Electrothermally actuated inline microfluidic valve P. Selvaganapathy * , E.T. Carlen, C.H. Mastrangelo Department of Electrical Engineering and Computer Science, Center for Wireless Integrated Microsystems, University of Michigan, Ann Arbor, MI 48109-2122, USA Abstract A normally open electrothermally actuated inline microvalve that has been developed, fabricated and tested with liquids is presented. These actuators use high volumetric expansion of a sealed patch of Paraffin heated above its melting point, providing large displacements and forces while using low power. The inline valve is surface micromachined on top of preformed flexible microfluidic channels using a low temperature fabrication process; therefore it is suitable for integration with microfluidic networks requiring actuation of a large number of independent valves under electrical control. Complete closure of sealed microchannels has been observed with power as low as 40 mW. Response times of 15 ms have been measured. Breakdown of the inline valve occurs at an upstream pressure of 23 psig. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Microfluidic; Valve; Paraffin; Parylene; Surface micromachined; Inline 1. Introduction Emerging microfluidic systems involve the integration and automation of many individual steps performed in macroscale biochemical analysis [1–3]. This requires the ability to control precisely and efficiently the transport of reagents and samples throughout different parts of the system. Typically these systems require many valves oper- ating simultaneously or in sequence. Therefore, the valve actuators must be small, operating with low power, and must integrate easily with the system components. Over the past decade, there has been wide variety of actuation mechanisms and methods employed for construc- tion of microvalves including electrostatic, magnetic, piezo- electric, bimorph and thermopneumatic actuation [1]. Some of these techniques employ bulk micromachining and ano- dic bonding, making the valves large, out of plane and difficult to integrate in large scale in microfluidic systems. In this paper, we present a simple microfluidic valve that is easily integrated with other fluidic components on the same die. Thermopneumatic actuation was chosen for this valve because it holds several advantages over other actua- tion schemes. Most actuation methods provide either large displacements or large forces. Shape memory alloys provide both large actuation and displacements but can be difficult to integrate and do not conform to the shape of the channel [4]. Thermopneumatic actuation provides both large displace- ments (2–10 mm) and forces (1 N), and with the recent published method [5] for batch fabrication compatible with microfluidic systems, is ideal for this application. The thermopneumatic actuators used as the active elements of the microvalves presented here are based on the phase change expansion of a sealed surface micromachined patch of Paraffin. 2. Device design The actuation mechanism depends on a thermally trig- gered phase change in the Paraffin actuation material result- ing in a volumetric expansion. Paraffin is a linear saturated hydrocarbon mixture of alkanes of varying chain lengths, with high volumetric expansion upon phase change from solid to liquid. The general chemical formula for Paraffins is C n H 2nþ2 , composed entirely of Carbon–Carbon single bonded chains. In the solid state, there is van der Waal’s forces of attraction are present between the hydrogen atoms in adjacent chains. Hence increased chain lengths increase the van der Waal’s forces, thereby increasing the melting temperature of the Paraffin. Fig. 1 shows the range of melting temperatures for Paraffins of different chain lengths. Hence the actuations temperature can be tailored to the needs of the application by the choice of the actuation material chain length. Paraffin also has other desirable properties such as low thermal and electrical conductivity, Sensors and Actuators A 104 (2003) 275–282 * Corresponding author. E-mail address: pselvaga@umich.edu (P. Selvaganapathy). 0924-4247/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-4247(03)00030-X