Dual-camera wireless capsule endoscopy improves diagnostic coverage of the gastrointestinal tract but introduces severe constraints on antenna placement, electromagnetic coupling, and in-body communication reliability. This paper presents a compact, camera-integrated, four-element dual-polarized multiple-input multiple-output (MIMO) antenna specifically designed for dual-camera capsule endoscopy operating at 868 MHz. The proposed architecture consists of two planar, dual-polarized MIMO antenna modules co-located with opposing cameras, enabling complementary polarization and radiation-pattern diversity to mitigate orientation-dependent fading in lossy biological environments. Each antenna element employs a slot-loaded half-annular patch structure to achieve $\pm$ 45° linear polarization while maintaining stable impedance under tissue loading. The integrated antenna occupies 61.84 mm2 and achieves inter-element isolation exceeding 23 dB, envelope correlation coefficients below 0.03, and diversity gain above 9.9 dB. Experimental validation in tissue-mimicking phantoms demonstrates a measured realized gain of -33.7 dBi per element, robust impedance stability across gastrointestinal tissues, and a link margin exceeding 20 dB. Specific absorption rate analysis confirms compliance with IEEE safety limits at power levels well above those used in practical capsule endoscopy systems. The proposed camera-aware antenna integration provides a compact and reliable solution for next-generation dual-camera capsule endoscopy, supporting robust in-body wireless telemetry under unpredictable orientation and propagation conditions.

Due to the varying electromagnetic properties of the gastrointestinal tissues-stomach, small intestine, and large intestineingestible antennas encounter variations in their operational parameters as they advance through the gastrointestinal tract. Previous studies have shown that tracking variations in the phase of the reflection coefficient enables differentiation between the gastrointestinal tissues. This paper proposes an antenna optimization strategy to increase the phase differences obtained across different tissues to improve the differentiation capability. First, analytical equations are derived to relate the rate of phase change to the real and imaginary parts of the antenna input impedance, and these equations are interpreted in the context of the proposed methodology. Next, the improvement achieved with the strategy is verified through numerical simulations using a microstrip patch antenna. The results indicate that the phase differences can be significantly increased with the proposed strategy, thereby improving the phase-based differentiation capability for gastrointestinal tissues.

Modern millimeter-wave (mmWave) systems such as on-body networks, automotive sensors, and short-range wireless links are increasingly expected to support both high-speed communication and precise sensing. For seamless integration into such platforms, antennas must be compact, lightweight, flexible, and low cost. However, existing antennas suffer from a rigid form factor and limited beam steering. This article presents a low-profile, fast-scanning leaky wave antenna (LWA) operating in the unlicensed V-band (57-64 GHz) offering wide-angle frequency-dependent beam steering without the need for complex phase-shifting networks. The novel meandering microstrip design allows independent control of gain and scanning rate (i.e., rate of change of main beam pointing direction with frequency). The experimental results demonstrate that the LWA achieves over 10 dB with a fan-beam steering range in the H-plane from -32 degrees to 45 degrees across the operating frequency band, while the half-power beamwidth (HPBW) is within 20 degrees in planar condition. Under strong bending conditions (e.g., for an on-body sensor placed on a human knee with an 80 mm radius), the antenna continues to operate effectively, with beam scanning from -25 degrees to 55 degrees with a realized gain degradation of 1.75 dB, and an increase of HPBW up to 25 degrees. These results confirm the LWA's robust beam-forming and scanning capabilities, even in highly conformal conditions.

Wireless smart insoles are increasingly used for motion analysis, providing critical insights for healthcare, sports performance, and injury prevention. A key challenge is accurately measuring and wirelessly transmitting foot movement and placement data in real-time, independently of inertial measurement units (IMUs) tied to biomechanical models, which only estimate gait parameters for a limited range of activities such as walking or running. Radio frequency (RF) phase-based ranging presents a promising solution for a broader range of gait analysis applications, including irregular movement of athletes in sport or people with diseases such as Parkinson's. However, wireless system design must address two key challenges as follows: 1) antenna robustness against the nondeterministic properties of the ground and the proximity of the body and 2) polarization mismatch due to foot movement. This study is the first to numerically and experimentally investigate how foot motion and insole operating conditions affect wave propagation and wireless communication at 2.45 GHz. We analyze the impact of soil properties and antenna polarization, demonstrating that vertical polarization significantly improves signal transmission compared with horizontal polarization, with a gain of 6-10 dB at a height of 5 cm. This improvement, driven by surface wave propagation, provides insights into the antenna design strategies for wireless insoles that can enhance the accuracy of phase-based ranging for foot distance measurement (DM), as confirmed through simulations and experimental validation. These findings contribute to the energy-efficient, real-time ranging and communication enabled by optimized antenna designs, reducing power consumption and improving motion analysis for a wide range of users, including athletes requiring precise performance metrics and patients undergoing diagnostics or rehabilitation.

