Improvement of Thermal Stability of Charges in Polylactic Acid Electret Films for Biodegradable Electromechanical Sensors.

Eco-friendly sensors fabricated from biocompatible and biodegradable materials are promising candidates for wearable and implantable electronics due to their environmental sustainability and biosafety. This article reports a fully biodegradable electromechanical sensor (FBES) utilizing a sandwich structure with macro ripple structured polylactic acid (PLA) electret films acting as sensitive layers and molybdenum (Mo) sheets serving as electrodes for a wearable device application. The stability of the space charge stored within the PLA film has been enhanced by introducing an internal cellular structure and improving the polarization process. A macro ripple structure of the PLA layer with higher deformation is a great guarantee for boosting the pressure sensitivity. The results indicate that inserting cell microstructures and optimizing the polarization process significantly improve the charge storage stability of PLA films by nearly 55%. This enhancement is attributed to several factors, including the extended charge drift path of the charges in cellular films, a synergy effect of surface charges, and "macroscopic" dipole charges distributed in the cells. The fabricated sensor achieves a high sensitivity of 1000 pC/kPa, a wide pressure detection range of 0.03-62.4 kPa, and satisfactory stability. Such sensors are not only sensitive to body movements but also to subtle physiological signals, satisfying the diverse needs of wearable healthcare. Importantly, all the composition materials of the sensor can be completely degraded after their service, aligning with the environmentally friendly principles of green development.

Contact-Electro-Catalysis Through Electret Behavior to Facilitate Electron Transfer.

Contact-electro-catalysis (CEC) usually uses polymer dielectrics as its catalysts under mechanical stimulation conditions, which although has a decent catalytic dye degradation effect still warrants performance improvement. A carrier separation promotion strategy based on an internal electric field by polarization can effectively improve ferroelectric material performance in photocatalysis and piezocatalysis. Therefore, carrier separation as a necessary process of CEC also can be promoted and is largely expected to improve CEC performance theoretically. However, the carrier separation enhancement by the internal electric field strategy has not been achieved in the CEC experiment yet, because of the difficulty of building an internal electric field in an inert polymer dielectric. Herein, a polytetrafluoroethylene (PTFE) dielectric was charged through an electret process, which was believed to establish an internal electric field for CEC catalysts proved by KPFM, XPS, and triboelectric nanogenerator voltage output analysis. The fastest degradation rate of methyl orange reached over 90% at 1.5 h, while the hydroxyl free radical (•OH) yield of the PTFE electret was nearly three times that of the original PTFE. Density functional theory (DFT) calculations verified that the potential barrier of interatomic electron transfer between PTFE and H2O was reduced by 37% under the internal electric field. The electret strategy used herein to optimize the PTFE catalyst provides a base for the use of other general plastics in CEC and facilitates the production of easily prepared, easily recyclable, and inexpensive polymer dielectric catalysts that can promote large-scale pollutant degradation via CEC.

Bioelectret Materials and Their Bioelectric Effects for Tissue Repair: A Review.

Biophysical and clinical medical studies have confirmed that biological tissue lesions and trauma are related to the damage of an intrinsic electret (i.e., endogenous electric field), such as wound healing, embryonic development, the occurrence of various diseases, immune regulation, tissue regeneration, and cancer metastasis. As exogenous electrical signals, such as conductivity, piezoelectricity, ferroelectricity, and pyroelectricity, bioelectroactives can regulate the endogenous electric field, thus controlling the function of cells and promoting the repair and regeneration of tissues. Materials, once polarized, can harness their inherent polarized static electric fields to generate an electric field through direct stimulation or indirect interactions facilitated by physical signals, such as friction, ultrasound, or mechanical stimulation. The interaction with the biological microenvironment allows for the regulation and compensation of polarized electric signals in damaged tissue microenvironments, leading to tissue regeneration and repair. The technique shows great promise for applications in the field of tissue regeneration. In this paper, the generation and change of the endogenous electric field and the regulation of exogenous electroactive substances are expounded, and the latest research progress of the electret and its biological effects in the field of tissue repair include bone repair, nerve repair, drug penetration promotion, wound healing, etc. Finally, the opportunities and challenges of electret materials in tissue repair were summarized. Exploring the research and development of new polarized materials and the mechanism of regulating endogenous electric field changes may provide new insights and innovative methods for tissue repair and disease treatment in biological applications.

