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TECHNICAL PAPERS: Soft Tissues

Altered Mechanical Behavior of Epicardium Due to Isothermal Heating Under Biaxial Isotonic Loads

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

Department of Biomedical Engineering

J. D. Humphrey

Department of Biomedical Engineering and The M.E. DeBakey Institute, Texas A&M University, College Station, TX 77843-3120

J Biomech Eng 125(3), 381-388 (Jun 10, 2003) (8 pages) doi:10.1115/1.1567754 History: Received August 01, 2002; Revised December 01, 2002; Online June 10, 2003
Copyright © 2003 by ASME
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Figures

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Biaxial stretching protocols used to investigate the behavior of epicardium at various equilibrium thermal damage states. Data are plotted as stretch ratio in the initially preferred (λ2) and orthogonal (λ1) directions. Note the level of control available from on-line calculations of the deformation gradient, F , at 30 Hz. Each protocol was performed cyclically at ∼0.05 Hz, and data are from the fifth cycle for each protocol.
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Schema of the configurations assumed by a specimen during testing. With the exception of βv, all configurations were recorded so that deformations gradients could be calculated. Double- and single-headed arrows denote reversibility and irreversibility, respectively.
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Representative equibiaxial data for one left ventricular epicardial specimen. Data are plotted as Cauchy membrane stress versus stretch ratio, which are calculated relative to βo. Panel (a) shows the native behavior from the fifth cycle of an equibiaxial test. Panel (b) shows the altered mechanical response for (T,τ,P)=(65,1200,6.0), also from the fifth cycle of an equibiaxial test. Circles denote the initially preferred direction and triangles correspond to the orthogonal direction.
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Representative fit of equations 345 to mechanical data from a native specimen. Panel (a) shows the fit to one of the two constant stretch tests used for the regressions. Panel (b) shows predictions for an equibiaxial stretch test, data that were not included in the regression. Stretches are calculated relative to βo. Circles denote the initially preferred direction and triangles correspond to the orthogonal direction.
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Representative fit of equations 345 to data from an isotonic (T,τ,P)=(65,1200,12.8) heating test. Panel (a) shows the fit to one of the two constant stretch tests used for the regressions. Panel (b) shows the prediction for an equibiaxial stretch test relative to βp, data that were not included in the regression analysis. Stretches are calculated relative to βt. Circles denote the initially preferred direction and triangles correspond to the orthogonal direction.
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Plot of a ratio of best-fit parameters per the results in equation 6. A line of identity has been included as a reference. Coefficients are plotted as native (circles) and damaged (triangles). Recall from equation 6 that a ratio greater than one indicates that the stiffer direction corresponds with the initially preferred direction. Notice that this is the case for all native specimens, but few damaged specimens.
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Data are plotted as |T | versus |V | for mean native (circles) and damaged (triangles) responses for P=12.8 N/m at various time-temperature combinations: Panels (a), (b), (c), and (d) correspond to (T,τ,P)=(65,7200,12.8), (70,900,12.8), (75,900,12.8), and (80,900,12.8), for which 1−λ1λ2=0.057, 0.078, 0.274, and 0.474, respectively (Table 1).
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A measure of extensibility versus areal shrinkage. Data from the various combinations of (T,τ,P) are plotted as |V | evaluated at |T|=25 N/m (triangles) and 50 N/m (circles) versus 1−λ1λ2 (Table 1), where λ1 and λ2 are stretch ratios from βo to βt.
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Representative stress resultant-stretch responses (Tij versus λi) at 37°C from the fifth cycle of equibiaxial testing for specimens immersed in a 65°C bath for selected accumulated heating times τ=0, 30, 150, and 360 seconds with associated equilibrium shrinkage ξ1e≅ξ2e=0, 14, 44, and 57%. Material responses in the circumferential and apex-to-base directions are shown in Panel (a) and Panel (b), respectively. Note the gradual increase in hysteresis with each level of thermal damage.

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