Nanocomposite Conductive Elastomer: Microfabrication Processes and Applications in Soft-Matter MEMS Sensors
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0947-A07-01
Nanocomposite Conductive Elastomer: Microfabrication Processes and Applications in Soft-Matter MEMS Sensors Chang Liu Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign, 208 North Wright Street, Urbana, IL, 61801
ABSTRACT 3D micro-molded PDMS (polydimethylsiloxane) elastomer is widely used in MEMS. However, traditional PDMS is non-conductive and as a result is used in mostly structural applications. It is difficult to embed elastomeric, “stretchy” conductors and transduction elements in molded PDMS matrix. We report general methods for monolithic fabrication of multi-layer PDMS structures with embedded conductive and non-conductive elastomer elements. Conductive PDMS parts, made of carbon-nanotube-filled composite PDMS, can form internal elastomer wires, electrodes, heaters, and sensors. The process uses a series of PDMS patterning, micromolding, and bonding techniques. In this work we demonstrate elastomer strain gauges, capacitive pressure sensors, as well as microfluidic channels with integrated heaters and sensors. INTRODUCTION Traditionally, elastomers such as PDMS have played a large role in MEMS. However, the role is largely structural – for example, serving as protective layers, encapsulants, valve diaphragms, fluidic channel structures, and so forth. To extend the use of PDMS in MEMS, it is necessary to embed wires and transduction elements. Conductive PDMS elastomer can be made by mixing/loading conductive particles, such as multi-wall carbon nanotubes (MWNT), carbon black, or metallic powerders, into regular PDMS matrix. For the devices presented here, 10% by weight MWNT are mixed with Sylgard184 PDMS to create functional elastomers. This composite polymer can achieve electrical conduction and a variety of transduction functions. For example, the resistance of patterned conductive PDMS piece is a function of chemical absorption [1], temperature [2], and strain [3]. It also forms the basis of actuation [4]. To develop a general new class of process and structures, we take advantage of a modified screen-printing process for precision patterning of thin film elastomers [5]. By combining these techniques for patterning functional elastomers with spin casting and molding, we have realized elastomer devices with embedded conductors and sensors as shown in Figure 1. The general fabrication process for a single layer is shown in Figure 2. Figure 2b-c is the precision patterning portion [5]. Multiple layers can be released and bonded.
EXPERIMENT In simplest form, a strip of conductive elastomer is embedded as a strain gauge in unmodified PDMS (Figure 3a). Applied strain alters the average spacing between conductive particles and therefore the resistance reading (Figure 3b). It is analogous to existing semiconductor and metal strain gauges, and can repeatably measure large strains (>1%). We have applied this approach to produce two more new devices – a soft capacitive tactile sensor and microfluidic channels with embedded flow-rate sensors. By combining two layers of ela
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