Enhanced Piezoelectric Performance of Electrospun PVDF-TrFE by Polydopamine-Assisted Attachment of ZnO Nanowires for Impact Force Sensing

In this work, piezoelectric PVDF-TrFE electrospun fibers (EFs) were fabricated using a high-throughput nozzle-free electrospinning process. Zinc oxide (ZnO) nanoparticles were robustly anchored to the PVDF-TrFE EFs, assisted by a self-polymerized polydopamine (PDA) layer, and subsequently grown into ZnO nanowires (NWs) using a low-temperature hydrothermal growth method. The EF mats were investigated for active impact force transduction and the piezoelectric voltage outputs of different combinations of PVDF-TrFE and ZnO nanomaterials were compared using two different impact force testing setups. The horizontal impact force test saw an increase in force sensitivity by a factor of 2.5 for the nanowires compared to the unmodified PVDF-TrFE EFs. Similarly, vertical drop impact testing demonstrated a 5.8-fold increase in sensitivity with a linear response (R² = 0.986) for a large range of impact forces up to 970 N. The EFs were also tested as a wearable impact force sensor to quantify soccer ball heading force, which was measured as 291.3 ± 51.0 N for a vertical ball speed of 7.8 ± 1.5 ms⁻¹¹ with an 8.2% average error compared to theoretical force values. It is believed the enhanced piezoelectric performance of these materials could provide a useful platform for wearable sensing and energy harvesting.     

Advances in Electrospun Fiber-Based Flexible Nanogenerators for Wearable Applications

In today's digital age, the need and interest in personal and portable electronics shows a dramatic growth trend in daily life parallel to the developments in sensors technologies and the internet. Wearable electronics that can be attached to clothing, accessories, and the human body are one of the most promising subfields. The energy requirement for the devices considering the reduction in device sizes and the necessity of being flexible and light, the existing batteries are insufficient and nanogenerators have been recognized a suitable energy source in the last decade. The mechanical energy created by the daily activities of the human body is an accessible and natural energy source for nanogenerators. Fiber-structured functional materials contribute to the increase in energy efficiency due to their effective surface to volume ratio while providing the necessary compatibility and comfort for the movements in daily life with its flexibility and lightness. Among the potential solutions, electrospinning stands out as a promising technique that can meet these requirements, allowing for simple, versatile, and continuous fabrication. Herein, wearable electronics and their future potential, electrospinning, and its place in energy applications are overviewed. Moreover, piezoelectric, triboelectric, and hybrid nanogenerators fabricated or associated with electrospun fibrous materials are presented.     

Piezoelectric and Electrostatic Properties of Electrospun PVDF‐TrFE Nanofibers and their Role in Electromechanical Transduction in Nanogenerators and Strain Sensors

Piezo‐ and ferroelectric nanofibers of the polymer poly(vinylidenefluoride) (PVDF) have been widely employed in strain and pressure sensors as well as nanogenerators for energy harvesting. Despite this interest, the mechanism of electromechanical transduction is under debate and a deeper knowledge about relevant piezoelectric or electrostatic properties of nanofibers is crucial to optimize transduction efficiency. Here poly(vinylidenefluoride‐trifluoroethylene) nanofibers at different electrospinning conditions are prepared. Macroscopic electromechanical response of fiber mats with microscopic analysis of single nanofibers performed by piezoelectric and electrostatic force microscopy are compared. The results show that electrospinning favors the formation of the piezoelectric β‐phase in the polymer and leads directly to piezoelectric properties that are comparable to annealed thin films. However, during electrospinning the electric field is not strong enough to induce direct ferroelectric domain polarization. Instead accumulation of triboelectric surface charges and trapped space charge is observed in the polymer that explain the electret like macroscopic electromechanical response.     

Effect of Geometrical Parameters on Piezoresponse of Nanofibrous Wearable Piezoelectric Nanofabrics Under Low Impact Pressure

Piezoelectric polymers are potential energizers for wearable electronics due to the possibility of developing their yarns for various textile products. The present study is aimed at understanding the effect of geometrical parameters, viz., yarn linear density (measured as Tex), twist per meter (TPM), plying, as well as weft and warp density on the piezoelectric voltage of electrospun yarns of polyvinylidene fluoride (PVDF) polymer and poly[(vinylidene fluoride)-co-trifluoroethylene] [P(VDF-TrFE)] copolymer. Yarns are developed by twisting and plying electrospun nanofibers and their mechanical and piezoelectric properties are systematically investigated. Relative advantages of the yarns of the copolymer with respect to PVDF in both aligned and random fiber geometries are evaluated. The studies show that piezoresponse of the woven nanogenerators can be enhanced by decreasing Tex and increasing the TPM, the plying number, and the fabric density. A record piezovoltage of ≈2.5 V is achieved through this work. The results of the present work can be used for the fabrication of flexible and breathable nanogenerators or sensors.     

