Ionic Liquid-Assisted 3D Printing of Self-Polarized β-PVDF for Flexible Piezoelectric Energy Harvesting

3D printed energy devices: generation, conversion, and storage

The energy devices for generation, conversion, and storage of electricity are widely used across diverse aspects of human life and various industry. Three-dimensional (3D) printing has emerged as a promising technology for the fabrication of energy devices due to its unique capability of manufacturing complex shapes across different length scales. 3D-printed energy devices can have intricate 3D structures for significant performance enhancement, which are otherwise impossible to achieve through conventional manufacturing methods. Furthermore, recent progress has witnessed that 3D-printed energy devices with micro-lattice structures surpass their bulk counterparts in terms of mechanical properties as well as electrical performances. While existing literature focuses mostly on specific aspects of individual printed energy devices, a brief overview collectively covering the wide landscape of energy applications is lacking. This review provides a concise summary of recent advancements of 3D-printed energy devices. We classify these devices into three functional categories; generation, conversion, and storage of energy, offering insight on the recent progress within each category. Furthermore, current challenges and future prospects associated with 3D-printed energy devices are discussed, emphasizing their potential to advance sustainable energy solutions.

Polyvinylidene Fluoride Based Piezoelectric Composites with Strong Interfacial Adhesion via Click Chemistry for Self-Powered Flexible Sensors

Achieving relatively uniform dispersion in organic-inorganic composites with overwhelming differences in surface energy is a perennial challenge. Herein, novel eliminated polyvinylidene fluoride (EPVDF)/EPVDF functionalized barium titanate nanoparticles (EPVDF@BT) flexible piezoelectric nanogenerators (PENGs) with strong interfacial adhesion are developed via thermal stretching following sequential click chemistry. Thanks to the strong interfacial adhesion, the optimal PENGs containing ultra-high β-phase content (97.2%) exhibit not only optimized mechanical and dielectric behaviors but also excellent piezoelectric properties with high piezoelectric output (V = 10.7 V, I = 216 nA), reliable durability (8000 cycles), ultrafast response time (20 ms), and good sensitivity (2.09 nA kPa-1), far outperforming most reported PVDF-based composites. Furthermore, COMSOL finite element simulations (FEM) confirm that the elevated stress transfer efficiency induced by the strong interfacial adhesion is the main driving force for enhanced piezoelectric performances. For practical applications, self-powered PENGs can simply but stably capture mechanical energy, drive tiny electronic devices, and serve as potential multifunctional and durable sensors for detecting human physiological motions. This work opens a pioneering avenue to break the trade-offs between piezoelectric and other properties, which is of great importance for developing self-powered flexible sensors.

3D Printing of Anisotropic Piezoresistive Pressure Sensors for Directional Force Perception

Anisotropic pressure sensors are gaining increasing attention for next-generation wearable electronics and intelligent infrastructure owing to their sensitivity in identifying different directional forces. 3D printing technologies have unparalleled advantages in the design of anisotropic pressure sensors with customized 3D structures for realizing tunable anisotropy. 3D printing has demonstrated few successes in utilizing piezoelectric nanocomposites for anisotropic recognition. However, 3D-printed anisotropic piezoresistive pressure sensors (PPSs) remain unexplored despite their convenience in saving the poling process. This study pioneers the development of an aqueous printable ink containing waterborne polyurethane elastomer. An anisotropic PPS featuring tailorable flexibility in macroscopic 3D structures and microscopic pore morphologies is created by adopting direct ink writing 3D printing technology. Consequently, the desired directional force perception is achieved by programming the printing schemes. Notably, the printed PPS demonstrated excellent deformability, with a relative sensitivity of 1.22 (kPa*wt. %)-1 over a substantial pressure range (2.8 to 8.1 kPa), approximately fivefold than that of a state-of-the-art carbon-based PPS. This study underscores the versatility of 3D printing in customizing highly sensitive anisotropic pressure sensors for advanced sensing applications that are difficult to achieve using conventional measures.

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Ionic liquid modified PVDF/BCZT nanocomposites for space charge induced mechanical energy harvesting performance

Mechanical energy harvesting performances of poly(vinylidene fluoride) (PVDF) based composites are most often correlated with their polar phase and the individual piezoelectricity of the used filler materials. Here we show that the significant enhancement of space charge polarization of the said composites can play the key dominant role in determining their mechanical energy harvesting performance regardless of their polar phase and individual piezoelectricity of the used fillers. For this purpose, ionic liquid has been incorporated into PVDF/0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Ti0.8Zr0.2)O3(BCZT) composites which led to a huge enhancement in space charge polarization. The gradual addition of ionic liquid into 10 wt% BCZT loaded PVDF (PBCZT) has helped in extraordinarily enhancing the conductivity gradually which has confirmed the huge enhancement of space charge polarization. However, after a certain limit of ionic liquid addition, the polar phase of the composite films is decreased. Despite this, the output voltages from the piezoelectric and piezo-tribo hybrid nanogenerators (PENGs and HNGs, respectively) fabricated by using the developed films have been found to be increased gradually with the increase in the ionic liquid amount in PBCZT composite. As the amount of BCZT filler was kept fixed for all the films, this result has confirmed the key role of space charge polarization of PVDF-based composites in determining their mechanical energy harvesting performances compared to the effect of polar phase and individual piezoelectricity of filler. The optimized PENG and HNG devices have shown the output voltage as high as 52 and 167 V, respectively, with power densities ∼85 and 152μW cm-2which predicted their excellent usability in real life energy conversion devices. This work also shows that the effect of extraordinarily enhanced space charge polarization is effective in improving the performance of all types of mechanical energy harvesting devices regardless of their mechanisms (piezoelectric or hybrid).

