Wearable Chemical Sensors: Present Challenges and Future Prospects

Recent advances in biofluid detection with micro/nanostructured bioelectronic devices

Most biofluids contain a wide variety of biochemical components that are closely related to human health. Analyzing biofluids, such as sweat and tears, may deepen our understanding in pathophysiologic conditions associated with human body, while providing a variety of useful information for the diagnosis and treatment of disorders and disease. Emerging classes of micro/nanostructured bioelectronic devices for biofluid detection represent a recent breakthrough development of critical importance in this context, including traditional biosensors (TBS) and micro/nanostructured biosensors (MNBS). Related biosensors are not restricted to flexible and wearable devices; solid devices are also involved here. This article is a timely overview of recent technical advances in this field, with an emphasis on the new insights of constituent materials, design architectures and detection methods of MNBS that support the necessary levels of biocompatibility, device functionality, and stable operation for component analysis. An additional section discusses and analyzes the existing challenges, possible solutions and future development of MNBS for detecting biofluids.

Extending Porous Silicone Capacitive Pressure Sensor Applications into Athletic and Physiological Monitoring

Porous polymer dielectric materials have been developed to increase the sensitivity of capacitive pressure sensors, so that they might expand capacitive sensor use, and promote the realization of the advantages of this class of sensor in further fields. However, their use has not been demonstrated in physiological monitoring applications such as respiration monitoring and body position detection during sleep; an area in need of unmet medical attention for conditions such as sleep apnea. Here, we develop and characterize a sensor comprised of a poly dimethylsiloxane (PDMS) sponge dielectric layer, and PDMS/carbon black (CB) blend electrode layers, with suitable compliance and sensitivity for integration in mattresses, pillows, and athletic shoe insoles. With relatively high pressure sensitivity (~0.1 kPa^-1) and mechanical robustness, this sensor was able to fulfill a wide variety of roles, including athletic monitoring in an impact mechanics scenario, by recording heel pressure during running and walking, and physiological monitoring, by detecting head position and respiration of a subject lying on a pad and pillow. The sensor detected considerably greater relative signal changes than those reported in recent capacitive sensor studies for heel pressure, and for a comparably minimal, resistive sensor during respiration, in line with its enhanced sensitivity.

Advances in self-powered chemical sensing via a triboelectric nanogenerator

Chemical sensors allow for continuous detection and analysis of underexplored molecules in the human body and the surroundings and have promising applications in human healthcare and environmental protection. With the increasing number of chemical sensors and their wide-range distribution, developing a continuous, sustainable, and pervasive power supply is vitally important but an unmet scientific challenge to perform chemical sensing. Self-powered chemical sensing via triboelectric nanogenerators (TENGs) could be a promising approach to this critical situation. TENGs can convert mechanical triggers from the surroundings into usable electrical signals for chemical sensing in a self-powered and environment-friendly manner. Moreover, their simple structure, low probability of failure, and wide choice of materials distinguish them from other chemical sensing technologies. This review article discusses the working principles of TENGs and their applications in chemical sensing with respect to the role of TENGs as either a self-powered sensor or a power source for existing chemical sensors. Advances in materials innovation and nanotechnology to optimize the chemical sensing performances are discussed and emphasized. Finally, the current challenges and future prospect of TENG enabled self-powered chemical sensing are discussed to promote interdisciplinary field development and revolutions.

A novel IoT-based health and tactical analysis model with fog computing

In sports competitions, depending on the conditions such as excitement, stress, fatigue, etc. during the match, negative situations such as disability or loss of life may occur for players and spectators. Therefore, it is extremely important to constantly check their health. In addition, some strategic analyzes are made during the match. According to the results of these analyzes, the technical team affects the course of the match. Effects can have positive and sometimes negative results. In this article, fog computing and an Internet of Things (IoT) based architecture are proposed to produce new technical strategies and to avoid disabilities. Players and spectators are monitored with sensors such as blood pressure, body temperature, heart rate, location etc. The data obtained from the sensors are processed in the fog layer and the resulting information is sent to the devices of the technical team and club doctors. In the architecture based on fog computing and IoT, priority processes are computed with low latency. For this, a task management algorithm based on priority queue and list of fog nodes is modified in the fog layer. Authentication and data confidentiality are provided with the Federated Lightweight Authentication of Things (FLAT) method used in the proposed model. In addition, using the Software Defined Network controller based on blockchain technology ensures data integrity.

Autonomous, Real-Time Monitoring Electrochemical Aptasensor for Circadian Tracking of Cortisol Hormone in Sub-microliter Volumes of Passively Eluted Human Sweat

The proposed work involves the development of an autonomous, label-free electrochemical sensor for real-time monitoring of cortisol levels expressed naturally in sub-microliter sweat volumes, for prolonged sensing periods of ∼8 h. Highly specific single-stranded DNA (ssDNA) aptamer is used for affinity capture of cortisol hormone eluted in sweat dynamically. The cortisol present in sweat binds to the aptamer capture probe that changes conformation and modulates electrochemical properties at the electrode-buffer interface, which was studied using dynamic light scattering studies for the entire physiological sweat pH. Attenuated total reflectance-Fourier transform infrared spectroscopy and UV-vis spectroscopy were used to optimize the binding chemistry of the elements of the sensor stack. Nonfaradaic electrochemical impedance spectroscopy was used to calibrate the sensor for a dynamic range of 1-256 ng/mL. An R² of 0.97 with an output signal range of 20-50% was obtained. Dynamic cortisol level variation tracking was studied using continuous dosing experiments to calibrate the sensor for temporal variation. The sensor did not show significant susceptibility to noise due to cross-reactive interferents and nonspecific buffer constituents. The performance of the developed aptasensor was compared with the previously established cortisol immunosensor in terms of surface charge behavior and nonfaradaic biosensing. The aptamer sensor shows a higher signal-to-noise ratio, better resolution, and has a larger output range for the same input range as the cortisol immunosensor. The feasibility of deploying the developed aptasensing scheme as continuous lifestyle and performance monitors was validated through human subject studies.

