Boundary conditions during biaxial testing of planar connective tissues. Part 1: dynamic behavior.

Current mechanical testing methods used to determine the biaxial properties of planar connective tissues may lead to artifactual observations of material behavior. The method of sample gripping affects the constraint on the extracellular fibers at the bounds of the sample. This applied constraint not only affects how the load is transferred to the sample, but also how the load is transmitted throughout the rest of the material - thereby influencing the resulting mechanical behavior of the tissue. In this study, we compared the dynamic biaxial mechanical response of pericardial tissue samples under two different gripping methods: (i) the common method of suturing sample edges and (ii) a new biaxial clamping method. Tissue samples were repeatedly testing using both gripping methods under the same conditions. The tissue samples appeared to be stiffer and less extensible when mechanically tested with clamped sample edges, as opposed to when tested with sutured sample edges. Thus, the influence of the sample boundaries affected the response of the material - precisely the situation to be avoided for reliable material testing. This casts doubt on whether any in vitro mechanical testing method can used to determine the "real" properties of the tissue since the boundary conditions of the tissue in situ are presently unknown.