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Mechanical energy harvesting technology converting mechanical energy wasted in our surroundings to electrical energy has been regarded as one of the critical technologies for self-powered sensor network and Internet of Things (IoT). Although triboelectric energy harvesters based on contact electrification have attracted considerable attention due to their various advantages compared to other technologies, a further improvement of the output performance is still required for practical applications in next-generation IoT devices. In recent years, numerous studies have been carried out to enhance the output power of triboelectric energy harvesters. The previous research approaches for enhancing the triboelectric charges can be classified into three categories: i) materials type, ii) device structure, and iii) surface modification. In this review article, we focus on various mechanisms and methods through the surface modification beyond the limitations of structural parameters and materials, such as surficial texturing/patterning, functionalization, dielectric engineering, surface charge doping and 2D material processing. This perspective study is a cornerstone for establishing next-generation energy applications consisting of triboelectric energy harvesters from portable devices to power industries. [ABSTRACT FROM AUTHOR]

A self-charging hybrid power unit has been developed by integrating a water-evaporation-induced nanogenerator with a flexible nano-patterned supercapacitor. The nanogenerator can harvest environmental thermal energy and mechanical energy through the water evaporation process, and the supercapacitor can be charged simultaneously. The former offers stable electrical power as output, whereas the Ppy-based supercapacitor shows a capacitance of 12.497 mF/cm2 with 96.42% retention after 4,000 cycles. After filling the power unit with water as the fuel, it can be fully charged in about 20 min. The power unit can be flexibly integrated with electronic devices such as sensor nodes and wireless transmitters employing the Internet of Things. This new approach can offer new possibilities in continuous future operation of randomly distributed electronic devices incorporated in the Internet of Things. [ABSTRACT FROM AUTHOR]

Since the nanogenerator (NG) was invented in 2006, it has been successfully developed and utilized to harvest various forms of mechanical energy in vivo. The NGs promote the progress of self-powered biomedical devices. Moreover, NGs can also be used as sensors to detect a variety of important physiological signals, which brings us closer to real-time, high-fidelity monitoring of physical and pathological information. This paper summarizes the in vivo applications of NGs as biomedical sensors, including in cardiac sensors, respiration sensors, blood pressure sensors, gastrointestinal sensors and bladder sensors. However, there are still many challenges in using NGs as sensors in vivo. For example, how can we minimize and encapsulate the NGs, how can we increase the stability and reliability during long-term detection, and how can we establish a corresponding relationship between the NG's electrical output and the physiological signals. It is also critical to follow the medical principles more closely in the development of self-powered sensors in the future. We believe that the self-powered sensors would promote the development of the next-generation healthcare monitoring systems. [ABSTRACT FROM AUTHOR]

• Lead-free (1− x) KNN- x BTO nanoparticles were prepared by solid state reaction method. • The composite films were made by mixing the nanoparticles into Polydimethylsiloxane. • Composite film made of 10 wt% nanoparticles generates the maximum electrical output. • The nanogenerator device is used as an active sensor for sleep monitoring system. A flexible composite film based piezoelectric nanogenerator has been explored using lead-free (1− x) KNaNbO 3 - x BaTiO 3 NPs (x = 0.02, 0.04, 0.06 and 0.08) (used as KNN- x BTO) nanoparticles (NPs). The composite films were made of KNN-x BTO NPs impregnated into a Polydimethylsiloxane (PDMS) matrix. BTO was doped into the KNN lattice without affecting the orthorhombic phase of KNN. Structural analysis such as XRD and Raman spectroscopy were used to confirm the phase formation and atomic vibrations of the piezoelectric materials. Further the electrical analysis of the piezoelectric composite nanogenerator (PCNG) has been performed and the device generates a maximum electrical output of 58 V, 450 nA. Then the PCNG was used as an active sensor for sleep monitoring system by placing the PCNG in the hands and legs of human body and monitor the body movements. This type of system helps to diagnose the sleeping disorder in humans and to be a future advancement in mobile health care monitoring. [ABSTRACT FROM AUTHOR]

The corrosion issue has been considered as the greatest challenge in metallic devices, which will not only degrade the performance but also cause safety concerns. Within the Blue Energy framework, the metallic electrode in nanogenerators will always face the risk of chloride attack through different approaches in the long-term applications within the extreme oceanic conditions. Here in this work, the chloride ion accumulation in the passive film that initiates from the surface adsorption via the surface defects towards the internal migration has been unravelled in our case study and analysis. The underlying chloride ion migration mechanism will be ascribed to the anion Frenkel pair-like mechanism as a defect induced energy transfer process. The molecule dynamic simulations under 300 K also support the evident damage of the passive film induced by the subtle chloride attack. The in-depth understanding of the corrosion process from a microscopic perspective can further assist for diagnosing and remediation of corrosion of the future nanogenerator electrode and other metallic devices. Synopsis: Illustration of the mechanisms of pitting corrosion that initiated from the damage to the surface Cr 2 O 3 as the protection layer of the metallic electrode soaking within the oceanic condition. Image 1 • For the Blue Energy, metallic electrode protection lacks attention for long-term application in harsh ocean environment. • A clear migration path of the chloride attack has been revealed from surface to the oxide/alloy interface. • MD simulations support the blister phenomenon and final breakdown of the passive film during the chloride ions migration. [ABSTRACT FROM AUTHOR]

