# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 5 Wireless Technologies
### Self-Powered System
#### 5.1.1 Wireless Power Transfer  
Electrical energy can be transferred wirelessly through inductive coupling between the antennas of a transmitter and a receiver [142]. In this system, the transmitting antenna converts the alternating current passing through it into oscillating magnetic fields, then, as the magnetic fields reach the adjacent receiving antenna, an alternating current is induced in the receiver device. The amount of electric current that is generated depends on the intensities of the magnetic fields reaching the receiving antenna, so, to achieve sufficient energy transfer, the system requires that the two antennas be located very close to each other. Resonant inductive coupling can be used to increase the distance over which sufficient power can be transferred. Resonant inductive coupling refers to the phenomenon in which the inductive coupling becomes stronger when the resonant frequencies of the two antennas are in the same predetermined range [143]. Through resonant inductive coupling, the efficiency of energy transfer can be enhanced significantly, but elaborate tunning of the resonant circuit is required to match the frequencies of the antenna circuits.  
[PERSON] et al. demonstrated a smart contact lens for monitoring glucose and intraocular pressure, in which the power supply and data transmission are acquired wirelessly through the inductive coupling between the antenna embedded into the lens and an external reader coil connected to a network analyzer, as shown in Figure 6a [144]. The reader coil was placed 10 mm away from the smart contact lens to inductively couple and power it. Glucose levels and intraocular pressure were measured simultaneously but independently by the variation in reflection coefficient and the shift of resonance frequency from the data recorded on the network analyzer, respectively. Then, [PERSON] et al. coupled a similar antenna circuit with a rectifier that was comprised of Si diodes and a capacitor to convert the wirelessly-received alternating current (AC) power to direct current (DC) power, as shown in Figure 6b [15]. Then, the DC power turned on the glucose sensor and the light-emitting diode (LED) pixel on the lens to provide a visualized sensing signal. In addition, the spiral antenna and the interconnect were made of Ag nanofibers to provide high transparency and stretchability. Later, to enable the continuous operation of a smart contact lens powered by inductive coupling, [PERSON] et al. incorporated a rechargeable solid-state supercapacitor into a wireless energy transfer circuit, as shown in Figure 6c [145]. The power that was received wirelessly was stored in the supercapacitor. The supercapacitor was made of carbon electrodes and a solid-state polymer electrolyte, and it was formed in an arc-shape on the smart contact lens using a computer-assisted, direct ink writing (DIW) technique. The arc-shaped form provided the efficient storage of energy within the limited area of the contact lens, and it provided durable operation even when being stretched. After 45 s of wireless charging, the supercapacitor turned on the LED pixel placed on the contact lens and it stayed on for more than 60 s. Also, the capacitor showed a negligible change in its charging/discharging behavior, and it retained more than 90% of its original capacity after 30 cycles of charging/discharging, thereby indicating its long-term reliability and durability.