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Technical Briefs

Minimal Preconditioning Effects Observed for Inflation Tests of Planar Tissues

[+] Author and Article Information
Baptiste Coudrillier

Department of Mechanical Engineering,
The Johns Hopkins University,
Baltimore, MD 21218

Stephen Alexander

Department of Biomedical Engineering,
Boston University,
Boston, MA 02215

Thao D. Nguyen

e-mail: vicky.nguyen@jhu.edu
Department of Mechanical Engineering,
The Johns Hopkins University,
Baltimore, MD 21218

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received December 19, 2012; final manuscript received July 17, 2013; accepted manuscript posted July 29, 2013; published online September 23, 2013. Assoc. Editor: Jeffrey Ruberti.

J Biomech Eng 135(11), 114502 (Sep 23, 2013) (14 pages) Paper No: BIO-12-1630; doi: 10.1115/1.4025105 History: Received December 19, 2012; Revised July 17, 2013; Accepted July 29, 2013

The purpose of this study is to investigate the effects of preconditioning on the deformation response of planar tissues measured by inflation tests. The inflation response of test specimens, including the bovine cornea, bovine and porcine sclera, and human skin, exhibited a negligible evolving deformation response when subjected to repeated pressure loading with recovery periods between cycles. Tissues obtained complete recovery to the reference state, and strain contours across the entire specimen were nearly identical at the maximum pressure of each load cycle. This repeatability was obtained regardless of strain history. These results suggest that negligible permanent change was induced in the microstructure by inflation testing. Additionally, we present data illustrating that a lack of a recovery period can result in an evolving deformation response to repeated loading that is commonly attributed to preconditioning. These results suggest that the commonly observed effects of preconditioning may be avoided by experimental design for planar tissues characterized by long collagen fibers arranged in the plane of the tissue. Specifically, if the test is designed to fully fix the specimen boundary during loading, adequate recovery periods are allowed after each load cycle, and loads are limited to avoid damage, preconditioning effects may be avoided for planar tissues.

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References

Figures

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Fig. 1

Tissue, fixture, and inflation chamber. (a) Skin specimen glued to the back of the fixture, scored through the thickness at the gluing site, and the scored cuts further filled with glue to create a rigid boundary; (b) skin specimen on inflation chamber; (c) bovine sclera similarly glued to fixture; and (d) bovine sclera on inflation chamber.

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Fig. 2

Schematic of additional loading regime for bovine sclera. After the three preconditioning cycles prescribed in Table 1, two additional load cycles and three creep tests were performed prior to a final load-unload cycle identical to the first.

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Fig. 3

Schematic of top view of specimens, showing points or regions where strains were reported. Strains were reported for a single point at the apex for human skin tissue. Average strains over a region were reported for ocular tissue to minimize the effects of noise. Strain contours are also reported over the entire surface for skin tissues and all quadrants for ocular tissues.

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Fig. 12

Uniaxial stress-strain curves measured for bovine cornea comparing the response from the loading portion of the first pressure cycle (–) of the preconditioning protocol and from the loading portion of a pressure cycle after preconditioning (-). Results show a large stiffening effect associated with preconditioning. The specimen was allowed to rest at the baseline pressure for an extended period of time after the preconditioning. Adapted from Fig. 7 of Boyce et al. [16].

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Fig. 11

Contour plots of the meridional strains at the maximum pressure of the first and final pressure cycles for (a) bovine cornea, (b) porcine sclera, and (c) bovine sclera. The bovine sclera plot in (c) included a final cycle after 4 hours of additional testing, including two creep tests and a slow load-unload test. The mean and standard deviation of the strains across the entire contour are reported above each figure.

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Fig. 10

Propagation of DIC uncertainty into strain calculation uncertainty for an idealized numerical sphere: (a) meridional and (b) circumferential strains over a 12.5-mm sphere subjected to a 200 -μm radial inflation with DIC uncertainty (described as a Gaussian distribution of displacements, 195 ± 12 μm, based on previous work [31]). The resulting strain variation of 1.61 ± 0.071 % for the meridional direction and 1.59 ± 0.056 % for the circumferential direction is nearly equivalent to the 1.61 % theoretical uniform strain.

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Fig. 9

Contour plots of the circumferential strains at the maximum pressure of the first and final pressure cycles for (a) bovine cornea, (b) porcine sclera, and (c) bovine sclera. The bovine sclera plot in (c) included a final cycle after 4 hours of additional testing, including two creep tests and a slow load-unload test. The mean and standard deviation of the strains across the entire contour are reported above each figure.

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Fig. 8

Pressure-strain response computed over an averaged region in the meridional and circumferential directions for three successive cycles for (a) bovine cornea, (b) porcine sclera, and (c) bovine sclera. The bovine sclera plot in (c) included a final cycle after 4 hours of additional testing, including two creep tests and a slow load-unload test.

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Fig. 7

Pressure-strain response for a skin specimen (M/83) subjected to pressure cycles at 0.07 kPa/s without intervening recovery periods, measured for the fiber and perpendicular directions

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Fig. 6

Contours of the strain in the fiber and perpendicular directions at the maximum pressure of the first and final cycle for the (M/61) human skin specimen tested at the slower 0.07 kPa/s loading rate with 15-min recovery periods. The mean and standard deviation of the strains across the entire contour are reported above each figure.

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Fig. 5

Pressure-strain response for two human skin specimens tested at tenfold faster loading rate of 0.70 kPa/s with 5-min recovery periods: (a) M/43 - fiber direction; (b) M/43 - perpendicular direction; (c) M/61 - fiber direction; (d) M/61 - perpendicular direction

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Fig. 4

Pressure-strain response for two human skin specimens tested at a slower rate of 0.07 kPa/s with 15-min recovery periods: (a) M/43 - fiber direction; (b) M/43 - perpendicular direction; (c) M/61 - fiber direction; (d) M/61 - perpendicular direction

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