Electrohydrodynamic (EHD) inkjet printing flexible pressure sensors with a multilayer structure and periodically patterned Ag nanoparticles

Flexible pressure sensors are widely employed for accurate pressure sensing on geometrically complex surfaces. As sensing materials, silver nanoparticles (AgNPs) have high electrical conductivity but relatively poor sensitivity as a trade-off. In this work, electrohydrodynamic (EHD) inkjet printing was utilized to directly write patterns of AgNPs tracks with periodic geometries on the flex-substrate surface. The patterns in which the as-printed AgNPs tracks, with a width of several tens of micrometres, exhibited a piezoresistive effect. This work confirmed that introducing multilayered structures into the flexible pressure sensors with AgNPs patterns was a practical path to improve the sensing sensitivity, with the assistance of soft packaging material of Polydimethylsiloxane (PDMS). The sensitivity was improved more than tenfold after fourfold overlapping of the as-printed single-layer sensor. Experimental tests, formula calculations, and numerical simulations of the sensors were conducted. It was concluded that the as-printed single-layer sensor with the AgNPs pattern of concave regular hexagonal structure (CRHTS) had better sensing performance than that of grid-type structure (GTS) or wave-type structure (WTS). For the two-layered CRHTS sensor, the dynamic and quasi-static sensing response characteristics, response recovery duration, cyclic stability, and ability to discriminate different strain frequencies were further measured and analysed. The working principle of the flex sensors was discussed based on the Percolation Theory and the Tunneling Effect. Some application demonstrations of the sensors were also exhibited. The structural design and EHD inkjet printing fabrication path facilitate the development of more versatile flex sensors.