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A new facile strategy to maximize the charge density over 1000 μC m−2 for a high-output TENG is demonstrated by introducing new physics in contact electrification, i.e. ‘Electric-field-driven interfacial trapping of drifting triboelectric charges’.

Acoustic energy transfer using ferroelectrically augmented triboelectric receivers can efficiently deliver energy to implantable medical devices, marine cable operation sensors, and electronic devices with electromagnetic interference shielding cases.

A pseudocubic-tetragonal-orthorhombic (PC-T-O) multi-structure was developed in the 0.96(K0.5Na0.47Li0.03) (Nb0.92Sb0.08)O3-0.04(Ca0.5Sr0.5)ZrO3 [(KN0.47L0.03)NS-CSZ] lead-free piezoceramic. The PC structure observed in this sample was deemed to have an R3m rhombohedral structure. This sample demonstrated a large d33 (560 pC/N) owing to the existence of the PC-T-O multi-structure. The (KN0.47L0.03)NS-CSZ thick film also exhibited a large d33 (560 pC/N) and strain (0.19% at 4.0 kV/mm). The large strain was sustained even after the application of 106 electric-field cycles. Therefore, the (KN0.47L0.03)NS-CSZ thick film exhibited excellent fatigue characteristics and was utilized to fabricate a planar-type actuator, which achieved a large displacement (300 μm) and acceleration (872 G) at 125 V/mm. A cantilever-type multilayer actuator was also prepared using five layers of the (KN0.47L0.03)NS-CSZ thick film and demonstrated a large displacement (3500 μm) and acceleration (42.96 G) at 250 V/mm. Therefore, the (KN0.47L0.03)NS-CSZ thick film is a good candidate for fabricating various piezoelectric actuators.

Wireless energy transfer (WET) is the transmission of electric power without any physical connections such as wires. Currently, inductive coupling mediated by electromagnetic (EM) waves is the most common method of WET and is widely used to charge portable devices such as smartphones, Bluetooth earphones, electric shavers, and visual prostheses. However, its application is still limited due to a number of issues including low efficiency, short charging distance, heating problem, and limited choice of transmission medium. Due to these issues, EM-based WET cannot be applied to implantable medical devices, marine cable operation sensors, and electronic devices with electromagnetic interference shielding. Recently, as an alternative to EM-based WET, acoustic energy transfer mediated by sound waves becomes more attractive. Ultrasound offers advantages for transmission in dense media such as liquids or solids and is regardless of electromagnetic shielding. In this review, we investigate recent progress in acoustic power transfer technology in terms of acoustic energy conversion mechanism and provide the future research direction of acoustic power transfer technology.

Enhanced pyroelectric response is achieved via domain engineering from [001] grain-oriented, tetragonal-phase, lead-free 0.2(2/3K0.5Bi0.5TiO3-1/3BaTiO3)-0.8Na0.5Bi0.5TiO3 (KBT-BT-NBT) ceramics prepared by a templated grain growth method. The [001] crystallographic orientation leads to large polarization in tetragonal symmetry; therefore, texturing along this direction is employed to enhance the pyroelectricity. X-ray diffraction analysis revealed a Lotgering factor (degree of texturing) of 93 % along the [001] crystallographic direction. The textured KBT-BT-NBT lead-free ceramics showed comparable pyroelectric figures of merit to those of lead-based ferroelectric materials at room temperature (RT). In addition to the enhanced pyroelectric response at RT, an enormous enhancement in the pyroelectric response (from 1750 to 90,900 μC m−2 K−1) was achieved at the depolarization temperature because of the sharp ferroelectric to antiferroelectric phase transition owing to coherent 180° domain switching. These results will motivate the development of a wide range of lead-free pyroelectric devices, such as thermal sensors and infra-red detectors.

Nonlinear mono-stable and multi-stable piezoelectric energy harvesters have attracted a lot of attention owing to their broadband frequency spectra and excellent energy harvesting performance. Herein, two types of nonlinear mono-stable, bi-stable, tri-stable, and quad-stable piezoelectric energy harvesters using cantilever structure and magnetic interaction are compared and analyzed. Based on the magnetizing current method, the magnetic force equations are obtained. Calculation results demonstrate that the stability of these harvesters is dependent on the equivalent linear elastic force and the vertical magnetic force. The equilibrium point occurs when the equivalent linear elastic force equals to the vertical magnetic force. The relationship between the number of stable equilibrium points E S and the number of the intersections of the two force curves N I is that E S= (N I + 1)/2. Experiments are carried out to verify the equivalent linear elastic force, vertical magnetic force, and the number of stable equilibrium points of the fabricated prototypes. The experimental results are consistent with the calculated results, which verifies the correctness of the stability mechanism. Moreover, it is found that the stability mechanism is also applicable to the harvesters with more stable equilibrium points, such as penta-stable and hexa-stable harvesters. This work reveals the stability mechanism of nonlinear mono-stable and multi-stable energy harvesters using cantilever structure and magnetic interaction, and provides technical methods for the design of multi-stable energy harvesters.

