Applications of PHAs
Besides being a thermoplastic with properties comparable to that of polyethylene, PHAs are also completely biodegradable. The ability to produce PHAs from renewable carbon sources also ensures a sustainable ‘green chemistry’ process. Much work has been directed to the production of various types of PHAs for applications as commodity plastics. The ability to further chemically modify the functional groups in these PHAs broadens their scope of application as biodegradable polymers as well as bioabsorbable materials for biomedical purposes.
For application as tissue engineering scaffolds, the suitable material must possess properties such as biocompatibility, support cell growth, guide and organise the cells, allow tissue ingrowth and should finally degrade to non-toxic products .
PHAs are also potential material for applications in controlled drug release systems [51, 52]. The biocompatibility and biodegradability properties of PHAs make them attractive as materials for drug delivery. A wide variety of monomers can be incorporated into PHAs, resulting in various physical properties that range from highly crystalline materials to strong elastomers. By judiciously controlling the monomer composition of PHAs, the degradation rate can be indirectly controlled. The enzymatic degradation of PHAs is usually catalysed by the bacterial PHA depolymerase.
The fact that P(3HB-co-4HB) and P(4HB) are also polymers with potential therapeutic applications have been pointed out in a recent review. The 4HB units are pharmacologically active compounds, which have been used in the treatment of alcohol withdrawal syndrome [54, 55]and narcolepsy . Other potential applications include the treatment of patients with chronic schizophrenia, catatonic schizophrenia, atypical psychoses, chronic brain syndrome, neurosis, drug addiction and withdrawal, Parkinson’s disease and other neuropharmacological illnesses, hypertension, ischemia, circulatory collapse, radiation exposure, cancer, and myocardial infarction .
Optically active compounds
Recently, PHAs are also gaining much attention as a source of enantiomerically pure compounds although this possibility was realized almost a decade ago [58, 59]. Due to the specificity of the PHA synthase, biosynthesized PHAs only contain the (R)-isomers of 3-hydroxyacids. Close to 150 different monomer constituents of PHAs have been identified to date and therefore various enantiomerically pure compounds can theoretically be obtained by depolymerising the polymers. The depolymerization can be performed either by chemical or biological methods. On the other hand, the biological method of depolymerization employs either the intracellular  or the extracellular depolymerase .