Paraffin Actuated Surface Micromachined Valves

A new, active, normally-open blocking microvalve that uses the thermal expansion of a sealed, thin paraffin patch for actuation has been fabricated and tested. The entire structure is batch-fabricated by surface micromachining the actuator and channel materials on top of a single substrate. The paraffin actuated microvalves are suitable for applications requiring many devices on a single die, low processing temperatures, and simple, nonbonded process technology. Gas flow rates in the 0.1-2.0 sccm range have been measured for several devices with actuation powers less than 50mW. INTRODUCTION Over the past decade elaborate microfluidic valves have been constructed based on electrostatic, magnetic, piezoelectric, bimorph and thermopneumatic actuation methods [1]. Because of their complexity, the majority of these devices are made by bonding many thick glass or silicon substrates together, some even requiring external cavity fills. This bulk micromachining construction technique makes the valves large and difficult to integrate with other components in microfluidic systems. Applications requiring many active valves on a single die are rapidly emerging. Integrated micro gas chromatography and mass spectrometry systems are being developed and will require effective valving devices [2–4]. In addition, microfluidic systems, such as DNA analysis systems [5, 6], require effective microvalves in order to control the transport of samples and reagents throughout different parts of the system. Typically these systems require many, independently operating valves in order to perform complex or parallel functions. Therefore, the actuators must operate independently, have a simple fabrication process and integrate easily with the other system components. In this paper, we present a simple valve that is easily integrated with other fluidic components on the same die. Key to our simple valve fabrication is the choice of actuation method. Most actuation methods provide either a large deflection but not a large force (or vice-versa); therefore they require complex construction methods that overcome their short comings (through mechanical advantage schemes for example). Thermopneumatic actuators can provide both large displacements and forces, but their fabrication and integration in large microsystems is cumbersome [7]. Shape memory alloy microactuators also provide both large displacements and forces, but are typically difficult to fabricate [8]. Recently, a new type of microactuator which provides large displacements ( m), high actuation force ( 1N), fabrication simplicity, and suitability for microfluidic valve applications has been reported [9]. These microactuators are based on the thermal expansion of a sealed, surface micromachined patch of high actuating power paraffin wax film. DESIGN Material: The microactuators used for active valves in this paper are based on the thermal expansion of a thin layer of paraffin wax. Paraffin belongs to a family of materials that experience a large volumetric expansion upon melting. Typically a volumetric expansion between 10 30% can be reached when heated to temperatures ranging from 65-150 C, depending on the melting temperature of the material. Figure 1 shows the pressure-volumetemperature (PVT) characteristics of the paraffin material used for the devices in this paper (bonding wax, Logitech, Ltd. (0CON-175) with a melting temperature of 75 C). S p ec if ic V o lu m e ( cm / g ) 3 20.0 40.0 60.0 80.0 100.0 120.0 140.0 1.30