Seung-Man Yang

Nanowire-based single-cell endoscopy

One-dimensional smart probes based on nanowires and nanotubes that can safely penetrate the plasma membrane and enter biological cells are potentially useful in high-resolution and high-throughput gene and drug delivery, biosensing and single-cell electrophysiology. However, using such probes for optical communication across the cellular membrane at the subwavelength level remains limited. Here, we show that a nanowire waveguide attached to the tapered tip of an optical fibre can guide visible light into intracellular compartments of a living mammalian cell, and can also detect optical signals from subcellular regions with high spatial resolution. Furthermore, we show that through light-activated mechanisms the endoscope can deliver payloads into cells with spatial and temporal specificity. Moreover, insertion of the endoscope into cells and illumination of the guided laser did not induce any significant toxicity in the cells.

Synthesis and assembly of structured colloidal particles

Synthesis and self-assembly of structured colloids is a nascent field. Recent advances in this area include the development of a variety of practical routes to produce robust photonic band-gap materials, colloidal lithography for nanopatterns, and hierarchically structured porous materials with high surface-to-volume ratios for catalyst supports. To improve their properties, non-conventional suprastructures have been proposed, which could be built up using binary or bimodal mixtures of spherical particles and particles with internal or surface nanostructures. This Feature Article will describe the state-of-the-art in colloidal particles and their assemblies. The paper consists of three main sections categorized by the type of colloid, namely shape-anisotropic particles, chemically patterned particles and internally structured particles. In each section, we will discuss not only synthetic routes to uniform colloids with a range of structures, features and shapes, but also self-organization of these colloids into macrocrystalline structures with varying nanoscopic features and functionalities. Finally, we will outline future perspectives for these colloidal suprastructures.

Characterizing and tracking single colloidal particles with video holographic microscopy

We use digital holographic microscopy and Mie scattering theory to simultaneously characterize and track individual colloidal particles. Each holographic snapshot provides enough information to measure a colloidal spheres radius and refractive index to within 1%, and simultaneously to measure its three-dimensional position with nanometer in-plane precision and 10 nanometer axial resolution.

Absorption of carbon dioxide through hollow fiber membranes using various aqueous absorbents

Abstract Microporous membrane absorbers were used for the separation of carbon dioxide–nitrogen mixtures. The membranes applied were made of polytetrafluoroethylene (PTFE) and the absorbents included aqueous solutions of different kinds of amines. We determined the separation efficiency and the overall mass transfer coefficient of carbon dioxide of the membrane modules, and evaluated the separation performance under different absorbent concentrations and temperatures. Absorption rates through the PTFE hollow fiber membranes were measured at temperatures ranging from 2 to 60°C, and the removal rate of carbon dioxide was found to increase with the increase of the volumetric flow rate of an absorbent. We also conducted a theoretical model analysis to predict the separation efficiency. The results showed that the theoretical model agreed well with the experimental observation for the case of aqueous amine solutions.

Flexible, Angle-Independent, Structural Color Reflectors Inspired by Morpho Butterfly Wings

Thin-film color reflectors inspired by Morpho butterflies are fabricated. Using a combination of directional deposition, silica microspheres with a wide size distribution, and a PDMS (polydimethylsiloxane) encasing, a large, flexible reflector is created that actually provides better angle-independent color characteristics than Morpho butterflies and which can even be bent and folded freely without losing its Morpho-mimetic photonic properties.

Optofluidic Encapsulation of Crystalline Colloidal Arrays into Spherical Membrane

