Plastics are everywhere. They have outdone many of the traditional materials with their durability, flexibility and water-resistance among numerous other useful characteristics.
However, despite their versatility in various domestic, industrial and medical applications, plastics bring forth a myriad of environment-related problems. These plastics are mostly synthetic and are derived from petroleum-based chemicals. The rate of exhaustion of the mineral oil resources is one of greatest concerns regarding the production of these synthetic plastics. Furthermore, owing to their non- or slow- biodegradability, the accumulation of the plastic wastes in the environment has become an increasing problem.
In the light of such situation, biomass-derived biodegradable bioplastics have gained the interest of many scientists as a more sustainable material. Bioplastics can be of many types and are obtained from various biological sources including plants and microbes. This article focuses on one such material, polyhydroxyalkanoates-commonly known as PHAs- and their microbial biosynthesis.
Polyhydroxyalkanoates are fully biodegradable and biocompatible. Their properties are similar to those of petrochemical-based traditional plastics and they are available in a variety of polymers depending on the number of monomer units, thus displaying a wide selection of physical and chemical properties.
PHAs are linear polyoxoesters of R-hydroxyalkanoic acid (HA) monomers, with a number of carbon atoms ranging from 4-14. They are classified based on the number of carbons and the type of monomeric units producing homopolymers or heteropolymers.
Short chain length PHAs (SCL-PHAs) such as poly(3-hydroxybutyrate) or P(3HB) and poly(4-hydroxybutyrate) or P(4HB) contain 3–5 carbon atoms. Medium chain length PHAs (MCL-PHAs) such as homopolymers poly (3-hydroxyhexanoate) or P(3HHx), poly(3-hydroxyoctanoate) or P(3HO) and heteropolymers such as P(3HHx-co-3HO) contain 6-14 carbon atoms.
Applications of PHAs in Industry
PHAs are currently being used in many industrial purposes such as packaging materials (mostly cosmetic containers and food packaging material), moisture barrier in sanitary towels and nappies, etc.
Owing to their immunological inertness, PHAs are considered a good candidate to be used in medical applications such as cardiovascular products, prodrugs, dental and maxillofacial treatment, drug delivery vehicles, wound sutures and dressings etc.
Biosynthesis of PHAs
Many living organisms, mainly plants and bacteria produce PHAs. However, microorganisms are more suitable for the industrial production of PHAs due to the fact that plant cells can only accumulate low yields of PHAs i.e. less than 10% (w/w) without adversely affecting their growth. In contrast, bacteria are known to store up PHAs at levels as high as 90% (w/w) of their dry cell weight.
A wide range of Gram-positive and Gram-negative bacteria including Aeromonas hydrophila, Alcaligenes latus, Pseudomonas sp, Bacillus sp, and Methylobacterium sp naturally synthesise PHAs. These bacteria produce PHA as a carbon and energy storage compounds under imbalanced nutritional conditions. When carbon is in excess and the other nutrients such as nitrogen or phosphorous or oxygen is limited, PHA is accumulated in the form of water-insoluble granules in their cytoplasms.
Industrial-Scale Production of PHA
Fermentation of PHA is carried out in a two-stage fed-batch process. The first stage, which is aimed to increase the cell density of the bacterial culture, is operated in nutrient-rich media that supports the growth. In the second–stage the aim is to increase the PHA concentrations by the depletion of a nutrient such as nitrogen. Other parameters such as temperature, pH, etc. depend on the choice of microorganisms.
After fermentation, bacterial cells containing PHAs are separated from the medium by centrifugation. Various methods for the recovery of intracellular PHA have been studied including solvent extraction, cell disruption, pre-treatment of cells, chemical or enzymatic digestion of substances other than polyhydroxyalkanoates in the system, extraction using supercritical CO2, however, an economical, safe and industrially feasible process is yet to be exploited in order to maximise the recovery yields.
Despite their environmental friendly characteristics and potential usage in many different purposes, the cost of production is still high compare to the conventional petrochemical-based plastics. Therefore, researches are being carried out on reducing the production cost allowing a more viable large-scale production of PHAs. The key areas under study are the use of cheap carbon sources including wastes and byproducts, recombinant microbial strains, increasing fermentation efficiency, efficient recovery and purification processes etc.
Keshavarz, T., & Roy, I. (2010). Polyhydroxyalkanoates: bioplastics with a green agenda. Current opinion in microbiology, 13(3), 321-326.