3D Printing of Flexible BaTiO3/Polydimethylsiloxane Piezocomposite with Aligned Particles for Enhanced Energy Harvesting

With the rapid development of human-machine interactions and artificial intelligence, the demand for wearable electronic devices is increasing uncontrollably all over the world; however, an unsustainable power supply for such sensors continues to restrict their applications. In the present work, piezoelectric barium titanate (BaTiO3) ceramic powder with excellent properties was prepared from milled precursors through a solid-state reaction. To fabricate a flexible device, the as-prepared BaTiO3 powder was mixed with polydimethylsiloxane (PDMS) polymer. The BaTiO3/PDMS ink with excellent rheological properties was extruded smoothly by direct ink writing technology (DIW). BaTiO3 particles were aligned due to the shear stress effect during the printing process. Subsequently, the as-printed composite was assembled into a sandwich-type device for effective energy harvesting. It was observed that the maximum output voltage and current of this device reached 68 V and 720 nA, respectively, for a BaTiO3 content of 6 vol %. Therefore, the material extrusion-based three-dimensional (3D) printing technique can be used to prepare flexible piezoelectric composites for efficient energy harvesting.

Energy harvesting from water impact using piezoelectric energy harvester

Energy, as an indispensable part of human life, has been a hot topic of research among scholars. The water kinetic energy generated by ocean currents, as a kind of clean energy, has high utilization rate, high power generation potential, and a broad prospect of powering microelectronic devices. As a result, the water kinetic piezoelectric energy harvester (WKPEH) has made significant progress in powering ocean sensors by harvesting ocean currents. This paper provides a comprehensive review of technologies that have been used in recent years to harvest energy from marine fluids using WKPEH. Detailed study of the energy harvesting mechanism of WKPEH. WKPEH can use the flutter-induced vibrations, vortex-induced vibrations, and wake oscillation principles to harvest water kinetic energy. The structural characteristics and output performance of each mechanism are also discussed and compared, and finally, a prospect on WKPEH is given.

Effects of Ionic Liquid Content on the Electrical Properties of PVDF Films by Fused Deposition Modeling

In this study, polyvinylidene fluoride (PVDF) composite films were prepared by fused deposition modeling, and the effects of ionic liquid (IL) content on the printability, crystallization behavior, and electrical properties of melted PVDF were systematically investigated. The results show that the addition of IL increased the temperature sensitivity of melted PVDF and decreased its viscosity, while IL acted as a plasticizer to lower the melting point of PVDF and improve its FDM printability. The imidazole cations in IL had electrostatic interactions with the -CF2- groups in PVDF, which promoted the transformation of the nonpolar phase to the polar phase in PVDF; thus, the addition of IL was beneficial to the increase in the polar β phase. The PVDF with 20 wt.% IL contained the highest proportion of β phase content (32.59%). Moreover, the increase in polar β-phase content also increased the polarization strength of PVDF and improved its ferroelectric properties. PVDF with 10 wt.% IL had the highest residual polarization strength (16.87 μC/m2).

Optimizing solvent dipole moment enables PVDF to improve piezoelectric performance

The all-trans conformation (β-phase) possesses a significant impact on the piezoelectric polymer polyvinylidene fluoride (PVDF). Inducing more molecular chain [-CH2-CF2-]nto form all-trans conformation is one of the biggest obstacles for manufacturing high-performance piezoelectric sensing devices. Herein, the continuous vacuum technology is used to modulate the polarity of binary solvents by the proportion of the lower solvent. The regulated solvent forms a high dipole moment, an interaction between the dipole ofβ-phase and the dipole moment makes the phase reversal in PVDF. Fourier transform infrared spectroscopy, piezoelectric constant test and other characterization results show that when the weakly polar acetone and the strongly polar solvent DMF reach a ratio of 4:6, the pure PVDF film possesses high piezoelectricity (d33∼ -44.8 pC N-1) and strong self-polarization. Additionally, the A4D6device exhibits high sensitivity (S1= 0.182 V/N, 0.5 N ∼ 30 N), driven capability (0.49 mW m-2), and reliability during the electrical tests as a pressure device. This work provides an effective and cost-effective route of optimizing the solvent's polarity to improve the piezoelectric characteristics of the polymer.

