The laser used for hair removal works by the principle of selective photothermolysis , whose objective is the elimination of the hair follicle without affecting surrounding structures .It does this by pulses of high energy light, having wavelengths that are selectively absorbed by the target chromophore using emission times that are less than the thermal relaxation time of the hair follicle (20-60 milliseconds).
The hair follicle contains a natural chromophore is melanin. Other chromophores in the skin are water and hemoglobin. Lasers are used as target for depilation melanin and must emit light with a wavelength suitable for absorption it is present but very little by other pigments. Light energy, being absorbed by melanin, is transformed into heat energy, ie heat that is going to damage the hair follicle.
In 2004 it was published the first isolation and staining of hair follicle stem cells. Today it is accepted that for hair removal of a hair follicle is necessary to destroy the pluripotent stem cells that are housed in the protuberance or bulge, a cluster of cells responsible for producing the input of the follicle in anagen phase and generate hair They are housed in stable portion thereof up to the insertion of the erector muscle of the hair at the interface between the outer root sheath and connective tissue.
There seem to stimulatory and inhibitory cytokines hair growth which are released from the dermal papilla (vascular feeding every hair structure) and therefore is also interesting destroy this structure.If the parent or injured bulb and papilla follicular growth retardation is observed while if the stem cells are injured extrusion would achieve permanent hair removal.
Parameters to consider selective photothermolysis of the hair follicle (laser hair removal)
It is the band of the electromagnetic spectrum where each laser emits. It is measured in nanometers (nm). A longer wavelength, higher skin penetration and less absorption by the chromophore. For melanin, the lengths of near-infrared wavelengths are most suitable (700-1400 nm).
Thermal relaxation time
Is the time required to reduce by 50% the temperature generated by the light emission structures over tissue or where it acts. Ideal pulse duration must be between the relaxation time of the epidermis (3-10 milliseconds) and hair follicle (40-100 milliseconds).
Dose or fluence
It is the relationship between the supplied energy and the beam irradiation surface and is measured in Joules / cm 2. The efficiency is proportional to the dose.
It is the duration of the power output. It is measured in nano, micro or milliseconds. The pulse duration has taken great importance since Altshuler postulated the extended theory of photothermolysis, which calculates the ideal pulse length for a “target” to distance “heater” considering the diffusion time of heat from the “heater “to the outermost part of the” target”.
The “heater” would be what absorbs the energy (melanin) and the “target” the aim to damage by heat (protuberance stem cells). The time of thermal damage by this theory is time irreversible damage to the outermost portion of the “target” and for a follicle of 200 microns is calculated to be 170 ms.
Number of Pulses
The ability to spread the energy in several pulses (pulse train), can apply higher doses preserving the epidermis.
Interval between pulses or “delay”
Time between one pulse and the other.
Size of the radiation beam or “spot”
A larger surface area greater penetration and greater efficiency. When a small spot is applied there is more scattering of photons in radial direction, not reaching the hair bulbs and yield declines. To avoid this effect of radial dissipation, the spot size should be larger than the penetration depth of light in the necessary to be effective (5-10 mm) tissue.
He is shooting speed (number of pulses per second) and helps reduce the duration of the sessions. It is measured in hertz.