Cooling efficiency of cryogen spray during laser therapy of skin.

Cryogen spray cooling (CSC) is used extensively for epidermal protection during laser-induced photothermolysis of port wine stains and other vascular skin lesions. The efficacy of CSC depends critically on the heat transfer coefficient (H) at the skin surface for which, however, no reliable values exist. Reported values for H, based on tissue phantoms, vary from 1,600 to 60,000 W/m(2) K. A simple experimental model was designed and constructed, consisting of a pure silver-measuring disk (diameter 10 mm, thickness approximately 1 mm), embedded in a thermal insulator. The disk was covered with a 10 microm thick stratum corneum layer, detached from in vivo human skin. The heat transfer coefficient of the stratum corneum/cryogen interface was measured during CSC with short spurts of atomized tetrafluoroethane. H was found to be dependent on the specific design of the cryogen valve and nozzle. With nozzles used in typical clinical settings, H was 11,500 W/m(2) K, when averaged over a 100 ms spurt, and 8,000 W/m(2) K when averaged over a 200 ms spurt. The presented model enables accurate prediction of H and thus improve control over temperature depth profile and cooling efficiency during laser therapy. Thereby, it may contribute to improvement of therapeutic outcome.