Single-wire RF transmission lines for implanted devices

Abstract

We consider the use of insulated single wires as transmission lines to carry 100 MHz - 3 GHz radio frequency (RF) signals among devices implanted in biological tissue. In contrast to near-field magnetically coupled links, the use of transmission lines to carry RF signals results in higher efficiency for a given implant package size once the antenna is included, albeit with the disadvantage of tissue displacement along the path of the wire. We present a theory based on the work of Goubau and Rao that describes the transmission line loss of a single insulated wire in a lossy dielectric medium. We experimentally verify the characteristic impedance and insertion loss of transmission lines formed by thin wires insulated with Teflon fluorinated ethylene propylene (FEP). We consider media including 0.91% saline (a homogeneous tissue proxy), muscle tissue, and brain tissue, and present a launcher design based on a dielectric loaded coaxial sleeve. For example, in the saline proxy, a single FEP-insulated conductor of only 0.127 mm diameter presents a measured return loss of 10 dB in a 50Ω system, with a measured insertion loss of only 1 dB/cm at 1 GHz.