What makes nanotechnology possible?
The Atomic Force Microscope
The Atomic Force Microscope (AFM) was not the first SPM technique to be invented, but it is the easiest to understand. In essence a very fine probe is used to read a surface. As the probe moves over the surface it encounters lumps and bumps, which cause it to move up and down. This movement is detected by the deflection of a laser beam that is reflected off the back of the probe - as shown right.
In practice this simple system would risk damage to both the probe and the surface being studied. The scanning tip would tend to plough through large bumps, but may not probe to the bottom of deep holes. To get round this the sample is mounted on a piezoelectric stage, which allows the sample to be adjusted up or down by as little as 0.01nm. As the AFM tip rides over the surface, electronics drive the stage to keep the deflection of the laser beam constant - so it stays at the same height throughout the process.
The reason AFM is so important is that it can probe surfaces in air, and even under water. Earlier imaging techniques were restricted to use in hard vacuum - and often samples had to be pre-treated with metallic coatings.AFM techniques have the ability to look at living cellular machinery!
Other techniques important in the development of nanotechnology include:
Electron microscopes: In our journey to the nano-world we found that, limited by the wavelength of visible light, we cannot see objects much smaller that a micrometre. One of the first technologies to overcome this limit was the electron microscope. This uses electrons as opposed to photons to probe matter, and while it has limitations, it is still a very important tool for examining nano-scale structures, as it can examine larger sample areas than is practical with SPM.
Epitaxial techniques: While SPM provides a new window on the nano-world, it is the advance in silicon-based electronic chip technology that drives a lot of the technical development in this field. Current processor architecture resolutions are to better than 45nm - see evolutionary nanotechnology.
Molecular biology: Life was there first - our cells are the archetypal nano-factories. Synthetic DNA has been used to build 'proof of purpose' models for molecular computing , as well as serving as the building blocks in 'molecular mecano'. As our control over organisms at the genetic level improves, simple bacteria may offer a quick leg-up to making nano-devices in bulk -
Incremental nanotechnology — rebranding science
We can change the properties of many existing materials by taking control of their nano-scale structure. One elegant way of doing this is to use the tendency of certain molecules to self-assemble into coherent architectures. Soap molecules are expert at self-assembly, and can capture water in structures such as films and bubbles.
In fact most molecules self-assemble, but when the forces driving this are very strong, we end up with hard, crystalline masses! This tendency to self-assemble is actually a problem if you want to disperse ultra-strong carbon buckytubes in a resin to form a useful engineering composite, for example. Here the buckytubes have a tendency to stick together rather than to the resin, so it is proving harder than expected to make 'the toughest materials ever'...
Superhydrophobic coatings are an example where structure at the nano-scale imparts new and useful properties to a material. If we increase the surface area of a material then any water drop resting on this surface has to greatly increaseits surface area in order to wet it. This results in a big increase in the surface energy of the water droplet. When the surface architecture gets down to nano-scale the water droplet tends to adopt a spherical or near spherical shape, and runs off the surface very quickly - taking any dirt with it!
Liposomes are an example of nano-structures that self-assemble from phospholipids. These form the membranes and much of the structural material of a living cell. They are not new to science, but perhaps their relationship to the radical claim that nanotechnology can make you immortal makes it attractive for cosmetics
evolutionary nanotechnology — top down design
Evolutionary nanotechnology involves scaling existing technologies down in size to the nano-scale. This engineering approach is most important with silicon chip technology. Smaller-scale components allow designers to pack more circuitry into a smaller area of silicon, and this has big advantages in terms of processing speed and memory capacity. When we want more in a smaller package, component size must be reduced, and this increasingly involves working with components at the nano-scale.
As an example we can look at the change in size from the original hard drives developed by IBM in the 1970's (which stored 5Mb on a stack of fifty 24-inch diameter discs) to the tiny hard disc drives in modern MP3 players like the iPod (which can store up to 80Gb on a single disc less than one inch in diameter!). If we look more closely, however, we see that this apparent continuum of device scale actually conceals important step changes in technology...
To get more data onto a magnetic disc we have to make the size of the magnetic 'ones and noughts' smaller. These are stored as magnetised patches on the disc surface, but as they get smaller they produce a weaker magnetic field. This means that the detector must be more sensitive to pick up the data.
'Giant Magneto Resistance' (GMR) is the most recent development in this technology. In a read head working on GMRprinciples, two magnetic layers are separated by a non-magnetic spacer layer a few nm wide. The technology allowed IBM to achieve a world record data density of 35 billion bits per square inch in 1999 - but it is now almost certainly working in a hard drive near you!
While GMR makes positive use of a nanoscale effect, it is more common to encounter problems when trying to make 'big world' devices smaller. One of these problems is the quantum mechanical 'tunnelling' effect. With smaller, and more sensitive devices, electrons are able to 'tunnel' across narrow insulating gaps, and cause spurious signals, which can lead to errors in digital circuits.
Chip manufacturers are working on a range of possible techniques for circumventing this problem, but scientist believe that chip architectures of 4nm may be possible.
The bio-silicon interface
Silicon microcircuitry technology has led to a number of important bioscience advances, with 'gene chips' being perhaps the most influential. While gene chips are not currently nano-scale devices, they are rapidly heading in that direction as manufacturers pack more analytical capacity onto each chip.
