Tiny sensor offers new medical, security uses

CHICAGO, Nov 2 (Reuters) US researchers have developed a magnetic sensor smaller than a grain of rice and sensitive enough to detect a fetal heart beat, offering the potential for a host of new medical and security uses.

The sensor, described on Thursday in the journal Nature Photonics, provides a low-cost and portable way to detect changes in a magnetic field, they said.

''What we've done is demonstrate a very good sensitivity with a very small cell,'' said John Kitching of the US Department of Commerce's National Institute of Standards and Technology, or NIST, who led the project.

The device is 1,000 times more sensitive than NIST's last microchip-sized mini-sensor. Right now it is just a prototype, but Kitching said the device could be used in a range of applications, from fetal heart monitoring to screening for explosives.

Because of its small size, it could run for several weeks on a single AA battery, he said in a telephone interview.

''Magnetic fields are all over the place,'' Kitching said.

''Anything that has iron in it has a magnetic field.'' Electric currents, such as those from power lines, emit a magnetic field, as do the electrical impulses that make the heart contract or brain cells fire.

Larger magnetometers are used by geologists to find iron deposits and by archeologists to find buried objects. They can also be used by satellites to track the Earth's magnetic field.

The most sensitive magnetic sensors -- superconducting quantum interference devices, or SQUIDS -- can detect very weak changes in magnetic fields but must be kept very cold, making them much bigger and far more power-hungry.

While the NIST device sacrifices a bit of that sensitivity, it makes up for it in portability.

''We are able to make something that is almost as sensitive without all the stuff that is needed to make it run,'' Kitching said.

The prototype consists of a tiny container holding about 100 billion atoms of rubidium. The NIST researchers shoot an infrared laser beam through the container and measure how much light the atoms absorb. The higher the absorption, the stronger the magnetic field.

Kitching said the device is highly adaptable. It could be used to measure electrical activity in the brain, helping to spot tumors or monitor brain function. Or it could be used in war zones to detect bombs that may not have exploded.

''It really does open up a wide range of possibilities,'' he said.

REUTERS SZ HS0909