Energy Autonomous Sweat-Based Wearable Systems

Advances in Functional Cellulose Hydrogels as Electrolytes for Flexible Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) emerge as leading candidates for a flexible energy storage system, distinguished by high capacity, affordability, and inherent safety. The integration of hydrogel electrolytes, particularly those with saturated aqueous solvents, has significantly enhanced the electrochemical performance of ZIBs while preserving their essential flexibility. Nonetheless, challenges in electrochemical performance under specific conditions highlight the nascent stage of this technology, with numerous technical hurdles awaiting resolution. Addressing these challenges, recent investigations have leveraged the unique properties of cellulose hydrogel-namely, its exceptional toughness, tensile strength, extreme temperature resilience, stimulus responsiveness, and self-healing capabilities-to innovate multifunctional flexible zinc-based batteries. This paper conducts a comprehensive review of the physicochemical attributes of cellulose hydrogel electrolytes within ZIBs. We thoroughly analyze their performance under diverse environmental conditions, offering insights into the current landscape and their future potential. By examining these aspects, we aim to underscore the developmental prospects and the challenges that lie ahead for hydrogel electrolytes in ZIBs, paving the way for further advancement in this promising field.

Optimizing Droplet-Based Electricity Generator via a Low Sticky Hydrophobic Droplet-Impacted Surface

Droplet-based electricity generators (DEGs) are increasingly recognized for their potential in converting renewable energy sources. This study explores the interplay of surface hydrophobicity and stickiness in improving DEG efficiency. It find that the high-performance C-WaxDEGs leverage both these properties. Specifically, DEGs incorporating polydimethylsiloxane (PDMS) with carnauba wax (C-wax) exhibit increased output as surface stickiness decreases. Through experimental comparisons, PDMS with 1wt.% C-wax demonstrated a significant power output increase from 0.07 to 1.2 W m- 2, which attribute to the minimized adhesion between water molecules and the polymer surface, achieved by embedding C-wax into PDMS surface to form microstructures. This improvement in DEG performance is notable even among samples with similar surface potentials and contact angles, suggesting that C-wax's primary contribution is in reducing surface stickiness rather than altering other surface properties. The further investigations into the C-WaxDEG variant with 1wt.% C-wax PDMS uncover its potential as a sensor for water quality parameters such as temperature, pH, and heavy metal ion concentration. These findings open avenues for the integration of C-WaxDEGs into flexible electronic devices aimed at environmental monitoring.

Intelligent Cockpits for Connected Vehicles: Taxonomy, Architecture, Interaction Technologies, and Future Directions

Highly integrated information sharing among people, vehicles, roads, and cloud systems, along with the rapid development of autonomous driving technologies, has spurred the evolution of automobiles from simple "transportation tools" to interconnected "intelligent systems". The intelligent cockpit is a comprehensive application space for various new technologies in intelligent vehicles, encompassing the domains of driving control, riding comfort, and infotainment. It provides drivers and passengers with safety, comfort, and pleasant driving experiences, serving as the gateway for traditional automobile manufacturing to upgrade towards an intelligent automotive industry ecosystem. This is the optimal convergence point for the intelligence, connectivity, electrification, and sharing of automobiles. Currently, the form, functions, and interaction methods of the intelligent cockpit are gradually changing, transitioning from the traditional "human adapts to the vehicle" viewpoint to the "vehicle adapts to human", and evolving towards a future of natural interactive services where "humans and vehicles mutually adapt". This article reviews the definitions, intelligence levels, functional domains, and technical frameworks of intelligent automotive cockpits. Additionally, combining the core mechanisms of human-machine interactions in intelligent cockpits, this article proposes an intelligent-cockpit human-machine interaction process and summarizes the current state of key technologies in intelligent-cockpit human-machine interactions. Lastly, this article analyzes the current challenges faced in the field of intelligent cockpits and forecasts future trends in intelligent cockpit technologies.

Wireless Technologies in Flexible and Wearable Sensing: From Materials Design, System Integration to Applications

Wireless and wearable sensors attract considerable interest in personalized healthcare by providing a unique approach for remote, noncontact, and continuous monitoring of various health-related signals without interference with daily life. Recent advances in wireless technologies and wearable sensors have promoted practical applications due to their significantly improved characteristics, such as reduction in size and thickness, enhancement in flexibility and stretchability, and improved conformability to the human body. Currently, most researches focus on active materials and structural designs for wearable sensors, with just a few exceptions reflecting on the technologies for wireless data transmission. This review provides a comprehensive overview of the state-of-the-art wireless technologies and related studies on empowering wearable sensors. The emerging functional nanomaterials utilized for designing unique wireless modules are highlighted, which include metals, carbons, and MXenes. Additionally, the review outlines the system-level integration of wireless modules with flexible sensors, spanning from novel design strategies for enhanced conformability to efficient transmitting data wirelessly. Furthermore, the review introduces representative applications for remote and noninvasive monitoring of physiological signals through on-skin and implantable wireless flexible sensing systems. Finally, the challenges, perspectives, and unprecedented opportunities for wireless and wearable sensors are discussed.

