Interaction of Cryogen Spray with Human Skin under Vacuum Pressures

Clinical results recently demonstrated improvement in lesion blanching using local vacuum to target small vessels in port wine stains (PWS) birthmarks laser therapy. 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, which may lead to enhanced heat transfer. The objective of our work is to study the time and space dependent thermal response of a skin phantom to cryogen sprays at different pressures and constant humidity conditions. For this purpose, liquid cryogen was sprayed onto a skin phantom within atmospheric and various local vacuum pressures ranging from 17–70 kPa at 15 % and 50 % local humidity. We used a linear array of sensors mounted on the phantom surface to measure temperature changes as a function of time along a 6 mm radius, and a direct heat conduction solution to calculate the correspondent local heat fluxes. Compared to atmospheric pressures, lower minimum surface temperatures and higher heat fluxes from skin phantoms were observed along a 4 mm radius under all vacuum pressures and 15 % humidity. At 6 mm, there are no considerable differences in the heat transferred with or without vacuum application. At 50 %, the advantages of spraying in vacuum are less significant. Comparisons of results at constant pressures show that the heat extraction is more efficient at lower humidity levels. In general, the application of vacuum inherently reduces the content of water vapour. It follows that overall, the release of cryogen sprays under vacuum pressure enhances the heat extraction from the skin surface and, therefore, should help improve epidermal protection provided by cryogen spray cooling during PWS laser therapy.