8.6.5 Eroding systems

Release of drug by erosion of the polymeric or macromolecular matrix in which a drug is dissolved or dispersed provides another mechanism for controlling drug absorption. A typical bioerodible system would be that achieved by the molecular association of a cellulose acetate phthalate (CAP) (a carboxylic acid polymer) with a poloxamer block copolymer such as Pluronic L101. This interaction is between the proton-donating CAP and the proton-accepting poloxamer. By varying the ratio of CAP to poloxamer, the erosion periods can be controlled from hours to days. Figure 8.35 shows the relationship between the percentage polymer eroded and the release of drugs from a 50 : 50 mixture of


the polymers containing 10% of the drug metronidazole.

Computer simulation of eroding matrices (Fig. 8.36) can give an accurate representation of the process and can predict the position of the erosion front and the weight of the matrix.

More precise control of release than is possible with matrices has recently been achieved by the application of several features of polymer physical chemistry, as discussed below.

8.6.6 Osmotic pump

Osmotic pumps for oral administration

A variety of osmotic pumps have been described in the patent literature.12 In the oral

Figure 8.37 Low power scanning electron micrographs of an oral osmotic pump (Osmet). (a) A section showing the semipermeable membrane, the osmotic core and the laser-drilled orifice. (b) The same 8 h after immersion in water. (c) The laser-drilled orifice. (d) Another view of the tablet via the orifice after 24 h immersion.

Semipermeable membrane Osmotic drug core

Delivery orifice

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