Nano Nightmares

Nanotechnology, like genetically modified food or nuclear power, often produces a knee-jerk reaction. It’s somehow ‘not natural’ and so is considered scary and dangerous. This is primarily a reaction to words, the same way that it easy for advertisers to push emotional buttons with ‘natural’ as good and ‘artificial’ as bad.

This is a silly distinction. There is a lot in nature that is very dangerous indeed – and much that is artificial protects us from that. If you doubt this, try removing everything artificial when you are flying in a plane over shark infested waters. For that matter, many of the most virulent poisons like ricin and botulinus toxin are natural. Water crammed with bacteria and faecal matter is natural. Clean, safe drinking water from a tap is artificial. Yet we can’t help reacting like puppets when the advertisers use those magic words.

Some concerns about nanotechnology are down to what is at best futurology and at worst science fiction. Prince Charles infamously caused headlines back in 2003, when newspapers reported ‘The prince has raised the spectre of the “grey goo” catastrophe in which sub-microscopic machines designed to share intelligence and replicate themselves take over and devour the planet.’

Charles later denied ever meaning this, commenting that he never used the expression ‘grey goo’ and saying ‘I do not believe that self-replicating robots, smaller than viruses, will one day multiply uncontrollably and devour our planet. Such beliefs should be left where they belong, in the realms of science fiction.’ But he certainly did express concerns that not enough was being done to assess and manage any risk associated with the use of nanotechnology.

Unlike the grey goo headlines, this is a perfectly reasonable attitude. The very nature of nanotechnology implies using substances in physical formats that our bodies might not have encountered, and hence we can’t make assumptions without appropriate testing and risk assessment.

If we are to be sensible about this, we need to first avoid a blanket response to nanotechnology. You would be hard pressed to find a reason for being worried about the impact of nanometer thin coatings, such as that used by P2i (sponsors of the Nature’s Nanotech series) There is a big difference between manipulating coatings at the nanoscale and manufacturing products with nanoparticles and small nanotubes.

We know that breathing in nanoparticles, like those found in soot in the air, can increase risk of lung disease, and there is no reason to think that manufactured nanoparticles would be any less dangerous than the natural versions. When some while ago the Soil Association banned artificial nanoparticles from products they endorsed, I asked them why only artificial particles. Their spokesperson said that natural ones are fine because ‘life evolved with these.’

This, unfortunately, is rubbish. You might as well argue it is okay to put natural salmonella into food because ‘life evolved with it.’ Life also evolved with cliffs, but it doesn’t make falling off them any less dangerous. There is no magic distinction between a natural and an artificial substance when it comes to chemical makeup, and in practice if there is risk from nanoparticles it is likely to be from the physics of their very small size, rather than anything about their chemistry.

There are three primary concerns about nanoparticles – what will happen if we breathe them, eat them and put them on our skin. The breathing aspect is probably the best understand and is already strongly legislated on in the UK – we know that particulates in the air can cause a range of diseases and have to be avoided. There is really no difference here between the need to control nanoparticles and any other particles and fibres we might breathe. Whenever a process throws particulates into the air it ought to be controlled. (And this applies to the ‘natural’ smoke from wood fires, say, which is high in dangerous particulates, as well as any industrial process.)

When it comes to food, we have good coverage from The House of Lords Science and Technology committee in a 2010 report. They point out that nanotechnologies have a range of possible applications in food that could benefit both consumers and industry. ‘These include creating foods with unaltered taste but lower fat, salt or sugar levels, or improved packaging that keeps food fresher for longer or tells consumers if the food inside is spoiled.’

The committee’s report sensibly argued ‘Our current understanding of how [nanoparticles] behave in the human body is not yet advanced enough to predict with any certainty what kind of impact specific nanomaterials may have on human health. Persistent nanomaterials are of particular concern, since they do not break down in the stomach and may have the potential to leave the gut, travel throughout the body, and accumulate in cells with long-term effects that cannot yet be determined.’

