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Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class that are of interest as bio-derived and biodegradable plastics.[1] The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but other polymers of this class are produced by a variety of organisms: these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and their copolymers.

Contents

BiosynthesisEdit

PHB is produced by microorganisms (such as Ralstonia eutrophus, Methylobacterium rhodesianum or Bacillus megaterium) apparently in response to conditions of physiological stress;[2] mainly conditions in which nutrients are limited. The polymer is primarily a product of carbon assimilation (from glucose or starch) and is employed by microorganisms as a form of energy storage molecule to be metabolized when other common energy sources are not available.

Microbial biosynthesis of PHB starts with the condensation of two molecules of acetyl-CoA to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB.[3] PHAs granules are then recovered by disrupting the cells.[4]

 
Structure of poly-(R)-3-hydroxybutyrate (P3HB), a polyhydroxyalkanoate
 
Chemical structures of P3HB, PHV and their copolymer PHBV

Thermoplastic polymerEdit

Most commercial plastics are synthetic polymers derived from petrochemicals. They tend to resist biodegradation. PHB-derived plastics are attractive because they are compostable and derived from renewables and are bio-degradable.

ICI had developed the material to pilot plant stage in the 1980s, but interest faded when it became clear that the cost of material was too high, and its properties could not match those of polypropylene.

In 1996 Monsanto (who sold PHB as a copolymer with PHV under the trade name Biopol) bought all patents for making the polymer from ICI/Zeneca. However, Monsanto's rights to Biopol were sold to the American company Metabolix in 2001[5] and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004. Monsanto began to focus on producing PHB from plants instead of bacteria.[6] But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB.[7]

In June 2005, a US company, Metabolix, received the Presidential Green Chemistry Challenge Award (small business category) for their development and commercialisation of a cost-effective method for manufacturing PHAs in general, including PHB.

Biopol is currently used in the medical industry for internal suture. It is nontoxic and biodegradable, so it does not have to be removed after recovery.[citation needed]

PropertiesEdit

Citation needed

  • Water-insoluble and relatively resistant to hydrolytic degradation. This differentiates PHB from most other currently available biodegradable plastics, which are either water-soluble or moisture-sensitive.
  • Good oxygen permeability.
  • Good ultra-violet resistance but poor resistance to acids and bases.
  • Soluble in chloroform and other chlorinated hydrocarbons.[8]
  • Biocompatible and hence is suitable for medical applications.
  • Melting point 175 °C., and glass transition temperature 2 °C.
  • Tensile strength 40 MPa, close to that of polypropylene.
  • Sinks in water (while polypropylene floats), facilitating its anaerobic biodegradation in sediments.
  • Nontoxic.
  • Less 'sticky' when melted

HistoryEdit

Polyhydroxybutyrate was first isolated and characterized in 1925 by French microbiologist Maurice Lemoigne.[9]

BiodegradationEdit

Firmicutes and proteobacteria can degrade PHB. Bacillus, Pseudomonas and Streptomyces species can degrade PHB. Pseudomonas lemoigne, Comamonas sp. Acidovorax faecalis, Aspergillus fumigatus and Variovorax paradoxus are soil microbes capable of degradation. Alcaligenes faecalis, Pseudomonas, and Illyobacter delafieldi, are obtained from anaerobic sludge. Comamonas testosteroni and Pseudomonas stutzeri were obtained from sea water. Few of these are capable of degrading at higher temperatures; notably excepting thermophilic Streptomyces sp. and a thermophilic strain of Aspergillus sp.[10]

Water soluble PHBEdit

Thermogelling polymers belong to a class of stimuli-responsive hydrogels that undergo a macroscopic sol-to-gel transition in response to temperature.[11][12][13] Much of the ongoing research in this field is focused on hydrogels for biomedical applications as an injectable sustained drug delivery matrix or scaffolds for tissue engineering. PHB-based thermogelling polymers was recently developed by Xian Jun Loh and his team in 2007.[14]|[15][16] These materials have been shown to sustain the release of BSA for nearly 3 months.[16] The gels have also been used as a sustained delivery drug depot for the delivery of chemotherapeutics in an mouse model.[17][18]

