Antibody-Coated Wearable Organic Electrochemical Transistors for Cortisol Detection in Human Sweat

Balancing performance and stability characteristics in organic electrochemical transistor

Nowadays organic electrochemical transistors (OECTs) are becoming a promising platform for bioelectronics and biosensing due to its biocompatibility, high sensitivity and selectivity, low driving voltages, high transconductance and flexibility. However, the existing problems associated with degradation processes within the OECT during long-term operation hinder their widespread implementation. Moreover, trade-offs often arise between OECT transconductance and speed, fast ion transport and electron mobility, electrochemical stability and sensitivity, cycling stability and signal amplification, and other metrics. Ensuring high performance characteristics and achieving enhanced stability in OECTs are distinct strategies that do not always align, as progress in one aspect often necessitates a trade-off with the other. This dynamic arises from the need to find a balance between reversible and irreversible processes in the behavior of OECT active layers, and providing simultaneously favorable conditions for ion and electron transport and their efficient charge coupling. This review article systematically summarizes the phenomenological and physical-chemical aspects associated with factors and mechanisms that determine both performance and long-term stability of OECT, paying special attention to the consideration of existing and promising approaches to extend the OECT lifespan, while maintaining (or even increasing) high effectiveness of its operation.

Engineering 3D microtip gates of all-polymer organic electrochemical transistors for rapid femtomolar nucleic-acid-based saliva testing

Point-of-care testing (POCT) of trace amount of biomarkers in biofluids is critical towards health monitoring and early diagnosis. In particular, to facilitate non-invasive saliva testing, the development of low-cost, lightweight and disposable biosensors is in urgent need, while the ultrahigh sensitivity beyond conventional clinical tests remains a great challenge. Herein, we demonstrate a simple and fully printable all-polymer organic electrochemical transistor (OECT) biosensor to detect femtomolar (fM)-level biomolecules in saliva within a few minutes by employing highly conducting lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-doped poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) serving as both the channel and gate. A 3D microtip-shaped and Au nanoparticle-decorated multiscale gate interface facilitates the sub-fM-level sensing of female hormones (e.g., progesterone) and oligonucleotide cancer biomarkers by aptamers and DNA probes, respectively. Compared to a planar gate, the micro-engineered interface endows the OECT biosensor with significantly lower detection limit by 10-100 times down to <0.1 fM and faster response of <5 min, accomplishing unprecedentedly high sensitivity while maintaining outstanding mechanical flexibility. Consequently, such microtip-gate all-polymer OECT (MAOECT) enables POCT directly in 1000-fold diluted human saliva samples without centrifugation or redox probes, benefiting female fertility monitoring and oral cancer diagnosis as proof-of-concept demonstrations. This straightforward approach presents great potentials in low-cost wearable health management, at-home monitoring and personalized medicine.

Smart Health Monitoring: Review of Electrochemical Biosensors for Cortisol Monitoring

Cortisol, also known as the stress hormone, is a crucial corticosteroid hormone that significantly increases secretion in the human body when facing notable stress. Monitoring cortisol levels is crucial for personal stress management and the diagnosis and treatment of certain diseases. Electrochemical biosensors combine the efficient sensitivity of electrochemical technology with the high specificity of biological recognition processes, making them widely applicable in the analysis of human body fluid components. This work outlines the working mechanism of cortisol electrochemical biosensors, focusing particularly on sensing elements such as antibodies, aptamers, and molecularly imprinted polymers. It provides detailed explanations of the operational principles of these different recognition elements. This work summarizes and evaluates the latest advancements in electrochemical biosensors for detecting cortisol in human body fluids, discussing the influence of different recognition elements on sensor design and electrochemical performance. Subsequently, through a comparative analysis of various sensor performances, the work further discusses the challenges in translating laboratory achievements into practical applications, including enhancing key metrics such as sensor reusability, reproducibility, long-term stability, continuous monitoring capability, and response time. Finally, it offers insights and recommendations for achieving real-time, continuous, and long-term monitoring with cortisol electrochemical biosensors.

