Use of Surface Penetration Technology to Fabricate Superhydrophobic Multifunctional Strain Sensors with an Ultrawide Sensing Range

Research Progress on Applying Intelligent Sensors in Sports Science

Smart sensors represent a significant advancement in modern sports science, and their effective use enhances the ability to monitor and analyze athlete performance in real time. The integration of these sensors has enhanced the accuracy of data collection related to physical activity, biomechanics, and physiological responses, thus providing valuable insights for performance optimization, injury prevention, and rehabilitation. This paper provides an overview of the research progress in the application of smart sensors in the field of sports science; highlights the current advances, challenges, and future directions in the deployment of smart sensor technologies; and anticipates their transformative impact on sports science and athlete development.

Superhydrophobic wearable sensor: fabrication, application, and perspective

Wearable sensors have attracted considerable interest due to their ability to detect a variety of information generated by human physiological activities through physical and chemical means. The performance of wearable sensors is limited by their stability, and endowing wearable sensors with superhydrophobicity is one of the means to enable them to maintain excellent performance in harsh environments. This review emphasizes the imperative progress in flexible superhydrophobic sensors for wearable devices. Besides, the wettability principle and the mechanism of wearable sensors are briefly introduced to propose the combination of superhydrophobicity and wearable sensors. Next, superhydrophobic substrates for wearable sensors, including but not limited to, polydimethylsiloxane, polyurethane, gel, rubber, and fabric, are described in depth, and also the respective fabrication processes and performances. Moreover, the utility of superhydrophobic wearable sensors in a normal intelligent environment is described, highlighting their application in monitoring physiological signals, such as physical movement, pulse, vibration, temperature, perspiration, respiration, and so on. Finally, this review evaluates the challenges and dilemmas that wearable sensors must be overcome for further development and improve the functional performance of wearable sensors, paving the way for their expansion into advanced wearable sensing systems.

Recent advances in hydrogel-based flexible strain sensors for harsh environment applications

Flexible strain sensors are broadly investigated in electronic skins and human-machine interaction due to their light weight, high sensitivity, and wide sensing range. Hydrogels with unique three-dimensional network structures are widely used in flexible strain sensors for their exceptional flexibility and adaptability to mechanical deformation. However, hydrogels often suffer from damage, hardening, and collapse under harsh conditions, such as extreme temperatures and humidity levels, which lead to sensor performance degradation or even failure. In addition, the failure mechanism in extreme environments remains unclear. In this review, the performance degradation and failure mechanism of hydrogel flexible strain sensors under various harsh conditions are examined. Subsequently, strategies towards the environmental tolerance of hydrogel flexible strain sensors are summarized. Finally, the current challenges of hydrogel flexible strain sensors in harsh environments are discussed, along with potential directions for future development and applications.

Hydrophobic TPU/CNTs-ILs Ionogel as a Reliable Multimode and Flexible Wearable Sensor for Motion Monitoring, Information Transfer, and Underwater Sensing

Ionogel-based sensors have gained widespread attention in recent years due to their excellent flexibility, biocompatibility, and multifunctionality. However, the adaptation of ionogel-based sensors in extreme environments (such as humid, acidic, alkaline, and salt environments) has rarely been studied. Here, thermoplastic polyurethane/carbon nanotubes-ionic liquids (TPU/CNTs-ILs) ionogels with a complementary sandpaper morphology on the surface were prepared by a solution-casting method with a simple sandpaper as the template, and the hydrophobic flexible TPU/CNTs-ILs ionogel-based sensor was obtained by modification using nanoparticles modified with cetyltrimethoxysilane. The hydrophobicity improves the environmental resistance of the sensor. The ionogel-based sensor exhibits multimode sensing performance and can accurately detect response signals from strain (0-150%), pressure (0.1-1 kPa), and temperature (30-100 °C) stimuli. Most importantly, the hydrophobic TPU/CNTs-ILs ionogel-based sensors can be used not only as wearable strain sensors to monitor human motion signals but also for information transfer, writing recognition systems, and underwater activity monitoring. Thus, the hydrophobic TPU/CNTs-ILs ionogel-based sensor offers a new strategy for wearable electronics, especially for applications in extreme environments.

Research Progress and Application of Multimodal Flexible Sensors for Electronic Skin

In recent years, wearable electronic skin has garnered significant attention due to its broad range of applications in various fields, including personal health monitoring, human motion perception, human-computer interaction, and flexible display. The flexible multimodal sensor, as the core component of electronic skin, can mimic the multistimulus sensing ability of human skin, which is highly significant for the development of the next generation of electronic devices. This paper provides a summary of the latest advancements in multimodal sensors that possess two or more response capabilities (such as force, temperature, humidity, etc.) simultaneously. It explores the relationship between materials and multiple sensing capabilities, focusing on both active materials that are the same and different. The paper also discusses the preparation methods, device structures, and sensing properties of these sensors. Furthermore, it introduces the applications of multimodal sensors in human motion and health monitoring, as well as intelligent robots. Finally, the current limitations and future challenges of multimodal sensors will be presented.

