Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Technology Roadmap for Flexible Sensors.

Wearable Sensors-Enabled Human–Machine Interaction Systems: From Design to Application

Funding information A*STAR-NCBR, Grant/Award Number: R263-000-C91-305; HIFES Seed Funding, Grant/Award Number: R263-501-012-133; National Key Research and Development Program of China, Grant/Award Numbers: 2019YFB2004800, R-2020-S-002; RIE Advanced Manufacturing and Engineering (AME) programmatic grant, Grant/Award Number: A18A4b0055 Abstract Triboelectric nanogenerator (TENG) technology is a promising research field for energy harvesting and nanoenergy and nanosystem (NENS) in the aspect of mechanical, electrical, optical, acoustic, fluidic, and so on. This review systematically reports the progress of TENG technology, in terms of energy-boosting, emerging materials, self-powered sensors, NENS, and its further integration with other potential technologies. Starting from TENG mechanisms including the ways of charge generation and energy-boosting, we introduce the applications from energy harvesters to various kinds of self-powered sensors, that is, physical sensors, chemical/gas sensors. After that, further applications in NENS are discussed, such as blue energy, human-machine interfaces (HMIs), neural interfaces/implanted devices, and optical interface/wearable photonics. Moving to new research directions beyond TENG, we depict hybrid energy harvesting technologies, dielectric-elastomer-enhancement, self-healing, shape-adaptive capability, and self-sustained NENS and/or internet of things (IoT). Finally, the outlooks and conclusions about future development trends of TENG technologies are discussed toward multifunctional and intelligent systems.

Progress in TENG technology—A journey from energy harvesting to nanoenergy and nanosystem

On-skin electrodes function as an ideal platform for collecting high-quality electrophysiological (EP) signals due to their unique characteristics, such as stretchability, conformal interfaces with skin, biocompatibility, and wearable comfort. The past decade has witnessed great advancements in performance optimization and function extension of on-skin electrodes. With continuous development and great promise for practical applications, on-skin electrodes are playing an increasingly important role in EP monitoring and human-machine interfaces (HMI). In this review, the latest progress in the development of on-skin electrodes and their integrated system is summarized. Desirable features of on-skin electrodes are briefly discussed from the perspective of performances. Then, recent advances in the development of electrode materials, followed by the analysis of strategies and methods to enhance adhesion and breathability of on-skin electrodes are examined. In addition, representative integrated electrode systems and practical applications of on-skin electrodes in healthcare monitoring and HMI are introduced in detail. It is concluded with the discussion of key challenges and opportunities for on-skin electrodes and their integrated systems.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202001938

Materials, Devices, and Systems of On‐Skin Electrodes for Electrophysiological Monitoring and Human–Machine Interfaces

We present gas-permeable, ultrathin, and stretchable electrodes enabled by self-assembled porous substrates and conductive nanostructures. An efficient and scalable breath figure method is employed to introduce the porous skeleton, and then silver nanowires (AgNWs) are dip-coated and heat-pressed to offer electric conductivity. The resulting film has a transmittance of 61%, sheet resistance of 7.3 Ω/sq, and water vapor permeability of 23 mg cm-2 h-1. With AgNWs embedded below the surface of the polymer, the electrode exhibits excellent stability in the presence of sweat and after long-term wear. We demonstrate the promising potential of the electrode for wearable electronics in two representative applications: skin-mountable biopotential sensing for healthcare and textile-integrated touch sensing for human-machine interfaces. The electrode can form conformal contact with human skin, leading to low skin-electrode impedance and high-quality biopotential signals. In addition, the textile electrode can be used in a self-capacitance wireless touch sensing system.

Gas-Permeable, Ultrathin, Stretchable Epidermal Electronics with Porous Electrodes

Nanomaterial-enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial-enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real-world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial-enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.

Nanomaterial-Enabled Flexible and Stretchable Sensing Systems: Processing, Integration, and Applications

Stretchable electronics based on nanomaterials has received much interest recently. However, it is challenging to print 1D nanomaterials (e.g., nanowires) with high resolution on stretchable elasto...

Achieving High-Resolution Electrohydrodynamic Printing of Nanowires on Elastomeric Substrates through Surface Modification

Direct Fabrication of VIA Interconnects by Electrohydrodynamic Printing for Multi-Layer 3D Flexible and Stretchable Electronics

Stretchable Organic Transistor Based Pressure Sensor Employing a Porous Elastomer Gate Dielectric

The demand of cost-effective fabrication of printed flexible transistors has dramatically increased in recent years due to the need for flexible interface devices for various application including e-skins, wearables, and medical patches. In this study, electrohydrodynamic (EHD) printing processes are developed to fabricate all the components of polymer-based organic thin film transistors (OTFTs), including source/drain and gate electrodes, semiconductor channel, and gate dielectrics, which streamline the fabrication procedure for flexible OTFTs. The flexible transistors with top-gate-bottom-contact configuration are fabricated by integrating organic semiconductor (i.e., poly(3-hexylthiophene-2,5-diyl) blended with small molecule 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), conductive polymer (i.e., poly (3,4-ethylenedioxythiophene) polystyrene sulfonate), and ion-gel dielectric. These functional inks are carefully designed with orthogonal solvents to enable their compatible printing into multilayered flexible OTFTs. The EHD printing process of each functional component is experimentally characterized and optimized. The fully EHD-printed OTFTs show good electrical performance with mobility of 2.86 × 10−1 cm2 V−1 s−1 and on/off ratio of 104, and great mechanical flexibility with small mobility change at bending radius of 6 mm and stable transistor response under hundreds of bending cycles. The demonstrated all printing-based fabrication process provides a cost-effective route toward flexible electronics with OTFTs. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/admt.202300410

All Electrohydrodynamic Printed Flexible Organic Thin Film Transistors

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