(J-394) Wireless Batteryless Soft Sensor for Ambulatory Cardiovascular Health Monitoring

Introduction:: Cardiovascular diseases (CVD) account for over 19 million deaths annually in the United States1. Early detection and diagnosis of CVD are imperative in their treatment. Along with electrocardiograms (ECGs), seismocardiography (SCG) offers a way to view heart function by characterizing fiducial points, traditionally through an accelerometer. However, accelerometer systems are rigid and bulky, which increases signal noise due to motion artifacts. Progressing to thin-film alternatives, devices that measure SCG still rely on rigid circuit components, low-throughput fabrication processes, and near-field communication. We aim to combine the comfort of thin film SCG sensors, a rapid, one-step fabrication method, and passive wireless capability. We propose a batteryless, wireless, chipless, and passive soft SCG sensor that can be measured using inductive coupling. The device shows a strong association with commercial standards and allows for greater access to CVD prevention through its affordability and comfort.

Materials and Methods:: To develop the sensor system, analytical and experimental studies were applied to optimize mechanical and resonant properties. First, a glass slide was spin-coated with silicone elastomer. On a separate glass slide, polydimethylsiloxane and copper foil were then laid across the slide. PI was spin-coated onto the copper foil and cured on a hot plate. Afterward, the copper and PI were transferred to the glass slide coated with elastomer. The sensor was then patterned on a laser micro-machining system. A bridge made of commercial magnet wire was soldered across the pads connecting the interior of the coil with the end of the capacitive sensor. Finally, elastomer was spin-coated across the device and the device outline was cut out using laser micromachining. The signal from the LC circuit was read through a vector network analyzer and a custom MATLAB program was used to record the S11 parameter at a stimulus frequency close to the resonant frequency. Reading the parameter then showed the SCG signal.

Results, Conclusions, and Discussions:: The device comprises a capacitive strain sensor and an inductive coil, forming an LC oscillator circuit^2,3. We tested the device through a vector network analyzer, which showed linear resonant frequency response in low strains. This is especially crucial for SCG measurements because skin response is on a micrometer scale (Figure 1A). At higher strains, the frequency response can be predicted through a quadratic model (Figure 1B) Since copper is not stretchable, the fingers are supported by connections with serpentine structures (Figure 2A). The serpentine pattern changes between the coil and the sensor to enable strain at the interface while minimizing stress on the copper (Figure 2B), and reliability was tested by stretching and unstretching for 100 cycles, in which the capacitances stayed constant (Figure 3). With the LC circuit, the device measures heart rate, pulse, and SCG data. The signal quality of the data decreases as the distance between the coils increases, and the highest quality factor occurs at 6mm between coils (Figure 4). SCG data compares similarly to clinical standards (Figure 5A), and five cardiac events can be shown in a cycle within the signal (Figure 5B).

Overall, the wireless and batteryless system (works) demonstrates highly accurate SCG, heart rate, and pulse measurements compared to clinical standards. The highly sensitive strain sensor can measure heart vibrations on the chest and identify fiducial points corresponding to cardiac events using inductive coupling. The stretchable thin-film sensor offers usefulness in ambulatory and remote healthcare applications where mechano-acoustic signals are essential, such as arrhythmia detection or obstructive sleep apnea. In future studies, a sensor array may provide benefits by offering high amplitude signals near important heart locations, such as near the atrial and mitral valves.

Acknowledgements (Optional): : I would like to thank the Yeo group and the Georgia Tech VIP program for their support.

References (Optional): : [1] Tsao CW, Aday AW, Almarzooq ZI, et al. American Heart Association Council on Epidemiology and Prevention Statistics Committee and Stroke Statistics Subcommittee. Heart disease and stroke statistics-2023 update: a report from the american heart association. Circulation 2023;147:e93-e621.
[2] Herbert R, Lim HR, Rigo B, Yeo WH. Fully implantable wireless batteryless vascular electronics with printed soft sensors for multiplex sensing of hemodynamics. Sci Adv 2022;8:eabm1175.
[3] Herbert R, Elsisy M, Rigo B, et al. Fully implantable batteryless soft platforms with printed nanomaterial-based arterial stiffness sensors for wireless continuous monitoring of restenosis in real time. Nano Today 2022;46:101557.