Rate dependent biomechanical properties of corneal stroma in unconfined compression

The corneal stroma is a highly ordered extracellular matrix and mainly responsible for the mechanical strength of the cornea. The rate dependent mechanical and rheological properties of the cornea are not completely understood and there is large variation in the reported estimates. In this work, the rate dependent mechanical behavior of the corneal stroma was investigated using experimental studies and theoretical models. Unconfined compression stress-relaxation experiments at different displacement rates and compressive strains were conducted. The unconfined compression material parameters, i.e. corneal out-of-plane modulus, in-plane modulus and permeability coefficient were determined from curve-fitting the experimental data with a transversely isotropic biphasic model. It was found that the maximum force reached during the step loading increased with both increasing magnitude and rate of the compressive strain. It was also observed that at all loading rates the in-plane Young's modulus increased with increasing strain, while the permeability coefficient decayed with increasing compressive strain. At a constant compressive strain, both the in-plane Young's modulus and the permeability coefficient increased with increasing the loading rate. Regardless of loading rates and compressive strains, a range of corneal out-of-plane modulus of 0.6 kPa to 13.8 kPa, in-plane modulus of 0.5 MPa to 4.8 MPa, and permeability coefficient of 1×10⁻¹⁴ m⁴/N·s to 7×10⁻¹⁴ m⁴/N·s was found.