Extended Review Concerning the Integration of Electrochemical Biosensors into Modern IoT and Wearable Devices

Electrochemical biosensors include a recognition component and an electronic transducer, which detect the body fluids with a high degree of accuracy. More importantly, they generate timely readings of the related physiological parameters, and they are suitable for integration into portable, wearable and implantable devices that are significant relative to point-of-care diagnostics scenarios. As an example, the personal glucose meter fundamentally improves the management of diabetes in the comfort of the patients' homes. This review paper analyzes the principles of electrochemical biosensing and the structural features of electrochemical biosensors relative to the implementation of health monitoring and disease diagnostics strategies. The analysis particularly considers the integration of the biosensors into wearable, portable, and implantable systems. The fundamental aim of this paper is to present and critically evaluate the identified significant developments in the scope of electrochemical biosensing for preventive and customized point-of-care diagnostic devices. The paper also approaches the most important engineering challenges that should be addressed in order to improve the sensing accuracy, and enable multiplexing and one-step processes, which mediate the integration of electrochemical biosensing devices into digital healthcare scenarios.

Microfluidic Sensing Textile for Continuous Monitoring of Sweat Glucose at Rest

Wearable sweat sensors have received considerable attention due to their great potential for noninvasive continuous monitoring of an individual's health status applications. However, the low secretion rate and fast evaporation of sweat pose challenges in collecting sweat from sedentary individuals for noninvasive analysis of body physiology. Here, we demonstrate wearable textiles for continuous monitoring of sweat at rest using the combination of a heating element and a microfluidic channel to increase localized skin sweat secretion rates and combat sweat evaporation, enabling accurate and stable monitoring of trace amounts of sweat. The Janus sensing yarns with a glucose sensing sensitivity of 36.57 mA cm-2 mM-1 are embroidered into the superhydrophobic heated textile to collect sweat directionally, resulting in improved sweat collection efficiency of up to 96 and 75% retention. The device also maintains a highly durable sensing performance, even in dynamic deformation, recycling, and washing. The microfluidic sensing textile can be further designed into a wireless sensing system that enables sedentary-compatible sweat analysis for the continuous, real-time monitoring of body glucose levels at rest.

Personal Thermoregulation by Moisture-Engineered Materials

Personal thermal management can effectively manage the skin microenvironment, improve human comfort, and reduce energy consumption. In personal thermal-management technology, owing to the high latent heat of water evaporation in wet-response textiles, heat- and moisture-transfer coexist and interact with each other. In the last few years, with rapid advances in materials science and innovative polymers, humidity-sensitive textiles have been developed for personal thermal management. However, a large gap exists between the conceptual laboratory-scale design and actual textile. Here, moisture-responsive textiles based on flap opening and closing, those based on yarn/fiber deformation, and sweat-evaporation regulation based on textile design for personal thermoregulation are reviewed, and the corresponding mechanisms and research progress are discussed. Finally, the existing engineering and scientific limitations and future developments are considered to resolve the existing issues and accelerate the practical application of moisture-responsive textiles and related technologies.

Biomimetic Wearable Sensors: Emerging Combination of Intelligence and Electronics

Owing to the advancement of interdisciplinary concepts, for example, wearable electronics, bioelectronics, and intelligent sensing, during the microelectronics industrial revolution, nowadays, extensively mature wearable sensing devices have become new favorites in the noninvasive human healthcare industry. The combination of wearable sensing devices with bionics is driving frontier developments in various fields, such as personalized medical monitoring and flexible electronics, due to the superior biocompatibilities and diverse sensing mechanisms. It is noticed that the integration of desired functions into wearable device materials can be realized by grafting biomimetic intelligence. Therefore, herein, the mechanism by which biomimetic materials satisfy and further enhance system functionality is reviewed. Next, wearable artificial sensory systems that integrate biomimetic sensing into portable sensing devices are introduced, which have received significant attention from the industry owing to their novel sensing approaches and portabilities. To address the limitations encountered by important signal and data units in biomimetic wearable sensing systems, two paths forward are identified and current challenges and opportunities are presented in this field. In summary, this review provides a further comprehensive understanding of the development of biomimetic wearable sensing devices from both breadth and depth perspectives, offering valuable guidance for future research and application expansion of these devices.

A Dual-Functional MXene-Based Bioanode for Wearable Self-Charging Biosupercapacitors

As a reliable energy-supply platform for wearable electronics, biosupercapacitors combine the characteristics of biofuel cells and supercapacitors to harvest and store the energy from human's sweat. However, the bulky preparation process and deep embedding of enzyme active sites in bioelectrodes usually limit the energy-harvesting process, retarding the practical power-supply sceneries especially during the complicated in vivo motion. Herein, a MXene/single-walled carbon nanotube/lactate oxidase hierarchical structure as the dual-functional bioanode is designed, which can not only provide a superior 3D catalytic microenvironment for enzyme accommodation to harvest energy from sweat, but also offers sufficient capacitance to store energy via the electrical double-layer capacitor. A wearable biosupercapacitor is fabricated in the "island-bridge" structure with a composite bioanode, active carbon/Pt cathode, polyacrylamide hydrogel substrate, and liquid metal conductor. The device exhibits an open-circuit voltage of 0.48 V and the high power density of 220.9 µW cm-2 at 0.5 mA cm-2 . The compact conformal adhesion with skin is successfully maintained under stretching/bending conditions. After repeatedly stretching the devices, there is no significant power attenuation in pulsed output. The unique bioelectrode structure and attractive energy harvesting/storing properties demonstrate the promising potential of this biosupercapacitor as a micro self-powered platform of wearable electronics.

