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hyoscine, estradiol, fentanyl, coniine and other drugs. The word 'ostensibly' is used as there is a debate about whether the barrier membrane in these devices is the rate-limiting step in absorption. The barrier properties of skin are so variable, however, that one advantage of rate-controlling systems is that they prevent too rapid dosing in patients with highly permeable skin. In those with less-permeable skin, the systems probably act only as reservoirs. The now wide range of systems is listed in Table 8.15, which includes a reference to the pressure-sensitive adhesives discussed earlier in this chapter.

Some of the devices and the bases of their design are shown in Fig. 8.40 (the Transiderm system is also shown in Fig. 8.34). There are two groups: membrane and matrix systems. Membrane systems generally consist of a reservoir, a rate-controlling membrane and an adhesive layer. Diffusion of the active principle through the controlling membrane governs release rate. The active principle is usually present in suspended form; liquids and gels are used as dispersion media. In matrix systems the active principle is dispersed in a matrix which consists either of a gel or of an adhesive film.

Table 8.15 Some transdermal drug-delivery devices"

Product

Drug

Adhesive

Adhesive useb

Marketing company

Anti-anginal

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