Hoon Eui Jeong

A nontransferring dry adhesive with hierarchical polymer nanohairs

We present a simple yet robust method for fabricating angled, hierarchically patterned high-aspect-ratio polymer nanohairs to generate directionally sensitive dry adhesives. The slanted polymeric nanostructures were molded from an etched polySi substrate containing slanted nanoholes. An angled etching technique was developed to fabricate slanted nanoholes with flat tips by inserting an etch-stop layer of silicon dioxide. This unique etching method was equipped with a Faraday cage system to control the ion-incident angles in the conventional plasma etching system. The polymeric nanohairs were fabricated with tailored leaning angles, sizes, tip shapes, and hierarchical structures. As a result of controlled leaning angle and bulged flat top of the nanohairs, the replicated, slanted nanohairs showed excellent directional adhesion, exhibiting strong shear attachment (≈26 N/cm2 in maximum) in the angled direction and easy detachment (≈2.2 N/cm2) in the opposite direction, with a hysteresis value of ≈10. In addition to single scale nanohairs, monolithic, micro-nanoscale combined hierarchical hairs were also fabricated by using a 2-step UV-assisted molding technique. These hierarchical nanoscale patterns maintained their adhesive force even on a rough surface (roughness <20 μm) because of an increase in the contact area by the enhanced height of hierarchy, whereas simple nanohairs lost their adhesion strength. To demonstrate the potential applications of the adhesive patch, the dry adhesive was used to transport a large-area glass (47.5 × 37.5 cm2, second-generation TFT-LCD glass), which could replace the current electrostatic transport/holding system with further optimization.

Plasmon-enhanced photocatalytic activity of iron oxide on gold nanopillars.

Photocatalytic water splitting represents a promising way to produce renewable hydrogen fuel from solar energy. Ultrathin semiconductor electrodes for water splitting are of particular interest because the optical absorption occurs in the region where photogenerated charge carriers can effectively contribute to the chemical reactions on the surface. It is therefore important to manipulate and concentrate the incident light so that more photons can be absorbed within the thin film. Here we show an enhanced photocurrent in a thin-film iron oxide photoanode coated on arrays of Au nanopillars. The enhancement can be attributed primarily to the increased optical absorption originating from both surface plasmon resonances and photonic-mode light trapping in the nanostructured topography. The resonances can be tuned to a desirable wavelength by varying the thickness of the iron oxide layer. A net enhancement as high as 50% was observed over the solar spectrum.

Si/InGaN Core/Shell Hierarchical Nanowire Arrays and their Photoelectrochemical Properties

Three-dimensional hierarchical nanostructures were synthesized by the halide chemical vapor deposition of InGaN nanowires on Si wire arrays. Single phase InGaN nanowires grew vertically on the sidewalls of Si wires and acted as a high surface area photoanode for solar water splitting. Electrochemical measurements showed that the photocurrent density with hierarchical Si/InGaN nanowire arrays increased by 5 times compared to the photocurrent density with InGaN nanowire arrays grown on planar Si (1.23 V vs RHE). High-resolution transmission electron microscopy showed that InGaN nanowires are stable after 15 h of illumination. These measurements show that Si/InGaN hierarchical nanostructures are a viable high surface area electrode geometry for solar water splitting.

Rational design and enhanced biocompatibility of a dry adhesive medical skin patch.

This work was supported by National Research Foundation of Korea (NRF) grant (No. 20110017530), WCU (World Class University) program (R31-2008-000-10083-0) on multiscale mechanical design, and Basic Science Research Program (2010-0027955) funded by the Ministry of Education, Science, and Technology (MEST). This work was supported in part by the Award No KUK-F1-037-02, made by King Abdullah University of Science and Technology (KAUST) and Institute of Advanced Machinery and Design (IAMD) of Seoul National University.

Stretchable, Adhesion-Tunable Dry Adhesive by Surface Wrinkling

We introduce a simple yet robust method of fabricating a stretchable, adhesion-tunable dry adhesive by combining replica molding and surface wrinkling. By utilizing a thin, wrinkled polydimethyl siloxane (PDMS) sheet with a thickness of 1 mm with built-in micropillars, active, dynamic control of normal and shear adhesion was achieved. Relatively strong normal (approximately 10.8 N/cm(2)) and shear adhesion (approximately 14.7 N/cm(2)) forces could be obtained for a fully extended (strained) PDMS sheet (prestrain of approximately 3%), whereas the forces could be rapidly reduced to nearly zero once the prestrain was released (prestrain of approximately 0.5%). Moreover, durability tests demonstrated that the adhesion strength in both the normal and shear directions was maintained over more than 100 cycles of attachment and detachment.

