Biocompatible nanogenerators through high piezoelectric coefficient 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 nanowires for in-vivo applications

Self-powered biomedical devices: biology, materials, and their interfaces

Integrating biomedical electronic devices holds profound promise for advancements in healthcare and enhancing individuals' quality of life. However, the persistent challenges associated with the traditional batteries' limited lifespan and bulkiness hinder these devices' long-term functionality and consistent power supply. Here, we delve into the biology and material interfaces in self-powered medical devices by summarizing the intrinsic electric demands in humans, analyzing material and biological mechanisms for electricity generation and storage, and discussing the pathways toward self-chargeable powering. As a result, the current challenges in material designs and biological integrations emerged to shape the future directions in advancing self-powered medical devices. This paper calls on the community to integrate biology and material science to develop self-powering medical devices and improve their clinical prospects.

Recent Progress on Flexible Self-Powered Tactile Sensing Platforms for Health Monitoring and Robotics

Over the past decades, tactile sensing technology has made significant advances in the fields of health monitoring and robotics. Compared to conventional sensors, self-powered tactile sensors do not require an external power source to drive, which makes the entire system more flexible and lightweight. Therefore, they are excellent candidates for mimicking the tactile perception functions for wearable health monitoring and ideal electronic skin (e-skin) for intelligent robots. Herein, the working principles, materials, and device fabrication strategies of various self-powered tactile sensing platforms are introduced first. Then their applications in health monitoring and robotics are presented. Finally, the future prospects of self-powered tactile sensing systems are discussed.

Large piezoelectric responses and ultra-high carrier mobility in Janus HfGeZ3H (Z = N, P, As) monolayers: a first-principles study

Breaking structural symmetry in two-dimensional layered Janus materials can result in enhanced new phenomena and create additional degrees of piezoelectric responses. In this study, we theoretically design a series of Janus monolayers HfGeZ3H (Z = N, P, As) and investigate their structural characteristics, crystal stability, piezoelectric responses, electronic features, and carrier mobility using first-principles calculations. Phonon dispersion analysis confirms that HfGeZ3H monolayers are dynamically stable and their mechanical stability is also confirmed through the Born-Huang criteria. It is demonstrated that while HfGeN3H is a semiconductor with a large bandgap of 3.50 eV, HfGeP3H and HfGeAs3H monolayers have narrower bandgaps being 1.07 and 0.92 eV, respectively. When the spin-orbit coupling is included, large spin-splitting energy is found in the electronic bands of HfGeZ3H. Janus HfGeZ3H monolayers can be treated as piezoelectric semiconductors with the coexistence of both in-plane and out-of-plane piezoelectric responses. In particular, HfGeZ3H monolayers exhibit ultra-high electron mobilities up to 6.40 × 103 cm2 V-1 s-1 (HfGeAs3H), indicating that they have potential for various applications in nanoelectronics.

The coexistence of high piezoelectricity and superior optical absorption in Janus Bi2X2Y (X = Te, Se; Y = Te, Se, S) monolayers

Bismuth chalcogenide and its derivatives have been attracting attention in various fields as semiconductors or topological insulators. Inspired by the high piezoelectric properties of Janus Bi2TeSeS monolayer and the excellent optical absorption properties of the Bi2X3 (X = Te, Se, S) monolayers, we theoretically predicted four new-type two-dimensional (2D) monolayers Janus Bi2X2Y (X = Te, Se; Y = Te, Se, S) using the first principles combined with density functional theory (DFT). The thermal, dynamic, and mechanical stabilities of Janus Bi2X2Y monolayers were confirmed based on ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constants calculations. Their elastic properties, band structures, piezoelectric, and optical properties were systematically investigated. It was found that Janus Bi2X2Y monolayers have a typical Mexican hat-shaped valence band edge structure and, therefore, have a ring-shaped flat band edge, which results in their indirect band gaps. The results show that Janus Bi2X2Y monolayers are semiconductors with moderate band gaps (0.62-0.98 eV at the HSE + SOC level). After considering the electron-phonon renormalization (EPR), the band gaps are reduced by less than 5% at 0 K under the zero-point renormalization (ZPR) and further reduced by approximately 10% at 300 K. Besides, Janus Bi2X2Y monolayers also exhibit excellent optical absorption properties in the blue-UV light region, with the peak values at the order of 8 × 105 cm-1. Particularly, the Janus Bi2Te2S monolayer was found to exhibit a piezoelectric strain coefficient d11 of up to 20.30 pm V-1, which is higher than that of most of the 2D materials. Our results indicate that Janus Bi2X2Y monolayers could be promising candidates in solar cells, optical absorption, and optoelectronic devices; especially, a Janus Bi2Te2S monolayer can also be an excellent piezoelectric material with great prospects in the fields of mechanical and electrical energy conversion.

Implantable Flexible Sensors for Health Monitoring

Flexible sensors, as a significant component of flexible electronics, have attracted great interest the realms of human-computer interaction and health monitoring due to their high conformability, adjustable sensitivity, and excellent durability. In comparison to wearable sensor-based in vitro health monitoring, the use of implantable flexible sensors (IFSs) for in vivo health monitoring offers more accurate and reliable vital sign information due to their ability to adapt and directly integrate with human tissue. IFSs show tremendous promise in the field of health monitoring, with unique advantages such as robust signal reading capabilities, lightweight design, flexibility, and biocompatibility. Herein, a review of IFSs for vital signs monitoring is detailly provided, highlighting the essential conditions for in vivo applications. As the prerequisites of IFSs, the stretchability and wireless self-powered properties of the sensor are discussed, with a special attention paid to the sensing materials which can maintain prominent biosafety (i.e., biocompatibility, biodegradability, bioresorbability). Furthermore, the applications of IFSs monitoring various parts of the body are described in detail, with a summary in brain monitoring, eye monitoring, and blood monitoring. Finally, the challenges as well as opportunities in the development of next-generation IFSs are presented.

