Ionogels is a kind of hybrid materials composed of ionic liquids (ILs) and solid polymer network matrix, has been extensively investigated in the most recent decade. Due to the excellent mechanical properties and ionic conductivity, their promising applications in flexible stress and strain sensors have been proposed and explosively developed. In this review, we briefly summarize research progresses on ionogel based flexible stress and strain sensors (IFSSs) from five aspects, including material synthesis, device fabrication, working principles, characteristics and performances, and potential applications. Some outlooks and perspectives are also proposed at the end of review. The review is expected to provide reference and new insights into the research of IFSS. Graphical abstract

Based on the magic substance dopamine (DA), a simple and effective one-step solution deposition strategy has been developed for constructing high-performance amorphous zirconium oxide (ZrO₂) nanocomposite thin film. DA can be auto-polymerized to form a polydopamine (PDA) coating, which adheres strongly to the silicon substrate and serves as an active platform to induce the subsequent deposition of ZrO₂. The obtained ZrO₂ nanocomposite thin films with the thickness of about one hundred nanometers presents outstanding mechanical and tribological performances even without a subsequent high-temperature annealing process. More importantly, the preparation process is definitely low-consumption and environmentally friendly. This work overcomes the claim that ZrO₂ films obtained by aqueous solution deposition process generally possess low mechanical properties and thus avoid the defects of focusing mainly on nature and performances of heat-treated and crystalline ZrO₂. Based on this work, it is believed that the as-deposited ZrO₂ nanocomposite film can be widely used as a protecting coating and lubricating material for micro- and nano-electromechanical systems (MEMS/NEMS) and many other systems working in similar conditions.

In this study, the tribological properties of polytetrafluoroethylene (PTFE) composites filled with polyetheretherketone (PEEK) and nano-Al₂O₃ particles were studied using a block-on-ring wear tester. The tribological performance of the composites was affected by the experimental parameters (sliding speed, normal load, and environmental temperature) and the composites achieved a high-speed sliding friction state. The results showed that the PEEK and nano-Al₂O₃ particles significantly improved the wear resistance of the PTFE composites. In addition, the nano-Al₂O₃ particles increased the hardness of the composites and enhanced the mechanical properties to enable applications in a wider range of industrial fields. The effects of the sliding speed and normal load on the tribological properties were more significant than that of the environmental temperature. In addition, the entire wear process was divided into three stages (the initial wear stage, severe wear transition stage, and ultralow stable wear stage), according to the evolution of the tribological characteristics (wear rate, morphology of the worn surface and transfer film, and wear debris morphology).

Soft electronic skins (e-skins) with superior sensing performance have attracted significant attention in numerous applications. However, most of the reported e-skins have been prepared based on an inert polymer substrate without completely considering its self-healing ability; this severely limits the practical application of these e-skins. In addition, the fabrication of most of the e-skins is conducted via photolithography and other tedious processes; therefore, it is difficult to equilibrate high sensing performance and preparation simplification. Herein, a simple and general fabrication strategy of self-healable and highly sensitive electrical sensor, consisting of a sensing component, i.e. Au-deposited toothbrush-hair micro-prick arrays, and a substrate component, i.e. a P-TDI-IP elastomer obtained via the block polymerization of polytetramethylene glycol (P), 2,4′-tolylene diisocyanate (TDI), and isophorone diisocyanate (IP), is reported. The self-healable P-TDI-IP elastomer acting as a substrate endows the reported electrical sensor with an excellent self-healing capability (96% with 6 h) and an outstanding stretchability (1200%). Importantly, due to the presence of unique micro-prick arrays, the electrical sensor could detect multiaxial tactile stimuli including pressing and shearing. Moreover, this device exhibited high sensitivities (3.32 kPa⁻¹ for pressing, GF = 2.82 for shearing), the rapid response time of 25 ms, and the low detection limit of 5 Pa. Owing to these prominent properties, the electrical sensor presented anticipated abilities for distinguishing ultrafine surfaces with different roughnesses and monitoring various human motions.