Biomechanical properties of porcine cornea; planar biaxial tests versus uniaxial tensile tests

The cornea is a transparent tissue whose mechanical properties are important for its optical and physiological functions. The mechanical properties of cornea depend on the composition and microstructure of its extracellular matrix, which is composed of collagen fibrils with preferential orientations. The present research was done in order to characterize corneal mechanical response using the biaxial mechanical testing method and to compare biaxial measurements with those found from uniaxial tensile tests. For this purpose, thirty square-shaped specimens excised from the center of porcine cornea were mounted into an ElectroForce TestBench device such that their superior/inferior (SI) and nasal/temporal (NT) meridians were aligned with motor axes. Furthermore, ten corneal strips dissected from the NT direction (n = 5) and SI direction (n = 5) were mounted into an RSA-G2 Solid Analyzer testing machine. The biaxial experiments were performed at stretch ratios of 1:1, 1:0.5, 0.5:1, 1:0.01, and 0.01:1 and displacement rates of 2 mm/min (n = 20) and 10 mm/min (n = 10). The uniaxial experiments were done using the displacement rate of 2 mm/min. The planar square-shaped samples tested under equibiaxial loading showed similar mechanical response in NT and SI directions. Furthermore, uniaxial experiments revealed no significant difference in tensile response of corneal strips excised from NT and SI directions. However, equibiaxial testing tensile stresses were significantly larger than those found from uniaxial tensile measurements. The mechanical behavior of cornea in biaxial tests was dependent on the applied stretch ratio. The differences and similarities between uniaxial and biaxial experimental measurements were discussed and it was concluded that the planar biaxial testing method characterized the mechanical response of cornea by mimicking its in vivo loading state more closely than uniaxial experiments.