Super long pulse hair removal

The field of laser hair removal has undergone rapid evolution since the first Q-switched Nd:YAG system combined with carbon-black sensitizer was developed by Thermolase and cleared by the FDA in 1995. Today, numerous coherent and incoherent light sources are available for this application. To thermally damage the hair follicle without injuring the surrounding dermis, these systems target melanin in the hair shaft/follicle with wavelengths in the 700-1100 nm range. The stem cells, which are responsible for hair-follicle regrowth, are located at the connective tissue sheath in a ring concentric with the hair shaft at a depth of 0.8-1.2 mm below the skin surface. According to one theory, the stem cells (which do not contain melanin) must be damaged to achieve permanent hair removal. Thus the entire follicular structure bounded by the connective tissue sheath should be heated to the 65-70/spl deg/C thermal denaturation temperature. Since only the hair shaft and matrix contain melanin, the rest of the follicle can be damaged by either explosive vaporization of the hair shaft/matrix or by thermal diffusion. Most light-based hair-removal devices currently on the market utilize relatively long (1-100 ms) pulses to damage hair follicles. For pulsewidths in this range (neglecting explosive vaporization), only part of the follicle is thermally damaged because the temperature at the connective tissue sheath remains below the denaturation temperature. Explosive vaporization of the hair shaft/matrix may create areas of damage out to the connective tissue sheath, but the damage patterns are unpredictable. For pulsewidths in the 0.1-1 second range (super long pulse), the temperature at the connective tissue sheath exceeds 65-70/spl deg/C, but the surrounding tissue remains below the denaturation temperature. Thus the entire follicle volume is thermally denatured while the surrounding dermal tissue remains viable.