Flexible Electrochemical Urea Sensor Based on Surface Molecularly Imprinted Nanotubes for Detection of Human Sweat

Role of wearable electrochemical biosensors in monitoring renal function biomarkers in sweat: a review

Real-time detection of renal biomarkers is crucial for immediate and continuous monitoring of kidney function, facilitating early diagnosis and intervention in kidney-related disorders. This proactive approach enables timely adjustments in treatment plans, particularly in critical situations, and enhances overall patient care. Wearable devices emerge as a promising solution, enabling non-invasive and real-time data collection. This comprehensive review investigates numerous types of wearable sensors designed to detect kidney biomarkers in body fluids such as sweat. It critically evaluates the precision, dependability, and user-friendliness of these devices, contemplating their seamless integration into daily life for continuous health tracking. The review highlights the potential influence of wearable technology on individualized renal healthcare and its role in preventative medicine while also addressing challenges and future directions. The review's goal is to provide guidance to academics, healthcare professionals, and technologists working on wearable solutions for renal biomarker detection by compiling the body of current knowledge and advancements.

Mechanism of high sensitivity proton acids doped polypyrrole molecularly imprinted electrochemical sensor and its application in urea detection

Molecularly imprinted electrochemical sensor is a kind of convenient, fast, and stable analyzer, but the conductivity of electrode materials and their affinity with the analyte affect its performance. A proton acid (PSS, SA, SSA) doping method was proposed to improve the electrochemical performance of the polypyrrole molecularly imprinted polymer (PPy-MIP), which promoted the electropolymerization of pyrrole, reduced the charge transfer resistance, and increased the electrochemical surface area. In terms of both improving conductivity and affinity, the response of the proton acids doped the polypyrrole molecularly imprinted electrochemical sensors (PPy-MIECS) to urea was improved by 25-fold (PSS), 5-fold (SA), and 3-fold (SSA) over that of PPy-MIECS. In addition, the PSS-PPy-MIECS was validated for the practical application with a linear detection range from 0.1 mM to 100 mM, high selectivity (α = 39.73), reusability (RSD% = 4.54 %), reproducibility (RSD% = 0.93 %), and stability (11 days). The advantage of proton acid doping method in PSS-PEDOT-MIECS to urea and PSS-PPy-MIECS to glucose extended its application in the performance enhancement of MIECS design.

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

Multifunctional laser-induced graphene circuits and laser-printed nanomaterials toward non-invasive human kidney function monitoring

Faced with the increasing prevalence of chronic kidney disease (CKD), portable monitoring of CKD-related biomarkers such as potassium ion (K+), creatinine (Cre), and lactic acid (Lac) levels in sweat has shown tremendous potential for early diagnosis. However, a rapidly manufacturable portable device integrating multiple CKD-related biomarker sensors for ease of sweat testing use has yet to be reported. Here, a portable electrochemical sensor integrated with multifunctional laser-induced graphene (LIG) circuits and laser-printed nanomaterials based working electrodes fabricated by fully automatic laser manufacturing is proposed for non-invasive human kidney function monitoring. The sensor comprises a two-electrode LIG circuit for K+ sensing, a three-electrode LIG circuit with a Kelvin compensating connection for Cre and Lac sensing, and a printed circuit board based portable electrochemical workstation. The working electrodes containing Cu and Cu2O nanoparticles fabricated by two-step laser printing show good sensitivity and selectivity toward Cre and Lac sensing. The sensor circuits are fabricated by generating a hydrophilic-hydrophobic interface on a patterned LIG through laser. This sensor recruited rapid laser manufacturing and integrated with multifunctional LIG circuits and laser-printed nanomaterials based working electrodes, which is a potential kidney function monitoring solution for healthy people and kidney disease patients.

Sweat gland morphology and physiology in diabetes, neuropathy, and nephropathy: a review

Context: Sweat glands (SGs) play a vital role in thermal regulation. The function and structure are altered during the different pathological conditions.Objective: These alterations are studied through three techniques: biopsy, sweat analytes and electrical activity of SG.Methods: The morphological study of SG through biopsy and various techniques involved in quantifying sweat analytes is focussed on here. Electrical activities of SG in diabetes, neuropathy and nephropathy cases are also discussed, highlighting their limitations and future scope.Results and Conclusion: The result of this review identified three areas of the knowledge gap. The first is wearable sensors to correlate pathological conditions. Secondly, there is no device to look for its structure and quantify its associated function. Finally, therapeutic applications of SG are explored, especially for renal failure. With these aspects, this paper provides information collection and correlates SG with pathologies related to diabetes. Hence this could help researchers develop suitable technologies for the gaps identified.

Flexible Wearable Electrochemical Sensors Based on AuNR/PEDOT:PSS for Simultaneous Monitoring of Levodopa and Uric Acid in Sweat

As a facile substitute for the invasive technique of blood testing, wearable electrochemical sensors exhibit high potential for the noninvasive and real-time monitoring of biomarkers in human sweat. However, owing to enzyme specificity, the simultaneous detection of multiple biomarkers by enzymatic analysis is challenging. Moreover, sweat accumulation under sensors causes sweat contamination, which hinders real-time biomarker detection from sweat. This study reports the design and fabrication of flexible wearable electrochemical sensors containing a composite comprising Au nanorods (AuNRs) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) for the nonenzymatic detection of levodopa (LD) and uric acid (UA) in sweat. Each sensor was integrated with a flexible three-electrode system and a microfluidic patch for sweat sampling. AuNRs immobilized by PEG-doped PEDOT:PSS showed excellent analytical performance for LD and UA at different potentials. Thus, the newly fabricated sensors could detect LD and UA over a broad detection range with high sensitivity and showed a low limit of detection for both species. On-body assessments confirmed the ability of these sensors to simultaneously detect LD and UA in real time. Therefore, this study could open new frontiers in the fabrication of wearable electrochemical sensors for the pharmacokinetic profile tracking of LD and gout management.

