Polymer nanocomposite meshes for flexible electronic devices

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Abstract

Flexible electronic devices featuring fashionable wearability, flexibility, and compatibility are drawing considerable attention for use in touch screens, healthcare monitoring, smart wound dressing, electronic skin, energy harvesting and storage devices, flexible displays and human-machine interfacing. Apart from the electrochemical performance, mechanical flexibility and even stretchability are considerable issues for electronic devices to ensure reliable data collection in applications. However, traditional inorganic materials such as metals and semiconductors possess poor mechanical flexibility and limited elasticity. Polymer nanocomposite mesh scaffolds assembled from polymer fibers offer fascinating opportunities for flexible electronic devices by virtue of their inherent flexibility, high surface area, the permeability to air and liquids of the porous mesh structure, and well-controlled composition. Emerging progress in high-performance flexible electronic devices using polymer mesh-based nanocomposites has been achieved through implementations as both passive and active components. In this review, the main design and fabrication strategies of functional polymer nanocomposite mesh scaffolds for flexible electronic devices will be first summarized in brief. Next, the application of polymer nanocomposite meshes as passive components (such as functional substrates, templates, and carbonized precursors) and active components (such as friction layers for nanogenerators, electroactive materials and separators for energy storage devices, and sensing layers for chemical sensors) in various high-performance flexible electronic devices will be reviewed in detail. Finally, the challenges and perspectives will be discussed to offer inspiration for the design of advanced polymer nanocomposite meshes, as well as to promote the utilization and integration of polymer nanocomposite mesh scaffolds into flexible electronic devices with outstanding performance and environmental friendliness.

Graphical abstract

Polymer nanocomposite mesh-based electronic devices, featuring high flexibility, well-controlled compositions, high surface areas, lightweight, and porous structures permeable to air and liquid, are becoming tremendously popular for applications in healthcare monitoring, electronic skin, energy harvesting and storage devices, flexible displays and implantable bioelectronics. The emerging progress in polymer nanocomposite mesh-based flexible electronic devices and in the related mechanisms behind the mesh structure-induced functionalities have been intensively described. The related critical challenges and prospects are provided to offer inspiration for the design of advanced polymer nanocomposite meshes, as well as to promote the utilization and integration of polymer nanocomposite mesh scaffolds into multifunctional flexible electronic devices with outstanding performance and environmental friendliness for various applications.

