Hyunsik Yoon

In this review, we highlight the properties, functions and applications of stimuli-responsive hydrogel patterns in bioanalytical applications. Stimuli-responsive hydrogel patterns can be realized by well-established micro- and nanofabrication technologies such as photolithography and micromolding, and are currently adopted as active components for manipulation of flow and biosamples in microchannel and microarray systems. We overview the properties of stimuli-responsive hydrogel materials and th

Capillary force lithography (CFL) with impermeable mold is presented. For the CFL to be operative over large area, either the mold or the substrate has to be flexible. With a silicon wafer mold and a flexible substrate, a repeated line and space pattern with a spacing of 30nm is shown to be well patterned. With a flexible mold and a hard substrate, a similar pattern with a spacing of 60nm is demonstrated by CFL. The flexibility is needed for the intimate contact that is required between the mold

Water‐repelling surfaces inspired by lotus leaves have been developed for their commercial needs in superhydrophobic and self‐cleaning coatings on glasses and windows. The extraordinary properties originate from their multiscale structures with waxy materials. To obtain high transparency as well as superhydrophobicity, microhair arrays are designed with large spacing to reduce optical scattering effects caused by microstructures, but with a trilevel hierarchical structure to compensate for the l

We raise issues regarding the 3D printing of complex structures using UV-curable materials. Models of failures based on the transparency of the UV-curable materials, high absorption not reaching the upper parts, and mechanical failure are discussed.

We present a simple approach to reversibly switch the direction of liquid flow on physically symmetric and chemically asymmetric prism structures by exploiting the reversibility of surface wetting properties of a thermo-responsive polymer, poly(N-isopropyl-acrylamide). Such an asymmetric prism array creates a flow path in the direction of the lower critical contact angle. This allows a unidirectional “step flow” across the ridges of prism channels, which can be made reversible with a suitable te

The morphology of conjugated polymers has critical influences on electronic and optical properties of optoelectronic devices. Even though lots of techniques and methods are suggested to control the morphology of polymers, very few studies have been performed inducing high charge transport along out-of-plane direction. In this study, the self-assembly of homo- and blended conjugated polymers which are confined in nanostructures is utilized. The resulting structures lead to high charge mobility al

We introduce a one-step procedure of bending nanopillars, which simply involves oblique metal deposition at a tilted angle of 45 degrees on the pillars by thermal evaporation. The face selection in the bending procedure was determined by the nature of residual stress generated in the metal film during evaporation. If the stress was tensile as with many metals (sigma(f) > 0), the Janus nanopillars were bent toward the metal face; if the residual stress was compressive as in the case of Al (sigma(

Abstract Superomniphobic surfaces inspired by nature have been studied for decades. Recently, the development of liquid‐repelling surfaces has moved from the fabrication of artificial structures to real applications that address friction associated with clothes, paper, and skin. To have superoleophobicity, re‐entrant structures such as mushrooms or inverse trapezoids have been suggested. However they can be mechanically fragile, especially under shear stress, because the bottom region is narrow.

Microneedles (MNs) are micron-sized needles that can penetrate the stratum corneum, enabling the non-invasive and painless administration of drugs and vaccines. In this work, fabrication conditions for high-aspect-ratio MNs by the photopolymerization of polyethylene glycol diacrylate (PEGDA) were investigated. Ultraviolet (UV) light was used to crosslink photocurable prepolymers in specific areas defined by a photomask. The aspect ratio of solidified MNs is too small to penetrate the stratum cor

Abstract The camouflage used by cephalopods is an interesting topic in biomimetics. Squid, part of the cephalopod family, have transformable skin that can be made transparent or darkened through control of the light‐absorption area. This is achieved using a muscular structure. A smart‐window scheme inspired by this is developed. Magnetic nanopigments dispersed within an asymmetric pyramidal array are used and the light‐absorption area is manipulated through use of a magnetic field. Refractive‐in

Sub-100nm structures can be fabricated in tens of seconds with an aspect ratio much larger than unity by the general purpose patterning method presented here. A flexible film mold and a rapid flash heating with an infrared lamp are used in this nonphotolithographic patterning technique. Unlike other unconventional methods, the substrate surface can be made exposed and the resulting pattern height is sufficiently high for subsequent etching of the substrate.

The lower part of polymeric nanopillars is made mechanically stiff by surrounding it with metal, while the top part of the pillars is soft. The metal-shell-free tops of the pillars could be used for fabricating hollow structures or decorated with functional materials. This mechanical reinforcement shows stability against capillary-force-induced clustering and enables applications in a water environment.

An asymmetric ratchet structure within microchannels is demonstrated by directionally guided light transmission for controlled liquid flow. A direct and facile method is presented to realize programmed asymmetric structures, which control the fluid direction and speed.