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POLYHYDROXYALKANOATE (PHA) PRODUCTION FROM METHANE & NATURAL GAS - LITERATURE REVIEW PART 5

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POLYHYDROXYALKANOATE (PHA) PRODUCTION FROM METHANE & NATURAL GAS - LITERATURE REVIEW PART 5

Wed, 04/03/2019 - 18:04
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PHA PRODUCTION FIELD
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Section 5: Applications and Economics of PHA(s)

With several improvements being made thus far to the production PHA(s), their applications can now be extended beyond everyday articles to the medical field, the fuel and agricultural industries. A detailed discussion of these applications is outlined below.

5.1 Industrial Applications

Several industries producing polymers have been able to commercially produce PHAs for wide variety of application. A USA-based company, Metabolix, produces PHA blends of P(3HB) and poly(3-hydroxyoctanoate) from a recombinant E. coli K12 which has been approved by the FDA for food additives. Metabolic also produces a PHA polymer call BIOPOL which is a co-polymer of poly(3-hydroxybutrate-co-3-hydroxyvalerate). BIOPOL can be used to coat paper and paperboards, and its thermoplastic properties also make it suitable for injections, blow molding and film production. BIOPOL can also be drawn into microfilaments or fibers for making fishing nets, ropes and crab cages, and when coated with polyvinyl alcohol, creates a good matrix for seaweed growth. BIOPOL has been used by cosmetic companies such as Baton Rouge, LA, USA to make bottles for hair care products while other have used it to make shampoo bottles and disposable razors. BIOPOL’s antistatic properties, makes it suitable for electric and electronic packaging [82].

 

Procter and Gamble developed Nodax, a PHA copolymer of 3-hydroxybutrate and medium chain length (mcl) hydrocarbon units such 3-hydroxyhexanoate, 3-hydroxyoctanoate and 3-hydroxydecanoate. This very flexible polymer can be used to make flushable that can degrade in the septic system under both aerobic and anaerobic conditions. Nodax has also been explored for the manufacturing of medical surgical garments, compostable bags, carpets and packaging materials. Furthermore, a German company – Biomer, which owns the technology to make P(3HB) in large scale from Alcaligens latus, uses sucrose as feedstock to achieve PHB accumulation of about 90% in cell dry weight. This polymer can be used to make combs, pens and bullets [82]. 

 

PHAs have piezoelectric properties and can therefore be uses to make a number of sensors, gas lighters, acoustics and also for material testing. The flexibility of copolymers makes them suitable for producing the plastic film moisture barrier in nappies and sanitary towels. PHA-based latex can be used to make water resistant surfaces for coating purposes [82].

 

5.2 Biofuel Applications

Biofuel production from PHAs was proposed for the first time in a study carried out by Zhang et al [95]. In this study, P(3HB) and mcl PHAs were converted respectively through an esterification process to obtain 3-hydroxybutyrate methyl ester (3HBME) and medium chain length hydroxyalkanoate methyl ester (3HAME) by acid-catalyzed hydrolysis. Their combustion heats alongside other fuels like ethanol, n-butanol, n-propanol, diesel, and gasoline were investigated. They found that 3HBME and 3HAME had combustion heats of 20 and 30 KJ/g respectively, compared to ethanol, which had a combustion heat of 27 KJ/g. A 10% addition of 3HBME and 3HAME respectively enhanced the combustion heat of ethanol to 30 and 35 KJ/g. Meanwhile addition of 3HBME and 3HAME into n-propanol and n-butanol decreased their combustion heat. The combustion heat of blends of 3HBME/diesel (or 3HBME/gasoline) and 3HAME/diesel (or 3HAME/gasoline) was much lower compared to pure gasoline and diesel. Even though the reason for the decrease is still unknown, it is hypothesized that the long chain carbons in diesel and gasoline resulted to incomplete burning, which could lead to a reduction in combustion effects. A cost estimate for this process was carried out and reported as $1200/ton which is much higher compared to the price of gasoline in the Chinese market of about $800/ton [95]. Even with the high cost of production, this research presents a promising application for the energy sector and a potential to produce a fuel will little to no greenhouse gas emissions.

