Dynamic behavior of cryogen spray cooling: Effects of spurt duration and spray distance

Abstract

Background and Objectives Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatologic surgery. Since optimization of CSC permits the safe use of higher light doses, which improves therapeutic outcome in many patients with superficial skin lesions, studies have focused on understanding spray–surface interactions and cooling dynamics. The objective of this study is to measure accurately temperature variations at the sprayed surface and the effects of spurt duration (Δt) and nozzle-to-sprayed surface distance (L) on cooling dynamics during CSC. Study Design/Materials and Methods A fast-response temperature measurement sensor is built using thin (20 μm) aluminum foil placed on top of a poly methyl-methacrylate resin (Plexiglass^®) with a 50 μm bead diameter thermocouple positioned in between. Liquid film residence time (t_r) and minimum surface temperature (T_min) are systematically measured as a function of Δt and L. Results Two distinct spray–surface interaction mechanisms are recognized. The transition between them occurs at a critical length L_c≈25–30 mm. Noticeable characteristics include: (1) for spurts at L < L_c, shorter t_r, and lower T_min are reached as compared to L > L_c, T_min is dependent on Δt and L, while t_r is a function of Δt only; (2) for spurts at L > L_c, T_min still depends on L but not on Δt, while t_r becomes a function of both Δt and L. Finally, for all Δt, t_r reaches a maximum at L = 40 mm. Conclusions Based on our results, a good choice to achieve low T_min and t_r for the treatment of superficial skin lesions may be met by using Δt of ≈30–50 milliseconds and the shortest spray distance that is tolerable by the patient. Spurt durations (Δt) of more than 30–50 milliseconds at spray distances (L) greater than L_c lead to higher T_min and longer t_r. These parameters may be appropriate for laser therapy of deeper targets. Lasers Surg. Med. 32:152–159, 2003.