In recent years, with the rapid development of laser science in medical cosmetology, laser cosmetology has become a new fashion in cosmetology, and some skin lesions that were difficult to cure or not effective in the past can now receive very good results. And as one of the laser beauty light hair removal technology with its wide publicity and application has also been gradually recognized by patients.
With the development of society, more and more new technologies have come to people’s lives, and light hair removal technology is becoming increasingly mature as one of the fastest developing technologies of laser cosmetic treatment in recent years. So what is the principle of light hair removal technology?
I. Overview
In 1996, the first laser hair removal equipment was introduced to the market (ruby laser equipment). At present, the laser devices involved in the clinic are Ruby laser (Ruby, 694nm), Emerald laser (Alexandrite, 755nm), Diode laser (Diode, 810nm), Nd:YAG (1064nm), Electro-optical Synergy, ELOS (combination of IPL and (a combination of IPL and RF electromagnetic waves) and the more recently used intense pulsed light (IPL, 600-1200nm).
II. Hair Biology
(A) Hair follicle
The hair follicle is composed of three parts. The first part is the funnel, which is the part between the pore and the opening of the sebaceous gland. The second part is the isthmus, which is the part between the opening of the sebaceous gland and the attachment point of the erector spinae. The third part is the inferior segment, which is the part between the attachment point of the erector spinae and the base of the hair follicle (neurovascular system).
(ii) Hair
The hairs are morphologically divided into three types, the first is the fetal hair, which is a fine hair covering the fetus and fades away in the neonatal period, the second is the fine hair, which is non-pigmented and its diameter is about 30 μm. The third is terminal hair, which is a common type of hair with a diameter of about 150-300μm. The three types of hairs can be transformed into each other; puberty is the change from fine hair to terminal hair, and androgenic alopecia is the change from terminal hair to fine hair. The color of the hair is determined by the amount of pigment in the hair shaft, containing two types of pigments, true melanin (brown-black pigment) and eumelanin (red pigment), with melanocytes located in the upper part of the hair bulb and in the outer myelin sheath of the funiculus.
There are two types of hirsutism, the first is hirsutism, which is characterized by an increase in hair on all parts of the body, independent of androgen levels. The second is hirsutism in women, which is characterized by excessive hair growth and distribution beyond the normal physiological range in women compared to women of the same ethnicity and age, with a masculine tendency, and is associated with androgen levels.
Hair growth is generally divided into three cycles. The first is the anagen phase, accounting for about 80%-85% of the total hair, which is the active hair growth phase, with rapid division of stromal cells, formation of hair trunk, and high melanin content, which determines the length of hair (hair can be as long as 6 years), and the second is the degenerative phase, accounting for about 2% of the total hair, with mitotic cessation of stromal cells and significant reduction of nutrition in capillaries, generally lasting 3 weeks. The third is the resting period, accounting for about 10%-15% of the total amount of hair, this period of hair loss generally lasts for 3 months, or even up to a year, to determine the number of hairs, the periodic growth of hair pigment content is different on the effect of hair removal has a great impact.
Three, the basic principle of hair removal
(A) Selective photothermolysis
Selective photothermolysis is the use of skin lesion tissue to a specific wavelength of light radiation selective absorption characteristics, the absorbed light energy into heat energy. In the process of hair removal treatment, the heat generated by the chromophore (mainly melanin) is transmitted to the biochemical cells, which will destroy the biochemical cells and thus play the role of permanent hair removal.
1. Chromophore: mainly melanin. The melanin distributed in the hair follicle and hair shaft is abundant, distributed between the cells of the hair bulb stroma and can be transferred to the structures of the hair shaft (such as the medulla, cortex and hair microdermis), absorbing red light or near-infrared light.
2. target base: it is the hair follicle stem cells. Located in the hair bulb and the bulge above it, i.e. in the outer root sheath of the hair follicle between the opening of the sebaceous gland and the attachment of the hair follicle of the erector spinae.
