Suitability of Common Ingestible Antennas for Multiplexed Gastrointestinal Biosensing

TL;DR

The paper tackles the challenge of selecting ingestible antennas for multiplexed GI sensing by comparing dipole, patch, and loop configurations in tissue-mimicking environments at the 434 MHz ISM band. It evaluates sensing capability via phase changes in the reflection coefficient, robustness via the $|S_{11}|< -10$ dB bandwidth $\Delta f_i$, center-frequency stability via $\Delta f_c$, and transmission performance via gain and radiation efficiency $\eta$, across GI tissues ST, SI, and LI. Simulations and measurements show the loop antenna generally offers the strongest sensing signals and highest $G$/$\eta$, while the patch provides better robustness to detuning due to dielectric loading but lower sensing performance; dipole lies in between with notable thickness-dependent trends. The work also demonstrates that shell thickness critically tunes near-field interaction and efficiency, and that operating frequency choices modulate performance in line with theoretical bounds, providing practical guidelines for designing multiplexed GI biosensor systems.

Abstract

Ingestible sensor devices, which are increasingly used for internal health monitoring, rely on antennas to perform sensing functions and simultaneously to communicate with external devices. Despite the development of various ingestible antennas, there has been no comprehensive comparison of their performance as biosensors. This paper addresses this gap by examining and comparing the suitability of three common types of ingestible antennas -- dipole, patch, and loop -- as biosensors for distinguishing gastrointestinal tissues (stomach, small intestine, and large intestine) based on their electromagnetic properties. The antennas studied in this work conform to the inner surface of biocompatible polylactic acid capsules with varying shell thicknesses and operate in the 433 MHz Industrial, Scientific, and Medical band. The comparison is performed in gastrointestinal tissues using several antenna parameters: 1) Sensing Capability: Changes in the phase of the reflection coefficient in the tissues are selected as the sensing parameter. 2) Robustness: The frequency interval (f_i) in which the antennas are matched (|S11| < -10 dB) in all the tissues and the maximum change in the center frequency (f_c) in different tissues are examined. 3) Radiation Performance: The gain and radiation efficiency of the antennas are examined. The effect of shell thickness on gain and radiation efficiency at 434 MHz is presented. Additionally, the radiation efficiency at various frequencies allocated for medical communications is compared with the theoretical maximum achievable efficiencies. These comprehensive data provide valuable information for making engineering decisions when designing multiplexed biosensor antennas for ingestible applications.

Figures (9)

• Figure 1: General overview of the concept examined in this paper. (a) Human GI system with the capsule including the ingestible sensor antenna. (b) General model of the wireless biosensor capsule showing required elements to operate it and two operations realized by the capsule: 1) sensing in reactive near-field region where near-fields radiated by the antenna extend into the tissues, resulting in changes in antenna parameters in different tissues and 2) communication with the outside of the body where collected data is transmitted to an off-body device via propagation of EM-fields through the lossy tissues.
• Figure 2: Model of the antennas, the capsule, and the phantom used in the study (units: mm). (a) Meandered dipole antenna. (b) Patch antenna with two identical low-impedance radiating elements connected with a high-impedance meandered microstrip line. (c) Meandered loop antenna. (d) Patch antenna conforming to the inner surface of the PLA capsule filled with a PLA fixing cylinder. (e) Capsule placed in the middle of the spherical homogeneous phantom for EM simulations.
• Figure 3: Prototype used in the measurements. (a) Fabricated patch antenna, capsule, and fixing cylinder. (b) Final prototype with the coaxial cable and epoxy sealing.
• Figure 4: Simulated magnitude and phase of the reflection coefficient of the antennas in three gastrointestinal tissues for $t = 0.2$ mm. Magnitude for the (a) dipole, (b) patch, and (c) loop. Phase for the (d) dipole, (e) patch, and (f) loop. Note that the measurements were conducted for $t = 0.4$ mm, and the measurement results are presented later in the paper (Fig. \ref{['fig:measresult']}).
• Figure 5: Simulated magnitude and phase of the reflection coefficient of the antennas in three gastrointestinal tissues for $t = 0.6$ mm. Magnitude for the (a) dipole, (b) patch, and (c) loop. Phase for the (d) dipole, (e) patch, and (f) loop.