S. Priyanka and R. Rajam *

School of Bio Sciences and Technology

VIT University, Vellore - 632 014, India.

e-mail: rajendran.n@vit.ac.in*

Introduction

Biomaterials are natural products synthesized and catabolised by different organisms and have broad biotechnological applications. They can be assimilated by many species and do therefore possess biocompatibility with the host. In this way, they confer upon them a considerable advantage with respect to other conventional synthetic products. Bioplastics are biomaterials that are polyesters produced by a range of microbial sources, and plants under different nutrient and environmental conditions. They are derived from renewable biomass sources, such as vegetable oil, corn starch, and pea starch. Table 1 summarizes the different types of bioplastics based on the chemical nature. The production and use of bioplastics is generally regarded ecofriendly as compared to plastic production from petroleum. The reason for this is former relies less on fossil fuel as a carbon source and also hazardous waste released is lesser or even negligible when compared to that of oil-derived plastics. In Europe, bioplastics account for 60% of the biodegradable materials market. The most common end use market is for packaging materials. Japan has also been a pioneer in bioplastics, incorporating them into electronics and automobiles.

Table 1 : Types and properties of bioplastics

Biological Sources

Plastics manufactured by conventional methods come from non-renewable hydrocarbon resources. They cannot be broken down easily by microorganisms as they consist of long polymer molecules tightly bound to one another. Biodegradable plastics can be made using polymers from bacteria, algae and plants as they consist of shorter, more easily degraded polymers and hence seem to be a fascinating option today. Following is a brief account of how each group of organisms can be utilized for the purpose.

1. Algae

Algae serve as an excellent feedstock for plastic production owing to its many advantages such as high yield and the ability to grow in a wide range of environments. Algae bioplastics mainly evolved as a byproduct of algae biofuel production. Algae based plastics have been a recent trend in the era of bioplastics compared to traditional methods of utilizing feedstocks of corn and potatoes as plastics. While algae-based plastics are in their infancy, once they are into commercialization they are likely to find applications in a wide range of industries. However, before commercialization is realized, many technical problems have to be negated. Cereplast the company which makes ‘Cereplast Algae Plastics’ produce the plastic that contains only 50% algae. Plastics that comprise material derived 100% from algae are still not a reality and require innovative developments. The use of biotechnological techniques can play a key role in conducting the feasibility and sustainability studies in algae bioplastics.

2. Fungi

The contribution of this group of organisms towards bioplastic production is still not perceptible.

3. Plants

Crop plants are capable of producing large amounts of a number of useful chemicals at a low cost compared to that of bacteria or yeast. Commercialization of plant derived bioplastics, particularly PHAs will require the creation of transgenic crop plants that in addition to high product yields have normal plant phenotypes and transgenes that are stable over several generations. In contrast to bacteria, plant cells are highly compartmentalized hence the desired genes for example phb must be targeted to the compartment of the plant cells where the concentration of the precursor molecule is high. Many oil crops such as rapeseed, sunflower and soybean could be potentially engineered for the production of PHA. The other plants currently in use for PHA production are Gossypium hirsutum and Zea mays. The advantage looks more with the starch-producing crops than oil crops in terms of yield (kg/hectare) but the diversion of precursor molecules towards PHB synthesis is likely to be more complex in starch crops since the flux of carbon is primarily directed towards sucrose.

4. Bacteria

Bacteria are so far the most widely studied organisms with regard to production of bioplastics. Particularly, PHAs are synthesized by many gram-positive and gram-negative bacteria from at least 75 different genera. PHAs extracted from bacterial cells show material properties that are similar to polypropylene (Braunegg et al.. 1998). These polymers are accumulated intracellularly (Fig. 1) to levels as high as 90% of the cell dry weight under conditions of nutrient stress and act as a carbon and energy reserve (Madison and Huisman, 1999). The occurrence of PHAs in bacteria has been known since 1920s, when Lemoigne reported the formation of poly 3-hydroxybutyrate (PHB) inside bacteria (Lemoigne, 1926). Non-storage PHA that are of low molecular weight, have been detected in the cytoplasmic membrane and cytoplasm of Escherichia coli. The following are some of the most important bacterial species utilized for the production of PHAs. They are Alcaligenes eutrophus, Bacillus megaterium QMB1551, Klebsiella aerogenes recombinants, Methylobacterium rhodesianum MB 1267, Pseudomonas aeruginosa, P. denitrificans, P. putida, P. oleovorans and Sphaerotilus natans (Reddy et al., 2003). Additional information on this aspect can be obtained from Verlinden et al. (2006).

