Emerging horizons of MXenes in wearable sensors for advanced health monitoring: an overview

MXene-based conductive hydrogels (MCHs) have emerged as highly promising materials for next-generation wearable electrochemical sensors, owing to their exceptional electrical conductivity, mechanical flexibility, and biocompatibility. This review provides a comprehensive and up-to-date overview of recent advances in the development of wearable sensors that incorporate Ti3C2Tx MXene hydrogels for the detection of a wide range of analytes in various biofluids. Diverse hydrogel matrices have been integrated with MXenes to fabricate highly sensitive platforms capable of monitoring key biomarkers such as glucose, dopamine, uric acid, lactate, norepinephrine, sodium, creatinine, and pH. These sensors have been successfully deployed at various locations in the body, including the forearm, chest, wrist, and head, using flexible formats such as skin patches, microfluidic devices, pantyliners, wearable caps, and attachable body accessories. Notably, several configurations demonstrate ultralow detection limits, reaching the nanomolar level, enabling real-time, noninvasive analysis of sweat, urine, and other physiological fluids. The wide range of functional additives and customizable design approaches underscores the modularity and tunability of MCH-based systems for specific applications. This review critically evaluates the design principles, sensing mechanisms, performance metrics, and practical limitations of current MCH-based wearable platforms, providing insights that can guide future innovations in smart wearable healthcare technologies.

Research on remote health monitoring through wearable sensors has attained popularity in recent decades mainly due to aging population and expensive health care services. Microfluidic wearable sweat sensors provide economical, non-invasive mode of sample collection, important physiological information, and continuous tracking of human health. Recent advances in wearable sensors focus on electrochemical monitoring of biomarkers in sweat and can be applicable in various fields like fitness monitoring, nutrition, and medical diagnosis. This review focuses on the evolution of wearable devices from benchtop electrochemical systems to microfluidic-based wearable sensors. Major classification of wearable sensors like skin contact-based and biofluidic-based sensors are discussed. Furthermore, sweat chemistry and related biomarkers are explained in addition to integration of microfluidic systems in wearable sweat sensors. At last, recent advances in wearable electrochemical sweat sensors are discussed, which includes tattoo-based, paper microfluidics, patches, wrist band, and belt-based wearable sensors.

Over the past decades, wearable and implantable devices have demonstrated great potential for a wide range of personalized health monitoring and therapeutic applications. This special issue primarily focuses on functional and electronic materials, sensors technologies and capabilities, and the associated energy solutions for wearable and implantable devices toward healthcare applications. We have collected 17 reviews, four research articles, and one perspective, all of which are within the scope of this area and cover the topics in breadth and depth.

Wearable electrochemical sensors have the potential to overcome the problem of infrequent clinical visits that leads to transient events of potential diagnostic importance being unduly overlooked. The promise of real-time, personalized health care has driven multidisciplinary work on fabricating various forms of wearable sensors. Although remarkable advances in device form factor and integrated circuit design have been achieved, notable hurdles, such as shelf life, reuseability, flex and sweat resistance, and longitudinal performance, remain unaddressed. This perspective seeks to summarize major advances in current wearable electrochemical sensors and to highlight the most pressing challenges that will benefit from collective research endeavors.

Wearable multimodal sensors could enable the continuous, non‐invasive, precise monitoring of vital human signals critical for remote health monitoring and telemedicine. Atomically thin materials with intriguing physical characteristics, rich chemistry, and extreme sensitivity to external stimuli are attractive for implementing high‐performance wearable sensors. Despite the increased interest and efforts in 2D materials‐based wearable sensors, reducing the manufacturing and integration costs while improving the product performance remains challenging. Previous review articles provided good coverage discussing the material and device aspects of 2D materials‐based wearable devices. However, few reviews discussed the status quo, prospects, and opportunities for the scalable nanomanufacturing of 2D materials wearable sensors for health monitoring. To fill this gap, the recent advances in 2D materials‐based wearable health sensors are reviewed. The structure design, fabrication processing, the mechanisms of 2D materials‐based wearable health sensors, and their applications for human health monitoring are discussed. More significantly, a systematic discussion of the state‐of‐the‐art and technological gaps for enabling future design and nanomanufacturing of 2D materials wearable health sensors are provided. Finally, the challenges and opportunities associated with the scalable nanomanufacturing of 2D wearable health sensors are discussed.

