**5.3 Muti-band antennas for wireless implanted devices**

In wireless implanted devices, the antenna plays a key role in managing the communication process as well as the transfer of power. Hence, it is a multi-task process that require more than one frequency band operating simultaneously. For example, a frequency band is needed for biotelemetry and another one for power transfer. In addition, a higher frequency might be needed for wakeup controller. Since the implanted device is needed to be miniaturized, it is preferable to employ one antenna that can operate efficiently at more than one frequency.

**Figure 12.** *Power gain (G) of both topologies at different tissue locations [34].*

**Figure 13.** *Variation of resonant frequency and bandwidth at different parts of human body [35].*

Several recent studies [36, 37] attempted to address the multi-band issue by designing a dual- or triple-band antennas. In [37], a dual band antenna operating at 915 MHz and 2.45 GHz is designed and fabricated for scalp-implanted devices. The fabricated meandered line antenna and the experimental setup is shown in **Figure 14**. For validation, normally the measurements must of antennas for wireless implanted devices are carried out in saline solution. **Figure 15** presents the simulated and measured return loss of the designed antenna. It is worth mentioning that the link margin decreases with increasing the transmission range, where the highest bit rate undergoes larger loss as shown in **Figure 16**.

Another compact multi-band antenna is proposed in [37]. The implemented antenna exhibits triple resonance behavior due to the employment of spiral structure. The antenna can be operated at 433.1–434.8 MHz, 1520–1693 MHz and 2400– 2483.5 MHz. The fabricated triple-band spiral antenna and the experimental set up is depicted in **Figure 17**. In addition, **Figure 18** illustrates the SAR values for all frequency bands.

Some studies in literature has proposed different frequency band [38]. **Figure 19** shows the return loss of the designed antenna showing the three resonant frequencies and the bandwidth for each band.

## **5.4 Employing RFID antennas in wireless implanted devices**

Some studies proposed the use of near-field inductively coupled implanted devices operating at low frequencies with two antennas (implanted and wearable); *Implantable Wireless Systems: A Review of Potentials and Challenges DOI: http://dx.doi.org/10.5772/intechopen.99064*

**Figure 14.** *Fabricated antenna and experimental set up [36].*

**Figure 15.** *Simulated and measured return loss [36].*

**Figure 16.** *Variation of link margin with communication distance for different bit rates [36].*

and an additional far-field antenna for the off-body data transmission system. RFID approach is suggested in [39], where the implant part carries a backscattering microsystem. On the other hand, the wearable antenna (outer ring) serves as the radiating part for the off-body data communication as shown in **Figure 20**.

One of the biggest challenges of the far-field antennas that are used in implanted devices is the large size of the antenna, which should proportional to the wavelength of the electromagnetic waves. In this application, the implanted device needs

**Figure 17.** *The Fabricated triple-band spiral antenna and experimental set up [37].*

**Figure 18.**

*Simulated averaged SAR surface (top row) and coronal (bottom row) distributions over 1-g of tissue in an anatomical human head model [37].*

#### **Figure 19.**

*Simulated and measured return loss for the designed triple-band antenna [38].*

to as small as possible, thus, it is important to design miniaturized antennas with acceptable efficiency. To address this issue, a compact electromagnetic antenna array with dimensions around 200 μm can be utilized as reported in [40]. The proposed antenna system can harvest electromagnetic energy to power up the RFID *Implantable Wireless Systems: A Review of Potentials and Challenges DOI: http://dx.doi.org/10.5772/intechopen.99064*

**Figure 20.** *Implantable and wearable antenna prototypes for brain RFID system [39].*

**Figure 21.** *Wireless implantable NanoNeuroRFID system reported in [39].*

system. In addition, the antenna array can sense the neuronal magnetic fields. The overall wireless implantable NanoNeuroRFID system is shown in **Figure 21**.
