Recent Advances in Ultra‐Thin Electronics

Abstract

Soft electronics is an exciting research field that aims to seamlessly integrate electronic components and devices onto non-rigid, non-planar, complex 3D surfaces and objects. In the last decade, the research on soft electronics has been mainly driven by the increasing demand and request for a new category of portable and ubiquitous electronic systems capable to bridge the gap between biology and electronics, and to be integrated with the human body, with non-conventional features such as conformability. Targeted applications include wearable electronics, human-machine interfacing, epidermal electrodes for biopotential signals acquisition, as well as personal health-care monitoring systems. Therefore, two main strategies can be followed to improve the conformability of materials and, therefore, electronics systems.[2] From one side it is possible to push on materials innovation, developing low Young Modulus, intrinsically soft and stretchable electronic materials (e.g., organic semiconductors, conductive polymers, soft substrates, etc.). On the other side, smart structural designs and architectures can enable the use of more traditional materials and components to develop conformable integrated devices. Obviously, both approaches can be pursued in parallel. Among others, the use of ultra-thin substrates and devices is one of the most powerful approaches to achieving full conformability onto target surfaces. This is because the strain (and thus the stress) locally induced on the surface of a sheet, by bending it at a certain radius of curvature, is directly proportional to the thickness of the same sheet.[3] Moreover, according to Equation 1, the bending stiffness (the most important physical quantity toward conformability) drops dramatically down by reducing the thickness, since this is proportional to the third power of the thickness.[4] Interestingly, under a certain threshold thickness (that can vary, depending on the materials involved and substrates), the thin sheet can adhere and conform spontaneously to surfaces due to physical interactions (i.e., dispersive forces, Van Der Waals interactions),[5, 6] enabling the intimate contact between such films and virtually any surface, including skin.[7-9] In this Special Issue, we collected several recent advances and focused reviews in which low thickness is the key approach to obtaining conformable and soft electronic components and devices. We thank all the invited contributors that make it possible with their wonderful contributions, to succeed in building up this. Below we identify papers via the name of the person invited. Last, we thank Dr. Gaia Tomasello for allowing us to be Guest Editors of this exciting Special Issue. It has been a real pleasure working with her. As introduced, one of the main drivers of conformal electronics development is the quest for unobtrusive monitoring of bio-electric signals. Four papers assess this topic, describing the use of thin and conformable electronics for various physiological signals monitoring, from the electrodes to complete systems. In article number 2200883, George Malliaras and co-workers reported a novel thin, flexible, high-density, and tissue-compatible multishank probe for high-resolution recordings. It is combined with an additively manufactured insertion shuttle that allows the probe to be implanted into the brain as a new tool for in vivo electrophysiology. Yohufan Hu and co-workers proposed in article number 2200916 a flexible wireless surface electromyography (sEMG) monitoring system. It consists of a stretchable epidermal patch and a flexible printed circuit board to provide high fidelity real-time evaluation of muscle strength and fatigue, thanks to the very low skin-electrode contact impedance and minimized background noise. In article number 2201279 Tsuyoshi Sekitani and co-workers presented an innovative printing process for the fabrication of low-noise electrodes for an organic pseudo-complementary metal-oxide-sem