# A Review of Multifunctional Antenna Designs for Internet of Things
## 2 Materials and Methods
### Biomedical Devices
#### 2.1.1 Challenges of Ingestible and Implantable Antennas  
In recent years, there has been a rapid development of wearable sensors and transmitters, making them an easily accessible and commercially available option where simple patch antennas are sufficient [4]. Different examples of biomedical antennas are presented in Figure 1. However, IoT antennas for implantable and ingestible devices present a significant challenge for the designer. The human body is conductive and presents high dielectric constants [27], primarily due to its water content and the electrodes. This electromagnetically hostile environment complicates the development of miniature and efficient antennas capable of penetrating human tissue to transmit data to an external receiver when the sensor is located inside the patient's body. For these applications, the antenna must achieve high efficiency, operate at low frequencies, and have a small footprint.  
High efficiency is crucial for these devices as their power supply is limited; they either use small batteries, which cannot be frequently replaced, or rely on energy harvesting or wireless power transfer techniques [4]. Additionally, to ensure that the signal can be transmitted outside of the human body, operating at a longer wavelength is beneficial, as it facilitates easier signal transmission through heterogeneous human tissues. Moreover, given the requirement for these devices to be implanted or ingested, it is imperative that the antennas have a low profile to fit within the confined spaces of a small capsule [18] or implantable device [28]. Based on the application, the size and the shape of the implantable antenna may differ, but in most cases, a few dozen mm\({}^{3}\) of volume is needed for their placement inside the human body. On the other hand, the data rate is not deemed a critical  
Figure 1: Architecture of communication systems involving implantable, ingestible, and wearable antennas.  
factor for these types of devices, as the data collection from measurements is expected to occur at relatively low sampling frequencies (less than 1 Hz), focusing instead on the reliable transmission of crucial health metrics. As a result, electrically small antennas have become increasingly attractive for such applications [4].  
Designing electrically small antennas presents significant challenges, primarily due to their extremely low efficiency, often a result of the platform effect seen in printed structures, and the complexity of their operation in the near field, which complicates the analysis [4]. Furthermore, due to their low operating frequencies, their bandwidth is limited to low-data-rate applications. The preferred operating frequency for most implantable antennas falls within the free bands allocated for industrial, scientific, and medical (ISM) applications (433, 915, and 2450 MHz). Among the various designs, printed planar loop antennas are the most commonly utilized for these applications [19; 29]. These antennas are favored for their compact size and high efficiency, attributed to their primary excitation of magnetic fields, which are more capable of penetrating the highly dielectric and lossy human tissue compared to electric fields. This makes printed loop antennas particularly well suited for implantable devices, where efficient communication through human tissue is essential. On the other hand, in a conformal configuration, the antenna is wrapped inside the device package to minimize the occupied space while maintaining a large electrical size [30]. This structure is observed in capsule-like ingestible antennas (endoscopy). Their cylindrical shape is optimal, providing a smooth curvature for antenna wrapping [31]. However, in recent years, different types of magnetoelectric antennas have been proposed to achieve low operating frequencies while maintaining a low footprint (<10 mm\({}^{3}\)) in order to fit inside leadless pacemakers and brain devices [32; 33].