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

Anisotropic Poly(vinylidene fluoride-co-trifluoroethylene)/MXene Aerogel-Based Piezoelectric Nanogenerator for Efficient Kinetic Energy Harvesting and Self-Powered Force Sensing Applications

Lightweight flexible piezoelectric devices have garnered significant interest over the past few decades due to their applications as energy harvesters and wearable sensors. Among different piezoelectrically active polymers, poly(vinylidene fluoride) and its copolymers have attracted considerable attention for energy conversion due to their high flexibility, thermal stability, and biocompatibility. However, the orientation of polymer chains for self-poling under mild conditions is still a challenging task. Herein, anisotropic poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE)/MXene aerogel-based piezoelectric generators with highly oriented MXene fillers are fabricated. The unidirectional freezing of a hybrid solution facilitates the strain-induced alignment of MXene nanosheets and polymer chains along the solvent crystal growth direction due to the robust interactions between the MXene nanosheets (O-H/F groups) and PVDF-TrFE chains (F-C/C-H groups). Consequently, this process fosters the development of abundant electroactive β crystals with preferred alignment characteristics, leading to the formation of intrinsic self-oriented dipoles within the PVDF-TrFE aerogel. As a result, the piezoelectric properties of PVDF-TrFE are fully harnessed without any complex poling process, resulting in an open-circuit voltage of around 40 V with MXene loading of 3 wt % in anisotropic aerogel, which is 2-fold higher than that of the corresponding isotropic aerogel where the MXene nanosheets and polymer chains are randomly aligned. Furthermore, the developed piezoelectric nanogenerator was demonstrated as a tactile sensor which showed a high sensitivity of 9.6 V/N for lower forces (less than 2 N) and a sensitivity of 1.3 V/N in the higher force regime (2 N < force < 10 N). The strategy adopted here not only provides the enhancement of the piezoelectric crystalline form for self-poling but also paves an avenue toward developing self-powered energy harvesters using piezoelectric polymers.

Waste cotton textile-derived cellulose composite porous film with enhanced piezoelectric performance for energy harvesting and self-powered sensing

Integrating flexible piezoelectric nanogenerators (PENGs) into wearable and portable electronics offers promising prospects for motion monitoring. However, it remains a significant challenge to develop environmentally friendly PENGs using biodegradable and cost-effective natural polymers for mechanical energy harvesting and self-powered sensing. Herein, reduced graphene oxide (rGO) and barium titanate (BTO) were introduced into regenerated cellulose pulp to fabricate a composite porous film-based PENG. The incorporation of rGO not only increased the electrical conductivity of the porous film but also enhanced the dispersibility of BTO. Moreover, the unique pore structure of the composite porous film improved the polarization effect of the air inside the pores, thereby greatly boosting the overall piezoelectric performance. The piezoelectric coefficient of the resulting composite porous film reaches up to 41.5 pC·N-1, which is comparable to or higher than those reported in similar studies. Consequently, the PENG assembled from this cellulose/rGO/BTO composite porous film (CGB-PENG) achieved an output voltage of 47 V, a current of 4.6 μA, and a power density of 30 μW·cm-2, approximately three times the output voltage and ten times the power density of similar studies. This work presents a feasible approach for the fabrication of high-performance cellulose-based PENGs derived from recycled waste cotton textiles.

Flexible piezoelectric materials and strain sensors for wearable electronics and artificial intelligence applications

With the rapid development of artificial intelligence, the applications of flexible piezoelectric sensors in health monitoring and human-machine interaction have attracted increasing attention. Recent advances in flexible materials and fabrication technologies have promoted practical applications of wearable devices, enabling their assembly in various forms such as ultra-thin films, electronic skins and electronic tattoos. These piezoelectric sensors meet the requirements of high integration, miniaturization and low power consumption, while simultaneously maintaining their unique sensing performance advantages. This review provides a comprehensive overview of cutting-edge research studies on enhanced wearable piezoelectric sensors. Promising piezoelectric polymer materials are highlighted, including polyvinylidene fluoride and conductive hydrogels. Material engineering strategies for improving sensitivity, cycle life, biocompatibility, and processability are summarized and discussed focusing on filler doping, fabrication techniques optimization, and microstructure engineering. Additionally, this review presents representative application cases of smart piezoelectric sensors in health monitoring and human-machine interaction. Finally, critical challenges and promising principles concerning advanced manufacture, biological safety and function integration are discussed to shed light on future directions in the field of piezoelectrics.