Lead-free alternative materials for converting mechanical energy into electrical energy through piezoelectric transduction are of significant value in a diverse range of technological applications. This review describes novel approaches to the fabrication of hybrid piezoelectric polymer-based materials with enhanced piezoelectric responses for biomedical energy-harvesting applications. The most promising routes toward significant improvements in the piezoelectric response and energy-harvesting performance of such materials are discussed. Background information, including definitions of increases in the piezoelectric charge coefficients, generated open circuit voltage, short circuit current, and power density are presented. The effects of the presence of various lead-free components in the structure of the piezoelectric polymers on their piezoresponse or energy-harvesting performance are reviewed. The piezocomposites described are mostly based on poly-(vinylidene fluoride) (PVDF), or its copolymer, poly-(vinylidene fluoride)-trifluoroethylene PVDF-TrFE, loaded with various nanofillers such as reduced graphene oxide (rGO), inorganic compounds (nanoparticles such as barium titanate BaTiO 3 , and potassium sodium niobate (KNaNbO 3)), salts (LaCl 3, ErCl 3, GdCl 3, etc), metal oxides (ZnO, MgO, TiO 2), metallic nanoparticles (Ag, Pt), and carbon nanotubes (CNTs). Non-biodegradable hybrid piezocomposites developed with potential or actually demonstrated applications in biomedical devices and sensors, including implantable nanogenerators and stimulatory materials for wound healing and tissue regeneration form the focus of the current research. Based on a literature survey, it is concluded that novel piezoelectric material systems and device architectures can support the development of flexible, self-powered, multifunctional e-skin for detecting of the rate of motion in humans, for degrading many kinds of organic pollutants, and for sterilizing bacteria on its surface. Various implantable devices that require no external energy input for biomedical energy-harvesting or bone defect repair applications in vivo , as well as sensors or detectors for human motion, health monitoring, independent temperature monitoring, and pressure control that can be prepared based on piezoelectric polymer materials are also discussed. Image 1 • Novel approaches to the fabrication of hybrid piezoelectric materials are discussed. • Novel piezoelectric material systems and device architectures are proposed. • Implantable devices for biomedical energy-harvesting or bone repair are reported. • Routes toward significant improvements in energy-harvesting performance are described. • Physical phenomena underlying the increased energy-harvesting performance are presented. [ABSTRACT FROM AUTHOR]

In this study, neat polyvinylidene fluoride (PVDF), PVDF/polyoctafluoropentyl acrylate (PFA, 90/10 w/w) and PVDF-PFA/hydroxyapatite (HAP, 0 to 20 wt% HAP w.r.t. PVDF-PFA content) were electrospun under controlled conditions, and studied for their piezo-responsive behaviours as a function of nanoweb stacking arrangement (constructive and destructive) and folding architectures (simple, series and parallel connection of electrodes between each folding). Influence of varying HAP content (0, 3, 7, 10, and 20 wt%) on the molecular orientation of CH2-CF2 dipoles in PVDF-PFA/HAP blends were investigated using FTIR, XRD and SEM analysis. Quantitative analysis showed increased β-crystalline content for the 7 wt% HAP sample. From the peak-to-peak output voltage, 7 wt% HAP sample exhibited significantly higher output voltage (Vp-p = 1.21 V) compared to neat PVDF-PFA sample (Vp-p = 0.46 V). Overall, the combinative effect of HAP addition and folding played a vital role in improving the piezoelectric output signals of PVDF-PFA/HAP, which signifies the importance of this study. [ABSTRACT FROM AUTHOR]

In recent years, triboelectric nanogenerator (TENG) has been attracting lots of attention for harvesting electrical energy from mechanical energy, because of its simplicity in designing, cost-effectiveness, and high output power. In the present trend, the performance of TENG is improved mainly by surface modification using complex techniques. In the present work, we have demonstrated enhanced triboelectrification between polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) by incorporating ZnO nanorods into PVDF polymer. The fabricated ZnO-PVDF/PTFE based TENG showed 21% enhanced output voltage and 60% enhanced short-circuit current in comparison to PVDF/PTFE based TENG with an instantaneous output power density of ∼10.6 μW/cm2. The increase in triboelectrification comes not only from the enhancement in β-phase content, which leads to increase polarizability, but also from the enhancement in surface roughness, hydrophobicity, and a decrease in the work function of PVDF after incorporating ZnO nanorods. Image 1079 • Achieved 65.6% more instantaneous output power in nanocomposite film as compared to bare film based TENG. • Surface roughness, polarizability and hydrophobicity plays an important role in the performance of TENG. • Simple yet effective way of enhancing the triboelectric property of ferroelectric film (PVDF). • The localised ferroelectric property of PVDF was studied using PFM. [ABSTRACT FROM AUTHOR]

Piezoelectric two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS 2) have recently attracted significant attention owing to their applicability for fabrication of flexible power generators. In this study, novel piezoelectric nanogenerators (PNGs) consisting of 2D piezoelectric MoS 2 shells are fabricated where an Al 2 O 3 thin layer deposited on the surface of polystyrene (PS) beads is used to avoid collapse of the spherical MoS 2 shells under the high growth temperature. In addition, the MoS 2 shell size is controlled by adjusting the PS bead size and the effects of the MoS 2 shell size on power generation characteristics are investigated. Our PNG based on the piezoelectric MoS 2 shells produces a peak output voltage of approximately 1.2 V at a pressure of 4.2 kPa. The minimum pressure for power generation by tapping is 0.3 kPa. This novel method is very promising for development of the next-generation PNGs based on 2D semiconductor piezoelectric materials. Image 1 • Novel piezoelectric nanogenerators (PNGs) consisting of 2D piezoelectric MoS 2 shells were fabricated. • The effect of the MoS 2 shell size on power generation characteristics was investigated. • The output voltage of 1.2 V was measured under the pressure of 4.2 kPa by using MoS 2 shell-based PNGs. • High-power generation properties and nonlinear hysteresis behavior were observed for the micrometer-scale MoS 2 shells. [ABSTRACT FROM AUTHOR]

