On-site Multiplexing of Mechanically Imperceptible Sensor Arrays

Flexible electronics is a rapidly growing research field which enables a wide range of applications, as for consumer electronics, healthcare, environmental monitoring and a smart skins [1-3]. There are already impressive demonstrations of different types of sensors developments prepared on flexible or elastomeric support [4-9]. However, to mimic the human skin sensory ability will require the combination of multiple, distributed sensor arrays (i.e. temperature, tactile, strain) integrated on a flexible substrate. One of the challenges here is to connect each sensor with the read-out electronics. For example, if 10 sensors are to be connected, one would require at least 20 wires, which would increase the overall complexity of whole system, make it bulky and diminish its flexibility. In electrical engineering the standard approach to solve this issue is to use a multiplexing unit to minimize the amount of wires to be addressed. Yet, as there are no flexible multiplexers available, the integration of their rigid counterparts into flexible wearable systems becomes crucial [10]. Therefore, it is key point is to have a small size yet high performance multiplexer which improves the conformability and integration level of the system. As an improvement over the smallest commercially available CMOS multiplexer (32 to 1 multiplexer, overall size 7 mm x 7 mm) we have developed a monolithic CMOS analog multiplexer comprised of 96 CMOS transfer gates arranged as 3 x 32:1 multiplexers. It allows addressing up to 32 sensors simultaneously usingby 9 wires going outside and has having a footprint of just 1 mm x 4 mm.

Here, we applied the developed multiplexer for interfacing ultra-thin and mechanically imperceptible resistive temperature sensor arrays realized on imperceptible 6-µm-thick polymeric foils. As a case study, we used temperature sensor arrays. The multiplexer was integrated on a flexible printed circuit board connectedand coupled to the ultrathin sensors part by using of Z-conductive tape ora novel combination of soldering and encapsulation processes to establish reliable contacts. Using this method, we could perform real time measurements on distributed arrays of on-skin sensors. By placing these arrays on the fingers of a human hand, wWe demonstrated their use ability of this sensor array to capture and quantify the temperature information of cold/hot objects as approached by a test subject. This concept is promising for robotics, rehabilitation and, human-machine interfaces, where precise determination of local variables (e.g. temperature, pressure) is of high importance.
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