REVIEW Single-cell microfluidic impedance cytometry: a review Tao Sun • Hywel Morgan Received: 10 January 2010 / Accepted: 2 February 2010 / Published online: 6 March 2010 Ó Springer-Verlag 2010 Abstract Lab-on-chip technologies are being developed for multiplexed single cell assays. Impedance offers a simple non-invasive method for counting, identifying and monitoring cellular function. A number of different microfluidic devices for single cell impedance have been developed. These have potential applications ranging from simple cell counting and label-free identification of dif- ferent cell types or detecting changes in cell morphology after invasion by parasites. Devices have also been devel- oped that trap single cells and continuously record impedance data. This technology has applications in basic research, diagnostics, or non-invasively probing cell func- tion at the single-cell level. This review will describe the underlying principles of impedance analysis of particles. It then describes the state-of-the-art in the field of microflu- idic impedance flow cytometry. Finally, future directions and challenges are discussed. 1 Introduction Microfluidic single-cell analysis systems require techno- logical solutions for counting, trapping, focusing, separat- ing, sorting, characterisation and identification of single cells (Brown and Audet 2008; Chao and Ros 2008). Whilst bulk measurements on large populations of cells provide average information, individual cells, which are identical in appearance, generally have heterogeneous behaviour (Sims and Allbritton 2007; Svahn and Berg 2007). Therefore, high-throughput single-cell analysis methods are being developed that offer new approaches for characterising large numbers of single cells at high speed. Flow cytometry is a well-established technique for counting, identifying and sorting cells (Davey and Kell 1996; Shapiro 2004). Modern commercial fluorescence-activated-cell-sorting machines can analyse thousands of cells per second, but are generally expensive complex machines that are unsuited to handling small sample volumes. Lab-on-chip technologies (Manz et al. 1992; Whitesides and Stroock 2001; Thorsen et al. 2002; Beebe et al. 2002; Stone et al. 2004; Squires and Quake 2005; Whitesides 2006) offer new approaches for cell assays, and new technologies are being developed for high-speed cell manipulation. Individual cells can be identified on the basis of dif- ferences in size and dielectric properties using electrical techniques which are non-invasive and label-free. Char- acterisation of the dielectric properties of biological cells is generally performed in two ways using AC electrokinetics or impedance spectroscopy. AC electrokinetic techniques are used to study the behaviour of particles (movement and/or rotation) and fluids subjected to an AC electric field. The electrical forces act on both the particles and the suspending fluid and have their origin in the charge and electric field distribution in the system. They are the basis of phenomena such as dielectrophoresis (Pohl 1978; Pethig 1979; Morgan and Green 2003; Voldman 2006; Sun et al. 2007a), travelling wave dielectrophoresis (Huang et al. 1993; Morgan et al. 1997), electrorotation (Arnold and Zimmermann 1988; Yang et al. 1999) and electroorienta- tion (Jones 1995). T. Sun (&)  H. Morgan Nano Research Group, School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK e-mail: ts5@ecs.soton.ac.uk H. Morgan e-mail: hm@ecs.soton.ac.uk 123 Microfluid Nanofluid (2010) 8:423–443 DOI 10.1007/s10404-010-0580-9