The rapid advancements of wearable electronics have caused a paradigm shift in consumer electronics, and the emerging development of stretchable electronics opens a new spectrum of applications for electronic systems. Playing a critical role as the power sources for independent electronic systems, energy harvesters with high flexibility or stretchability have been the focus of research efforts over the past decade. A large number of the flexible energy harvesters developed can only operate at very low strain level (≈0.1%), and their limited flexibility impedes their application in wearable or stretchable electronics. Here, the development of highly flexible and stretchable (stretchability >15% strain) energy harvesters is reviewed with emphasis on strategies of materials synthesis, device fabrication, and integration schemes for enhanced flexibility and stretchability. Due to their particular potential applications in wearable and stretchable electronics, energy-harvesting devices based on piezoelectricity, triboelectricity, thermoelectricity, and dielectric elastomers have been largely developed and the progress is summarized. The challenges and opportunities of assembly and integration of energy harvesters into stretchable systems are also discussed.

Inkjet printing, known as digital writing technique, can directly deposit functional materials to form pattern onto substrate. This paper provides an overview of inkjet printing technologies for flexible electronics. Firstly, we highlight materials challenges in implementing flexible devices into practical application, especially for inkjet printing process. Then the micro/nano-patterning technologies of inkjet printing are discussed, including conventional inkjet printing techniques and electrohydrodynamic printing techniques. Thirdly, the related equipments on inkjet printing are shown. Finally, challenges for its future development are also discussed. The main purpose of the work is to condense the basic knowledge and highlight the challenges associated with the burgeoning and exciting field of inkjet printing for flexible electronics.flexible electronics, nanomanufacturing, organic thin film transistor, micro/nano-patterning, inkjet printing, electrohydynamics, roll-to-roll Citation: Overview of flexible electronics technologyFlexible electronics, also known as printable/organic electronics, represent a technology for building electronic circuits by depositing electronic devices onto flexible substrates. Realization of flexible electronics with performance equal to conventional microelectronics built on brittle semiconductor wafers, but in high mobilities, optical transparency, light-weight, stretchable/bendable formats and easy to print rapidly over large areas would enable many new applications [1-4] not satisfied by a traditional rigid electronics. The applications vary from medicine and biology to energy technology and space science [1,2], such as flexible display [4,5], thin film solar cell [6,7], large area sensors and actuators [8,9], shown in Figure 1. These applications are based on thin film transistors (TFTs) which have strong materials and processes contents. Flexible electronics *Corresponding authors (have open boundaries that move with its development and applications and is a highly interdisciplinary field. As a result, there are considerable opportunities for innovation and basic scientific research into new types of electronic materials, processes and equipments.The flexibility, a critical issue in flexible electronics, is one of the most important differences from traditional microelectronics. Polymer organics and inorganic materials are the two kind of materials adopted in flexible electronics. The polymer organics are generally believed to be well suited for these applications and naturally compatible with polymeric substrates. However, the electrical properties is not ideal when devices are fabricated with polymer organics. This leads to interest in the possibility of inorganic based flexible electronics [10]. It is a challenge to design a bendable and stretchable electronics based on inorganic materials due to their small fracture strain. The most basic realization is thin films of the inorganics are adopted as semiconductors, conductors and/or insulators on substrates to mini-

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Assembling multifunctional thin devices onto arbitrary curvilinear surfaces allows widespread and innovative applications in artificial intelligence and advanced healthcare industries.

Hybrid perovskites show enormous potential for display due to their tunable emission, high color purity, strong photoluminescence and electroluminescence. For display applications, full-color and high-resolution patterning is compulsory, however, current perovskite processing such as spincoating fails to meet these requirements. Here, electrohydrodynamic (EHD) printing, with the unique advantages of high-resolution patterning and large scalability, is introduced to fabricate full-color perovskite patterns. Perovskite inks via simple precursor mixing are prepared to in situ crystallize tunableand bright-photoluminescence perovskite arrays without adding antisolvent. Through optimizing the EHD printing process, a high-resolution dot matrix of 5 µm is achieved. The as-printed patterns and pictures show full color and high controllability in micrometer dimension, indicating that the EHD printing is a competitive technique for future halide perovskite-based high-quality display.

Piezoelectric structures, in forms that allow mere in-surface deformations under large strains, are attractive for bio-integrated systems. Here, mechano-electrospinning (MES) is presented to direct-write straight nanofibers of polyvinylidene fluoride onto a prestrained poly(dimethylsiloxane) (PDMS) substrate, to position and polarize a piezoelectric nanofiber array in one-step. Wrinkled/non-wrinkled buckling modes are found when the substrates are released, and the morphology of the direct-written fiber proved the key to determine the buckling modes, which can be tuned precisely by MES parameters. The non-wrinkled, stretchable piezoelectric devices with a highly synchronized serpentine fiber array exhibit their in-surface deformation and stable piezoelectric performance up the failure strain of PDMS (∼110% in our study), which may be used as stretchable sensors and energy converters/providers.

Electrically conductive composites (ECCs) hold great promise in stretchable electronics because of their printability, facile preparation, elasticity, and possibility for large-area fabrication. A high conductivity at steady state and during mechanical deformation is a critical property for ECCs, and extensive efforts have been made to improve the conductivity. However, most of those approaches are exclusively functional to a specific polymer matrix, restricting their capability to meet other requirements, such as mechanical, adhesive, and thermomechanical properties. Here, we report a generic approach to prepare ECCs with conductivity close to that of bulk metals and maintain their conductivity during stretching. This approach iodizes the surfactants on the commercial silver flakes, and subsequent photo exposure converts these silver iodide nanoparticles to silver nanoparticles. The ECCs based on silver nanoparticle-covered silver flakes exhibit high conductivity because of the removal of insulating surfactants as well as the enhanced contact between flakes. The treatment of silver flakes is independent of the polymer matrix and provides the flexibility in matrix selection. In the development of stretchable interconnects, ECCs can be prepared with the same polymer as the substrate to ensure strong adhesion between interconnects and the substrate. For the fabrication of on-skin electrodes, a polymer matrix of low modulus can be selected to enhance conformal contact with the skin for reduced impedance.

Hyper-stretchable self-powered sensors with high sensitivity and excellent stability using low-cost, printable, organic nanomaterials are attractive for immense applications. Here we present self-similar piezoelectric nano/ microfibers for a hyper-stretchable self-powered sensor that demonstrates high stretchability (> 300%), low detection limit (0.2 mg), and excellent durability (> 1400 times at strain 150%). A proposed helix electrohydrodynamic printing technique (HE-Printing) in combination with in-surface self-organized buckling is used to fabricate aligned self-similar poly[vinylidene fluoride] (PVDF) nano/microfibers with in situ mechanical stretch and electrical poling to produce excellent piezoelectric properties. The hyper-stretchable self-powered sensors have shown repeatable and consistent electrical outputs with detection limit an order of magnitude smaller than the other stretchable sensors. Additionally, such sensors can simultaneously measure the own status and the extra multiply physical quantities, such as lateral pressure, impulse rate and applied strain. The high sensitivity can be further utilized to remotely detect human motion in addition to sensing a piece of paper with 1 mm × 1 mm. Further the fiber-based sensors can avoid the catastrophic collapse or wrinkling of serpentine film-based structure during stretching.