G Wayne Brodland

Publications (2)4.55 Total impact

• Source

  Available from: utoronto.ca

  Article: Biaxial mechanical testing of human sclera.

  Armin Eilaghi, John G Flanagan, Inka Tertinegg, Craig A Simmons, G Wayne Brodland, C Ross Ethier
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  ABSTRACT: The biomechanical environment of the optic nerve head (ONH), of interest in glaucoma, is strongly affected by the biomechanical properties of sclera. However, there is a paucity of information about the variation of scleral mechanical properties within eyes and between individuals. We thus used biaxial testing to measure scleral stiffness in human eyes. Ten eyes from 5 human donors (age 55.4+/-3.5 years; mean+/-SD) were obtained within 24h of death. Square scleral samples (6mm on a side) were cut from each ocular quadrant 3-9 mm from the ONH centre and were mechanically tested using a biaxial extensional tissue tester (BioTester 5000, CellScale Biomaterials Testing, Waterloo). Stress-strain data in the latitudinal (toward the poles) and longitudinal (circumferential) directions, here referred to as directions 1 and 2, were fit to the four-parameter Fung constitutive equation W=c(e(Q)-1), where Q=c(1)E(11)(2)+c(2)E(22)(2)+2c(3)E(11)E(22) and W, c's and E(ij) are the strain energy function, material parameters and Green strains, respectively. Fitted material parameters were compared between samples. The parameter c(3) ranged from 10(-7) to 10(-8), but did not contribute significantly to the accuracy of the fitting and was thus fixed at 10(-7). The products cc(1) and cc(2), measures of stiffness in the 1 and 2 directions, were 2.9+/-2.0 and 2.8+/-1.9 MPa, respectively, and were not significantly different (two-sided t-test; p=0.795). The level of anisotropy (ratio of stiffness in orthogonal directions) was 1.065+/-0.33. No statistically significant correlations between sample thickness and stiffness were found (correlation coefficients=-0.026 and -0.058 in directions 1 and 2, respectively). Human sclera showed heterogeneous, near-isotropic, nonlinear mechanical properties over the scale of our samples.
  Journal of biomechanics 06/2010; 43(9):1696-701. · 2.66 Impact Factor
• Article: Strain uniformity in biaxial specimens is highly sensitive to attachment details.

  Armin Eilaghi, John G Flanagan, G Wayne Brodland, C Ross Ethier
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  ABSTRACT: Biaxial testing has been used widely to characterize the mechanical properties of soft tissues and other flexible materials, but fundamental issues related to specimen design and attachment have remained. Finite element models and experiments were used to investigate how specimen geometry and attachment details affect uniformity of the strain field inside the attachment points. The computational studies confirm that increasing the number of attachment points increases the size of the area that experiences sensibly uniform strain (defined here as the central sample region where the ratio of principal strains E(11)/E(22)<1.10), and that the strains experienced in this region are less than nominal strains based on attachment point movement. Uniformity of the strain field improves substantially when the attachment points span a wide zone along each edge. Subtle irregularities in attachment point positioning can significantly degrade strain field uniformity. In contrast, details of the apron, the region outside of the attachment points, have little effect on the interior strain field. When nonlinear properties consistent with those found in human sclera are used, similar results are found. Experiments were conducted on 6 x 6 mm talc-sprinkled rubber specimens loaded using wire "rakes." Points on a grid having 12 x 12 bays were tracked, and a detailed strain map was constructed. A finite element model based on the actual geometry of an experiment having an off-pattern rake tine gave strain patterns that matched to within 4.4%. Finally, simulations using nonequibiaxial strains indicated that the strain field uniformity was more sensitive to sample attachment details for the nonequibiaxial case as compared to the equibiaxial case. Specimen design and attachment were found to significantly affect the uniformity of the strain field produced in biaxial tests. Practical guidelines were offered for design and mounting of biaxial test specimens. The issues addressed here are particularly relevant as specimens become smaller in size.
  Journal of Biomechanical Engineering 09/2009; 131(9):091003. · 1.90 Impact Factor