3D Printed Polymer Piezoelectric Materials: Transforming Healthcare through Biomedical Applications

Three-dimensional (3D) printing is a promising manufacturing platform in biomedical engineering. It offers significant advantages in fabricating complex and customized biomedical products with accuracy, efficiency, cost-effectiveness, and reproducibility. The rapidly growing field of three-dimensional printing (3DP), which emphasizes customization as its key advantage, is actively searching for functional materials. Among these materials, piezoelectric materials are highly desired due to their linear electromechanical and thermoelectric properties. Polymer piezoelectrics and their composites are in high demand as biomaterials due to their controllable and reproducible piezoelectric properties. Three-dimensional printable piezoelectric materials have opened new possibilities for integration into biomedical fields such as sensors for healthcare monitoring, controlled drug delivery systems, tissue engineering, microfluidic, and artificial muscle actuators. Overall, this review paper provides insights into the fundamentals of polymer piezoelectric materials, the application of polymer piezoelectric materials in biomedical fields, and highlights the challenges and opportunities in realizing their full potential for functional applications. By addressing these challenges, integrating 3DP and piezoelectric materials can lead to the development of advanced sensors and devices with enhanced performance and customization capabilities for biomedical applications.

Electroactive properties and piezo-tribo hybrid energy harvesting performances of PVDF-AlFeO3 composites: role of crystal symmetry and agglomeration of fillers

Inorganic filler-loaded PVDF-based composites have been very widely used for electrical and energy harvesting applications in recent times. In this regard, the effects of different parameters of fillers like size, shape, chemical states, distribution, functional properties, and many others on the output performance of PVDF have been widely studied. However, the effect of another important parameter, namely the crystal symmetry of the filler, in tuning the energy harvesting performance of PVDF has been rarely explored. Therefore, to explore this fact, here we develop PVDF-based composite films by using two types of AlFeO3 fillers, one with rhombohedral R3̄c symmetry (AFRH) and another with an orthorhombic Pc21n structure. Ferrite-based oxides have been chosen here as fillers due to their good dielectric compatibility with PVDF. On the other hand, AlFeO3 has been chosen due to the simplicity of synthesizing it with both centrosymmetric and non-centrosymmetric crystal structures and the scarcity of reports exploring the energy-harvesting performance of AlFeO3-based polymer composites. A significant difference in particle agglomeration has also been observed here between the mentioned two types of AlFeO3 fillers which was mainly due to their specific synthesis conditions. The electroactive properties of PVDF have been observed to be mostly dependent on filler agglomeration. However, the crystal symmetry has shown a strong effect on the piezoelectric energy harvesting performances. As a result of these facts, the piezo-tribo hybrid energy harvesting performance, which depends on both the dielectric permittivity and piezoelectric activity, has been observed to be better for the AFRH5-based hybrid device (AFRH5H) (with ∼72 V open circuit voltage and ∼45 μW cm-2 power density) compared to that of the AFOR5-based hybrid device (AFOR5H). The real-life applications of all the energy harvesting devices have also been demonstrated here.

Electrospinning of Highly Bi-Oriented Flexible Piezoelectric Nanofibers for Anisotropic-Responsive Intelligent Sensing

Flexible piezoelectric energy harvesters (PEHs) have gained substantial attention owing to their wearability, breathability, and sustainable self-powered supply. However, existing film PEHs cannot identify forces in different bending directions, limiting their applications in wearable electronics and artificial intelligence. This study constructs a fabric PEH for the first time by introducing piezoelectric anisotropic BaTi2 O5 nanorods (BT2-nr) into piezoelectric polyvinylidene fluoride (PVDF) nanofibers with a bi-oriented architecture, in which BT2-nr uniformly aligns in the PVDF nanofiber during electrospinning. The dual-orientation feature endows the flexible PEH with anisotropy, which can sensitively identify the forces at different bending directions (e.g., bent vertically, parallelly, or twisted by 45° along the fiber orientations). Simultaneously, the composite PVDF/BT2 PEH containing 15 wt.% BT2-nr delivers an optimal piezoelectric output of 31.2 V with a high sensitivity of 5.22 V N-1 . The developed anisotropic PEH can be used as a self-powered pressure sensor for multimodal intelligent biomonitoring of human movement. This study provides a feasible strategy for fabricating self-powered flexible PEHs with high electromechanical conversion efficiency and multifunctionality for wearable piezoelectric pressure sensors.

