Small but deadly

Nanotechnology, using microscopic particles, may offer huge benefits to mankind. But let them loose in the human body, studies have found, and they will wreak havoc

In his novel Prey, Michael Crichton portrayed a future threatened by minuscule, self-replicating robots that begin to consume the planet. That's still in the realm of science fiction, but not everyone believes that nanotechnology is inherently safe. The Prince of Wales, for instance, warned us a year ago that the unleashing of small-scale "nano" particles on an unprepared world could result in a Thalidomide-like health disaster.

It is not easy to dismiss such fears over the possible health effects of nanotechnology - the science of the very small, at the scale of a billionth of a metre. There may be no evidence of risk, but that does not mean that the risk is zero. In fact, only now are scientists beginning to shed some light on exactly how the specially engineered nanoparticles, which are the basic ingredients of nanotechnology, might affect our health if they managed to end up in the wider environment.

Much of this research is based on the round assembly of 60 carbon atoms known as the buckyball, or fullerene. This nanoparticle is considered important because it is a building block for all sorts of new materials and medicines. Buckyballs have remarkable characteristics. If you shoot one of these virus-sized particles at a steel plate at 15,000mph, it bounces back unharmed. Squash one, and it becomes twice as hard as diamond. They're hollow, so you can put other molecules, such as drugs, inside them.

But studies suggest that the properties that may make fullerenes useful may also make them toxic. The first evidence came earlier this year when Eva Oberdorster, an American toxicologist at the Southern Methodist University in Dallas, published a study showing how, after two days of swimming in water containing buckyballs, largemouth bass fish suffered damage to the fat membranes in their brains. Their livers had responded as though there was a toxin present.

Now, scientists at Rice University, Houston, have pinpointed what could be the mechanism causing this damage. Rice's breakthrough study, published in the journal Nano Letters, is the first to look at the toxic effects on individual human cells exposed to fullerenes, and the first to indicate the cause. "People have shown there's a hazard, but this is the first work about how that hazard comes to be. It's important for the community to understand how, because then you can change it," says Kristen Kulinowski, the director of Rice's Centre for Biological and Environmental Nanotechnology.

The Rice researchers exposed human liver and skin cells to solutions containing different concentrations of fullerenes. Four types of solution were tested. One contained plain buckyballs; in the other three, researchers modified the buckyballs by attaching other molecules to their sides.

They measured how many cells died within 48 hours, and repeated the tests until they found the exposure level for each solution that killed half the cells. The plain buckyballs destroyed half the cells in a concentration of about 20 parts per billion, but a concentration of 10 million times more was needed to make the modified fullerenes as toxic.

All atoms and molecules are surrounded by their own particular halo of electrons, but in the buckyball's case, the structure of this halo seems to be very disruptive to biological systems. "The fullerene has what is known as a high electron affinity, which means it likes to pluck electrons from other molecules it comes into contact with," says Dr Kulinowski.

When a molecule loses one electron, it's often left with a lonely partner electron. Such a molecule - a free radical - is highly reactive in its desperation to pair off. "The proposed mechanism is that these free radicals attack the cells in their search for extra electrons and damage the cell membranes," Kulinowski says. In essence, free radicals punch holes in the membranes.

The buckyballs, however, remain stable because the energy from the extra electrons they pick up seems to spread evenly over the ball-like structure. This is one of the reasons why fullerenes are so incredibly strong.

A research team at the University of Michigan has discovered a similar hole-punching effect with another, larger nanoparticle called a dendrimer, which is used in nanomedicine to deliver drugs or imaging agents to specific parts of the body via the blood. The team found that the size of the nanoparticle seemed to make a difference, with the smallest dendrimers having no adverse effects.

A member of the team, Professor Mark Banaszak Holl, says that if the presence of nanoparticles can produce holes in cell membranes, it makes it easy for all sorts of molecules, including the nanoparticle itself, to get inside the cell. "This provides a physical mechanism via which nanoparticles can be toxic to cells - even if the material the nanoparticle is made from would normally not cause chemical toxicity," he explains. This work was published in the journal Bioconjugate Chemistry last summer.

