Hydrogel-Based Sensors for Human-Machine Interaction

Surfactant-Enhanced Anti-Swelling Hydrogel Flexible Sensor for Machine Learning-Assisted Underwater Gesture Recognition

Hydrogels hold great promise as flexible sensors. However, the development of hydrogel sensors with exceptional anti-swelling properties and stable performance remains a significant challenge. This study introduced a novel design strategy based on surfactant-assisted hydrophobic associative hydrogels. Through free-radical copolymerization in the presence of the surfactant cetyltrimethylammonium bromide, hydrogels with outstanding mechanical properties, excellent anti-swelling capabilities, and superior sensing performance are successfully fabricated. The resulting hydrogel demonstrated remarkable anti-swelling behavior (swelling ratio: -2.3%), exceptional mechanical strength (breaking strain: 3594.5%), and sustained fatigue resistance during repeated underwater loading-unloading cycles (1000 cycles). Integrating the hydrogel sensor with machine learning, precise and stable underwater gesture recognition and motion monitoring are achieved with an accuracy of 98.3%. This study provides a new perspective for advancing flexible underwater sensor technologies and underscores their broad potential in smart wearable devices, health monitoring, and underwater exploration.

Recent progress of nanomaterials-based composite hydrogel sensors for human-machine interactions

Hydrogel-based flexible sensors have demonstrated significant advantages in the fields of flexible electronics and human-machine interactions (HMIs), including outstanding flexibility, high sensitivity, excellent conductivity, and exceptional biocompatibility, making them ideal materials for next-generation smart HMI sensors. However, traditional hydrogel sensors still face numerous challenges in terms of reliability, multifunctionality, and environmental adaptability, which limit their performance in complex application scenarios. Nanomaterial-based composite hydrogels significantly improve the mechanical properties, conductivity, and multifunctionality of hydrogels by incorporating conductive nanomaterials, thereby driving the rapid development of wearable sensors for HMIs. This review systematically summarizes the latest research progress on hydrogels based on carbon nanomaterials, metal nanomaterials, and two-dimensional MXene nanomaterials, and provides a comprehensive analysis of their sensing mechanisms in HMI, including triboelectric nanogenerator mechanism, stress-resistance response mechanism, and electrophysiological acquisition mechanism. The review further explores the applications of composite hydrogel-based sensors in personal electronic device control, virtual reality/augmented reality (VR/AR) game interaction, and robotic control. Finally, the current technical status and future development directions of nanomaterial composite hydrogel sensors are summarized. We hope that this review will provide valuable insights and inspiration for the future design of nanocomposite hydrogel-based flexible sensors in HMI applications.

Hydrogel-Based Biointerfaces: Recent Advances, Challenges, and Future Directions in Human-Machine Integration

Human-machine interfacing (HMI) has emerged as a critical technology in healthcare, robotics, and wearable electronics, with hydrogels offering unique advantages as multifunctional materials that seamlessly connect biological systems with electronic devices. This review provides a detailed examination of recent advancements in hydrogel design, focusing on their properties and potential applications in HMI. We explore the key characteristics such as biocompatibility, mechanical flexibility, and responsiveness, which are essential for effective and long-term integration with biological tissues. Additionally, we highlight innovations in conductive hydrogels, hybrid and composite materials, and fabrication techniques such as 3D/4D printing, which allow for the customization of hydrogel properties to meet the demands of specific HMI applications. Further, we discuss the diverse classes of polymers that contribute to hydrogel conductivity, including conducting, natural, synthetic, and hybrid polymers, emphasizing their role in enhancing electrical performance and mechanical adaptability. In addition to material design, we examine the regulatory landscape governing hydrogel-based biointerfaces for HMI applications, addressing the key considerations for clinical translation and commercialization. An analysis of the patent landscape provides insights into emerging trends and innovations shaping the future of hydrogel technologies in human-machine interactions. The review also covers a range of applications, including wearable electronics, neural interfaces, soft robotics, and haptic systems, where hydrogels play a transformative role in enhancing human-machine interactions. Thereafter, the review addresses the challenges hydrogels face in HMI applications, including issues related to stability, biocompatibility, and scalability, while offering future perspectives on the continued evolution of hydrogel-based systems for HMI technologies.

