Ionic flexible sensors (IFS) usually consist of an ionomer matrix and two conductive electrodes, the failure of which mostly originates from interfacial debonding between matrix and electrode layers. To improve electrode's adhesion and impedance matching with matrix, polymer binder or plasmonic heating technology is used to enhance the adhesion of electrodes, but there are technical challenges such as high resistance and harsh conditions. Herein, inspired by biological hair, we proposed a reliable and facile method to form AgNWs@Au-embedded Nafion flexible electrodes (AN FEs) for IFS without rigorous temperature and harsh conditions. Through integrating the spraying and electrodepositing Au method, we achieved that the AgNWs are partly embedded in the matrix layer for forming the embedded layer, similar to the root of biological hair, which is used to fix the FEs and collect the ion charges. The other parts of AgNWs exposed on the surface form the conductive mesh layer for transmitting the signal, analogous to the tip of biological hair. Compared with other AgNWs FEs, AN FEs exhibit high adhesion (∼358 kPa) and low sheet resistance (∼ 3.7 Ω/□), and high stabilities after 100 washing cycles, 200 s H 2 O 2 corrosion or 1500s HCl corrosion. A self-powered IFS prepared by AN FEs can achieve dual sensing of mechanical strain and ambient humidity and still has promising sensing performance after being exposed to air for 2 months, which further indicates potential applications of the prepared FEs in next-generation multifunctional flexible electronic devices.

Ionic polymer–metal composites (IPMCs) have attracted attention in recent years due to their integration of actuation and sensing functions. As one of the main sensing functions of IPMCs, humidity sensing has been of consistent interest in wearable health monitors and artificial skin. However, there are still some technical challenges in that classical IPMCs have poor humidity sensing performance due to their dense surface electrode, and IPMCs are damaged easily due to an electrode/membrane mismatch. In this work, through the spraying and electrodepositing process, we developed an efficient method to rapidly prepare a Au-shell-Ag-NW (silver nanowire)-based IPMC with high strength, low surface resistance and excellent humidity sensing performance. Meanwhile, we optimized the preparation method by clarifying the influence of solvent type and electrodepositing time on the performance of the Au-shell-Ag-NW-based IPMC, thus effectively improving the humidity sensing effect and strength of the IPMC. Compared with previous research, the humidity electrical response (~9.6 mV) of the Au-shell-Ag-NW-based IPMC is at least two orders of magnitude higher than that of the classical IPMC (~0.41 mV), which is mainly attributed to the sparse gap structure for promoting the exchange of water molecules in the environment and Nafion membrane, a low surface resistance (~3.4 Ohm/sq) for transmitting the signal, and a seamless connection between the electrode and Nafion membrane for fully collecting the ion charges in the Nafion membrane. Additionally, the Au-shell-Ag-NW-based IPMC could effectively monitor the human breathing process, and the humidity sensing performance did not change after being exposed to the air for 4 weeks, which further indicates that the Au-shell-Ag-NW-based IPMC has good application potential due to its efficient preparation technology, high stability and good reproducibility.

Ionic polymer metal composite (IPMC) has been extensively studied as a pressure sensor. Nevertheless, few works have focused on the size effects of external loads on IPMC pressure sensing. Herein, we investigated the effects of contact area on pressure sensing of an IPMC sensor. By placing a soft substrate behind the IPMC, we enlarged the strain when IPMC was pressed and revealed that the signal generation of IPMC sensor depends on the compression and bending coupling effects, which even change from negative to positive (from -0.184 mV to 0.102 mV) with the increase of the radius of the contact area (from 1 mm to 4 mm). The experimental results were qualitatively proved by the theoretical compression model and finite element simulation. It may provide a new and effective way to simplify the measurement of the contact area.

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Inspired by the mechanism of touch and pain in human skin, we integrated two ion-sensing films and a polydimethylsiloxane (PDMS) layer together to achieve a bionic artificial receptor with the capacity of distinguishing touch or pain perception through ion-electrical effect. The ion-sensing film provides the carrier of touch or pain perception, while the PDMS layer as a soft substrate is used to regulate the perception ability of receptor. Through a series of experiments, we investigated the effects of physical properties of the PDMS layer on the sensing ability of an artificial receptor. Further, contact area tests were performed in order to distinguish touch or pain under a sharp object. It is revealed that the pressure threshold triggering the touch and pain feedback of the artificial receptor presented an increasing trend when the elastic modulus and thickness of the PDMS substrate increase. The distinction ability of touch and pain becomes more pronounced under higher elastic modulus and larger thickness. Furthermore, the induced pain feedback becomes more intense with the decrease of the loading area under the same load, and the threshold of pain drops down from 176.68 kPa to 54.57 kPa with the decrease of the radius from 3 mm to 1 mm. This work potentially provides a new strategy for developing electronic skin with tactile sensing and pain warning. The pressure threshold and sensing range can be regulated by changing the physical properties of the middle layer, which would be advantageous to robotics and healthcare fields.

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