Highly Stretchable and Sensitive SBS/Graphene Composite Fiber for Strain Sensors

Flexible strain sensors have attracted tremendous interests due to the emergence of intelligent wearable technology. Electrically conductive fibers are desirable candidates for flexible strain sensors, but up til now, there still exist enormous challenges to obtain conductive fibers exhibiting simultaneously high stretchability and high strain sensitivity. This paper introduces a poly (styrene‐butadiene‐styrene) (SBS)/graphene (Gr) composite fiber‐based flexible strain sensor fabricated by a facile and highly scalable wet spinning method. The results demonstrate that the graphene content has significant influence on the morphology, mechanical properties, and electromechanical properties of the composite fibers. The fibers with 5 wt% graphene have a wide response range of up to 100% strain, a high electrical sensitivity with the gauge factor of 10083.98 at 100% strain, and meanwhile, a high level of stability for 2100 stretching–releasing cycles under an applied strain of 20%. Furthermore, the SBS‐5%Gr composite fibers display excellent sensing performance in detecting human upper limb movements at different joints including hand joints, wrist joints, elbow joints, and shoulder joints.     

A Skin-Inspired Triboelectric Nanogenerator with an Interpenetrating Structure for Motion Sensing and Energy Harvesting

Rapid advancements in wearable electronics impose the challenge on power supply devices. Herein, a flexible single-electrode triboelectric nanogenerator (SE-TENG) that enables both human motion sensing and biomechanical energy harvesting is reported. The SE-TENG is fabricated by interpenetrating Ag-coated polyethylene terephthalate (PET) nanofibers within a polydimethylsiloxane (PDMS) elastomer. The Ag coating and PDMS are performed as the electrode and dielectric material for the SE-TENG, respectively. The Ag-coated PET nanofibers enlarge the electrode surface area, which is beneficial to increase sensing sensitivity. The flexible SE-TENG sensor shows the capability of outputting alternating electrical signals with an open-circuit voltage up to 50 V and a short-circuit current up to 200 nA in response to externally applied pressure. It is used to sense various types of human motions and harvest electric energy from body motion. The harvested energy can successfully power wearable electronics, such as an electronic watch and light-emitting diode. Therefore, the as-prepared SE-TENG sensor with a pressure response and self-powered capability provides potential applications in wearable sensors or flexible electronics for personal healthcare and human–machine interfaces.     

Highly Stretchable,Tough, and Conductive Ag@Cu Nanocomposite Hydrogels for Flexible Wearable Sensors and Bionic Electronic Skins

Flexible conductive materials and flexible electronic devices are driving the development of the next generation of cutting-edge wearable electronics. However, the existing hydrogel-based flexible conductive materials have limited tensile capacity, low toughness, and poor anti-fatigue performance, resulting in narrow sensing area and insufficient durability. In this paper, a conductive nanocomposite hydrogel with high ductility, toughness, and fatigue resistance is prepared by combining silver coated copper (Ag@Cu) nanoparticles with gelatin followed by one-step immersion in sodium sulfate (Na₂SO₄) solution. The salting-out of gelatin in Na₂SO₄ solution greatly improve the mechanical properties of this gelatin-based hydrogel. The uniform distribution of Ag@Cu nanoparticles inside the whole hydrogel endow the composite hydrogel with excellent electrical conductivity (1.35 S m⁻¹). In addition, it displayed high and stable tensile strain sensitivity over a wide strain range (gauge factor = 2.08). Therefore, the Ag@Cu-Gel hydrogel is sensitive and stable enough to be successfully utilized as flexible wearable sensor for detecting human motion signals in real time, such as bending of human joints, swallowing, and throat vocalization. Furthermore, this hydrogel is also suitable for application as electronic skin for bionic robots. The above results demonstrate the promising application of Ag@Cu-Gel hydrogel for wearable electronics.     

