Biosensors and Bioelectronics 21 (2006) 1644–1648 Short communication Sample flow switching techniques on microfluidic chips Yu-Jen Pan a , Jin-Jie Lin a , Win-Jet Luo b , Ruey-Jen Yang a,∗ a Department of Engineering Science, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan b Department of Electronic Engineering, Far East College, Tainan 744, Taiwan Received 4 May 2005; received in revised form 8 June 2005; accepted 5 July 2005 Available online 19 August 2005 Abstract This paper presents an experimental investigation into electrokinetically focused flow injection for bio-analytical applications. A novel microfluidic device for microfluidic sample handling is presented. The microfluidic chip is fabricated on glass substrates using conventional photolithographic and chemical etching processes and is bonded using a high-temperature fusion method. The proposed valve-less device is capable not only of directing a single sample flow to a specified output port, but also of driving multiple samples to separate outlet channels or even to a single outlet to facilitate sample mixing. The experimental results confirm that the sample flow can be electrokinetically pre-focused into a narrow stream and guided to the desired outlet port by means of a simple control voltage model. The microchip presented within this paper has considerable potential for use in a variety of applications, including high-throughput chemical analysis, cell fusion, fraction collection, sample mixing, and many other applications within the micro-total-analysis systems field. © 2005 Elsevier B.V. All rights reserved. Keywords: Microfluidics; Electrokinetic forces; Flow switching 1. Introduction Lab-on-a-chip devices involve the miniaturization and integration of various chemical processes onto a single chip. The literature contains many reports of the miniaturization of traditional laboratory systems onto single microchip sub- strates (Manz et al., 1990; Gravesen et al., 1993). Micro-total- analysis systems (-TAS) combine the operations of sample preparation, mixing, separation and detection. Recently, sig- nificant progress has been made in developing microfluidic component, such as mixers (Moorthy et al., 2001; Oddy et al., 2001) and flow detectors (Schrum et al., 1999). Various rudi- mentary microfluidic systems have also been demonstrated (Norlin et al., 1998; Fu et al., 2004). Compared to their large- scale counterparts, these systems reduce the cost and increase the throughput of bio-analytical assays. Continuous sample injection is an essential requirement for bio-analytical assays. Accordingly, the aim of the present study is to develop a ∗ Corresponding author. Tel.: +886 6 275 7575x63343; fax: +886 6 276 6549. E-mail address: rjyang@mail.ncku.edu.tw (R.-J. Yang). microfluidic device capable of providing a high-throughput continuous injection of single and multiple samples. When performing bio-analytical assays it is necessary to guide the sample fluid flow precisely to the specified outlet port. The purpose of such operations is to deliver accurate amounts of liquid to the reservoir where the bio- chemical reaction is to take place. In the past, researches were devoted to flow switch development in microfluidic systems. Various designs have been reported for microchip- based flow-switching devices in which some form of active structure was integrated with fluid-carrying microchannels. For example, D¨ oring et al. (1992) demonstrated that a lam- inar flow could be steered into one of two outlet ports by using a thermal bimorph cantilever with a Si/Al bimetal structure. In an alternative approach, Lemoff and Lee (2000) developed an ac magneto-hydrodynamic (MHD) microflu- idic switch, in which the Lorentz force was used to pump an electrolytic solution. In their design, the sample flow was switched between two outlets by integrating two ac MHD pumps with a Y-shaped fluidic circuit. In the two studies cited above, an active structure was integrated with a system of microchannels to guide the sam- 0956-5663/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2005.07.013