Highly Dispersed, Adhesive Carbon Nanotube Ink for Strain and Pressure Sensors

Biomaterials for reliable wearable health monitoring: Applications in skin and eye integration

Recent advancements in biomaterials have significantly impacted wearable health monitoring, creating opportunities for personalized and non-invasive health assessments. These developments address the growing demand for customized healthcare solutions. Durability is a critical factor for biomaterials in wearable applications, as they must withstand diverse wearing conditions effectively. Therefore, there is a heightened focus on developing biomaterials that maintain robust and stable functionalities, essential for advancing wearable sensing technologies. This review examines the biomaterials used in wearable sensors, specifically those interfaced with human skin and eyes, highlighting essential strategies for achieving long-lasting and stable performance. We specifically discuss three main categories of biomaterials-hydrogels, fibers, and hybrid materials-each offering distinct properties ideal for use in durable wearable health monitoring systems. Moreover, we delve into the latest advancements in biomaterial-based sensors, which hold the potential to facilitate early disease detection, preventative interventions, and tailored healthcare approaches. We also address ongoing challenges and suggest future directions for research on material-based wearable sensors to encourage continuous innovation in this dynamic field.

Revolutionizing human healthcare with wearable sensors for monitoring human strain

With the rapid advancements in wearable sensor technology, healthcare is witnessing a transformative shift towards personalized and continuous monitoring. Wearable sensors designed for tracking human strain offer promising applications in rehabilitation, athletic performance, occupational health, and early disease detection. Recent advancements in the field have centered on the design optimization and miniaturization of wearable biosensors. Wireless communication technologies have facilitated the simultaneous, non-invasive detection of multiple analytes with high sensitivity and selectivity through wearable biosensors, significantly enhancing diagnostic accuracy. This review meticulously chronicles noteworthy advancements in wearable sensors tailored for healthcare and biomedical applications, spanning the current market landscape, challenges faced, and prospective trends, including multifunctional smart wearable sensors and integrated decision-support systems. The domain of flexible electronics has witnessed substantial progress over the past decade, particularly in flexible strain sensors, which are crucial for contemporary wearable and implantable devices. These innovations have broadened the scope of applications in human health monitoring and diagnostics. Continuous advancements in novel materials and device architectural methodologies aim to expand the utility of these sensors while meeting the increasingly stringent demands for enhanced sensing performance. This review explores the diverse array of wearable sensors-from piezoelectric, piezoresistive, and capacitive sensors to advanced optical and bioimpedance sensors-each distinguished by unique material properties and functionalities. We analyzed these technologies' sensitivity, accuracy, and response time, which were crucial for reliably capturing strain metrics in dynamic, real-world conditions. Quantitative performance comparisons across various sensor types highlighted their relative effectiveness, strengths, and limitations regarding detection precision, durability, and user comfort. Additionally, we discussed the current challenges in wearable sensor design, including energy efficiency, data transmission, and integration with machine learning models for enhanced data interpretation. Ultimately, this review emphasized the revolutionary potential of wearable strain sensors in advancing preventative healthcare and enabling proactive health management, ushering in an era where real-time health insights could lead to more timely interventions and improved health outcomes.

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.

High-Performance Stretchable Strain Sensors Based on Auxetic Fabrics for Human Motion Detection

Wearable strain sensors play a pivotal role in real-time human motion detection and health monitoring. Traditional fabric-based strain sensors, typically with a positive Poisson's ratio, face challenges in maintaining sensitivity and comfort during human motion due to conflicting resistance changes in different strain directions. In this work, high-performance stretchable strain sensors are developed based on graphene-modified auxetic fabrics (GMAF) for human motion detection in smart wearable devices. The proposed GMAF sensors, with a negative Poisson's ratio achieved through commercially available warp-knitting technology, exhibit an 8-fold improvement in sensitivity compared to conventional plain fabric sensors. The unique auxetic fabric structure enhances sensitivity by synchronizing resistance changes in both wale and course directions. The GMAF sensors demonstrate excellent washability, showing only slight degradation in auxeticity and an acceptable increase in resistance after 10 standard wash cycles. The GMAF sensors maintain stability under different strain levels and various motion frequencies, emphasizing their dynamic performance. The sensors exhibit superior conformability to joint movements, which effectively monitor a full range of motions, including joint bending, sports activities, and subtle actions like coughing and swallowing. The research underscores a promising approach to achieve industrial-scale production of wearable sensors with improved performance and comfort through fabric structure design.

