analytical/experimental results.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 2

Stress-stretch curves from plane strain simulations for varying fiber orientations. Note that due to D2=0 specimens have no lateral contraction except for specimen 2 (±45 deg), where lateral contraction due to rotation of the fibers toward the principal stretch axis occurred (inner box).

Grahic Jump Location
Figure 3

Experimental force relaxation curve for unconfined compression of an agarose hydrogel. Nonlinear viscoelasticity can fit the data very well. Biphasic simulation without solid matrix viscoelasticity could not describe the relaxation and constant viscosities can only partially fit the experimental curve.

Grahic Jump Location
Figure 4

(a) Pore pressure profile during impaction within the cartilage; (b) section of the impacted cartilage after 14 days of culture. The cells directly underneath the indenter are dead (red stain). Note how the pattern of live cells (green stain) appears to resemble the calculated pore pressure pattern (from Ref. 42).

Grahic Jump Location
Figure 1

Axial Cauchy stress over the axial stretch for one compressible (ν=0.0) and one nearly incompressible (ν=0.4995) cylindrical specimen undergoing a uniaxial test. Fibers are only active in tension (λ1>1).

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