Ingenious Structure Engineering to Enhance Piezoelectricity in Poly(vinylidene fluoride) for Biomedical Applications

The future development of wearable/implantable sensing and medical devices relies on substrates with excellent flexibility, stability, biocompatibility, and self-powered capabilities. Enhancing the energy efficiency and convenience is crucial, and converting external mechanical energy into electrical energy is a promising strategy for long-term advancement. Poly(vinylidene fluoride) (PVDF), known for its piezoelectricity, is an outstanding representative of an electroactive polymer. Ingeniously designed PVDF-based polymers have been fabricated as piezoelectric devices for various applications. Notably, the piezoelectric performance of PVDF-based platforms is determined by their structural characteristics at different scales. This Review highlights how researchers can strategically engineer structures on microscopic, mesoscopic, and macroscopic scales. We discuss advanced research on PVDF-based piezoelectric platforms with diverse structural designs in biomedical sensing, disease diagnosis, and treatment. Ultimately, we try to give perspectives for future development trends of PVDF-based piezoelectric platforms in biomedicine, providing valuable insights for further research.

The Latest Advances in Ink-Based Nanogenerators: From Materials to Applications

Nanogenerators possess the capability to harvest faint energy from the environment. Among them, thermoelectric (TE), triboelectric, piezoelectric (PE), and moisture-enabled nanogenerators represent promising approaches to micro-nano energy collection. These nanogenerators have seen considerable progress in material optimization and structural design. Printing technology has facilitated the large-scale manufacturing of nanogenerators. Although inks can be compatible with most traditional functional materials, this inevitably leads to a decrease in the electrical performance of the materials, necessitating control over the rheological properties of the inks. Furthermore, printing technology offers increased structural design flexibility. This review provides a comprehensive framework for ink-based nanogenerators, encompassing ink material optimization and device structural design, including improvements in ink performance, control of rheological properties, and efficient energy harvesting structures. Additionally, it highlights ink-based nanogenerators that incorporate textile technology and hybrid energy technologies, reviewing their latest advancements in energy collection and self-powered sensing. The discussion also addresses the main challenges faced and future directions for development.

Self-Powered Pressure-Temperature Bimodal Sensing Based on the Piezo-Pyroelectric Effect for Robotic Perception

Multifunctional sensors have played a crucial role in constructing high-integration electronic networks. Most of the current multifunctional sensors rely on multiple materials to simultaneously detect different physical stimuli. Here, we demonstrate the large piezo-pyroelectric effect in ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals for simultaneous pressure and temperature sensing. The outstanding piezoelectric and pyroelectric properties of PMN-PT result in rapid response speed and high sensitivity, with values of 46 ms and 28.4 nA kPa-1 for pressure sensing, and 1.98 s and 94.66 nC °C-1 for temperature detection, respectively. By leveraging the distinct differences in the response speed of piezoelectric and pyroelectric responses, the piezo-pyroelectric effect of PMN-PT can effectively detect pressure and temperature from mixed-force thermal stimuli, which enables a robotic hand for stimuli classification. With appealing multifunctionality, fast speed, high sensitivity, and compact structure, the proposed self-powered bimodal sensor therefore holds significant potential for high-performance artificial perception.

