Micro-impedance cytometry

Electrical impedance spectroscopy is a non-invasive and label free technique that allows for rapid counting and characterisation of particles in suspension based on their response to applied AC potentials. In recent years, lab-on-a-chip technologies have been developed to enable single-cell impedance detection and a wide range of impedance-based microfluidic devices have been reported. Despite the number of contributions and the achievements of this field, micro-impedance cytometry still suffers from a lack of sensitivity and specificity compared to traditional flow cytometry, which limits the potential commercialization of microfluidic impedance devices. While impedance measurements of beads and cells are well established, discrimination between particles that are very similar in size or detection of small particles (around 1 ?m in diameter) such as bacteria, still represents a difficult task. A number of issues limit the sensitivity and specificity of these microfluidic systems. Primarily, the sensitivity is governed by the dimension of the sample analysis volume. A small volume gives a high sensitivity, but this can lead to practical problems, including fabrication and clogging of the device. In addition, the spatial location of each particle needs to be controlled accurately within the field. Therefore, an efficient and accurate method for focussing the particles in the centre of the electric field is important. In this thesis, a micro-impedance cytometer for the detection of small particles and bacteria and for the discrimination of particles that are very similar in size is presented. The device consists of a microfluidic channel where two pairs of microfabricated electrodes are provided to perform differential measurements of single particles in suspension at high speed. Different electrode configurations and different techniques for focussing the sample within the detection region of the device are investigated in order to improve the sensitivity of the system without reducing the dimensions of the microfluidic channel. Detection at a volume ratio of particle to an estimated sensing volume of 0.007% and discrimination of 1 ?m and 2 ?m diameter polystyrene beads and E. coli are demonstrated. The micro-impedance cytometer is also proven to be a reliable and effective system to investigate and determine the unknown dielectric properties of particles in suspension, such as polyelectrolyte microcapsule