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PHOTOEPILATION Photoepilation — hair removal with laser or lamp light sources — is considered nowadays to be the most effective and safe way to remove excess or unwanted hair. This method of hair removal works using the principle of selective photothermolysis in which a carefully timed pulse of laser energy is delivered to hair shafts and bulbs in such a manner as to destroy hair follicle while sparing the surrounding skin structures. A scheme of hair structure is shown in Figure below. The bottom part of the hair follicle contains papilla which is fed by numerous blood-vessels. The hair follicle is sided with a sebaceous gland, a sweat gland and an arrector pilii muscle which lifts the hair. Both hair shaft and follicle contain melanin – a pigment which gives hair its color and can absorb emission of certain wavelengths.
Effective absorption of melanin occurs within the range between 600-1100 nm. Wavelengths under 600 nm are strongly absorbed by blood microvessels and therefore cannot be used for hair removal; wavelengths above 1100 nm are strongly absorbed by water in tissue. The wavelengths available in the 600-1100 nm spectral range are: 694 nm (Ruby laser), 755 nm (Alexandrite laser), 800 nm (diode lasers), 1064 nm (Nd:YAG laser) and wideband pulsed light sources (550-1200 nm). The present-day epilation market offers all the above light sources
thus emphasizing ambiguity of approaches applied to hair removal
by laser manufacturers, dealers and practicing cosmetologists.
Commercial models of hair removal light sources possess extremely
wide range of parameters: wavelength, pulse duration, fluence,
pulse repetition rate and beam diameter differ from model to model,
each individual system having practical experience in treatment
of patients with certain skin types, hair color and hair follicle
depth. Table 1
Does a universal system to treat a larger number of patients really exist? No reply can be given to this question without studying some basic principles on which skin and hair structures absorb light. Under the theory of selective photothermolysis, selective light absorption is mainly influenced by a wavelength applied. The first and most significant condition in choosing laser wavelength is penetration depth which should be sufficient to target hair bulbs typically resting at 2-4 mm depth. Literature studies show that penetration of laser energy into the skin is influenced by the following major factors: a) light dissipation in the skin; authors (1, 8, 9) claim that laser energy dissipation coefficient teadily decreases with increasing wavelength (see Fig.1) thus providing deeper penetration of longer wavelengths. 
Fig.1. Absorption and scattering coefficient of
dermis with 1% of blood content b) laser energy absorption by melanin present in skin and hair structures; absorption coefficient is also decreasing with increasing wavelength (Fig.1 and 2). There is a slight peak at 980 nm due to absorption of energy by water. While at short wavelengths the dependence of laser light absorption on hair color is strong enough, at wavelengths higher than 1000 nm it diminishes significantly (see Fig.2). 
Fig.2. Light absorption by black and blond hair as a function of wavelength. Thus, shorter wavelengths (Ruby, Alexandrite, major spectrum of a xenon lamp) are actively absorbed by hair what makes such photoepilators more preferable at first sight. However, melanin contained in superficial skin layers, although less concentrated than in hair, absorbs short wavelengths strong enough to cause side effects during epilation. Because of strong absorption in superficial skin layers short wavelengths can only reach hair shafts and not deeply laid bulbs. Therefore Ruby and Alexandrite lasers are better used for targeting undeep hair bulbs in treatment of light-skinned patients (Fitzpatrick type I and II). Furthermore, when dark-skinned patients (Fitzpatrick types IV-V) are treated with 500–800 nm laser energy competitive absorption of light by hair and skin may cause singes and pigmentation problems in the targeted tissue. The aforesaid equally refers to lamp epilators - pulsed light sources generating in the whole spectrum range of maximum absorption by melanin-bearing skin components 500 (590)…1200 nm. Long wavelengths emitted by a lamp photoepilator (900 … 1200 nm) penetrate into skin deeply enough. Yet the main part of lamp epilator energy falls within 500–900 nm, and this range is very strongly absorbed by melanin in the epidermal skin layer. Thus, lamp epilators as well as Ruby, Alexandrite or diode lasers may be dangerous in treatment of IV-VI Fitzpatrick types, and their use is restricted by seasonal limitations and probable pigmentation disorders in treatment sites. Filters used to "cut off" short wavelengths considerably degrade lamp performance.
Nd:YAG systems generate 1064 nm wavelength which is much less absorbed by melanin of skin and therefore much safer. On the other hand, Nd:YAG laser energy is capable to pass deep into the skin (up to 4 mm) and target hair bulbs at different depth regardless the skin color. Curve approximation at 1064 nm area (Fig.2) allows to speculate that absorption coefficient of 1064 nm laser energy is not greatly dependent on hair color providing an opportunity to unify treatment modes for patients with different hair color and skin types. This factor encourages wide use of Nd:YAG lasers as versatile devices for hair removal which may be applied to people with various phototypes, without any seasonal or race limitations. Another important factor influencing selection of a light source for photoepilation is the level of photothermolysis selectivity, i.e. proportion between temperature of a heated hair follicle and adjacent epidermis layers. The main reason for difference in temperature of hair and skin is unequal concentration of absorbing chromophore — melanin; hair usually contains more. To efficiently remove hair with different follicle depth the photothermolysis selectivity level should be sufficiently large over the whole site of light diffusion in the skin. Research shows (Fig.3a and 3b, (1)) that the photothermolysis selectivity level rises with wavelength increase, and if the discussed range within 690-1000 nm best fits for removing hair much darker than skin, only wavelengths higher than 800 nm are able to make follicle temperature exceed temperature of epidermis in cases when the hair is only slightly darker than the skin. This primarily applies to hair with deeply laid follicles. Analysis of the stated factors shows that only wavelengths longer than 1000 nm (Nd:YAG systems) allow the ratio between follicle and epidermis temperatures to be higher than 1 at 3-4 mm. 