High-Throughput Automatic Laser Printing Strategy toward Cost-effective Portable Integrated Urea Tele-Monitoring System

A portable sweat urea sensing system is a promising solution to satisfy the booming requirement of kidney function tele-monitoring. However, the complicated manufacturing route and the cumbersome electrochemical testing system still need to be improved to develop the urea point-of-care testing (POCT) and tele-monitoring devices. Here, a universal technical route based on a high-throughput automatic laser printing strategy for fabricating the portable integrated urea monitoring system is proposed. This integrated system includes a high-performance laser-printed urea sensing electrode, a planar three-electrode system, and a self-developed wireless mini-electrochemical workstation. A precursor donor layer is activated by laser scribing and in situ transferred into functional nanoparticles for the drop-on-demand printing of the urea sensing electrode. The obtained electrodes show high sensitivity, low detection limit, fast response time, high selectivity, good average recovery, and long-term stability for urea sensing. Additionally, a laser-induced graphene circuit-based miniature planar three-electrode system and a wireless mini-electrochemical workstation are designed for sensing data collection and transmitting, achieving real-time urea POCT and tele-monitoring. This scalable method provides a universal solution for high-throughput and ultra-fast fabrication of urea-sensing electrodes. The portable integrated urea monitoring system is a competitive option to achieve cost-effective POCT and tele-monitoring for kidney function.

Electrochemical sensing fibers for wearable health monitoring devices

Real-time monitoring of health conditions is an emerging strong issue in health care, internet information, and other strongly evolving areas. Wearable electronics are versatile platforms for non-invasive sensing. Among a variety of wearable device principles, fiber electronics represent cutting-edge development of flexible electronics. Enabled by electrochemical sensing, fiber electronics have found a wide range of applications, providing new opportunities for real-time monitoring of health conditions by daily wearing, and electrochemical fiber sensors as explored in the present report are a promising emerging field. In consideration of the key challenges and corresponding solutions for electrochemical sensing fibers, we offer here a timely and comprehensive review. We discuss the principles and advantages of electrochemical sensing fibers and fabrics. Our review also highlights the importance of electrochemical sensing fibers in the fabrication of "smart" fabric designs, focusing on strategies to address key issues in fiber-based electrochemical sensors, and we provide an overview of smart clothing systems and their cutting-edge applications in therapeutic care. Our report offers a comprehensive overview of current developments in electrochemical sensing fibers to researchers in the fields of wearables, flexible electronics, and electrochemical sensing, stimulating forthcoming development of next-generation "smart" fabrics-based electrochemical sensing.

Explainable Deep Learning-Assisted Self-Calibrating Colorimetric Patches for In Situ Sweat Analysis

Sweat has emerged as a compelling analyte for noninvasive biosensing technology because it contains a wealth of important biomarkers in hormones, organic biomacromolecules, and various ionic mixtures. These components offer valuable insights and can reflect an individual's physiological conditions. Here, we introduced an explainable deep learning (DL)-assisted wearable self-calibrating colorimetric biosensing analysis platform to efficiently and precisely detect the biomarker's concentration in sweat. Specifically, we have integrated the advantages of the colorimetric sensing method, adsorbing-swelling hydrogel, and explainable DL algorithms to develop an enzyme/indicator-immobilized colorimetric patch, which has reliable colorimetric sensing ability and excellent adsorbing-swelling function. A total of 5625 colorimetric images were collected as the analysis data set and assessed two DL algorithms and seven machine learning (ML) algorithms. Zn2+, glucose, and Ca2+ in human sweats could be facilely classified and quantified with 100% accuracy via the convolutional neural network (CNN) model, and the testing results of actual sweats via the DL-assisted colorimetric approach are 91.7-97.2% matching with the classical UV-vis spectrum. Class activation mapping (CAM) was utilized to visualize the inner working mechanism of CNN operation, which contributes to verify and explicate the design rationality of the noninvasive biosensing technology. An "end-to-end" model was established to ascertain the black box of the DL algorithm, promoted software design or principium optimization, and contributed facile indicators for health monitoring, disease prevention, and clinical diagnosis.

Simultaneous detection of urea and lactate in sweat based on a wearable sweat biosensor

Urea and lactate are biomarkers in sweat that is closely associated with human health. This study introduces portable, rapid, sensitive, stable, and high-throughput wearable sweat biosensors utilizing Au-Ag nanoshuttles (Au-Ag NSs) for the simultaneous detection of sweat urea and lactate. The Au-Ag NSs arrays within the biosensor's microfluidic cavity provide a substantial surface-enhanced Raman scattering (SERS) enhancement effect. The limit of detection (LOD) for urea and lactate are 2.35 × 10-6 and 8.66 × 10-7 mol/L, respectively. This wearable sweat biosensor demonstrates high resistance to compression bending, repeatability, and stability and can be securely attached to various body parts. Real-time sweat analysis of volunteers wearing the biosensors during exercise demonstrated the method's practicality. This wearable sweat biosensor holds significant potential for monitoring sweat dynamics and serves as a valuable tool for assessing bioinformation in sweat.

A Review of Electroactive Nanomaterials in the Detection of Nitrogen-Containing Organic Compounds and Future Applications

Electrochemical and impedimetric detection of nitrogen-containing organic compounds (NOCs) in blood, urine, sweat, and saliva is widely used in clinical diagnosis. NOC detection is used to identify illnesses such as chronic kidney disease (CKD), end-stage renal disease (ESRD), cardiovascular complications, diabetes, cancer, and others. In recent years, nanomaterials have shown significant potential in the detection of NOCs using electrochemical and impedimetric sensors. This potential is due to the higher surface area, porous nature, and functional groups of nanomaterials, which can aid in improving the sensing performance with inexpensive, direct, and quick-time processing methods. In this review, we discuss nanomaterials, such as metal oxides, graphene nanostructures, and their nanocomposites, for the detection of NOCs. Notably, researchers have considered nanocomposite-based devices, such as a field effect transistor (FET) and printed electrodes, for the detection of NOCs. In this review, we emphasize the significant importance of electrochemical and impedimetric methods in the detection of NOCs, which typically show higher sensitivity and selectivity. So, these methods will open a new way to make embeddable electrodes for point-of-detection (POD) devices. These devices could be used in the next generation of non-invasive analysis for biomedical and clinical applications. This review also summarizes recent state-of-the-art technology for the development of sensors for on-site monitoring and disease diagnosis at an earlier stage.

