Whether they are detecting toxic molecules in the air or pathogenic bacteria in a vat of yoghurt, many microscopic sensors share a crucial weakness: the ultra-thin wires that relay signals from the physical sensing components to the circuitry. If those fragile wires break or corrode, the sensor becomes useless.
Now a wireless sensor has been developed that detects substances through their effect on an applied magnetic field. “This is a wireless technology you can embed in any environment where sub-millimetre wiring is a risk, either due to spark risk or because of its fragility,” says the sensor’s developer, Mike Gibbs, an expert in engineering materials at the University of Sheffield in the UK.
Typical molecular sensors, for example, use 1-micrometre-long silicon cantilevers that ping up and down like a diving board when the target molecule lands on them. This vibrates a piezoelectric crystal, producing a fluctuating current of the same frequency. Wires connecting the crystal to a sensing circuit allow the device to register the cantilever’s motion.
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To build his wireless sensor, Gibbs applied a 100-nanometre-thick magnetic film to a silicon nitride cantilever. “Pinging” the cantilever with a field from an external electromagnetic coil sets it vibrating. That vibration in turn induces a current in the coil at the same frequency. When a molecule or pathogen of interest lands on the cantilever it will alter this frequency by a telltale amount. Because there are no wires to the cantilever, it can be placed in a spark-free environment, with the coil at a safe remove. Gibbs has filed a patent on the idea, which he presented at an Institute of Nanotechnology conference in London last week.
The absence of exposed electrical connections could be very useful in some applications, says Ian Eastwood of Authentix, a security technology firm based in Dunnington, UK. “Airlines have asked us to look into ways of sensing contaminated aviation fuel to prevent the equivalent of someone putting sugar in a car’s fuel tank. Removing spark risk could be one way we could do this.”
“The absence of exposed electrical connections could be very useful”
