Beyond Metallic Electrode: Spontaneous Formation of Fluidic Electrodes from Operational Liquid in Highly Functional Droplet-Based Electricity Generator.

The droplet-based electricity generator (DEG) has facilitated efficient droplet energy harvesting, yet diversifying its applications necessitates the incorporation of various to the DEG. This study first proposes a methodology for advancing the DEG by substituting its conventional metallic electrode with electrically conductive water electrode (WE), which is spontaneously generated during the operation of the DEG with operating liquid. Due to the inherent conductive and fluidic nature of water, the introduction of the WE maintains the electrical output performance of the DEG while imparting functionalities such as high transparency and flexibility. So, the resultant WE applied DEG (WE-DEG) exhibits high optical transmittance (≈99%) and retains its electricity-generating capability under varying deformations, including bending and stretching. This innovation expands the versatility of the DEG, and especially, a sun-raindrop dual-mode energy harvester is demonstrated by hybridizing the WE-DEG and photovoltaic (PV) cell. This hybridization effectively addresses the weather-dependent limitations inherent in each energy harvester and enhances the temperature-induced inefficiencies typically observed in PV cells, thereby enhancing the overall efficiency. The introduction of the WE will be poised to catalyze new developments in DEG research, paving the way for broader applicability and enhanced efficiency in droplet energy harvesting technologies.

Sustainable Thermal Regulation of Electronics via Mitigated Supercooling of Porous Gallium-Based Phase Change Materials.

Gallium liquid metal is one of the promising phase change materials for passive thermal management of electronics due to their high thermal conductivity and latent heat per volume. However, it suffers from severe supercooling, in which molten gallium does not return to solid due to the lack of nucleation. It may require 28.2 °C lower temperature than the original freezing point to address supercooling, leading to unstable thermal regulation performance along fluctuations of cooling condition. Here, gallium is infused into porous copper in an oxide-free environment, forming intermetallic compound impurities at the interfaces to reduce the activation energy for heterogeneous nucleation. The porous-shaped gallium provides ≈63% smaller supercooling than that of the bulk type due to large specific surface area (≈9,070 cm2 per cm3) and high wetting characteristics (≈16° of contact angle) on CuGa2 intermetallic layer. During repetitive heating-cooling cycles, porous-shaped gallium consistently shows propagation of crystallization at even near room temperature (≈25 °C) while maintaining stable performance as thermal buffer, whereas droplet-shaped gallium is gradually degraded due to partial-supercooled state. The findings will improve the responsive thermal regulation performance to relieve a rapid increase in temperature of semiconductors/batteries, and also have a potential for energy storage applications.

Environmentally Robust Triboelectric Tire Monitoring System for Self-Powered Driving Information Recognition via Hybrid Deep Learning in Time-Frequency Representation.

Developing a robust artificial intelligence of things (AIoT) system with a self-powered triboelectric sensor for harsh environment is challenging because environmental fluctuations are reflected in triboelectric signals. This study presents an environmentally robust triboelectric tire monitoring system with deep learning to capture driving information in the triboelectric signals generated from tire-road friction. The optimization of the process and structure of a laser-induced graphene (LIG) electrode layer in the triboelectric tire is conducted, enabling the tire to detect universal driving information for vehicles/robotic mobility, including rotation speeds of 200-2000 rpm and contact fractions of line. Employing a hybrid model combining short-term Fourier transform with a convolution neural network-long short-term memory, the LIG-based triboelectric tire monitoring (LTTM) system decouples the driving information, such as traffic lines and road states, from varied environmental conditions of humidity (10%-90%) and temperatures (50-70 °C). The real-time line and road state recognition of the LTTM system is confirmed on a mobile platform across diverse environmental conditions, including fog, dampness, intense sunlight, and heat shimmer. This work provides an environmentally robust monitoring AIoT system by introducing a self-powered triboelectric sensor and hybrid deep learning for smart mobility.