Sweat analysis for urea sensing: trends and challenges

With increasing population there is a rise in pathological diseases that the healthcare facilities are grappling with. Sweat-based wearable technologies for continuous monitoring have overcome the demerits associated with sweat sampling and sensing. Hence, sweat as an alternative biofluid holds great promise for the quantification of a host of biomarkers and understanding the functioning of the body, thereby deducing ailments quickly and economically. This comprehensive review accounts for recent advances in sweat-based LOCs (Lab-On-Chips), which are a likely alternative to the existing blood-urea sample testing that is invasive and time-consuming. The present review is focused on the advancements in sweat-based Lab-On-Chips (LOCs) as an alternative to invasive and time-consuming blood-urea sample testing. In addition, different sweat collection methods (direct skin, near skin and microfluidic) and their mechanism for urea sensing are explained in detail. The mechanism of urea in biofluids in protein metabolism, balancing nitrogen levels and a crucial factor of kidney function is described. In the end, research and technological advancements are explained to address current challenges and enable its widespread implementation.

Determination of urea, phosphate, and potassium in agricultural runoff waters using electrochemical impedance spectroscopy

Large-scale use of chemical fertilizers has resulted in the contamination of agricultural runoff waters by soil macronutrients NPK, whose detection is of significant interest. This work reports the determination of macronutrients in the form of urea (N), orthophosphate PO43- (P), and potassium K+ (K) in simulated agricultural runoff waters. Their solutions were prepared by extracting water-soluble constituents of soil. This 'base' solution contains high concentrations of various species, including Cl-, SO42-, NO3-, PO43-, Na+, K+, and NH4+ along with natural organic matter. Predetermined amounts of urea (4 to 22.5 ppm), PO43- (7 to 50 ppm), and potassium K+ (25 to 250 ppm) were added to the base simulated runoff water to prepare standard stock solutions. Using stainless steel working and counter electrodes, a small AC perturbation (±10 mV vs. OCP, vs. Ag/AgCl) was applied and the frequency response of the working electrode-solution interface was measured from 1 Hz to 1 MHz. The interface itself was modeled as a suitable equivalent electrical circuit, and the magnitudes of its components were fitted from experimental data using nonlinear regression. It is observed that PO43- concentration is a linear function of charge transfer resistance arising from redox reaction, K+ concentration is a quadratic function of double-layer capacitance arising from its higher mobility, and urea concentration can be correlated as a linear function of constant phase element arising from its polarization in the presence of an applied electric field. The sensor exhibits good sensitivity, repeatability, and excellent performance against interfering species. These preliminary results show significant potential for development of a real-time or on-site sensing device.

Tremella polysaccharide-based conductive hydrogel with anti-freezing and self-healing ability for motion monitoring and intelligent interaction

The application of conductive hydrogels in flexible wearable devices has garnered significant attention. In this study, a self-healing, anti-freezing, and fire-resistant hydrogel strain sensor is successfully synthesized by incorporating sustainable natural biological materials, viz. Tremella polysaccharide and silk fiber, into a polyvinyl alcohol matrix with borax cross-linking. The resulting hydrogel exhibits excellent transparency, thermoplasticity, and remarkable mechanical properties, including a notable elongation (1107.3 %) and high self-healing rate (91.11 %) within 5 min, attributed to the dynamic cross-linking effect of hydrogen bonds and borax. A strain sensor based on the prepared hydrogel sensor can be used to accurately monitor diverse human movements, while maintaining exceptional sensing stability and durability under repeated strain cycles. Additionally, a hydrogel touch component is designed that can successfully interact with intelligent electronic devices, encompassing functions like clicking, writing, and drawing. These inherent advantages make the prepared hydrogel a promising candidate for applications in human health monitoring and intelligent electronic device interaction.

Flexible and Stretchable Electrochemical Sensors for Biological Monitoring

The rise of flexible and stretchable electronics has revolutionized biosensor techniques for probing biological systems. Particularly, flexible and stretchable electrochemical sensors (FSECSs) enable the in situ quantification of numerous biochemical molecules in different biological entities owing to their exceptional sensitivity, fast response, and easy miniaturization. Over the past decade, the fabrication and application of FSECSs have significantly progressed. This review highlights key developments in electrode fabrication and FSECSs functionalization. It delves into the electrochemical sensing of various biomarkers, including metabolites, electrolytes, signaling molecules, and neurotransmitters from biological systems, encompassing the outer epidermis, tissues/organs in vitro and in vivo, and living cells. Finally, considering electrode preparation and biological applications, current challenges and future opportunities for FSECSs are discussed.

Novel graphene electrochemical transistors incorporating zirconia inorganic molecular imprinted layer：Design, construction and application for highly sensitive and selective detection of acetaminophen

Owing to their intrinsic amplifying effect together with chemical stability, graphene electrochemical transistor sensors (GECTs) are gaining momentum for sensing applications. However, the surface of GECTs for different detection substances must be modified with different recognition molecules, which was cumbersome and lack a universal method. Molecularly imprinted polymer (MIP) is a kind of polymer with specific recognition function for given molecules. Here, MIP and GECTs were combined to effectively solve the problem of weak selectivity of GECTs, and achieve the high sensitivity and selectivity of MIP-GECTs equipment in detecting acetaminophen (AP) in complex urine environment. A novel molecular imprinting sensor based on Au nanoparticles modified zirconia (ZrO₂) inorganic molecular imprinting membrane on reduced graphene oxide (ZrO₂-MIP-Au/rGO) was proposed. ZrO₂-MIP-Au/rGO was synthesized by one-step electropolymerization using AP as template, ZrO₂ precursor as the functional monomer. The -OH group on ZrO₂ and the -OH/-CONH- group on AP were easily bonded by hydrogen bonding to form a MIP layer on the surface, which allows the sensor to have a large number of imprinted cavities for AP specific adsorption. As a proof of method, the GECTs based on ZrO₂-MIP-Au/rGO functional gate electrode has the characteristics of wide linear range (0.1 nM-4 mM), low detection limit (0.1 nM) and high selectivity for AP detection. These achievements highlight the introduction of specific and selective MIP to GECTs with unique amplification function, which could effectively solve the problem of selectivity of GECTs in complex environments, suggesting the potential of MIP-GECTs in real-time diagnosis.

