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TECHNICAL PAPERS: Soft Tissue

# Fiber Kinematics of Small Intestinal Submucosa Under Biaxial and Uniaxial Stretch

[+] Author and Article Information
Thomas W. Gilbert, Jonathan S. Grashow, Savio L.-Y. Woo

Department of Bioengineering, McGowan Institute of Regenerative Medicine,  University of Pittsburgh, Pittsburgh, PA 15219

Michael S. Sacks1

Department of Bioengineering, McGowan Institute of Regenerative Medicine,  University of Pittsburgh, Pittsburgh, PA 15219msacks@pitt.edu

Departments of Bioengineering and Surgery, McGowan Institute of Regenerative Medicine,  University of Pittsburgh, Pittsburgh, PA 15219

Michael B. Chancellor

Department of Urology, McGowan Institute of Regenerative Medicine,  University of Pittsburgh, Pittsburgh, PA 15219

1

Corresponding author: W.K. Whiteford Professor of Bioengineering, Engineering Tissue Mechanics Laboratory, McGowan Institute for Regenerative Medicine, University of Pittsburgh, 100 Technology Drive, Pittsburgh, PA 15219.

J Biomech Eng 128(6), 890-898 (May 13, 2006) (9 pages) doi:10.1115/1.2354200 History: Received January 05, 2005; Revised May 13, 2006

## Abstract

Improving our understanding of the design requirements of biologically derived collagenous scaffolds is necessary for their effective use in tissue reconstruction. In the present study, the collagen fiber kinematics of small intestinal submucosa (SIS) was quantified using small angle light scattering (SALS) while the specimen was subjected to prescribed uniaxial or biaxial strain paths. A modified biaxial stretching device based on Billiar and Sacks (J. Biomech., 30, pp. 753–7, 1997) was used, with a real-time analysis of the fiber kinematics made possible due to the natural translucency of SIS. Results indicated that the angular distribution of collagen fibers in specimens subjected to 10% equibiaxial strain was not significantly different from the initial unloaded condition, regardless of the loading path $(p=0.31)$. Both 10% strip biaxial stretch and uniaxial stretches of greater than 5% in the preferred fiber direction led to an increase in the collagen fiber alignment along the same direction, while 10% strip biaxial stretch in the cross preferred fiber direction led to a broadening of the distribution. While an affine deformation model accurately predicted the experimental findings for a biaxial strain state, uniaxial stretch paths were not accurately predicted. Nonaffine structural models will be necessary to fully predict the fiber kinematics under large uniaxial strains in SIS.

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## Figures

Figure 8

Representative statistical distribution functions of the angular distribution of collagen fibers for specimens subjected to increasing magnitudes of uniaxial stretch up to 25% the XD. The initial collagen fiber distribution is represented as R(θ) and the deformed collagen fiber distributions are represented as R5(θ), R10(θ), R15(θ), R20(θ), and R25(θ), where the subscript indicates the percentage of uniaxial stretch. (a) A dramatic shift in the preferred fiber orientation toward the stretching direction was observed, while the degree of orientation initially decreased and then tended to approach the NOI for the unloaded condition. (b) The prediction of the deformed statistical fiber distribution at 25% stretch, R′(β25), is also shown. The predicted distribution did not accurately predict the rotation or degree of alignment observed experimentally.

Figure 7

Representative statistical distribution functions of the angular distribution of collagen fibers for specimens subjected to increasing magnitudes of uniaxial stretch up to 25% of the PD. The initial collagen fiber distribution is represented as R(θ) and the deformed collagen fiber distributions are represented as R5(θ), R10(θ), R15(θ), R20(θ), and R25(θ), where the subscript indicates the percentage of uniaxial stretch. (a) The distribution tends to become more aligned and shifts toward the direction of stretch. (b) The prediction of the deformed statistical fiber distribution at 25% stretch, R′(β25), is also shown. The predicted distribution predicted a degree of alignment much greater than that observed experimentally.

Figure 6

Representative statistical distribution functions of the angular distribution of collagen fibers for specimens subjected to 10% equbiaxial stretch. The initial collagen fiber distribution is represented as R(θ) and the collagen fiber distribution for equibiaxial stretch is represented as REB(θ). No detectable change in the collagen fiber distribution was observed. The average change in normalized orientation index was 0.3%±1.8%. The prediction of the deformed statistical fiber distribution, R′(βEB), is also shown in the graph. The predicted distribution accurately predicted the results observed experimentally.

Figure 5

Representative statistical distribution functions of the angular distribution of collagen fibers for specimens subjected to (a) 10% strip biaxial stretch along the preferred fiber direction and then to (b) 10% equibiaxial stretch. The initial collagen fiber distribution is represented as R(θ), the collagen fiber distribution for strip biaxial stretch in the PD is represented as RPD(θ), and the collagen fiber distribution for equibiaxial stretch is represented as REB(θ). Strip biaxial stretch of 10% in the PD led to an increase in the collagen fiber alignment, as indicated by the increase in normalized intensity. The average change in normalized orientation index was 2.7%±1.8%. The prediction of the deformed statistical fiber distributions, R′(βPD) and R′(βEB), are also shown in each graph. The predicted distribution accurately predicted the results observed experimentally.

Figure 4

Representative statistical distribution functions of the angular distribution of collagen fibers for specimens subjected to (a) 10% strip biaxial stretch along the XD and then to (b) 10% equibiaxial stretch. The initial collagen fiber distribution is represented as R(θ), the collagen fiber distribution for strip biaxial stretch in the XD is represented as RXD(θ), and the collagen fiber distribution for equibiaxial stretch is represented as REB(θ). Strip biaxial stretch along the XD led to a widening of the collagen fiber distribution, as indicated by the decrease in normalized intensity. The average change in normalized orientation index was −2.9%±1.0%. The prediction of the deformed statistical fiber distributions, R′(βXD) and R′(βEB), are also shown in each graph. The predicted distribution accurately predicted the results observed experimentally.

Figure 3

A graphical representation of the biaxial stretching protocols utilized. Angular fiber distributions were obtained at β0 and βEB for each test of each specimen, and at βXD and βPD when strip biaxial stretch was applied.

Figure 2

(a) Schematic and (b) photograph of the SALS – Biaxial Fiber Kinematics Testing System. The biaxial stretching device is mounted on the SALS device in a reservoir for holding PBS or other media. The laser passes through a 1cm hole in the mirror to impact the specimen. The mirror is mounted at 45 deg to the specimen to reflect the image to a CCD camera, which is set up orthogonal to the path of the laser to allow simultaneous fiber architecture measurement and tracking of the strain markers. This allows the user to obtain real-time measurements of the collagen fiber distribution under load.

Figure 1

Photograph of the mechanical stretching device. The biaxial stretching device consisted of four lead screws coupled via a series of gears to two computer-controlled stepper motors. The specimens were mounted using sutures that were threaded over pulleys that were free to rotate. The geometry of the specimens is also shown. The extensions of the specimen were folded to increase the suture retention strength, and four graphite markers were fixed to the specimen for strain tracking.

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