The effects of preconditioning strain on measured tissue properties

Characterizing the mechanical properties of soft biological tissues presents a formidable challenge. In order to ensure that the structure of the specimens is at a repeatable reference state, preconditioning is commonly performed before the actual test. However, the exact mechanisms of preconditioning remain unclear and more research on this issue needs to be undertaken so that the methods for preconditioning can be refined to reduce experimental variability. Previous studies have suggested that the choice of preconditioning strain may influence the measured properties. In this study, uniaxial tests were performed on three groups of spinal cord tissues. Two groups (Groups 1 and 2) were preconditioned at 5% strain and tested to 5% and 2% each, while the third (Group 3) was preconditioned at 2% strain and tested to 2%. The peak stress measured at 2% strain for group 3 was 0.0054+/-0.0035MPa and this was significantly higher than group 2 (134%; p=0.015) and group 1 (p=0.005). However, the ratio of peak to equilibrium stress and the relaxation time were similar for both preconditioning strains. This study suggests that in future work, the preconditioning strain should be reported and the highest strain being used in the study should be used for preconditioning. This is important to allow meaningful comparison of test data within the study and also with other studies.