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The natural permeabilities of polymers vary over a wide range and this widens the choice, provided one can select a polymer which is compatible with the tissues with which it comes into contact. Once a polymer has been chosen that gives a flux of drug sufficient to provide adequate circulating levels, use of fillers and plasticisers can give fine control of permeability (Table 8.7).

The method of preparation also influences the properties of the film. Cast films of varying properties can be prepared by variation inter alia of the solvent power of the casting solution containing the polymer, although the complex processes involved in film formation are not yet fully understood. It is clear, however, that the conformation of the polymer chains in concentrated solution just prior to solvent evaporation will determine the density of the film, and the number and size of pores and voids. Drug flux through dense (nonporous) polymer membranes is by diffusion; flux through porous membranes will be by diffusion and by transport in solvent through pores in the film. With

Table 8.7 Factors that influence

diffusivity in

polymers

Factor

Net effect on D

Increased polymer molecular weight

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