Ultrasensitive aflatoxin B1 detection based on vertical organic electrochemical transistor

Herein, we presented an ultrasensitive Aflatoxin B1 (AFB1) detection platform based on vertical organic electrochemical transistor (vOECT) first time. Chitosan-graphene nanosheets nanocomposites and AFB1 antibodies were modified on commercial electrodes as immunosensors, in series with gate electrodes of vOECT, operated at enhancement mode with ultrahigh transconductance gm 94 mS to amplify current signals. When AFB1 is added, the impedance of the immunosensors increased due to antigen-antibody immune binding, resulting in a potential decrease in reaction cell. Then, the potential decrease leads to an effective gate voltage VGeff increase, contributing to a significant drain-source current IDS decrease as a consequence of ultrahigh gm of vOECT. As a result, the presented vOECT platform exhibited an ultrahigh sensitivity of ∼1 mA/dec, and an ultralow detection limit of 0.01 fg/mL (S/N = 3), superior to all previous reported values. Furthermore, the platform exhibited satisfactory stability and specificity, and was applied to detect AFB1 in corn samples.

Current Strategies and Future Directions of Wearable Biosensors for Measuring Stress Biochemical Markers for Neuropsychiatric Applications

Most wearable biosensors aimed at capturing psychological state target stress biomarkers in the form of physical symptoms that can correlate with dysfunction in the central nervous system (CNS). However, such markers lack the specificity needed for diagnostic or preventative applications. Wearable biochemical sensors (WBSs) have the potential to fill this gap, however, the technology is still in its infancy. Most WBSs proposed thus far target cortisol. Although cortisol detection is demonstrated as a viable method for approximating the extent and severity of psychological stress, the hormone also lacks specificity. Multiplex WBSs that simultaneously target cortisol alongside other viable stress-related biochemical markers (SBMs) can prove to be indispensable for understanding how psychological stress contributes to the pathophysiology of neuropsychiatric illnesses (NPIs) and, thus, lead to the discovery of new biomarkers and more objective clinical tools. However, none target more than one SBM implicated in NPIs. Till this review, cortisol's connection to dysfunctions in the CNS, to other SBMs, and their implication in various NPIs has not been discussed in the context of developing WBS technology. As such, this review is meant to inform the biosensing and neuropsychiatric communities of viable future directions and possible challenges for WBS technology for neuropsychiatric applications.

Toward At-Home and Wearable Monitoring of Female Hormones: Emerging Nanotechnologies and Clinical Prospects

Steroid hormones, especially progesterone (P4), estradiol (E2), and testosterone (T), are key bioactive regulators in various female physiological processes, including growth and development, ovulation, and the reproductive cycle, as well as metabolism and mental health. As lipophilic molecules produced in sex glands, these steroid female hormones can be transported through blood vessels into various body fluids such as saliva, sweat, and urine. However, the ultralow concentration of steroid hormones down to picomolar (pM) level necessitates great demands for ultrasensitive but low-cost analytic tools to implement accurate, point-of-care or even continuous monitoring in a user-friendly fashion. This review focuses on the latest advances in materials and nanotechnologies to allow the rapid detection of female hormones at the pM level or below and the potentials in at-home and wearable hormone monitoring. We specifically summarize the optical and electrochemical strategies in this category, particularly those affording low cost and portable signal readout for at-home use. Furthermore, emerging flexible/wearable innovations are highlighted, which allow the continuous hormone cycle tracking in a noninvasive manner. The potential of these techniques is discussed to address the need for real-time acquisition of the hormone fluctuation, facilitating health monitoring at home. Lastly, we provide a comprehensive introduction to the prospects of female hormone monitoring in clinical diagnosis and treatment, from the perspective of gynecology and reproductive medicine clinicians.

Smart Bioelectronic Nanomesh Face Masks with Permeability and Flexibility for Monitoring Cortisol in Saliva

Bioelectronic face masks can easily collect biomarkers in saliva, in which free cortisol is abundant. However, conventional bioelectronic face masks involve significant challenges in terms of permeability and inhalation due to their nonpermeable film-type structure. Herein, we introduce a flexible and permeable nanomesh-based wearable biosensor designed for bioelectronic face masks that monitor cortisol levels. The diameter of the nanofiber matrix has a range of 200 to 500 nm and offers outstanding flexibility (2% resistance change at a bending radius of 2 mm), reliability (0.3% resistance change at a bending radius of 5 mm after 1000 bending cycles), and permeability (116.91 g m-2 h-1 at 18 °C with 40% humidity, which is 10 times higher compared with film) based on the nanoporous structure. We evaluated the electrochemical responses of functionalized interdigitated electrodes on a flexible and permeable poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanomesh. Our nanomesh cortisol biosensors demonstrated exceptional sensitivity to cortisol, even at low concentrations, with a detection limit as low as 10 pM. Furthermore, we measured cortisol in clinical samples, such as artificial saliva and human saliva, using nanomesh-based bioelectronic face masks. This study highlights the potential for further applications of bioelectronic face masks for detecting numerous biomarkers.

