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producing biopolymers with thioester linkages in the polymer backbone using
C. necator in media containing 3-mercaptopropionate (3MP) or 3-mercaptobutyrate (3MB) in addition to 3-hydroxybutyrate as constituents [219–221]. Many factors affect the PHA’s chemical composition like the microbial strain, the substrate, the cultivation condition, the extraction method, the number of
phaC,
phaB genes, the regulator
phaP (phasin) and the presence of inhibitors. They inhibit different pathways, especially those which supply the synthases with different kinds of monomer or inhibit other pathways, which consume these monomers for their own or degrade it to shorter units like
β oxidation pathway. In general, the PHA composition depends on the PHA synthases, the carbon source and the metabolic routes involved. The molecular weights of PHAs were established by light scattering, gel permeation chromatograph and sedimentation analysis. Their monomer composition was determined by gas chromatography (GC), mass spectroscopy (MS) and nuclear magnetic resonance (NMR) analysis [222]. PHAs show material properties that are similar to some common plastics such as polypropylene [223]. The bacterial origins of the PHAs make these polyesters a natural material, and many microorganisms have the ability to degrade these macromolecules [224]. The molecular mass of PHAs varies per PHA producer but is generally in the order of 50 × 10
3 to 1 × 10
6 Da. Inside the cell, P(3HB) exists in a fluid, amorphous state. However, after extraction from the cell with organic solvents, P(3HB) becomes highly crystalline [225] and in this state it is stiff but brittle material. Because of its brittleness, P(3HB) is not very stress resistant. The high melting temperature of P(3HB) (around 170
oC) is close to the temperature in which this polymer decomposes thermally and thus limits the ability to process the homopolymer. The incorporation of 3-hydroxyvalerate (3HV) into the P(3HB) resulted in P(3HB-
co-3HV) copolymer that is less stiff and brittle than P(3HB), that can be used to prepare films that exert excellent water and gas barrier properties like polypropylene, and that can be processed at lower temperature while retaining most of the other excellent mechanical properties of P(3HB) [226]. (P(3HB-
co-3HV)) has also low crystallinity and is more elastic than P(3HB) [227, 228]. The latex-like PHAs (PHA
MCL) display physical properties, which differ significantly from the PHA
SCL, such as P(3HB). Particularly with respect to the melting temperature and the extension to break value the two types of PHAs showed, mainly because of the lower crystallinity of PHA
MCL, striking differences. PHAs have been processed into fibers, which were then used to construct materials such as non-woven fabrics [229]. Moreover P(3HB) and P(3HB-
co-3HV) were described as hot-melt adhesives [230]. They are considered for several applications in the packaging industry, medicine, pharmacy, agriculture and food industry or as raw materials for the synthesis of enantiomerically pure chemicals and the production of paints [231]. Possible applications of P(3HB) and copolymers are as packaging materials or agricultural foil [232]. As in the Biopol
TM recovery process, the fermentor contents are heat treated to break down the nucleic acids, and proteases and detergents are added to solubilize the cells. Subsequent washing (i.e., removal of the solubilized cell material) and concentration of the resulting PHA latex is established by cross-flow microfiltration. To produce a coating, the PHA latex is sprayed onto a substrate such as paper. After evaporation of the water, the PHA latex particles readily coalesce into a film [218]. Due to the relatively high cost of PHA production, it is wise to apply PHAs for some cost-effective applications like medicinal instruments. PHAs were proved to be biocompatible in tissue engineering, implantations, etc. Many prokaryotic and eukaryotic organisms are able to produce LMW PHB molecules that are complexed with other biomolecules such as polyphosphates and that are present at low concentrations [233]. Over recent years, PHAs were used to develop many devices and material useful for clinical purposes such as suture fasteners, meniscus repair devices, rivets, tacks, staples, screws, (including interference screws), bone plates and bone plating systems, surgical mesh, repair patches, orthopedic pins (including bone filling augmentation material), adhesion barriers, stents, guided tissue repair regeneration devices, articular cartilage repair devices, nerve guides, tendon repair devices, pericardial patches, bulking and filling agents, vein valves, bone marrow scaffolds, meniscus regeneration devices, ligament, tendon grafts, ocular cell