PCB-C4D coupled with paper-based microfluidic sampling for the rapid detection of liquid conductivity.

The detection of liquid electrical conductivity has board applications in food safety testing, water quality monitoring, and agricultural soil analysis. Electrodes used in traditional liquid electrical conductivity detection come into direct contact with liquid, leading to electrode contamination and affecting the accuracy of the detection results. The capacitively coupled contactless conductivity detection (C4D) method effectively addresses this issue. However, impurity particles present in the solution can compromise the consistency and repeatability of detection results. This study combines paper-based microfluidic technology with printed circuit board-capacitively coupled contactless conductivity detection (PCB-C4D) to address this issue. Prior to sample detection, in situ rapid filtration is employed to remove impurity particles from the raw solution sample, significantly enhancing detection consistency and reliability. Simultaneously, Optimization of PCB-C4D parameters, channel size, filtration time, and solution drop rate ensures optimal detection conditions. A compact kit design facilitates reliable assembly of the PCB-C4D electrodes and paper-based channel, enhancing practicality. Practical measurements on the conductivity of orange juice, cucumber, and soil solution further validate the method's accuracy, rapidity, and effectiveness in in situ conductivity detection. This work advances the practical application of PCB-C4D technology.

Thread-Embedded-in-PDMS Wearable Strain Sensor for Real-Time Monitoring of Human Joint Motion.

Real-time monitoring of human joint motion holds paramount importance in assessing joint health status, preventing and treating joint diseases, and evaluating physical flexibility and coordination. However, traditional strain sensors face limitations in meeting the substantial strain requirements associated with human joint motion. Recently, there has been considerable attention directed towards flexible strain sensors prepared using pliable substrates combined with silk and cotton fabrics. Nonetheless, these sensors exhibit insufficient linearity across the entire measurement range, thereby compromising the predictability of real joint motion based on the output signal. This paper introduced a flexible strain sensor designed to address this issue by offering an enhanced range and high linearity. Specifically, the core wire of the strain sensor was produced by coating a polybutylene terephthalate thread with conductive carbon ink integrated with carbon nanotubes, encapsulated in a thin layer of polydimethylsiloxane in an "S" configuration. The proposed strain sensor maintained excellent linearity within its strain range of 60%, along with advantages such as rapid response speed and robust durability. On-trial tests further affirmed the sensor's capability to effectively monitor the motion of human joints.

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 Sandwich-Structured Wearable Sensor with Sebum Filtering for Continuous Detection of Sweat pH.

Wearable sweat sensors, a product of the development of flexible electronics and microfluidic technologies, can continuously and noninvasively monitor abundant biomarkers in human sweat; however, sweat interferences, such as sebum, can reduce sensor reliability and accuracy. Herein, for the first time, the influence of sebum on the potentiometric response of an all-solid-state pH sensor was studied, and the obtained experimental results show that sebum mixed in sweat can decrease the potential response of the sensor and the slope of its calibration curve. A paper-based sandwich-structured pH sensor that can filter the sebum mixed in sweat was proposed based on commonly used oil-control sheets. Moreover, the hydrophilic properties, microstructure, and microfluidic performance of the sensor were investigated. The detection performance of the paper-based sandwich-structured pH sensor was comprehensively evaluated in terms of calibration in the presence of sebum and potentiometric response upon the addition of sebum. Furthermore, the anti-interference ability of the sensor was evaluated using different analytes under various deformation conditions. On-body trials were conducted to verify the performance, and their results showed that the proposed sensor can filter over 90% of the sebum in sweat, significantly enhancing sensor reliability and accuracy. Additionally, microfluidic channels could be simply fabricated using a scissor and paper, obviating the need for complex micromachining processes, such as photolithography and laser engraving. Overall, this work illustrates the influence of sebum on the detection performance of traditional potentiometric wearable sensors and paves the way for their development for real-world applications.

Advances in Non-Electrochemical Sensing of Human Sweat Biomarkers: From Sweat Sampling to Signal Reading.

Sweat, commonly referred to as the ultrafiltrate of blood plasma, is an essential physiological fluid in the human body. It contains a wide range of metabolites, electrolytes, and other biologically significant markers that are closely linked to human health. Compared to other bodily fluids, such as blood, sweat offers distinct advantages in terms of ease of collection and non-invasive detection. In recent years, considerable attention has been focused on wearable sweat sensors due to their potential for continuous monitoring of biomarkers. Electrochemical methods have been extensively used for in situ sweat biomarker analysis, as thoroughly reviewed by various researchers. This comprehensive review aims to provide an overview of recent advances in non-electrochemical methods for analyzing sweat, including colorimetric methods, fluorescence techniques, surface-enhanced Raman spectroscopy, and more. The review covers multiple aspects of non-electrochemical sweat analysis, encompassing sweat sampling methodologies, detection techniques, signal processing, and diverse applications. Furthermore, it highlights the current bottlenecks and challenges faced by non-electrochemical sensors, such as limitations and interference issues. Finally, the review concludes by offering insights into the prospects for non-electrochemical sensing technologies. By providing a valuable reference and inspiring researchers engaged in the field of sweat sensor development, this paper aspires to foster the creation of innovative and practical advancements in this domain.