GaN/V 2 O 5 core-shell nanowire (NW) piezoelectric generators of controlled crystallographic orientations were fabricated, and the device performance was characterized. Catalyst-assisted c- and m-axis GaN NWs were grown on a c-plane GaN thin film by varying the NH 3 flow rate and reactor pressure. The grown NWs were then utilized to fabricate the flexible piezoelectric nanogenerators (PNGs) to practically investigate the impact of the c- and m-axis GaN NWs on the piezoelectric response. The c-axis GaN NWs exhibited a higher piezoelectric output than m-axis GaN NWs. Furthermore, the GaN/V 2 O 5 core-shell structure was utilized in the NWs to suppress the internal carrier screening that degrades the piezoelectric output. The maximum output voltage (27 V) exhibited by core-shell c-axis NWs was thrice the voltage exhibited by pristine c-axis GaN NWs (9 V). A stability test was performed for one hour to verify the feasibility of using flexible PNGs for real applications. The high stability of PNGs was attributed to the flexibility and high crystallinity of the NWs. GaN/V 2 O 5 core-shell nanowire (NW) piezoelectric generators of controlled crystallographic orientations are reported. Catalyst-assisted c- and m-axis GaN NWs grown by MOCVD are utilized to fabricate the flexible piezoelectric nanogenerators to investigate the impact of the c- and m-axis GaN NWs on the piezoelectric response. The c-axis GaN NWs exhibited the higher piezoelectric output than m-axis GaN NWs. The GaN/V 2 O 5 core-shell structure is utilized in the NWs which further enhanced the output voltage to 27 V and output current to 590 nA. Image 1 • C- & m-axis GaN NWs are grown by MOCVD for piezoelectric generators. • The piezoelectric output for pristine c-axis NWs is twice the output of m-axis NWs. • The piezoelectric output of the GaN/V 2 O 5 core-shell NWs is three times of pristine NWs. • PNGs exhibited long-term stability due to excellent mechanical properties of NWs. • GaN/V 2 O 5 heterojunction suppressed the junction current screening effect. [ABSTRACT FROM AUTHOR]

Abstract The origin of unique piezotronic properties within low dimensional nanomaterial systems will enable an in-depth understanding of nanogenerators for broad applications in the future. Notably, the low dimensional ZnO exhibits stronger temperature sensitivity than the bulk ZnO, which will be proved by the extreme phonon instability at room temperature 300 K. The temperature dependence shows a nearly Fermi-Dirac δ- function. We have proposed the selection criteria for the nanogenerator material screening based on the theoretical derivation of elastic perturbation entropy (EP-S). The resulted phonon conservation behaviors induced by the discontinuity in phonon dispersion dominates the highly sensitive response to the local electrical field change. The Kohn Anomalies (KA) has been identified in the wide bandgap semiconductor ZnO, in which discontinuous energy conversions induced by such KA or boundary discontinuity will not only minimize the degeneration effect but also induce the high piezotronic response. Moreover, we have carefully examined the ZnO surface on both structural and electronic structures, which identified the surface metallic properties. Thus, these theoretical results have supplied sound phonon evidence for the unique piezotronic properties in ZnO, which will facilitate the new insight in the future research of nanogenerators. Graphical abstract The generalized diagram of the KA-assisted pizeotronic mechanism in ZnO. Image 1 Highlights • KA in phonon dispersion of low-dimensional ZnO is firmly correlated to the high response of piezotronics property. • A quantum-phonon-based selection criterion is proposed to screen the piezoelectrical material for future nanogenerator. • The metallicity of ZnO surface under 300 K has been observed with the defective structure with oxygen extrusion. [ABSTRACT FROM AUTHOR]

Abstract Remarkable progress has been made recently in the development of soft materials and devices for human-machine interface. Emerging soft-electronics with skin-inspired stretchable, healable and bio-degradable properties have diffused the boundaries of electronics and biology. Practical utilisation of these electronics relies on the development of a skin-inspired soft energy harvester to power them. Mechanical energy harvesters including triboelectric and piezoelectric nanogenerators have emerged as promising energy harvesters for soft-electronics. In this review, we highlight the recent progress in stretchable, self-healing, and bio-compatible soft triboelectric nanogenerators. We explicitly discussed the operating mechanism, design strategies, fabrication techniques, and performances of the stretchable and self-healing energy harvesters. Additionally, the progress of bio-compatible energy harvesters for effective interaction with biological systems is discussed. Finally, we present the perspectives, existing challenges and future road-map for the development of skin-inspired soft mechanical energy harvesters. Graphical abstract We summarized the recent advances of skin-inspired triboelectric nanogenerator with stretchability, self-healability and bio-compatibility. The available materials, device design, and working mechanism are discussed. Presenting the perspectives, existing challenges and future road-map for the development of skin-inspired soft mechanical energy harvesters. fx1 Highlights • The recent progress of materials, conductors and strategies developed for stretchable TENGs are discussed. • The mechanisms and strategies to realise self-healing TENGs are highlighted. • The materials and methods for biocompatible and implantable TENGs are summarized and suggested. [ABSTRACT FROM AUTHOR]

Two-dimensional materials with high-strength and stress-sensitive characteristics are promising for application in piezoelectric energy harvesting. We have synthesized two-dimensional Li-doped ZnO microdisks via a simple one-step hydrothermal method combined with low-temperature calcination. The Li-doped ZnO was self-poled with d33 of ∼ 15 pm V−1. The effect of the lithium concentration, precursor concentration, Li-doped ZnO weight ratio, and poling process on the output of nanogenerators based on the Li-doped ZnO microdisks was fully investigated. An outstanding output (∼ 20 V, ∼ 18 μA cm−2, and ∼ 5.33 mW cm−3) was obtained without a poling process. The stability was confirmed by fatigue testing over 36,000 cycles. This work provides a feasible solution to address the longstanding aim of achieving a balance between high output and cost-effective manufacturing of piezoelectric energy harvesters. [ABSTRACT FROM AUTHOR]

