Another UK government statement on nanotechnology

As I mentioned on Wednesday, the UK government took the opportunity of Thursday’s nano-summit organised by the consumer advocate group Which? to release a statement about nanotechnology. The Science Minister’s speech didn’t announce anything new or dramatic – the minister did “confirm our commitment to keep nanotechnology as a Government priority”, though as the event’s chair, Nick Ross, observed, the Government has a great many priorities. The full statement (1.3 MB PDF) is at least a handy summary of what otherwise would be a rather disjointed set of measures and activities.

The other news from the Which? event was the release of the report from their Citizen’s Panel. Some summaries, as well as a complete report, are available from the Which? website. Some flavour of the results can be seen in this summary: “Panellists were generally excited about the potential that nanotechnologies offer and were keen to move ahead with developing them. However, they also recognised the need to balance this with the potential risks. Panellists identified many opportunities for nanotechnologies. They appreciated the range of possible applications and certain specific applications, particularly for health and medicine. The potential to increase consumer choice and to help the environment were also highlighted, along with the opportunity to ‘start again’ by designing new materials with more useful properties. Other opportunities they highlighted were potential economic developments for the UK (and the jobs this might create) and the potential to help developing countries (with food or cheaper energy).” Balanced against this generally positive attitude were concerns about safety, regulation, information, questions about the accessibility of the technology to the poor and the developing world, and worries about possible long-term environmental impacts.

The subject of nanotechnology was introduced at the meeting with this short film.

Which nanotechnology?

It seems likely that nanotechnology will move a little higher up the UK news agenda towards the end of this week – tomorrow sees the launch event for the results of a citizens’ panel run by the consumer group Which?. This will be quite a high profile event, with a keynote speech by the Science Minister, Ian Pearson, outlining current UK nanotechnology policy. This will be the first full statement on nanotechnology at Ministerial level for some time. I’m one the panel responding to the findings, which I will describe tomorrow.

Drew Endy on Engineering Biology






Carbon nanotubes as engineering fibres

Carbon nanotubes have become iconic symbols of nanotechnology, promising dramatic new breakthroughs in molecular electronics and holding out the possibility of transformational applications like the space elevator. Another perspective on these materials places them, not as a transformational new technology, but as the continuation of incremental progress in the field of high performance engineering fibres. This perhaps is a less dramatic way of positioning this emerging technology, but it may be more likely to bring economic returns in the short term and thus keep the field moving. A perspective article in the current issue of Science magazine – Making strong fibres (subscription required), by Han Gi Chae and Satish Kumar from Georgia Tech, nicely sets current achievements in developing carbon nanotube based fibres in the context of presently available high strength, high stiffness fibres such as Kevlar, Dyneema, and carbon fibres.

The basic idea underlying all these fibres is the same, and is easy to understand. Carbon-carbon covalent bonds are very strong, so if you can arrange in a fibre made from a long-chain molecule that all the molecules are aligned along the axis of the fibre, then you should end up pulling directly on the very strong carbon-carbon bonds. Kevlar is spun from a liquid crystal precursor, in which its long, rather rigid molecules spontaneously line up like pencils in a case, while Dyneema is made from very long polyethylene molecules that are physically pulled out straight during the spinning process. Carbon fibres are typically made by making a highly aligned fibre from a polymer like polyacrylonitrile, that is then charred to leave graphitic carbon in the form of bundles of sheets, like a rolled up newspaper. If you could make a perfect bundle of carbon nanotubes, all aligned along the direction of the fibre, it would be almost identical to a carbon fibre chemically, but with a state of much greater structural perfection. This idea of structural perfection is crucial. The stiffness of a material pretty much directly reflects the strength of the covalent bonds that make it up, but strength is actually a lot more complicated. In fact, what one needs to explain about most materials is not why they are as strong as they are, but why they are so weak. It is all the defects in materials – and the weak spots they lead to – which mean they rarely get even close to their ideal theoretical values. Carbon nanotubes are no different, so the projections of ultra-high strength that underlie ideas like the space elevator are still a long way off when it comes to practical fibres in real life.

