Han Eol Lee

Self-powered fully-flexible light-emitting system enabled by flexible energy harvester

Energy-harvesting technology utilising mechanical energy sources is a promising approach for the sustainable, independent, and permanent operation of a variety of flexible electronics. A new concept of a fully-flexible light-emitting system, self-powered by a high-performance piezoelectric thin-film energy harvester has been first established by manipulating highly-robust, flexible, vertically structured light emitting diodes (f-VLEDs). The f-VLEDs fabricated by anisotropic conductive film bonding and entire wafer etching show stable and durable performances during periodic mechanical deformations. A high-output energy harvester capable of generating up to 140 V and 10 μA can be fabricated via laser lift-off (LLO) process widely used in industries, in a safe and robust manner. In particular, this LLO process is of great benefit for the fabrication of mechanically stable, flexible piezoelectric devices, without causing any degradation of piezoelectric properties. In this process, self-powered all-flexible electronic system with light emittance can be spontaneously achieved by the electricity produced from flexible thin-film generator by applying slight biomechanical energy without any externally applied energy storage. This conceptual technology of self-powering based on the conversion of mechanical energy to electrical energy can open a facile and robust avenue for diverse, self-powered, bio-implantable applications, as well as commercial display applications.

High-Performance Flexible Thermoelectric Power Generator Using Laser Multiscanning Lift-Off Process

Flexible thermoelectric generators (f-TEGs) are emerging as a semipermanent power source for self-powered sensors, which is an important area of research for next-generation smart network monitoring systems in the Internet-of-things era. We report in this paper a f-TEG produced by a screen-printing technique (SPT) and a laser multiscanning (LMS) lift-off process. A screen-printed TEG was fabricated on a SiO2/a-Si/quartz substrate via the SPT process, and the LMS process completely separated the rigid quartz substrate from the original TEG by selective reaction of the XeCl excimer laser with the exfoliation layer (a-Si). Using these techniques, we fabricate a prototype f-TEG composed of an array of 72 TE couples that exhibits high flexibility at various bending radii, together with excellent output performance (4.78 mW/cm2 and 20.8 mW/g at ΔT = 25 °C). There is no significant change in the device performance even under repeated bending of 8000 cycles.

Laser–Material Interactions for Flexible Applications

The use of lasers for industrial, scientific, and medical applications has received an enormous amount of attention due to the advantageous ability of precise parameter control for heat transfer. Laser-beam-induced photothermal heating and reactions can modify nanomaterials such as nanoparticles, nanowires, and two-dimensional materials including graphene, in a controlled manner. There have been numerous efforts to incorporate lasers into advanced electronic processing, especially for inorganic-based flexible electronics. In order to resolve temperature issues with plastic substrates, laser-material processing has been adopted for various applications in flexible electronics including energy devices, processors, displays, and other peripheral electronic components. Here, recent advances in laser-material interactions for inorganic-based flexible applications with regard to both materials and processes are presented.

Optogenetic Mapping of Functional Connectivity in Freely Moving Mice via Insertable Wrapping Electrode Array Beneath the Skull

Spatiotemporal mapping of neural interactions through electrocorticography (ECoG) is the key to understanding brain functions and disorders. For the entire brain cortical areas, this approach has been challenging, especially in freely moving states, owing to the need for extensive craniotomy. Here, we introduce a flexible microelectrode array system, termed iWEBS, which can be inserted through a small cranial slit and stably wrap onto the curved cortical surface. Using iWEBS, we measured dynamic changes of signals across major cortical domains, namely, somatosensory, motor, visual and retrosplenial areas, in freely moving mice. iWEBS robustly displayed somatosensory evoked potentials (SEPs) in corresponding cortical areas to specific somatosensory stimuli. We also used iWEBS for mapping functional interactions between cortical areas in the propagation of spike-and-wave discharges (SWDs), the neurological marker of absence seizures, triggered by optogenetic inhibition of a specific thalamic nucleus. This demonstrates that iWEBS represents a significant improvement over conventional ECoG recording methodologies and, therefore, is a competitive recording system for mapping wide-range brain connectivity under various behavioral conditions.

Flash-Induced Stretchable Cu Conductor via Multiscale-Interfacial Couplings

Abstract Herein, a novel stretchable Cu conductor with excellent conductivity and stretchability is reported via the flash‐induced multiscale tuning of Cu and an elastomer interface. Microscale randomly wrinkled Cu (amplitude of ≈5 µm and wavelength of ≈45 µm) is formed on a polymer substrate through a single pulse of a millisecond flash light, enabling the elongation of Cu to exceed 20% regardless of the stretching direction. The nanoscale interlocked interface between the Cu nanoparticles (NPs) and the elastomer increases the adhesion force of Cu, which contributes to a significant improvement of the Cu stability and stretchability under harsh yielding stress. Simultaneously, the flash‐induced photoreduction of CuO NPs and subsequent Cu NP welding lead to outstanding conductivity (≈37 kS cm−1) of the buckled elastic electrode. The 3D structure of randomly wrinkled Cu is modeled by finite element analysis simulations to show that the flash‐activated stretchable Cu conductors can endure strain over 20% in all directions. Finally, the wrinkled Cu is utilized for wireless near‐field communication on the skin of human wrist.

Trichogenic Photostimulation Using Monolithic Flexible Vertical AlGaInP Light-Emitting Diodes

Alopecia is considered an aesthetic, psychological, and social issue among modern people. Although laser-induced skin stimulation is utilized for depilation treatment, such treatment has significant drawbacks of high energy consumption, huge equipment size, and limited usage in daily life. Here, we present a wearable photostimulator for hair-growth applications using high-performance flexible red vertical light-emitting diodes (f-VLEDs). Flexible microscale LEDs were effectively fabricated by a simple monolithic fabrication process, resulting in high light output (∼30 mW mm-2), low forward voltage (∼2.8 V), and excellent flexibility for wearable biostimulation. Finally, trichogenic stimulation of a hairless mouse was achieved using high-performance red f-VLEDs with high thermal stability, device uniformity, and mechanical durability.