Yong Whan Choi

Shape-Controllable Microlens Arrays via Direct Transfer of Photocurable Polymer Droplets

A simple method is presented to form an array of shape-controllable microlenses by partial photocuring of an UV-curable polymer and direct transfer. Using the transferred lens array, nanoscale metal patterns as small as 130-nm gaps are detected under an optical microscope with a distinguishable resolution.

Ultra-sensitive Pressure sensor based on guided straight mechanical cracks.

Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.

Transparent ITO mechanical crack-based pressure and strain sensor

Sensors to detect motion with high precision have been extensively studied in diverse engineering research fields. Among them, transparent devices, which have strong adaptability in various fields such as display panels, have not gained much academic interest. In this study, we present a highly sensitive pressure and strain sensor based on a cracked transparent epilayer, indium-tin oxide (ITO), deposited on a transparent PET substrate. This sensor system, with which we demonstrate how to detect pressure and finger motions, exhibits ultra-sensitivity to strain (gauge factor about 4000 at 2% strain), pressure (sensitivity is about 1.91 kPa−1 at pressures from 30 to 70 kPa), and transparency (up to 89% at a wavelength of 560 nm). Also, durability has been validated over 5000 cycles. The sensor thus boasts broad applications including touchscreens and motion detectors.

Robust microzip fastener: repeatable interlocking using polymeric rectangular parallelepiped arrays.

We report a highly repeatable and robust microzip fastener based on the van der Waals force-assisted interlocking between rectangular parallelepiped arrays. To investigate zipperlike interlocking behaviors, various line arrays were fabricated with three different spacing ratios (1, 3, and 5 of 800 nm in width) and width of parallelepipeds (400 nm, 800 nm, and 5 μm with the spacing ratio of 1). In addition, the different rigidity of line arrays was inspected for a repeatable microzip fastener. The normal and shear locking forces were measured with variation of the material rigidity as well as geometry of the array, in good agreement with a proposed theory based on the contact area and force balance. The maximum adhesion forces as high as ∼8.5 N cm(-2) in the normal direction and ∼29.6 N cm(-2) in the shear direction were obtained with high stability up to 1000 cycles. High stability of our fastening system was confirmed for preventing critical failures such as buckling and fracture in practical applications.

Facile Multiscale Patterning by Creep-Assisted Sequential Imprinting and Fuel Cell Application

The capability of fabricating multiscale structures with desired morphology and incorporating them into engineering applications is key to realizing technological breakthroughs by employing the benefits from both microscale and nanoscale morphology simultaneously. Here, we developed a facile patterning method to fabricate multiscale hierarchical structures by a novel approach called creep-assisted sequential imprinting. In this work, nanopatterning was first carried out by thermal imprint lithography above the glass transition temperature (Tg) of a polymer film, and then followed by creep-assisted imprinting with micropatterns based on the mechanical deformation of the polymer film under the relatively long-term exposure to mechanical stress at temperatures below the Tg of the polymer. The fabricated multiscale arrays exhibited excellent pattern uniformity over large areas. To demonstrate the usage of multiscale architectures, we incorporated the multiscale Nafion films into polymer electrolyte membrane fuel cell, and this device showed more than 10% higher performance than the conventional one. The enhancement was attributed to the decrease in mass transport resistance because of unique cone-shape morphology by creep-recovery effects and the increase in interfacial surface area between Nafion film and electrocatalyst layer.

Hybrid Microfabrication of Nanofiber-Based Sheets and Rods for Tissue Engineering Applications

Electrospun nanofibers have been developed into a variety of forms for tissue engineering scaffolds to regulate the cellular functions guided by nanotopographical cues. Here, we have successfully fabricated nanofiber-based scaffold complexes of rod and sheet type by combining the three microfabrication techniques of electrospinning, spin coating, and polymer melt deposition. It was demonstrated that this hybrid fabrication could produce uniaxially aligned nanofiber scaffolds supported by a thin film, allowing for a mechanically enforced substrate for cell culture as well as facile scaffold manipulation. The results of cell analysis indicated that nanofibers on spin-coated films could provide contact guidance effects on cells and retain them even after manipulation. As an application of the cell-laden nanofiber film, we built a rod-type structure by rolling up the film around a mechanically supporting core microfiber, which was incorporated by polymer melt deposition. A biocompatible and biodegradable polymer, polycaprolactone, was used throughout the processes and thus could be used as a directly implantable substitute in tissue regeneration.

Crack-based strain sensor with diverse metal films by inserting an inter-layer

Various sensory systems to detect human motions have been developed for wearable healthcare and artificial electronic skins. Recently, an ultrasensitive mechanical crack-based strain sensor inspired by a spiders slit organ has been proposed. In spite of its high sensitivity, flexibility, and fascinating sensing ability to vibration, the materials that can be used to manufacture the sensor are limited to certain kinds because of the low adhesion between the substrate and a metal film. Therefore, the compatibility of materials with the substrate is a crucial issue in developing a practical sensor system. Here, we present a mechanical crack-based strain sensor with diverse metal (Au, Ag and Pt) films by introducing an inter-layer. Two inter-layers are used; a Cr layer is for generating cracks and MoO3 layer for enhancing the adhesion between the substrate and the metal layer. When cracks are generated on the Cr layer, they are propagated to the conductive metal layers (Au, Ag and Pt). Our crack-based strain sensor exhibited reproducibility and durability with high sensitivity to strain (GF = ∼1600 for Au and Ag layered crack sensors at 2% strain, GF = ∼850 for Pt layered sensor at 2% strain).

Repetitive Cleavage of Elastomeric Membrane via Controlled Interfacial Fracture

Here, we report a method of fabricating thin layer of polydimethylsiloxane (PDMS), with a thickness in the range of 60-80 nm, which can be repeatedly generated (more than 10 times) from the same block of PDMS via controlled interfacial fracture. The thin layers can be transferred to various substrates by peeling off from the bulk PDMS. The cleavage is attributed to the built-in stress at the fracture interface due to plasma treatment, resulting in the repetitive formation of the thin membranes, with no residue from processing, and with a surface roughness of ∼5 nm. We were able to demonstrate transferred patterns with controlled thickness by varying the oxygen plasma treatment conditions and the composition of bulk PDMS stamp. Using the method, we achieved residual-free patterns with submicrometer resolution for applications in biomolecule array templates.

Photocurable PUA (Poly Urethaneacrylat) cantilever integrated with ultra-high sensitive crack-based sensor

This paper describes the fabrication and characterization of ultra-high sensitive polymer cantilever to precisely monitor the change in contraction force of cardiomyocytes. The mechanical crack-based sensor inspired from a spider showed enhanced sensitivity towards strain and vibration in nature. In spite of all these interesting characteristics, the crack-based sensor has not yet been used for biomedical applications such as detecting a small force generated from cells. Herein we made successful attempt to develop a novel sensing technique for measuring the contraction force of cardiomyocytes. This idea can be applied for the development of cantilever-based cardiac toxicity screening systems.