Advances in Wireless, Batteryless, Implantable Electronics for Real-Time, Continuous Physiological Monitoring

This review summarizes recent progress in developing wireless, batteryless, fully implantable biomedical devices for real-time continuous physiological signal monitoring, focusing on advancing human health care. Design considerations, such as biological constraints, energy sourcing, and wireless communication, are discussed in achieving the desired performance of the devices and enhanced interface with human tissues. In addition, we review the recent achievements in materials used for developing implantable systems, emphasizing their importance in achieving multi-functionalities, biocompatibility, and hemocompatibility. The wireless, batteryless devices offer minimally invasive device insertion to the body, enabling portable health monitoring and advanced disease diagnosis. Lastly, we summarize the most recent practical applications of advanced implantable devices for human health care, highlighting their potential for immediate commercialization and clinical uses.

Accelerated Osteogenic Response of Electrodynamically Stimulated Mg1-xCaxSi1-xZrxO3 (x = 0-0.4) Bioelectrets

MgSiO3-based biodegradable ceramics demonstrated remarkable potential for treating small-scale bone defects and temporary bone replacement. In addition, the dissolution behavior of MgSiO3 bioceramics can be tuned by doping of Ca and Zr elements at Mg and Si sites, respectively. The present study reported the influence of formation of Ca- and Zr-codoped Mg1-xCaxSi1-xZrxO3 (x = 0, 0.1, 0.2, 0.3, and 0.4) bioelectrets and electrodynamic stimulation toward improving their osteogenic response. Mg1-xCaxSi1-xZrxO3 electrets were successfully synthesized by a solid-state route. A detailed X-ray photoelectron spectroscopy (XPS) analyses revealed that the electrets produced oxygen-deficient active sites. The formation of Mg1-xCaxSi1-xZrxO3 electrets significantly increased the surface hydrophilicity. Inductively coupled plasma (ICP) analyses were used to examine the leaching behavior of Ca/Zr-codoped MgSiO3 bioceramics. In vitro cell culture analyses indicated that the osteogenesis of MG-63 cells was remarkably enhanced on the electrodynamic field-treated Mg1-xCaxSi1-xZrxO3 bioelectrets as compared to hydroxyapatite (HA). Moreover, a better osteogenic response was observed for higher concentrations of Ca (0.3 and 0.4) and Zr (0.3 and 0.4) doping in the MgSiO3 bioelectrets. Further, the mechanism of enhanced cellular functionality was revealed by the measurement of intracellular Ca2+.

Unleashing the piezoelectric potential of PVDF: a study on phase transformation from gamma (γ) to beta (β) phase through thermal contact poling

Polyvinylidene fluoride (PVDF) is known for its piezoelectric properties. This material has different crystalline phases, alpha (α), beta (β) and gamma (γ), where the β-phase, in particular, is related to the piezoelectric behavior of PVDF. While the transformation from the α-phase to β-phase in PVDF is well-documented and widely studied, the transformation from γ- to β-phase has not yet been fully explored. However, when PVDF is produced by certain solution-based methods it can adopt its γ-form, which is not as piezoelectric as the β-phase. Hence, this study aims to bridge this gap by investigating the transformation from γ- to β-phase in PVDF nanocomposites films obtained from solution-based techniques. Our PVDF nanocomposite is made by solvent evaporation-assisted 3D printing of PVDF's nanocomposite with barium-titanate nanoparticles (BTO). To achieve the γ- to β-phase transformation, we first highlight the importance of annealing in the successful poling of PVDF samples. We then perform an in-depth analysis of the α-, β- and γ-crystallographic phases of PVDF-BTO using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). We observed that after annealing but before poling, the PVDF-BTO nanocomposite contains 76% of β + γ phases, the majority of which is the γ-phase. Poling of these samples resulted in the combination of the β + γ phases reaching 93% with the appearance of 40% of absolute fraction of the β-phase. We then demonstrated that the fraction of β-phase in the nanocomposite - as indicated by the 1275 cm-1 peak in PVDF's FTIR spectra - is not uniform on the surface area of the film. Additionally, the value of the absolute β-phase content also depends on the poling field's direction. Our work reveals that while considering PVDF's piezoelectric behavior, it is critical to be aware of these nuances and this article offers essential insights on how to address them. Overall, this study provides a step-by-step guideline to enhance the piezoelectricity of PVDF-based nanocomposites for sensing applications.

Preparation of Nanocomposites for Antibacterial Orthodontic Invisible Appliance Based on Piezoelectric Catalysis

Compared to fixed orthodontic appliances with brackets, thermoplastic invisible orthodontic aligners offer several advantages, such as high aesthetic performance, good comfort, and convenient oral health maintenance, and are widely used in orthodontic fields. However, prolonged use of thermoplastic invisible aligners may lead to demineralization and even caries in most patients' teeth, as they enclose the tooth surface for an extended period. To address this issue, we have created PETG composites that contain piezoelectric barium titanate nanoparticles (BaTiO₃NPs) to obtain antibacterial properties. First, we prepared piezoelectric composites by incorporating varying amounts of BaTiO₃NPs into PETG matrix material. The composites were then characterized using techniques such as SEM, XRD, and Raman spectroscopy, which confirmed the successful synthesis of the composites. We cultivated biofilms of Streptococcus mutans (S. mutans) on the surface of the nanocomposites under both polarized and unpolarized conditions. We then activated piezoelectric charges by subjecting the nanocomposites to 10 Hz cyclic mechanical vibration. The interactions between the biofilms and materials were evaluated by measuring the biofilm biomass. The addition of piezoelectric nanoparticles had a noticeable antibacterial effect on both the unpolarized and polarized conditions. Under polarized conditions, nanocomposites demonstrated a greater antibacterial effect than under unpolarized conditions. Additionally, as the concentration of BaTiO₃NPs increased, the antibacterial rate also increased, with the surface antibacterial rate reaching 67.39% (30 wt% BaTiO₃NPs). These findings have the potential for application in wearable, invisible appliances to improve clinical services and reduce the need for cleaning methods.