Selective photothermolysis requires an appropriate pulse width for the diffusion of heat, so a pulse width greater than or equal to the thermal relaxation time of the hair follicle is generally chosen for treatment during hair removal. Thermal relaxation time (TRT) is the time required for half of the heat to be dissipated, which is about 10-100ms, and thermal damage time (TDT, TDT≥TRT) is the time for the heat to continue to cause damage to a target base.
The energy density setting in the treatment process is mainly related to the skin color, the lighter the skin color the higher the energy density used, the darker the skin color the lower the energy density used. In the treatment process, choose as large a spot as possible, so that it can reduce the scattering of light in the dermis can reduce scattering, with faster speed and better hair removal effect.
(B) Epidermal cooling
Epidermal cooling protects the epidermis and allows the application of higher optical parameters while relieving the pain during the treatment. The most common form of epidermal cooling is the application of ice or cold gel to the skin surface to cool the skin. Contact Cooling (Contact Cooling) is the close contact with the skin by means of a cold sapphire tip (10°C-10°C), which allows the skin temperature to be reduced from 30°C to 20°C. Convective air cooling (Convective air cooling), through the device to give high-speed convection of cold air (about 10 ℃) can make the skin temperature from 30 ℃ to 20 ℃. Dynamic cooling (Dynamic Cooling, Cryogen Spray Cooling), can make the skin temperature from 30 ℃ down to 0 ℃, spray coolant generally choose tetrafluoroethane (Tetrafluoroethane, HFC134, boiling point 26 ℃).
(iii) Photomechanical action
Photomechanical action is achieved by Q-switched Nd:YAG 1064nm laser and toner (selective addition), which can heat the chromophore (melanin) very quickly and produce photoacoustic shock waves, thus destroying melanocytes, but often cannot completely destroy the hair follicle, and white hair rather than permanent hair reduction occurs.
(iv) Photochemical action
Photochemical action is the method taken for patients with lighter hair color. Photodynamic therapy is absorbed by the hair follicle sebaceous glands through a photosensitizer (ALA), which produces monomorphic oxygen and destroys the hair follicle, using 20% ALA (δ-aminolevulinic acid) for 3 hours, followed by a single treatment with a 630 nm laser can achieve a 40% hair removal rate, independent of hair color and growth cycle, but requires long-term observation and a large number of case studies.
Four, hair removal equipment
(A) Traditional hair removal equipment
1. Ruby laser (694nm): characterized by high melanin absorption rate, suitable for fair complexion (Ⅰ-Ⅲ), fine hair or blonde patients, high risk of treatment for dark complexion. For the original hair removal laser with large volume and slower treatment speed.
2. Emerald laser (755nm): more commonly used hair removal laser, with lower melanin absorption than ruby laser, provides the best balance of efficacy and safety for dark-skinned patients (I-IV). Has a higher repetition frequency, larger spot to reduce fatigue and pain for both doctor and patient.
3. Diode laser (800-810nm): the gold standard of laser hair removal, with simple structure, extremely high energy (2900W), equipped with contact cooling and wider pulse width (5-400ms), good hair removal effect and safer for dark-skinned patients (I-V). Slightly more painful during treatment than the former.
4. Long pulse laser Nd:YAG (1064nm): low melanin absorption, to offset this deficiency need to increase the laser energy, requires good skin cooling, can safely treat patients with various skin tones (especially type VI skin), can be used to treat pseudofolliculitis.
5. Intense pulsed light IPL (640-1200nm): millisecond pulse width, large spot, single pulse or multi-pulse mode, various different pulse delays, flexible treatment parameter settings, can be effectively applied to the treatment of various skin tones and hairs. Compared with laser, the hair removal effect is similar, but easy to cause epidermal damage. It can be applied in combination with 1064nm laser.
