Determination of particle distributions in microfluidic systems under the influence of electric fields Andreas Heeren * , Monika Fleischer, Dieter Kern Institute of Applied Physics, University of Tuebingen, Auf der Morgenstelle 10, 72076 Tuebingen, Germany Received 5 October 2007; received in revised form 14 January 2008; accepted 15 January 2008 Available online 26 January 2008 Abstract One of the challenges in biological applications of nanotechnology is the manipulation of micro and nanoparticles in microfluidic systems. In one approach, the short-range forces exerted on particles by electric fields, e.g. via dielectrophoresis, can be utilized for this purpose. By a combination of dielectrophoresis and electroosmosis it is possible to act on particles in larger volumes. In this work a new method for examining particle distributions in microfluidic devices is presented together with results of experiments in which the method was used to investigate the combination of dielectrophoresis and electroosmosis. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Electroosmosis; Dielectrophoresis; Microfluidics 1. Introduction One of the challenges in biological applications of nano- technology is the manipulation of micro- and nanoparticles in microfluidic systems. For the capture of particles near electrodes exerting dielectrophoretic forces and for their subsequent local release, a new concept called pulsed dielectrophoresis has been developed [1]. Since the interac- tion range of dielectrophoretic forces is very limited, a com- bination of dielectrophoresis (DEP) and electroosmosis (EO) has recently been presented [2] for the manipulation of particles over dimensions typical for microfluidic sys- tems. In this case, the flow generated by EO is used to affect particles at larger distances. The dynamics of particles affected by electric fields has previously been studied by recording the processes close to the electrodes [3]. For fur- ther optimization of the above manipulation concepts, an improved measurement setup and analyzing method has been developed in this work, that allows the characteriza- tion of particle dynamics across the whole microfludic channel. 2. Determination of particle distributions Fluorescently labeled 1 lm particles in a buffer solution are investigated in an inverted fluorescence microscope. In order to determine the distribution of the particles in a microfluidic system in three dimensions, a series of micro- graphs of different image planes in the z-direction is taken at a typical spacing of 10 lm, employing fully automated microscope control and image processing. An example of such an image can be seen in Fig. 1. The software Image- Pro Plus (MediaCybernetics) makes it possible to detect particles along with their positions. With this method the rearrangement of particles in the channels can be traced. From the number of particles in each image and the known volume of the corresponding layer in the liquid the distribution of the total particle concentration can be deter- mined. A typical error of overestimation by a constant fac- tor is to be expected with this procedure, resulting from the fact that the particles are not only visible in one level of the z-stack, but in a few micrographs. In order to correct for this error, it is possible to compare the positions of the par- ticles in each plane with those in the next one. Particles at nearly the same position are counted only once. In Fig. 2, the merged image of the particles detected in two levels is 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.047 * Corresponding author. E-mail address: andreas.heeren@uni-tuebingen.de (A. Heeren). www.elsevier.com/locate/mee Available online at www.sciencedirect.com Microelectronic Engineering 85 (2008) 1294–1297