Polymer Microvalve Based on Anisotropic Expansion of Polypyrrole
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Polymer Microvalve Based on Anisotropic Expansion of Polypyrrole Yevgeny Berdichevsky, Y.-H. Lo Electrical and Computer Engineering Department, University of California, San Diego ABSTRACT An actuator fabricated from the conductive polymer polypyrrole using microfabrication techniques is presented. This actuator utilizes the anisotropic volume change, which occurs under application of an electric field in polypyrrole grown in the presence of NaDBS. This volume change is in direction normal to the substrate, and is several times larger than lateral volume change utilized in polypyrrole microactuators to date. The actuator is applied in a microfluidic valve where membrane and microchannels are fabricated from a transparent elastomer using soft lithography. The use of elastomer enabled a good valve seal and encapsulation of the electrolyte used for polypyrrole actuation. INTRODUCTION Conducting polymers, distinguished by conjugated polymer backbones, are used for a variety of applications, including fabrication of photonic and electronic devices such as LEDs, transistors, and chemical sensors [1, 2]. Some conducting polymers, such as polypyrrole, have also been used as actuators, or as “artificial muscles” [3]. Under the stimulus of an electric potential, polypyrrole undergoes a reduction-oxidation reaction involving movement of ions in and out of the polymer matrix. This migration of ions leads to volume change. Most commonly, the volume change is converted into mechanical work by employing a bilayer structure, typically a film of polypyrrole a few tens of micrometers thick on a conducting flexible substrate. When the polymer expands or contracts, the bilayer bends, and electrochemical energy is converted into mechanical energy. These polypyrrole bilayers have been employed at both macro- and micro- dimensions [4], and it has been shown that the volume change in polypyrrole of 2-3% along the surface of the bilayer can produce very large displacement. However, there are a few problems associated with this mode of actuation. The mechanical force that can be applied by the tip of the bilayer, which is the point that undergoes the most physical displacement, is very small – some orders of magnitude smaller than the pressure generated in the polypyrrole film. Also, delamination of the polymer from the substrate is very common, since the bonding between polymer film and electrode is mostly due to noncovalent interactions [5]. It has been reported in the literature that polypyrrole electropolymerized in the presence of sodium dodecylbenzenesulfonate (NaDBS), can undergo reversible volume change of 30% or more in the direction normal to the substrate [6]. The authors hypothesized that this is due to anisotropic morphology of polypyrrole (PPy) film with incorporated DBS ions. This volume change, if realized in a thick micropatterned PPy film, can be directly applied in a variety of micromechanical systems. In this work, a microvalve utilizing anisotropic volume change of PPy(DBS) is fabricated.
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THE CONCEPT A
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