Wearable Electrochemical Biosensors for Advanced Healthcare Monitoring

Recent advancements in wearable electrochemical biosensors have opened new avenues for on-body and continuous detection of biomarkers, enabling personalized, real-time, and preventive healthcare. While glucose monitoring has set a precedent for wearable biosensors, the field is rapidly expanding to include a wider range of analytes crucial for disease diagnosis, treatment, and management. In this review, recent key innovations are examined in the design and manufacturing underpinning these biosensing platforms including biorecognition elements, signal transduction methods, electrode and substrate materials, and fabrication techniques. The applications of these biosensors are then highlighted in detecting a variety of biochemical markers, such as small molecules, hormones, drugs, and macromolecules, in biofluids including interstitial fluid, sweat, wound exudate, saliva, and tears. Additionally, the review also covers recent advances in wearable electrochemical biosensing platforms, such as multi-sensory integration, closed-loop control, and power supply. Furthermore, the challenges associated with critical issues are discussed, such as biocompatibility, biofouling, and sensor degradation, and the opportunities in materials science, nanotechnology, and artificial intelligence to overcome these limitations.

An in silico design method of a peptide bioreceptor for cortisol using molecular modelling techniques

Cortisol is established as a reliable biomarker for stress prompting intensified research in developing wearable sensors to detect it via eccrine sweat. Since cortisol is present in sweat in trace quantities, typically 8-140 ng/mL, developing such biosensors necessitates the design of bioreceptors with appropriate sensitivity and selectivity. In this work, we present a systematic biomimetic methodology and a semi-automated high-throughput screening tool which enables rapid selection of bioreceptors as compared to ab initio design of peptides via computational peptidology. Candidate proteins from databases are selected via molecular docking and ranked according to their binding affinities by conducting automated AutoDock Vina scoring simulations. These candidate proteins are then validated via full atomistic steered molecular dynamics computations including umbrella sampling to estimate the potential of mean force using GROMACS version 2022.6. These explicit molecular dynamic calculations are carried out in an eccrine sweat environment taking into consideration the protein dynamics and solvent effects. Subsequently, we present a candidate baseline peptide bioreceptor selected as a contiguous sequence of amino acids from the selected protein binding pocket favourably interacting with the target ligand (i.e., cortisol) from the active binding site of the proteins and maintaining its tertiary structure. A unique cysteine residue introduced at the N-terminus allows orientation-specific surface immobilization of the peptide onto the gold electrodes and to ensure exposure of the binding site. Comparative binding affinity simulations of this peptide with the target ligand along with commonly interfering species e.g., progesterone, testosterone and glucose are also presented to demonstrate the validity of this proposed peptide as a candidate baseline bioreceptor for future cortisol biosensor development.

Biosensors for psychiatric biomarkers in mental health monitoring

Psychiatric disorders are associated with serve disturbances in cognition, emotional control, and/or behavior regulation, yet few routine clinical tools are available for the real-time evaluation and early-stage diagnosis of mental health. Abnormal levels of relevant biomarkers may imply biological, neurological, and developmental dysfunctions of psychiatric patients. Exploring biosensors that can provide rapid, in-situ, and real-time monitoring of psychiatric biomarkers is therefore vital for prevention, diagnosis, treatment, and prognosis of mental disorders. Recently, psychiatric biosensors with high sensitivity, selectivity, and reproducibility have been widely developed, which are mainly based on electrochemical and optical sensing technologies. This review presented psychiatric disorders with high morbidity, disability, and mortality, followed by describing pathophysiology in a biomarker-implying manner. The latest biosensors developed for the detection of representative psychiatric biomarkers (e.g., cortisol, dopamine, and serotonin) were comprehensively summarized and compared in their sensitivities, sensing technologies, applicable biological platforms, and integrative readouts. These well-developed biosensors are promising for facilitating the clinical utility and commercialization of point-of-care diagnostics. It is anticipated that mental healthcare could be gradually improved in multiple perspectives, ranging from innovations in psychiatric biosensors in terms of biometric elements, transducing principles, and flexible readouts, to the construction of 'Big-Data' networks utilized for sharing intractable psychiatric indicators and cases.

Organic Electrochemical Transistors for Biomarker Detections

The improvement of living standards and the advancement of medical technology have led to an increased focus on health among individuals. Detections of biomarkers are feasible approaches to obtaining information about health status, disease progression, and response to treatment of an individual. In recent years, organic electrochemical transistors (OECTs) have demonstrated high electrical performances and effectiveness in detecting various types of biomarkers. This review provides an overview of the working principles of OECTs and their performance in detecting multiple types of biomarkers, with a focus on the recent advances and representative applications of OECTs in wearable and implantable biomarker detections, and provides a perspective for the future development of OECT-based biomarker sensors.

