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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

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 ]

3D Hierarchical Architectures Prepared by Single Exposure Through a Highly Durable Colloidal Phase Mask

Three-dimensional hierarchical architectures are fabricated using a simple, cost-effective, durable colloidal phase mask containing a colloidal monolayer embedded in a flexible polydimethylsiloxane (PDMS) membrane. These structures give rise to a photonic bandgap that can be tuned over a wide spectral range from the visible to the near-infrared regions.

High-throughput optofluidic platforms for mosaicked microfibers toward multiplex analysis of biomolecules

We describe high-throughput optofluidic platforms for mosaic-patterned microfibers by generating stratified laminar flows. An inert carrier liquid flow near PDMS channel walls conveyed a photopolymerizable liquid which permitted stable production of microfibers with particular morphologies and compositional patterns. Finally, mosaicked microfibers were prepared with desired configurations toward multiplex biomolecular analysis.

Spatially Selective Nucleation and Growth of Water Droplets on Hierarchically Patterned Polymer Surfaces

On a hierarchical polymer surface consisting of microposts and nanopillar arrays, water droplets are nucleated and grown selectively in the grooves between the microposts as the vapor pressure increases, whereas water droplets are randomly nucleated on a flat surface and surfaces consisting of microposts or nanopillars only.

Lithographic Design of Overhanging Microdisk Arrays Toward Omniphobic Surfaces

Omniphobic surfaces are created by designing an array of overhanging microdisks on a polymer film through two steps of photolithography. Two distinct edges and the large height of the microdisks relative to their separation ensure the formation of an air mat under the microdisks, providing an omniphobic property. Moreover, the freestanding omniphobic films are transparent and flexible, potentially serving as liquid-repellent surfaces in various applications.

Dynamic designing of microstructures by chemical gradient-mediated growth

Shape is one of the most important determinants of the properties of microstructures. Despite of a recent progress on microfabrication techniques, production of three-dimensional micro-objects are yet to be fully achieved. Nature uses reaction–diffusion process during bottom-up self-assembly to create functional shapes and patterns with high complexity. Here we report a method to produce polymeric microstructures by using a dynamic reaction–diffusion process during top-down photolithography, providing unprecedented control over shape and composition. In radical polymerization, oxygen inhibits reaction, and therefore diffusion of oxygen significantly alters spatial distribution of growth rate. Therefore, growth pathways of the microstructures can be controlled by engineering a concentration gradient of oxygen. Moreover, stepwise control of chemical gradients enables the creation of highly complex microstructures. The ease of use and high controllability of this technology provide new opportunities for microfabrication and for fundamental studies on the relationships between shape and function for the materials.

Magnetic-Nanoflocculant-Assisted Water-Nonpolar Solvent Interface Sieve for Microalgae Harvesting.

Exploitation of magnetic flocculants is regarded as a very promising energy-saving approach to microalgae harvesting. However, its practical applicability remains limited, mainly because of the problem of the postharvest separation of magnetic flocculants from microalgal flocs, which is crucial both for magnetic-flocculant recycling and high-purity microalgal biomasses, but which is also a very challenging and energy-consuming step. In the present study, we designed magnetic nanoflocculants dually functionalizable by two different organosilane compounds, (3-aminopropyl)triethoxysilane (APTES) and octyltriethoxysilane (OTES), which flocculate negatively charged microalgae and are readily detachable at the water-nonpolar organic solvent (NOS) interface only by application of an external magnetic field. APTES functionalization imparts a positive zeta potential charge (29.6 mV) to magnetic nanoflocculants, thereby enabling microalgae flocculation with 98.5% harvesting efficiency (with a dosage of 1.6 g of dMNF/g of cells). OTES functionalization imparts lipophilicity to magnetic nanoflocculants to make them compatible with NOS, thus effecting efficient separation of magnetic flocculants passing through the water-NOS interface sieve from hydrophilic microalgae. Our new energy-saving approach to microalgae harvesting concentrates microalgal cultures (∼1.5 g/L) up to 60 g/L, which can be directly connected to the following process of NOS-assisted wet lipid extraction or biodiesel production, and therefore provides, by simplifying multiple downstream processes, a great potential cost reduction in microalgae-based biorefinement.

Regenerative Astaxanthin Extraction from a Single Microalgal (Haematococcus pluvialis) Cell Using a Gold Nano-Scalpel.

Milking of microalgae, the process of reusing the biomass for continuous production of target compounds, can strikingly overcome the time and cost constraints associated with biorefinery. This process can significantly improve production efficiency of highly valuable chemicals, for example, astaxanthin (AXT) from Haematococcus pluvialis. Detailed understanding of the biological process of cell survival and AXT reaccumulation after extraction would be of great help for successful milking. Here we report extraction of AXT from a single cell of H. pluvialis through incision of the cell wall by a gold nanoscalpel (Au-NS), which allows single-cell analysis of wound healing and reaccumulation of AXT. Interestingly, upon the Au-NS incision, the cell could reaccumulate AXT at a rate two times faster than the control cells. Efficient extraction as well as minimal cellular damage, keeping cells alive, could be achieved with the optimized shape and dimensions of Au-NS: a well-defined sharp tip, thickness under 300 nm, and 1-3 μm of width. The demonstration of regenerative extraction of AXT at a single cell level hints toward the potential of a milking process for continuous recovery of target compounds from microalgae while keeping the cells alive.