F-VLEDs Reduce Heat Generation, Opening Way for Biomedical Applications

Flexible vertical micro-LEDs (f-VLEDs) have been developed using anisotropic conductive film (ACF)-based transfer and interconnection technology. Thermal reliability and lifetime were improved by reducing heat generation within the thin film LEDs.

Researchers from the Korea Advanced Institute of Science and Technology (KAIST) fabricated a f-VLED array using simultaneous transfer and interconnection, which was possible through the precise alignment of an ACF bonding process. These f-VLEDs, which were 5-µm thick and less than 80-µm large, achieved an optical power density about three times higher than that of lateral micro LEDs. Using optogenetic stimulation of the f-VLEDs, the researchers succeeded in controlling behavior in mouse models. Specifically, the f-VLEDs showed an optical power density of more than 25 mW/mm² — enough to stimulate the mouse motor neurons below layer III.

This is comparison of micro-LEDs technology. Courtesy of KAIST.

Researchers inserted the f-VLEDs into the narrow space between the skull and the brain surface of the mice. The f-VLEDs illuminated neurons on the frontal motor cortex of the mice, enabling researchers to control body movements with minimal tissue damage.

While several biomedical tools have used flexible optoelectronic devices combined with optogenetic mouse models to modulate specific brain activity, most current applications are limited to activation of small functional regions using blue-light driven channelrhodopsin. The KAIST team introduced f-VLEDs for perturbation of specific functional areas of mouse cortex.

Micro-scaled LEDs effectively compressed the conductive balls dispersed in ACF, resulting in red light emissions with high optical power density, capable of stimulating motor neurons deep below the brain surface. Selective operation of pulsed red light from f-VLEDs induced mouse body movements and synchronized electromyogram (EMG) signals. The expression of chrimson, a red-shifted channelrhodopsin, enabled red-light excitation of targeted functional areas of the motor cortex.

These experiments could open new opportunities for entire cortical mapping that would allow exploration of the connectivity between undefined motor areas in the mouse brain.

The advancement of micro-LED technology has been impeded by issues that include poor device efficiency, low thermal reliability, and the lack of interconnection technology for high-resolution micro LED displays.

“The flexible vertical micro LED can be used in low-power smart watches, mobile displays and wearable lighting," professor Keon Jae Lee said. "In addition, these flexible optoelectronic devices are suitable for biomedical applications such as brain science, phototherapeutic treatment and contact lens biosensors.”

The research was published in Nano Energy (doi: 10.1016/j.nanoen.2017.12.011).

Professor Lee's team developed flexible vertical micro LEDs using anisotropic ACF-based transfer and interconnection technology. Courtesy of KAIST.