Biofluid-Activated Biofuel Cells, Batteries, and Supercapacitors: A Comprehensive Review

Recent developments in wearable and implanted devices have resulted in numerous, unprecedented capabilities that generate increasingly detailed information about a user's health or provide targeted therapy. However, options for powering such systems remain limited to conventional batteries which are large and have toxic components and as such are not suitable for close integration with the human body. This work provides an in-depth overview of biofluid-activated electrochemical energy devices, an emerging class of energy sources judiciously designed for biomedical applications. These unconventional energy devices are composed of biocompatible materials that harness the inherent chemistries of various biofluids to produce useable electrical energy. This work covers examples of such biofluid-activated energy devices in the form of biofuel cells, batteries, and supercapacitors. Advances in materials, design engineering, and biotechnology that form the basis for high-performance, biofluid-activated energy devices are discussed. Innovations in hybrid manufacturing and heterogeneous integration of device components to maximize power output are also included. Finally, key challenges and future scopes of this nascent field are provided.

Paper Supercapacitor Developed Using a Manganese Dioxide/Carbon Black Composite and a Water Hyacinth Cellulose Nanofiber-Based Bilayer Separator

Flexible and green energy storage devices have a wide range of applications in prospective electronics and connected devices. In this study, a new eco-friendly bilayer separator and primary and secondary paper supercapacitors based on manganese dioxide (MnO2)/carbon black (CB) are developed. The bilayer separator is prepared via a two-step fabrication process involving freeze-thawing and nonsolvent-induced phase separation. The prepared bilayer separator exhibits superior porosity of 46%, wettability of 46.5°, and electrolyte uptake of 194% when compared with a Celgard 2320 trilayer separator (39%, 55.58°, and 110%). Moreover, lower bulk resistance yields a higher ionic conductivity of 0.52 mS cm-1 in comparison to 0.22 mS cm-1 for the Celgard separator. Furthermore, the bilayer separator exhibits improved mean efficiency of 0.44% and higher specific discharge capacitance of 13.53%. The anodic and cathodic electrodes are coated on a paper substrate using MnO2/CB and zinc metal-loaded CB composites. The paper supercapacitor demonstrates a high specific capacitance of 34.1 mF cm-2 and energy and power density of 1.70 μWh cm-2 and 204.8 μW cm-2 at 500 μA, respectively. In summary, the concept of an eco-friendly bilayer cellulose separator with paper-based supercapacitors offers an environmentally friendly alternative to traditional energy storage devices.

Advanced Self-Powered Biofuel Cells with Capacitor and Nanogenerator for Biomarker Sensing

Self-powered biofuel cells (BFCs) have evolved for highly sensitive detection of biomarkers such as noncodon micro ribonucleic acids (miRNAs) in the presence of interfering substrates. Self-charging supercapacitive BFCs for in vivo and in vitro cellular microenvironments represent the most prevalent sensing mechanism for diagnosis. Therefore, self-powered biosensing (SPB) with a capacitor and contact separation with a triboelectric nanogenerator (TENG) offers electrochemical and colorimetric dual-mode detection via improved electrical signal intensity. In this review, we discuss three major components: stretchable self-powered BFC design, miRNA sensing, and impedance spectroscopy. A specific focus is given to 1) assembling of sensors for biomarkers, 2) electrical output signal intensification, and 3) role of supercapacitors and nanogenerators in SPBs. We outline the key features of stretchable SPBs and the sequence of miRNA sensing by SPBs. We have emphasized the need of a supercapacitor and nanogenerator for SPBs in the context of advanced assembly of the sensing unit. Finally, we outline the role of impedance spectroscopy in the detection and estimation of biomarkers. We highlight key challenges in SPBs for biomarker sensing, which needs improved sensing accuracy, integration strategies of electrochemical biosensing for in vitro and in vivo microenvironments, and the impact of miRNA sensing on cancer diagnostics. This article attempts a specific focus on the accuracy and limitations of sensing unit for miRNA biomarkers and associated tool for boosting electrical signal intensity for a potential big step further.

Progress in optical sensors-based uric acid detection

The escalating number of patients affected by various diseases, such as gout, attributed to abnormal uric acid (UA) concentrations in body fluids, has underscored the need for rapid, efficient, highly sensitive, and stable UA detection methods and sensors. Optical sensors have garnered significant attention due to their simplicity, cost-effectiveness, and resistance to electromagnetic interference. Notably, research efforts have been directed towards UA on-site detection, enabling daily monitoring at home and facilitating rapid disease screening in the community. This review aims to systematically categorize and provide detailed descriptions of the notable achievements and emerging technologies in UA optical sensors over the past five years. The review highlights the advantages of each sensor while also identifying their limitations in on-site applications. Furthermore, recent progress in instrumentation and the application of UA on-site detection in body fluids is discussed, along with the existing challenges and prospects for future development. The review serves as an informative resource, offering technical insights and promising directions for future research in the design and application of on-site optical sensors for UA detection.

Multifunctional MOF-Derived Au, Co-Doped Porous Carbon Electrode for a Wearable Sweat Energy Harvesting-Storage Hybrid System

As an efficient alternative for harnessing the energy from human's biofluid, a wearable energy harvesting-storage hybrid supercapacitor-biofuel cell (SC-BFC) microfluidic system is established with one multifunctional electrode. The electrode integrates metal-organic framework (MOF) derived carbon nanoarrays with embedded Au, Co nanoparticles on a flexible substrate, and is used for the symmetric supercapacitor as well as the enzyme nanocarriers of the biofuel cell. The electrochemical performance of the proposed electrode is evaluated, and the corresponding working mechanism is studied in depth according to the cyclic voltammetry and density functional theory calculation. The multiplexed microfluidic system is designed to pump and store natural sweat to maintain the continuous biofuel supply in the hybrid SC-BFC system. The biofuel cell module harvests electricity from lactate in sweat, and the symmetric supercapacitor module accommodates the bioelectricity for subsequent utilization. A numerical model is developed to validate the normal operation in poor and rich sweat under variable situations for the microfluidic system. One single SC-BFC unit can be self-charged to ≈0.8 V with superior mechanical durability in on-body testing, as well as energy and power values of 7.2 mJ and 80.3 µW, respectively. It illustrates the promising scenery of energy harvesting-storage hybrid microfluidic system.