Double emulsion droplets encapsulating crystalline colloidal arrays (CCAs) with a narrow size distribution were produced using an optofluidic device. The shell phase of the double emulsion was a photocurable resin that was photopolymerized downstream of the fluidic channel within 1 s after drop generation. The present optofluidic synthesis scheme was very effective for fabricating highly monodisperse spherical CCAs that were made structurally stable by in situ photopolymerization of the encapsulating shells. The shell thickness and the number of core emulsion drops could be controlled by varying the flow rates of the three coflowing streams in the dripping regime. The spherical CCAs confined in the shell exhibited distinct diffraction patterns in the visible range, in contrast to conventional film-type CCAs. As a result of their structure, the spherical CCAs exhibited photonic band gaps for normal incident light independent of the position on the spherical surface. This property was induced by heterogeneous nucleation at the smooth wall of the spherical emulsion drop during crystallization into a face-centered cubic (fcc) structure. On the other hand, the solidified shells did not permit the penetration of ionic species, enabling the CCAs to maintain their structure in a continuous aqueous phase of high ionic strength for at least 1 month. In addition, the evaporation of water molecules inside the shell was slowed considerably when the core-shell microparticles were exposed to air: It took approximately 6 h for a suspension encapsulated in a thick shell to evaporate completely, which is approximately 1000 times longer than the evaporation time for water droplets with the same volume. Finally, the spherical CCAs additionally exhibited enhanced stability against external electric fields. The spherical geometry and high dielectric constant of the suspension contributed to reducing the electric field inside the shell, thereby inhibiting the electrophoretic movement of the charged particles.

Droplet Microfluidics for Producing Functional Microparticles

Isotropic microparticles prepared from a suspension that undergoes polymerization have long been used for a variety of applications. Bulk emulsification procedures produce polydisperse emulsion droplets that are transformed into spherical microparticles through chemical or physical consolidation. Recent advances in droplet microfluidics have enabled the production of monodisperse emulsions that yield highly uniform microparticles, albeit only on a drop-by-drop basis. In addition, microfluidic devices have provided a variety of means for particle functionalization through shaping, compartmentalizing, and microstructuring. These functionalized particles have significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications. Finally, we outline the current limitations and future perspectives on microfluidic techniques used to produce microparticles.

Controlled Origami Folding of Hydrogel Bilayers with Sustained Reversibility for Robust Microcarriers

Microencapsulation and controlled release have long been studied because of the high demand for practical delivery systems in the pharmaceutics and cosmetics fields. Multiphase emulsion drops have provided efficient templates for microcapsules, and various feasible methods have been developed for controlled release. However, the emulsion-based approach has limitations for the in situ control of membrane permeability. Micro-origami has emerged as one of the most promising alternative approaches for producing tunable microcapsules with the potential to be applied, for example as drug carriers, actuators, microcontainers, and microrobots. Inspired by living organisms in nature such as the ice plant and Venus flytrap, two different micro-origami approaches have been employed to make various microstructures. One approach uses solid patches connected by active hinge materials. Typical examples use various metal– metal, metal–polymer, and polymer–polymer combinations. The patch and hinge system has enabled the capture, release, and gripping of target materials, showing the feasibility of micro-origami structures. However, the microcapsule is limited to polyhedral shapes in this approach, and complete sealing of the gaps between patches requires exquisite control of the folding angles. Moreover, the delicate and complex fabrication processes make practical applications difficult. The second approach uses a bilayer structure composed of two different materials. For example, a metal– polymer bilayer can show bending/unbending when the polymeric active layer suffers significant volume change, but the metal layer remains unchanged. 13] Polymer materials have been employed in both layers to make biocompatible microcapsules. 15] However, complete sealing of the gaps in the bilayer contact regions remains an important, yet unmet, need. In addition, a simple and effective method for the fabrication of practical microcapsules has not yet been developed, and remains highly desirable. This is the main thrust of the present study. Herein, we report the use of biocompatible bilayer structures for the fabrication of tunable microcapsules based on micro-origami. Monodisperse bilayer microstructures were prepared using a facile photolithographic procedure, without employing photomask alignment. In addition, highly flexible hydrogels were selected as both active and passive layers, facilitating tight contact between patches. The bilayer structure therefore enabled in situ encapsulation, through a reversible transformation to microcapsules with a closed compartment. The resultant microcapsules showed negligible leakage of encapsulants and triggered release of the encapsulants could be achieved simply by inducing the unfolding of the hydrogel bilayer. The essential strategy of our approach relies on the anisotropic volume change of a hydrogel bilayer. As shown in Scheme 1a, the active hydrogel layer shows significant volume expansion under external stimuli by swelling, whereas the passive hydrogel layer remains in a constant volume. Therefore, mechanical stress drives the bending of the bilayer, resulting in microcapsules with a closed compartment. The hydrogel swelling behavior is highly reversible, enabling repeated transformations. The hydrogel bilayer structure was prepared on a glass substrate, using photolithography with an amorphous silicon photomask, as shown in Scheme 1b. Here, we propose poly(2hydroxyethyl methacrylate-co-acrylic acid), p(HEMA-coAA), and poly(2-hydroxyethyl methacrylate), p(HEMA), as model components because they are widely used, FDAapproved (FDA = Food and Drug Administration) biocompatible materials. One monomer solution for p(HEMA-coAA) was infiltrated into the space between the photomask and a polydimethylsiloxane (PDMS) microchannel of 25 mm thickness; this monomer solution was then polymerized by UV irradiation through the photomask. The second monomer solution for p(HEMA) was infiltrated into the space between the same photomask and a PDMS microchannel 50 mm in thickness, after washing out the previously unpolymerized solution. Upon the second round of UV irradiation, bilayer structures consisting of a p(HEMA) layer on a p(HEMA-coAA) layer were formed; alignment of the photomask was unnecessary, because each layer was fabricated on the photomask. The resultant bilayer structures were released from the photomask through immersion in a pH 9 buffer solution. To exploit the structural transformation of the bilayer microparticles, we used two different shapes of microparticle: snowman-shaped and flower-shaped. The shape and dimensions of these microparticles were carefully determined to ensure a fully closed compartment in the swollen state; both the snowmanand flower-shaped microparticles were 50 mm [*] T. S. Shim, Dr. S.-H. Kim, Dr. C.-J. Heo, H. C. Jeon, Prof. S.-M. Yang National Creative Research Initiative Center for Integrated Optofluidic Systems and Department of Chemical and Biomolecular Engineering KAIST Daejeon, 305-701 (Korea) E-mail: smyang@kaist.ac.kr Homepage: http://msfl.kaist.ac.kr