Flexible and Wearable Biosensors for Monitoring Health Conditions

Flexible and wearable biosensors have received tremendous attention over the past decade owing to their great potential applications in the field of health and medicine. Wearable biosensors serve as an ideal platform for real-time and continuous health monitoring, which exhibit unique properties such as self-powered, lightweight, low cost, high flexibility, detection convenience, and great conformability. This review introduces the recent research progress in wearable biosensors. First of all, the biological fluids often detected by wearable biosensors are proposed. Then, the existing micro-nanofabrication technologies and basic characteristics of wearable biosensors are summarized. Then, their application manners and information processing are also highlighted in the paper. Massive cutting-edge research examples are introduced such as wearable physiological pressure sensors, wearable sweat sensors, and wearable self-powered biosensors. As a significant content, the detection mechanism of these sensors was detailed with examples to help readers understand this area. Finally, the current challenges and future perspectives are proposed to push this research area forward and expand practical applications in the future.

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.

Wearable potentiometric biosensor for analysis of urea in sweat

Herein, we introduce wearable potentiometric biosensors on screen-printed carbon electrodes (SPCEs) for on-body and on-site monitoring of urea in sweat. The biosensor architecture was judiciously designed to detect urea at different pHs and incorporate a pH sensor, thus containing polyaniline ink, urease bioink and a polyvinylchloride membrane. Urea detection could be performed in the wide range from 5 to 200 mM at pH 7.0, encompassing urea levels in human sweat. The biosensor response was fast (incubation time 5 min), with no interference from other substances in sweat. Reliable urea detection could be done in undiluted human sweat with a skin-worn flexible device using the pH correction strategy afforded by the pH sensor. The performance of the epidermal biosensor was not affected by severe bending strains. The feasibility of mass production was demonstrated by fabricating epidermal flexible biosensors using slot-die coating with a roll-to-roll technique.

Multiplex Chroma Response Wearable Hydrogel Patch: Visual Monitoring of Urea in Body Fluids for Health Prognosis

Visual wearable devices can rapid intuitively monitor biomarkers in body fluids to indicate the human health status and provide valuable reference for further medical diagnosis. However, unavoidable interference factors such as skin color, natural light, and background luminescence can interfere with the visualization accuracy of flexible wearable devices, limiting their practical sensing application. Here, we designed a wearable sensing patch via an embedded upconversion optical probe in a 3D porous polyacrylamide hydrogel, exhibiting a multiplex chroma response to urea based on the inner filter effect, which overcomes the susceptibility to external conditions due to its near-infrared excited luminescence and improves the resolution and accuracy of visual sensing. Furthermore, a highly compatible portable sensing platform combined with a smartphone was designed to achieve in situ rapid quantitative analysis of urea. The limit of detection values of the upconversion optical probe and hydrogel sensor are as low as 1.4 and 30 μM respectively, exhibiting the practicality in different scenarios. The designed sensing patch provides a convenient and accurate sensing strategy for the detection of biomarkers in body fluids and has the potential to be developed into a point-of-care device to provide disease early warning and clinical diagnosis.

Nonenzymatic Sweat Wearable Uric Acid Sensor Based on N-Doped Reduced Graphene Oxide/Au Dual Aerogels

Sweat wearable sensors enable noninvasive and real-time metabolite monitoring in human health management but lack accuracy and wearable applicability. The rational design of sensing electrode materials will be critical yet challenging. Herein, we report a dual aerogel-based nonenzymatic wearable sensor for the sensitive and selective detection of uric acid (UA) in human sweat. The three-dimensional porous dual-structural aerogels composed of Au nanowires and N-doped graphene nanosheets (noted as N-rGO/Au DAs) provide a large active surface, abundant access to the target, rapid electron transfer pathways, and a high intrinsic activity. Thus, a direct UA electro-oxidation is demonstrated at the N-rGO/Au DAs with a much higher activity than those at the individual gels (i.e., Au and N-rGO). Moreover, the resulting sensing chip displays high performance with a good anti-interfering ability, long-term stability, and excellent flexibility toward the UA detection. With the assistance of a wireless circuit, a wearable sensor is successfully applied in the real-time UA monitoring on human skin. The obtained result is comparable to that evaluated by high-performance liquid chromatography. This dual aerogel-based nonenzymatic biosensing platform not only holds considerable promise for the reliable sweat metabolite monitoring but also opens an avenue for metal-based aerogels as flexible electrodes in wearable sensing.

Three-dimensional flexible and stretchable gold foam scaffold for real-time electrochemical sensing in cells and in vivo

Compared with typical two-dimensional electrodes, the three-dimensional (3D) cell culture platform can simulate the real cell survival environment for cell growth to accurately reproduce cell functions. Moreover, considering that living cells are exposed to various of mechanical force in the microenvironment, the construction of 3D electrodes with excellent flexible, stretchable, and biocompatibility is of great significance to real-time monitor mechanically evoked biomolecule release from cells. Herein, we demonstrated a straightforward and effective three-step approach to fabricate three-dimensional flexible and stretchable gold foam scaffold (3D Au foam scaffold) for construction of 3D cell culture integrated electrochemical sensing platform. The excellent biological and electrical properties of Au nanostructures and porous networks of the 3D scaffold endow the platform with desirable biocompatibility and sensitive electrochemical sensing performance. As a proof of concept, the 3D Au foam scaffold functionalized with cobalt based nanocubes (Co NCs/Au foam scaffold) was validated to provide 3D culture for human umbilical vein endothelial cells (HUVECs), and synchronously real-time monitor superoxide anion (O₂^•-) released by HUVECs under mechanical stretching. Furthermore, 3-mercaptopropionic acid (3-MPA) modified 3D Au foam (3-MPA/Au foam scaffold) was successfully used for real-time monitoring of catecholamines in rat brain. The results demonstrate the great potential of this 3D Au foam scaffold for real-time electrochemical monitoring biomolecules in vitro and in vivo, providing convenience for future research on mechanotransduction relevant processes.