Synergistic Combination of Dual Clays in Multilayered Nanocomposites for Enhanced Flame Retardant Properties.

In an effort to reduce the flammability of synthetic polymeric materials such as cotton fabrics and polyurethane foam (PUF), hybrid nanocoatings are prepared by layer-by-layer assembly. Multilayered nanocomposites of a cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA), are paired with two kinds of clay nanoplatelets, montmorillonite (MMT) and vermiculite (VMT). The physical properties such as thickness and mass and thermal behaviors in clay-based nanocoatings with and without incorporation of tris buffer are compared to assess the effectiveness of amine salts on flame retardant (FR) performances. A PDDA-tris/VMT-MMT system, in which tris buffer is introduced into the cationic PDDA aqueous solution, produces a thicker and heavier coating. Three different systems, including PDDA/MMT, PDDA/VMT-MMT, and PDDA-tris/VMT-MMT, result in conformal coating, retaining the weave structure of the fabrics after being exposed to a vertical and horizontal flame test, while the uncoated sample is completely burned out. The synergistic effects of dual clay-based hybrid nanocoatings are greatly improved by adding amine salts. Cone calorimetry reveals that the PDDA-tris/VMT-MMT-coated PUF eliminates a second peak heat release rate and significantly reduces other FR performances, compared to those obtained from the clay-based multilayer films with no amine salts added. Ten bilayers of PDDA-tris/VMT-MMT (≈250 nm thick) maintain the shape of foam after exposure to a butane torch flame for 12 s. As for practical use of these nanocomposites in real fire disasters, spray-assisted PDDA-tris/VMT-MMT multilayers on woods exhibit high resistance over flammability. Improved fire resistance in PDDA-tris/VMT-MMT is believed to be due to the enhanced char yield through the addition of tris buffer that promotes the deposition of more clay particles while retaining a highly ordered deposition of a densely packed nanobrick wall structure. This work demonstrates the ability to impart significant fire resistance to synthetic polymer materials in a fully renewable nanocoating that uses environmentally benign chemistry.

Green synthesis of ZnO nanocubes from Ceropegia omissa H. Huber extract for photocatalytic degradation of bisphenol An under visible light to mitigate water pollution.

Plastic pollution has become a major environmental problem because it does not break down and poses risks to ecosystems and human health. This study focuses on the environmentally friendly synthesis of ZnO nanocubes using an extract from Ceropegia omissa H. Huber plant leaves. The primary goal is to investigate the viability of these nanocubes as visible-light photocatalysts for the degradation of bisphenol A (BPA). The synthesized ZnO nanocubes have a highly crystalline structure and a bandgap of 3.1 eV, making them suitable for effective visible-light photocatalysis. FTIR analysis, which demonstrates that the pertinent functional groups are present, demonstrates the chemical bonding and reducing processes that take place in the plant extract. The XPS method also studies zinc metals, oxygen valencies, and binding energies. Under visible light irradiation, ZnO nanocubes degrade BPA by 86% in 30 min. This plant-extract-based green synthesis method provides a long-term replacement for traditional procedures, and visible light photocatalysis has advantages over ultraviolet light. The study's results show that ZnO nanocubes may be good for the environment and can work well as visible light photocatalysts to break down organic pollutants. This adds to what is known about using nanoparticles to clean up the environment. As a result, this study highlights the potential of using environmentally friendly ZnO nanocubes as a long-lasting and efficient method of reducing organic pollutant contamination in aquatic environments.

Advancing Energy Harvesting Efficiency from a Single Droplet: A Mechanically Guided 4D Printed Elastic Hybrid Droplet-Based Electricity Generator.

