Biodegradable/bioerodible polymers

This chapter focuses on biodegradable polymers used in the health domain. Some examples of biodegradable and bioerodible polymers are presented in Table 4.1, and some of their thermomechanical properties are given in Table 4.2. 

To prepare PEG-coated nanospheres as depicted in Fig. 1, first, amphiphilic bioerodible polymers (symbolized as PEG R), composed of a PEG block and a hydrophobic biodegradable block (R = PLA or PLGA), were synthesized. PEG-coated nanospheres were then formed by taking advantage of the different solubdities of R and PEG in aqueous and organic solutions (Spenlehauer et al., 1992 Grefet al., 1993a). Nanospheres were also formed with diblock PEG poly(E-caprolactone) (PEG PCL) and PEG-poly(sebacic acid) (PEG PSA) (Peracchia et al, 1996). 

The meaning and definition of the words biodegradable, bioerodable, bioresorbable and bioabsorbable, which are often used misleadingly in the tissue engineering literature, are of primary importance in discussing the rationale, function and chemical and physical properties of polymer-based scaffolds [58]. In this paper, the biorelated polymer properties are based on the definitions given by Vert etal. [58, 59] ... 

Hydrogels, 13 729-759. See also Microgels Superabsorbent polymers (SAPs) AMPS polymer, 23 721 applications for, 13 747-753 biodegradable, 13 739-742 bioerodible, 9 63 conducting, 7 524 cross-linked poly (ethylene oxide),... 

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

Toxic degradation products this effect is applicable to biodegradable polymers for example, degradation of poly(alkylcyanoacrylate) leads to the formation of formaldehyde which is considered toxic in humans. In the case of a bioerodible polyvinylpyrrolidone), the accumulation of the dissolved polymer in the liver raises a longterm toxicity issue. 

A second class of biodegradable polymers of interest are those used in the human (or animal) body. These polymers include those used in artificial organs, other implants, and controlled release devices for delivery of pharmaceuticals. Being placed in contact with the tissue environment, they can potentially biodegrade. In products such as biodegradable sutures and bioerodible drug-delivery matrices, such breakdown in the body may be undesirable. 

Numerous synthetic or natural polymers have been used as matrices for micro- and nanoparticles or -capsules, most of which are biodegradable or bioerodible. Microparticles (or microspheres) are systems in which the drug is dispersed throughout the particle whereas capsules are vesicular systems in which the drug is contained in a cavity surrounded by the polymeric membrane (Couvreur and Puisieux, 1993). 

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