Design and Characterization of a Miniaturized Implantable Antenna in a Seven-Layer Brain Phantom

In this paper, we propose a miniaturized implantable antenna exhibiting a broadside radiation pattern and wide operating bandwidth. Previously reported small implantable antennas often display omnidirectional radiation patterns which are not suitable for in-to-off wireless body area network. The proposed design overcomes this problem by optimizing the antenna structure inside a realistic brain implant environment, a seven-layer brain phantom including skin, fat, bone, dura, cerebrospinal fluid (CSF), gray and white matters. The seven-layer phantom was modeled in a full-wave simulation software, and then the antenna was embedded in dura layer. The antenna has a circular shape with a diameter of 10 mm and a thickness of 0.5 mm. The top and bottom insulating layers share the same dimensions of the antenna. With the given location and surrounding materials, the antenna geometry was optimized to resonate at 2.4 GHz and to radiate broadside. The optimal design was fabricated using a low-loss biocompatible PCB material, Taconic RF-35 (εr = 3.5, tanδ = 0.0018), and tested in a seven-layer brain phantom implemented with semi-solid artificial tissue emulating (ATE) materials. The results of both the simulation and measurement revealed similar -10-dB impedance bandwidths of 13.8% and 14.9%, respectively, which are wider than those of most single-band implantable antennas operating at 2.4 GHz. The proposed antenna also displayed a measured peak realized gain of -20.75 dBi and an acceptable radiation efficiency of 0.24%, which are within the typical range. Furthermore, we calculated the specific absorption rate (SAR) and assessed its compliance with the IEEE safety guidelines.