Fig. 1: Intracellular accumulation of PHAs in bacterial cells

(Source: www.technewsdaily.com)

The main candidates for the large-scale production of PHAs are plants and bacteria. However, plant cells give very low yields [<10% w⁄w dry weight]. High levels [10 - 40% w⁄w dry weight] have been shown to have a negative effect on growth and development of plant. In bacteria they can be accumulated up to 90% of the cell dry weight. Accumulating PHAs is a natural way for bacteria to store carbon and energy, when nutrient supplies are imbalanced. These polyesters are accumulated when bacterial growth is limited by depletion of nitrogen, phosphorous (Shang et al., 2003) or oxygen and an excess amount of a carbon source is still present. While the most common limitation is nitrogen, for some bacteria, such as Azotobacter sp., the most effective limitation is oxygen (Dawes, 1990). The first PHA to be discovered and therefore the most studied is PHB. In their metabolism, bacteria produce acetyl-coenzyme-A (acetyl-CoA), which is converted into PHB by three biosynthetic enzymes(Fig. 2)

Fig. 2. Metabolic pathway for PHB

Petroplastics vs Bioplastics : Pros and Cons

Although a lot of expectations have been pinned on bioplastics, but many aspects have to be dealt to make the future of these commercially viable. Most important of all is the cost feasibility of bioplastics, apart from this, there is a concern about genetically modified organisms, sustainably grown biomass, there is an urgent need to develop composting programs and infrastructure, also there is a lack of adequate labeling and concern over contamination of recycling systems. Inspite of all these points, bioplastics have many merits over the petroplastics as depicted in figure 3.

Fig. 3. Bioplastics vs Petroplastics

Applications

Apart from the general applications like manufacture of polythene bags, trays, containers and bottles for soft drinks and dairy products, blister foils for fruit, vegetables and medicines are manufactured from bioplastics. However, now this trend is expected to change with the innovation and upgradation in technology and these biomaterials will intervene in the manufacture of products such as mobile phones, cameras, medical devices, electronics, as well as automotive parts. PHAs vary in toughness and flexibility, depending on their formulation. As such they can be used either in pure form or as additives to oil derived plastics such as polyethylene. However, these bioplastics are currently far more expensive than petrochemically based plastics and are therefore used mostly in applications that conventional plastics cannot perform, such as medical applications. Most interesting application to come up is the application of these bioplastics in tissue engineering. PHAs are immunologically inert and are only slowly degraded in human tissue, which means they can be used as devices inside the body. Scientists have envisaged the use of these for making artificial bones, pacemakers, shunts etc. Additionally, they have the potential to be used as drug delivery agents for constant and uniform release of drug over a period of time in human body. Studies related to these aspects are still very preliminary and a lot of research has to be done in this area.

References

Braunegg, G., Lefebvre, G. and Genser, K. F. (1998) Polyhydroxyalkanoates, biopolyesters from renewable resources: Physiological and engineering aspects. Journal of Biotechnology 65, 127 - 161.

Dawes, E. (1990) Novel Biodegradable Microbial Polymers. Dordrecht, The Netherlands: Kluwer Academic Publishers.

Lemoigne, M. (1926) Produits de dehydration et de polymerisation de l’acide ß- oxobutyrique. Bulletin de la Société de Chimie Biologique. 8, 770 - 782.

Madison, L. L. and Huisman, G. W. (1999) Metabolic engineering of poly (3-hydroalkanoates): from DNA to plastic. Microbiology and Molecular Biology Reviews. 63, 21 - 53.

Reddy, C. S. K., Ghai, R., Rashmi and Kalia, V. C. (2003) Polyhydroxyalkanoates: an overview. Bioresource Technology. 87, 137 - 146.

Shang, L., Jiang, M. and Chang, H. N. (2003) Poly (3-hydroxybutyrate) synthesis in fed-batch culture of Ralstonia eutropha with phosphate limitation under different glucose concentrations. Biotechnology Letters. 25, 1415 - 1419.

Verlinden, R. A. J., Hill, D. J., Kenward, M. A., Williams, C. D. and Radecka, I. (2007) Bacterial synthesis of biodegradable Polyhydroxyalkanoates. Journal of Applied Microbiology 102, 1437 – 1449.