Electrochemical-based remote biomarker monitoring: Toward Internet of Wearable Things in telemedicine

Internet of Wearable Things (IoWT) will be a major breakthrough for remote medical monitoring. In this scenario, wearable biomarker sensors have been developing not only to diagnose point-of-care (POC) of diseases, but also to continuously manage them. On-body tracking of biomarkers in biofluids is regarded as a proper substitution of conventional biomarker sensors for dynamic sampling and analyzing due to their high sensitivity, conformability, and affordability, creating ever-rising the market demand for them. In a wireless body area network (WBAN), data is captured from all sensors on the body to a smartphone/laptop, and sent the sensed data to a cloud for storing, processing, and retrieving, and ultimately displayed the data on custom applications (Apps). Wearable IoT biomarker sensors are used for early diseases diagnosis and continuous monitoring in developing countries in which people hardly access to healthcare systems. In this review, we aim to highlight a wide range of wearable electrochemical biomarker sensors, accompanied by microfluidics for continuous sampling, which will pave the way toward developing wearable IoT biomarker sensors to track health status. The current challenges and future perspective in skin-conformal biomarker sensors will be discussing their potential applicability for IoWT in cloud-based telemedicine.

Hydrogel-Integrated Multimodal Response as a Wearable and Implantable Bidirectional Interface for Biosensor and Therapeutic Electrostimulation

A hydrogel that fuses long-term biologic integration, multimodal responsiveness, and therapeutic functions has received increasing interest as a wearable and implantable sensor but still faces great challenges as an all-in-one sensor by itself. Multiple bonding with stimuli response in a biocompatible hydrogel lights up the field of soft hydrogel interfaces suitable for both wearable and implantable applications. Given that, we proposed a strategy of combining chemical cross-linking and stimuli-responsive physical interactions to construct a biocompatible multifunctional hydrogel. In this hydrogel system, ureidopyrimidinone/tyramine (Upy/Tyr) difunctionalization of gelatin provides abundant dynamic physical interactions and stable covalent cross-linking; meanwhile, Tyr-doped poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) acts as a conductive filler to establish electrical percolation networks through enzymatic chemical cross-linking. Thus, the hydrogel is characterized with improved conductivity, conformal biointegration features (i.e., high stretchability, rapid self-healing, and excellent tissue adhesion), and multistimuli-responsive conductivity (i.e., temperature and urea). On the basis of these excellent performances, the prepared multifunctional hydrogel enables multimodal wearable sensing integration that can simultaneously track both physicochemical and electrophysiological attributes (i.e., motion, temperature, and urea), providing a more comprehensive monitoring of human health than current wearable monitors. In addition, the electroactive hydrogel here can serve as a bidirectional neural interface for both neural recording and therapeutic electrostimulation, bringing more opportunities for nonsurgical diagnosis and treatment of diseases.

Electrochemical Nanosensors for Sensitization of Sweat Metabolites: From Concept Mapping to Personalized Health Monitoring

Sweat contains a broad range of important biomarkers, which may be beneficial for acquiring non-invasive biochemical information on human health status. Therefore, highly selective and sensitive electrochemical nanosensors for the non-invasive detection of sweat metabolites have turned into a flourishing contender in the frontier of disease diagnosis. A large surface area, excellent electrocatalytic behavior and conductive properties make nanomaterials promising sensor materials for target-specific detection. Carbon-based nanomaterials (e.g., CNT, carbon quantum dots, and graphene), noble metals (e.g., Au and Pt), and metal oxide nanomaterials (e.g., ZnO, MnO2, and NiO) are widely used for modifying the working electrodes of electrochemical sensors, which may then be further functionalized with requisite enzymes for targeted detection. In the present review, recent developments (2018-2022) of electrochemical nanosensors by both enzymatic as well as non-enzymatic sensors for the effectual detection of sweat metabolites (e.g., glucose, ascorbic acid, lactate, urea/uric acid, ethanol and drug metabolites) have been comprehensively reviewed. Along with this, electrochemical sensing principles, including potentiometry, amperometry, CV, DPV, SWV and EIS have been briefly presented in the present review for a conceptual understanding of the sensing mechanisms. The detection thresholds (in the range of mM-nM), sensitivities, linear dynamic ranges and sensing modalities have also been properly addressed for a systematic understanding of the judicious design of more effective sensors. One step ahead, in the present review, current trends of flexible wearable electrochemical sensors in the form of eyeglasses, tattoos, gloves, patches, headbands, wrist bands, etc., have also been briefly summarized, which are beneficial for on-body in situ measurement of the targeted sweat metabolites. On-body monitoring of sweat metabolites via wireless data transmission has also been addressed. Finally, the gaps in the ongoing research endeavors, unmet challenges, outlooks and future prospects have also been discussed for the development of advanced non-invasive self-health-care-monitoring devices in the near future.

γ-Irradiation crosslinking of graphene oxide/cellulose nanofiber/poly (acrylic acid) hydrogel as a urea sensing patch

Poly (acrylic acid) (PAA) nanocomposite hydrogel was fabricated as a sensing patch for non-invasive dual detection of urea in sweat. The hydrogel was prepared by γ-irradiation crosslinking of PAA solution incorporated with graphene oxide (GO) and cellulose nanofiber (CNF). With high water-sorption capacity and transparency, the hydrogel was suitable to accommodate coloring reagents and enzymes for colorimetric sensing of urea in sweat. The colorimetric sensor exhibited vivid color change towards the increase of urea concentration in a linear range of 40-80 mM covering a cut-off value (60 mM) for chronic kidney disease (CKD) indication. Furthermore, the hydrogel could be directly applied as a substrate for direct quantitation of urea in sweat by laser desorption ionization mass spectroscopy (LDI-MS). While CNF improved the mechanical properties of the hydrogel, GO played a key role in enhancing laser desorption ionization of urea in LDI-MS and increased the hydrogel functionalities. LDI-MS verified that GO/CNF/PAA hydrogel could act as a direct matrix for promoting urea ionization and these results corresponded well with the colorimetric sensor. Hence, this hydrogel patch might be a potential material to be applied in non-invasive and dual-detection of CKD in medical diagnosis.

