A ‘Symmetrical’ Approach Improves Wireless Microsensors

Billy Hurley, Digital Editorial Manager

Networks of wireless microsensors enable the monitoring of areas that have previously been considered off limits — including toxic environments, hard-to-reach vehicle components, or remote parts of the human body.

To enable the large-scale implementation of sensing devices, however, requires a greater sensitivity from the tiny detectors.

A team of researchers are hoping that a “symmetrical” approach to microsensors will do the trick, enhancing readings in an even smaller footprint.

In a new technique called isospectral parity-time-reciprocal scaling, or PTX symmetry, a “reader,” paired with a passive microsensor, receives highly sensitive radio frequencies in a miniaturized setup.

Andrea Alù, director of the ASRC’s Photonics Initiative and Einstein Professor of Physics at The Graduate Center, and Pai-Yen Chen, professor at Wayne State University, demonstrated the phenomenon in a telemetric sensor system.

The researchers hope that the PTX approach will enable ubiquitous networks of long-lasting, unobtrusive microsensors — the kind used to monitor smart cities, wireless health, and cyber-physical systems.

“While there has been continuous progress in miniature micro-machined sensors, the basics of telemetric readout technique remains essentially unchanged since its invention,” said Professor Chen. “This new telemetry approach will make possible the long-sought goal of successfully detecting tiny physical or chemical actuation from contactless microsensors.”

Chen spoke with Tech Briefs about how his team’s symmetrical sensor technique can be applied to a variety of industrial and medical applications.

Tech Briefs: Why is it so important to improve the sensitivity of microsensors?

Professor Pai-Yen Chen: The sensitivity of a sensor indicates how much its output signal — typically, the electrical signal — changes with the variation of physical input or chemical quantity.

For instance, for wireless pressure sensors, a small pressure change in a cavity can drastically modulate frequency responses (such as a large shift in the resonance frequency or a clear difference in magnitude of the backscattered signal).

Improving sensitivity of wireless, battery-free microsensors is often hindered by the increased power dissipation in it (inversely proportional to the sensor’s size), which lead to low Q-factor , poor resolution, and unclear spectral shift in response to variations of the property to be measured.

Tech Briefs: How does your new microsensor design improve upon traditional designs?

Prof. Chen: In recent years, although there has been continuous progress in micromachining technology and sensor design, the basics of sensor telemetry remains essentially the same since its invention in ‘60s. In this work, instead of developing a new structure or material for the microsensor itself, we propose a new telemetry system obeying the parity-time (PT) symmetry (or spatial and time reflection symmetry) to improve both sensitivity and resolution.

To achieve a PT-symmetric telemetric system, an active RF reader is used to interrogate the passive microsensor, providing balanced gain and loss, which interact in a contactless manner. If this PT-symmetric system is operated around a critical point, a so-called exceptional point in physics terms, largely bifurcating resonance frequencies with respect to the physical property perturbation will enhance the sensitivity, compared to conventional telemetric approaches.

Tech Briefs: Where do you see this new microsensor design being most useful? In what kinds of applications?

Prof. Chen: Technological progress in microelectromechanical system (MEMS) and scalable manufacturing allow inexpensive microscopic sensors to replace costly macroscopic sensors, supporting a variety of medical and industrial applications. Besides, the need for massive numbers of sensors is critical for the Internet of Things (IoT), including everything from industrial equipment, infrastructures, automobiles, to healthcare.

The explosive growth is predicted to begin. Our PT-symmetric sensor telemetry technique can be potentially applied to various types of IoT sensors. For instance, the ultrasensitive MEMS-based pressure sensor demonstrated in our paper  can be extended to bioimplantable devices measuring intraocular, intracranial, intravascular, and blood pressure inside the human body, as well as tire-pressure monitoring.

Tech Briefs: What is isospectral parity-time-reciprocal scaling, or PTX symmetry, and how does the PTX symmetry improve microsensor design?

Prof. Chen: Compared to PT-symmetry, parity-time-reciprocal scaling (PTX)-symmetry introduces a new reciprocal scaling operation, which allows breaking the balance between gain and loss coefficients, and also the mirror symmetry between the reactive components (such as the capacitor and inductor in the circuit).

In this scenario, more flexibilities are allowed to enhance the resonance linewidth (proportional to Q-factor), boosting the spectral resolution that represents the ability of a sensor to distinguish small changes in the physical property. On the other hand, PT- and PTX-symmetry telemetry systems exhibit exactly the same resonance frequency (e.g., center frequency of reflection dip) and its shift with respect to perturbation of the physical property to be sensed.

What do you think? Will “symmetrical” microsensors improve the IoT? Share your comments and questions below.