Muscle Fiber-Inspired High-Performance Strain Sensors for Motion Recognition and Control

The rapid development of wearable technology, flexible electronics, and human-machine interaction has brought about revolutionary changes to the fields of motion analysis and physiological monitoring. Sensors for detecting human motion and physiological signals have become a hot topic of current research. Inspired by the muscle fiber structure, this paper proposed a highly stable strain sensor that was composed of stretchable Spandex fibers (SPF), multiwalled carbon nanotubes (MWCNTs), and silicone rubber (Ecoflex). This sensor adopted an immersion coating process in which MWCNTs were conformally deposited on SPF, and Ecoflex was filled into the fiber interstices, completing the encapsulation and filling of the SPF to construct a stable three-dimensional conductive network. Thanks to the filling of Ecoflex, contact between conductive fibers during the stretching process was avoided, resulting in a significant change in the resistance. The sensitivity of the sensor reached 54.84, which is 10 times higher than before the Ecoflex filling with a stretchable strain range of up to 70%. The encapsulation of Ecoflex also prevented the detachment of MWCNTs on the fibers during stretching, improving the mechanical stability. The sensor can be easily attached to the surface of human skin to rapidly monitor various human motion signals. Furthermore, the sensor was related to the manipulator through wireless Bluetooth to realize the intelligent control of the manipulator. This work not only provided a more precise data monitoring method for medical and motion analysis fields but also offered an innovative solution for manipulator control.

Advancements in Binary Solvent-Assisted Hydrogel Composites for Wearable Sensing Applications

The advancement of wearable sensing technologies has been pivotal in revolutionizing healthcare, environmental monitoring, and personal fitness. Among the diverse materials employed in these technologies, multifunctional hydrogel composites have emerged as critical components due to their unique properties, including high water content, flexibility, and biocompatibility. This review provides a comprehensive overview of the state-of-the-art in binary solvent-assisted hydrogel composites for wearable sensing applications. It begins by defining hydrogel composites and their essential attributes for wearable sensors, specifically focusing on binary solvent-assisted methods that enhance their performance and functionality. The review then delves into the applications of these composites in health monitoring, environmental detection, and sports and fitness, highlighting their role in advancing wearable technologies. Despite their promising features, there are significant challenges related to durability, sensitivity, and integration that need to be addressed to fully exploit these materials in wearable devices. This review discusses these challenges and presents potential solutions, including the development of new materials, improvement in fabrication processes, and strategies for achieving multifunctionality and sustainable design. Looking forward, the paper outlines future directions for research in this field, emphasizing the need for innovative materials and technologies that can lead to more effective, reliable, and eco-friendly wearable sensors. This review aims to inspire further research and development in the field of wearable sensing, paving the way for new applications and advancements in healthcare, environmental monitoring, and personal fitness technologies.

Conducting polymer hydrogels based on supramolecular strategies for wearable sensors

Conductive polymer hydrogels (CPHs) are gaining considerable attention in developing wearable electronics due to their unique combination of high conductivity and softness. However, in the absence of interactions, the incompatibility between hydrophobic conductive polymers (CPs) and hydrophilic polymer networks gives rise to inadequate bonding between CPs and hydrogel matrices, thereby significantly impairing the mechanical and electrical properties of CPHs and constraining their utility in wearable electronic sensors. Therefore, to endow CPHs with good performance, it is necessary to ensure a stable and robust combination between the hydrogel network and CPs. Encouragingly, recent research has demonstrated that incorporating supramolecular interactions into CPHs enhances the polymer network interaction, improving overall CPH performance. However, a comprehensive review focusing on supramolecular CPH (SCPH) for wearable sensing applications is currently lacking. This review provides a summary of the typical supramolecular strategies employed in the development of high-performance CPHs and elucidates the properties of SCPHs that are closely associated with wearable sensors. Moreover, the review discusses the fabrication methods and classification of SCPH sensors, while also exploring the latest application scenarios for SCPH wearable sensors. Finally, it discusses the challenges of SCPH sensors and offers suggestions for future advancements.