Self-chargeable supercapacitor made with MXene-bacterial cellulose nanofiber composite for wearable devices

The development of wearable electronics is restricted by the developments of supporting energy storage devices, especially flexible supercapacitors. Nowadays, miniaturized supercapacitors based on MXenes due to their obvious advantages in the specific capacity have received extensive attention. The energy existing in the surrounding environment has been used to directly charge energy storage devices. However, the hybrid wearable electronics integrated supercapacitors are mechanically connected through metal wires leading to non-compact devices. Thus, it is urgent to develop a general and universal method to fabricate high-performance robust MXene-based flexible electrodes with high electrical conductivity and apply them to self-chargeable supercapacitors and compact wearable devices. Herein, the bacterial cellulose (BC) nanofibers are used as a crosslinking agent to connect two-dimensional MXene nanosheets through the hydrogen bond, which greatly improves the mechanical strength of MXene-bacterial cellulose (MXene-BC) composite films (Young's modulus reaching 6.8 GPa). The supercapacitors made with the electrodes of MXene-BC composite films (BC content is 10%) present high capacitance behavior (areal capacitance up to 346 mF cm^-2) because the introduction of BC nanofibers increases the interlayer spacing of MXene nanosheets, providing more storage space for the ions in the electrolyte. Then, a self-chargeable supercapacitor is proposed based on the combination of a zinc-air (Zn-air) battery and a supercapacitor. The self-chargeable supercapacitor can realize self-charging after dropping a drop of electrolyte solution into the Zn-air battery. The charging voltage of a single self-chargeable supercapacitor can reach 0.6 V after adding artificial sweat as the electrolyte. Finally, a smart wristband with the function of self-charging is proposed, which can absorb the sweat generated by the human for self-chargeable supercapacitors to drive the pedometer integrated within the smart wristband to work. The proposed self-chargeable supercapacitors are simple and effective, not restricted by the use environment, providing a promising way for self-powered wearable electronics.

Weavable yarn-shaped supercapacitor in sweat-activated self-charging power textile for wireless sweat biosensing

The yarn-based sweat-activated battery (SAB) is a promising energy source for textile electronics due to its excellent skin compatibility, great weavability, and stable electric output. However, its power density is too low to support real-time monitoring and wireless data transmission. Here, we developed a scalable, high-performance sweat-based yarn biosupercapacitor (SYBSC) with two symmetrically aligned electrodes made by wrapping hydrophilic cotton fibers on polypyrrole/poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate)-modified stainless steel yarns. Once activated with artificial sweat, the SYBSC could offer a high areal capacitance of 343.1 mF cm^-2 at 0.5 mA cm^-2. After 10,000 times of bending under continuous charge-discharge cycles and 25 cycles of machine washing, the device could retain the capacitance at rates of 68% and 73%, respectively. The SYBSCs were integrated with yarn-shaped SABs to produce hybrid self-charging power units. The hybrid units, pH sensing fibers, and a mini-analyzer were woven into a sweat-activated all-in-one sensing textile, in which the hybrid, self-charging units could power the analyzer for real-time data collection and wireless transmission. The all-in-one electronic textile could be successfully employed to real-time monitor the pH values of the volunteers' sweat during exercise. This work can promote the development of self-charging electronic textiles for monitoring human healthcare and exercise intensity.

Evolution of nanostructured skin patches towards multifunctional wearable platforms for biomedical applications

Recent advances in the field of skin patches have promoted the development of wearable and implantable bioelectronics for long-term, continuous healthcare management and targeted therapy. However, the design of electronic skin (e-skin) patches with stretchable components is still challenging and requires an in-depth understanding of the skin-attachable substrate layer, functional biomaterials and advanced self-powered electronics. In this comprehensive review, we present the evolution of skin patches from functional nanostructured materials to multi-functional and stimuli-responsive patches towards flexible substrates and emerging biomaterials for e-skin patches, including the material selection, structure design and promising applications. Stretchable sensors and self-powered e-skin patches are also discussed, ranging from electrical stimulation for clinical procedures to continuous health monitoring and integrated systems for comprehensive healthcare management. Moreover, an integrated energy harvester with bioelectronics enables the fabrication of self-powered electronic skin patches, which can effectively solve the energy supply and overcome the drawbacks induced by bulky battery-driven devices. However, to realize the full potential offered by these advancements, several challenges must be addressed for next-generation e-skin patches. Finally, future opportunities and positive outlooks are presented on the future directions of bioelectronics. It is believed that innovative material design, structure engineering, and in-depth study of fundamental principles can foster the rapid evolution of electronic skin patches, and eventually enable self-powered close-looped bioelectronic systems to benefit mankind.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.

