Nondegradable polymers

These facts are different demonstrations of the same event degradation reactions occur simultaneously with electropolymerization.49-59 These reactions had also been called overoxidation in the literature. The concept is well established in polymer science and consists of those reactions between the pristine polymer and the ambient that promote a deterioration of the original polymeric properties. The electrochemical consequence of a strong degradation is a passivation of the film through a decrease in the electrical conductivity that allows a lower current flow at the same potential than the pristine and nondegraded polymer film did. Passivation is also a well-established concept in the electrochemistry of oxide films or electropainting. 

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. 

Wood and, polymers. Natural materials as well as materials manufactured from plant or animal origin, such as wood, cotton, paper products, wool, and leather, etc., are fully biodegradable under aerobic conditions. (Dexter)5 Plastics are materials that consist mainly of highly polymeric, organic compounds. Also, nondegradable polymer may become degradable by a combined chemical, physical, and biological attack.64,67... 

The explanation of the origin of the broad visible luminescence, however, has been a very controversial issue 10, 24). Two kinds of luminescence should be distinguished the luminescence measured after photodegradation and the luminescence from nondegraded polymers at very low temperatures (4-70 K). In nondegraded samples, the intensity of this broad luminescence decreases very rapidly with an increase in temperature from 4 to 70 K, and it is reversible in the heat cycle (11). On the other hand, the sharp peak... 

Nondegradable polymers. These are stable in biological systems. They are mostly used as components of implantable devices for drug delivery. 

Figure 7 Principle of a Monte Carlo-based approach to mathematically model polymer degradation and drug diffusion in PLGA-based microparticles. Scheme of the iimer structure of the system (one-quarter of a spherical cross section) (A) at time t = 0 (before exposure to the release medium) and (B) during dmg release. Gray, dotted, and white pixels represent nondegraded polymer, drag and pores, respectively. Source From Ref. 48.

TABLE 18.1 Nondegradable Polymers That Have Been Used for Various Clinical Applications ... 

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