Cooling the Epidermis

To reduce the damage to the upper layers of the skin, surface cooling is directed at the dermo-epidermal junction where the highest amounts of epidermal melanin reside [68]. The heat generated by the laser treatment must be exchanged between the cooling device and skin stratum corneum [69]. Four types of cooling have been developed to cool the skin during laser treatment: Clear gel (usually chilled), contact cooling (through a window cooled by circulating water), cryogen spray (immediately before/after the laser pulse), and air-cooling [71] (Table 8.1). Topically applied gels are the least expensive cooling method [68]. The cooled gels have a smaller temperature gradient than active cooling devices, so only a small amount of heat can be removed from the skin before the temperature gradient decays, and heat transfer ceases. A sapphire window with 2-6°C circulating water is also used to protect the skin from overheating [72]. These types of systems can drop the tem­perature of the skin’s basal layer by 20°C with a 0.5 s exposure. Evaporative cooling using cryogen sprays has also been used [73,74]. With this method, one is able to create larger temperature gradients between the dermis and epidermis, which allows for greater protec­tion of the epidermis, and reduced risk of inadvertent dermal cooling. For example, the cryogen spray, which is applied for 10-50 ms, followed by the delivery of the laser pulse within 5-10 ms has been used for skin protection. The epidermis is cooled to -10°C for a short period of time (<100 ms), and is limited to cooling to a depth of about 200 um. The cryogen spray relies on the atomization of the spray for uniform dispersal of the droplets; any irregularities in the droplet size may lead to variable localization of the cooling [74]. Although the heat extraction is up to twenty times more efficient than conductive devices, the lack of optical coupling at the skin surface may increase backscatter. In a humid envi­ronment, condensation may occur, impeding the subsequent laser pulse.