A number of lasers systems are now available for hair removal (Table 7.2), as the absorption spectrum of melanin within hair follicles extends from the ultraviolet to the near-infrared wavelengths [1,15,23,53]. The most simple classification system divides the lasers according to wavelength: short, intermediate, or long. The laser surgeon must choose a short wavelength technology (500-800 nm) for light skin phenotypes with light brown/blond hair of thin diameter, whereas longer wavelengths (800-1200 nm) would be used for darker skin phenotypic individuals who have coarse dark brown/black hair [9].
Most of the methods intended for long-term hair removal use optical energy delivered to the tissue and absorbed mostly by the hair shaft, while the epidermis and surrounding tissue have minimal absorption [24]. There are two kinds of light sources that provide sufficient energy to achieve thermal destruction of hair: lasers and intense pulsed light. Laser light energy is monochromatic, and can be selectively absorbed by the hair shaft at the skin depth of a few millimeters. Intense pulsed light is composed of a broad spectrum white light of output wavelengths (500-1200 nm) that can be tuned to achieve better results by filtering out some parts of the spectrum generated by the light source in accordance with skin pigmentation [25-27].
However, various limitations on the laser and IPL modalities curtail their utilization capacity. The effective removal of unwanted hair using optical energy has been essentially limited to black, dark, and medium tones of brown hair [28]. Light energy must penetrate
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Table 7.2 Indications for Short, Intermediate, and Long Wavelength Laser Hair Removal
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through the epidermis first to reach the hair follicle, thereby creating a major barrier for penetration. The benefits of high energy fluences are countered by the risks of damaging the epidermis. One limitation is the treatment of darker skin types. Even though advances in technology and development of longer wavelength lasers such as the 1064 nm Nd:YAG have enhanced the ability to treat darker skin types, the high concentration of endogenous melanin within the epidermis of these individuals increases the risk of crusting, blistering, and dyschromia [29-31]. An additional drawback of these technologies is the inability to treat light pigmented hair, such as red, blond, gray, and white, which do not contain high concentrations of melanin [8].
Light-based technologies have become restricted by the limitations placed on their methodology. The innovation of radiofrequency energy (RF), however, has brought new life to the optical-based systems, as this energy modality is not sensitive to melanin concentration in the shaft or the epidermis. High-frequency current in the range of 0.3-10 Megahertz, or RF current, produces a pure thermal effect on biological tissue that is dependent on the electrical properties of the tissue [32]. The high efficiency of RF current for tissue heating has made it useful for electrosurgery, and an attractive source of energy for various dermatologic applications [33,34]. The mechanism of tissue heating is based on generating joules of heat by electrical current. The heat generated is described by Joule’s law:
H=fb,
where jis the density of electrical current, and s is electrical conductivity [35]. Impedance is the value that opposes conductivity. The RF component generates heat from a current of ions that acts according to the physical principle of impedance, that is, electrical current will always follow the path of least resistance [33,36]. For example, blood has a very high electrical conductivity; therefore, it has low impedance. Bone has a very low electrical conductivity or high impedance. Electrical current will always follow the path of highest conductivity (lower impedance); therefore, it does not penetrate bone, but will flow around it. Impedance is also directly, but inversely, correlated with heat, as shown by the equation mentioned earlier [20]. Higher temperatures produce lower impedance and therefore direct the flow of current [37].
Electrical conductivity depends on the frequency of electrical current, type of tissue, and its temperature. The distribution of electrical current depends on the geometry of the electrodes. Typically, two configurations are used in medicine, either monopolar or bipolar. The major difference is how the RF current is controlled and directed at the target. However, there is no difference in the ultimate effect at the same RF fluence [ 20 ].
The bipolar system is most commonly used in conjunction with light energy during laser hair removal, examples which include the Aurora and Polaris systems (Syneron Medical Ltd., Yokneam, Israel). A bipolar system passes an electrical current between two electrodes at a fixed distance. Both electrodes are applied to the treated area, and electrical current propagation is limited by the area between electrodes. The behavior of electrical current in a bipolar system is depicted in Fig. 7.1. The penetration of electrical current can be estimated as half the distance between the electrodes, for instance if the electrodes are placed 8mm apart, the penetration depth is about 4mm [20].