Self-Powered Flexible Sour Sensor for Detecting Ascorbic Acid Concentration Based on Triboelectrification/Enzymatic-Reaction Coupling Effect

Artificial sensory substitution systems can mimic human sensory organs through replacing the sensing process of a defective sensory receptor and transmitting the sensing signal into the nervous system. Here, we report a self-powered flexible gustation sour sensor for detecting ascorbic acid concentration. The material system comprises of Na2C2O4-Ppy with AAO modification, PDMS and Cu wire mesh. The working mechanism is contributed to the triboelectrification/enzymatic-reaction coupling effect, and the device can collect weak energy from body movements and directly output triboelectric current without any external power-units. The triboelectric output is affected by AA concentration, and the response is up to 34.82% against 15.625 mM/L of AA solution. Furthermore, a practical application in detecting ascorbic acid concentration of different drinks has been demonstrated. This work can encourage the development of wearable flexible electronics and this self-powered sour sensor has the potential that can be acted as a kind of gustatory receptors to build electronic tongues.

Challenges and opportunities for small volumes delivery into the skin

Each individual's skin has its own features, such as strength, elasticity, or permeability to drugs, which limits the effectiveness of one-size-fits-all approaches typically found in medical treatments. Therefore, understanding the transport mechanisms of substances across the skin is instrumental for the development of novel minimal invasive transdermal therapies. However, the large difference between transport timescales and length scales of disparate molecules needed for medical therapies makes it difficult to address fundamental questions. Thus, this lack of fundamental knowledge has limited the efficacy of bioengineering equipment and medical treatments. In this article, we provide an overview of the most important microfluidics-related transport phenomena through the skin and versatile tools to study them. Moreover, we provide a summary of challenges and opportunities faced by advanced transdermal delivery methods, such as needle-free jet injectors, microneedles, and tattooing, which could pave the way to the implementation of better therapies and new methods.

Recent advances in graphene-based nanobiosensors for salivary biomarker detection

As biosensing research is rapidly advancing due to significant developments in materials, chemistry, and electronics, researchers strive to build cutting-edge biomedical devices capable of detecting health-monitoring biomarkers with high sensitivity and specificity. Biosensors using nanomaterials are highly promising because of the wide detection range, fast response time, system miniaturization, and enhanced sensitivity. In the recent development of biosensors and electronics, graphene has rapidly gained popularity due to its superior electrical, biochemical, and mechanical properties. For biomarker detection, human saliva offers easy access with a large variety of analytes, making it a promising candidate for its use in point-of-care (POC) devices. Here, we report a comprehensive review that summarizes the most recent graphene-based nanobiosensors and oral bioelectronics for salivary biomarker detection. We discuss the details of structural designs of graphene electronics, use cases of salivary biomarkers, the performance of existing sensors, and applications in health monitoring. This review also describes current challenges in materials and systems and future directions of the graphene bioelectronics for clinical POC applications. Collectively, the main contribution of this paper is to deliver an extensive review of the graphene-enabled biosensors and oral electronics and their successful applications in human salivary biomarker detection.

Big data and machine learning for materials science

Herein, we review aspects of leading-edge research and innovation in materials science that exploit big data and machine learning (ML), two computer science concepts that combine to yield computational intelligence. ML can accelerate the solution of intricate chemical problems and even solve problems that otherwise would not be tractable. However, the potential benefits of ML come at the cost of big data production; that is, the algorithms demand large volumes of data of various natures and from different sources, from material properties to sensor data. In the survey, we propose a roadmap for future developments with emphasis on computer-aided discovery of new materials and analysis of chemical sensing compounds, both prominent research fields for ML in the context of materials science. In addition to providing an overview of recent advances, we elaborate upon the conceptual and practical limitations of big data and ML applied to materials science, outlining processes, discussing pitfalls, and reviewing cases of success and failure.

Point-of-Care Drug of Abuse Testing in the Opioid Epidemic

CONTEXT.—

The United States is experiencing an opioid overdose epidemic. Point-of-care (POC) drug of abuse testing is a useful tool to combat the intensified opioid epidemic.

OBJECTIVES.—

To review commercially available POC drug of abuse testing involving opioids, to review opportunities and challenges for POC opioid testing and emerging testing methods in research literature, and finally to summarize unmet clinical needs and future development prospects.

DATA SOURCES.—

The Google search engine was used to access information for commercial opioid POC devices and the Google Scholar search engine was used to access research literature published from 2000 to 2019 for opioid POC tests.

CONCLUSIONS.—

The opioid epidemic provides unprecedented opportunities for POC drug testing, with significant clinical needs. Compared with gold standard tests, limitations for commercially available opioid POC testing include lower analytical sensitivity, lower specificity, and cross-reactivity. In response to unmet clinical needs, novel methods have emerged in research literature, such as microfluidics and miniature mass spectrometry. Future prospects include the development of quantitative POC devices and smarter and real-time drug testing.

Reusable Surface-Enhanced Raman Spectroscopy Membranes and Textiles via Template-Assisted Self-Assembly and Micro/Nanoimprinting

Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful tool for ultrasensitive fingerprint recognition of molecules with considerable potential in wearable biochemical sensing. However, previous efforts to fabricate wearable SERS devices by directly treating fabrics with plasmonic nanoparticles have generated a nonuniform assembly of nanoparticles, weakly adsorbed on fabrics via van der Waals forces. Here, we report the creation of washing reusable SERS membranes and textiles via template-assisted self-assembly and micro/nanoimprinting approaches. Uniquely, we employ the capillary force driven self-assembly process to generate micropatch arrays of Au nanoparticle (NP) aggregates within hydrophobic microstructured templates, which are then robustly bonded onto semipermeable transparent membranes and stretchable textiles using the UV-resist based micro/nanoimprinting technique. A mild reactive ion etching (RIE) treatment of SERS membranes and textiles can physically expose the SERS hotspots of Au NP-aggregates embedded within the polymer UV resist for further improvement of their SERS performance. Also, we demonstrate that the semipermeable transparent SERS membranes can keep the moisture content of meat from evaporating to enable stable in situ SERS monitoring of biochemical environments at the fresh meat surface. By contrast, stretchable SERS textiles can allow the spreading, soaking, and evaporation of solution analyte samples on the fabric matrix for continuous enrichment of analyte molecules at the hotspots in biochemical SERS detection. Due to the mechanical robustness of the UV-resist immobilized Au NP aggregates, simple detergent-water washing with ultrasound sonication or mechanical stirring can noninvasively clean contaminated hot spots to reuse SERS textiles. Therefore, we envision that washing reusable SERS membranes and textiles by template-assisted self-assembly and micro/nanoimprinting fabrication are promising for wearable biochemical sensing applications, such as wound monitoring and body fluid monitoring.