Their recommendation was not to abandon these technologies, but rather that it was essential to perform appropriate research, preferably across the EU, to check the impact of such nanomaterials when consumed, and to ensure that all such materials that interact differently with the body from ordinary foodstuffs are assessed for risk before they are allowed onto the market. This seems eminently sensible.

The final area, applying nanoparticles to the skin, is perhaps most urgent, because most of apply them on a regular basis. Most sun defence products, and a number of cosmetics contain them. It is hard to find a good reason to allow for any risk in a pure cosmetic, and arguably they should be prevented from containing nanoparticles. But the story is more nuanced with sun creams.

Most sunscreens contain particles of titanium dioxide or zinc oxide. These invisible particles, ranging from nanoscale to significantly larger, provide most of the sunscreen’s protection against dangerous ultraviolet. What has to be weighed up is the benefits of using products to prevent a cancer that kills over 65,000 people a year worldwide – and would kill many more if sunscreens weren’t used – against a risk that has not been associated with any known deaths.

The potential for these nanoparticles to cause harm depends on them penetrating through the outer layers of the skin to reach cells where they could cause damage. In theory a nanoparticle is capable of doing this. But the current evidence is that the particles remain on the surface of the skin and do not reach viable skin cells. Skin cancer is a particular risk in Australia, so this is a topic that has been studied in depth there. As Cancer Council Australia concludes: ‘there is no credible evidence that sunscreens containing nanoparticles pose a health risk. There is plenty of evidence however, proving that sunscreen can help reduce the risk of skin cancer, in particular non-melanoma skin cancer.’

Overall, then, we should not be lax about nanoparticles and their effect on our bodies. We need careful testing and where necessary regulation. But equally we should not be swayed into knee-jerk reactions by emotional words carrying little meaning.

Written by Brian Clegg - Popular Science

The Importance of Being Wet - Nature's Nanotech (4)

Image from Wikipedia

The image that almost always springs to mind when nanotechnology is mention is Drexler’s tiny army of assemblers and the threat of being overwhelmed by grey goo. But what many forget is that there is a fundamental problem in physics facing anyone building invisibly small robots (nanobots) – something that was spotted by the man who first came up with the concept of working on the nanoscale.

That man was Richard Feynman. His name may not be as well known outside physics circles as, say, Stephen Hawking, but ask a physicist to add a third to a triumvirate of heroes with Newton and Einstein and most would immediately choose Feynman. It didn’t hurt that Richard Feynman was a bongo-playing charmer whose lectures delighted even those who couldn’t understand the science, helped by an unexpected Bronx accent – imagine Tony Curtis lecturing on quantum theory.

Feynman became best known to the media for his dramatic contribution to the Challenger inquiry, when in front of the cameras he plunged an O-ring into iced water to show how it lost its elasticity. But on an evening in December 1959 he gave a lecture that laid the foundation for all future ideas of nanobots. His talk at the annual meeting of the American Physical Society was titled There’s Plenty of Room at the Bottom, and his subject was manipulating and controlling things on a small scale.

Feynman pointed out that people were amazed by a device that could write the Lord’s Prayer on the head of a pin. But ‘Why cannot we write the entire 24 volumes of the Encyclopedia Britannica on the head of a pin?’ As he pointed out, the dots that make up a printed image, if reduced to a scale that took the area of paper in the encyclopedia down to pinhead size, would still contain 1,000 atoms each – plenty of material to make a pixel. And it could be read with technology they had already.

Feynman went on to describe how it would be possible to write at this scale, but also took in the idea that the monster computers of his day would have to become smaller and smaller to cram in the extra circuits required for sophisticated computation. Then he described how engineering could be undertaken on the nanoscale, and to do so, he let his imagination run a little wild.

What Feynman envisaged was making use of the servo ‘hands’ found in nuclear plants to act remotely, but instead of making the hands the same size as the original human hands, building them on a quarter scale. He would also construct quarter size lathes to produce scaled down parts for new devices. These quarter scale tools would be used to produce sixteenth scale hands and lathes, which themselves would produce sixty-fourth scale items… and so on, until reaching the nanoscale.

