Their current work also demonstrates the possibility of a switchable system that responds to local conditions. In the experiments, the spheres glowed only when the solution had the same acidity as structures called lysosomes located within cells. This is due in part to the pH sensitivity of the enzyme they used; but it is also, Hunziker says, because the pores are open at this level of acidity. This sensitivity could theoretically ensure that the fluorescent signal only switches on inside a cell.

The Swiss researchers are now testing the toxicity of the nanocarriers in animals and working on developing a system that could deliver an appropriate drug to targeted cells, perhaps by using synthetic channels, rather than the current bacterial proteins, which would open to deliver the drug once inside target cells.

Robert Langer, professor of chemical engineering at MIT, says the work is interesting, but also cautions that it is still at a very early stage--animal tests have yet to demonstrate its usefulness. Meanwhile, Theresa Allen, professor of pharmacology at the University of Alberta in Canada, is concerned that the use of bacterial proteins could trigger an immune reaction. But she also says the current nanocarrier system might be a useful diagnostic tool to analyze lab samples.

If the researchers are able to develop a working drug-delivery platform, they'll still face stiff competition. Already-approved drug-delivery systems, for example, are now being modified to break down and release their cargo when they reach lysosomes.

But the new system demonstrates the ability to engineer a complex, smart nanocarrier, which could open the way for more powerful diagnostics and treatments.