title: Effects of hypobaric pressure on human skin: implications for cryogen spray cooling (part II).
abstract: OBJECTIVE
  Clinical results have demonstrated that dark purple port wine stain (PWS) birthmarks respond favorably to laser induced photothermolysis after the first three to five treatments. Nevertheless, complete blanching is rarely achieved and the lesions stabilize at a red-pink color. In a feasibility study (Part I), we showed that local hypobaric pressure on PWS human skin prior to laser irradiation induced significant lesion blanching. The objective of the present study (Part II) is to investigate the effects of hypobaric pressures on the efficiency of cryogen spray cooling (CSC), a technique that assists laser therapy of PWS and other dermatoses.
  
  
  METHODS
  Experiments were carried out within a suction cup and vacuum chamber to study the effect of hypobaric pressure on the: (1) interaction of cryogen sprays with human skin; (2) spray atomization; and (3) thermal response of a model skin phantom. A high-speed camera was used to acquire digital images of spray impingement on in vivo human skin and spray cones generated at different hypobaric pressures. Subsequently, liquid cryogen was sprayed onto a skin phantom at atmospheric and 17, 34, 51, and 68 kPa (5, 10, 15, and 20 in Hg) hypobaric pressures. A fast-response temperature sensor measured sub-surface phantom temperature as a function of time. Measurements were used to solve an inverse heat conduction problem to calculate surface temperatures, heat flux, and overall heat extraction at the skin phantom surface.
  
  
  RESULTS
  Under hypobaric pressures, cryogen spurts did not produce skin indentation and only minimal frost formation. Sprays also showed shorter jet lengths and better atomization. Lower minimum surface temperatures and higher overall heat extraction from skin phantoms were reached.
  
  
  CONCLUSIONS
  The combined effects of hypobaric pressure result in more efficient cryogen evaporation that enhances heat extraction and, therefore, improves the epidermal protection provided by CSC.
authors.affiliation: Department of Mechanical Engineering, University of California, Riverside, California 92521, USA. gaguilar@engr.ucr.edu
authors.forename: Guillermo
authors.initials: G
authors.lastname: Aguilar
authors.forename: Walfre
authors.initials: W
authors.lastname: Franco
authors.forename: Jie
authors.initials: J
authors.lastname: Liu
authors.forename: Lars O
authors.initials: LO
authors.lastname: Svaasand
authors.forename: J Stuart
authors.initials: JS
authors.lastname: Nelson
mesh_terms: D000336:Aerosols / Q000494:pharmacology; D003080:Cold Temperature; D006801:Humans; D006845:Hydrocarbons, Fluorinated / Q000494:pharmacology*; D053685:Laser Therapy; D028022:Low-Level Light Therapy / Q000379:methods; D019339:Port-Wine Stain / Q000503:physiopathology / Q000532:radiotherapy*; D011312:Pressure; D012680:Sensitivity and Specificity; D012881:Skin Temperature / Q000187:drug effects / Q000528:radiation effects; D013814:Thermal Conductivity; D013817:Thermography; D014741:Video Recording
chemical_list: D000336:Aerosols; D006845:Hydrocarbons, Fluorinated