Dong Jae Kim, Tae Yoon Jeon, Sung-Gyu Park, Hye Ji Han, Sang Hyuk Im, Dong-Ho Kim and Shin-Hyun Kim, "Uniform Microgels containing Agglomerates of Silver Nanocubes for Molecular Size-Selectivity and High SERS Activity", Small, accepted for publication (2017).
Microgels containing agglomerates of silver nanocubes are microfluidically created to provide SERS-active substrates. The microgels permit selective infusion of small molecules and nanogaps among the nanocubes serve as hot spots for SERS. Therefore, the microgels enable us to selectively analyze Raman spectra of small molecules dissolved in complex mixtures with high sensitivity in the absence of interruption from large adhesives.
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 Materials, accepted for publication (2017).
Photonic microcapsules with onion-like topology are microfluidically designed to have cholesteric liquid crystals with opposite handedness in their core and shell. The microcapsules exhibit structural colours caused by dual photonic bandgaps, resulting in a rich variety of colour on the optical palette. Moreover, the microcapsules can switch the colours from either core or shell depending on the selection of light-handedness.
Soojeong Cho, Tae Soup Shim, Ju Hyeon Kim, Dong-Hyun Kim, and Shin-Hyun Kim, "Selective Coloration of Melanin Nanospheres through Resonant Mie Scattering", Advanced Materials, accepted for publication (2017).
Black melanin inks are prepared to selectively exhibit colors under strong light, inspired by human hair. High absorbance of melanin suppresses multiple scattering, causing resonant Mie scattering predominant. Various colors can be developed as the resonant wavelength dictated by nanosphere diameter. Therefore, the melanin inks can be used to encrypt and selectively disclose multicolor patterns for anti-counterfeiting applications.
Ju Hyeon Kim, Kwanghwi Je, Tae Soup Shim, and Shin-Hyun Kim, "Reaction-Diffusion-Mediated Photolithography for Designing Pseudo-3D Microstructures", Small, accepted for publication (2017).
Microstructures with three-dimensional (3D) features provide advanced functionalities in many applications. We have employed reaction-diffusion process in photolithography to produce pseudo-3D microstructures in a reproducible manner. In this work, we investigate the influences of various parameters on growth behavior of polymeric structures and expand the use of the reaction-diffusion-mediated photolithography (RDP) to a wide range of structural dimensions. In addition, we study how a lens effect alters the growth behavior of microstructures in conjunction with reaction-diffusion process. For small separation between reaction sites in the array, ultraviolet (UV) exposure time is optimized along with the separation to avoid film or plateau formation. We further prove that the RDP process is highly reproducible and applicable to various photocurable resins. In a demonstrative purpose, we show the use of microdomes created by the RDP process as microlens arrays. The RDP process enables the production of pseudo-3D microstructures even with collimated UV light in the absence of complex optical setups, thereby potentially serving as a useful means to create micropatterns and particles with unique structural features.
Seung Yeol Lee, Jongkook Choi, Jong-Ryul Jeong, Jung H. Shin, and Shin-Hyun Kim, "Magnetoresponsive Photonic Microspheres with Structural Color Gradient", Advanced Materials, accepted for publication (2016).
Photonic Janus particles are created by alternately sputtering silica and titania on microspheres to have a structural color gradient. In addition, the microspheres are rendered magnetoresponsive. The Janus microspheres with optical and magnetic anisotropy enable on-demand control over orientation and structural color through manipulation of an external magnetic field, thereby being useful as active color pigments for reflection-mode display.
Juwoon Park, Sang Seok Lee, Young Hoon Sohn, Shin-Hyun Kim, and Yutaek Seo, "Hydrate Formation in Water-Laden Microcapsules for Temperature-Sensitive Release of Encapsulants", RSC Advances, accepted for publication (2016). (Park & Lee equally contributed, Co-corresponding author)
Microcapsules have been widely used to store and release active materials for various purposes. In this work, we design microcapsules that separate an inner water phase from guest molecules in the surrounding medium with a polymeric shell. The water and guest molecules are brought into contact within the shell, where a hydrate is formed when the temperature is lower than hydrate formation condition. A steady supply of water and guest molecules through the shell matrix into the hydrates yields local cracks in the shell. As the hydrates continue to grow in the absence of external shear flow, the cracks slowly propagate along the whole shell. In contrast, in the presence of external shear, the cracks formed by the hydrate formation are rapidly widened by the shear. This is the first direct evidence presenting the effects of the hydrate formation on water-laden microcapsules. We believe that the microcapsules can be further engineered to produce temperature-sensitive microcarriers for controlled delivery of specialty chemicals.
Tae Yong Lee, Tae Min Choi, Tae Soup Shim, Raoul A.M. Frijns, and Shin-Hyun Kim, "Microfluidic production of multiple emulsions and functional microcapsules", Lab on a Chip, accepted for publication (2016).
Recent advances in microfluidics have enabled the controlled production of multiple-emulsion drops with onion-like topology. The multiple-emulsion drops possess an intrinsic core-shell geometry, which makes them useful as templates to create microcapsules with a solid membrane. High flexibility in the selection of materials and hierarchical order, achieved by microfluidic technologies, has provided versatility in the membrane properties and microcapsule functions. The microcapsules are now designed not just for storage and release of encapsulants but for sensing microenvironments, developing structural colours, and many other uses. This article reviews the current state of the art in the microfluidic-based production of multiple-emulsion drops and functional microcapsules. The three main sections of this paper discuss distinct microfluidic techniques developed for the generation of multiple emulsions, four representative methods used for solid membrane formation, and various applications of functional microcapsules. Finally, we outline the current limitations and future perspectives of microfluidics and microcapsules.