Poly hydroxybutyrate

Poly(hydroxybutyrate) was first discovered in 1926 as a constituent of the bacterium Bacillus megaterium (7). Since then, poly(hy-droxybutyrate) has been found in a wide variety of different genera of gram-negative and gram-positive bacteria (8). 

The microorganisms produce poly(hydroxyalkanoate)s using R- -hydroxyacyl-Coenzym-A enz5mcies as the direct metabolic substrate for a poly(hydroxyalkanoate) S5mthase. 

Poly(j3-hydroxy butyric acid) can be extracted from a suspension of bacterial cells, i.e., Alcaligens eutrophus, by causing the cells to flocculate, by pH modification, and then extracting the polymer 

After agitation of the aqueous slurry with the extraction solvent, the two liquid phases may be separated The cell debris remains in the aqueous phase while the poly(/3-hydroxy butyric acid) is dissolved in the solvent phase. Suitable non aqueous solvents can be chloroform, 1,2-dichloroethane, and methylene chloride (6,9). 

In addition, Aeromonas caviae can produce a random copol5nner of 3-hydroxybutyric acid and 3-hydroxyhexanoic acid under aerobic conditions, when sodium salts of alkanoic acids of even carbon numbers and oUve oil are feeded (10). 

Poly(3-hydroxybutyrate) (1.8) is a bacterial polyester that behaves as an acceptable thermoplastic, yet can be produced from renewable agricultural feedstocks and is biodegradable. It is typically produced not in the pure state. 

The carbon atom which carries the methyl group is chiral, but biosynthesis is stereoselective, and gives rise to a natural polymer with the R configuration. The polymer is a partially crystalline thermoplastic which melts at about 80 °C. 

5 Some Commercially Attractive PHAs 2.5.1 Poly(3-hydroxybutyrate) [P(3HB)J 

P(3HB) is the best characterized and most extensively studied of all the known PHAs. In its native granule, P(3HB) exists in amorphous (Barnard and Sanders 1989) state while extracted granules have 55-80 % crystallinity (Holmes 1988). 

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SCHEME 8.10 Chemical stmcture of poly-4-hydroxybutyrate (P4HB). 

Martin DP and Williams SE. Medical appUcations of poly-4-hydroxybutyrate A strong flexible absorbable biomaterial. Biochem Eng, 2003, 16, 97-105. 

Many other polymers of this class are produced by a variety of organisms these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), poly-hydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers. 

It was found meanwhile that nearly every slim unbranched polymer chain, such as poly(trimethylene oxide) [224], poly(l,3-dioxolane) [225], poly(tetramethylene oxide) [226], polyethylene imine) [227], poly(3-hydroxy propionate), poly (4-hydroxybutyrate) and poly(6-hydroxyhexanoate) [228,229], poly(butylene succinate) [229], polyadipates [230], nylon-6 [231], and even oligomers of polyethylene [232], form a-CD ICs with channel structures. In all of these cases, inclusion is a heterogeneous process, since the guest polymer and its CD complex are almost insoluble in water. Therefore, extensive sonication had to be applied to accelerate the diffusion process. The polymer was also dissolved in an organic solvent, e.g., nylon-6 in formic acid, and this solution was added to the solution of a-CD [231], Alternatively, a monomer, such as 11-aminoundecanoic acid, was included in a-CD and polymerized to nylon-11 by solid state polycondensation within the channels of the IC. Thus, the IC of nylon-11 was formed under conservation of the crystal packing [233-235],... 

Poly(4-hydroxybutyrate) [P(4HB)] is a highly ductile, flexible polymer withstanding an extension of around 1,(XX)% before breaking, compared to P(3HB), which can only extend to less than 10% before breaking. Combining these different monomers to form copolymers, as in P(3HB-co-4HB), been described as one of the most useful PHAs by Sudesh et al. [5], produces a family of materials with mechanical properties that can be tailored to specific needs. P(3HB-co-4HB) has been found to have desirable mechanical properties for applications in the medical and pharmaceutical field [11]. The biocompatibility and bioabsorbable nature of P(3HB-co-4HB) makes it the most valuable type of biopolymer among the vast number of different PHAs synthesized by microorganisms. To date, five wild-type bacteria, which can produce P(3HB-co HB), i.e. R. eutropha... 

Choi MH, Yoon SC, Lenz RW (1999) Production of poly(3-hydroxybutyric acid-co-4-hydroxybutyiic acid) and poly(4-hydroxybutyric acid) without subsequent degradation by Hydrogenophaga pseudoflava. Appl Environ Microbiol 65 1570-1577 Chowdhury AA (1963) Poly-/3-hydroxybuttersaure abbauende bakteiien und exo-enzyme. Arch Microbiol 47 167-200... 

