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Research Papers

Direct Measurement of Nonuniform Large Deformations in Soft Tissues During Uniaxial Extension

[+] Author and Article Information
Todd C. Doehring1

School of Biomedical Engineering, Science, and Health Systems, Drexel University, Philadelphia, PA 19104tcdoe@drexel.edu

Michael Kahelin

School of Biomedical Engineering, Science, and Health Systems, Drexel University, Philadelphia, PA 19104

Ivan Vesely

Saban Research Institute, Childrens Hospital Los Angeles, Keck School of Medicine, University of Southern California, Los Angeles, CA

1

Corresponding author.

J Biomech Eng 131(6), 061001 (Apr 21, 2009) (6 pages) doi:10.1115/1.3116155 History: Received September 18, 2007; Revised November 24, 2008; Published April 21, 2009

Understanding the complex relationships between microstructural organization and macromechanical function is fundamental to our knowledge of the differences between normal, diseased/injured, and healing connective tissues. The long-term success of functional tissue-engineered constructs or scaffolds may largely depend on our understanding of the structural organization of the original tissue. Although innovative techniques have been used to characterize and measure the microstructural properties of collagen fibers, a large gap remains in our knowledge of the behavior of intermediate scale (i.e., “mesostructural”) groups of fiber bundles in larger tissue samples. The objective of this study was to develop a system capable of directly measuring deformations of these smaller mesostructures during application of controlled loads. A novel mesostructural testing system (MSTS) has been developed to apply controlled multiaxial loads to medium (meso-) scale tissue specimens, while directly measuring local nonuniform deformations using synchronized digital video capture and “markerless” image correlation. A novel component of the MSTS is the use of elliptically polarized light to enhance collagen fiber contrast, providing the necessary texture for accurate markerless feature tracking of local fiber deformations. In this report we describe the components of the system, its calibration and validation, and the results from two different tissues: the porcine aortic valve cusp and the bovine pericardium. Validation tests on prepared samples showed maximum error of direct strain measurement to be 0.3%. Aortic valve specimens were found to have larger inhomogeneous strains during tensile testing than bovine pericardium. Clamping effects were more pronounced for the valve specimens. A new system for direct internal strain measurement in connective tissues during application of controlled loads has been developed and validated. The results from the two different tissues show that significant inhomogeneous deformations can occur even in simple tensile testing experiments.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 6

Comparison of clamp-to-clamp versus midsubstance strains for (a) the bovine pericardium and (b) the aortic valve cusp

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Figure 7

Stress-strain curves for the aortic valve cusp and pericardium tissue, computed using the clamp-to-clamp and midsubstance strains from Fig. 6

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Figure 8

Markerless tracking and Lagrangian strains (x-direction) for both specimens (a)(1)–(d)(1) unloaded, (a)(2)–(d)(2) ∼50% loaded, and (a)(3)–(d)(3) fully loaded. The maximum local strain in the red areas was 35%. Note the difference in specimen size (the valve specimen is actually smaller than the pericardium specimen).

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Figure 1

MSTS, control GUI, and loading jig

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Figure 2

(a) Schematic of the elliptically polarized light imaging system, with an aortic valve specimen viewed in (b) normal white-light and (c) elliptically polarized light

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Figure 3

Markerless tracking of a printed pattern as it moves vertically. The red dots are the “virtual markers” and the green lines are their paths of motion.

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Figure 4

Images of (a) a tissue specimen with initial virtual marker mesh and (b) DIC tracking of the tissue specimen as it is moved horizontally using a precision micrometer. Plot (c) shows the DIC measured versus micrometer displacement with linear fit results. Plot (d) shows the mean (±std. dev.) of the DIC measured first principal strain. Noted on the plot (*) is the typical displacement used during an actual test. Since no strain is applied to the specimen in this validation test, the strain shown here represents error in the DIC tracking method.

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Figure 5

Results from stress-relaxation validation tests on known materials

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