Passive Wireless Strain and pH Sensing Using Carbon Nanotube-Gold Nanocomposite Thin Films Kenneth J. Loh a , Jerome P. Lynch *a,b , and Nicholas A. Kotov c a University of Michigan, Dept. of Civil & Environmental Engineering; b University of Michigan, Dept. of Electrical Engineering & Computer Science; c University of Michigan, Dept. of Chemical Engineering, Ann Arbor, MI 48109. ABSTRACT The recent development of wireless sensors for structural health monitoring has revealed their strong dependency on portable, limited battery supplies. Unlike current wireless sensors, passive radio frequency identification (RFID) systems based on inductive coupling can wirelessly receive power from a portable reader while transmitting collected data back. In this paper, preliminary results of a novel inductively coupled strain and corrosion sensor based upon material fabrication techniques from the nanotechnology field are presented. By varying polyelectrolyte species during a layer-by-layer fabrication process, carbon nanotube-polyelectrolyte multilayer thin film sensors sensitive to different mechanical (e.g. strain) and chemical (e.g. pH) stimuli can be produced. Validation studies conducted with different carbon nanotube thin films designed as either strain or pH sensors reveal high sensitivity and linear performance. When coupled with a copper inductive coil antenna, resulting RFID-based sensors exhibit wirelessly readable changes in resonant frequency and bandwidth. Furthermore, a carbon nanotube-gold nanocomposite thin film is fabricated and patterned into a highly conductive coil structure to realize a novel thin film inductive antenna. Preliminary results indicate that nanotube-gold nanocomposites exhibit resonance conditions, holding great promise for future RFID applications. Keywords: Carbon nanotube, gold nanoparticles, layer-by-layer assembly, pH, RFID, strain, wireless. 1. INTRODUCTION Over the lifespan of civil infrastructures (namely bridges, buildings, pipelines, among many others), unanticipated extreme loading (e.g. earthquakes) and harsh environmental conditions (e.g. deicing salt on roadways) can adversely affect long-term structural integrity and performance. As a result, researchers in the structural health monitoring (SHM) community have utilized tethered sensor networks to measure structural responses that can be used to identify structural deterioration. While traditional cabled monitoring systems are reliable, extensive cabling leads to high capital, installation, and maintenance costs. For instance, a recent instrumentation on the Tsing Ma suspension bridge in Hong Kong (2001) intended to monitor bridge behavior to strong wind and seismic loading costs $27,000 per sensing channel [1]. Such high costs prevent many users from installing dense sensor networks that would be needed to identify local damage. In recent years, a variety of low cost academic and commercial wireless sensor prototypes have been developed in hopes to outperform traditional tethered monitoring systems [2]. By providing reliable wireless data communication and embedded computing power, many wireless sensor prototypes have been validated in the laboratory and in the field to reveal performance levels just as good, if not better, than cabled systems [3]. Unfortunately, one significant disadvantage of wireless sensors is that they typically require portable power supplies (i.e. batteries) coupled with them. Nevertheless, their lower costs permit dense sensor network installation to allow the transitioning from global-based (e.g. modal analysis) to component-level monitoring. Since wireless sensors suffer from finite power constraints, many researchers have developed low-cost wireless sensors based on inductive coupling, also known as radio frequency identification (RFID) [2]. In particular, RFID sensors for monitoring strain and corrosion environments (pH) have been proposed. Both types of sensors are monitoring the structure at the local-level with strain correlated to some damage processes and pH changes suggesting environments sensitive to the corrosion of steel materials [4]. Early RFID sensor investigation has been performed by Mita and * jerlynch@umich.edu ; phone 1 734 615-5290; http://www-personal.umich.edu/~jerlynch/index.html Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2007, edited by Masayoshi Tomizuka, Chung-Bang Yun, Victor Giurgiutiu, Proc. of SPIE Vol. 6529, 652919, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.715826 Proc. of SPIE Vol. 6529 652919-1