But maybe we shouldn’t be disappointed by the failure of nanotubes (so far) to live up to these very high expectations, but instead compare them to existing strong fibres. This has been the approach of Cambridge’s Alan Windle, whose group probably is as far ahead as anyone in developing a practical process for making useful nanotube fibres. Their experimental rig (see this recent BBC news report for a nice description, with videos) draws a fibre out from a chemical vapour deposition furnace, essentially pulling out smoke. The resulting nanotubes are far from being the perfect tubes of the typical computer visualisation, typically looking more like dog-bones than perfect cylinders (see picture below). Their strength is a long way below the ideal values – but it is still 2.5 times greater than the strongest currently available fibres. They are very tough, as well, suggesting that early applications might be in things like bullet proof vests and flak jackets, for which, sadly, there seems to be growing demand. Another interesting early application of nanotubes highlighted by the Science article is as processing aids for conventional carbon fibres, where it seems that the addition of only 1% of carbon nanotubes to the precursor fibre can increase the strength of the resulting carbon fibre by 64%.

Nanotubes from the Windle group
“Dogbone” carbon nanotubes produced by drawing from a CVD furnace. Transmission electron micrograph by Marcelo Motta, from the Cambridge research group of Alan Windle. First published in M. Motta et al. “High Performance Fibres from ‘Dog-Bone’ Carbon Nanotubes”. Advanced Materials, 19, 3721-3726, 2007.

Scooby Doo, nano too

Howard Lovy returns to his coverage of nanotechnology in popular culture with news of a forthcoming film, Nano Dogs the Movie, in which some lovable family pets acquire super abilities after scoffing some carelessly abandoned nanobots. Not to be outdone, I’ve been conducting my own in-depth cultural research, which has revealed that no less an icon of saturday morning children’s TV than Scooby Doo has fully entered the nanotechnology age.

In the current retooling of this venerable cartoon, Shaggy and Scooby Doo Get a Clue, the traditional plot standbys (it was the janitor, back-projecting the ghostly figures onto the clouds, and he’d have got away with it if it hadn’t been for those meddling kids) have been swept away to be replaced by an evil nanobot wielding scientist. But the nanobots aren’t all bad; Scooby Doo’s traditionally energising Scooby snacks have themselves been fortified with nanobots, giving him a number of super-dog powers.

I wasn’t able to follow all the plot twists on Sunday morning, as I had to cook the children’s porridge, but it seems that the imprudent nano-scientist had attempted to mis-use his nanobots in order to make his appearance (formerly plump, ageing, balding and with a bad haircut, as you’d expect) more, well, Californian. Naturally, this all ended badly. I’ve seen some less incisive commentaries on the human (or, indeed, canine) enhancement debate.

The rain it raineth on the just

I’m optimistic in general about the prospects of solar energy; as should be well known, the total amount of energy arriving at the earth from the sun is orders of magnitude more than is needed to supply all our energy needs. The problem currently is reducing the price and hugely scaling up the production areas of photovoltaics. But, as I live in the not notoriously sunny country of Britain, someone will always want to make some sarcastic comment about how we’d be better off trying to harvest energy from rain rather than sun here. So I was pleased to see, in the midst of a commentary from Philip Ball on the general concept of scavenging energy from the environment, a reference to generating energy from falling raindrops.

The research, described in an article in Rain power: harvesting energy from the sky, was done by Thomas Jager, Romain Guigon, Jean-Jacques Chaillout, and Ghislain Despesse, from Minatec in Grenoble. The original work is described in two articles in the journal Smart Materials and Structures, Harvesting raindrop energy: theory and Harvesting raindrop energy: experimental study (subscription required for full article). The basic idea is very simple; it uses a piezoelectric material, which generates a voltage across its faces when it is deformed, to convert the energy imparted on the impact of a raindrop onto a surface into a pulse of electrical current. The material chosen is a polymer called poly(vinylidene fluoride), which is already extensively exploited for its piezoelectric properties in applications such as microphones and loudspeakers.

So, should we abandon plans to coat our roofs with solar cells and instead invest in rain-energy harvesting panels? It’s worth doing a back of an envelope sum. The article claims that a typical raindrop’s velocity is about 3 m/s. Taking Sheffield’s average annual rainfall of about 80 cm, we can estimate the total kinetic energy of the rain landing on a square meter in a year as 3600 J, corresponding to a power per unit area of about 3 milliwatts. This isn’t very impressive; even at these dismal northern latitudes the sun supplies about 100 W per square meter, averaged over the year. So, even accounting for the fact that PVDF is likely to be a lot cheaper than any photovoltaic material in prospect, and energy conversion efficiencies might be higher, its difficult to see any circumstances in which it would make sense to try and collect rainwater energy rather than sunlight.

