Polysaccharides, natural degradable polymers

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]. 

Among all the polymers available to be used for drug delivery systems, (bio) degradable polymers are highly recommended. Indeed, one of the key points of this kind of system is the removal of the carrier after the release of the active pharmaceutical ingredients. Moreover, to avoid side effects, in particular when the carrier is injected, the polymer must be biocompatible. For aU of these reasons, natural polymers such as polysaccharides, polypeptides, or phospholipids are generally used as building blocks for the formulations [1]. 

Polymers with hydrolyzable linkages in the backbone can also be used as degradable materials. So far, most of them are too expensive and do not have the desirable combination of mechanical and chemical properties. Well-known synthetic hydrolyzable polymers are polyesters [1], polycarbonates [2], polyanhydrides [2], polyamides [2] and poly(amino acids) [2]. Hydrolyzable natural polymers may be cheaper and are believed to be representative of the future development in degradable polymers. Many scientists today are looking for new possibilities using such traditional natural polymers as polysaccharides, proteins and lipids. Special interest is focused on poly(j8-hydroxybutyrate) and its copolymers [3,4]. Well-known natural products... 

Nature provides a chemically diverse variety of degradable polymers. These polymers were the first materials used by humankind. Early men dressed themselves in hides (proteins, polysaccharides), later in cotton (polysaccharide), silk and wool (proteins). Early men used wood (polysaccharides, polyphenols) for tools and construction materials. Where available natural rubber (polyisoprene) was used for a variety of daily-life fimctions, from construction to water-proofing storage containers. 

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