A Smartwatch Integrated with a Paper‐based Microfluidic Patch for Sweat Electrolytes Monitoring

Worth your sweat: wearable microfluidic flow rate sensors for meaningful sweat analytics

Wearable microfluidic sweat sensors could play a major role in the future of monitoring health and wellbeing. Sweat contains biomarkers to monitor health and hydration status, and it can provide information on drug intake, making it an interesting non-invasive alternative to blood. However, sweat is not created in excess, and this requires smart sweat collection strategies to handle small volumes. Microfluidic solutions are commonly employed which use capillary action or evaporation to drive flow. In current literature about sweat analytics, the emphasis lies predominantly on developing the sensors for measuring the composition of sweat. Yet, solely measuring sweat composition does not suffice, because the composition varies due to inter- and intra-individual differences in sweat rate. The measurement of sweat rate is thus crucial for enabling a reliable interpretation and standardisation of this data. Recently, more wearable sweat sensors, also integrating a means of measuring flow, have been developed. This manuscript reviews state-of-the-art sweat collection strategies and flow rate measuring techniques. Generally, flow rate measurements are performed by impedimetric or capacitive methods. However, these techniques can be impaired due to limited lifetime and signal interference from changing ionic contents in sweat. Discrete measurement techniques, such as impedance measurements of an advancing fluid front with interdigitated electrodes, calorimetric and colorimetric techniques can be very reliable, because they selectively measure flow. However, one should take the available size, intended application and compatibility with other sensors into account. Overall, accurate flow rate sensors integrated in reliable microfluidic sweat sensor platforms will enable the standardisation of sweat measurements to unlock the potential of sweat analytics in advancing physiological research, personalized diagnostics and treatment of diseases.

Recent Advances in the Fabrication and Application of Electrochemical Paper-Based Analytical Devices

In response to growing environmental concerns, the scientific community is increasingly incorporating green chemistry principles into modern analytical techniques. Electrochemical paper-based analytical devices (ePADs) have emerged as a sustainable and efficient alternative to conventional analytical devices, offering robust applications in point-of-care testing, personalized healthcare, environmental monitoring, and food safety. ePADs align with green chemistry by minimizing reagent use, reducing energy consumption, and being disposable, making them ideal for eco-friendly and cost-effective analyses. Their user-friendly interface, alongside sensitive and selective detection capabilities, has driven their popularity in recent years. This review traces the evolution of ePADs from simple designs to complex multilayered structures that optimize analyte flow and improve detection. It also delves into innovative electrode fabrication methods, assessing key advantages, limitations, and modification strategies for enhanced sensitivity. Application-focused sections explore recent advancements in using ePADs for detecting diseases, monitoring environmental hazards like heavy metals and bacterial contamination, and screening contaminants in food. The integration of cutting-edge technologies, such as wearable wireless devices and the Internet of Things (IoT), further positions ePADs at the forefront of point-of-care testing (POCT). Finally, the review identifies key research gaps and proposes future directions for the field.

Multi-dimensional microfluidic paper-based analytical devices (μPADs) for noninvasive testing: A review of structural design and applications

The rapid emergence of microfluidic paper-based devices as point-of-care testing (POCT) tools for early disease diagnosis and health monitoring, particularly in resource-limited areas, holds immense potential for enhancing healthcare accessibility. Leveraging the numerous advantages of paper, such as capillary-driven flow, porous structure, hydrophilic functional groups, biodegradability, cost-effectiveness, and flexibility, it has become a pivotal choice for microfluidic substrates. The repertoire of microfluidic paper-based devices includes one-dimensional lateral flow assays (1D LFAs), two-dimensional microfluidic paper-based analytical devices (2D μPADs), and three-dimensional (3D) μPADs. In this comprehensive review, we provide and examine crucial information related to paper substrates, design strategies, and detection methods in multi-dimensional microfluidic paper-based devices. We also investigate potential applications of microfluidic paper-based devices for detecting viruses, metabolites and hormones in non-invasive samples such as human saliva, sweat and urine. Additionally, we delve into capillary-driven flow alternative theoretical models of fluids within the paper to provide guidance. Finally, we critically examine the potential for future developments and address challenges for multi-dimensional microfluidic paper-based devices in advancing noninvasive early diagnosis and health monitoring. This article showcases their transformative impact on healthcare, paving the way for enhanced medical services worldwide.