Mobility at the surface of polymer glasses

Hard, transparent plastics like plexiglass, polycarbonate and polystyrene resemble glasses, and technically that’s what they are – a state of matter that has a liquid-like lack of regular order at the molecular scale, but which still displays the rigidity and lack of ability to flow that we expect from a solid. In the glassy state the polymer molecules are locked into position, unable to slide past one another. If we heat these materials up, they have a relatively sharp transition into a (rather sticky and viscous) liquid state; for both plexiglass and polystyrene this happens around 100 °C, as you can test for yourself by putting a plastic ruler or a (polystyrene) yoghourt pot or plastic cup into a hot oven. But, things are different at the surface, as shown by a paper in this week’s Science (abstract, subscription needed for full paper; see also commentary by John Dutcher and Mark Ediger). The paper, by grad student Zahra Fakhraai and Jamie Forrest, from the University of Waterloo in Canada, demonstrates that nanoscale indentations in the surface of a glassy polymer smooth themselves out at a rate that shows that the molecules near the surface can move around much more easily than those in the bulk.

This is a question that I’ve been interested in for a long time – in 1994 I was the co-author (with Rachel Cory and Joe Keddie) of a paper that suggested that this was the case – Size dependent depression of the glass transition temperature in polymer films (Europhysics Letters, 27 p 59). It was actually a rather practical question that prompted me to think along these lines; at the time I was a relatively new lecturer at Cambridge University, and I had a certain amount of support from the chemical company ICI. One of their scientists, Peter Mills, was talking to me about problems they had making films of PET (whose tradenames are Melinex or Mylar) – this is a glassy polymer at room temperature, but sometimes the sheet would stick to itself when it was rolled up after manufacturing. This is very hard to understand if one assumes that the molecules in a glassy polymer aren’t free to move, as to get significant adhesion between polymers one generally needs the string-like polymers to mix themselves up enough at the surface to get tangled up. Could it be that the chains at the surface had more freedom to move?

We didn’t know how to measure chain mobility directly near a surface, but I did think we could measure the glass transition temperature of a very thin film of polymer. When you heat up a polymer glass, it expands, and at the transition point where it turns into a liquid, there’s a jump in the value of the expansion coefficient. So if you heated up a very thin film, and measured its thickness you’d see the transition as a change in slope of the plot of thickness against temperature. We had available to us a very sensitive thickness measuring technique called ellipsometry, so I thought it was worth a try to do the measurement – if the chains were more free to move at the surface than in the bulk, then we’d expect the transition temperature to decrease as we looked at very thin films, where the surface had a disproportionate effect.

I proposed the idea as a final year project for the physics undergraduates, and a student called Rachel Cory chose it. Rachel was a very able experimentalist, and when she’d got the hang of the equipment she was able to make the successive thickness measurements with a resolution of a fraction of an Ångstrom, as would be needed to see the effect. But early in the new year of 1993 she came to see me to say that the leukemia from which she had been in remission had returned, that no further treatment was possible, but that she was determined to carry on with her studies. She continued to come into the lab to do experiments, obviously getting much sicker and weaker every day, but nonetheless it was a terrible shock when her mother came into the lab on the last day of term to say that Rachel’s fight was over, but that she’d been anxious for me to see the results of her experiments.

Looking through the lab book Rachel’s mother brought in, it was clear that she’d succeeded in making five or six good experimental runs, with films substantially thinner than 100 nm showing clear transitions, and that for the very thinnest films the transition temperatures did indeed seem to be significantly reduced. Joe Keddie, a very gifted young American scientist then working with me as a postdoc, (he’s now a Reader at the University of Surrey) had been helping Rachel with the measurements and followed up these early results with a large-scale set of experiments that showed the effect, to my mind, beyond doubt.

Despite our view that the results were unequivocal, they attracted quite a lot of controversy. A US group made measurements that seemed to contradict ours, and in the absence of any theoretical explanation of them there were many doubters. But by the year 2000, many other groups had repeated our work, and the weight of evidence was overwhelming that the influence of free surfaces led to a decrease in the temperature at which the material changed from being a glass to being a liquid in films less than 10 nm or so in thickness.

But this still wasn’t direct evidence that the chains near the surface were more free to move than they were in the bulk, and this direct evidence proved difficult to obtain. In the last few years a number of groups have produced stronger and stronger evidence that this is the case; Jamie and Zahra’s paper I think nails the final uncertainties, proving that polymer chains in the top few nanometers of a polymer glass really are free to move. Among the consequences of this are that we can’t necessarily predict the behaviour of polymer nanostructures on the basis of their bulk properties; this is going to become more relevant as people try and make smaller and smaller features in polymer resists, for example. What we don’t have now is a complete theoretical understanding of why this should be the case.