Antenna Designs for Wireless Medical Implants.

Abstract

Active medical implants are devices that are surgically implanted inside the body. They have been developed to treat a wide range of ailments and many require some form of communications link with the outside world for diagnostics and maintenance. Radio links promise a wide range of benefits over low frequency inductive coupling. However, body tissue is a challenging environment for both the device and the radio signal that it transmits. Innovation in medical technology is creating a demand for higher bandwidth and resilient implant links, and this in turn is driving the development of implantable antennas. Many factors constrain antenna design choices. These include low signal power levels, minimal space availability and the effect that tissue characteristics have on the antenna. Most body tissues are highly dissipative and antennas must be specifically designed for a high dielectric environment. A review of spectrum band availability indicates that the 401 – 406 MHz and 2.4 GHz bands are most suitable for implants. Multiband antennas were explored in the research, but the design priority focussed on 401 – 406 MHz. Custom antenna designs were simulated in software and prototypes were later produced and tested in a range of different materials that closely imitate body tissue. These tests revealed a discrepancy in the software simulations. It affected all the prototypes, causing them to resonate at circa 150 MHz higher than the expected frequency. The simulation software’s transient domain solver was eventually identified as a likely cause of the frequency offset, but in the interim the antenna designs were progressed using empirical methods. The frequency domain solver has been found to predict the resonant frequencies of dielectrically loaded implant antennas with better accuracy than the time domain solver. Two final designs were developed. Validation tests confirmed a good return loss of 10 to 20 dB in a replicated implant environment. The designs are meander line variants of an inverted-L antenna. The design process, test results and detailed specifications are set out hereafter. Antenna Designs for Wireless Medical Implants Page 5