Flexibility is the most attractive and challenging issue in the biomedical application of wearable sensors. Even after achieving a selective, sensitive, durable, miniaturized, and robust medical sensor, flexibility is needed to provide efficient and continuous interaction of sensors with living tissues. Flexible and wearable sensors can provide real-time and continuous monitoring of both physical and chemical properties of the human body during sports training. Wearable biomedical sensors consist of sensors, data analyzers, data processors, and energy sources that are placed on stretchable natural fabric or synthetic flexible polymer to achieve efficient and comfortable interaction with the human body. The smart, remote, and real-time monitoring of the physiological processes of the patient’s body, the early detection of diseases, controlled drug delivery, and participation in artificial skin construction are the latest advances in flexible biomedical sensors. In this chapter, first, the materials of flexible sensors, types of sensing, and the mechanisms of sensing are briefly explained. Then, the application of flexible sensors in health monitoring is discussed and exemplified. In the conclusion, the recent progress of flexible sensors in drug delivery and artificial skins is illustrated.

Wearable sensors for continuous monitoring of vital signs for extended periods of weeks or months are expected to revolutionize healthcare services in the home and workplace as well as in hospitals and nursing homes. This invited paper describes recent research progress in wearable health monitoring technology and its clinical applications, with emphasis on blood pressure and circulatory monitoring. First, a finger ring-type wearable blood pressure sensor based on photo plethysmogram is presented. Technical issues, including motion artifact reduction, power saving, and wearability enhancement, will be addressed. Second, sensor fusion and sensor networking for integrating multiple sensors with diverse modalities will be discussed for comprehensive monitoring and diagnosis of health status. Unlike traditional snap-shot measurements, continuous monitoring with wearable sensors opens up the possibility to treat the physiological system as a dynamical process. This allows us to apply powerful system dynamics and control methodologies, such as adaptive filtering, single- and multi-channel system identification, active noise cancellation, and adaptive control, to the monitoring and treatment of highly complex physiological systems. A few clinical trials illustrate the potentials of the wearable sensor technology for future heath care services.

Wearable sensors have received a major recent attention owing to their considerable promise for monitoring the wearer's health and wellness. The medical interest for wearable systems arises from the need for monitoring patients over long periods of time. These devices have the potential to continuously collect vital health information from a person's body and provide this information to them or their healthcare provider in a timely fashion. Unlike early efforts aimed at monitoring mobility and vital signs, our recent efforts aim at filling the gaps by providing continuous biochemical information. The new chemical sensing wearable platforms provide new avenues to continuously and non-invasively monitor individuals and can thus tender crucial real-time information regarding a wearer's health. This presentation will discuss recent developments in the field of wearable electrochemical sensors integrated directly on the epidermis or within the mouth for various non-invasive biomedical monitoring applications [1-3]. Particular attention will be given to non-invasive monitoring of metabolites and electrolytes using flexible amperometric and potentiometric sensors, respectively, along with related materials, energy and integration considerations. Microneedle sensor arrays for multiplexed ISF monitoring will also be discussed also with wearable energy harvesters aimed to power these devices. The preparation and characterization of such wearable electrochemical sensors will be described, along with their current status and future prospects and challenges.REFERENCES[1] "Wearable Chemical Sensors: Present Challenges and Future Prospects" A. J. Bandodkar, I. Jeerapan, J. Wang, ACS Sensors, 2016, 1, 464.[2] A. J. Bandodkar and J. Wang, "Non-invasive wearable electrochemical sensors: a review", Trends Biotechnol., 2014, 32, 363.[3] "Wearable biosensors for healthcare monitoring", J. Kim, A. S. Campbell, B. Esteban-Fernández de Ávila, and J. Wang, Nature Biotechnology, 2019, 37, 389.

Wearable sensors have received a major recent attention owing to their considerable promise for monitoring the wearer’s health and wellness. The medical interest for wearable systems arises from the need for monitoring patients over long periods of time. These devices have the potential to continuously collect vital health information from a person’s body and provide this information to them or their healthcare provider in a timely fashion. Unlike early efforts aimed at monitoring mobility and vital signs, our recent efforts aim at filling the gaps by providing continuous biochemical information. The new chemical sensing wearable platforms provide new avenues to continuously and non-invasively monitor individuals and can thus tender crucial real-time information regarding a wearer’s health. This presentation will discuss recent developments in the field of wearable electrochemical sensors integrated directly on the epidermis or within the mouth for various non-invasive biomedical monitoring applications [1-3]. Particular attention will be given to non-invasive monitoring of metabolites and electrolytes using flexible amperometric and potentiometric sensors, respectively, along with related materials, energy and integration considerations. Microneedle sensor arrays for multiplexed ISF monitoring will also be discussed also with wearable energy harvesters aimed to power these devices. The preparation and characterization of such wearable electrochemical sensors will be described, along with their current status and future prospects and challenges.REFERENCES [1] "Wearable Chemical Sensors: Present Challenges and Future Prospects" A. J. Bandodkar, I. Jeerapan, J. Wang, ACS Sensors, 2016, 1, 464.[2] A. J. Bandodkar and J. Wang, “Non-invasive wearable electrochemical sensors: a review”, Trends Biotechnol., 2014, 32, 363.[3] “Wearable biosensors for healthcare monitoring”, J. Kim, A. S. Campbell, B. Esteban-Fernández de Ávila, and J. Wang, Nature Biotechnology, 2019, 37, 389.