ReferencesEdit

  1. ^ Lichtenthaler, Frieder W. (2010). "Carbohydrates as Organic Raw Materials". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.n05_n07. ISBN 978-3-527-30673-2. 
  2. ^ Ackermann, Jörg-uwe; Müller, Susann; Lösche, Andreas; Bley, Thomas; Babel, Wolfgang (1995). "Methylobacterium rhodesianum cells tend to double the DNA content under growth limitations and accumulate PHB". Journal of Biotechnology. 39 (1): 9–20. doi:10.1016/0168-1656(94)00138-3. 
  3. ^ Steinbüchel, Alexander (2002). Biopolymers, 10 Volumes with Index. Wiley-VCH. ISBN 3-527-30290-5. [page needed]
  4. ^ Jacquel, Nicolas; Lo, Chi-Wei; Wei, Yu-Hong; Wu, Ho-Shing; Wang, Shaw S. (2008). "Isolation and purification of bacterial poly(3-hydroxyalkanoates)". Biochemical Engineering Journal. 39 (1): 15–27. doi:10.1016/j.bej.2007.11.029. 
  5. ^ "METABOLIX PURCHASES BIOPOL ASSETS FROM MONSANTO". Archived from the original on February 4, 2007. Retrieved February 17, 2007. 
  6. ^ Poirier, Yves; Somerville, Chris; Schechtman, Lee A.; Satkowski, Michael M.; Noda, Isao (1995). "Synthesis of high-molecular-weight poly([r]-(-)-3-hydroxybutyrate) in transgenic Arabidopsis thaliana plant cells". International Journal of Biological Macromolecules. 17 (1): 7–12. doi:10.1016/0141-8130(95)93511-U. PMID 7772565. 
  7. ^ "Plastics You Could Eat". Retrieved November 17, 2005. 
  8. ^ Jacquel, Nicolas; Lo, Chi-Wei; Wu, Ho-Shing; Wei, Yu-Hong; Wang, Shaw S. (2007). "Solubility of polyhydroxyalkanoates by experiment and thermodynamic correlations". AIChE Journal. 53 (10): 2704–14. doi:10.1002/aic.11274. INIST:19110437. 
  9. ^ Lemoigne, M (1926). "Produits de dehydration et de polymerisation de l'acide ß-oxobutyrique" [Dehydration and polymerization product of β-oxy butyric acid]. Bull. Soc. Chim. Biol (in French). 8: 770–82. 
  10. ^ Tokiwa, Yutaka; Calabia, Buenaventurada P.; Ugwu, Charles U.; Aiba, Seiichi (2009). "Biodegradability of Plastics". International Journal of Molecular Sciences. 10 (9): 3722–42. doi:10.3390/ijms10093722. PMC 2769161 . PMID 19865515. 
  11. ^ Dou, Qing Qing; Liow, Sing Shy; Ye, Enyi; Lakshminarayanan, Rajamani; Loh, Xian Jun (2014). "Biodegradable Thermogelling Polymers: Working Towards Clinical Applications". Advanced Healthcare Materials. 3 (7): 977–88. doi:10.1002/adhm.201300627. PMID 24488805. 
  12. ^ https://www.research.a-star.edu.sg/feature-and-innovation/7194/turning-up-the-heat-on-hydrogels[full citation needed]
  13. ^ https://phys.org/news/2014-05-hydrogel-video.html[full citation needed]
  14. ^ https://www.google.ch/patents/US20100080795[full citation needed]
  15. ^ Loh, Xian Jun; Goh, Suat Hong; Li, Jun (2007). "New Biodegradable Thermogelling Copolymers Having Very Low Gelation Concentrations". Biomacromolecules. 8 (2): 585–93. doi:10.1021/bm0607933. PMID 17291082. 
  16. ^ a b Loh, Xian Jun; Goh, Suat Hong; Li, Jun (2007). "Hydrolytic degradation and protein release studies of thermogelling polyurethane copolymers consisting of poly[(R)-3-hydroxybutyrate], poly(ethylene glycol), and poly(propylene glycol)". Biomaterials. 28 (28): 4113–23. doi:10.1016/j.biomaterials.2007.05.016. PMID 17573109. 
  17. ^ Wu, Yun-Long; Wang, Han; Qiu, Ying-Kun; Liow, Sing Shy; Li, Zibiao; Loh, Xian Jun (2016). "PHB-Based Gels as Delivery Agents of Chemotherapeutics for the Effective Shrinkage of Tumors". Advanced Healthcare Materials. 5 (20): 2679–2685. doi:10.1002/adhm.201600723. PMID 27594657. 
  18. ^ http://www.advancedsciencenews.com/injectable-hydrogels-for-precise-chemotherapy/[full citation needed]

External linksEdit