In recent years, with the increasing attention to body hair, there are more and more plastic methods of laser hair removal. The following is the background knowledge and current research in this field.
In 1963, Goldman first proposed the method of destroying colored hair follicles with ruby laser. Ohshiro and Maruyama also found that hair removal can occur after treatment of giant nevus with ruby laser, but it is also accompanied by serious epidermal damage. The theory of “selective photopyrolysis” has solved this problem and become the theoretical basis of laser hair removal.
Selecting the laser with appropriate wavelength and ensuring that the action time is shorter than the time when heat diffuses from the target tissue to the surrounding tissue can selectively destroy the target tissue. There are structures in the skin called chromophores, which are responsible for absorbing light energy. Endogenous chromophores include melanin, hemoglobin and oxygenated hemoglobin. Exogenous chromophores, such as tattoo pigments. Chromophores can absorb light energy and produce thermodynamic, chemical and mechanical effects on the skin.
In order to achieve the effect of permanent hair removal, it is necessary to destroy two areas of hair follicle: hair ball and swelling part, because these two areas include hair mother cells, which can form a complete hair follicle. Melanocytes are in or around these two areas. Laser can effectively target melanin and destroy these two regions. In addition, there are ways to make hair follicles absorb exogenous chromophores to increase the sensitivity of hair follicles to laser. Pigment absorption of light energy will lead to local hair follicle thermal coagulation necrosis.
The photothermal effect of laser is affected by three factors: wavelength, pulse duration and optical energy density. The wavelength should be the longest light in the range with the strongest absorption of melanin, because the wavelength of the laser determines the depth of penetrating the skin. The longer the laser wavelength, the deeper the depth. The pulse duration should be shorter than the thermal relief time required by hair follicles to minimize indirect thermal damage. The optical energy density must be greater than or equal to the threshold of tissue destruction.
The melanin of the epidermis will absorb energy competitively with the melanin of the hair follicle, so the cooling of the epidermis should be done well, especially for patients with darker skin. Contact skin cooling should be performed before, during and after operation. Refrigerant spray can also be used before treatment. Cooling measures can effectively reduce the occurrence of postoperative pigmentation, blisters and scars.
On the premise of reasonably protecting the epidermis and superficial dermis, higher energy laser can be used. The absorption spectra of melanin and oxygenated hemoglobin to different wavelengths of light were studied. It was found that the absorption rates of melanin and oxygenated hemoglobin in longer wavelength laser for hair removal were similar. Therefore, the skin should be squeezed during hair removal to eliminate the interference of blood in capillaries.