A Microfluidic Sensor for Continuous Sweat Flow Rate Monitoring
Brief Introduction
Sweating is fundamental to human physiology, regulating body temperature and maintaining skin barrier function and hydration. The loss of substantial sweat, particularly during intense activity or in extreme environments, is accompanied by a critical loss of electrolytes, which can lead to severe health conditions such as hyponatremia. Continuous monitoring of sweat dynamics is therefore crucial for providing timely insights into physiological status and health risks. However, conventional sensing technologies are incapable of continuously monitoring flow rate in filled microchannels, a major constraint for dynamic analysis. To address this, we introduce a microfluidic flow sensor based on a strain-responsive mechanism. By engineering a curved microfluidic channel, the sensor effectively converts tangential forces generated by fluid flow into a quantifiable strain signal, thus breaking the core bottleneck of traditional methods. The sensor’s performance is significantly amplified by a fabrication process that precisely controls the PDMS wall thickness via in situ electrothermal tuning and selectively functionalizes the channel wall with a CNT sensing network. Our sensor achieves precise, continuous monitoring of sweat flow rates within a dynamic range of 0.05–5 mL/min, validated by a high correlation between experimental results and COMSOL simulations. This technology not only offers a highly robust new paradigm for wearable health monitoring but also paves the way for preemptive medical alerts, precision healthcare, and enhanced occupational safety.
How it is being developed
Materials
Polydimethylsiloxane (PDMS) was purchased from Dow Corning (USA). Single-walled carbon nanotubes of ultra-high purity were obtained from XFNano (China), and dichloromethane was supplied by Aladdin (China). Polytetrafluoroethylene (PTFE) film was obtained from Daoguan (China). Polyvinyl alcohol (PVA) was obtained from Merck (Germany). Both Silica nanoparticles and ammonium dichromate were supplied by Aladdin (China).
Device fabrication
Initially, PDMS was prepared by mixing the base and curing agent at a 20:1 ratio. The mixture was degassed under a vacuum of -0.1 MPa for 30 minutes to eliminate entrapped air bubbles. An iron-chromium-aluminum alloy wire (OCr25Al5) with a diameter of 500 μm and a length of 15 cm was embedded into the PDMS mixture, followed by initial in-situ electric heating with a current of 2A. After 5s centrifugation, post-electric heating was performed. The microfluidic structure was then immersed in dichloromethane for demolding and subsequently treated with UV light for 20 minutes. The UV-treated microchannel structure was then positioned in a mold lined with PTFE film. Next, 1mL of CNT solution was precisely introduced into the mold using a pipette. Finally, the CNT was cured in an oven at 60°C to obtain the strain-based microfluidic flow rates sensor.
Device characterization
The mechanical properties of PDMS were characterized using a computer-controlled tensile and peeling tester (Guangdong Zhongye Precision Technology, China). For modulus measurement, PDMS samples were cut into specific geometries as shown in Supplementary Figure 1. To investigate the tensile resistance characteristics of the sensors, the microfluidic devices were mounted on a horizontal tensile testing platform and stretched to a fixed length. The electrical characterization of CNT networks was performed using a source measurement unit (Keithley 2450, USA) to evaluate current-voltage (I-V) characteristics. The morphology of the CNT was characterized using a scanning electron microscope (Zeiss Sigma 300, Germany). Prior to imaging, the PDMS substrate was sputter-coated for 60 seconds to enhance surface conductivity. A micro-flow pump (Duke SPLab01) was used to inject liquids with flow rates of 0.1, 0.5, 1, 2, 3, 4, 5, 6, and 7 mL/min into the microchannel. All mechanical simulations were conducted using COMSOL Multiphysics?.
Expected outcome
The low-cost and easily fabricated sweat flow rate sensor developed in this project, characterized by its high reproducibility and significant potential for commercialization, is poised to overcome the critical bottleneck of continuous flow rate monitoring in existing wearable technologies. This work establishes an innovative, data-driven solution guided by precise physiological metrics. It not only fills a crucial technological gap in precision wearable sensors for continuous sweat analysis but also provides key enabling technology for the design, manufacturing, and market deployment of downstream health monitoring devices. The applications aim to enhance personal health management and optimize performance in high-risk occupational settings. By facilitating early health warnings to reduce healthcare expenditures, improving the efficiency of pharmaceutical R&D, and minimizing economic losses in specialized industries due to personnel exceeding physiological limits, this technology is expected to significantly advance the national “Big Health” and high-tech manufacturing sectors, serving as a powerful economic driver for industrial upgrading.Furthermore, the technological breakthroughs of this project will substantially support the establishment of proactive health management models and the enhancement of health and safety systems for personnel in high-risk roles. For key personnel operating in extreme or high-load environments, such as firefighters and military personnel, the sensor offers reliable, real-time physiological monitoring and alerts. It effectively bridges the gap between physiological data acquisition and timely intervention, enabling “real-time safeguarding” and “precision management” of individual health and pioneering a new paradigm of individualized health assurance. Consequently, the project’s outcomes will strongly contribute to the implementation of the “Healthy China 2030” national strategy, improve public health literacy, and safeguard human resources in critical national sectors, thereby making a vital technological contribution to the development of a harmonious and healthy society.
