Temporary tattoo as unconventional substrate for conformable and transferable electronics on skin and beyond

In the growing field of conformable electronics, tattoo technology has emerged from among the various approaches so farHere, temporary tattoo paper is adopted as unconventional substrate to build up transferable body compliant devices, which establishes a stable and long-lasting interface with the skin. Tattoo-based devices have shown their capabilities in multiple fields, with the main application in human health biomonitoring. Such an approach is advancing to become state-of-the-art, overcoming some limits of existing technologies, as in the case of skin-contact electrodes and sweat analysis. Temporary tattoo has also been adopted in other fields, such asorganic electronics, the development of organic solar cells, and transferable edible transistors. Multiple and complementary fabrication approaches on temporary tattoos have been demonstrated, spanning from traditional vacuum-based deposition methods to various printing technologies. In this review, together with reporting and discussing the main fabrication methods and applications of tattoo technology, we describe the main features of the tattoo substrate. New insights into its material composition and properties are given, discussing the pros and cons in comparison to other approaches adopted in conformable electronics. Together with providing a comprehensive and up to date review of advancements in tattoo technology, this review aims to contribute in a better understanding of the capabilities offered by such a low cost and versatile substrate. This can help in opening up new research for emerging applications, like in the relevant field of sustainable electronics.