A droplet possesses the ubiquity and potential to harvest a vast amount of energy. To exploit droplets effectively, a novel output enhancement strategy that can coexist and create synergy with the recently studied droplet-based electricity generator (DEG) and material/surface structure modification must be investigated. In this study, a mechanical buckling-based 4D printed elastic hybrid droplet-based electricity generator (HDEG) consisting of a DEG and solid-solid triboelectric nanogenerator (S-S TENG) is first presented. During the electricity generation process of the DEG by droplet impact, the HDEG structure, which is merged via a simple 4D printing technique, permits the conversion of dissipated energy into elastic energy, resulting in an S-S TENG output. The HDEG outputs are naturally integrated owing to the simultaneous activation of a single droplet, resulting in an approximately 30% improvement over the output of a single DEG. Internal and external parametric studies are performed as HDEG design guidelines. The HDEG exhibits a 25% better energy supply performance than that of a single DEG, demonstrating its applicability as a power source. This research proposes the way toward a hybrid system that efficiently harvests energy from ubiquitous droplets.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications.

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

A Liquid Triboelectric Series.

The triboelectric series is a generally accepted method for describing the triboelectric effect. It provides a way to control the double face of the ubiquitous triboelectric effect: causes of unpredictable accidents and the resultant surface charge as energy sources. However, previous studies have been biased in solids despite being observed in liquids (liquid-solid contact electrification). Therefore, a liquid triboelectric series is necessary to be established to manipulate the liquid triboelectric effect according to the appropriate goal. In this study, a liquid triboelectric series is first established to describe the triboelectric properties of each liquid when contact electrification occurs with a solid surface. The liquid triboelectric series covers electrolytes, organic solvents, oxidants, and higher sugar alcohols. Common chemical groups can be derived from the liquid triboelectric series that hydroxyl groups enhance, and benzene groups suppress the liquid triboelectric effect. The results are demonstrated by the amplified efficiency of an energy harvester and particle contamination after surface washing. This study will play a pivotal role in understanding the liquid-solid contact electrification phenomenon and providing new perspectives on the applications of the liquid triboelectric effect.

Microfluidic nanodevices for drug sensing and screening applications.

The outbreak of pandemics (e.g., severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 in 2019), influenza A viruses (H1N1 in 2009), etc.), and worldwide spike in the aging population have created unprecedented urgency for developing new drugs to improve disease treatment. As a result, extensive efforts have been made to design novel techniques for efficient drug monitoring and screening, which form the backbone of drug development. Compared to traditional techniques, microfluidics-based platforms have emerged as promising alternatives for high-throughput drug screening due to their inherent miniaturization characteristics, low sample consumption, integration, and compatibility with diverse analytical strategies. Moreover, the microfluidic-based models utilizing human cells to produce in-vitro biomimetics of the human body pave new ways to predict more accurate drug effects in humans. This review provides a comprehensive summary of different microfluidics-based drug sensing and screening strategies and briefly discusses their advantages. Most importantly, an in-depth outlook of the commonly used detection techniques integrated with microfluidic chips for highly sensitive drug screening is provided. Then, the influence of critical parameters such as sensing materials and microfluidic platform geometries on screening performance is summarized. This review also outlines the recent applications of microfluidic approaches for screening therapeutic and illicit drugs. Moreover, the current challenges and the future perspective of this research field is elaborately highlighted, which we believe will contribute immensely towards significant achievements in all aspects of drug development.

Polypyrrole-coated nanocellulose for solar steam generation: A multi-surface photothermal ink with antibacterial and antifouling properties.

Solar energy-based steam generation holds immense potential to tackle the problem of 1.1 billion people lacking access to freshwater and 2.7 billion experiencing freshwater scarcity at least one month a year. Efficient, portable, and universal photothermal materials are required for popularity of solar-driven evaporation systems. Herein, a facile one-pot process based on solution-processed vapor phase polymerization is adopted to fabricate polypyrrole-coated cellulose nanocrystals (CNC-PPy). The CNC-PPy dispersed in water is used as an ink (CNC-PPy ink) to create photothermal layers. The developed ink is readily laminated on diverse substrates utilizing a common paintbrush that firmly attached without any delamination after drying. The optimized cellulose membrane (6 coating cycles) presents an excellent evaporation rate of 1.96 Kg m-2 h-1 with corresponding light-to-vapor efficiency of 88.92 % at 1 sun. In addition, the CNC-PPy display excellent antibacterial and antifouling properties in powder and laminated forms against E. coli and S. aureus.

