Effect of skin indentation on heat transfer during cryogen spray cooling.
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BACKGROUND AND OBJECTIVES: Cryogen spray cooling (CSC) is used to pre-cool the epidermis during dermatological laser procedures such as treatment of port wine stain (PWS) birthmarks, hair removal, and non-ablative photorejuvenation. Thus far, heat transfer studies related to CSC optimization have assumed a flat surface but clinical observation suggests that human skin indents due to the force of an impinging cryogen spray. STUDY DESIGN/MATERIALS AND METHODS: High-speed videos of cryogen spray impingement on in vivo human skin were taken and the resulting indentations characterized as a function of both nozzle-to-skin distance and anatomic location. Detectors with pre-formed indentations were constructed and heat flux measurements were performed at two nozzle-to-surface distances. RESULTS: Indentation causes cryogen accumulation that reduces the efficiency of heat transfer when compared to spray impingement on a flat surface. Large indentations, however, encourage convective flow within the cryogen pool that mitigates many of the negative effects of liquid layer thickening and improves the heat flux. CONCLUSIONS: Flat surfaces produce the most efficient heat transfer, but once indentation exists (as it does in all clinically relevant cases), larger indentations produce a higher maximum heat flux. This suggests that higher momentum sprays (which produce larger skin indentations for identical spurts) than those in current clinical use may improve CSC efficiency.