3D-printed polymer composite devices based on a ferroelectric chiral ammonium salt for high-performance piezoelectric energy harvesting

Three-dimensional printing (3DP) is an emerging technology to fabricate complex architectures, necessary to realize state-of-the-art flexible and wearable electronic devices. In this regard, top-performing devices containing organic ferro- and piezoelectric compounds are desired to circumvent significant shortcomings of conventional piezoceramics, e.g. toxicity and high-temperature device processibility. Herein, we report on a 3D-printed composite of a chiral ferroelectric organic salt {[Me3CCH(Me)NH3][BF4]} (1) with a biodegradable polycaprolactone (PCL) polymer that serves as a highly efficient piezoelectric nanogenerator (PENG). The ferroelectric property of 1 originates from its polar tetragonal space group P42, verified by P-E loop measurements. The ferroelectric domain characteristics of 1 were further probed by piezoresponse force microscopy (PFM), which gave characteristic 'butterfly' and hysteresis loops. The PFM amplitude vs. drive voltage measurements gave a relatively high magnitude of the converse piezoelectric coefficient for 1. PCL polymer composites with various weight percentages (wt%) of 1 were prepared and subjected to piezoelectric energy harvesting tests, which gave a maximum open-circuit voltage of 36.2 V and a power density of 48.1 μW cm-2 for the 10 wt% 1-PCL champion device. Furthermore, a gyroid-shaped 3D-printed 10 wt% 1-PCL composite was fabricated to test its practical utility, which gave an excellent output voltage of 41 V and a power density of 56.8 μW cm-2. These studies promise the potential of simple organic compounds for building PENG devices using advanced manufacturing technologies.

Advances in Wearable Piezoelectric Sensors for Hazardous Workplace Environments

Recent advances in wearable energy harvesting technology as solutions to occupational health and safety programs are presented. Workers are often exposed to harmful conditions-especially in the mining and construction industries-where chronic health issues can emerge over time. While wearable sensors technology can aid in early detection and long-term exposure tracking, powering them and the associated risks are often an impediment for their widespread use, such as the need for frequent charging and battery safety. Repetitive vibration exposure is one such hazard, e.g., whole body vibration, yet it can also provide parasitic energy that can be harvested to power wearable sensors and overcome the battery limitations. This review can critically analyze the vibration effect on workers' health, the limitations of currently available devices, explore new options for powering different personal protective equipment devices, and discuss opportunities and directions for future research. The recent progress in self-powered vibration sensors and systems from the perspective of the underlying materials, applications, and fabrication techniques is reviewed. Lastly, the challenges and perspectives are discussed for reference to the researchers who are interested in self-powered vibration sensors.

Mechanoluminescent-Triboelectric Bimodal Sensors for Self-Powered Sensing and Intelligent Control

Self-powered flexible devices with skin-like multiple sensing ability have attracted great attentions due to their broad applications in the Internet of Things (IoT). Various methods have been proposed to enhance mechano-optic or electric performance of the flexible devices; however, it remains challenging to realize the display and accurate recognition of motion trajectories for intelligent control. Here, we present a fully self-powered mechanoluminescent-triboelectric bimodal sensor based on micro-nanostructured mechanoluminescent elastomer, which can patterned-display the force trajectories. The deformable liquid metals used as stretchable electrode make the stress transfer stable through overall device to achieve outstanding mechanoluminescence (with a gray value of 107 under a stimulus force as low as 0.3 N and more than 2000 cycles reproducibility). Moreover, a microstructured surface is constructed which endows the resulted composite with significantly improved triboelectric performances (voltage increases from 8 to 24 V). Based on the excellent bimodal sensing performances and durability of the obtained composite, a highly reliable intelligent control system by machine learning has been developed for controlling trolley, providing an approach for advanced visual interaction devices and smart wearable electronics in the future IoT era.

Self-Powered Wireless Flexible Ionogel Wearable Devices

Ionogels are promising soft materials for flexible wearable devices because of their unique features such as ionic conductivity and thermal stability. Ionogels reported to date show excellent sensing sensitivity; however, they suffer from a complicated external power supply. Herein, we report a self-powered wearable device based on an ionogel incorporating poly(vinylidene fluoride) (PVDF). The three-dimensional (3D) printed PVDF-ionogel exhibits amazing stretchability (1500%), high conductivity (0.36 S/m at 105 Hz), and an extremely low glass transition temperature (-84 °C). Moreover, the flexible wearable devices assembled from the PVDF-ionogel can precisely detect physiological signals (e.g., wrist, gesture, running, etc.) with a self-powered supply. Most significantly, a self-powered wireless flexible wearable device based on our PVDF-ionogel achieves monitoring healthcare of a human by transmitting obtained signals with a Bluetooth module timely and accurately. This work provides a facile and efficient method for fabricating cost-effective wireless wearable devices with a self-powered supply, enabling their potential applications for healthcare, motion detection, human-machine interfaces, etc.

