Flexible Tactile Sensor Based on Patterned Ag-Nanofiber Electrodes through Electrospinning

Recent Advances in Flexible Temperature Sensors: Materials, Mechanism, Fabrication, and Applications

Flexible electronics is an emerging and cutting-edge technology which is considered as the building blocks of the next generation micro-nano electronics. Flexible electronics integrate both active and passive functions in devices, driving rapid developments in healthcare, the Internet of Things (IoT), and industrial fields. Among them, flexible temperature sensors, which can be directly attached to human skin or curved surfaces of objects for continuous and stable temperature measurement, have attracted much attention for applications in disease prediction, health monitoring, robotic signal sensing, and curved surface temperature measurement. Preparing flexible temperature sensors with high sensitivity, fast response, wide temperature measurement interval, high flexibility, stretchability, low cost, high reliability, and stability has become a research target. This article reviewed the latest development of flexible temperature sensors and mainly discusses the sensitive materials, working mechanism, preparation process, and the applications of flexible temperature sensors. Finally, conclusions based on the latest developments, and the challenges and prospects for research in this field are presented.

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

Metallic Micro-Nano Network-Based Soft Transparent Electrodes: Materials, Processes, and Applications

Soft transparent electrodes (TEs) have received tremendous interest from academia and industry due to the rapid development of lightweight, transparent soft electronics. Metallic micro-nano networks (MMNNs) are a class of promising soft TEs that exhibit excellent optical and electrical properties, including low sheet resistance and high optical transmittance, as well as superior mechanical properties such as softness, robustness, and desirable stability. They are genuinely interesting alternatives to conventional conductive metal oxides, which are expensive to fabricate and have limited flexibility on soft surfaces. This review summarizes state-of-the-art research developments in MMNN-based soft TEs in terms of performance specifications, fabrication methods, and application areas. The review describes the implementation of MMNN-based soft TEs in optoelectronics, bioelectronics, tactile sensors, energy storage devices, and other applications. Finally, it presents a perspective on the technical difficulties and potential future possibilities for MMNN-based TE development.

Highly Sensitive Flexible Pressure Sensors with Hybrid Microstructures Similar to Volcano Sponge

Preparing hybrid microstructures on flexible substrates is a crucial approach to achieving highly sensitive flexible pressure sensors. However, the preparation of hybrid microstructures on soft materials often faces complex, time-consuming, and costly problems, which hampers the advancement of highly sensitive flexible sensors. Herein, based on a 3D-printing template and a household microwave oven, a simple, green, and one-step microwave irradiation process using glucose porogen is applied to develop a flexible pressure sensor with a volcano-sponge-like porous dome structure based on porous polydimethylsiloxane (PDMS). Due to the easily deformable porous dome on the porous PDMS substrate, the flexible pressure sensor showcases exceptional sensitivity of 611.85 kPa-1 in 0-1 and 50.31 kPa-1 over a wide range of 20-80 kPa. Additionally, the sensor takes only 43 ms to respond, 123 ms to recover, and presents excellent stability (>1100 cycles). In application testing, the sensor effectively captures pulse signals, speech signals, tactile signals from a mechanical gripper, and gesture signals, demonstrating its potential applications in medical diagnosis and robotics. In conclusion, the microwave irradiation method based on template and glucose porogen provides a new way for the simple, low-cost, and green preparation of porous-surface hybrid microstructures on polymers and high-performance flexible pressure sensors.

A Nonlinear Impact-Driven Triboelectric Vibration Energy Harvester for Frequency Up-Conversion

Energy harvesting effectively powers micro-sensors and wireless applications. However, higher frequency oscillations do not overlap with ambient vibrations, and low power can be harvested. This paper utilizes vibro-impact triboelectric energy harvesting for frequency up-conversion. Two magnetically coupled cantilever beams with low and high natural frequencies are used. The two beams have identical tip magnets at the same polarity. A triboelectric energy harvester is integrated with the high-frequency beam to generate an electrical signal via contact-separation impact motion between the triboelectric layers. An electrical signal is generated at the low-frequency beam range achieving frequency up-converter. The two degrees of freedom (2DOF) lumped-parameter model system is used to investigate the system's dynamic behavior and the corresponding voltage signal. The static analysis of the system revealed a threshold distance of 15 mm that divides the system into monostable and bistable regimes. In the monostable and bistable regimes, softening and hardening behaviors were observed at low frequencies. Additionally, the threshold voltage generated was increased by 1117% in comparison with the monostable regime. The simulation findings were experimentally validated. The study demonstrates the potential of using triboelectric energy harvesting in frequency up-converting applications.

