Double Split Rings as Extremely Small and Tuneable Antennas for Brain Implantable Wireless Medical Microsystems

Wireless intracranial implantable microsystems are believed to potentially innovate the management of brain disorders and the treatment of neurological diseases. The fundamental challenge in the development of the wireless implantable system is the attainment of miniature antennas achieving adequately high efficiency for signaling and wireless power transfer in the presence of the dissipative intracranial tissues. Here, we demonstrate and evaluate an effective approach that utilizes the coupled split rings to develop the miniature implantable antenna. With the proposed approach, the antenna size can be decreased to $\pi \times (3 \times 1.5) \times 1$ mm3 which is less than 2% of the operating wavelength at 915 MHz, while the antenna maintains the gain of −25 dBi when placed 16 mm deep in the anatomical human head model. As a proof of concept, we developed RFID tags based on the proposed antenna and verified their performance with tissue mimicking liquid and in vivo experiment with rats. The measured maximum read range of the tags reaches 0.7 and 1.1 m when immersed 30 mm in the tissue mimicking liquid and implanted 6 mm in a rat’s cranial cavity, respectively.