Recent Announcement

Our work on SERS-active microgels is selected as 100 R&Ds for 2025

posted Dec 27, 2017, 4:12 PM by Shin-Hyun Kim

Our work on SERS-active microgels is selected as 100 R&Ds for 2025 by the national academy of engineering of Korea (2025년, 대한민국을 이끌 100대 기술과 주역, 한국공학한림원).

New paper in Adv. Mater. Interfaces

posted Dec 18, 2017, 3:06 AM by Shin-Hyun Kim

Bomi Kim, Sangmin Lee, and Shin-Hyun Kim, "Double-Emulsion-Templated Anisotropic Microcapsules for pH-Triggered Release", Advanced Materials InterfacesAccepted for publication (2017). (Corresponding author)

Biocompatible microcapsules with a function of pH-triggered release are promising for the site-specific release of bio-actives in the human body. In this work, we produce smart microcapsules with three distinct configurations of membranes to control the rate of pH-triggered release. The microcapsules are prepared with a template of water-in-oil-in-water double-emulsion drops. The oil shells of drops contain two polymers dissolved in an organic solvent: one is a biodegradable polymer selected from poly(lactic acid), poly(lactic-co-glycolic acid), and polycaprolactone and the other is a pH-responsive cationic polymer. Upon the depletion of organic solvent, two polymers confined in the shell undergo phase separation and double-emulsion drops transform to microcapsules with the solid membrane. Three different configurations of the membranes—spherical, snowman-like, and eyeball-like structures—are exclusively produced according to the selection of biodegradable polymers. Although all the microcapsules release hydrophilic encapsulant loaded in the core when they are subjected to a weakly acidic condition, the release kinetics strongly depends on the configuration of the membrane. Larger surface coverage and thinner membrane of pH-responsive domains result in faster release.

New Article in Chem. Mater.

posted Dec 10, 2017, 4:38 AM by Shin-Hyun Kim

Jaehoon Oh, Bomi Kim, Sangmin Lee, Shin-Hyun Kim, and Myungeun Seo, "Semipermeable Microcapsules with a Block Polymer-Templated Nanoporous Membrane", Chemistry of MaterialsAccepted for publication (2017). (J. Oh & B. Kim contributed equally, Co-corresponding author) [pdf]

Microcapsule with nanoporous membranes can regulate transmembrane transport in a size-dependent fashion while protecting active materials in the core from the surrounding, thereby being useful as artificial cell models, carriers for cells and catalyst, and microsensors. In this work, we report a pragmatic microfluidic approach to producing such semipermeable microcapsules with precise control of the cut-off threshold of permeation. Using a homogeneous polymerization mixture for polymerization-induced microphase separation (PIMS) process as the oil phase of water-in-oil-in-water (W/O/W) double emulsions, a densely crosslinked shell composed of a bicontinuous nanostructure that percolates through the entire thickness is prepared, which serves as a template for a monolithic nanoporous membrane of microcapsules with size-selective permeability. We demonstrate the nanopores with precisely controlled size by the block polymer self-assembly governs molecular diffusion through the membrane and renders manipulation of the cut-off threshold.

New Article in ACS Applied Materials and Interfaces

posted Nov 22, 2017, 4:51 PM by Shin-Hyun Kim

Yongjoon Heo, Su Yeon Lee, Ji-Won Kim, Tae Yoon Jeon, and Shin-Hyun Kim, "Controlled Insertion of Planar Defect in Inverse Opals for Anti-counterfeiting Applications", ACS Applied Materials & InterfacesAccepted for publication (2017). 

Inverse opals have been used for structural coloration and photonic applications owing to their photonic bandgap properties. When the photonic structures contain planar defects, they provide defect modes, which are useful for lasing, sensing, and waveguiding. However, it remains a challenge to insert a planar defect into inverse opals in a reproducible manner. Here, we report a new method for producing planar-defect-inserted inverse opals using sequential capillary wetting of colloidal crystals and creating micropatterns through photolithography. Three cycles of deposition and thermal embedding of colloidal crystals into the underlying film of negative photoresist are performed. In the three cycles, opal, particle monolayer, and opal are sequentially employed, which yields the monolayer-templated planar defect sandwiched by two inverse opals after particle removal. The planar defect provides a passband whose wavelength can be controlled by adjusting the diameter of particles for the defect layer. Moreover, the defect-inserted inverse opals can be micropatterned by photolithography as the negative photoresist is used as a matrix. The resulting micropatterns deliver a unique spectral code featured by a combination of stop band and defect mode and a graphical code dictated by photolithography, being useful for anti-counterfeiting applications.

