Biocompatibility of PHAs

The biocompatibility of PHAs originates from the fact that some monomers incorporated into the polymer chain occur naturally in the human body. The monomer (i )-3-hydroxybutyric acid is a normal metabohte found in human blood. This hydroxy acid is present at concentrations of 3 10 mg per 100 ml blood in healthy adults. Also low molecular weight PHAs are found com-plexed to other cellular macromolecules - hence they are called complexed PHAs (cPHAs). For example, cPHAs have been found in human tissues complexed with low-density lipoproteins, carrier protein albumin and in the potassium channel (KcsA) of Streptomyces lividans. Biocompatibility of PHAs, like any other biomaterial, is dependent on factors such as shape, surface porosity, surface hydrophilicity, surface energy, chemistry of the material and its degradation product. In tissue engineering, it is important that the 

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Biocompatibility. In special fields of application (especially for medical purposes), PHAs are superb compared with conventional plastics owing to their biocompatibility. The ideal biocompatibility of PHAs is underUned by the natural occurrence of (/ )-3-hydroxybutyric acid (3HB) and its low molecular weight oligomers and polymers in human blood and tissue (Agus et al. 2006 Steinbiichel and Hein 2001 Steinbiichel and Liitke-Eversloh 2003 Zinn et al. 2001). Table 1 provides an overview of the PHA-producing genera that have been reported in the literature. 

PHA solutions of various densities were used to prepare transparent flexible films. The surface properties of PHB and P(HB-co-HV) fllm scaffolds were similar to each other and to those of synthetic polyesters (polyethylene terephthalate, poly (methyl methacrylate), polyvinyl chloride, and polyethylene) (Shishatskaya 2(X)7X The scaffold s surface properties are important for cell attachment and proliferation. To enhance cell adhesion to the surface, improve the gas-dynamic properties of scaffolds, and increase their permeability for substrates and cell metabolites, the scaffolds can be treated by physical factors or by chemical reagents. Biocompatibility of PHA scaffolds has been enhanced by immobilizing collagen fllm matrices on the scaffold surface and coating with chitosan and chitosan/polysaccharides (Hu et al. 2003). 

Apart from modifications in the bulk, also surface modification of PHAs has been reported. Poly(3HB-co-3HV) film surfaces have been subjected to plasma treatments, using various (mixtures of) gases, water or allyl alcohol [112-114]. Compared to the non-treated polymer samples, the wettability of the surface modified poly(3HB-co-3HV) was increased significantly [112-114]. This yielded a material with improved biocompatibility, which is imperative in the development of biomedical devices. 

However, in addition to their thermoplasticity, representatives of PHAs have optical activity, increase induction period of oxidation, exhibit the piezoelectric effect and, what is most important, they are characterized as being biodegradable and biocompatible. At the same time, the PHAs have disadvantages (high cost, brittleness), which can be partially or completely compensated by using composite materials based on blends with other polymers, with dispersed fillers or plasticizers. Taking into account all the above, we have suggested to create a mixed polymer composite based on poly-3-hydroxybutyrate (PHB) and polyisobutylene (PIB). 

Poly(hydroxyalkanoates) (PHAs) are a very common class of bacterial reserve materials, that have attracted considerable industrial attention (Anderson and Dawes, 1990). These polyesters are biodegradable and biocompatible thermoplastics with physical and mechanical properties dependent on their monomeric composition. The production of PHAs is a typical biotechnological process whose development requires the involvement of several scientific disciplines, i.e. genetics, biochemistry, microbiology, bioprocess engineering, polymer chemistry, and polymer engineering. 

Electrospinning of PHA is still relatively new in scaffold fabrication. To date, P(3HB) and P(3HB-co-3HV) are the most common microbial polyesters to be electrospun into tissue-engineering scaffolds. Suwantong et al. (2007) prepared ultrafine electrospun fiber mats of P(3HB) and P(3HB-co-3HV) as scaffolding materials for skin and nerve generation, hi their study, they evaluated the in vitro biocompatibility of these fibers using mouse fibroblasts and Schwann cells... 

Aliphatic polyesters are the most representative examples of biodegradable polymeric materials. Poly(3-hydroxyalkanoate)s, PHA, are well known biocompatible and biodegradable polyesters that are produced by various microorganisms as carbon and energy reserves. The physical properties of PHAs vary from crystalline-brittle to soft-sticky materials depending on the length of the side aliphatic chain on p carbon ... 

Totally biodegradable blends are polymer mixtures containing PHAs and another (or other) biodegradable polymer(s) as components. The blending of PHAs with certain biodegradable polymers usually improves the biocompatibility and the... 

With the wide variety of applications of PHA, medical applications of PHA seem to be the most economically practical area. It Is vital to exploit and develop the application of PH As in the medical field. Most of the PHAs available in sufficient quantities, including PHB, PHBV, PHBHHx, P4HB, P3HB4HB, and PHO, have been studied for bio-implant applications. All of them showed good biocompatibility and some bio degradability. Of these, P4HB has been approved by the FDA for suture application with the trade name TephaFLEX marketed by Tepha Inc., of Cambridge, Mass., USA. Future efforts have been directed to develop more medical applications for PHA, mostly, three-dimensional scaffolds for implant purposes. 

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