• Laser Hair Removal
• General Information
• Hair Removal Methods

HOW DOES A LASER HAIR REMOVAL SYSTEM WORK?

The principle is selective photothermolysis, the objective of which is to eliminate the hair follicle without damaging adjacent structures. This is achieved by means of high energy light pulses, with wavelengths that are selectively absorbed by the target chromofore, using emission durations shorter than the thermal relaxation time of the hair follicle (20-60 milliseconds).

The hair follicle contains melanin, a natural chromophore. Other chromophores in the skin are water and haemoglobin. The lasers used in hair removal target melanin and must emit light with an appropriate wavelength for high absorption by this natural chromophore, but minimal absorption by the rest. As light energy is absorbed by the melanin it is transformed into thermal energy, meaning heat which will destroy the hair follicle.

Nowadays, it is still unclear which specific site has to be destroyed to avoid hair regeneration. Studies postulate that the destruction of the papilla and matrix cause a delay in follicle growth. On the other hand, the destruction of stem cells in the protuberance area at the interface between the external radicular sheath and connective tissue causes a permanent epilation.

Parameters to be considered in photothermolysis:

Wavelength: is the electromagnetic spectrum frequency band where the laser emits. It is measured in nanometres (nm). The greater the wavelength the greater penetration in the skin and the lesser absorption by chromophore. Wavelengths closer to infrared (700-1400nm) are more appropriate for melanin.

Thermal relaxation time: is the amount of time necessary to reduce by 50% the temperature generated by light emission on the targeted tissue or structures. Optimal pulse duration must be between epidermal relaxation time (3-10 milliseconds) and hair follicle relaxation time (40-100 milliseconds).

Dosage or fluency: is the relationship between available energy and the surface covered by the irradiation beam, measured in Joules/cm2. Efficacy is proportional to dose.

Pulse duration: is the duration of energy emission. It is measured in nano, micro or milliseconds. Pulse duration has become greatly important after Altshuler postulated the amplified theory of photothermolysis. This theory calculates the ideal pulse length for a target at a distance from the heater, considering how long it takes for heat to scatter from the heater to the outermost part of the target. The heater would be what absorbs the energy (melanin), and the target, the object to be damaged by the heat (protuberance stem cells). According to this theory, thermal damage time is the irreversible damage time of the most external part of the target. For a follicle of 200 micras it would be 170 ms.

Number of pulses: the fractioning of energy into several pulses allows the application of larger doses while preserving the epidermis.

Pulse intervals or “pulse delay”: time between pulses.

Size of the irradiation beam or “spot”: the greater the surface, the greater the penetration and efficacy. When a small spot is delivered there is greater scattering of photons in radial direction, without reaching the hair bulbs with a decrease of fluency. To avoid the effect of radial scattering the spot size must be bigger than the light’s penetration depth into the tissue (5-10 mm).

Pulse frequency: shot speed (number of pulses per second triggered), which reduces the duration of the sessions. It is measured in Hertz.