The use of graphene and two-dimensional materials for industrial, scientific, and medical applications has recently received an enormous amount of attention due to their exceptional physicochemical properties. There have been numerous efforts to incorporate these two-dimensional materials into advanced flexible electronics, especially aimed for wearable biomedical applications. Here, recent advances in two-dimensional materials-based flexible electronic sensors for wearable biomedical applications with regard to both materials and devices are presented.

An innovative autonomous resonance-tuning (ART) energy harvester is reported that utilizes adaptive clamping systems driven by intrinsic mechanical mechanisms without outsourcing additional energy. The adaptive clamping system modulates the natural frequency of the harvester's main beam (MB) by adjusting the clamping position of the MB. The pulling force induced by the resonance vibration of the tuning beam (TB) provides the driving force for operating the adaptive clamp. The ART mechanism is possible by matching the natural frequencies of the TB and clamped MB. Detailed evaluations are conducted on the optimization of the adaptive clamp tolerance and TB design to increase the pulling force. The energy harvester exhibits an ultrawide resonance bandwidth of over 30 Hz in the commonly accessible low vibration frequency range (100 Hz) owing to the ART function. The practical feasibility is demonstrated by evaluating the ART performance under both frequency and acceleration-variant conditions and powering a location tracking sensor.

In this study, it is demonstrated that the giant self-biased magnetoelectric (SME) response can be achieved from a center-clamped magnetostrictive–piezoelectric laminate composite by employing magnetic tip masses. An asymmetric laminate structure consisting of two different magnetostrictive layers (Metglas and nickel) with opposite signs of piezomagnetic coefficient is introduced to promote structural bending resonance, and the effect of layout change of attaching the magnetic tip masses on SME responses is systematically investigated. The highest SME effect is observed when all magnetic tip masses are loaded on the Metglas layer and their magnetization directions are normal to the Metglas surface. It is proposed that not only the parallel magnetic domains to external magnetic field but also the non-parallel magnetic domains effectively contribute to the total magnetostriction. The fabricated SME laminates exhibit giant SME voltage coefficients ranging from 14.11 to 52.35 V cm−1 Oe−1, depending on the direction of the fields of the tip magnets. These high SME voltage output values and their controllability are promising for precision field sensors, magnetic energy harvesters and field-tunable devices.

Triboelectric generators (TEG), based on contact electrification and electro-static induction, has received a significant attention because of their numerous applications. To improve the electrified surface charge density in TEG, increasing the surface area of dielectric materials or forming internal hollow structures are typically employed to increase capacitance. However, the fabrication processes of such structures are complex and time-consuming. Here, we provide a facile and cost-effective synthesis method for the porous PDMS based TEG via a novel vapor encapsulation casting (VEC). The double dielectric layer composed of the porous and dense PDMS layers are formed in-site through VEC. The thickness and the thickness ratio of the double dielectric layer can be precisely controlled by adjusting the uncured PDMS thickness and vapor penetration depth. The double dielectric layer TEG (DTEG) exhibits the improved harvesting performance because the porous dielectric layer increases the capacitance and compressibility, while the dense layer passivates the fully open pores which reduce the charging surface area as completely opening through the dielectric layer without contacting the bottom electrode. We obtain the maximum output voltage of 345 V and short circuit current of 3 μA/cm2 from DTEG having 0.95 porous thickness ratio, resulting 330% enhancement in the power output as compared to the dense PDMS based TEG. We further investigate the performance of DTEG under various operating conditions. We also demonstrate the operation of Bluetooth distance/temperature sensors using capacitors charged by DTEG.

Conventional artificial muscles induce bending by aligning large-sized ions within the electrolyte upon bias application. Such design, alike many other actuator types, suffer from volatile actuation where the actuated position gets lost upon switch-off. Here, we develop a non-volatile artificial muscle with ion insertion electrode materials. Upon bias application, the inserted ions pose stress on the electrodes that sustain even after power shut-off. The demonstrated actuator consists of lithium germanide (LixGe) thin films deposited on both sides of a flexible polyimide (PI) substrate. The device exhibits 35.2 mm displacement when operated at 2 V and generates the blocking force of 0.67 mN. The observed stress and volume expansion reach 248 MPa and 8.2% for the 284 nm Li3Ge thin films, respectively. The actuated position is maintained against gravity with 12.1% decay in the actuated distance after 10 min. The novel actuator type proves the potential use of lithium insertion materials as actuation materials and shows that non-volatile actuation can be realized with ion-insertion electrodes.

