RF interferometric wearable wrist pulse detection system

Abstract

This study presents a novel wrist pulse detection system based on the variation of transmission coefficient (S21) of coupled resonators. Radiated signal from one resonator is modulated by the wrist pulse and transmitted to the other resonator. To improve sensitivity of the system, a radio‐frequency (RF) interferometer concept is employed and its validity is mathematically analysed. Full‐wave electromagnetic simulation is conducted with the wrist phantom model to predict the transmission coefficient between resonators. The peak level of the S21 is around −10 dB and the resonance frequency shifts to higher frequency as the distance between the resonator and wrist increases. A prototype of the system is fabricated and tested to show sensitivity enhancement around the 2.4 GHz industry‐science‐medical (ISM) band. Experimental results reveal that the wrist pulse signal measured by the RF interferometer system is in exact coincidence with the reference finger sensor. Also, the sensitivity is enhanced about 23 dB with the RF interferometer system based on the measurement results.

A Flexible Near-Field Biosensor for Multisite Arterial Blood Flow Detection

“…The physics of near-field perturbations can be described as the detuning effect of RLC tank circuits at resonance, where the system variables influence the effective permittivity of the capacitively coupled medium, in turn modulating the RF attributes (e.g., S 11 and phase responses at the resonance band) and the impedances of the resonator [ 1 , 5 , 13 , 14 , 16 , 17 , 25 ]. In our chosen application, the coupling capacitance between the resonator and the dielectric tissue under test varies at resonance during field perturbation [ 1 , 5 , 25 , 26 , 27 ].…”

“…The utilities of nearfield RF resonators have been studied for several decades in the fields of wearable technologies with different transceiver architectures for detecting cardiopulmonary motion, precordial displacements for respiration rate and heartbeat, radial pulse, intraocular pressure as well as intracranial pressure (ICP) [ 1 , 2 , 5 , 6 , 7 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. However, most applications require rigid antennas as a sensing element or bulky benchtop readout equipment for signal transduction, which lacks epidermal conformance and a wearable form factor.…”

“…Previous benchmark works have demonstrated miniature readout systems for transducing specific RF attributes such as resonant frequency shift and reflection coefficient modulation. However, those are typically power-intensive solutions since the resonators resonate at very high frequency (VHF) and ultra-high frequency bands (UHF) [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 22 , 23 , 24 ], which demand a relatively high power-consuming RF transmitter coupled with a power amplifier. On the other hand, the receiver circuit at microwave frequencies becomes more complex and power-intensive because it requires more filters and low-noise amplifiers to condition the intermediate frequencies to extract the desired demodulated waveform.…”

“…For example, a readout circuit based on free-running voltage-controlled oscillators with surface acoustic wave filters suffers from the frequency drift of the oscillator due to the aging of components and temperature variation. Prior studies have also reported a wrist pulse detection scheme using a phase-locked loop system and an array resonator with an enhanced sensitivity based on a phase shifter, attenuator, and RF coupler [ 13 , 16 , 17 , 18 ]. However, in a practical application, the sensitivity of the readout circuit depends on the subjective calibration of the attenuator and phase shifter, which limits the usability of the sensor on different humans [ 23 ].…”