Today, wearable healthcare systems play an important role in disease prediction and diagnosis. Among them, sweat analysis has attracted great attention from academia and industry because sweat contains abundant body secretions, which can feed back necessary health information and are easier to collect compared to other body fluids. Common current sweat collection methods include the use of absorbent materials, superhydrophobic/superhydrophilic surfaces, perspiration guides, epidermal microfluidics, etc. However, these methods inevitably introduce environmental pollution during the sweat collection process. Therefore, collecting and driving sweat in droplets directly on the epidermis is an effective way to reduce unnecessary contact with the environment.

Based on this, Professor Chen Liguo, Associate Professor Huang Haibo, Associate Researcher Wen Zhen and others from Soochow University reported a wearable human sweat monitoring platform (WSMP) based on a dielectric electrowetting (EWOD) device and a triboelectric nanogenerator (TENG). The platform collects and transports sweat droplets in different chambers through the dielectric wetting effect, and allows them to combine and react with pH indicators. The high-voltage electric field generated by TENG can change the contact angle of electrolyte droplets by more than 30% when the triboelectric voltage is 5 kV, and realize the control of droplet motion by changing the wettability of the solid-liquid interface. Finally, the researchers demonstrated the application of the wearable WSMP, realizing the preliminary detection of the pH value of human sweat. The research was published in Advanced Functional Materials in a paper titled “A Wearable Electrowetting on Dielectrics Sensor for Real-Time Human Sweat Monitor by Triboelectric Field Regulation.”

Specifically, the WSMP consists of the EWOD device as the main detection element of sweat and the TENG as the voltage source. The insole-made TENG can provide a high-voltage electric field for sweat drop drive in EWOD by harvesting human motion energy. When people walk or move, the two electrodes of the TENG sense and transfer alternating charges of opposite polarities. In TENG, aluminum foil and fluorinated ethylene propylene film are used as friction layers for the top and bottom plates, and porous polyurethane is used as the substrate. The top and bottom of the sandwich-structured EWOD device are ITO electrodes coated with hydrophobic materials, and the middle is an electrowetting dielectric layer. The EWOD device is flexible and transparent, easy to wear and observe, and can be attached to the epidermis of the calf or ankle, and its upper electrodes are respectively connected with the aluminum foils of the upper and lower friction layers of the TENG. When sweat flows toward the edge of the EWOD device, the high voltage generated by the TENG is applied to the electrodes, and the contact angle of the sweat droplets on the electrodes decreases, resulting in an inward total surface tension. Therefore, the sweat droplets can smoothly flow into the chamber of the EWOD device, and the pH indicator moves in the opposite direction and reacts with the sweat droplets, realizing the pH detection of the sweat droplets.

Figure 1 The structure and working principle of WSMP

The principle of the droplet moving under the action of the dielectric wetting effect is that the contact angle changes with the change of the applied voltage. When a triboelectric field is formed between the two plates in the EWOD device, the contact angle of the droplet decreases. The variation in the droplet contact angle during the AC cycle can be explained by the periodic adsorption of ions of different polarities in the droplet due to the electric field of changing direction. To study the effect of the output of TENG on the wetting of the medium, the researchers showed the change process of the contact angle. When the TENG periodically contacts and separates to generate an electric potential, the droplets vibrate repeatedly, and the contact angle hysteresis on the Teflon hydrophobic layer is about 7°, so the driving force of the triboelectric field is sufficient to move the droplets with different dissolved species. In addition, the researchers achieved the movement and merging of droplets in the EWOD device. The droplets are distributed on both sides of the middle electrode. When the middle electrode is energized, the droplets on both sides move relative to each other at the same time, and then merge for microreaction.

Fig. 2 The principle of EWOD effect produced by triboelectric field

Figure 3. Droplets move in an EWOD device driven by a triboelectric field

Finally, the researchers demonstrated a WSMP device worn on the lower leg to test pH through the color response of sweat and a pH indicator. The TENG integrated in the insole of sports shoes is connected to the EWOD device to provide a triboelectric field for driving sweat droplets, and the mechanical energy of human motion is the main source of TENG power generation. The reaction part is a parallel plate structure, which can effectively avoid pollution. Sweat droplets get stuck when they are located at the transition zone between the unipolar and bipolar plates, and the electrodes are then energized by the TENG, causing the droplets to flow in the chamber, which then enters the chamber and reacts with the pH indicator. The researchers used ordinary least squares with RGB features to accurately reflect the pH detected from the wearer. According to the fitting equation, the sweat color after reaction is extracted and analyzed, and then the data is transmitted to the mobile APP to realize real-time sweat monitoring. The researchers used WSMP to wear experiments on three healthy volunteers, and the results showed that the three volunteers’ sweat pH values ​​were 6.57, 6.80 and 7.01.

Figure 4 Demonstration of WSMP worn on the calf for real-time sweat monitoring

In conclusion, the researchers designed a wearable human sweat monitoring platform based on contact-separated TENG and flexible EWOD devices to explore the active and efficient detection of sweat to reflect human health status. Sweat droplets can be controlled by the electrostatic field provided by the contact separation TENG. The WSMP can be integrated with mobile phones and has the advantages of self-power supply, low pollution, and active control. By wearing the WAMP, the researchers successfully achieved pH monitoring of human sweat. This work has great potential in realizing highly integrated and complex detection systems, and has great application prospects in the field of wearable sensors, which can be used for parallel measurement of multiple parameters.

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Reviewing Editor: Li Qian

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