Enhanced Piezocatalytic Reactive Oxygen Species Production Activity and Recyclability of the Dual Piezoelectric Cu3B2O6/PVDF Composite Membrane

Piezocatalysts have attracted considerable attention due to their ability to convert natural mechanical energy into chemical energy. However, the inefficient chemical reactions of the free charges and the poor mechanical endurance of the powder piezoelectric materials have largely restricted their wide application. Here, by combining piezocatalyst Cu₃B₂O₆ (CBO) and polyvinylidene fluoride (PVDF), a composite membrane CBO/PVDF with superior stability and excellent piezo-performance is prepared for the first time. This composite membrane shows a high efficiency for the degradation of antibiotics and organic dyes under ultrasonication; particularly, the removal efficiency is 33.9 times higher than that of a pure PVDF membrane for amoxicillin degradation, and it maintains a high efficiency after 16 cycling tests. The polarization electric field in the dual piezoelectric composite membrane significantly enhances the redox reaction of the intrinsic free carrier with dissolved oxygen and water molecules to generate reactive oxygen species. The results provide a strategy for combining the borate with the polymer membrane to lead piezocatalysis to real future 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.

Surface Wrinkling for Flexible and Stretchable Sensors

Recent advances in nanolithography, miniaturization, and material science, along with developments in wearable electronics, are pushing the frontiers of sensor technology into the large-scale fabrication of highly sensitive, flexible, stretchable, and multimodal detection systems. Various strategies, including surface engineering, have been developed to control the electrical and mechanical characteristics of sensors. In particular, surface wrinkling provides an effective alternative for improving both the sensing performance and mechanical deformability of flexible and stretchable sensors by releasing interfacial stress, preventing electrical failure, and enlarging surface areas. In this study, recent developments in the fabrication strategies of wrinkling structures for sensor applications are discussed. The fundamental mechanics, geometry control strategies, and various fabricating methods for wrinkling patterns are summarized. Furthermore, the current state of wrinkling approaches and their impacts on the development of various types of sensors, including strain, pressure, temperature, chemical, photodetectors, and multimodal sensors, are reviewed. Finally, existing wrinkling approaches, designs, and sensing strategies are extrapolated into future applications.

Screen Printing of Surface-Modified Barium Titanate/Polyvinylidene Fluoride Nanocomposites for High-Performance Flexible Piezoelectric Nanogenerators

Piezoelectric energy harvesters are appealing for the improvement of wearable electronics, owing to their excellent mechanical and electrical properties. Herein, screen-printed piezoelectric nanogenerators (PENGs) are developed from triethoxy(octyl)silane-coated barium titanate/polyvinylidene fluoride (TOS-BTO/PVDF) nanocomposites with excellent performance based on the important link between material, structure, and performance. In order to minimize the effect of nanofiller agglomeration, TOS-coated BTO nanoparticles are anchored onto PVDF. Thus, composites with well-distributed TOS-BTO nanoparticles exhibit fewer defects, resulting in reduced charge annihilation during charge transfer from inorganic nanoparticles to the polymer. Consequently, the screen-printed TOS-BTO/PVDF PENG exhibits a significantly enhanced output voltage of 20 V, even after 7500 cycles, and a higher power density of 15.6 μW cm^-2, which is 200 and 150% higher than those of pristine BTO/PVDF PENGs, respectively. The increased performance of TOS-BTO/PVDF PENGs is due to the enhanced compatibility between nanofillers and polymers and the resulting improvement in dielectric response. Furthermore, as-printed devices could actively adapt to human movements and displayed excellent detection capability. The screen-printed process offers excellent potential for developing flexible and high-performance piezoelectric devices in a cost-effective and sustainable way.

Wearable Sensors for Healthcare: Fabrication to Application

This paper presents a substantial review of the deployment of wearable sensors for healthcare applications. Wearable sensors hold a pivotal position in the microelectronics industry due to their role in monitoring physiological movements and signals. Sensors designed and developed using a wide range of fabrication techniques have been integrated with communication modules for transceiving signals. This paper highlights the entire chronology of wearable sensors in the biomedical sector, starting from their fabrication in a controlled environment to their integration with signal-conditioning circuits for application purposes. It also highlights sensing products that are currently available on the market for a comparative study of their performances. The conjugation of the sensing prototypes with the Internet of Things (IoT) for forming fully functioning sensorized systems is also shown here. Finally, some of the challenges existing within the current wearable systems are shown, along with possible remedies.

Combining Solid-State Shear Milling and FFF 3D-Printing Strategy to Fabricate High-Performance Biomimetic Wearable Fish-Scale PVDF-Based Piezoelectric Energy Harvesters