Highly efficient patterning technique for silver nanowire electrodes by electrospray deposition and its application to self-powered triboelectric tactile sensor.

A patterned transparent electrode is a crucial component of state-of-the-art wearable devices and optoelectronic devices. However, most of the patterning methods using silver nanowires (AgNWs), which is one of the outstanding candidate materials for the transparent electrode, wasted a large amount of unused AgNWs during the patterning process. Here, we report a highly efficient patterning of AgNWs using electrospray deposition with grounded electrolyte solution (EDGE). During electrospray deposition, a patterned electrolyte solution collector attracted AgNWs by strong electrostatic attraction and selectively deposited them only on the patterned collector, minimizing AgNW deposited elsewhere. The enhanced patterning efficiency was verified through a comparison between the EDGE and conventional process by numerical simulation and experimental validation. As a result, despite the same electrospray deposition conditions for both cases except for the existence of the electrolyte solution collector, the coverage ratio of AgNWs fabricated by the EDGE process was at least six times higher than that of AgNWs produced by the conventional process. Furthermore, the EDGE process provided high design flexibility in terms of not only the material of the substrate, including a polymer and a ceramic but also the shape of the substrate, including a 2D flat and 3D curved surface. As an application of the EDGE process, a self-powered touch sensor exploiting the triboelectric effect was demonstrated. Thus, the EDGE process would be utilized in further application in wearable or implantable devices in the field of biomedicine, intelligent robots, and human-machine interface.

Complex 3D microfluidic architectures formed by mechanically guided compressive buckling.

Microfluidic technologies have wide-ranging applications in chemical analysis systems, drug delivery platforms, and artificial vascular networks. This latter area is particularly relevant to 3D cell cultures, engineered tissues, and artificial organs, where volumetric capabilities in fluid distribution are essential. Existing schemes for fabricating 3D microfluidic structures are constrained in realizing desired layout designs, producing physiologically relevant microvascular structures, and/or integrating active electronic/optoelectronic/microelectromechanical components for sensing and actuation. This paper presents a guided assembly approach that bypasses these limitations to yield complex 3D microvascular structures from 2D precursors that exploit the full sophistication of 2D fabrication methods. The capabilities extend to feature sizes <5 µm, in extended arrays and with various embedded sensors and actuators, across wide ranges of overall dimensions, in a parallel, high-throughput process. Examples include 3D microvascular networks with sophisticated layouts, deterministically designed and constructed to expand the geometries and operating features of artificial vascular networks.

Direct Fabrication of Freestanding and Patterned Nanoporous Junctions in a 3D Micro-Nanofluidic Device for Ion-Selective Transport.

In the field of micro-nanofluidics, a freestanding configuration of a nanoporous junction is highly demanded to increase the design flexibility of the microscale device and the interfacial area between the nanoporous junction and microchannels, thereby improving the functionality and performance. This work first reports direct fabrication and incorporation of a freestanding nanoporous junction in a microfluidic device by performing an electrolyte-assisted electrospinning process to fabricate a freestanding nanofiber membrane and subsequently impregnating the nanofiber membrane with a nanoporous precursor material followed by a solidification process. This process also enables to readily control the geometry of the nanoporous junction depending on its application. By these advantages, vertically stacked 3D micro-nanofluidic devices with complex configurations are easily achieved. To demonstrate the broad applicability of this process in various research fields, a reverse electrodialysis-based energy harvester and an ion concentration polarization-based preconcentrator are produced. The freestanding Nafion-polyvinylidene fluoride nanofiber membrane (F-NPNM) energy harvester generates a high power (59.87 nW) owing to the enlarged interfacial area. Besides, 3D multiplexed and multi-stacked F-NPNM preconcentrators accumulate multiple preconcentrated plugs that can increase the operating sample volume and the degree of freedom of handling. Hence, the proposed process is expected to contribute to numerous research fields related to micro-nanofluidics in the future.