Hydrovoltaic Nanogenerators for Self-Powered Sweat Electrolyte Analysis

Human sweat comprises various electrolytes that are health status indicators. Conventional potentiometric electrolyte sensors require an electrical power source, which is expensive, bulky, and requires a complex architecture. Herein, this work demonstrates an electric nanogenerator fabricated using silicon nanowire (SiNW) arrays comprising modified carbon nanoparticles. The SiNW arrays platform is demonstrated as an effective self-powered sensor for sweat electrolyte analysis. It has been shown that an evaporation-induced water flow in nanochannels can yield an open-circuit voltage (V_oc ) of 0.45 V and a short-circuit current of 10.2 µA at room temperature as a result of overlapped electric double layers. The electrolyte in the water flow results in a V_oc decrease due to the charge shielding effect. The V_oc is inversely proportional to the electrolyte concentration. The fabricated hydrovoltaic device shows the capability for sensing electrolytes in human sweat, which is useful in evaluating the hydration status of volunteers following intense physical exercise. The device depicts a novel response mechanism compared to conventional electrochemical sensors. Furthermore, the hydrovoltaic device shows a maximum output power of 1.42 µW, and as such has been successfully shown to drive various electronic devices including light-emitting diodes, a calculator, and an electronic timer.

Universal Fully Integrated Wearable Sensor Arrays for the Multiple Electrolyte and Metabolite Monitoring in Raw Sweat, Saliva, or Urine

Fully integrated wearable sensors are capable of dynamically, directly, and independently tracking biomarkers in raw noninvasive biofluids without any other equipment or accessories by integrating the unique on-body monitoring feature with the special complete functional implementation attribute. Sweat, saliva, and urine are three important noninvasive biofluids, and changes in their biomarkers hold great potential for revealing physiological conditions. However, it is still a challenge to design single fully integrated wearable sensor arrays (FIWSAs) that are universally able to concurrently measure electrolytes and metabolites in three of the most common noninvasive biofluids including sweat, saliva, and urine. Here, we propose the first single universal FIWSAs for wirelessly, noninvasively, and simultaneously measuring various metabolites (i.e., uric acid) and electrolytes (i.e., Na⁺ and H⁺) in raw sweat, saliva, or urine under subjects' exercise by integrating the specifically designed microfluidic, sensing, and electronic modules in a seamless manner. We evaluate its utility for noninvasive gout management in healthy subjects and in gout patients through a purine-rich meal challenge and with a medicine-treatment control, respectively. Noninvasive monitoring of multiple electrolytes and metabolites in a variety of raw noninvasive biofluids via such single universal FIWSAs may enrich the understanding of the biomarkers' levels in the body and would also facilitate self-health management.

Device integration of electrochemical biosensors

Electrochemical biosensors incorporate a recognition element and an electronic transducer for the highly sensitive detection of analytes in body fluids. Importantly, they can provide rapid readouts and they can be integrated into portable, wearable and implantable devices for point-of-care diagnostics; for example, the personal glucose meter enables at-home assessment of blood glucose levels, greatly improving the management of diabetes. In this Review, we discuss the principles of electrochemical biosensing and the design of electrochemical biosensor devices for health monitoring and disease diagnostics, with a particular focus on device integration into wearable, portable and implantable systems. Finally, we outline the key engineering challenges that need to be addressed to improve sensing accuracy, enable multiplexing and one-step processes, and integrate electrochemical biosensing devices in digital health-care pathways.

Flexible Textile-Based Sweat Sensors for Wearable Applications

The current physical health care system has gradually evolved into a form of virtual hospitals communicating with sensors, which can not only save time but can also diagnose a patient's physical condition in real time. Textile-based wearable sensors have recently been identified as detection platforms with high potential. They are developed for the real-time noninvasive detection of human physiological information to comprehensively analyze the health status of the human body. Sweat comprises various chemical compositions, which can be used as biomarkers to reflect the relevant information of the human physiology, thus providing references for health conditions. Combined together, textile-based sweat sensors are more flexible and comfortable than other conventional sensors, making them easily integrated into the wearable field. In this short review, the research progress of textile-based flexible sweat sensors was reviewed. Three mechanisms commonly used for textile-based sweat sensors were firstly contrasted with an introduction to their materials and preparation processes. The components of textile-based sweat sensors, which mainly consist of a sweat transportation channel and collector, a signal-selection unit, sensing elements and sensor integration and communication technologies, were reviewed. The applications of textile-based sweat sensors with different mechanisms were also presented. Finally, the existing problems and challenges of sweat sensors were summarized, which may contribute to promote their further development.

Triboelectric Nanogenerators as Power Sources for Chemical Sensors and Biosensors

The recent advances of portable sensors in flexible and wearable form factors are drawing increasing attention worldwide owing to their requirement applications ranging from health monitoring to environment monitoring. While portability is critical for these applications, real-time data gathering also requires a reliable power supply-which is largely met with batteries. Besides the need for regular charging, the use of toxic chemicals in batteries makes it difficult to rely on them, and as a result different types of energy harvesters have been explored in recent years. Among these, triboelectric nanogenerators (TENGs) provide a promising platform for harnessing ambient energy and converting it into usable electric signals. The ease of fabrication and possibility to develop TENGs with a diverse range of easily available materials also make them attractive. This review focuses on the TENG technology and its efficient use as a power source for various types of chemical sensors and biosensors. The paper describes the underlying mechanism, various modes of working of TENGs, and representative examples of their utilization as power sources for sensing a multitude of analytes. The challenges associated with their adoption for commercial solutions are also discussed to stimulate further advances and innovations.

