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TECHNICAL PAPERS: Soft Tissue

Effects of Boundary Conditions on the Estimation of the Planar Biaxial Mechanical Properties of Soft Tissues

[+] Author and Article Information
Wei Sun

Engineered Tissue Mechanics Laboratory, Department of Bioengineering,  University of Pittsburgh, Pittsburgh, PA

Michael S. Sacks2

Engineered Tissue Mechanics Laboratory, Department of Bioengineering,  University of Pittsburgh, Pittsburgh, PAmsacks@pitt.edu

Michael J. Scott

 Edwards Lifesciences, Irvine, CA

2

Corresponding author. Department of Bioengineering, McGowan Institute for Regenerative Medicine, Room 234, 100 Technology Drive, University of Pittsburgh, Pittsburgh, PA 15219.

J Biomech Eng 127(4), 709-715 (Mar 03, 2005) (7 pages) doi:10.1115/1.1933931 History: Received January 04, 2004; Revised March 03, 2005

Evaluation and simulation of the multiaxial mechanical behavior of native and engineered soft tissues is becoming more prevalent. In spite of this growing use, testing methods have not been standardized and methodologies vary widely. The strong influence of boundary conditions were recently underscored by Waldman [2002, J. Materials Science: Materials in Medicine13, pp. 933–938] wherein substantially different experimental results were obtained using different sample gripping methods on the same specimens. As it is not possible to experimentally evaluate the effects of different biaxial test boundary conditions on specimen internal stress distributions, we conducted numerical simulations to explore these effects. A nonlinear Fung-elastic constitutive model (Sun, 2003, JBME125, pp. 372–380, which fully incorporated the effects of in-plane shear, was used to simulate soft tissue mechanical behavior. Effects of boundary conditions, including varying the number of suture attachments, different gripping methods, specimen shapes, and material axes orientations were examined. Results demonstrated strong boundary effects with the clamped methods, while suture attachment methods demonstrated minimal boundary effects. Suture-based methods appeared to be best suited for biaxial mechanical tests of biological materials. Moreover, the simulations demonstrated that Saint-Venant’s effects depended significantly on the material axes orientation. While not exhaustive, these comprehensive simulations provide experimentalists with additional insight into the stress–strain fields associated with different biaxial testing boundary conditions, and may be used as a rational basis for the design of biaxial testing experiments.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 2

Stress–strain output of the FE simulations with different numbers of suture attachments. (a) 45deg material axes, normal stresses; (b) shear stress; (c) stresses for 0deg material axes.

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Figure 3

Stress (in kPa) along the BB lines of the FE simulations with different numbers of suture attachments, (a) for 45deg material axes and (b) for 0deg material axes.

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Figure 4

Von Mises stresses (in kPa) and deformations of FE models for different gripping methods at (a)–(d) 45deg material axes; (e)–(g) at 0deg material axes.

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Figure 5

Peak stress of inner central regions for (a) 45deg and (b) 0deg material axes for different gripping methods.

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Figure 6

Stress (in kPa) along the BB lines of the FE simulations for different gripping methods, (a) for 45deg material axes and (b) for 0deg material axes.

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Figure 1

(a) A schematic of the biaxial testing setup with four suture attachments, indicating the material axes X1–X2; (b) biaxial testing setup with gripping method as using clamp on each side of a square sample; and (c) biaxial testing setup with clamps on each side of a cruciform sample. Dimensions of a, b, and c are listed in Table 3. Note that the BB line is used for measuring the stress variation within the specimen.

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