A KAIST research team has developed flexible vertical micro LEDs (f-VLEDs) using anisotropic conductive film (ACF)-based transfer and interconnection technology. The team, led by Professor Keon Jae Lee from the Department of Materials Science and Engineering and Professor Daesoo Kim from the Department of Biological Sciences, also succeeded in controlling animal behavior via optogenetic stimulation of the f-VLEDs.

The flexible micro LEDs show excellent flexibility and retain their optical performance under bent and twisted conditions. (Credit: KAIST)

“The flexible vertical micro LED can be used in low-power smart watches, mobile displays, and wearable lighting,” says Lee. “In addition, these flexible optoelectronic devices are suitable for biomedical applications such as brain science, phototherapeutic treatment, and contact lens biosensors.” The optogenetic LED stimulator could also facilitate the exploration of unknown brain functions and provide a therapeutic tool for neurological behavior disorders.

Flexible micro LEDs have become a strong candidate for the next generation of displays due to their ultra-low power consumption, fast response speed, and excellent flexibility. However, the previous micro LED technology had critical issues such as poor device efficiency, low thermal reliability, and lack of interconnection technology for high-resolution micro LED displays.

The team designed new transfer equipment and fabricated the f-VLED array (50 × 50) using simultaneous transfer and interconnection through the precise alignment of ACF bonding process. These f-VLEDs (thickness: 5 μm, size: below 80 μm) achieved optical power density (30 mW/mm2) — three times higher than that of lateral micro LEDs, improving thermal reliability and lifetime by reducing heat generation within the thin film LEDs.

Fig. 1 - Comparison of μ-LED technologies. (Credit: KAIST)

The f-VLEDs can be applied to optogenetics for controlling the behavior of neurons in brains. In contrast to the electrical stimulation that activates all of the neurons in the brain, optogenetics can stimulate specific excitatory or inhibitory neurons within the localized cortical areas of the brain, which facilitates precise analysis, high-resolution mapping, and neuron modulation of animal brains. (See the researchers’ previous ACS Nano paper, “Optogenetic Mapping of Functional Connectivity in Freely Moving Mice via Insertable Wrapping Electrode Array Beneath the Skull.”)

In their current work, they inserted the innovative f-VLEDs into the narrow space between the skull and the brain surface and succeeded in controlling mouse behavior by illuminating motor neurons on two-dimensional cortical areas located deep below the brain surface. Study results were published in their paper, “Optogenetic Control of Body Movements via Flexible Vertical Light-Emitting Diodes on Brain Surface” in the February 2018 issue of Nano Energy.

Lee has established a startup company called Fronics Inc. based on the micro LED technology. He is looking for global partnerships for commercialization.

A video of the technology is available here . For more Information, visit here .