Electrically Transduced Gas Sensors Based on Semiconducting Metal Oxide Nanowires

Semiconducting metal oxide-based nanowires (SMO-NWs) for gas sensors have been extensively studied for their extraordinary surface-to-volume ratio, high chemical and thermal stabilities, high sensitivity, and unique electronic, photonic and mechanical properties. In addition to improving the sensor response, vast developments have recently focused on the fundamental sensing mechanism, low power consumption, as well as novel applications. Herein, this review provides a state-of-art overview of electrically transduced gas sensors based on SMO-NWs. We first discuss the advanced synthesis and assembly techniques for high-quality SMO-NWs, the detailed sensor architectures, as well as the important gas-sensing performance. Relationships between the NWs structure and gas sensing performance are established by understanding general sensitization models related to size and shape, crystal defect, doped and loaded additive, and contact parameters. Moreover, major strategies for low-power gas sensors are proposed, including integrating NWs into microhotplates, self-heating operation, and designing room-temperature gas sensors. Emerging application areas of SMO-NWs-based gas sensors in disease diagnosis, environmental engineering, safety and security, flexible and wearable technology have also been studied. In the end, some insights into new challenges and future prospects for commercialization are highlighted.

Real-Time Monitoring of Heavy Metals in Healthcare via Twistable and Washable Smartsensors

The wearable and integrated sensing platform is a promising choice for developing real-time analytic electronics with clear advantages but still poses challenges, such as the realization of high precision, low limit of detection (LOD), moderate mechanical capacity, integration, and miniaturization. In this work, a simple printed wearable smartsensor has been fabricated with the aid of electrochemical plating methods with bismuth (Bi) films. The excellent sensing behaviors, including linear relationship, selectivity, stability, repeatability, and the LOD at ppb levels, have been obtained by this smartsensor. Additionally, the highly flexible textile-based sensor exhibits potential application on the substrates of daily cloth, sports T-shirt, and sports wristbands, and it maintains good stability under repeated deformations of washing and twisting. Importantly, integrated with printed circuit board, single chip micyoco, and Bluetooth modules, a smartsensing platform is successfully acquired for real-time detection of heavy metals (e.g., Zn, Cd, Pb, etc.). Finally, actual samples of human sweat, seawater, cosmetics, and drinking water have been remotely successfully demonstrated for detection by this smartsensor, enabling a great promise for fast on-site screening of samples in practical application.

Potential Scenarios and Hazards in the Work of the Future: A Systematic Review of the Peer-Reviewed and Gray Literatures

It would be useful for researchers, practitioners, and decision-makers to anticipate the hazards that workers will face in the future. The focus of this study is a systematic review of published information to identify and characterize scenarios and hazards in the future of work. Eleven bibliographic databases were systematically searched for papers and reports published from 1999 to 2019 that described future of work scenarios or identified future work-related hazards. To compile a comprehensive collection of views of the future, supplemental and ad hoc searches were also performed. After screening all search records against a set of predetermined criteria, the review yielded 36 references (17 peer-reviewed, 4 gray, and 15 supplemental) containing scenarios. In these, the future of work was described along multiple conceptual axes (e.g. labor market changes, societal values, and manual versus cognitive work). Technology was identified as the primary driver of the future of work in most scenarios, and there were divergent views in the literature as to whether technology will create more or fewer jobs than it displaces. Workforce demographics, globalization, climate change, economic conditions, and urbanization were also mentioned as influential factors. Other important themes included human enhancement, social isolation, loneliness, worker monitoring, advanced manufacturing, hazardous exposures, sustainability, biotechnology, and synthetic biology. Pandemics have not been widely considered in the future of work literature, but the recent COVID-19 pandemic illustrates that was short-sighted. Pandemics may accelerate future of work trends and merit critical consideration in scenario development. Many scenarios described 'new' or 'exacerbated' psychosocial hazards of work, whereas comparatively fewer discussed physical, chemical, or biological hazards. Various preventive recommendations were identified. In particular, reducing stress associated with precarious work and its requirements of continual skill preparation and training was acknowledged as critical for protecting and promoting the health and well-being of the future workforce. In conclusion, the future of work will be comprised of diverse complex scenarios and a mosaic of old and new hazards. These findings may serve as the basis for considering how to shape the future of work.

Minimally Invasive Electrochemical Patch-Based Sensor System for Monitoring Glucose and Lactate in the Human Body-A Survey-Based Analysis of the End-User's Perspective

Background: Wearable electrochemical sensors that detect human biomarkers allow a comprehensive analysis of a person’s health condition. The “electronic smart patch system for wireless monitoring of molecular biomarkers for health care and well-being” (ELSAH) project aims to develop a minimally invasive sensor system that is capable of continuously monitoring glucose and lactate in the dermal interstitial fluid in real time. It is the objective of the present study to compare the intended ELSAH-patch specifications with the expectations and requirements of potential end-users at an early stage during the development phase. Methods: A questionnaire addressing different aspects of the ELSAH-patch was filled out by 383 respondents. Results: The participants stated a high general demand for such a system, and they would use the ELSAH-patch in different health care and physical fitness applications. The preferred terminal device for communication with the sensor would be the smartphone. An operating time of 24 hours would be sufficient for 55.8% of the users (95%-CI: 50.3–61.3%), while 43.5% of them (95%-CI: 38.0–48.9%) would prefer a lifetime of several days or more. The software should have a warning function, especially for critical health conditions. Since the measured personal data would be highly sensitive, the participants called for high standards for data security and privacy. Conclusion: In general, the participants’ responses on their expectations and requirements were well in line with the intended specifications of the ELSAH-patch system. However, certain technical aspects such as the lifetime, data security and accuracy require special attention during its development.