The first of exogenous chromophore is toner, and the second is photosensitizer ALA, which can be applied in combination with laser. According to the literature, single treatment can achieve 30% hair removal rate, while the clinical treatment effect of applying other non-laser light sources is not satisfactory. The third is MeladineTM (melanin-encased phosphatidylcholine-based liposome solution), which is an iron solution containing melanin, applied on the skin can make light colored hair follicles temporarily enriched with melanin to increase the melanin content thus improving the effect of hair removal.J De Leeuw Tanzi & Alster,2003ASDS meeting that it can retard the growth and non-permanent hair removal.
(II) Special hair removal equipment
1. Special semiconductor laser
Traditional hair removal requires the highest possible energy density, single spot (single pass) treatment, but the treatment pain is greater, the risk is higher; and special semiconductor laser has: low energy density (10_J/cm2), multiple spot (multiple pass) repeated scanning (10Hz) treatment, can reduce the treatment pain and risk, the clinical results achieved with the traditional laser Similar to traditional laser.
2. Photoelectricity integrated technology
The above figure is the photoelectricity integrated technology, as shown in the figure is the strong pulse light bipolar radio frequency collection device. Equipped with contact cooling, less dependent on the absorption of melanin, effective for white or gray hair, the respective energy is reduced to increase safety, suitable for the treatment of all types of skin, and also can be used in combination with photosensitizers to increase the effect of hair removal.
3.Photonics technology (PPT)
It was first applied to acne treatment, and now it is clinically applied to hair removal. As shown in the picture above by the airbag combined with 810nm semiconductor laser combined application.
As you can see in the figure above, the light source is far away from both the color base and the target base during traditional laser hair removal, while the photo-air technology can absorb the skin in the cavity close to the light source, shortening the distance between the target base and the light source, and at the same time reducing the concentration of skin melanin and hemoglobin due to the extension of the skin, so that the competitive absorption of light is reduced. Reduce the energy density during the treatment process, thus reducing the pain, while the larger light spot design can accelerate the treatment speed, while increasing the light penetration depth and efficacy.
(iii) Home hair removal equipment
General application of semiconductor laser, IPL destruction of hair follicles, there is no international standard specification equipment, low energy density, high safety, the results of the study was done under strictly controlled conditions, but for the lack of training, improper operation of the patient is easy to cause eye and skin damage.
(D) treatment parameter settings
Laser treatment parameters need to be set according to the quality of hair and skin conditions, generally after the completion of the settings need to first test the light spot to observe whether the parameter settings are reasonable, in the laser hair removal treatment process the ideal light spot response (as shown in the right figure) for the redness and swelling around the hair follicle, no epidermal damage performance, after treatment if the patient’s skin reaction is obvious, the need for skin cooling to reduce damage to the epidermis.
(V) Treatment effect
1, temporary hair loss: hair growth is delayed, usually lasting 1-3 months, related to the length of the resting period.
2.permanent hair reduction：After treatment, the number of terminal hairs is reduced steadily in the long term (greater than one complete hair growth cycle), which can be 4-12 months depending on the body part. Some studies also show that the treatment eventually makes the terminal hair recede into fine hair, not make the density of hair reduction.
3, permanent hair removal (permanent hair removal): generally difficult to achieve.
Hair removal effect depends on the quality of hair and energy density (skin condition), the ideal candidate for clinical hair removal: white skin, black hair, dense. Generally each treatment can reduce 30±% hair, so several treatments are needed. The treatment protocol is a total of 3-6 sessions at 1-3 month intervals (6 weeks) and most can achieve 80-89% hair removal. Animal studies show that the effect is related to the growth cycle, humans seem to be unaffected by this.
V. Complications
The first common complication is pigmentation changes, including permanent or temporary hyperpigmentation and hypopigmentation. The cause is mainly due to improper setting of laser parameters, including wavelength, pulse width and energy density, and also includes improper epidermal cooling and tanned skin of the patient. The second is scar formation, mainly caused by high energy density in the laser parameter settings, and the third is the more common folliculitis, which is also a normal reaction.