With the development of nano-scale chip architectures it will be possible to build devices that have direct interactions with cellular machinery. For the moment, however, this is still largely the domain of radical nanotechnology.
adical nanotechnology — dreams and nightmares
Nanotechnology represents an important interface between 'big world' science and technology, and molecular structures. Radical nanotechnology envisages the possibilities of a mature technology that allows us to exercise control with engineering precision down to the atomic level.
A number of possible routes to enhance our ability to manipulate matter at the molecular, and even the atomic level, are currently being explored:
Bio-nanotechnology
Perhaps the biggest breakthrough of nanotechnology to date is in showing us quite how competent nature is at this scale, working with soft materials like proteins in aqueous solutions. These are not the sorts of materials that engineers are familiar with, but cellular machinery offers a glimpse as to what is possible.
Nature offers us an extremely competent nano-scale toolbox, with control over the design and construction of proteins. These molecules can self-assemble into a wide range of shapes and devices, from rotary engines to ion pumps.
Left: Existing cellular machinery driven by biochemicals such as ATP can serve as templates for new nanotech devices.
Acknowledgement: Professor J.B.C.Findlay and Dr M.A.Harrison, Biochemistry and Molecular Biology, University of Leeds.
In a process that might be termed 'biokleptic' nanotechnology, a number of scientists are working to include these devices into nano-scale devices, and/or using DNA synthesis methods to build them.
Biokleptic nanotechnology is restricted to working with proteins, which have a restricted operating environment. There is, however, a wide range of other 'soft materials' - such as polymers. Biomimetic nanotechnology, therefore, is the study of how devices that mimic biological systems can be made using these rather more familiar materials.
A problem that some have seen with nanotechnology is that of making enough nano-devices to be useful. Certainly positioning each atom using an AFM would be laborious in the extreme! One of the big dreams of nanotechnology is, therefore, a replicator capable of generating copies and useful products. This is, after all, exactly what a living cell does...
What is the possibility of a nano-machine running amuck, and converting the whole world into replicas of itself - a fine grey goo?
Synthetic life: Currently molecular biologists are working on creating 'stripped down' organisms, with the minimum coding required to allow them to reproduce. The hope is that these might serve as nano factory units, into which code for doing whatever we want might be installed.
Converting the world to grey goo: What is the possibility of a nano-machine running amuck, and converting the whole world into replicas of itself - a fine grey goo? Any such goo-bot will have pretty strong competition from very well entrenched organisms, with several billion years evolutionary experience at deterring just such attacks. We think it is more likely that, like H.G. Wells' Martians in the book The War of the Worlds, our goo-bot would make a tasty snack for the first passing bacterium!
Health and nanotechnology
The first medicine that specifically mentions nanotechnology in its patent has recently been licensed in the USA for treatment of patients with breast cancer. It is based on combining an active anti-cancer drug with nano-sized particles of the protein albumin. The nanoparticle acts as an inert carrier for the drug in place of a solvent that was having unpleasant side effects.
In general, anti-cancer drugs work by killing cancerous cells; unfortunately they are not as picky as to which cells they kill as we would like, and this results in a range of unpleasant side effects which limits the dosages that can be given, and may result in treatment having to be stopped before the cancer has been killed. An important focus of a lot of research into nano-medicine is therefore to make drug delivery systems smart enough to recognise cancer cells and deliver their active payload just to them.
Above right: Scientists at Sheffield have been making polymer particles that can carry peptides and other molecules into cells. Here fluorescent polymer particles show up blue against the red-stained cellular material. (Image courtesy Steve Rimmer.)
At present we are a long way away from nanobot surgeons that can travel through blood vessels, repairing damage and excising malignancies at the cellular level.
The likely effect of nano-toxins is difficult to quantify; we don't in general expect materials to become toxic simply because they have been prepared as a nano-particle formulation. Powders, however, have lots of surface compared to their volume, and this can increase the reactivity and toxicity of some materials. This could be compounded in certain environments by the smaller particle size being able to penetrate further into the lungs.
Common sources of nano-particulates in the environment are hydrocarbon combustion (engine emissions) and gas to particulate reactions in the atmosphere (aerosol formation). There is certainly some evidence that urban environments with higher levels of engine emissions and other aerosols are associated with increased prevalence of diseases.
Molecular nanotechnology (M.N.T)
Combining components with different properties at the nano-scale allows us to make a new class of 'meta materials.
One area that is receiving considerable attention is the possibility of software control over the assembly of molecules. This is already possible with DNA and protein synthesisers, and there is a small range of chemical linkers that can be used in combination with other monomer units (i.e. not amino acids or nucleic acids).
Combining components with different properties at the nano-scale allows us to make a new class of 'metamaterials'. At this scale an electron experiences a combination of the properties of all of the components, potentially allowing us to manufacture materials with very unusual properties. GMR devices are an example of metamaterials which are in widespread use. Being able to do this sort of thing with individual molecules, however, will result in a blurring of the distinction between materials and devices.
Hard nano-engineering:
Based on what nature can do with soft materials, can something similar be done with the steel, glass, ceramics and silicon that engineers currently prefer? To realise this dream, we will need nano-machinery capable of handling a wide range of substrates. It is worth bearing in mind that there is biological machinery for working with metals, ceramics and silica (see Nature's Nanotechnologists ), but it will be hard work to adapt this to making motorcars, computers or aeroplanes...
Above right: A hypothetical nano-fabricator would be able to build a wide range of objects - given the software blueprint
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