Electrochemical glucose sensors in diabetes management: an updated review (2010-2020)

While over half a century has passed since the introduction of enzyme glucose biosensors by Clark and Lyons, this important field has continued to be the focus of immense research activity. Extensive efforts during the past decade have led to major scientific and technological innovations towards tight monitoring of diabetes. Such continued progress toward advanced continuous glucose monitoring platforms, either minimal- or non-invasive, holds considerable promise for addressing the limitations of finger-prick blood testing toward tracking glucose trends over time, optimal therapeutic interventions, and improving the life of diabetes patients. However, despite these major developments, the field of glucose biosensors is still facing major challenges. The scope of this review is to present the key scientific and technological advances in electrochemical glucose biosensing over the past decade (2010-present), along with current obstacles and prospects towards the ultimate goal of highly stable and reliable real-time minimally-invasive or non-invasive glucose monitoring. After an introduction to electrochemical glucose biosensors, we highlight recent progress based on using advanced nanomaterials at the electrode-enzyme interface of three generations of glucose sensors. Subsequently, we cover recent activity and challenges towards next-generation wearable non-invasive glucose monitoring devices based on innovative sensing principles, alternative body fluids, advanced flexible materials, and novel platforms. This is followed by highlighting the latest progress in the field of minimally-invasive continuous glucose monitoring (CGM) which offers real-time information about interstitial glucose levels, by focusing on the challenges toward developing biocompatible membrane coatings to protect electrochemical glucose sensors against surface biofouling. Subsequent sections cover new analytical concepts of self-powered glucose sensors, paper-based glucose sensing and multiplexed detection of diabetes-related biomarkers. Finally, we will cover the latest advances in commercially available devices along with the upcoming future technologies.

Self-Powered Potentiometric Sensor Transduction to a Capacitive Electronic Component for Later Readout

Potentiometric sensors operate as galvanic cells where the voltage is spontaneously generated as a function of the sample composition. We show here that energy can be harvested, stored during the sensing process without external power, and physically isolated from the sensor circuit for later readout. This is accomplished by placing an electronic capacitor as a portable transduction component between the indicator and the reference electrode at the point where one would ordinarily connect the high-input-impedance voltmeter. The voltage across this isolated capacitor indicates the originally measured ion activity and can be read out conveniently, for example, using a simple handheld multimeter. The capacitor is shown to maintain the transferred charge for hours after its complete disconnection from the sensor. The concept is demonstrated to detect the physiological concentrations of K⁺ in artificial sweat samples. The methodology provides a readout principle that could become very useful in portable form factors and opens possibilities for potentiometric detection in point-of-care applications and inexpensive sensing devices where an external power source is not desired.

Wearable Electrochemical Sensors for the Monitoring and Screening of Drugs

Wearable electrochemical sensors capable of noninvasive monitoring of chemical markers represent a rapidly emerging digital-health technology. Recent advances toward wearable continuous glucose monitoring (CGM) systems have ignited tremendous interest in expanding such sensor technology to other important fields. This article reviews for the first time wearable electrochemical sensors for monitoring therapeutic drugs and drugs of abuse. This rapidly emerging class of drug-sensing wearable devices addresses the growing demand for personalized medicine, toward improved therapeutic outcomes while minimizing the side effects of drugs and the related medical expenses. Continuous, noninvasive monitoring of therapeutic drugs within bodily fluids empowers clinicians and patients to correlate the pharmacokinetic properties with optimal outcomes by realizing patient-specific dose regulation and tracking dynamic changes in pharmacokinetics behavior while assuring the medication adherence of patients. Furthermore, wearable electrochemical drug monitoring devices can also serve as powerful screening tools in the hands of law enforcement agents to combat drug trafficking and support on-site forensic investigations. The review covers various wearable form factors developed for noninvasive monitoring of therapeutic drugs in different body fluids and toward on-site screening of drugs of abuse. The future prospects of such wearable drug monitoring devices are presented with the ultimate goals of introducing accurate real-time drug monitoring protocols and autonomous closed-loop platforms toward precise dose regulation and optimal therapeutic outcomes. Finally, current unmet challenges and existing gaps are discussed for motivating future technological innovations regarding personalized therapy. The current pace of developments and the tremendous market opportunities for such wearable drug monitoring platforms are expected to drive intense future research and commercialization efforts.

Flexible Capacitive Humidity Sensors Based on Ionic Conductive Wood-Derived Cellulose Nanopapers

With the growing requirements for the renewability and sustainability of electronic products, environmentally friendly cellulose-based materials have attracted immense research interests and gained increasing prominence for electronic devices. Humidity sensors play an essential role in industries, agriculture, climatology, medical services, and daily life. Here, for the first time, we fabricate capacitive humidity sensors based on ionic conductive wood-derived cellulose nanopapers (WCNs). The WCN-based humidity sensors exhibited ultrahigh sensitivity, fast response, small hysteresis, and more importantly, a wide working range of relative humidity (RH). The sensors showed >10⁴ times increase in the sensing signal over the 7-94% RH range at 20 Hz, while many reported humidity sensors with high sensitivity often have the working range limited to high RH levels. Our sensors can realize the distinction of nuances in humidity and exhibit outstanding noncontact skin humidity sensing properties. Flexible WCN-based humidity sensors were also fabricated, and they displayed excellent sensing properties with long-time stability, endowing them with multifunctional applications. The contrast humidity sensing experiment compared to the existing commercial humidity sensor further demonstrated the higher and faster response of our WCN-based sensors. Thus, this work provides effective guidance for the design of high-performance humidity sensors using nanopapers and opens a new dimension for a variety of future applications.

