As manufacturing technology has improved over the past few years, it is inevitable that the size of components to be manufactured has been affected, and the desire to reduce the size of such components is the driving force behind the move towards micro-and nanotechnology. One of the problems is the electrode breakdown at electrode separations for less than a millimetre separation. At large separation, the behaviour of the electrodes has been widely studied and is reasonably well understood. However, some fundamental problems have not been properly addressed such as maximum safe operating voltages and critical dimensions required at the small separations between the different types of materials. A systematic study of electrical breakdown at sub-millimetre separations using materials commonly used in the fabrication of microdevices is being undertaken. Specimens for examination at electrode separations from 500 nm to 25 µm have been made with different electrode configurations, such as flat to flat and flat to point.

The separation of red blood cells from plasma flowing in microchannels is possible by biophysical effects such as the Zweifach-Fung bifurcation law. In the present study, daughter channels are placed alongside a main channel such that cells and plasma are collected separately. The device is aimed to be a versatile but yet very simple module producing high-speed and high-efficiency plasma separation. The resulting lab-on-a-chip is manufactured using biocompatible materials. Purity efficiency is measured for mussel and human blood suspensions as different parameters, such as flow rate and geometries of the parent and daughter channels are varied. The issues of blood plasma separation at the microscale are discussed in relation to the different regimes of flow. Results are compared with those obtained by other researchers in the field of micro-separation of blood.

A microfluidic system was developed for blood plasma separation at high flow rate. This system uses only hydrodynamic forces to separate plasma from whole blood. The microfluidic network features a series of constrictions and bifurcations to enhance the product yield and purity. A maximum purity efficiency of 100% is obtained on blood with entrance hematocrit level up to 30% with a flow rate of 2 mL h(-1). Flow cytometry was performed on the extracted plasma to evaluate the separation efficiency and to assess cell damage. A core target of this study was the detection of cell-free DNA from the on-chip extracted plasma. To this effect, PCR was successfully carried out off-chip on the cell-free DNA present in the plasma extracted on-chip. A house-keeping gene sequence (GAPDH) was amplified without the need for a purification after the separation, thereby showing the high quality of the plasma sample. The resulting data suggests that the system can be used as a preliminary module of a total analysis system for cell-free DNA detection in human plasma.

Recent advances in microparticle separation in continuous flow are presented. It is intended for scientists in the field of separation science in biology, chemistry and microsystems engineering. Recent techniques of micron-sized particle separation within microsystems are described with emphasis on five different categories: optical, magnetic, fluidic-only, electrical and minor separation methods. Examples from the growing literature are explained with insights on separation efficiency and microengineering challenges. Current applications of the techniques are discussed.

For efficient operation, micromotors and microactuators, such as those employed in microsystems, are required to operate with high electric fields at electrode separations of the order of micrometres. An apparatus was built to accurately measure the breakdown voltage for electrode spacings as low as 0. 5~. Breakdown voltage measurements in air and nitrogen are presented and discussed for the gap range 0.5 to 1 5~. Energy dispersive analysis of X-rays (EDAX) confirms the transfer of material from cathode to anode and vice V L ' I :~ during the breakdown mechanism. The Paschen law has been confirmed not to be applicable at gap settings of less than 4 p. The shape of the curve and the breakdown voltage values are found to be the same for different gases and different high pressures up to 4 p separation. Below this value, an analytical explanation of the breakdown voltage based on quantum tunnelling of electrons is obtained in terms of electrical field enhancement at microprotrusions and the work function of the electrode material.