## 3 Wireless Communication and Power Transmission
### Wireless Charge  
Inductive coupling: Energy is transferred to implantable medical devices, including cardiac pacemakers, defibrillators, neuromuscular stimulators, etc., via inductive coupling. [PERSON] et al. designed a compact stimulator with dimensions of merely 5 mm \(\times\) 7.5 mm [105]. Operating at a resonant frequency of 198 MHz, it can be positioned up to 14 cm away from the external transmitter and stores energy through a switched capacitor. Once the voltage reaches the threshold, the stored energy is released as a stimulation output. By employing a positive feedback trigger circuit, the requirement for a stimulation control circuit block is eliminated, thereby saving valuable space. In the in vivo experiment, two electromyogram (EMG) recording electrodes were implanted into the gastrocnemius muscle of rats, while the ground electrode was attached to the skin, successfully demonstrating the functionality of the stimulator.  
Magnetic resonance coupling: Magnetic resonance coupling realizes energy transmission through transmitting and receiving resonators, featuring high efficiency and rel atively long transmission distances. [PERSON] proposed a wireless magnetic resonance energy transmission system for miniature implantable medical sensors [106], achieving efficient, stable, and lossless energy transmission (Figure 3a). Through experiments, it was verified that this system exhibits good performance and stability under different distance and load conditions.  
Ultrasonic transmission: Ultrasonic waves achieve energy transfer by means of sound waves. Given that biological tissues have a relatively low absorption rate of ultrasonic waves, this characteristic gives it distinct advantages in energy transmission for tiny implants or deep tissues. For example, [PERSON] et al. developed an implantable pressure sensing system driven by mechanical vibration [107]. This system utilizes the piezoelectric effect and can collect ultrasonic energy with a great power density. Specifically, it converts acoustic vibration harmonics into electrical energy and then charges a capacitor. During the period when the cantilever beam is in a non-vibrating state, the stored charge is discharged through an LC resonant circuit equipped with a pressure-sensitive inductor, providing the required energy for deep brain stimulation.  
Triboelectric nanogenerator: Leveraging the triboelectric effect, it is capable of converting the mechanical energy within the human body into electrical energy, thereby endowing devices with the self-power supply capacity. As the pioneer of the triboelectric nanogenerator, [PERSON] put forward a self-powered implantable ligament strain sensor grounded in triboelectric nanogenerator technology [108]. Powered by the triboelectric nanogenerator, upon being implanted into the patellar ligament, this sensor can generate electrical signals during human locomotion. These signals are utilized to assess the damage of muscles or ligaments (Figure 3b). Furthermore, this implant elicits only a mild inflammatory response.  
Table 3 lists the power, distance and size of different wireless charging solutions.  
Figure 3: (**a**) Schematic diagram of the wireless resonant energy transmission system and the implanted receiver coil and IC chip packaged in PDMS. (**b**) Application and architecture of the OFS-TENG. (**c**) Capacitive pulse sensors installed around the arteries measure the changes in vascular diameter caused by arterial pulsation. The change in capacitance causes the resonant frequency of the LCR circuit to shift. This change was measured by the external reading coil.