Strain-rate sensitivity of porcine and ovine corneas

Knowledge of strain-rate sensitivity of corneal tissue is important for improving the understanding of the tissue's response to mechanical actions and the accurate numerical simulation of corneal biomechanical behaviour under the effects of disease and surgery. In the study, fresh and well-preserved porcine and ovine corneal buttons were subjected to uniaxial tension loads with seven different strain rates ranging between 0.8 and 420% per minute. All specimens exhibited increased stiffness (as measured by the tangent modulus) with higher strain rates. However, clear differences in their behaviour were observed. While ovine corneas showed gradual, consistent and mostly statistically significant increases in stiffness with all elevations in strain rate, porcine corneas' response was significant over only a limited range of low strain rates. The effect of strain rate on the material's stress-strain behaviour was considered in the formation of three sets of constitutive models including: (i) a model based on a simple exponential stress-strain relationship, (ii) the Ogden model that considers the tissue's hyperelasticity but not anisotropy, and (iii) a third model by Holzapfel, Gasser and Ogden that considers both hyperelasticity and anisotropy. The three models are introduced to enable consideration of the strain rate effects in simulations employing finite element programs with varying capabilities or in modelling applications in corneal biomechanics which may or may not require consideration of mechanical anisotropy.