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, which range from 13.8 degrees to 174.4 degrees, are selected as the sensing parameter. 2) Robustness in Different GI Tissues: the frequency interval (Delta f(i)) in which the antennas are matched (|S-11| -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. 4) Robustness against Internal Components: impedance detuning that may be caused by the presence of other internal components is analyzed. Additionally, the effect of shell thickness on gain and radiation efficiency at 434 MHz is presented, and 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.

Wireless power transfer enables battery-free operation of implantable bioelectronic devices, ensuring long-term functionality without relying on bulky batteries. This paper provides design guidelines for efficient electromagnetic WPT to miniature deep-body implants. It compares near-field, mid-field, and far-field methods, focusing on mid-field WPT for improved efficiency for miniature devices at greater depths. Key factors such as frequency selection, transmitter and receiver design, and methods to reduce tissue losses are discussed. Strategies like wavefront shaping and metasurfaces are examined to enhance power delivery while meeting safety constraints.

This study investigates the impact of antenna polarization on RF signal propagation for body-centric devices and, in particular, smart insole applications. A numerical analysis was conducted to evaluate the performance of vertical and horizontal polarizations across different soil electromagnetic properties at 2.45 GHz. The results reveal that vertical polarization enables more effective signal transmission with more than 8 dB improvement over horizontal polarization. The signal wavefront remains relatively stable over the soil for vertical polarization, making it suitable for communication between nodes. These findings provide valuable guidance for the design of efficient antennas for the next generation of body-centric devices. In this study, we demonstrate the applicability of this approach for smart insoles. However, surface-wave guiding principles can be translated to other applications such as networks of ingestible, implantable, and wearable devices.

Miniaturized wireless capsule endoscopes offer minimally invasive real-time monitoring of vital parameters of the gastrointestinal tract. Although various ingestible capsules are commercially available for wireless endoscopy, the precise real-time localization of these capsules within the tract remains a challenge. This work proposes the utilization of a pH -sensitive biodegradable superstrate for gastrointestinal segment tracking in the 433 MHz Industrial, Scientific, and Medical (ISM) band. The superstrate consists of three rings engineered to degrade at specific pH levels that correspond to the distinct environments of the GI tissues - stomach, small intestine, and large intestine. This superstrate covers the outer surface of a capsule which features an integrated conformal dipole antenna. Hence, biodegradation of each ring of the superstrate in its respective GI segment leads to the alteration of the near-field of the antenna, changing its center frequency and reflection coefficient at 434 MHz in each segment. In this way, biodegradation process links the location data to the input parameters of the antenna, enabling segment tracking within the tract. Simulation results show that a shift of at least 5 MHz can be created in the center frequency as the capsule advances along each segment while the antenna is kept operational over the 433 MHz ISM band throughout the entire tract. Finally, preliminary tests using liquid gastrointestinal phantoms are conducted to validate the proposed technique.

This paper introduces a novel 2-D beam-steerable rectenna designed for mm-wave RF wireless power transfer (RF-WPT) applications. The 2-D beamsteering capability of the proposed rectenna is achieved by integrating an on-chip 4 x 4 Butler matrix, which is co-integrated with a rectifier array in a 22 nm FD-SOI CMOS, and a leaky wave antenna array. The proposed solution incorporates dual rectifier units simultaneously to enhance the receiver's power receptivity. In this manner, the rectenna is capable of receiving power from the same direction while taking advantage of frequency diversity. This is accomplished using the novel integration scheme of the single leaky wave array antenna with two rectifier units. The proposed solution maintains the system's compactness and reduces the overall cost while offering a fully passive 2-D beam scanning for the receiver unit. These merits make the proposed solution promising for RF-WPT applications in the Internet of Things (IoT) ecosystem.

