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The Ring-Pull Assay for Mechanical Properties of Fibrous Soft Tissues – an Analysis of the Uniaxial Approximation and a Correction for Nonlinear Thick-Walled Tissues

  • Sp Iss: Experimental Advances in Cardiovascular Biomechanics
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Abstract

Background: The ring-pull test, where a ring of tissue is hooked via two pins and stretched, is a popular biomechanical test, especially for small arteries. Although convenient and reliable, the ring test produces inhomogeneous strain, making determination of material parameters non-trivial. Objective: To determine correction factors between ring-pull-estimated and true tissue properties. Methods: A finite-element model of ring pulling was constructed for a sample with nonlinear, anisotropic mechanical behavior typical of arteries. The pin force and sample cross-section were used to compute an apparent modulus at small and large strain, which were compared to the specified properties. The resulting corrections were validated with experiments on porcine and ovine arteries. The correction was further applied to experiments on mouse aortic rings to determine material and failure properties. Results: Calculating strain based on centerline stretch rather than inner-wall or outer-wall stretch afforded better estimation of tissue properties. Additional correction factors were developed based on ring wall thickness H, centerline ring radius Rc, and pin radius a. The corrected estimates for tissue properties were in good agreement with uniaxial stretch experiments. Conclusions: The computed corrections improved estimation of tissue material properties for both the small-strain (toe) modulus and the large-strain (lockout) modulus. When measuring tensile strength, one should minimize H/a to ensure that peak stress occurs at the sample midplane rather than near the pin. In this scenario, tensile strength can be estimated accurately by using inner-wall stretch at the midplane and the corrected properties.

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Notes

  1. The large-strain portion of the stress-strain curve is often called the linear regime, but we use lockout in this work to avoid confusion with linear material models.

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Acknowledgments

The authors acknowledge funding from the National Science Foundation Graduate Research Fellowship Program (NSF GRFP) under Grant No. 00039202. The authors also acknowledge funding from the National Institutes of Health under Grant No. U01-HL139471. The authors acknowledge and thank the University Imaging Center (UIC) at the University of Minnesota for the use of the small animal ultrasound system, Dr. Paul A. Iaizzo and the Visible Heart Lab for the porcine and ovine tissue used in this study, and Dr. Paulo P. Provenzano and the Provenzano Research Group for the mice used in this study. The authors also thank Drs. Neeta Adhikari and Jennifer L. Hall for their technical advice on dissection and ring testing of mouse tissues, Shannen B. Kizilski for her assistance in design and fabrication of the ring pull apparatus, and Elizabeth Gacek for her technical assistance in setting up contact in the ring pull simulations.

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  1. Department of Biomedical Engineering, University of Minnesota, 7-105 Church St SE, Minneapolis, MN, 55455, USA

    R. R. Mahutga, C. T. Schoephoerster & V. H. Barocas

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  1. R. R. Mahutga
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  2. C. T. Schoephoerster
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  3. V. H. Barocas
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Correspondence to V. H. Barocas.

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The authors declare that they have no conflict of interest. All animal tissue used in the current study was obtained from animals previously sacrificed as part of other, institutionally-approved studies; no live animals were used or sacrificed for the sole purpose of the current study. This study involved no human subjects.

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Mahutga, R.R., Schoephoerster, C.T. & Barocas, V.H. The Ring-Pull Assay for Mechanical Properties of Fibrous Soft Tissues – an Analysis of the Uniaxial Approximation and a Correction for Nonlinear Thick-Walled Tissues. Exp Mech 61, 53–66 (2021). https://doi.org/10.1007/s11340-020-00623-3

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