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• concentration (C1) and the receiving region at a lower concentration (C2) per unit distance

(x) by the following expression:

• The flux is proportional to the concentration gradient, dC/dx:

• J = -D x

• where:

• D is the diffusion coefficient of a penetrant, in cm2/s

• C is the concentration, in g/cm3 or g/mL

• x is the distance perpendicular to the surface of the barrier, in cm

• Thus,

• 1= -D x

• = -D x S x

• The negative sign in this equation signifies that diffusion occurs in a direction of

decreasing concentration.

• The flux is always a positive quantity.

https://www.youtube.com/watch?v=ZHhb9JFo01k

Although the diffusion coefficient, D, or diffusivity appears to be a proportionality constant, it

does not remain constant.

It is affected by changes in concentration, temperature, pressure, solvent properties, molecular

weight, and the chemical nature of the diffusant.

For example, the larger the molecular weight, the lower the diffusion coefficient.

8. Maximizing the bioavailability of drug applied to the skin

Various technologies have been developed to bypass or modulate the barrier function of the
skin and to allow easier passage of drugs into the dermal microcirculation
Enhancement of transdermal permeation
A. Chemical Approaches:

• Penetration enhancers
Incorporation of penetration enhancers facilitates the absorption of drugs by altering the
barrier property of the stratum corneum.
Permeation enhancers should be:

a) Pharmacologically inert
b) Nontoxic, non-irritating, nonallergic

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