Nanotechnoogy used to be a playground for speculation and science fiction. It is now one of the most exciting research fields in contemporary science. It is true that nanotechnology has been heralded as the "next big thing" for years and the excitement surrounding it certainly increased after the dotcom bubble burst and people started looking for another wonder technology. Nevertheless, despite the hype, big strides have been taken and more are imminent.
Nanotechnology is an umbrella term for approaches from different scientific disciplines which share the defining element of operating within the very small world of molecules, where distances are measured in nanometres, or millionths of a millimetre.
The computer industry has already entered the nanoworld. Chip production operates with features as small as 100 nanometres, and they are getting smaller. But can this success be extended to other fields? Will there be houses, cars, space stations built by manipulation on the nanometre scale?
Evolution shows the way
One good reason to believe that this nanotech revolution will happen is everywhere around us: nature. Look in the mirror and you will see an incredibly complex structure, which has self-assembled from the genetic instructions encoded in the molecules of a single cell (the fertilised egg). Life has many layers of complexity, but all of them ultimately arise from what happens on the nanometre scale. Molecules interact with each other to form cells, cells form tissues, tissues make animals.
The living cell can do many things that are way beyond our technical abilities. It can read out the information of a single strand of DNA (we would have to "amplify" it to millions of copies before we can read it); it can produce complex molecular machines which self-assemble from many different parts; it can recognise faults on the molecular scale and fix them efficiently; and it can produce structures on the nanoscale that eventually combine to make amazing macroscale structures, such as a tree, an elephant or a human.
Over the last five decades, scientists have learnt to understand the basic principles and details of how the nanotechnology of nature works. Mechanistic understanding of biological systems has progressed rapidly from the structure of DNA through to the genome sequences of homo sapiens and a dozen other higher organisms. Some of the most important molecular machines of the cell have become open books. Scientists can download their structures and work out how they would interact, for instance, with a new drug. Even highly complex systems, like the photosynthesis machinery and the protein factory of the cell, can now be studied this way.
While there are still many blank spaces on the map, the principles of the molecular machinery of life are now clear. They differ from the way we make small structures (such as computer chips) in a number of ways:
l Nature builds complex architecture from the bottom up, while computer engineers work from the large scale downwards.
l To achieve complexity, nature starts from simple building blocks, which can be lined up to chain molecules (such as DNA), then fold up to three-dimensional structures, then assemble to molecular machines.
l While chemists have been building molecular structures by making or breaking solid bonds between atoms (known as covalent bonds), nature does so by making use of so-called weak interactions, which can be formed and broken much more readily.
l Finally, when it comes to assembling a complex piece of equipment from a number of parts, there is no mechanic in the cell to put the parts together. The molecular parts are designed in such a clever way that they snap together by themselves.
Chemists and biochemists working in the nanotechnology field typically use the bottom-up approach, borrowing one or several of these principles from nature. By designing more and more complex molecular assemblies, they aim at the same size range that chip manufacturers try to reach from the other side, by further miniaturisation of their technology.
Where do we stand today?
As recently as 20 years ago, the idea of manipulating things on the nanometre scale was a distant dream. It had first been raised by the physicist Richard Feynman in his famous 1959 after-dinner speech ("There is plenty of room at the bottom"). More than two decades later, K Eric Drexler developed it further with his highly speculative book Engines of Creation. Drexler's vision focused on what might become possible once scientists could move atoms around and position them at will. He painted a future where nanorobots would do all the work for us, but he also considered the possibility that such things might get out of control and take over the world, a scenario known as the "grey goo" theory and often used in science fiction (see below).
Since the publication of Engines of Creation, a number of developments have transformed nanotechnology from science fiction to a real field of experimental research:
l New techniques, including laser tweezers and the atomic force microscope, have been developed to a point where they allow researchers to handle nanoscale objects (such as protein molecules) individually, position them, and probe their response to experimentally-induced changes of conditions.
l A new kind of chemistry based on the clever use of non-covalent interaction, known as supramolecular chemistry, grew rapidly and is now at the point where it can produce not only molecular toys, but even things that can become useful in real life products.
l The field of protein design, known as a heroically hopeless endeavour in the 1980s, has recently reached the point where it can create artificial proteins for real applications.
l The discovery of methods to produce football and tube-shaped carbon molecules, known as fullerenes, and carbon nanotubes in large quantities have given access to a whole new world of nanoscale structures.
Nanotechnology is now moving from exploration to application. For instance, nanoscale electronics has seen rapid progress in the past couple of years thanks to competing approaches based on carbon nanotubes (championed by Cees Dekker at Delft, in the Netherlands) and nanowires (taken up by Charles Lieber at Harvard), in developing the first electronic components (transistors, simple circuits) made from nanoscale elements. The nanotube and nanowire may hold the key to the electronics of the future when the silicon chip reaches the limits of miniaturisation.
What will be the first nanotech products?
