Implantable Antennas: Illuminating the Path to Health Innovation
Imagine small medical devices inside your body monitoring your health. These tiny helpers can prevent, diagnose, and treat diseases without the need for frequent hospital visits. They function like health detectives, sending updates about your well-being. Devices such as glucose monitors, pacemakers, and temperature trackers use radio waves to communicate and play a vital role in improving millions of lives worldwide. In the medical field, a telemetry unit communicates with an external receiver using a built-in antenna, helping to share crucial information. To ensure effective operation within the body, a small yet powerful antenna must be integrated into or around the device. A broad-spectrum implantable antenna is essential for seamless data transmission, as the body's varied tissues can otherwise reduce efficiency and bandwidth.
The meander line antenna (MLA) is highly promising due to its simple design. Currently, MLA is used in capsule endoscopy devices as a small implantable antenna for biomedical bone implants within the Medical Device Radiocommunications Service (401-406 MHz). However, the impact of human tissue on the electrical characteristics of the MLA has not yet been fully established. Designing a meander line antenna for in-body conditions is crucial to determine the resonant structure. Zero-reactance, a self-resonant condition, is essential for effective radiation generation. Therefore, this research proposes the self-resonant state, investigates the performance of MLA in in-body conditions based on electromagnetic simulation results, and validates the findings through measurement results.
Simulation and Analysis
The simulation was conducted using FEKO 7.0. The objective was to establish a design methodology for MLA in both free-space and in-body conditions. First, the MLA was simulated in free space to confirm the design principles. Once these principles were verified, simulations were carried out for implantable applications within the human body. Three tissue types—skin, fat, and muscle—were selected to examine the performance of MLA across different conductivity and permittivity values. Using the Method of Moments (MoM) computational technique, the electrical characteristics of MLA inside body tissue-equivalent material were calculated.
Based on the simulation results, self-resonant equations were derived. A self-resonant equation was proposed for the MLA in free space. This research involves both simulations and theoretical equations to validate the proposed equation. Previously, researchers had not solved two key problems:
In the structural equation of self-resonance, capacitive reactance (XD) and inductive reactance (XC) were not explicitly represented.
The antenna quality factor (Q) had not been discussed in relation to reactance values.
In this study, XC is derived from the self-inductance of the MLA’s crank section, while XD is determined from the stored charge at the edge of the MLA. The electromagnetic simulation results revealed areas where the electric fields converge and diverge. By applying Gauss's theorem, the stored charge was calculated from the electric field distributions. The capacitance was then obtained by determining the ratio of charge to the applied voltage. By equating XD and XC, the self-resonant equation was derived. The self-resonant structures obtained using this equation align well with the results of electromagnetic simulations.
Experimental Validation
The validity of the equation was confirmed through measurement results. For experimental purposes, muscle, fat, and skin phantoms were fabricated using a chemical formulation to achieve accurate permittivity and conductivity values. Small antennas were then fabricated and inserted into these phantoms for measurement. The antenna input resistances, voltage standing wave ratio (VSWR), and antenna gains were observed. Furthermore, Q factors derived from reactance values were compared with simulation results for the VSWR. The close agreement between the two sets of Q factor values confirms the usefulness of the proposed equation.
This research provides antenna engineers with a practical tool to determine antenna length, width, input impedance, capacitive reactance, and inductive reactance values for implantable applications.
Research Collaboration
This research was conducted with the support of Prof. Dr Yoshihide Yamada and Ir. Ts. Dr Kamilia Kamardin from the Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia (UTM).
Ts. Dr. Ngu War Hlaing
School of Engineering and Technology
Email: @email