A Precursor Approach to Electrospun Polyaniline Nanofibers for Gas Sensors

Polyaniline (PANI) has served as one of the most promising conducting materials in a variety of fields including sensors, actuators, and electrodes. Fabrication of 1D PANI fibers using electrospinning methods has gained a significant amount of attention. Due to the extremely poor solubility of PANI in common organic solvents, fabrication of electrospun PANI fiber has been carried out either by using corrosive solvents such as H₂SO₄ or by electrospinning in the presence of other matrix polymers. Herein, a new approach to the fabrication of PANI fibers using tert‐butyloxycarbonyl‐protected PANI (t‐Boc PANI) as the conducting polymer precursor is reported. The t‐Boc PANI is soluble in common organic solvents (e.g., chloroform and tetrahydrofuran), and electrospinning of t‐Boc PANI in those solvents affords nano/micrometer‐sized t‐Boc PANI fibers. Treatment of the electrospun t‐Boc PANI fibers with HCl results in the removal of the acid labile t‐Boc group and the generation of conducting (≈20 S cm^?1) PANI fibers. The HCl‐doped PANI fibers are successfully used in the detection of gaseous ammonia with a detection limit of 10 ppm.

     

Polyampholyte Polymers-Based Sensors: A Review on Stimuli and Applications

Polyampholytes are a specific type of zwitterionic materials composed of monomers with both positive and negative charges. These materials can be synthesized through various methods, such as free radical, anionic, or cationic polymerization, or by modifying existing polymers through postpolymerization processes. Polyampholytes possess unique properties that make them attractive for a wide range of applications, particularly in sensor technology. They can undergo conformational changes in response to external stimuli like pH variations, temperature fluctuations, or changes in salt concentration. These properties have led to their application in biosensors, salt and ion sensors, pH sensors, fluorescence-based sensors, strain sensors, and thermosensitive sensors. It is worth noting that in some cases polyampholytes can respond to multiple stimuli simultaneously. Overall, polyampholytes are drawing great attention for their excellent mechanical properties including self-healing, high toughness, and fatigue resistance. Thus, this review is focused on advances that are made to develop polyampholyte polymer-based sensors in different applications.     

Electrospun Nanofibers for Sensing and Biosensing Applications—A Review

Sensors and biosensors have found applications in many areas, e.g., in medicine and clinical diagnostics, or in environmental monitoring. To expand this field, nanotechnology has been employed in the construction of sensing platforms. Because of their properties, such as high surface area to volume ratio, nanofibers (NFs) have been studied and used to develop sensors with higher loading capacity, better sensitivity, and faster response time. They also allow to miniaturize designed platforms. One of the most commonly used techniques of the fabrication of NFs is electrospinning. Electrospun NFs can be used in different types of sensors and biosensors. This review presents recent studies concerning electrospun nanofiber-based electrochemical and optical sensing platforms for the detection of various medically and environmentally relevant compounds, including glucose, drugs, microorganisms, and toxic metal ions.     

Piezoelectric Electrospun Fibrous Scaffolds for Bone,Articular Cartilage and Osteochondral Tissue Engineering

Osteochondral tissue (OCT) related diseases, particularly osteoarthritis, number among the most prevalent in the adult population worldwide. However, no satisfactory clinical treatments have been developed to date to resolve this unmet medical issue. Osteochondral tissue engineering (OCTE) strategies involving the fabrication of OCT-mimicking scaffold structures capable of replacing damaged tissue and promoting its regeneration are currently under development. While the piezoelectric properties of the OCT have been extensively reported in different studies, they keep being neglected in the design of novel OCT scaffolds, which focus primarily on the tissue’s structural and mechanical properties. Given the promising potential of piezoelectric electrospun scaffolds capable of both recapitulating the piezoelectric nature of the tissue’s fibrous ECM and of providing a platform for electrical and mechanical stimulation to promote the regeneration of damaged OCT, the present review aims to examine the current state of the art of these electroactive smart scaffolds in OCTE strategies. A summary of the piezoelectric properties of the different regions of the OCT and an overview of the main piezoelectric biomaterials applied in OCTE applications are presented. Some recent examples of piezoelectric electrospun scaffolds developed for potentially replacing damaged OCT as well as for the bone or articular cartilage segments of this interfacial tissue are summarized. Finally, the current challenges and future perspectives concerning the use of piezoelectric electrospun scaffolds in OCT regeneration are discussed.     

