Advancements in Flexible Nanogenerators: Polyvinylidene Fluoride-Based Nanofiber Utilizing Electrospinning

With the gradual miniaturization of electronic devices and the increasing interest in wearable devices, flexible microelectronics is being actively studied. Owing to the limitations of existing battery systems corresponding to miniaturization, there is a need for flexible alternative power sources. Accordingly, energy harvesting from surrounding environmental systems using fluorinated polymers with piezoelectric properties has received significant attention. Among them, polyvinylidene fluoride (PVDF) and PVDF co-polymers have been researched as representative organo-piezoelectric materials because of their excellent piezoelectric properties, mechanical flexibility, thermal stability, and light weight. Electrospinning is an effective method for fabricating nanofibrous meshes with superior surface-to-volume ratios from polymer solutions. During electrospinning, the polymer solution is subjected to mechanical stretching and in situ poling, corresponding to an external strong electric field. Consequently, the fraction of the piezoelectric β-phase in PVDF can be improved by the electrospinning process, and enhanced harvesting output can be realized. An overview of electrospun piezoelectric fibrous meshes composed of PVDF or PVDF co-polymers to be utilized is presented, and the recent progress in enhancement methods for harvesting output, such as fiber alignment, doping with various nanofillers, and coaxial fibers, is discussed. Additionally, other applications of these meshes as sensors are reviewed.

Recent developments in wearable piezoelectric energy harvesters

Wearable piezoelectric energy harvesters (WPEHs) have gained popularity and made significant development in recent decades. The harvester is logically built by the movement patterns of various portions of the human body to harvest the movement energy and immediately convert it into usable electrical energy. To directly power different microelectronic devices on the human body, a self-powered device that does not require an additional power supply is being created. This Review provides an in-depth review of WPEHs, explaining the fundamental concepts of piezoelectric technology and the materials employed in numerous widely used piezoelectric components. The harvesters are classed according to the movement characteristics of several portions of a person's body, such as pulses, joints, skin, and shoes (feet). Each technique is introduced, followed by extensive analysis. Some harvesters are compared, and the benefits and drawbacks of each technique are discussed. Finally, this Review presents future goals and objectives for WPEH improvement, and it will aid researchers in understanding WPEH to the point of more efficient wireless energy delivery to wearable electronic components.

Stimulation strategies for electrical and magnetic modulation of cells and tissues

Electrical phenomena play an important role in numerous biological processes including cellular signaling, early embryogenesis, tissue repair and remodeling, and growth of organisms. Electrical and magnetic effects have been studied on a variety of stimulation strategies and cell types regarding cellular functions and disease treatments. In this review, we discuss recent advances in using three different stimulation strategies, namely electrical stimulation via conductive and piezoelectric materials as well as magnetic stimulation via magnetic materials, to modulate cell and tissue properties. These three strategies offer distinct stimulation routes given specific material characteristics. This review will evaluate material properties and biological response for these stimulation strategies with respect to their potential applications in neural and musculoskeletal research.

Electrospun PVDF-based piezoelectric nanofibers: materials, structures, and applications

Polyvinylidene fluoride (PVDF) has been considered as a promising piezoelectric material for advanced sensing and energy storage systems because of its high dielectric constant and good electroactive response. Electrospinning is a straightforward, low cost, and scalable technology that can be used to create PVDF-based nanofibers with outstanding piezoelectric characteristics. Herein, we summarize the state-of-the-art progress on the use of filler doping and structural design to enhance the output performance of electrospun PVDF-based piezoelectric fiber films. We divide the fillers into single filler and double fillers and make comments on the effects of various dopant materials on the performance and the underlying mechanism of the PVDF-based piezoelectric fiber film. The effects of highly oriented structures, core-shell structures, and multilayer composite structures on the output properties of PVDF-based piezoelectric nanofibers are discussed in detail. Furthermore, the perspectives and opportunities for PVDF piezoelectric nanofibers in the fields of health care, environmental monitoring, and energy collection are also discussed.

Acoustic Core-Shell Resonance Harvester for Application of Artificial Cochlea Based on the Piezo-Triboelectric Effect

The demand for flexible, efficient, and self-powered cochlear implants applied to remedy sensorineural hearing loss caused by dysfunctional hair cells remains urgent. Herein, we report an acoustic core-shell resonance harvester for the application of artificial cochleae based on the piezo-triboelectric effect. Integrating dispersed BaTiO₃ particles as cores and porous PVDF-TrFE as shells, the acoustic harvest devices with ingenious core-shell structures exhibit outstanding piezo-triboelectric properties (V_oc = 15.24 V, DA_sc = 9.22 mA/m²). The acoustic harvest principle reveals that BaTiO₃ nanocores resonate with sound waves and bounce against porous PVDF-TrFE microshells, thereby generating piezo-triboelectric signals. By experimental measurement and numerical modeling, the vibration process and resonance regulation of acoustic harvest devices were intensively investigated to regulate the influential parameters. Furthermore, the acoustic harvesters exhibit admirable feasibility and sensitivity for sound recording and show potential application for artificial cochlea.

