Skin Electroporation: Modeling Perspectives - Porous Media: Applications in Biological Systems and Biotechnology - page 336

impermeable to the solute. This is the basic idea upon which most porous
medium representation of the SC has been developed. The representation of
the SC architecture consisting of a symmetric brick and mortar model was
introduced by Michaels et al. (1975) in which the interstitial horizontal lengths
between vertical spaces are all of the same length (see Figure 9.2(a)). Varia-
tions of this model have been proposed to better represent the SC geometry.
A nonsymmetric concept of the SC has been introduced by Johnson et al.
(1997), which more accurately describes the lateral diffusive path by using
an offset that is defined as the ratio of long to short lateral diffusion path
lengths (see Figure 9.2(b)). A trapezoidal representation of the corneocyte is
presented by Wang et al. (2006), which attempts to more realistically repre-
sent the SC microstructure and includes diffusion through the corneocytes.
The study finds that this route does play a minor role. Further developments
of this model include those of Frasch and Barbero (2005) and Barbero and
Frasch (2005), which use irregular description of the SC architecture based on
actual micrograph of mouse SC with the commercial software package ANSYS
to more accurately capture transient transdermal diffusion.
An excellent presentation and comparison of the models based on the
geometry of Figures 9.2(a) and 9.2(b) is presented by Kushner et al. (2007a).
This study derives a two-tortuosity model that accounts for the total lipid-
filled space as well as lateral diffusion pathways along. The study compares
the model results with the more traditional model descriptions of porosity
and tortuosity (see Table 9.1). In the descriptions of Table 9.1, N refers to
the number of layers within the SC, and the other geometric parameters of
the SC representation are shown in Figures 9.2(a) and 9.2(b). The corneocyte
offset, ω , is defined as the ratio of long to short lateral paths ω = d L / d S . The

(a)

h

g

d

g

(b)

h

g

d

g

d L

d s

FIGURE 9.2
Brick and mortar representations of SC geometry: (a) symmetric, (b) asym-
metric.