Ionic tactile sensors as promising biomaterials for artificial skin: Review of latest advances and future perspectives

Ionic tactile sensors (ITS) are an emerging subfield of wearable electronics, capable of mimicking the human skin, including not only the typical anisotropic structure, mechanical behaviour, and tactile functions but even the mechanosensitive ionic channels that are crucial for the human sense of touch. With the rapid development of intelligent technology, such bioinspired materials constitute the core foundation of intelligent systems and are a candidate to be the next generation e-skins, offering a more accurate and evolved biointerface. In the latest years, a wealth of novel ultra-stretchable ITS was proposed, progressively refining the choice of soft materials, including ion gels, ionic liquids and hydrogels, and fabrication techniques. Regardless of materials and methods adopted, all these tactile sensors can feel mechanical solicitations and external stimuli, thus behaving as – or even better than – human skin. In this review, an overview of the very latest advances in high-performance ITS applied in intelligent systems is reported. First, generality of ITS will be summarized. After, ion gel, ionic liquid, hydrogel, and elastomer ITS will be discussed focusing first on composition, fabrication, type and mode of sensing and then on their characteristics and application. In this perspective, the advantages that biomimetic approaches brought in terms of sensitivity, speed of response and multimodality of sensing will be highlighted, with a particular focus on the development of electrochromic, thermochromic, self-powered and self-healing devices. In conclusion, the prospects of tactile sensors for intelligent systems in biomedicine and robotics will be discussed, along with the possible strategies to overcome the current shortcomings, in terms of biocompatibility, durability, mechanical performance, adhesion to biological substrates, which represent the future challenges.