Ph.D Thesis

Ph.D StudentParatore Federico
SubjectCoupling Electrokinetics with Patterned Surfaces: from
Enhanced Immunoassays to Microscale Flow Control
DepartmentDepartment of Mechanical Engineering
Supervisors ASSOCIATE PROF. Moran Bercovici
DR. Govind Kaigala
Full Thesis textFull thesis text - English Version


This dissertation is concerned with the coupling of isotachophoresis (ITP) focusing with surface immunoassays and the study of non-uniform electroosmotic flow (EOF) as a mechanism to control microscale flows.

In the first part of the dissertation, we present a new class of surface-based immunoassays in which protein-antibody reaction is accelerated by ITP. We demonstrate the use of ITP to pre-concentrate and deliver target proteins to a surface decorated with specific antibodies, where effective utilization of the focused sample is achieved by modulating the driving electric field (stop-and-diffuse ITP mode) or applying a counter flow that opposes the ITP motion (counterflow ITP mode). Using enhanced green fluorescent protein (EGFP) as a model protein, we carry out an experimental optimization of the ITP-based immunoassay and demonstrate a 1,300-fold improvement in limit of detection compared to a standard immunoassay, in a 6 min protein-antibody reaction. We discuss the design of buffer chemistries for other protein systems, and in concert with experiments provide full analytical solutions for the two operation modes, elucidating the interplay between reaction, diffusion and accumulation time scales, and enabling the prediction and design of future immunoassays.

In the second part, we experimentally demonstrate the phenomenon of electroosmotic dipole flow that occurs around a localized surface charge region under the application of an external electric field in a Hele-Shaw cell. We use localized deposition of polyelectrolytes to create well-controlled surface charge variations, and show that for a disk-shaped spot, the internal pressure distribution that arises, results in uniform flow within the spot and dipole flow around it. We further suggest that the superposition of surface charge spots can be used to generate complex flow patterns.

In the third part, we leverage this fundamental work and demonstrate the use of localized field effect electroosmosis (FEEO) to create dynamic flow patterns, allowing fluid manipulation without the use of physical walls. We control a set of gate electrodes embedded in the floor of a fluidic chamber using an AC voltage in sync with an external electric field, creating non-uniform EOF distributions. These give rise to a pressure field that drives the flow throughout the chamber. We demonstrate a range of unique flow patterns that can be achieved, including regions of recirculating flow surrounded by quiescent fluid and volumes of complete stagnation within a moving fluid. We also demonstrate the interaction of multiple gate electrodes with an externally generated flow field, allowing spatial modulation of streamlines in real time. Furthermore, we provide a characterization of the system in terms of time response and dielectric breakdown, as well as engineering guidelines for its robust design and operation. We believe that the ability to create tailored microscale flow using solid state actuation will open the door to entirely new on-chip functionalities.