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J. A. Last, P. Russell, S. J. Liliensiek, C. R. Croasdale, P. F. Nealey, C. J. Murphy; Measuring the Compliance Profile of the Human Cornea by Atomic Force Microscopy. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2408.
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To obtain a complete compliance profile of the human cornea. The modulus of each layer of the human cornea, including the corneal anterior basement membrane, Bowman’s layer, stroma and Descemet’s membrane was measured.
Atomic force microscopy (AFM, Nanoscope IIIA, Veeco Metrology) was used to determine the elastic modulus. The elastic modulus is a measure of the tissue stiffness and is inversely proportional to the compliance. A spherical tip was indented into the surface of the tissue and the cantilever deflection monitored. The elastic behavior allows analysis of the data with the Hertz equation, a model that provides a relationship between the indentation force and depth and is a function of the tip radius and the modulus of the substrate.
Values obtained for the modulus of the anterior basement membrane were 7.5 ± 4.2 kPa, while modulus values obtained on Descemet’s membrane were 47.6 ± 16.5 kPa, slightly higher than observed for the anterior basement membrane. The modulus values obtained for both Bowman’s layer and the stroma are higher than for either of the basement membranes, ranging from 130 to 500 kPa.
The biophysical properties, including structure and elastic modulus, of each layer of the human cornea are unique. The ability to accurately determine the elastic modulus of each layer is a crucial step in the design of better prosthetics. The topography of Descemet’s membrane has characteristics similar to that of the anterior basement membrane, but with smaller pore sizes in a more tightly packed structure. This important structural difference may lead to the observed differences in elastic modulus. The modulus values observed for Bowman’s layer and the stroma are much higher, reflecting the structural differences between these layers and the basement membranes. The stroma is typically 90% of the thickness of the cornea and values obtained for this layer are consistent with literature values obtained using bulk experimental techniques for the cornea including all layers.
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