Self-Powered Autonomous Electrostatic Dust Removal for Solar Panels by an Electret Generator.

Solar panels often suffer from dust accumulation, significantly reducing their output, especially in desert regions where many of the world's largest solar plants are located. Here, an autonomous dust removal system for solar panels, powered by a wind-driven rotary electret generator is proposed. The generator applies a high voltage between one solar panel's output electrode and an upper mesh electrode to generate a strong electrostatic field. It is discovered that dust particles on the insulative glass cover of the panel can be charged under the high electrical field, assisted by adsorbed water, even in low-humidity environments. The charged particles are subsequently repelled from the solar panel with the significant Coulomb force. Two panels covered with sand dust are cleaned in only 6.6 min by a 15 cm diameter rotary electret generator at 1.6 m s-1 wind speed. Experimental results manifest that the system can work effectively in a wide range of environmental conditions, and doesn't impact the panel performance for long-term operation. This autonomous system, with its high dust removal efficiency, simplicity, and low cost, holds great potential in practical applications.

Alkyl-π Functional Molecular Gels: Control of Elastic Modulus and Improvement of Electret Performance.

The development of optoelectronically-active soft materials is drawing attention to the application of soft electronics. A room-temperature solvent-free liquid obtained by modifying a π-conjugated moiety with flexible yet bulky alkyl chains is a promising functional soft material. Tuning the elastic modulus (G') is essential for employing optoelectronically-active alkyl-π liquids in deformable devices. However, the range of G' achieved through the molecular design of alkyl-π liquids is limited. We report herein a method for controlling G' of alkyl-π liquids by gelation. Adding 1 wt % low-molecular-weight gelator formed the alkyl-π functional molecular gel (FMG) and increased G' of alkyl-π liquids by up to seven orders of magnitude while retaining the optical properties. Because alkyl-π FMGs have functional π-moieties in the gel medium, this new class of gels has a much higher content of π-moieties of up to 59 wt % compared to conventional π-gels of only a few wt %. More importantly, the gel state has a 23 % higher charge-retention capacity than the liquid, providing better performance in deformable mechanoelectric generator-electret devices. The strategy used in this study is a novel approach for developing next-generation optoelectronically-active FMG materials.

An ultra-compact mechanical antenna based on PTFE highly charged electret for extremely low frequency communications.

-Extremely-Low Frequencies (ELF, 30∼300Hz) transmitting antennas in wireless communications are often limited by antenna size and complex impedance matching networks. In this paper, we propose an ultra-small Artificial Electret Type Mechanical Antenna (AETMA), which is composed of a single charge electret and a driving structure, with high radiation efficiency and small size. In order to improve the electric dipole moment of the mechanical antenna, we employ a pin-plate corona polarization technique and a unidirectional stretching treatment to obtain a porous thin-film electret that can stably store a large amount of charge. Its surface charge density can reach 5.355 mC/m2 and we analyze its surface potential stability. To assess the radiation capability of AETMA, the radiation field models of three kinds of mechanical antennas are established and verified by simulation. Additionally, we simulate and compare the planar electret and curved electret configurations to determine the optimal form of AETMA. The radiation intensity of the planar electret is found to be superior under the same moment of inertia. Finally, a 1m-scale artificial electret antenna is designed based on the optimal model. Comparative analysis with existing rotary mechanical antenna schemes confirms the great potential of the proposed AETMA for portable, miniaturized and high-performance wireless communication devices.