High-performance flexible piezoelectric polymer-ceramic composites are in high demand for increasing wearable energy-harvesting applications. In this work, a strategy combining solid-state shear milling (S³M) and fused filament fabrication (FFF) 3D-printing technology is proposed for the fabrication of high-performance biomimetic wearable piezoelectric poly(vinylidene fluoride) (PVDF)/tetraphenylphosphonium chloride (TPPC)/barium titanate (BaTiO₃) nanocomposite energy harvesters with a biomimetic fish-scale-like metamaterial. The S³M technology could greatly improve the dispersion of BaTiO₃ sub-micrometer particles and the interfacial compatibility, resulting in better processability and piezoelectric performance of the nanocomposites. Typically, the FFF 3D printed energy harvester incorporating 30 wt % BaTiO₃ showed the highest piezoelectric outputs with an open-circuit voltage of 11.5 V and a short-circuit current of 220 nA. It could hence drive nine green LEDs to work normally. In addition, a 3D-printed biomimetic wearable energy harvester inspired by an environmentally adaptive fish-scale-like metamaterial was further fabricated. The fish-scale-like energy harvester could harvest energy through different deformation motions and successfully recharge a 4.7 μF capacitor by being mounted on a bicycle tire and the tire's rolling. This work not only provides a 3D printing strategy for designing diversified and complex geometric structures but also paves the way for further applications in flexible, wearable, self-powered electromechanical energy harvesters.

Electrohydrodynamic Pulling Consolidated High-Efficiency 3D Printing to Architect Unusual Self-Polarized β-PVDF Arrays for Advanced Piezoelectric Sensing

Piezoelectric pressure sensors are important for applications in robotics, artificial intelligence, communication devices, etc. The hyperboloid is theoretically predicted to be an unusual 3D structure that allows concerted piezoelectric enhancement owing to its synergistic effects of geometrical stress confinement and stress concentration, but has not been experimentally fulfilled due to a lack of efficient architecting techniques. In this work, a 3D hyperboloidal arrayed self-polarized PVDF piezoelectric energy harvester (PEH) is successfully fabricated by incorporating electrohydrodynamic (EHD) pulling technology into fused deposition modeling (FDM) 3D printing. This strategy not only simplifies the layer-by-layer constructing procedure for arrays, but simultaneously realizes a self-polarized and high β-phase (92%) PVDF PEH in a single electric-pulling step, saving posttreatment such as poling and removing excessive additives. Such a PEH delivers a significantly enhanced piezoelectric potential which is around 8 times that of a 2D flat film sensor. Moreover, this PEH featuring excellent linearity within a wide pressure regime, enables the sensing of human activities in a relatively large force range, which is otherwise difficult for traditional film sensors to differentiate. This work demonstrates a potential roadmap to advanced piezoelectric sensors exploiting unusual 3D structures enabled by the unique EHD pulling coupled 3D printing technique.

3D Printing-Enabled In-Situ Orientation of BaTi2O5 Nanorods in β-PVDF for High-Efficiency Piezoelectric Energy Harvesters

Piezoelectric energy harvesters (PEHs) with a three-dimensional (3D) structure are arousing increasing interest because of the ability to efficiently convert mechanical energy into electricity catering for self-powered systems. Among them, 3D PEHs composed of 1-3-type piezoelectric composites which exploit one-dimensional (1D) piezoceramic fillers rather than conventional powders are particularly attractive. However, an issue involving the orientation of the 1D fillers to utilize the piezoelectric effect renders the 3D structural design for high-efficiency energy conversion more challenging. Herein, for the first time, we introduce the fused deposition modeling (FDM) 3D printing to the flexible construction of poly(vinylidene fluoride) (PVDF)-based 3D PEHs by incorporating 1D BaTi₂O₅ (BT2) nanorods as piezoelectric fillers. The shearing force generated by FDM successfully realizes the in situ uniform orientation of BT2 nanorods in the PVDF (98% β crystals) matrix along the nozzle extrusion direction. Besides, by coupling 3D printing with the appealing piezoelectric anisotropy feature of BT2 nanorods, the 3D PEH is able to generate different piezoelectric responses to the same applied external force from X, Y, and Z directions. Furthermore, an optimized 3D conical array structure is constructed to amplify the effective deformation of the PEH to enhance its piezoelectric output. As expected, customized PEH can continuously power commercial electronic devices and monitor various human motions, indicating 3D printing as a multifunctional strategy to fabricate 3D PEHs with 1-3-type piezoelectric composite materials for self-powering microelectronic applications.

High-Resolution 3D Printing for Electronics

The ability to form arbitrary 3D structures provides the next level of complexity and a greater degree of freedom in the design of electronic devices. Since recent progress in electronics has expanded their applicability in various fields in which structural conformability and dynamic configuration are required, high-resolution 3D printing technologies can offer significant potential for freeform electronics. Here, the recent progress in novel 3D printing methods for freeform electronics is reviewed, with providing a comprehensive study on 3D-printable functional materials and processes for various device components. The latest advances in 3D-printed electronics are also reviewed to explain representative device components, including interconnects, batteries, antennas, and sensors. Furthermore, the key challenges and prospects for next-generation printed electronics are considered, and the future directions are explored based on research that has emerged recently.

