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TECHNICAL PAPERS: Soft Tissue

Altered Mechanical Behavior of Epicardium Under Isothermal Biaxial Loading

[+] Author and Article Information
P. B. Wells, J. L. Harris, J. D. Humphrey

Department of Biomedical Engineering, Texas A&M University, College Station, TX 77843-3120

J Biomech Eng 126(4), 492-497 (Sep 27, 2004) (6 pages) doi:10.1115/1.1785807 History: Received December 19, 2003; Revised March 22, 2004; Online September 27, 2004
Copyright © 2004 by ASME
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Figures

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Typical Cauchy membrane stress versus stretch response for native bovine epicardium in the stiffer direction. Shown, too, are the approximate isometric constraints, relative to the native tissue equibiaxial response, imposed during heating. These constraints are called 1.03, Low, Elbow, and High; see text for specific definitions. Note that the High constraint is the only one that imposes a significant pre-stress prior to heating.
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Schema of the overall testing protocol. The specimen is shown with four central tracking markers. βX denotes various traction-free reference configurations. Arrows denote load-induced stretching, and hatched ends denote an isometric biaxial constraint.
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Representative data from a 1.03 test. Panel (a) shows the evolution of forces along both axes during the course of heating (T=75°C,τ=900 s). The direction that achieves the greatest force is generally the mechanically stiffer direction. Recall, therefore, that we incorporated a SALS device to align the preferred fiber direction along the y-axis of our device. The elliptical images were not well defined, in general, when using the right ventricular epicardium, which creates a large degree of uncertainty with respect to the orientation. Therefore, in some cases we actually found the x-axis to be the stiffer of the two. Experimentally, having the stiffer direction aligned with the x-axis yields sub-optimal control and therefore a slightly larger variation between data sets. Nonetheless, trends are not affected by a misalignment of the tissue, and therefore during the analysis we can think of the directions as merely the stiffer and the less-stiff directions irrespective of the x- or y-axis. Panel (b) illustrates the altered mechanical behavior due to heating. Note the non-zero initial slope for the damaged specimen, the loss of extensibility, and in this case a tendency towards decreased anisotropy. All data are plotted relative to βN.
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Plot of the mean (equibiaxial) mechanical data following the 1.03 isometric heating tests (n=5). Circles represent the native tissue mechanical response, whereas the squares represent that of the heated tissue. The stretch magnitudes are calculated relative to the native undeformed configuration, βN. Note that |V|=(√2) in the reference configuration.
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Plot of the mean equibiaxial data from the four test groups (n=5 to 7 per group) and a representative mean native curve (* ). In comparison to Figure 4, the magnitude of V is normalized here to the maximum value in the mean native data. The squares (□), pluses (+), circles (○), and triangles (▵) represent the 1.03, low stretch, elbow, and high constraints, respectively. As the isometric constraint increased (i.e., greater stretch was imposed), given the same temperature level and duration of exposure, the equibiaxial response of the heated tissue differed less from that of the native tissue, suggesting that stretch delays thermal damage.
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Possible linear correlation between the relative extensibility of the damaged tissue (Ψ) at various levels of |T| (15, 25, 35, 45 and 55 N/m) and the relative amount of isometric stretch imposed during heating (Φ). Because physiologic loads are more likely to be high, we chose the data corresponding to |T|=55 N/m (‘filled’ circles) as the basis for a best-fit linear trend-line, which has an R2 value of 0.687. Open circles correspond to |T|=15, 25, 35, and 45 N/m, and as can be seen, they too correspond well to the linear trend.
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Relation between resulting equibiaxial mechanical responses in isometric and isotonic tests. The asterisks (* ) represent the mean native response, the open squares (□), circles (○), and triangles (▵) represent the 1.03, low stretch, and high isometric constraints, respectively. The filled squares (▪), circles (•), and triangles (▴) represent the (T,τ,P) combinations of (65, 900, 0.0), (75, 900, 12.8) and (75, 900, 21.3) from 17, respectively. The latter two were chosen for consistency with respect to the temperature and duration of heating, while the former represents a bound. The solid line shows the combined isotonic/isometric constraint.
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Representative data showing a lack of shrinkage during the test in which high isotonic loads are applied to the tissue during heating. This illustrates that the stretches in both directions remain constant during heating, and therefore an isotonic/isometric constraint is enforced.

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