Multimodal Wireless Wound Sensors via Higher-Order Parity-Time Symmetry

Abstract

Chronic wounds have emerged as a significant healthcare burden, affecting millions of patients worldwide and presenting a substantial challenge to healthcare systems. The diagnosis and management of chronic wounds are notably intricate, with inappropriate management contributing significantly to the amputation of limbs. In this work, we propose a compact, wireless, battery-free, and multimodal wound monitoring system to facilitate timely and effective wound treatment. The design of this monitoring system draws on the principles of higher-order parity-time symmetry, which incorporates spatially balanced gain, neutral, and loss, embodied by an active -RLC reader, an LC intermediator, and a passive RLC sensor, respectively. Our experimental results demonstrate that this wireless wound sensor can detect temperature (T), relative humidity (RH), pressure (P), and pH with exceptional sensitivity and robustness, which are critical biomarkers for assessing wound healing status. Our in vitro experiments further validate the reliable sensing performance of the wound sensor on human skin and fish. This multifunctional monitoring system may provide a promising solution for the development of futuristic wearable sensors and integrated biomedical microsystems.

Keywords: biomedical sensors; parity-time symmetry; wearable sensors; wireless sensors; wound healing.

Fig. 1.

(a) Schematic of the wireless wound monitoring system, where the parameters to be detected (e.g., temperature, relative humidity, pressure, and pH) are obtained from the shifts of narrow resonant dips. (b) Equivalent circuit diagram of the third-order PTX-symmetric sensing system, which consists of an active −RLC reader, an LC intermediator and passive RLC sensors.

Fig. 2.

(a) Real parts of eigenfrequencies of the PT/PTX-symmetric system in Fig. 1(b), where the calculated and measured results are represented by clolor countours and scattered points, respectively. (b) Contours of reflection coefficients as a function of the gain-loss parameter $\gamma$ and the normalized angular frequency $\omega ∕{\omega }_0$; here $\kappa =0.5$.

Fig. 3.

(a) Photograph of experimental setup for the wound monitoring system (left) and the compact, flexible, and wearable wound sensor (right). Measured reflection spectra for two scenarios: (b) when both $R_s$ and $C_s$ are unknown, and one first tries to determine $C_s$, and (c) after $C_s$ is known, one tries to find the exact value of $R_s$.

Fig. 4.

Reflection spectra of the PT/PTX-symmetric wound monitoring system with (a) 30% RH and different temperature, and (b) 50 °C and different RH. Here, the solid and dashed lines respectively represent the measured and calculated results.

Fig. 5.

(a) Measured capacitance of the capacitive humidity sensor versus relative humidity. (b) Frequency shift ${\Delta f}_2$ as a function of relative humidity. (c) Measured equivalent resistance of the NTC thermistor versus temperature. (d) Frequency ratio $f_3∕f_2$ as a function of temperature. The error bars are generated from three measurements.

Fig. 6.

Reflection spectra of the PT/PTX-symmetric wound monitoring system with (a) pH=10 and different pressures, and (b) P = 200 mmHg and different pH values. Here, the solid and dashed lines respectively represent the measured and calculated results.

Fig. 7.

(a) Potential difference between the pH electrodes (left axis) and equivalent capacitance of the sensor (right axis) versus pH level. (b) Frequency shift ${\Delta f}_2$ as a function of pH level. Inset: schematic of the tank sensor when performing pressure and pH measurement. (c) Equivalent resistance of the force sensing resistor versus pressure. (d) Frequency ratio $f_3∕f_2$ as a function of pressure. The error bars are generated from three measurements.

Fig. 8.

Photographs of the wound sensor (a) affixed to human skin and (b) affixed to a fish. The reader setup is exactly the same as that shown in Fig. 3(a). (c) Measured reflection spectrum of the wound sensing system with T = 33.2 °C and RH = 30%. (d) Frequency shift ${\Delta f}_2$ as a function of relative humidity with T = 25 °C (top). Frequency ratio $f_3∕f_2$ as a function of temperature with RH = 30% (bottom). (c) and (d) indicate that the resonant frequency and sensing efficacy of the wound sensor exhibit remarkable consistency when deployed in air, adhered to human skin, and adhered to a fish.