In recent years, with the rapid development of Laser Science in medical cosmetology, laser cosmetology has become a new fashion of cosmetology. Some skin lesions that were difficult to cure or had poor curative effect in the past can now receive good curative effect. As one of the laser cosmetology, light hair removal technology has been gradually recognized by patients with its wide publicity and application.
With the development of society, more and more new technologies have come to people’s life. As one of the fastest developing technologies of laser beauty in recent years, light depilation technology is becoming more and more mature. So what is the principle of light depilation technology?
In 1996, the first laser hair removal equipment was launched (ruby laser equipment). At present, the laser equipment involved in clinic include ruby laser (694nm), Alexandrite laser (755nm), diode laser (810nm), Nd: YAG (1064nm), electro optical synergy, ELOS (combined with IPL and RF electromagnetic wave) and recently commonly used intense pulsed light (IPL, 600-1200nm).
2、 Hair biology
（1） Hair follicle
Hair follicle consists of three parts. The first part is the funnel, which is the part between the pores and the opening of the sebaceous gland. The second part is the isthmus, which is the part from the opening of sebaceous gland to the attachment point of bristle muscle. The third part is the lower segment, which is between the attachment point of the erector pili muscle and the base of the hair follicle (neurovascular system).
Hair can be divided into three types in terms of morphology. The first is fetal hair, which is the fine hair covering the fetus and fades out in the neonatal period. The second is pigmented hair, with a diameter of 30 μ M or so. The third is terminal hair, which is a common kind of hair with a diameter of 150-300 μ M or so. The three types of hair can be transformed into each other. Puberty changes from terminal hair to terminal hair, and male hair loss changes from terminal hair to terminal hair. The color of hair is determined by the amount of pigment in the hair stem. It contains two pigments: eumelanin (brownish black pigment) and eumelanin (red pigment). Melanocytes are located in the upper part of the hair ball and the outer myelin sheath of the infundibulum.
There are two types of hirsutism. The first is hirsutism, which is characterized by increased hair in all parts of the body, which has nothing to do with androgen levels. The second is women’s hirsutism. Compared with women of the same nationality and age, women’s characteristic hair growth is excessive, distributed beyond the normal physiological range, showing a masculine tendency, which is related to the level of androgen.
Hair growth is generally divided into three cycles. The first is the growth stage, accounting for about 80% – 85% of the total hair. It is an active hair growth stage. Stromal cells divide rapidly and form hair stems. The content of melanin is high, which determines the length of hair (hair can be up to 6 years). The second is the degradation stage, accounting for about 2% of the total hair. It is the cessation of mitosis of stromal cells and the obvious reduction of nutrition in capillaries, which generally lasts for 3 weeks. The third is the rest period, accounting for about 10% – 15% of the total hair. The hair loss in this period generally lasts for 3 months or even a year, which determines the number of hair. The periodic growth of hair and the different content of pigment have a great impact on the effect of hair removal.
3、 Basic principles of hair removal
（1） Selective photopyrolysis
Selective photopyrolysis is to convert the absorbed light energy into heat energy by using the selective absorption characteristics of skin damaged tissues to a specific wavelength of light radiation. In the process of hair removal treatment, the heat generated by the color base (mainly melanin) is transmitted to the biochemical cells to destroy the biochemical cells, so as to play the role of permanent hair removal.
- Color base: mainly melanin. Melanin distributed in hair follicles and hair stems is rich, distributed among the cells of hair bulb matrix, and can transfer to the structure of hair stem (such as medulla, cortex and hair cuticle) to absorb red light or near-infrared light;
- Target: hair follicle stem cells. It is located in the outer root sheath of the hair follicle between the hair ball and the bulge above it, that is, the opening of the sebaceous gland and the attachment of the hair follicle of the erector pili.
There is heat diffusion in selective photopyrolysis, which requires an appropriate pulse width. Therefore, in the process of hair removal treatment, the pulse width greater than or equal to the thermal relaxation time of hair follicle is generally selected for treatment. Thermal relaxation time (TRT) is the time required for half of the heat to dissipate, about 10-100ms. Thermal damage time (TDT, TDT ≥ TRT) is the time when the heat continues to cause some target damage.
In the treatment process, the setting of energy density is mainly related to the color of the skin. The lighter the color of the skin, the higher the energy density used, and the darker the color of the skin, the lower the energy density used. In the treatment process, choose a large spot as much as possible, which can reduce the scattering of light in the dermis, reduce the scattering, and have faster speed and better hair removal effect.
（2） Epidermal cooling
Epidermal cooling can protect the epidermis and allow the application of higher optical parameters while alleviating the pain during treatment. The most common epidermal cooling is to cool the skin by ice or cold gelatin smear on the skin surface. Contact cooling is to make close contact with the skin through the cold head of sapphire (10 ℃ – 10 ℃), which can reduce the skin temperature from 30 ℃ to 20 ℃. Convective air cooling: the skin temperature can be reduced from 30 ℃ to 20 ℃ by giving high-speed convective cold air (about 10 ℃) through the equipment. Dynamic cooling (cryogen spray cooling) can reduce the skin temperature from 30 ℃ to 0 ℃, and tetrafluoroethane (HFC134, boiling point 26 ℃) is generally selected as the jet coolant.
