Dongha Tahk

Fabrication of antireflection and antifogging polymer sheet by partial photopolymerization and dry etching.

We present a simple method to fabricate a polymer optical sheet with antireflection and antifogging properties. The method consists of two consecutive steps: photocross-linking of UV-curable polyurethane acrylate (PUA) resin and reactive ion etching (RIE). During photopolymerization, the cured PUA film is divided into two domains of randomly distributed macromers and oligomers due to a relatively short exposure time of 20 s at ambient conditions. Using the macromer domain as an etch-mask, dry etching was subsequently carried out to remove the oligomer domain, leaving behind a nanoturf surface with tunable roughness. UV-vis spectroscopy measurements demonstrate that transmittance of a nanoturf surface is enhanced up to 92.5% as compared to a flat PUA surface (89.5%). In addition, measurements of contact angle (CA) reveal that the etched surface shows superhydrophilicity with a CA as small as 5 degrees. To seek potential applications, I-V characteristics of a thin film organic solar cell were measured under various testing conditions. It is shown that the efficiency can be increased to 2.9% when a nanoturf film with the surface roughness of 34.73 nm is attached to indium tin oxide (ITO) glass. More importantly, the performance is maintained even in the presence of water owing to superhydrophilic nature of the film.

Wettability-Controllable Super Water- and Moderately Oil-Repellent Surface Fabricated by Wet Chemical Etching

A facile fabrication method is presented for a super-repellent surface, in which a silicon wafer is etched with a wet chemical method and treated by a fluorinated self-assembled monolayer. This surface is composed of harshly rough nanostructures and highly dense nanoholes. The contact angle of both water and oil with the surface is larger than 150 degrees. The self-cleaning capability of the surface allows for the removal of sticky powders with glycerin droplet. Any desired part(s) of the super-repellent surface can be turned superamphiphilic by simply exposing the desired part(s) to ultraviolet light.

Continuous and Scalable Fabrication of Bioinspired Dry Adhesives via a Roll-to-Roll Process with Modulated Ultraviolet-Curable Resin

A simple yet scalable strategy for fabricating dry adhesives with mushroom-shaped micropillars is achieved by a combination of the roll-to-roll process and modulated UV-curable elastic poly(urethane acrylate) (e-PUA) resin. The e-PUA combines the major benefits of commercial PUA and poly(dimethylsiloxane) (PDMS). It not only can be cured within a few seconds like commercial PUA but also possesses good mechanical properties comparable to those of PDMS. A roll-type fabrication system equipped with a rollable mold and a UV exposure unit is also developed for the continuous process. By integrating the roll-to-roll process with the e-PUA, dry adhesives with spatulate tips in the form of a thin flexible film can be generated in a highly continuous and scalable manner. The fabricated dry adhesives with mushroom-shaped microstructures exhibit a strong pull-off strength of up to ∼38.7 N cm(-2) on the glass surface as well as high durability without any noticeable degradation. Furthermore, an automated substrate transportation system equipped with the dry adhesives can transport a 300 mm Si wafer over 10,000 repeating cycles with high accuracy.

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.

A Low Permeability Microfluidic Blood-Brain Barrier Platform with Direct Contact between Perfusable Vascular Network and Astrocytes

A novel three dimensional blood brain barrier (BBB) platform was developed by independently supplying different types of media to separate cell types within a single device. One channel (vascular channel, VC) is connected to the inner lumen of the vascular network while the other supplies media to the neural cells (neural channel, NC). Compared to co-cultures supplied with only one type of medium (or 1:1 mixture), best barrier properties and viability were obtained with culturing HUVECs with endothelial growth medium (EGM) and neural cells with neurobasal medium supplemented with fetal bovine serum (NBMFBS) independently. The measured vascular network permeability were comparable to reported in vivo values (20 kDa FITC-dextran, 0.45 ± 0.11 × 10−6 cm/s; 70 kDa FITC-dextran, 0.36 ± 0.05 × 10−6 cm/s) and a higher degree of neurovascular interfacing (astrocytic contact with the vascular network, GFAP-CD31 stain overlap) and presence of synapses (stained with synaptophysin). The BBB platform can dependably imitate the perivascular network morphology and synaptic structures characteristic of the NVU. This microfluidic BBB model can find applications in screening pharmaceuticals that target the brain for in neurodegenerative diseases.