Recent Trends of Bioanalytical Sensors with Smart Health Monitoring Systems: From Materials to Applications

Smart biosensors attract significant interest due to real-time monitoring of user health status, where bioanalytical electronic devices designed to detect various activities and biomarkers in the human body have potential applications in physical sign monitoring and health care. Bioelectronics can be well integrated by output signals with wireless communication modules for transferring data to portable devices used as smart biosensors in performing real-time diagnosis and analysis. In this review, the scientific keys of biosensing devices and the current trends in the field of smart biosensors, (functional materials, technological approaches, sensing mechanisms, main roles, potential applications and challenges in health monitoring) will be summarized. Recent advances in the design and manufacturing of bioanalytical sensors with smarter capabilities and enhanced reliability indicate a forthcoming expansion of these smart devices from laboratory to clinical analysis. Therefore, a general description of functional materials and technological approaches used in bioelectronics will be presented after the sections of scientific keys to bioanalytical sensors. A careful introduction to the established systems of smart monitoring and prediction analysis using bioelectronics, regarding the integration of machine-learning-based basic algorithms, will be discussed. Afterward, applications and challenges in development using these smart bioelectronics in biological, clinical, and medical diagnostics will also be analyzed. Finally, the review will conclude with outlooks of smart biosensing devices assisted by machine learning algorithms, wireless communications, or smartphone-based systems on current trends and challenges for future works in wearable health monitoring.

Laser patterned graphene pressure sensor with adjustable sensitivity in an ultrawide response range

Flexible pressure sensors have attracted wide attention because of their applications in wearable electronic, human-computer interface, and healthcare. However, it is still a challenge to design a pressure sensor with adjustable sensitivity in an ultrawide response range to satisfy the requirements of different application scenarios. Here, a laser patterned graphene pressure sensor (LPGPS) is proposed with adjustable sensitivity in an ultrawide response range based on the pre-stretched kirigami structure. Due to the out-of-plane deformation of the pre-stretched kirigami structure, the sensitivity can be easily tuned by simply modifying the pre-stretched level. As a result, it exhibits a maximum sensitivity of 0.243 kPa-1, an ultrawide range up to 1600 kPa, a low detection limit (6 Pa), a short response time (42 ms), and excellent stability with high pressure of 1200 kPa over 500 cycles. Benefiting from its high sensitivity and ultrawide response range, the proposed sensor can be applied to detect physiological and kinematic signals under different pressure intensities. Additionally, taking advantage of laser programmable patterning, it can be easily configured into an array to determine the pressure distribution. Therefore, LPGPS with adjustable sensitivity in an ultrawide response range has potential application in wearable electronic devices.

MXene-Based Chemo-Sensors and Other Sensing Devices

MXenes have received worldwide attention across various scientific and technological fields since the first report of the synthesis of Ti3C2 nanostructures in 2011. The unique characteristics of MXenes, such as superior mechanical strength and flexibility, liquid-phase processability, tunable surface functionality, high electrical conductivity, and the ability to customize their properties, have led to the widespread development and exploration of their applications in energy storage, electronics, biomedicine, catalysis, and environmental technologies. The significant growth in publications related to MXenes over the past decade highlights the extensive research interest in this material. One area that has a great potential for improvement through the integration of MXenes is sensor design. Strain sensors, temperature sensors, pressure sensors, biosensors (both optical and electrochemical), gas sensors, and environmental pollution sensors targeted at volatile organic compounds (VOCs) could all gain numerous improvements from the inclusion of MXenes. This report delves into the current research landscape, exploring the advancements in MXene-based chemo-sensor technologies and examining potential future applications across diverse sensor types.

