Biaxial Mechanical Testing


Introduction

Improving the clinical potential of tissue-engineered technologies by significantly improving the biomimetic behavior of tissue-engineered constructs. In 2011, it was reported that since the late 1980’s over $5B had been invested in tissue engineering and regenerative medicine (1). The following year it was reported that there were over 100 companies (a three-fold growth since 2008) in the tissue engineering industry generating $3.6B in sales (2). A PubMed search of the term “tissue engineering” conducted at the time of this writing produced over 74,000 hits. Yet according to the NIH NIBIB Fact Sheet on Tissue Engineering and Regenerative Medicine published in July 2013 “Currently, tissue engineering plays a relatively small role in patient treatment.” In consideration of the resources expended and extensive activity in the field, viable tissue-engineering based clinical solutions are conspicuously limited.

A variety of experiments has been performed to obtain the mechanical properties of biological tissue and tissue-engineered constructs in all relevant deformation. Uniaxial studies have been widely utilized to determine the mechanical properties of soft biological tissues and tissue engineered constructs since it is convenient to control the boundary condition in one dimension. This material testing method was founded from linear-elasticity theorems and was originally conceived to investigate the mechanical properties of isotropic, linearly-elastic materials under small deformations. However, biologic tissues are inhomogeneous, anisotropic, non-linear materials that typically undergo large deformations and are often subjected to complex multi-axial loading conditions in vitro. Thus, biaxial testing methods are proposed to mimic the physiological-loading state to fully understand the mechanical behavior of the tissues.

 

Biaxial Testing System

image2  image5

 

Testing Methods

 

biaxial-test-schematic-drawingweiss-shear-test

                     Biaxial Test                                               Previous Shear Test (Image reprinted from Weiss et al, 2002)

sheartestschematicdrawing

                  Our shear Test

In previous simple shear tests, an in-plane moment is usually created when the two clamps are pulled towards different directions. In the current design, we propose a methodology to compensate this moment.

 

Multi-photon Microscopy (MPM)

MPM can be used to non-destructively visualize molecule-specific intrinsic optical signals, including elastin autofluorescence and collagen second harmonic generation (SHG), allowing the detailed characterization of three-dimensional engineered tissue microstructure, both pre- and post- implantation, as well as a comparison of engineered to native tissue microstructure.

multi-photon-images-of-a-strattice-based-hernia-mesh-post-implant

MPM is used to evaluate (1) ratio of collagen to elastin, and (2) collagen integrity and fiber orientation.

 

 

Strain Measurement

Digital Image Correction (DIC) has been widely used in biological tissues as a non-contact strain measurement technique.

rubber3strainrate0-01_exx_final rubber3strainrate0-01_exy_final rubber3strainrate0-01_eyy_final

Preliminary uniaxial tensile test performed on biaxial system

 

Modeling

example-model

Figure 13. An example model parameterization of complex membrane deformation exhibiting dilatation, squeeze, and shear responses.
Left graphic: reference state prior to deformation.
Right graphic: deformed state. Parameter a is the extension of a material element along the 1-direction in a material basis. Parameter b is an extension along its 2-direction. While parameter γ is the magnitude of shear in that γ = tan(φ) with φ being the angle of shear in the 1-2 plane. Clockwise rotations are positive valued; counterclockwise rotations are negative valued.

 

Preliminary Tests

  1. Rubber

Sorbathane Rubber

Polyurethane

References

Weiss, J.A., J.C. Gardiner, and C. Bonifasi-Lista, Ligament material behavior is nonlinear, viscoelastic and rate-independent under shear loading. Journal of Biomechanics, 2002. 35(7): p. 943-950.