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Technical Briefs

Inelastic Behavior in Repeated Shearing of Bovine White Matter

[+] Author and Article Information
Taylor S. Cohen

Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104

Andrew W. Smith, Panagiotis G. Massouros, Amy Q. Shen

Department of Mechanical, Aerospace, and Structural Engineering, Washington University, St. Louis, MO 63130

Philip V. Bayly

Department of Mechanical, Aerospace, and Structural Engineering, and Department of Biomedical Engineering, Washington University, St. Louis, MO 63130

Guy M. Genin1

Department of Mechanical, Aerospace, and Structural Engineering, Washington University, Campus Box 1185, St. Louis MO 63130genin@wustl.edu

1

Corresponding author.

J Biomech Eng 130(4), 044504 (Jun 12, 2008) (4 pages) doi:10.1115/1.2939290 History: Received October 26, 2005; Revised April 01, 2008; Published June 12, 2008

Understanding the brain’s response to multiple loadings requires knowledge of how straining changes the mechanical response of brain tissue. We studied the inelastic behavior of bovine white matter and found that when this tissue is stretched beyond a critical strain threshold, its reloading stiffness drops. An upper bound for this strain threshold was characterized, and was found to be strain rate dependent at low strain rates and strain rate independent at higher strain rates. Results suggest that permanent changes to tissue mechanics can occur at strains below those believed to cause physiological disruption or rupture of axons. Such behavior is characteristic of disentanglement in fibrous-networked solids, in which strain-induced mechanical changes may result from fiber realignment rather than fiber breakage.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Example of analysis. The curves are normalized by the value of torque on the first (top) loading curve at a 1.5% peak engineering shear strain. The first reloading results in a normalized torque at γmax=1.5% of about 0.9, and the second in a normalized torque of about 0.8. The threshold strain at this particular strain rate would be found by interpolating halfway between the peak strain of the virgin curve and that of the first reloading curve. The dotted line represents a 15% drop in the normalized torque, which was the threshold at which significant changes were determined to have occurred.

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Figure 2

Typical torque versus angle data for the reloading of specimens tested at 37°C

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Figure 3

Typical torque versus angle data for the specimens tested at 37°C. (a) Reloading cycles at low strain levels caused no permanent mechanical changes: The curves for the ninth and tenth cycles (pictured) followed those of the eight previous cycles. (b) Higher strain levels in later reloading cycles led to reduced secant moduli upon reloading.

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Figure 4

Bovine white matter (here tested at 22°C) exhibited a reduction in secant modulus upon reloading, with the subsequent twisting of the same magnitude resulting in relatively minor reductions of the tissue’s mechanical resistance.

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Figure 5

Estimated strains at which permanent mechanical changes occur in bovine white matter. Data are shown for 18 specimens. The data appear to exhibit a rate-dependent mechanism for mechanical changes at low strain rates, and a rate-independent mechanism at higher strain rates.

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