Abstract A bouncing water droplet not only contains mechanical energy, but also electrostatic energy due to the triboelectric charges on its surface that are generated by its contacts with the surrounding environment. Here, we report development of contact electrification-based water droplet-driven triboelectric nanogenerator (Wd-TENG) for harvesting energy from the water-droplet bouncing between two superhydrophobic surfaces. The Wd-TENG consists of polytetrafluoroethylene balls on the bottom and zinc oxide nanosheets on the top; two surfaces of the device obtain electric energy during the contact electrification from the bouncing water, and additionally, maximize the bouncing motions of the droplets. The device produced a short-circuit current and an open-circuit voltage of 1.3 μA and 1.4 V, respectively. Besides, power-generating performances of the Wd-TENG at various angles of inclination were also investigated, and it showed voltage outputs of 5.5 V, 16 V, 9.8 V, and 6.8 V at the inclination angles of 52°, 58°, 64°, and 70°, respectively. These results demonstrate that the Wd-TENG is potentially a strong candidate for scavenging energy from raindrops. Graphical abstract We report development of contact electrification-based water droplet-driven triboelectric nanogenerator (Wd-TENG) for harvesting energy from the water-droplet bouncing between two superhydrophobic surfaces. The Wd-TENG consists of polytetrafluoroethylene balls on the bottom and zinc oxide nanosheets on the top; two surfaces of the device obtain electric energy during the contact electrification from the bouncing water, and additionally, maximize the bouncing motions of the droplets. It is suggested that the potential of the Wd-TENG demonstrated in this study for harvesting raindrop energy is promising. fx1 Highlights • We report development of contact electrification-based water droplet-driven triboelectric nanogenerator (Wd-TENG). • The Wd-TENG consists of polytetrafluoroethylene balls and zinc oxide nanosheets to effectively harvest from dropping water. • It is suggested that Wd-TENG is potentially a strong candidate for scavenging energy from raindrops. [ABSTRACT FROM AUTHOR]

Abstract Harvesting environmental thermal energy from a natural evaporation process is a promising method for acquiring renewable and sustainable energy. Here, we present an evaporation-driven water flow nanogenerator based on a piece of flexible carbon nanoparticle film. The open-circuit voltage of one single-piece device can be tuned from − 3 to 3 V by manipulating the surface functional groups on the carbon nanoparticles. The effects of different modification molecules and solution concentrations were also investigated. By combining the modified carbon films with opposite surface charges, we enhanced the output of the generator to a maximum open-circuit voltage of 5 V and a short-circuit current of 1.5 μA with a film size of 5 cm × 5 cm. Furthermore, the device can be easily integrated with carbon nanotube-based supercapacitors (CNT-SCs) to assemble the energy-storing and energy-harvesting devices into a self-sustaining power system. The electrical output of the device can directly power a liquid crystal display in an ambient environment and a blue LED with help of self-charged built-in SCs. This eco-friendly device, with its low cost and simple fabrication, shows great potential for future clean energy utilization and for extending the field of portable electronics. Graphical abstract fx1 Highlights • By chemical modification, voltage output of an evaporation-induced flow nanogenerator can be tuned from ~ −3 V to 3 V. • Voltage output of a piece of hybrid flexible device can reach to 5.0 V. • A self-charged energy harvesting system is demonstrated with the combination of the nanogenerator and CNTs supercapacitors. [ABSTRACT FROM AUTHOR]

Abstract The piezoelectric charge coefficients d 33 of single BaTiO 3 (BT) nanoparticles (NPs) were characterized using a transmission electron microscope (TEM) that is equipped with a precise charge meter and an in-situ TEM indentation holder that enables controlled compression experiments. An exceptionally high d 33 of 1775 pC/N was obtained in NPs that are smaller than the critical diameter (D ; typically known as < 100 nm) that has been regarded as the lower limit to permit for ferroelectricity in BT. The mechanical conversion efficiency of piezoelectric BT nanogenerators enhanced as D of BT NPs was decreased; this result corresponds with the single-NP compression measurements of d 33. This quantification of the effect of D in ferroelectric materials may guide development of efficient and high-powered nanostructured piezoelectric energy devices such as piezoelectric nanogenerators. Graphical abstract fx1 Highlights • The piezoelectric d33 of BT nanoparticles are characterized in a transmission electron microscope (TEM) with an in-situ stage. • Exceptionally high d33 of 1775 pC/N by direct piezoelectric effect from a 50 nm BT nanoparticle is monitored. • The piezoelectric nanogenrators with BT nanoparticles are fabricated to confirm the scaling effect of BT nanoparticles. • The nanogenerator including BT nanoparticles shows an outstanding output performance compare to ordinal BT particles. [ABSTRACT FROM AUTHOR]

With the rapid development of the internet of things (IoT), sustainable self-powered wireless sensory systems and diverse wearable and implantable electronic devices have surged recently. Under such an opportunity, nanogenerators, which can convert continuous mechanical energy into usable electricity, have been regarded as one of the critical technologies for self-powered systems, based on the high sensitivity, flexibility, and biocompatibility of piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs). In this review, we have thoroughly analyzed the materials and structures of wearable and implantable PENGs and TENGs, aiming to make clear how to tailor a self-power system into specific applications. The advantages in TENG and PENG are taken to effectuate wearable and implantable human-oriented applications, such as self-charging power packages, physiological and kinematic monitoring, in vivo and in vitro healing, and electrical stimulation. This review comprehensively elucidates the recent advances and future outlook regarding the human body’s self-powered systems.

Structural optimizations of the piezoelectric layer in nanogenerators have been predicted to enhance the output performance in terms of the figure of merit. Here, we report the effect of dielectric constant on electrical outputs of piezoelectric nanogenerator using ZnO/PDMS composites with varied ZnO coverages. The dielectric constant of piezoelectric layers was adjusted from 3.37 to 6.75. The electrical output voltage of 9 mV was achieved in the nanogenerator containing the ZnO/PDMS composite with the dielectric constant of 3.46, which is an 11.3-fold enhancement compared to the value of the nanogenerator featuring the composite with high dielectric constants. Significantly, lowering the dielectric constant of the piezoelectric layer improves the electrical output performance of piezoelectric nanogenerators.

