Implantable MIMO Antenna Design for High Data Rate Wireless Capsule Endoscopy

Abstract

Antenna performance is closely tied to physical size, which presents major challenges
in wireless capsule endoscopy (WCE) due to its limited internal space. WCE systems
require antennas that are compact, wideband, and easy to integrate—requirements
that conventional designs often fail to meet. This research proposes a multistage
antenna design strategy for WCE, focusing on conformal, miniaturized, and transparent structures.
In the first stage, a conformal antenna is designed using a left–right symmetrical tri-arm dipole structure on a flexible substrate, allowing it to fit along the
capsule’s inner cylindrical wall. Multiple resonant frequencies are introduced and
optimized to improve impedance matching and achieve ultra-wideband (UWB) operation. The final antenna measures 30.5 mm×9 mm×0.04 mm, with a bandwidth of
0.59–1.36 GHz and a fractional bandwidth of 78.97%. The second stage introduces
a miniaturization technique based on dielectric hollowing, which reduces effective
permittivity without changing the external shape. This allows the integration of
a dual-element multiple-input multiple-output (MIMO) system inside the capsule.
Each antenna occupies just 7.192 mm3 and operates from 0.61 to 1.51 GHz, achieving 84.91% fractional bandwidth with low envelope correlation and good isolation.
In the final stage, a transparent antenna is developed to share space with the frontfacing camera. A conductivity–frequency model is built for inkjet-printed silver
mesh structures with 50–90% transparency over 0–6 GHz. Based on this, a circularly polarized CPW-fed circular patch antenna is designed, operating from 2–3 GHz
with 70% measured transparency.
All antennas are simulated using Ansys HFSS and CST Studio Suite, fabricated
with appropriate materials, and tested using pork tissue under controlled lab conditions to approximate body-like environments. Results confirm improvements in
bandwidth, compactness, MIMO performance, and optical transparency. This study
presents a complete and multi-stage antenna design framework tailored to the unique
constraints of WCE, providing a solid foundation for high-capacity, space-efficient,
and visually integrated biomedical devices.