Self-powered system with wireless data transmission

Fundamentals of Flexoelectricity, Materials and Emerging Opportunities Toward Strain-Driven Nanocatalysts

Flexoelectricity, an intrinsic property observed in materials under nonuniform deformation, entails a coupling between polarization and strain gradients. Recent catalyst advancements have reignited interest in flexoelectricity, particularly at the nanoscale, where pronounced strain gradients promote robust flexoelectric effects. This paper comprehensively examines flexoelectricity, encompassing methodologies for precise measurement, elucidating its distinctions from related phenomena, and exploring its potential applications in augmenting catalytic properties. So far, the greatest potentials are based on lead strontium titanate (PST) and other metallic titanates such as titania (TiO2), strontium titanate (STO), barium strontium titanate (BST) sulfates (MoS2, ZnS) and halide perovskites (with archetype XPbI3). This review explores the promise of flexoelectric properties in addressing material and photocatalytic challenges, such as charge carrier recombination and ineffective surface charge separation. Additionally, it sheds light on the synergy with emerging paradigms like photo-flexo catalysis and synergistic flexo-piezo catalysis, specifically focusing on selective chemical transformations like green hydrogen production. Current limitations related to the usage of photoflexoelectricity for photocatalysis are mostly the stability of the used substance (susceptibility to photodegradation) or the voltage values, which represent the inferior potential for specific practical applications. This work underscores the indispensable role of flexoelectricity in catalysis and its capacity to steer future research and technological advancement.

Advancing High-Performance Piezoelectric Nanogenerators: Simple Electric Field Switching for Orientated and Aligned BaTiO3/PDMS Composite

Barium titanate (BaTiO3) is renowned for its high dielectric constant and remarkable piezoelectric attributes, positioning it as a key element in the advancement of environmentally sustainable devices. Nevertheless, the effectiveness of piezoelectric nanogenerators (PENGs) that integrate BaTiO3 nanoparticles (NPs) and poly(dimethylsiloxane) (PDMS) poses a challenge, thereby restricting their utility in energy harvesting applications. This study presents a direct approach involving the cyclic manipulation of direct current (DC) power supply terminals to achieve unidirectional alignment of BaTiO3 NPs within a PDMS matrix, aiming to enhance the performance of the PENGs. Examination of the morphology and evaluation of diffraction planes, notably (111) and (200), in the aligned BaTiO3 PENGs exhibited well-oriented structures resulting from the repetitive switching between two electrodes, leading to improved piezoelectric properties. The BaTiO3 PENGs manifested notably higher output power (∼15 V and 1.91 μA) in contrast to devices containing randomly distributed polarized BaTiO3-PDMS composite films. The generated power was sufficient to directly operate six light-emitting diodes (LEDs) connected in series, with a collective nominal voltage of around 14 V, encompassing red, green, and blue LEDs. Nanoindentation verified the enhanced piezoelectric characteristics attributed to the alignment, sensitivity to bending, and energy-cohesive effects of clustered BaTiO3 one-dimensional (1D) pillars. These findings suggest a widely applicable technique for aligning and situating nanoparticles vertically within a polymer matrix, exploiting the intrinsic dielectric properties of the nanoparticles through a straightforward electric field switching mechanism.

Wearable fabric-based ZnO nanogenerator for biomechanical and biothermal monitoring

Wearable devices that can mechanically conform to human skin are a necessity for reliable monitoring and decoding of biomechanical activities through skin. Most inorganic piezoelectrics, however, lack deformability and damage tolerance, impeding stable motion monitoring. Here, we present an air-permeable fabric-based ZnO nanogenerator with mechanical adaptivity to diverse deformations for wearable piezoelectric sensors, collecting biomechanical health data. We fabricate ZnO nanorods incorporated throughout the entire nylon fabric, with a strategically positioned neutral mechanical plane, for bending-sensitive electronics (2.59 μA mm). Its hierarchically interlocked geometry also permits sensitive tactile sensing (0.15 nA kPa-1). Various physiological information about activities, including pulse beating, breathing, saliva swallowing, and coughing, is attained using skin-mounted sensors. Further, the pyroelectric sensing capability of a mask-attached device is demonstrated by identifying specific respiratory patterns. Our wearable healthcare sensors hold great promise for real-time monitoring of health-related vital signs, informing individuals' health status without disrupting their daily lives.

Smart Contact Lenses as Wearable Ophthalmic Devices for Disease Monitoring and Health Management

The eye contains a complex network of physiological information and biomarkers for monitoring disease and managing health, and ocular devices can be used to effectively perform point-of-care diagnosis and disease management. This comprehensive review describes the target biomarkers and various diseases, including ophthalmic diseases, metabolic diseases, and neurological diseases, based on the physiological and anatomical background of the eye. This review also includes the recent technologies utilized in eye-wearable medical devices and the latest trends in wearable ophthalmic devices, specifically smart contact lenses for the purpose of disease management. After introducing other ocular devices such as the retinal prosthesis, we further discuss the current challenges and potential possibilities of smart contact lenses.

