arXiv subjects

Denys Nikolayev

Publications and source records attributed to Denys Nikolayev.

11 recordsLinked to original sources

arXiv2026

Articulatory movements influence electromagnetic wave transmission through the vocal tract

This study experimentally validates a numerical model of electromagnetic propagation through the human head during the pronunciation of different vowels, with the goal of improving our understanding of the underlying physical phenomena. A realistic finite element model was created from magnetic resonance images acquired while pronouncing the vowels /a/, /i/, and /u/. The model was validated against scattering matrix measurements obtained from two subjects whose geometries were modeled. Despite several potential sources of discrepancy, the simulations and measurements showed good qualitative agreement, confirming the validity of the approach. Similar transmission coefficient patterns were observed across subjects for the same vowels. Within the investigated frequency range of (1-6~GHz), the electric field exhibited a Mie scattering pattern. Local minima and maxima in the transmission coefficient, characterizing different articulatory configurations, were correlated with local variations in the electric field amplitude. The transmission coefficient's shape results from an interplay between resonance patterns and antenna placement, while the degree of mouth opening influences the shape of scattering modes. Although technically challenging, this numerical approach proved effective for studying electromagnetic propagation in the human head. The resulting robust numerical model and improved understanding of the underlying physics are expected to facilitate the development of radio-frequency-based silent speech interfaces.

physics.app-ph↗

arXiv2024

Physical Insights into Electromagnetic Efficiency of Wireless Implantable Bioelectronics

Autonomous implantable bioelectronics rely on wireless connectivity, necessitating highly efficient electromagnetic (EM) radiation systems. However, limitations in power, safety, and data transmission currently impede the advancement of innovative wireless medical devices, such as tetherless neural interfaces, electroceuticals, and surgical microrobots. To overcome these challenges and ensure sufficient link and power budgets for wireless implantable systems, this study explores the mechanisms behind EM radiation and losses, offering strategies to enhance radiation efficiency in wireless implantable bioelectronics. Using analytical modeling, the EM waves emitted by the implant are expanded as a series of spherical harmonics, enabling a detailed analysis of the radiation mechanisms. This framework is then extended to approximate absorption losses caused by the lossy and dispersive properties of tissues through derived analytical expressions. The radiation efficiency and in-body path loss are quantified and compared in terms of three primary loss mechanisms. The impact of various parameters on the EM efficiency of implantable devices is analyzed and quantified, including operating frequency, implant size, body-air interface curvature, and implantation location. Additionally, a rapid estimation technique is introduced to determine the optimal operating frequency for specific scenarios, along with a set of design principles aimed at improving radiation performance. The design strategies derived in this work - validated through numerical and experimental demonstrations on realistic implants - reveal a potential improvement in implant radiation efficiency or gain by a factor of five to ten, leading to a corresponding increase in overall link efficiency compared to conventional designs.

physics.bio-ph↗

arXiv2024

Conformal Wide-Angle Scanning Leaky-Wave Antenna for V-Band On-Body Applications

Wearable on-body millimeter-wave (mmWave) radars can provide obstacle detection and guidance for visually impaired individuals. The antennas, being a crucial component of these systems, must be lightweight, flexible, low-cost, and compact. However, existing antennas suffer from a rigid form factor and limited reconfigurability. This article presents a low-profile, fast scanning leaky-wave antenna (LWA) operating in the unlicensed V-band (57-64 GHz) for on-body applications such as lightweight portable frequency modulated continuous wave (FMCW) radars. The novel meandering microstrip design allows independent control of gain and scanning rate (rate of change of main beam pointing direction with frequency). Experimental results show that the LWA achieves a realized gain above 10 dB with a fan-beam steering range in the H-plane from -35{deg} to 45{deg} over the operating frequency band, while the half power beamwidth (HPBW) is within 20{deg} in planar condition. To assess the on-body applicability, the antenna's performance is evaluated under bending. When placed on the knee (corresponding to 80 mm radius), the beam steers from -25{deg} to 55{deg} with a maximum realized gain degradation of 1.75 dB, and an increase of HPBW up to 25{deg}. This demonstrates the LWA's robustness in conformal conditions, while maintaining beam-forming and beam-scanning capabilities. Simulations confirm that the LWA's ground plane minimizes user exposure, adhering to international guidelines. Finally, we demonstrate a 2-D spatial scanning by employing an array of twelve LWAs with phased excitation, enabling beam-forming in the E-plane from -50{deg} to 50{deg}, while the HPBW remains below 20{deg}. Mutual coupling analysis reveals that isolation loss and active reflection coefficient remain below 15 dB throughout the operating band.

physics.app-ph↗

arXiv2024

Suitability of Common Ingestible Antennas for Multiplexed Gastrointestinal Biosensing

