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J. A. Last, S. S. Soofi, S. J. Liliensiek, P. Russell, P. F. Nealey, C. J. Murphy; Measuring the Mechanical Properties of Constituents of the Human Cornea by Atomic Force Microscopy. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4800.
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To determine the local elastic modulus of the corneal anterior basement membrane, Descemet’s membrane and the commercially available basement membrane-like material MatrigelTM. The elastic modulus is a measure of the tissue stiffness and is inversely proportional to the compliance.
Atomic force microscopy (AFM, Nanoscope IIIA, Veeco Metrology) was used to determine the elastic modulus. 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.
The resulting force curves generally consist of a straight line as the tip approaches the sample and a gradual increase in deflection as the tip comes into contact with the surface, as expected for a soft material. In addition, the approach and retract curves overlap, indicating elastic behavior for the indentation rate used (2 µm/sec). Values obtained for the modulus of the anterior basement membrane range from 2 kPa to 15 kPa with a mean of 7.5 ± 4.2 kPa. Modulus values obtained on Descemet’s membrane ranged from 20 - 67 kPa with a mean of 47.6 ± 16.5 kPa, slightly higher than observed for the anterior basement membrane. MatrigelTM was found to have a modulus similar to that of the anterior basement membrane with a mean of 1.9 kPa.
Recently, the mechanical properties of the extracellular matrix have been shown to be an important intrinsic property for determining cellular response to extracellular cues. The modulus of these soft tissues has been successfully measured with AFM. The modulus for Descemet’s membrane was found to be higher than the modulus of the anterior basement membrane of the cornea. The topography of Descemet’s membrane has been shown to be similar to that of the anterior basement, but with smaller pore sizes resulting in a more tightly packed structure. This structural difference may account for the observed modulus differences. In addition, MatrigelTM, with pore sizes similar to that of the anterior basement membrane, was found to have a similar modulus value. G.A. Abrams, S.S. Schaus, S.L. Goodman, P.F. Nealey, C.J. Murphy: Nanoscale Topography of the Corneal Epithelial Basement Membrane and Descemet’s Memrane of the Human. Cornea, 19(1), 57, (2000).
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