Material Properties of Porcine Corneal Stroma in Unconfined Compression

The tensile properties of the cornea have been extensively studied while there are fewer studies on its compressive stiffness. The mechanical properties and structure of the cornea like many other connective tissues are derived from the function and properties of their extracellular matrix. The corneal extracellular matrix, stroma, is a polyelectrolyte gel composed of collagenous fibers embedded in an aqueous matrix. The cornea has two different functions: optical and mechanical. It is the main refractive component of the visual system and it is an effective barrier resisting the deformation caused by external and internal stresses. A necessary condition for corneal optical properties and transparency is the maintenance of a pseudo hexagonal arrangement of the collagen fibers inside the extracellular matrix. This regular arrangement is attributed to the interaction of collagen fibers with the proteoglycans. Under physiological conditions, the proteoglycans are ionized and form a hydrated gel in the empty space between the collagen fibrils by attracting the water and solutes. The interaction of the negatively fixed charges of the proteoglycans with themselves and with the free ions inside the interstitial fluid contributes to the corneal swelling pressure and subsequently to its compressive properties.