Ingestible sensor devices, which are increasingly used for internal health monitoring, rely on antennas to perform sensing functions and simultaneously to communicate with external devices. Despite the development of various ingestible antennas, there has been no comprehensive comparison of their performance as biosensors. This paper addresses this gap by examining and comparing the suitability of three common types of ingestible antennas -- dipole, patch, and loop -- as biosensors for distinguishing gastrointestinal tissues (stomach, small intestine, and large intestine) based on their electromagnetic properties. The antennas studied in this work conform to the inner surface of biocompatible polylactic acid capsules with varying shell thicknesses and operate in the 433 MHz Industrial, Scientific, and Medical band. The comparison is performed in gastrointestinal tissues using several antenna parameters: 1) Sensing Capability: Changes in the phase of the reflection coefficient in the tissues are selected as the sensing parameter. 2) Robustness: The frequency interval (f_i) in which the antennas are matched (|S11| < -10 dB) in all the tissues and the maximum change in the center frequency (f_c) in different tissues are examined. 3) Radiation Performance: The gain and radiation efficiency of the antennas are examined. The effect of shell thickness on gain and radiation efficiency at 434 MHz is presented. Additionally, the radiation efficiency at various frequencies allocated for medical communications is compared with the theoretical maximum achievable efficiencies. These comprehensive data provide valuable information for making engineering decisions when designing multiplexed biosensor antennas for ingestible applications.

eess.SY↗

arXiv2024

Quasistatic approximation in neuromodulation

We define and explain the quasistatic approximation (QSA) as applied to field modeling for electrical and magnetic stimulation. Neuromodulation analysis pipelines include discrete stages, and QSA is applied specifically when calculating the electric and magnetic fields generated in tissues by a given stimulation dose. QSA simplifies the modeling equations to support tractable analysis, enhanced understanding, and computational efficiency. The application of QSA in neuro-modulation is based on four underlying assumptions: (A1) no wave propagation or self-induction in tissue, (A2) linear tissue properties, (A3) purely resistive tissue, and (A4) non-dispersive tissue. As a consequence of these assumptions, each tissue is assigned a fixed conductivity, and the simplified equations (e.g., Laplace's equation) are solved for the spatial distribution of the field, which is separated from the field's temporal waveform. Recognizing that electrical tissue properties may be more complex, we explain how QSA can be embedded in parallel or iterative pipelines to model frequency dependence or nonlinearity of conductivity. We survey the history and validity of QSA across specific applications, such as microstimulation, deep brain stimulation, spinal cord stimulation, transcranial electrical stimulation, and transcranial magnetic stimulation. The precise definition and explanation of QSA in neuromodulation are essential for rigor when using QSA models or testing their limits.

physics.med-ph↗

arXiv2023

A Miniature Non-Uniform Conformal Antenna Array Using Fast Synthesis for Wide-Scan UAV Application

To overcome the limited payload of lightweight vehicles such as unmanned aerial vehicle (UAV) and the aerodynamic constraints on the onboard radar, a compact nonuniform conformal array is proposed in order to achieve a wide beamscanning range and to reduce the sidelobes of the planar array. The non-uniform array consists of 7x4 elements where the inner two rows follow a geometric sequence while the outer two rows follow an arithmetic sequence along the x axis. The element spacing along the y axis is gradient from the center as well. This geometry not only provides more degrees of freedom to optimize the array radiation, but also reduces the computation cost when synthesizing the excitation and the configuration of the array for a specific beam pattern. As field cancellation may happen due to the convex and concave features of the non-canonical UAV surface, a fast and low-cost in-house code to calculate the radiation pattern of a large scale conformal array for an arbitrary surface and element pattern is employed to optimize the array structure. As a proof of concept, the proposed array with a total volume of 142x93x40 mm3 is implemented at ISM band (5.8 GHz) using a miniature widebeam single-layer patch antenna with a dimension of 0.12lambdax0.12lambdax0.025lambda. By using the beamforming technique, an active onboard system is measured, which achieves the maximum gain of 21.8 dBi and a scanning range of >50deg and -28deg~28deg with a small scan loss of 2.2 and 0.5 dB in elevation and azimuth, respectively. Therefore, our design has high potential for wireless communication and sensing on UAV.

eess.SP↗

arXiv2023

Analytical Model for Calculating Gain Pattern of Antennas Implanted in Large Host Bodies

This paper presents a method for the fast and accurate estimation of the gain pattern and maximum gain of an implanted antenna including the effect of the host body, under the assumption that the latter is electrically large. The estimation procedure is based on the radiation of an elementary dipole source placed in a planar body model. The derivation of closed-form expressions is based on spherical wave analysis and the Green's functions for layered media. The validity of this approximation for practical cases is shown on different implanted antennas, where the results are compared to full wave simulations and measurements.

physics.med-ph↗

arXiv2023

Meandering microstrip leaky-wave antenna with dual-band linear-circular polarization and suppressed open stopband

