Reconfigurable Ultra-Miniaturized MIMO Antenna for Tissue-Independent Communication in Injectable Medical Implants

초록

This article introduces a compact, reconfigurable multiple-input-multiple-output (MIMO) antenna tailored for injectable in-body implants, designed to operate efficiently in the 2.45 GHz industrial, scientific, and medical (ISM) band, ensuring consistent performance regardless of the host tissue type, such as muscle or fat. The antenna design achieves miniaturization using meandered lines and shorting pins, while reconfigurability is obtained by adjusting the meandered length with an ON-OFF switch (diode). When the diode is ON, the antenna's meandered length is increased, thereby extending its effective length and enabling resonance in the 2.42-2.48 GHz range for fat tissues. Conversely, when the diode is OFF, the effective length is reduced, allowing the same antenna to operate within the 2.38-2.48 GHz range in muscle tissues. The proposed antenna, fabricated on a flexible polyimide substrate (epsilon(r) =3.5), with overall dimensions of 5 mm x 2 mm x 0.05 mm, is enclosed within a biocompatible polyimide casing. The antenna's performance is evaluated through simulations in homogeneous single-layer (fat/muscle) and three-layer (skin, fat, and muscle) phantoms, along with a realistic heterogeneous hand model, and validated through an ex-vivo setup, which includes fabricating the antenna prototype and measuring its performance in pork meat and a phantom solution. This innovative, miniaturized, tissue-independent MIMO antenna achieves isolation greater than 15 dB via orthogonal arrangement and 1 mm element spacing, with impedance bandwidths of 60 MHz in fat and 100 MHz in muscle. Furthermore, MIMO characteristics are evaluated and antenna robustness and safety are verified through specific absorption rate measurements using voxel models, highlighting the antenna's suitability for injectable applications.

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