Insung S. Choi

One-step modification of superhydrophobic surfaces by a mussel-inspired polymer coating.

A bio-inspired approach for superhydrophobic surface modification was investigated. Hydrophilic conversion of the superhydrophobic surface was easily achieved through this method, and the superhydrophobic-hydrophilic alternating surface was generated by the method combined with soft-lithography. The resulting patterned surface showed high water adhesion property in addition to superhydrophobic property.

Highly‐Efficient, Flexible Piezoelectric PZT Thin Film Nanogenerator on Plastic Substrates

A highly-efficient, flexible piezoelectric PZT thin film nanogenerator is demonstrated using a laser lift-off (LLO) process. The PZT thin film nanogenerator harvests the highest output performance of ∼200 V and ∼150 μA·cm(-2) from regular bending motions. Furthermore, power sources generated from a PZT thin film nanogenerator, driven by slight human finger bending motions, successfully operate over 100 LEDs.

Mussel-inspired encapsulation and functionalization of individual yeast cells.

The individual encapsulation of living cells has a great impact on the area of cell-based sensors and devices as well as fundamental studies in cell biology. In this work, living yeast cells were individually encapsulated with functionalizable, artificial polydopamine shells, inspired by an adhesive protein in mussels. Yeast cells maintained their viability within polydopamine, and the cell cycle was controlled by the thickness of the shells. In addition, the artificial shells aided the cell in offering much stronger resistance against foreign aggression, such as lyticase. After formation of the polydopamine shells, the shells were functionalized with streptavidin by utilizing the chemical reactivity of polydopamine, and the functionalized cells were biospecifically immobilized onto the defined surfaces. Our work suggests a biomimetic approach to the encapsulation and functionalization of individual living cells with covalently bonded, artificial shells.

Norepinephrine: Material-Independent, Multifunctional Surface Modification Reagent

A facile approach for material-independent surface modification using norepinephrine was investigated. pH-induced oxidative polymerization of norepinephrine forms adherent films on vastly different types of material surfaces of noble metals, metal oxides, semiconductors, ceramics, shape-memory alloys, and synthetic polymers. Secondary biochemical functionalizations such as immobilization of proteins and growth of biodegradable polyester on the poly(norepinephrine) films were demonstrated.

Microfluidics Section: Design and Fabrication of Integrated Passive Valves and Pumps for Flexible Polymer 3-Dimensional Microfluidic Systems

This paper describes the fabrication of flexible, polymeric 3-dimensional microfluidic systems with integrated check valves (flap and diaphragm valves) and a pump by stacking patterned poly(dimethylsiloxane) (PDMS) layers containing microchannels and vias. We describe this procedure for fabricating, manipulating, and bonding of PDMS membranes and bas-relief plates into multilayer microfluidic devices. The fabrication and demonstration of integrated check valves and a pump in a prototype polymer 3-dimensional microfluidic system is a step toward practical realization of all-polymer, flexible, low-cost, disposable microfluidic devices for biochemical applications.

Patterned polymer growth on silicon surfaces using microcontact printing and surface-initiated polymerization

Patterned polymer films were grown on SiO2/Si surfaces by a process starting with microcontact printing (μCP) of octadecyltrichlorosilane (OTS), formation of a monolayer derived from norbornenyl trichlorosilane (Nbn–SiCl3) in areas not protected by OTS, activation of the surfaces derived from Nbn–SiCl3 with a ruthenium catalyst, and surface-initiated ring-opening metathesis polymerization of derivatives of norbornene by the catalytically active ruthenium species. These patterned polymer films were successfully used as reactive ion etching resists. The combination of μCP and surface-initiated polymerization makes possible molecular-level control of polymer composition and thickness in both lateral and vertical directions: the smallest patterned lateral features were 2 μm lines; this width was determined by the features of the stamp used in μCP and is not the intrinsic limit of the method. The thickness of the polymer film was, typically, 5–100 nm and could be controlled by monomer concentration and reactio...

Synthesis of PAMAM Dendrimer Derivatives with Enhanced Buffering Capacity and Remarkable Gene Transfection Efficiency

In this study, we introduced histidine residues into l-arginine grafted PAMAM G4 dendrimers to enhance proton buffering capacity and evaluated the physicochemical characteristics and transfection efficacies in vitro. The results showed that the synthesized PAMAM G4 derivatives effectively delivered pDNA inside cells and the transfection level improved considerably as the number of histidine residues increased. Grafting histidine residues into the established polymer vector PAMAM G4-arginine improved their proton buffering capacity. The cytotoxicity of PAMAM G4 derivatives was tested and it was confirmed that they displayed relatively lower cytotoxicity compared to PEI25KD in various cell lines. Also, confocal microscopy results revealed that PAMAM G4 derivatives effectively delivered pDNA into cells, particularly into the nucleus. These PAMAM dendrimer derivatives conjugated with histidines and arginines may provide a promising polymeric gene carrier system.