ZnO nanowires are excellent candidates for energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. The key parameters governing the general performance of the integrated devices include the dimensions of the ZnO nanowires used, their doping level, and surface trap density. However, although the method used to grow these nanowires has a strong impact on these parameters, its influence on the performance of the devices has been neither elucidated nor optimized yet. In this paper, we implement numerical simulations based on the finite element method combining the mechanical, piezoelectric, and semiconducting characteristic of the devices to reveal the influence of the growth method of ZnO nanowires. The electrical response of vertically integrated piezoelectric nanogenerators (VING) based on ZnO nanowire arrays operating in compression mode is investigated in detail. The properties of ZnO nanowires grown by the most widely used methods are taken into account on the basis of a thorough and comprehensive analysis of the experimental data found in the literature. Our results show that the performance of VING devices should be drastically affected by growth method. Important optimization guidelines are found. In particular, the optimal nanowire radius that would lead to best device performance is deduced for each growth method.

• The basic principles of acoustic physics are discussed. • The typical ultrasonic response TENG and PENG designs and materials are summarized. • The structural design scheme of flexible photoelectric device is summarized. • The biomedical applications of the devices are classified and discussed. This paper provides a comprehensive review on the acoustoelectric materials and devices in biomedicine. Prominent development has been achieved in minimally invasive medicine and high-efficacy medical treatment. Nevertheless, substantial challenges persist, encompassing sustainable power provisioning, effective drug administration, tissue regeneration, and real-time monitoring. From a foundational and material design perspective, this paper delves into the distinctive contributions and inventive solutions offered by ultrasound technology in tackling these pivotal concerns. It first discusses the fundamentals of ultrasound physics, followed by the typical ultrasound responsive TENG and PENG designs and materials. Structural design plots for flexible aoustoelectric device are then summarized. The biomedical applications of these devices are categorized and discussed. The review article furnishes a systematic documentation and timely examination of the latest advancements in relevant research fields of biomedical acoustoelectric mateirals & devices. The paper concludes with an assessment of current challenges and prospects in the field, with a view to further enhancing the precision, specificity, and system-level integration of personalized healthcare. [ABSTRACT FROM AUTHOR]

Piezoelectric inks have emerged as a promising technological innovation in the field of flexible electronics, sensors, and energy-harvesting devices. Current review aims to provide a comprehensive overview of the key aspects related to piezoelectric inks, including their definition, synthesis methods, composition, raw materials, commercial ink products, sustainability aspects, advantages, limitations, and printing techniques. A detailed exploration of various synthesis approaches for piezoelectric inks has been presented, encompassing methods such as sol-gel, solid-state synthesis, chemical precipitation, spray pyrolysis, electrospinning, and hydrothermal synthesis. Additionally, this study also addresses the critical components required to achieve piezoelectricity in inks, focusing on the integration of piezoelectric nanoparticles or nanomaterials into the ink matrix. Considering the diverse range of applications for piezoelectric inks, the study delves into the composition and selection of raw materials. Special emphasis is placed on exploring eco-friendly and sustainable materials that can contribute to ink formulations with reduced environmental footprint. The study also evaluates the current state of commercial piezoelectric inks available in the market, analyzing their performance, compatibility with different printing techniques, and potential for mass production. The advantages of utilizing piezoelectric inks in various applications are highlighted, including their high sensitivity, mechanical flexibility, low power consumption, and compatibility with flexible substrates. However, acknowledging that no technology is without limitations, the paper discusses the challenges associated with piezoelectric inks, such as limited material availability, processing complexities, and potential reliability issues. Lastly, the review provides insights into the printing techniques employed for depositing piezoelectric inks onto different substrates, such as inkjet printing, screen printing, and aerosol jet printing, among others, while discussing their respective advantages and limitations. [Display omitted] [ABSTRACT FROM AUTHOR]

Massive free electrons in metals could escape from the metal surface to generate a convection current when colliding with high-energy photons (e.g. UV light). Namely, metals with specific work functions could transduce light energy into direct-current (DC) electricity via the famous external photoelectric effect. However, obtaining alternating-current (AC) electricity via a non-Faraday's law of electromagnetic induction directly using metal foil is challenging. Inspired by our recent work, we herein for the first time report a subtle method to directly transduce UV light energy into AC electricity by metal foil nanogenerators and propose a new mechanism regarding the transduction of UV light into AC electricity entitled the neglected internal photoelectric effect (NIPE). According to the NIPE, a zinc foil-based NIPE nanogenerator (NIPEG) can output an optimal open-circuit voltage (V oc) and a short-circuit current (I sc) of 108 V and 320 μA, respectively. The maximum output power and the photoelectric conversion efficiency of the fabricated nanogenerator approach 23.9 W m−2 and 77.02%, respectively. Meantime, the electrical energy stored in the capacitor connected to the Zn foil-based nanogenerator after rectification is several orders of magnitude higher than that of the conventional Cu/PTFE/PET/Cu-based triboelectric nanogenerator and Cu/PTFE/Al-based water droplet nanogenerator. The herein approach potentially offers a novel energy harvesting and conversion avenue to develop self-powered and wearable electronic devices. [Display omitted] • UV-light can be transduced into AC electricity using a metal foil nanogenerator. • A neglected internal photoelectric effect of metal foil is proposed. • The optimal V oc and I sc of the nanogenerator under UV reach 108 V and 320 μA, respectively. • The optimal output power and photoelectric conversion efficiency reach 23.9 W m−2 and 77.0%, respectively. • The energy stored in a 2200 μF capacitor is several orders of magnitude higher than typical TENGs. [ABSTRACT FROM AUTHOR]

