A Passive Inertial Switch Using MWCNT–Hydrogel Composite With Wireless Interrogation Capability

This paper presents the development of a passive inertial switch using multiwall carbon nanotube (MWCNT)–hydrogel composite integrated with an inductor/capacitor ( $L$ – $C$ ) resonator. The device consists of a polydimethylsiloxane (PDMS) microfluidic chip containing MWCNT–hydrogel composite and water droplet and a glass substrate with a capacitor plate and an inductor coil. When the acceleration exceeds the designed threshold level, the water passes through the channel to the hydrogel cavity. The hydrogel swells and changes the capacitance of the integrated $L$ – $C$ resonator, which, in turn, changes the resonant frequency that can be remotely detected. Each sensor unit does not require onboard power and circuitry for operation, so the proposed device is disposable and is suitable for low-cost applications. All PDMS structures were fabricated using soft lithography. The $L$ – $C$ resonator was fabricated using a lift-off process to pattern metal layers on a glass substrate. The response time of the device is considerably reduced by introducing MWCNTs into the hydrogel composites. The dimensions of the device are $15 \hbox {mm} \times 10 \hbox {mm} \times 1.5 \hbox {mm}$ . The characterization of the proposed device was also demonstrated. The threshold $g$ -values, which differ for various applications, were strongly affected by the channel widths. The phase-dip measurement shows that the resonant frequencies shi- t from 164 to approximately 148 MHz when the device is activated by acceleration. $\hfill$ [2012-0290]