Interfacing Imperceptible Electronics with the Human Body—New Solutions and Approaches

The advent of epidermal electronics, which can be traced back to the beginning of the 2010 decade, radically changed the idea of electronics/human body interfacing, blurring the boundaries between the system to be analyzed (the human body) and the tools that are necessary to effectively transduce its multifaceted biosignals. In particular, the newborn possibility of a seamless integration of electronic devices and the human skin quickly led to the rise of new perspectives and outlooks that ultimately lead to imperceptible electronics, a concept that literally break the obsolete dichotomy between the “soft” human body on one side and the “rigid” recording device on the other side.<br/>Nowadays, two of the most interesting fields of applications of epidermal electronics are undoubtedly the development of sensing systems for robotic skin, and the engineering of bioelectrodes for the monitoring of biopotential, such as electrocardiography (ECG), electromyography (EMG) and electroencefalography (EEG) signals from the surface of the skin. Regarding the first application, we show here an approach based on Parylene C sub-micrometric substrates and the piezo(and piro) electric polymer PVDF coupled to an organic transistor called Organic Charge Modulated FET (OCMFET) for the development of tactile sensors for robotic skin applications. Thanks to the versatility of the OCMFET structure, the low-cost materials and the low-resolution fabrication processes, this method allows to easily obtain ultrasensitive temperature and force sensors on ultra-conformable substrates that can be easily transferred onto a target surface (including a robotic end effector or the human skin) without losing their characteristics. As pertains to the second application (which represents one of the most appealing example of the possible integration between this novel branch of electronics and the human body), despite the numerous different approaches that have been proposed so far, some of the intrinsic weaknesses of using ultrathin electrodes within the biomedical domain are still yet to be solved. The first issue is related to the breathability of these epidermal patches, an important aspect when on-body measurements are taken into consideration. With the intent of overcoming this issue and thus improving the comfort of the subject during the measurement, we developed a simple yet effective solution for the development of tattoo electrodes for biopotential recordings, which relies on biocompatible materials and low-resolution fabrication techniques. In particular, Parylene C-based submicrometer-thick electrodes have been rendered breathable through a large area, oxygen plasma-based perforation technique, surpassing the breathability of common textiles like jeans and approaching that of knitted clothing, thus making these electrodes suitable for long-term biopotential recordings. The second important aspect that we recently tackled is related to the methods of connection of such imperceptible films to the external electronics, a problem that effectively voids some of the main advantages of having an ultra-thin and conformable device. The adopted solution relies on free-standing, ultra-thin PEDOT:PSS based functional films with conductive and ferrimagnetic properties, thanks to the integration of inexpensive ferrite powder layers within the film itself. The obtained electrodes can be transferred on the skin as temporary tattoos and directly contacted using magnetic connectors without compromising their conformability.<br/>The proposed approaches, being at the edge between the fervent fields of epidermal and organic electronics, represent interesting examples of how it is possible to obtain high-performing and conformable tactile sensors, while at the same time overcoming some of the more important issues and limitations that stem from interfacing electronics and the human body.