Recent Advances, Opportunities, and Challenges in Developing Nucleic Acid Integrated Wearable Biosensors for Expanding the Capabilities of Wearable Technologies in Health Monitoring

Wearable biosensors are becoming increasingly popular due to the rise in demand for non-invasive, real-time monitoring of health and personalized medicine. Traditionally, wearable biosensors have explored protein-based enzymatic and affinity-based detection strategies. However, in the past decade, with the success of nucleic acid-based point-of-care diagnostics, a paradigm shift has been observed in integrating nucleic acid-based assays into wearable sensors, offering better stability, enhanced analytical performance, and better clinical applicability. This narrative review builds upon the current state and advances in utilizing nucleic acid-based assays, including oligonucleotides, nucleic acid, aptamers, and CRISPR-Cas, in wearable biosensing. The review also discusses the three fundamental blocks, i.e., fabrication requirements, biomolecule integration, and transduction mechanism, for creating nucleic acid integrated wearable biosensors.

Hydrogels Enable Future Smart Batteries

The growing trend of intelligent devices ranging from wearables and soft robots to artificial intelligence has set a high demand for smart batteries. Hydrogels provide opportunities for smart batteries to self-adjust their functions according to the operation conditions. Despite the progress in hydrogel-based smart batteries, a gap remains between the designable functions of diverse hydrogels and the expected performance of batteries. In this Perspective, we first briefly introduce the fundamentals of hydrogels, including formation, structure, and characteristics of the internal water and ions. Batteries that operate under unusual mechanical and temperature conditions enabled by hydrogels are highlighted. Challenges and opportunities for further development of hydrogels are outlined to propose future research in smart batteries toward all-climate power sources and intelligent wearables.

A Comprehensive Review of the Recent Developments in Wearable Sweat-Sensing Devices

Sweat analysis offers non-invasive real-time on-body measurement for wearable sensors. However, there are still gaps in current developed sweat-sensing devices (SSDs) regarding the concerns of mixing fresh and old sweat and real-time measurement, which are the requirements to ensure accurate the measurement of wearable devices. This review paper discusses these limitations by aiding model designs, features, performance, and the device operation for exploring the SSDs used in different sweat collection tools, focusing on continuous and non-continuous flow sweat analysis. In addition, the paper also comprehensively presents various sweat biomarkers that have been explored by earlier works in order to broaden the use of non-invasive sweat samples in healthcare and related applications. This work also discusses the target analyte's response mechanism for different sweat compositions, categories of sweat collection devices, and recent advances in SSDs regarding optimal design, functionality, and performance.

A Self-Powered Wearable Sensor for Continuous Wireless Sweat Monitoring

Wireless wearable sweat analysis devices can monitor biomarkers at the molecular level continuously and in situ, which is highly desired for personalized health care. The miniaturization, integration, and wireless operation of sweat sensors improve the comfort and convenience while also bringing forward new challenges for power supply technology. Herein, a wireless self-powered wearable sweat analysis system (SWSAS) is designed that effectively converts the mechanical energy of human motion into electricity through hybrid nanogenerator modules (HNGMs). The HNGM shows stable output characteristics at low frequency with a current of 15 mA and a voltage of 60 V. Through real-time on-body sweat analysis powered by HNGM, the SWSAS is demonstrated to selectively monitor biomarkers (Na⁺ and K⁺ ) in sweat and wirelessly transmit the sensing data to the user interface via Bluetooth.

A Portable Sweat Sensor Based on Carbon Quantum Dots for Multiplex Detection of Cardiovascular Health Biomarkers

The future of personalized diagnostics and treatment of cardiovascular diseases lies in the use of portable sensors. Portable sensors can acquire biomarker information in biological fluids such as sweat, an approach that mitigates the shortcomings of conventional hospital-centered healthcare. Low sensitivity, selectivity, and specificity remain bottlenecks for the widespread use of portable sensors. Herein, we demonstrate a portable sensor that simultaneously detects Na⁺, ascorbic acid, and human neuropeptide Y in sweat, all useful biomarkers to index cardiovascular health. The portable sensor comprises a six-electrode system containing three working electrodes, two reference electrodes, and one counter electrode. The working electrodes were prepared by depositing sensing components on carbon quantum dot (CQD) electrodes. The sensing mechanisms were based on selective ion recognition, enzyme catalytic reaction, and immune response, which guarantees specificity to corresponding targets. The CQDs offer massive reactive sites and effectively reduce the interfacial impedance during the sensing reaction, thereby enhancing the three biomarkers' detection sensitivity. As evidence of portable sensor capability, we demonstrate herein its effective simultaneous detection of the three biomarkers in a real sweat from healthy volunteers during routine activities including exercise, extra ascorbic acid ingestion, and extra Na⁺ ingestion. As such, the sensor shows promise for real-time noninvasive personalized medical diagnostics and metabolic wellness management.

Design and Testing of Autonomous Chargeable and Wearable Sweat/Ionic Liquid-Based Supercapacitors

This work demonstrates ionic liquid electrolyte-inscribed sweat-based dual electrolyte functioning supercapacitors capable of self-charging through sweat electrolyte function under a non-enzymatic route. The supercapacitor electrodes are fabricated from TREN (tris(2-aminoethyl)amine), poly-3,4-ethylenedioxythiophene, and a graphene oxide mixture with copper-mediated chelate, and this polymer-GO-metal chelate film can produce excellent energy harvest/storage performance from a sweat and ionic liquid integrated electrolyte system. The fabricated device is specifically designed to reduce deterioration using a typical planar structure. In the presence of sweat with ionic liquid, the dual electrolyte mode supercapacitor exhibits a maximum areal capacitance of 3600 mF cm-2 , and the energy density is 450 mWhcm-2 , which is more than 100 times greater than that from previously reported supercapacitors. The supercapacitors were fabricated/attached directly to textile fabrics as well as ITO-PET (Indium tin oxide (ITO)-polyethylene terephthalate (PET) film to study their performance on the human body during exercise. The self-charging performance with respect to sweat wetting time for the sweat@ionic liquid dual electrolyte showed that the supercapacitor performed well on both fabric and film. These devices exhibited good response for pH effect and biocompatibility, and as such present a promising multi-functional energy system as a stable power source for next-generation wearable smart electronics.

