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H. Loree, R. Pertaub, D. Borja, D. Muller; Studies of Corneal Thermal Response to Cooling and Heating Using a Porcine Gelatin Phantom. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4218.
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Keraflex (KFX), a microwave-based approach to thermal keratoplasty for refractive correction of myopia and treatment of keratoconus, uses cryogen spray cooling to reduce or eliminate disruption of the corneal epithelium and Bowman’s membrane while shrinking stromal collagen. We studied the corneal thermal response to cooling and KFX using a porcine gelatin phantom.
A scaled-up bench top model of KFX applies heating via parallel copper plates spaced with a foam dielectric. Cooling is provided via 2 nozzles connected to a pressurized tank of R134a cryogen. A phantom is prepared using Type A gelatin from porcine skin (10% w/v in 0.1M PBS) and crosslinked by soaking for 72 hrs in 3mg/ml nordihydroguaiaretic acid (NDGA) solution. This simple and inexpensive phantom has electrical and thermal properties similar to those of human cornea, while its high melting point (~80°C) provides stability during KFX. The phantom is cast in a custom fixture which holds 6 thermocouples -- placed at the surface and at 50 and 200 µm below the surface -- providing temperature measurements at the epithelial and stromal levels. Heating and cooling pulse lengths are controlled via custom LabVIEW software, which also records and stores temperature waveforms for analysis.
At the surface, the thermal response to cooling is well represented by a square root function of time, suggesting a diffusion-based heat transfer process. Thermal recovery obeys a more complex exponential function. The curve-fit model agrees with experimental results for longer cooling pulses (~100ms) but deviates as pulses get shorter (~10ms).
The corneal phantom is a useful tool to validate numerical models of KFX heating and cooling, and to optimize clinical cooling pulse sequences to protect the epithelium.
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