The concept of flexible electronics has been around for several decades. In principle, anything thin or very long can become flexible. While cables and wiring are the prime example for flexibility, it was not until the space race that silicon wafers used for solar cells in satellites were thinned to increase their power per weight ratio, thus allowing a certain degree of warping. This concept permitted the first flexible solar cells in the 1960s (Crabb and Treble, 1967). The development of conductive polymers (Shirakawa et al., 1977), organic semiconductors, and amorphous silicon (Chittick et al., 1969; Okaniwa et al., 1983) in the following decades meant huge strides toward flexibility and processability, and thus these materials became the base for electronic devices in applications that require bending, rolling, folding, and stretching, among other properties that cannot be fulfilled by conventional electronics (Cheng and Wagner, 2009) (Figure 1). Presently there is great interest in new materials and fabrication techniques which allow for highperformance scalable electronic devices to be manufactured directly onto flexible substrates. This interest has also extended to not only flexibility but also properties like stretchability and healability which can be achieved by utilizing elastomeric substrates with strong molecular interactions (Oh et al., 2016; Kang et al., 2018). Likewise, biocompatibility and biodegradability has been achieved through polymers that do not cause adverse effect to the body and can be broken down into smaller constituent pieces after utilization (Bettinger and Bao, 2010; Irimia-Vladu et al., 2010; Liu H. et al., 2019). This new progress is now enabling devices which can conform to complex and dynamic surfaces, such as those found in biological systems and bioinspired soft robotics. These next-generation flexible electronics open up a wide range of exciting new applications such as flexible lighting and display technologies for consumer electronics, architecture, and textiles, wearables with sensors that help monitor our health and habits, implantable electronics for improved medical imaging and diagnostics, as well as extending the functionality of robots and unmanned aircraft through lightweight and conformable energy harvesting devices and sensors. While conventional electronics are very capable of these functions, flexible electronics are intended to expand the mechanical features to adhere to novel form factors through hybrid strategies, or as standalone solutions where the application does not require high computation power, intended to be highly robust to deformation, low cost, thin, or disposable. The definition of flexibility differs from application to application. From bending and rolling for easier handling of large area photovoltaics, to conforming onto irregular shapes, folding, twisting, stretching, and deforming required for devices in electronic skin, all while maintaining device performance and reliability. While early progress and many important innovations have already been achieved, the field of flexible electronics has many challenges before it becomes part of our daily life. This represents a huge opportunity for scientific research and development to rapidly and considerably advance this area (Figure 2). In this article the status, key challenges and opportunities for the field of nextgeneration flexible devices are elaborated in terms of materials, fabrication and specific applications. Edited and Reviewed by: Jhonathan Prieto Rojas, King Fahd University of Petroleum and Minerals, Saudi Arabia

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Flexible Electronics: Status, Challenges and Opportunities

Recent Progress in Manufacturing Techniques of Printed and Flexible Sensors: A Review.

Recent developments in nanotechnology-based printing electrode systems for electrochemical sensors.

Cellulose: A Contribution for the Zero e-Waste Challenge

Electrochemical sensors appear as low-cost, rapid, easy to use, and in situ devices for determination of diverse analytes in a liquid solution. In that context, conducting polymers are much-explored sensor building materials because of their semiconductivity, structural versatility, multiple synthetic pathways, and stability in environmental conditions. In this state-of-the-art review, synthetic processes, morphological characterization, and nanostructure formation are analyzed for relevant literature about electrochemical sensors based on conducting polymers for the determination of molecules that (i) have a fundamental role in the human body function regulation, and (ii) are considered as water emergent pollutants. Special focus is put on the different types of micro- and nanostructures generated for the polymer itself or the combination with different materials in a composite, and how the rough morphology of the conducting polymers based electrochemical sensors affect their limit of detection. Polypyrroles, polyanilines, and polythiophenes appear as the most recurrent conducting polymers for the construction of electrochemical sensors. These conducting polymers are usually built starting from bifunctional precursor monomers resulting in linear and branched polymer structures; however, opportunities for sensitivity enhancement in electrochemical sensors have been recently reported by using conjugated microporous polymers synthesized from multifunctional monomers.

Electrochemical Sensors Based on Conducting Polymers for the Aqueous Detection of Biologically Relevant Molecules.

Hydrazine is widely used in industries as a precursor for blowing agents, pharmaceuticals, and pesticides. It is a highly toxic compound; therefore, it is of paramount interest to develop new analytical methods for the detection and control of hydrazine exposure. In this work, we describe the fabrication of an all inkjet-printed paper sensor composed of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) electrode functionalized with zinc oxide (ZnO) and encapsulated in a Nafion matrix for the amperometric determination of low concentrations of hydrazine. The electrochemical properties of the fully inkjet-printed PEDOT:PSS/Nafion and PEDOT:PSS/ZnO/Nafion sensors are compared in the presence and absence of different concentrations of hydrazine. The stability and sensitivity of these electrodes are significantly enhanced after modification with ZnO particles. The layer-by-layer deposition of the materials on the electrode surface is characterized by SEM, XRD, and AFM. The printed sensor exhibits a linear response in the 10–500 μM hydrazine concentration range and a ∼5 μM detection limit (at S/N = 3). The electrochemical sensitivity is 0.14 μA μM−1 cm−2, and the best working voltage is 0.5 V. The developed sensor was applied successfully for the determination of hydrazine content in tap, sea, and mineral water samples validating the accuracy of this sensor.

Aminta N. Castillo

Papers

A paper-based inkjet-printed PEDOT:PSS/ZnO sol-gel hydrazine sensor