Green One-Step Strategy of Conductive Ink for Active Health Monitoring in Rehabilitation and Early Care

Conductive ink deposited on flexible substrates through simple methods such as dyeing or printing is one of the most promising approaches for scalable fabrication of wearable electronics. However, excessive chemical additives or a complex preparation process has limited the practical applications of conductive inks. Herein, a highly stable and antibacterial AgNPs/CNT/rGO (SACR) conductive ink with the only assistance of sustainable silk sericin (SS) is developed through a green one-step strategy. SS functions as not only the reductant of silver ions and GO by donating electrons but also the dispersant and stabilizer of CNTs through strong noncovalent interactions. The universality of SACR ink is demonstrated by depositing on various flexible substrates through handwriting, screen-printing, and dyeing techniques; meanwhile, the mechanical reliability between SACR ink and substrates is validated by peeling, bending, and twisting measurements. In addition, the synergistic effects of the multilevel hierarchical 0D/1D/2D structure and abundant interfacial interactions in SACR ink are advantageous to enhancing sensing performance. An SACR ink-based strain sensor and hydrogen peroxide (H2O2) sensor are fabricated to detect physical and biochemical indicators, demonstrating the enormous potential of SACR ink in intelligent wearables for active health monitoring in early care.

An Electroactive and Self-Assembling Bio-Ink, based on Protein-Stabilized Nanoclusters and Graphene, for the Manufacture of Fully Inkjet-Printed Paper-Based Analytical Devices

Hundreds of new electrochemical sensors are reported in literature every year. However, only a few of them makes it to the market. Manufacturability, or rather the lack of it, is the parameter that dictates if new sensing technologies will remain forever in the laboratory in which they are conceived. Inkjet printing is a low-cost and versatile technique that can facilitate the transfer of nanomaterial-based sensors to the market. Herein, an electroactive and self-assembling inkjet-printable ink based on protein-nanomaterial composites and exfoliated graphene is reported. The consensus tetratricopeptide proteins (CTPRs), used to formulate this ink, are engineered to template and coordinate electroactive metallic nanoclusters (NCs), and to self-assemble upon drying, forming stable films. The authors demonstrate that, by incorporating graphene in the ink formulation, it is possible to dramatically improve the electrocatalytic properties of the ink, obtaining an efficient hybrid material for hydrogen peroxide (H2 O2 ) detection. Using this bio-ink, the authors manufactured disposable and environmentally sustainable electrochemical paper-based analytical devices (ePADs) to detect H2 O2 , outperforming commercial screen-printed platforms. Furthermore, it is demonstrated that oxidoreductase enzymes can be included in the formulation, to fully inkjet-print enzymatic amperometric biosensors ready to use.

Amphibious epidermal area networks for uninterrupted wireless data and power transfer

The human body exhibits complex, spatially distributed chemo-electro-mechanical processes that must be properly captured for emerging applications in virtual/augmented reality, precision health, activity monitoring, bionics, and more. A key factor in enabling such applications involves the seamless integration of multipurpose wearable sensors across the human body in different environments, spanning from indoor settings to outdoor landscapes. Here, we report a versatile epidermal body area network ecosystem that enables wireless power and data transmission to and from battery-free wearable sensors with continuous functionality from dry to underwater settings. This is achieved through an artificial near field propagation across the chain of biocompatible, magneto-inductive metamaterials in the form of stretchable waterborne skin patches-these are fully compatible with pre-existing consumer electronics. Our approach offers uninterrupted, self-powered communication for human status monitoring in harsh environments where traditional wireless solutions (such as Bluetooth, Wi-Fi or cellular) are unable to communicate reliably.