Conformal phase masks made of polyurethane acrylate with optimized elastic modulus for 3D nanopatterning

The pattern resolution of soft lithographic techniques is critically determined by the elastic modulus of the soft mold that can support fine and high-aspect-ratio features with conformal adhesion to target substrates. We present a strategy to fine-tune the elastic modulus of conformal molds made of polyurethane acrylate by optimizing the chemical structures and the composition of prepolymer and modulator. Trimethylolpropane ethoxylated (15) triacrylate plays a key role as a delicate modulator for increasing the elastic modulus of soft aliphatic urethane diacrylate oligomer with its low cross-linking density. The optimized molds have sufficiently high elastic modulus (>23 MPa) for defect-free replication of dense, high-aspect-ratio (>2) nanopillars and nanotrench structures while still preserving their conformality. The conformal mold with good mechanical and optical properties can serve as a semi-permanently usable optical phase mask with a wide range of phase modulations for generating three-dimensional (3D) nanostructures with high precision.

Fabrication of a hierarchical structure by oxygen plasma etching of a photocured microstructure containing a silicon moiety

We present a simple and straightforward method for creating a hierarchical structure in which the nanoscale roughness is derived from a microstructure. This hierarchical structure is at the backbone of almost all biomimetic functions. A liquid blend of a photocurable prepolymer and a functionalized polysiloxane is moulded by photocuring, and then the moulded film is simply exposed to a blanket oxygen plasma to produce the hierarchical structure. The nanoscale roughness is controlled by varying the weight ratio of acrylate-functionalized polysiloxane to acrylated prepolymer. To demonstrate the efficacy of the fabrication method, a superhydrophobic surface was produced by coating the hierarchical structure with a self-assembled monolayer (SAM).

Spontaneous dewetting-induced residue-free patterning at room temperature

A lithographic patterning method is presented that is based on dewetting induced by sequential molding under an applied pressure. Because of spontaneous dewetting taking place, the window to be opened is free from any residue and the surface exposure is instantaneously assured. This residue-free patterning can be accomplished without any heating process and surface treatment, irrespective of pattern duty ratio. The residue-free patterning is made possible with the use of a rigiflex mold and a roller that is used to bring about pressure-induced thinning leading to spontaneous dewetting. A necessary condition for the method is that the spreading coefficient of spin-coated liquid be negative. The exposed surface can be utilized as a sacrificial layer for etching of underlying layer and/or thin film deposition in a fabrication of electronic and biological devices.

Design rules for a tunable merged-tip microneedle

This publication proposes the use of an elasto-capillarity-driven self-assembly for fabricating a microscale merged-tip structure out of a variety of biocompatible UV-curable polymers for use as a microneedle platform. In addition, the novel merged-tip microstructure constitutes a new class of microneedles, which incorporates the convergence of biocompatible polymer micropillars, leading to the formation of a sharp tip and an open cavity capable of both liquid trapping and volume control. When combined with biocompatible photopolymer micropillar arrays fabricated with photolithography, elasto-capillarity-driven self-assembly provides a means for producing a complex microneedle-like structure without the use of micromolding or micromachining. This publication also explores and defines the design rules by which several fabrication aspects, such as micropillar dimensions, shapes, pattern array configurations, and materials, can be manipulated to produce a customizable microneedle array with controllable cavity volumes, fracture points, and merge profiles. In addition, the incorporation of a modular through-hole micropore membrane base was also investigated as a method for constitutive payload delivery and fluid-sampling functionalities. The flexibility and fabrication simplicity of the merged-tip microneedle platform holds promise in transdermal drug delivery applications.Drug delivery: self-assembly of microneedlesThe fabrication of merged-tip microneedles by elasto-capillarity-driven self-assembly is investigated in photocurable polymers, revealing design parameters for their potential use in drug delivery. Microneedles can be used for delivery of drug payloads by perforating the skin. However, existing needle fabrication processes typically rely on expensive approaches such as etching, embossing, and molding. Now, a team from Seoul National University led by Noo Li Jeon, explore design parameters for fabrication by elastic-capillary-driven self-assembly: photolithography is used to fabricate closely spaced polymer micropillars, the tips of which crosslink to form a microneedle. A number of microneedle geometries are studied, for different UV-curable polymers, suggesting the possibility of their use in drug delivery.

Surface energy-tunable iso decyl acrylate based molds for low pressure-nanoimprint lithography

We presented surface energy-tunable nanoscale molds for unconventional lithography. The mold is highly robust, transparent, has a minimized haze, does not contain additives, and is a non-fluorinated isodecyl acrylate and trimethylolpropane triacrylate based polymer. By changing the mixing ratio of the polymer components, the cross-linking density, mechanical modulus, and surface energy (crucial factors in low pressure ((1-2) × 105 N m-2) low pressure-nanoimprint lithography (LP-NIL)), can be controlled. To verify these properties of the molds, we also characterized the surface energy by measuring the contact angles and calculating the work of adhesion among the wafer, polymer film, and mold for successful demolding in nanoscale structures. Moreover, the molds showed high optical clarity and precisely tunable mechanical and surface properties, capable of replicating sub-100 nm patterns by thermal LP-NIL and UV-NIL.