Chemically tailored molecular surface modification of bamboo pulp fibers for manipulating the electret performance of electret filter media.

The surface chemical composition of materials is essential for regulating their charge trapping and storage capabilities, which directly affect their electret performance. Although chemical modification of materials to alter electret performance has been investigated, the mechanism through which electret properties are regulated more systematically via chemical customization has not been elucidated in detail. Herein, p-phenylenediamine, benzidine and 4,4'-diaminotriphenyl, which have different conjugated strength functional groups, were selected to chemically tailor the surface of bamboo pulp fibers to regulate the electret properties and elucidate the regulatory mechanism more systematically. The results showed that the charge trapping and storage properties of materials could be regulated by introducing functional groups with different conjugated strengths to their surfaces, realizing the regulation of the electret properties. Moreover, the charge trapping and storage ability could be tailored more specifically by regulating the number of functional groups. By chemical customization to provide electrostatic effects to the materials, the purification time was reduced by approximately 45 %-52 %. More importantly, a relatively systematic mechanism was proposed to elucidate the effect of the conjugate group strength on the charge trapping and charge storage properties of the material. These findings will provide guidance for the investigation of chemical modifications to regulate the electret performance of materials.

Frequency-Dependent Multistep Ferroelectric Polarization Switching Mechanism in P(VDF-TrFE)-Based Capacitors Induced by Polystyrene Electret-like Modulation.

In this work, we investigate multistep ferroelectric polarization switching dynamics of a series of poly(vinylidene fluoride-trifluoroethylene)/polystyrene, P(VDF-TrFE)/PS, as active layers in ferroelectric capacitors with variable P(VDF-TrFE)/PS thickness ratios and a wide range of driving voltage frequencies (1-1000 Hz). The PS electret-like modulation effects on the depolarized field fluctuation are proven to be responsible for this multistep ferroelectric polarization switching process. To be specific, the switching current density peak splits into two peaks in both positive and negative voltage ranges according to the stimulus-response (S-R) data from the metal-ferroelectric-electret-metal capacitor driven by a periodic triangular voltage wave. The double-peak current trough appears when the transitorily suppressed ferroelectric polarization switching occurs while the discharge and recharge of the PS electret by external voltage brings a specific dynamic change in the electric field across ferroelectric (EFE). We also propose a theoretical model to simulate the ferroelectric polarization switching process at a current trough zone. This phenomenon provides new concepts on the electret-modulated multistep ferroelectric switching dynamics, and such switching mechanisms are critical for realizing reliable nonvolatile memory applications in flexible electronics.

Enhancing electrostatic charge stability of corona charged Teflon electret films for radiation dosimetry by optimizing metal electrode backing material.

Metal electrode backing (MEB) material was found to have a significant role on the electrostatic surface charge stability of Teflon polytetrafluoroethylene (PTFE) electret films. PTFE films of different thicknesses were positively and negatively charged by using our home-made modified point-to-plane corona poling rotating systems. Different MEB materials and thicknesses; aluminum, copper, stainless steel, zinc, silver, and gold were applied. The electrostatic surface charge stability of charged PTFE films was monitored for 200 h at similar storage conditions. Proper MEB material enhances the electrostatic surface charge stability of electret films due to the work function of the metal electrodes and high potential barrier formation at the interface of MEB material and electret film. The studies show that thinner MEB materials provide higher electrostatic surface charge stability in PTFE films. Therefore, thinner MEB material with higher work function is an effective compromise for producing electret films with higher electrostatic surface charge stability. The findings are extremely important for the applications of highly stable electret films for different applications in particular for radiation dosimetry with special regards to radon monitoring.

Electret Modulation Strategy to Enhance the Photosensitivity Performance of Two-Dimensional Molybdenum Sulfide.

