J 1

where J is the solute flux, D is the solute diffusion coefficient in the stratum corneum

Figure 9.21 'Bricks and mortar' model of the stratum corneum, illustrating possible pathways of drug permeation through intact stratum corneum (transcellular and tortuous intercellular pathways) and the lamellar structure of intercellular lipids.

Reproduced from H. R. Moghimi etal., Int. J. Pharm., 131, 117 (1996).

Figure 9.21 'Bricks and mortar' model of the stratum corneum, illustrating possible pathways of drug permeation through intact stratum corneum (transcellular and tortuous intercellular pathways) and the lamellar structure of intercellular lipids.

Reproduced from H. R. Moghimi etal., Int. J. Pharm., 131, 117 (1996).

(values range from 1 x 10 12 to 1 x 10-17 m2 s 1 for human stratum corneum), P is the solute partition coefficient between vehicle and skin, and 6 is the thickness of the stratum corneum. A Cv is the difference in solute concentration between vehicle and tissue. This relation is obtained as shown in Box 9.2.

Box 9.2 Derivation of equation (9.10)

Fick's law of diffusion shows that (for a given vehicle)

A proportionality constant Kp may be added. Thus

where Kp is the permeability constant, which provides a means of expressing absorption measurements for comparing different vehicles and conditions. The units of a permeability constant are m s01, the concentration term being mol m~3, so that J has the correct units of mol m02 s_1. It has been shown that PD

Kp=T

so that equation (9.10) is readily obtained.

DP ACv

0 0

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