Imagine a Packaging Material that Kills Bacteria and Keeps Food Longer

Their day job is to keep trees upright. But now the forest's tiniest building blocks are on their way into fancy products for the future.

Imagine a packaging material that kills bacteria and keeps food longer in good condition. Or a disposable duvet cover that keeps infection away from you when you lie in a hospital bed.

Scientists in Trondheim believe that a lot of exciting new products can be created if we can manage to make use of some of Nature's tiniest construction materials. They are called “fibrils”; a word you have probably never heard of. But in fact, there are millions of them in the paper you have in your hands just now.

A wonder of Nature
Midsummer night, 2005: a steady stream of print journalists and TV teams arrive on the SINTEF/NTNU campus, where they are greeted by proud metallurgists in lab-coats and safety helmets. They have achieved large-scale production of carbon nanotubes, a material with a tensile strength ten times as high as the strongest steel, but weighing only one tenth as much.

This super-material was created in a 30 000 degree plasma arc.

Little do the reporters know of what is going on in the building next door, which belong to the Paper and Fibre Institute (PFI). There, and in the adjacent laboratories, a handful of busy people from PFI, SINTEF and NTNU are working on fibrils – nanocomponents that Nature creates all by itself, with the help of sunlight, air and water.

Fibrils form continuously in all growing trees. In terms of strength, they cannot be compared with carbon nanotubes, but they are strong enough for SINTEF's Bjørn Steinar Tanem to regard them as potential reinforcement materials in plastics, as he sits and admires them in the electron microscope.

Reinforcement rods of sugar
Fibrils are Nature's own mini-mini-reinforcement rods. They consist of long sugar molecules (cellulose), arranged in bundles These bundles make up the wall of the drinking straw-like wood cells that tree-trunks consist of.

It is the tough strength of the fibrils that keeps the giants of the forest swaying but upright in the strongest gusts of wind. The material has evolved in one of the world's biggest nano-laboratories: the forest.

“And Nature has taken millions of years to perfect the process,” says Tanem.

In paper mills, the cells are beaten and squashed flat, to re-appear in the form of paper fibres; or by boiling them, they can be turned into cellulose.

It is already quite possible to separate the fibrils from wood cells, and to extract bundles of molecules that are measured in nanometres; i.e. millionths of a millimetre. But the process is expensive.

Now scientists in many laboratories in the western world, including Trondheim, are trying to make the process more energy- and cost-efficient. But this is no easy job, according to Kristin Syverud of the Paper and Fibre Institute.

“It has taken a lot of energy to build up these wood cells in Nature, and then we come along and want to use as little energy as possible to tear them apart again.”

Valuable strength
PFI has been working for three years on a basic research project on fibrils, with financial support from the Research Council of Norway and in close cooperation with scientists from SINTEF and NTNU. According to Syverud, there is no lack of scientific challenges, but she believes that the topic is worth a bit of effort, since fibrils possess many qualities that fascinate the scientists, one of them being their strength.

Fibrils are long in comparison with their diameter, which makes them good at absorbing forces. According to SINTEF's Tanem, they are therefore very suitable as reinforcements for plastics. He predicts, for example, that they could enable plastics to be used in automotive components.

The Trondheim scientists wish to use fibrils in biopolymers; materials produced from natural products such as maize starch. The aim is to develop composite structures whose life cycle will have the least possible impact on the environment.

“Fibrils can give biopolymers new, improved properties which, in conjunction with good design, could form the basis of thinner-walled moulded products, for example, thus reducing the amount of raw material needed,” says Tanem.

However, the first necessity is for more research. For one thing, getting the fibrils into plastic is no simple task. But according to Tanem, the group has already made progress in this aspect.

As a parallel activity, PhD student Martin Andersen at NTNU and SINTEF's Per Martin Stenstad have been manipulating the surface of the fibrils, producing the alterations that are needed to make them “comfortable” within the plastic matrix.

PFI's Kristin Syverud is particularly taken by the results of a quite different application.

Combating bacteria
The surface of fibrils makes it easy to link them to other active substances, and here too, surface scientists Andersen and Stenstad have been using their expertise. Stenstad, in fact, has worked on similar projects with the famous “Ugelstad microspheres”. For this “fibrils with attachments” variant, the Trondheim scientists selected a chemical that kills micro-organisms, which they have managed to make stick tightly to the fibrils.

“This is important, for substances of this sort must not leach out and end up in the wrong place,” says Syverud.

She explains that these results have spawned exciting product concepts within the project group, including the idea of using fibrils to make bactericidal food wrappings, disposable duvet covers and water filters.

The list of potential applications for fibrils is long, ranging over several branches of industry (see fact-box). However, the scientists still have a good deal to work on before fibrils are ready to hit the shelves.

Processing fever
“Controlling the size distribution of the fibrils once they have been separated out is one of the challenges that still make us tear our hair,” says Syverud, who has been leading the project together with Per Stenius, an adjunct professor at NTNU.

Two different mechanical techniques are in use today to extract the fibrils from the wood cells: a mill, and a nozzle that produces a large pressure drop. Both are energy-intensive. However, according to Syverud,there is already know-how, for example at PFI's Swedish owners, that will significantly reduce energy consumption. She is also quite certain that this research will lead to commercial products.

“However, which of all the potential areas of application will take off is something we don't know yet. And I am sure that not all of our ideas will end up as products.”

The western world's cellulose industry is the driving force behind fibril research. The industrialised world has realised that it is difficult to compete on price for traditional cellulose, and is looking around for applications for processed cellulose. Kristin Syverud believes that the global focus on the environment will contribute to the demand for fibrils.

“They are a renewable resource that is being created by Nature around us every day. And they are certainly cheap,” she says.

As for me, I will be looking at trees with a bit more respect next time I am walking in the woods.