Semiconducting Titanate Supported Ruthenium Clusterzymes for Ultrasound-Amplified Biocatalytic Tumor Nanotherapies

The external-stimulation-induced reactive-oxygen-species (ROS) generation has attracted increasing attention in therapeutics for malignant tumors. However, engineering a nanoplatform that integrates with efficient biocatalytic ROS generation, ultrasound-amplified ROS production, and simultaneous relief of tumor hypoxia is still a great challenge. Here, we create new semiconducting titanate-supported Ru clusterzymes (RuNC/BTO) for ultrasound-amplified biocatalytic tumor nanotherapies. The morphology and chemical/electronic structure analysis prove that the biocatalyst consists of Ru nanoclusters that are tightly stabilized by Ru-O coordination on BaTiO₃ . The peroxidase (POD)- and halogenperoxidase-like biocatalysis reveals that the RuNC/BTO can produce abundant •O₂ ^- radicals. Notably, the RuNC/BTO exhibits the highest turnover number (63.29 × 10^-3 s^-1 ) among the state-of-the-art POD-mimics. Moreover, the catalase-like activity of the RuNC/BTO facilitates the decomposition of H₂ O₂ to produce O₂ for relieving the hypoxia of the tumor and amplifying the ROS level via ultrasound irradiation. Finally, the systematic cellular and animal experiments have validated that the multi-modal strategy presents superior tumor cell-killing effects and suppression abilities. We believe that this work will offer an effective clusterzyme that can adapt to the tumor microenvironment-specific catalytic therapy and also provide a new pathway for engineering high-performance ROS production materials across broad therapeutics and biomedical fields.

Flexible PVDF-TrFE Nanocomposites with Ag-decorated BCZT Heterostructures for Piezoelectric Nanogenerator Applications

Flexible piezoelectric nanogenerators are playing an important role in delivering power to next-generation wearable electronic devices due to their high-power density and potential to create self-powered sensors for the Internet of Things. Among the range of available piezoelectric materials, poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)-based piezoelectric composites exhibit significant potential for flexible piezoelectric nanogenerator applications. However, the high electric fields that are required for poling cannot be readily applied to polymer composites containing piezoelectric fillers due to the high permittivity contrast between the filler and matrix, which reduces the dielectric strength. In this paper, novel Ag-decorated BCZT heterostructures were synthesized via a photoreduction method, which were introduced at a low level (3 wt %) into the matrix of PVDF-TrFE to fabricate piezoelectric composite films. The effect of Ag nanoparticle loading content on the dielectric, ferroelectric, and piezoelectric properties was investigated in detail, where a maximum piezoelectric energy-harvesting figure of merit of 5.68 × 10^-12 m²/N was obtained in a 0.04Ag-BCZT NWs/PVDF-TrFE composite film, where 0.04 represents the concentration of the AgNO₃ solution. Modeling showed that an optimum performance was achieved by tailoring the fraction and distribution of the conductive silver nanoparticles to achieve a careful balance between generating electric field concentrations to increase the level of polarization, while not degrading the dielectric strength. This work therefore provides a strategy for the design and manufacture of highly polarized piezoelectric composite films for piezoelectric nanogenerator applications.

Long-term in vivo operation of implanted cardiac nanogenerators in swine

Implantable nanogenerators (i-NG) provide power to cardiovascular implantable electronic devices (CIEDs) by harvesting biomechanical energy locally eliminating the need for batteries. However, its long-term operation and biological influences on the heart have not been tested. Here, we evaluate a soft and flexible i-NG system engineered for long-term in vivo cardiac implantation. It consisted of i-NG, leads, and receivers, and was implanted on the epicardium of swine hearts for 2 months. The i-NG system generated electric current throughout the testing period. Biocompatibility and biosafety were established based on normal blood and serum test results and no tissue reactions. Heart function was unchanged over the testing period as validated by normal electrocardiogram (ECG), transthoracic ultrasound, and invasive cardiac functional measures. This research demonstrates the safety, long term operation and therefore the feasibility of using i-NGs to power the next generation CIEDs.

Bimodal Nanocomposite Platform with Antibiofilm and Self-Powering Functionalities for Biomedical Applications

Advances in microelectronics and nanofabrication have led to the development of various implantable biomaterials. However, biofilm-associated infection on medical devices still remains a major hurdle that substantially undermines the clinical applicability and advancement of biomaterial systems. Given their attractive piezoelectric behavior, barium titanate (BTO)-based materials have also been used in biological applications. Despite its versatility, the feasibility of BTO-embedded biomaterials as anti-infectious implantable medical devices in the human body has not been explored yet. Here, the first demonstration of clinically viable BTO-nanocomposites is presented. It demonstrates potent antibiofilm properties against Streptococcus mutans without bactericidal effect while retaining their piezoelectric and mechanical behaviors. This antiadhesive effect led to ∼10-fold reduction in colony-forming units in vitro. To elucidate the underlying mechanism for this effect, data depicting unfavorable interaction energy profiles between BTO-nanocomposites and S. mutans using the classical and extended Derjaguin, Landau, Verwey, and Overbeek theories is presented. Direct cell-to-surface binding force data using atomic force microscopy also corroborate reduced adhesion between BTO-nanocomposites and S. mutans. Interestingly, the poling process on BTO-nanocomposites resulted in asymmetrical surface charge density on each side, which may help tackle two major issues in prosthetics-bacterial contamination and tissue integration. Finally, BTO-nanocomposites exhibit superior biocompatibility toward human gingival fibroblasts and keratinocytes. Overall, BTO-embedded composites exhibit broad-scale potential to be used in biological settings as energy-harvestable antibiofilm surfaces.