Metal Ion-induced Gelation of High-concentration Graphite-like Crystalline Nanosheet Aqueous Suspensions

The stability and transformation of nanomaterial aqueous suspensions are essential for their applications. Preparation of high-concentration carbon nanomaterials suspensions remains challenging due to their nonpolar nature. Herein, 200 mg mL-1 carbon nanomaterial aqueous suspensions are achieved by using graphite-like crystalline nanosheets (GCNs) with high hydrophilicity. Furthermore, these high-concentration GCN aqueous suspensions spontaneously transform into gels when induced by mono-, di-, and trivalent metal salt electrolytes at room temperature. Theoretical calculation of potential energy by DLVO theory reveals that the gelatinized GCNs is a new and metastable state between two usual forms of solution and coagulation. It is shown that the gelation of GCNs is due to the preferential orientation of nanosheets in an edge-edge arrangement, which differs from the case of solution and coagulation. High-temperature treatment of GCN gels produces metal/carbon materials with pore structures. This work provides a promising opportunity to create various metal/carbon functional materials.

Hierarchical Synergistic Structure for High Resolution Strain Sensor with Wide Working Range

Strain sensors have been attracting tremendous attention for the promising application of wearable devices in recent years. However, the trade-off between high resolution, high sensitivity, and broad detection range is a great challenge for the application of strain sensors. Herein, a novel design of hierarchical synergistic structure (HSS) of Au micro cracks and carbon black (CB) nanoparticles is reported to overcome this challenge. The strain sensor based on the designed HSS exhibit high sensitivity (GF > 2400), high strain resolution (0.2%) even under large loading strain, broad detection range (>40%), outstanding stability (>12000 cycles), and fast response speed simultaneously. Further, the experiments and simulation results demonstrate that the carbon black layer greatly changed the morphology of Au micro-cracks, forming a hierarchical structure of micro-scale Au cracks and nano-scale carbon black particles, thus enabling synergistic effect and the double conductive network of Au micro-cracks and CB nanoparticles. Based on the excellent performance, the sensor is successfully applied to monitoring tiny signals of the carotid pulse during body movement, which illustrates the great potential in the application of health monitoring, human-machine interface, human motion detection, and electronic skin.

Soft Electronics for Health Monitoring Assisted by Machine Learning

Due to the development of the novel materials, the past two decades have witnessed the rapid advances of soft electronics. The soft electronics have huge potential in the physical sign monitoring and health care. One of the important advantages of soft electronics is forming good interface with skin, which can increase the user scale and improve the signal quality. Therefore, it is easy to build the specific dataset, which is important to improve the performance of machine learning algorithm. At the same time, with the assistance of machine learning algorithm, the soft electronics have become more and more intelligent to realize real-time analysis and diagnosis. The soft electronics and machining learning algorithms complement each other very well. It is indubitable that the soft electronics will bring us to a healthier and more intelligent world in the near future. Therefore, in this review, we will give a careful introduction about the new soft material, physiological signal detected by soft devices, and the soft devices assisted by machine learning algorithm. Some soft materials will be discussed such as two-dimensional material, carbon nanotube, nanowire, nanomesh, and hydrogel. Then, soft sensors will be discussed according to the physiological signal types (pulse, respiration, human motion, intraocular pressure, phonation, etc.). After that, the soft electronics assisted by various algorithms will be reviewed, including some classical algorithms and powerful neural network algorithms. Especially, the soft device assisted by neural network will be introduced carefully. Finally, the outlook, challenge, and conclusion of soft system powered by machine learning algorithm will be discussed.

