Justin P. McMurray

Many commercial wearable sensor systems typically rely on a single continuous cardiorespiratory sensing modality, photoplethysmography (PPG), which suffers from inherent biases (i.e., differences in skin tone) and noise (e.g., motion and pressure artifacts). In this research, we present a wearable device that provides robust estimates of cardiorespiratory variables by combining three physiological signals from the upper arm: multiwavelength PPG, single-sided electrocardiography (SS-ECG), and bioimpedance plethysmography (BioZ), along with an inertial measurement unit (IMU) providing 3-axis accelerometry and gyroscope information. We evaluated the multimodal device on 16 subjects by its ability to estimate heart rate (HR) and breathing rate (BR) in the presence of various static and dynamic noise sources (e.g., skin tone and motion). We proposed a hierarchical approach that considers the subject’s skin tone and signal quality to select the optimal sensing modality for estimating HR and BR. Our results indicate that, when estimating HR, there is a trade-off between accuracy and robustness, with SS-ECG providing the highest accuracy (low mean absolute error; MAE) but low reliability (higher rates of sensor failure), and PPG/BioZ having lower accuracy but higher reliability. When estimating BR, we find that fusing estimates from multiple modalities via ensemble bagged tree regression outperforms single-modality estimates. These results indicate that multimodal approaches to cardiorespiratory monitoring can overcome the accuracy–robustness trade-off that occurs when using single-modality approaches.

Cardiovascular disease (CVD) is the leading cause of global mortality, with hypertension affecting over one billion people. Current noninvasive blood pressure (BP) systems, like cuffs, suffer from discomfort and placement errors and lack continuous monitoring. Wearable solutions promise improvements, but technologies like photoplethysmography (PPG) and bioimpedance (BIOZ) face usability and clinical accuracy limitations. PPG is sensitive to skin tone and body mass index (BMI) variability, while BIOZ struggles with electrode contact and reusability. We present a novel, strain gauge-based wearable BP device that directly quantifies pressure via a dual transducer system, compensating for tissue deformation and external forces to enable continuous, accurate BP measurement. The reusable, energy-efficient, and compact design suits long-term daily use. A novel leg press protocol across 10 subjects (systolic: 71.04–241.42 mmHg, diastolic: 53.46–123.84 mmHg) validated its performance under dynamic conditions, achieving mean absolute errors of 2.45 ± 3.99 mmHg (systolic) and 1.59 ± 2.08 mmHg (diastolic). The device showed enhanced robustness compared to the Finapres, with less motion-induced noise. This technology significantly advances current methods by delivering continuous, real-time BP monitoring without reliance on electrodes, independent of skin tone, while maintaining a high accuracy and user comfort.

To combat the unmet need of early detection and monitoring of postpartum hemorrhage (PPH), the leading cause of maternal death, a wearable short-wave infrared (SWIR) photoplethysmography (PPG) sensor was developed for real-time detection and monitoring of hemodilution, a compensatory response to PPH. The device uses wavelengths at 900 and 1300nm, ideal for monitoring hemoglobin and water contributions in blood, respectively. These wavelengths minimally absorb melanin, which has been shown to bias PPG results in pulse oximetry. High signal-to-noise PPG signals have been acquired from both the 900nm and 1300nm channels in human volunteers. Further, the hemodilution sensing capabilities were tested by flowing sheep blood through a pulsatile flow phantom. The ratio of the peak amplitudes of the PPG waveforms from both LEDs showed a linear correlation, demonstrating feasibility to detect hemodilution and provide a low-cost, fully wireless, continuous monitoring tool for early diagnosis and monitoring of PPH.

