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in formulations can influence the behaviour of drugs even when the drug is present as a physical mixture with the polymer. For example, combination of polyoxyethylene glycol 4000 with sulfathiazole increases the solution rate of sulfonamides (Fig. 8.22). Probable mechanisms include an increase in drug solubility or increased wetting of the drug surrounded by the hydrophilic polymer.

range of polymers, many of which have been discussed in this chapter. Of these, two classes have been approved by the FDA: anionic poly(acrylic acid) (carbophil) derivatives and the cationic chitosans. Polycarbophil and carbomer (Carbopol 934P) have pKa values of about 4.5 and display maximum muco-adhesivity at pH values where they are mostly undissociated6 (see Fig. 8.23b).

8.4.8 Bioadhesivity of water-soluble polymers 8.4.9 Polymers as wound dressings

Adhesion between a surface of a hydrophilic polymer, or a surface to which a hydrophilic polymer has been grafted or adsorbed, and a biological surface arises from interactions between the polymer chains and the macro-molecules on the mucosal surface. From Fig. 8.23(a) it is clear that to achieve maximum adhesion there should be maximum interaction between the polymer chains of the bioadhesive (A) and the mucus (B). The charge on the molecules will be important, and for two anionic polymers maximum interaction will occur when they are not charged. Penetration and association must be balanced. Table 8.5 shows the adhesive performance of a

Several polymers are now used in the preparation of synthetic wound dressings. Synthaderm is a 'synthetic skin' of a modified polyurethane foam, hydrophilic on one side and hydrophobic on the other. The hydro-philic side is placed in contact with the wound. The system has been described as an 'environmental dressing'7 as it (a) maintains a high humidity at the dressing interface, (b) removes excess exudate, (c) allows gaseous exchange and (d) provides insulation; moreover, it is impermeable to bacteria, the outer surface remaining dry unlike many saturable dressings. Lyofoam is a similar product.8 Laminates of polypeptides and elastomers have

Figure 8.23 (a) Schematic representation of two phases, adhesive (A) and mucus (B), which adhere due to chain adsorption and consecutive chain entanglement during mucoadhesion. (Reproduced from N. A. Peppas and A. G. Mikos, in Bioadhesion (ed. R. Gurney and H. Junginger), Wiss. Verlagsgesellschaft, Stuttgart, 1990.) (b) Effect of pH on in vitro bioadhesion of polycarbophil to rabbit gastric tissue. (From reference 6.)

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