POTHOLES are a scourge of rich and poor countries alike. The American Automobile Association recently calculated that 16m drivers in the United States suffered pothole damage to their vehicles in the past five years. That damage ranged from punctures, via bent wheels, to broken suspensions. The bill to fix it was about $3 billion a year. In India, meanwhile, the cost of potholes is often paid in a harsher currency than dollars. There, more than 3,000 people a year are killed in accidents involving them. Yet cash-strapped governments often ignore the problem, letting roads deteriorate. In Britain, for example, some £12 billion ($17 billion) would be needed to make all roads pothole-free. Ways of repairing potholes more cheaply and enduringly would thus be welcome. And several groups of researchers are working on it.

The most common cause of potholes is water penetrating cracks in a road’s surface and weakening its foundation. This is a particular problem with asphalt surfaces. These are made from an aggregate of materials bound together by sticky bitumen. The constant pounding of traffic disintegrates the road surface above the weakened area. In cold climates the destruction is aggravated by water in the cracks freezing and thawing. The shattered asphalt then peels away, leaving a pothole.

To make matters worse, any repairs that do happen are usually a lash-up. To save money, the material used for the patch is frequently “worked cold”. This means it is not heated with specialist equipment to make the bitumen in it soft enough to flow into the shape required and meld properly with the edges of the pothole. Instead the stuff is simply shovelled off the back of a lorry and pounded down. That can work as a temporary fix until the road can be resurfaced properly, but often as not this job gets delayed almost indefinitely, which results in more cracks appearing around the fill and yet more potholes.

What is needed is a material that can be used as readily as a cold patch, but which works as well as a hot one. Larry Zanko and his colleagues at the University of Minnesota Duluth, think they know what it is. They are mixing asphalt with ground iron ore that contains magnetite—an iron oxide which, as its name suggests, is magnetic. A phenomenon called ferromagnetic resonance means that when magnetite is zapped with microwaves of an appropriate frequency it gets hot.

Dr Zanko and his colleagues built an experimental repair vehicle equipped with a microwave generator on the end of a hydraulic arm. Using this on asphalt that contained between 1% and 2% magnetite, he found he could heat the material in a patch to 100°C in about ten minutes. At that temperature it could be tamped down to produce a more effective repair. The heat also drives out moisture, further improving adhesion, says Dr Zanko. He and his team are now trying to raise the money needed to develop the technology into a commercial pothole-fixing system.

An even better approach, however, would be to stop potholes forming in the first place, by sealing the cracks that cause them before any damage is done. Etienne Jeoffroy of ETH Zurich, a Swiss university, has been attempting to do just that in collaboration with André Studart and Manfred Partl of EMPA, a Swiss research group. He also mixes iron oxide into the asphalt, but in this case it does not start off magnetic. Rather, he uses a magnetic field to heat it.

The process he employs is one also used to treat certain tumours. The tumour in question is injected with iron-oxide nanoparticles, which are less than 100 nanometres (billionths of a metre) across. These are then subjected to an alternating magnetic field, which heats them up and cooks the diseased tissue. In his experiments, Dr Jeoffroy found that it takes just a few seconds to heat pieces of nanoparticle-containing bitumen this way. Thus softened, the bitumen seeps into incipient cracks, sealing them up.

Maintenance of roads containing such nanoparticles might therefore require no more than driving over them once a year with a special vehicle which generates an appropriate magnetic field. That would, though, require building roads this way in the first place—or, at least, resurfacing them with nanoparticle-containing asphalt when such maintenance falls due. Stopping potholes growing in existing surfaces, by contrast, means eternal vigilance. And that is what Phil Purnell of the University of Leeds, in Britain, hopes to automate.

As part of a wider project of automating the inspection and maintenance of infrastructure, Dr Purnell and his colleagues are looking at automatic systems which might be fitted to vehicles that ply regular routes, such as buses, to examine roads routinely for signs of damage. In one version of the future such a system would then activate a robotic repair vehicle when it came across a crack that needed fixing. This robot would come to the crack and fill it with a fast-setting bonding material (asphalt would not be needed, since no hole would yet have formed). That is not quite as neat as using nanoparticles and magnetic fields to create a self-healing highway. But if it does the job, who cares?