The technology presented in this chapter utilizes a new approach combining either IPL or diode laser with conductive RF modalities simultaneously applied to tissues (Table 7.3) [37].
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Electrodes Figure 7.1 Schematic representation of the flow of electrical current through the epidermis using a bipolar system. The geometry of the radiofrequency (RF) electrodes is designed to deliver the RF current at a depth of 4 mm, which can target the deepest hair follicles in all anatomic locations. |
Its advantages in initial clinical trials show results in areas where purely light-based systems have not shown efficacy; that is in the treatment of light hair and dark skin phenotypes [20]. Both forms of energy are pulsed and delivered to the tissues with a hand-held device. In the case of the Aurora, the light source is a high-power xenon-lamp that is filtered to transmit the wavelength range of 680-980 nm. The Polaris, on the other hand, manipulates an 810 nm diode laser pulse. The conducted RF electrical energy is bipolar, and can generate energy up to 50J/cm [3,38]. The bipolar RF generator consists of flashlamp pulsed light delivered through a contact sapphire light guide, and the bipolar RF energy is delivered through electrodes embedded in the system applicator when brought into contact with the skin surface [39]. The device also includes an active dermal monitoring system that measures changes in the skin impedance, which is adjustable by the user to provide an integrated safety mechanism (impedance safety limit) to prevent overheating of the dermis. A thermoelectric cooling hand piece provides contact cooling at a temperature of approximately 5°C before, during, and after energy delivery [27].
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Table 7.3 Comparison of Combined Optical and RF Hair Removal Technologies: System Specifications
RF: radiofrequency. |
The synergistic effect of combining RF and optical energy allows for lower optical fluences to be utilized creating a safe treatment for darker skin types [34]. The treatment efficacy is not compromised due to the addition of conducted bipolar RF energy that selectively heats the hair follicle. The conducted RF selectivity mechanism is not affected by the absorption of melanin in the skin or hair shaft [31,45,46].
The combined modality system operates by using pulsed light energy to heat the hair shaft through a selective absorption of energy by the melanin in the hair shaft. Heat then dissipated to the follicle damages the hair shaft (Fig. 7.2a). The conducted RF energy selectivity mechanism is different than light, and the chromophore-independence of RF is perhaps its greatest advantage in allowing for treatment of any hair follicle, regardless of color [20,40]. The RF field for the bipolar system is controlled by impedance properties of the tissue; the current will always flow to the area of minimal impedance between the electrodes. There are two major factors that control the tissue impedance. The material of the hair shaft is mostly keratin which is not conductive; therefore the RF energy will go around the follicle directly (Fig. 7.2b). Second, as the temperature increases, the impedance of the tissue decreases. The light energy creates a preheated area inside the tissue, and directs the RF field into this preheated area versus the surrounding tissue of lower impendence. While the pulse durations are usually initiated at the same time, the RF pulse is set at a longer duration versus that of the optical energy to preheat the target and increase RF selectivity [21,27]. Using the thermal damage time over the thermal relaxation time of the hair follicle allows for longer pulses to extend the zone of thermal damage from melanin-laden structures in the central portion of the follicle to the outermost layers of the root and connective tissue sheath [41]. This heat profile created is uniform across the targeted hair follicle and shaft giving excellent results with minimal risk to the surrounding tissue (Fig. 7.2c) [42].
Based on the mechanisms described earlier, the optimal method utilized is a nearsimultaneous application of the optical and bipolar RF energies with a precooling of the epidermis [27]. The steps are summarized as follows:
1. Hydrate and cool the epidermis.
2. Apply optical energy to selectively heat the target and bipolar RF energy to the heated target. The applied energy should be at the level at which the temperature of the epidermis does not exceed the target temperature.
3. Discontinue optical pulse and continue RF pulse for additional selective heating of the target [10,15,27,43,44].
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Figure 7.2 Temperature profile of hair structure: (a) shaft is heated by light energy; (b) follicle is heated by RF energy; (c) follicle and shaft heated by combined optical and RF energies. |
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Table 7.4 Summary of Clinical Studies with Combined Optical and RF Hair Removal Systems
RF: radiofrequency; 5-ALA: topical aminolevulinic acid. |