Wearable Sensors
December 1,2011, Printed Electronics USA 2011, Santa Clara, CA—Joshua Windmiller from UC San Diego illustrated their efforts in wearable printed sensors. As electronics and sensors continue their progression of shrinking and adding new capabilities, eventually a full laboratory will be reduced to the size of a postage stamp.
This reduction will allow sensors to be part of (protective) clothing in security and environmental monitoring. The sensors will depend upon the specific nature of chemical compounds and reactions, and on specialized electrodes printed on materials. The current technology uses screen printing to apply layers of application-specific materials like conductors, reactive sensor compounds which change parameters with contact to specific compounds, and insulators to a substrate.
The inks are formulated for good adhesion and flexibility while being the carrier for the various layer chemistries. For security functions like explosives sensing, carbon ink is screen printed on Gore-Tex fabric, which offers better surface properties than most other materials. The hysterisis curves for a printed sensor on Gore-Tex is more open and repeatable than on the same materials on cotton or polyester, and is stable over multiple bending cycles.
Test structures on fabric have shown good sensitivity and linearity to nitroaromatic explosives through multiple exposure cycles. The performance does not change much even after multiple laundry cycles. The sensors have a distinctive response for vapor-phase detection that gets more pronounced over time of exposure.
Another application is for a lab on a wetsuit. In this function, sensors are applied to neoprene for sensing chemicals under water. Sensors on a neoprene substrate demonstrate similar performance to ones on solid Al2O3 and show no effects from mechanical deformation. The neoprene-based sensors are capable of high-fidelity detection of phenol, and 4-chlorophenol in seawater. The sensor is attached to an encapsulated controller that illuminates a red LED if concentrations of pollutants exceed a safe level.
The sensors have detected heavy metals and nitroaromatic explosives in untreated seawater samples with good stability and sensitivity.
The works have demonstrated textile sensing via electrochemistry with printed sensors. Some potential applications for these printed sensors include combat and security, and environmental monitoring. While the sensors show good sensitivity, stability, and flexibility, it is important to understand the morphology and effects of mechanical strain and memory on their performance. By combining the sensors with the on-going developments in microelectronics, many new areas become open for development in the area of wearable sensors.
