The vacuum pressure in the microdermabrasion device provides multiple functions. Importantly, the vacuum pressure provides a driving force to create a continuous flow of microparticles to bombard the skin surface and to aspirate the abraded skin microfragments into a waste container. The vacuum pressure also controls the kinetic energy of the particles that hit the skin surface, thus providing a means to modulate microdermabrasion to suit different skin types. Various in vitro and in vivo studies have demonstrated the need to optimize the operating pressure to different skin types. A study by Andrews et al. (2011c) using porcine skin in vitro demonstrated that vacuum pressure (30–60 kPa) does not significantly affect the degree of tissue removal from porcine skin. In contrast a study by Gill et al. (2009) performed in vivo in humans and rhesus macaques showed a dependence of stratum corneum and viable epidermis removal on pressure. Figure 16.4 reproduced from this study shows that while at a vacuum pressure of 30 kPa (3 s exposure), there is little to no removal of the stratum corneum, an increase in vacuum pressure to 45 kPa (exposure time of 3 s) results in complete abrasion of the stratum corneum and the viable epidermis. A similar effect of pressure is seen in rhesus macaques (Gill et al. 2009). This species-dependent effect of vacuum pressure may be due to species-dependent differences in adhesion between the viable epidermis and the dermis. As seen in Fig. 16.4, microblisters can be formed at high vacuum pressures for human and rhesus macaque skin (Gill et al. 2009) and hairless rats (Zhou and Banga 2011). Microblisters may represent an early stage in the formation of macroscopic suction-based blisters, a technique which is clinically used to harvest the epidermis for transplantation or to study inflammation of the skin (Gupta et al. 1999; Gupta and Kumar 2000; Suthar et al. 2010). Thus, at high vacuum pressures, the viable epidermis may simply be tearing away from the dermis rather than being removed by particle-based abrasion effects. Microblisters were however not seen in pig skin (Andrews et al. 2011a, b, c). It is thus critical to optimize the microdermabrasion vacuum pressure for use in humans versus other laboratory animal models.

The flow rate of the device as a function of pressure is dependent on the device used and can be controlled via an external knob, which internally controls a valve in the flow path of the microparticles. In a study that used a MegaPeel® (DermaMed Solutions, LLC, USA) microdermabrasion device, the particle flow rate was found to be constant at all times at a specific pressure (Andrews et al. 2011a, b, c). Figure 16.5 shows the flow rate at a vacuum pressure of 40 kPa at three, six, and nine turns of the microparticle flow rate knob. Three and six turns of the flow rate knob induced a mass flow rate of 0.36 ± 7.70 × 10⁻³ g/s and 0.21 ± 2.1 × 10⁻² g/s, respectively. The microparticle flow rate at nine turns was zero. Higher flow rates resulted in more tissue removal.

The hand tip can be held stationary (stationary mode), or it can be moved on the skin surface at a fixed speed (mobile mode). In either case, the time of contact can affect rate and degree of microdermabrasion (depth of skin abraded). In the mobile mode, the speed of hand tip movement and the number of times it is repeatedly moved across the treatment area can further modulate the degree of microdermabrasion.

The effect of stationary vs mobile mode of the hand tip has been compared in vitro (Andrews et al. 2011a, b, c) and in vivo in humans, rhesus macaques, and hairless rats (Gill et al. 2009; Zhou and Banga 2011). The in vitro and in vivo studies offer similar conclusions. The different parameters investigated in the two in vivo studies are summarized in Table 16.1.

Using histological analysis of the skin biopsies from humans and macaques, Gill et al. (2009) quantified the effect of different microdermabrasion operating parameters on skin layers. It was found that in the mobile mode, the degree of tissue removal increases with vacuum applied, number of passes, and the speed of tip movement, while in a stationary mode, the degree of skin removal increases with an increase in vacuum pressure and time of application. Figure 16.4 shows the representative effects observed at the different microdermabrasion conditions in humans. In stationary mode, microblisters (separation of the viable epidermis from the dermis) were also observed (Fig. 16.4, double-lined arrows), which are undesirable for most drug delivery applications. A study by Fujimoto et al. has proposed a model to predict the flux of drug across the skin based on the different microdermabrasion conditions (Fujimoto et al. 2005).

Microdermabrasion has proven to be a safe procedure for routine cosmetic applications wherein the stratum corneum is only partially removed from the skin. In contrast, for drug delivery applications, it is desirable to remove full thickness of the stratum corneum. However, for safety concerns full-thickness removal of the stratum corneum should be limited to microscopic islands rather than bulk areas of the skin. In principle because the stratum corneum is the major rate-limiting barrier, even if the stratum corneum can be removed from microscopic areas of the skin, a significant enhancement in skin permeability should be observed. Indeed creation of discrete micron-sized holes in the stratum corneum using microneedles (Gill and Prausnitz 2007a, b; Wermeling et al. 2008), lasers (Fang et al. 2004b), and thermal ablation (Lee et al. 2011) has been shown to increase skin permeability by orders of magnitude. To create such discrete and localized areas of microdermabrasion, a mask with micron-sized holes has recently been developed (Andrews et al. 2011a, b, c). This mask when placed between the skin and the hand tip protects the skin from particles except in areas where the skin is exposed via the mask holes. The masks can be fabricated using metals or polymers and can have different hole sizes. An example of a mask used with microdermabrasion is shown in Fig. 16.6a. The mask is fabricated out of stainless steel and has holes that are 250 μm in diameter. The abraded areas of the skin were stained with a dye and are shown in Fig. 16.6b. A histological section of the skin of one of the sites of abrasion is shown in Fig. 16.6c. Using a mask allows greater control over the area, amount, and depth of skin abrasion and improves reproducibility. The mask approach has been used in vivo to achieve a therapeutically effective delivery of insulin in diabetic rats (Andrews et al. 2011a).

Transepidermal water loss (TEWL) is a measure of the amount of water permeating from the skin outward toward the surroundings. TEWL increases with loss of skin integrity and thus offers a convenient method to study microdermabrasion. TEWL has been used in different microdermabrasion studies (Lee et al. 2003; Rajan and Grimes 2002; Zhou and Banga 2011) and has been shown to increase and to correlate with the extent of microdermabrasion performed.