ABSTRACT
Nina Otberg, Heike Richter, Hans Schaefer, Ulrike Blume-Peytavi, Wolfram Sterry, and Ju¨rgen Lademann Humboldt University Berlin, Berlin, Germany
I. INTRODUCTION
The knowledge of permeation and penetration processes is a prerequisite for the development and optimization of drugs and cosmetics. In the past, percutaneous absorption was described as diffusion though the lipid domains of the stratum corneum. It was presumed that skin appendages, which mean hair follicles and sweat glands, play a subordinate role in absorption processes. The amount of appendages of the total skin surface was estimated to represent up to 0.1% (1). However, previous studies show higher absorption rates in skin areas with higher follicle density (2-6). On the other hand, hair follicle size and density and the amount of the absorbed drug have never been correlated. The variation in the thickness of the stratum corneum in different body areas was considered to be the main reason for the varying absorption rates. Feldman and Maibach (2) and Maibach et al. (3) found regional variations of percutaneous absorption in different skin areas. They assumed that the density and size of hair follicles might be the reason for their findings. More recent studies more strongly suggest that skin appendages play an important role in permeation and penetration processes of topically applied substances. Tenjarla et al. (4) and Hueber et al. (5,6) found significant differences in percutaneous absorption of appendage-free scarred skin and normal skin. Turner and Guy (7) found a significant iontophoretic drug delivery across the skin via follicular structures. Essa et al. (8) performed an in vitro Franz cell experiment for iontophoretic drug delivery. A new technique involving a stratum corneum/epidermis sandwich method was used for blocking the follicular orifice. A five times lower absorption rate was found when the potential shunt routes were blocked.
