ABSTRACT
The electrical properties of the skin have been discussed in Section 2.2. Above a certain threshold value, the resistance of the skin is progressively reduced. A retrospective investigation using data from several studies has shown that the resistance of the skin depends most strongly on voltage, followed by current density and power density. Since electroporation involves the use of very short pulses, much higher voltages (compared with iontophoresis) can be used without causing sensation. The minimum current that is required to be applied to the skin to evoke a sensation is termed the perception threshold, while the minimum to cause a painful sensation is called the pain threshold. The latter is more variable as it depends on both physical and sociological factors and is typically 3-25 times greater than the former. The perception and pain thresholds do not scale directly with either current (I) or current density (Ij) but exhibit more complex functionalities. Power-function least-squares fit of the data resulted in the following relationships for perception thresholds:
and the following for pain thresholds (Prausnitz, 1996a, 1997):
where A is the electrical contact area. For single pulses of about 90 V across human stratum corneum, the recovery of the skin resistance has been shown to be very complete, returning to about 90 per cent of the pre-pulse value. The recovery consisted of four phases, with about 60 per cent recovery in phase I (20 ms), 40-70 per cent in phase II (0.4-0.8 s) and 60-90 per cent in phase III (10 s). This is followed by a slow phase of recovery of over a minute to a few hours. For higher voltage pulses (> 130 V), the recovery was typically less than 50 per cent (Pliquett et al., 1995a). The recovery time for these conditions may be quicker if full-thickness skin is used. Using excised full-thickness porcine skin, the recovery was shown to take place almost instantaneously or within the time required to switch to the measuring instrument. This was claimed to be due to the stratum corneum being still attached to the epidermis and underlying tissue, and being relaxed, unlike the former study where the stratum corneum was heat stripped, hydrated and mounted in a chamber. The permeabilization was found to depend on the electrical exposure dose, which is the product of the pulse voltage and the cumulative pulsing exposure time. Skin resistance was observed to drop to 20 per cent of its pre-pulsing value when pulsed beyond a critical dosage of 0.4 V/s, but recovered rapidly. When the dose exceeded 200 V/s, the recovery was slow and incomplete (Gallo et al., 1997).
