Only Skin Deep: Inkjet-Printed Zero-Power Sensors for Large-Scale RFID-Integrated Smart Skins

The ability to monitor and sense environmental conditions in real time over large areas is a difficult and expensive task, especially when it comes to monitoring in harsh environments. Whether it is monitoring suspension bridges that experience immense forces from storms and earthquakes for structural integrity, detecting noxious gases in manufacturing facilities, or making sure the vegetables on the supermarket shelf are still fresh and being kept at the correct temperature and humidity level, these sensors and sensor networks have the ability to greatly improve cognitive intelligence and knowledge of the environment around us, that is, as long as they come at the right price. Current methods for deploying large-scale sensor networks involve miles of cabling that source power and collect data, or battery operated wireless sensors, which pose a serious environmental risk with the disposal of billions of batteries every year. While these methods are necessary in some situations where real-time data or harsh environments prohibit manual monitoring of critical environment parameters, the cost, installation difficulty, and maintenance rarely justify their use over manual inspections and monitoring. This is where the concept of smart skins comes in. Smart skins are cognitive, intelligent skins that sense, wirelessly communicate, and, in the future, will be able to modify environmental parameters using simple passive RFID technology. These skins can be applied everywhere be it a shelf lining in a grocery store or the outside of a Boeing 787, all while maintaining an unobtrusive and lightweight form factor similar to the application of a decal sticker. Smart skins are zero-power devices meaning they scavenge their own energy using ambient electromagnetic, solar, thermal, mechanical, or RFID/Radar-based interrogation techniques. In short, smart skins could prove to be the ultimate sensing tool that could potentially allow for the mass implementation of perpetual wireless networks even in extremely rugged environments. The system overview of a conventional smart skin is shown in Figure 1, which shows a skin consisting of several types of sensors that can be uniquely identified in a sensing matrix, and an interrogation network that is used to poll/interrogate the sensors and relay the data back to a processing hub. This allows realtime knowledge of various sensed parameters, such as the stress gradients due to trucks passing over bridges, or of the propagation rate of a gas leak or fire within a building. The smart skin concept can also be extended to that of body-wearable skins for continuous monitoring and reporting of critical biosignals utilizing novel liquid antenna principles. Biocompatibility and wearability requirements further push the need for autonomous, self-powering sensors.