As the smallest structures in computer chips are now well below the one micrometre size, one could count every new computer and mobile phone as a nanotechnology product. On the other hand, these products result from the steady progress of miniaturisation that has followed Moore's Law (the doubling of computer performance every 18 months) over the last three decades. They arise from gradual improvements more than from new thinking.
The first nanotechnology breakthrough outside the information technology market is the microfabricated impact sensor to trigger airbags in cars. The new kind of sensor is based on a Mems (micro-electromechanical system) device, which means that it is fabricated by the same kind of technology as a computer chip, only that its function is mainly mechanical rather than electronic. When it was introduced in 1995, it turned out to be not only smaller and more efficient than the sensors previously available, but also 100 times cheaper. Understandably, it took over the world market in a matter of months.
But how about products designed from molecules upwards? There is at least one that you can buy already. It is the self-cleaning window. It uses a combination of two clever molecular tricks. First, it contains a catalyst that uses the energy of light to oxidise common kinds of dirt, to convert them into smaller, more soluble molecules that wash away with rain water. At this point, the second trick comes in. Ordinary glass is fairly water-repellent (hydrophobic), which means that water does not cover it smoothly, but tends to form droplets. The surface of self-cleaning glass, however, is coated in molecules that attract water and encourage it to spread out. So, instead of sitting around as drops which leave drying spots when they evaporate, the rain will cover the surface evenly, dissolve what the photocatalyst made of the dirt, and run off. Simple. Yet it would not be possible without molecular design on the nanometre scale.
Generally, it can be said that three-dimensional nanomachinery is still far away, while two-dimensional applications are already with us. Molecular self-assembly is much easier to control in two dimensions than in three. Using a wafer-thin gold foil as a basis, you can get molecules to stand on it side by side, in patterns that you can define at excellent resolution. Such arrangements can be used in the laboratory, but also in, say, hospitals.
What next?
Further miniaturisation will mean that equipment can increasingly shrink out of sight, so a mobile phone might be no bigger than a shirt button (or be built into a hollow tooth). And after 25 years in which computers have become ubiquitous in our lives, they will now spend the next 25 years becoming invisible.
But the biggest impact of forthcoming nanotechnology applications will probably be in medicine. Disease typically arises from malfunctioning at the cellular scale. Treating cells with a scalpel is like fixing a computer chip with an axe, and treating them with drugs that invade the whole body is like immersing a computer in a bath to clean up one chip.
While many drugs work on the principle of addressing a property that is specific to the targeted cell type, unwanted side effects are still common in most drug treatments. Efficient therapy would address the right group of cells at the right time with the right doses. Ideally, a nanoscale device that could be implanted or worn on the skin near the organ in question, should contain sensors that assess the physiological state of the malfunctioning organ, a primitive computer that assesses the correct response, and compartments that release the drug molecules at the right time in the right place.
There are at least two areas in which we might see such developments in the near future. One is post-surgery pain treatment, used for example after hip replacements. It should soon become possible to implant a Mems-style drug release chip together with the artificial joint, which could take care of the patient's drug needs in a localised way and over a time scale of months. Secondly, diabetics could soon benefit from devices that combine insulin sensors and dispensers. As this disease is common, and the insulin supply from injections is never optimal, there would be a huge demand for such a device.
A further important trend that will affect both the information technology and the biomedical applications of nanotechnology is the disappearance of the distinction we now make between biological and electronic systems. With the advent of molecular scale technology, electronic components will begin to resemble living cells more than silicon chips. On the other hand, faulty body parts will more and more easily be fixed by implants that might be on the micro or the nano scale. With very similar molecules being used on both sides, there will be a continuum of molecular devices linking the (as yet) distinct domains of biology and technology.
What about the "grey goo" problem?
For all the potential benefits to be gained from nanotechnology, there is, of course, the possibility that nanoscale systems-like all new scientific developments-might be misused. The main fear relating to nanoscale technology is its ability to create systems that could behave and spread like living beings. This scenario is based on Drexler's proposal of making "assemblers," or nanomachines, that can rearrange atoms, and then "replicators," or assemblers that can make copies of themselves. If such a road were to be taken, so the scare story goes, it might lead to a replicator that could feed on organic matter while being itself indigestible to animals, so it would place itself on top of the food web and essentially turn our biosphere into "grey goo"-grey because all the information and differentiation has been drained out of it. This danger, technically known as "global ecophagy by biovorous nanoreplicators," was highlighted in a notorious article in Wired magazine a few years ago by Bill Joy of Sun Microsystems.
While it is generally dangerous to underestimate human stupidity, I believe that nanotechnology will take a route rather different from Drexler's assembler/replicator scheme based on the idea of picking atoms and putting them into place. It now appears that nanotechnology will proceed in the direction of molecular technology, using molecules that are not so far remote from biomolecules. Thus, even if somebody should be foolish enough to create a roving replicator in the distant future, this new life form might actually find a natural place in the biosphere, rather than destroying it. Again, the distinction between biology and technology might become meaningless.
Meanwhile, it is our responsibility to make sure that the imminent progress in nanotechnology will help to develop efficient medical devices and smart houses, perhaps even to expand into space. It is all up to us.