Flexible Organic Transistors for Biosensing: Devices and Applications

Flexible and stretchable biosensors can offer seamless and conformable biological-electronic interfaces for continuously acquiring high-fidelity signals, permitting numerous emerging applications. Organic thin film transistors (OTFTs) are ideal transducers for flexible and stretchable biosensing due to their soft nature, inherent amplification function, biocompatibility, ease of functionalization, low cost, and device diversity. In consideration of the rapid advances in flexible-OTFT-based biosensors and their broad applications, herein, a timely and comprehensive review is provided. It starts with a detailed introduction to the features of various OTFTs including organic field-effect transistors and organic electrochemical transistors, and the functionalization strategies for biosensing, with a highlight on the seminal work and up-to-date achievements. Then, the applications of flexible-OTFT-based biosensors in wearable, implantable, and portable electronics, as well as neuromorphic biointerfaces are detailed. Subsequently, special attention is paid to emerging stretchable organic transistors including planar and fibrous devices. The routes to impart stretchability, including structural engineering and material engineering, are discussed, and the implementations of stretchable organic transistors in e-skin and smart textiles are included. Finally, the remaining challenges and the future opportunities in this field are summarized.

Redox-Concatenated Aptamer Integrated Skin Mimicking Electrochemical Patch for Noninvasive Detection of Cortisol

This research focuses on developing and validating a wearable electrochemical biosensor called the concatenated aptamer integrated skin patch, also known as the Captain Patch. The main objective is to detect cortisol levels in sweat, which can provide valuable insights into an individual's health. The biosensor utilizes a corrugated surface that mimics the skin, allowing for better attachment and an improved electrochemical performance. The study demonstrates the successful application of Captain Patch on the human body by using artificially spiked sweat samples. The results indicate good measurement accuracy and conformity when the patch is worn on the body. However, for long-term usage, the patch needs to be changed every 3-4 h or worn three times a day to enable monitoring of cortisol levels. Despite the need for frequent patch changes, the cost-effectiveness and ease of operation make these skin patches suitable for longitudinal cortisol monitoring and other sweat analytes. By customization of the biorecognition probe, the developed biowearable can be used to monitor a variety of vital biomarkers. Overall, Captain Patch, with its capability of detecting specific health markers such as cortisol, hints at the future potential of wearables to offer valuable data on various other biomarkers. Our approach presents the first step in integrating a cost-effective wearable electrochemical patch integrated with a redox-concatenated aptamer for noninvasive biomarker detection. This personalized approach to monitoring can lead to improved patient outcomes and increased patient engagement in managing their health.

Carboxyl graphene modified PEDOT:PSS organic electrochemical transistor for in situ detection of cancer cell morphology

Circulating tumor cells in human peripheral blood play an important role in cancer metastasis. In addition to the size-based and antibody-based capture and separation of cancer cells, their electrical characterization is important for rare cell detection, which can prove fatal in point-of-care testing. Herein, an organic electrochemical transistor (OECT) biosensor made of solution-gated carboxyl graphene mixed with PEDOT:PSS for the detection of cancer cells in situ is reported. Carboxyl graphene was used in this work to modulate cancer cell morphology, which differs significantly from normal blood cells, to achieve rare cancer cell detection. When the concentration of carboxyl graphene mixed in PEDOT:PSS was increased from 0 to 5 mg mL-1, the cancer cell surface area increased from 218 μm2 to 530 μm2, respectively. A change in cell morphology was also detected by the OECT. Negative charges in the cancer cells induced a positive shift in gate voltage, which was approximately 40 mV for spherical-shaped cells. When the cell surface area increased, transfer curves of transistor revealed a negative shift in gate voltage. Therefore, the sensor can be used for in situ detection of cancer cell morphology during the cell capture process, which can be used to identify whether the captured cells are deformable.