Utilization of waste material for value-added applications is a sustainable approach towards reduction of global waste. Herein, the waste rust iron is transformed into useful Fe 2 O 3 nanoparticles (FNPs). Here for the first time, FNPs were uniquely utilized in multiple applications, including resistive switching (RS), tribo and piezo-electric nano-generators, magnetic hyperthermia, and toxic dyes adsorption from water. The RS characteristics of the Ag/FNPs/ITO device emphasized bipolar switching with excellent cyclic stability, endurance, and memory retention. Afterward, the FNPs based tribo and piezo-electric nanogenerator device demonstrated greater instantaneous power (5200 µW) at 10 MΩ, surpassing previously reported inorganic or garbage materials. Furthermore, the pertinency of FNPs was explored for biomedical application by induction heating studies for magnetic fluid hyperthermia (MFH). MFH revealed an increase in temperature of 14.9 °C and 23.4 °C for 5 mg and 10 mg FNPs ferrofluid, respectively, thus indicating the potential of FNPs for possible cancer treatment with biocompatibility suggestions. Finally, the adsorption study reveals that the FNPs are a promising candidate for the adsorption of safranin M.S., methylene blue, malachite green, and lemon yellow powder, (77, 78, 59, and 56 mg/g, respectively). We believe that our current study will result in a significant impact to reduce the future burden of global waste and thus encourage its utilization for value-added applications. [Display omitted] • Waste rust iron converted to Fe 2 O 3 nanoparticles (FNP) via. grinding and annealing. • The FNPs first time utilized for four different applications. • Resistive switching of the FNPs emphasized excellent performance. • Tribo/piezo-electric nanogenerator showed greater instantaneous power (5200 µW). • FNPs hold application in magnetic hyperthermia and adsorption of toxic dyes. [ABSTRACT FROM AUTHOR]

In recent years, wearable electronic devices have made considerable progress thanks to the rapid development of the Internet of Things. However, even though some of them have preliminarily achieved miniaturization and wearability, the drawbacks of frequent charging and physical rigidity of conventional lithium batteries, which are currently the most commonly used power source of wearable electronic devices, have become technical bottlenecks that need to be broken through urgently. In order to address the above challenges, the technology based on triboelectric effect, i.e., triboelectric nanogenerator (TENG), is proposed to harvest energy from ambient environment and considered as one of the most promising methods to integrate with functional electronic devices to form wearable self-powered microsystems. Benefited from excellent flexibility, high output performance, no materials limitation, and a quantitative relationship between environmental stimulation inputs and corresponding electrical outputs, TENGs present great advantages in wearable energy harvesting, active sensing, and driving actuators. Furthermore, combined with the superiorities of TENGs and fabrics, textile-based TENGs (T-TENGs) possess remarkable breathability and better non-planar surface adaptability, which are more conducive to the integrated wearable electronic devices and attract considerable attention. Herein, for the purpose of advancing the development of wearable electronic devices, this article reviews the recent development in materials for the construction of T-TENGs and methods for the enhancement of electrical output performance. More importantly, this article mainly focuses on the recent representative work, in which T-TENGs-based active sensors, T-TENGs-based self-driven actuators, and T-TENGs-based self-powered microsystems are studied. In addition, this paper summarizes the critical challenges and future opportunities of T-TENG-based wearable integrated microsystems.

In recent decades, nanogenerators based on several techniques such as triboelectric effects, piezoelectric effects, or other mechanisms have experienced great developments. The nanoenergy generated by nanogenerators is supposed to be used to overcome the problem of energy supply problems for portable electronics and to be applied to self-powered microsystems including sensors, actuators, integrated circuits, power sources, and so on. Researchers made many attempts to achieve a good solution and have performed many explorations. Massive efforts have been devoted to developing self-powered electronics, such as self-powered communication devices, self-powered human–machine interfaces, and self-powered sensors. To take full advantage of nanoenergy, we need to review the existing applications, look for similarities and differences, and then explore the ways of achieving various self-powered systems with better performance. In this review, the methods of applying nanogenerators in specific circumstances are studied. The applications of nanogenerators are classified into two categories, direct utilization and indirect utilization, according to whether a treatment process is needed. We expect to offer a line of thought for future research on self-powered electronics.

With the rapid development of wearable electronic systems, the need for stretchable nanogenerators becomes increasingly important for autonomous applications such as the Internet-of-Things. Piezoelectric nanogenerators are of interest for their ability to harvest mechanical energy from the environment with its inherent polarization arising from crystal structures or molecular arrangements of the piezoelectric materials. In this work, 3D printing is used to fabricate a stretchable piezoelectric nanogenerator which can serve as a self-powered sensor based on synthesized oxide–polymer composites.

With the increase of interest in the application of piezoelectric polyvinylidene fluoride (PVDF) in nanogenerators (NGs), sensors, and microdevices, the most efficient and suitable methods of their synthesis are being pursued. Electrospinning is an effective method to prepare higher content β-phase PVDF nanofiber films without additional high voltage poling or mechanical stretching, and thus, it is considered an economically viable and relatively simple method. This work discusses the parameters affecting the preparation of the desired phase of the PVDF film with a higher electrical output. The design and selection of optimum preparation conditions such as solution concentration, solvents, the molecular weight of PVDF, and others lead to electrical properties and performance enhancement in the NG, sensor, and other applications. Additionally, the effect of the nanoparticle additives that showed efficient improvements in the PVDF films was discussed as well. For instance, additives of BaTiO3, carbon nanotubes, graphene, nanoclays, and others are summarized to show their contributions to the higher piezo response in the electrospun PVDF. The recently reported applications of electrospun PVDF films are also analyzed in this review paper.