Fig.3a. Ratio between hair follicle temperature
(TH) and epidermis temperature (TE) 
Fig.3b. Ratio between hair follicle temperature
(TH) and epidermis temperature (TE) Thus, thermal features of 1064 nm laser energy absorption by skin and hair prove true the eventual wide use of Nd:YAG laser systems as versatile devices for hair removal which may target unwanted hair in patients with any skin type and follicle depth, without seasonal or race limitations. The aforesaid encouraged SOLAR LS - a company manufacturing various laser systems on ruby, alexandrite and yttrium-aluminum garnet - to choose an Nd:YAG laser medium as the most versatile and safest hair removal system. The result of scientific and design research performed by SOLAR LS together with practicing cosmetologists is the DeLight — a laser epilator on yttrium-aluminum garnet which proves to be a unique combination of optimal characteristics for efficient hair removal, versatility of use on a wide range of patients, and moderate cost. It is obvious that any laser medium fulfills its advantages only if the optimum treatment conditions such as spot size, pulse duration and fluence value are observed. We will try to give a brief explanation on factors that determined a selection of the above mentioned parameters for DeLight. Spot size. The selection of a spot size is influenced by two major reasons: a) a large spot size allows to reduce a number of flashes and lessens treatment time; b) authors (5) claim that penetration into skin depends not only on wavelength applied and skin color treated but also on the size of a light spot. If a treated spot is smaller than penetration depth light is dissipated and penetration decreases. Figure 4 reveals that Ruby laser energy fluence is reduced to 20% at 2,6 mm with 1 mm spot diameter and at 3,5 mm with 5 mm spot. 
Fig.4. A 694 nm light fluence distribution at two different spot sizes. Spots with 5 mm diameter are therefore to be used in order to reduce dissipation effect. The Nd:YAG laser system is powerful enough to meet this requirement without any complications. Due to the high output DeLight offers a possibility of varying treatment spots within 5-13 mm range, this parameter being the highest among currently available laser epilators. Energy fluence. Laser energy per pulse should be sufficient to provide 25-50 J/cm2 fluence for maximum spot size (1). The DeLight laser system features maximum energy value in 80 J pulse thus satisfying the above requirement. Pulse duration is a parameter which allows fine adjustment of "milder" treatment modes ensuring maximum efficacy and making the treatment absolutely painless. The point consists in making treatment more selective due to optimization of pulse duration. A choice of pulse width is determined by the thermal relaxation time (TRT) — the time it takes for 50% of energy to be conducted away from a target tissue. When treatment is aimed at damaging a target then pulse duration should be less or equal to the target's TRT. To avoid overheat the tissue should be treated with pulses greatly exceeding its TRT. Epidermis is the first to be exposed during laser epilation and may be easily damaged by high-power energies so pulse duration should exceed its thermal relaxation time. Yet to destroy hair follicles laser energy should be applied in a period shorter than the follicle TRT. The TRT for the follicle is estimated to be 40-100 msec (4, 6, 7) depending on its diameter while epidermis has 1-10 msec TRT. Therefore, a hair removing device must use pulses longer than 10 msec but shorter than 100 msec. The optimum pulse duration will be 40-50 msec. Authors (1) illustrate time dependence of epidermis and hair bulbs temperature when targeted by 3 short pulses <7 msec with 20 msec interval (see Fig.5). 
Fig.5. Temperature behavior in the epidermis and in the hair bulb during a triple pulse. The figure clearly shows that by the end of the third pulse the bulb temperature increases to TH~80°C and epidermis temperature is TE~58°C. Most laser and lamp epilators currently available at the market provide one-digit millisecond pulse duration, while "long-pulsed" systems are more advantageous as they allow best results with minimum heating of superficial skin layers. This, on one hand, makes skin cooling almost unnecessary, on the other hand, allows to treat patients whose skin and hair colour is least contrasted. The DeLight laser epilator gives a possibility to select pulse duration within 1 to 50 msec. Thus, specific features of the DeLight laser epilator present an ideal combination ensuring effective hair removal in I-V phototypes due to the right choice of energy fluence, spot diameter and "long" pulses. Thermal safety, minimum treatment time and clinical efficiency are characteristic for hair removal process performed with the DeLight laser system. For more information on the new DeLight long-pulse Nd:YAG laser system click here References: 1. G. Lask, S. Eckhouse, M. Slatkim et.al.
The role of laser and intense light sources in |