Cutting-Edge Hydrogel Technologies in Tissue Engineering and Biosensing: An Updated Review

Hydrogels, known for their unique ability to retain large amounts of water, have emerged as pivotal materials in both tissue engineering and biosensing applications. This review provides an updated and comprehensive examination of cutting-edge hydrogel technologies and their multifaceted roles in these fields. Initially, the chemical composition and intrinsic properties of both natural and synthetic hydrogels are discussed, highlighting their biocompatibility and biodegradability. The manuscript then probes into innovative scaffold designs and fabrication techniques such as 3D printing, electrospinning, and self-assembly methods, emphasizing their applications in regenerating bone, cartilage, skin, and neural tissues. In the realm of biosensing, hydrogels' responsive nature is explored through their integration into optical, electrochemical, and piezoelectric sensors. These sensors are instrumental in medical diagnostics for glucose monitoring, pathogen detection, and biomarker identification, as well as in environmental and industrial applications like pollution and food quality monitoring. Furthermore, the review explores cross-disciplinary innovations, including the use of hydrogels in wearable devices, and hybrid systems, and their potential in personalized medicine. By addressing current challenges and future directions, this review aims to underscore the transformative impact of hydrogel technologies in advancing healthcare and industrial practices, thereby providing a vital resource for researchers and practitioners in the field.

An interfacial host-guest inclusion complex regulated supramolecular nanocomposite hydrogel showing tunable mechanical strength, self-healing, strain sensitivity and NIR responsiveness

The development of supramolecular nanocomposite hydrogels with good mechanical properties and multifunctional characteristics remains challenging. The reinforced role of interfacial weak interactions is important for the mechanical properties of nanocomposite hydrogels. Here, a dynamic host-guest inclusion complex from the host molecule CB[7] and guest units was employed to prepare Fe3O4 hybrid supramolecular nanocomposite hydrogels. The results show that the as-obtained hydrogel with a porous structure was prepared. The CB[7]-modified Fe3O4 (Fe3O4@CB[7]) nanoparticles severed as a cross-linker for fabricating the hydrogel's network. By changing the Fe3O4@CB[7] content, their tensile stress ranged from 0.102 to 0.403 MPa and their compression stress ranged (70% compression strain) from 0.059 to 0.775 MPa. By changing the guest units, their tensile stress ranged from 0.3 MPa to 0.403 MPa. The self-healing efficiency of the hydrogels was 99% after 48 h at room temperature. The as-obtained hydrogels with strain sensitivity can be applied for detecting the movement of an elbow and finger. The supramolecular hydrogel exhibits NIR responsiveness, self-healing, injectability, tunable mechanical strength and conductive ability, and can be used in flexible electronics.

From Fluorescence-Transfer-Lightening-Printing-Assisted Conductive Adhesive Nanocomposite Hydrogels toward Wearable Interactive Optical Information-Electronic Strain Sensors

The interactive flexible device, which monitors the human motion in optical and electrical synergistic modes, has attracted growing attention recently. The incorporation of information attribute within the optical signal is deemed advantageous for improving the interactive efficiency. Therefore, the development of wearable optical information-electronic strain sensors holds substantial promise, but integrating and synergizing various functions and realizing strain-mediated information transformation keep challenging. Herein, an amylopectin (AP) modified nanoclay/polyacrylamide-based nanocomposite (NC) hydrogel and an aggregation-induced-emission-active ink are fabricated. Through the fluorescence-transfer printing of the ink onto the hydrogel film in different strains with nested multiple symbolic information, a wearable interactive fluorescent information-electronic strain sensor is developed. In the sensor, the nanoclay plays a synergistic "one-stone-three-birds" role, contributing to "lightening" fluorescence (≈80 times emission intensity enhancement), ionic conductivity, and excellent stretchability (>1000%). The sensor has high biocompatibility, resilience (elastic recovery ratio: 97.8%), and strain sensitivity (gauge factor (GF): 10.9). Additionally, the AP endows the sensor with skin adhesiveness. The sensor can achieve electrical monitoring of human joint movements while displaying interactive fluorescent information transformation. This research poses an efficient strategy to develop multifunctional materials and provides a general platform for achieving next-generation interactive devices with prospective applications in wearable devices, human-machine interfaces, and artificial intelligence.