Laser hair removal is one of the most commonly performed cosmetic procedures in the U.S. and around the world. At least a dozen of aesthetic laser companies developed hair removal systems for use in dermatology clinics. Recently, the FDA also approved a number of home-use devices: from Tria, which uses laser diodes, to Remington, Philips and others, which use the Intense Pulse Light (IPL). Despite claims that many of these companies make with regard to the efficacy of the laser hair removal using their particular system, none of them produces “permanent” hair removal. Moreover, the outcome of the treatment will depend strongly on the skin type and hair color of the patient. In order to understand the differences between various systems, let’s take a deeper look at the growth cycle of hair and the scientific foundation of the hair removal process.
There are three phases of hair growth (figure 1):
The active growth phase called anagen is characterized by rapid division of cells in the root of the hair, adding to the hair shaft. Scalp hair stays in this active phase of growth for two to six years. From 80% to 90% of the hairs on the head are in the anagen phase.
The next is a short (2-3 weeks) catagen phase during which the hair detaches from its blood supply and active growth stops.
And finally, the telogen phase, where the hair is released and the hair follicle rests for three months. After three months, the follicle goes back into the anagen phase and begins to grow new hair.
Because of the intense division of the cells, the bulb at the bottom of the follicle during anagen is much bigger than in the other 2 stages (Figure 2). Histological observations show that all current laser systems inflict damage to the follicle only during anagen, while hair follicles with small (<25mm) shafts do not demonstrate any morphological change. This is why laser hair removal must be done in several separate treatments over 3 to 6 months spaced out over several weeks since, at any given time, hair follicles are in one of these three stages.
There are three different mechanisms that can destroy hair follicles upon laser light exposure: 1) Photothermal, 2) Photomechanical, and 3) Photochemical. All three had been utilized for hair removal, with varying degrees of success.
Photothermal destruction
The idea behind this method is to deposit enough energy into the hair to destroy it without collateral damage to the surrounding tissue (a.k.a. selective photothermolysis). Therefore, there are three specifications of the laser system that govern this process: a) output power of the laser – it should be powerful enough to destroy the hair follicle, b) wavelength of the laser – it should be strongly absorbed by the hair follicle and shaft, but not by the surrounding tissue, and finally c) pulse duration of light emitted by the laser – it should be short enough to destroy the hair without causing the surrounding tissue to be heated to unsafe temperature levels.
The mean length of a scalp hair follicle is 4.16mm (Figure 3), therefore the laser light should be able to reach down to the bulb ~5mm. The penetration depth of light into the tissue strongly depends on the wavelength, and it has its maximum in the near-infrared (NIR) range of wavelengths from 650nm to 1350nm, also known as a therapeutic window. On the other hand, melanin, a natural pigment present in the hair, has a relatively strong absorption of light in this range (Figure 4).
Lasers or light sources that operate in the red or near-infrared wavelength region (694-nm ruby laser, 755-nm alexandrite laser, diode lasers, 1064-nm Nd:YAG laser, and noncoherent light sources (Intense Pulse Light, IPL) with band-pass filters) all lie in this window of the spectrum in which selective absorption by melanin is combined with deep penetration into the dermis. Therefore, selective heating of the hair shaft, the hair follicle epithelium, and the heavily pigmented matrix is possible in the 600 nm to 1350 nm region.
Besides the wavelength, the duration of the light pulse plays an important role. In order to achieve thermal damage, the pulse duration should be shorter or equal to the thermal relaxation time of the hair follicle, which is estimated to be approximately 10-100 milliseconds, depending on size.
One has to keep in mind, though, that melanin is also present in the epidermis. Therefore, the light will be absorbed by the skin as well. The thermal relaxation time of 80–100 μm thick epidermis is in the range of 5-10 ms. Thus, selecting the pulse duration for the treatment can have a significant effect on the efficacy and pain level. Recent studies of Super–long-pulse (>100 milliseconds) appears to allow for long-term hair removal.
Therefore, ideal devices for hair removal should have the ability to select the wavelength and the pulse duration. None of the solid-state lasers available in the market can fulfill these requirements, while semiconductor laser diodes can be developed for a particular wavelength, and their pulse duration and repetition rate are easily controlled by electronics.
Photomechanical destruction
The physical mechanism behind photomechanical destruction of hair is based on the absorption of very short pulses of light (nano- and picosecond in duration). When such a short pulse is absorbed by endogenous (melanin) or exogenous (carbon black particles, or hair dye) chromophore, the rapid heating of the chromophore occurs causing a quick expansion in its volume that creates shock waves, which in turn destroy a small volume of the surrounding tissue. However, studies show that this mechanism destroys only the melanocytes but not complete follicle. The use of exogenous chromophores is even less efficient, as the penetration of these chromophores into the hair shaft is limited in depth. This explains why, despite their ubiquity and wide use for tattoo removal and other aesthetic procedures, Q-switched ruby and Nd:YAG lasers have never demonstrated good results in hair removal.
