# Wearable Flexible Antenna for UWB On-Body and Implant Communications
## 5 Discussion  
This paper presents a compact-sized wearable flexible monopole antenna designed for in-body and on-body communications at ISM and UWB frequency bands. Compared to the recently published wearable flexible antennas structures, the proposed monopole antenna is a very simple, more compact in size, and represents good radiation performance in terms of total efficiency. Additionally, this is the very first flexible antenna paper presenting evaluations for in-body communications by studying radio channel characteristics between the proposed antenna and an UWB capsule endoscope with simulations. The antenna evaluations were performed via electromagnetic simulations using CST Studio Suite simulation software, utilizing a tissue layer model for the abdominal tissues as well as an anatomical voxel model. Additionally, the antenna prototype was prepared, and measurements were carried out with a hand phantom as well as with a volunteer. It was found that this antenna operates well both in ISM and UWB bands. The closer the antenna is to the skin surface, the better the antenna matching. This is an important characteristic for flexible antennas designed for body area networks and it also provides potential for many different applications. Another important characteristic is that the bending of the antenna has only minor effect on the S11 results since the antenna would naturally bend when attached on the human skin. The power flow analysis show that the propagation depth in the abdominal tissue layer model and voxel model is good up to 5.8 GHz. Besides, the channel evaluations between the flexible antenna and a UWB capsule model using the layer model are in line with the power flow results. With the proposed flexible antenna, the signal could be detected easily up to 5.8 GHz if the receiver's sensitivity limit is around \(-65\) dBm [61]. With receivers having better sensitivity [62], even the signals transmitted at 7 GHz could be detected smoothly in this case. Naturally, as the propagation depth increases, also power loss increases and thus, 7 GHz frequency will become excessive to detect signals from the capsule which is deep inside the intestine area especially with persons having thicker muscle and fat layers. Overall, the presented results indicate that this flexible antenna would operate well as an on-body antenna in the capsule endoscopy if lower frequencies as well as multiple receiving antennas are used. At higher frequencies, the antenna is suitable for on-body communications as well as for in-body communications with lower propagation depth requirements. The properties of the capsule endoscope antenna also have a strong impact on the radio channel evaluations between the on-body antenna and the implant antenna, especially in the practical scenarios where the capsule can rotate inside the intestine. Being in arbitrary position, the channel attenuation may become excessively high in unfavorable capsule positions if both on-body and implant antennas are linearly polarized. Hence, the use of circularly polarized efficient implant antennas, as presented, e.g., in [13; 14; 15], could significantly improve the channel condition in certain capsule positions. Additionally, enhancement of the power transmission link in the implant communications requiring deep propagation depths, as presented in [14], could also improve the performance of implant communications in practical scenarios. As a future work, we will evaluate radio channel characteristics between the flexible antenna and a capsule endoscope model with simulations using several different anatomical voxel models having different sizes and body constitutions. Additionally, we will verify the simulation results measuring human tissue mimicking phantoms as well as pork tissue. Besides, our future work will include evaluations of the antenna's usability for different medical monitoring and diagnostic applications in different body parts. We also aim to use more efficient and circularly polarized implant antennas for realistic evaluations.