Dielectrophoresis can discriminate distinct cellular identities in heterogeneous populations, and monitor cell state changes associated with activation and clonal expansion, apoptosis, and necrosis, without the need for biochemical labels. Demonstrated capabilities include the enrichment of haematopoetic stem cells from bone marrow and peripheral blood, and adult stem cells from adipose tissue. Recent research suggests that this technique can predict the ultimate fate of neural stem cells after differentiation before the appearance of specific cell-surface proteins. This review summarises the properties of cells that contribute to their dielectrophoretic behaviour, and their relevance to stem cell research and translational applications.

Cell identification and enumeration are essential procedures within clinical and research laboratories. For over 150 years, quantitative investigation of body fluids such as counts of various blood cells has been an important tool for diagnostic analysis. With the current evolution of point-of-care diagnostics and precision medicine, cheap and precise cell counting technologies are in demand. This article reviews the timeline and recent notable advancements in cell counting that have occurred as a result of improvements in sensing including optical and electrical technology, enhancements in image processing capabilities, and contributions of micro and nanotechnologies. Cell enumeration methods have evolved from the use of manual counting using a hemocytometer to automated cell counters capable of providing reliable counts with high precision and throughput. These developments have been enabled by the use of precision engineering, micro and nanotechnology approaches, automation and multivariate data analysis. Commercially available automated cell counters can be broadly classified into three categories based on the principle of detection namely, electrical impedance, optical analysis and image analysis. These technologies have many common scientific uses, such as hematological analysis, urine analysis and bacterial enumeration. In addition to commercially available technologies, future technological trends using lab-on-a-chip devices have been discussed in detail. Lab-on-a-chip platforms utilize the existing three detection technologies with innovative design changes utilizing advanced nano/microfabrication to produce customized devices suited to specific applications.

Measurements are reported of the main factors, namely the AC voltage frequency and magnitude, that were observed to influence the number of cells destroyed during dielectrophoresis (DEP) experiments on Jurkat T cells and HL60 leukemia cells. Microelectrodes of interdigitated and quadrupolar geometries were used. A field-frequency window has been identified that should be either avoided or utilised, depending on whether or not cell damage is to be minimised or is a desired objective. The width and location of this frequency window depends on the cell type, as defined by cell size, morphology and dielectric properties, and is bounded by two characteristic frequencies. These frequencies are the DEP cross-over frequency, where a cell makes the transition from negative to positive DEP, and a frequency determined by the time constant that controls the frequency dependence of the field induced across the cell membrane. When operating in this frequency window, and for the microelectrode designs used in this work, cell destruction can be minimised by ensuring that cells are not directed by positive DEP to electrode edges where fields exceeding 30-40 kV/m are generated. Alternatively, this field-frequency window can be exploited to selectively destroy specific cell types in a cell mixture.

The manipulation of ribosomal RNA ͑rRNA͒ extracted from E. coli cells by dielectrophoresis ͑DEP͒ has been demonstrated over the range of 3 kHz-50 MHz using interdigitated microelectrodes. Quantitative measurement using total internal reflection fluorescence microscopy of the time dependent collection indicated a positive DEP response characterized by a plateau between 3 kHz and 1 MHz followed by a decrease in response at higher frequencies. Negative DEP was observed above 9 MHz. The positive DEP response below 1 MHz is described by the ClausiusMossotti model and corresponds to an induced dipole moment of 3300 D with a polarizability of 7.8ϫ 10 −32 F m 2 . The negative DEP response above 9 MHz indicates that the rRNA molecules exhibit a net moment of Ϫ250 D, to give an effective permittivity value of 78.5 0 , close to that of the aqueous suspending medium, and a relatively small surface conductance value of ϳ0.1 nS. This suggests that our rRNA samples have a fairly open structure accessible to the surrounding water molecules, with counterions strongly bound to the charged phosphate groups in the rRNA backbone. These results are the first demonstration of DEP for fast capture and release of rRNA units, opening new opportunities for rRNA-based biosensing devices.

Human African trypanosomiasis or sleeping sickness is a deadly disease endemic in sub-Saharan Africa, caused by single-celled protozoan parasites. Although it has been targeted for elimination by 2020, this will only be realized if diagnosis can be improved to enable identification and treatment of afflicted patients. Existing techniques of detection are restricted by their limited field-applicability, sensitivity and capacity for automation. Microfluidic-based technologies offer the potential for highly sensitive automated devices that could achieve detection at the lowest levels of parasitemia and consequently help in the elimination programme. In this work we implement an electrokinetic technique for the separation of trypanosomes from both mouse and human blood. This technique utilises differences in polarisability between the blood cells and trypanosomes to achieve separation through opposed bi-directional movement (cell counterflow). We combine this enrichment technique with an automated image analysis detection algorithm, negating the need for a human operator.