A programmable epidermal microfluidic valving system for wearable biofluid management and contextual biomarker analysis

Active biofluid management is central to the realization of wearable bioanalytical platforms that are poised to autonomously provide frequent, real-time, and accurate measures of biomarkers in epidermally-retrievable biofluids (e.g., sweat). Accordingly, here, a programmable epidermal microfluidic valving system is devised, which is capable of biofluid sampling, routing, and compartmentalization for biomarker analysis. At its core, the system is a network of individually-addressable microheater-controlled thermo-responsive hydrogel valves, augmented with a pressure regulation mechanism to accommodate pressure built-up, when interfacing sweat glands. The active biofluid control achieved by this system is harnessed to create unprecedented wearable bioanalytical capabilities at both the sensor level (decoupling the confounding influence of flow rate variability on sensor response) and the system level (facilitating context-based sensor selection/protection). Through integration with a wireless flexible printed circuit board and seamless bilateral communication with consumer electronics (e.g., smartwatch), contextually-relevant (scheduled/on-demand) on-body biomarker data acquisition/display was achieved.

Wearable biosensors have been used successfully for biomarker analysis, however, a lack of control over sampling limits applications. Here, the authors report a programmable microfluidic valve to control flow rate, sampling times and allow for biofluid routing and compartmentalisation.

Surface Immobilized Nucleic Acid-Transcription Factor Quantum Dots for Biosensing

Immobilization of biosensors on surfaces is a key step toward development of devices for real-world applications. Here the preparation, characterization, and evaluation of a surface-bound transcription factor-nucleic acid complex for analyte detection as an alternative to conventional systems employing aptamers or antibodies are described. The sensor consists of a gold surface modified with thiolated Cy5 fluorophore-labeled DNA and an allosteric transcription factor (TetR) linked to a quantum dot (QD). Upon addition of anhydrotetracycline (aTc)-the analyte-the TetR-QDs release from the surface-bound DNA, resulting in loss of the Förster resonance energy transfer signal. The sensor responds in a dose-dependent manner over the relevant range of 0-200 µm aTc with a limit of detection of 80 nm. The fabrication of the sensor and the subsequent real-time quantitative measurements establish a framework for the design of future surface-bound, affinity-based biosensors using allosteric transcription factors for molecular recognition.

Strain-Controlled Flexible Graphene/GaN/PDMS Sensors Based on the Piezotronic Effect

Using simple graphene transfer and the laser lift-off process for a non-centrosymmetric GaN layer on a flexible polydimethylsiloxane (PDMS) substrate, the piezotronic effect by strain-induced current-voltage measurements at the two end points is studied. By inducing compressive strain on the flexible graphene/GaN/PDMS sensor, the Schottky barrier between the graphene and GaN/PDMS heterojunction can be electro-mechanically modulated by the piezotronic effect. It is observed that the flexible graphene/GaN/PDMS sensor is sensitive to various applied compressive and tensile strains in the positive/negative bias scans. The sensor is extremely sensitive to a compressive strain of -0.1% with a gauge factor of 13.48, which is 3.7 times higher than that of a standard metal strain gauge. Furthermore, the sharp response of the flexible graphene/GaN/PDMS sensor under the -0.1% compressive strain is also investigated. The results of this study herald the development of commercially viable large-scale flexible/wearable strain sensors based on the strain-controlled piezotronic effect in future investigations.

Anti-Biofouling Strategies for Long-Term Continuous Use of Implantable Biosensors

The growing trend for personalized medicine calls for more reliable implantable biosensors that are capable of continuously monitoring target analytes for extended periods (i.e., >30 d). While promising biosensors for various applications are constantly being developed in the laboratories across the world, many struggle to maintain reliable functionality in complex in vivo environments over time. In this review, we explore the impact of various biotic and abiotic failure modes on the reliability of implantable biosensors. We discuss various design considerations for the development of chronically reliable implantable biosensors with a specific focus on strategies to combat biofouling, which is a fundamental challenge for many implantable devices. Briefly, we introduce the process of the foreign body response and compare the in vitro and the in vivo performances of state-of-the-art implantable biosensors. We then discuss the latest development in material science to minimize and delay biofouling including the usage of various hydrophilic, biomimetic, drug-eluting, zwitterionic, and other smart polymer materials. We also explore a number of active anti-biofouling approaches including stimuli-responsive materials and mechanical actuation. Finally, we conclude this topical review with a discussion on future research opportunities towards more reliable implantable biosensors.

Fabrication of fluorescent pH-responsive protein-textile composites

Wearable pH sensors are useful tools in the healthcare and fitness industries, allowing consumers to access information related to their health in a convenient manner via the monitoring of body fluids. In this work, we tailored novel protein-textile composites to fluorescently respond to changing pH. To do so, we used amyloid curli fibers, a key component in the extracellular matrix of Escherichia coli, as genetic scaffold to fuse a pH-responsive fluorescent protein, pHuji. Engineered amyloids form macroscopic and environmentally resistant aggregates that we isolated to use as stand-alone hydrogel-based sensors, and that we trapped within textile matrices to create responsive bio-composites. We showed that these composites were mechanically robust and vapor-permeable, thus exhibiting favorable characteristics for wearable platforms. CsgA-pHuji fibers integrated in the textile allowed the final device to respond to pH changes and distinguish between alkaline and acidic solutions. We demonstrated that the resulting composites could sustain their fluorescence response over days, and that their sensing ability was reversible for at least 10 high/low pH cycles, highlighting their potential for continuous monitoring. Overall, we introduced a biosynthesized amyloid-based textile composite that could be used as biosensing patch for a variety of applications in the smart textile industry.

