Hydrophilic polymers implantable membranes

Nondegradable polymers are also useful as matrices for ocular implants. This application requires the polymer to be hydrophilic, to minimize local tissue irritation. Need for ocular implants stems from the challenges posed to conventional ocular medicines (i.e., eye drops) such as rapid dilution, tear washout, poor patient compliance, and limited bioavailability. Ocular implants from hydrophilic polymer matrices that provide localized sustained release may overcome the above limitations. The first polymeric sustained release product to reach the market was Ocusert , a pilocarpin sustained release ocular implant developed by Alza. Ocusert has the drug reservoir as a thin disc of pilocarpine-alginate complex sandwiched between two transparent discs of microporous membrane fabricated from ethylene-vinyl acetate copolymer. The microporous membranes permit the tear fluid to penetrate into the drug reservoir compartment to dissolve pilocarpine from the complex. Pilocarpine molecules are then released at a constant rate of 20 or 40 pg/hr for a four- to seven-day management of glaucoma. 

As a silicone mbber membrane is not permeable to hydrophilic or high molecular weight compounds, concerted efforts were made to develop other biocompatible polymers for use in implantable devices. Such polymers include poly(ethylene-co-vinyl acetate), poly (ethylene), poly(propylene), poly(hydroxymethyl methacrylate), poly(lactide-co-glycolide), poly (anhydrides) and poly(ortho esters). The characteristics and applications of each important polymer family will be discussed later in this chapter. 

---