SQUIDS are finding new applications in some unexpected places. SQUIDS, the ultra-sensitive magnetic field detectors currently used for brain scans, are now monitoring aircraft corrosion and spotting bacteria, the American Physical Society March Meeting heard this week in Seattle.

The secret of the SQUID is a small loop of superconducting wire, interrupted by at least one narrow strip of electrical insulator to form a 'Josephson Junction'. Current flowing around this loop is extremely sensitive to changes in the magnetic field within it. The best SQUIDS can detect magnetic fields a hundred billion times smaller than the Earth's feeble magnetic pull.

SQUID detectors are increasing the sensitivity and speed of immunoassays -- the use of specific antibodies to detect small amounts of material in a sample, in this case bacteria or viruses -- Helene L. Grossman of the University of California at Berkeley told the meeting.

Small magnetic particles are attached to the antibodies, which are mixed into the sample and bind to the pathogen. The whole sample is briefly exposed to a strong magnetic field to align the magnetic particles, and then quickly examined with the SQUID detector.

Magnetic particles bound to pathogens remain aligned longer than unbound antibodies, so the SQUID can instantly detect the presence and approximate concentration of the pathogen.

It is early days for this procedure but Grossman's team has already tripled the sensitivity of existing tests. "We hope to increase the sensitivity a further 4,000 times, and may ultimately be able to detect individual bacteria and even watch them move," she adds.

The method can be easily adapted to any bacteria or virus using commercially available antibodies and magnetic particles and in the future could be customized to detect specific molecules or antibodies.

Meanwhile, the American Air Force is funding research into using SQUIDs to track corrosion in ageing aeroplanes. There is currently no way to examine corrosion below the surface of metals, and the only way to find out whether rivets and welds are degrading is to take them apart and look.

So John Wikswo of Vanderbilt University, Nashville Tennessee and colleagues have been probing corroding aircraft parts in the lab using SQUID detectors, with considerable success.

Corrosion involves the movement of charged particles, which creates a magnetic field that is minute -- but large enough for a SQUID to detect easily. The technique can also monitor the corrosion in situ, as it happens. "This is an application of SQUIDS for which there is no competition," says Wikswo.

Wikswo's team dried samples from old aircraft, then immersed them in water or salt solution. They expected to detect differing rates of decay, but came in for a surprise.

"It doesn't make a whole lot of difference [whether] I park my old aircraft near the sea or away from it," Wikswo told the meeting. The salts and oxide remnants left behind as a metal degrades seem to be far more corrosive than even strong salt water.

Jeremy Thomson is the Assistant Web Editor of Nature