Flexible sensors find a wide range of applications in different fields, including human-machine interfaces, electronic devices and robotics. The design and manufacturing of these sensors often closely follow the trends in miniaturization, functionality and compatibility fundamental to the design of integrated sensing materials. To date, a wide range of nano- and micro-scale flexible sensors have been developed using various materials that possess the necessary transducing principles. Among them, piezoelectric materials occupy a vital position in the design of sensing systems owing to their improved mechanical properties and energy conversion efficiency at the nanoscale level.
Particularly, piezoelectric polymers like polyvinylidene fluoride (PVDF) and constituent co-polymers have attracted research attention as promising sensing materials. However, most PVDF and related co-polymers are assembled via a multistep and tedious process, leading to costly and bulk sensing devices that cannot meet the current miniaturization and industrialization requirements. To overcome this challenge, there have been considerable attempts to improve the interfacial interactions of functional and electrode elements by exploring different piezoelectric sensor-based flexible electrodes. Compared with traditional metal electrodes, flexible conductive electrodes produce superior energy conversion efficiency and hold potential applications in next-generation flexible sensors.
Nevertheless, flexible electrodes require multiple machining, which increases the size, cost and complexity of manufacturing the sensors. In addition, most piezoelectric sensing materials rely on the bottom and top planar electrodes to facilitate the assembly of piezoelectric sensors. This is characterized by flexibility limitations, high cost and time-consuming manufacturing processes that limit their practical applications. Electrospinning technique that has gained popularity for fabricating nanofiber materials with higher energy conversion efficiency and larger piezoelectric coefficient has proved insufficient for fabricating flexible nanofibers.
To address these challenges, Wenbo Jiang (Master degree), Kongsen Hu (PhD candidate) and Dr. Nan Lv from Northeast Electric Power University in collaboration with Dr. Zhiwei Lyu from Shenzhen Wenirune Electronic Technologies Company Limited, proposed a feasible strategy for constructing nanoresistance networks and piezoelectric elements through fabricating single PAN/PANI/PVP piezoelectric-conductive nanofibers by electrospinning technology. Piezoelectric polymer PAN served as functional elements, while PANI and matrix PVP served as nanoresistance networks. The tiny force-sensing capability of the PAN/PANI/PVP integrated nanofibers membrane (INFM) was systematically evaluated and discussed. Their work is currently published in the journal, Sensors and Actuators A: Physical.
The researchers demonstrated the ability of the PAN/PANI/PVP piezoelectric-conductive nanofibers to timely collect and output voltage via the nanoresistance networks based on the principle of Wheatstone bridge. The integration nanoresistance networks and piezoelectric elements was beneficial in improving polarization and collection of the induced charges, contributing to enhanced piezoelectric conversion and output performance. This enabled the INFM to accurately perceive pressure, especially tiny force, with high-precision sensitivity of approximately 667 mVN-1 and an extremely low detection limit of about 0.05 while maintaining the linear relationship.
In a nutshell, a novel PAN/PANI/PVP piezoelectric-conductive nanofibers capable of constructing nanoresistance networks and piezoelectric elements simultaneously via electrospinning technique was reported. The presented strategy eliminated the constraints of the sandwich structure associated with traditional devices to achieve single nanofibers, thus paving the way for efficient construction of nanoscale, lightweight, low cost and highly sensitive integrated sensing materials. Regarding their outstanding performance, the authors, in a statement to Advances in Engineering, observed that the PAN/PANI/PVP INFM could pioneer novel breakthroughs in the design and fabrication of higher-performance flexible micro/nanoscale sensors for different practical applications.
Jiang, W., Hu, K., Lv, N., & Lyu, Z. (2021). Single flexible nanofibers to achieve simultaneous construction of piezoelectric elements and nanoresistance networks for tiny force sensing. Sensors and Actuators A: Physical, 332, 113203.