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Polyhydroxyalkanoates orPHAs arepolyesters produced in nature by numerous microorganisms, including throughbacterialfermentation ofsugars orlipids.[1] When produced by bacteria they serve as both a source of energy and as a carbon store. More than 150 differentmonomers can be combined within this family to give materials with extremely different properties.[2] These plastics are biodegradable and are used in the production ofbioplastics.[3]
They can be eitherthermoplastic orelastomeric materials,[citation needed] withmelting points ranging from 40 to 180 °C.[citation needed]
The mechanical properties andbiocompatibility of PHA can also be changed by blending, modifying the surface or combining PHA with other polymers, enzymes and inorganic materials, making it[clarification needed] possible for a wider range of applications.[4]
To induce PHA production in a laboratory setting, a culture of a micro-organism such asCupriavidus necator can be placed in a suitable medium and fed appropriate nutrients so that it multiplies rapidly. Once the population has reached a substantial level, the nutrient composition can be changed to force the micro-organism to synthesize PHA. The yield of PHA obtained from the intracellular granule[further explanation needed] inclusions can be as high as 80% of the organism's dry weight.[citation needed]
The biosynthesis of PHA is usually caused by deficiency conditions (e.g. lack of macro elements such as phosphorus, nitrogen, trace elements, or lack of oxygen) and the excess supply of carbon sources.[5] However, the prevalence of PHA production within either a mono-culture or a set of mixed-microbial organisms can also be dependent on overall nutrient limitation, not just macro elements. This is especially the case in the 'feast/famine' cycle method for induction of PHA production, wherein carbon is periodically added and depleted to cause famine, which encourages cells to produce PHA during 'feast' as a storage method for periods of famine.[citation needed]
Polyesters are deposited in the form of highly refractive granules in the cells. Depending upon the microorganism and the cultivation conditions, homo- orcopolyesters with different hydroxyalkanoic acids are generated. PHA granules are then recovered by disrupting the cells.[6] RecombinantBacillus subtilis str. pBE2C1 andBacillus subtilis str. pBE2C1AB were used in production of polyhydroxyalkanoates (PHA) and it was shown that they could usemalt waste as carbon source for lower cost of PHA production.[citation needed]
PHA synthases are the key enzymes of PHA biosynthesis. They use the coenzyme A - thioester of (r)-hydroxy fatty acids as substrates. The two classes of PHA synthases differ in the specific use of hydroxy fatty acids of short or medium chain length.
The resulting PHA is of the two types:
A few bacteria, includingAeromonas hydrophila andThiococcus pfennigii, synthesize copolyester from the above two types of hydroxy fatty acids, or at least possess enzymes that are capable of part of this synthesis.
Another even larger scale synthesis can be done with the help of soil organisms. For lack of nitrogen and phosphorus they produce a kilogram of PHA per three kilograms of sugar.
The simplest and most commonly occurring form of PHA is the fermentative production ofpoly-beta-hydroxybutyrate [poly(3-hydroxybutyrate), P(3HB)], which consists of 1000 to 30000 hydroxy fatty acid monomers.
In the industrial production of PHA, the polyester is extracted and purified from the bacteria by optimizing the conditions of microbial fermentation ofsugar,glucose, orvegetable oil.
In the 1980s,Imperial Chemical Industries developedpoly(3-hydroxybutyrate-co-3-hydroxyvalerate) obtained via fermentation that was named "Biopol". It was sold under the name "Biopol" and distributed in the U.S. byMonsanto and laterMetabolix.[7]
As raw material for the fermentation, carbohydrates such as glucose and sucrose can be used, but also vegetable oil or glycerine from biodiesel production. Researchers in industry are working on methods with which transgenic crops will be developed that express PHA synthesis routes from bacteria and so produce PHA as energy storage in their tissues. Several companies are working to develop methods of producing PHA from waste water, includingVeolia subsidiary Anoxkaldnes.[8] and start-ups, Micromidas,[9] Mango Materials,[10][11] Full Cycle Bioplastics,[12] Newlight andPaques Biomaterials.[13][14]
PHAs are processed mainly via injection molding, extrusion and extrusion bubbles into films and hollow bodies.
PHA polymers are thermoplastic, can be processed on conventional processing equipment, and are, depending on their composition, ductile and more or less elastic.[15] They differ in their properties according to their chemical composition (homo-or copolyester, contained hydroxy fatty acids).
They areUV stable, in contrast to other bioplastics from polymers such aspolylactic acid, partial ca. temperatures up to180 °C, and show a low permeation of water. Thecrystallinity can lie in the range of a few to 70%. Processability, impact strength and flexibility improves with a higher percentage ofvalerate in the material. PHAs are soluble in halogenated solvents suchchloroform,dichloromethane ordichloroethane.[16]
PHB is similar in its material properties topolypropylene (PP), has a good resistance to moisture and aroma barrier properties. Polyhydroxybutyric acid synthesized from pure PHB is relatively brittle and stiff. PHB copolymers, which may include other fatty acids such as beta-hydroxyvaleric acid, may be elastic.
Due to itsbiodegradability and potential to createbioplastics with novel properties, much interest exists to develop the use of PHA-based materials. PHA fits into thegreen economy as a means to create plastics from non-fossil fuel sources. Furthermore, active research is being carried out for thebiotransformation "upcycling" ofplastic waste (e.g.,polyethylene terephthalate andpolyurethane) into PHA usingPseudomonas putida bacteria.[17]
A PHA copolymer calledPHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)) is less stiff and tougher, and it may be used as packaging material.
In June 2005, US companyMetabolix, Inc. received the USPresidential Green Chemistry Challenge Award (small business category) for their development and commercialisation of a cost-effective method for manufacturing PHAs.[18]
There are potential applications for PHA produced by micro-organisms[2] within the agricultural,[19] medical and pharmaceutical industries, primarily due to their biodegradability.
Fixation and orthopaedic applications have includedsutures, suture fasteners,meniscus repair devices,rivets, tacks, staples, screws (including interference screws), bone plates and bone plating systems, surgical mesh, repair patches, slings, cardiovascular patches, orthopedic pins (including bone.lling augmentation material),adhesion barriers,stents, guided tissue repair/regeneration devices, articularcartilage repair devices, nerve guides,tendon repair devices,atrial septal defect repair devices, pericardial patches, bulking and filling agents,vein valves,bone marrow scaffolds, meniscus regeneration devices,ligament and tendon grafts,ocular cell implants, spinal fusion cages, skin substitutes,dural substitutes, bone graft substitutes, bone dowels, wound dressings, andhemostats.[20]
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