Fig. 2: Electrical characteristics of wireless rechargeable optoelectronic systems.

a, b Schematic diagram (a) and simulated magnetic field density (B) (b) of a rat cage (39.6 cm (W) × 34.6 cm (L) × 21.3 cm (H)) with three loop antennas (top, side, and bottom antenna). c Measurement of rectified voltage (blue line) and power delivered to the load (red line) of a wireless device with different load resistances (from 7–30 kΩ), which was placed at the center with a 6 cm height above the ground of the rat cage. The maximum output power (~5.3 mW) was obtained with a 4.7 kΩ load resistance, when an input power of 12.5 W was transmitted. d Normalized power at the output load (4.7 kΩ) of wireless devices at various locations (center, edge, and corner), heights (3, 6, 9, and 12 cm) and orientations (0°, 30°, 60°, and 80°) inside a rat cage. e Variation of energy-harvesting efficiency with respect to location and angular orientation of a device at a height of 6 cm in the rat cage. f Wireless battery charging characteristics in saline water (0.9%) at various heights (0, 3, 6, 9, and 12 cm) for the device located at the center (left) and the corner (right) of the cage. Battery charging was started after the battery was fully discharged. g Real-time monitoring of the battery level during the wireless closed-loop auto-charging operation of the optoelectronic system in two different sequential scenarios: wireless auto-charging (i) without and (іі) with LED operation. When the battery voltage level reaches the preset maximum, charging is automatically turned off temporarily for 10 s to confirm whether the battery is actually fully charged (see small dimple in the recorded signal at 22 min). The small fluctuation of the battery voltage level appearing between 58 and 80 min is due to electrical noise caused by the LED operation. The proof-of-principle operation was carried out with a device immersed in saline water (0.9%), which was located at the center of the rat cage floor.