implants, spinal fusion cages, skin substitutes, dural substitutes, bone graft substitutes, bone dowels, wound dressing and hemostats [234–240]. Many biochemical engineering, molecular biology experiments and other tools were used to change the end products of the polyhydroxyalkanoate or to produce copolymers. To assess the biocompatibility of PHB, the structural organization of cellular molecules involved in adhesion was studied using osteoblastic and epithelial cell lines. On PHB, both cell lines revealed a rounded cell shape due to reduced spreading. The filamentous organization of the actin cytoskeleton was impaired. In double immunofluorescence, analyses the co-localization of the fibronectin with the fibril actin was demonstrated [241]. The investigated properties of PHB and PHB-co-PHV films proved to be fundamentally similar [242–246]. PHB-co-PHV film was chosen as a temporary substrate for growing retinal pigment epithelium cells as an organized monolayer before their subretinal transplantation. The surface of the PHB-co-PHV film was rendered hydrophilic by oxygen plasma treatment to increase the reattachment of D407 cells on the film surface. The cells were also grown to confluency as an organized monolayer suggesting PHB-co-PHV film as a potential temporary substrate for subretinal transplantation to replace diseased or damaged retinal pigment epithelium [247]. Tesema et al. and Malm et al. implanted PHB non-woven patches as transannular patches into the right ventricular outflow tract and pulmonary artery in 13 weanling sheep [248–250].
PHB non-woven patches can be used as a scaffold for tissue regeneration in low-pressure systems. The regenerated vessel had structural and biochemical qualities in common with the native pulmonary artery [250]. PHAs were used in tissue engineering, as antibiotic carriers, and many other medicinal applications [238, 251, 252]. Chen and Wu recently reported that PHAs possesses the biodegradability, biocompatibility and thermo-processibility for not only implant applications but also controlled drug release uses. PHAs show a promising future in pharmaceutical application such as drug delivery, which open a new approach. The many possibilities to tailor-make PHAs for medical implant applications have shown that this class of materials has a bright future as tissue engineering materials [253]. Different types of mutagenesis were applied for changing the substrate specificity, study the catalytic residues and to overproduce the PHAs [254–257].
2.6 Biopolymer Type Number 5: Polyisoprenoides
2.6.1 Natural Rubber
Natural rubber is a cis-1,4-polyisoprene-based biopolymer that has good resilience and damping behavior, but poor chemical resistance and processing capacity. It is collected from the milky secretion (latex) of individual trees, but the Hevea brasiliensis tree is the only important commercial source of natural rubber (sometimes called Pará rubber). Guayuleule is the only other plant under cultivation as a commercial source of rubber (Parthenium argentatum). Tyres, computer components, gloves, toys, shoe soles, elastic bands, flippers, erasers and athletic equipment are well-known uses of natural rubber. It is typically used for applications that need resistance to abrasion/wear; elastic resistance and properties that absorb damping or shock. In the production of synthetic rubber, oil is one of the necessary substituents. Natural rubber has enjoyed a rising market share due to the cost of oil and has become an attractive replacement for synthetic rubber. Since natural rubber has better properties compared to other synthetically manufactured rubber, rubber industries usually use it to enhance properties and extend applications of other rubber materials by blending. A very low level of adherence to other materials has also been documented. Natural rubber blended with virgin and recycled ethylene-propylene-diene monomer has been reported by Hayeemasae et al., the curing rate of natural rubber vulcanization was decreased. This was due to the incompatibility of these materials with natural rubber being cured. However, the maximum torque for the recycled material was increased with the addition of both virgin and recycled EPDM and was even higher. This was due to the higher density of cross-linking implemented by EPDM. Natural rubber is an economically important biopolymer with unparalleled performance characteristics, such as high elasticity, durability and efficient heat dispersion [258, 259]. Normal rubber is poly (cis-1,4-isoprene) 300 to 70000 isoperene molecules are coupled to form an irregular structure that cannot crystallize under normal conditions that mediate the amorphous rubber texture. Normal rubber is poly (cis-1,4-isoprene). Currently, the major rubber
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