Simulation of heat distribution and thermal damage patterns of diode hair-removal lasers: an applicable method for optimizing treatment parameters.

Objective: We simulated the heat distribution and thermal damage patterns of diode hair-removal lasers for different spot sizes, pulse durations, and fluences as a guide for optimization.

Background: Recently, the concept of thermal damage time as a reference for pulse duration has become a subject of debate.

Methods: Laser-Induced-Temperature-Calculation-In-Tissue (LITCIT) was used for the simulations. Skin was modeled as two homogenous layers of epidermis/dermis and two coaxial cylinders as the hair shaft/ follicle. Opto-thermal coefficients of the components and the radiant parameters of the laser (diode, 810 nm) were defined.

Results: At constant fluences and pulse durations, the damage occurred deeper when larger spot sizes were used. At constant pulse duration, high fluences caused significant damage to the hair follicle and epidermis. By using longer pulse durations (≤ 400 ms) at constant fluences, there was more effective damage to the hair follicle while sparing the adjacent epidermis and dermis. Because of the time-dependent temperature profiles, an increased pulse duration creates a moderate, gradual rise in the target's temperature. Pulse durations > 400 ms are accompanied by unwanted dermis damage.

Conclusions: Our results show that using very long pulse durations near the tissue damage time (≤ 400 ms) creates better efficacy in treating unwanted hairs while avoiding unwanted damage.