Wearable electrochemical device based on butterfly-like paper-based microfluidics for pH and Na+ monitoring in sweat

A wearable potentiometric device is reported based on an innovative butterfly-like paper-based microfluidic system, allowing for continuous monitoring of pH and Na+ levels in sweat during physical activity. Specifically, the use of the butterfly-like configuration avoids evaporation phenomena and memory effects, enabling precise and timely biomarker determination in sweat. Two ad hoc modified screen-printed electrodes were embedded in the butterfly-like paper-based microfluidics, and the sensing device was further integrated with a portable and miniaturized potentiostat, leveraging Bluetooth technology for efficient data transmission. First, the paper-based microfluidic configuration was tested for optimal fluidic management to obtain optimized performance of the device. Subsequently, the two electrodes were individually tested to detect the two biomarkers, namely pH and Na+. The results demonstrated highly promising near-Nernstian (0.056 ± 0.002 V/dec) and super-Nernstian (- 0.080 ± 0.003 V/pH) responses, for Na+ and pH detection, respectively. Additionally, several important parameters such as storage stability, interferents, and memory effect by hysteresis study were also investigated. Finally, the butterfly-like paper-based microfluidic wearable device was tested for Na+ and pH monitoring during the physical activity of three volunteers engaged in different exercises, obtaining a good correlation between Na+ increase and dehydration phenomena. Furthermore, one volunteer was tested through a cardiopulmonary test, demonstrating a correlation between sodium Na+ increase and the energetic effort by the volunteer. Our wearable device highlights the high potential to enable early evaluation of dehydration and open up new opportunities in sports activity monitoring.

Paper-Based Electrochemical (Bio)Sensors for the Detection of Target Analytes in Liquid, Aerosol, and Solid Samples

The last decade has been incredibly fruitful in proving the multifunctionality of paper for delivering innovative electrochemical (bio)sensors. The paper material exhibits unprecedented versatility to deal with complex liquid matrices and facilitate analytical detection in aerosol and solid phases. Such remarkable capabilities are feasible by exploiting the intrinsic features of paper, including porosity, capillary forces, and its easy modification, which allow for the fine designing of a paper device. In this review, we shed light on the most relevant paper-based electrochemical (bio)sensors published in the literature so far to identify the smart functional roles that paper can play to bridge the gap between academic research and real-world applications in the biomedical, environmental, agrifood, and security fields. Our analysis aims to highlight how paper's multifarious properties can be artfully harnessed for breaking the boundaries of the most classical applications of electrochemical (bio)sensors.

The use of biological fluids in microfluidic paper-based analytical devices (μPADs): Recent advances, challenges and future perspectives

The use of microfluidic paper-based analytical devices (μPADs) for aiding medical diagnosis is a growing trend in the literature mainly due to their low cost, easy use, simple manufacturing, and great potential for application in low-resource settings. Many important biomarkers (proteins, ions, lipids, hormones, DNA, RNA, drugs, whole cells, and more) and biofluids are available for precise detection and diagnosis. We have reviewed the advances μPADs in medical diagnostics have achieved in the last few years, focusing on the most common human biofluids (whole blood/plasma, sweat, urine, tears, and saliva). The challenges of detecting specific biomarkers in each sample are discussed, along with innovative techniques that overcome such limitations. Finally, the difficulties of commercializing μPADs are considered, and future trends are presented, including wearable devices and integrating multiple steps in a single platform.

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.

Influence of the Composition of Plasticizer-Free Silicone-Based Ion-Selective Membranes on Signal Stability in Aqueous and Blood Plasma Samples

Solid-contact ion-selective electrodes (SC-ISEs) in direct long-term contact with physiological samples must be biocompatible and resistant to biofouling, but most wearable SC-ISEs proposed to date contain plasticized poly(vinyl chloride) (PVC) membranes, which have poor biocompatibility. Silicones are a promising alternative to plasticized PVC because of their excellent biocompatibility, but little work has been done to study the relationship between silicone composition and ISE performance. To address this, we prepared and tested K+ SC-ISEs with colloid-imprinted mesoporous (CIM) carbon as the solid contact and three different condensation-cured silicones: a custom silicone prepared in-house (Silicone 1), a commercial silicone (Dow 3140, Silicone 2), and a commercial fluorosilicone (Dow 730, Fluorosilicone 1). SC-ISEs prepared with each of these polymers and the ionophore valinomycin and added ionic sites exhibited Nernstian responses, excellent selectivities, and signal drifts as low as 3 μV/h in 1 mM KCl solution. All ISEs maintained Nernstian response slopes and had only very slightly worsened selectivities after 41 h exposure to porcine plasma (log KK,Na values of -4.56, -4.58, and -4.49, to -4.04, -4.00, and -3.90 for Silicone 1, Silicone 2, and Fluorosilicone 1, respectively), confirming that these sensors retain the high selectivity that makes them suitable for use in physiological samples. When immersed in porcine plasma, the SC-ISEs exhibited emf drifts that were still fairly low but notably larger than when measurements were performed in pure water. Interestingly, despite the very similar structures of these matrix polymers, SC-ISEs prepared with Silicone 2 showed lower drift in porcine blood plasma (-55 μV/h, over 41 h) compared to Silicone 1 (-495 μV/h) or Fluorosilicone 1 (-297 μV/h).