Breathing-Driven Self-Powered Pyroelectric ZnO Integrated Face Mask for Bioprotection

Rapid spread of infectious diseases is a global threat and has an adverse impact on human health, livelihood, and economic stability, as manifested in the ongoing coronavirus disease 2019 (COVID-19) pandemic. Even though people wear a face mask as protective equipment, direct disinfection of the pathogens is barely feasible, which thereby urges the development of biocidal agents. Meanwhile, repetitive respiration generates temperature variation wherein the heat is regrettably wasted. Herein, a biocidal ZnO nanorod-modified paper (ZNR-paper) composite that is 1) integrated on a face mask, 2) harvests waste breathing-driven thermal energy, 3) facilitates the pyrocatalytic production of reactive oxygen species (ROS), and ultimately 4) exhibits antibacterial and antiviral performance is proposed. Furthermore, in situ generated compressive/tensile strain of the composite by being attached to a curved mask is investigated for high pyroelectricity. The anisotropic ZNR distortion in the bent composite is verified with changes in ZnO bond lengths and OZnO bond angles in a ZnO₄ tetrahedron, resulting in an increased polarization state and possibly contributing to the following pyroelectricity. The enhanced pyroelectric behavior is demonstrated by efficient ROS production and notable bioprotection. This study exploring the pre-strain effect on the pyroelectricity of ZNR-paper might provide new insights into the piezo-/pyroelectric material-based applications.

Perovskite Piezoelectric-Based Flexible Energy Harvesters for Self-Powered Implantable and Wearable IoT Devices

In the ongoing fourth industrial revolution, the internet of things (IoT) will play a crucial role in collecting and analyzing information related to human healthcare, public safety, environmental monitoring and home/industrial automation. Even though conventional batteries are widely used to operate IoT devices as a power source, these batteries have a drawback of limited capacity, which impedes broad commercialization of the IoT. In this regard, piezoelectric energy harvesting technology has attracted a great deal of attention because piezoelectric materials can convert electricity from mechanical and vibrational movements in the ambient environment. In particular, piezoelectric-based flexible energy harvesters can precisely harvest tiny mechanical movements of muscles and internal organs from the human body to produce electricity. These inherent properties of flexible piezoelectric harvesters make it possible to eliminate conventional batteries for lifetime extension of implantable and wearable IoTs. This paper describes the progress of piezoelectric perovskite material-based flexible energy harvesters for self-powered IoT devices for biomedical/wearable electronics over the last decade.

Engineering the Defects and Microstructures in Ferroelectrics for Enhanced/Novel Properties: An Emerging Way to Cope with Energy Crisis and Environmental Pollution

In the past century, ferroelectrics are well known in electroceramics and microelectronics for their unique ferroelectric, piezoelectric, pyroelectric, and photovoltaic effects. Nowadays, the advances in understanding and tuning of these properties have greatly promoted a broader application potential especially in energy and environmental fields, by harvesting solar, mechanical, and heat energies. For example, high piezoelectricity and high pyroelectricity can be designed by defect or microstructure engineering for piezo- and pyro-catalyst, respectively. Moreover, highly piezoelectric and broadband (UV-Vis-NIR) light-responsive ferroelectrics can be designed via defect engineering, giving rise to a new concept of photoferroelectrics for efficient photocatalysis, piezocatalysis, pyrocatalysis, and related cocatalysis. This article first summarizes the recent developments in ferroelectrics in terms of piezoelectricity, pyroelectricity, and photovoltaic effects based on defect and microstructure engineering. Then, the potential applications in energy generation (i.e., photovoltaic effect, H2 generation, and self-powered multisource energy harvesting and signal sensing) and environmental protection (i.e., photo-piezo-pyro- cocatalytic dye degradation and CO2 reduction) are reviewed. Finally, the outlook and challenges are discussed. This article not only covers an overview of the state-of-art advances of ferroelectrics, but also prospects their applications in coping with energy crisis and environmental pollution.

Electrogenic Bacteria Promise New Opportunities for Powering, Sensing, and Synthesizing

Considerable research efforts into the promises of electrogenic bacteria and the commercial opportunities they present are attempting to identify potential feasible applications. Metabolic electrons from the bacteria enable electricity generation sufficient to power portable or small-scale applications, while the quantifiable electric signal in a miniaturized device platform can be sensitive enough to monitor and respond to changes in environmental conditions. Nanomaterials produced by the electrogenic bacteria can offer an innovative bottom-up biosynthetic approach to synergize bacterial electron transfer and create an effective coupling at the cell-electrode interface. Furthermore, electrogenic bacteria can revolutionize the field of bioelectronics by effectively interfacing electronics with microbes through extracellular electron transfer. Here, these new directions for the electrogenic bacteria and their recent integration with micro- and nanosystems are comprehensively discussed with specific attention toward distinct applications in the field of powering, sensing, and synthesizing. Furthermore, challenges of individual applications and strategies toward potential solutions are provided to offer valuable guidelines for practical implementation. Finally, the perspective and view on how the use of electrogenic bacteria can hold immeasurable promise for the development of future electronics and their applications are presented.

Efficient purification of wastewater by applying mechanical force and BaCO3/TiO2 and BaTiO3/TiO2 piezocatalysts

In typical advanced oxidation catalysis, a semiconductor should have a robust capacity to generate separated electron-hole pairs on a material's surface under irradiation of photons with energy more than the material's bandgap. However, rapid charge carrier recombination and low photon to current yield of semiconductor photocatalysts and low percentages of UV light in sunlight leads to a low level of photocatalytic efficiency for practical application. Mechanical energy is a natural energy that can be considered as a form of rich, clean and renewable energy which can be harvested by using piezoelectric materials. Here, we developed BaCO₃/TiO₂ and BaTiO₃/TiO₂ composites as mechanical harvesting materials to decontaminate pollutants. Results showed that BaCO₃ has a great effect on the piezocatalytic activity of products. The control sample (sample without Ba) only degraded 11.2% of Acid Red 151 (AR151) , while the sample containing Ba degraded 96.7% of AR151. Besides, the effects of several parameters, including the natural surfactant, reaction time and temperature, calcination, and ultrasonic power and pulse on the catalytic activity of the as-prepared piezocatalysts were studied. Results showed that it is possible to degrade 99.1% of AR151 by controlling ultrasonic parameters during 2 h of mechanical energy force.

