Laser pulse width seems to play an important role in laser-assisted hair removal. Thermal conduction during the laser pulse heats a region around each microscopic site of optical energy absorption. The spatial scale of thermal confinement and resultant thermal or thermome-chanical damage is therefore strongly related to the laser pulse width. Q-switched laser nanosecond pulses effectively damage individual pigment cells within a hair follicle by confinement of heat at the spatial level of melano-somes (Zenzie et al. 2000). They can induce leukotrichia and cause a temporary hair growth delay, but do not inactivate the follicle itself.
On the other hand, lasers with longer pulse durations not only allow gentle heating of the melanosomes, but also target the entire follicu-lar epithelium by allowing thermal conduction from the pigmented hair shaft and pigmented epithelial cells to the entire follicular structure.
Therefore, lasers emitting longer pulse durations can achieve two goals: (1) Epidermal melanosomes are preserved. This then helps to preserve the epidermis. (2) Adequate heat diffusion occurs to the surrounding follicle from the light-absorbing melanized bulb and shaft.
The use of a longer laser-emitted pulse width may necessitate the use of higher fluences because the longer pulse now heats a larger volume of tissue. This may be of some benefit in allowing higher fluences to be used on dark skin types with both less risk of epidermal injury and increased chances of transfollicular damage.
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