Investigation of Conformal MIMO Antenna for Implantable Devices Based on Theory of Characteristic Modes

To meet an implantable device for human disease detection, a conformal multiple-input–multiple-output (MIMO) antenna is investigated in this article. The layout and decoupling of the antenna are designed according to the inherent electromagnetic properties of the implanted device based on the theory of characteristic modes (TCM). Compared with traditional methods, the TCM provides a method and theory to design a conformal implantable antenna with MIMO properties. The designed antenna has the advantage of simple structure, low space occupation, and little multipath reflection. The newly designed structure adopts two planar helical radiators to achieve size miniaturization. Some I-type patches are located between two radiators and slotting structures on the ground plane to reduce coupling. The biocompatible flexible polyimide is used as a dielectric substrate, enabling the conformal property and compatibility with the human tissue. Measurements of the fabricated prototype have been finished in the minced pork and skin-mimicking gel. Results show very good agreement with the simulations, which validate the suitability of the proposed antenna for the implantable devices. To the best of our knowledge, this is the first time that the TCM has been applied to guiding the design of implantable MIMO antennas.