Radiation Diversity Enabled Self-Isolated Compact Dual-Band Cubic MIMO Antenna for Wireless Biomedical Implants in Variable and Dynamic Environment

Abstract

This study presents a compact dual-band cubic multi-input-multi-output (MIMO) antenna specifically designed for gastrointestinal (GI) tract capsule endoscopy and cardiac leadless pacemaker systems. The proposed cubic MIMO antenna operates across two frequency bands: 1.395 to 1.4 GHz and 2.4 to 2.4835 GHz. Comprising four individual antennas, it has overall dimensions of 5.12 × 5.12 × 4.6 ${\text{mm}}^{\text{3}}$, which makes it a compact cubic design, achieved by employing symmetrically embedded radiating patch slots. The strategic relocation of port, pin, and ground slot not only resulted in reduced coupling due to opposite current flow but also contributed to achieving excellent frequency tuning for all antenna elements in cubic configuration. Encapsulated within wireless implants with batteries, sensors, and device circuitry, the proposed MIMO antenna was simulated in both homogeneous and heterogeneous body phantoms, including the small intestine, large intestine, stomach, and heart. Experimental validation also conducted using minced pork yielded results that agree with simulations, demonstrating the MIMO antenna effective performance, including measured reflection coefficient (−22 dB, −19 dB), gain ($-$28.17 dBi, $-$18.15 dBi), −10 dB bandwidth (390 MHz, 670 MHz), minimal coupling (−23 dB, −24 dB), and fractional bandwidth (27%, 26%) at 1.3975 and 2.45 GHz, respectively. Each cubic element radiates in four opposite directions, enabling radiation diversity in all four directions, crucial for various body postures during movement. The specific absorption rate (SAR) is also calculated and confirmed to remain within very safe limits for human implantation. Furthermore, a communication link analysis established the reliability of the antenna in maintaining stable communication with an external device over an 10 m and 15 m radius at the respective resonant frequencies, achieving a high data transmission rate of 100 Mbps. Further evaluation, including envelope correlation coefficient (ECC), diversity gain (DG), channel capacity loss (CCL), and total active reflection coefficient (TARC), confirms the usefulness of the proposed MIMO. Consequently, this MIMO antenna emerges as a highly promising candidate with radiation diversity, high compactness, and self-isolation ability for several wireless biomedical implants.