  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.
