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Abstract We present a broadband impedance matching model for traveling waves on lossy magnetic metamaterials known as magnetoinductive waveguides (MIWs) in conductive media. Thus far, broadband impedance matching has only been demonstrated in the case of a lossless MIW in free-space due to complexities introduced by losses and eddy current effects. As such, current studies in conductive environments have been limited to utilizing narrow-band matching techniques or relying on attenuation to mitigate reflections, thus limiting the system performance in terms of bandwidth and transmission loss. The proposed model overcomes these limitations by utilizing the nearest neighbor coupling and binomial approximations to generate transducer design criteria in terms of equivalent circuit parameters for broadband impedance matching. To validate the model, a transducer is designed for a 40-MHz lossy MIW submerged in an ocean water phantom. Reflection coefficient results demonstrate a 15.5% fractional bandwidth and a maximum value of − 9.0 dB in the propagation band of the MIW, indicating excellent performance. This model expands the potential design space of MIWs to include complex environments such as underwater, underground, or on the human body. 

Despite cognitive workload (CW) being a critical metric in several applications, no technology exists to seamlessly and reliably quantify CW. Previously, we demonstrated the feasibility of a wearable MagnetoCardioGraphy (MCG) sensor to classify high vs. low CW based on MCG-derived heart rate variability (mHRV). However, our sensor was unable to address certain critical operational requirements, resulting in noisy signals, often to the point of being unusable. In addition, test conditions for the participants were not decoupled from motion (i.e., physical activity (PA)), raising questions as to whether the noted changes in mHRV were attributed to CW, PA, or both. This study reports software and hardware advancements to optimize the MCG data quality, and investigates whether changes in CW (in the absence of PA) can be reliably detected. Performance is validated for healthy adults (n = 10) performing three types of CW tasks (one for low CW and two for high CW to eliminate the memory effect). Results demonstrate the ability to retrieve MCG R-peaks throughout the recordings, as well as the ability to differentiate high vs. low CW in all cases, confirming that CW does modulate the mHRV. A paired Bonferroni t-test with significance 𝛼=0.01 confirms the hypothesis that an increase in CW decreases mHRV. Our findings lay the groundwork toward a seamless, practical, and low-cost sensor for monitoring CW. 

Despite cognitive workload (CW) being a critical metric in several applications, no technology exists to seamlessly and reliably quantify CW. Previously, we demonstrated the feasibility of a wearable MagnetoCardioGraphy (MCG) sensor to classify high vs. low CW based on MCG-derived heart rate variability (mHRV). However, our sensor was unable to address certain critical operational requirements, resulting in noisy signals, often to the point of being unusable. In addition, test conditions for the participants were not decoupled from motion (i.e., physical activity (PA)), raising questions as to whether the noted changes in mHRV were attributed to CW, PA, or both. This study reports software and hardware advancements to optimize the MCG data quality, and investigates whether changes in CW (in the absence of PA) can be reliably detected. Performance is validated for healthy adults (n = 10) performing three types of CW tasks (one for low CW and two for high CW to eliminate the memory effect). Results demonstrate the ability to retrieve MCG R-peaks throughout the recordings, as well as the ability to differentiate high vs. low CW in all cases, confirming that CW does modulate the mHRV. A paired Bonferroni t-test with significance α=0.01 confirms the hypothesis that an increase in CW decreases mHRV. Our findings lay the groundwork toward a seamless, practical, and low-cost sensor for monitoring CW. 

We have recently demonstrated a wearable MagnetoCardioGraphy (MCG) sensor capable of classifying high vs. low cognitive work, i.e., the amount of mental effort a person is exerting when performing a task during a given period of time. However, a major limitation of our previous work was the requirement to eliminate any type of motion for the participants. Here, we explore the effect of motion by employing three (3) different experimental setups, each with a different amount of physical motion and cognitive load exerted. To better understand the effect of motion, an inertial measurement unit (IMU) and a breathing rate sensor are employed in addition to the MCG sensor. Our results show that heart rate variability (HRV), demonstrated through the mean difference in duration between consecutive heartbeats, is at its highest when neither cognitive workload nor motion are exerted. HRV drops when the subject involves cognitive workload and motion. Our results pave the way for additional research in the field, with an utmost goal of catering to specific clinical applications. 

Abstract The propagation of magnetoinductive (MI) waves across magnetic metamaterials known as magnetoinductive waveguides (MIWs) has been an area of interest for many applications due to the flexible design and low-loss performance in challenging radio-frequency (RF) environments. Thus far, the dispersion behavior of MIWs has been limited to mono- and diatomic geometries. In this work, we present a recursive method to generate the dispersion equation for a general poly-atomic MIW. This recursive method greatly simplifies the ability to create closed-form dispersion equations for unique poly-atomic MIW geometries versus the previous method. To demonstrate, four MIW geometries that have been selected for their unique symmetries are analyzed using the recursive method. Using applicable simplifications brought on by the geometric symmetries, a closed-form dispersion equation is reported for each case. The equations are then validated numerically and show excellent agreement in all four cases. This work simultaneously aids in the further development of MIW theory and offers new avenues for MIW design in the presented dispersion equations.