Fabrication of Unidirectional Water Permeable PS/PET Composite Nanofibers Modified with Silver Nanoparticles via Electrospinning

In this study, the composite nanofiber membranes (AgNPs-PS/PET) composed of hydrophobic polystyrene (PS) embedded with different additions of silver nanoparticles (AgNPs) and hydrophilic hydrolyzed polyethylene terephthalate (PET) were prepared via electrospinning technology to achieve the function of unidirectional water penetration. The addition of AgNO₃ was at 0 wt%, 0.5 wt%, 1.0 wt% and 1.5 wt% as the variables. The surface morphology and structure of AgNPs-PS/PET composite nanofibers were characterized by scanning electron microscopy (SEM), x-ray energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The SEM image showed that the fibers of the composite materials were continuous and uniform as a result of electrospinning. The presence and content of Ag nanoparticles dispersed in the nanofibers were investigated using EDS and TEM. The contact angle (CA) was tested to illustrate the wettability of the composite nanofiber membranes using a static contact angle measuring instrument and the process of unidirectional water penetration was recorded. Meanwhile, the mechanism of unidirectional water penetration was analyzed. Moreover, the electrospinning solution's viscosity and conductivity were also investigated. Eventually, the optimal addition of AgNO₃ (1.0 wt%) was confirmed and the prepared AgNPs-PS/PET composite nanofiber membranes were able to achieve the function of unidirectional water penetration. These membranes have the potential to be applied in smart textiles, unidirectional water collection and wound dressing.

Rational Design of Advanced Triboelectric Materials for Energy Harvesting and Emerging Applications

The properties of materials play a significant role in triboelectric nanogenerators (TENGs). Advanced triboelectric materials for TENGs have attracted tremendous attention because of their superior advantages (e.g., high specific surface area, high porosity, and customizable macrostructure). These advanced materials can be extensively applied in numerous fields, including energy harvester, wearable electronics, filtration, and self-powered sensors. Hence, designing triboelectric materials as advanced functional materials is important for the development of TENGs. Herein, the structural modification methods based on electrospinning to improve the triboelectric properties and the latest research progress in this kind of TENGs are systematically summarized. Preparation methods and design trends of nanofibers, microspheres, hierarchical structures, and doping nanomaterials are highlighted. The factors influencing the formation and properties of triboelectric materials are considered. Furthermore, the latest progress on the applications of TENGs is systematically elaborated. Finally, the challenges in the development of triboelectric materials are discussed, thereby guiding researchers in the large-scale application of TENGs.

PDMS Microfabrication and Design for Microfluidics and Sustainable Energy Application: Review

The polydimethylsiloxane (PDMS) is popular for wide application in various fields of microfluidics, microneedles, biology, medicine, chemistry, optics, electronics, architecture, and emerging sustainable energy due to the intrinsic non-toxic, transparent, flexible, stretchable, biocompatible, hydrophobic, insulating, and negative triboelectric properties that meet different requirements. For example, the flexibility, biocompatibility, non-toxicity, good stability, and high transparency make PDMS a good candidate for the material selection of microfluidics, microneedles, biomedical, and chemistry microchips as well as for optical examination and wearable electronics. However, the hydrophobic surface and post-surface-treatment hydrophobic recovery impede the development of self-driven capillary microchips. How to develop a long-term hydrophilicity treatment for PDMS is crucial for capillary-driven microfluidics-based application. The dual-tone PDMS-to-PDMS casting for concave-and-convex microstructure without stiction is important for simplifying the process integration. The emerging triboelectric nanogenerator (TENG) uses the transparent flexible PDMS as the high negative triboelectric material to make friction with metals or other positive-triboelectric material for harvesting sustainably mechanical energy. The morphology of PDMS is related to TENG performance. This review will address the above issues in terms of PDMS microfabrication and design for the efficient micromixer, microreactor, capillary pump, microneedles, and TENG for more practical applications in the future.