Construction of Bio-Piezoelectric Platforms: From Structures and Synthesis to Applications

Piezoelectric materials, with their unique ability for mechanical-electrical energy conversion, have been widely applied in important fields such as sensing, energy harvesting, wastewater treatment, and catalysis. In recent years, advances in material synthesis and engineering have provided new opportunities for the development of bio-piezoelectric materials with excellent biocompatibility and piezoelectric performance. Bio-piezoelectric materials have attracted interdisciplinary research interest due to recent insights on the impact of piezoelectricity on biological systems and their versatile biomedical applications. This review therefore introduces the development of bio-piezoelectric platforms from a broad perspective and highlights their design and engineering strategies. State-of-the-art biomedical applications in both biosensing and disease treatment will be systematically outlined. The relationships between the properties, structure, and biomedical performance of the bio-piezoelectric materials are examined to provide a deep understanding of the working mechanisms in a physiological environment. Finally, the development trends and challenges are discussed, with the aim to provide new insights for the design and construction of future bio-piezoelectric materials.

A Fully Self-Healing Piezoelectric Nanogenerator for Self-Powered Pressure Sensing Electronic Skin

As an important way of converting mechanical energy into electric energy, a piezoelectric nanogenerator (PENG) has been widely applied in energy harvesting as well as self-powered sensors in recent years. However, its robustness and durability are still severely challenged by frequent and inevitable mechanical impacts in real application environments. Herein, a fully self-healing PENG (FS-PENG) as a self-powered pressure sensing electronic skin is reported. The self-healing piezoelectric composite and self-healing Ag NW electrode fabricated through mixing piezoelectric PZT particles and conductive Ag NWs into self-healing polydimethylsiloxane (H-PDMS) are assembled into the sandwich structure FS-PENG. The FS-PENG could not only effectively convert external stimulation into electrical signals with a linear response to the pressure but also retain the excellent self-healing and stable sensing property after multiple cycles of cutting and self-healing process. Moreover, a self-healing pressure sensor array composed of 9 FS-PENGs was attached on the back of the human hand to mimic the human skin, and accurate monitoring of the spatial position distribution and magnitude of the pressure was successfully realized.

Wearable and Implantable Electroceuticals for Therapeutic Electrostimulations

Wearable and implantable electroceuticals (WIEs) for therapeutic electrostimulation (ES) have become indispensable medical devices in modern healthcare. In addition to functionality, device miniaturization, conformability, biocompatibility, and/or biodegradability are the main engineering targets for the development and clinical translation of WIEs. Recent innovations are mainly focused on wearable/implantable power sources, advanced conformable electrodes, and efficient ES on targeted organs and tissues. Herein, nanogenerators as a hotspot wearable/implantable energy-harvesting technique suitable for powering WIEs are reviewed. Then, electrodes for comfortable attachment and efficient delivery of electrical signals to targeted tissue/organ are introduced and compared. A few promising application directions of ES are discussed, including heart stimulation, nerve modulation, skin regeneration, muscle activation, and assistance to other therapeutic modalities.

Polysaccharides and proteins-based nanogenerator for energy harvesting and sensing: A review

Nanogenerator is a promising energy harvesting device that can scavenge tiny mechanical energy from the surrounding environment, and then convert it into electricity. Natural bio-polymers are the potential candidates for the design of nanogenerators due to their excellent characteristics like piezoelectricity, triboelectricity, non-toxicity, biocompatibility and biodegradability. Especially, nanogenerators using bio-sourced polymers as the core raw materials are suitable for wearable and implantable devices. In this review, major advancements in the sensing field of nanogenerators based on natural polysaccharides and proteins (cellulose, chitosan, alginate, agarose, starch, lignin, silk fibroin, collagen, gelatin, keratin, peptide, M13 bacteriophage, β-cyclodextrin, spider silk, etc.) are summarized. Also, challenges in the improvement of electric output performance, flexibility, anti-humidity and energy management for natural polymers based-nanogenerators are proposed. In the future, they will be applied in daily life as an alternative for traditional power source after addressing issues mentioned above.

Energy Harvesting towards Self-Powered IoT Devices

The internet of things (IoT) manages a large infrastructure of web-enabled smart devices, small devices that use embedded systems, such as processors, sensors, and communication hardware to collect, send, and elaborate on data acquired from their environment. Thus, from a practical point of view, such devices are composed of power-efficient storage, scalable, and lightweight nodes needing power and batteries to operate. From the above reason, it appears clear that energy harvesting plays an important role in increasing the efficiency and lifetime of IoT devices. Moreover, from acquiring energy by the surrounding operational environment, energy harvesting is important to make the IoT device network more sustainable from the environmental point of view. Different state-of-the-art energy harvesters based on mechanical, aeroelastic, wind, solar, radiofrequency, and pyroelectric mechanisms are discussed in this review article. To reduce the power consumption of the batteries, a vital role is played by power management integrated circuits (PMICs), which help to enhance the system’s life span. Moreover, PMICs from different manufacturers that provide power management to IoT devices have been discussed in this paper. Furthermore, the energy harvesting networks can expose themselves to prominent security issues putting the secrecy of the system to risk. These possible attacks are also discussed in this review article.

Electroconductive and free-shapeable nanocomposite hydrogels with an ultrafast self-healing property and high stretchability performance

Conductive self-healing hydrogels as a fascinating class of materials have received much attention in recent years and been widely used in many fields. However, a long healing time and poor electrical conductivity have limited their extended applications. To overcome these shortcomings, we fabricated an excellent conductive self-healing hydrogel by embedding a nanocomposite of Ag nanoparticles and reduced graphene oxide (Ag/RGO) in PVA-borax dynamic networks, which exhibits a relatively high conductivity (4.43 S m-1), good flexibility and excellent self-healing properties without any external stimuli. The multifunctional hydrogel could self-heal within 3 s at room temperature. It also exhibits an excellent free-shapeable property like clay such that it can be modeled into any different complex geometrical shape as desired. It is expected to have potential applications in many fields such as flexible electronic wearable devices, sensors, rechargeable batteries, and biomaterials.

