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Cristina Labate, Janice Dias, Vivian Sueiras, Maria De Santo, Giuseppe Lombardo, Marco Lombardo, Noel Marysa Ziebarth; Depth-dependent mechanical anisotropy of the cornea assessed by atomic force microscopy. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3709.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the mechanical properties of the porcine corneal stroma at different depths using Atomic Force Microscopy (AFM).
Experiments were carried out on five porcine corneal tissues (1 day postmortem). The eyes were retrieved from an abattoir and shipped to the laboratory overnight. Upon arrival, the epithelium was gently removed using a cotton swab, and the cornea was excised with a generous scleral rim. Before experiments, the samples were placed in 20% dextran solution to restore the corneal thickness to physiological levels (600-700µm). Thereafter, each sample was mounted onto an artificial chamber and a microkeratome (CB, Moria) was used to section the cornea into an anterior and posterior lenticule. Two different heads, 90µm and 250µm, were used. A custom-built Atomic Force Microscope designed for the mechanical testing of ophthalmic tissues was used to characterize the depth-dependent mechanics of the stroma. Measurements were performed at room temperature and corneal specimens were placed in a custom sample holder filled with 15% dextran solution to maintain hydration. A spherical AFM cantilever tip (38µm tip radius, nominal elastic constant of 15N/m) was used to probe the posterior lenticule in all samples. All measurements were repeated 15 times per sample, on the central part of the stroma. The data obtained were analyzed with a custom Matlab program. Young’s modulus (E) of the posterior lenticules was determined by fitting force curve data to the Hertz model for a spherical indenter.
The average thickness of the intact porcine corneas was 675±16μm. The microkeratome removed 232-510μm of tissue. Young’s modulus of elasticity significantly decreased non-linearly as measurement depth in the cornea increased (E ranged from 105 to 632kPa) (R2=0.64823; p<0.01). At the most superficial layer (155 μm depth), Young’s modulus was 623±51kPa, while at the deepest layer (471µm depth), it was 163±34kPa.
AFM was reliable to ascertain the depth-dependent mechanical anisotropy of the corneal stroma, showing that Young’s modulus of elasticity decreases non linearly with increasing depth in the stroma.
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