Chip-Scale Magnetic Sensor Offers Cardio Possibilities

Originally Published MDDI March 2005

R&D DIGEST

Heather Thompson

Engineers at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) have produced a low-power magnetic sensor designed to detect field changes as small as 50 pT. The sensor may ultimately be used in cardiovascular instruments for measuring heartbeats and possibly even for diagnosing heart disease.

What is unusual about the magnetometer is that it is extremely small, barely larger than a grain of rice. And when packed with associated electronics, it could measure as small as 1 cm3. Researchers for the project used chip-scale technology to achieve such accuracy in a device so small.

“This technology really came about from our work with a chip-scale atomic clock,” explains Peter Schwindt, PhD, who is part of the team at NIST. “The two devices are basically the same, with slightly different magnetic sensitivities.” The research team members have been working on developing the atomic clock for nearly three years. They recently expanded their efforts to explore magnetometer applications as well.

The motion of electrons produces magnetic fields either in the form of electrical current or as magnetism in certain metals such as iron, cobalt, and nickel. The sensor works by detecting minute changes in the energy levels of electrons in the presence of a magnetic field. Within a sealed transparent cell on the device, a small sample of rubidium is heated to form a vapor. As light from a semiconductor laser passes through the vapor, changes are detected. The amount of laser light that is absorbed by the atoms is detected by a photocell. Larger magnetic fields produce proportionally bigger changes in the energy absorption.

The researchers believe this method is as sensitive as larger flux-gate-style magnetometers, but may have better accuracy. Further, a sensor with such accuracy typically weighs about 6 lb.

The NIST chip-scale magnetometer can be fabricated and assembled on a semiconductor wafer. About the size of a computer chip, the sensor is applicable for use in handheld and otherwise-mobile devices.

The U.S. Defense Advanced Research Project Agency funded the project, and the device was primarily engineered for navigation, resource location, or ordnance sensors. However, several medical instrument applications are possible.

“The main use we've been exploring is for measuring hearts with a magnetic field,” Schwindt says. “Some research has indicated that certain diseases may be easier to detect with a magnetocardiogram.” Still, he explains, “the magnetometer's strength needs to increase about two magnitudes in order to be effective, and we are working on that.”

Schwindt also believes that the magnetometer could eventually be used to measure fetal heartbeats, since the human body does not distort magnetic fields.

Researchers at NIST believe the sensor could be on the market within 2–3 years, but it depends on several factors. A clear goal for the team, says Schwindt, is to first increase the magnetometer's sensitivity. “We are also working on creating a market for the atomic clock,” Schwindt adds. “And then, once there is a clear route, we'll look to market the magnetic sensor.”

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