Implantable bioelectronics require highly efficient wireless connectivity for autonomous operation and closed-loop control, yet power constraints, safety regulations, and data transmission limitations continue to hinder advancements in medical device innovation. This study systematically investigates radiation loss mechanisms and proposes strategies to improve electromagnetic efficiency in wireless implantable systems. Utilizing spherical harmonic analysis, we quantify radiation efficiency and in-body path loss through rigorous closed-form electromagnetic modeling, identifying three primary loss mechanisms. On this basis, we introduce a rapid and accurate estimation technique to optimize the operating frequency, complemented by design principles aimed at augmenting radiation performance for robust wireless links. The proposed strategies, substantiated through comprehensive numerical and experimental validation with realistic implants, demonstrate a potential 5-to 10-fold improvement in implant radiation efficiency or gain, offering significant benefits for early-stage implantable device development.

Objectives The proposed mechanisms of spinal cord stimulation (SCS) follow the polarization of dorsal column axons; however, the development of subparesthesia SCS has encouraged the consideration of different targets. Given their relative proximity to the stimulation electrodes and their role in pain processing (eg, synaptic processing and gate control theory), spinal cord dorsal horn interneurons may be attractive stimulation targets. Materials and Methods We developed a computational modeling pipeline termed “quasiuniform-mirror assumption” and applied it to predict polarization of dorsal horn interneuron cell types (islet type, central type, stellate/radial, vertical-like) to SCS. The quasiuniform-mirror assumption allows the prediction of the peak and directional axes of dendrite polarization for each cell type and location in the dorsal horn, in addition to the impact of the stimulation pulse width and electrode configuration. Results For long pulses, the peak polarization per milliampere of SCS with a spaced bipolar configuration was islet type 3.5mV, central type 1.3mV, stellate/radial 1.4mV, and vertical-like 1.6mV. For stellate/radial, the peak dendrite polarization was dorsal-ventral, and for islet-type, the peak dendrite polarization was in the rostral-caudal axis. For islet type and central type cells, peak dendrite polarization was between stimulation electrodes, whereas for stellate/radial and vertical-like cells, peak dendrite polarization was under the stimulation electrodes. The impact of the pulse width depends on the membrane time constants. Assuming a 1-millisecond time constant, for a 1-millisecond or 100-μs pulse width, the peak dendrite polarization decreases (from direct current values) by approximately 33% and approximately 88%, respectively. Increasing the interelectrode distance beyond approximately 3 cm did not significantly increase the peak polarization but expanded the region of interneuron polarization. Conclusions Predicted maximum polarization of islet-cells in the superficial dorsal horn at locations between electrodes is 4.6mV for 2 mA, 1-millisecond pulse SCS. A polarization of a few millivolts is sufficient to modulate synaptic processing through subthreshold mechanisms. Our simulations provide support for SCS approaches optimized to modulate the dendrites of dorsal horn neurons.

Background: Weak alternating electric fields (∼1 V/m) modulate nervous system activity. Yet, the exact mechanism by which such low amplitude electric fields can modulate neural activity is still unknown despite the use of specific protocols aiming at quantifying neural sensitivity. Objective: To characterize how the measurement protocol can impact neural sensitivity to weak electric fields and bias in vivo sensitivity. Methods: We considered a variety of somatic clamp stimulation to drive the activity of biophysical morpho-realistic reconstructed neurons during extracellular alternating stimulation to quantify the sensitivity to the field depending on the nature of cells’ activity. Results: Cells sensitivity to alternating current stimulation depended on the type of input used to drive their activity, with a different frequency response for each protocol, with a trend for inhibitory neurons to be more sensitive to higher stimulation frequencies. Even with the same clamp protocol, sensitivity depended on the statistics of the input used. Significance: Neuronal sensitivity to alternating current stimulation is highly input-dependent, which has been largely neglected so far, and depends on the current statistic of the received inputs, and is not reliably represented by simple current clamps protocols. ### Competing Interest Statement The authors have declared no competing interest. Agence Nationale de la Recherche, https://ror.org/00rbzpz17, ANR-10-LABX-07-01

