Abstract
Fast heat extraction is critically important to obtain the maximal benefit of cryogen spray cooling (CSC) during laser therapy of shallow skin lesions, such as port wine stain birthmarks. However, a film of liquid cryogen can build up on the skin surface, impairing heat transfer due to the relatively low thermal conductivity and higher temperature of the film as compared to the impinging spray droplets. In an attempt to optimize the cryogen mass flux, while minimally affecting other spray characteristics, we apply a series of 10 ms spurts with variable duty cycles. Heat extraction dynamics during such intermittent cryogen sprays were measured using a custom-made metal-disc detector. The highest cooling rates were observed at moderate duty cycle levels. This confirms the presence, and offers a practical way to eliminate the adverse effect of liquid cryogen build-up on the sprayed surface. On the other hand, lower duty cycles allow a substantial reduction in the average rate of heat extraction, enabling less aggressive and more efficient CSC for treatment of deeper targets, such as hair follicles.
Highlights
In laser dermatologic surgery and cosmetic treatments, non-specific absorption by epidermal melanin competes with absorption of radiation in subsurface target chromophores
For some clinical indications, such as port wine stain birthmarks (PWS), this limitation adversely affects the success of laser therapy in many patients
Pre-cooling of the epidermis with cryogenic sprays permits the safe application of laser pulses with higher energy (Welch et al 1983, Nelson et al 1995), leading to an improved outcome in laser therapy of PWS (Nelson et al 1995, Waldorf et al 1997, Chang and Nelson 1999) and hemangiomas (Chang et al 1998, 2001), rhytides (Kelly et al 1999), and hair removal
Summary
In laser dermatologic surgery and cosmetic treatments, non-specific absorption by epidermal melanin competes with absorption of radiation in subsurface target chromophores. Pre-cooling of the epidermis with cryogenic sprays permits the safe application of laser pulses with higher energy (Welch et al 1983, Nelson et al 1995), leading to an improved outcome in laser therapy of PWS (Nelson et al 1995, Waldorf et al 1997, Chang and Nelson 1999) and hemangiomas (Chang et al 1998, 2001), rhytides (Kelly et al 1999), and hair removal. Despite the wide clinical use of cryogen spray cooling (CSC) and considerable research efforts in recent years, our understanding of the involved heat-transfer mechanisms and dynamics remains incomplete. It is not certain whether existing CSC devices have been optimized for their respective applications
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