The Progress of Research into Flexible Sensors in the Field of Smart Wearables

In modern society, technology associated with smart sensors made from flexible materials is rapidly evolving. As a core component in the field of wearable smart devices (or 'smart wearables'), flexible sensors have the advantages of excellent flexibility, ductility, free folding properties, and more. When choosing materials for the development of sensors, reduced weight, elasticity, and wearer's convenience are considered as advantages, and are suitable for electronic skin, monitoring of health-related issues, biomedicine, human-computer interactions, and other fields of biotechnology. The idea behind wearable sensory devices is to enable their easy integration into everyday life. This review discusses the concepts of sensory mechanism, detected object, and contact form of flexible sensors, and expounds the preparation materials and their applicability. This is with the purpose of providing a reference for the further development of flexible sensors suitable for wearable devices.

Recent Advances of Nanomaterials-Based Molecularly Imprinted Electrochemical Sensors

Molecularly imprinted polymer (MIP) is illustrated as an analogue of a natural biological antibody-antigen system. MIP is an appropriate substrate for electrochemical sensors owing to its binding sites, which match the functional groups and spatial structure of the target analytes. However, the irregular shapes and slow electron transfer rate of MIP limit the sensitivity and conductivity of electrochemical sensors. Nanomaterials, famous for their prominent electron transfer capacity and specific surface area, are increasingly employed in modifications of MIP sensors. Staying ahead of traditional electrochemical sensors, nanomaterials-based MIP sensors represent excellent sensing and recognition capability. This review intends to illustrate their advances over the past five years. Current limitations and development prospects are also discussed.

Chemical-Based Surface Plasmon Resonance Imaging of Fingerprints

Fingerprints are extremely useful in personal identification; however, they are usually based on physical rather than chemical images because it remains a challenge to reveal a clear chemical fingerprint easily and sensitively. Herein, a surface plasmon resonance imaging (SPRi) method, combined with a chemically selective stepwise signal amplification (CS³A) strategy, is proposed to chemically image fingerprints with adjustable sensitivity and clarity. High-fidelity glucose-associated fingerprint images were obtained at five to seven cycles of CS³A based on the recognition reaction of concanavalin A (ConA) with dextran. The method is also extendable to image substances that possess and/or can be tagged with ConA- or dextran-recognizable groups. For demonstration, SPRi of carboxylic substances was conducted by amidating the carboxyl group with glucosamine to enable the ConA-based CS³A. Glucose- and carboxyl-based fingerprints were simultaneously and clearly imaged, allowing us to perform quantitative analysis of the representative of either glucose or amino acid (e.g., serine) or both. The curves measured from the standard spots were linear in the ranges of 1-4000 μM for glucose and 3.2-4000 μM for serine, with linear correlated coefficients of 0.9979 and 0.9962, respectively. It was then applied to the study of metabolic secretions in fingerprints during running exercise, yielding variation tendencies similar to those measured from sweat samples in the literature. As a noninvasive tool, the CS³A-coupled SPRi reveals both clear images of fingerprints and quantitative chemical information, and it is anticipated to become a competitive new method for chemically imaging fingerprints.

Specificity recognition for a target protein, cytochrome c using molecularly imprinted hydrogels

Protein imprinted hydrogel, which is one form of protein imprinted molecularly imprinted polymers (MIPs), is an important material for enzyme-linked immunosorbent assay, drug delivery materials, sensors, separation materials, etc. To...

Simultaneous and visual detection of multiple dopes by an aptamer/AuNPs sensor

Through introducing multiple aptamers in a suitable ratio, we achieved the simultaneous and visual detection of three dopes in one sensor.

Electronic Textiles for Wearable Point-of-Care Systems

Traditional public health systems are suffering from limited, delayed, and inefficient medical services, especially when confronted with the pandemic and the aging population. Fusing traditional textiles with diagnostic, therapeutic, and protective medical devices can unlock electronic textiles (e-textiles) as point-of-care platform technologies on the human body, continuously monitoring vital signs and implementing round-the-clock treatment protocols in close proximity to the patient. This review comprehensively summarizes the research advances on e-textiles for wearable point-of-care systems. We start with a brief introduction to emphasize the significance of e-textiles in the current healthcare system. Then, we describe textile sensors for diagnosis, textile therapeutic devices for medical treatment, and textile protective devices for prevention, by highlighting their working mechanisms, representative materials, and clinical application scenarios. Afterward, we detail e-textiles' connection technologies as the gateway for real-time data transmission and processing in the context of 5G technologies and Internet of Things. Finally, we provide new insights into the remaining challenges and future directions in the field of e-textiles. Fueled by advances in chemistry and materials science, textile-based diagnostic devices, therapeutic devices, protective medical devices, and communication units are expected to interact synergistically to construct intelligent, wearable point-of-care textile platforms, ultimately illuminating the future of healthcare system in the Internet of Things era.

Progress of Wearable and Flexible Electrochemical Biosensors With the Aid of Conductive Nanomaterials

Conductive nanomaterials have recently gained a lot of interest due to their excellent physical, chemical, and electrical properties, as well as their numerous nanoscale morphologies, which enable them to be fabricated into a wide range of modern chemical and biological sensors. This study focuses mainly on current applications based on conductive nanostructured materials. They are the key elements in preparing wearable electrochemical Biosensors, including electrochemical immunosensors and DNA biosensors. Conductive nanomaterials such as carbon (Carbon Nanotubes, Graphene), metals and conductive polymers, which provide a large effective surface area, fast electron transfer rate and high electrical conductivity, are summarized in detail. Conductive polymer nanocomposites in combination with carbon and metal nanoparticles have also been addressed to increase sensor performance. In conclusion, a section on current challenges and opportunities in this growing field is forecasted at the end.

