Taek Dong Chung

A graphene-based electrochemical device with thermoresponsive microneedles for diabetes monitoring and therapy

Owing to its high carrier mobility, conductivity, flexibility and optical transparency, graphene is a versatile material in micro- and macroelectronics. However, the low density of electrochemically active defects in graphene synthesized by chemical vapour deposition limits its application in biosensing. Here, we show that graphene doped with gold and combined with a gold mesh has improved electrochemical activity over bare graphene, sufficient to form a wearable patch for sweat-based diabetes monitoring and feedback therapy. The stretchable device features a serpentine bilayer of gold mesh and gold-doped graphene that forms an efficient electrochemical interface for the stable transfer of electrical signals. The patch consists of a heater, temperature, humidity, glucose and pH sensors and polymeric microneedles that can be thermally activated to deliver drugs transcutaneously. We show that the patch can be thermally actuated to deliver Metformin and reduce blood glucose levels in diabetic mice.

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.

Graphene-incorporated chitosan substrata for adhesion and differentiation of human mesenchymal stem cells†

A simple method that uses graphene to fabricate nanotopographic substrata was reported for stem cell engineering. Graphene-incorporated chitosan substrata promoted adhesion and differentiation of human mesenchymal stem cells (hMSCs). In addition, we proposed that nanotopographic cues of the substrata could enhance cell-cell and cell-material interactions for promoting functions of hMSCs.

Electrochemical analysis based on nanoporous structures

Analytical applications and the underlying principles of unique electrochemistry in nanoporous structures are reviewed and discussed. In addition to the conventional concept of enlarged surface area, the structural effects of nanoporous materials can play significant roles such as discriminative electrokinetics, the nano-confinement effect, electrical double layer overlapping, ion-selective impedance, etc. The applications described in this review article include solid-state pH sensors, miniaturized pseudo-reference electrodes, nonenzymatic glucose monitoring, ion diodes, transistors, extracellular neural probes, and a few more. Further intensive research is required to develop creative analytical tools based on nanoporous structures and to unravel the underlying physicochemical principles.

Glucose sensor using a microfabricated electrode and electropolymerized bilayer films.

A new type miniaturized glucose sensor with good selectivity and stable current response has been developed. The structure consists of a recessed rectangular microfabricated platinum electrode, inner layer of two electropolymerized nonconducting films, and outer bilayer of poly(tetrafluoroethylene) (Teflon) and polyurethane (PU) films. Glucose oxidase (GOx) is entrapped during the electropolymerization of a poly(m-phenylenediamine) (PMPD) film in an acetate buffer (AB) solution, on which a highly interference-resistive PMPD film is deposited in a phosphate buffered saline (PBS) solution. The second PMPD film causes no significant decrease in accessibility of glucose to GOx. The inner layer maintains less than 1% permeability to acetaminophen for 12 days. The fairly adhesive outer layer allows stable current response. Due to high permeability, the information about enzyme activity can be obtained without serious error in spite of outer layer intervening between enzymes and solution. The apparent Michaelis-Menten constant and the maximum steady-state current density were 24 mM and 80 microA cm(-2), respectively.

Ionic Circuits Based on Polyelectrolyte Diodes on a Microchip

Green means go: A polyelectrolyte diode on a microchip exhibits well-defined nonlinear rectifying behavior. This system visualizes the dynamic distribution of ions in a charged polymer phase under an electric field on a real-time basis using fluorescence images (see picture). Multiple polyelectrolyte diodes are integrated on a microchip to produce a variety of logic gates based on ionic circuits.

High yield sample preconcentration using a highly ion-conductive charge-selective polymer.

The development and analysis of a microfluidic sample preconcentration system using a highly ion-conductive charge-selective polymer [poly-AMPS (2-acrylamido-2-methyl-1-propanesulfonic acid)] is reported. The preconcentration is based on the phenomenon of concentration polarization which develops at the boundaries of the poly-AMPS with buffer solutions. A negatively charged polymer, poly-AMPS, positioned between two microchannels efficiently extracts cations through its large cross section, resulting in efficient anion sample preconcentration. The present work includes the development of a robust polymer that is stable over a wide range of buffers with varying chemical compositions. The sample preconcentration effect remains linear to over 3 mM (0.15 pmol) and 500 microM (15 fmol) for fluorescein and TRITC-tagged albumin solutions, respectively. The system can potentially be used for concentrating proteins on microfluidic devices with subsequent analysis for proteomic applications.

A new organic modifier for anti-stiction

The chemical and mechanical characteristics of a new surface modifier, dialkyldichloromethylsilane (DDMS, CH/sub 3/)/sub 2/SiCl/sub 2/, for stiction-free polysilicon surfaces are reported. The main strategy is to replace the conventional monoalkyl-trichlorosilane (MTS, RSiCl/sub 3/) such as octadecyltrichlorosilane (ODTS) or 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) with dichlorodisilane (DDS, R/sub 2/SiCl/sub 2/) with two short chains, especially DDMS. DDMS, with shorter chains in aprotic media, rapidly deposits on the chemically oxidized polysilicon surface at room temperature and successfully prevents long cantilevers 3 mm in length from in-use as well as release stiction. DDMS-modified polysilicon surfaces exhibit satisfactory hydrophobicity, long term stability and thermal stability, which are comparable to those of FDTS. DDMS as an alternative to FDTS and ODTS provides a few valuable advantages; ease in handling and storage of the solution, low temperature-dependence and low cost. In addition to the new modifier molecule, the simplified process of direct release right after washing the modified surface with isooctane was proposed to cut the processing time.

Microfluidic approaches for gene delivery and gene therapy

Recent advances in microfluidics have created new and exciting prospects for gene delivery and therapy. The micro-scaled environment within microfluidic systems enables precise control and optimization of multiple processes and techniques used in gene transfection and the production of gene and drug transporters. Traditional non-viral gene transfection methods, such as electroporation, microinjection and optical gene transfection, are improved from the use of innovative microfluidic systems. Additionally, microfluidic systems have also made the production of many viral and non-viral vectors controlled, automated, and reproducible. In summary, the development and application of microfluidic systems are producing increased efficiency in gene delivery and promise improved gene therapy results.

A portable microfluidic flow cytometer based on simultaneous detection of impedance and fluorescence

A portable microfluidic flow cytometer with dual detection ability of impedance and fluorescence was developed for cell analysis and particle-based assays. In the proposed system, fluorescence from microparticles and cells is measured through excitation by a light emitting diode (LED) and detection by a solid-stated photomultiplier (SSPM). Simultaneous impedometric detection provides information on the existence and size of microparticles and cells through polyelectrolyte gel electrodes (PGEs) operated by custom designed circuits for signal detection, amplification, and conversion. Fluorescence and impedance signals were sampled at 1 kHz with 12 bit resolution. The resulting microfluidic cytometer is 15x10x10 cm(3) in width, depth, and height, with a weight of about 800 g. Such a miniaturized and battery powered system yielded a portable microfluidic cytometer with high performance. Various microbeads and human embryonic kidney 293 (HEK-293) cells were employed to evaluate the system. Impedance and fluorescence signals from each bead or cell made classification of micro particles or cells easy and fast.