In this research, we examine the potential of measuring physiological variables, including heart rate (HR) and respiration rate (RR) on the upper arm using a wireless multimodal sensing system consisting of an accelerometer, a gyroscope, a three-wavelength photoplethysmography (PPG), single-sided electrocardiography (SS-ECG), and bioimpedance (BioZ). The study included collecting HR data when the subject was at rest and typing, and RR data when the subject was at rest. The data from three wavelengths of PPG and BioZ were collected and compared to the SS-ECG as the standard. The accelerometer and gyro signals were used to exclude data with excessive noise due to motion. The results showed that when the subject remained sedentary, the mean absolute error (MAE) for the HR calculation for all three wavelengths of the PPG modality was less than two bpm, while the BioZ was 3.5 bpm compared with SS-ECG HR. The MAE for typing increased for both modalities and was less than three bpm for all three wavelengths of the PPG but increased to 7.5 bpm for the BioZ. Regarding RR, both modalities resulted in RR within one breath per minute of the SS-ECG modality for the one breathing rate. Overall, all modalities on this upper arm wearable worked well when the subject was sedentary. Still, the SS-ECG and PPG showed less variability for the HR signal in the presence of motion during micro-motions such as typing.

Commercial and industry-standard pulse oximeters exhibit limitations in accurately measuring blood oxygen saturation (SpO2) across diverse skin tones, particularly in those with darker pigmentation. This discrepancy can lead to erroneous SpO2 readings due to the absorption of light by melanin, thereby placing those with darker skin tones at heightened risk. In this study, we designed, built, and tested a multimodal system capable of simultaneously measuring blood oxygen levels (SpO2) and contact pressure. Our primary objective was to evaluate the effects of contact pressure on mitigating inherent pulse oximetry inaccuracies across diverse skin tones. To achieve this, we calibrated and tested the device using five skin types in the range of 3–9 on the monk skin tone (MST) scale and seven contact pressure levels across the physiologic range of 10–70 mmHg. The research outcomes of this preliminary study showed that the most accurate results across a range of skin tones appear around the contact pressure range of 50 mmHg. This research has potential implications for healthcare providers, as accurate monitoring of blood oxygen levels is crucial for timely intervention and effective disease management. Moreover, addressing disparities in pulse oximetry readings that come from differences in skin tone across populations will help provide equitable healthcare outcomes for individuals from such diverse populations.

Wearable devices for continuous monitoring of arterial pulse waves have the potential to improve the diagnosis, prognosis, and management of cardiovascular diseases. These pulse wave signals are often affected by the contact pressure between the wearable device and the skin, limiting the accuracy and reliability of hemodynamic parameter quantification. Here, a continuous hemodynamic monitoring device that enables the simultaneous recording of dual‐channel bioimpedance and quantification of pulse wave velocity (PWV) and blood pressure (BP) is reported. The investigations demonstrate the effect of contact pressure on bioimpedance and PWV. The pulsatile bioimpedance magnitude reached its maximum when the contact pressure approximated the mean arterial pressure of the subject. PWV is employed to continuously quantify BP while maintaining comfortable contact pressure for prolonged wear. The mean absolute error and standard deviation of the error compared to the reference value are determined to be 0.1 ± 3.3 mmHg for systolic BP, 1.3 ± 3.7 mmHg for diastolic BP, and −0.4 ± 3.0 mmHg for mean arterial pressure when measurements are conducted in the lying down position. This research demonstrates the potential of wearable dual‐bioimpedance sensors with contact pressure guidance for reliable and continuous hemodynamic monitoring.

As wearable devices continue to advance, the implementation of additional sensors and device health monitoring capabilities has become integral to their development. To explore noninvasive health monitoring capabilities, in this study, we developed a multimodal system consisting of multiwavelength photoplethysmography (PPG), and bio-impedance (Bio-Z) used to extract signals simultaneously from the wrist above the radial artery. We then evaluated the systolic rising edges of each modality and the systolic temporal peak-to-peak separation between the Bio-Z and PPG signals. The preliminary results show a signal time delay with a correlation to penetration depth between the systolic peaks across modalities within and across subjects. The trend showed the systolic peak of Bio-Z arriving first, with PPG systolic peaks arriving milliseconds after in the order of infrared, red, and green, each with its own time delay and decreasing slope intensity, likely resulting from differences in light penetration depth and impedance changes due to continuum mechanics associated with the cardiac pulse wave.