Many people want to remove the hair on certain parts of the body, including shaving, plucking, etc. Temporary hair removal methods include plucking, beeswax hair removal, chemical hair removal and shaving. These methods are relatively cheap, convenient, and easy to use, but the effect is short-lived and requires repeated treatment on a regular basis. With the advancement of technology, laser hair removal and IPL hair removal have become the main means to permanently reduce and remove unwanted hair.
Any phototherapy relies on the absorption of photons by a specific target (also called a "chromophore"). In the skin, these chromophores can include tattoo ink, hemoglobin, melanin, water, and collagen. For full body laser hair removal, the chromophore is the melanin located in the hair follicle and hair shaft. In the permanent laser hair removal process, the high-intensity pulsed laser beam (IPL laser hair removal is a broad-spectrum light source) is absorbed by melanin, and the light energy is converted into intense heat energy and diffuses to damage the hair follicles.
Although the stem cells located in the hair bulge do not contain melanin, they are also thermally damaged and destroyed due to the diffusion of heat energy in the hair follicles. This damage to the stem cells can prevent hair regeneration. For dark-skinned patients, melanin in the epidermis and the melanin in the hair follicles will compete to absorb photons, leading to poor hair removal and may also induce adverse reactions.
The effect of laser hair removal permanent depends on 3 important parameters:
1. Select the appropriate wavelength (1100-1200nm) in the visible light to near infrared (NIR) region
2. Pulse duration (in ms)
3. Energy density related to total exposure time (J/cm²)
By choosing the correct parameters, the thermal damage can be limited to the target chromophore.
Another important concept is the thermal relaxation time (TRT). TRT is defined as the time required for the target to cool to 50% of the initial temperature. In order to cause damage to the target tissue, it is necessary for the chromophore rather than the adjacent tissue to absorb photons. To achieve this, the pulse duration should be less than TRT. If the pulse duration exceeds TRT, not only will the target be damaged, but the heat energy will also spread to adjacent tissues and cause damage.
In addition, the growth stage of the hair also affects the effect of hair removal device. The ideal hair removal phase of the hair growth cycle is the active growth phase, because melanin is abundant at this time. It is estimated that if the hair is removed during the growth phase, a 20% reduction can be observed with each hair removal laser treatment.
Laser hair removal deals and IPL permanent hair removal provide a long-lasting and effective method to remove unwanted hair, with few adverse reactions and related to different skin types. Since epidermal melanin competes for energy absorption, the darker the skin is, the greater the risk of burns and abnormal skin tone is. In clinical application, attention should be paid to the standardization of operation and the prevention of adverse reactions.