Impact of Multi-Walled CNT Incorporation on Dielectric Properties of PVDF-BaTiO3 Nanocomposites and Their Energy Harvesting Possibilities

The current study investigated the fabrication of multi-walled carbon nanotubes (MWCNTs) adhering to Barium titanate (BaTiO3) nanoparticles and poly(vinylidene fluoride) (PVDF) nanocomposites, as well as the impact of MWCNT on the PVDF-BaTiO3 matrix in terms of dielectric constant and dielectric loss with a view to develop a high performance piezoelectric energy harvester in future. The capacity and potential of as-prepared nanocomposite films for the fabrication of high-performance flexible piezoelectric nanogenerator (PNG) were also investigated in this work. In particular, five distinct types of nanocomposites and films were synthesized: PB (bare PVDF–BaTiO3), PBC-1 (PVDF–BaTiO3-0.1 wt% CNT), PBC-2 (PVDF–BaTiO3-0.3 wt% CNT), PBC-3 (PVDF–BaTiO3-0.5 wt% CNT), and PBC-4 (PVDF–BaTiO3-1 wt% CNT). The dielectric constant and dielectric loss increased as MWCNT concentration increased. Sample PBC-3 had the optimum dielectric characteristics of all the as-prepared samples, with the maximum output voltage and current of 4.4 V and 0.66 μA, respectively, with an applied force of ~2N. Fine-tuning the BaTiO3 content and thickness of the PNGs is likely to increase the harvester’s performance even more. It is anticipated that the work would make it easier to fabricate high-performance piezoelectric films and would be a suitable choice for creating high-performance PNG.

Mass transfer analysis of Boron-doped Carbon Nanotubes Cathode for Dual-electrolyte Lithium-air Batteries

Dual-electrolyte Li-air batteries (LABs) have the advantages of high specific energy density and low overpotential, but the mass transfer mechanism is still unclear. Its mass transfer is essential to battery...

Recent advances in the 3D printing of ionic electroactive polymers and core ionomeric materials

The recent advances in the 3D printing, or additive manufacturing, of ionic electroactive polymers (EAP) and their future applications.

Boosted Mechanical Piezoelectric Energy Harvesting of Polyvinylidene Fluoride/Barium Titanate Composite Porous Foam Based on Three-Dimensional Printing and Foaming Technology

The popularity of intelligent and green electronic devices means that the use of renewable mechanical energy has gradually become an inevitable choice for social development. However, it is difficult for the existing energy harvesters to meet the requirement for efficient collection of discrete mechanical energy due to the limitation of traditional two-dimensional (2D) film deformation. In this research, a green and convenient supercritical carbon dioxide foaming (Sc-CO2)-assisted selective laser sintering method was developed, and piezoelectric energy harvesters with a 3D porous structure of polyvinylidene fluoride (PVDF)/barium titanate (BaTiO3) were successfully constructed. The 3D structure combined with the porous structure made full use of the normal space, amplified the stress-strain effect, and improved the piezoelectric output capability. Under the synergistic effect of BaTiO3, the foams exhibited high output with an output voltage of 20.9 V and a current density of 0.371 nA/mm2, which exceeded most of the known PVDF/BaTiO3 energy harvesters, and the prepared piezoelectric energy harvester could directly light up 11 green light-emitting diodes and charge a 1 μF commercial capacitor to 4.98 V within 180 s. This work emphasizes the key role of 3D printing and Sc-CO2 foaming in fabricating 3D piezoelectric energy harvesters.

Progress in Piezoelectric Nanogenerators Based on PVDF Composite Films

In recent years, great progress has been made in the field of energy harvesting to satisfy increasing needs for portable, sustainable, and renewable energy. Among piezoelectric materials, poly(vinylidene fluoride) (PVDF) and its copolymers are the most promising materials for piezoelectric nanogenerators (PENGs) due to their unique electroactivity, high flexibility, good machinability, and long–term stability. So far, PVDF–based PENGs have made remarkable progress. In this paper, the effects of the existence of various nanofillers, including organic–inorganic lead halide perovskites, inorganic lead halide perovskites, perovskite–type oxides, semiconductor piezoelectric materials, two–dimensional layered materials, and ions, in PVDF and its copolymer structure on their piezoelectric response and energy–harvesting properties are reviewed. This review will enable researchers to understand the piezoelectric mechanisms of the PVDF–based composite–film PENGs, so as to effectively convert environmental mechanical stimulus into electrical energy, and finally realize self–powered sensors or high–performance power sources for electronic devices.