A Bifunctional Wearable Sensor Based on a Nanoporous Membrane for Simultaneous Detection of Sweat Lactate and Temperature

Wearable sensors for non-invasive, real-time detection of sweat lactate have far-reaching implications in the fields of health care and exercise physiological responses. Here, we propose a wearable electrochemical sensor with gold nanoelectrode arrays fabricated on the nanoporous polycarbonate (PC) membrane by encapsulating lactate oxidase (LOx) in chitosan (CS) hydrogel for detecting body temperature and sweat lactate concurrently. Flexible gold nanoporous electrodes not only enhance electrode area but also offer a nanoconfined space to accelerate the catalytic reaction of LOx and control substrate concentration on the surface of LOx to decrease substrate inhibition. The proposed sensor has a long durability of 13 days and better selectivity for the detection of sweat lactate over a wide linear range (0.01-35 mM) with a low detection limit (0.144 μM). Furthermore, temperature-dependent transmembrane currents passing through the sensor are used to estimate body temperature. We then use multiple linear regression to adjust the effect of temperature on lactate detection and succeed in monitoring lactate molecules in sweat and body temperature during exercise.

Healthcare Monitoring Sensors Based on Organic Transistors: Surface/Interface Strategy and Performance

Organic transistors possess inherent advantages such as flexibility, biocompatibility, customizable chemical structures, solution-processability, and amplifying capabilities, making them highly promising for portable healthcare sensor applications. Through convenient and diverse modifications at the material and device surfaces or interfaces, organic transistors allow for a wide range of sensor applications spanning from chemical and biological to physical sensing. In this comprehensive review, the surface and interface engineering aspect associated with four types of typical healthcare sensors is focused. The device operation principles and sensing mechanisms are systematically analyzed and highlighted, and particularly surface/interface functionalization strategies that contribute to the enhancement of sensing performance are focused. An outlook and perspective on the critical issues and challenges in the field of healthcare sensing using organic transistors are provided as well.

An integrated wearable sticker based on extended-gate AlGaN/GaN high electron mobility transistors for real-time cortisol detection in human sweat

Cortisol hormone imbalances can be detected through non-invasive sweat monitoring using field-effect transistor (FET) biosensors, which provide rapid and sensitive detection. However, challenges like skin compatibility and integration with sweat collection have hindered FET biosensors as wearable sensing platforms. In this study, we present an integrated wearable sticker for real-time cortisol detection based on an extended-gate AlGaN/GaN high electron mobility transistor (HEMT) combined with a soft bottom substrate and flexible channel for sweat collection. The developed devices exhibit excellent linearity (R2 = 0.990) and a high sensitivity of 1.245 μA dec-1 for cortisol sensing from 1 nM to 100 μM in high-ionic-strength solution, with successful cortisol detection demonstrated using authentic human sweat samples. Additionally, the chip's microminiature design effectively reduces bending impact during the wearable process of traditional soft binding sweat sensors. The extendedgate structure design of the HEMT chip enhances both width-to-length ratio and active sensing area, resulting in an exceptionally low detection limit of 100 fM. Futhermore, due to GaN material's inherent stability, this device exhibits long-term stability with sustained performance within a certain attenuation range even after 60 days. These stickers possess small, lightweight, and portable features that enable real-time cortisol detection within 5 minutes through direct sweat collection. The application of this technology holds great potential in the field of personal health management, facilitating users to conveniently monitor their mental and physical conditions.

Advanced and personalized healthcare through integrated wearable sensors (versatile)

Applications of integrated wearable sensors for the monitoring of human vital signs and clinically relevant biomarkers.

Wearable and Implantable Cortisol-Sensing Electronics for Stress Monitoring

Cortisol is a steroid hormone that is released from the body in response to stress. Although a moderate level of cortisol secretion can help the body maintain homeostasis, excessive secretion can cause various diseases, such as depression and anxiety. Conventional methods for cortisol measurement undergo procedures that limit continuous monitoring, typically collecting samples of bodily fluids, followed by separate analysis in a laboratory setting that takes several hours. Thus, recent studies demonstrate wearable, miniaturized sensors integrated with electronic modules that enable wireless real-time analysis. Here, the primary focus is on wearable and implantable electronic devices that continuously measure cortisol concentration. Diverse types of cortisol-sensing techniques, such as antibody-, DNA-aptamer-, and molecularly imprinted polymer-based sensors, as well as wearable and implantable devices that aim to continuously monitor cortisol in a minimally invasive fashion are discussed. In addition to the cortisol monitors that directly measure stress levels, other schemes that indirectly measure stress, such as electrophysiological signals and sweat are also summarized. Finally, the challenges and future directions in stress monitoring and management electronics are reviewed.

