Degradable polyesters medical applications

Biodegradable polyurethanes have been proposed and studied before (9-72). The difference in our study is the inclusion of a phosphoester linkage instead of the commonly used polyester component. This seems to provide more flexibility as the side chain of the phosphate or phosphonate can be varied. For controlled drug delivery applications, drugs can be linked to this site to form a pendant delivery system. Moreover, for certain medical applications, fast degradation rate is obtainable by the introduction of these hydrolyzable phosphoester bonds. With the LDI based polyurethanes, drugs or other compounds of interest can also be coupled to the ester side chain of the lysine portion. 

More recently, polyesters with beneficial degradation products (salicylic acid) have been produced to promote healing through enhanced regeneration of tissue [10]. Degradation mechanisms relevant to medical applications include... 

There are two principal ways by which polymer chains can be hydrolyzed, passively by chemical hydrolysis or actively by enzymatic reaction. The latter method is most important for naturally occurring polymers such as polysaccharides and polyfhydroxy alkanoate)s, e.g., polyhydroxybutyrate and polyhydroxyvaler-ate [121,125]. Many synthetic aliphatic polyesters utilized in medical applications degrade mainly by pure hydrolysis [121]. 

At the time of writing, the applications of biodegradable polymers are confined mostly to the field of agriculture, where they are used in products with limited lifetimes, such as mulch films and pellets for the controlled release of herbicides. The synthetic polyesters used in medical applications, principally polylactide and poly(lactide-co-glycolide), while claimed to be biodegradable, are degraded in the body mainly, if not entirely, by chemical hydrolysis. There is little evidence that the hydrolysis of these polyesters of a-hydroxyacids can be catalyzed by hydrolase or depolymerase enzymes. 

Biodegradable polyester-based composites have been extensively studied for use in medical applications owing to their biocompatible and degradable properties in the human body. The major reported examples in biomedical products are fracture-fixation devices, such as sutures, screws, micro titration plates, and delivery systems [77]. Cellulosic nanofiber reinforced PLA composite materials... 

The fabrics used in these devices also differ. Polyester is a commonly used polymer in medical applications. This polymer is used in the ASO, the BCSO, the CS/SF and the DAW devices. The GHSO device, on the other hand, is composed of ePTFE. Both of these polymers are known to be reasonably inert, flexible, durable and resistant to degradation. Additionally, their creep resistance is acceptable and they are therefore used in vascular applications where the fabric s creep behavior is an important consideration. The fabric construction of the ASO device is unknown, but it appears to be a nonwoven. This specific construction provides a mesh, in which pore sizes can easily be controlled. This is of importance when considering tissue in-growth. Additionally, if the fabric is made with very small pores, it... 

Table 10.1 gives an overview of different aliphatic-aromatic copolyesters synthesised as degradable materials during the last few years. Part of the work reported in the literature dealt with hydrolytic degradation mechanisms which do not involve enzymic catalysis (chemical hydrolysis). This kind of degradation is often present in medical applications of polyesters, e.g., as implants in living tissues. Enzymic catalysed hydrolysis, in contrast, is usually connected to microbial degradation in the environment. 

PH As are a family of linear polyesters of 3, 4, 5, and 6-hydroxyacids, synthesized by a wide variety of bacteria through the fermentation of sugars, hpids, alkanes, alkenes, and aUsanoic acids. They are recyclable, natural materials, and can be easily degraded to carbon dioxide and water. This makes them as excellent replacements for petroleum-derived plastics in terms of processability, physical characteristics, and biodegradability. In addition, these polymers are biocompatible and hence have several medical applications [238], leading to vast interest in PHAs in bionanocomposites as well. The main polymers studied are poly(3-hydroxybutyrate), PHB and poly(hydroxybutyrate-cohydroxyvalerate). 

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