How important is sample alignment in planar biaxial testing of anisotropic soft biological tissues? A finite element study

Abstract

Finite element models of biomedical applications increasingly use anisotropic hyperelastic material formulations. Appropriate material parameters are essential for a reliable outcome of these simulations, which is why planar biaxial testing of soft biological tissues is gaining importance. However, much is still to be learned regarding the ideal methodology for performing this type of test and the subsequent parameter fitting procedure. This paper focuses on the effect of an unknown sample orientation or a mistake in the sample orientation in a planar biaxial test using rakes. To this end, finite element simulations were conducted with various degrees of misalignment. Variations to the test method and subsequent fitting procedures are compared and evaluated. For a perfectly aligned sample and for a slightly misaligned sample, the parameters of the Gasser-Ogden-Holzapfel model can be found to a reasonable accuracy using a planar biaxial test with rakes and a parameter fitting procedure that takes into account the boundary conditions. However, after a certain threshold of misalignment, reliable parameters can no longer be found. The level of this threshold seems to be material dependent. For a sample with unknown sample orientation, material parameters could theoretically be obtained by increasing the degrees of freedom along which test data is obtained, e.g. by adding the data of a rail shear test. However, in the situation and the material model studied here, the inhomogeneous boundary conditions of the test set-ups render it impossible to obtain the correct parameters, even when using the parameter fitting method that takes into account boundary conditions. To conclude, it is always important to carefully track the sample orientation during harvesting and preparation and to minimize the misalignment during mounting. For transversely isotropic samples with an unknown orientation, we advise against parameter fitting based on a planar biaxial test, even when combined with a rail shear test.

Development of an improved parameter fitting method for planar biaxial testing using rakes

“…This method has been applied to a virtual rake‐based planar biaxial test and showed an improved approximation of the ground truth material parameters (see Table ) . The importance of the method was highlighted again when misalignment of the sample with respect to the test axes was investigated …”

An improved parameter fitting approach of a planar biaxial test including the experimental prestretch

“…Homogeneity was assumed in terms of the tissue's thickness and microstructure. Moreover, the BCC methods were illustrated using a basic FE model adopted from [12] where the preload is applied evenly over the five rake holes without tear and the artery axes are perfectly aligned with the test directions [17]. To apply the BCC(p,F p ) method on real experimental data, we suggest to use a more elaborate FE model in which the rakes are explicitly modeled and the sample's contour is defined using image processing, see [18].…”

“…To overcome this, studies have been conducted to gain more insight in various aspects of the experimental protocol and data analysis. The influence of the gripping mechanism and placement on different sample shapes has been studied [12,13,15,16,17,18], as well as other aspects of the protocol such as the effect of preconditioning [19,20], the effect of flattening an excised sample prior to sample mounting [21], the difference between a force-and displacement-based protocol, the sample side on which the deformation is tracked [14] and the variations of the experimental stress calculation and parameter fitting [12].…”

Biaxial testing system for characterization of mechanical and rupture properties of small samples

“…However, commercial material testing systems (e.g. Tytron 250 Microforce Testing System, ElectroForce Planar Biaxial TestBench, Cell Scale BioTester [3] , [4] , etc.) are often expensive ($50 K-$100 K + ) and several research laboratories have developed custom, less expensive, uniaxial or biaxial devices [5] , [6] , [7] , [8] , [9] , [10] , [11] , [12] , or open source [13] .…”