Moisture-induced autonomous surface potential oscillations for energy harvesting

A variety of autonomous oscillations in nature such as heartbeats and some biochemical reactions have been widely studied and utilized for applications in the fields of bioscience and engineering. Here, we report a unique phenomenon of moisture-induced electrical potential oscillations on polymers, poly([2-(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide-co-acrylic acid), during the diffusion of water molecules. Chemical reactions are modeled by kinetic simulations while system dynamic equations and the stability matrix are analyzed to show the chaotic nature of the system which oscillates with hidden attractors to induce the autonomous surface potential oscillation. Using moisture in the ambient environment as the activation source, this self-excited chemoelectrical reaction could have broad influences and usages in surface-reaction based devices and systems. As a proof-of-concept demonstration, an energy harvester is constructed and achieved the continuous energy production for more than 15,000 seconds with an energy density of 16.8 mJ/cm2. A 2-Volts output voltage has been produced to power a liquid crystal display toward practical applications with five energy harvesters connected in series.

Piezoelectric Materials for Energy Harvesting and Sensing Applications: Roadmap for Future Smart Materials

Piezoelectric materials are widely referred to as "smart" materials because they can transduce mechanical pressure acting on them to electrical signals and vice versa. They are extensively utilized in harvesting mechanical energy from vibrations, human motion, mechanical loads, etc., and converting them into electrical energy for low power devices. Piezoelectric transduction offers high scalability, simple device designs, and high-power densities compared to electro-magnetic/static and triboelectric transducers. This review aims to give a holistic overview of recent developments in piezoelectric nanostructured materials, polymers, polymer nanocomposites, and piezoelectric films for implementation in energy harvesting. The progress in fabrication techniques, morphology, piezoelectric properties, energy harvesting performance, and underpinning fundamental mechanisms for each class of materials, including polymer nanocomposites using conducting, non-conducting, and hybrid fillers are discussed. The emergent application horizon of piezoelectric energy harvesters particularly for wireless devices and self-powered sensors is highlighted, and the current challenges and future prospects are critically discussed.

Miniature microbial solar cells to power wireless sensor networks

Conventional wireless sensor networks (WSNs) powered by traditional batteries or energy storage devices such as lithium-ion batteries and supercapacitors have challenges providing long-term and self-sustaining operation due to their limited energy budgets. Emerging energy harvesting technologies can achieve the longstanding vision of self-powered, long-lived sensors. In particular, miniature microbial solar cells (MSCs) can be the most feasible power source for small and low-power sensor nodes in unattended working environments because they continuously scavenge power from microbial photosynthesis by using the most abundant resources on Earth; solar energy and water. Even with low illumination, the MSC can harvest electricity from microbial respiration. Despite the vast potential and promise of miniature MSCs, their power and lifetime remain insufficient to power potential WSN applications. In this overview, we will introduce the field of miniature MSCs, from early breakthroughs to current achievements, with a focus on emerging techniques to improve their performance. Finally, challenges and perspectives for the future direction of miniature MSCs to self-sustainably power WSN applications will be given.

Emerging Pyroelectric Nanogenerators to Convert Thermal Energy into Electrical Energy

Pyroelectric energy harvesting systems have recently received substantial attention for their potential applications as power generators. In particular, the pyroelectric effect, which converts thermal energy into electrical energy, has been utilized as an infrared (IR) sensor, but upcoming sensor technology that requires a miniscule amount of power is able to utilize pyroelectric nanogenerators (PyNGs) as a power source. Herein, an overview of the progress in the development of PyNGs for an energy harvesting system that uses environmental or artificial energies such as the sun, body heat, and heaters, is provided. It begins with a brief introduction of the pyroelectric effect, and various polymer and ceramic materials based PyNGs are reviewed in detail. Various approaches for developing polymer-based PyNGs and various ceramic materials-based PyNGs are summarized in particular. Finally, challenges and perspectives regarding the PyNGs are described.

Piezoelectric Current Generator Based on Bismuth Ferrite Nanoparticles

Bismuth ferrite nanoparticles with an average particle diameter of 45 nm and spatial symmetry R3c were obtained by combustion of organic nitrate precursors. BiFeO₃-silicone nanocomposites with various concentrations of nanoparticles were obtained by mixing with a solution of M10 silicone. Models of piezoelectric generators were made by applying nanocomposites on a glass substrate and using aluminum foil as contacts. The thickness of the layers was about 230 μm. There was a proportional relationship between the different concentrations of nanoparticles and the detected potential. The output voltages were 0.028, 0.055, and 0.17 V with mass loads of 10, 30, and 50 mass%, respectively.

Self-charging power system for distributed energy: beyond the energy storage unit

Power devices for the smart sensor networks of Internet of things (IoT) are required with minimum or even no maintenance due to their enormous quantities and widespread distribution. Self-charging power systems (SCPSs) refer to integrated energy devices with simultaneous energy harvesting, power management and effective energy storage capabilities, which may need no extra battery recharging and can sustainably drive sensors. Herein, we focus on the progress made in the field of nanogenerator-based SCPSs, which harvest mechanical energy using the Maxwell displacement current arising from the variation in the surface-polarized-charge-induced electrical field. Prototypes of different nanogenerator-based SCPSs will be overviewed. Finally, challenges and prospects in this field will be discussed.

