Junghoon Lee

Electrowetting and electrowetting-on-dielectric for microscale liquid handling

This paper deals with electrowetting (EW) and electrowetting-on-dielectric (EWOD) principles applied to microfluidic devices. EW and EWOD are principles that can control wettability of liquids on solid surfaces using electric potential. While EW is controlling wettability of a certain electrolyte on a metal electrode by varying electric energy across the electrical double layer (EDL), EWOD applies to virtually any aqueous liquid by varying electric energy across the thin dielectric film between the liquid and conducting substrate. These driving mechanisms have many advantages. By electrically changing the wettability of each of the electrode patterns on a surface, a liquid on these electrodes can be shaped and driven along the active electrodes, making microfluidics extremely simple both for device fabrication and operation. It is also worth noting that, driven by surface tension, the mechanism becomes more effective as the size of the device becomes smaller. This paper describes fundamental concepts and the proof-of-concept experiments, modeling and design, microfabrication processes, and initial testing results for the microfluidic devices based on the EW and EWOD principles.

The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite

One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.

Surface-tension-driven microactuation based on continuous electrowetting

This paper describes the first microelectromechanical systems (MEMS) demonstration device that adopts surface tension as the driving force. A liquid-metal droplet can be driven in an electrolyte-filled capillary by locally modifying the surface tension with electric potential. We explore this so-called continuous electrowetting phenomenon for MEMS and present crucial design and fabrication technology that reduce the surface-tension-driving principle, inherently powerful in microscale, into practice. The key issues that are identified and investigated include the problem of material compatibility, electrode polarization, and electrolysis, as well as the micromachining process. Based on the results from the initial test devices and the design concept for a long-range movement of the liquid-metal droplet, we demonstrate a liquid micromotor, an electrolyte and liquid-metal droplets rotating along a microchannel loop. Smooth and wear-free rotation of the liquid system is shown at a speed of /spl sim/40 mm/s (or 420 r/min along a 2-mm loop) with a driving voltage of only 2.8 V and little power consumption (10-100 /spl mu/W).

Design of Surface Hierarchy for Extreme Hydrophobicity

An extreme water-repellent surface is designed and fabricated with a hierarchical integration of nano- and microscale textures. We combined the two readily accessible etching techniques, a standard deep silicon etching, and a gas phase isotropic etching (XeF2) for the uniform formation of double roughness on a silicon surface. The fabricated synthetic surface shows the hallmarks of the Lotus effect: durable super water repellency (contact angle>173 degrees) and the sole existence of the Cassie state even with a very large spacing between roughness structures (>1:7.5). We directly demonstrate the absence of the Wenzels or wetted state through a series of experiments. When a water droplet is squeezed or dropped on the fabricated surface, the contact angle hardly changes and the released droplet instantly springs back without remaining wetted on the surface. We also show that a ball of water droplet keeps bouncing on the surface. Furthermore, the droplet shows very small contact angle hysteresis which can be further used in applications such as super-repellent coating and low-drag microfludics. These properties are attributed to the nano/micro surface texture designed to keep the nonwetting state energetically favorable.

Targeting C-type lectin receptors with multivalent carbohydrate ligands

C-type lectin receptors (CLRs) represent a large receptor family including collectins, selectins, lymphocyte lectins, and proteoglycans. CLRs share a structurally homologous carbohydrate-recognition domain (CRD) and often bind carbohydrates in a Ca²⁺-dependent manner. In innate immunity, CLRs serve as pattern recognition receptors (PRRs) and bind to the glycan structures of pathogens and also to self-antigens. In nature, the low affinity of CLR/carbohydrate interactions is overcome by multivalent ligand presentation at the surface of cells or pathogens. Thus, multivalency is a promising strategy for targeting CLR-expressing cells and, indeed, carbohydrate-based targeting approaches have been employed for a number of CLRs, including asialoglycoprotein receptor (ASGPR) in the liver, or DC-SIGN expressed by dendritic cells. Since CLR engagement not only mediates endocytosis but also influences intracellular signaling pathways, CLR targeting may allow for cell-specific drug delivery and also the modulation of cellular functions. Glyconanoparticles, glycodendrimers, and glycoliposomes were successfully used as tools for CLR-specific targeting. This review will discuss different approaches for multivalent CLR ligand presentation and aims to highlight how CLR targeting has been employed for cell specific drug delivery. Major emphasis is directed towards targeting of CLRs expressed by antigen-presenting cells to modulate immune responses.

