Thermal Interaction of Cryogen Spray with Human Skin under Hypobaric Pressures

Clinical results of port wine stain (PWS) birthmarks laser therapy have shown that dark purple lesions usually respond well to the first 3–5 treatments. However, for most PWS, complete blanching is never achieved, and the lesion stabilizes at a red-pink color. We have recently demonstrated that with the aid of a local vacuum applied to the lesion site prior to laser irradiation, photocoagulation of the smaller vessels in PWS birthmarks can be successfully achieved. We have then hypothesized that the release of cryogen spurts under vacuum pressures may also offer many advantages to this therapeutic procedure due to different spray-surface interactions under vacuum conditions. The objective of our work is to study the thermal response of a skin phantom to CSC at vacuum pressures. For this purpose, liquid cryogen was sprayed onto a skin phantom under atmospheric and various vacuum pressures, ranging from -17 to -70 kPa, at two nozzle-surface distances, 30 and 40 mm. We used a temperature sensor mounted on the phantom surface to measure temperature changes as a function of time, and an analytical solution, based on Fourier’s law and Duhamel’s theorem, to calculate heat flux and overall heat removal. Compared to atmospheric pressures, lower minimum surface temperatures and higher heat removal from the skin phantom were observed under all vacuum pressures and both surface-nozzle distances used in this study. This approach enhances the heat extraction from the skin surface and, therefore, should help improving epidermal protection provided by CSC during PWS laser therapy.