A new method to determine rate-dependent material parameters of corneal extracellular matrix

The cornea protects internal ocular contents against external insults while refracting and transmitting the incoming light onto the lens. The biomechanical properties of the cornea are largely governed by the composition and structure of the stromal layer which is an extracellular matrix composed of collagen fibrils embedded in a hydrated soft matrix. The mechanical behavior of the corneal stroma has commonly been characterized using uniaxial tensile tests and inflation experiments. In the present study, unconfined compression experiments were used to investigate the influence of loading rates on compressive behavior of nineteen porcine corneal specimens. The experiments were performed at ramp displacement rates 0.15 μm/s (eight samples), 0.5 μm/s (six samples), and 1.0 μm/s (five samples). For all tests, a maximum compressive strain of 50% (five strain increments of 4% followed by three strain increments of 10%) was selected. The experimental data was analyzed by a transversely isotropic biphasic model and material parameters, i.e., the in-plane Young's modulus, the out-of-plane Young's modulus, and the permeability coefficient were calculated. It was observed that while the permeability coefficient decreased exponentially with increasing compressive strain, the in-plane and out-of-plane Young's moduli increased exponentially with increasing strain. Furthermore, it was found that the equilibrium stress was almost rate independent.