A Triaxial-Measurement Shear-Test Device for Soft Biological Tissues

[+] Author and Article Information
Socrates Dokos, Ian J. LeGrice, Bruce H. Smaill

Department of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand

Julia Kar

Department of Engineering Science, School of Engineering, University of Auckland, Auckland, New Zealande-mail: jk1@cec.wustl.edu

Alistair A. Young

Department of Physiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealande-mail: a.young@auckland.ac.nz

J Biomech Eng 122(5), 471-478 (Mar 28, 2000) (8 pages) doi:10.1115/1.1289624 History: Received February 24, 1999; Revised March 28, 2000
Copyright © 2000 by ASME
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Schematic overview of Shear-Test Device (drawn to scale). See text for further explanation.
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(a) Top view of X-Y force transducer used to separate the horizontal X,Y forces generated on the upper surface of the tissue due to horizontal displacement of its lower surface. A tissue platform that makes contact with top surface of the tissue sample is fixed to the center of the inner ring. (b) Displacement of inner ring due to force applied in the X direction. The asterisk marks the location of strain gages that separate out and measure this X force. (c) Displacement of middle and inner rings due to force applied in the Y direction. As in (b), the asterisk marks the location of strain gages that selectively measure this Y force. Note that in diagrams (b) and (c), the extent of ring deflection is exaggerated for illustrative purposes.
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Typical calibration curves obtained following the protocols described in the text. The principal axis calibrations (left panel) were obtained using the manual calibration procedure outlined in the text. Linear regression fits (dashed lines) were used to determine the principal calibration coefficients comprising the diagonal terms of the 3×3 calibration matrix. Cross-coupling plots (right panels) were then generated by fixing a rigid sample between the upper and lower tissue platforms of the device. The lower platform was then translated in each of the three principal directions separately, with forces recorded and shown in the right panel plots. For these rigid tests, forces were determined using the principal axis coefficients of the preceding manual calibration routine. The rigid tests were a direct way to measure the cross-influences of each principal force on the other two axes. From top to bottom on the right, the panels show the magnitude of cross-talk of an X force referred to the Y and Z directions, a Y force referred to X and Z, and a Z force referred to X and Y. Linear regression fits to these test results were then used to determine the remaining off-diagonal terms of the calibration matrix.
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Test protocols to ascertain independently the material properties of gel samples used to validate the shear test device. In the tensile test (a), a thin cylindrical sample of gel (diameter 5.3 mm) was suspended vertically with weights attached to its lower end. A video camera was used to record and measure the distance between evenly spaced material markers to ascertain strain. In the rotational shear test (b), a concentric cylindrical sample of gel (outer diameter 38 mm, inner diameter 16 mm, height 24 mm) was cast between two cylindrical tubes and mounted so that the inner cylinder was held stationary while the outer surface was free to rotate under the action of an applied couple. The outer cylinder was supported by a thrust-bearing base to minimize the effects of friction and gravitational load on the sample. Two strings (shown as F arrows) attached within grooves on the outer cylinder were used to apply a known couple. Material markers on the top surface of the sample, as recorded by video, were used to measure the angle of rotation of each material point as a function of its radial position in the sample.
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Gel data from (a) tensile and (b) rotational shear tests from batch 1 samples. Dashed curve on each graph is the least squares fit of neo-Hookean analytical solution to the data.
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Shear properties of a cuboid gel sample from batch 1 measured using the shear test device. Sinusoidal shear strains of 0.25 magnitude, 30 s period were applied over two cycles in the X and Y directions separately, as shown in the inset. The sample was not compressed in the vertical Z direction. Plots of corresponding shear force against shear strain are shown in the main graph. Analytical plots using a neo-Hookean material parameter of C1=10.2 kPa obtained from the rotational shear test (see Table 1) are also superimposed on the graphs.
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Shear properties of rat septum measured using the shear test device. Sinusoidal time-varying shear displacements were applied to the RV face of a BDM-treated block of septal tissue in two orthogonal directions corresponding to the apex-base direction and the circumferential direction (anterior-posterior in intact heart). In both tests, stress-strain curves stabilized after two cycles, with stable single cycles shown in these plots. The inset graph is a plot of downward compressive force arising due to the shear strain in both directions of the sample. With the upper surface of the sample held fixed, positive base-apex strains correspond to shear displacements of the lower surface (RV side) toward the base, while positive transverse shear strains are posteriorly directed displacements.




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