Advances in electrochemical biosensor design for the detection of the stress biomarker cortisol

The monitoring of stress levels in humans has become increasingly relevant, given the recent incline of stress-related mental health disorders, lifestyle impacts, and chronic physiological diseases. Long-term exposure to stress can induce anxiety and depression, heart disease, and risky behaviors, such as drug and alcohol abuse. Biomarker molecules can be quantified in biological fluids to study human stress. Cortisol, specifically, is a hormone biomarker produced in the adrenal glands with biofluid concentrations that directly correlate to stress levels in humans. The rapid, real-time detection of cortisol is necessary for stress management and predicting the onset of psychological and physical ailments. Current methods, including mass spectrometry and immunoassays, are effective for sensitive cortisol quantification. However, these techniques provide only single measurements which pose challenges in the continuous monitoring of stress levels. Additionally, these analytical methods often require trained personnel to operate expensive instrumentation. Alternatively, low-cost electrochemical biosensors enable the real-time detection and continuous monitoring of cortisol levels while also providing adequate analytical figures of merit (e.g., sensitivity, selectivity, sensor response times, detection limits, and reproducibility) in a simple design platform. This review discusses the recent developments in electrochemical biosensor design for the detection of cortisol in human biofluids. Special emphasis is given to biosensor recognition elements, including antibodies, molecularly imprinted polymers (MIPs), and aptamers, as critical components of electrochemical biosensors for cortisol detection. Furthermore, the advantages and limiting factors of various electrochemical techniques and sensing in complex biofluid matrices are overviewed. Remarks on the current challenges and future perspectives regarding electrochemical biosensors for stress monitoring are provided, including matrix effects (pH dependence and biological interferences), wearability, and large-scale production.

Highly sensitive detection of malaria biomarker through matching channel and gate capacitance of integrated organic electrochemical transistors

Organic electrochemical transistors (OECTs) possess versatile advantages for biochemical and electrophysiological applications due to electrochemical gating and ion-to-electron conversion capability. Although OECTs have been successfully applied for biochemical sensing, the effect of relative capacitance for specific sensing events is still unclear. In the present work, we design integrated interdigitated OECTs (iOECTs) with on-plane gold gate and different channel geometries for point-of-care diagnosis of malaria using aptamer as receptor. The transconductance of the iOECTs gated with micro-size gold electrodes decreased with increasing the channel thicknesses, especially for devices with large channel areas, which is inconsistent with devices gated by typical Ag/AgCl electrodes, attributing to the limited gating efficiency of the micro-size gate electrode. The capacitance of gate electrode was heavily suppressed by receptors but increased with the incubation of targets. In addition, the integrated iOECTs with thin channels exhibited superior sensitivity for malaria detection with the detection limit as low as 3.2 aM, much lower than their thick channel counterpart and other state-of-the-art biosensors. These deviations could be caused by the high relative capacitances, with respect to the gate and channel capacitance (Cg/Cch), resulting in a high gate potential drop over the organic channel and thus entirely gating on the thin channel device. These findings provide guidance to optimize the geometry of OECT devices with on-chip integrated gates and the fabrication of miniaturized OECTs for biosensing applications.

Access and Management of Sweat for Non-Invasive Biomarker Monitoring: A Comprehensive Review

Sweat is an important biofluid presents in the body since it regulates the internal body temperature, and it is relatively easy to access on the skin unlike other biofluids and contains several biomarkers that are also present in the blood. Although sweat sensing devices have recently displayed tremendous progress, most of the emerging devices primarily focus on the sensor development, integration with electronics, wearability, and data from in vitro studies and short-term on-body trials during exercise. To further the advances in sweat sensing technology, this review aims to present a comprehensive report on the approaches to access and manage sweat from the skin toward improved sweat collection and sensing. It is begun by delineating the sweat secretion mechanism through the skin, and the historical perspective of sweat, followed by a detailed discussion on the mechanisms governing sweat generation and management on the skin. It is concluded by presenting the advanced applications of sweat sensing, supported by a discussion of robust, extended-operation epidermal wearable devices aiming to strengthen personalized healthcare monitoring systems.

Dual detection of human motion and glucose in sweat with polydopamine and glucose oxidase doped self-healing nanocomposite hydrogels

