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Implantable Medical Devices (IMDs) with wireless telemetry functionalities in the radio-frequency (RF) range are nowadays used to perform an expanding variety of diagnostic and therapeutic functions. Example applications include temperature monitors (Scanlon, 1997), pacemakers and cardioverter defibrillators (Wessels, 2002), functional electrical stimulators (FES) (Guillory & Normann, 1999), blood-glucose sensors (Shults, 1994), cochlear (Buchegger, 2005), gastric and bladder controllers (Sani, 2009), glucose monitors (Karacolak, 2008), retinal implants (Gosalia, 2004) etc. As technology continues to evolve, new implantable medical devices are being developed, and their use is expected to rapidly increase from an already large base.
A key and critical component of RF-enabled IMDs is the integrated implantable antenna, which enables bidirectional wireless communication between the IMD and exterior monitoring/control equipment. Designers of implantable antennas need to deal with a number of challenges, including miniaturization, biocompatibility, impedance matching, radiation performance, and compliance with international safety guidelines for the specific absorption rate (SAR). Patch designs are usually preferred, because of their flexibility in design, shape, and conformability. Furthermore, patch antennas lend themselves easily to a number of miniaturization techniques, including use of high-permittivity dielectric materials, lengthening of the current flow path excited on the radiating patch, addition of shorting pins between the ground and patch planes, and vertical stacking of multiple patches (Kiourti & Nikita, 2012a).
Medical implant communications most commonly take place in the medical implant communications service (MICS) band (402.0–405.0 MHz), which is regulated by the United States Federal Communications Commission (FCC, 1999) and the European Radiocommunications Committee (ERC, 1997). The 433.1-434.8, 868.0-868.6 and 902.8-928.0 MHz industrial, scientific and medical (ISM) bands are additionally suggested for biotelemetry in some countries (ITU-R). However, focus is on the MICS band, because of its advantages to be available worldwide and feasible with low power and low cost circuits, reliably support high data rate transmissions, fall within a relatively low noise portion of the spectrum, and acceptably propagate through human tissue.