Bioinspired Functionalization of Silica-Encapsulated Yeast Cells†

Cell-surface modification is usually achieved by sophisticated but complicated methods, such as the introduction of nonbiogenic functional groups by metabolic or genetic engineering. Although such methods have evolved into biocompatible and bioorthogonal strategies, the possibility that the direct insertion of functional moieties causes significant perturbations to cell membranes still remains. For a decade, encapsulation methods have been developed as an alternative, indirect approach to cell-surface modifications, as it is thought that the cell integrity would not be perturbed by the encapsulation methods where functional moieties are introduced onto the cell surface without any direct contact with cell membranes. For example, the noncovalent adsorption of macromolecules, mostly by layer-by-layer (LbL) processes, has been utilized to introduce various functionalities, including fluorescent and magnetic properties, catalytic moieties, and supporting templates, to the living cells. On the other hand, recently reported artificial shells, which robustly encapsulate individual living cells, have attracted a great deal of attention as a new approach to cell-surface modifications and formation of artificial spores, because the artificial shells were reported to enhance cell viability and also to control cell division; these factors would be beneficial in the development of biosensor circuits, lab-ona-chip systems, and bioreactors, as well as for fundamental studies in cell biology. It is therefore anticipated that the synergistic combination of the protective encapsulation and the cell-surface functionalization would make a significant step towards the aforementioned applications. Despite the advantages of physically protective shells, the utilization of the artificial shells for practical applications still remains a challenge. The mechanical robustness and chemical inertness of the artificial shells prove beneficial for protecting living cells, but, contradictorily, these properties limit chemical functionalizations of the shells in terms of reactivity. For example, calcium carbonate or calcium phosphate shells lack chemical reactivity. Although the chemistry of silicon is well established, the functionalization of silica shells requires harsh conditions, such as high pH values and harmful solvents. Therefore, it is a prerequisite for any application that the functionalizabilty of the artificial shells is ensured along with the mechanical robustness of the protective shells. Herein we report a bioinspired method for the encapsulation of individual living yeast cells with functionalizable silica shells. Specifically, we used biomimetic silicification, which was inspired by the biosilicification of diatoms. Biomimetic silicification is achieved by specific interactions between silicic acid derivatives and cationic polyamines, such as natural and synthetic peptides, and synthetic polymers: the self-assembled structure of polyamines is thought to act as a catalytic template for the in vivo polycondensation of silicic acid derivatives. We reasoned that chemical functional groups would be introduced directly to the biomimetically formed silica by adding silanol derivatives that contain functional groups in the course of biomimetic polycondensation of silicic acid derivatives. (3-Mercaptopropyl)trimethoxysilane (MPTMS) was selected as a model additive because it was reported to be polycondensed simultaneously with silicic acid under physiologically mild conditions. 12] The functionalizable silica shells formed in this work would expand the utility of artificial shells, because the thiol group in the silica shell can be used for introducing various functions through specific reactions of the thiol moiety with maleimide derivatives under biocompatible conditions (aqueous solution, pH 7.4; Figure 1). The polyelectrolyte multilayer of poly(ethyleneimine) (PEI, Mw: 750 000) and poly(sodium 4-styrenesulfonate) (PSS, Mw: 70000) was used as a catalytic template for biomimetic silicification because previous studies indicated that PEI was biocompatible and acts as a catalyst for biomimetic silica formation. PEI and PSS were alternately deposited onto the surface of Saccharomyces cerevisiae (S. cerevisiae ; baker s yeast). The layer-by-layer processes were initiated with PEI so that electrostatic interactions occur with the negatively charged cell surfaces, and terminated with PEI so that catalytic interactions occur with silicic acid derivatives at the outer interface. For the individual encapsulation of yeast cells with thiol-functionalized silica (SiO2 ; i.e., formation of yeast@SiO2 ), the PEI/PSS multilayercoated cells were placed for 30 min in a silicic acid derivative solution (100 mm), which had been prepared by adding [*] Dr. S. H. Yang, E. H. Ko, Prof. Dr. I. S. Choi Molecular-Level Interface Research Center Department of Chemistry, KAIST, Daejeon 305-701 (Korea) Fax: (+ 82)42-350-2810 E-mail: ischoi@kaist.ac.kr Homepage: http://cisgroup.kaist.ac.kr

In Vitro Developmental Acceleration of Hippocampal Neurons on Nanostructures of Self-Assembled Silica Beads in Filopodium-Size Ranges†

Abstract Topographical cues play an important role in in vitro neuronal development. In their Communication (DOI: 10.1002/anie.201106271), Y. Nam, I. S. Choi, J. S. Lee, and co-workers show that neuritogenetic acceleration occurs on silica-bead monolayers made up of beads with a diameter of more than 200 nm, but not on monolayers of beads with smaller diameters. The biochemical study indicates neurons sense topographical differences in nanostructures and alter their behavior accordingly.

Cytoprotective Silica Coating of Individual Mammalian Cells through Bioinspired Silicification

The cytoprotective coating of physicochemically labile mammalian cells with a durable material has potential applications in cell-based sensors, cell therapy, and regenerative medicine, as well as providing a platform for fundamental single-cell studies in cell biology. In this work, HeLa cells in suspension were individually coated with silica in a cytocompatible fashion through bioinspired silicification. The silica coating greatly enhanced the resistance of the HeLa cells to enzymatic attack by trypsin and the toxic compound poly(allylamine hydrochloride), while suppressing cell division in a controlled fashion. This bioinspired cytocompatible strategy for single-cell coating was also applied to NIH 3T3 fibroblasts and Jurkat cells.