Encouragingly, both the Michigan and Rice groups have found that modifying the surface of these nanoparticles by attaching other molecules makes them far more body-friendly, and this may be a way to tune toxicity. "Attaching small molecules to the buckyball surface disrupts the electronic structure and therefore makes it less accommodating to extra electrons, so it doesn't produce as many free radicals," Kulinowski says.

However, she warns against drawing any general conclusions about engineered nanoparticles. We need to wait, she says, until scientists have properly investigated the relationship between structure and function for a wide range of nanoparticles.

A great deal more work is also needed to see what happens inside the whole body, where cellular repair mechanisms, whole-organ and whole-body processes come into play. "Every cell in the body is surrounded by a membrane. If you put therapeutic nanoparticles in the bloodstream and they started punching holes in the cells lining the blood vessels, it would seem to be a bad thing. But we just don't know," says Ken Donaldson, Professor of Respiratory Toxicology at the University of Edinburgh. "Some membranes have pores in them already. It could mean nothing or it could mean a lot," he says.

Donaldson's interest in engineered nanoparticles comes by way of his work on asbestos, coal-mining dust and car pollution particles. It is well established that inhaling very small particles can cause inflammation in the lungs. This immune response, Donaldson suggests, is to do with the large surface area of the lungs that is exposed. It may have evolved as a way to deal with invasion by bacteria. "Inflammation in the lungs, unlike in the skin, for example, is not a trivial matter as it doesn't take too much lung inflammation to compromise your breathing," he says.

Nanoparticles add another dimension because, being a similar size to viruses, they can get into the bloodstream through the lungs and can enter the brain directly via inhalation through the nose. This is quite new, and there is not a lot of hard data, according to Donaldson. He's concerned about links that have been found between welding and Parkinson's disease, because there is research showing that inhaled metals from welding-fume nanoparticles end up in the brain.

In Oberdorster's experiment, it seems that the buckyballs got into the fish brains by entering the bloodstream through the gills. However, she points out that despite the changes she found in their bodies, the fish appeared to be functioning normally. "They could eat just fine, and when I tried to catch them with a net, they were doing all the proper escape behaviours," she says. "We think the damage was as debilitating as having a very bad migraine."

She says it is possible that the brain tissue might repair itself because there are cells in the brain that help the mending process, but the only way to find this out would be to carry out a follow-up study.

In the fish livers, the buckyballs had switched on genes related to inflammation and involved in breaking down contaminants. This is a standard response to a toxic substance, and it suggests that the fish were trying to change the fullerenes into their constituent parts to get rid of them. "If the body could deal with buckyballs like any other toxicant, that would be really nice; you wouldn't have to worry about them accumulating in the body," says Oberdorster. This is being investigated.

With the nanotechnology industry poised to make all kinds of nanoparticles in huge quantities, these toxicology results are timely, for the public and for the thousands of workers in the industry exposed to such materials.

But a lot more needs to be understood before we can relax about the possible effects of nanoparticles on human health.

THE BENEFITS OF DOWNSIZING

Nanotechnology is the science of the very small, on a scale of millionths of a millimetre - a thousand times smaller than the micro-devices used in electronics.

Nano-scale materials are already used for some mobile phones and for the ultra-thin coating on self-cleaning glass. There are also plans to develop nanotechnology for delivering drugs to the target tissues in the body, to build biology implants to help the blind to see, and to clean contaminated ground.

Scientists can already manipulate atoms with "nano" microscopes. They have also made nano-scale machines - even works of art - smaller than the width of a human hair.

A reasonable fear is that when particles are constructed on the nanoscale, it will be easier for them to penetrate the body - through the skin and lungs, say - and so pose new health threats. Viruses are an example of naturally occurring nano-scale particles.

Steve Connor