Progress in lead-free piezoelectric nanofiller materials and related composite nanogenerator devices

Current piezoelectric device systems need a significant reduction in size and weight so that electronic modules of increasing capacity and functionality can be incorporated into a great range of applications, particularly in energy device platforms. The key question for most applications is whether they can compete in the race of down-scaling and an easy integration with highly adaptable properties into various system technologies such as nano-electro-mechanical systems (NEMS). Piezoelectric NEMS have potential to offer access to a parameter space for sensing, actuating, and powering, which is inflential and intriguing. Fortunately, recent advances in modelling, synthesis, and characterization techniques are spurring unprecedented developments in a new field of piezoelectric nano-materials and devices. While the need for looking more closely at the piezoelectric nano-materials is driven by the relentless drive of miniaturization, there is an additional motivation: the piezoelectric materials, which are showing the largest electromechanical responses, are currently toxic lead (Pb)-based perovskite materials (such as the ubiquitous Pb(Zr,Ti)O3, PZT). This is important, as there is strong legislative and moral push to remove toxic lead compounds from commercial products. By far, the lack of viable alternatives has led to continuing exemptions to allow their temporary use in piezoelectric applications. However, the present exemption will expire soon, and the concurrent improvement of lead-free piezoelectric materials has led to the possibility that no new exemption will be granted. In this paper, the universal approaches and recent progresses in the field of lead-free piezoelectric nano-materials, initially focusing on hybrid composite materials as well as individual nanoparticles, and related energy harvesting devices are systematically elaborated. The paper begins with a short introduction to the properties of interest in various piezoelectric nanomaterials and a brief description of the current state-of-the-art for lead-free piezoelectric nanostructured materials. We then describe several key methodologies for the synthesis of nanostructure materials including nanoparticles, followed by the discussion on the critical current and emerging applications in detail.

Keggin and Dawson polyoxometalates as electrodes for flexible and transparent piezoelectric nanogenerators to efficiently utilize mechanical energy in the environment

In recent years, piezoelectric nanogenerators (PENGs) have been developed as a promising energy-harvesting electronic device. However, the electrodes of most PENGs devices are precious metals, thus increasing the production cost. Here, we propose a flexible transparent PENGs with polyoxometalates (POMs) as the electrodes; it can effectively utilize ambient mechanical energy to generate electricity. Five types of polyoxometalates with different structures and compositions are selected as the electrode materials for PENGs for the first time, and the output performance of different PENGs electrode devices is tested. The PENG device with (NH₄)₆P₂Mo₁₈O₆₂ as the electrode can steadily provide a high electric output with an open-circuit voltage of 2.8 mV and a short-circuit current of 8.5 µA at the bending degree of 90°. At the same time, the transmission spectrum shows that the average visible transmittance (AVT) of PENG can reach 31%, thus outperforming the benchmark for window applications. Finally, the working mechanism, force analysis, repeatability, and stability of PENG are systematically evaluated. All the studies show that this flexible transparent device has potential application prospect in wearable electronic devices.

Piezoelectric nanofiber/polymer composite membrane for noise harvesting and active acoustic wave detection

Being one of the most common forms of energy existing in the ambient environment, acoustic waves have a great potential to be an energy source. However, the effective energy conversion of an acoustic wave is a great challenge due to its low energy density and broad bandwidth. In this work, we developed a new piezoelectric nanogenerator (PENG), which is mainly composed of a piece of piezoelectric nanofiber/polymer composite membrane. As an energy harvester, the PENG can effectively scavenge a broad low-frequency (from 50 Hz to 400 Hz) acoustic energy from the ambient environment, and it can even scavenge a very weak acoustic energy with a minimum pressure of only 0.18 Pa. When a drum was used as an excitation source, the maximum open-circuit voltage and short-circuit current density of the PENG reached 1.8 V and 1.67 mA m^-2, respectively. In addition, the PENG had a good stability and its output frequency and amplitude were closely related to the driving sound wave, which made the PENG capable of detecting acoustic signals in the living environment and have the potential to be applied as a self-powered active acoustic detector.

Multifunctional Glycerol-Water Hydrogel for Biomimetic Human Skin with Resistance Memory Function

Biomimetic human skinlike materials with preferably self-healing ability, high sensitivity for external stimuli, and good adhesiveness against diverse substrates under a wide range of temperatures are of great importance in various applications such as wearable devices, human-motion devices, and soft robotics. However, most of the reported biomimetic human skinlike materials lack memory function, i.e., they cannot memorize the external stimuli once the stimuli disappear. This drawback hinders their applications in mimicking the human skin in real world. Here, a polyacrylamide/Au@polydopamine glycerol-water (GW) hydrogel has been designed to address this challenge. The as-prepared GW hydrogel exhibits a fast self-healing efficiency and good adhesiveness against diverse substrates under a wide range of temperatures (from -15 to 37 °C). Additionally, our GW hydrogel also possesses good perceived ability for external stimuli and subtle/large human motions. Most importantly, resistance memory function has been realized based on our GW hydrogel. These outstanding properties make it potentially significant in mimicking the human skin in real world.

Effect of anacardic acid against echinococcosis through inhibition of VEGF-induced angiogenesis

Echinococcosis is a zoonotic infection caused by cestode species of the genus Echinococcus, with limited treatment options. It is urgent to develop new anti-hydatid agent. In this paper, we reported anacardic acid (AA), a natural product isolated from the Brazilian cashew-nut shell liquid, which presented a high activity against metacestodes of Echinococcus multilocularis (E. multilocularis) and Echinococcus granulosus sensu stricto (E. granulosus s.s.) in vitro and in vivo. AA exerted a better efficacy on E. granulosus s.s. protoscoleces and E. multilocularis metacestodes than that of albendazole (ABZ) and dihydroartemisinin (DHA) in vitro, and an inhibition on the growth of Echinococcus metacestode as effective as ABZ in vivo. Moreover, we also found that one of the mechanisms of AA against Echinococcus could be the suppression of angiogenesis on/in the metacestode mass through inhibiting vascular endothelial growth factor (VEGF)—induced signalling pathways. This work finds that AA is a new promising potential candidate drug for echinococcosis treatment.