Developing a generally applicable electrochemical sensor for detecting macrolides in water with thiophene-based molecularly imprinted polymers

Our screening data revealed the threat macrolide antibiotics, especially azithromycin (AZN), posed to human health with its increasing occurrence in water environment. The electrochemical sensor based on molecularly imprinted polymer (MIP) is a promising platform that caters for the next generation of intelligent wastewater treatment plants (WWTPs) by virtue of its wide tolerance to water from all sources and in-situ monitoring. However, low initiation potentials of cross-linking monomers contributed by the electron-rich circumstance allowed them to usurp sites designed for functional monomers when electrically stimulated, leading to an unsatisfactory binding capacity. Another uncertainty is that multiple reaction sites of cross-linking monomers granted them complex polymerization routes and made it difficult to ensure the consistency of preparation. Serval monomers had been investigated with electrochemical tools and the performance of sensors constructed with these monomers were compared in this study. Based on the results, we proposed a protocol in which a novel functional monomer possessing a stronger electron-donating group, phenyl, was adopted to compete for the dominance in electropolymerization. Beyond that, the cross-linking monomer was modified with electron-withdrawing groups to raise its initiation potential. A monothiophene with a moderate initiation potential was also recruited as the linker to address the steric hindrance. In this way, polymerization proceeded in a specific order. It is worth mentioning that the Marangoni flow is an ideal tool to deal with the Coffee-ring deposition while drop-casting. The resulting sensor showed good performance with a limitation of detection (LOD) of 0.120 μM for AZN and a satisfactory selectivity, and the design can be applied to constructing sensors for a variety of macrolide antibiotics.

Biomimetic electrochemical sensors: New horizons and challenges in biosensing applications

The urge to meet the ever-growing needs of sensing technology has spurred research to look for new alternatives to traditional analytical methods. In this scenario, the glucometer is the flagship of commercial electrochemical sensing platforms, combining selectivity, reliability and portability. However, other types of enzyme-based biosensors seldom achieve the market, in spite of the large and increasing number of publications. The reasons behind their commercial limitations concern enzyme denaturation, and the high costs associated with procedures for their extraction and purification. In this sense, biomimetic materials that seek to imitate the desired properties of natural enzymes and biological systems have come out as an appealing path for robust and sensitive electrochemical biosensors. We herein portray the historical background of these biomimicking materials, covering from their beginnings until the most impactful applications in the field of electrochemical sensing platforms. Throughout the discussion, we present and critically appraise the major benefits and the most significant drawbacks offered by the bioinspired systems categorized as Nanozymes, Synzymes, Molecularly Imprinted Polymers (MIPs), Nanochannels, and Metal Complexes. Innovative strategies of fabrication and challenging applications are further reviewed and evaluated. In the end, we ponder over the prospects of this emerging field, assessing the most critical issues that shall be faced in the coming decade.

Integrated hand-held electrochemical sensor for multicomponent detection in urine

Electrochemical sensors have shown great advantage and application potential in point-of-care testing (POCT) related scenarios. However, some fatal problems plague its widespread utilization, which include the susceptibility of sensors to interference in real samples (e.g. pH), the contradiction between the limited objects detectable for most sensors and the requirement of multi-target analysis in most cases, and the complicated procedures in sensor preparation as well as in routine use. This paper contributed a tip-like electrochemical sensor prototype. By integrated with a commercial pipettor, it fulfilled semi-automatic assay procedure of sampling, detection and rinsing, thus saving operational time and manual work. The tip sensor owns the property of simple fabrication and is free from any modification of extra bio/chem materials. Moreover, built on multiple electrochemical signal outputs including open circuit potential, peak current and potential of specific electrochemical reaction, this work established a novel multi-component sensing strategy, wherein detection of uric acid (UA), urea and pH in urine samples was realized by using one single working electrode. The detection range for the above targets is 5.0~600 μM for UA, 4.0~8.0 for pH and 0.5~7.0 mM for urea with the detection limits (S/N = 3) of 0.05 μM for UA and 5.4 μM for urea, and the sensitivity of pH assay is 73 mV/pH. Notably, as variation of sample pH has impact on electrochemical analysis, the pH-related parameter was introduced for calibration to diminish such interference. The developed tip sensor and the novel sensing strategy may open a new window for electrochemical technology and broaden its application in POCT.

3-D printed electrode integrated sensing chip and a PoC device for enzyme free electrochemical detection of blood urea

Herein we report a novel electrochemical sensing chip and a point-of-care device (PoC) for enzyme-free electrochemical detection of urea in human blood. The electrochemical sensing chip was developed by 3-D printing of conductive Ag ink and subsequent electrodeposition of AuNP-rGO nanocomposite. Material characterization of the sensing chip was conducted to find a plausible mechanism for the electrochemical reaction with urea. Subsequently, the response with varying concentrations of urea in solution and human blood samples was tested. High peak response current (~5 times than that of the highest reported value), low impedance, rapid sensor fabrication procedure, high selectivity towards urea, excellent linear response (R² = 0.99), high sensitivity of 183 μA mM^-1 cm^-2, the fast response indicated by high diffusion coefficient, the limit of detection of 0.1 µM, tested shelf life of more than 6 months and recovery rate of >99% ensured the application of the developed sensor chip towards PoC urea detection test kit. A PoC device housing an electronic circuitry following the principles of linear sweep voltammetry and compatible with a sensing chip was developed. A maximum percentage error of 4.86% and maximum RSD of 3.63% confirmed the use of the PoC device for rapid urea measurements in human blood.

Molecularly imprinted membrane for transport of urea, creatinine, and vitamin B12 as a hemodialysis candidate membrane

High levels of urea and creatinine in the blood are a sign of decreased kidney function. To remove these substances from the blood, hemodialysis which utilizes membranes could be used. In this study, a molecularly imprinted membrane (MIM) was synthesized for the selective transport of urea. The synthesis is initiated with the polymerization of eugenol into polyeugenol and then into polyeugenoxy acetate (PA). The PA is then contacted with urea and then used as the functional polymer in the synthesis of MIM with polysulfone as the membrane base, and polyethylene glycol as the cross-linking agent. The result was later analyzed with FTIR and SEM-EDX. The membrane is then used in the transport of urea, creatinine, and vitamin B12 and then compared with the non-imprinted membrane (NIM) performance. By using UV-Vis spectrophotometry, the results showed that the membrane with 10 h heating variation is able to transport more urea and is more selective than NIM; this proves that the urea template on the MIM enables it to recognize urea molecules better than creatinine and vitamin B12. The order of transport from the best results is urea > creatinine > vitamin B12.