New Article in ACS Nano

posted Nov 2, 2017, 4:57 PM by Shin-Hyun Kim

Gun Ho Lee, Tae Min Choi, Bomi Kim, Sang Hoon Han, Jung Min Lee, and Shin-Hyun Kim, "Chameleon-Inspired Mechanochromic Photonic Films Composed of Nonclose-Packed Colloidal Arrays", ACS NanoAccepted for publication (2017).

Chameleon uses a nonclose-packed array of guanine nanocrystals in iridophores to develop and tune skin colors in full visible range. Inspired from the biological process uncovered in panther chameleons, we design photonic films containing a nonclose-packed face-centered cubic (fcc) array of silica particles embedded in an elastomer. The nonclose-packed array is formed by interparticle repulsion exerted by solvation layers on the particle surface, which is rapidly captured in the elastomer by photo-curing of the dispersion medium. The artificial skin exhibits the structural color that shifts from red to blue under stretching or compression. The separation between inelastic particles enables the tuning without experiencing significant rearrangement of particles, providing elastic deformation and reversible color change, as chameleons do. The simple fabrication procedure consists of film casting and UV irradiation, potentially enabling the continuous high-throughput production. The mechanochromic property of the photonic films enables the visualization of deformation or stress with colors, which is potentially beneficial for various applications, including mechanical sensors, sound-vision transformers, and color display.

New paper in Adv. Mater. Interfaces

posted Oct 13, 2017, 6:30 AM by Shin-Hyun Kim

Kwanghwi Je, Ju Hyeon Kim, Tae Soup Shim, Minhee Ku, Jaemoon Yang, and Shin-Hyun Kim, "Lithographically-Designed Conical Microcarriers for Programed Release of Multiple Actives", Advanced Materials InterfacesAccepted for publication (2017).  (Corresponding author)

The programmed release of multiple ingredients is important in the therapeutics and pharmaceutical fields. A variety of core–shell microcarriers have been designed to fulfill the release function; however, encapsulating multiple actives in their own compartments and releasing them in a programmed manner remains a challenge due to restrictions on the material sets that may be used to form the compartments. In this work, we report the development of lithographically featured core–shell microcarriers composed of double cones and a cap that encapsulate and release various combinations of multi-actives in a pre-defined fashion. Active-free caps were first prepared on a photomask using conventional photolithography. Onto each cap were formed, sequentially, an active-loaded small cone and large cone in two steps of reaction diffusion-mediated photolithography (RDP). The release kinetics of the actives stored in the inner and outer cones were controlled by tailoring the cross-linking density of the photocured polymers that composed each compartment. The cap prevented direct diffusion from the inner cone to the surrounding. The RDP-based lithographic means for creating core–shell microcarriers provides new opportunities for delivering synergistic combinations of drugs in pharmacotherapy.

New Article in ACS AM&I

posted Sep 25, 2017, 4:48 PM by Shin-Hyun Kim

Hyelim Kang, Yong Joon Heo, Dong Jae Kim, Ju Hyeon Kim, Tae Yoon Jeon, Soojeong Cho, Hye-Mi So, Won Seok Chang, and Shin-Hyun Kim, "Droplet-Guiding Superhydrophobic Arrays of Plasmonic Microposts for Molecular Concentration and Detection", ACS Applied Materials & Interfaces Accepted for publication (2017).  (Corresponding author) [pdf]

Droplet-guiding superhydrophobic SERS substrates are created by a combinatorial lithographic technique. Photolithography defines the pattern of a micropillar array with a radial density gradient, whereas colloidal lithography features a nanotip array on the top surface of each micropillar. The nanotip array renders the surface superhydrophobic and the pattern of micropillars endows the radial gradient of contact angle, enabling the spontaneous droplet migration toward the center of the pattern. Water droplets containing target molecules are guided to the center and the molecules dissolved in the droplets are concentrated at the surface of the central micropillar during droplet evaporation. Therefore, the molecules can be analyzed at the predefined position by Raman spectra without scanning the entire substrate. At the same time, SERS-active nanotip array provides high sensitivity of Raman measurement.

