Figure 8.25 Permeation of oxygen through several polymer films. This illustrates the diversity of the properties of available polymers and shows the relatively high permeability of polydimethylsiloxane, a common component of prolonged-action drug devices.

Reproduced from R. Kesting, Synthetic Polymeric Membranes, McGraw-Hill, New York, 1971.

hydrophilicity of the cellulose acetates is directly proportional to their -OH content and inversely proportional to the hydrophobic acetyl content (Table 8.8). Alternative approaches to alteration of characteristics of water permeability include the use of block copolymers where one can alter the ratio of hydrophilic polymer to increase transport rates of polar materials.

The affinity of drugs for plastics

The affinity of drugs for plastics will vary with the structure of the drug. Chlorpromazine, for example, has a very high affinity for some materials used as tubing (Table 8.9). Silicone is very permeable and this can be put to good use in other areas. Steroids are adsorbed from solutions passing through polyethylene tubing. In analytical techniques such interactions can also be important. Glyceryl tri-nitrate, which has a high affinity for lipophilic plastics, migrates from tablets in contact with plastic liners in packages, causing a reduction of the active content of many tablets to zero. This peculiar migratory behaviour is due to the volatility of the drug; normally the drug molecules would only be able to be significantly affected by such transfer when in the solution state. This topic is dealt with also in section 10.8.

Table 8.8 Effect of hydroxyl and acetyl content on water permeability and sorption of moisture by cellulose acetates0

Hydroxyl content (%)

Acetyl content (%)

Moisture sorption (%) at 95% relative humidity at 25°C

Water permeability D1 C (10 07gcms 01)

0 0

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