# A Miniaturized Circularly-Polarized Antenna for In-Body Wireless Communications
## 1 Introduction  
Implantable medical devices (IMDs) have increasingly caught the attention of the scientific community due to their wireless capabilities of detecting bio-medical information and transmitting health data much more flexibly and conveniently than traditional wired sensors placed exterior to the body [1; 2; 3]. These devices have been widely adopted in many applications including neural recording [4], glucose monitoring [5], and intracranial pressure monitoring [6], etc. Implantable antennas act as a key factor to assure wireless communications between the implantable devices and the external equipment [7], and have been assigned at the industrial, scientific, and medical (ISM) bands with the operating frequency ranges of 433.1-434.8 MHz [8; 9], 902-928 MHz [10], and 2.40-2.48 GHz [11; 12]. Additionally, the 402-405 MHz medical implant communication services (MICS) band [13] and the 1395-1400 MHz wireless medical telemetry services (WMTS) band [14] are also designated for the implantable antennas. Implantable devices used for the biomedical telemetry are typical microsystem and therefore cannot accommodate large antennas. Due to the relatively short electromagnetic wavelength, the implantable antenna working at the ISM higher frequency band of 2.40-2.48 GHz is adopted by some researchers, making the dimension electrically small enough. Moreover, considerable design efforts, such as employing high-permittivity dielectric substrates [15], loading shorting pins to connect the patch and the ground [16], and extending the current flow path on the patch surface [17], have been made to realize the compact dimension of the implantable antennas. Many crucial factors such as biocompatibility, specific absorption rate (SAR), far-field radiation, and operating bandwidth, should be also considered during the designing of implantable antennas. The planar inverted-F antenna (PIFA) was demonstrated as a useful prototype in the design of implantable antennas because of its structural simplicity and low profile [18]. The monopole antenna with the advantages of omnidirectional radiation pattern and wide bandwidth has been integrated with the implantable system [19]. In [20], a differentially-fed dual-band flexible antenna was proposed for ingestible capsule system. Nevertheless, the above-mentioned antennas are linearly polarized and dependent on the relative orientations between the transmitters and the external receivers. A circularly-polarized (CP) antenna is preferred for the implantable devices because it can reduce multipath distortion and provide flexible mobility, compared with a linearly polarized antenna. However, only a few works focus on the miniaturized CP implantable antennas. Capacitive loadings were introduced in a square patch with a central squared slot to realize circular polarization for an implantable antenna [21]. An axial-mode multilayer helical circularly-polarized implantable antenna for ingestible capsule endoscope system was presented in [22]. Additionally, by cutting rectangular slots into the patch and adding open stubs in the annular ring, a CP bio-friendly implantable annular ring antenna was realized in [23]. A miniaturized CP squared patch antenna for implantable devices at the ISM band of 2.40-2.48 GHz was preliminarily introduced in [24]. After that, future works have been carried out. In this article, an in-depth study on the working principle and a more detailed analysis on the performance of the proposed antenna are presented while a proof-of-concept fabricated prototype was fully characterized, verifying the good performance of the proposed antenna topology. Through introducing reactive loading, slow wave effect is formed on the radiator, making the dimension of the proposed antenna compact. The contents of this article are organized as follows: In Section 2, a simulation environment is set up, the geometry of the proposed implantable antenna is described, and the simulation results are studied.