Anisotropic time-dependant behaviour of the aortic valve.

The complex tri-layered structure of the aortic valve (AV) results in anisotropic quasi-static mechanical behaviour. However, its influence on AV viscoelasticity remains poorly understood. Viscoelasticity may strongly influence AV dynamic mechanical behaviour, making it essential to characterise the time-dependent response for designing successful substitutes. This study attempts to characterise the time-dependent behaviour of the AV at different strain and load increments, and to gain insight into the contribution of the microstructure to this behaviour. Uniaxial incremental stress-relaxation and creep experiments were undertaken, and the experimental data analysed with a generalised Maxwell model, to determine the characteristic time-dependent parameters. Results showed that the time dependent response of the tissue differed with the loading direction, and also with the level of applied load or strain, in both stress-relaxation and creep phenomena. Both phenomena were consistently more pronounced in the radial loading direction. Fitting of the Maxwell model highlighted that the time dependent modes required to model the data also varied in different increments, and additionally with the loading direction. These results suggest that different micro-structural mechanisms may be activated in stress-relaxation and creep, determined by the microstructural organisation of the valve matrix in each loading direction, at each strain or load increment.