Enhancing wireless in vivo sensor readouts with 'divergent exceptional points'
(Nanowerk Spotlight) Radio-frequency (RF) wireless sensors are essential components of smart objects and internet-of-things components. For instance, wireless battery-free inductor-capacitor (LC) sensors have been ubiquitously used in monitoring vital physiological factors of human bodies (e.g., microsensors for monitoring eye disease, congestive heart failures, or brain injury) or assessing the structural health of civil infrastructures such as bridges.
The way these systems work in general is that an external wireless reader is exploited to interrogate a fully-passive, chip-less microsensor by transmitting an RF signal and processing the received backscattered signal. The information to be sensed is encoded in the backscattered RF signals and can be decoded by, for example, reading shifts in resonant frequency.
However, it has long been known that these passive microsensors suffer from poor quality of data and sensitivity due to the environments they operate in and the need for sensors with extremely small footprints.
If the complex electronic systems operate with balanced, spatially-separated gain and loss, they may satisfy parity-time (PT)-symmetry. Mathematically, these systems can be described by a non-Hermitian effective Hamiltonian. One of the properties of such a system is that there can exist certain conditions – called exceptional points – which are branch point singularities in the parameter space of a system at which two or more eigenvalues, and their corresponding eigenvectors, coalesce and become degenerate.
"Standard EPs are usually observed in two-state open systems with PT-symmetry, causing the Taylor series expansion to become singular and fail to converge," Pai-Yen Chen, an Associate Professor in the Department of Electrical and Computer Engineering at the University of Illinois at Chicago, explains to Nanowerk.
In research published in Physical Review Letters ("Experimental Observation of PT Symmetry Breaking near Divergent Exceptional Points"), a research team led by professor Chen at the University of Illinois at Chicago and professor Ramy El-Ganainy at Michigan Technological University, in collaboration with professor Demetrios Christodoulides at CREOL, University of Central Florida, has theoretically introduced and experimentally demonstrated a new class of PT-symmetric RF electronic and telemetric systems, which combine EPs with divergent points (that is another type of mathematical singularity related to poles of complex functions).
These nearly divergent exceptional points (DEPs) can have an unprecedented large eigenfrequency bifurcation beyond those obtained by standard EPs, as can be seen in the figure below:
Left: Schematic of the dual-links three-stages PT-symmetric telemetric sensing system. Right: A plot of the real eigenfrequencies, varying a function of non-Hermiticity parameter. An enhanced bifurcation of eigenfrequencies is observed. (click on image to enlarge)
Higher-order PT-symmetry has recently been investigated in quantum and optical systems, which show that an optical PT trimer comprising a neutral element sandwiched between gain/loss sites can possess a third-order exceptional point. In these systems, the level of eigenvalue bifurcation increases with an increasing number of neutral elements. In order to achieve an ultrasensitive optical PT sensor, it sometimes requires a large array of neutral optical elements between gain/loss sites.
However, in this work, the research team led by Chen and El-Ganainy (which included graduate students Maryam Sakhdari, Mehdi Hajizadegan, and Qi Zhong) show that a simple RF electronic PT trimer that forms the telemetric sensing system is sufficient to achieve the maximum bifurcation effect. This is because the appearance of DEP provides a giant multiplication factor to the bifurcation effect, and such results do not have an analogue in a PT-symmetric optical or quantum system.
"Our proposed technique will dramatically increase sensitivity of microsensors by exploiting the striking bifurcation effect around the DEP," says Chen. "Under operation, any small change in the microsensor’s impedance – as a function of targeted properties – can cause a large shift of resonant frequency in the measured backscattering spectrum."
"In short, we translated the concept of PT-symmetry, first proposed in quantum mechanics by Bender in 1998, into RF telemetric sensing systems," he elaborates. "Arranging the telemetry circuit into a quantum analogue of PT-symmetric non-Hermitian Hamiltonian inspired us to figure out the condition of DEP that plays a critical role in improving the sensitivity of wireless microsensors."
The team's transformative research results pave the way towards building a new generation of telemetering and battery-free wireless micro- and nanosensors with extraordinary sensitivity and resolvability. This could result in significant benefits for wireless implants and wearable electronics, soft robotics, and RF sensors in vehicles, aircraft, civil infrastructures and harsh environments.
According to the researchers, this work can also be extended to other microwave, millimeter-wave and terahertz wireless systems.
One of the most important applications of this work may be point-of-care monitoring and diagnostics. Wearable and implantable microsensors need greater accuracy and quality of data, as well as higher sampling rate, in order to build an effective and trustworthy healthcare internet-of-things.
This telemetry technique can improve the performance of microsensors for monitoring pressure, strain, temperature, humidity, biochemical, gas, and so on.
The team is currently working with the University of Illinois College of Medicine for demonstration and commercialization of a PT-symmetric wireless intraocular pressure (IOP) sensor.
Eye pressure (or IOP) is the only known sign of glaucoma, which has been a leading cause of blindness for people over 60 and affects approximately 3 million people in the USA alone.
"Based on the DEP-enhanced PT-symmetric telemetry method, we are building a minimally invasive implantable wireless sensor for real-time, continuous IOP monitoring," Sakhdari notes. "We expect that our new telemetry technique will greatly improve sensor sensitivity, resolvability, clinical safety, and patient comfort."
One of the current challenges for the team is to engineer wideband and stabilized circuits capable of mitigating deterioration of signal-to-noise ratio at a DEP, because the large frequency drift and the amplification of signals in active circuits are accompanied with increased noise and instability. Hajizadegan says that this requires a multidisciplinary effort involving applied physics and electrical engineering for maximizing the potential of DEP-associated, wireless, fully-passive microsensors.
"Our proposed multi-elements PT-symmetric telemetric sensing system can be applied to various real-world health monitoring systems and in clinical practice that require minimally-invasive and continuously-functioning wireless micro- and nanosensors," Chen concludes. "There are plenty of opportunities for continued research in the areas of DEPs and PT-symmetry in the area of next-generation wireless wearable and implantable electronics. At the moment, the fundamental limits of sensitivity enhancement, optimal circuit topologies, and optimal device engineering at an DEP are open-ended research questions."