Many pulmonary diseases, such as asthma, cystic fibrosis and lung cancer, need drugs to be inhaled so that they can reach the affected area. To do this, patients have to gasp on an inhaler that emits the particulate drugs into the windpipe.

But the effectiveness of these inhalers is not great: typically only 4% of the drug makes it through the windpipe, forcing doctors to administer higher doses, which can exacerbate unwanted side effects.

A better way, according to Carsten Rudolph at Ludwig-Maximilians University in Munich and co-workers from elsewhere in Germany, is to mix the drugs with iron-oxide magnetic nanoparticles and microdroplets of water, or so-called "nanomagnetosols". These nanomagnetosols can then be guided directly to problem areas using a magnetic field. The idea is not new, but Rudolph's group show for the first time that it can be performed in a real organism – in this case, a mouse.

The researchers began by creating a computer simulation of a mouse's airways where the windpipe forks into two bronchi, taking into account air flow rates measured in previous physiological studies. Assuming they were to use nanomagnetosol droplets with an average diameter of 3.5 µm, they predicted that they could use a magnetic probe placed close to a bronchus to get up to 16% coverage of the microdroplets.

Rudolph's group tested their prediction by opening up the chest of a mouse, and placed a specially designed magnetic tip probe with a high flux gradient of 100 Tm-1 next to one of the lungs. When they squirted the nanomagnetosol droplets into the mouse's airways, they found that the lung next to the probe received eight times more drug coverage than the one without. Upon placing the probe on another mouse with its chest intact, the benefit was reduced, with just two and a half times more coverage.

Performing the same feat in humans will not be so straightforward, however. Human lungs are much larger and more intricate, so it will be difficult to guide the nanomagnetosol droplets with the same accuracy. Moreover, a much more powerful magnetic probe will be required to overcome the additional distance between the probe and inner lung.