Geometrically encoded magnetic sensors (GEMs), developed by researchers from National Institute of Standards and Technology (NIST) and the National Institutes of Health (NIH), react to local biochemical conditions by changing their shape and response to radio frequencies.

The shape-shifting probe, about one-hundredth as wide as a human hair, could have a major impact on clinical diagnostics. The device detects and measures localized conditions on the molecular scale deep within tissues, such as a difference in acidity near an inflammation site, and observes the changes in real time.

Instead of optically based sensing, the shape-changing probes are designed to operate in the radio frequency (RF) spectrum, specifically to be detectable with standard nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) equipment. The GEMs receive strong, distinctive signals from very small dimensions at substantial depths, or in other locations impossible to probe with optically based sensors.

The microengineered metal-gel sandwich structures are about 5 to 10 times smaller than a single red blood cell. Each consists of two separate magnetic disks that range from 0.5 to 2 micrometers (millionths of a meter) in diameter and are tens of nanometers (billionths of a meter) thick.

Between the disks is a spacer layer of hydrogel, a polymer network that can absorb water and expand significantly; the amount of expansion depends on the chemical properties of the gel and the environment around it. Conversely, the gel can also shrink in response to changing local conditions. Swelling or shrinking of the gel changes the distance (and hence, the magnetic field strength) between the two disks, and that, in turn, changes the frequency at which the protons in water molecules around and inside the gel resonate in response to radio-frequency radiation.