Strip Extensiometry for Comparison of the Mechanical Response of Bovine, Rabbit, and Human Corneas

Abstract

Specimens of bovine, rabbit, and human corneas were systematically tested in uniaxial tension to experimentally determine their effective nonlinear stress-strain relations, and hysteresis. Cyclic tensile tests were performed over the physiologic load range of the cornea, up to a maximum of 10 percent strain beyond slack strain. Dimensional changes to corneal test specimens, due to varying laboratory environmental conditions, were also assessed. The measured stress-strain data was found to closely fit exponential power function relations typical of collagenous tissues when appropriate account was taken of specimen slack strain. These constitutive relations are very similar for rabbit, human and bovine corneas; there was no significant difference between the species after preconditioning by one cycle. The uniaxial stress strain curves for all species behave similarly in that their tangent moduli increase at high loads and decrease at low loads as a function of cycling. In the bovine and rabbit data, there is a general trend towards more elastic behavior from the first to second cycles, but there is little variation in these parameters from the second to third cycles. In comparison, the human data demonstrates relatively little change between cycles. Increases in width of corneal test specimens, up to a maximum of 2 percent were found to occur under 95 percent relative humidity test conditions over 10 minutes elapsed time test periods, while specimens which were exposed to normal laboratory conditions (45 percent RH) were found to shrink in width up to a maximum of 9.5 percent over the same elapsed time period. The thickness of the test specimens were observed to decrease by 3 percent in 95 percent relative humidity and by 12 percent in 45 percent relative humidity over the same elapsed time period.

The mechanics of soft biological composites.

“…A more direct comparison was made by extracting the stress-strain response from our first preconditioning cycle. This un-preconditioned data (triangles in Figure 2.7) exhibited similar behavior to the bovine experiments reported in [35]. Unlike the present preconditioning methodology, simple single-ramp experiments combine recoverable viscoelastic strain with inelastic strain.…”

“…of cornea tensile strips from this study and Refs. [35,45] and in the vicinity of 10%/min. Tensile strip experiments of bovine corneas have shown that the tissue exhibits a nonlinear rate-dependent stress-strain response and a highly nonlinear creep response that depends on the applied hold stress.…”

“…Hoeltzel [35] suggests that this effect is negligible, noting that the cornea behaves like a membrane, with very little resistance to out-of-plane bending, presumably because the lamellae of the cornea only weakly transmit in-plane shear. In contrast, [25] computed corrections to the stress-strain response for the flattening effect and variations in the corneal thickness.…”

“…Previous studies on cornea mechanics have typically employed a tensile strip method [64,3,35,45,89,86], or a more physiologically relevant inflation method [88,44,34,77]. These measurements have been used, for example, to study the efficacy of cross-linking agents as means to counteract keratoconous [86,80].…”

Biomechanical Properties of Corneal Extracellular Matrix Using Experimental and Computational Methods

“…The uniaxial tensile tests are done using rectangular corneal strips subjected to tensile loading [1][2][3][4][5][6][7][8]. Because of the well-known limitations of this testing technique [9], it has often been used for comparative studies [3,6,10].…”