Due to the limited light absorption efficiency of atomic thickness layers and the existence of quenching effects, photodetectors solely made of transition metal dichalcogenides (TMDs) have exhibited an unsatisfactory detection performance. In this article, electret/TMD hybridized devices were proposed by vertically coupling a MoS2 channel and the PTFE film, which reveals an optimized photodetection behavior. Negative charges were generated in the PTFE layer through the corona charging method, akin to applying a negative bias on the MoS2 channel in lieu of a traditional voltage-driven back gate. Under a charging voltage of -6 kV, PTFE/MoS2 devices reveal improved photodetection performance (Rhybrid = 67.95A/W versus Ronly = 3.37 A/W, at 470 nm, 1.20 mW cm-2) and faster recovery speed (τd(hybrid) = 2000 ms versus τd(only) = 2900 ms) compared to those bare MoS2 counterparts. The optimal detection performance (2 orders of magnitude) was obtained when the charging voltage was -2 kV, limited by the minimum of the carrier density in MoS2 channels. This study provides an alternative strategy to optimize optoelectronic devices based on the 2D components through non-voltage-driven gating.

Exploring Microphone Technologies for Digital Auscultation Devices.

The aim of this work is to present a preliminary study for the design of a digital auscultation system, i.e., a novel wearable device for patient chest auscultation and a digital stethoscope. The development and testing of the electronic stethoscope prototype is reported with an emphasis on the description and selection of sound transduction systems and analog electronic processing. The focus on various microphone technologies, such as micro-electro-mechanical systems (MEMSs), electret condensers, and piezoelectronic diaphragms, intends to emphasize the most suitable transducer for auscultation. In addition, we report on the design and development of a digital acquisition system for the human body for sound recording by using a modular device approach in order to fit the chosen analog and digital mics. Tests were performed on a designed phantom setup, and a qualitative comparison between the sounds recorded with the newly developed acquisition device and those recorded with two commercial digital stethoscopes is reported.

Accelerated Osteogenic Response of Electrodynamically Stimulated Mg1-xCaxSi1-xZrxO3 (x = 0-0.4) Bioelectrets.

MgSiO3-based biodegradable ceramics demonstrated remarkable potential for treating small-scale bone defects and temporary bone replacement. In addition, the dissolution behavior of MgSiO3 bioceramics can be tuned by doping of Ca and Zr elements at Mg and Si sites, respectively. The present study reported the influence of formation of Ca- and Zr-codoped Mg1-xCaxSi1-xZrxO3 (x = 0, 0.1, 0.2, 0.3, and 0.4) bioelectrets and electrodynamic stimulation toward improving their osteogenic response. Mg1-xCaxSi1-xZrxO3 electrets were successfully synthesized by a solid-state route. A detailed X-ray photoelectron spectroscopy (XPS) analyses revealed that the electrets produced oxygen-deficient active sites. The formation of Mg1-xCaxSi1-xZrxO3 electrets significantly increased the surface hydrophilicity. Inductively coupled plasma (ICP) analyses were used to examine the leaching behavior of Ca/Zr-codoped MgSiO3 bioceramics. In vitro cell culture analyses indicated that the osteogenesis of MG-63 cells was remarkably enhanced on the electrodynamic field-treated Mg1-xCaxSi1-xZrxO3 bioelectrets as compared to hydroxyapatite (HA). Moreover, a better osteogenic response was observed for higher concentrations of Ca (0.3 and 0.4) and Zr (0.3 and 0.4) doping in the MgSiO3 bioelectrets. Further, the mechanism of enhanced cellular functionality was revealed by the measurement of intracellular Ca2+.

Biomedical Applications of Electrets: Recent Advance and Future Perspectives.

