Electrically interfacing the skin for monitoring personal health condition is the basis of skin‐contact electrophysiology. In the clinical practice the use of stiff and bulky pregelled or dry electrodes, in contrast to the soft body tissues, imposes severe restrictions to user comfort and mobility while limiting clinical applications. Here, in this work dry, unperceivable temporary tattoo electrodes are presented. Customized single or multielectrode arrays are readily fabricated by inkjet printing of conducting polymer onto commercial decal transfer paper, which allows for easy transfer on the user's skin. Conformal adhesion to the skin is provided thanks to their ultralow thickness (<1 µm). Tattoo electrode–skin contact impedance is characterized on short‐ (1 h) and long‐term (48 h) and compared with standard pregelled and dry electrodes. The viability in electrophysiology is validated by surface electromyography and electrocardiography recordings on various locations on limbs and face. A novel concept of tattoo as perforable skin‐contact electrode, through which hairs can grow, is demonstrated, thus permitting to envision very long‐term recordings on areas with high hair density. The proposed materials and patterning strategy make this technology amenable for large‐scale production of low‐cost sensing devices.

Temporary tattoo electrodes are the most recent development in the field of cutaneous sensors. They have successfully demonstrated their performances in the monitoring of various electrophysiological signals on the skin. These epidermal electronic devices offer a conformal and imperceptible contact with the wearer while enabling good quality recordings over time. Evaluations of brain activity in clinical practice face multiple limitations, where such electrodes can provide realistic technological solutions and increase diagnostics efficiency. Here we present the performance of inkjet-printed conducting polymer tattoo electrodes in clinical electroencephalography and their compatibility with magnetoencephalography. The working mechanism of these dry sensors is investigated through the modeling of the skin/electrode impedance for better understanding of the biosignals transduction at this interface. Furthermore, a custom-made skin phantom platform demonstrates the feasibility of high-density recordings, which are essential in localizing neuropathological activities. These evaluations provide valuable input for the successful application of these ultrathin electronic tattoos sensors in multimodal brain monitoring and diagnosis.

We present an innovative fabrication process for the production of low cost fully-plastic flexible MEAs and prove that they are a valid proof-of-concept for a platform for the electrophysiological analysis of cardiac cell cultures.

In the growing field of conformable electronics, among the various approaches so far, tattoo technology has emerged. Here temporary tattoo paper is adopted as unconventional substrate to build up transferable body compliant devices, which establish a stable and long-lasting interface with the skin. Tattoo-based devices have shown their capabilities in multiple fields, with main application in human health biomonitoring. Such approach is advancing the 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 as in organic electronics, within 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 on its material composition and properties are given, discussing the pros and cons in comparison with 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 to ain a better understanding of the capabilities offered by such low cost and versatile substrate. This can help in opening up new research for emerging applications, like in the relevant field of sustainable electronics.

In bioelectronics, conducting polymer coatings allow the reduction of the impedance of metallic electrodes and facilitate the translation of bioelectrical signals at their interface. Such coatings can be made using thin film deposition from a solution or direct synthesis via electrodeposition. The electrical control over the deposition offers the possibility for a fine‐tuning of the film's thickness and structure. However, the mechanical stability of such coatings mainly suffer from their poor adhesion to the electrode surface and film cracking. Here, an extended study on the kinetics of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electropolymerization and the evolution of its physicochemical properties is provided. The impedance spectroscopy closely follows the electrochemical variations during the PEDOT:PSS's film growth, described by modeled equivalent circuits. The film's properties change during polymerization in relation to the supporting electrode size, its surface chemistry, and the deposition time. The film growth structures polymeric morphology in a confluent layer with a strong thickness increase before reaching its mechanical surface failure. Before this point, the film remains stable over a hundred cycles of applied potential strain in a defined redox window. These evaluations benchmark the PEDOT:PSS film properties during its electropolymerization toward electrochemically tunable transducers for bioelectronics.

Inkjet printing remains one of the most cost-efficient techniques for device prototyping and manufacturing, offering considerable freedom of digital design, non-contact, and additive fabrication. When developing novel wearable devices, a balanced approach is required between functional, user-safe materials and scalable manufacturing processes. Here, we propose a tailor-made ink formulation, based on non-hazardous materials, to develop green electronic devices aimed at interfacing with humans. We demonstrate that developed ink exhibits high-resolution inkjet printability, in line with theoretical prediction, on multiple wearable substrates. The ink's chemical composition ensures the pattern's enhanced electrical properties, mechanical flexibility, and stability in water. The cytocompatibility evaluations show no noxious effects from printed films in contact with human mesenchymal stem cells. Finally, we fabricated a printed wearable touch sensor on a non-woven fabric substrate, capable of tracking human steps. This is a step toward the development of green wearable electronics manufacturing, demonstrating a viable combination of materials and processes for biocompatible devices.

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Tattoo electronics is one of the emerging technologies in skin compliant biosensing. The growing interest in their large application in health monitoring raises several interrogations on how these sensors interface with the skin. In this paper, the bioimpedance at the interface of the skin and ultra‐conformable tattoo electrodes made of conducting polymers are focused on. The electrochemical characteristics of these electrodes differ from traditional gelled Ag/AgCl electrodes. The modeling of equivalent circuits in different skin‐electrode configurations proposes the explanation of the biopotentials transduction mechanism. The strong agreement between the circuit model and experimental values reveals the capacitive coupling of conducting polymer tattoo electrodes where circuit's values reflect the electrodes’ and skin physical characteristics. Additional studies underline an enhanced signal stability in inter/intra‐subject evaluations using dry tattoos beneficial for broad long‐term recordings. This study provides a comprehensive explanation of the skin/tattoo electrode interface model. The understanding of this interface is essential when designing next generation wearable biomonitoring devices using imperceptible interfaces.