Microfluidic and Electrokinetic Manipulation of Single Cells

Abstract

Traditional cell assays report on the average results of a cell population. However, a wide range of new tools are being developed for a fundamental understanding of single cell's functionality. Nonetheless, the current tools are either limited in their throughput or the accuracy of the analysis. One such technology is electrorotation. Although it is known to be unique in its capability for single-cell characterization, it is commonly a slow technique with a processing time of about 30 minutes per cell. For this reason, this thesis focuses on the development of a 3D electrode based electrorotation setup for fast and automatic extraction of a single cell's spectrum. For this purpose, new fabrication processes for 3D electrodes were developed to achieve high-resolution patterning of 3D metal electrodes. The first process we developed was a subtractive one based on passivated silicon structures and the second process was an additive one based on SU-8 photolithography. The additive nature of the second process enables high patterning resolution of electrodes and connection layers, while providing high conductivity thanks to the use of standard metal films. The electrodes have been characterized by different electrical measurements to ensure a proper connection and side-wall exposure. Furthermore, we characterized and compared the sheet resistance of planar and vertical layers. A further microfabrication process was developed for integrating the electrodes into microfluidic channels. The process was designed to enable the use of high numerical aperture lenses; for that purpose, a PDMS-mediated bonding process was engineered to seal the channels with a thin glass coverslip. Moreover, the development of a process to realize microfluidic access holes on the back of the wafer reduces the footprint of the chips and facilitates access for the microscope optics. Finally, a pressure-driven system was used together with the chips to achieve high control of liquid injections and to enable fast and precise flow stop. The combination of such a system, together with the dielectrophoretic forces that can be applied by the 3D electrodes, allows accurate positioning of single cells inside the 3D electrode quadrupole. The particles can then be analyzed by electrorotation. For this purpose, a custom Labview interface was built to coordinate the full setup and to acquire a full electrorotation spectrum in less than 3 minutes.