The detection of human motion and sweat composition are important for human health or sports training, so it is necessary to develop flexible sensors for monitoring exercise processes and sweat detection. Mussel secretion of adhesion proteins enables self-healing of byssus and adhesion to surfaces. We prepared Au nanoparticles@polydopamine (AuNPs@PDA) nanomaterials based on mussel-inspired chemistry and compounded them with polyvinyl alcohol (PVA) hydrogels to obtain PVA/AuNPs@PDA self-healing nanocomposite hydrogels. Dopamine (DA) was coated on the surface of AuNPs to obtain AuNPs based composite (AuNPs@PDA) and the AuNPs@PDA was implanted into the PVA hydrogels to obtain nanocomposite hydrogel through facile freeze-thaw cycle. Glucose oxidase (GOD) was added to the hydrogel matrix to achieve specific detection of glucose in sweat. The obtained hydrogels exhibit high deformability (573.7 %), excellent mechanical strength (550.3 KPa) and self-healing properties (85.1 %). The PVA/AuNPs@PDA hydrogel sensors exhibit quick response time (185.0 ms), wide strain sensing range (0-500 %), superior stability and anti-fatigue properties in motion detection. The detection of glucose had wide concentration detection range (1.0 μmol/L-200.0 μmol/L), low detection limits (0.9 μmol/L) and high sensitivity (24.4 μA/mM). This work proposes a reference method in dual detection of human exercise and sweat composition analysis.

Electrochemical sensors for cortisol detection: Principles, designs, fabrication, and characterisation

Psychological stress is a major factor contributing to health discrepancies among individuals. Sustained exposure to stress triggers signalling pathways in the brain, which leading to the release of stress hormones in the body. Cortisol, a steroid hormone, is a significant biomarker for stress management due to its responsibility in the body's reply to stress. The release of cortisol in bloodstream prepares the body for a "fight or flight" response by increasing heart rate, blood pressure, metabolism, and suppressing the immune system. Detecting cortisol in biological samples is crucial for understanding its role in stress and personalized healthcare. Traditional techniques for cortisol detection have limitations, prompting researchers to explore alternative strategies. Electrochemical sensing has emerged as a reliable method for point-of-care (POC) cortisol detection. This review focuses on the progress made in electrochemical sensors for cortisol detection, covering their design, principle, and electroanalytical methodologies. The analytical performance of these sensors is also analysed and summarized. Despite significant advancements, the development of electrochemical cortisol sensors faces challenges such as biofouling, sample preparation, sensitivity, flexibility, stability, and recognition layer performance. Therefore, the need to develop more sensitive electrodes and materials is emphasized. Finally, we discussed the potential strategies for electrode design and provides examples of sensing approaches. Moreover, the encounters of translating research into real world applications are addressed.

Flexible and Stretchable Organic Electrochemical Transistors for Physiological Sensing Devices

Flexible and stretchable bioelectronics provides a biocompatible interface between electronics and biological systems and has received tremendous attention for in situ monitoring of various biological systems. Considerable progress in organic electronics has made organic semiconductors, as well as other organic electronic materials, ideal candidates for developing wearable, implantable, and biocompatible electronic circuits due to their potential mechanical compliance and biocompatibility. Organic electrochemical transistors (OECTs), as an emerging class of organic electronic building blocks, exhibit significant advantages in biological sensing due to the ionic nature at the basis of the switching behavior, low driving voltage (<1 V), and high transconductance (in millisiemens range). During the past few years, significant progress in constructing flexible/stretchable OECTs (FSOECTs) for both biochemical and bioelectrical sensors has been reported. In this regard, to summarize major research accomplishments in this emerging field, this review first discusses structure and critical features of FSOECTs, including working principles, materials, and architectural engineering. Next, a wide spectrum of relevant physiological sensing applications, where FSOECTs are the key components, are summarized. Last, major challenges and opportunities for further advancing FSOECT physiological sensors are discussed.

Introducing Nanoscale Electrochemistry in Small-Molecule Detection for Tackling Existing Limitations of Affinity-Based Label-Free Biosensing Applications

Electrochemical sensing techniques for small molecules have progressed in many applications, including disease diagnosis and prevention as well as monitoring of health conditions. However, affinity-based detection for low-abundance small molecules is still challenging due to the imbalance in target-to-receptor size ratio as well as the lack of a highly sensitive signal transducing method. Herein, we introduced nanoscale electrochemistry in affinity-based small molecule detection by measuring the change of quantum electrochemical properties with a nanoscale artificial receptor upon binding. We prepared a nanoscale molecularly imprinted composite polymer (MICP) for cortisol by electrochemically copolymerizing β-cyclodextrin and redox-active methylene blue to offer a high target-to-receptor size ratio, thus realizing "bind-and-read" detection of cortisol as a representative target small molecule, along with extremely high sensitivity. Using the quantum conductance measurement, the present MICP-based sensor can detect cortisol from 1.00 × 10-12 to 1.00 × 10-6 M with a detection limit of 3.93 × 10-13 M (S/N = 3), which is much lower than those obtained with other electrochemical methods. Moreover, the present MICP-based cortisol sensor exhibited reversible cortisol sensing capability through a simple electrochemical regeneration process without cumbersome steps of washing and solution change, which enables "continuous detection". In situ detection of cortisol in human saliva following circadian rhythm was carried out with the present MICP-based cortisol sensor, and the results were validated with the LC-MS/MS method. Consequently, this present cortisol sensor based on nanoscale MICP and quantum electrochemistry overcomes the limitations of affinity-based biosensors, opening up new possibilities for sensor applications in point-of-care and wearable healthcare devices.