Advances in Sweat Wearables: Sample Extraction, Real-Time Biosensing, and Flexible Platforms

Wearable biosensors for sweat-based analysis are gaining wide attention due to their potential use in personal health monitoring. Flexible wearable devices enable sweat analysis at the molecular level, facilitating noninvasive monitoring of physiological states via real-time monitoring of chemical biomarkers. Advances in sweat extraction technology, real-time biosensors, stretchable materials, device integration, and wireless digital technologies have led to the development of wearable sweat-biosensing devices that are light, flexible, comfortable, aesthetic, affordable, and informative. Herein, we summarize recent advances of sweat wearables from the aspects of sweat extraction, fabrication of stretchable biomaterials, and design of biosensing modules to enable continuous biochemical monitoring, which are essential for a biosensing device. Key chemical components of sweat, sweat capture methodologies, and considerations of flexible substrates for integrating real-time biosensors with electronics to bring innovations in the art of wearables are elaborated. The strategies and challenges involved in improving the wearable biosensing performance and the perspectives for designing sweat-based wearable biosensing devices are discussed.

H2-gas sensing and discriminating actions of a single-yarn sensor based on a Pd/GO multilayered thin film using FFT

A single-yarn H2-gas sensor was fabricated by self-assembling poly(allylamine hydrochloride) (PAH), poly(styrenesulfonic acid) sodium salt (PSS), graphene oxide (GO) and Pd-based complex thin films layer-by-layer on a single-yarn and then in situ reducing the Pd-based complex to a Pd/GO/PAH/PSS/PAH multilayered thin film. The H2-gas sensing properties, effect of bending and humidity influence on this sensor were investigated. The sensor exhibited a high response and good linearity over the range of 1000 to 10 000 ppm of H2 gas. The response of the sensor decreased under both conditions of a bending angle up to 20° and ambient humidity above 50% RH. A fast Fourier transform (FFT) analyzer was employed to disperse the signals of the sensor under the conditions of bending and ambient humidity influence in the presence of H2 gas. Differentiation of the amplitude of FFT from the first-order to second-order frequency spectra effectively increased the discrimination capability of the sensor under the conditions of bending and humidity influence.

Screen-Printed Electrodes: Promising Paper and Wearable Transducers for (Bio)Sensing

Screen-printing technology has revolutionized many fields, including that of electrochemical biosensing. Due to their current relevance, this review, unlike other papers, discusses the relevant aspects of electrochemical biosensors manufactured using this technology in connection to both paper substrates and wearable formats. The main trends, advances, and opportunities provided by these types of devices, with particular attention to the environmental and biomedical fields, are addressed along with illustrative fundamentals and applications of selected representative approaches from the recent literature. The main challenges and future directions to tackle in this research area are also pointed out.

Wearable piezoelectric mass sensor based on pH sensitive hydrogels for sweat pH monitoring

Colorimetric and electrochemical (bio)sensors are commonly employed in wearable platforms for sweat monitoring; nevertheless, they suffer from low stability of the sensitive element. In contrast, mass-(bio)sensors are commonly used for analyte detection at laboratory level only, due to their rigidity. To overcome these limitations, a flexible mass-(bio)sensor for sweat pH sensing is proposed. The device exploits the flexibility of piezoelectric AlN membranes fabricated on a polyimide substrate combined to the sensitive properties of a pH responsive hydrogel based on PEG-DA/CEA molecules. A resonant frequency shift is recorded due to the hydrogel swelling/shrinking at several pH. Our device shows a responsivity of about 12 kHz/pH unit when measured in artificial sweat formulation in the pH range 3–8. To the best of our knowledge, this is the first time that hydrogel mass variations are sensed by a flexible resonator, fostering the development of a new class of compliant and wearable devices.

Employing Conductive Metal-Organic Frameworks for Voltammetric Detection of Neurochemicals

This paper describes the first implementation of an array of two-dimensional (2D) layered conductive metal-organic frameworks (MOFs) as drop-casted film electrodes that facilitate voltammetric detection of redox active neurochemicals in a multianalyte solution. The device configuration comprises a glassy carbon electrode modified with a film of conductive MOF (M₃HXTP₂; M = Ni, Cu; and X = NH, 2,3,6,7,10,11-hexaiminotriphenylene (HITP) or O, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP)). The utility of 2D MOFs in voltammetric sensing is measured by the detection of ascorbic acid (AA), dopamine (DA), uric acid (UA), and serotonin (5-HT) in 0.1 M PBS (pH = 7.4). In particular, Ni₃HHTP₂ MOFs demonstrated nanomolar detection limits of 63 ± 11 nM for DA and 40 ± 17 nM for 5-HT through a wide concentration range (40 nM-200 μM). The applicability in biologically relevant detection was further demonstrated in simulated urine using Ni₃HHTP₂ MOFs for the detection of 5-HT with a nanomolar detection limit of 63 ± 11 nM for 5-HT through a wide concentration range (63 nM-200 μM) in the presence of a constant background of DA. The implementation of conductive MOFs in voltammetric detection holds promise for further development of highly modular, sensitive, selective, and stable electroanalytical devices.

Layered Double Hydroxide-Modified Organic Electrochemical Transistor for Glucose and Lactate Biosensing

Biosensors based on Organic Electrochemical Transistors (OECTs) are developed for the selective detection of glucose and lactate. The transistor architecture provides signal amplification (gain) with respect to the simple amperometric response. The biosensors are based on a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) channel and the gate electrode is functionalised with glucose oxidase (GOx) or lactate oxidase (LOx) enzymes, which are immobilised within a Ni/Al Layered Double Hydroxide (LDH) through a one-step electrodeposition procedure. The here-designed OECT architecture allows minimising the required amount of enzyme during electrodeposition. The output signal of the biosensor is the drain current (I_d), which decreases as the analyte concentration increases. In the optimised conditions, the biosensor responds to glucose in the range of 0.1–8.0 mM with a limit of detection (LOD) of 0.02 mM. Two regimes of proportionality are observed. For concentrations lower than 1.0 mM, a linear response is obtained with a mean gain of 360, whereas for concentrations higher than 1.0 mM, I_d is proportional to the logarithm of glucose concentration, with a gain of 220. For lactate detection, the biosensor response is linear in the whole concentration range (0.05–8.0 mM). A LOD of 0.04 mM is reached, with a net gain equal to 400.

Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices

Next-generation wearable technology needs portable flexible energy storage, conversion, and biosensor devices that can be worn on soft and curved surfaces. The conformal integration of these devices requires the use of soft, flexible, light materials, and substrates with similar mechanical properties as well as high performances. In this review, we have collected and discussed the remarkable research contributions of recent years, focusing the attention on the development and arrangement of soft and flexible materials (electrodes, electrolytes, substrates) that allowed traditional power sources and sensors to become viable and compatible with wearable electronics, preserving or improving their conventional performances.

A Self-Healable Bifunctional Electronic Skin

For the mimicry of human skin, one of the challenges is how to detect and recognize different stimulus by electronic device, while still has the ability of skin self-recovery at the same time. Because of the excellent elasticity and flexibility, strong self-healing ability, in this paper, we reported a bifunctional self-healing e-skin with polyurethane (PU) and polyurethane@multiwalled carbon nanotubes (PU@CNT) as the sensing materials by integrating a resistance temperature sensor on top of a capacitive pressure sensor on the same flexible cellulose nanocrystals@carboxylated nitrile rubber@polyethylenimine (CNC@XNBR) substrate. Studies found that each type of sensor exhibited fast and superior response to only the target stimuli. Meanwhile, due to the self-recovery properties of PU and CNC@XNBR, as-fabricated e-skin has the self-healing ability after damage and remains excellent sensitivity to temperature and pressure after healing. A 5 × 5 device array was also fabricated, which can simultaneously image the pressure and temperature distribution.

Portable and field-deployed surface plasmon resonance and plasmonic sensors

Plasmonic sensors are ideally suited for the design of small, integrated, and portable devices that can be employed in situ for the detection of analytes relevant to environmental sciences, clinical diagnostics, infectious diseases, food, and industrial applications. To successfully deploy plasmonic sensors, scaled-down analytical devices based on surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) must integrate optics, plasmonic materials, surface chemistry, fluidics, detectors and data processing in a functional instrument with a small footprint. The field has significantly progressed from the implementation of the various components in specifically designed prism-based instruments to the use of nanomaterials, optical fibers and smartphones to yield increasingly portable devices, which have been shown for a number of applications in the laboratory and deployed on site for environmental, biomedical/clinical, and food applications. A roadmap to deploy plasmonic sensors is provided by reviewing the current successes and by laying out the directions the field is currently taking to increase the use of field-deployed plasmonic sensors at the point-of-care, in the environment and in industries.

Wearable Battery-Free Perspiration Analyzing Sites Based on Sweat Flowing on ZnO Nanoarrays

We fabricated wearable perspiration analyzing sites for actively monitoring physiological status during exercises without any batteries or other power supply. The device mainly consists of ZnO nanowire (NW) arrays and flexible polydimethylsiloxane substrate. Sweat on the skin can flow into the flow channels of the device through capillary action and flow along the channels to ZnO NWs. The sweat flowing on the NWs (with lactate oxidase modification) can output a DC electrical signal, and the outputting voltage is dependent on the lactate concentration in the sweat as the biosensing signal. ZnO NWs generate electric double layer (EDL) in sweat, which causes a potential difference between the upper and lower ends (hydrovoltaic effect). The product of the enzymatic reaction can adjust the EDL and influence the output. This device can be integrated with wireless transmitter and may have potential application in constructing sports big data. This work promotes the development of next generation of biosensors and expands the scope of self-powered physiological monitoring system.

Disruptive, Soft, Wearable Sensors

The wearable industry is on the rise, with a myriad of technical applications ranging from real-time health monitoring, the Internet of Things, and robotics, to name but a few. However, there is a saying "wearable is not wearable" because the current market-available wearable sensors are largely bulky and rigid, leading to uncomfortable wearing experience, motion artefacts, and poor data accuracy. This has aroused a world-wide intensive research quest for novel materials, with the aim of fabricating next-generation ultra-lightweight and soft wearable devices. Such disruptive second-skin-like biosensing technologies may enable a paradigm shift from current wearable 1.0 to future wearable 2.0 products. Here, the state-of-the-art progress made in the key phases for future wearable technology, namely, wear → sense → communicate → analyze → interpret → decide, is summarized. Without a doubt, materials innovation is the key, which is the main focus of the discussion. In addition, emphasis is also given to wearable energy, multicomponent integration, and wireless communication.

Novel Graphene Planar Architecture with Ultrahigh Stretchability and Sensitivity

Graphene has attracted increasing attention for strain sensing due to its unique electrical and mechanical properties by tailoring and assembling functional macrostructures with a well-defined configuration. Here a novel graphene-based planar network (GPN) with highly stretchable strain sensing is developed by direct ink writing. The integrated and regulated structure of GPN indicates an excellent response sensitivity and cyclic stability to various strain modes compared with the traditional graphene-based woven fabric (GWF) structure. An equivalent resistance network is introduced to analyze the resistance change mechanism and fracture failure mode of the network structures, in which the difference can be mainly attributed to the interfacial resistance at the crosspoints of the crossed ribbons. The tunable and interconnected GPN shows a significant difference in the response sensitivity under stretching strain in different directions, and the relative resistance change is up to 20 and 3 in horizontal and vertical directions after 1000 cycles for a 20% stretching strain, respectively, which can be explained by the transformation of the stretching mode from macro-structural stretching to micromaterial stretching. The controllable fabrication of GPN can be utilized not only for the detection of full-range human activities but monitoring external stress distribution in real-time by integration.

Microfabricated electrochemical sensing devices

Electrochemistry provides possibilities to realize smart microdevices of the next generation with high functionalities. Electrodes, which constitute major components of electrochemical devices, can be formed by various microfabrication techniques, and integration of the same (or different) components for that purpose is not difficult. Merging this technique with microfluidics can further expand the areas of application of the resultant devices. To augment the development of next generation devices, it will be beneficial to review recent technological trends in this field and clarify the directions required for moving forward. Even when limiting the discussion to electrochemical microdevices, a variety of useful techniques should be considered. Therefore, in this review, we attempted to provide an overview of all relevant techniques in this context in the hope that it can provide useful comprehensive information.

