Scalable Fabrication of Flexible Single‐Layer Strain and Double‐Layer Pressure Sensors by Inkjet Printing for Subtle Vibration Detection

Abstract

Flexible sensors have attracted tremendous attentions in subtle force/vibration detection, human motion/health monitoring, and other related application fields. Although various sensors are developed to meet this growing demand, it is still a huge challenge to prepare satisfactory sensors with ultrahigh sensitivity and superior stability. In this work, a double‐layer flexible sensor is successfully prepared via an efficient and designable inkjet printing approach. Metallic silver layers are deposited onto polyurethane acrylate substrates with or without microstructures to form the flexible electrodes. The combination of a flat electrode and a microstructured electrode finally becomes a durable double‐layer flexible sensor with ultrahigh sensitivity and superior stability. Besides, the flexible flat electrodes can also be applied as single‐layer sensors. An ultrahigh sensitivity (maximum gauge factor of >1100), favorable durability (>1000 cycles), and fast response time (≈90 ms) are achieved simultaneously in a flat single‐layer sensor, while the combined double‐layer sensor presents quick response capability to small pressures (≈250 Pa). Moreover, these single‐layer and double‐layer sensors can monitor a variety of physiological signals (e.g., human pulse waveform, breathing behavior, and voice recognition), indicating their promising applications in wearable electronics.

Printed flexible mechanical sensors

“…Too low of a concentration will result in disconnections in the conductive path, while too high will lead to a low GF as the change in the interactions between the AgNPs or AgNWs will be too small. 126 The highest GF reached among the flat silver-based sensors 43,123,126,127 is in the 4.32-5% strain range obtained by Gai et al 127 Their sensor was fabricated by inkjet printing silver ink onto a polyurethane acrylate (PUA) substrate. When a tensile strain is applied, the silver particles will be pulled apart, which leads to a resistance increase.…”

“…2, Agbased sensors provide good sensing performance (GFs of mostly 10 and above), especially if used in tandem with an appropriate structure. 43,69,123,[126][127][128] The GF is defined by the change in electrical resistance under stretching over strain: GF = (ΔR/R 0 )/ε, where R 0 is the initial electrical resistance, ΔR is the change in resistance, and ε is the strain. 128 CNT-based sensors have a large sensing range and high GFs; the sensing range of graphene-based sensors is larger than most other sensors as well, and CB-based sensors have an average performance when it comes to the sensing range and GF.…”

“…As shown in Fig. 2, a large proportion of printed flexible strain sensors with high GFs is silverbased, 43,69,123,[126][127][128] largely due to silver's excellent conductivity. 69 The majority of them are fabricated by printing a conductive silver ink, 43,127 that may contain either silver nanowires (AgNWs) 126 or silver nanoparticles (AgNPs), 123 on a flat flexible substrate.…”

“…2, a large proportion of printed flexible strain sensors with high GFs is silverbased, 43,69,123,[126][127][128] largely due to silver's excellent conductivity. 69 The majority of them are fabricated by printing a conductive silver ink, 43,127 that may contain either silver nanowires (AgNWs) 126 or silver nanoparticles (AgNPs), 123 on a flat flexible substrate. The concentrations of the Ag-based inks were tuned in these studies such that the interactions between the AgNPs or AgNWs during deformation are in the right range to cause detectable resistance changes.…”

Recent advances in inkjet-printing technologies for flexible/wearable electronics

“…104 With the advanced technology, more and more mechanical sensors, such as strain sensors, pressure sensors, and tactile sensors, based on inkjet printing were fabricated. 105–113…”