It is a fundamental assumption that a repaired mitral valve (MV) or MV replacement should mimic the functionality of the native MV as closely as possible. Thus, improvements in valvular treatments are dependent on the establishment of a complete understanding of the mechanical properties of the native MV. In this work, the biaxial mechanical properties, including the viscoelastic properties, of the MV anterior leaflet (MVAL) were explored. A novel high-speed biaxial testing device was developed to achieve stretch rates both below and beyond in-vitro values reported for the MVAL (Sacks et al, ABME, Vol. 30,pp. 1280-90, 2002). Experiments were performed with this device to assess the effects of stretch rate (from quasi-static to physiologic) on the stress-stretch response in the native leaflet. Additionally, stress-relaxation and creep tests were performed on the MVAL under physiologic biaxial loading conditions.The results of these tests showed that the stress-stretch responses of the MVAL during the loading phases were remarkably independent of stretch rate. The results of the creep and relaxation experiments revealed that the leaflet exhibited significant relaxation, but unlike traditional viscoelastic biological materials, exhibited negligible creep.These results suggested that the MVAL may be functionally modeled as an anisotropic quasi-elastic material and highlighted the importance of performing creep experiments on soft tissues. Additionally, this study underscored the necessity of performing biaxial experiments in order to appropriately determine the mechanical properties of membranous tissues