BackgroundHospital stays after major surgery are shorter than ever before. Although enhanced recovery and early discharge have many benefits, some complications will now first manifest themselves in home settings. Remote patient monitoring with wearable sensors in the first days after hospital discharge may capture clinical deterioration earlier but is largely uncharted territory.ObjectiveThis study aimed to assess the technical feasibility of patients, discharged after esophagectomy, being remotely monitored at home with a wireless patch sensor and the experiences of these patients. In addition, we determined whether observing vital signs with a wireless patch sensor influences clinical decision making.MethodsIn an observational feasibility study, vital signs of patients were monitored with a wearable patch sensor (VitalPatch, VitalConnect Inc) during the first 7 days at home after esophagectomy and discharge from hospital. Vital signs trends were shared with the surgical team once a day, and they were asked to check the patient’s condition by phone each morning. Patient experiences were evaluated with a questionnaire, and technical feasibility was analyzed on a daily basis as the percentage of data loss and gap durations. In addition, the number of patients for whom a change in clinical decision was made based on the results of remote vital signs monitoring at home was assessed.ResultsPatients (N=20) completed 7 days each of home monitoring with the wearable patch sensor. Each of the patients had good recovery at home, and remotely observed vital signs trends did not alter clinical decision making. Patients appreciated that surgeons checked their vital signs daily (mean 4.4/5) and were happy to be called by the surgical team each day (mean 4.5/5). Wearability of the patch was high (mean 4.4/5), and no reports of skin irritation were mentioned. Overall data loss of vital signs measurements at home was 25%; both data loss and gap duration varied considerably among patients.ConclusionsRemote monitoring of vital signs combined with telephone support from the surgical team was feasible and well perceived by all patients. Future studies need to evaluate the impact of home monitoring on patient outcome as well as the cost-effectiveness of this new approach.

The increased prevalence of chronic disease in aging population entails health risks and imposes significant economic and social burden. It is essential to provide comfortable, cost-effective, and easy-to-use unobtrusive and wearable systems for personal well-being and healthcare. Novel flexible material-based non-invasive and wearable sensors offer an efficient and cost-effective solution, which enables the continuous and real-time monitoring of important physiological signs of the human beings, the assessment of personal health conditions and that provides feedback from remote and home monitoring. In this paper, novel flexible material-based wearable sensors, devised into body sensor networks to capture and monitor vital bio-signals, including electroencephalography (EEG), electrocardiography (ECG) and respiratory, are proposed. Silver nanowires (Ag NWs) and polydimethylsiloxane composite material, carbon foam, and graphene-based fiber are used to sense the EEG, ECG, and respiratory, respectively. With different flexible materials, the smart hat and smart jacket are designed to affix the sensors, which enable long-term health monitoring of vital signals seamlessly. Meanwhile, the corresponding acquisition circuits are developed and mounted with the proposed electrodes on the garments. More importantly, a comprehensive protocol is designed to validate the performance of the proposed system, while some standard sensors and commercial devices are used for comparison. The evaluation results demonstrate the proposed system represents a comparable performance with the existing system. In summary, the proposed sensing system offers an unobtrusive, detachable, expandable, user-friendly, and comfortable solution for physiological signal monitoring. It can be expected to use for the remote healthcare monitoring and provide personalized information of health, fitness, and diseases.

BackgroundContinuous vital sign monitoring using wearable sensors may enable early detection of patient deterioration and sepsis.ObjectiveThis study aimed to explore patient experiences with wearable sensor technology and carry out continuous monitoring through questionnaire and interview studies in an acute hospital setting.MethodsPatients were recruited for a wearable sensor study and were asked to complete a 9-item questionnaire. Patients responses were evaluated using a Likert scale and with continuous variables. A subgroup of surgical patients wearing a Sensium Vital Sign Sensor was invited to participate in semistructured interviews. The Sensium wearable sensor measures the vital signs: heart rate, respiratory rate, and temperature. All interview data were subjected to thematic analysis.ResultsOut of a total of 500 patients, 453 (90.6%) completed the questionnaire. Furthermore, 427 (85.4%) patients agreed that the wearable sensor was comfortable, 429 (85.8%) patients agreed to wear the patch again when in hospital, and 398 (79.6%) patients agreed to wear the patch at home. Overall, 12 surgical patients consented to the interviews. Five main themes of interest to patients emerged from the interviews: (1) centralized monitoring, (2) enhanced feelings of patient safety, (3) impact on nursing staff, (4) comfort and usability, and (5) future use and views on technology.ConclusionsOverall, the feedback from patients using wearable monitoring sensors was strongly positive with relatively few concerns raised. Patients felt that the wearable sensors would improve their sense of safety, relieve pressure on health care staff, and serve as a favorable aspect of future health care technology.

