Recent Structure Development of Poly(vinylidene fluoride)-Based Piezoelectric Nanogenerator for Self-Powered Sensor

Smart and Multifunctional Materials Based on Electroactive Poly(vinylidene fluoride): Recent Advances and Opportunities in Sensors, Actuators, Energy, Environmental, and Biomedical Applications

From scientific and technological points of view, poly(vinylidene fluoride), PVDF, is one of the most exciting polymers due to its overall physicochemical characteristics. This polymer can crystalize into five crystalline phases and can be processed in the form of films, fibers, membranes, and specific microstructures, being the physical properties controllable over a wide range through appropriate chemical modifications. Moreover, PVDF-based materials are characterized by excellent chemical, mechanical, thermal, and radiation resistance, and for their outstanding electroactive properties, including high dielectric, piezoelectric, pyroelectric, and ferroelectric response, being the best among polymer systems and thus noteworthy for an increasing number of technologies. This review summarizes and critically discusses the latest advances in PVDF and its copolymers, composites, and blends, including their main characteristics and processability, together with their tailorability and implementation in areas including sensors, actuators, energy harvesting and storage devices, environmental membranes, microfluidic, tissue engineering, and antimicrobial applications. The main conclusions, challenges and future trends concerning materials and application areas are also presented.

Comparative Studies on Electrodes for Rumen Bacteria Microbial Fuel Cells

Microbial fuel cells (MFCs) using rumen bacteria have been proposed as a power source for running devices inside cattle. In this study, we explored the key parameters of the conventional bamboo charcoal electrode in an attempt to improve the amount of electrical power generated by the microbial fuel cell. We evaluated the effects of the electrode's surface area, thickness, and rumen content on power generation and determined that only the electrode's surface area affects power generation levels. Furthermore, our observations and bacterial count on the electrode revealed that rumen bacteria concentrated on the surface of the bamboo charcoal electrode and did not penetrate the interior, explaining why only the electrode's surface area affected power generation levels. A Copper (Cu) plate and Cu paper electrodes were also used to evaluate the effect of different electrodes on measuring the rumen bacteria MFC's power potential, which had a temporarily higher maximum power point (MPP) compared to the bamboo charcoal electrode. However, the open circuit voltage and MPP decreased significantly over time due to the corrosion of the Cu electrodes. The MPP for the Cu plate electrode was 775 mW/m² and the MPP for the Cu paper electrode was 1240 mW/m², while the MPP for bamboo charcoal electrodes was only 18.7 mW/m². In the future, rumen bacteria MFCs are expected to be used as the power supply of rumen sensors.

Piezoelectric Enhancement of Piezoceramic Nanoparticle-Doped PVDF/PCL Core-Sheath Fibers

Electrospinning is considered to be an efficient method to prepare piezoelectric thin films because of its ability to transform the phase of the polymers. A core-sheath structure can endow fibers with more functions and properties. In this study, fibers with a core-sheath structure were prepared using polyvinylidene fluoride (PVDF) included with nanoparticles (NPs) as the shell layer and polycaprolactone (PCL) as the core layer. Their mechanical and piezoelectric properties were studied in detail. During the course of the electrospinning process, PVDF was demonstrated to increase the amount of its polar phase, with the help of nanoparticles acting as a nucleating agent to facilitate the change. PCL was chosen as a core material because of its good mechanical properties and its compatibility with PVDF. Transmission electron microscope (TEM) assessments revealed that the fibers have a core-sheath structure, and shell layers were loaded with nanoparticles. Mechanical testing showed that the core layer can significantly improve mechanical properties. The XRD patterns of the core-sheath structure fibers indicated the β phase domain the main component. Piezoelectric testing showed that the doped nanoparticles were able to enhance piezoelectric performances. The increases of mechanical and piezoelectric properties of core-sheath structure fibers provide a feasible application for wearable electronics, which require flexibility and good mechanical properties.

Facile One-Step Synthesis of PVDF Bead-on-String Fibers by Pressurized Gyration for Reusable Face Masks

Single-use face masks pose a threat to the environment and are not cost-effective, which prompts the need for developing reusable masks. In this study, pressurized gyration (PG) successfully produced bead-on-string polyvinylidene fluoride (PVDF) fibers with fiber diameters ranging from 2.3 μm to 26.1 μm, and bead diameters ranging from 60.9 μm to 88.5 μm by changing the solution parameters. The effect of the solution parameters on the crystalline phase was studied by Fourier-transform infrared spectroscopy (FT-IR), where the β-phase contents of PG PVDF fibers reached over 75%. The fiber morphology and β-phase contents of PG PVDF fibers indicated the potential mechanical and electrostatic filtration efficiency of PG PVDF fibers, respectively. Additionally, the hydrophobicity was investigated by static water contact angle tests, and the PVDF fibers showed superior hydrophobicity properties (all samples above 125°) over commercial polypropylene (PP) single-use masks (approximately 107°). This study supports the notion that the PG PVDF fiber mats are a promising candidate for future reusable face masks.

Piezoelectric Fibers: Processing and Challenges

Integration of piezoelectric materials in composite and textile structures is promising for creating smart textiles with sensing or energy harvesting functionalities. The most direct integration that combines wearability, comfort, and piezoelectric efficiency consists of using fibers made of piezoelectric materials. The latter include inorganic ceramics or organic polymers. Ceramics have outstanding piezoelectric properties but can not be easily melted or solubilized in a solvent to be processed in the form of fibers. They have to be spun from precursor materials and thermally treated afterward for densification and sintering. These delicate processes have to be carefully controlled to optimize the piezoelectric properties of the fibers. On the other hand, organic piezoelectric polymers, such as polyvinylidene fluoride (PVDF), can be spun by more conventional textile fibers technologies. In addition to enjoy an easier manufacturing, organic piezoelectric fibers display flexibility that facilitates their integration and use in smart textiles. However, organic fibers suffer from a low piezoelectric efficiency. This reviews looks at the processing techniques and their specific limitations and advantages to realize single-component or coaxial piezofibers. Fundamental challenges related to the use of composite fibers are discussed. The latter include challenges for poling and electrically wiring the fibers to collect charges under operation or to apply electrical fields. The electromechanical properties of these fibers processed by different manufacturing techniques are compared. Recent studies of structures used to integrate such fibers in textiles and composites with conventional techniques and their potential applications are discussed.

Ferroelectric Polymer Nanofibers Reminiscent of Morphotropic Phase Boundary Behavior for Improved Piezoelectric Energy Harvesting

Ferroelectric and piezoelectric polymers have attracted great attention from many research and engineering fields due to its mechanical robustness and flexibility as well as cost-effectiveness and easy processibility. Nevertheless, the electrical performance of piezoelectric polymers is very hard to reach that of piezoelectric ceramics basically and physically, even in the case of the representative ferroelectric polymer, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). Very recently, the concept for the morphotropic phase boundary (MPB), which has been exclusive in the field of high-performance piezoelectric ceramics, has been surprisingly confirmed in P(VDF-TrFE) piezoelectric copolymers by the groups. This study demonstrates the exceptional behaviors reminiscent of MPB and relaxor ferroelectrics in the feature of widely utilized electrospun P(VDF-TrFE) nanofibers. Consequently, an energy harvesting device that exceeds the performance limitation of the existing P(VDF-TrFE) materials is developed. Even the unpoled MPB-based P(VDF-TrFE) nanofibers show higher output than the electrically poled normal P(VDF-TrFE) nanofibers. This study is the first step toward the manufacture of a new generation of piezoelectric polymers with practical applications.