A Microfluidic Sensor for Continuous Sweat Flow Rate Monitoring
Brief Introduction
Sweating is fundamental to human physiology, regulating body temperature and maintaining skin barrier function and hydration. The loss of substantial sweat, particularly during intense activity or in extreme environments, is accompanied by a critical loss of electrolytes, which can lead to severe health conditions such as hyponatremia. Continuous monitoring of sweat dynamics is therefore crucial for providing timely insights into physiological status and health risks. However, conventional sensing technologies are incapable of continuously monitoring flow rate in filled microchannels, a major constraint for dynamic analysis. To address this, we introduce a microfluidic flow sensor based on a strain-responsive mechanism. By engineering a curved microfluidic channel, the sensor effectively converts tangential forces generated by fluid flow into a quantifiable strain signal, thus breaking the core bottleneck of traditional methods. The sensor’s performance is significantly amplified by a fabrication process that precisely controls the PDMS wall thickness via in situ electrothermal tuning and selectively functionalizes the channel wall with a CNT sensing network. Our sensor achieves precise, continuous monitoring of sweat flow rates within a dynamic range of 0.05–5 mL/min, validated by a high correlation between experimental results and COMSOL simulations. This technology not only offers a highly robust new paradigm for wearable health monitoring but also paves the way for preemptive medical alerts, precision healthcare, and enhanced occupational safety.
How it is being developed
Materials
Polydimethylsiloxane (PDMS) was purchased from Dow Corning (USA). Single-walled carbon nanotubes of ultra-high purity were obtained from XFNano (China), and dichloromethane was supplied by Aladdin (China). Polytetrafluoroethylene (PTFE) film was obtained from Daoguan (China). Polyvinyl alcohol (PVA) was obtained from Merck (Germany). Both Silica nanoparticles and ammonium dichromate were supplied by Aladdin (China).
Device fabrication
Initially, PDMS was prepared by mixing the base and curing agent at a 20:1 ratio. The mixture was degassed under a vacuum of -0.1 MPa for 30 minutes to eliminate entrapped air bubbles. An iron-chromium-aluminum alloy wire (OCr25Al5) with a diameter of 500 μm and a length of 15 cm was embedded into the PDMS mixture, followed by initial in-situ electric heating with a current of 2A. After 5s centrifugation, post-electric heating was performed. The microfluidic structure was then immersed in dichloromethane for demolding and subsequently treated with UV light for 20 minutes. The UV-treated microchannel structure was then positioned in a mold lined with PTFE film. Next, 1mL of CNT solution was precisely introduced into the mold using a pipette. Finally, the CNT was cured in an oven at 60°C to obtain the strain-based microfluidic flow rates sensor.
Device characterization
The mechanical properties of PDMS were characterized using a computer-controlled tensile and peeling tester (Guangdong Zhongye Precision Technology, China). For modulus measurement, PDMS samples were cut into specific geometries as shown in Supplementary Figure 1. To investigate the tensile resistance characteristics of the sensors, the microfluidic devices were mounted on a horizontal tensile testing platform and stretched to a fixed length. The electrical characterization of CNT networks was performed using a source measurement unit (Keithley 2450, USA) to evaluate current-voltage (I-V) characteristics. The morphology of the CNT was characterized using a scanning electron microscope (Zeiss Sigma 300, Germany). Prior to imaging, the PDMS substrate was sputter-coated for 60 seconds to enhance surface conductivity. A micro-flow pump (Duke SPLab01) was used to inject liquids with flow rates of 0.1, 0.5, 1, 2, 3, 4, 5, 6, and 7 mL/min into the microchannel. All mechanical simulations were conducted using COMSOL Multiphysics?.
Expected outcome
The low-cost and easily fabricated sweat flow rate sensor developed in this project, characterized by its high reproducibility and significant potential for commercialization, is poised to overcome the critical bottleneck of continuous flow rate monitoring in existing wearable technologies. This work establishes an innovative, data-driven solution guided by precise physiological metrics. It not only fills a crucial technological gap in precision wearable sensors for continuous sweat analysis but also provides key enabling technology for the design, manufacturing, and market deployment of downstream health monitoring devices. The applications aim to enhance personal health management and optimize performance in high-risk occupational settings. By facilitating early health warnings to reduce healthcare expenditures, improving the efficiency of pharmaceutical R&D, and minimizing economic losses in specialized industries due to personnel exceeding physiological limits, this technology is expected to significantly advance the national “Big Health” and high-tech manufacturing sectors, serving as a powerful economic driver for industrial upgrading.Furthermore, the technological breakthroughs of this project will substantially support the establishment of proactive health management models and the enhancement of health and safety systems for personnel in high-risk roles. For key personnel operating in extreme or high-load environments, such as firefighters and military personnel, the sensor offers reliable, real-time physiological monitoring and alerts. It effectively bridges the gap between physiological data acquisition and timely intervention, enabling “real-time safeguarding” and “precision management” of individual health and pioneering a new paradigm of individualized health assurance. Consequently, the project’s outcomes will strongly contribute to the implementation of the “Healthy China 2030” national strategy, improve public health literacy, and safeguard human resources in critical national sectors, thereby making a vital technological contribution to the development of a harmonious and healthy society.