Synergy of Light and Radiofrequency Energy for Hair Removal

Neil S. Sadick1 and Rita V. Patel2

department of Dermatology, Weill Medical College of Comell University,

New York, NY, USA

2Department of Dermatology, University of Miami Miller School of Medicine,

Miami, FL, USA

7.1 I ntroduction

The process of unwanted hair removal has made technological strides over the past decade to the point where permanent results can be accomplished by irreversibly damaging hair follicles via the use of lasers [1]. While less expensive methods of hair removal such as plucking, waxing, and chemical dermabrasion are still popular, there has been a strong consumer movement to longer-lasting techniques [2-5]. The search for a more enduring method with compelling results across all hair colors and skin types remains the main challenge facing the cosmetic dermatologist today. However, with the advent of nonablative devices, which have been successfully engineered to treat various aesthetic demands such

Gurpreet S. Ahluwalia (ed.), Cosmetic Applications of Laser and Light-Based Systems, 181-194,

© 2009 William Andrew Inc.

as wrinkles and displeasing pigmented lesions [6], hair removal using lasers and intense pulsed light (IPL) have begun to yield promising results and are set to become a safe, efficient, and reliable method for long-term hair removal [7-12].

Hair follicles have three main phases of growth. Anagen is the period of active growth, catagen is the involutionary stage, and telogen is the resting period. The anagen phase represents the best targeted stage for treatment, as the degree of melanin in the follicle is maximal, allowing for greater energy absorption with subsequent heat production that damages the hair follicle [14]. Melanin, the main target chromophore of light energy, is located in follicles of the terminal hair that extend deep into adipose tissue lying 2-7 mm below the skin surface, depending on the body site [13,14].

Removal of hair by light can be accomplished, in theory, by three mechanisms: photo­thermal destruction through local heating, photomechanical destruction through the gen­eration of shock waves, or photochemical destruction through the creation of toxic mediators such as singlet oxygen or free radicals [15]. Laser hair removal is accomplished though follicular unit destruction [16]. The ability to remove hair without damaging the surround­ing skin is based on selective photothermolysis: the theory that at a particular wavelength, pulse duration, and fluence, thermal injury is confined to the target, containing a light­absorbing molecule, known as a chromophore [17].

Photoepilatory devices are guarded under the principle of thermokinetic selection by which, target structures of large volume (i. e., hair) are not able to transmit absorbed energy to secondary structures, versus smaller-volume structures of the same chromophore (i. e., epidermis) [18]. Therefore, an appropriate pulse length of laser or light energy must be chosen that will selectively heat the target structure and the neighboring desired targets, for example the hair papillae, the germinative cell layer, and bulge areas of the hair follicle [7]. The duration of impulse must be selected to be above the thermal relaxation time of the epidermis, and below the thermal relaxation time of the targeted cell, the hair follicle [19].

The average thermal relaxation time of the epidermis is 3-10 ms, whereas the hair fol­licle thermal relaxation time varies from 80 to 100 ms [19]. In addition, hair follicle depth, diameter, density, and cyclical phase variations differ depending on the anatomic site of the body; for instance, 85% of the hair on the scalp is in the anagen phase [19]. The wave­length, fluence, depth of penetration, pulse duration, and spot size are variables character­istic to each laser system, and must be taken into account when choosing the appropriate nonablative modality for permanent hair removal [19].

Permanent hair reduction associated with long-term photoepilation requires light – induced interaction with the primary bulge and secondary matrix germinative regions of the pilosebaceous unit, which include areas from the bulge down to the matrix [7]. It has been emphasized that both the hair follicle and the bulge area are important for hair growth, as studies have shown that the mere destruction of the bulge area results in only temporary hair reduction [20,21]. Pantrichodestruction or damage to the entire germina – tive area of the follicle must therefore occur for permanent removal [19]. Partial germina – tive zone injury will lead to the formation of dystrophic hair that have the potential to regrow, whereas nongerminative zone injury will lead to an exogen shedding of hair that will regrow as a nonaffected terminal hair during the subsequent anagen follicular cycle (Table 7.1) [7,19,22].