Recent advances in lithography technology and the spread of 3D printers allow us a facile fabrication of special materials with complicated microstructures. The materials are called “designed materials” or “architectured materials” and provide new opportunities for material development. These materials, which owing to their rationally designed architectures exhibit unusual properties at the micro- and nano-scales, are being widely exploited in the development of modern materials with customized and improved performance. Meta-materials are found to possess superior and unusual properties as regards static modulus (axial stress divided by axial strain), density, energy absorption, smart functionality, and negative Poisson’s ratio (NPR). However, in spite of recent developments, it has only been feasible to fabricate a few such meta-materials and to implement them in practical applications. Against such a backdrop, a broad review of the wide range of cellular auxetic structures for mechanical metamaterials available at our disposal and their potential application areas is important. Classified according to their geometrical configuration, this paper provides a review of cellular auxetic structures. The structures are presented with a view to tap into their potential abilities and leverage multidimensional fabrication advances to facilitate their application in industry. In this review, there is a special emphasis on state-of-the-art applications of these structures in important domains such as sensors and actuators, the medical industry, and defense while touching upon ways to accelerate the material development process.

Piezoelectric energy harvesters (PEHs) aim to generate sufficient power to operate targeting device from the limited ambient energy. PEH includes mechanical-to-mechanical, mechanical-to-electrical, and electrical-to-electrical energy conversions, which are related to PEH structures, materials, and circuits, respectively; these should be efficient for increasing the total power. This critical review focuses on PEH structures and materials associated with the two major energy conversions to improve PEH performance. First, the resonance tuning mechanisms for PEH structures maintaining continuous resonance, regardless of a change in the vibration frequency, are presented. Based on the manual tuning technique, the electrically- and mechanically-driven self-resonance tuning (SRT) techniques are introduced in detail. The representative SRT harvesters are summarized in terms of tunability, power consumption, and net power. Second, the figure-of-merits of the piezoelectric materials for output power are summarized based on the operating conditions, and optimal piezoelectric materials are suggested. Piezoelectric materials with large kij , dij , and gij values are suitable for most PEHs, whereas those with large kij and Qm values should be used for on-resonance conditions, wherein the mechanical energy is directly supplied to the piezoelectric material. This comprehensive review provides insights for designing efficient structures and selection of proper piezoelectric materials for PEHs.

The transition of autonomous vehicles into fleets requires an advanced control system design that relies on continuous feedback from the tires. Smart tires enable continuous monitoring of dynamic parameters by combining strain sensing with traditional tire functions. Here, we provide breakthrough in this direction by demonstrating tire-integrated system that combines direct mask-less 3D printed strain gauges, flexible piezoelectric energy harvester for powering the sensors and secure wireless data transfer electronics, and machine learning for predictive data analysis. Ink of graphene based material was designed to directly print strain sensor for measuring tire-road interactions under varying driving speeds, normal load, and tire pressure. A secure wireless data transfer hardware powered by a piezoelectric patch is implemented to demonstrate self-powered sensing and wireless communication capability. Combined, this study significantly advances the design and fabrication of cost-effective smart tires by demonstrating practical self-powered wireless strain sensing capability., Designing efficient sensors for smart tires for autonomous vehicles remains a challenge. Here, the authors present a tire-integrated system that combines direct mask-less 3D printed strain gauges, flexible piezoelectric energy harvester for powering the sensors and secure wireless data transfer electronics, and machine learning for predictive data analysis.

We demonstrate the feasibility of the strain energy harvesting from the automobile tires, powering wireless data transfer with enhanced frame rates, and self-powered strain sensing. For this, we used a flexible organic piezoelectric material for continuous power generation and monitoring of the variable strain experienced by a tire under different driving conditions. Power output of ∼580 µW at 16 Hz (∼112 km/h) from the energy-harvester and mounted on a section of a tire, is sufficient to power 78 LEDs. We further demonstrate that the stored energy was sufficient to power the wireless system that transmits tire deformation data with an enhanced frame rate to control system of a vehicle. Using sensors mounted on a tire of a mobile test rig, measurements were conducted on different terrains with varying normal loads and speeds to quantify the sensitivity and self-powered sensing operation. These results provide a foundation for self-powered real-time sensing and energy efficient data transfer in autonomous vehicles.