Multifunctional Iontronic Sensor Based on Liquid Metal-Filled Ho llow Ionogel Fibers in Detecting Pressure, Temperature, and Proximity

Fiber-based pressure/temperature sensors are highly desired in wearable electronics because of their natural advantages of good breathability and easy integrability. However, it is still a great challenge to fabricate reliable and highly sensitive fiber-based pressure/temperature sensors via a scalable and facile strategy. Herein, a novel fiber-based iontronic sensor with excellent pressure- and temperature-sensing capabilities is designed by assembling two crossed hollow and porous ionogel fibers filled with liquid metal. Serving as a pressure sensor, a high detection resolution (1.16 Pa), a high sensitivity of 13.30 kPa^-1 (0-2 kPa), and a wide detection range (∼207 kPa) are realized owing to its novel hierarchical structure and the selection of deformable liquid electrodes. As a temperature sensor, it exhibits a high temperature sensitivity of 25.99% °C^-1 (35-40 °C), high resolution of 0.02 °C, and good repeatability and reliability. On the basis of these excellent sensing capabilities, the as-prepared sensor can detect not only pressure signals varied from weak pulse to large joint movements but also the proximity of different objects. Furthermore, a large-area fiber array can be easily woven for acquiring the pressure mapping to intuitively distinguish the location, magnitude, and shape of the loaded object. This work provides a universal strategy to design fiber-shaped iontronic sensors for wearable electronics.

Effect of surfactant concentration on the evaporation-driven deposition of carbon nanotubes: from coffee-ring effect to strain sensing

Carbon nanotubes (CNTs) as electrically conductive materials are of great importance in the fabrication of flexible electronic devices and wearable sensors. In this regard, the evaporation-driven self-assembly of CNTs has attracted increasing attention. CNT-based applications are mostly concerned with the alignment of CNTs and the density of CNT films. In the present work, we focus on the latter by trying to achieve an optimal evaporation-driven deposition with the densest CNT ring. Although surfactants are used for effective dispersion and colloidal stabilization of CNTs in the aqueous phase, their excessive usage induces Marangoni eddies in the evaporating sessile droplets, leading to poor ring depositions. Thus, there is an optimum surfactant concentration that contributes to CNTs deagglomeration and results in the densest ring-like deposition with relatively high thickness. We report that this optimum concentration for sodium dodecyl sulfate (SDS) as a surfactant can be approximately considered as much as the concentration of multi-walled carbon nanotubes (MWCNTs) as the colloidal nanoparticles. Optimal depositions show the lowest electrical resistances for each CNT concentration, making them suitable for electronic applications. We also propose the multiple depositions method in which a new droplet is printed after the complete evaporation of the previous droplet. This method can lead to denser rings with a higher conductivity using lower concentrations of CNTs. Lastly, we fabricate strain sensors based on the optimal evaporation-driven deposition of CNTs which show higher gauge factors than the commercial strain gauges, corroborating the applicability of our method.

Carbon Nanotube Ink Dispersed by Chitin Nanocrystals for Thermoelectric Converter for Self-Powering Multifunctional Wearable Electronics

The screen-printing process of conductive ink can realize simple and large-scale manufacture of micro/nano patterns for producing wearable electronic products. Herein, chitin nanocrystals (ChNCs) are used as a dispersant for the preparation of multiwalled carbon nanotube (MWCNT) ink with high viscosity and uniformity by ultrasound treatment. ChNCs can interact with MWCNT in noncovalent ways, including π-π and hydrophobic interactions. ChNCs/MWCNT (CCNT) ink does not aggregate even after standing for 3 months with a maximum MWCNT concentration of 33 mg mL^-1 and dispersion efficiency of 91.1%. Using CCNT ink, a paper-based thermoelectric generator (TEG) is manufactured by screen-printing technology. With good thermoelectric and strain sensing properties, CCNT coated paper can stably collect human energy at room temperature to realize self-powering. The CCNT coated paper-based TEG can convert thermal voltage signals into musical notes, monitor the changes in human behavior and respiratory rate, and monitor joint movements. Moreover, CCNT coated paper has no cytotoxicity by CCK-8 and live/dead staining. This work puts forward a strategy of green preparation of MWCNT-based ink by adding renewable chitin, which opens up a new way to apply MWCNT-based ink in self-powering wearable multifunctional sensors.