Facile Tailoring of Contact Layer Characteristics of the Triboelectric Nanogenerator Based on Portable Imprinting Device.

Renewable energy harvesting technologies have been actively studied in recent years for replacing rapidly depleting energies, such as coal and oil energy. Among these technologies, the triboelectric nanogenerator (TENG), which is operated by contact-electrification, is attracting close attention due to its high accessibility, light weight, high shape adaptability, and broad applications. The characteristics of the contact layer, where contact electrification phenomenon occurs, should be tailored to enhance the electrical output performance of TENG. In this study, a portable imprinting device is developed to fabricate TENG in one step by easily tailoring the characteristics of the polydimethylsiloxane (PDMS) contact layer, such as thickness and morphology of the surface structure. These characteristics are critical to determine the electrical output performance. All parts of the proposed device are 3D printed with high-strength polylactic acid. Thus, it has lightweight and easy customizable characteristics, which make the designed system portable. Furthermore, the finger tapping-driven TENG of tailored PDMS contact layer with microstructures is fabricated and easily generates 350 V of output voltage and 30 µA of output current with a simple finger tapping motion-related biomechanical energy.

Water-Stable Flexible Nanocellulose Chiral Nematic Films through Acid Vapor Cross-Linked Glutaraldehyde for Chiral Nematic Templating.

Cellulose nanocrystals (CNCs) have drawn considerable attention for their use in optical and sensor applications due to their appealing properties of chiral nematic photonic structures. However, the flexibility and water instability of neat CNC chiral nematic films are questionable and compromise their outstanding properties. We propose a room-temperature process for fabricating flexible, water-stable chiral nematic CNC films. Aqueous glutaraldehyde (GA) was first mixed with CNCs, and then free-standing films were formed by evaporation-induced self-assembly. The chiral nematic dry films that formed were then exposed to hydrochloric acid vapor for subsequent GA cross-linking with CNCs. The GA cross-linked CNC films had a highly ordered chiral nematic organization. The enhanced water stability of the films was demonstrated by using GA cross-linked CNC films as freestanding template substrates for conducting polymers (polypyrrole) and metal oxides (iron oxide) to form flexible chiral nematic photonic hybrids.

Increased Interfacial Area between Dielectric Layer and Electrode of Triboelectric Nanogenerator toward Robustness and Boosted Energy Output.

Given the operation conditions wherein mechanical wear is inevitable, modifying bulk properties of the dielectric layer of a triboelectric nanogenerator (TENG) has been highlighted to boost its energy output. However, several concerns still remain in regards to the modification due to high-cost materials and cumbersome processes being required. Herein, we report TENG with a microstructured Al electrode (TENG_ME) as a new approach to modifying bulk properties of the dielectric layer. The microstructured Al electrode is utilized as a component of TENG to increase the interfacial area between the dielectric layer and electrode. Compared to the TENG with a flat Al electrode (TENG_F), the capacitance of TENG_ME is about 1.15 times higher than that of TENG_F, and the corresponding energy outputs of a TENG_ME are 117 µA and 71 V, each of which is over 1.2 times higher than that of the TENG_F. The robustness of TENG_ME is also confirmed in the measurement of energy outputs changing after sandpaper abrasion tests, repetitive contact, and separation (more than 105 cycles). The results imply that the robustness and long-lasting performance of the TENG_ME could be enough to apply in reliable auxiliary power sources for electronic devices.

