Usually burnt to heat the facilities at pulp and paper-making operations after valuable carbohydrate components such as cellulose have been separated from the woody feedstock—and sometimes cursed for its tendency to stick like glue to the other components of the wood—the polymer lignin, making up almost a third of the wood in trees, has become hot property in research and development (R&D) geared at making bio-based products.
So what’s new, you might wonder. Biorefinery operators such as Borregaard of Norway and Domsjö Fabriker of Sweden have been using lignin for other products than energy for some time, mainly as a component of cement. Carbon fibres have been developed for various applications, for instance by the Swedish research institute Innventia; and there is Borregaard occupying a niche with the way in which it makes vanilla flavouring from the lignin polymer. Still, as biomass researcher John Ralph of the U.S.-based University of Wisconsin-Madison said in a recent interview with Swedish science journalists, 'Nothing has come to the top yet as being a winner application' made from lignin.
Part of the reason for that is likely the complexity of lignin—making it hard to break away from the rest of the wood and perhaps even to understand—and its tendency to cling to the carbohydrates cellulose and hemicellulose inside the wood, like a cement holding them together. After all, lignin is what gives plants their sturdiness and allow them to reach their stems towards the sky despite gravity's pulling the other way.
Either way, Ralph should know. On 27 August he presented a Lignin 2014 conference with groundbreaking fundamental research on how to alter trees from within, by introducing a modification designed to make its lignin content more malleable (watch a video excerpt of his presentation on http://bio4energy.se). The result would be a tree, say a poplar tree, with additional readily cleavable bonds in a part of its lignin content (in the so-called lignin backbone). The new lignin present in a tree thus modified should be easier to cleave into smaller pieces and to break away from the rest of the wood. Lignin researchers refer to this method, or rather its result, as 'zip' lignin.
'The bonds in the lignin polymer as it exists are quite difficult to break. They require very harsh conditions. So the idea is to put some simple bonds in… Someone named the method 'zip' lignin way back. It’s not a perfect term but it just [means that the lignin is made] easier to zip apart into pieces', Ralph explained.
Still, Ralph and many others with him say they believe in the potential of using lignin, notably as a replacement in products currently made from petrochemicals and so ultimately from fossilised oil. The belief is upheld by the fact that lignin distinguishes itself by being an aromatic substance, such as those also found in products refined from crude oil. If those latter products could be at least partly replaced with renewable alternatives, much could be gained in terms of lessening society’s reliance on fossil oil and the environmental footprint of a number of widely used consumer products. With a touch of luck, at least some of tomorrow’s plastics and similar frequent-use products could be made from the natural plant polymer lignin.
'It is the only significant natural source of aromatic compounds at the moment so it is something we are really underexploiting', Ralph said.
Foams, antioxidants, carbon fibres…
On the applied side, world-leading bioenergy scientists are working with commercial companies to make it happen. Art Ragauskas, having shifted affiliations recently to the University of Tennessee-Knoxville, U.S.A., and whose group develops polyurethanes for use in foam insulation in cars and antioxidants in car tyres, is a notable example. Another U.S.-based authority, Simo Sarkanen of the University of Minnesota, urged the 165 participants of the Lignin 2014 conference, held at Umeå, Sweden in August, to take another look at finding ways to make plastics from lignin.
'There are a lot start-up firms both in Europe, North and South America that are looking for applications of lignin. We work with several that are using lignin for polyurethanes. There is a strong effort in the U.S., and especially in Sweden, to use lignin for carbon-based fibres', according to professor Ragauskas;
'[Those firms] are all being driven by the fact that the forest products’ industry is becoming more energy efficient. So there is an abundance of lignin there. The cellulosic ethanol industry has a real abundance of lignin as they start becoming, leading, demonstration plants… On both sides of the ocean and in some parts of the Far East this whole issue of how can we have greater value from lignin is being pursued'.
… and medical applications
On the applied side too people are breaking new ground. Researchers at the University of Florida are developing a biomedical application from bioenergy sorghums, a grass species. The application, lignin nanotubes designed for use as DNA transporters, as a form of gene therapy, carries such novelty that even Ralph billed it as being a 'very novel' and 'quite farsighted' feature of the conference.
What associate professor Wilfred Vermerris and his group have done is to show in tests on human cell cultures that lignin nanotubes—tubular structures on the micro millimeter scale—are better tolerated by living cells than the current alternatives, synthetically-made carbon nanotubes or viruses, as transporters of functional copies of genes. The synthetic carbon nanotubes also happen to be hugely expensive.
'We hope that by using a natural polymer we can circumvent some of these challenges', Vermerris said, adding: 'When you subject the living materials to the tubes they tolerate the lignin at much higher levels than the carbon'.
The root of the problem in genetic disease is generally a single dysfunctional gene, Vermerris explained, meaning that a protein is not being made as it should. Therefore, if it were possible to give patients a targeted treatment—such as introducing a functional copy of the deficient gene—by means that were proven to be safe and well tolerated by the human body, medicine could go from helping patients’ keeping their disease at arm’s length by taking pharmaceutical drugs to restoring gene function.
'The big question is whether this approach will work in living organisms', Vermerris said. If it did, however, the application 'might potentially' be less expensive than that based on carbon nanofibres, selling today at U.S. $500 per gramme, according to Vermerris.
'The market value for such an application [of lignin] is high', he added.
To find out, the Florida-based researchers would go onto to test their lignin application on animals, procedures for which were 'very strict' in terms of safety for the animals, Vermerris said, 'but we are getting ready to do this'.
'Smart' food packaging
Yet another use for lignin could be as a component of packaging to prolong the shelf-life of food, potentially doing away with plastic and aluminum content of food containers. Sandra Winestrand of Umeå University said scientists in the Sweden-based research environment Bio4Energy had been investigating whether lignin treated with a copper-containing enzyme could effectively keep oxygen out of sealed food containers. Promising results had been obtained on behalf of Bio4Energy in an EU-funded research project, in collaboration with industrial partners such as Domsjö Fabriker.
Chatting to reporters after being interviewed, Ragauskas seemed to sum up the mood not only of the Lignin 2014 conference, but also the international community of actors busily chasing after the next big hit in lignin research and development:
'The whole field is being redefined. If we had talked 15 years ago we would have talked primarily about pulp and paper… It’s gone very fast'.