High Performance Triboelectric Nanogenerators of Reduced Graphene Oxide-Based Poly(vinyl alcohol) Nanocomposites toward Biomechanical Energy Harvesting and an IoT System for Human–Machine Interface Application

Flexible and stretchable triboelectric nanogenerators (TENGs) have been rapidly advanced owing to the demand for portable and wearable electronic devices that can work under universal or motional circumstances. While versatile materials can be applied in a TENG as dielectric materials, flexible and cost-effective electrodes are crucially important for the output performance of TENGs. Herein, we developed a poly(vinyl alcohol) (PVA) hydrogel TENG doped with a novel two-dimensional material, graphitic carbon nitride (g-C3N4), which could act as both a cost-effective flexible electrode and a positive dielectric for TENG with different morphologies. The measured peak-to-peak open-circuit voltage of the TENG reached 80 V at a dopant concentration of 2.7 wt % in single-electrode mode, which is far higher than that of the pristine PVA hydrogel TENG. As a demonstration of the application, the g-C3N4/PVA hydrogel TENG can be adopted as electronic skin to monitor the movement of the human body. Low-frequency mechanical energy-harvesting devices in different morphologies including discoid flake shape, tube shape, and spiral shape in the single-electrode mode or contact-separation mode have been designed, fabricated, and evaluated. All of these merits of the proposed hydrogel TENG after doping two-dimensional (2D) material g-C3N4 have demonstrated their promising potential for versatile applications in biomechanical energy harvesting and self-powered sensing.

Oral healthcare monitoring is a vital aspect of identifying and addressing oral dental problems including tooth decay, gum pain, and oral cancer. Day by day, healthcare facilities and regular checkups are becoming more costly and time-consuming. In this context, consumers are moving toward advanced technology, such as bite sensors, to obtain regular data about their occlusal chewing patterns and strength. The triboelectric nanogenerator (TENG) can potentially eliminate the need for a battery by simply converting abundant vibrations from nature or human motion into electrical energy. In this work, biomaterials are obtained from biowastes such as cellulose from wood waste, chitosan from crab shells, and gelatin from fish scales. All wastes are biodegradable, and our work aims at sustainability and waste hierarchy. The single electrode mode-based TENG was designed and fabricated using biodegradable poly(vinyl alcohol) (PVA)-biomaterial composites, rice paper as a substrate, and edible silver leaf as an electrode. The highest electrical output was obtained for PVA/chitosan 10 wt % composite-based TENG (PC10) of about 20 V, 200 nA, and 12 nC. The biomechanical energy harvesting was measured, and powering of LED was demonstrated using a PC10 TENG device. A biocompatible bite sensor based on the TENG was used to measure the biting force of a dummy teeth model to demonstrate its potential use in dental health applications. It indicates the promising future value of disposable oral medication devices without any invasive surgery or injection.

Regenerative braking is a well-known technology applied in electric vehicles to achieve high energy efficiency through an energy-recovery mechanism. The same concept has been applied to robotic applications, such as legged robots, lower-limb prostheses, and biomechanical energy harvesters. In particular, a biomechanical energy harvester enables humans to generate watts of power while simultaneously assisting in the braking of human joints during walking. In this study, a systematic analysis of a biomechanical regenerative braking energy harvester was conducted. First, we reviewed the design considerations of each harvester component and designed an energy-harvester prototype with high power density through a systematic design process. Subsequently, the dynamics of the designed harvester and its effect on human biomechanics were analyzed through device testing and human testing. The designed harvester demonstrated a power density of 3.3 W/kg for level-ground walking during device testing. We evaluated muscle activities and joint kinematics in versatile walking scenarios such as sloped walking. In level-ground and downhill walking, the hamstring muscle activity was assisted by the braking torque simultaneously generating 1.2 W and 0.7 W, respectively, during negative work phase. Meanwhile, we confirmed that the braking torque was generated rather in the positive work phase interfering the quadriceps muscle activity. Comparing previous knee-joint-driven biomechanical regenerative braking energy harvesters, our harvester shows relatively high power density level even with slower walking speed and without any special mechanism.Supplementary InformationThe online version contains supplementary material available at 10.1007/s40684-022-00472-6.

Biomechanical energy harvesters are designed to generate electrical energy from human locomotion (e.g., walking) with minimal or no additional effort by the users. These harvesters aim to carry out the work of the muscles during phases in locomotion where the muscles are acting as brakes. Currently, many harvesters focus on the knee joint during late swing, which is only one of three phases available during the gait cycle. For the device to be successful, there is a need to consider design components such as the motor/generator and the gear ratio. These components influence the amount of electrical energy that could be harvested, metabolic power during harvesting, and more. These various components make it challenging to achieve the optimal design. This paper presents a design of a knee harvester with a direct drive that enables harvesting both in flexion and extension using optimization. Subsequently, two knee devices were built and tested using five different harvesting levels. Results show that the 30% level was the best, harvesting approximately 5 W of electricity and redacting 8 W of metabolic energy compared to walking with the device as a dead weight. Evaluation of the models used in the optimization showed a good match to the system model but less for the metabolic power model. These results could pave the way for an energy harvester that could utilize more of the negative joint power during the gait cycle while reducing metabolic effort.

Zirconium metal-organic framework and hybridized Co-NPC@MXene nanocomposite-coated fabric for stretchable, humidity-resistant triboelectric nanogenerators and self-powered tactile sensors

Recently, triboelectric nanogenerator (TENG), working based on triboelectric effect and electrical induction, emerge as one kind of energy harvesting technology. The TENG is mainly composed of dielectric layer where frictional contact occurs and conductive layer where current flows. Because of the characteristic of triboelectric effect, strategies to make efficient TENG are focused on material selection and effective contact area increment. Given materials, micro/nanostructure fabrication corresponds to the latter strategy. Several fabrication methods are already reported such as surface etching with nanoparticle and surface replication, but they require high fabrication cost. Furthermore, they need additional process such as integration micro/nanotophographical layer with conductive layer, resulting in a multistep fabrication. The fabrication environment hinders high productivity and commercialization of TENG. In this study, we used thermal nanoimprinting to fabricate a transparent and flexible TENG named a Pattern Assisted TriboElectric Replicable Nanogenerator (PATERN). From the method, we can achieve nanostructure replication and integration with conductive layer simultaneously, which are two essential steps to fabricate high performance TENG. The fabrication cycle was within 6 min. The PATERN has maximum transmittance about 88% at the 850-1600 nm range, and bending radius about 3.2 mm. From modifying the shape of electrode layer, we can also achieve a bi-directional mechanical energy harvesting ability, which is essential for efficiently harvest the energy from complex real mechanical motions. Given that TENG can be cost-effectively fabricated by utilizing conventionally available films (less than $1) with fast fabrication process, we believed the technology lead a new concept of disposable TENG and its accompanying application, such as hybrid energy harvester or biomechanical energy harvester. Figure 1

We have been investigating the preparation of macroporous gel particles of poly(vinyl alcohol) [1-3]. These particles are obtained by the saponification of particles of poly(vinyl acetate) formed in the polymerization of vinyl acetate in a suspension process. The poly(vinyl alcohol) gel particles have been used as a column packing for aqueous separations. However, the gels have a disadvantage in pressure-resistant property because of poor mechanical strength. We expected that a gel having excellent mechanical stability might be obtained by reinforcing poly(vinyl alcohol) with an inorganic component. We report here that reinforcement using silica as an inorganic component is useful for the improvement of performance of poly(vinyl alcohol) gel particles used as a column packing for gel permeation chromatography in aqueous media. Tetraethoxy silane (TEOS) was used as the inorganic component of the hybrid gels throughout this study. The sol-gel method was applied to the preparation of hybrid gel [4-6]. The process is divided into two steps: hydrolysis of metal alkoxides to produce metal hydroxides, followed by polycondensation of hydroxyl groups. By carrying out this reaction in a poly(vinyl alcohol) gel matrix containing many hydroxyl groups, it is possible to copolymerize metal alcoxide with poly(vinyl alcohol). Thus a silica network may be incorporated into the poly (vinyl alcohol) gel matrix. The gel particles of poly(vinyl acetate) (DP = 1000) obtained by suspension polymerization were classified to give a particle size distribution with particle diameters in the range 350-400 #m. The saponification of poly(vinyl acetate) to poly(vinyl alcohol) was carried out at 30 ~ for 1 month by immersing the gel particles in a solution containing sodium hydroxide and methanol in an aqueous saturated sodium sulfate solution [7]. The gel particles of poty(vinyl alcohol) obtained above have sufficient mechanical stability in water below 55 ~ without crosslinking treatment. The gel particles (1 g) obtained above were immersed in 10 ml of ethanol for 24 h and then tetraethoxy silane and hydrochloric acid as catalyst were added to the particles. The reaction was carried out at 30 ~ For the purpose of removal of TEOS homopolymer the reaction product was then subjected to a Soxhlet extraction using tetrahydrofuran. Swelling behaviour and gel permeation chromatographic properties of the hybrid gel particles obtained above were investigated. The ratio of the volume of the particles in ethanol to that on swelling with water (swelling ratio) was evaluated from observations by optical microscopy. As shown in Fig. 1, swelling ratio decreases with increase of reaction time and levels off at about 2 h. This is due to the formation of the silica network in the poly(vinyl alcohol) gel matrix. The swelling behaviour is mainly controlled by the amount of TEOS added. The slurry of hybrid gel particles was packed into a 150 x 4 mm stainless column at a pressure of about 5 MPa. High-performance gel permeation chromatography (HP-GPC) separations were performed with a Shimazu LC-6AD, employing distilled water as eluent. Solutions (40 #1) of individual solutes were injected with an off-column syringe-septum arrangement. Detection of solutes was performed with a Shimazu Refracto Monitor Model RID-6A (cell volume = 10 ~tl, aqueous reference). The samples of poly(ethylene glycol) are designated PEG with a number which is the molecular weight provided by the suppliers (Kanto Chemicals, Tokyo). The calibration curve was established at a flow rate of 0.4cm3/min with a PEG concentration of 2.0% (w/v). The calibration curve of the hybrid gel particles established with PEG samples is shown in Fig. 2, in comparison with the poly(vinyl alcohol) gel particles. The value of the excluded molecular weight is almost the same for the hybrid gel particles and the poly(vinyl alcohol) gel particles. However, the calibration curve for the column packing containing the hybrid gel particles shifts to higher elution volumes. Since separation power is inversely proportional to the slope of the plot of molecular weight versus elution volume, it is clear that the column containing the hybrid gel particles shows better resolution separations. The shape of the slope

We report human skin based triboelectric nanogenerators (TENG) that can either harvest biomechanical energy or be utilized as a self-powered tactile sensor system for touch pad technology. We constructed a TENG utilizing the contact/separation between an area of human skin and a polydimethylsiloxane (PDMS) film with a surface of micropyramid structures, which was attached to an ITO electrode that was grounded across a loading resistor. The fabricated TENG delivers an open-circuit voltage up to -1000 V, a short-circuit current density of 8 mA/m(2), and a power density of 500 mW/m(2) on a load of 100 MΩ, which can be used to directly drive tens of green light-emitting diodes. The working mechanism of the TENG is based on the charge transfer between the ITO electrode and ground via modulating the separation distance between the tribo-charged skin patch and PDMS film. Furthermore, the TENG has been used in designing an independently addressed matrix for tracking the location and pressure of human touch. The fabricated matrix has demonstrated its self-powered and high-resolution tactile sensing capabilities by recording the output voltage signals as a mapping figure, where the detection sensitivity of the pressure is about 0.29 ± 0.02 V/kPa and each pixel can have a size of 3 mm × 3 mm. The TENGs may have potential applications in human-machine interfacing, micro/nano-electromechanical systems, and touch pad technology.

An enzyme catalyzing the degradation of secondary alcohol oxidase-oxidized poly(vinyl alcohol), in which hydroxyl groups of poly(vinyl alcohol) are partially converted to keto groups, was purified to an electrophoretically homogeneous state from a mixed culture broth of at least three different soil bacteria. The enzyme was active to the oxidized poly(vinyl alcohol), but not to intact poly(vinyl alcohol) and to a variety of examined low molecular weight keto compounds. The enzyme was, therefore, tentatively called oxidized poly(vinyl alcohol)-degrading enzyme. The enzyme was a single polypeptide having a molecular weight of38, 000. The N- and C-terminal amino acids were alanine and threonine, respectively. The isoelectric point was pH 10.0. The optimum pH for activity was 6.5 and the optimum temperature 45°C. The enzyme activity was inhibited by Hg2+ and recovered by reduced glutathione, although p-chloromercuribenzoate had no effect. The enzyme reaction on oxidized poly(vinyl alcohol) required neither oxygen nor other electron acceptors, and resulted in a rapid decrease in viscosity, a fall of pH and an increase in compounds that are positive to a color reaction specific to carboxylic acids. The infrared spectrum of reaction products also showed the presence of carboxylic acids. Based on these results, the reaction catalyzed by the enzyme has been suggested to be hydrolytic cleavages of the main chain of oxidized poly(vinyl alcohol) as in the equation: An enzymatic system of poly(vinyl alcohol) degradation was reconstructed by use of purified samples of secondary alcohol oxidase and oxidized poly(vinyl alcohol)-degrading enzyme.

This article focuses on the development of triboelectric nanogenerator (TENG) by utilizing advanced 2D nanomaterials with innovative design and easy fabrication method to achieve durable TENG with high-power density and improved cycling performance. Herein, we fabricated a layer-by-layer stacked vertical contact-separation (CS) mode TENG. In this unique design, we incorporated a thin film of micron-sized Ti3C2Tx-MXene ultrathin sheets (TMSs) into a polyethylene terephthalate (PET) based tribo-negative electrode. A tribo-positive layer was prepared by integrating an optimized amount of NaCl into a polyvinyl alcohol (PVA) matrix. After optimization of both triboelectric layers, the optimized TMS-TENG showed an open-circuit voltage (Voc) ∼ 390 V, short-circuit current (Isc) ∼ 96 μA, and power density of 6.66 W·m−2. The boosted performance is due to the synergistic effect of TMSs, used as a charge trapping layer on the electronegative side, and the impact of an NaCl:PVA impregnated layer on the electropositive side of the TENG. The ultrathin-layered structures of sandwiched TMS film serve bifunctionally as a charge accumulation and charge trapping entity simultaneously, increasing the charge separation due to high dielectric constant, and thus increasing the overall output power of the fabricated TENG. The prepared TMS-TENG was tested as a pressure sensor to monitor different sensitive physiological movements of the human body. Further applications of the designed TMS-TENGs have been revealed by powering more than 500 LEDs, an electronic calulator, and swiftly charging micro-capacitors by utilizing direct output power. By engineering electrodes, we can gain insight into the specific role that TMSs play in enhancing the performance of TENGs. This research provides a new avenue for designing self-powered pressure/motion sensors in robotics and harvesting biomechanical energy as electrical energy for sustainable electronics.

Abstract: This study examines the effects of the lactide/glycolide molar ratio on the synthesis of polymeric nanoparticles using a PVA-SDS stabilizers solution. PLA was obtained with a ratio of 100/0, while PLGA was synthesized at ratios of 88/12 and 64/36. Polymerizations employed ROP at 130°C with stannous oc-tanoate and 1-dodecanol as catalysts. Polymerizations were conducted through ring opening at 130°C with stannous octanoate and 1-dodecanol as catalyst and co-catalyst. Characterized polymers were used to pre-pare emulsions stabilized with polyvinyl alcohol (15 g/L) alone or mixed with sodium dodecyl sulfate of various molecular weights (MW=13-23, 31-50 and 85-124 KDa). These emulsions were stored in phos-phate-buffered saline for 28 days at 37.4°C. Colloidally stable emulsions were achieved using different poly (vinyl alcohol) concentrations, with the lactide/glycolide molar ratio influencing particle diameter. The elec-trostatic stabilizer formed by the poly (vinyl alcohol)-sodium dodecyl sulfate mixture demonstrated .Superior stabilization compared to poly (vinyl alcohol) alone, representing a novel finding. Moreover, the poly (vinyl alcohol-sodium dodecyl sulfate) mixture showed reduced water diffusion into the nanoparticles compared to poly (vinyl alcohol) alone, as evidenced by molecular weight and pH measurements. Addition-ally, the degradation of poly (lactic acid) and poly (lactic-co-glycolic acid) films was investigated; pH measurements in the immersed solutions showed that the degradation was increased with higher glycolide content.

We here reported that hyperbranched poly(ether amine) (hPEA) and poly(vinyl alcohol) (PVA) interpenetrating network (hPEA/PVA-IPN) can be used for the selective adsorption and separation of guest homologues. A series of hyperbranched poly(ether amine) and poly(vinyl alcohol) interpenetrating networks (hPEA/PVA-IPNs) were fabricated by introducing poly(vinyl alcohol) chains into network of hyperbranched poly(ether amine), in which two independent networks of hyperbranched poly(ether amine) and poly(vinyl alcohol) were cross-linked through photodimerization of coumarin groups of hyperbranched poly(ether amine) and aldol condensation reaction between hydroxyl groups of poly(vinyl alcohol) and glutaraldehyde, respectively. The mechanical strength of interpenetrating networks can be enhanced by the introduction of poly(vinyl alcohol), and the tensile strength of interpenetrating networks increased with tens of times in compared with the pure hyperbranched poly(ether amine) network. The adsorption behavior of seven fluorescein dyes sharing with the same backbone and charge state onto hyperbranched poly(ether amine) and poly(vinyl alcohol) interpenetrating networks was then investigated in detail. Regardless of their charge states, these interpenetrating networks exhibited the quick adsorption to Rose Bengal (RB), Erythrosin B (ETB), Eosin B (EB), 4',5'-dibromofluorescein (DBF), and 4,5,6,7-tetrachlorofluorescein (TCF), with high adsorption capacity (Qeq) and very low adsorption of Calcein (Cal) and fluorescein (FR). The adsorption process was found to follow the pseudo-second-order kinetics, and the introduction of poly(vinyl alcohol) has no obvious effect on the adsorption behavior in this study. The big difference in the adsorption is indicative of the selective adsorption of hyperbranched poly(ether amine) and poly(vinyl alcohol) interpenetrating networks to fluorescein dyes. Based on the unique selective adsorption, the separation of several mixtures of fluorescein dyes such as RB/Cal, RB/FR, ETB/FR, and ETB/Cal was achieved by using hPEA/PVA-IPN as adsorbent.

Metal-organic framework-derived nanoporous carbon incorporated nanofibers for high-performance triboelectric nanogenerators and self-powered sensors

Currently is observed a great interest in the use of bioactive natural products for modification and functionalization of fibers to produce antimicrobial protective medical textiles. One of the areas is related to the ability to obtain electrospun nanofibers with potential bioactive properties. The aim of this study was to produce and characterize nanofibers from an aqueous solution of poly (vinyl alcohol (PVA) and beeswax (BW). To investigate the possibility of obtaining nanofibers with addition of beeswax is done in two forms - as solution and micro emulsion. Beeswax has a rich chemical composition, a mixture of proteins, vitamins, trace elements, esters, fatty acids, carbohydrates, lipids. Itself smoothies and moisturizes the skin, helps in the treatment recovery of burned skin, slows aging and has antibacterial activity. The fibers were produced in laboratory conditions with single nozzle spin-draw device. The nanofibers are based on 9% PVA solution and 2% (by weight) beeswax as additive. As a pad for the electrospun nanofibers has been used thermo bonded medical nonwoven textile. The structure of the nanofiber layers is investigated by scanning electron microscope (SEM) and atom force microscope (AFM). The fabrication of poly (vinyl alcohol) non-woven materials by electrospinning of polymer solutions, containing various concentrations of cationic, anionic amphoteric and nonionic surfactants is a complicated process. The type of additive which is used for the functionalization of the fibers changes an electroconductivity, surface tension, viscosity, therefore rheological method for controlling the process was used. The properties of the electrospun materials like air permeability, water vapor permeability, mass and thickness are examined as well. The average diameters of the produced bicomponent fibers were in the range 100-420 nm. Water-resistant nano fibrous materials were obtained by thermal crosslinking at 100 oC for 12 h. Fourier transformed infrared spectroscopy (FTIR) showed that PVA/BW nanofibers are present in a stable form. A further project employs to examine the received bilayer material to determine their biological activity and their potential use as plasters for regeneration of skin injuries.

Synthesis and transport properties of poly(vinyl alcohol) acrylonitrile/2-hydroxyethyl methacrylate membranes—I. Synthesis of poly(vinyl alcohol) acrylonitrile 2-hydroxyethyl methacrylate graft copolymer