Efficient and Wideband Implantable Antenna Based on Magnetic Structures

Implantable devices are becoming prevalent in the field of biotelemetry. However, designing implantable antennas is challenging, since compact structure, large bandwidth, and relatively high gain are required at the same time in the lossy human tissue. In this paper, we analytically derive the electromagnetic (EM) fields of the electric and magnetic dipoles in the lossy human tissue and compare their radiation performance. Based on the analysis, a coupled small loop antenna is designed to obtain a large bandwidth of 74.1% when embedded in the muscle. The antenna is composed of an inner loop and an outer loop with printed capacitive structures, and it can also be rolled up into a capsule using flexible substrate. The overall dimension is 30 × 16 × 0.5 mm3and π × (5)2× 16 mm3for the planar and the capsule structure, respectively. Compared with other implantable antennas, a relatively high gain of -23.19 dBi has been achieved for the planar small loop, as the magnetic antenna suffers from less power dissipation in the human tissue. The specific absorption rate (SAR) of the antenna is 177 W/kg with an input power of 1 W, which enables a maximum transmitting power of 9.03 mW according to the IEEE standard. The high transmitting power ensures the reliable communication link. The antenna prototypes were fabricated and measured in the minced pork, which has similar EM properties as the human muscle. The simulated and measured results basically agree with each other. The proposed implantable antenna is competitive in its balanced performance of wide band, high gain, low profile, and low SAR.