As the internet of things (IoT) era approaches, various sensors, and wireless electronic devices such as smartphones, smart watches, and earphones are emerging. As the types and functions of electronics are diversified, the energy consumption of electronics increases, which causes battery charging and maintenance issues. The piezoelectric nanogenerator (PENG) received great attention as an alternative to solving the energy issues of future small electronics. In particular, polyvinylidene fluoride (PVDF) piezoelectric polymer-based PENGs are strong potential candidate with robust mechanical properties and a high piezoelectric coefficient. In this review, we summarize the recent significant advances of the development of PVDF-based PENGs for self-powered energy-harvesting systems. We discuss the piezoelectric properties of the various structures of PVDF-based PENGs such as thin film, microstructure, nanostructure, and nanocomposite.

Energy harvesting technologies have found significant importance over the past decades due to the increasing demand of energy and self-powered design of electronic and implantable devices. Herein, we demonstrate the design and application of in situ poled highly flexible piezoelectric poly vinylidene fluoride (PVDF) graphene oxide (GO) hybrid nanofibers in aligned mode for multifaceted applications from locomotion sensors to self-powered motion monitoring. Here we exploited the simplest and most versatile method, called electrospinning, to fabricate the in situ poled nanofibers by transforming non-polar α-phase of PVDF to polar β- phase structures for enhanced piezoelectricity under high bias voltage. The flexible piezoelectric device fabricated using the aligned mode generates an improved output voltage of 2.1 V at a uniform force of 12 N. The effective piezoelectric transduction exhibited by the proposed system was tested for its multiple efficacies as a locomotion detector, bio-e-skin, smart chairs and so on.

Nanogenerators (NGs) based on Ni-doped ZnO (NZO) nanorod (NR) arrays were fabricated and explored in this study. The ZnO films were grown on indium tin oxide (ITO) glass substrates, and the NZO NRs were prepared by the chemical bath deposition (CBD) method. The samples were investigated via field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) spectral analysis. The results showed that the growth of NRs presented high-density single crystalline structures and were preferentially oriented in the c-axis direction. The optical characteristics of the NZO NRs were also measured by photoluminescence (PL) spectra. All samples exhibited two different emissions, including ultraviolet (UV) and green emissions. ITO etching paste was used to define patterns, and an electrode of Au film was evaporated onto the ITO glass substrates by the electron beam evaporation technique to assemble the NG device. In summary, ZnO NRs with Ni dopant (5 mM) showed significantly excellent performance in NGs. The optimal measured voltage, current, and power for the fabricated NGs were 0.07 V, 10.5 µA, and 735 nW, respectively.

The advent of liquid-solid triboelectric nanogenerators (LS-TENGs) has ushered in a new era for harnessing and using energy derived from water. To date, extensive research has been conducted to enhance the output of LS-TENGs, thereby improving water utilization efficiency and facilitating their practical application. However, in contrast to intricate chemical treatment methods and specialized structures, a straightforward operational process and cost-effective materials are more conducive to the widespread adoption of LS-TENGs in practical applications. This work presents a novel method to enhance the output of LS-TENGs by increasing the liquid-solid contact area. The approach involves creating roughness on the solid surface through sandpaper grinding, which is simple in design and easy to operate and significantly reduces the cost of the experiment. The theory is applied to the solid triboelectric layer commonly used in the LS-TENG, demonstrating its universality and wide applicability to improve the output of the LS-TENG. The practical performance of the device is demonstrated by charging the capacitor and external load and driving the hygrometer and commercial 5 W LED light bulb, which can directly light up 300 commercial light-emitting diodes (LEDs) driven by a drop of water. This work provides a new method for the optimization of LS-TENGs and contributes to the wide application of LS-TENGs. This is a significant step forward in the field of energy harvesting and utilization.

Gas therapy is emerging as a highly promising therapeutic strategy for cancer treatment. However, there are limitations, including the lack of targeted subcellular organelle accuracy and spatiotemporal release precision, associated with gas therapy. In this study, we developed a series of photoactivatable nitric oxide (NO) donors NRh-R-NO (R = Me, Et, Bn, i Pr, and Ph) based on an N -nitrosated upconversion luminescent rhodamine scaffold. Under the irradiation of 808 nm light, only NRh-Ph-NO could effectively release NO and NRh-Ph with a significant turn-on frequency upconversion luminescence (FUCL) signal at 740 nm, ascribed to lower N-N bond dissociation energy. We also investigated the involved multistage near-infrared-controlled cascade release of gas therapy, including the NO released from NRh-Ph-NO along with one NRh-Ph molecule generation, the superoxide anion O 2 ⋅- produced by the photodynamic therapy (PDT) effect of NRh-Ph, and highly toxic peroxynitrite anion (ONOO ‒ . After mild nano-modification, the nanogenerator (NRh-Ph-NO NPs) empowered with superior biocompatibility could target mitochondria. Under an 808 nm laser irradiation, NRh-Ph-NO NPs could induce NO/ROS to generate RNS, causing a decrease in the mitochondrial membrane potential and initiating apoptosis by caspase-3 activation, which further induced tumor immunogenic cell death (ICD). 2 ⋅- . After mild nano-modification, the nanogenerator (NRh-Ph-NO NPs) empowered with superior biocompatibility could target mitochondria. Under an 808 nm laser irradiation, NRh-Ph-NO NPs could induce NO/ROS to generate RNS, causing a decrease in the mitochondrial membrane potential and initiating apoptosis by caspase-3 activation, which further induced tumor immunogenic cell death (ICD). In vivo therapeutic results of NRh-Ph-NO NPs showed augmented RNS-potentiated gas therapy, demonstrating excellent biocompatibility and effective tumor inhibition guided by real-time FUCL imaging. Collectively, this versatile strategy defines the targeted RNS-mediated cancer therapy., Competing Interests: The authors declared no conflicts of interest., (© 2024 The Authors.)

