Mechanical properties of the human sclera under various strain rates: Elastic, hyperelastic, and viscoelastic models

The sclera is the white outer shell of the eye which its material properties have a key asset for preserving the form of the eye against the exterior and interior musculature and fluctuations forces induce by Intraocular Pressure (IOP). Besides, the biomechanical atmosphere of the head of the optic nerve is profoundly affected by the mechanical properties of the sclera. Computational modeling is a useful way to understand various types of the injuries to the eye, such as trauma. In order to produce reliable numerical results, it is necessary to input precise material properties into the numerical models. However, so far there is a paucity of knowledge about the dissimilarity of the scleral mechanical properties/behaviors under various loading conditions. Therefore, the aim of this study was to evaluate the linear elastic and nonlinear hyperelastic mechanical properties of the human sclera at eight different strain rates. The experimental results revealed a significant role of the strain rate on the elastic modulus and maximum stress in a way that the lowest and highest elastic modulus and stress were observed at the strain rate of 5 and 200 mm/min, respectively. Due to the nonlinear mechanical behavior of the sclera, three different constitutive strain energy density functions examined and their coefficients were calculated thru the linear/nonlinear optimization method. Finally, the agreement of the constitutive data with that of the experimental ones was verified using a FE model of the tensile test. Due to the strain rate dependency of the results, the viscoelastic time-dependentmechanical behavior of the sclera was also assessed through the relaxation Prony-series formulation. Although all these three material models can be used to address the mechanical response of the sclera, according to the objective of a numerical model and its possible applications, each of which can be selected. The outcomes of the concurrent study may have implications not only for understanding the linear elastic and nonlinear hyperelastic mechanical properties of the sclera tissue under various loading rates, but also for knowing the time-dependent viscoelastic mechanical properties of the sclera under the relaxation loading.