Soft Material-Enabled, Flexible Hybrid Electronics for Medicine, Healthcare, and Human-Machine Interfaces
Flexible hybrid electronics (FHE), designed in wearable and implantable configurations, have enormous applications in advanced healthcare, rapid disease diagnostics, and persistent human-machine interfaces. Soft, contoured geometries and time-dynamic deformation of the targeted tissues require high flexibility and stretchability of the integrated bioelectronics. Recent progress in developing and engineering soft materials has provided a unique opportunity to design various types of mechanically
... compliant and deformable systems. Here, we summarize the required properties of soft materials and their characteristics for configuring sensing and substrate components in wearable and implantable devices and systems. Details of functionality and sensitivity of the recently developed FHE are discussed with the application areas in medicine, healthcare, and machine interactions. This review concludes with a discussion on limitations of current materials, key requirements for next generation materials, and new application areas. Materials 2018, 11, 187 2 of 33 material properties such as adhesion, flexibility, stretchability, and biocompatibility is critical for conformal and intimate integration with targeted human tissues. In general, FHE involve various types of sensors, circuits, and substrate materials; the combination of multiple components decides the overall mechanical and material properties of the electronics. Recent advances in soft materials, nano-microfabrication, and low-profile electronics have enabled the development of wearable and implantable electronics. The progress in materials processing and engineering of low-modulus functional materials allowed to design mechanically compliant electronics. Furthermore, hybrid integration of nanomaterials enhanced the electrical properties of the flexible and stretchable devices. As an example, carbon nanotubes (CNTs), first discovered in 1991 and synthesized in 1993, were embedded in soft elastomers for improved conductivity    . Graphene, a more recently observed material, is an emerging choice for creating conductive composites, while offering enhanced stretchability  . These materials, and others to be discussed in this review, offered versatility to manipulate a device's electrical and mechanical properties, depending on target applications. In particular, the first generation of skin-like electronics, introduced in 2011 , opened a new era to develop unobtrusive and wearable sensors, actuators, antennas, and other components. These advances in materials and associated integration technologies have ignited the development of new FHE where high-performance electronic components are hybridized with soft materials. Existing reviews of wearable and implantable electronics typically offer narrow scopes. For example, recent reviews [18, 19] focus only on specific wearable devices such as strain or tactile sensors, while others [20, 21] include expansion of wearable sensor types but still miss the coverage of important aspects of implantable FHE. Similarly, a few reviews summarize limited areas of implantable devices for neural applications [22, 23] , while emphasizing characteristics of particular materials, such as graphene, nanomaterials, hydrogels, and typical polymers      . Here, we provide a broader view of FHE via the comprehensive summary of the widely used functional materials to design soft wearable and implantable electronics, device integration strategies, and their applications in the areas of medicine, healthcare, and human-machine interfaces. In addition, we review the characteristics of those soft-material based electronics in terms of device type, sensing material, target signal, sensitivity and more. Overall, this review delivers a comprehensive summary of key materials, design criteria, and associated device performance to design high performance, wearable and implantable FHE. Flexible Hybrid Electronics (FHE) Definition FHE refer to mechanically flexible and stretchable electronic devices and systems, enabled by hybrid combination of soft functional materials, compliant membranes, and integrated functional chip components. With the use of such materials, mechanical reliability with high flexibility and stretchability in the electronic devices can be achieved unlike traditional rigid electronics. As a result, FHE have found new applications in medicine, healthcare, and machine interfaces, via intimate and conformal interactions with soft and deformable human tissues. Recent advancement and current research in the development, characterization, and applications of a variety of FHE are discussed in the following sections. Material Characteristics for Wearable and Implantable Electronics Low-profile, wearable and implantable electronics can realize continuous, real-time monitoring of biomarkers for rapid, point-of-care disease diagnosis and therapeutics [29, 30] . In terms of the device lamination on the human skin, poor contact leads to unreliable signal quality, contamination, and discomfort [7, 9] . Thus, a careful selection of materials along with the mechanical analysis is critical to ensure the conformal and intimate contact of the device with the skin. Utilization of soft materials offers enhancement of the device's overall flexibility. A systematic engineering of a thickness, stiffness, and modulus of materials provides proper mechanical compliance to make the device lamination on Materials 2018, 11, 187 4 of 33 Materials 2017, 10, x FOR PEER REVIEW 4 of 33 Figure 1. Soft functional materials. (a) Graphene-CNT-nickel hetero-nanostructure. Reprinted with permission from Reference , Copyright 2017, John Wiley and Sons; (b) Cross-stacked graphene-CNT (carbon nanotube) films. Reprinted from Reference , Copyright (2017), with permission from Elsevier; (c) SWCNT-based nanowire. Reproduced from Reference  with permission of The Royal Society of Chemistry; (d) Graphene oxide foam with low-effective elastic modulus for high sensitivity.