Implantable Energy-Harvesting Devices

The sustainable operation of implanted medical devices is essential for healthcare applications. However, limited battery capacity is a key challenge for most implantable medical electronics (IMEs). The human body abounds with mechanical and chemical energy, such as the heartbeat, breathing, blood circulation, and the oxidation-reduction of glucose. Harvesting energy from the human body is a possible approach for powering IMEs. Many new methods for developing in vivo energy harvesters (IVEHs) have been proposed for powering IMEs. In this context energy harvesters based on the piezoelectric effect, triboelectric effect, automatic wristwatch devices, biofuel cells, endocochlear potential, and light, with an emphasis on fabrication, energy output, power management, durability, animal experiments, evaluation criteria, and typical applications are discussed. Importantly, the IVEHs that are discussed, are actually implanted into living things. Future challenges and perspectives are also highlighted.

Materials and Wearable Devices for Autonomous Monitoring of Physiological Markers

Wearable devices are gaining considerable attention owing to the ease with which they can collect crucial information in real-time, both continuously and noninvasively, regarding a wearer's health. A concise summary is given of the three main elements that enable autonomous detection and monitoring of the likelihood or the existence of a health-risk state in continuous and real-time modes, with an emphasis on emerging materials and fabrication techniques in the relevant fields. The first element is the sensing technology used in the noninvasive detection of physiological markers relevant to the state of health. The second element is self-powered devices for longer periods of use by drawing energy from bodily movement and temperature. The third element is the self-healing properties of the materials used in the wearable devices to extended usage if they become scratched or cut. Promises and challenges of the separately reviewed parts and the combined parts are presented and discussed. Ideas regarding further improvement of skin-based wearable devices are also presented and discussed.

Energy Harvesting Research: The Road from Single Source to Multisource

Energy harvesting technology may be considered an ultimate solution to replace batteries and provide a long-term power supply for wireless sensor networks. Looking back into its research history, individual energy harvesters for the conversion of single energy sources into electricity are developed first, followed by hybrid counterparts designed for use with multiple energy sources. Very recently, the concept of a truly multisource energy harvester built from only a single piece of material as the energy conversion component is proposed. This review, from the aspect of materials and device configurations, explains in detail a wide scope to give an overview of energy harvesting research. It covers single-source devices including solar, thermal, kinetic and other types of energy harvesters, hybrid energy harvesting configurations for both single and multiple energy sources and single material, and multisource energy harvesters. It also includes the energy conversion principles of photovoltaic, electromagnetic, piezoelectric, triboelectric, electrostatic, electrostrictive, thermoelectric, pyroelectric, magnetostrictive, and dielectric devices. This is one of the most comprehensive reviews conducted to date, focusing on the entire energy harvesting research scene and providing a guide to seeking deeper and more specific research references and resources from every corner of the scientific community.

Nanogenerator for Biomedical Applications

In the past 10 years, the development of nanogenerators (NG) has enabled different systems to operate without external power supply. NG have the ability to harvest the mechanical energies in different forms. Human body motions and activities can also serve as the energy source to drive NG and enable self-powered healthcare system. In this review, a summary of several major actual applications of NG in the biomedical fields is made including the circulatory system, the neural system, cell modulation, microbe disinfection, and biodegradable electronics. Nevertheless, there are still many challenges for NG to be actually adopted in clinical applications, including the miniaturization, duration, encapsulation, and output performance. It is also very important to further combine the NG development more precisely with the medical principles. In future, NG can serve as highly promising complementary or even alternative power suppliers to traditional batteries for the healthcare electronics.

One-Step Solvent Evaporation-Assisted 3D Printing of Piezoelectric PVDF Nanocomposite Structures

Development of a 3D printable material system possessing inherent piezoelectric properties to fabricate integrable sensors in a single-step printing process without poling is of importance to the creation of a wide variety of smart structures. Here, we study the effect of addition of barium titanate nanoparticles in nucleating piezoelectric β-polymorph in 3D printable polyvinylidene fluoride (PVDF) and fabrication of the layer-by-layer and self-supporting piezoelectric structures on a micro- to millimeter scale by solvent evaporation-assisted 3D printing at room temperature. The nanocomposite formulation obtained after a comprehensive investigation of composition and processing techniques possesses a piezoelectric coefficient, d₃₁, of 18 pC N^-1, which is comparable to that of typical poled and stretched commercial PVDF film sensors. A 3D contact sensor that generates up to 4 V upon gentle finger taps demonstrates the efficacy of the fabrication technique. Our one-step 3D printing of piezoelectric nanocomposites can form ready-to-use, complex-shaped, flexible, and lightweight piezoelectric devices. When combined with other 3D printable materials, they could serve as stand-alone or embedded sensors in aerospace, biomedicine, and robotic applications.

Theoretical Study of the BaTiO₃ Powder's Volume Ratio's Influence on the Output of Composite Piezoelectric Nanogenerator

The combination of the piezoelectric materials and polymer is an effective way to make the piezoelectric nanogenerator (PENG) possess both the polymer's good flexibility and ferroelectric material's high piezoelectric coefficient. The volume ratio of ferroelectric material in the composite is an important factor that determines the PENG's output performance. In this paper, the BaTiO₃/polydimethylsiloxane (PDMS) composite PENG was demonstrated as having an optimal volume ratio (46%) at which the PENG can output its highest voltage, and this phenomenon can be ascribed to the trade-off between the composite PENG's top electrode charge and its capacitance. These results are of practical importance for the composite PENG's performance optimization.

Research Update: Materials design of implantable nanogenerators for biomechanical energy harvesting

Implantable nanogenerators are rapidly advanced recently as a promising concept for harvesting biomechanical energy in vivo. This review article presents an overview of the most current progress of implantable piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG) with a focus on materials selection, engineering, and assembly. The evolution of the PENG materials is discussed from ZnO nanostructures, to high-performance ferroelectric perovskites, to flexible piezoelectric polymer mesostructures. Discussion of TENGs is focused on the materials and surface features of friction layers, encapsulation materials, and device integrations. Challenges faced by this promising technology and possible future research directions are also discussed.

Facile hydrothermal synthesis of BaZrxTi1−xO3 nanoparticles and their application to a lead-free nanocomposite generator

Piezoelectric BaZr_xTi_1−xO₃ nanoparticles synthesized via a facile hydrothermal reaction were embedded in a flexible lead-free nanocomposite generator.

