Mesoscale Thin Film Actuator for Promoting Fluid Motion in Microfluidic and Nanofluidic Channels
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Mesoscale Thin Film Actuator for Promoting Fluid Motion in Microfluidic and Nanofluidic Channels Daniel J. Sadler, Gaurav Singh1, Frederic Zenhausern, Ravi F. Saraf1 Motorola Labs Solid State Research Center Tempe, AZ 85284, U.S.A. 1 Virginia Tech Department of Chemical Engineering Blacksburg, VA 24061, U.S.A. ABSTRACT Microfluidic and nanofluidic devices often require actuators to induce fluid motion for applications such as pumping and mixing in small channels. Mixing, for instance, is important in systems where channel or chamber dimensions are on the order of 100 µm or larger as diffusive mixing can be prohibitively slow at these dimensions. In this work, a new mesoscale thin film polymer electromechanical actuator is introduced for use in the aforementioned applications. Unlike inorganic piezoelectric actuators, the devices based on these materials will be relatively easy to fabricate involving no high temperature processing, crystal growth, or microlithography. Fabrication of an array of actuators is simply achieved by spin casting the polymer over top of lithographically patterned gold electrodes at a thickness of less than 50 nm. This simple process enables a microfluidic device based on these actuators to be an integral part of a microfluidic channel rather than a separate unit operation. Depending on the application, the actuator array can be designed and controlled for random perturbations of the fluid flow field as required for mixing or for systematic actuation as required for pumping. These thin-film mesoscale actuators have been characterized and show extremely favorable properties such as a high electrostrictive response (compared to none in the bulk) and a frequency response of up to 50 kHz. In addition, finite element simulations show feasibility of these actuators for use in microfluidic mixing applications. INTRODUCTION Spurred from Silicon microfabrication, Lab-on-a-chip technology has the potential of becoming a highly pervasive approach to perform future medical diagnostics and personalized drug design. Combined with microarray technology, one of the primary drivers of the technology is the ability to simultaneously perform multitudes of molecular analytics on small volumes of biofluids [1]. A plethora of devices from basic unit operations such as micropumps [2-4] and microvalves [5-6], to complex devices such as polymerase chain reactors (PCR) [7-9] and capillary electrophoresis devices [10] have been demonstrated. Complex operation in microfluidic devices is limited, however, by the inability to mix fluids from two or more streams. Due to low Reynolds number, mixing due to convective transport is small. The mixing primarily occurs due to the diffusion and thermal fluctuations, which are relatively slow processes. To illustrate the (poor) mixing, consider DNA in a microfluidic PCR device. For mixing, the DNA must translate at least about the width of the channel, W ~ 100 microns. Typically, for a 1000 base DNA, the diffusion constant, D ~ 10-7 cm2/s. Thus the diffusion limited mixing
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