Breathable and Stretchable Organic Electrochemical Transistors with Laminated Porous Structures for Glucose Sensing

Dynamic glucose monitoring is important to reduce the risk of metabolic diseases such as diabetes. Wearable biosensors based on organic electrochemical transistors (OECTs) have been developed due to their excellent signal amplification capabilities and biocompatibility. However, traditional wearable biosensors are fabricated on flat substrates with limited gas permeability, resulting in the inefficient evaporation of sweat, reduced wear comfort, and increased risk of inflammation. Here, we proposed breathable OECT-based glucose sensors by designing a porous structure to realize optimal breathable and stretchable properties. The gas permeability of the device and the relationship between electrical properties under different tensile strains were carefully investigated. The OECTs exhibit exceptional electrical properties (gm ~1.51 mS and Ion ~0.37 mA) and can retain up to about 44% of their initial performance even at 30% stretching. Furthermore, obvious responses to glucose have been demonstrated in a wide range of concentrations (10-7-10-4 M) even under 30% strain, where the normalized response to 10-4 M is 26% and 21% for the pristine sensor and under 30% strain, respectively. This work offers a new strategy for developing advanced breathable and wearable bioelectronics.

A wearable sweat electrochemical aptasensor based on the Ni-Co MOF nanosheet-decorated CNTs/PU film for monitoring of stress biomarker

Monitoring cortisol, a hormone released by the adrenal cortex in response to stress, is essential to evaluate the endocrine response to stress stimuli. While the current cortisol sensing methods require large laboratory settings, complex assay, and professional personnel. Herein, a novel flexible and wearable electrochemical aptasensor based on a Ni-Co metal-organic frameworks (MOF) nanosheet-decorated carbon nanotubes (CNTs)/polyurethane (PU) film is developed for rapid and reliable detection of cortisol in sweat. First, the CNTs/PU (CP) film was prepared by a modified wet spinning technology, and the CNTs/polyvinyl alcohol (PVA) solution was thermally deposited on the surface of CP film to form the highly flexible CNTs/PVA/CP (CCP) film with excellent conductivity. Then aminated Ni-Co MOF nanosheet prepared by a facile solvothermal method was conjugated with streptavidin and modified on the CCP film. Biofunctional MOF can effectively capture cortisol aptamer due to its excellent specific surface area. In addition, the MOF with peroxidase activity can catalytic oxidization of hydroquinone (HQ) by hydrogen peroxide (H₂O₂), which could amplify the peak current signal. The catalytic activity of Ni-Co MOF was substantially suppressed in the HQ/H₂O₂ system due to the formation of the aptamer-cortisol complex, which reduced the current signal, thereby realizing highly sensitive and selective detection of cortisol. The sensor has a linear range of 0.1-100 ng/mL and a detection limit of 0.032 ng/mL. Meanwhile, the sensor showed high accuracy for cortisol detection under mechanical deformation conditions. More importantly, the prepared MOF/CCP film based three-electrode was assembled with the polydimethylsiloxane (PDMS) substrate, and the sweat-cloth was used as the sweat collection channel to fabricate a wearable sensor patch for monitoring of cortisol in volunteers' sweat in the morning and evening. This flexible and non-invasive sweat cortisol aptasensor shows great potential for quantitative stress monitoring and management.

Advances in wearable electrochemical antibody-based sensors for cortisol sensing

Cortisol is a crucial hormone involving many physiological processes. Hence, cortisol detection is essential. This review highlights the key progress made on wearable electrochemical sensors using antibodies. It covers the design, principle, and electroanalytical methodology for detecting cortisol noninvasively. This article also analyzes and collects the analytical performances of electrochemical cortisol sensors. The development of these sensors continues to face challenges such as biofouling, sample management, sensitivity, flexibility, stability, and recognition layer performance. It is also necessary to develop a sensitive electrode and material. This article also presents potential strategies for designing antibody electrodes and provides examples of sensing systems. Additionally, it discusses the challenges in translating research into practical applications.