Showing lots of fibre

Nanofibres of pure cellulose can be made from bacteria that live on food waste and extrude them as mats. The fibres are very stiff and, although nobody seems to have measured the strengths of individual fibres, they are likely to be very sturdy. Researchers around the world have, for more than a century, been studying bacteria that produce cellulose. However, development has now reached the point where first products are being produced commercially and a major breakthrough has just been achieved in producing the material in a useful form on an industrial scale, at much lower cost. First mainstream engineering industrial applications are likely to be in filtration, closely followed by composites, but potential applications are being studied in industries ranging from wound care to cosmetics. Emiliano Bilotti, research assistant in the Department of Materials at Queen Mary, University of London, told Eureka at the recent Interplas Show that the bacteria live on sugars, fruit juices and waste from the fruit industry. “They expel very highly crystalline cellulose fibres,” he explained. And, whereas cellulose from wood and plants contains a multitude of things, cellulose from bacteria is very pure. Typical fibres are tens of nanometres across and about 0.5 micrometres long. Bilotti unveiled a Japanese snack food product, in which gelatinous cubes of bacteria and fibres are grown in pineapple juice. “Strengths have not been characterised, but stiffness is 40, 50 or 60 Gpa,” he states. He has been creating films and prototype filters, and is looking at ways of embedding it in polymers to create composites, which is his research speciality. More than one type of bacterium produces cellulose, but the species that has been most studied is Acetobacter xylinum, which extrudes glucan chains from pores on its sides into the growth medium. These aggregate into microfibrils, which bundle to form microbial cellulose ribbons. Some strains of the bacterium are much better than others in producing useful material, both in terms of production quantity and quality of end product. Pure crystals of cellulose, according to articles published by other researchers at Queen Mary, have a Young’s Modulus of 130 to 150 GPa and a tensile strength of 10Gpa - about three times that of Aramid Fibre. To be useful in bulk, however, there has to be some interaction between individual fibres, either in sheet form, spun together or used as reinforcement in a matrix of something else. Claims for its bulk material strength vary enormously in the scientific literature, which probably reflects differences in strains of bacteria used as sources, varying levels of technical skill, and who is trying to justify their research and who is trying to discredit an academic rival. Nonetheless, in bulk, it seems to be possible to produce bacterial cellulose in a form that has a Young’s Modulus similar to that of aluminium - about 70 Gpa - and is about as strong as steel (around 400 Mpa), because the fibres have a greater density of hydrogen bonds than cellulose from, say, wood, resulting in much better adhesion between fibres, whether they be in the form of yarn or film. They are also about one hundredth of the thickness of cellulose fibres from wood, opening up new applications, especially in products that require engineering on a very small scale. In film form, Sony already uses it for diaphragms in its top-end headphones, because it offers the best combination of high Young’s Modulus and damping coefficient of any material known, and as a microfiltration membrane. The main drawback is its cost, since at present it is mainly produced in trays, based on traditional processes for making it as an edible delicacy, and grows mostly at the air-liquid interface, because of the high oxygen demand of the bacterium. Meanwhile, Dr Ida Idayu Muhamad, a senior lecturer in the bioprocess engineering department at the Universiti Teknologi Malaysia in Johor, has taken the production process several stages farther, developing a rotary disk reactor, plus dryer, to produce the material on an industrial scale at lower cost and with lowered water content. Eureka caught up with her at the British Invention Show and learned she is working with pineapple waste, which represents 80% of the total material after the juice has been extracted. The breakthrough in production technology is important, because the device increases production yield, reduces labour requirements and is “easy to scale up”, she says. The basic idea is inspired by rotating biological contactors, used in wastewater treatment. It employs rotating disks that alternately soak the organisms in nutrient and expose them to air. In the re-engineered design, it is able to reduce water content by heating. The process works well with a number of other fruit juices and fruit processing wastes and the production, “Is easy to scale up”. As well as being faster than traditional tray growing methods, reducing production cycle time to seven days, the yield of useful product is tripled. Her methodologies are already attracting interest from one pineapple canning company. Apart from filters and composites, applications are seen in food industries, healthcare and pharmaceuticals, cosmetics, textiles, wound care, skin cream, repairing antique documents and artificial leather. In particular, it is seen as having potential for use as artificial skin, artificial blood vessels, liquid loaded medical pads and speciality membranes, because of its inherent biocompatibility and biodegradability. Potential markets for the material produced as pellets in air-agitated cultures - which seems to be the favoured production method in Japan - include: the mining industry, the oil industry, foods, and the pulp and top-end paper industry. If it could ever be made cheaply enough to be supplied as film for plastic supermarket bags, it would allow everyone to use them again with a clear conscience. Back in 1998, shortly before he retired, Don von Schriltz, DuPont’s global technology director for engineering polymers, told Eureka how the company’s polymer business was originally founded on cellulose and that he could see it returning to those roots in the future. It seems he could well be right. Pointers * Bacterial cellulose fibres are very pure, tens of nm across and about 0.5 micron long * Their stiffness is up to 70 GPa. Fundamental strengths should be at least as great than those of the best commercial polymers, but best bulk strength seems to about the same as that of steel * They can potentially be produced in industrial quantities at greatly lowered cost and water content, and with triple the yield, by a new process