The principle of laser hair removal equipment is still based on the theory of selective photothermal action. There is abundant melanin in hair follicles and hair shafts. Melanin is distributed between cells of the hair bulb matrix and can also be transferred to the structure of the hair shaft (such as medulla, cortex, and trichome). Lasers can target melanin. Precise and selective hair removal treatment. After the melanin absorbs the energy of the laser, the temperature rises sharply, which causes the destruction of the surrounding hair follicle tissue and removes the hair.
The 808 laser hair removal device and the theory of selective photothermal action. The principle of 808 laser hair removal equipment is to select a specific wavelength laser that can be selectively absorbed by the target tissue but not absorbed by the surrounding tissue according to the biological characteristics of different tissues, and the pulse width is shorter than or equal to the thermal relaxation time of the target tissue (Note) , When the energy density is greater than the threshold required for target tissue damage, the most effective treatment of the diseased target tissue can be ensured, while the damage to the surrounding normal tissue is minimal. This theory achieves a perfect unification of the efficacy and safety of laser therapy. Selective photothermal action includes three basic conditions: Condition one, select the wavelength that can act on the target tissue and be strongly absorbed by the target tissue. Two aspects should be considered: (1) the selective absorption of the wavelength of laser light by the target tissue; (2) the degree of laser energy of this wavelength acting on the target group line. Condition 2, the laser pulse duration (pulse width) should be less than or equal to the thermal relaxation time of the target tissue. In this way, the target tissue does not have enough time to transfer heat to the surrounding normal tissue, and the thermal damage to the surrounding normal tissue is minimal. The third condition is to select the appropriate energy that can destroy the target tissue without causing complications, which is actually to determine the endpoint of treatment.
Human hair is cyclical, and each cycle includes a growth phase, a regression phase, and a resting phase. Generally speaking, the growth period of hair is about 3 years, the regression period is about 3 weeks, and the resting period is about 3 months. The growth of hair follicles in different parts is not synchronized and has its own cycle. The length of hair in different parts is related to the different growth cycle time. The growth period of eyebrows and eyelashes is only about 2 months in the growth period. The hair grows actively, the hair matrix cells divide rapidly, and the melanin is the most. It is extremely sensitive to lasers. During the degenerative phase, the hair matrix degenerates, cell division stops, capillary nutrition is significantly reduced, dermal papilla atrophy, melanocytes in the hair bulb stop producing and transporting black Qin to keratinocytes, which can last for several weeks. In the telogen phase, the hair follicles separate from the dermal papilla, and the hair falls out, which can last from several months to a year. With the reconstruction of the undifferentiated epithelial cells of the dermal papilla, the hair begins the growth phase, and new hair matrix and hair bulbs are formed, and the melanocytes in the hair bulbs also return to normal functions. The hair follicles in the early stage of growth are small, the growth part is shallow, and the most fragile. In the middle and later stages of growth, the hair follicle is the largest and deepest. The hair follicles that have just emerged from the telogen phase and the hair bulbs are shallower, which will affect the number and duration of laser hair removal treatments. The balls stop producing melanin in the degenerative and resting phases, and the laser has no obvious effect on the hair in these two phases. The laser can only work after the melanocytes have recovered their function in the anagen phase, so laser hair removal requires multiple treatments. If the proportion of hair in the growth period of a certain part is small, the number of treatments should be relatively more, on the contrary, the number of treatments can be reduced. Hair in different parts has different growth cycles, so the interval between each treatment is also different. For example, the hair on the upper lip has a relatively short resting period, so the interval can be as short as a month, and the resting period of the trunk and limbs is relatively long. Therefore, the treatment interval should be about 2 months, and the depth of hair follicles also varies according to the location. If the depth is deeper, a longer wavelength laser treatment should be used. On the contrary, if the depth is shallower, a shorter wavelength laser treatment can be used. If the hair follicle density is too high, the laser energy should be appropriately reduced during treatment.
1. According to the three basic conditions of the theory of selective photothermal action, we first determine the wavelength of hair removal.
1. The hair bulb and the hair bulge are where the hair follicle stem cells are located. The photon must be able to reach this depth to be effective. As shown in the figure above, only red light and infrared light match.
2. If the wavelength is too short, the melanin of the epidermis will compete with the melanin of the hair follicle to absorb more energy, thus easily causing epidermal damage.
3. Therefore, the wavelength of the hair removal laser equipment is preferably above 600nm. Commonly used clinical wavelengths are 755nm, 808nm, 1064nm. In addition, the photon hair removal technology is the treatment in the band of 640nm-1200nm. These wavelengths selectively heat the deep hair shaft, follicle epidermis, and pigment-rich matrix.
Second, the choice of pulse width of laser hair removal equipment.
Hair Removal Laser Pulse Width Determination
①The pulse width of the hair removal laser should be greater than the thermal relaxation time of the epidermis (0-10ms), otherwise it will easily cause epidermal damage.
②By lengthening the laser pulse width, the thermal relaxation time close to the hair follicle can improve the curative effect. The melanin in it is more susceptible to damage.
③The diameter of the hair follicle is 200-300um, and the thermal relaxation time of the hair follicle is 40-100ms. Therefore, the ideal laser pulse width should be in the range of tens of milliseconds to 100ms
④ The hair follicle stem cells lining the outer hair root sheath do not contain pigments, but are important targets for long-term hair removal. The thermal damage range is extended to the entire hair follicle with pulses with a width greater than the heat relaxation time of the hair shaft, so as to be more completely destroyed. Hair follicle stem cells.
Laser hair removal equipment appropriately selects the pulse width and energy of the laser to heat the target tissue. During this process, the larger tissue is heated and destroyed, while the smaller tissue is only slightly heated and remains intact. When the pulse width is short, the temperature of both large and small particles increases after laser irradiation. Small particles dissipate heat relatively quickly. The temperature of large particles is higher than that of small particles, but neither reaches the breaking point
When the pulse width is long, the temperature of large and small particles continues to rise, the temperature of large particles (hair follicle melanin particles) exceeds the damage point and is damaged, while the temperature of small particles (epidermal melanin particles) does not reach the damage point and remains intact.
When the pulse width is short, the energy released by the laser machine in an instant causes the temperature of the tissue around the hair follicle to rise rapidly, and because it is too late to dissipate heat, it will be destroyed at the same time, which is the result of skin treatment damage. In this same energy state, the pulse width used is longer, which can avoid this situation. Only when the temperature of the hair follicle exceeds its destruction point, and the temperature of its surrounding tissue is still lower than its destruction point to remain intact, the real purpose of the treatment can be achieved.
According to the second condition in the theory of selective photothermal action, the laser pulse duration (pulse width) should be less than or equal to the thermal relaxation time of the target tissue. So the most effective pulse width should be about 30ms. However, some studies have found that for dark-skinned patients (Indian skin or African skin), using a laser with a pulse width of more than 100 ms not only has better efficacy but also has fewer adverse reactions. Therefore, the actual pulse width range of the hair removal instrument should be 10ms-200ms (no matter how large the pulse width is).
Determination of clinical endpoints: Selecting the appropriate energy that can destroy the target tissue without causing complications is actually the determination of the treatment endpoint. The choice of laser energy density for hair removal should be based on the principle of "individualized dose", starting with a small dose of treatment, and gradually increasing the energy, so that after treatment, the patient has a mild burning sensation, and local skin appears reddish and other hyperemia. The small edematous papules around the hair follicles with an "orange peel-like" appearance are the end points of the treatment. If there is a large erythema, it means that the energy density is too high.
To sum up: 808 laser hair removal equipment hair removal is mainly: wavelength selection, pulse width and energy density settings, as well as the judgment of clinical endpoints. It is very important to adjust the corresponding parameters according to the skin color and different parts of different patients.