ELECTROSTATICALLY-ACTUATED RECONFIGURABLE ELASTOMER MICROFLUIDICS

We present a user-programmable reconfigurable elastomer microfluidic system which employs electrostatic actuation of water-filled elastomer microfluidic channels. Device actuation was achieved by applying 5 MHz, 15-20 V voltages to induce “wet” electrostatic gap-closing of nano-liter PDMS microfluidic chambers embedded with metal flexure electrodes. The primary contributions of this technology are: a) elastomer microfluidics that do not require external pneumatics to actuate when filled with air, oil, or water, b) a fabrication process compatible with standard PDMS microfluidics, and c) actuation voltages low enough to be driven by off-the-shelf RF IC’s. INTRODUCTION For the past fifteen years, microfluidics has been a research field of intense scientific and engineering interest [1]. Technologies based on elastomers such as polydimethylsiloxane (PDMS) and parylene have become very popular due to ease of fabrication and use [2, 3, 4]. Most these technologies require external pneumatic connections for liquid control, but this manipulation becomes problematic as the system advances toward high-density large-scale integration. The lack of local low power actuation also prevents in vivo applications of these devices [5]. In this paper, we present a completely reconfigurable distributed elastomer microfluidic network system, as shown in Fig. 1. The operation of the device and two designs of the fabricated chips are shown in Fig. 2. The device consists of 7 (hexagon) or 9 (square) micro chambers which can be actuated independently by electrostatic pull-in of a PDMS-metal membrane [6]. The embedded metal flexure forms the top capacitor plate and the ITO electrode forms the bottom plate. When a voltage above the pull-in voltage is applied, the PDMS-metal “roof” collapses onto the “floor” (Fig. 2 a) and liquid within this actuated chamber is expelled into adjacent open micro chambers. This applied signal had a 5 MHz frequency to prevent electrical double layer screening of the electrostatic forces [7]. This makes possible the design of VLSI fluidic systems with many actuated components each driven by digitally synthesized signals. The device can actuate in air, water, and oil without external fluidic connections. These micro devices are also scalable to nanofluidic regimes and are compatible with general PDMS microfluidics. THEORY AND DEVICE DESIGN A key issue of designing this micro device is the pull-in voltage which is affected by the device geometry, mechanical properties and dielectric constants of the membranes and working liquids. A closed form of the pull-in voltage can be expressed as [7, 8]: where k = spring constant; tox = oxide thickness; g = channel height; εox= oxide permittivity; σR = membrane residual stress, A= capacitor area; εL = liquid dielectric permittivity; ν = Poisson’s ratio; E = Young’s modulus, and t = membrane thickness. Devices with square elements measured 300 μm × 300 μm and hexagonal elements had 200 μm edges, as shown in Fig. 2b. Hexagonal chambers were found preferable due to 1) compact footprint 2) shorter liquid transport path and 3) more uniform membrane deflection. The patterned spiral metal line was only for electric conduction of the PDMS, and its stiffness could be neglected when compared to that of PDMS membrane. The spring constant [9] of the devices and the pull-in voltage were calculated to be 20-30 N/m and 15-20 V. We previously characterized the pull-in behavior of this type of device [6]. The inlets connecting to each micro chamber were 50 μm in width and 5 mm in length. Table 1: Parameter values used in equation (1).