Enhanced piezoresponse of highly aligned electrospun poly(vinylidene fluoride) nanofibers

Advanced Materials for Energy Harvesting and Soft Robotics: Emerging Frontiers to Enhance Piezoelectric Performance and Functionality

Piezoelectric energy harvesting captures mechanical energy from a number of sources, such as vibrations, the movement of objects and bodies, impact events, and fluid flow to generate electric power. Such power can be employed to support wireless communication, electronic components, ocean monitoring, tissue engineering, and biomedical devices. A variety of self-powered piezoelectric sensors, transducers, and actuators have been produced for these applications, however approaches to enhance the piezoelectric properties of materials to increase device performance remain a challenging frontier of materials research. In this regard, the intrinsic polarization and properties of materials can be designed or deliberately engineered to enhance the piezo-generated power. This review provides insights into the mechanisms of piezoelectricity in advanced materials, including perovskites, active polymers, and natural biomaterials, with a focus on the chemical and physical strategies employed to enhance the piezo-response and facilitate their integration into complex electronic systems. Applications in energy harvesting and soft robotics are overviewed by highlighting the primary performance figures of merits, the actuation mechanisms, and relevant applications. Key breakthroughs and valuable strategies to further improve both materials and device performance are discussed, together with a critical assessment of the requirements of next-generation piezoelectric systems, and future scientific and technological solutions.

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.

A Self-Powered Hydrogel/Nanogenerator System Accelerates Wound Healing by Electricity-Triggered On-Demand Phosphatase and Tensin Homologue (PTEN) Inhibition

Electrical stimulation therapy (EST) has been established as an effective strategy to accelerate wound healing by stimulating cell proliferation and migration, ultimately promoting re-epithelialization and vascularization, two key processes that significantly influence the rate of wound healing. Phosphatase and tensin homologue (PTEN), a widely expressed protein in somatic cells, works as a "brake" regulating cell differentiation, proliferation, and migration. Given that this "brake" also works in cell electrical responses, there is a hypothesis that PTEN inhibition may amplify the efficacy of EST in wound treatment. However, long-term inhibition of PTEN may result in DNA damage and reduce DNA repair, which poses a significant challenge to the safe use of PTEN inhibitors. To address this issue, we developed a system that combines PTEN inhibitor loaded electro-responsive hydrogel (BPV@PCP) with a wearable direct current pulse piezoelectric nanogenerator (PENG). The PENG converts the rat's motions into electric fields that synchronously charge the wound edge tissue and BPV@PCP. Electric field intensity was lower when the rat was quiet or anesthetized, which is insufficient to trigger an effective PTEN inhibitor release. However, when the rat was in action, the electric field intensity exceeded 625 mV/mm, resulting in a rapid drug release. This on-demand PTEN inhibition accelerated wound healing by amplifying cell electric responsiveness while avoiding negative effects associated with continuous overinhibition of PTEN. Notably, this system improves vascularization not only by improving endothelial cell electric responsiveness but also through the paracrine pathway, in which electrical stimulation and PTEN inhibition synergically promote VEGF secretion.

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.

Improved Piezoelectric Performance of Electrospun PVDF Nanofibers with Conductive Paint Coated Electrode

Fabrication of Nanogenerators (NGs) using Electrospun polyvinylidene fluoride (PVDF) nanofibers for sensing and energy harvesting applications is a trending research due to its flexibility, biocompatibility, low-cost, etc. Different electrode materials, polymer composites had been proposed to increase the energy output. However, the contact area between the electrode material and nanofiber mat which helps to conduct more piezoelectric charges to the electrode surface are still unexplored especially at nanoscale level. In this paper, authors have proposed the use of low-cost carbon conductive paint to increase the contact area between the electrode and nanofiber mat. The electrode material is coated with conductive paint and the NG was fabricated with that electrode to compare the performances with conventional NG. Piezoelectric performance of the proposed NG has increased substantially as it generates an open circuit voltage [Formula: see text]) of 4.5[Formula: see text]V and short circuit current [Formula: see text]) of 25[Formula: see text]nA, whereas the conventional NG can only produce 1.6 [Formula: see text]) and 1.5[Formula: see text]nA [Formula: see text]). A drop test experiment was conducted, and the device consistency was verified experimentally.