 

PHA Biofuel

 

5.3 Medical Applications

PHAs such as P(3HB), PHBV, P(4HB), P(3HO) and P(3HB-co-3HHx) are gaining traction in the medical field after being tested in animals. They have been used to develop devices such as sutures, meniscus repair discs, screws, bone plating systems, surgical patches, tissue regeneration devices, stents, scaffolds, bone grafts, just to name a few. They are also very suitable for the slow and controlled release of drugs and hormones [84,96]. It is important to note that commercially available PHAs are industrial grade rather than medical grade, since they contain some impurities such as endotoxins or residual chemicals. Therefore, for PHAs to be suitable for medical applications, their extraction procedures need to produce very pure PHA products without residual proteins or traces of toxic chemicals.

 

A Boston based company-Tepha, now specializes in the production of several medical implants/ devices from PHAs such as heart valves, dressings, stents, dusting powders, just to name few. Tepha also markets P(4HB) under the name PHA4400 for medical application and a recombinant E. coli stain K12 is used to produce the product. However, the γ-irradiation used to sterilize the P(4HB) has a potential to decrease the  polymer’s molecular weight and hence increase the polydispersity index. It is important to note that P(4HB) when compared to other PHAs, conveys better mechanical properties with a very high tensile strength and is the first PHA polymer to be approved by the FDA for biomedical implant material [82]. In a recent study, PHBHHx piezoelectric properties have made it suitable for use in the production of oestosynthetic materials. These materials are used for stimulating bone growth and for repairing damaging nerves [96]. 

 

Biodegradable polymers of lactate and glycolate are commonly used as release products for drug delivery. However, since these polymers can only degrade by bulk hydrolysis, their drug release cannot be fully controlled. Therefore the introduction of PHAs as drug release products in early 90s made them excellent candidates due to biodegradability, biocompatibility and their ability to degrade by surface erosion [82]. In a study, where P(3HB) and PLA were tested as a drug carriers for a specific target, P(3HB) was found to rapidly release the drugs within 24 h compared to PLA which took about 7 days. Some major factors that affect drug release rate include the matrix porosity, polymer composition, polymer crystallinity and the polymer molecular weight. It has also been reported that the high melting temperature of most PHA homo- and co-polymers could be disadvantageous for drug delivery and therefore Mcl PHAs with lower melting temperatures could resolve the problem [82]. Studies so far have only been able to test the drug delivery potential of PHB and PHBV. With the rapid development of new PHA polymers possessing even better properties, their drug delivery capacity could be exploited further. Some researchers recently developed a system consisting of a PHA nanoparticles used to package hydrophobic drugs alongside its associated binding protein, PhaP phasin, which was able to bind the polymer through strong hydrophobic interactions and fused to a polypeptide ligand. This drug delivery system was used to target cancer cells or macrophages and they found that the ligand-PhaP-PHA nanoparticle system was taken up by the targeted macrophages both in vivo and in vitro proving targeted drug delivery [97]. 

 

5.4 Agricultural Applications

PHAs also have several applications in the agricultural industry. PHBV could be used to administer insecticides when crops are sown. When grown alongside the crops, these insecticides within the PHBV polymer would be released at a controlled rate depending on the level of pest infestation [82]. However, the release rate could vary depending on the environmental conditions in the soil. PHAs could also be used as carriers for bacterial inoculants to improve nitrogen fixation since bacterial cells used to prepare long-term inoculants are able to withstand certain harsh environment conditions. This concept was proven both in laboratory scale experiments and on field sites in Mexico where crop yields were observed to increase consistently with PHA-rich inoculants [82]. 