High-Performance Poly(vinylidene difluoride)/Dopamine Core/Shell Piezoelectric Nanofiber and Its Application for Biomedical Sensors

Fabrication of soft piezoelectric nanomaterials is essential for the development of wearable and implantable biomedical devices. However, a big challenge in this soft functional material development is to achieve a high piezoelectric property with long-term stability in a biological environment. Here, a one-step strategy for fabricating core/shell poly(vinylidene difluoride) (PVDF)/dopamine (DA) nanofibers (NFs) with a very high β-phase content and self-aligned polarization is reported. The self-assembled core/shell structure is believed essential for the formation and alignment of β-phase PVDF, where strong intermolecular interaction between the NH2 groups on DA and the CF2 groups on PVDF is responsible for aligning the PVDF chains and promoting β-phase nucleation. The as-received PVDF/DA NFs exhibit significantly enhanced piezoelectric performance and excellent stability and biocompatibility. An all-fiber-based soft sensor is fabricated and tested on human skin and in vivo in mice. The devices show a high sensitivity and accuracy for detecting weak physiological mechanical stimulation from diaphragm motions and blood pulsation. This sensing capability offers great diagnostic potential for the early assessment and prevention of cardiovascular diseases and respiratory disorders.

Design of high conductive and piezoelectric poly (3,4-ethylenedioxythiophene)/chitosan nanofibers for enhancing cellular electrical stimulation

Electroactive nanofibrous scaffold is a vital tool for the study of the various biological research fields from bioelectronics to regenerative medicine, which can provide cell preferable 3D nanofiber architecture and programmed electrical signal. However, intrinsic non-biodegradability is a major problem that hinders its widespread application in the clinic. Herein, we designed, synthesized, and characterized shell/core poly (3,4-ethylenedioxythiophene) (PEDOT)/chitosan (CS) nanofibers by combining the electrospinning and recrystallization processes. Upon incorporating a trace amount of PEDOT (1.0 wt%), the resultant PEDOT/CS nanofibers exhibited low interfacial charge transfer impedance, high electrochemical stability, high electrical conductivity (up to 0.1945 S/cm), and ultrasensitive piezoelectric property (output voltage of 22.5 mV by a human hair prodding). With such unique electrical and conductive properties, PEDOT/CS nanofibers were further applied to brain neuroglioma cells (BNCs) to stimulate their adhesion, proliferation, growth, and development under an optimal external electrical stimulation (ES) and a pulse voltage of 400 mV/cm. ES-responsive PEDOT/CS nanofibers indeed promoted BNCs growth and development as indicated by a large number and density of axons. The synergetic interplay between external ES and piezoelectric voltage demonstrates new PEDOT-based nanofibers as implantable electroactive scaffolds for numerous applications in nerve tissue engineering, human health monitoring, brain mantle information extraction, and degradable microelectronic devices.

Core/Shell Piezoelectric Nanofibers with Spatial Self-Orientated β-Phase Nanocrystals for Real-Time Micropressure Monitoring of Cardiovascular Walls

Implantable pressure biosensors show great potential for assessment and diagnostics of pressure-related diseases. Here, we present a structural design strategy to fabricate core/shell polyvinylidene difluoride (PVDF)/hydroxylamine hydrochloride (HHE) organic piezoelectric nanofibers (OPNs) with well-controlled and self-orientated nanocrystals in the spatial uniaxial orientation (SUO) of β-phase-rich fibers, which significantly enhance piezoelectric performance, fatigue resistance, stability, and biocompatibility. Then PVDF/HHE OPNs soft sensors are developed and used to monitor subtle pressure changes in vivo. Upon implanting into pig, PVDF/HHE OPNs sensors demonstrate their ultrahigh detecting sensitivity and accuracy to capture micropressure changes at the outside of cardiovascular walls, and output piezoelectric signals can real-time and synchronously reflect and distinguish changes of cardiovascular elasticity and occurrence of atrioventricular heart-block and formation of thrombus. Such biological information can provide a diagnostic basis for early assessment and diagnosis of thrombosis and atherosclerosis, especially for postoperative recrudescence of thrombus deep within the human body.