This paper proposes a dual-band frequency scanning meandering microstrip leaky-wave antenna with linear polarization in the Ku-band and circular polarization in the K-band. This is achieved by making use of two spatial harmonics for radiation. The unit cell of the periodic microstrip antenna contains three meanders with mitred corners. To ensure circular polarization, a theoretical formulation is developed taking into account the delay caused by microstrip length intervals. It defines the unit cell geometry by determining the length of the meanders to ensure that axial ratio remains below 3 dB throughout the operational band. Moreover, the meanders are used to provide better control over scanning rate (the ratio of change of angle of maximum radiation with frequency) and reduce spurious radiation of harmonics by ensuring single harmonic operation within the operational band. To guarantee continuous scanning through broadside direction, open stopband is suppressed using mitered angles. The antenna is designed on a 0.254-mm substrate making it suitable for conformal applications. The fabricated antenna shows a backward to forward beam steering range of 72 deg (-42 deg to 30 deg) in the K-band (19.4-27.5 GHz) with circular polarization and of 75 deg (-15 deg to 60 deg) in the Ku-band (11-15.5 GHz) with linear polarization.

physics.app-ph↗

arXiv2023

Wireless powering efficiency of deep-body implantable devices

The wireless power transfer efficiency to implanted bioelectronic devices is constrained by several frequency-dependent physical mechanisms. Recent works have developed several mathematical formulations to understand these mechanisms and predict the optimal operating conditions. However, existing approaches rely on simplified body models, which are unable to capture important aspects of wireless power transfer. Therefore, this paper proposes the efficiency analysis approach in anatomical models that can provide insightful information on achieving the optimum operation conditions. First, this approach is validated with a theoretical spherical wave expansion analysis, and the results for a simplified spherical model and a human pectoral model are compared. The results show that although a magnetic receiver outperforms an electric one for near-field operation and both sources could be equally employed in far-field range, it is in mid-field that the maximum efficiency is achieved with an optimum frequency between 1-5 GHz depending on the implantation depth. The receiver orientation is another factor that affects the efficiency, with a maximum difference between the best and worst-case scenarios around five times for the electric source and over 13 times for the magnetic one. This approach is used to analyze the case of a deep-implanted pacemaker wirelessly powered by an on-body transmitter and subjected to stochastic misalignments. We evaluate the efficiency and exposure, and we demonstrate how a buffered transmitter can be tailored to achieve maximum powering efficiency. Finally, design guidelines that lead to optimal implantable wireless power transfer systems are established from the results obtained with the proposed approach.

physics.app-ph↗

arXiv2022

Quasi-Static Approximation Error of Electric Field Analysis for Transcranial Current Stimulation

Objective: Numerical modeling of electric fields induced by transcranial alternating current stimulation (tACS) is currently a part of the standard procedure to predict and understand neural response. Quasi-static approximation for electric field calculations is generally applied to reduce the computational cost. Here, we aimed to analyze and quantify the validity of the approximation over a broad frequency range. Approach: We performed electromagnetic modeling studies using an anatomical head models and considered approximations assuming either a purely ohmic medium (i.e., static formulation) or a lossy dielectric medium (quasi-static formulation). The results were compared with the solution of Maxwell's equations in the cases of harmonic and pulsed signals. Finally, we analyzed the effect of electrode positioning on these errors. Main Results: Our findings demonstrate that the quasi-static approximation is valid and produces a relative error below 1% up to 1.43 MHz. The largest error is introduced in the static case, where the error is over 1% across the entire considered spectrum and as high as 20% in the brain at 10 Hz. We also highlight the special importance of considering the capacitive effect of tissues for pulsed waveforms, which prevents signal distortion induced by the purely ohmic approximation. At the neuron level, the results point a difference of sense electric field as high as 22% at focusing point, impacting pyramidal cells firing times. Significance: Quasi-static approximation remains valid in the frequency range currently used for tACS. However, neglecting permittivity (static formulation) introduces significant error for both harmonic and non-harmonic signals. It points out that reliable low frequency dielectric data are needed for accurate tCS numerical modeling.

physics.app-ph↗

arXiv2022

On the use of impedance detuning for gastrointestinal segment tracking of ingestible capsules

During their travel through the gastrointestinal tract, ingestible antennas encounter detuning in their impedance response due to varying electromagnetic properties of the surrounding tissues. This paper investigates the possibility of using this impedance detuning to detect in which segment of the gastrointestinal tract - stomach, small intestine, or large intestine - the capsule is located. Meandered dipole antennas operating in the 433 MHz Industrial, Scientific, and Medical Band are designed for this purpose. The antennas conform to the inner surface of 3D-printed polylactic-acid capsules with a shell thickness of 0.6 or 0.4 mm. The impedance response is first optimized numerically in a homogeneous cylindrical phantom with time-averaged electromagnetic properties. The magnitude and the phase of the reflection coefficient are then obtained in different tissues and compared with simulations and measurements. The experimental demonstration is carried out first using tissue-mimicking liquids and then in a recently deceased ex vivo porcine model. The minimum change in the phase between different gastrointestinal tissues was determined to be around 10 degrees in the porcine model, indicating that the changes in the impedance response, particularly the changes in the phase, provide sufficient information to follow the position of the capsule in the gastrointestinal tract.

physics.app-ph↗