Numerical analysis of cold injury of skin in cryogen spray cooling for laser dermatologic surgery

In laser dermatologic surgery, cryogen spray cooling (CSC) is used to avoid unwanted thermal damage such as scars from skin burning due to the melanin absorption of the laser beam. As the cryogen is fully atomized from the nozzle, temperature of the droplets can quickly drop below-60 oC because of evaporation. Such low temperature may lead to cold injury of skin. Therefore, spray cooling process should be accurately controlled during clinical practice to achieve sufficient protection and avoid cold injury. This study presents a numerical analysis for cold injury of skin in cryogen spray cooling for dermatologic laser surgery by a newly developed hest transfer model. For the freezing of skin cells, heat conduction equation was used to describe frozen and unfrozen zones, and heat capacity method was utilized for mushy zone to consider the phase change of tissue. A realistic boundary condition was implemented to simulate the cooling effect during cryogen spray cooling by a generalized correlation for the dynamic heat transfer coefficient. By tracking the front of the tissue phase change, the model can be used to predict the movement of the harmful isothermals. With this model, the severity of cold injury is quantified under various clinical conditions and the effects of initial temperature as well as the spurt duration on possible cold injury of skin are investigated. The results show that 100∼150ms spray cooling duration is appropriate to avoid non-uniform cooling along the radial direction and also prevent potential cold injury. Lower room temperature (10∼20°C) is recommended to achieve a deep penetration protection. Further development of new candidate cryogens with lower boiling point (e.g. R407C or R404a) are highly recommended to achieved a better cooling effect.