Professor Takakuni Douseki, at the College of Science and Engineering, Ritsumeikan University, has developed a wireless power transmitter for wearable devices. The new power transmitter controls the transmission of electric power in response to varying distance from a portable power receiver.

As part of a joint research project with Shiseido Company, Limited, Prof. Douseki demonstrated that the power transmitter turns on a light of a false eyelash fitted with an ultra-small power receiver without means of a battery or wiring at a distance of 1.5 m. The receiver integrates fiber optics, a rectenna and an LED light.

The research team aims to further improve the “light-emitting false eyelashes” and hopes to make the ultra-small wearable device a popular cosmetic accessory. The technology is expected to find application not only in false eyelashes but in other wearable devices too.

Read on for more technical details…

The newly-developed power transmitter measures a distance to a human body with a distance sensor using ultrasonic technology and thereby controls the transmission of electric power through a CPU in response to the measured distance. This configuration allows the power transmitter to emit controlled electromagnetic waves with an electric field strength below that of national protection guidelines, whilst maintaining a viable electric power supply to a target device; remaining effective even if the distance between the human body and the power transmitter changes.

The power receiver includes a built in LED, luminous optical fiber (1 mm in diameter and 30 mm in length), and a rectenna (a combination of an antenna, matching circuit, and rectifier circuit). Prof. Douseki fitted the integrated power receiver on a false eyelash and implemented the "light-emitting false eyelash", an ultra-small wearable device, through the joint research.

If a wearable device is wirelessly supplied with electricity by electromagnetic waves from a source at a short distance of several meters, a user can use the wearable device all the time in an environment of the electromagnetic waves without worrying about battery exhaustion.

Proximity-type wireless electricity supply systems have been implemented previously, in which a wearable device has to come close to a power source so that a secondary battery in the device is charged by a coil- or capacitor-dependent change in magnetic or electric fields. However, no technology has been put to practical use in terms of supplying electric power to a device at a short distance by electromagnetic waves so as to directly operate it. Wirelessly supplying electricity at long distance by electromagnetic waves requires an increase in transmitted electric power. Meanwhile, a human body wearing a wearable device moves and if the human body comes close to a power transmitter, the electric field strength unfavorably exceeds a level of 61.4 V/m specified in the guidelines for human body protection against electromagnetic waves. Another challenge is that rectennas incorporated in power receivers need to be smaller with a decrease in the size of wearable devices.

The tests verified that the LED in the device fitted on the false eyelash without battery and wiring turns on with 10 W of electric power transmitted from a distance of 1.5 m. The tests also verified that the brightness of the LED does not alter with a change in the distance between the power transmitter and the power receiver.

Research results were announced at IEEE SENSORS 2017, an international conference held in Glasgow, Scotland, the U.K. in October 2017.