Photochemical destruction
Photodynamic therapy (PDT) is an active area of clinical research which uses drugs called photosensitizers in conjunction with irradiation of the treated area with light to produce therapeutic effects. PDT is an FDA-approved technique that has been successfully applied for treating certain forms of cancer. The mechanism of action involves selective bonding of the drug molecule to the cancer cells, activation of the photosensitizer that results in the generation of toxic reactive oxygen species, which in turn kill the adjacent cells. PDT may become a very useful approach for hair removal. Because photosensitizers are localized in the follicular epithelium, photochemical destruction of all hair follicles, regardless of hair color or growth cycle, could potentially be achieved. A distinctive feature of PDT for hair removal is that, unlike cancer treatment, where systemic agents are used, the hair-removal photosensitizers can be applied topically. The discovery of aminolevulinic acid (5-ALA) as a topical photosensitizer has opened a potentially very fertile area of therapeutic options using PDT. In a pilot study, a mean hair loss of 40% was reported in 12 volunteers subjected to a single exposure (200 J/cm2) of 630 nm light 3 hours after an application of 5- ALA to the skin. In another study, a different photosensitizer called Rose Bengal was used to selectively destroy lighter or white hairs. 15 volunteers were subjected to three sessions of PDT within a 4- to 6-week span. Six months after the last treatment, the average reduction of white hairs was observed to be 40%. A host of other porphyrins, chlorins, phthalocyanines, purpurins, and phenothiazine dyes can act as photodynamic agents and are under development as drugs for photodynamic therapy.
However, this technology is still in its nascent stage, and long-term clinical studies are necessary to assess the safety and long-term efficacy of this modality.
Various light sources were employed for hair removal. These include ruby, alexandrite, Nd:YAG, and diode lasers, as well as intense pulsed light sources. While all of them had demonstrated utility in hair removal, there is a significant difference in the efficacy, and there is no system that works equally well for different patients.
Intense Pulse Light
These pulsed, noncoherent broadband light sources were touted by many manufacturers as the “ultimate” hair-removal tool suitable for all patients. By placing appropriate filters in front of this light source, wavelengths from 590-1200 nm can be selected. Also, the pulse durations can be selected from a few milliseconds to a fraction of a second, and the repetition rate can be easily chosen. The wide choice of wavelengths, pulse durations, and repetition rates allow for a potentially effective treatment for a wide range of skin types and hair colors. However, the power density of these sources is significantly lower than that of the lasers. In most cases, only the hair shaft is destroyed without a debilitating injury to the follicle. It appears that the main motivation of the system manufacturers for employing these sources is their cost, which is significantly lower than the laser.
Ruby lasers. Grossman had pioneered laser hair removal using this type of laser. Thirteen patients with fair skin and dark hair received a single treatment with fluences in the 20-60 J/cm2 range. All subjects demonstrated a delay in hair growth for 1 to 3 months, and more than 50% had experienced a persistent hair loss, which was greatest in sites treated at the highest fluence. Additional studies with larger numbers of patients have confirmed that hair counts are reduced by approximately 30% after a single treatment with the ruby laser. The effect of multiple treatment sessions is additive; hair counts are reduced by approximately 60% after 3-4 treatments. However, because of a rather short wavelength (694.3 nm) of the ruby laser, it is only efficient for patients with fair skin (Fitzpatrick phototype I-III). The occurrence of cutaneous side-effects such as blistering, hypopigmentation and hyperpigmentation is common with the darker tone skin patients. Also, a slow repetition rate makes for longer treatment, and realistically only suitable for small areas.
Alexandrite lasers emit at longer wavelengths (755 nm) than the ruby laser, which results in greater depth of penetration. Therefore, more energy is deposited in the dermis compared with the epidermis. The risk for epidermal damage in persons with darker skin types is therefore reduced, and they can be used for patients with olive tone skin. It also can have much faster repetition rate. These features made alexandrite lasers, the staple of hair removal technology in the early 2000s. However, it can cause hypopigmentation in darker skin patients, and the procedure is more painful.
Nd:YAG lasers emit deeply penetrating 1064-nm wavelength. They are available as Q-switched lasers that output a few nanosecond pulses of light as well as long-pulse lasers with pulse duration of 15–30 ms. The reduced melanin absorption at this wavelength requires high fluences up to 50 J/cm2 in order to adequately damage hair. However, poor melanin absorption at this wavelength coupled with epidermal cooling makes the long-pulsed Nd:YAG potentially a safe laser treatment for darker skin types, up to VI. Although capable of inducing a growth delay, it appears that Nd:YAG lasers are ineffective for long-term hair removal.
Diode lasers. Over the past two decades, the technology of high power diode lasers has advanced to the point where their operational parameters have surpassed the ruby, alexandrite, and Nd:YAG lasers. They are also far less expensive, far more efficient, and have longer lifetimes.
Currently, only 800nm diode lasers are used for hair removal. They have proven to be very effective for removal of dark hair. Because these lasers operate at a longer wavelength, darker skin types can be treated more safely especially when coupled with active cooling devices.
The biggest advantage of diode lasers is that they can be made for any wavelengths in the therapeutic window, thus allowing for selecting the most efficient wavelength for the patient’s hair color and skin tone. In addition, a combination of several wavelengths can be used to achieve better efficacy. They can be manufactured with the output power levels of several hundred Watts, and their pulse duration can be easily adjusted between 0.1 ms and 300 ms, which is the typical pulse width for laser hair removal.
AKELA laser is proud to offer a wide range of high-power laser diode products for different modalities of laser hair removal. We offer single- and multi-wavelength high-power laser modules that will meet the most challenging requirements of system manufacturers. We also have products specifically designed for use in Photodynamic Therapy.
For detailed technical specifications on the diode lasers offered by AKELA, send us a request at [email protected]. Our laser experts will be happy to discuss your application.