Quantitative investigation of thermal damage in Er:YAG laser skin resurfacing

Feasibility of deep coagulation of skin with superficially absorbed Er:YAG lasers is investigated using a numerical model. Unlike most previous models, the skin is treated as a two-component material: water trapped in spherical cavities inside an infinite elastic medium. In describing the interaction of mid-IR laser light with skin, thermodynamic behavior of pressurized hot water and steam is combined with elastic response of the surrounding medium. A one-dimensional treatment of heat diffusion and the Arrhenius model of the protein denaturation process are also included in the model. Temperature evolution, profile and coagulation depth are analyzed as a function of the pulse duration, number of applied pulses and repetition frequency of the sequence. The results indicate that the depth of coagulated layer, which amounts to 15 - 40 micrometer with a single free-generated Er:YAG laser pulse, can be extended up to 200 micrometer with no surface ablation by using a repetitive pulse sequence of suitable single-pulse fluence, repetition frequency and duration.