A flexible energy harvester based on a lead-free and piezoelectric BCTZ nanoparticle-polymer composite

Lead-free piezoelectric 0.5(Ba_0.7Ca_0.3)TiO₃-0.5Ba(Zr_0.2Ti_0.8)O₃ (BCTZ) nanoparticles (NPs) composed of earth-abundant elements were adopted for use in a flexible composite-based piezoelectric energy harvester (PEH) that can convert mechanical deformation into electrical energy. The solid-state synthesized BCTZ NPs and silver nanowires (Ag NWs) chosen to reduce the toxicity of the filler materials were blended with a polydimethylsiloxane (PDMS) matrix to produce a piezoelectric nanocomposite (p-NC). The naturally flexible polymer-based p-NC layers were sandwiched between two conductive polyethylene terephthalate plastic substrates to achieve a flexible energy harvester. The BCTZ NP-based PEH effectively generated an output voltage peak of ∼15 V and a current signal of ∼0.8 μA without time-dependent degradation. This output was adequate to operate a liquid crystal display (LCD) and to turn on six blue light emitting diodes (LEDs).

Flexible Lead-Free BiFeO3/PDMS-Based Nanogenerator as Piezoelectric Energy Harvester

Perovskite ferroelectric BiFeO₃ has been extensively researched in many application fields, but has rarely been investigated for the energy conversion of tiny mechanical motions in electricity in spite of its large theoretical remnant polarization. Here we demonstrate the fabrication of a flexible piezoelectric nanogenerator based on BiFeO₃ nanoparticles (NPs), which were synthesized using a sol-gel process. The BiFeO₃ NPs-PDMS composite device exhibits an output open circuit voltage of ∼3 V and short circuit current of ∼250 nA under repeated hand pressing. The output generation mechanism from the PNG is discussed on the basis of the alignment of electric dipoles in the composite film. It is demonstrated that the output power from the PNG can directly drive the light-emitting diode (LED) and charge capacitor. These results demonstrate that BiFeO₃ nanomaterials have the potential for large-scale lead-free piezoelectric nanogenerator applications.

Mesoporous Piezoelectric Polymer Composite Films with Tunable Mechanical Modulus for Harvesting Energy from Liquid Pressure Fluctuation

Harvesting mechanical energy from biological systems possesses great potential for in vivo powering implantable electronic devices. In this paper, a development of flexible piezoelectric nanogenerator (NG) is reported based on mesoporous poly(vinylidene fluoride) (PVDF) films. Monolithic mesoporous PVDF is fabricated by a template-free sol-gel-based approach at room temperature. By filling the pores of PVDF network with poly(dimethylsiloxane) (PDMS) elastomer, the composite's modulus is effectively tuned over a wide range down to the same level of biological systems. A close match of the modulus between NG and the surrounding biological component is critical to achieve practical integration. Upon deformation, the composite NG exhibits appreciable piezoelectric output that is comparable to or higher than other PVDF-based NGs. An artificial artery system is fabricated using PDMS with the composite NG integrated inside. Effective energy harvesting from liquid pressure fluctuation (simulating blood pressure fluctuation) is successfully demonstrated. The simple and effective approach for fabricating mesoporous PVDF with tunable mechanical properties provides a promising route toward the development of self-powered implantable devices.

In Vivo Self-Powered Wireless Cardiac Monitoring via Implantable Triboelectric Nanogenerator

Harvesting biomechanical energy in vivo is an important route in obtaining sustainable electric energy for powering implantable medical devices. Here, we demonstrate an innovative implantable triboelectric nanogenerator (iTENG) for in vivo biomechanical energy harvesting. Driven by the heartbeat of adult swine, the output voltage and the corresponding current were improved by factors of 3.5 and 25, respectively, compared with the reported in vivo output performance of biomechanical energy conversion devices. In addition, the in vivo evaluation of the iTENG was demonstrated for over 72 h of implantation, during which the iTENG generated electricity continuously in the active animal. Due to its excellent in vivo performance, a self-powered wireless transmission system was fabricated for real-time wireless cardiac monitoring. Given its outstanding in vivo output and stability, iTENG can be applied not only to power implantable medical devices but also possibly to fabricate a self-powered, wireless healthcare monitoring system.

Efficacy of osthole for Echinococcus granulosus in vitro and Echinococcus multilocularis in vivo

Echinococcosis is a zoonotic infection caused by cestode species of the genus Echinococcus; in addition, this zoonosis has long been neglected as a parasitic disease and has limited treatment options. Clinical drugs such as benzimidazole derivatives have limited treatment efficacy. The current study evaluated a novel drug, osthole, with low toxicity and high activity against Echinococcus in vitro and in vivo. The results in vitro indicated that the viability of Echinococcus granulosus protoscoleces in the group treated with osthole (120μM) decreased by 100% within 3days. In vivo experiments were conducted using parasite-infected mice. For this purpose, three groups of infected mice were treated daily for 6 weeks with albendazole (ABZ, 100mg/kg, positive control group), osthole (100mg/kg, experimental group), or honey/PBS (100mg/kg, negative control group), respectively. The osthole- and ABZ-treated groups presented a significant reduction in wet weight of metacestodes, increase in the level of interleukin (IL)-4 and the percentage of eosinophils compared with the control group. Osthole exhibited a high activity against echinococcosis in vivo. In addition, the toxicity of osthole was evaluated via an in vitro 3-(4,5)-dimethylthiahiazo(-z-y1)-3,5-di-phenytetrazoliumromide (MTT) assay, as well as via morphological observation and calculation of liver and kidney function indexes in vivo. No obvious toxic effects of osthole were observed in our study. Therefore, this novel drug may be a promising alternative to benzimidazole in anti-echinococcosis chemotherapy.