25th Anniversary Article: Scalable Multiscale Patterned Structures Inspired by Nature: the Role of Hierarchy

Multiscale, hierarchically patterned surfaces, such as lotus leaves, butterfly wings, adhesion pads of gecko lizards are abundantly found in nature, where microstructures are usually used to strengthen the mechanical stability while nanostructures offer the main functionality, i.e., wettability, structural color, or dry adhesion. To emulate such hierarchical structures in nature, multiscale, multilevel patterning has been extensively utilized for the last few decades towards various applications ranging from wetting control, structural colors, to tissue scaffolds. In this review, we highlight recent advances in scalable multiscale patterning to bring about improved functions that can even surpass those found in nature, with particular focus on the analogy between natural and synthetic architectures in terms of the role of different length scales. This review is organized into four sections. First, the role and importance of multiscale, hierarchical structures is described with four representative examples. Second, recent achievements in multiscale patterning are introduced with their strengths and weaknesses. Third, four application areas of wetting control, dry adhesives, selectively filtrating membranes, and multiscale tissue scaffolds are overviewed by stressing out how and why multiscale structures need to be incorporated to carry out their performances. Finally, we present future directions and challenges for scalable, multiscale patterned surfaces.

Bio-inspired slanted polymer nanohairs for anisotropic wetting and directional dry adhesion

Slanted polymer nanohairs possess a number of attractive properties in terms of anisotropic wetting and directional adhesion. This highlight provides an overview of the recent progress in the development of bio-inspired slanted polymer nanohairs and their applications towards anisotropic wetting and directional dry adhesion properties. With the advanced nano-fabrication techniques, it is possible to fabricate angled, directionally bent polymer nanohairs in a highly reproducible and geometry-controllable manner. The fabrication methods can be categorized into two streams: direct replica molding from a master with slanted structure or nanofabrication (photolithography or molding) with post treatment such as e-beam exposure, thermal annealing and mechanical compression. In this highlight, the fabrication methods for angled, high aspect ratio polymer nanohairs are briefly described along with their potential applications in anisotropic wetting and directional adhesion. Particular emphasis is given to recent achievements and future directions in biomimetic functional surfaces.

Wetting transition and optimal design for microstructured surfaces with hydrophobic and hydrophilic materials

We present wetting transition of a water droplet on microstructured polymer surfaces using materials with different hydrophilicity or hydrophobicity: hydrophobic polydimethyl siloxane (PDMS) (theta(water) approximately 110 degrees) and hydrophilic Norland Optical Adhesive (NOA) (theta(water) approximately 70 degrees). The microstructures were fabricated by replica molding and self-replication with varying pillar geometry [diameter: 5 microm, spacing-to-diameter ratio (s/d): 1-10 (equal interval), height-to-diameter ratio (h/d): 1-5] over an area of 100 mm(2) (10 mm x 10 mm). Measurements of contact angle (CA) and contact angle hysteresis (CAH) demonstrated that wetting state was either in the homogeneous Cassie regime or in the mixed regime of Cassie and Wenzel states depending on the values of s/d and h/d. These two ratios need to be adjusted to maintain stable superhydrophobic properties in the Cassie regime; s/d should be smaller than approximately 7 (PDMS) and approximately 6 (NOA) with h/d being larger than approximately 2 to avoid wetting transition by collapse of a water droplet into the microstructure. Based on our observations, optimal design parameters were derived to achieve robust hydrophobicity of a microstructured surface with hydrophobic and hydrophilic materials.

Generation and Self‐Replication of Monolithic, Dual‐Scale Polymer Structures by Two‐Step Capillary‐Force Lithography

Inhibition effects in UV radiation curing by oxygen were utilized for fabricating monolithic, micro/nanoscale hierarchical polymer structures via two-step UVassisted capillary force lithography (CFL). It was found that the UV exposure time for the partial curing of microstructure was a crucial parameter; a shorter exposure time induced collapse of the underlying microstructure while a longer time gave rise to non-fluidity of the microstructure. The partial curing is attributed to inhibition of UV crosslinking by trapped or permeated oxygen within mold cavities. Using this method, various dual-scale hierarchical structures were fabricated with minimum resolution to 50 nm over a large area (5×5 cm 2 ) in a fast and reproducible manner.

Anisotropic Adhesion Properties of Triangular‐Tip‐Shaped Micropillars

Directional dry adhesive microstructures consisting of high-density triangular-tip-shaped micropillars are described. The wide-tip structures allow for unique directional shear adhesion properties with respect to the peeling direction, along with relatively high normal adhesion.