Recently, electrical stimulation, as a non-pharmacological physical stimulus, has been widely exploited in biomedical and clinical applications due to its ability to significantly enhance cell proliferation and differentiation. As a kind of dielectric material with permanent polarization characteristics, electrets have demonstrated tremendous potential in this field owing to their merits of low cost, stable performance, and excellent biocompatibility. This review provides a comprehensive summary of the recent advances in electrets and their biomedical applications. We first provide a brief introduction to the development of electrets, as well as typical materials and fabrication methods. Subsequently, we systematically describe the recent advances of electrets in biomedical applications, including bone regeneration, wound healing, nerve regeneration, drug delivery, and wearable electronics. Finally, the present challenges and opportunities have also been discussed in this emerging field. This review is anticipated to provide state-of-the-art insights on the electrical stimulation-related applications of electrets.

Electroactive, Antibacterial, and Biodegradable Poly(lactic acid) Nanofibrous Air Filters for Healthcare.

Poly(lactic acid) (PLA)-based nanofibrous membranes (NFMs) hold great potential in the field of biodegradable filters for air purification but are largely limited by the relatively low electret properties and high susceptibility to bacteria. Herein, we disclosed a facile approach to the fabrication of electroactive and antibacterial PLA NFMs impregnated with a highly dielectric photocatalyst. In particular, the microwave-assisted doping (MAD) protocol was employed to yield Zn-doped titanium dioxide (Zn-TIO), featuring the well-defined anatase phase, a uniform size of ∼65 nm, and decreased band gap (3.0 eV). The incorporation of Zn-TIO (2, 6, and 10 wt %) into PLA gave rise to a significant refinement of the electrospun nanofibers, decreasing from the highest diameter of 581 nm for pure PLA to the lowest value of 264 nm. More importantly, dramatical improvements in the dielectric constants, surface potential, and electret properties were simultaneously achieved for the composite NFMs, as exemplified by a nearly 94% increase in surface potential for 3-day-aged PLA/Zn-TIO (90/10) compared with that of pure PLA. The well regulation of morphological features and promotion of electroactivity contributed to a distinct increase in the air filtration performance, as demonstrated by 98.7% filtration of PM0.3 with the highest quality factor of 0.032 Pa-1 at the airflow velocity of 32 L/min for PLA/Zn-TIO (94/6), largely surpassing pure PLA (89.4%, 0.011 Pa-1). Benefiting from the effective generation of reactive radicals and gradual release of Zn2+ by Zn-TIO, the electroactive PLA NFMs were ready to profoundly inactivate Escherichia coli and Staphylococcus epidermidis. The exceptional combination of remarkable electret properties and excellent antibacterial performance makes the PLA membrane filters promising for healthcare.

Recent Advanced Applications of Ionic Liquid for Future Iontronics.

Recently, electronic devices that make use of a state called the electric double layers (EDL) of ion have opened up a wide range of research opportunities, from novel physical phenomena in solid-state materials to next-generation low-power consumption devices. They are considered to be the future iontronics devices. EDLs behave as nanogap capacitors, resulting the high density of charge carriers is induced at semiconductor/electrolyte by applying only a few volts of the bias voltage. This enables the low-power operation of electronic devices as well as new functional devices. Furthermore, by controlling the motion of ions, ions can be used as semi-permanent charge to form electrets. In this article, we are going to introduce the recent advanced application of iontronics devices as well as energy harvesters making use of ion-based electrets, leading to the future iontronics research.

Synthesis of 2,5,8-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films.

To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] trithiophene (TPBTT) and its ethyl derivative (m-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m-ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m-ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m-ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films.

Fabrication and Analysis of Microcapsule Electrets with a Tunable Flexoelectric-like Response.

The electret has drawn considerable attention as an emerging flexible energy collector. In this work, a charged microcapsule is designed which can provide a stable storage space for electric charge in the electret. The flexoelectric-like response is achieved by embedding a layer of charged microcapsules in the middle plane of the flexible polymer to form an electret. The results of Fourier transform infrared spectroscopy and energy-dispersive X-ray spectroscopy verified the successful preparation of microcapsules. Zeta potential analysis showed the negative electrical properties of the microcapsules. The prepared microcapsule electret has a significant flexoelectric effect under loading conditions. This work presents a preliminary theoretical study of the microcapsule electret to optimize the output characteristics of the electret by varying the parameters, including the number of microcapsules, the size of the electret, and the external load. Good agreement was achieved with the experimental results, which verified the validity of the theoretical study. This work provides a new method for preparing electret and further promotes its application in electromechanical transducers.