Song S, Hein S, Steinbuchel A (1999) Production of poly(4-hydroxybutyric acid) by fed-batch cultures of recombinant strains of Escherichia coli. Biotechnol Lett 21 193-197 Sozer N, Kokini JL (2009) Nanotechnology and its applications in the food sector. Trends Biotechnol 27 82-89... 

Several types of bacterial polyesters that are produced by biosynthesis are poly-3-hydroxybutyrate, poly-4-hydroxybutyrate, poly-3-hydroxyvalerate, poly-3-hydroxyhexanoate, poly-3-hydroxy-heptanoate, etc., and their respective copolymer combinations. Due to their ability to degrade naturally in variety of environments, they will find a lot of applications in disposal items, short-term packaging, and also considered biocompatible in contact with living tissues and can be used for biomedical applications (e.g., drug encapsulation, tissue engineering) (Chauhan, 2012). 

Besides the common polyhydroxybutyrate (PHB), other polymers of this class are produced by a variety of organisms such as poly-4-hydroxybutyrate, PHV, and polyhy-droxyhexanoate. PHV is a naturally occurring bacterial polyester, which was first isolated by Wallen and coworkers [172,173]. Also, PHV/PHB copolymers have been studied to make a wide range of thermally processable polyesters, which exhibit the necessary long-term degradation profile required for a degradable fracture fixation device [174]. 

Abate, R., Ballistreri, A., Impallomeni, G., and Montaudo, G., Thermal Degradation of Microbial Poly(4-hydroxybutyrate), Macromolecules, 71, 332,1994. Ballistreri, A., Garozzo, D., Giuffrida, M., and Montaudo, G., Microstructure of Bacterial Poly( 8-hydroxybutyrate-co-/3-hydroxyvalerate) by Fast Atom... 

To date, various block copolymers have been produced using biological systems. This includes poly(3HB-fo-4HB) [17], P(3HB)-f>-poly(3-hydroxyvalerate-co-3-hydroxyheptanoate) [18], PHB-f -poly(hydroxyhexanoate) [19], poly 3HB-fc-poly(3-hydroxyheptanoate) [P(3HP)] [20], P(3HP)-f -poly(4-hydroxybutyrate) [P(4HB)] [21], poly(3-hydroxyhexanoate)-fe-poly(3-hydroxydecanoate)-co-[3-hydroxydodecanoate (3HDD)] [22] and poly[3-HDD-f -poly(3-hydroxy-9-decanoate)] [23]. These studies were motivated by the fact that although random copolymers, such as poly(3HB-co-3HV) and poly(3HB-co-4HB), exhibit useful mechanical and thermal properties they suffer from a deterioration of polymer properties due to the effect of ageing. It was found that all block copolymers exhibited improved properties compared with the two relative homopolymers, random and blend polymers. Various... 

P(3HB) poly[(R)-3-hydroxybutyrate], UHMW-P(3HB) ultra-high-molecular-weight poly[(/J)-3-hydroxybutyrate], P(3HB-co-3HV) poly[(l )-3-hydroxybutyrate-co-(/f)-3-hydroxyvalerate], P(4HB) poly[(R)-4-hydroxybutyrate], poly(4-hydroxybutyrate) P(3HB-co-3HH) poly[(/J)-3-hy-droxybutyrate-co-(R)-3-hydroxyhexanoate]... 

Martin, D.P. and Williams, S.F. (2003) Medical applications of poly-4-hydroxybutyrate a strong flexible absorbable biomaterial. Biochem. Eng. 

Figure 12.4 A stent prototype made of a poly(L-lactic acid)(PLLA)/poly(4-hydroxybutyrate) blend. The stent is shown before (top) and after...

C., Schmohl, K., Martin, D.P., Ydlliams, S.E., Sternberg, K., and Schmitz, K.P. (2007) A biodegradable slotted tube stent based on poly(l-lactide) and poly(4-hydroxybutyrate) for rapid... 

The latest addition to the synthetic absorbable suture materials is TephaFLEX which is thermally melt-spun from poly-4-hydroxybutyrate, a member of the class of absorbable biomaterials known as polydroxyal-kanoates, or PHA (FDA, 2007 Martin and Williams, 2003). Studies have shown that TephaFLEX is both biocompatible and noninflammatory. Their biodegradation occurs through normal processes and the products of the breakdown are metabolites that already exist in the body. 

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