New Article in ACS Nano

posted Jul 18, 2017, 12:17 AM by Shin-Hyun Kim

Jaeho Choi, Wonhee Cho, Seung Yeol Lee, Yeon Sik Jung, Shin-Hyun Kim, and Hee-Tak Kim, "Flexible and Robust Superomniphobic Surfaces created by Localized Photofluidization of Azopolymer Pillars", ACS Nano Accepted for publication (2017).  (Co-corresponding author)

Springtails, insects which breathe through their skins, possess mushroom-shaped nanostructures. As doubly re-entrant geometry in the mushroom head enhances the resistance against liquid invasion, the springtails have robust, liquid-free omniphobic skins. Although omniphobic surfaces are promising for various applications, it remains an important challenge to mimic the structural feature of springtails. This paper presents a pragmatic method to create doubly re-entrant nanostructures and robust super-omniphobic surfaces by exploiting localized photofluidization of azopolymers. Irradiation of circularly-polarized light reconfigures azopolymer micropillars to have a mushroom-like head with a doubly re-entrant nano-geometry through protrusion and inward bending of polymer film from the top edge. The light-driven reconfigured micropillars facilitate the pining of triple line as the springtails do. In particular, the unique geometry exhibits super-omniphobicity even for liquids whose equilibrium contact angles are almost zero in the presence of a practical level of external pressure. In addition, the simple fabrication process is highly reproducible, scalable, and compatible with various substrate materials including flexible polymeric film. Our results suggest that our photofluidization technology will provide a practical route to develop robust super-omniphobic surfaces.

New Article in Journal of Materials Chemistry C

posted Jul 2, 2017, 4:39 PM by Shin-Hyun Kim

Hyeon Jin Seo, Sang Seok Lee, Jieun Noh, Jae Won Ka, Cheolmin Park, Shin-Hyun Kim, and Yun Ho Kim, "Robust Photonic Microparticles comprised of Cholesteric Liquid Crystals for Anti-forgery Materials", Journal of Materials Chemistry C Accepted for publication (2017)  (Seo & Lee contributed equally, Co-corresponding author)

Cholesteric liquid crystals (CLCs) possess photonic bandgap owing to the helical arrangement of molecules. The CLCs reflect circularly-polarized light of a specific handedness and wavelength, exhibiting colors. The wavelength of the selective reflection, or the structural color, can be easily controlled by varying the concentration of a chiral dopant. Although such a unique optical property renders CLCs promising for various applications, their fluidity severely limits the ease of processing and structural stability. To overcome the limitation, we design CLC microparticles (CLC-MPs) by photopolymerization of reactive mesogens (RMs) in CLC droplets. With capillary microfluidic devices, highly uniform emulsion drops of CLC-RM mixtures are prepared in an aqueous phase drops, which are then exposed to ultraviolet (UV) to yield solid microparticles. The diameter of the CLC-MPs is precisely controlled by either manipulating the flow rates of dispersed and continuous phases or varying the diameter of the capillary orifice in the microfluidic devices. The wavelength of reflection and handedness of helical structure are selected by the composition of the dispersed phase. The photo-polymerization of RMs leads to the formation of a three-dimensional rigid network, thereby yielding the CLC-MPs with high mechanical stability. The CLC-MPs could be further assembled to form two-dimensional hexagonal arrays on flat surfaces or deposited in pre-defined trenches or holes by a mechanical rubbing. Two distinct CLC-MPs with opposite handedness can be patterned to show different color graphics depending on the selection of handedness of circularly-polarized light, which are appealing for anti-forgery patches.

Frontispiece in Advanced Materials

posted Jun 18, 2017, 6:23 AM by Shin-Hyun Kim   [ updated Jun 18, 2017, 6:24 AM ]

Sang Seok Lee, Hyeon Jin Seo,Yun Ho Kim, and Shin-Hyun Kim, "Structural Color Palettes of Core-Shell Photonic Ink Capsules containing Cholesteric Liquid Crystals", Advanced Materials29, 1606894 (2017).(Co-corresponding author

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