Freestanding 3D Mesostructures, Functional Devices, and Shape-Programmable Systems Based on Mechanically Induced Assembly with Shape Memory Polymers.

Capabilities for controlled formation of sophisticated 3D micro/nanostructures in advanced materials have foundational implications across a broad range of fields. Recently developed methods use stress release in prestrained elastomeric substrates as a driving force for assembling 3D structures and functional microdevices from 2D precursors. A limitation of this approach is that releasing these structures from their substrate returns them to their original 2D layouts due to the elastic recovery of the constituent materials. Here, a concept in which shape memory polymers serve as a means to achieve freestanding 3D architectures from the same basic approach is introduced, with demonstrated ability to realize lateral dimensions, characteristic feature sizes, and thicknesses as small as ≈500, 10, and 5 µm simultaneously, and the potential to scale to much larger or smaller dimensions. Wireless electronic devices illustrate the capacity to integrate other materials and functional components into these 3D frameworks. Quantitative mechanics modeling and experimental measurements illustrate not only shape fixation but also capabilities that allow for structure recovery and shape programmability, as a form of 4D structural control. These ideas provide opportunities in fields ranging from micro-electromechanical systems and microrobotics, to smart intravascular stents, tissue scaffolds, and many others.

Development of the Triboelectric Nanogenerator Using a Metal-to-Metal Imprinting Process for Improved Electrical Output.

Triboelectric nanogenerators (TENG), which utilize contact electrification of two different material surfaces accompanied by electrical induction has been proposed and is considered as a promising energy harvester. Researchers have attempted to form desired structures on TENG surfaces and successfully demonstrated the advantageous effect of surface topography on its electrical output performance. In this study, we first propose the structured Al (SA)-assisted TENG (SA-TENG), where one of the contact layers of the TENG is composed of a structured metal surface formed by a metal-to-metal (M2M) imprinting process. The fabricated SA-TENG generates more than 200 V of open-circuit voltage and 60 µA of short-circuit current through a simple finger tapping motion. Given that the utilization of the M2M imprinting process allows for the rapid, versatile and easily accessible structuring of various metal surfaces, which can be directly used as a contact layer of the TENG to substantially enhance its electrical output performance, the present study may considerably broaden the applicability of the TENG in terms of its fabrication standpoint.

A Simple Approach to Characterize Gas-Aqueous Liquid Two-phase Flow Configuration Based on Discrete Solid-Liquid Contact Electrification.

In this study, we first suggest a simple approach to characterize configuration of gas-aqueous liquid two-phase flow based on discrete solid-liquid contact electrification, which is a newly defined concept as a sequential process of solid-liquid contact and successive detachment of the contact liquid from the solid surface. This approach exhibits several advantages such as simple operation, precise measurement, and cost-effectiveness. By using electric potential that is spontaneously generated by discrete solid-liquid contact electrification, the configurations of the gas-aqueous liquid two-phase flow such as size of a gas slug and flow rate are precisely characterized. According to the experimental and numerical analyses on parameters that affect electric potential, gas slugs have been verified to behave similarly to point electric charges when the measuring point of the electric potential is far enough from the gas slug. In addition, the configuration of the gas-aqueous liquid two-phase microfluidic system with multiple gas slugs is also characterized by using the presented approach. For a proof-of-concept demonstration of using the proposed approach in a self-triggered sensor, a gas slug detector with a counter system is developed to show its practicality and applicability.

One-Step Fabrication of Transparent and Flexible Nanotopographical-Triboelectric Nanogenerators via Thermal Nanoimprinting of Thermoplastic Fluoropolymers.

Using thermal nanoimprinting, a novel transparent and flexible nanotopographical triboelectric nanogenerator (TENG), with simultaneous nanoreplication and integration of the contact layer with the electrode layer, is first demonstrated. It is expected that the present rapid one-step fabrication methodology well give "disposability" to the TENG with extremely reduced manufacturing costs, which may allay commercialization concerns.