Durable,Sensitive, and Wide‐Range Wearable Pressure Sensors Based on Wavy‐Structured Flexible Conductive Composite Film

Flexible pressure sensors have potential applications in human motion monitoring and electronic skins. To satisfy the practical applications, pressure sensors with a high sensitivity, a low detection limit, a broad response range, and an excellent stability are highly needed. Here, a piezoresistive pressure sensor based on wavy‐structured single‐walled carbon nanotube/graphite flake/thermoplastic polyurethane (SWCNT/GF/TPU) composite film is fabricated by a prestretching process. Due to the random wavy structure, high conductivity, and good flexibility, the prepared sensor displays a low detection limit of 2 Pa, a wide sensing range of 0–60 kPa, and a high sensitivity of 5.49 kPa^?1 for 0–50 Pa. Furthermore, the sensor shows a remarkable repeatability of over 1.1 × 10⁴, 9.0 × 10³, and 2.0 × 10³ pressure loading/unloading cycles at 50 Pa, 500 Pa, and 30 kPa, respectively, and a fast responsibility of 100–150 ms of loading response time and 400–600 ms of relaxation time. Therefore, the pressure sensor is successfully adopted to monitor both the large‐scale human activities (e.g., walk and jump) and the small‐scale signals (e.g., wrist pulse). Furthermore, a sensor array is assembled to map the weight and shape of an object, indicating its various potential applications including human–machine interactions, human health monitoring, and other wearable electronics.     

Global View and Trends in Electrospun Nanofiber Membranes for Particulate Matter Filtration: A Review

Numerous experimental works for particulate matter (PM) filtration by electrospun nanofiber membranes (ESNFMs) are published in the last 10 years (2010–2021). Organizing and comparing the large amount of the available information to identify the best trends constitutes a big challenge. This review classifies all kinds of ESNFMs considering their physical, chemical, or electrical characteristics. All of them are obtained by modifying several parameters during a specific stage associated to the electrospinning process (ES). In this review, each of these stages is considered a "moment” as a particular instant in time. According to that, three modifications are made: Moment 1—before ES, which refers to changes in polymeric solution composition; moment 2—during ES, which refers to modifying parameters while ES is performed; and moment 3—after ES, which involves applying post-treatments directly on the membrane. After classifying all kinds of filters by moments, a detailed comparison of ESNFMs with the highest quality factors for PM_0.3 is presented, finding out the best trends and comparing their main filtration parameters as well, where the most promising ones correspond to charged and nanofiber/nets membranes, due to their high capture efficiencies (>95%) while maintaining low pressure drops (<100 Pa).     

Highly‐Aligned Electrospun Luminescent Nanofibers Prepared from Polyfluorene/PMMA Blends: Fabrication,Morphology, Photophysical Properties and Sensory Applications

Highly‐aligned luminescent electrospun nanofibers were successfully prepared from two binary blends of PFO/PMMA and PF⁺/PMMA. The PFO/PMMA aligned electrospun fibers showed a core/shell structure but the PF⁺/PMMA fibers exhibited periodic aggregate domains in the fibers. The aligned fibers had polarized steady‐state luminescence with a polarized ratio as high as 4, much higher than the non‐woven electrospun fibers or spin‐coated film. Besides, the PF⁺/PMMA aligned electrospun fibers showed an enhanced sensitivity to plasmid DNA. Such aligned electrospun fibers could have potential applications in optoelectronic or sensory devices.

     

溶胶-凝胶法制备防腐耐磨杂化涂层的研究进展

有机无机杂化涂层兼有机涂层和无机涂层的双重特点和性能,具有优良的机械性能和阻隔作用,成为材料防护涂层研究的热点。阐述了溶胶-凝胶法合成有机/无机杂化涂层材料的原理和步骤,介绍了有机/无机杂化涂层在材料防腐耐磨方面的研究现状,提出了利用溶胶-凝胶法制备防腐耐磨杂化涂层存在的一些问题。     

Self‐Powered Flexible Sensor Based on the Graphene Modified P(VDF‐TrFE) Electrospun Fibers for Pressure Detection

Polymer P(VDF‐TrFE) has been extensively applied in modern flexible electronics, such as nanogenerators and pressure sensors. In this study, a repolarization method is proposed to exploit the piezoelectric properties of the P(VDF‐TrFE) electrospinning film modified by the reduced graphene oxide (rGO). Then, the repolarized composite film is applied as the self‐powered flexible pressure sensor. Notably, the piezoelectric output voltage and current of the repolarized composite film are up to 1.5 V and 0.125 µA, respectively. Typically, the piezoelectric voltage of the composite film is three times as high as that of the pure spinning film. Meanwhile, this composite film also exhibits piezoresistive effect, which is ascribed to the 3D network structure of the electrospun nanofibers. In addition, the highest piezoresistive sensitivity of the pressure sensor is 0.072 kPa^?1. To sum up, the pressure sensor fabricated in this study allows to simultaneously detect the static and dynamic pressure loads, which thereby has great application potentials in electronic skins (e‐skins) for human motion monitoring, such as motion state and finger bending.     

