Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class that was first isolated and characterized in 1925 by French microbiologist Maurice Lemoigne. PHB is produced by microorganisms (like Ralstonia eutrophus or Bacillus megaterium) apparently in response to conditions of physiological stress. The polymer is primarily a product of carbon assimilation (from glucose or starch) and is employed by micro-organisms 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.[1]
The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but many 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.
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Currently, plastics and synthetic polymers are mainly produced from hydrocarbons, which do not decompose and they are stored, burnt or recycled at end of life. However, it is possible to produce compostable plastic using PHB. Fermentation can also be used to produce the alcohol needed to produce plastic, although this renewable source of energy is expensive and is not used commercially.
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[2] 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.[3] But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB.[4]
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.
Another group of researchers at Micromidas Inc. have begun to produce PHB from the bacteria in municipal waste water. This approach shows promise for the future of human waste disposal and biodegradable plastic production.[5]
Biopol is currently used in the medical industry for internal suture. It is non toxic and biodegradable, so it does not have to be removed after recovery.