When using the principle of "selective photothermolysis" for laser hair removal, the laser wavelength and pulse width should ensure that the laser hits the hair follicle without damaging the adjacent tissue. Clinical studies have shown that the longer the wavelength, the deeper the penetration and the smaller the scattering, but the worse the absorption of melanin, the stronger the energy, but the epidermis cannot be damaged; the pulse width is smaller than the TRT (thermal relaxation) of the hair follicle. Time) is effective, and TRT larger than the epidermis is safe; the larger the spot size, the deeper the penetration; at the same time, active skin cooling should be used to protect the epidermis and relieve pain. When the wavelength is less than 600nm, the laser is mainly absorbed strongly by hemoglobin and protein, and when the wavelength is greater than 1100nm, the laser is mainly absorbed by the water in the body tissue, and the light is transparent when the wavelength is in the range of 600nm~1100nm. Therefore, people usually choose lasers with wavelengths in the range of 600nm to 1100nm for hair removal. Lasers and intense pulsed light sources used for hair removal mainly include long-pulse ruby laser (694nm), long-pulse alexandrite laser (755nm), semiconductor laser (800nm), long-pulse Nd:YAG laser (l064nm), intense pulse incoherent broadband Light source (590~1200nm), etc.
1. Long Pulse Ruby Laser
694nm ruby laser is strongly absorbed by melanin. This laser has 15% to 20% of the energy that can penetrate the entire dermis layer, and the hair with melanin in the dermis and deep layers can absorb the energy of the laser, causing the hair follicles to overheat and be damaged. The normal tissue around the hair follicle has a very low light absorption rate at 694 nm. When the pulse width is constant, the selective photothermal damage to the hair follicle can be achieved without damaging the surrounding normal tissue.
2. Long-pulse alexandrite laser
The 755nm alexandrite laser is similar to the ruby laser, except that it has slightly stronger penetration and lower melanin absorption because it has a slightly longer wavelength.
3. Semiconductor laser
The penetration depth of the semiconductor laser system in the skin tissue is deep, more than 5mm, the scope of action can reach the deep dermis and subcutaneous adipose tissue, and can effectively penetrate the epidermis to the dermis hair follicle tissue, thereby destroying any part of the human body and depth of hair. The epidermis absorbs very little laser energy, so no hyperpigmentation occurs.
4. Long-pulse Nd:YAG laser
The wavelength of the long-pulse Nd:YAG laser is 1064nm and can penetrate 5-7mm. Due to the low affinity of melanin for the 1064 nm laser, exogenous chromophores (such as carbon smears) are often used for treatment. The laser spot scans the entire treatment area until visible carbon particles are removed. The disadvantage is that sometimes the carbon particles cannot reach the hair bulb and affect the therapeutic effect. For example, if the sweat glands and sebaceous glands are contaminated with carbon particles, they are easily damaged, and the clinical effect is not ideal. Recently, a long-pulse-width laser with a wavelength of 1064 nm has been clinically reported for hair removal without the need for carbon smears before surgery. Because epidermal melanosomes are very sensitive to short-pulse (<1ms) lasers, and long-pulse (≤200ms) lasers can allow the application of high-energy lasers to effectively heat the inner and outer hair root sheaths and hair growth areas without affecting it. Epidermal melanosome, so it can be safely used in the treatment of black skin.
5. Intense pulsed light
High-intensity pulsed light is not a laser light source, but a non-continuous light source with multiple wavelength combinations. Its wavelength is between 550 and 1200 nm, and it can penetrate 1.3 mm. The treatment parameters can be adjusted according to skin color or hair color. Theoretically, broad-spectrum wavelength light source systems increase the absorption of light by non-target objects, thus increasing adverse reactions.