Dissolution Arrest and Stability of Particle-Covered Bubbles

Experiments show that bubbles covered with monodisperse polystyrene particles, with particle to bubble radius ratios of about 0.1, evolve to form faceted polyhedral shapes that are stable to dissolution in air-saturated water. We perform Surface Evolver simulations and find that the faceted particle-covered bubble represents a local minimum of energy. At the faceted state, the Laplace overpressure vanishes, which together with the positive slope of the bubble pressure-volume curve, ensures phase stability. The repulsive interactions between the particles cause a reduction of the curvature of the gas-liquid interface, which is the mechanism that arrests dissolution and stabilizes the bubbles.

Dynamic Modulation of Photonic Bandgaps in Crystalline Colloidal Arrays Under Electric Field

The self-organization of colloidal building blocks into threedimensional periodic nanostructures has attracted considerable attention in materials chemistry and soft condensed-matter physics. [ 1–3 ] In particular, the spatial regularity of colloidal crystals at subwavelength periods induces photonic bandgaps (PBGs), which is of practical importance for photonic and biological applications. [ 4–6 ] To prepare and tailor close-packed colloidal crystals, evaporation-induced crystallization in various confi ning geometries has typically been used. [ 7–9 ] Vertical deposition of colloidal particles on a substrate during evaporation produces photonic crystals with high crystallinity over large areas. [ 10 , 11 ] Also, direct assembly of colloids under an electric fi eld has been used to create close-packed colloidal crystals on electrodes. [ 12–14 ] On the other hand, non-close-packed colloidal crystals can be prepared in systems with strong electrostatic or steric interactions between particles. [ 15–17 ] When particles have like charges, they repel each other through electrostatic interactions and minimize the total repulsive energy by forming ordered structures known as crystalline colloidal arrays (CCAs). [ 15 ] Many researchers have developed chemical sensors [ 18 ]