Phase Inversion in PVDF Films with Enhanced Piezoresponse Through Spin-Coating and Quenching

In the present work, poly(vinylidene fluoride) (PVDF) films were produced by spin-coating, and applying different conditions of quenching, in order to investigate the dominant mechanism of the β-phase formation. The influence of the polymer/solvent mass ratio of the solution, the rotational speed of the spin-coater and the crystallization temperature of the film on both the β-phase content and the piezoelectric coefficient (d₃₃) were investigated. This study demonstrates that the highest values of d₃₃ are obtained when thinner films, produced with a lower concentration of polymer in the solvent (i.e., 20 wt.%), go through quenching in water, at room temperature. Whereas, in the case of higher polymer concentration (i.e., 30 wt.%), the best value of d₃₃ (~30 pm/V) was obtained through quenching in liquid nitrogen, at the temperature of 77 K. We believe that in the former case, phase inversion is mainly originated by electrostatic interaction of PVDF with the polar molecules of water, due to the low viscosity of the polymer solution. On the contrary, in the latter case, due to higher viscosity of the solution, mechanical stretching induced on the polymer during spin-coating deposition is the main factor inducing self-alignment of the β-phase. These findings open up a new way to realize highly efficient devices for energy harvesting and wearable sensors.

Self-Powered Well-Aligned P(VDF-TrFE) Piezoelectric Nanofiber Nanogenerator for Modulating an Exact Electrical Stimulation and Enhancing the Proliferation of Preosteoblasts

Electric potential plays an indispensable role in tissue engineering and wound healing. Piezoelectric nanogenerators based on direct piezoelectric effects can be self-powered energy sources for electrical stimulation and have attracted extensive attention. However, the accuracy of piezoelectric stimuli on piezoelectric polymers membranes in vitro during the dynamic condition is rarely studied. Here, a self-powered tunable electrical stimulation system for assisting the proliferation of preosteoblasts was achieved by well-aligned P(VDF-TrFE) piezoelectric nanofiber membrane (NFM) both as a nanogenerator (NG) and as a scaffold. The effects of electrospinning and different post-treatments (annealing and poling) on the surface wettability, piezoelectric β phase, ferroelectric properties, and sensing performance of NFMs were evaluated here. The polarized P(VDF-TrFE) NFM offered an enhanced piezoelectric value (d31 of 22.88 pC/N) versus pristine P(VDF-TrFE) NFM (d31 of 0.03 pC/N) and exhibited good sensing performance. The maximum voltage and current output of the P(VDF-TrFE) piezoelectric nanofiber NGs reached -1.7 V and 41.5 nA, respectively. An accurate electrical response was obtained in real time under dynamic mechanical stimulation by immobilizing the NGs on the flexible bottom of the culture plate, thereby restoring the real scene of providing electrical stimulation to the cells in vitro. In addition, we simulated the interaction between the piezoelectric nanofiber NG and cells through an equivalent circuit model. To verify the feasibility of P(VDF-TrFE) nanofiber NGs as an exact electrical stimulation, the effects of different outputs of P(VDF-TrFE) nanofiber NGs on cell proliferation in vitro were compared. The study realized a significant enhancement of preosteoblasts proliferation. This work demonstrated the customizability of P(VDF-TrFE) piezoelectric nanofiber NG for self-powered electrical stimulation system application and suggested its significant potential application for tissue repair and regeneration.

Stretchable and colorless freestanding microwire arrays for transparent solar cells with flexibility

Transparent solar cells (TSCs) are emerging devices that combine the advantages of visible transparency and light-to-electricity conversion. Currently, existing TSCs are based predominantly on organics, dyes, and perovskites; however, the rigidity and color-tinted transparent nature of those devices strongly limit the utility of the resulting TSCs for real-world applications. Here, we demonstrate a flexible, color-neutral, and high-efficiency TSC based on a freestanding form of n-silicon microwires (SiMWs). Flat-tip SiMWs with controllable spacing are fabricated via deep-reactive ion etching and embedded in a freestanding transparent polymer matrix. The light transmittance can be tuned from ~10 to 55% by adjusting the spacing between the microwires. For TSCs, a heterojunction is formed with a p-type polymer in the top portion of the n-type flat-tip SiMWs. Ohmic contact with an indium-doped ZnO film occurs at the bottom, and the side surface has an Al₂O₃ passivation layer. Furthermore, slanted-tip SiMWs are developed by a novel solvent-assisted wet etching method to manipulate light absorption. Finite-difference time-domain simulation revealed that the reflected light from slanted-tip SiMWs helps light-matter interactions in adjacent microwires. The TSC based on the slanted-tip SiMWs demonstrates 8% efficiency at a visible transparency of 10% with flexibility. This efficiency is the highest among Si-based TSCs and comparable with that of state-of-the-art neutral-color TSCs based on organic-inorganic hybrid perovskite and organics. Moreover, unlike others, the stretchable and transparent platform in this study is promising for future TSCs.

