Mitigation of Tissue-Induced Frequency Detuning in Skin Implantable Antennas

This paper investigates the tissue-induced detuning of resonant frequency in a skin-implantable microstrip patch antenna and explores potential remedial design solutions using dielectric superstrate loading. The antenna is designed for the 2.45 GHz ISM band on a 0.254 mm-thick Rogers RT/Duroid 6010 LM substrate (ε<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf>_<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sub</inf>= 10.2), providing an approximate footprint of 76 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (20 mm × 15 mm × 0.254 mm) suitable for implantation in skin tissue. Simulations are conducted using CST Microwave Studio<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sup>, with the antenna positioned within the skin layer of a three-layer tissue phantom comprising skin, fat, and muscle, each with specific thickness and material properties. The frequency detuning tolerance range for the 2.4-2.484 GHz ISM band is determined by simulating dry and wet skin of 2 mm and 4 mm thicknesses, reflecting typical variations in human skin layers. Subsequently, the impact of superstrate loading on the antenna is explored utilizing popular biocompatible dielectric materials (ε<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_sup</inf>), including Rogers RT/Duroid 6010 LM, Zirconia, Macor, halogen-free FR–4, and Teflon, with air considered as a reference superstrate. Results indicate that superstrates with permittivity significantly lower than that of the substrate (ε<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_sup</inf>/ε<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r_sup</inf><<1) satisfy the predefined detuning criteria, with Teflon demonstrating optimal performance characterized by minimal detuning of 24 MHz and robust impedance matching, where return loss at resonance ranges from –18 dB (worst-case) to –42.7 dB (best-case) across all simulated tissue conditions.