Touch-Based Stressless Cortisol Sensing

Tracking fluctuations of the cortisol level is important in understanding the body's endocrine response to stress stimuli. Traditional cortisol sensing relies on centralized laboratory settings, while wearable cortisol sensors are limited to slow and complex assays. Here, a touch-based non-invasive molecularly imprinted polymer (MIP) electrochemical sensor for rapid, simple, and reliable stress-free detection of sweat cortisol is described. The sensor readily measures fingertip sweat cortisol via highly selective binding to the cortisol-imprinted electropolymerized polypyrrole coating. The MIP network is embedded with Prussian blue redox probes that offer direct electrical signaling of the binding event to realize sensitive label-free amperometric detection. Using a highly permeable sweat-wicking porous hydrogel, instantaneously secreted fingertip sweat can be conveniently and rapidly collected without any assistance. By eliminating time lags, such rapid (3.5 min) fingertip assay enables the capture of sharp variations in cortisol levels, compared to previous methods. Such advantages are demonstrated by tracking cortisol response in short cold-pressor tests and throughout day-long circadian rhythm, along with gold-standard immunoassay validation. A stretchable epidermal MIP sensor is also described for directly tracking cortisol in exercise-induced sweat. The rapid touch-based cortisol sensor offers an attractive, accessible, stressless avenue for quantitative stress management.

Solution Blowing Spinning Technology towards Green Development of Urea Sensor Nanofibers Immobilized with Hydrazone Probe

Cellulose has been one of the most widespread materials due to its renewability, excellent mechanical properties, biodegradability, high absorption ability, biocompatibility and cheapness. Novel, simple and green colorimetric fibrous film sensor was developed by immobilization of urease enzyme (U) and tricyanofuran hydrazone (TCFH) molecular probe onto cellulose nanofibers (CNF). Cellulose acetate nanofibers (CANF) were firstly prepared from cellulose acetate using the simple, green and low cost solution blowing spinning technology. The produced CANF was exposed to deacetylation to introduce CNF, which was then treated with a mixture of TCFH and urease enzyme to introduce CNF-TCFH-U nanofibrous biosensor. CNF were reinforced with tricyanofuran hyrazone molecular probe and urease enzyme was encapsulated into calcium alginate biopolymer to establish a biocomposite film. This CNF-TCFH-U naked-eye sensor can be applied as a disposable urea detector. CNF demonstrated a large surface area and was utilized as a carrier for TCFH, which is the spectroscopic probe and urease is a catalyst. The biochromic CNF-TCFH-U nanofibrous biosensor responds to an aqueous medium of urea via a visible color signal changing from off-white to dark pink. The morphology of the generated CNF and CNF-TCFH-U nanofibrous films were characterized by different analytical tools, including energy-dispersive X-ray patterns (EDX), polarizing optical microscope (POM), Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM). SEM images of CNF-TCFH-U nanofibers demonstrated diameters between 800 nm and 2.5 μm forming a nonwoven fabric with a homogeneous distribution of TCFH/urease-containing calcium alginate nanoparticles on the surface of CNF. The morphology of the cross-linked calcium alginate nanoparticles was also explored using transmission electron microscopy (TEM) to indicate an average diameter of 56-66 nm. The photophysical performance of the prepared CNF-TCFH-U was also studied by CIE Lab coloration parameters. The nanofibrous film biosensor displayed a relatively rapid response time (5-10 min) and a limit of detection as low as 200 ppm and as high as 1400 ppm. Tricyanofuran hydrazone is a pH-responsive disperse dye comprising a hydrazone detection group. Determination of urea occurs through a proton transfer from the hydrazone group to the generated ammonia from the reaction of urea with urease.

Sweat-Based Noninvasive Skin-Patchable Urea Biosensors with Photonic Interpenetrating Polymer Network Films Integrated into PDMS Chips

A wearable noninvasive biosensor for in situ urea detection and quantification was developed using a urease-immobilized photonic interpenetrating polymer network (IPN_urease) film. The photonic IPN film was intertwined with solid-state cholesteric liquid crystals (CLC_solid) and a poly(acrylic acid) (PAA) network on a flexible poly(ethylene terephthalate) substrate adhered to a poly(dimethylsiloxane) (PDMS) chip that was fabricated using an aluminum mold. The presence of urea in the chemical matrix of human sweat red-shifted the reflected color of the photonic IPN_urease film, and quantification was achieved by observing the wavelength at the photonic band gap (λ_PBG) with a limit of detection of 0.4 mM and a linear range of 0.9-50 mM. The color changes observed in the photonic IPN film were digitalized using the CIE 1931 xy coordinates on a cell phone image, thereby enabling fast, direct diagnosis via a downloadable app. This novel PDMS chip can be expanded for use with other biosensors.

Recent Progress in Wearable Biosensors: From Healthcare Monitoring to Sports Analytics

Recent advances in lab-on-a-chip technology establish solid foundations for wearable biosensors. These newly emerging wearable biosensors are capable of non-invasive, continuous monitoring by miniaturization of electronics and integration with microfluidics. The advent of flexible electronics, biochemical sensors, soft microfluidics, and pain-free microneedles have created new generations of wearable biosensors that explore brand-new avenues to interface with the human epidermis for monitoring physiological status. However, these devices are relatively underexplored for sports monitoring and analytics, which may be largely facilitated by the recent emergence of wearable biosensors characterized by real-time, non-invasive, and non-irritating sensing capacities. Here, we present a systematic review of wearable biosensing technologies with a focus on materials and fabrication strategies, sampling modalities, sensing modalities, as well as key analytes and wearable biosensing platforms for healthcare and sports monitoring with an emphasis on sweat and interstitial fluid biosensing. This review concludes with a summary of unresolved challenges and opportunities for future researchers interested in these technologies. With an in-depth understanding of the state-of-the-art wearable biosensing technologies, wearable biosensors for sports analytics would have a significant impact on the rapidly growing field-microfluidics for biosensing.