Study on the visco-hyperelastic behavior of the zona pellucida

The zona pellucida (ZP) is a specialized extracellular matrix surrounding the developing oocyte. This thick matrix consists of different types of glycoprotein, which have different roles in fertilization. Nowadays several techniques are developed and refined to establish the ZP mechanical response. The assumption at the basis of these methods is that the ZP behaves like an elastic body, dissipative forces are neglected, and thus the Young modulus value remains unaffected by probe dynamics. On the contrary dissipative force are strongly regulated by the slippage of ZP chains past one another whereas the absolute reaction force value is mainly due to the architecture of the ZP structure (number of cross-links and distances between knots). Elastic deflection is then due to the ability of each chain to stretch, whereas viscous flow is caused by the sliding of the molecules over one another. Therefore viscous reaction forces generated by the ZP have to be considered one of the main player in regulating the sperm transit but their peculiar behavior along the ZP structure is still poorly understood. In this context, for the first time, we developed and verified a visco-hyperelastic model able to reproduce the ZP reaction force stressed at different probe rate