Natural gum-based electronic ink with water-proofing self-healing and easy-cleaning properties for directly on-skin electronics

On-skin electronic systems, which can facilitate noninvasive continuous acquisition of low-artifact physiological signals, are a promising technique for future wearable devices in healthcare. Inspired by the nature of Arabic gum (AG), we developed a costless, easy-to-prepare, easy-to-use, and environment-friendly electronic ink (E-ink) that can be used to construct multiform on-skin electronic systems through simple painting or stamping. In addition to its competitive electrical properties, the E-ink has the following advantages: waterproof (0.5 m/s water flushing for 10 s), self-healing (1.5 mm wide wound), and easy-cleaning (can be easily removed using cotton ball with 5% surfactant), making it environmentally tolerant and highly reliable for practical use. We demonstrated that our E-ink can act as electric wires for epidermal circuits, sensors to handle a variety of physiological data measurements. This research provides an effective strategy for direct integration of electronics and skin, which can accelerate the realization of the next generation of imperceptible, scalable, cost-effective and customized wearable devices.

Advances in the Robustness of Wearable Electronic Textiles: Strategies, Stability, Washability and Perspective

Flexible electronic textiles are the future of wearable technology with a diverse application potential inspired by the Internet of Things (IoT) to improve all aspects of wearer life by replacing traditional bulky, rigid, and uncomfortable wearable electronics. The inherently prominent characteristics exhibited by textile substrates make them ideal candidates for designing user-friendly wearable electronic textiles for high-end variant applications. Textile substrates (fiber, yarn, fabric, and garment) combined with nanostructured electroactive materials provide a universal pathway for the researcher to construct advanced wearable electronics compatible with the human body and other circumstances. However, e-textiles are found to be vulnerable to physical deformation induced during repeated wash and wear. Thus, e-textiles need to be robust enough to withstand such challenges involved in designing a reliable product and require more attention for substantial advancement in stability and washability. As a step toward reliable devices, we present this comprehensive review of the state-of-the-art advances in substrate geometries, modification, fabrication, and standardized washing strategies to predict a roadmap toward sustainability. Furthermore, current challenges, opportunities, and future aspects of durable e-textiles development are envisioned to provide a conclusive pathway for researchers to conduct advanced studies.

A Three-Dimensional-Printed Recyclable, Flexible, and Wearable Device for Visualized UV, Temperature, and Sweat pH Sensing

Wearable devices are now recognized as a powerful tool to collect physiological and environmental information in a smart, noninvasive, and real-time manner. Despite the rapid progress of wearable devices especially wearable electronic devices, there are still several challenges that limit their further development, for example, a complicated electrical signal acquisition and processing process to eliminate the interference from the surrounding signals, bulky power supply, inevitable e-waste, and environmental pollution. Herein, we report a 3D-printed recyclable, flexible, and wearable device for visualized UV, temperature, and sweat pH sensing. Compared with wearable electronic devices, our visualized wearable device senses environmental (UV light, ambient temperature), biophysical (skin temperature), and biochemical (sweat pH) signals via stimuli-responsive color change, which does not require complicated electronic circuit design/assembly, time-consuming data processing and additional power source. In addition, this visualized wearable device is fabricated via a 3D support bath printing technology by printing UV-, temperature-, and sweat pH-sensing inks containing photochromic, thermochromic, and pH-chromic materials, respectively, into/onto sustainable starch solution, resulting in a multi-functional, recyclable, and flexible sensing device with high reproducibility. Our results reveal that UV light intensities under sunlight (0-2500 μW/cm2), ambient, and skin temperatures (0-38 °C) as well as sweat pH (4.0-7.0) can be successfully monitored.