Triboelectric touch sensor for position mapping during total hip arthroplasty

Objective

In this research, a triboelectric nanogenerator (TENG) was utilized to determine if a pressure-based sensor could detect bearing friction in a total hip arthroplasty (THA) and detect the contact of specific areas during ROM checks.

Results

The pressure-based sensor shows capability to sense bearing friction. In more detail, the TENG embedded in four different sides of the trial exhibits up to 1 V from peak-to-peak. Moreover, these flexible touch sensors with TENG describes a peak signal in output voltage which should lead to extremely sensitive detection of bearing friction induced by the THA.

Piezoelectric Materials for Controlling Electro-Chemical Processes

Piezoelectric materials have been analyzed for over 100 years, due to their ability to convert mechanical vibrations into electric charge or electric fields into a mechanical strain for sensor, energy harvesting, and actuator applications. A more recent development is the coupling of piezoelectricity and electro-chemistry, termed piezo-electro-chemistry, whereby the piezoelectrically induced electric charge or voltage under a mechanical stress can influence electro-chemical reactions. There is growing interest in such coupled systems, with a corresponding growth in the number of associated publications and patents. This review focuses on recent development of the piezo-electro-chemical coupling multiple systems based on various piezoelectric materials. It provides an overview of the basic characteristics of piezoelectric materials and comparison of operating conditions and their overall electro-chemical performance. The reported piezo-electro-chemical mechanisms are examined in detail. Comparisons are made between the ranges of material morphologies employed, and typical operating conditions are discussed. In addition, potential future directions and applications for the development of piezo-electro-chemical hybrid systems are described. This review provides a comprehensive overview of recent studies on how piezoelectric materials and devices have been applied to control electro-chemical processes, with an aim to inspire and direct future efforts in this emerging research field.

A facile hot-pressing process for fabricating flexible top electrodes of piezoelectric ZnO nanowire nanogenerators

We report a facile and effective hot-pressing strategy for fabricating the flexible top electrode for a piezoelectric nanogenerator (PENG). Flexible stainless steel (SUS) foil was employed as the bottom electrode and substrate of the device. Zinc oxide nanowires (ZnO NWs) were grown on SUS substrate through the hydrothermal synthesis method. The top electrode of Zn foil was combined with dielectric polydimethylsiloxane (PDMS) film using the hot-pressing process. The resulting top electrode is thick enough to enable the device to generate piezoelectric output differently under bending conditions. The PENG devices generated an output voltage of about 2.2 V and an output current of 8 nA under the optimum operating conditions. The devices fabricated by the hot-pressing process were robust enough to retain their generating ability after thousands of bending and releasing cycles.

High-efficient synergy of piezocatalysis and photocatalysis in bismuth oxychloride nanomaterial for dye decomposition

In this work, it is found that the hydrothermally-synthesized bismuth oxychloride can behave both the piezocatalysis and photocatalysis for the Rhodamine B dye decomposition. ∼99% decomposition efficiency is achieved after both vibrating and lighting the Rhodamine B dye solution for ∼96 min with the addition of bismuth oxychloride catalyst, while the ∼72% and ∼26% decomposition efficiencies are obtained for only photocatalysis or only piezocatalysis respectively. In bi-catalysis, the mechanical strain produced due to vibration will directly provide an electric field that will increase the separation between the photo-induced electron-hole pairs, yielding to the enhanced decomposition performance of bi-catalysis. There is no significant change in the bi-catalytic performance of bismuth oxychloride nanomaterial observed after being recycled four times. Bismuth oxychloride catalyst is potential for the bi-catalytic decomposition treatment of wastewater through harvesting both the environmental vibration energy and light energy.

A self-charging cyanobacterial supercapacitor

Microliter-scale photosynthetic microbial fuel cells (micro-PMFC) can be the most suitable power source for unattended environmental sensors because the technique can continuously generate electricity from microbial photosynthesis and respiration through day-night cycles, offering a clean and renewable power source with self-sustaining potential. However, the promise of this technology has not been translated into practical applications because of its relatively low performance. By creating an innovative supercapacitive micro-PMFC device with maximized bacterial photoelectrochemical activities in a well-controlled, tightly enclosed micro-chamber, this work established innovative strategies to revolutionize micro-PMFC performance to attain stable high power and current density (38 μW/cm² and 120 μA/cm²) that then potentially provides a practical and sustainable power supply for the environmental sensing applications. The proposed technique is based on a 3-D double-functional bio-anode concurrently exhibiting bio-electrocatalytic energy harvesting and charge storing. It offers the high-energy harvesting functionality of micro-PMFCs with the high-power operation of an internal supercapacitor for charging and discharging. The performance of the supercapacitive micro-PMFC improved significantly through miniaturizing innovative device architectures and connecting multiple miniature devices in series.

An Experimental and Computational Study on Inverted Flag Dynamics for Simultaneous Wind–Solar Energy Harvesting

This paper presents results from experiments and simplified numerical simulations on the flow-induced dynamics and power generation of inverted flags that combine flexible piezoelectric strips with photovoltaic cells to simultaneously harvest kinetic wind energy and solar radiant energy. Experiments were conducted in a wind tunnel under controlled wind excitation and light exposure, focusing in particular on the dynamics and power generation of the inverted flag harvester. Numerical simulations were carried out using a lattice-Boltzmann fluid solver coupled with a finite element structural solver via the immersed-boundary method, focusing in particular on minimizing the simulation run time. The power generated during the tests shows that the proposed inverted flag harvester is a promising concept, capable of producing enough power (on the order of 1 mW) to supply low-power electronic devices in a range of applications where distributed power generation is needed. Notwithstanding key simplifications implemented in the numerical model to achieve a fast execution, simulations and measurements are in good agreement, confirming that the lattice-Boltzmann method is a viable and time-effective alternative to classic Navier–Stokes-based solvers when dealing with strongly coupled fluid–structure interaction problems characterized by large structural displacements.

