On the Modeling and Characterization of Cryogen Spray Cooling for Application to Port Wine Stain Laser Therapy

The combination of cryogen spray cooling (CSC) and laser irradiation has proven to be a viable solution to treat various dermatologic vascular disorders, such as port wine stain birthmarks (PWS). PWS patients are treated with laser pulses that induce permanent thermal damage to the target blood vessels. However, absorption of laser energy by melanin causes localized heating of the epidermis, which may result in complications, such as hypertrophy, scarring, or dyspigmentation. By applying a cryogen spurt to the skin surface for an appropriately short period of time (10 to 100 milliseconds), the epidermis can be precooled prior to the application of the laser pulse and, therefore, reduce or eliminate undesirable skin damage. Despite the commercial use of cryogen spray cooling (CSC) during PWS laser therapy, there is still a limited understanding of the fluid dynamics, thermodynamics, and heat transfer characteristics of cryogen sprays. Optimization of CSC has become necessary after clinical studies have shown that current CSC commercial nozzles provide insufficient epidermal protection to patients with higher epidermal melanin concentration. To achieve this goal, it is necessary to explore the influence that fundamental spray properties have on the overall heat extraction from skin (Q) and the instantaneous heat flux (q). In this work we present recent experimental studies targeted towards the characterization of cryogen sprays and the optimization of cooling procedures. To do that, we measure the average droplet diameter (d), velocity (v), and droplet density (C) with the aid of a phase Doppler particle analyzer (PDPA). We also measure the overall mass flux (m), and average spray temperature (T). Finally, using custom-built temperature sensor devices, we compute Q and q. We show that q is largely dominated by m and v, and not so much by other spray characteristics, such as d and T, as previously believed. Moreover, show that less dense sprays appear to induce higher q. We go on to show that among m and v, the former has an even stronger influence on Q and q than the latter. INTRODUCTION During dermatologic laser treatments of vascular lesions, such as port-wine stains (PWS), laser light absorbed by the epidermal melanin poses two obstacles. First, if a significant portion of energy from the laser is absorbed within the epidermis, it may cause irreversible thermal damage to the epidermis and, second, light absorption in the epidermis results in less energy reaching the target vessels [1]. Cryogen spray cooling (CSC) is used effectively on patients with low melanin concentration (skin types I-IV) to prevent epidermal thermal damage, while allowing sufficient energy to reach the target lesion [2,3,4]. However, CSC fails to provide sufficient thermal protection for patients with darker skin types (V and VI), who have higher levels of epidermal melanin. Therefore, to extend the benefit of CSC to all patients, it is necessary to improve cooling efficiency: this can be achieved by an in depth understanding of the fundamental spray parameters that influence heat extraction from human skin. In recent studies, it has been shown that q can be increased by changing nozzle geometry, such as nozzle diameter [3,5,6], spray conditions, such as nozzle-to-skin distance [7], and/or sequential deposition of cryogen in multiple spurts [8]. The reason is that changes of these parameters influence the fundamental spray properties (m, d, v, C, and T). Depending on how these properties combine, the net