Indoor Radon Levels in Homes and Schools in the Western Cape, South Africa-Results from a Schools Science Outreach Initiative and Corresponding Model Predictions.

We describe a school science outreach initiative that introduced learners to applied nuclear physics research by means of a two-day workshop that involved learners and teachers from 5 schools in the Western Cape province of South Africa. During this workshop, the participants were introduced to the naturally occurring, inert, colorless, and tasteless radioactive gas radon (222Rn). During the first day of the workshop, the participants were informed about the detrimental health impacts of inhaling radon and its daughter radionuclides and were shown how indoor radon activity concentrations can be measured using the electret ion chamber (EIC) technology. The learners were then each supplied with a short-term electret (E-PERM, Radelec, Frederick, MD, USA) and associated ion chamber to enable them to make radon measurements in their homes. The teachers in turn were supplied with EICs to enable them make radon measurements in their schools. The participants returned the EICs on the second day of the workshop, one week later. Here, the drop in the potential difference across each electret was measured in order to calculate the average indoor radon activity concentration. A total of 49 indoor radon concentrations were measured. The average indoor radon concentrations were 36 ± 26 Bqm-3 in homes and 41 ± 36 Bqm-3 in schools, while the highest concentration was found to be 144 Bqm-3. These levels were compared to predictions from a model that uses input information about the uranium content associated with the surface geology at each measurement location. The predictions compared well with the measured values.

Washable and Breathable Electret Sensors Based on a Hydro-Charging Technique for Smart Textiles.

Flexible electromechanical sensors based on electret materials have shown great application potential in wearable electronics. However, achieving great breathability yet maintaining good washability is still a challenge for traditional electret sensors. Herein, we report a washable and breathable electret sensor based on a hydro-charging technique, namely, hydro-charged electret sensor (HCES). The melt-blown polypropylene (MBPP) electret fabric can be charged while washing with water. The surface potential of MBPP electret fabric can be improved by optimizing the type of water, water pressure, water temperature, drying temperature, drying time, ambient air pressure, and ambient relative humidity. It is proposed that the single fiber has charges of different polarities on the upper and lower surfaces due to contact electrification with water, thereby forming electric dipoles between fibers, which can lead to better surface potential stability than the traditional corona-charging method. The HCES can achieve a high air permeability of ∼215 mm/s and sensitivity up to ∼0.21 V/Pa, with output voltage remaining stable after over 36,000 working cycles and multiple times of water washing. As a demonstration example, the HCES is integrated into a chest strap to monitor human respiration conditions.

Understanding the Role of Soft X-ray in Charging Solid-Film and Cellular Electrets.

Solid-film electrets and cellular electrets are defined as promising insulating dielectric materials containing permanent electrostatic and polarizations. High-performance charging methods are critical for electret transducers. Unlike dielectric barrier discharge (DBD) charging, the soft X-ray charging method, with its strong penetration ability, has been widely used in electrets after packaging and has even been embedded in high-aspect-ratio structures (HARSs). However, the related charging model and the charging effect of the soft X-ray irradiation remain unclear. In this study, the charge carrier migration theory and the one-dimensional electrostatic model were employed to build the soft X-ray charging models. The influence of soft X-ray irradiation under deferent poling voltages was investigated theoretically and experimentally. The conducted space charge measurement based on a pulsed electro-acoustic (PEA) system with a soft X-ray generator revealed that soft X-ray charging can offer higher surface charge densities and piezoelectricity to cellular electrets under the critical poling voltage lower than twice the breakdown voltage.