High Tensile, Antibacterial, and Conductive Hydrogel Sensor with Multiple Cross-Linked Networks Based on PVA/Sodium Alginate/Zinc Oxide

Hydrogel sensors have attracted a lot of attention due to their great significance for biosensors and human detection, especially their antibacterial properties when in direct contact with the human body. However, it is challenging to improve mechanical and antibacterial performance simultaneously. In this study, by using ultrasonic dispersion technology to attach zinc oxide to cellulose and adding sodium alginate, a multiple cross-linking network is generated, which effectively solves this problem. The proposed poly(vinyl alcohol)/sodium alginate/zinc oxide/hydrogel sensor exhibits not only excellent biocompatibility but also high tensile properties (strain above 2000%). Besides, the sensor also has an antibacterial function (against Escherichia coli and Staphylococcus aureus). The hydrogel acts as a strain sensor and biosensor; it can also be used as a human health detection sensor; its high tensile properties can detect large tensile deformation and small changes in force, such as finger bending, knee bending, and other joint movements, and can also be used as a sound detection sensor to detect speech and breathing. This study provides a simple method to prepare hydrogel sensors that can be useful for human health detection and biosensor development.

Ultrasensitive and ultrastretchable metal crack strain sensor based on helical polydimethylsiloxane

The majority of crack sensors do not offer simultaneously both a significant stretchability and an ultrahigh sensitivity. In this study, we present a straightforward and cost-effective approach to fabricate metal crack sensors that exhibit exceptional performance in terms of ultrahigh sensitivity and ultrahigh stretchability. This is achieved by incorporating a helical structure into the substrate through a modeling process and, subsequently, depositing a thin film of gold onto the polydimethylsiloxane substrate via sputter deposition. The metal thin film is then pre-stretched to generate microcracks. The sensor demonstrates a remarkable stretchability of 300%, an exceptional sensitivity with a maximum gauge factor reaching 107, a rapid response time of 158 ms, minimal hysteresis, and outstanding durability. These impressive attributes are attributed to the deliberate design of geometric structures and careful selection of connection types for the sensing materials, thereby presenting a novel approach to fabricating stretchable and highly sensitive crack-strain sensors. This work offers a universal platform for constructing strain sensors with both high sensitivity and stretchability, showing a far-reaching significance and influence for developing next-generation practically applicable soft electronics.

Superhydrophobic Flexible Strain Sensors Constructed Using Nanomaterials: Their Fabrications and Sustainable Applications

Superhydrophobic flexible strain sensors, which combine superhydrophobic coatings with highly sensitive flexible sensors, significantly enhance sensor performance and expand applications in human motion monitoring. Superhydrophobic coatings provide water repellency, surface self-cleaning, anti-corrosion, and anti-fouling properties for the sensors. Additionally, they enhance equipment durability. At present, many studies on superhydrophobic flexible sensors are still in the early research stage; the wear resistance and stability of sensors are far from reaching the level of industrial application. This paper discusses fundamental theories such as the wetting mechanism, tunneling effect, and percolation theory of superhydrophobic flexible sensors. Additionally, it reviews commonly used construction materials and principles of these sensors. This paper discusses the common preparation methods for superhydrophobic flexible sensors and summarizes the advantages and disadvantages of each method to identify the most suitable approach. Additionally, this paper summarizes the wide-ranging applications of the superhydrophobic flexible sensor in medical health, human motion monitoring, anti-electromagnetic interference, and de-icing/anti-icing, offering insights into these fields.

An Ultrastretchable Gradient Ionogel Induced by a Self-Floating Strategy for Strain Sensing

The fabrication of gradient ionogels for flexible strain sensors remains challenging because of the complex preparation procedures, and it is still difficult to prepare highly stretchable ionogels (strain > 10000%). In this study, a strategy is proposed to successfully fabricate gradient ionogels and apply them to flexible strain sensors by utilizing the self-floating character of the polysiloxane cross-linker. A gradient ionogel with ultrahigh stretchability (>14000%) is prepared via a one-step in situ photopolymerization process of the precursor with long-chain poly(dimethylsiloxane) bis(2-methyl acrylate) (PDMSMA). PDMSMA, which has a self-floating ability and excellent flexibility, induces a gradient composition distribution in the ionogel, thereby endowing the ionogel with superior stretchability and gradient changes in conductivity and adhesivity from the top to the bottom layer. Because of multiple molecular interactions, the bottom surface of the ionogel possesses good resilience and self-adhesion, whereas the top surface, which has a high PDMSMA content, shows a nonsticky performance. As a result, a singular gradient ionogel having both a sticky bottom surface and a nonsticky top surface is achieved. Furthermore, the flexible strain sensor that is created based on these gradient ionogels exhibits high sensitivity (its gauge factor reaching 5.08), a wide detection range (1-1500%), fast response times, and good linearity. Notably, the detection signal remains repeatable over 1000 uninterrupted strain cycles. The fabricated strain sensor was further utilized to monitor joint movements and physiological signals. This work provides a facile strategy for fabricating gradient ionogels and shows their application potential in the field of flexible electronics.

