Lap Man Lee

Self-aligned immobilization of proteins utilizing PEG patterns.

A novel self-aligned method to selectively immobilize proteins on a silicon dioxide surface is developed in conjunction with a standard lift-off patterning technique of a PEG layer. The approach is designed to photolithographically pattern regions that specifically bind target proteins and particles, surrounded by regions that suppress non-specific attachment of bio-species. The physical and biological properties of the derivatized surfaces at the end of the fabrication process are characterized.

In-plane vortex flow in microchannels generated by electroosmosis with patterned surface charge

Electrokinetically driven in-plane vortex flows in a microchannel are studied utilizing a patterned surface charge technique requiring both positively and negatively charged regions on the same substrate. In the first part, a periodic flow pattern consisting of counter-rotating vortex pairs is analyzed experimentally and numerically; this is a relatively easy flow to experimentally realize in the lab since no charge-free region is necessary. The good agreement between the measured and computed flow fields demonstrates that: (i) the surface charge patterning technique can be used for driving electrokinetically complex vortex flow patterns in microchannels, and (ii) the applied CFD code can be used for calculating reliably such flow fields. In the second part, the numerical scheme is utilized to study a single, in-plane vortex in order to reveal the proper length and velocity scales as well as the dominant control parameters. This flow field, although simpler, is very difficult to realize experimentally due to the need for a large surface area carrying no charge. The resulting 3D flow field features a coherent vortex with its axis perpendicular to the symmetrically charged regions on the top and bottom surfaces of the microchannel. Three length scales, the active-region length and width as well as the channel height, and a velocity scale, the speed of the electroosmotic flow, have been identified as the relevant variables. The strength of the in-plane vortex along with several flow patterns has been characterized on the basis of these four independent variables.

Two-Dimensional Analysis of Electrokinetically Driven Out-of-Plane Vortices in a Microchannel Liquid Flow Using Patterned Surface Charge

The technology developed for photolithographically patterning the electric surface charge to be negative, positive, or neutral enables the realization of complex liquid flows even in straight and uniform microchannels with extremely small Reynolds number. A theoretical model to analyze a steady incompressible electrokinetically driven two-dimensional liquid flow in a microchannel with an inhomogeneous surface charge under externally applied electric field is derived. The flow field is obtained analytically by solving the biharmonic equation with the Helmholtz-Smoluchowski slip boundary condition using the Fourier series expansion method. The model has been applied to study three basic out-of-plane vortical flow fields: single vortex and a train of corotating and a series of counterrotating vortex pairs. For model verification, the solution for the single vortex has been tested against numerical computations based on the full Navier-Stokes equations revealing the dominant control parameters. Two interesting phenomena have been observed in out-of-plane multivortex dynamics: merging of corotating vortices and splitting of counterrotating vortices. The criteria for the onset of both phenomena are discussed

Fabrication of microchannels with patterned bio-active layers

Patterns of bio-active coatings on the inner surfaces of microchannels have been realized using a novel low-temperature, UV-epoxy glass-to-silicon bonding technique. Sucrose is applied as a protection layer for the immobilized bio-functional films during the bonding step. The bio-functional layer is composed of antibody patterns, for binding specific targets, next to polyethylene glycol (PEG) coated regions for preventing non-specific absorption. The activity of the patterned bio layer is tested, after the removal of the sucrose protection layer, utilizing fluorescent microscopy. A solution of fluorescent-labelled antigens is injected into the microchannels for incubation with the immobilized antibodies. Upon exposure to proper radiation, light is emitted only from the antibody patterns while the PEG regions remain dark. Hence, the sucrose-protection and UV-bonding techniques have not significantly compromised the functionality of the patterned antibodies, in binding to their counter receptors, and PEG molecules, in preventing non-specific adsorption, at the end of the fabrication process.

Unsteady in-plane vortex motion in a microchannel liquid flow

An unsteady flow can dramatically enhance the mixing efficiency in a highly localized region, as the flow would become chaotic if time is an independent variable. In this work, the response of uniform electroosmotic flow to an oscillating electric field is first examined experimentally and numerically as a function of the driving frequency. Then a steady in-plane micro vortex flow pattern, traced by microparticles, is realized and compared to numerical simulations. Upon confirmation of the simulations for uniform but unsteady and steady but non-uniform flows, the CFD code has finally been applied to study unsteady non-uniform flow field, for which it is difficult to measure flow properties. The time evolution of liquid vortex motion in a microchannel, due to either sinusoidal or sudden electric field reversal, is numerically investigated revealing the relationship between length and time scales dominating momentum transfer in electrokinetically-driven unsteady liquid flow.

Microfluidics: Device Science and Technology

This paper presents the basic concepts related to the newly emerging field of microfluidics. Following a brief introduction of the general conservation and particular laws, three size effects are introduced. The velocity slip boundary condition for gas flows as well as the electrokinetic and polar-mechanics effects for liquid flow in microdomains are introduced. Pressure-driven gas flows and electrokinetically-driven liquid flows in microchannels are analyzed in details. Finally, several flow diagnostic techniques and fabrication of microfluidic systems are described.

Electrokinetically-driven micro mixer with a novel surface-charge pattern

An electrokinetically-driven micro mixer with a special surface-charge pattern was designed, fabricated and characterized using fluorescence video microscopy. Zeta potential of the working fluids was measured to facilitate the numerical simulation and optimization of the proposed mixer. The mixing flow of the electrolyte with and without microparticle was digitally recorded and analyzed in terms of concentration profile and mixing index. The experimental mixing enhancement is consistent with the simulation result