Screen-Printed Wearable Sweat Sensor for Cost-Effective Assessment of Human Hydration Status through Potassium and Sodium Ion Detection

Human sweat is intricately linked to human health, and unraveling its secrets necessitates a substantial volume of experimental data. However, conventional sensors fabricated via complex processes such as photolithography offer high detection precision at the expense of prohibitive costs. In this study, we presented a cost-effective and high-performance wearable flexible sweat sensor for real-time monitoring of K+ and Na+ concentrations in human sweat, fabricated using screen printing technology. Initially, we evaluated the electrical and electrochemical stability of the screen-printed substrate electrodes, which demonstrated good consistency with a variation within 10% of the relative standard deviation (RSD), meeting the requirements for reliable detection of K+ and Na+ in human sweat. Subsequently, we employed an "ion-electron" transduction layer and an ion-selective membrane to construct the sensors for detecting K+ and Na+. Comprehensive tests were conducted to assess the sensors' sensitivity, linearity, repeatability, resistance to interference, and mechanical deformation capabilities. Furthermore, we evaluated their long-term stability during continuous monitoring and storage. The test results confirmed that the sensor's performance indicators, as mentioned above, met the requirements for analyzing human sweat. In a 10-day continuous and regular monitoring experiment involving volunteers wearing the sensors, a wealth of data revealed a close relationship between K+ and Na+ concentrations in human sweat and hydration status. Notably, we observed that consistent and regular physical exercise effectively enhanced the body's resistance to dehydration. These findings provided a solid foundation for conducting extensive experiments and further exploring the intricate relationship between human sweat and overall health. Our research paved a practical and feasible path for future studies in this domain.

Paper-based microfluidics in sweat detection: from design to application

Sweat, as a sample that includes a lot of biochemical information, is good for non-invasive monitoring. In recent years, there have been an increasing number of studies on in situ monitoring of sweat. However, there are still some challenges for the continuous analysis of samples. As a hydrophilic, easy-to-process, environmentally friendly, inexpensive and easily accessible material, paper is an ideal substrate material for making in situ sweat analysis microfluidics. This review introduces the development of paper as a sweat analysis microfluidic substrate material, focusing on the advantages of the structural characteristics of paper, trench design and equipment integration applications to expand the design and research ideas for the development of in situ sweat detection technology.

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

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

Microfluidic wearable electrochemical sweat sensors for health monitoring

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

Porous carbon-based robust, durable, and flexible electrochemical device for K+ detection in sweat

Ion sensors are attracting attention for real-time ion monitoring in biological fluids, which requires the development of sensitive, stable, flexible, robust and durable ion-selective electrodes (ISEs) and reference electrodes (REs). In this paper, a highly robust and durable ion sensor was prepared by coating polymer membranes on porous carbon electrodes. A high sensitivity of 58.6 mV per decade with a rapid response time of 0.8 s, and a negligible potential drift less than 1.4 mV h^-1 were obtained simultaneously. In addition, after six washing cycles, the K⁺ ion sensors still have an average sensitivity of 53.2 mV per decade. Importantly, the polymer membrane permeated and packed the porous structure tightly, and thus the ion sensors presented outstanding robustness and durability. The Nernst slope of the K⁺ ion response fluctuated from 60.2 to 57.9 mV per decade between 0° and 60° bending angles. Repeated bending for 8000 cycles did not result in the delamination of the sensing and reference membranes or reduction of the sensitivity (57.4 mV per decade). Furthermore, five kinds of flexible reference electrodes (LEC, bare Ag, bare Ag/AgCl, PVB + NaCl on Ag/AgCl, PVC/agarose + NaCl on Ag/AgCl) were fabricated and evaluated in terms of the sensitivity for Cl^- and long-term stability. Finally, the flexible K⁺ ion sensor was integrated with microfluidic channels and connected to a portable electrochemical workstation to realize the real-time analysis of human sweat.

Dual-mode ion-selective electrodes and distance-based microfluidic device for detection of multiple urinary electrolytes

Here, we developed a microfluidic paper device by combining ion-selective electrodes (ISE) and a distance-based paper device (dPAD) for simultaneous potentiometric and colorimetric detection of urine electrolytes including K+, Na+ and Cl−.

Solid-Contact Potentiometric Cell with Symmetry

By its nature, a traditional potentiometric cell composed of an Ag/AgCl-based reference electrode and a solid-contact indicating electrode is not symmetric. This results in undesirable potential drifts in response to a common perturbation such as a temperature change of the sample. We propose here an approach to restore symmetry by constructing a cell with two identical solid-contact ISEs used as reference and indicating electrodes. In this arrangement, the reference electrode is immersed in a compartment containing a constant background of an ion of interest, while the indicating electrode is directly immersed in the sample solution. This approach was successfully demonstrated for a cell composed of nitrate-selective electrodes with the hydrophobic derivative of poly(3,4-ethylenedioxythiophene) as a transducer layer. In particular, the symmetric setup is shown to lower by 4-5 times the observed potential drift resulting from temperature changes between +25 and +5 °C.