A Robust Triboelectric Impact Sensor with Carbon Dioxide Precursor-Based Calcium Carbonate Layer for Slap Match Application

As an urgent international challenge, the sudden change in climate due to global warming needs to be addressed in the near future. This can be achieved through a reduction in fossil fuel utilization and through carbon sequestration, which reduces the concentration of CO2 in the atmosphere. In this study, a self-sustainable impact sensor is proposed through implementing a triboelectric nanogenerator with a CaCO3 contact layer fabricated via a CO2 absorption method. The triboelectric polarity of CaCO3 with the location between the polyimide and the paper and the effects of varying the crystal structure are investigated first. The impact sensing characteristics are then confirmed at various input frequencies and under applied forces. Further, the high mechanical strength and strong adherence of CaCO3 on the surface of the device are demonstrated through enhanced durability compared to the unmodified device. For the intended application, the as-fabricated sensor is used to detect the turning state of the paper Ddakji in a slap match game using a supervised learning algorithm based on a support vector machine presenting a high classification accuracy of 95.8%. The robust CaCO3-based triboelectric device can provide an eco-friendly advantage due to its self-powered characteristics for impact sensing and carbon sequestration.

Application of piezoelectric materials in the field of bone: a bibliometric analysis

In the past 4 decades, many articles have reported on the effects of the piezoelectric effect on bone formation and the research progress of piezoelectric biomaterials in orthopedics. The purpose of this study is to comprehensively evaluate all existing research and latest developments in the field of bone piezoelectricity, and to explore potential research directions in this area. To assess the overall trend in this field over the past 40 years, this study comprehensively collected literature reviews in this field using a literature retrieval program, applied bibliometric methods and visual analysis using CiteSpace and R language, and identified and investigated publications based on publication year (1984-2022), type of literature, language, country, institution, author, journal, keywords, and citation counts. The results show that the most productive countries in this field are China, the United States, and Italy. The journal with the most publications in the field of bone piezoelectricity is the International Journal of Oral & Maxillofacial Implants, followed by Implant Dentistry. The most productive authors are Lanceros-Méndez S, followed by Sohn D.S. Further research on the results obtained leads to the conclusion that the research direction of this field mainly includes piezoelectric surgery, piezoelectric bone tissue engineering scaffold, manufacturing artificial cochleae for hearing loss patients, among which the piezoelectric bone tissue engineering scaffold is the main research direction in this field. The piezoelectric materials involved in this direction mainly include polyhydroxybutyrate valerate, PVDF, and BaTiO3.

Applications and Challenges of Supercritical Foaming Technology

With economic development, environmental problems are becoming more and more prominent, and achieving green chemistry is an urgent task nowadays, which creates an opportunity for the development of supercritical foaming technology. The foaming agents used in supercritical foaming technology are usually supercritical carbon dioxide (ScCO₂) and supercritical nitrogen (ScN₂), both of which are used without environmental burden. This technology can reduce the environmental impact of polymer foam production. Although supercritical foaming technology is already in production in some fields, it has not been applied on a large scale. Here, we present a detailed analysis of the types of foaming agents currently used in supercritical foaming technology and their applications in various fields, summarizing the technological improvements that have been made to the technology. However, we have found that today's supercritical technologies still need to address some additional challenges to achieve large-scale production.

A Review on Mechanochemistry: Approaching Advanced Energy Materials with Greener Force

Mechanochemistry with solvent-free and environmentally friendly characteristics is one of the most promising alternatives to traditional liquid-phase-based reactions, demonstrating epoch-making significance in the realization of different types of chemistry. Mechanochemistry utilizes mechanical energy to promote physical and chemical transformations to design complex molecules and nanostructured materials, encourage dispersion and recombination of multiphase components, and accelerate reaction rates and efficiencies via highly reactive surfaces. In particular, mechanochemistry deserves special attention because it is capable of endowing energy materials with unique characteristics and properties. Herein, the latest advances and progress in mechanochemistry for the preparation and modification of energy materials are reviewed. An outline of the basic knowledge, methods, and characteristics of different mechanochemical strategies is presented, distinguishing this review from most mechanochemistry reviews that only focus on ball-milling. Next, this outline is followed by a detailed and insightful discussion of mechanochemistry-involved energy conversion and storage applications. The discussion comprehensively covers aspects of energy transformations from mechanical/optical/chemical energy to electrical energy. Finally, next-generation advanced energy materials are proposed. This review is intended to bring mechanochemistry to the frontline and guide this burgeoning field of interdisciplinary research for developing advanced energy materials with greener mechanical force.

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.

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.

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.

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.