Liquid micromotor driven by continuous electrowetting

This paper aims to acclaim surface tension as a practical microactuation force for MEMS, by developing the ground micromachining technologies and demonstrating a few prototypes including a liquid micromotor. A mechanism called continuous electrowetting is used to drive liquid-metal droplets along microchannels by controlling surface tension with electric potential. The paper first identifies and addresses key issues in employing this powerful surface-tension drive for MEMS throughout the design, fabrication, and testing of a simple test device. The development eventually leads us to a device called liquid micromotor, which demonstrates continuous travel of a mercury drop along a loop of microchannel. Wear-free and smooth rotation is achieved with a speed of 9.4 cm/s at a driving voltage of 10 V.

Addressable micro liquid handling by electric control of surface tension

This paper presents two types of electrowetting principles - regular electrowetting (EW) and electrowetting on dielectric (EWOD), applied for microactuation and correspondingly designed test devices for addressable micro liquid handling. The devices demonstrated sequential microactuation of liquid on electrodes with (EWOD) or without (EW) hydrophobic coating on them.

Biomolecular detection with a thin membrane transducer

We present a thin membrane transducer (TMT) that can detect nucleic acid based biomolecular reactions including DNA hybridization and protein recognition by aptamers. Specific molecular interactions on an extremely thin and flexible membrane surface cause the deflection of the membrane due to surface stress change which can be measured by a compact capacitive circuit. A gold-coated thin PDMS membrane assembled with metal patterned glass substrate is used to realize the capacitive detection. It is demonstrated that perfect match and mismatch hybridizations can be sharply discriminated with a 16-mer DNA oligonucleotide immobilized on the gold-coated surface. While the mismatched sample caused little capacitance change, the perfectly matched sample caused a well-defined capacitance decrease vs. time due to an upward deformation of the membrane by a compressive surface stress. Additionally, the TMT demonstrated the single nucleotide polymorphism (SNP) capabilities which enabled a detection of mismatching base pairs in the middle of the sequence. It is intriguing that the increase of capacitance, therefore a downward deflection due to tensile stress, was observed with the internal double mismatch hybridization. We further present the detection of thrombin protein through ligand-receptor type recognition with 15-mer thrombin aptamer as a receptor. Key aspects of this detection such as the effect of concentration variation are investigated. This capacitive thin membrane transducer presents a completely new approach for detecting biomolecular reactions with high sensitivity and specificity without molecular labelling and optical measurement.

Electrostatic actuation of microscale liquid-metal droplets

This paper reports sliding of micro liquid-metal droplets by electrostatic actuation for MEMS applications, bi-stable switching in particular. Basic theory concerning droplets on a plane solid surface is exposed followed by experimental study. Being a major parameter in the modeling of sliding droplets, the contact angle has been characterized in the case of mercury on an oxidized silicon wafer. The method used involves both traditional optical microscope and confocal laser imaging. The contact angle is found to be around 137/spl deg/ with an associated standard deviation of 8/spl deg/. The sample preparation is detailed. The droplets deposition method is based on selective condensation of mercury vapor on gold dots acting as preferred nucleation sites. This technique provides control of droplet dimensions and locations and is suitable for batch fabrication. Experimental study of electrostatic actuation coupled with finite-element method (FEM) analysis is described, leading to the determination of the sliding condition parameter, which represents a contact angle hysteresis of about 6/spl deg/. Experimental results also confirm the proportionality between minimum driving force and droplet dimension. Finally, a design optimization methodology is proposed, based on the use of finite-element model simulations.

Multi-walled carbon nanotubes experiencing electrical breakdown as gas sensors

This paper presents a new approach to gas sensing using a multi-walled carbon nanotube (MWCNT) subject to electrical breakdown. The electrical resistances of large-diameter MWCNTs were found to decrease in the presence of air after experiencing electrical breakdown, while pristine MWCNTs were not appreciably sensitive. The sensitivity could be controlled by manipulating the level of the electrical breakdown, and larger-diameter MWCNTs showed better sensitivity because they possess more damaged shells that can create more adsorption sites for oxygen molecules. It was suggested by theoretical calculations that the oxygen sensitivity might be associated with an oxidized junction that exists between the outer and inner shells of the nanotubes.