Epilation today: physiology of the hair follicle and clinical photo-epilation.

Despite the variations of length and type of hair (vellus or terminal), the growth of human hair in all body sites is cyclic. Phases of active hair growth, or anagen, are separated by periods of quiescence, or telogen. The duration of both phases varies greatly depending on the body site. Whether hairs are in anagen/telogen at the time of hair removal is important because only anagen hairs are particularly sensible to physical insults. Photo-epilation is a technique for long-term removal of unwanted hair by thermal destruction of the hair follicle and its reproductive system (stems cells). As melanin is the main chromophor existing in hair follicles the corresponding wavelength spectrum would range from ultraviolet up to infrared light. Furthermore longer wavelengths are preferred as the cromophor lies deep in the skin and the penetration of light is increasing with the wavelength. Thus, in the range of 600-1100 nm melanin absorption may be used for selective photothermolysis of hair follicles. Yet to be resolved questions for permanent destruction are the location of the key follicular target and the possible influence of the hair growth cycle on photothermolysis-induced hair removal. An overview on the individual physiology of the hair follicle is given to discuss the latest strategies for photo-epilation.