The second component of Feynman’s vision was a corresponding multiplication of quantity, as you would need billions of nanobots to do anything practical. So he would not make one set of quarter scale hands, but ten. Each of those would produce 10 sixteenth scale devices, so there would be 100 of them – and so on. Feynman points out there would not be a problem of space or materials, because one billion 1/4000 scale lathes would only take up two percent of the space and materials of a conventional lathe.

When he discussed running nanoscale machines, Feynman even considered the effect on lubrication. The mechanical devices we are familiar with need oil to prevent them ceasing up. As he pointed out, the effective viscosity of oil gets higher and higher in proportion as you go down in scale. It stops being a lubricant and starts being like attempting to operate in a bowl of tar. But, he argues, you may well not need lubricants, as the bearings won’t run hot because the heat would escape very rapidly from such a small device.

So far, so good, but what is the problem Feynman mentions? He points out that ‘As we go down in size there are a number of interesting problems that arise. All things do not simply scale down in proportion.’ Specifically, as things get smaller they begin to stick together. If you unscrewed a nanonut from a nanobolt it wouldn’t fall off – the Van der Waals force we met on the gecko’s foot is stronger than the force of gravity on this scale. Small things stick together in a big way.

Feynman is aware there would be problems. ‘It would be like those old movies of a man with his hands full of molasses, trying to get rid of a glass of water.’ But he does effectively dismiss the problems. In reality, the nano-engineer doesn’t just have Van der Waals forces to deal with. Mechanical engineering generally involves flat surfaces briefly coming together to transfer force from one to the other, as when the teeth of a pair of gears mesh. But down at the nanoscale a new, almost magical, force springs into life – the Casimir effect.

If two plates get very close, they are attracted towards each other. This has nothing to do with electromagnetism, like the Van der Waals force, but is the result of a weird aspect of quantum theory. All the time, throughout all of space, quantum particles briefly spring into existence, then annihilate each other. An apparently empty vacuum is, in fact, a seething mass of particles that exist for such a short space of time that we don’t notice them.

However, one circumstance when these particles do come to the fore is when there are two sheets of material very close to each other. If the space separating the sheets is close enough, far fewer of these ‘virtual’ particles can appear between them than outside them. The result is a real pressure that pushes the plates together. Tiny parallel surfaces slam together under this pressure.

The result of these effects is that even though toy nanoscale gears have been constructed from atoms, a real nanotechnology machine – a nanobot – would simply not work using conventional engineering. Instead the makers of nanobots need to look to nature. Because the natural world has plenty of nanoscale machines, moving around, interacting and working. What’s the big difference? Biological machines are wet and soft.

By this I don’t mean they use water as a lubricant rather than oil, but rather they are not usually a device made up of a series of interlocking mechanical components like our machines but rather use a totally different approach to mechanisms and interaction that results in a ‘wet’, soft environment lacking flat surfaces and the opportunities for small scale stickiness to get in the way of their workings.

If we are to build nanomachines, our engineers need to think in a totally different way. We need to dismiss Feynman’s picture of miniature lathes, nuts, bolts and gears. Instead our model has to be the natural world and the mechanisms that evolution has generated to make our, admittedly inefficient, but still functioning nanoscale technology work and thrive. The challenge is huge – but so is the potential.

In the next article in this series we will look at the lessons we can learn from a specific example of nature’s ability to manufacture technology on the nanoscale – the remarkable virus.

Hanging with the Gecko - Nature's Nanotech (3)

Image from Wikipedia

If you’ve ever seen gecko walking up a wall, it’s an uncanny experience. Okay, it’s not a 40 kilo golden retriever, but we are still talking about an animal weighing around 70 grams that can suspend itself from a smooth wall as if it were a fly. For a gecko, even a surface like glass presents no problems. This is nature’s Spiderman.