Electrospun nanofiber-based glucose sensors for glucose detection

Diabetes is a chronic, systemic metabolic disease that leads to multiple complications, even death. Meanwhile, the number of people with diabetes worldwide is increasing year by year. Sensors play an important role in the development of biomedical devices. The development of efficient, stable, and inexpensive glucose sensors for the continuous monitoring of blood glucose levels has received widespread attention because they can provide reliable data for diabetes prevention and diagnosis. Electrospun nanofibers are new kinds of functional nanocomposites that show incredible capabilities for high-level biosensing. This article reviews glucose sensors based on electrospun nanofibers. The principles of the glucose sensor, the types of glucose measurement, and the glucose detection methods are briefly discussed. The principle of electrospinning and its applications and advantages in glucose sensors are then introduced. This article provides a comprehensive summary of the applications and advantages of polymers and nanomaterials in electrospun nanofiber-based glucose sensors. The relevant applications and comparisons of enzymatic and non-enzymatic nanofiber-based glucose sensors are discussed in detail. The main advantages and disadvantages of glucose sensors based on electrospun nanofibers are evaluated, and some solutions are proposed. Finally, potential commercial development and improved methods for glucose sensors based on electrospinning nanofibers are discussed.

Highly Thermoelectric ZnO@MXene (Ti3C2Tx) Composite Films Grown by Atomic Layer Deposition

Due to its unique high conductivity and flexibility, the two-dimensional MXene material (Ti₃C₂T_x) is expected to possess great potential in the thermoelectric field. However, the low thermoelectric performance from high thermal conductivity and a low Seebeck coefficient has limited its practical application. In this report, we demonstrate the uniform growth of ZnO layers on the laminar Ti₃C₂T_x membrane by atomic layer deposition (ALD). Benefiting from the low-temperature deposition characteristics of the ALD technique, the ZnO@Ti₃C₂T_x composite films maintain the basic apparent morphology of the original films after the deposition. We reveal that the Schottky barrier formed between ZnO and Ti₃C₂T_x exhibits an energy-filtering effect, significantly enhancing the Seebeck coefficient to result in more than a double increase in the power factor. Meanwhile, the strong phonon-interface scattering between ZnO and Ti₃C₂T_x is found to reduce the thermal conductivity of the composite films by a factor of four as compared to pure Ti₃C₂T_x ones, further improving the overall thermoelectric properties of the ZnO@Ti₃C₂T_x composite films. Our investigation provides an ALD-based strategy for growing wide band gap layers on the narrow band gap films to improve the thermoelectric performance of various MXene materials.

Self-Powered Active Sensing Based on Triboelectric Generators

The demand for portable and wearable chemical or biosensors and their expeditious development in recent years has created a scientific challenge in terms of their continuous powering. As a result, mechanical energy harvesters such as piezoelectric and triboelectric generators (TEGs) have been explored recently either as sensors or harvesters to store charge in small, but long-life, energy-storage devices to power the sensors. The use of energy harvesters as sensors is particularly interesting, as with such multifunctional operations it is possible to reduce the number devices needed in a system, which also helps overcome the integration complexities. In this regard, TEGs are promising, particularly for energy autonomous chemical and biological sensors, as they can be developed with a wide variety of materials, and their mechanical energy to electricity conversion can be modulated by various analytes. This review focuses on this interesting dimension of TEGs and presents various self-powered active chemical and biological sensors. A brief discussion about the development of TEG-based physical, magnetic, and optical sensors is also included. The influence of environmental factors, various figures of merit, and the significance of TEG design are explained in context with the active sensing. Finally, the key applications, challenges, and future perspective of chemical and biological detection via TEGs are discussed with a view to drive further advances in the field of self-powered sensors.

End-to-end design of wearable sensors

Wearable devices provide an alternative pathway to clinical diagnostics by exploiting various physical, chemical and biological sensors to mine physiological (biophysical and/or biochemical) information in real time (preferably, continuously) and in a non-invasive or minimally invasive manner. These sensors can be worn in the form of glasses, jewellery, face masks, wristwatches, fitness bands, tattoo-like devices, bandages or other patches, and textiles. Wearables such as smartwatches have already proved their capability for the early detection and monitoring of the progression and treatment of various diseases, such as COVID-19 and Parkinson disease, through biophysical signals. Next-generation wearable sensors that enable the multimodal and/or multiplexed measurement of physical parameters and biochemical markers in real time and continuously could be a transformative technology for diagnostics, allowing for high-resolution and time-resolved historical recording of the health status of an individual. In this Review, we examine the building blocks of such wearable sensors, including the substrate materials, sensing mechanisms, power modules and decision-making units, by reflecting on the recent developments in the materials, engineering and data science of these components. Finally, we synthesize current trends in the field to provide predictions for the future trajectory of wearable sensors.

