Modern wearable devices show promising results in terms of detecting vital bodily signs from the wrist. However, there remains a considerable need for a device that can conform to the human body’s variable geometry to accurately detect those vital signs and to understand health better. Flexible radio frequency (RF) resonators are well poised to address this need by providing conformable bio-interfaces suitable for different anatomical locations. In this work, we develop a compact wearable RF biosensor that detects multisite hemodynamic events due to pulsatile blood flow through noninvasive tissue–electromagnetic (EM) field interaction. The sensor consists of a skin patch spiral resonator and a wearable transceiver. During resonance, the resonator establishes a strong capacitive coupling with layered dielectric tissues due to impedance matching. Therefore, any variation in the dielectric properties within the near-field of the coupled system will result in field perturbation. This perturbation also results in RF carrier modulation, transduced via a demodulator in the transceiver unit. The main elements of the transceiver consist of a direct digital synthesizer for RF carrier generation and a demodulator unit comprised of a resistive bridge coupled with an envelope detector, a filter, and an amplifier. In this work, we build and study the sensor at the radial artery, thorax, carotid artery, and supraorbital locations of a healthy human subject, which hold clinical significance in evaluating cardiovascular health. The carrier frequency is tuned at the resonance of the spiral resonator, which is 34.5 ± 1.5 MHz. The resulting transient waveforms from the demodulator indicate the presence of hemodynamic events, i.e., systolic upstroke, systolic peak, dicrotic notch, and diastolic downstroke. The preliminary results also confirm the sensor’s ability to detect multisite blood flow events noninvasively on a single wearable platform.

The alteration of the hydrostatic pressure gradient in the human body has been associated with changes in human physiology, including abnormal blood flow, syncope, and visual impairment. The focus of this study was to evaluate changes in the resonant frequency of a wearable electromagnetic resonant skin patch sensor during simulated physiological changes observed in aerospace applications. Simulated microgravity was induced in eight healthy human participants (n = 8), and the implementation of lower body negative pressure (LBNP) countermeasures was induced in four healthy human participants (n = 4). The average shift in resonant frequency was −13.76 ± 6.49 MHz for simulated microgravity with a shift in intracranial pressure (ICP) of 9.53 ± 1.32 mmHg, and a shift of 8.80 ± 5.2097 MHz for LBNP with a shift in ICP of approximately −5.83 ± 2.76 mmHg. The constructed regression model to explain the variance in shifts in ICP using the shifts in resonant frequency (R² = 0.97) resulted in a root mean square error of 1.24. This work demonstrates a strong correlation between sensor signal response and shifts in ICP. Furthermore, this study establishes a foundation for future work integrating wearable sensors with alert systems and countermeasure recommendations for pilots and astronauts.

Low vagal tone is common in osteoarthritis (OA) comorbidities and results in greater peripheral inflammation. Characterizing vagal tone's role in OA pathogenesis may offer insights into OA's influences beyond the articular joint. We hypothesized that low vagal tone would accelerate onset of OA-related gait changes and worsen joint damage in a rat knee OA model. Knee OA was induced in male Sprague Dawley rats by transecting the medial collateral ligament and medial meniscus. Then, left cervical vagus nerve transection (VGX, n ​= ​9) or sham VGX (non-VGX, n ​= ​6) was performed. Gait and tactile sensitivity were assessed at baseline and across 12 weeks, with histology and systemic inflammation evaluated at endpoint. At week 4, VGX animals showed limping gait characteristics through shifted stance times from their OA to non-OA limb (p ​= ​0.055; stance time imbalance ​= ​1.6 ​± ​1.6%) and shifted foot strike locations (p ​< ​0.001; spatial symmetry ​= ​48.4 ​± ​0.835%), while non-VGX animals walked with a balanced and symmetric gait. Also at week 4, while VGX animals had a mechanical sensitivity (50% withdrawal threshold) of 13.97 ​± ​7.70 compared to the non-VGX animal sensitivity of 29.74 ​± ​9.43, this difference was not statistically significant. Histologically, VGX animals showed thinner tibial cartilage and greater subchondral bone area than non-VGX animals (p ​= ​0.076; VGX: 0.80 ​± ​0.036 ​mm²; non-VGX: 0.736 ​± ​0.066 ​mm²). No group differences in systemic inflammation were observed at endpoint. VGX resulted in quicker onset of OA-related symptoms but remained unchanged at later timepoints. VGX also had thinner cartilage and abnormal bone remodeling than non-VGX. Overall, low vagal tone had mild effects on OA symptoms and joint remodeling, and not at the level seen in common OA comorbidities.

Osteoarthritis (OA) often presents with comorbidities such as hypertension, potentially accelerating OA pathogenesis. We hypothesized that hypertension would exacerbate joint-level pathogenesis and OA-related symptoms in a sex-dependent manner. Male and female Spontaneously Hypertensive rats (hypertensive) and Sprague Dawley rats (normotensive) underwent either medial collateral ligament and medial meniscus transection (OA) or skin incision (sham) in the right knee (N ​= ​80; n ​= ​10/group/sex). Symptoms were measured monthly (gait) and bi-weekly (tactile sensitivity) for 8 weeks. Endpoint histology assessed joint-level damage, neurovascular changes, and cytokine levels in synovial fluid. At endpoint, hypertensive-OA rats had thinner cartilage across the medial tibial plateau than normotensive-OA rats (non-overlapping 95 ​% CI), regardless of sex. While not observed in males, hypertensive-OA females developed larger osteophytes (Q1–Q3 ​= ​0.049–0.124 ​mm²) than normotensive-OA females (Q1–Q3 ​= ​0.026–0.047 ​mm², p ​= ​0.02) and ranked higher for CD31⁺ vasculature in the subchondral bone plate (Q1–Q3 ​= ​18.2–21.5) than normotensive-OA females (Q1–Q3 ​= ​1; p ​= ​0.01). Hypertensive-OA females developed a limping gait, shifting stance times from their OA to non-OA limb (stance time imbalance ​= ​1.20 ​± ​1.15 ​%, p ​= ​0.04), offloaded their injured limb quicker (temporal symmetry ​= ​52.5 ​± ​1.4 ​%, p ​< ​0.001), and reduced stride lengths (weeks 4 and 8; hypertensive-OA ​< ​normotensive-OA, p ​< ​0.001). These gait changes were not observed in normotensive-OA females or males, nor in hypertensive-OA males. Hypertension worsened OA pathogenesis at the joint level, more substantially affecting joint remodeling and gait compensations in females. Our results encourage further investigation into the pathophysiologic drivers linking hypertension and OA, particularly vascular changes and sex differences.