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Introduction

Flexible electronic devices are attracting considerable attention regarding their wearability, flexibility, and compatibility with various applications including touch screens [1,2], healthcare monitoring [[3], [4], [5], [6], [7], [8]], electronic skin [[9], [10], [11], [12], [13], [14], [15], [16]], energy harvesting and storage devices [[17], [18], [19], [20]], and implantable bioelectronics [[21], [22], [23], [24], [25], [26], [27]]. Studies committed to improve the structures, functionalities and performances of flexible electronic devices are rapidly increasing worldwide since these devices will have a profound effect on people’s daily life. The general term “flexible electronic devices” means that the devices can still function well during bending, folding, compression or stretching [28]. Two main strategies have been presented for the construction of electronic devices with excellent flexibility and stretchability: using intrinsically soft materials and devising special geometrical configurations [[29], [30], [31], [32]]. Existing flexible electronic devices are generally based on inorganic/organic composites, such as metals [33], metal oxide semiconductors [[34], [35], [36]], carbon materials [37,38], and polymers [[39], [40], [41], [42], [43]]. Inorganic semiconductors and metals are considered to be a reasonable choice for the fabrication of electronic devices owing to their high conductivity and reproducable performance. However, the relatively high-density and rigidity of these materials oppose their ability to withstand repeated bending or stretching [44]. Abundant advanced structural designs and optimizations are devoted to endowing metal-based materials with enhanced flexibility. The task is generally feasible through the physical or chemical deposition of bendable ultrathin metal films with particular layouts (e.g., wavy structures, buckling, serpentine patterns, coiled springs and others) on flexible polymer substrates [[45], [46], [47], [48]]. To date, polymers have played considerable roles in the development of strategies and materials for high-performance flexible electronics [[49], [50], [51], [52], [53], [54], [55], [56], [57], [58]]. The internal rotations among single bonds facilitate the conformation changes of molecular chains and lead to the inherent high flexibility of polymers [59] which therefore highlight the broad space to readily tune the properties and functionalities of polymers through engineering molecular structures. In general, polymers may serve as supporting substrates for conductive components, protective encapsulants, adhesives, matrix, or active materials. Polydimethylsiloxane (PDMS), poly(ethylene terephthalate) (PET), polyimide (PI) and Ecoflex are among the most intensively used substrates for flexible electronic devices due to their remarkable flexibility. Conductive polymers, particularly those based on unique π-conjugated structures, such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PT), and so on, have attracted great interest by virtue of their tunable electronic properties, solution processability and excellent mechanical compliance for large-area processing [60,61]. Conjugated polymers are still considered to be the ideal candidates for flexible electronics even though inorganic materials exhibit better performance in terms of the conductivity and charge transport characteristics. The Young’s modulus of a semiconducting polymer is comparable to that of human skin (0.34 MPa), which greatly underlines the potential of these materials for implementation in attachable and wearable on-skin electronics [62,63]. In flexible device engineering, the appropriate geometrical designs of high-performance materials constitute another valid strategy for improved flexibility and stretchability. In this respect, polymer materials can be readily processed into various two-dimensional (2D) or three-dimensional (3D) micro/macrostructures, such as porous [64], fibrous [65], woven [66], coiled [67], wavy [68] and other hierarchical and patterned configurations [69], via different fabrication techniques. Compared to conventional polymer thin films with mechanical mismatches and limited permeability, the fibrous mesh scaffolds, assembled from interlacing fibers, filaments or yarns, offer fascinating opportunities for flexible electronic devices with well-controlled compositions, softness, higher surface areas, porous structures permeable to air and liquid, and high tolerance to damage [[70], [71], [72]]. In addition, the mesh structure is expected to play a vital role in devising ultrathin, lightweight, wearable and conformal electronic devices, which are promising for enhancing the properties of existing applications and for extending the range of new applications [73,74]. There are many delicate mesh structures assembled from natural polymer fibers, such as spider webs woven from silk fibers, and paper and textiles processed from cellulose fibers. Additionally, many kinds of meshes based on synthetic polymers are available thanks to the contributions of polymer science and engineering that benefited fiber-forming and weaving technologies [75]. Moreover, nanoscale conductive materials promote flexibility and electrical performance [76,77]. Therefore, the combination of soft polymer materials and active nanomaterials to prepare polymer nanocomposite mesh-based electronic devices with improved performance has sparked interest over the past two decade, preparing a solid foundation for new routes towards mesh-based polymer nanocomposite electronic materials (Fig. 1). Recently, several reviews have summarized the applications of fibers and textiles based on metals, carbon materials, and conductive polymers in energy harvesting and storage systems as well as wearable electronics [72,[78], [79], [80], [81]]. However, a review that highlights the applicability of polymer mesh composites in various kinds of flexible electronics including strain/pressure sensors, electronic skin, transparent conductors, energy harvesting and storage systems, flexible displays and bioelectronics is lacking. In this review, we will first focus on the strategies for preparing polymer nanocomposite meshes, which can be further integrated into flexible electronic devices via natural-resource-based meshes (cellulose paper, cotton fabric, and silk fabric), electrospinning, in situ polymerization with simultaneous deposition and photolithography (Fig. 2). The related mechanisms of the polymer mesh structure-induced functionalities, as well as device performances, will be elaborated. Then, the versatile methods for engineering polymer nanocomposite mesh scaffolds as passive components (such as functional substrates, templates, precursors, etc.) and active components (such as friction layers for nanogenerators, electrolyte separators for energy storage devices, sensing layers for chemical sensors, etc.) for various high-performance flexible electronic devices will be described in details by highlighting the most relevant and recent advances with the most representative examples (Fig. 3). Finally, the future perspectives for developing polymer nanocomposite mesh-based flexible electronic devices will be discussed to provide understanding of the design and synthesis of polymer mesh nanocomposites and of facilitating the design and assembly of polymer nanocomposite mesh-based wearable electronic devices exhibiting excellent performances, flexibility, compatibility, and environmental friendliness [82].

Section snippets

Material design and fabrication strategies for polymer nanocomposite meshes

Mesh structures, assembled from interconnected fibers, can be fabricated from various polymer nanocomposite components [103,104]. Fibers, referring to the materials consisting of continuous or discontinuous filaments, are ubiquitous in plants, animals and minerals in the nature of cellulose, proteins and minerals. Through well-established natural ingenuity, these fibers are further assembled into one-, two- and three-dimensional fiber meshes. For instance, proteinaceous silk fibers have been

Polymeric nanocomposite meshes as passive components for electronic devices

Flexible electronic devices have attracted attention for on-skin electronics, electronic skin, energy harvesting devices, environmental monitoring and implantable bioelectronics. Conventional flexible electronic devices are generally assembled from conductive material layers on supporting flexible substrates, such as PDMS and PET. However, the mechanical mismatch and limited permeability of conventional flexible substrates have restricted the applications of these electronic devices. Compared

Friction layer for nanogenerators

With the concerns regarding fossil fuel exhaustion and environmental pollution, the interest in renewable energy and energy conversion has been increasing [227,228]. Notably, energy harvesting that aims to capture, accumulate and store small amounts of energy resulting from motion, vibration, or heat is highly desirable [229]. It was since 2006 when piezoelectric nanogenerators (PENGs) were successfully demonstrated to convert mechanical energy into electrical energy on the basis of

Conclusions and perspectives

Interdisciplinary research initiatives have led to significant advancements in the field of flexible electronic devices for flexible sensors, electronic skin, energy harvesting and storage devices, flexible displays and bioelectronics. Polymers, featuring inherent flexibility, synthetic viability, and tunable physical and chemical properties, are one of the most promising candidates for the realization of flexible electronics. Versatile methods for polymer processing have been developed to

Declaration of Competing Interest

There is no conflict of interest to be considered.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (2017YFB0306905, 2016YFC0801302), the National Natural Science Foundation of China (21404006, 21774012 and 51973008), the Major Program of the National Nature Science and Foundation of China (51790500), the Beijing Natural Science Foundation (2152023 and 2202042), and the Fundamental Research Funds for the Central Universities.

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