Electrohydrodynamic Control Jit Kang’s Homepage

At present, the capability of performing reactions in micro-reactors is still mainly limited to a single reaction step. My goal here is to establish control mechanisms that utilized external applied alternating current (AC) electric fields in order to achieve enzymatic reactions in controllable manner within the microfluidic systems. Currently, most control mechanisms being employed on microfluidic systems have not different from the conventional practices, mainly in manipulating the reactants flow rate into reaction zone. All these techniques do not really taking any advantages on the most unique feature of microfluidic system with extremely large surface-to-volume ratio.

This project presents two novel control mechanisms for reaction control in microfluidic devices. In the first part of the thesis, electrophoretic deposition (EPD) is being employed to assemble enzyme-coated polystyrene colloids into highly ordered arrays adjacent to an electrode. The proposed mechanism is such that enzymes responsible for catalyzing each step of a 2-step series reaction A → B → C are immobilized on different particles. In order to promote conversion from intermediate B to final product C, particles are assembled into dense clusters where diffusion of B produced on one type of particle is sufficient for it to reach the 2^nd particle type and be enzymatically converted to C. To controllably decrease the C:B ratio, the distance between particles is manipulated. Bovine serum albumin (BSA), and immunoglobulin G (IgG) coated particles were first used as a preliminary test of the effect of adsorbed proteins on colloidal aggregation in AC EPD. Rapid two-dimensional assembly and controlled disassembly of protein-coated polystyrene particles was achieved at low applied electric field frequency. In addition to experimentally identifying electric field conditions that provide the most rapid control over particle spacing, a coupled reaction and diffusion model was constructed and solved numerically to test the feasibility of the proposed control strategy. Model calculations help choose the particle size ratios that provide for the greatest effect of interparticle distance on selectivity.

In the second part, we introduce the idea of spatial control by incorporating liposome into microfluidic system. The manipulation of colloidal system, both CML polystyrene particles and liposomes, in nonuniform electric field was achieved by dielectrophoresis (DEP). This mechanism describes reaction I with reactant A → Product catalyzed by enzyme B can be attained by confining reactant A in liposomes and transporting the liposomes over certain distances to enzyme B-coated polystyrene with DEP technique. Once the liposomes and polystyrene are in close proximity, a low frequency-high field strength AC electric field is then applied to rupture the liposomes and triggering the release of reactant A. Instead of diffuse away, most of the escaping reactant A will be enzymatically converted to product by enzyme B on neighboring particles. With this the reaction locus can be restricted in certain confining region and reaction I can be spatially controlled in any desired location within the microfluidic system. This idea was presented by showing the simple form of reaction – adsorption can be carried out by our proposed mechanism. Experimentally, we further explore the rupturing behavior of liposomes under our working condition and have identified the key parameters for this mechanism to work. All these works were then supported by theoretical explanations which are logically sound and mathematically viable.

This project was carried out under the supervision of Prof. Robert D. Tilton. The full thesis can be downloaded here.