Lead-free 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 nanowires for energy harvesting

Lead-free piezoelectric nanowires (NWs) show strong potential in sensing and energy harvesting applications due to their flexibility and ability to convert mechanical energy to electric energy. Currently, most lead-free piezoelectric NWs are produced through low yield synthesis methods and result in low electromechanical coupling, which limit their efficiency as energy harvesters. In order to alleviate these issues, a scalable method is developed to synthesize perovskite type 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) NWs with high piezoelectric coupling coefficient. The piezoelectric coupling coefficient of the BZT-BCT NWs is measured by a refined piezoresponse force microscopy (PFM) testing method and shows the highest reported coupling coefficient for lead-free piezoelectric nanowires of 90 ± 5 pm V(-1). Flexible nanocomposites utilizing dispersed BZT-BCT NWs are fabricated to demonstrate an energy harvesting application with an open circuit voltage of up to 6.25 V and a power density of up to 2.25 μW cm(-3). The high electromechanical coupling coefficient and high power density demonstrated with these lead-free NWs produced via a scalable synthesis method shows the potential for high performance NW-based devices.

Flexible High-Performance Lead-Free Na0.47K0.47Li0.06NbO3 Microcube-Structure-Based Piezoelectric Energy Harvester

Lead-free piezoelectric nano- and microstructure-based generators have recently attracted much attention due to the continuous demand of self-powered body implantable devices. We report the fabrication of a high-performance flexible piezoelectric microgenerator based on lead-free inorganic piezoelectric Na0.47K0.47Li0.06NbO3 (NKLN) microcubes for the first time. The composite generator is fabricated using NKLN microcubes and polydimethylsiloxane (PDMS) polymer on a flexible substrate. The flexible device exhibits excellent performance with a large recordable piezoelectric output voltage of 48 V and output current density of 0.43 μA/cm(2) under vertical compressive force of 2 kgf, for which an energy conversion efficiency of about 11% has been achieved. Piezoresponse and ferroelectric studies reveal that NKLN microcubes exhibited high piezoelectric charge coefficient (d33) as high as 460 pC/N and a well-defined hysteresis loops with remnant polarization and coercive field of 13.66 μC/cm(2) and 19.45 kV/cm, respectively. The piezoelectric charge generation mechanism from NKLN microgenerator are discussed in the light of the high d33 and alignment of electric dipoles in polymer matrix and dielectric constant of NKLN microcubes. It has been demonstrated that the developed power generator has the potential to generate high electric output power under mechanical vibration for powering biomedical devices in the near future.

High output power density nanogenerator based on lead-free 0.96(K0.48Na0.52)(Nb0.95Sb0.05)O3–0.04Bi0.5(Na0.82K0.18)0.5ZrO3 piezoelectric nanofibers

Piezoelectric nanogenerators that power micro/nano devices by converting surrounding tiny mechanical vibration into electrical energy and getting rid of batteries and power cables is attracting increasing attention in recent years.

Recent Advancements in Nanogenerators for Energy Harvesting

Nanomaterial-based generators are a highly promising power supply for micro/nanoscale devices, capable of directly harvesting energy from ambient sources without the need for batteries. These generators have been designed within four main types: piezoelectric, triboelectric, thermoelectric, and electret effects, and consist of ZnO-based, silicon-based, ferroelectric-material-based, polymer-based, and graphene-based examples. The representative achievements, current challenges, and future prospects of these nanogenerators are discussed.

Effects of mechanical deformation on energy conversion efficiency of piezoelectric nanogenerators

Piezoelectric nanogenerators (PNGs) are capable of converting energy from various mechanical sources into electric energy and have many attractive features such as continuous operation, replenishment and low cost. However, many researchers still have studied novel material synthesis and interfacial controls to improve the power production from PNGs. In this study, we report the energy conversion efficiency (ECE) of PNGs dependent on mechanical deformations such as bending and twisting. Since the output power of PNGs is caused by the mechanical strain of the piezoelectric material, the power production and their ECE is critically dependent on the types of external mechanical deformations. Thus, we examine the output power from PNGs according to bending and twisting. In order to clearly understand the ECE of PNGs in the presence of those external mechanical deformations, we determine the ECE of PNGs by the ratio of output electrical energy and input mechanical energy, where we suggest that the input energy is based only on the strain energy of the piezoelectric layer. We calculate the strain energy of the piezoelectric layer using numerical simulation of bending and twisting of the PNG. Finally, we demonstrate that the ECE of the PNG caused by twisting is much higher than that caused by bending due to the multiple effects of normal and lateral piezoelectric coefficients. Our results thus provide a design direction for PNG systems as high-performance power generators.

Flexible, Hybrid Piezoelectric Film (BaTi(1-x)Zr(x)O3)/PVDF Nanogenerator as a Self-Powered Fluid Velocity Sensor

We demonstrate a flexible piezoelectric nanogenerator (PNG) constructed using a hybrid (or composite) film composed of highly crystalline BaTi(1-x)Zr(x)O3 (x = 0, 0.05, 0.1, 0.15, and 0.2) nanocubes (abbreviated as BTZO) synthesized using a molten-salt process embedded into a poly(vinylidene fluoride) (PVDF) matrix solution via ultrasonication. The potential of a BTZO/PVDF hybrid film is realized in fabricating eco-friendly devices, active sensors, and flexible nanogenerators to interpret its functionality. Our strategy is based on the incorporation of various Zr(4+) doping ratios into the Ti(4+) site of BaTiO3 nanocubes to enhance the performance of the PNG. The flexible nanogenerator (BTZO/PVDF) exhibits a high electrical output up to ∼11.9 V and ∼1.35 μA compared to the nanogenerator (BTO/PVDF) output of 7.99 V and 1.01 μA upon the application of cyclic pushing-releasing frequencies with a constant load (11 N). We also demonstrate another exciting application of the PNG as a self-powered sensor to measure different water velocities at an outlet pipe. The average maximum peak power of the PNG varies from 0.2 to 15.8 nW for water velocities ranging from 31.43 to 125.7 m/s during the water ON condition. This study shows the compositional dependence approach, fabrication of nanostructures for energy harvesting, and self-powered devices in the field of monitoring for remote area applications.