Squid Detectives Could Save US Billions

When it comes to measuring the magnetic fields associated with so many processes in nature, from brain activity to oil deposits, one of physicists' best friends is the SQUID, or Superconducting Quantum Interference Device. SQUIDs have been around for several decades, but continuing improvements have opened new applications for them. At the March APS meeting, John Wikswo of Vanderbilt described SQUID studies of hidden corrosion in aging aircraft parts.

According to a 1996 Battelle study, corrosion costs the US a staggering $300 billion per year in infrastructure maintenance, and up to a third of this cost can be prevented with the proper anti-corrosion measures. Whereas traditional techniques provide month-to-month information on surface corrosion activity, SQUIDs can provide hour-by-hour pictures of subsurface corrosion occurring at microscopic rates. (SQUIDs can detect corrosion rates as small as 70 millionths of an inch per year in aluminum, Wikswo says.) The Vanderbilt team studied corrosion in aircraft lap joints, pieces of overlapping metal fastened with rivets or spot welds. While humid air did not increase corrosion appreciably in the lap joints, they determined that distilled water increased it significantly, presumably by activating dried chemical deposits within the metal. Contrary to common wisdom, however, salt water did not increase corrosion appreciably compared to distilled water. The researchers envision the SQUID as a lab tool that can provide advice on aircraft maintenance and the effectiveness of various anti-corrosion compounds.

Helene Grossman of LBL/UC-Berkeley demonstrated the use of SQUIDs to perform faster and more sensitive immunoassays, the detection of small levels of bacteria, viruses, or other proteins and chemicals in biological or industrial samples. In the SQUID technique, one adds magnetic particles to the sample of interest. The particles have specific antibodies or other binding compounds attached to them.

In addition, the particles are superparamagnetic, meaning that they line up with an applied magnetic field even for a short time after the field turns off. Exposed to such a field in the sample, particles which attach to the microorganism or molecule of interest stay aligned longer than unattached particles, providing a signal that can reveal as few as 30,000 attached magnetic particles. By contrast, the widely used ELISA immunoassay only detects as few as 100,000 labeled particles. The researchers are working to improve the SQUID's sensitivity by a factor of 4000 so that the detection of single microorganisms comes within reach.

— Phillip F. Schewe and Benjamin P. Stein