Polyhydroxybutyrate crystallinity

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

Metabolix s PHBV (polyhydroxybutyrate valerate) was initially developed by ICI. PHBV and related copolymers are made in a pilot plant using different bacteria to create compositions with up to 70% crystallinity. Elongation can be manipulated from 5% to 100%, and melting points range between 135 and 185 °C (275-365 °F). 

Polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV) are also being researched for use in medical devices. The PHB homopolymer is crystalline and brittle, whereas the copolymers of PHB with PHV are less crystalline, more flexible, and easier to process. These polymers typically require... 

It is known that PLA forms miscible blends with polymers such as PEG [53]. PLA and PEG are miscible with each other when the PLA fraction is below 50 per cent [53]. The PLA/PEG blend consists of two semi-miscible crystalline phases dispersed in an amorphous PLA matrix. PHB/PLA blends are miscible over the whole range of composition. The elastic modulus, stress at yield, and stress at break decrease, whereas the elongation at break increases, with increasing polyhydroxybutyrate (PHB) content [54]. Both PLA/PGA and PLA/PCL blends give immiscible components [55], the latter being susceptible to compatibilization with P(LA-co-CL) copolymers or other coupling agents. 

PolyhydroxyaUcanoates (PHAs) are the biopolymers possessing the material properties ranging from rigid and highly crystalline to flexible, amorphous, and elastomeric. Because of such properties and inherent biodegradability, PHAs have attracted the world-wide attention of scientists and researchers as environment-friendly alternative to the conventional petroleum-based polymers. Polyhydroxybutyrate (PHB) and polyhydroxyoctanoate (PHO) have been found to possess biocompatibility in mammalian systems. Such biomaterials have got great potential as medical implantation devices [78-81]. 

Polyhydroxyalkanoates and polyhydroxybutyrate in particular may be considered as rather controversial polymers. PHB is sometimes referred to as a material that appeared too early, before its time has came. To summarize, PHAs are prepared from renewable resources and are fully biodegradable. They outperform most of the other biodegradable polymers and many synthetic plastics in properties, notably mechanical strength and modulus, resistance to water and moisture due to high hydrophobicity, high crystallinity and several other physical properties such as barrier behavior or piezoelectricity. 

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