Fabrication of Textured Ni-Coated Carbon Tubes for a Flexible Strain Sensor: Effect of the Device Elastic Modulus on Sensor Performance

The exploration of flexible resistive sensors with excellent performance remains a challenge. In this paper, a nickel-coated carbon tube with a textured structure was prepared as a conductive sensitive material and inserted into the poly(dimethylsiloxane) (PDMS) polymer; interestingly, the sensor performance was controlled by the elastic modulus of the matrix resin. The results show that Pd²⁺ may be adsorbed by the active groups on the surface of a plant fiber as a catalytic center for the reduction of Ni²⁺. After 300 °C annealing, the inner plant fiber would be carbonized and attached to the outside of the nickel tube; to be precise, the textured Ni-encapsulated C tube was fabricated successfully. It is worth noting that the C tube serves as a layer of support for the external Ni coating, providing sufficient mechanical strength. In addition, resistance sensors with different properties were prepared by controlling the elasticity modulus of the PDMS polymer by introducing different contents of curing agents. The limit uniaxial tensile strain was enhanced from 42 to 49% and sensitivity reduced from 0.2 to 2.0% with the elasticity modulus of the matrix resin increasing from 0.32 to 2.2 MPa. As expected, the sensor is obviously appropriate for the detection of elbow joints, human speaking, and human joints with the reduction of the elasticity modulus of the matrix resin. To be precise, the optimal elastic modulus of the sensor matrix resin would facilitate the improvement of its sensitivity to monitor different human behaviors.

Recent Progress in Conducting Polymer Composite/Nanofiber-Based Strain and Pressure Sensors

The Conducting of polymers belongs to the class of polymers exhibiting excellence in electrical performances because of their intrinsic delocalized π- electrons and their tunability ranges from semi-conductive to metallic conductive regime. Conducting polymers and their composites serve greater functionality in the application of strain and pressure sensors, especially in yielding a better figure of merits, such as improved sensitivity, sensing range, durability, and mechanical robustness. The electrospinning process allows the formation of micro to nano-dimensional fibers with solution-processing attributes and offers an exciting aspect ratio by forming ultra-long fibrous structures. This review comprehensively covers the fundamentals of conducting polymers, sensor fabrication, working modes, and recent trends in achieving the sensitivity, wide-sensing range, reduced hysteresis, and durability of thin film, porous, and nanofibrous sensors. Furthermore, nanofiber and textile-based sensory device importance and its growth towards futuristic wearable electronics in a technological era was systematically reviewed to overcome the existing challenges.

Ultrathin Superhydrophobic Flexible Tactile Sensors for Normal and Shear Force Discrimination

Flexible tactile sensors, with the ability to sense and even discriminate between different mechanical stimuli, can enable real-time and precise monitoring of dexterous and complex robotic motions. However, making them ultrathin and superhydrophobic for practical applications is still a great challenge. Here, superhydrophobic flexible tactile sensors with hierarchical micro- and nanostructures, that is, warped graphene nanosheets adhered to micron-height wrinkled surfaces, were constructed using ultrathin medical tape (40 μm) and graphene. The tactile sensor enables the discrimination of normal and shear forces and senses sliding friction and airflow. Moreover, the tactile sensor exhibits high sensitivity to normal and shear forces, extremely low detection limits (15 Pa for normal forces and 6.4 mN for shear forces), and cyclic robustness. Based on the abovementioned characteristics, the tactile sensor enables real-time and accurate monitoring of the robotic arm's motions, such as moving, gripping, and lifting, during the process of picking up objects. The superhydrophobicity even allows the sensor to monitor the motions of the robotic arm underwater in real time. Our tactile sensors have potential applications in the fields of intelligent robotics and smart prosthetics.

Robust Superhydrophobic rGO/PPy/PDMS Coatings on a Polyurethane Sponge for Underwater Pressure and Temperature Sensing

Flexible wearable pressure sensors have attracted great interest from researchers in recent years because of their important applications in human-machine interaction, human behavior detection, medical diagnosis, and other fields. At present, integrating multiple functions such as pressure and temperature sensing and self-cleaning into a single material remains a challenging task. Here, by in situ reduction of graphene oxide (GO) grown on a sponge surface and deposition of polypyrrole (PPy) nanoparticles, we have built a highly sensitive, stable, and multifunctional rGO/PPy/poly(dimethylsiloxane) (PDMS) polyurethane (PU) sponge (GPPS) sensor for the detection of pressure, water level, and temperature. This multifunctional sensor shows excellent pressure-sensing performance, ultrasensitive loading sensing of a leaf (98 mg), and outstanding reproducibility over 5000 cycles. Due to the stability of the superhydrophobic surface water contact angle (WCA) = 153.3°, our sensor can work in an underwater environment, which can sense water levels from 1 cm (∼98 Pa) to 40 cm and also a variety of underwater behaviors (knock, ultrasonication, blow, etc.) with high stability. In addition, the sensor can be integrated into a circuit for the water level and pressure detection. The sensor can also be used as a smart underwater-temperature sensor; it shows a linear temperature coefficient of resistance (TCR) of 0.48% °C-1 in a temperature range of 35-80 °C. This multifunctional sensor shows potential application prospects in wearable electronic devices for sensing.