（3） Photo mechanical action
The photomechanical effect is realized by Q-switched Nd: yag1064nm laser and toner (selective addition), which can heat the color base (melanin) very quickly and produce photoacoustic shock wave, so as to destroy the melanocytes, but it often can not completely destroy the hair follicle, resulting in white hair rather than permanent hair reduction.
（4） Photochemical action
The method of light color is adopted for patients. Photodynamic therapy is absorbed by hair follicle sebaceous gland through photosensitizer (ALA) to produce singlet oxygen and destroy hair follicle. The use method is 20% ala（ δ- After a single treatment with 630nm laser for 3 hours, the hair removal rate can reach 40%, which is not affected by the color and growth cycle of hair, but it needs long-term observation and large case study.
4、 Depilation equipment
（1） Traditional hair removal equipment
- Ruby laser (694nm): characterized by high absorption rate of melanin, it is suitable for patients with fair complexion (Ⅰ – Ⅲ), fine hair or blonde hair, and the risk of dark complexion treatment is high. It is the original hair removal laser, with huge volume and slow treatment speed;
- Emerald laser (755nm): compared with the commonly used hair removal laser, the absorption rate of melanin is lower than that of ruby laser, which can provide the best balance between efficacy and safety for patients with dark skin color (ⅰ – ⅳ). It has high repetition frequency and large light spot, so as to reduce the fatigue and pain of both doctors and patients;
- Diode laser (800-810nm): the gold standard of laser hair removal. It has simple structure, high energy (2900W), contact cooling and wide pulse width (5-400ms). It has good hair removal effect and is safer for patients with dark skin color (I-V). The pain was slightly heavier than the former during treatment;
- Long pulse laser Nd: YAG (1064nm): the absorption rate of melanin is low. In order to offset this deficiency, it is necessary to increase laser energy and require good skin cooling. It can safely treat patients with various skin colors (especially type VI skin), and can be used to treat pseudofolliculitis;
- Intense pulsed light IPL (640-1200nm): millisecond pulse width, large spot, single pulse or multi pulse mode, various pulse delays, flexible setting of treatment parameters, which can be effectively applied to the treatment of various skin colors and hair. Compared with laser, the depilation effect is similar, but it is easy to cause epidermal damage. It can be combined with 1064nm laser.
The first is toner and the second is photosensitizer ala, which can be combined with laser. According to the literature, a single treatment can achieve 30% hair removal rate, while the clinical treatment effect of other non laser light sources is not ideal. The third is melanin encoded phosphatidylcholine based liposome solution, which is an iron solution containing melanin. Applying it to the skin can temporarily enrich the light hair follicles with melanin and increase the content of melanin, so as to improve the effect of hair removal. J de Leeuw, the 2002 eadv conference believes that the removal rate of light colored hair can reach 75% after treatment, while the removal rate of meladine is very low. Tanzi & Alster, 2003 ASDs conference believes that it can delay growth and not permanent hair removal.
（2） Special hair removal equipment
- Special semiconductor laser
Traditional hair removal requires as high energy density as possible. Single pass treatment is painful and risky; The special semiconductor laser has the advantages of low energy density (10_j / cm2) and multiple pass repeated scanning (10Hz), which can reduce the pain and risk of treatment. The clinical effect is similar to that of traditional laser.
- Photoelectric integrated technology
The above figure shows the photoelectric integrated technology. As shown in the figure, it is a bipolar RF assembly device with strong pulse light. Equipped with contact cooling, it is less dependent on the absorption of melanin. It is effective for white or gray hair. Its energy is reduced and its safety is increased. It is suitable for the treatment of various types of skin. At the same time, it can also be combined with photosensitizer to increase the hair removal effect.
- Phosgene Technology (PPT)
It was first used in the treatment of acne. At present, it has been used in depilation clinically. As shown in the figure above, it is combined with 810nm semiconductor laser through airbag.
As shown in the figure above, the light source of traditional laser hair removal is far away from the color base and target base, while phosgene technology can absorb the skin in the cavity close to the light source, shorten the distance between the target base and the light source, and reduce the concentration of melanin and hemoglobin in the skin due to the extension of the skin, so as to reduce the competitive light absorption. In the treatment process, reduce the energy density, so as to reduce the pain, and the larger spot design can speed up the treatment speed and increase the penetration depth and curative effect of light at the same time.
（3） Household hair removal equipment
Generally, semiconductor laser and IPL are used to destroy hair follicles. At present, there is no international standard equipment. The energy density is low and the safety is high. The research results are completed under strict control, but it is easy to cause eye and skin damage for patients with lack of training and improper operation.