In this study, a zinc oxide (ZnO) film was deposited by sputtering on an indium tin oxide (ITO) glass substrate. ZnO nanorods were then grown on the film by the hydrothermal method, then assembled with a gold electrode to fabricate a nanogenerator. The ZnO nanostructure and nanogenerator were analyzed by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and the measurement of current-voltage characteristics. The results of FESEM show that the length of the ZnO nanorods increased with the growth time, and the optimal dimensions of the ZnO nanorods were a length of 2 µm and a diameter of 130 nm at the growth time of 6 h. In the XRD pattern, ZnO (002) and (103) peaks were observed at 2θ = 34.45 and 62.51°, respectively, confirming that the ZnO nanorods were grown on the substrate. The nanogenerator was driven by an ultrasonic wave to measure its voltage and current. The highest average current and voltage were 3.46 × 10-6 A and 5.63 × 10-2 V, respectively. These results indicate that the ZnO nanorods prepared by the hydrothermal method are suitable for the fabrication of a nanogenerator. [ABSTRACT FROM AUTHOR]

Abstract From the multitude of nanostructures under active research, Zinc Oxide (ZnO) nanowires (NWs) have attracted enormous attention due to the materials' unique electrical, optical, mechanical and piezoelectric properties. Since 10 years, piezoelectric nanocomposites based nanogenerators (NGs) have gained extensive attention for their applications in mechanical energy harvesters and self-powered tactile sensors. Other emerging applications of ZnO single-crystalline NWs are high performance field-effect transistors (FETs) targeting low power applications, or highly sensitive FET based biosensors. A possible route to reduce the price of these devices is using low cost manufacturing over large-area substrates, and hydrothermal synthesis appears as a promising solution. First we will present here a facile, cost-effective and industrially scalable process flow for the fabrication of high performance stretchable nanogenerators (SNG) on polydimethylsiloxane (PDMS) substrate. The SNG device exhibits excellent performance with a 35 µW peak output power achieved from a 8 cm2 device under a pressure of 100 kPa. Moreover, we investigated ZnO nanostructures for FETs on both conventional rigid (Si/SiO 2) and unconventional flexible substrates (polyethylene terephthalate PET). The electrical characterization results (field-effect mobility, on/off current ratio, sub-threshold swing) reveal the potential of the present nanomaterials for high performance electronics. The key issues of efficient NGs and FETs will be presented, taking into account the technological constraints. This opens horizons for integrating high quality ZnO nanostructures as active semiconducting elements for autonomous flexible electronic circuits. [ABSTRACT FROM AUTHOR]

Abstract Biofouling has been a long-last problem in a variety of marine systems which causes energy waste and device damage. In this study, we present a self-activated anti-biofouling system enabled by electrical double layer disturbance, which could effectively suppress the initial formation of conditioning layers and subsequent microbe attachment. The small and low-frequency alternating electrical fields were produced by a triboelectric nanogenerator under water wave impacts. Systematic analyses confirmed that the anti-biofouling efficacy was directly related to the strength of the electric field and was effective in both fresh lake and sea water environments. An on-site demonstration was implemented at a calm lake shore for three weeks. The water wave-driven anti-biofouling exhibited excellent surface protection, which was significantly superior to several commercial anti-biofouling coatings. This development brings an effective and eco-friendly solution for protecting a broad range of surfaces against biofouling. Graphical abstract fx1 Highlights • This is the first presentation of a new anti-biofouling principle that is based on surface electrical double layer disturbance activated by nanogenerator. • The nanogenerator-enabled anti-biofouling mechanism was studied in detail and experimentally proved at lab scale. • The anti-fouling performance is superior to most commercial passive anti-biofouling techniques in natural lake. • Scaling-up feature is proved experimentally and supported by numerical calculation. [ABSTRACT FROM AUTHOR]

Abstract This comprehensive review focuses on recent advances in energy harvesting of micro-scale and nano-scale generators based on piezoelectric and triboelectric effects. The development of flexible and hybrid devices for a variety of energy harvesting applications are systematically reviewed. A fundamental understanding of the important parameters that determine the performance of piezoelectric, triboelectric and hybrid devices are summarized. Current research directions being explored and the emerging factors to improve harvester functionality and advance progress in achieving high performance and durable energy conversion are provided. Investigations with regard to integrating flexible matrices and optimizing the composition of the piezoelectric and triboelectric materials are examined to enhance device performance and improve cost-effectiveness for the commercial arena. Finally, future research trends, emerging device structures and novel materials in view of imminent developments and harvesting applications are presented. [ABSTRACT FROM AUTHOR]

Abstract Inspired by recent developments in the field of nanogenerators and piezotronics, two different approaches for the improvement of magnetic field sensing are discussed and their feasibility has been experimentally demonstrated. In the first concept, it could be shown that a piezotronic read-out eliminates the need for an external pre-amplifier in MEMS type cantilever sensors. This leads to reduced noise and lower power consumption, which makes the sensors interesting for self-powered applications. Furthermore, the lower noise is measured at short integration times, which reveals further application strategies for piezotronic sensor read-out. In the second concept, a new nanogenerator inspired utilization of electret film allows a reduction of the resonance frequency of cantilever sensors. This reduction is accompanied by higher amplitudes and lower Q-factors allowing energy efficient broadband measurements. A theoretical model has been developed based on an anharmonic oscillator, which is in good agreement with the observed experimental results. Both the demonstrated strategies can help to improve the energy efficiency of the presented magnetic field sensors, allowing lower power consumption and a decreased noise level. Graphical abstract fx1 Highlights • Two new strategies to enhance performance of magnetic field sensors. • Piezotronic sensor read-out for lower noise and higher energy efficiency. • Electret based sensor for large bandwidth operation at low frequencies with high amplitude. [ABSTRACT FROM AUTHOR]