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TECHNICAL PAPERS: Soft Tissue

Mechanical Characterization of Anisotropic Planar Biological Soft Tissues Using Large Indentation: A Computational Feasibility Study

[+] Author and Article Information
Martijn A. Cox1

Department of Biomedical Engineering, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

Niels J. Driessen, Carlijn V. Bouten, Frank P. Baaijens

Department of Biomedical Engineering, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

1

Corresponding author; e-mail: m.a.j.cox@tue.nl

J Biomech Eng 128(3), 428-436 (Oct 27, 2005) (9 pages) doi:10.1115/1.2187040 History: Received May 13, 2005; Revised October 27, 2005

Traditionally, the complex mechanical behavior of planar soft biological tissues is characterized by (multi)axial tensile testing. While uniaxial tests do not provide sufficient information for a full characterization of the material anisotropy, biaxial tensile tests are difficult to perform and tethering effects limit the analyses to a small central portion of the test sample. In both cases, determination of local mechanical properties is not trivial. Local mechanical characterization may be performed by indentation testing. Conventional indentation tests, however, often assume linear elastic and isotropic material properties, and therefore these tests are of limited use in characterizing the nonlinear, anisotropic material behavior typical for planar soft biological tissues. In this study, a spherical indentation experiment assuming large deformations is proposed. A finite element model of the aortic valve leaflet demonstrates that combining force and deformation gradient data, one single indentation test provides sufficient information to characterize the local material behavior. Parameter estimation is used to fit the computational model to simulated experimental data. The aortic valve leaflet is chosen as a typical example. However, the proposed method is expected to apply for the mechanical characterization of planar soft biological materials in general.

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Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Microscopy image of a porcine aortic valve leaflet, showing large collagen fiber bundles (modified from Sauren (see Ref. 38), with permission). The white square and circle illustrate the dimensions of the modeled tissue sample and indenter, respectively.

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Figure 2

A schematic view of the simulated setup. Spherical indentation tests are performed on top of an inverted confocal microscope.

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Figure 3

Top view (left) and front view (right) of the 3D model, showing indenter and boundary conditions

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Figure 4

Indentation force (middle row) and first (λ1) and second (λ2) principal stretches (bottom row) as a function of indentation depth, for three different fiber distributions (top row): a single fiber direction (left column, μ=0deg, σ→0deg), native valve fiber distribution (see Refs. 13,37) (middle column, μ=0deg, σ=10.7deg) and a uniform fiber distribution (right column, μ=0deg, σ→∞deg)

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Figure 5

(a) Angular fiber distribution in the undeformed configuration (solid line) and at 0.30mm indentation (dashed line); (b) relative fiber stress distribution at four levels of indentation: 0.20mm (solid line), 0.22mm (dashed line), 0.24mm (dash-dotted line), and 0.26mm (dotted line)

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Figure 6

Indentation forces as a function of indentation depth for simulation A. The experimental data (cross) is fit very well by the model (solid line).

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Figure 7

First principal strain (a), second principal strain (b), and indentation force (c) as a function of indentation depth for simulation B. Experimental data (cross) are fit very well by the model (solid line).

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