Optimization of a Piezoelectric Energy Harvester and Design of a Charge Pump Converter for CMOS-MEMS Monolithic Integration

The increasing interest in the Internet of Things (IoT) has led to the rapid development of low-power sensors and wireless networks. However, there are still several barriers that make a global deployment of the IoT difficult. One of these issues is the energy dependence, normally limited by the capacitance of the batteries. A promising solution to provide energy autonomy to the IoT nodes is to harvest residual energy from ambient sources, such as motion, vibrations, light, or heat. Mechanical energy can be converted into electrical energy by using piezoelectric transducers. The piezoelectric generators provide an alternating electrical signal that must be rectified and, therefore, needs a power management circuit to adapt the output to the operating voltage of the IoT devices. The bonding and packaging of the different components constitute a large part of the cost of the manufacturing process of microelectromechanical systems (MEMS) and integrated circuits. This could be reduced by using a monolithic integration of the generator together with the circuitry in a single chip. In this work, we report the optimization, fabrication, and characterization of a vibration-driven piezoelectric MEMS energy harvester, and the design and simulation of a charge-pump converter based on a standard complementary metal–oxide–semiconductor (CMOS) technology. Finally, we propose combining MEMS and CMOS technologies to obtain a fully integrated system that includes the piezoelectric generator device and the charge-pump converter circuit without the need of external components. This solution opens new doors to the development of low-cost autonomous smart dust devices.

Effect of TC(002) on the Output Current of a ZnO Thin-Film Nanogenerator and a New Piezoelectricity Mechanism at the Atomic Level

Understanding the piezoelectricity mechanism is crucial for developing new materials for better performance. Here, we developed a nanogenerator based on the ZnO thin films having various TC(002) values. The output current well correlated to the magnitude of (002) texture coefficient (TC(002)). Additionally, the TC(002)-dependent photovoltaic and rectification properties are observed. When the film is subjected to persistent compression, the photovoltaic, rectification, and piezoelectric properties fade away. Based on our observation that the ZnO polar structure always shows a spontaneous electron field (SEF), we thus propose a new piezoelectricity mechanism. The [001]-orientated ZnO thin film with the SEF is equivalent to a capacitor, the compression functions as a discharging process, and the removal of the external stress serves as a charging process. The physical mechanism provides an insight into various energy conversion processes that will inspire advanced designs of high-performance nanogenerators, solar cells, and other optoelectronic devices.

Nanowire-Based Biosensors: From Growth to Applications

Over the past decade, synthesized nanomaterials, such as carbon nanotube, nanoparticle, quantum dot, and nanowire, have already made breakthroughs in various fields, including biomedical sensors. Enormous surface area-to-volume ratio of the nanomaterials increases sensitivity dramatically compared with macro-sized material. Herein we present a comprehensive review about the working principle and fabrication process of nanowire sensor. Moreover, its applications for the detection of biomarker, virus, and DNA, as well as for drug discovery, are reviewed. Recent advances including self-powering, reusability, sensitivity in high ionic strength solvent, and long-term stability are surveyed and highlighted as well. Nanowire is expected to lead significant improvement of biomedical sensor in the near future.

3D Bioprinting of Cyanobacteria for Solar-driven Bioelectricity Generation in Resource-limited Environments

We demonstrate a hybrid biological photovoltaic device by forming a 3D cooperative biofilm of cyanobacteria and heterotrophic bacteria. 3D bioprinting technique was applied to engineer a cyanobacterial encapsulation in hydrogels over the heterotrophic bacteria. The device continuously generated bioelectricity from the heterotrophic bacterial respiration with the organic biomass supplied by the cyanobacterial photosynthesis. This innovative device platform can be the most suitable power source for unattended sensors, especially for those deployed in remote and resource-limited field locations.

Flexible Sandwich Piezoelectric Nanogenerators based ZnO Nanorods for Mechanical Energy Harvesting

We present a flexible sandwich piezoelectric nanogenerators (PENGs) device with gold-coated ZnO nanorods (Au@ ZNRs) as an efficient top electrode; this device was used to harvest energy from the human walking motion. ZNRs were synthesised on the two-piece of ZnO seed layer coated gold/flexible polyethylene terephthalate (Au/PET) substrates through a simple hydrothermal method of low temperature and low cost at molar concentration (0.01M). X-ray diffraction and field emission scanning electron microscopy images revealed that the as-grown ZNRs have high crystallinity and apparent vertical growth with hexagonal shapes, the average diameter of NRs is 120 nm. Flexible sandwich PENGs based ZNRs was fabricated with gold-coated one piece of ZNRs by DC-sputtering method as an efficient top electrode, which was placed on the uncoated ZNRs as-grown on another piece of substrate. The maximum output potential voltage (Vmax) under a periodic of pressing and releasing of human walking is 5.76 V. The results confirmed the top efficient electrode has created more contact area with uncoated NR when it is pressed, which increases the transfer efficiency effectively of piezoelectric potential that generated from uncoated ZNRs.