Nanomaterial-Enabled Flexible and Stretchable Sensing Systems: Processing, Integration, and Applications

Nanomaterial-enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial-enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real-world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial-enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.

O. Microfluidic Point-of-Care Devices: New Trends and Future Prospects for eHealth Diagnostics

Point-of-care (PoC) diagnostics is promising for early detection of a number of diseases, including cancer, diabetes, and cardiovascular diseases, in addition to serving for monitoring health conditions. To be efficient and cost-effective, portable PoC devices are made with microfluidic technologies, with which laboratory analysis can be made with small-volume samples. Recent years have witnessed considerable progress in this area with "epidermal electronics", including miniaturized wearable diagnosis devices. These wearable devices allow for continuous real-time transmission of biological data to the Internet for further processing and transformation into clinical knowledge. Other approaches include bluetooth and WiFi technology for data transmission from portable (non-wearable) diagnosis devices to cellphones or computers, and then to the Internet for communication with centralized healthcare structures. There are, however, considerable challenges to be faced before PoC devices become routine in the clinical practice. For instance, the implementation of this technology requires integration of detection components with other fluid regulatory elements at the microscale, where fluid-flow properties become increasingly controlled by viscous forces rather than inertial forces. Another challenge is to develop new materials for environmentally friendly, cheap, and portable microfluidic devices. In this review paper, we first revisit the progress made in the last few years and discuss trends and strategies for the fabrication of microfluidic devices. Then, we discuss the challenges in lab-on-a-chip biosensing devices, including colorimetric sensors coupled to smartphones, plasmonic sensors, and electronic tongues. The latter ones use statistical and big data analysis for proper classification. The increasing use of big data and artificial intelligence methods is then commented upon in the context of wearable and handled biosensing platforms for the Internet of things and futuristic healthcare systems.

Controlled synthesis of ultrathin MoS2 nanoflowers for highly enhanced NO2 sensing at room temperature

Fabrication of a high-performance room-temperature (RT) gas sensor is important for the future integration of sensors into smart, portable and Internet-of-Things (IoT)-based devices. Herein, we developed a NO2 gas sensor based on ultrathin MoS2 nanoflowers with high sensitivity at RT. The MoS2 flower-like nanostructures were synthesised via a simple hydrothermal method with different growth times of 24, 36, 48, and 60 h. The synthesised MoS2 nanoflowers were subsequently characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy. The petal-like nanosheets in pure MoS2 agglomerated to form a flower-like structure with Raman vibrational modes at 378 and 403 cm-1 and crystallisation in the hexagonal phase. The specific surface areas of the MoS2 grown at different times were measured by using the Brunauer-Emmett-Teller method. The largest specific surface area of 56.57 m2 g-1 was obtained for the MoS2 nanoflowers grown for 48 h. This sample also possessed the smallest activation energy of 0.08 eV. The gas-sensing characteristics of sensors based on the synthesised MoS2 nanostructures were investigated using oxidising and reducing gases, such as NO2, SO2, H2, CH4, CO and NH3, at different concentrations and at working temperatures ranging from RT to 150 °C. The sensor based on the MoS2 nanoflowers grown for 48 h showed a high gas response of 67.4% and high selectivity to 10 ppm NO2 at RT. This finding can be ascribed to the synergistic effects of largest specific surface area, smallest crystallite size and lowest activation energy of the MoS2-48 h sample among the samples. The sensors also exhibited a relative humidity-independent sensing characteristic at RT and a low detection limit of 84 ppb, thereby allowing their practical application to portable IoT-based devices.

Printed Organic Transistor-based Biosensors for Non-invasive Sweat Analysis

This review describes recent advances in biosensors for non-invasive human healthcare applications, especially focusing on sweat analysis, along with approaches for fabricating these biosensors based on printed electronics technology. Human sweat contains various kinds of biomarkers. The relationship between a trace amount of sweat biomarkers partially partitioned from blood and diseases has been investigated by omic analysis. Recent progress in wearable or portable biosensors has enabled periodic or continuous monitoring of some sweat biomarkers while supporting the results of the omic analysis. In this review, we particularly focused on a transistor-based biosensor that is highly sensitive in quantitatively detecting the low level of sweat biomarkers. Furthermore, we showed a new approach of flexible hybrid electronics that has been applied to advanced sweat biosensors to realize fully integrated biosensing systems wirelessly connected to a networked IoT system. These technologies are based on uniquely advanced printing techniques that will facilitate mass fabrication of high-performance biosensors at low cost for future smart healthcare.

A wearable lab-on-a-patch platform with stretchable nanostructured biosensor for non-invasive immunodetection of biomarker in sweat

Conformable, wearable biosensor-integrated systems are a promising approach to non-invasive and quantitative on-body detection of biomarkers in body fluids. However, realizing such a system has been slowed by the difficulty of fabricating a soft affinity-based biosensor patch capable of precise on-body fluid handling with minimal wearer intervention and a simple measurement protocol. Herein, we demonstrate a conformable, wearable lab-on-a-patch (LOP) platform composed of a stretchable, label-free, impedimetric biosensor and a stretchable microfluidic device for on-body detection of the hormone biomarker, cortisol. The all-in-one, stretchable microfluidic device can precisely collect and deliver sweat for cortisol quantitation and offers one-touch operation of reagent delivery for simultaneous electrochemical signal generation and washing. Three-dimensional nanostructuring of the Au working electrode enables the high sensitivity required to detect the pM-levels of cortisol in sweat. Our integrated LOP detected sweat cortisol quantitatively and accurately during exercise. This LOP will open a new horizon for non-invasive, highly sensitive, and quantitative on-body immunodetection for wearable personal diagnostics.