Biaxial Mechanical Properties of the Natural and Glutaraldehyde Treated Aortic Valve Cusp—Part I: Experimental Results

[+] Author and Article Information
K. L. Billiar, M. S. Sacks

Department of Biomedical Engineering, University of Miami, Coral Gables, FL 33124

J Biomech Eng 122(1), 23-30 (Jul 28, 1999) (8 pages) doi:10.1115/1.429624 History: Received November 24, 1998; Revised July 28, 1999
Copyright © 2000 by ASME
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A partially polarized image of an aortic cusp highlighting the heterogeneous fibrous structure, and main anatomical and biaxial specimen regions. Also shown are the locations of the optical markers as white circles, with the large circles denoting the nine markers used for structure–strain measurements and the smaller four markers used for strain measurements for all subsequent tests.
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Peak principal Green’s strains under equibiaxial tension (60 N/m) for the AV cusp superimposed over the preferred collagen fiber directions (as determined by SALS) for an (a) on-axis and (b) 45 deg specimen. Lines represent local collagen fiber preferred directions and arrows the principal strains, with lengths proportional to strain magnitude (see scale). The principal strains were closely oriented to the local preferred fiber directions throughout most of the region for both orientations. This result suggests that the local fiber architecture dominates the local strain field and that the boundary tethering effects were minimal in the interior of the specimen.
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(a) Plot of collagen fiber orientation (lines) superposed over the local degree of orientation (OI, gray scale in units of degrees) for an AV biaxial test specimen. Here, the white “holes” in the plot correspond to the displacement markers. (b) Corresponding tension–strain plots for different portions of the AV cusp shown in (a), with N=nodulus, M=midregion, B=belly. Although the stress tensor may be slightly different at each point due to the proximity of the tethering points (black lines), these results clearly highlight the nonhomogeneity of the mechanical response of the test specimen.
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Representative circumferential and radial stress–strain curves from a fresh and fixed AV cusp, demonstrating the pronounced mechanical anisotropy of both tissues. While the radial curves were similar, the circumferential “stiffness” is negative for the fresh specimen due to the strong in-plane coupling for this state.
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Average AV cuspal stress–strain data for both fresh and GL treated specimens, with data presented as mean ±SEM, n=11 for both groups. GL fixation appears to affect the tissue differently if the strains were calculated with respect to (a) the unconstrained configuration or the (b) preloaded and preconditioned configuration. Note the difference in the stretch ratio scales between (a) and (b).
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AV cuspal stress–strain data for the (a) circumferential and (b) radial directions for a GL treated cusp demonstrating the effects of transverse loading (in-plane coupling). Number adjacent to curves indicate biaxial test protocol number (Fig. 2). Note the non-monotonic relationship between tension and stretch ratio in the circumferential direction in all but the protocol 1 for this specimen. In contrast, in many fixed and most fresh samples, the circumferential strain increased monotonically with increasing stress.
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(a) A schematic of the biaxial test specimen, with the fibrous structure of the cusp depicting the large collagen cords, which undergo large rotations with loading (b–d). As the radial loads become larger with respect to the circumferential loads, the collagen fibers undergo large rotations. This causes contraction along the circumferential axis without buckling and allows for very large radial strains.




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