A comparative study on a flexible ZnO-based nano-generator using Schottky and p-n junction contact for energy harvesting applications

Vibration based piezoelectric energy harvesting from unused ambient sources is an efficient approach for a battery-free, sustainable and green power source for self-powered electronics. This paper presents the performance enhancement of a p-n junction based flexible nano-generator (NG). A stable, non-hazardous spiro and zinc oxide (ZnO) p-n junction based NG is proposed. The device performance is compared with a Schottky contact based NG. The experimental study is carried out using the tip excitation method. Most importantly, the contact modification with spiro improves the performance of the device by five times, thereby yielding an open circuit voltage of 300 mV, a short-circuit current density of 220 μA cm-2 and a maximum power density up to 48 μW cm-2 at 0.5 N. Furthermore, performance enhancement is achieved by reducing the external screening and the internal impedance. The results are validated using an RC time constant value derived from the impedance analysis.

Piezo-Potential Generation in Capacitive Flexible Sensors Based on GaN Horizontal Wires

We report an example of the realization of a flexible capacitive piezoelectric sensor based on the assembly of horizontal c&macr;-polar long Gallium nitride (GaN) wires grown by metal organic vapour phase epitaxy (MOVPE) with the Boostream^&reg; technique spreading wires on a moving liquid before their transfer on large areas. The measured signal (<0.6 V) obtained by a punctual compression/release of the device shows a large variability attributed to the dimensions of the wires and their in-plane orientations. The cause of this variability and the general operating mechanisms of this flexible capacitive device are explained by finite element modelling simulations. This method allows considering the full device composed of a metal/dielectric/wires/dielectric/metal stacking. We first clarify the mechanisms involved in the piezo-potential generation by mapping the charge and piezo-potential in a single wire and studying the time-dependent evolution of this phenomenon. GaN wires have equivalent dipoles that generate a tension between metallic electrodes only when they have a non-zero in-plane projection. This is obtained in practice by the conical shape occurring spontaneously during the MOVPE growth. The optimal aspect ratio in terms of length and conicity (for the usual MOVPE wire diameter) is determined for a bending mechanical loading. It is suggested to use 60⁻120 &micro;m long wires (i.e., growth time less than 1 h). To study further the role of these dipoles, we consider model systems with in-plane 1D and 2D regular arrays of horizontal wires. It is shown that a strong electrostatic coupling and screening occur between neighbouring horizontal wires depending on polarity and shape. This effect, highlighted here only from calculations, should be taken into account to improve device performance.

Fully Rollable Lead-Free Poly(vinylidene fluoride)-Niobate-Based Nanogenerator with Ultra-Flexible Nano-Network Electrodes

A fully rollable nanocomposite-based nanogenerator (NCG) is developed by integrating a lead-free piezoelectric hybrid layer with a type of nanofiber-supported silver nanowire (AgNW) network as electrodes. The thin-film nanocomposite is composed of electroactive polyvinylidene fluoride (PVDF) polymer matrix and compositionally modified potassium sodium niobate-based nanoparticles (NPs) with a high piezoelectric coefficient ( d₃₃) of 53 pm/V, which is revealed by the piezoresponse force microscopy measurements. Under periodical agitation at a compressive force of 50 N and 1 Hz, the NCG can steadily render high electric output up to an open-circuit voltage of 18 V and a short-circuit current of 2.6 μA. Of particular importance is the decent rollability of the NCG, as indicated by the negligible decay in the electric output after it being repeatedly rolled around a gel pen for 200 cycles. Besides, the biocompatible NCG can potentially be used to scavenge biomechanical energy from low-frequency human motions, as demonstrated by the scenarios of walking and elbow joint movement. These results rationally expand the feasibility of the developed NCG toward applications in lightweight, diminutive, and multifunctional rollable or wearable electronic devices.

Self-Powered Temperature-Mapping Sensors Based on Thermo-Magneto-Electric Generator

We demonstrate a thermo-magneto-electric generator (TMEG) based on second-order phase transition of soft magnetic materials that provides a promising pathway for scavenging low-grade heat. It takes advantage of the cyclic magnetic forces of attraction and repulsion arising through ferromagnetic-to-paramagnetic phase transition to create mechanical vibrations that are converted into electricity through piezoelectric benders. To enhance the mechanical vibration frequency and thereby the output power of the TMEG, we utilize the nonlinear behavior of piezoelectric cantilevers and enhanced thermal transport through silver (Ag) nanoparticles (NPs) applied on the surface of a soft magnet. This results in large enhancement of the oscillation frequency reaching up to 9 Hz (300% higher compared with that of the prior literature). Optimization of the piezoelectric beam and Ag NP distribution resulted in the realization of nonlinear TMEGs that can generate a high output power of 80 μW across the load resistance of 0.91 MΩ, which is 2200% higher compared with that of the linear TMEG. Using a nonlinear TMEG, we fabricated and evaluated self-powered temperature-mapping sensors for monitoring the thermal variations across the surface. Combined, our results demonstrate that nonlinear TMEGs can provide additional functionality including temperature monitoring, thermal mapping, and powering sensor nodes.

Enhancement of ZnO-based flexible nano generators via a sol-gel technique for sensing and energy harvesting applications

Zinc oxide (ZnO) is a remarkable inorganic semiconductor with exceptional piezoelectric properties compared to other semiconductors. However, in comparison to lead-based hazardous piezoelectric materials, its properties have undesired limitations. Here we report a 5∼6 fold enhancement in piezoelectric features via chemical doping of copper matched to intrinsic ZnO. A flexible piezoelectric nanogenerator (F-PENG) device was fabricated using an unpretentious solution process of spin coating, with other advantages such as robustness, low-weight, improved adhesion, and low cost. The device was used to demonstrate energy harvesting from a standard weight as low as 4 gm and can work as a self-powered mass sensor in a broad range of 4 to 100 gm. The device exhibited a novel energy harvesting technique from a wind source due to its inherent flexibility. At three different velocities (10∼30 m s-1) and five different angles of attack (0∼180 degrees), the device validated the ability to discern different velocities and directions of flow. The device will be useful for mapping the flow of air apart from harvesting the energy. The simulation was done to verify the underlining mechanism of aerodynamics involved.

