Biopolymers - Bacteria as plastic manufacturers

A team of researchers is hard at work trying to stimulate bacteria obtained from the ground and sewage plants to produce valuable biopolymer mass. When fed with nutrients and carbon substrates, some of the microorganisms in the bioreactor will, under carefully controlled conditions, produce a biomaterial that is both environmentally compatible and sustainable. Its great advantage compared with synthetic materials manufactured from crude oil is that it is derived from a perpetually renewable source and is completely biodegraded by fungi or bacteria at the end of its useful life.

To date more than 90 microorganisms are known to synthesise a carbon and energy storage compound consisting of natural polyesters known as polyhydroxyalkanoates (PHA) inside their cells. The granules of PHA can be separated from the dried cells using organic solvents and are finally precipitated using ice-cold methanol or ethanol. What is left is a plastic-like polymer mass that possesses highly specific properties and is ready for use in a wide variety of industrial and medical applications.

The most PHA mass is synthesized in the cells when the bacteria are given a surfeit of carbon substrates, for example in the form of fatty acids. If another nutrient is limited at the same time, the microorganisms cease replication and start to produce biopolymers.

Thus the composition and physical properties of the product can be controlled by the use of carbon substrate mixtures in ideal growth conditions. Whether the biopolymer mass turns out water-repellent, brittle, elastic or sticky depends on the type of microorganisms and the growth conditions in the bioreactor.

In their efforts to obtain biopolymers with customized properties, scientists are studying these growth conditions in minute detail. Continuous cultivation in a chemostat is essential for total control of the cell physiology. By fully understanding of the nature of the cell physiology and the ways it can be influenced, the team is able to work out countless new formulae for the biosynthesis of PHA in bacteria cultures.

The biopolymers produced with these formulae are rated not only on its physical properties, but also on their biological and chemical features. Short-chain-length PHAs (sclPHA) are thermoplastics and can be used for medical applications and disposable items such as laboratory materials or bodycare products. Medium-chain-length PHAs (mclPHA) have a fairly low melting point and are therefore more flexible than the brittle and stiff short-chain-length PHAs. mclPHAs are also ideal for medical applications. The possible applications for biopolymers include sutures, prosthetic skin, implants, nerve support tissue, artificial arteries, veins and heart valves.

But this is not all. PHAs also possess chemical properties that enable them to act as substrates for other substances. Thus it will one day be possible to deposit drugs in implants. As the human body degrades the implant, accurately measured doses of drugs will be released over precisely defined periods of time. Basic research into this special application is currently in progress in vitro at Empa.

PHAs are also suitable for industrial use. For example, zosteric acid, a non-toxic anti-rot agent derived from seaweed, can be bonded to the biopolymer. The biopolymers carrying the zosteric acid could then be used as a surface impregnation in the battle against the problem of biofilm.

For further information please contact:

Empa, Biocompatible Materials Section, Dr Manfred Zinn, tel. (+41) (0)71 274 76 98, E-mail:

Empa, Functional Polymers Section, Dr Roland Hany, tel. (+41) (0)1 823 40 84, E-mail: