Ultrasound-Driven On-Demand Transient Triboelectric Nanogenerator for Subcutaneous Antibacterial Activity

Electrotherapy and health monitoring with piezoelectric and triboelectric technologies

The growing need for advanced therapeutic and diagnostic technologies has spurred interest in self-powered systems for electrotherapy and health monitoring. Traditional battery-dependent medical devices (MDs) face limitations, including short operational lifespans and patient discomfort, underscoring the importance of sustainable alternatives. Mechanical energy harvesting technologies, such as piezoelectric and triboelectric nanogenerators (PENGs and TENGs), have emerged as promising solutions, enabling real-time health monitoring and non-invasive electrotherapy by converting biomechanical energy into electricity. This review explores the latest advancements in PENG and TENG technologies, emphasizing their applications in wound healing, neural regeneration, and real-time physiological monitoring. Strategies to overcome hurdles are discussed, demonstrating the transformative potential of PENG and TENG systems in next-generation MDs. By integrating energy harvesting with therapeutic and monitoring functionalities, piezoelectric and triboelectric systems offer a path toward non-invasive, efficient, and patient-centric medical solutions.

Advances in Triboelectric Nanogenerators for Microbial Disinfection

The global COVID-19 pandemic has highlighted the pivotal role of microbial disinfection technologies, driving the demand for innovative, efficient, and sustainable solutions. Triboelectric technology, known for efficiently converting ambient mechanical energy into electrical energy, has emerged as a promising candidate to address these needs. Self-powered electro-based microbial disinfection using triboelectric nanogenerators (TENGs) has emerged as a promising solution. TENGs have demonstrated effective disinfection capabilities in various settings, including water, air, surfaces, and wounds. This review explores the advancements in TENG-based microbial disinfection, highlighting its mechanisms and applications. By utilizing triboelectric technology, it provides comprehensive insights into the development of sustainable and efficient solutions for microbial control across diverse environments.

Bio-inspired e-skin with integrated antifouling and comfortable wearing for self-powered motion monitoring and ultra-long-range human-machine interaction

Electronic skin (e-skin) inspired by the sensory function of the skin demonstrates broad application prospects in health, medicine, and human-machine interaction. Herein, we developed a self-powered all-fiber bio-inspired e-skin (AFBI E-skin) that integrated functions of antifouling, antibacterial, biocompatibility and breathability. AFBI E-skin was composed of three layers of electrospun nanofibrous films. The superhydrophobic outer layer Poly(vinylidene fluoride)-silica nanofibrous films (PVDF-SiO2 NFs) possessed antifouling properties against common liquids in daily life and resisted bacterial adhesion. The polyaniline nanofibrous films (PANI NFs) were used as the electrode layer, and it had strong "static" antibacterial capability. Meanwhile, the inner layer Polylactic acid nanofibrous films (PLA NFs) served as a biocompatible substrate. Based on the triboelectric nanogenerator principle, AFBI E-skin not only enabled self-powered sensing but also utilized the generated electrical stimulation for "dynamic" antibacterial. The "dynamic-static" synergistic antibacterial strategy greatly enhanced the antibacterial effect. AFBI E-skin could be used for self-powered motion monitoring to obtain a stable signal output even when water was splashed on its surface. Finally, based on AFBI E-skin, we constructed an ultra-long-range human-machine interaction control system, enabling synchronized hand gestures between human hand and robotic hand in any internet-covered area worldwide theoretically. AFBI E-skin exhibited vast application potential in fields like smart wearable electronics and intelligent robotics.

Stimuli-responsive and targeted nanomaterials: Revolutionizing the treatment of bacterial infections

Bacterial infections have emerged as a major threat to global public health. The effectiveness of traditional antibiotic treatments is waning due to the increasing prevalence of antimicrobial resistance, leading to an urgent demand for alternative antibacterial technologies. In this context, antibacterial nanomaterials have proven to be powerful tools for treating antibiotic-resistant and recurring infections. Targeting nanomaterials not only enable the precise delivery of bactericidal agents but also ensure controlled release at the infection site, thereby reducing potential systemic side effects. This review collates and categorizes nanomaterial-based responsive and precision-targeted antibacterial strategies into three key types: exogenous stimuli-responsive (including light, ultrasound, magnetism), bacterial microenvironment-responsive (such as pH, enzymes, hypoxia), and targeted antibacterial action (involving electrostatic interaction, covalent bonding, receptor-ligand mechanisms). Furthermore, we discuss recent advances, potential mechanisms, and future prospects in responsive and targeted antimicrobial nanomaterials, aiming to provide a comprehensive overview of the field's development and inspire the formulation of novel, precision-targeted antimicrobial strategies.

BaTiO3 Doping Enhances Ultrasound-Driven Piezoelectric Bactericidal Effects of Fibrous Poly(L-Lactic Acid) Dressings to Accelerate Septic Wound Healing

Bacterial invasion in infected skin wounds triggers inflammation and impedes healing. Current therapeutic strategies incorporating drug interventions within wound dressings often result in drug resistance and delayed healing. Here, we developed a comprehensive therapeutic modality integrating piezoelectric fibrous dressing with controlled ultrasound stimulation for efficient healing in an infected wound model. The electrospun fibrous dressings composed of barium titanate (BaTiO3) doped poly(L-lactic acid) (PLLA) possess improved piezoelectric properties due to the aligned structure and high crystallinity, which achieved superior bactericidal efficacy upon ultrasound-mechanical-electric conversion that results in the production of reactive oxygen species (ROS). There were 88.72% and 90.43% killing rates of Staphylococcus aureus and Escherichia coli respectively upon ultrasound stimulation without any need for exogenous drugs, and a wound closure rate of 95.5% within 10 days. The in vivo results confirmed that this dressing effectively shortened wound closure time by about 2 days, with a much-improved healing rate of 14% compared with previously reported therapeutic strategies. This was accompanied by reduced inflammation and increased re-epithelialization and angiogenesis. Hence, our synergistic treatment by piezoelectric materials and controlled ultrasound stimulation provides a drug-free alternative approach in regenerative tissue engineering for simultaneously enhancing antibacterial effects and promoting wound healing.

High-frequency ultrasound modulation of Zn2+ release from nanoclay supported ZnO antibacterial composites

Bacterial infections pose considerable health risks, emphasising the critical need for effective and biocompatible antibacterial drugs. Considerably, we developed an efficient antimicrobial system incorporating the combined potential of high-frequency ultrasound and antimicrobial drugs against bacterial infections. A ZnO-kaolinite (Kaol) composite with antibacterial properties was synthesised by growing ZnO on the Kaol nano-clay surface using the co-precipitation method. High-frequency ultrasound efficiently promotes the release of Zn2+, which enhances the antibacterial properties. Furthermore, in-depth in vitro antibacterial studies and bacterial live/dead staining experiments validate the exceptionally high antibacterial performance of the composite. Therefore, owing to the synergistic effects of high-frequency ultrasound and antibacterial properties, the as-prepared novel antibacterial composite is a promising potential substitute for conventional antibacterial agents.

Biocompatible triboelectric energy generators (BT-TENGs) for energy harvesting and healthcare applications

Electronic waste (e-waste) has become a significant environmental and societal challenge, necessitating the development of sustainable alternatives. Biocompatible and biodegradable electronic devices offer a promising solution to mitigate e-waste and provide viable alternatives for various applications, including triboelectric nanogenerators (TENGs). This review provides a comprehensive overview of recent advancements in biocompatible, biodegradable, and implantable TENGs, emphasizing their potential as energy scavengers for healthcare devices. The review delves into the fabrication processes of self-powered TENGs using natural biopolymers, highlighting their biodegradability and compatibility with biological tissues. It further explores the biomedical applications of ultrasound-based TENGs, including their roles in wound healing and energy generation. Notably, the review presents the novel application of TENGs for vagus nerve stimulation, demonstrating their potential in neurotherapeutic interventions. Key findings include the identification of optimal biopolymer materials for TENG fabrication, the effectiveness of TENGs in energy harvesting from physiological movements, and the potential of these devices in regenerative medicine. Finally, the review discusses the challenges in scaling up the production of implantable TENGs from biomaterials, addressing issues such as mechanical stability, long-term biocompatibility, and integration with existing medical devices, outlining future research opportunities to enhance their performance and broaden their applications in the biomedical field.

Ultrasonic nanotechnology for the effective management of Staphylococcus aureus skin infections: an update

Ultrasonic nanotechnology represents a groundbreaking advancement in the management of Staphylococcus aureus skin infections, addressing the significant limitations of conventional treatments. S. aureus poses substantial challenges, including antibiotic resistance and biofilm formation, necessitating novel and effective approaches. By harnessing the power of ultrasonic waves and nanostructures, this technology offers a precise mechanism to disrupt bacterial cells, enhancing antibiotic susceptibility and facilitating the eradication of bacterial colonies. This innovative approach not only improves treatment outcomes, but also offers a non-invasive and highly efficient alternative to traditional methods. Recent studies have demonstrated the remarkable efficacy of ultrasonic nanotechnology, showcasing its ability to revolutionize the treatment paradigm for S. aureus infections. Ongoing research is dedicated to refining treatment protocols, developing new nanostructures, and assessing clinical applicability, with a focus on overcoming challenges such as scalability and long-term effectiveness. This review provides a comprehensive overview of the current state of ultrasonic nanotechnology in combating S. aureus skin infections, detailing its mechanism of action, summarizing key research findings, and highlighting its superiority over conventional modalities. Accumulating evidence underscores its potential as a pivotal development in modern science and technology, promising significant advancements in infection management strategies. As research continues to evolve, the optimization of protocols, exploration of innovative applications, and translation into clinical practice are poised to further solidify the transformative impact of ultrasonic nanotechnology in the medical field.

Advanced Ultrasound Energy Transfer Technologies using Metamaterial Structures

Wireless energy transfer (WET) based on ultrasound-driven generators with enormous beneficial functions, is technologically in progress by the valuation of ultrasonic metamaterials (UMMs) in science and engineering domains. Indeed, novel metamaterial structures can develop the efficiency of mechanical and physical features of ultrasound energy receivers (US-ETs), including ultrasound-driven piezoelectric and triboelectric nanogenerators (US-PENGs and US-TENGs) for advantageous applications. This review article first summarizes the fundamentals, classification, and design engineering of UMMs after introducing ultrasound energy for WET technology. In addition to addressing using UMMs, the topical progress of innovative UMMs in US-ETs is conceptually presented. Moreover, the advanced approaches of metamaterials are reported in the categorized applications of US-PENGs and US-TENGs. Finally, some current perspectives and encounters of UMMs in US-ETs are offered. With this objective in mind, this review explores the potential revolution of reliable integrated energy transfer systems through the transformation of metamaterials into ultrasound-driven active mediums for generators.

Enhancing Ultrasound Power Transfer: Efficiency, Acoustics, and Future Directions

Implantable medical devices (IMDs), like pacemakers regulating heart rhythm or deep brain stimulators treating neurological disorders, revolutionize healthcare. However, limited battery life necessitates frequent surgeries for replacements. Ultrasound power transfer (UPT) emerges as a promising solution for sustainable IMD operation. Current research prioritizes implantable materials, with less emphasis on sound field analysis and maximizing energy transfer during wireless power delivery. This review addresses this gap. A comprehensive analysis of UPT technology, examining cutting-edge system designs, particularly in power supply and efficiency is provided. The review critically examines existing efficiency models, summarizing the key parameters influencing energy transmission in UPT systems. For the first time, an energy flow diagram of a general UPT system is proposed to offer insights into the overall functioning. Additionally, the review explores the development stages of UPT technology, showcasing representative designs and applications. The remaining challenges, future directions, and exciting opportunities associated with UPT are discussed. By highlighting the importance of sustainable IMDs with advanced functions like biosensing and closed-loop drug delivery, as well as UPT's potential, this review aims to inspire further research and advancements in this promising field.

Recent Progress in Implantable Drug Delivery Systems

In recent years, tremendous effort is devoted to developing platforms, such as implantable drug delivery systems (IDDSs), with temporally and spatially controlled drug release capabilities and improved adherence. IDDSs have multiple advantages: i) the timing and location of drug delivery can be controlled by patients using specific stimuli (light, sound, electricity, magnetism, etc.). Some intelligent "closed-loop" IDDS can even realize self-management without human participation. ii) IDDSs enable continuous and stable delivery of drugs over a long period (months to years) and iii) to administer drugs directly to the lesion, thereby helping reduce dosage and side effects. iv) IDDSs enable personalized drug delivery according to patient needs. The high demand for such systems has prompted scientists to make efforts to develop intelligent IDDS. In this review, several common stimulus-responsive mechanisms including endogenous (e.g., pH, reactive oxygen species, proteins, etc.) and exogenous stimuli (e.g., light, sound, electricity, magnetism, etc.), are given in detail. Besides, several types of IDDS reported in recent years are reviewed, including various stimulus-responsive systems based on the above mechanisms, radio frequency-controlled IDDS, "closed-loop" IDDS, self-powered IDDS, etc. Finally, the advantages and disadvantages of various IDDS, bottleneck problems, and possible solutions are analyzed to provide directions for subsequent research.

Accelerated Wound Healing by Electrospun Multifunctional Fibers with Self-Powered Performance

Wound healing has been a persistent clinical challenge for a long time. Electrical stimulation is an effective therapy with the potential to accelerate wound healing. In this work, the self-powered electrospun nanofiber membranes (triples) were constructed as multifunctional wound dressings with electrical stimulation and biochemical capabilities. Triple was composed of a hydrolyzable inner layer with antiseptic and hemostatic chitosan, a hydrophilic core layer loaded with conductive AgNWs, and a hydrophobic outer layer fabricated by self-powered PVDF. Triple exhibited presentable wettability and acceptable moisture permeability. Electrical performance tests indicated that triple can transmit electrical signals formed by the piezoelectric effect to the wound. High antibacterial activities were observed for triple against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, with inhibition rates of 96.52, 98.63, and 97.26%, respectively. In vitro cell assays demonstrated that triple cells showed satisfactory proliferation and mobility. A whole blood clotting test showed that triple can enhance hemostasis. The innovative self-powered multifunctional fibers presented in this work offer a promising approach to addressing complications and expediting the promotion of chronic wound healing.

Collaborative Optimization Design of Self-Powered Sterilizer with Highly Efficient Synergistic Antibacterial Effect

The development of self-powered sterilizers has garnered significant attention in the scientific and engineering fields. However, there remains an urgent need to improve their sterilization efficiency. In this study, we present a self-powered sterilizer with superior antibacterial capability by maximizing the utilization of breakdown discharge generated by a soft-contact freestanding rotary triboelectric nanogenerator (FR-TENG). To achieve this, a collaborative optimization strategy is proposed, encompassing the structural design of the FR-TENG, the implementation of double voltage rectification, and manipulation of the gaseous phase. Through a comprehensive analysis of antibacterial rates and microscopic images, the effectiveness of the self-powered sterilizer against various types of bacteria, including Gram-positive and Gram-negative species, as well as mixed bacteria in natural seawater, is demonstrated. Further investigations into bacterial morphologies and solution compositions reveal that the synergistic effect between electroporation and the generation of reactive oxygen/nitrogen species contributes to efficient sterilization. Additionally, controlled trials and molecular dynamics simulations are conducted to quantitatively elucidate the synergistic antibacterial effect between electroporation and reactive oxygen/nitrogen species. This study highlights the effectiveness of the collaborative optimization strategy in enhancing the sterilization efficiency of self-powered sterilizers while providing valuable insights into the synergistic antibacterial mechanisms of physical and chemical sterilization.

Bulk Schottky Junctions-Based Flexible Triboelectric Nanogenerators to Power Backscatter Communications in Green 6G Networks

This work introduces a novel method to construct Schottky junctions to boost the output performance of triboelectric nanogenerators (TENGs). Perovskite barium zirconium titanate (BZT) core/metal silver shell nanoparticles are synthesized to be embedded into electrospun polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers before they are used as tribo-negative layers. The output power of TENGs with composite fiber mat exhibited >600% increase compared to that with neat polymer fiber mat. The best TENG achieved 1339 V in open-circuit voltage, 40 µA in short-circuit current and 47.9 W m-2 in power density. The Schottky junctions increased charge carrier density in tribo-layers, ensuring a high charge transfer rate while keeping the content of conductive fillers low, thus avoiding charge loss and improving performance. These TENGs are utilized to power radio frequency identification (RFID) tags for backscatter communication (BackCom) systems, enabling ultra-massive connectivity in the 6G wireless networks and reducing information communications technology systems' carbon footprint. Specifically, TENGs are used to provide an additional energy source to the passive tags. Results show that TENGs can boost power for BackCom and increase the communication range by 386%. This timely contribution offers a novel route for sustainable 6G applications by exploiting the expanded communication range of BackCom tags.

From Body Monitoring to Biomolecular Sensing: Current Progress and Future Perspectives of Triboelectric Nanogenerators in Point-of-Care Diagnostics

In the constantly evolving field of medical diagnostics, triboelectric nanogenerators (TENGs) stand out as a groundbreaking innovation for simultaneously harnessing mechanical energy from micromovements and sensing stimuli from both the human body and the ambient environment. This advancement diminishes the dependence of biosensors on external power sources and paves the way for the application of TENGs in self-powered medical devices, especially in the realm of point-of-care diagnostics. In this review, we delve into the functionality of TENGs in point-of-care diagnostics. First, from the basic principle of how TENGs effectively transform subtle physical movements into electrical energy, thereby promoting the development of self-powered biosensors and medical devices that are particularly advantageous for real-time biological monitoring. Then, the adaptable design of TENGs that facilitate customization to meet individual patient needs is introduced, with a focus on their biocompatibility and safety in medical applications. Our in-depth analysis also covers TENG-based biosensor designs moving toward exceptional sensitivity and specificity in biomarker detection, for accurate and efficient diagnoses. Challenges and future prospects such as the integration of TENGs into wearable and implantable devices are also discussed. We aim for this review to illuminate the burgeoning field of TENG-based intelligent devices for continuous, real-time health monitoring; and to inspire further innovation in this captivating area of research that is in line with patient-centered healthcare.

Polyvinyl alcohol-based economical triboelectric nanogenerator for self-powered energy harvesting applications

Triboelectric nanogenerators (TENGs) have emerged as a promising alternative for powering small-scale electronics without relying on traditional power sources, and play an important role in the development of the internet of things (IoTs). Herein, a low-cost, flexible polyvinyl alcohol (PVA)-based TENG (PVA-TENG) is reported to harvest low-frequency mechanical vibrations and convert them into electricity. PVA thin film is prepared by a simple solution casting technique and utilized to serve as the tribopositive material, polypropylene film as tribonegative, and aluminum foil as electrodes of the device. The dielectric-dielectric model is implemented with an arch structure for the effective working of the PVA-TENG. The device showed promising electrical output by generating significant open-circuit voltage, short-circuit current, and power . Also, PVA-TENG is subjected to a stability test by operating the device continuously for 5000 cycles. The result shows that, the device is mechanically durable and electrically stable. Further, the as-fabricated PVA-TENG is demonstrated to show feasible applications, such as charging two commercial capacitors with capacitances 1.1 and 4.7μF and powering green light-emitting diodes. The stored energy in the 4.7μF capacitor is utilized to power a digital watch and humidity and temperature sensor without the aid of an external battery. Thus, the PVA-TENG facilitates ease of fabrication, robustness, and cost-effective strategy in the field of energy harvesting for powering lower-grid electronics by demonstrating their potential as a sustainable energy source.

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 Bioresorbable Triboelectric Nanogenerators

With the growing demand for next-generation health care, the integration of electronic components into implantable medical devices (IMDs) has become a vital factor in achieving sophisticated healthcare functionalities such as electrophysiological monitoring and electroceuticals worldwide. However, these devices confront technological challenges concerning a noninvasive power supply and biosafe device removal. Addressing these challenges is crucial to ensure continuous operation and patient comfort and minimize the physical and economic burden on the patient and the healthcare system. This Review highlights the promising capabilities of bioresorbable triboelectric nanogenerators (B-TENGs) as temporary self-clearing power sources and self-powered IMDs. First, we present an overview of and progress in bioresorbable triboelectric energy harvesting devices, focusing on their working principles, materials development, and biodegradation mechanisms. Next, we examine the current state of on-demand transient implants and their biomedical applications. Finally, we address the current challenges and future perspectives of B-TENGs, aimed at expanding their technological scope and developing innovative solutions. This Review discusses advancements in materials science, chemistry, and microfabrication that can advance the scope of energy solutions available for IMDs. These innovations can potentially change the current health paradigm, contribute to enhanced longevity, and reshape the healthcare landscape soon.

Implantable, Biodegradable, and Wireless Triboelectric Devices for Cancer Therapy through Disrupting Microtubule and Actins Dynamics

Electric-field-based stimulation is emerging as a new cancer therapeutic modality through interfering with cell mitosis. To address its limitations of complicated wire connections, bulky devices, and coarse spatial resolution, an improved and alternative method is proposed for wirelessly delivering electrical stimulation into tumor tissues through designing an implantable, biodegradable, and wirelessly controlled therapeutic triboelectric nanogenerator (ET-TENG). With the excitation of ultrasound (US) to the ET-TENG, the implanted ET-TENG can generate an alternating current voltage and concurrently release the loaded anti-mitotic drugs into tumor tissues, which synergistically disrupts the assembly of microtubules and filament actins, induces cell cycle arrest, and finally enhances cell death. With the assistance of US, the device can be completely degraded after the therapy, getting free of a secondary surgical extraction. The device can not only work around those unresectable tumors, but also provides a new application of wireless electric field in cancer therapy.

Sandwich-type double-layer piezoelectric nanogenerators based on one- and two-dimensional ZnO nanostructures with improved output performance

Piezoelectric nanogenerators (PENGs) have attracted great interest owing to their broad range application in environmental mechanical energy harvesting to power small electronic devices. In this study, novel flexible and high-performance double-layer sandwich-type PENGs based on one-dimensional (1-D) and two-dimensional (2-D) zinc oxide (ZnO) nanostructures and Ni foam as the middle layer have been developed. The morphology and structure of 1- and 2-D ZnO nanostructures have been studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). To investigate the effect of structural design on the piezoelectric performance, single-layer PENGs were also fabricated. The piezoelectric output of all prepared PENGs were evaluated under different human impacts at various forces and frequencies. The double-layer designed PENGs showed a two times larger voltage output compared to the single-layer PENGs, and the use of Ni foam as middle-layer and of 2-D ZnO nanosheets (compared to 1-D nanorods) was also found to increase the performance of the designed PENGs. The working mechanism of the prepared PENGs is also discussed. The design of nanogenerators as double-layer sandwich structures instead of two integrated single-layer devices reduces the overall preparation time and processing steps and enhances their output performance, thus opening the gate for widening their practical applications.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Strategies for enhancing low-frequency performances of triboelectric, electrochemical, piezoelectric, and dielectric elastomer energy harvesting: recent progress and challenges

Mechanical energy harvesting transforms various forms of mechanical energy, including ocean waves, wind, and human motions, into electrical energy, providing a viable solution to address the depletion of fossil fuels and environmental problems. However, one major obstacle for the direct conversion of mechanical energy into electricity is the low frequency of the majority of mechanical energy sources (≤5 Hz), resulting in low energy conversion efficiency, output power and output current. Over recent years, a numerous innovative technologies have been reported to enable improved energy harvesting utilizing various mechanisms. This review aims to present an in-depth analysis of the research progress in low-frequency energy harvesting technologies that rely on triboelectric, electrochemical, piezoelectric, and dielectric elastomer effects. The discussion commences with an overview of the difficulties associated with low-frequency energy harvesting. The critical aspects that impact the low-frequency performance of mechanical energy harvesters, including working mechanisms, environmental factors, and device compositions, are elucidated, while the advantages and disadvantages of different mechanisms in low-frequency operation are compared and summarized. Moreover, this review expounds on the strategies that can improve the low-frequency energy harvesting performance through the modulations of material compositions, structures, and devices. It also showcases the applications of mechanical energy harvesters in energy harvesting via waves, wind, and human motions. Finally, the recommended choices of mechanical energy harvesters with different mechanisms for various applications are offered, which can assist in the design and fabrication process.

Self-Repairing and Energy-Harvesting Triboelectric Sensor for Tracking Limb Motion and Identifying Breathing Patterns

The increasing prevalence of health problems stemming from sedentary lifestyles and evolving workplace cultures has placed a substantial burden on healthcare systems. Consequently, remote health wearable monitoring systems have emerged as essential tools to track individuals' health and well-being. Self-powered triboelectric nanogenerators (TENGs) have exhibited significant potential for use as emerging detection devices capable of recognizing body movements and monitoring breathing patterns. However, several challenges remain to be addressed in order to fulfill the requirements for self-healing ability, air permeability, energy harvesting, and suitable sensing materials. These materials must possess high flexibility, be lightweight, and have excellent triboelectric charging effects in both electropositive and electronegative layers. In this work, we investigated self-healable electrospun polybutadiene-based urethane (PBU) as a positive triboelectric layer and titanium carbide (Ti3C2Tx) MXene as a negative triboelectric layer for the fabrication of an energy-harvesting TENG device. PBU consists of maleimide and furfuryl components as well as hydrogen bonds that trigger the Diels-Alder reaction, contributing to its self-healing properties. Moreover, this urethane incorporates a multitude of carbonyl and amine groups, which create dipole moments in both the stiff and the flexible segments of the polymer. This characteristic positively influences the triboelectric qualities of PBU by facilitating electron transfer between contacting materials, ultimately resulting in high output performance. We employed this device for sensing applications to monitor human motion and breathing pattern recognition. The soft and fibrous-structured TENG generates a high and stable open-circuit voltage of up to 30 V and a short-circuit current of 4 μA at an operation frequency of 4.0 Hz, demonstrating remarkable cyclic stability. A significant feature of our TENG is its self-healing ability, which allows for the restoration of its functionality and performance after sustaining damage. This characteristic has been achieved through the utilization of the self-healable PBU fibers, which can be repaired via a simple vapor solvent method. This innovative approach enables the TENG device to maintain optimal performance and continue functioning effectively even after multiple uses. After integration with a rectifier, the TENG can charge various capacitors and power 120 LEDs. Moreover, we employed the TENG as a self-powered active motion sensor, attaching it to the human body to monitor various body movements for energy-harvesting and sensing purposes. Additionally, the device demonstrates the capability to recognize breathing patterns in real time, offering valuable insights into an individual's respiratory health.