An enhanced low-frequency vibration ZnO nanorod-based tuning fork piezoelectric nanogenerator

In this paper, a piezoelectric nanogenerator (PENG) based on a tuning fork-shaped cantilever was designed and fabricated, aiming at harvesting low frequency vibration energy in the environment. In the PENG, a tuning fork-shaped elastic beam combined with ZnO nanorods (NRs), instead of conventional rectangular cantilever beams, was adopted to extract vibration energy. Benefiting from the high flexibility and the controllable shape of the substrate, this PENG was extremely sensitive to vibration and can harvest weak vibration energy at a low frequency. Moreover, a series of simulation models were established to compare the performance of the PENG with that of different shapes. On this basis, the experimental results further verify that this designed energy harvester could operate at a low frequency which was about 13 Hz. The peak output voltage and current could respectively reach about 160 mV and 11 nA, and a maximum instantaneous peak power of 0.92 μW cm^-3 across a matched load of 9 MΩ was obtained. Evidently, this newly designed PENG could harvest vibration energy at a lower frequency, which will contribute to broaden the application range of the PENG in energy harvesting and self-powered systems.

Toward Wearable Self-Charging Power Systems: The Integration of Energy-Harvesting and Storage Devices

One major challenge for wearable electronics is that the state-of-the-art batteries are inadequate to provide sufficient energy for long-term operations, leading to inconvenient battery replacement or frequent recharging. Other than the pursuit of high energy density of secondary batteries, an alternative approach recently drawing intensive attention from the research community, is to integrate energy-generation and energy-storage devices into self-charging power systems (SCPSs), so that the scavenged energy can be simultaneously stored for sustainable power supply. This paper reviews recent developments in SCPSs with the integration of various energy-harvesting devices (including piezoelectric nanogenerators, triboelectric nanogenerators, solar cells, and thermoelectric nanogenerators) and energy-storage devices, such as batteries and supercapacitors. SCPSs with multiple energy-harvesting devices are also included. Emphasis is placed on integrated flexible or wearable SCPSs. Remaining challenges and perspectives are also examined to suggest how to bring the appealing SCPSs into practical applications in the near future.

Pure OPM nanofibers with high piezoelectricity designed for energy harvesting in vitro and in vivo

Pure OPM nanofibers with unprecedented high piezoelectricity are successfully fabricated and applied on the skin as a motion sensor and in arterial blood vessels as a nanogenerator for energy harvesting.

Modulating Thin Film Transistor Characteristics by Texturing the Gate Metal

The development of reliable, high performance integrated circuits based on thin film transistors (TFTs) is of interest for the development of flexible electronic circuits. In this work we illustrate the modulation of TFT transconductance via the texturing of the gate metal created by the addition of a conductive pattern on top of a planar gate. Texturing results in the semiconductor-insulator interface acquiring a non-planar geometry with local variations in the radius of curvature. This influences various TFT parameters such as the subthreshold slope, gate voltage at the onset of conduction, contact resistance and gate capacitance. Specific studies are performed on textures based on periodic striations oriented along different directions. Textured TFTs showed upto ±40% variation in transconductance depending on the texture orientation as compared to conventional planar gate TFTs. Analytical models are developed and compared with experiments. Gain boosting in common source amplifiers based on textured TFTs as compared to conventional TFTs is demonstrated.

Self-Powered Active Sensor with Concentric Topography of Piezoelectric Fibers

In this study, we demonstrated a flexible and self-powered sensor based on piezoelectric fibers in the diameter range of nano- and micro-scales. Our work is distinctively different from previous electrospinning research; we fabricated this apparatus precisely via near-field electrospinning which has a spectacular performance to harvest mechanical deformation in arbitrary direction and a novel concentrically circular topography. There are many piezoelectric devices based on electrospinning polymeric fibers. However, the fibers were mostly patterned in parallel lines and they could be actuated in limited direction only. To overcome this predicament, we re-arranged the parallel alignment into concentric circle pattern which made it possible to collect the mechanical energy whenever the deformation is along same axis or not. Despite the change of topography, the output voltage and current could still reach to 5 V and 400 nA, respectively, despite the mechanical deformation was from different direction. This new arbitrarily directional piezoelectric generator with concentrically circular topography (PGCT) allowed the piezoelectric device to harvest more mechanical energy than the one-directional alignment fiber-based devices, and this PGCT could perform even better output which promised more versatile and efficient using as a wearable electronics or sensor.

Output optimized electret nanogenerators for self-powered long-distance optical communication systems

High-speed optical communication systems are built for real-time, massive and remote information exchange. However, any power outage will paralyse the systems and cause a huge loss. Here we constructed a self-powered long-distance optical communication system (SLOCS) utilizing output enhanced parallel connected electret nanogenerators (NGs) as a backup power for the power outage. The output current of the electret NG was promoted from 1.45 μA to 8.14 μA through optimizing the thickness of the electret film. In the SLOCS, a coded message was successfully transmitted for 50 meters by pressing electret NGs. The as-fabricated SLOCS paves the way to a simple and cost-effective strategy for developing a reliable emergency communication system in case of power outage, simultaneously promoting the progress of self-powered electronics.

