## 3 Wireless Communication and Power Transmission
### Wireless Information Transmission  
Common data transmission methods for wireless implantable devices include Bluetooth, Radio Frequency Identification (RFID), Near Field Communication (NFC), ultrasonic waves, magnetic induction coupling, and more (for detailed data, refer to Table 2). Bluetooth: Bluetooth communication is commonly seen in mobile phones and wearable electronic devices (such as bracelets and smart watches). It possesses several advantages, including low power consumption, user--friendliness, and high data transmission efficiency. However, the typical communication frequency band of Bluetooth is 2.4 GHz, which is susceptible to interference from wireless devices such as WIFI, smartphones, or microwave ovens. After years of development, BLE (Bluetooth Low Energy) has been developed specifically for low-power devices. It reduces the energy consumed in data transmission and extends the usage time of implantable devices [88]. At present, the commercially available Eversense E3 continuous glucose monitoring system is implanted beneath the patient's skin to monitor the glucose content in subcutaneous fluid. A wireless transmitter worn externally on the patient's body acquires information from the sensor and transmits glucose readings to the user's mobile device via Bluetooth every five minutes [89]. The Nalu Neurostimulation System developed by another company involves the implantation of a micro-stimulator under the skin. This system utilizes gentle electrical pulses to block pain signals prior to reaching the brain, offering a drug-free therapeutic approach. It enables wireless control through Bluetooth Low Energy technology. Patients can utilize a smartphone remote control application to select treatment options and regulate the stimulation, while doctors can also perform remote management via a clinical programming application [90]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Range** & **Transfer Rate** \\ \hline Radio Frequency & 100 m & 1–10 kbps \\ Bluetooth & 10 m & 1 Mbps \\ Near Field Communication & 4–20 cm & 106–424 kbps \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Distances and rates of some wireless communication technologies. Radio Frequency Identification: RFID is a better choice when it is difficult to power the object to be identified. RFID tags can work without a power source. They obtain energy from the reader, require no physical contact, and the wireless reading distance is relatively compatible with the depth of implantable devices [91]. Simultaneously, RFID technology is capable of concurrently reading the information of multiple sensors without interference, rendering it suitable for the simultaneous implantation of multiple sensors. Nevertheless, this technology does have certain limitations. High-frequency RF signals, such as those at 2.45 GHz, exhibit substantial scattering and absorption within the human body. For an antenna designed for external radiation, the calculated maximum Specific Absorption Rate (SAR) value at 2.45 GHz is 0.0369 W/kg. When an antenna designed for in-body communication is employed, the maximum SAR value at the same frequency escalates to 0.4479 W/kg. The significantly elevated SAR value of the antenna during in-body transmission poses certain challenges to signal reading [92]. Moreover, owing to the absence of unified RFID standards, issues regarding system interoperability among different suppliers exist. Seamless integration with existing systems is arduous, and there is also a risk of data leakage. In the commercial domain, RFID finds extensive applications. The Raumedic intracranial pressure sensor, a precision measurement catheter employed in the neurosurgical domain, is primarily utilized for monitoring parameters including intracranial pressure (ICP), temperature, and partial pressure of oxygen. By leveraging technologies such as semiconductor pressure sensors and thermistors, it enables direct and accurate in situ measurement of intracranial pressure.