Rubik's Cube as Reconfigurable Microfluidic Platform for Rapid Setup and Switching of Analytical Devices.

Microfluidics technology plays an important role in modern analytical instruments, while the modular design of microfluidics facilitates the reconfiguration of analytical instrument functions, making it possible to deploy on-demand systems in the field. However, modular design also faces the challenges such as connection reliability and reconfiguration convenience. Inspired by the self-locking structure of the Rubik's cube, a modular, reconfigurable microfluidic instrument architecture is proposed in this paper. The system has a self-locking structure of Rubik's cube components and an O-ring-based alignment and sealing mechanism, which enables reliable interconnection and rapid rearrangement of microfluidic modules by simply rotating the faces of the microfluidic cube. In addition, the system is capable of integrating a variety of customized modules to perform analysis tasks. A proof-of-concept application of detecting multiple pollutants in water is demonstrated to show the reconfigurable characteristics of the system. The findings of this paper provide a new idea for the design of microfluidic analytical instrument architectures.

Ion-selective membrane modified microfluidic paper-based solution sampling substrates for potentiometric heavy metal detection.

Paper-based microfluidic solution sampling is a viable option for potentiometric sensors to be used for the determination of analytes in samples with high solid-to-liquid ratios. Unfortunately, heavy metal sensitive electrodes cannot be easily integrated with paper-based solution sampling as heavy metals have strong physicochemical adsorption affinity towards paper substrates. In this work, paper substrates were modified with an ion-selective membrane (ISM) cocktail (used for the preparation of Pb2+-ion-selective electrodes (ISEs)) and coupled with model heavy metal Pb2+-ISEs. It was found that the super-Nernstian response of Pb2+-ISEs was eliminated when 10 to 50 mg ml-1 of the ISM cocktail was used for the modification of paper substrates. The modification of the paper substrates by Pb2+-ISM allowed the elimination of adsorption sites. In addition, it resulted in an improvement of sensor performance in terms of their detection limits to be similar to those for conditioned electrodes in standard beaker-based measurements. It is believed that the elimination of super-Nernstian response of the electrodes and improving the potentiometric responses and detection limits of ISEs were attributed to the compatibility improvement of the paper substrates and Pb2+-ISEs to the same type of ISM.

Modular Microfluidics: Current Status and Future Prospects.

This review mainly studies the development status, limitations, and future directions of modular microfluidic systems. Microfluidic technology is an important tool platform for scientific research and plays an important role in various fields. With the continuous development of microfluidic applications, conventional monolithic microfluidic chips show more and more limitations. A modular microfluidic system is a system composed of interconnected, independent modular microfluidic chips, which are easy to use, highly customizable, and on-site deployable. In this paper, the current forms of modular microfluidic systems are classified and studied. The popular fabrication techniques for modular blocks, the major application scenarios of modular microfluidics, and the limitations of modular techniques are also discussed. Lastly, this review provides prospects for the future direction of modular microfluidic technologies.

Emergence of debubblers in microfluidics: A critical review.

Bubbles in microfluidics-even those that appear to be negligibly small-are pervasive and responsible for the failure of many biological and chemical experiments. For instance, they block current conduction, damage cell membranes, and interfere with detection results. To overcome this unavoidable and intractable problem, researchers have developed various methods for capturing and removing bubbles from microfluidics. Such methods are multifarious and their working principles are very different from each other. In this review, bubble-removing methods are divided into two broad categories: active debubblers (that require external auxiliary equipment) and passive debubblers (driven by natural processes). In each category, three main types of methods are discussed along with their advantages and disadvantages. Among the active debubblers, those assisted by lasers, acoustic generators, and negative pressure pumps are discussed. Among the passive debubblers, those driven by buoyancy, the characteristics of gas-liquid interfaces, and the hydrophilic and hydrophobic properties of materials are discussed. Finally, the challenges and prospects of the bubble-removal technologies are reviewed to refer researchers to microfluidics and inspire further investigations in this field.

A Novel Solution-auto-introduction Electrophoresis Microchip Based on Capillary Force.