Stretchable organohydrogel fibers are attracting considerable interest for next-generation flexible and wearable soft strain sensors due to their excellent stability in harsh environments. However, due to the uniformly distributed ions and reduced number of carriers in the whole material, the sensitivity of organohydrogel fibers under subzero temperature is not desirable, which significantly hinders their practical application. Herein, a newly competitive proton-trapping strategy was designed to obtain anti-freezing organohydrogel fibers for high-performance wearable strain sensors via a simple freezing-thawing process, in which tetraaniline (TANI), serving as the proton trapper, and representing the shortest repeated structural unit of polyaniline (PANI), was physically crosslinked with polyvinyl alcohol (PVA) (PTOH). The as-prepared PTOH fiber exhibited an outstanding sensing performance at -40 °C due to the unevenly distributed ion carriers and the highly breakable proton-migration pathways, with a high gauge factor of 24.6 at a strain of 200-300%. Moreover, the existence of hydrogen bonds between the TANI and PVA chains endowed PTOH with a high tensile strength (1.96 MPa) and toughness (8.0 MJ m-3). Accordingly, strain sensors made from PTOH fibers and knitted textiles could monitor human motions rapidly and sensitively, demonstrating their potential as wearable anti-freezing anisotropic strain sensors.

Emerging plant diseases, caused by pathogens, pests, and climate change, are critical threats to not only the natural ecosystem but also human life. To mitigate crop loss due to various biotic and abiotic stresses, new sensor technologies to monitor plant health, predict, and track plant diseases in real time are desired. Wearable electronics have recently been developed for human health monitoring. However, the application of wearable electronics to agriculture and plant science is in its infancy. Wearable technologies mean that the sensors will be directly placed on the surfaces of plant organs such as leaves and stems. The sensors are designed to detect the status of plant health by profiling various trait biomarkers and microenvironmental parameters, transducing bio‐signals to electric readout for data analytics. In this perspective, the recent progress in wearable plant sensors is summarized and they are categorized by the functionality, namely plant growth sensors, physiology, and microclimate sensors, chemical sensors, and multifunctional sensors. The design and mechanism of each type of wearable sensors are discussed and their applications to address the current challenges of precision agriculture are highlighted. Finally, challenges and perspectives for the future development of wearable plant sensors are presented.

Wearable sensors have garnered considerable recent interest in recent years owing to their tremendous promise for a wide range of healthcare, fitness or military applications. These wearable sensing platforms provide new avenues to monitor individuals continuously and can thus provide rich real-time information regarding a wearer's health and performance. While most early efforts on wearable fitness and biomedical devices have been devoted to the continuous monitoring of vital signs (such as heart rate, respiration rate, skin temperature) from physical signals, wearable chemical sensors have received limited attention. The recent introduction of new non-invasive chemical sensors aims at filling current gaps in wearable sensor technology, as desired for mobile health monitoring and remote diagnostics. Wearable electrochemical sensors have been integrated recently onto both textile materials and directly on the epidermis for various monitoring applications owing to their unique ability to process chemical analytes in a non-invasive and non-obtrusive fashion. These recent efforts have led to tremendous attention to body-worn electrochemical devices, including potentiometric electrolyte sensors and amperometric biosensors for metabolites. The successful realization of wearable sensing devices requires the migration of electrochemical systems from traditional rigid and planar substrates to flexible, wearable, and conformal sheets. The field of wearable electrochemical sensors offers exciting collaborative opportunities and its future growth and success will rely on the abilities of the researchers to continue to innovate and collaborate to address the existing challenges and opportunities. The present Special Issue of Electroanalysis sheds useful insights into the latest advances and current trends in the field of wearable electrochemical sensors and biosensors. Given the rapid development of such body-worn sensors, the studies reported in this Special Issue are just the beginning of a fantastic topic and journey. I would like to thank all the authors who contributed to this issue. Joseph Wang, Chief Editor