IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 17 (2007) 1467–1478 doi:10.1088/0960-1317/17/8/008 In situ analysis of bacterial capture in a microfluidic channel Ashwin K Balasubramanian 1 , Ali Beskok 2 and Suresh D Pillai 3 1 Mechanical Engineering Department, Texas A&M University, College Station, TX, USA 2 BioMicroFluidics Laboratory, Old Dominion University, Norfolk, VA, USA 3 Poultry Science Department, Texas A&M University, College Station, TX, USA E-mail: abeskok@odu.edu Received 7 March 2007, in final form 10 May 2007 Published 29 June 2007 Online at stacks.iop.org/JMM/17/1467 Abstract We present a microfluidic approach for the continuous capture of Salmonella Newport cells suspended in a phosphate buffer using externally applied electric fields. The effects of flow rate, applied electric field and wall shear stress on cell capture in the device are analyzed using particle tracking via fluorescent microscopy techniques. Analyzing capture across multiple locations on the electrode surface enabled the estimation of average capture over the entire electrode area as a function of time. The device exhibits approximately a constant capture rate over an extended time frame, which is verified independently using the cell culture methods. An increased capture rate with an increased electric field is observed. The capture rate dependence on the flow rate and capture rate at various locations with different wall shear stress magnitudes does not exhibit statistically significant variations. The capture trends presented in this study can be utilized for designing microfluidic systems for biosensors, designed bacterial bio-films and devices for bacterial sample concentration from large volumes. (Some figures in this article are in colour only in the electronic version) Introduction Microorganisms adversely impact man-made and natural ecosystems by forming biofilms and bio-fouling [1–3]. Rapid isolation and detection of bacterial cells are crucial in medical diagnosis, water distribution lines, space exploration missions and bioterrorism-related events. Initial attachment of bacterial cells to the substrate is obviously the primary step in the formation of biofilms. When bacterial cells are in close proximity to a solid surface (<100 nm from surface), various interaction processes including van der Waals interactions, hydrophobic interactions and electrostatic interactions play an important role in determining the adhesion process. However, electrophoretic transport and electrostatic interactions between charged surfaces play a vital role in attraction of bacterial cells toward the surfaces, so that other interactions and adhesion can occur. Systematic studies of capture and immobilization of bacterial cells on a surface are essential to gain a better understanding of bacterial attachment to charged substrates in flow-based systems. Several researchers have used flow-based systems to study bacterial adhesion to surfaces in micro-scale devices. These include parallel plate and stagnation point flow chambers, rotating disk systems and cylindrical channels [4–9]. Design of such systems requires careful consideration of the time scales involved in fluid and particle motion. These systems often neglect various electrochemistry effects that arise due to the presence of electric fields. Quantification of adhesion in these systems was primarily based on microscopy, which requires an efficient particle tracking algorithm [10]. In situ quantification methods are often more reliable than indirect methods that involve removing the substrate from the flow chamber, washing off cells that did not adhere to the electrode and counting the captured cells under a microscope. One of the main advantages of the in situ microscopy analysis is that adhesion and desorption characteristics of cells can be studied as a function of flow parameters in detail. In this study, in situ quantification of microbial capture was studied by applying pressure-driven flow in a parallel plate microfluidic chamber with an external potential difference. 0960-1317/07/081467+12$30.00 © 2007 IOP Publishing Ltd Printed in the UK 1467