A novel solution-auto-introduction electrophoresis microchip based on capillary force aimed at improving portability is proposed in this paper. Two kinds of materials with micropores, poly(vinyl alcohol) (PVA)-sponge and nano-sponge, were employed as suction pumps that realized the introduction of a running buffer into the reservoirs. The surfaces of the microchannels in the microchips were modified by PVA to improve the moving velocity of the running buffer and the detection performance of the microchip. The introduction velocity of a running buffer in the PVA-coated microchannels was increased by two times compared with that in the native microchannels. The electrophoresis detection performance of several microchips composed of different microchannels and suction materials were evaluated comparatively. The results indicated that the surface coating of PVA can significantly improve the repeatability of the detection results by 20 - 40%, and the noise of the detected signals in the PVA-coated microchips is much lower than that in the native microchips. The proposed solution-auto-introduction electrophoresis microchip is a successful attempt that completely avoids the external connectors to accomplish the auto-introduction of running buffer. The solution-auto-introduction method provides a new train of thought for portable detection instruments with electrophoresis microchips in the future.

An Effective Capillary Valve Based on Micro-hole Array for Microfluidic Systems.

With the development of microfluidic systems for portable devices where the capillary force as the driving force drove the liquid in a microchannel. An effective capillary microvalve based on a micro-hole array is proposed in this paper for the purpose of good performance, and programmable control, and easy-to-fabricate. The Wenzel model was used to slow down the velocity of fluid flow. The microvalve was made by a hot-embossing and hot-bonding process. The valve function was assessed by liquid flowing experiments. The results showed that the valve employs an abruptly changing contact angle to slow down the fluid flow. The controlling time is approximately 177 - 213 s. The best control effect was obtained when the contact angle equaled to 90°. The effect of the roughness of the microvalve was also evaluated. Therefore, this work has a great potential and broad prospect for both research and applications in microfluidic systems, such as the portable devices in the medical testing field.

Simultaneous Determination of Inorganic Cations and Anions in Microchip Electrophoresis Using High-voltage Relays.

A new and simple method for the simultaneous determination of cations and anions by microchip electrophoresis with capacitively coupled contactless conductivity detection (ME-C4D) is described. The best analytical performance was found by applying a sinusoidal wave with 800 kHz frequency and 20 Vpp amplitude. An optimized background electrolyte (BGE) composed of 20 mM His/MES and 0.01 mM CTAB was chosen for the simultaneous analysis. Samples containing K+, Na+, and Li+ as the cations and Cl-, F- and PO43- as anions were analyzed simultaneously in a single run (within 3 min). The reproducibility obtained by the method was compared with those obtained in previous studies that had employed simultaneous analysis of anions and cations by ME-C4D. The proposed simultaneous determination method is inexpensive, simple, fast, easy to operate, and offers a high degree of integration.

A plug-in electrophoresis microchip with PCB electrodes for contactless conductivity detection.

A plug-in electrophoresis microchip for large-scale use aimed at improving maintainability with low fabrication and maintenance costs is proposed in this paper. The plug-in microchip improves the maintainability of a device because the damaged microchannel layer can be changed without needing to cut off the circuit wires in the detection component. Obviously, the plug-in structure reduces waste compared with earlier microchips; at present the whole microchip has to be discarded, including the electrode layer and the microchannel layer. The fabrication cost was reduced as far as possible by adopting a steel template and printed circuit board electrodes that avoided the complex photolithography, metal deposition and sputtering processes. The detection performance of our microchip was assessed by electrophoresis experiments. The results showed an acceptable gradient and stable detection performance. The effect of the installation shift between the microchannel layer and the electrode layer brought about by the plug-in structure was also evaluated. The results indicated that, as long as the shift was controlled within a reasonable scope, its effect on the detection performance was acceptable. The plug-in microchip described in this paper represents a new train of thought for the large-scale use and design of portable instruments with electrophoresis microchips in the future.

Concurrent determination and separation of inorganic cations and anions in microchip electrophoresis with precisely controlled high-voltage.

An improved method for the concurrent determination and separation of cations and anions by microchip electrophoresis with capacitively coupled contactless conductivity detection (ME-C4 D) is described. Two kinds of microchip structures were designed. The first microchip has a long bent separation channel. And for the defects of the first microchip, the second microchip with a Y-type separation channel has been proposed. The background electrolyte (BGE) composed of 20 mm His/MES and 0.01 mm CTAB was optimized for inhibiting the electroosmotic flow (EOF). Due to the low electroosmotic flow, the cations and anions migrate in opposite directions and can be separated from each other. With the precisely controlled high-voltage, cations and anions can be migrated in microchannels according to our requirements and sequentially detected by a C4 D detector built in-house. Samples containing K+ , Na+ , Li+ , Cl- , F- and PO43- were analyzed simultaneously in a single run (within 140 s) by both methods. The reproducibility obtained by both methods remained below 5% for migration time and within 3.5-9.1% for peak areas. The proposed concurrent determination methods are inexpensive, simple, fast, ease of operation, high degree of integration.