Objective. Transcranial alternating current stimulation (tACS) enables non-invasive modulation of brain activity, holding promise for clinical and research applications. Yet, it remains unclear how the stimulation frequency differentially impacts various neuron types. Here, we aimed to quantify the frequency-dependent behavior of key neocortical cell types. Approach. We used both detailed (anatomical multicompartments) and simplified (three compartments) single-cell modeling approaches based on the Hodgkin-Huxley formalism to study neocortical excitatory and inhibitory cells under various tACS intensities and frequencies within the 5-50 Hz range at rest and during basal 10 Hz activity. Main results. L5 pyramidal cells (PCs) exhibited the highest polarizability at direct current, ranging from 0.21 to 0.25 mm and decaying exponentially with frequency. Inhibitory neurons displayed membrane resonance in the 5-15 Hz range with lower polarizability, although bipolar cells had higher polarizability. Layer 5 PC demonstrated the highest entrainment close to 10 Hz, which decayed with frequency. In contrast, inhibitory neurons entrainment increased with frequency, reaching levels akin to PC. Results from simplified models could replicate phase preferences, while amplitudes tended to follow opposite trends in PC. Significance. tACS-induced membrane polarization is frequency-dependent, revealing observable resonance behavior. Whilst optimal phase entrainment of sustained activity is achieved in PC when tACS frequency matches endogenous activity, inhibitory neurons tend to be entrained at higher frequencies. Consequently, our results highlight the potential for precise, cell-specific targeting for tACS.

Non-invasive brain stimulation (NIBS) technologies have the potential to positively impact the treatment of neurological disorders. However, NIBS techniques suffer from a lack of penetration depth and focality, thereby restraining their full potential. Here, we investigate the feasibility of a novel technique based on sinusoidal radiofrequency electromagnetic fields (RF-EMF) that are amplitude-modulated in the extremely low frequency (ELF) range. To assess the neuromodulatory effects of ELF-modulated RF-EMF, we exposed primary neuronal cultures placed on Multi-Electrode Arrays (MEAs) and quantified their spiking activity based on phase entrainment. Our results failed to reveal consistent phase entrainment across cultures, while replicating a decrease of neuronal activity with purely sinusoidal 1.4 GHz stimulation. Finally, we suggest protocol improvements that might result in more reproducible effects, which is indispensable for translational applications.

The development and widespread adoption of body-implanted bioelectronics face significant challenges due to limited wireless performance and reliance on batteries. This contribution provides an overview of recent theoretical advancements and practical applications in antenna design for body-implanted bioelectronics. It explores performance indicators, including bandwidth, robustness, radiation efficiency, and the impact of loss mechanisms. Additionally, it discusses the role of antennas in wireless sensing and strategies to mitigate reflection losses, offering insights into efficient and safe power transfer.

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.

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.

Wearable on-body millimeter-wave (mmWave) radars can provide obstacle detection and guidance for visually impaired people. However, their everyday performance is hindered by the rigid form factor and limited scanning range. In this article, we propose a low-profile, fast-scanning leaky-wave antenna (LWA) operating in the unlicensed V-band (57-64 GHz) to be integrated for on-body applications such as lightweight portable frequency-modulated continuous wave (FMCW) radars. The proposed LWA consists of meandering microstrips that can conform to the human body curvatures while maintaining beam-forming and beam-scanning properties. Experimental results demonstrate that the planar LWA achieves a realized gain above 10 dB with a fan-beam steering range in the H-plane from -40 to 43 over the operating frequency band while the half power beam-width (HPBW) is within 20. Since for the foreseen application the antenna is supposed to conform to the user's body, the performance is also analyzed for a bent condition. The beam steering range changes to -32 to 50 when placed on the knee (corresponding to 80 mm radius). Under bending conditions, the LWA exhibits a maximum degradation of 1.75 dB, while the HPBW increases to 25. This shows that due to the small size of the antenna, the impact of bending is low and the beam-forming and beam-scanning property of the designed LWA remain intact. Furthermore, we enable 2-D spatial scanning by employing an array of twelve LWAs with phased excitation, extending the scanning range in the E-plane from -40 to 40, while the HPBW remains below 20 across the operational frequency range.

In this work, we propose a novel dual band antenna achieved with the help of meandering microstrips. The structure is optimized for operating in Ku- and K band. Circularly polarization is maintained throughout the K band, while the antenna is linearly polarized in the Ku-band. The open stopband is suppressed at the frequency corresponding to broadside radiation to improve the gain and maintain continuous scanning in both the bands. Theoretical formulations are provided to scale the antenna to higher or lower bands of operation. The two-band operation is achieved by 2(nd) and 1(st) spatial harmonic operation. The designed antenna demonstrates beam steering capabilities from backward to forward directions, spanning a range of 72 degrees, from -42 degrees to 30 degrees, in the K band frequency range 19.4 GHz to 27.5 GHz. Additionally, in the Ku- band, which covers frequencies from 11 GHz to 15.5 GHz, the antenna offers a beam steering range of 75 degrees, from -15 degrees to 60 degrees.

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.