Flexible ZnO nanorod-based piezoelectric nanogenerators on carbon papers

We report on the fabrication of ZnO nanorod (NR)-based flexible piezoelectric nanogenerators (PENGs) on carbon paper (CP). Structural investigations indicate that the ZnO NRs grew well along the porous CP surface. Optical investigation shows that the crystal quality of the ZnO NRs on the CP was comparable to that of NRs grown on Si substrate. As the molar concentration increased from 10-70 mM, the output voltage and current increased consistently from 3.6-6.8 V and 0.79-1.45 μA, respectively. The enhancements of the voltage and current were attributed to the enhanced accumulation of the potentials generated by the increased number of ZnO NRs in the PENG devices. Therefore, the porous CP enhanced the PENG performance due to the higher surface area, and provided a super-flexible self-powering platform.

Self-Powered Pressure Sensor with fully encapsulated 3D printed wavy substrate and highly-aligned piezoelectric fibers array

Near-field electrospinning (NFES) is capable of precisely deposit one-dimensional (1D) or two-dimensional (2D) highly aligned micro/nano fibers (NMFs) by electrically discharged a polymer solution. In this paper, a new integration of three-dimensional (3D) architectures of NFES electrospun polyvinylidene fluoride (PVDF) NMFs with the 3D printed topologically tailored substrate are demonstrated in a direct-write and in-situ poled manner, called wavy- substrate self-powered sensors (WSS). The fabrication steps are composed of the additive manufacture of 3D printed flexible and sinusoidal wavy substrate, metallization and NFES electrospun fibers in the 3D topology. This 3D architecture is capable of greatly enhancing the piezoelectric output. Finally, the proposed piezoelectrically integrated 3D architecture is applied to the self-powered sensors such as foot pressure measurement, human motion monitoring and finger-induced power generation. The proposed technique demonstrates the advancement of existing electrospinning technologies in constructing 3D structures and several promising applications for biomedical and wearable electronics.

A flat-panel-shaped hybrid piezo/triboelectric nanogenerator for ambient energy harvesting

Recently, many researchers have been paying attention to nanogenerators (NGs) as energy sources for self-powered mirco-nano systems, and studying how to achieve their higher power generation. Hence, we propose a hybrid-type NG for harvesting both the piezoelectric and triboelectric effect simultaneously. In the proposed hybrid NG, the piezoelectric NG (PNG) and triboelectric NG (TENG) are fabricated using polydimethylsiloxane (PDMS) and perovskite zinc stannite (ZnSnO₃) nanocubes with a high charge polarization of 59 uC cm^-2 composite (PDMS + ZnSnO₃) and UV surface-treated PDMS, respectively. To effectively combine a high output current of PNG and a high voltage of TENG, these two NGs are stacked upon each other, and separated by sponge spacers providing a uniform air gap for the triboelectric effect. In particular, this fabricated structure has a low Young's modulus for piezoelectricity. The proposed hybrid NG device effectively achieves a combined peak voltage of 300 V on an open circuit, a power density of 10.41 mW cm^-2 at 1 MΩ load, and a maximum short circuit current density of 16 mA cm^-2 at 50 Ω load. It is feasible that the proposed NG can be utilized as a source for various self-powered systems.

Enhanced performance of ZnO microballoon arrays for a triboelectric nanogenerator

In recent years, triboelectric nanogenerators (TENGs), harvesting energy from the environment as a sustainable power source, have attracted great attention. Currently, many reports focus on the effect of surface modification on the electrical output performance of the TENG. In this work, we have fabricated vertically grown ZnO microballoon (ZnOMB) arrays on top of pyramid-featured PDMS patterned film, contacted with PTFE film to construct the TENG. The electrical output performances of the designed TENG are presented under external forces with different frequencies. The corresponding output open-circuit voltage with ZnOMBs could reach about 57 V the current density about 59 mA m^-2 at 100 Hz, which was about 2.3 times higher than without any ZnO. The global maximum of the instantaneous peak power could reach 1.1 W m^-2 when the external load resistance was about 2 MΩ. Furthermore, the electrical output of the fabricated device could light 30 commercial LED bulbs without any rectifier circuits or energy-storage elements. This clearly suggests that this kind of surface modification can dramatically enhance the output performance of the TENG. Moreover, the design of TENG demonstrated here can be applied to various energy harvesting applications.

Orientation-dependent piezoresponse and high-performance energy harvesting of lead-free (K,Na)NbO3 nanorod arrays

High-quality (K,Na)NbO3 nanorod arrays with [110]-oriented spontaneous polarization and piezoelectric response were utilized for building high-output piezoelectric energy harvesters.

Conceptual Design of a Nano-Networking Device

Nanotechnology is an emerging scientific area whose advances, among many others, have a positive direct impact on the miniaturization of electronics. This unique technology enables the possibility to design and build electronic components as well as complete devices (called nanomachines or nanodevices) at the nano scale. A nanodevice is expected to be an essential element able to operate in a nanonetwork, where a huge number of them would coordinate to acquire data, process the information gathered, and wirelessly transmit those data to end-points providing innovative services in many key scenarios, such as the human body or the environment. This paper is aimed at studying the feasibility of this type of device by carefully examining their main component parts, namely the nanoprocessor, nanomemory, nanoantenna, and nanogenerator. To this end, a thorough state-of-the-art review is conveyed to discuss, substantiate, and select the most suitable current technology (commercial or pre-commercial) for each component. Then, we further contribute by developing a complete conceptual nanodevice layout taking into consideration its ultra-small size (similar to a blood cell) and its very restricted capabilities (e.g., processing, memory storage, telecommunication, and energy management). The required resources as well as the power consumption are realistically estimated.