Treatment of many pulmonary diseases, such as asthma, cystic fibrosis and lung cancer, requires 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.

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

A more effective approach, according to Carsten Rudolph at Ludwig-Maximilians-University in Munich and co-workers from elsewhere in Germany, is to mix the drugs with so-called "nanomagnetosols" (aerosol droplets comprising superparamagnetic iron-oxide nanoparticles dispersed in water). These nanomagnetosols can then be guided directly to problem areas using a magnetic field.

The idea is not new, but Rudolph's group has shown 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. Their calculations assumed spherical nanomagnetosol droplets (average geometric mean diameter of 3.5 microns) made up of iron-oxide nanoparticles (core diameter 50 nm) in water.

The simulation predicts that when a magnetic probe is placed close to a bronchus, up to 16% of the nanomagnetosols were deposited on the magnetized airway surface (versus 4% on the unmagnetized surface).

Rudolph's group tested this prediction by opening up the chest of a mouse, and placed a specially designed magnetic-tip probe with a high flux gradient of 100 T/m next to one of the lungs. When the researchers squirted the microdroplets 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. As well as being much larger, human lungs are more intricate, so it will be difficult to guide the microdroplets with the same accuracy. A much more powerful magnetic probe will also be required to overcome the additional distance between the probe and inner lung.