 

PHAs have also been proven to sustain aquaculture. Reports have shown that short chain fatty acids (SCFA) could increase the growth rate of aquatic animals. However, SCFA are highly soluble in water, which could result to low uptake efficiency. Fortunately, PHAs are biodegradable polymers of fatty acids, which are insoluble in water. Therefore, if PHAs are supplemented to the feed, they could subsequently degrade to their fatty acid derivatives in the animal’s intestinal tracts, releasing the fatty acid locally and hence improving upon the uptake efficiency and growth of the aquaculture. A study carried out to investigate the effect of supplementing the feed of European sea bass juveniles with PHB showed that after a 6-week period, PHB acted as an energy source for the fish. When their diets were supplemented with 2% and 5% (w/w) of PHB, the fish weight increased to a factor of 2.4 and 2.7, respectively. The pH of the gut decrease form 7.7 to 7.2 in both cases indicating the presence of fatty acids in the gut. This report also pointed out a difference in the microbial community of the gut on day 14 and 42 suggesting that the changes could be caused both by the fish enzymatic degradation and PHA degrading bacterial potentially present in the gut [98]. Another study isolated the bacteria from the gut of 3 aquatic animals; Siberian sturgeon, European sea bass and Giant River prawn, to investigate their PHA degradation capabilities. This study showed that all isolates were able to degrade PHB but the isolates from sturgeon showed low PHA-degrading capabilities possibly due to the lack of diversity in the bacterial culture compared to the others. When the bacterial isolates and PHB were added to a medium with brine shrimp larvae infected with a virulent pathogenic strain, the larvae’s survival rate increased [99]. These studies go to prove that inclusion of PHB in the fish diet has several benefits in both improving their growth performances and protecting their gut microbes from pathogenic infections.

 

5.5 Economics of PHAs

Bioplastic production has been at an upward rise in the past decade covering close to 15% of the global plastic market to date and its market share is projected to increase up to 30% in 2020. With PHAs being an attractive alternative for current synthetic plastics, these polymers could potentially become a key player in this market. However, PHA production has to become cheaper and cost effective to achieve this goal [8]. 

 

The major factors that affect the economics of PHA production include the polymer yield, productivity of the microbial cultures, the cost of raw materials and the method of PHA recovery from the cell [100]. Choi and Lee carried out an economic evaluation to analyze the PHA production process in 4 different bacterial strains utilizing different substrates for a target production capacity of 2,850 and one million tons of PHB/year. This process employed two PHB recovery methods: a surfactant-hypochlorite digestion and a dispersion treatment of chloroform and hypochlorite, and they showed that the annual operating costs were much lower with the former recovery method. The lowest price of PHB obtained for the lower annual production scale of 2850 tons was $5.58/kg while the price of PHB dropped to $4.75/kg when the production scale was increased to one million tons/year. They also concluded that the cost of the carbon substrate significantly affected the overall economics in large-scale production, and therefore cheap and cost effective substrates could potentially lower the cost of production [101]. 

 

Even with the PHA production process developing at such a fast pace over time, its cost of production is still about three times higher than polypropylene and therefore, in order to dominate the polymer market, more improvements need to be done on the overall production process to make the PHA polymer as competitive as its petroleum-derived counterparts [8]. Table (11) compares the market prices of current commercially available PHA with some standards petroleum bases polymers.

 

Table 11: Market Prices for Biopolymers and Synthetic Polymers. Adapted from L.R. Castilho et al. (2009)

Polymer

Market Prices

P(3HB) form Biomer - Germany

€12/kg

P(3HB-co-3HV) from Metabolix - USA

€10-12/kg

PLA from Cargill Dow - USA

€2-3/kg

Polypropylene (PP)

€0.74/kg

High Density Polyethylene (HDPE)

€0.78/kg

Low Density Polyethylene (LDPE)

€0.74/kg

Polystyrene (PS)

€0.70/kg

 

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