It might be reasonable to assume that the gecko’s gravity defying feats were down to sucker cups on its feet, a bit like a lizard version of a squid, but the reality is much more interesting. Take a look at a gecko’s toes and you’ll see a series of horizontal pads called setae. Seen close up they look like collections of hairs, but in fact they are the confusingly named ‘processes’ – very thin extensions of the tissue of toe which branch out into vast numbers of nanometer scale bristles.

These tiny projections add up to a huge surface area that is in contact with the wall or other surface the gecko decides to encounter. And that’s the secret of their glue-free adhesion. Because the gecko’s setae are ideally structured to make the most of the van der Waals force. This is a quantum effect resulting from interaction between molecules in the gecko’s foot and the surface.

We are used to atoms being attracted to each other by the electromagnetic force between different charged particles. So, for example, water molecules are attracted to each other by the hydrogen bonding we saw producing spherical water droplets in the previous feature. The relative positive charge on one of the hydrogen atoms is attracted to the relative negative charge on an oxygen. But the van der Waals force is a result of additional attraction after the usual forces that bond atoms together in molecules and hydrogen bonding have been accounted for.

Because of the strange quantum motion of electrons around the outside of an atom, the charge at any point undergoes small fluctuations – van der Waals forces arise when these fluctuations pair up with opposite fluctuations in a nearby atom. The result is a tiny attraction between each of the nanoscale protrusions on the foot and the nearby surface, which add up over the whole of the foot to provide enough force to keep the gecko in place.

Remarkably, if every single protrusion on a typical gecko’s foot was simultaneously in contact with a surface it could keep a heavy human in place – up to around 133 kg. In fact the biggest problem a gecko has is not staying on a surface, but getting its foot off. To make this possible its toes are jointed unusually and it seems to secrete a lubricating fluid that makes it easier to detach its otherwise dry but sticky pads.

Not surprisingly, there is a lot of interest in making use of gecko-style technology. After all, master this approach and you have a form of adhesion that is extremely powerful, yet doesn’t deteriorate with repeated attaching and detaching like a conventional adhesive. A number of universities have been researching the subject.

The first publication seems to have been from the University of Akron in Ohio, where a paper in 2007 described a gecko technology sticky tape with four times the sticking power of a gecko’s foot, meaning fully deployed gecko-sized pads could hold up around half a tonne. With these on its feet, a 40 kilogram golden retriever would have no problem walking up walls – the only difficulty would be managing to apply enough force to detach its paws as it walked. In the tape, the gecko’s setae are replaced by nanotubes of carbon fibre which are attached to a sheet of flexible polymer, acting as the tape.

The great thing about carbon nanotubes, which are effectively long, thin, flexible carbon crystals, is that they can be significantly narrower than the smallest protrusions from a gecko’s foot. A typical nanotube has a diameter of a single nanometer – pure nanotechnology – maximising the opportunity for van der Waals attraction. Within a year, other researchers at the University of Dayton (Ohio again!) were announcing a glue with ten times the sticking power of the gecko’s foot.

Such adhesives are available commercially on a small scale, offering the ability to stick under extreme temperature conditions and to surfaces that are wet or flexible that would defeat practically any conventional adhesive. We can expect to see a lot more gecko tapes (like the Geckskin product) and gecko glues in the future.

There have been other theories to explain the mechanism of the gecko’s foot, including a form of capillary attraction, but the best evidence at the moment is in favour of van der Waals forces. This seems to be borne out by the problem geckos have sticking to Teflon – PTFE has very low van der Waals attractiveness. To find out more about the gecko’s foot (and other technological inspirations from nature) I would recommend the aptly titled The Gecko’s Foot by Peter Forbes.

The action that keeps a gecko in place is a dry application of natural nanotechnology, but the more you look at the nanotech biological world, the more you realize it’s mostly a wet world. In the next feature in this series we’ll look at why conventional ‘dry’ engineering often won’t work on nanoscales and how we need to take a different look at the way we build our technology, bringing liquids into the mix.

 

----

 

You can also read this post on the Popular Science website.