Yi Ren

Sensors are core elements to directly obtain information from surrounding objects for further detecting, judging and controlling purposes. With the rapid development of soft electronics, flexible sensors have made considerable progress, and can better fit the objects to detect and, thus respond to changes more sensitively. Recently, as a newly emerging electronic ink, liquid metal is being increasingly investigated to realize various electronic elements, especially soft ones. Compared to conventional soft sensors, the introduction of liquid metal shows rather unique advantages. Due to excellent flexibility and conductivity, liquid-metal soft sensors present high enhancement in sensitivity and precision, thus producing many profound applications. So far, a series of flexible and wearable sensors based on liquid metal have been designed and tested. Their applications have also witnessed a growing exploration in biomedical areas, including health-monitoring, electronic skin, wearable devices and intelligent robots etc. This article presents a systematic review of the typical progress of liquid metal-enabled soft sensors, including material innovations, fabrication strategies, fundamental principles, representative application examples, and so on. The perspectives of liquid-metal soft sensors is finally interpreted to conclude the future challenges and opportunities.

Owing to the development of soft electronics enabled from liquid metal, flexible and wearable sensors have been attracting more and more interest recently. So far, several types of liquid metal sensors based on soft substrate have been gradually developed. However, there is still too much space to explore regarding fabrication efficiency including resolution etc. which in fact determines further development and applications of liquid-metal flexible sensors. In this article, we proposed and demonstrated a novel method to fabricate liquid-metal circuits on PDMS (Polydimethylsiloxane) by spraying and wiping processing, which simultaneously offers simpler manufacturing process and smaller feature size as compared with existing approaches. According to various desired needs, the PDMS substrate can be prepared with specifically designed micro-channels in advance and then liquid metal entity can be patterned via subsequent spraying and wiping. As experiments indicate, the whole process is quite implementable and convenient. Except for PDMS, we further showed that PDMS mixed up with Ecoflex can also serve as a benign substrate. In order to illustrate the applications of this technique, we successfully produced several typical functional circuits and tested their working performances. Their roles to develop further electrical or thermal resistive sensors were discussed. Overall, the feature size with high reliability has reached 50 μm. The present method is expected to contribute to the development of liquid-metal sensors with high resolution and sensitivity.