Dual-Polarized Water-Substrate-Based MIMO Implantable Antenna for IoMT-Enabled Biotelemetric Devices

Abstract

This paper introduces a water-substrate-based implantable antenna (WSIA) that can be easily adapted to 2-and 4-elements multiple-input multiple-output (MIMO) configurations for high-data-rate Internet of Medical Things (IoMT) devices. The proposed antenna operates in the 2.45-GHz industrial, scientific, and medical band for biotelemetry and in the 1.47-GHz midfield band for the seamless wireless powering of IoMT devices. Compared with previously reported implantable antennas, the proposed MIMO antenna element is ultra-miniaturized, achieved through high water permittivity and a circular ring-shaped radiator. Additionally, the antenna elements are strategically positioned for 2-and 4-ports MIMO configurations to achieve polarization diversity (PD), improving isolation and overall system efficiency. The antennas were optimized within a realistic capsule containing electronic components, sensors, and batteries and were subsequently fabricated and tested. In addition to satisfactory performance in terms of scattering parameters, isolation, gain, and polarization diversity, the proposed MIMO antenna systems exhibited significantly lower specific absorption rates (SARs) owing to water substrate, thereby offering improved user safety. Evaluation of the MIMO performance, focusing on the envelope correlation coefficient and diversity gain, yielded results within acceptable limits, even in diverse body environments. Moreover, a link-budget analysis revealed that the antennas can establish a reliable communication link at distances of 20 and 15 m at 1.47 and 2.45 GHz, respectively with high-data-rate of 100 Mb/s. Owing to the similarity between the electrical properties of water and human tissue, the proposed WSIAs efficiently transform radiated energy and, being 3D printable, offer significantly lower fabrication costs than conventional substrate-based designs. With advantages such as compact size, multi-band operations, PD, low SAR, and reduced fabrication costs, these antennas offer substantial benefits for IoMT implants, particularly for wireless capsule endoscopy.