Design and Development of Bio-Compatible Miniaturized Antenna for Wireless Neural Monitoring

Abstract

The findings of various investigations have unveiled that chronic diseases are the primary factor contributing to mortality among the elderly population worldwide. Consequently, there is a need to revamp public healthcare systems using emerging technology to address this issue. Swift diagnosis and perpetual monitoring of patients is best possible solution especially in the case of chronic diseases. But it is impractical for patient to visit diagnostic centres on a regular basis for continuous monitoring. The inclusion of body area networks (BAN) functioning within the vicinity of patients could provide a way to meet such demand. BAN networks contain implantable medical devices (IMDs) and external interrogator units which coordinate with IMDs in the collection of data. IMDs through sensors for the acquisition of various physiological parameters of the patients through deployment near or even inside the patient’s body in a few cases. However, powering such devices with minimal discomfort to patients is a tough ask for researchers. Hence, main focus of this research work is to design and develop an antenna for passive neural monitoring with the implant using mixer which operates with no power. The proposed antenna aids in making the the implant size smaller to enhance the patient’s comfort. Firstly, a comprehensive literature survey was undertaken to explore the different methodologies proposed by researchers for the implementation of an antenna in neural implants. Various approaches to power the implant, including the utilization of power cables, batteries, and RF power harvesting, passive monitoring using microwave back scattering were studied. Among these, the microwave back scattering technique, employing an Anti Parallel Diode Pair (APDP), emerged as the preferred choice due to its ability to make the harmonic mixer fully passive, thereby reducing the overall power requirement of the implant. In order to effectively work with the harmonic mixer, the antenna needs to resonate at harmonic frequencies. The key attributes of a neural implant antenna were identified as dual band operability, low profile, compact size, and bio-compatibility. Secondly, a Vivaldi antenna was developed to operate in two frequency bands with harmonic resonant frequencies, resulting in a highly directive end fire radiation pattern. However, in order to achieve these desired characteristics, the antenna had a maximum dimension of 35 mm. To address this limitation, a miniaturized antenna was designed using a modified micro-strip patch structure. This new design had a compact size of 15.5 mm x 13 mm while still achieving the optimum frequency and radiation characteristics. Due to its smaller size, the peak gain of the antenna in the lower and higher resonant bands was only 0.98 and 1.09 dB, respectively. As a result, further refinement of the antenna design was necessary. Thirdly, a tiny hat-shaped antenna with a defected ground was developed to meet the specific requirements of a neural implant antenna. To ensure its bio-compatibility, the antenna was covered with a dielectric polymer material (PDMS) and its performance was evaluated in both in-vitro and in-vivo setups during experimentation. The antenna exhibited an impedance bandwidth of 1.15 GHz, supporting high data rates, and achieved a directive gain of 1.29 dB at 3.84 GHz and 1.39 dB at 7.68 GHz. Finally, a miniature antenna measuring 9 mm x 11 mm was designed with elliptical resonators to enable dual-band functionality. The two elliptical resonators allowed the antenna to receive at 7.15 GHz and transmit at 14.3 GHz. The antenna’s impedance bandwidths of 1 GHz and 1.45 GHz facilitated communication at higher data rates. With peak gain values of 1.7 dBi and 2.2 dBi, the link budget was calculated considering data rates of 1 Mbps and 20 Mbps. To evaluate the Specific Absorption Rate, the antenna was simulated within a six-layer head model with a penetration depth of 10 mm. The performance metrics were compared, leading to necessary modifications to enhance the antenna’s capability and overall implant performance.