Ti2C3Tx/Polyurethane Constructed by Gas-Liquid Interface Self-Assembly for Underwater Sensing

Nowadays, multifunctional, easily prepared, and highly sensitive flexible sensors have attracted extensive attention and are gradually used in various scenarios. Here, we report the design of the Ti₂C₃T_x/polyurethane composites prepared by a facile gas-liquid interface self-assembly. The obtained flexible sensor has a wide detection range (∼900%), a low-stress detection limit (<1%), a high sensitivity (GF = 1.3, strain from 0 to 100%), and a fast response time (<140 ms). The multifunctional stress sensor can be applied to not only wearable motion monitoring and detection of various signals but also the detection of underwater human motion, as well as different motion states and swimming frequencies of toy fish in water, demonstrating its great prospects in a variety of applications, such as human movement monitoring and marine biological detection and research.

Skin bioelectronics towards long-term, continuous health monitoring

Skin bioelectronics are considered as an ideal platform for personalised healthcare because of their unique characteristics, such as thinness, light weight, good biocompatibility, excellent mechanical robustness, and great skin conformability. Recent advances in skin-interfaced bioelectronics have promoted various applications in healthcare and precision medicine. Particularly, skin bioelectronics for long-term, continuous health monitoring offer powerful analysis of a broad spectrum of health statuses, providing a route to early disease diagnosis and treatment. In this review, we discuss (1) representative healthcare sensing devices, (2) material and structure selection, device properties, and wireless technologies of skin bioelectronics towards long-term, continuous health monitoring, (3) healthcare applications: acquisition and analysis of electrophysiological, biophysical, and biochemical signals, and comprehensive monitoring, and (4) rational guidelines for the design of future skin bioelectronics for long-term, continuous health monitoring. Long-term, continuous health monitoring of advanced skin bioelectronics will open unprecedented opportunities for timely disease prevention, screening, diagnosis, and treatment, demonstrating great promise to revolutionise traditional medical practices.

Cu2O-Based Electrochemical Biosensor for Non-Invasive and Portable Glucose Detection

Electrochemical voltammetric sensors are some of the most promising types of sensors for monitoring various physiological analytes due to their implementation as non-invasive and portable devices. Advantages in reduced analysis time, cost-effectiveness, selective sensing, and simple techniques with low-powered circuits distinguish voltammetric sensors from other methods. In this work, we developed a Cu2O-based non-enzymatic portable glucose sensor on a graphene paste printed on cellulose cloth. The electron transfer of Cu2O in a NaOH alkaline medium and sweat equivalent solution at very low potential (+0.35 V) enable its implementation as a low-powered portable glucose sensor. The redox mechanism of the electrodes with the analyte solution was confirmed through cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy studies. The developed biocompatible, disposable, and reproducible sensors showed sensing performance in the range of 0.1 to 1 mM glucose, with a sensitivity of 1082.5 ± 4.7% µA mM-1 cm-2 on Cu2O coated glassy carbon electrode and 182.9 ± 8.83% µA mM-1 cm-2 on Cu2O coated graphene printed electrodes, making them a strong candidate for future portable, non-invasive glucose monitoring devices on biodegradable substrates. For portable applications we demonstrated the sensor on artificial sweat in 0.1 M NaOH solution, indicating the Cu2O nanocluster is selective to glucose from 0.0 to +0.6 V even in the presence of common interference such as urea and NaCl.

Wearable electrochemical biosensors to measure biomarkers with complex blood-to-sweat partition such as proteins and hormones

Smart electronic devices based on micro-controllers, also referred to as fashion electronics, have raised wearable technology. These devices may process physiological information to facilitate the wearer's immediate biofeedback in close contact with the body surface. Standard market wearable devices detect observable features as gestures or skin conductivity. In contrast, the technology based on electrochemical biosensors requires a biomarker in close contact with both a biorecognition element and an electrode surface, where electron transfer phenomena occur. The noninvasiveness is pivotal for wearable technology; thus, one of the most common target tissues for real-time monitoring is the skin. Noninvasive biosensors formats may not be available for all analytes, such as several proteins and hormones, especially when devices are installed cutaneously to measure in the sweat. Processes like cutaneous transcytosis, the paracellular cell–cell unions, or even reuptake highly regulate the solutes content of the sweat. This review discusses recent advances on wearable devices based on electrochemical biosensors for biomarkers with a complex blood-to-sweat partition like proteins and some hormones, considering the commented release regulation mechanisms to the sweat. It highlights the challenges of wearable epidermal biosensors (WEBs) design and the possible solutions. Finally, it charts the path of future developments in the WEBs arena in converging/emerging digital technologies.

Recent Advances of Point-of-Care Devices Integrated with Molecularly Imprinted Polymers-Based Biosensors: From Biomolecule Sensing Design to Intraoral Fluid Testing

Recent developments of point-of-care testing (POCT) and in vitro diagnostic medical devices have provided analytical capabilities and reliable diagnostic results for rapid access at or near the patient's location. Nevertheless, the challenges of reliable diagnosis still remain an important factor in actual clinical trials before on-site medical treatment and making clinical decisions. New classes of POCT devices depict precise diagnostic technologies that can detect biomarkers in biofluids such as sweat, tears, saliva or urine. The introduction of a novel molecularly imprinted polymer (MIP) system as an artificial bioreceptor for the POCT devices could be one of the emerging candidates to improve the analytical performance along with physicochemical stability when used in harsh environments. Here, we review the potential availability of MIP-based biorecognition systems as custom artificial receptors with high selectivity and chemical affinity for specific molecules. Further developments to the progress of advanced MIP technology for biomolecule recognition are introduced. Finally, to improve the POCT-based diagnostic system, we summarized the perspectives for high expandability to MIP-based periodontal diagnosis and the future directions of MIP-based biosensors as a wearable format.