Highly Stretchable and Compressible Carbon Nanofiber–Polymer Hydrogel Strain Sensor for Human Motion Detection

With the development of technology and the improvement of living standards, wearable electronic devices have attracted more attention. Here, both stretchable and compressible hydrogel strain sensors based on carbon nanofiber powder (CFP) and polyvinyl alcohol (PVA) are prepared by freezing–thawing cycles. The PVA/CFP hydrogel exhibits excellent stretchable (366%) and compressible strains (70%). During 1000 loading–unloading cycles, the PVA/CFP hydrogel has a low plastic deformation (<10%, for both stretching and compressing), small energy loss efficiency (5.62% under stretching and 12.13% under compressing), and stable mechanical strength and excellent sensitivity, at conditions whether it is stretched to 100% or compressed to 50% strains. The stretchable and compressible PVA/CFP hydrogel can not only accurately detect multiple stretching behaviors of human activity, such as bending of joints, swallowing or breathing, but also detect the changes of pressure during walking. Besides, the PVA/CFP hydrogel can operate electronic screens due to its internal ions, with more potential application in soft robotics and electronic skin.     

Electrospun nanoyarns: A comprehensive review of manufacturing methods and applications

The global smart textile industry is estimated to reach 5.9 billion by 2026. However current textiles are limited due to the number of materials that can be incorporated into textiles that provide functionality and due to the use of bulky and rigid electronics. Nanofibers provide an avenue to address these limitations. Additionally, by incorporating nanofibers into textiles, functionality, and properties can be controlled from the nanoscale to the macroscale. A new method of incorporating nanofibers into textiles is by transforming nanofibers into nanoyarns. Nanoyarns would expand the applications of smart textiles to include bioactive scaffolds, tissue engineering, sensors, solar cells, and batteries. This review paper provides a comprehensive overview of different manufacturing methods for producing nanoyarns and their applications in tissue engineering and energy.     

PVDF and P(VDF-TrFE) Electrospun Scaffolds for Nerve Graft Engineering: A Comparative Study on Piezoelectric and Structural Properties,and In Vitro Biocompatibility

Polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)) are considered as promising biomaterials for supporting nerve regeneration because of their proven biocompatibility and piezoelectric properties that could stimulate cell ingrowth due to their electrical activity upon mechanical deformation. For the first time, this study reports on the comparative analysis of PVDF and P(VDF-TrFE) electrospun scaffolds in terms of structural and piezoelectric properties as well as their in vitro performance. A dynamic impact test machine was developed, validated, and utilised, to evaluate the generation of an electrical voltage upon the application of an impact load (varying load magnitude and frequency) onto the electrospun PVDF (15–20 wt%) and P(VDF-TrFE) (10–20 wt%) scaffolds. The cytotoxicity and in vitro performance of the scaffolds was evaluated with neonatal rat (nrSCs) and adult human Schwann cells (ahSCs). The neurite outgrowth behaviour from sensory rat dorsal root ganglion neurons cultured on the scaffolds was analysed qualitatively. The results showed (i) a significant increase of the β-phase content in the PVDF after electrospinning as well as a zeta potential similar to P(VDF-TrFE), (ii) a non-constant behaviour of the longitudinal piezoelectric strain constant d₃₃, depending on the load and the load frequency, and (iii) biocompatibility with cultured Schwann cells and guiding properties for sensory neurite outgrowth. In summary, the electrospun PVDF-based scaffolds, representing piezoelectric activity, can be considered as promising materials for the development of artificial nerve conduits for the peripheral nerve injury repair.     

Advances in Piezoelectric Polymer Composites for Energy Harvesting Applications: A Systematic Review

Polymeric piezoelectric composites for energy harvesting applications are considered a significant research field which provides the convenience of mechanical flexibility, suitable voltage with sufficient power output, lower manufacturing cost, and rapid processing compared to ceramic‐based composites. This review focuses majorly on the basic theory and principles behind piezoelectric energy harvesting (PEH) devices, followed by specified materials used for the different devices. Different structural configurations associated with fabrication of PEH devices are discussed in detail along with their major advantages and drawbacks. Numerous classes of piezoelectric polymers such as polyvinylidene fluoride, polylactic acid, cellulose, polyamides, polyurea, polyurethanes, and their composites used for energy harvesting applications as a productive alternative of lead‐based piezo‐ceramics, are extensively addressed and explored. Additionally, current global and Indian scenarios associated with PEH devices, major challenges associated with them, and the future perspective of such devices are also reported in this review.