Broadband mechanical energy harvesting implies stable output power over a wide range of source frequency. Here we present a cost-effective solution towards achieving broadband response by designing a magnetically coupled piezoelectric energy harvester array that exhibits a large power density of 243 μW/cm3 g2 at natural frequency and bandwidth of more than 30 Hz under 1 g acceleration. The magnetically coupled piezoelectric energy harvester array exhibits dual modes of energy harvesting, responding to both stray magnetic field as well as ambient vibrations, and is found to exhibit the output power density of 36.5 μW/cm3 Oe2 at 79.5 Hz under the ambient magnetic field while maintaining the broadband nature. The magnetically coupled piezoelectric energy harvester array was demonstrated to harvest continuous power from a rotary pump vibration, an automobile engine vibration and a parasitic magnetic field surrounding a cable of an electric kettle. These demonstrations suggest that the magnetically coupled piezoelectric energy harvester array could serve the role of a standalone power source for wireless sensor nodes and small electronic devices.

In the emerging era of Internet of Things (IoT), power sources for wireless sensor nodes in conjunction with efficient and secure wireless data transfer are required. Energy harvesting technologies are promising solution toward meeting the requirements for sustainable power sources for the IoT. In this review, we focus on approaches for harvesting stray vibrations and magnetic field due to their abundance in the environment. Piezoelectric materials and piezoelectric–magnetostrictive [magnetoelectric (ME)] composites can be used to harvest vibration and magnetic field, respectively. Currently, such harvesters use modified lead zirconate titanate (or lead-based) piezoelectric materials and ME composites. However, environmental concerns and government regulations require the development of a suitable lead-free replacement for lead-based piezoelectric materials. In the past decade, several lead-free piezoelectric compositions have been developed and demonstrated with promising piezoelectric response. This paper reviews the significant results reported on lead-free piezoelectric materials with respect to high-density energy harvesting, covering novel processing techniques for improving the piezoelectric response and temperature stability. The review of the state-of-the-art studies on vibration and magnetic field harvesting is provided and the results are used to discuss various strategies for designing high-performance energy harvesting devices.

Enhanced and self-biased magnetoelectric (ME) coupling is demonstrated in a laminate heterostructure comprising 4 μm-thick Pb(Zr,Ti)O3 (PZT) film deposited on 50 μm-thick flexible nickel (Ni) foil. A unique fabrication approach, combining room temperature deposition of PZT film by granule spray in vacuum (GSV) process and localized thermal treatment of the film by laser radiation, is utilized. This approach addresses the challenges in integrating ceramic films on metal substrates, which is often limited by the interfacial chemical reactions occurring at high processing temperatures. Laser-induced crystallinity improvement in the PZT thick film led to enhanced dielectric, ferroelectric, and magnetoelectric properties of the PZT/Ni composite. A high self-biased ME response on the order of 3.15 V/cm·Oe was obtained from the laser-annealed PZT/Ni film heterostructure. This value corresponds to a ∼2000% increment from the ME response (0.16 V/cm·Oe) measured from the as-deposited PZT/Ni sample. This result is also one of the highest reported values among similar ME composite systems. The tunability of self-biased ME coupling in PZT/Ni composite has been found to be related to the demagnetization field in Ni, strain mismatch between PZT and Ni, and flexural moment of the laminate structure. The phase-field model provides quantitative insight into these factors and illustrates their contributions toward the observed self-biased ME response. The results present a viable pathway toward designing and integrating ME components for a new generation of miniaturized tunable electronic devices.

We demonstrate a microscale vibration energy harvester exhibiting an ultra-low resonance frequency and high power density. A spiral shaped microelectromechanical system (MEMS) energy harvester was designed to harvest ambient vibrations at a low frequency ( $\mu \text{m}$ -thickness exhibiting remanent polarization of 36.2 $\mu \text{C}$ /cm2 and longitudinal piezoelectric constant of 155 pm/V was synthesized to achieve high efficiency mechanical to electrical conversion. The experimental results demonstrate an ultra-low natural frequency of 48 Hz for MEMS harvester. This is one of the lowest resonance frequency reported for the piezoelectric MEMS energy harvester. Further, the position of the natural frequency was controlled by modulating the number of spiral turns and weight of the proof mass. The vibration mode shape and stress distribution were validated through a finite element analysis. The maximum output power of 23.3 nW was obtained from the five turns spiral MEMS energy harvester excited at 0.25 g acceleration and 68Hz. The normalized area and the volumetric energy density were measured to be $5.04\times 10^{-4}~ \mu \text{W}$ /mm $^{2}~\cdot ~\text{g}^{2}~\cdot$ Hz and $4.92\times 10^{-2} ~ \mu \text{W}$ /mm $^{3}~\cdot ~\text{g}^{2}~\cdot$ Hz, respectively. [2017-0018]