Self-Poled Poly(vinylidene fluoride)/MXene Piezoelectric Energy Harvester with Boosted Power Generation Ability and the Roles of Crystalline Orientation and Polarized Interfaces

Micropiezoelectric devices have become one of the most competitive candidates for use in self-powered flexible and portable electronic products because of their instant response and mechanic-electric conversion ability. However, achievement of high output performance of micropiezoelectric devices is still a significant and challenging task. In this study, a poly(vinylidene fluoride) (PVDF)/MXene piezoelectric microdevice was fabricated through a microinjection molding process. The synergistic effect of both an intense shear rate (>10⁴ s^-1) as well as numerous polar C-F functional groups in MXene flakes promoted the formation of β-form crystals of PVDF in which the crystallinity of β-form could reach as high as 59.9%. Moreover, the shear-induced shish-kebab crystal structure with a high orientation degree (f_h = ∼0.9) and the stacked MXene acted as the driving force for the dipoles to regularly arrange and produce a self-polarizing effect. Without further polarization, the fabricated piezoelectric microdevices exhibited an open-circuit voltage of 15.2 V and a short-circuit current of 497.3 nA, under optimal conditions (400 mm s^-1 and 1 wt % MXene). Impressively, such piezoelectric microdevices can be used for energy storage and for sensing body motion to monitor exercise, and this may have a positive impact on next-generation smart sports equipment.

Fabrication of PVDF/BaTiO3/CNT Piezoelectric Energy Harvesters with Bionic Balsa Wood Structures through 3D Printing and Supercritical Carbon Dioxide Foaming

Piezoelectric energy harvesters have received widespread attention in recent decades due to their inimitable electrical energy conversion methods. However, traditional polymer/piezoceramic materials and 2D thin-film structures have limited output performance, making them difficult to be efficiently applied in the collection of discrete mechanical energy. Here, new ternary composite powders were successfully developed by the ultrasonic coating method, and array structural devices with the construction of micropores were prepared using selective laser sintering (SLS) and supercritical carbon dioxide foaming (Sc-CO₂) technologies. Coating carbon nanotubes improved the polarization efficiency of poly(vinylidene fluoride)/barium titanate (PVDF/BaTiO₃) composites, which made it easy to perfectly combine the BaTiO₃ piezoelectric constant and the flexibility of PVDF, promoting d₃₃ from 0.7 to 2.6 pc/N. In addition, simulations and experiments simultaneously proved that SLS parts with high array densities amplified piezoelectric outputs because of a greater compression deformation in the vertical direction. Meanwhile, under the synergistic effect of SLS and Sc-CO₂, 3D bionic balsa wood structure foams were successfully fabricated, which took advantage of the normal space, expanded the stress-strain effect, and improved the piezoelectric output capability. Excitingly, the prepared foam could directly produce 19.3 V and 415 nA piezoelectric output to charge a 1 μF commercial capacitor to 5.03 V within 180 s, which surpassed most of the PVDF piezoelectric energy harvesters reported thus far. This work has an excellent innovative and practical value in enriching the types of piezoelectric materials for SLS 3D printing and the design of 3D piezoelectric structures.

Hierarchically Architected Polyvinylidene Fluoride Piezoelectric Foam for Boosted Mechanical Energy Harvesting and Self-Powered Sensor

With the rapid development of wearable electronics, piezoelectric materials have received great attention owing to their potential solution to the portable power source. To enhance the output capability and broaden the application, it is highly desired for the design of piezoelectric materials with a three-dimensional and porous structure to facilitate strain accumulation. Herein, enlightened by hierarchical structures in nature, a hierarchically nested network was constructed in polyvinylidene fluoride (PVDF) foam via solid-state shear milling and salt-leaching technology. The as-prepared foam exhibited two hierarchical levels of pores with diameters of 20∼50 μm and 0.3∼4 μm, by which the porosity and flexibility were significantly enhanced, while the highest piezoelectric output reached 11.84 V and 217.78 nA. As a proof-of-concept, the PVDF piezoelectric foam can also be used to monitor human movement toward the different magnitude of strain and frequency, and simultaneously collect energy in a multidimensional stress field for energy harvesting. This work provides a simple and convenient design idea for the preparation of energy harvesters, which have great application potential as a mechanical energy harvester or self-powered sensor in wearable electronic devices.

Facile preparation of high loading filled PVDF/BaTiO3 piezoelectric composites for selective laser sintering 3D printing

3D printed piezoelectric devices, due to their sufficient multidimensional deformation and excellent piezoelectric properties, are one of the most promising research directions. However, the lack of high loaded piezoelectric composites is the key bottleneck restricting the enhancement of the piezoelectric output. In this work, we successfully prepared a novel high loaded polyvinylidene fluoride (PVDF)/barium titanate (BaTiO₃) piezoelectric composite suitable for selective laser sintering (SLS) 3D printing via solid state shear milling (S³M) technology. The 50 wt% BaTiO₃ filling made the most outstanding contribution to the piezoelectric properties of the composites. The 3D printed cymbal parts with a stress amplification effect exhibited outstanding piezoelectric conversion efficiency and responsiveness, whose open circuit voltage and short circuit current could reach 20 V and 1.1 μA, respectively. This work not only contributed a new high loaded piezoelectric composite for SLS processing, but also provided a novel piezoelectric performance enhancement strategy by the construction of 3D structure.

A novel PVDF/BaTiO₃ cymbal part with excellent piezoelectric properties and responsiveness is designed and manufactured by selective laser sintering 3D printing technology.