Piezoelectric Effect and Electroactive Phase Nucleation in Self-Standing Films of Unpoled PVDF Nanocomposite Films

Novel polymer-based piezoelectric nanocomposites with enhanced electromechanical properties open new opportunities for the development of wearable energy harvesters and sensors. This paper investigates how the dissolution of different types of hexahydrate metal salts affects β-phase content and piezoelectric response (d₃₃) at nano- and macroscales of polyvinylidene fluoride (PVDF) nanocomposite films. The strongest enhancement of the piezoresponse is observed in PVDF nanocomposites processed with Mg(NO₃)₂⋅6H₂O. The increased piezoresponse is attributed to the synergistic effect of the dipole moment associated with the nucleation of the electroactive phase and with the electrostatic interaction between the CF₂ group of PVDF and the dissolved salt through hydrogen bonding. The combination of nanofillers like graphene nanoplatelets or zinc oxide nanorods with the hexahydrate salt dissolution in PVDF results in a dramatic reduction of d₃₃, because the nanofiller assumes a competitive role with respect to H-bond formation between PVDF and the dissolved metal salt. The measured peak value of d₃₃ reaches the local value of 13.49 pm/V, with an average of 8.88 pm/V over an area of 1 cm². The proposed selection of metal salt enables low-cost production of piezoelectric PVDF nanocomposite films, without electrical poling or mechanical stretching, offering new opportunities for the development of devices for energy harvesting and wearable sensors.

Morphology-Controlled Aluminum-Doped Zinc Oxide Nanofibers for Highly Sensitive NO2 Sensors with Full Recovery at Room Temperature

Room-temperature (RT) gas sensitivity of morphology-controlled free-standing hollow aluminum-doped zinc oxide (AZO) nanofibers for NO2 gas sensors is presented. The free-standing hollow nanofibers are fabricated using a polyvinylpyrrolidone fiber template electrospun on a copper electrode frame followed by radio-frequency sputtering of an AZO thin overlayer and heat treatment at 400 °C to burn off the polymer template. The thickness of the AZO layer is controlled by the deposition time. The gas sensor based on the hollow nanofibers demonstrates fully recoverable n-type RT sensing of low concentrations of NO2 (0.5 ppm). A gas sensor fabricated with Al2O3-filled AZO nanofibers exhibits no gas sensitivity below 75 °C. The gas sensitivity of a sensor is determined by the density of molecules above the minimum energy for adsorption, collision frequency of gas molecules with the surface, and available adsorption sites. Based on finite-difference time-domain simulations, the RT sensitivity of hollow nanofiber sensors is ascribed to the ten times higher collision frequency of NO2 molecules confined inside the fiber compared to the outer surface, as well as twice the surface area of hollow nanofibers compared to the filled ones. This approach might lead to the realization of RT sensitive gas sensors with 1D nanostructures.

Large-Scale Direct-Writing of Aligned Nanofibers for Flexible Electronics

Nanofibers/nanowires usually exhibit exceptionally low flexural rigidities and remarkable tolerance against mechanical bending, showing superior advantages in flexible electronics applications. Electrospinning is regarded as a powerful process for this 1D nanostructure; however, it can only be able to produce chaotic fibers that are incompatible with the well-patterned microstructures in flexible electronics. Electro-hydrodynamic (EHD) direct-writing technology enables large-scale deposition of highly aligned nanofibers in an additive, noncontact, real-time adjustment, and individual control manner on rigid or flexible, planar or curved substrates, making it rather attractive in the fabrication of flexible electronics. In this Review, the ground-breaking research progress in the field of EHD direct-writing technology is summarized, including a brief chronology of EHD direct-writing techniques, basic principles and alignment strategies, and applications in flexible electronics. Finally, future prospects are suggested to advance flexible electronics based on orderly arranged EHD direct-written fibers. This technology overcomes the limitations of the resolution of fabrication and viscosity of ink of conventional inkjet printing, and represents major advances in manufacturing of flexible electronics.