Liquid-piston based triboelectric device operates in real-time
Fluid pressure sensors are used in thousands of everyday applications, from monitoring oil lines and detecting water leaks, to ventilation systems and measuring blood pressure. Many existing pressure sensors require large, costly, and complex systems to power them, to send and receive signals, and to analyse data. This has led to a growing interest in using triboelectric nanogenerators (TENGs) as sensors. Such devices are lightweight, low-cost, display high sensitivity, and can generate their own power.
TENGs based on tubes containing fixed volumes of liquid have been frequently proposed for real-time fluid flow monitoring. Writing in Nano Energy [DOI: 10.1016/j.nanoen.2023.108419], a group of researchers, led by Georgia Tech materials scientist Prof. Zhong Lin Wang, has proposed their design for a portable, self-powered fluid pressure sensor powered by “…a liquid piston triboelectric nanogenerator (LP-TENG)”.
The LP-TENG is structurally simple. A PDMS support holds a 3D-printed air reservoir in a central position. This is attached to a PTFE tube, arranged into a simple radial pattern. The other end of the tube is connected to the pressure source under analysis. A series of copper strips attached onto the outer surface of the tube act as electrodes. The liquid column – considered to be a piston in the tube – is deionized water. It acts as one half of the TENG’s friction pair, with the PTFE tube itself acting as the other half. When the LP-TENG is connected to a pressure source, the liquid column moves forward or backward into the reservoir until the pressure in the system equalises, i.e., the pressure of the source is equal to the air pressure within the PTFE tube. The motion of the liquid column can generate charges on the inner wall of the tube via the triboelectric effect. If this happens far from the electrodes, there is no electrical output. However, if it happens close to the electrodes, electrostatic induction (between the PTFE and copper) causes a current to be generated.
Factors that influence the measured electrical signal include the velocity of the liquid column, as well as its length, the tube’s inner diameter, and both the distance between electrodes and their length. The researchers tested the LP-TENG while varying these factors, and produced a final device with a 2 mm tube diameter, and an electrode length, liquid column length, and distance between electrodes all set at 3 mm. By altering the specifics of the electrode array, e.g., positive-negative-positive followed by negative-positive-negative (PNP+NPN), they could detect different parameters of the fluid flow, e.g., direction, pressure, flow rate, etc. The authors say that they achieved an accuracy of the LP-TENG of 0.4 kPa (between 0 kPa and 30 kPa), “…which is superior to most commercial sensors.”
In addition, they made an LP-TENG with six NPN electrode arrays, and connected it to a stepper motor and syringe to stimulate a blood pressure measurement. They compared its performance to that of a standard set-up – a mercury sphygmomanometer, connected to an inflatable cuff. They found that the LP-TENG has a resolution of 10 mmHg, “…which is adequate to accurately identify the blood pressure (BP) level.”
They conclude that their device “is capable of real-time monitoring the fluid status, which can fit the need for diagnosing and detecting the BP level as a reliable POCT device in healthcare products.” They also suggest a host of other potential application areas, including hydrogen production.
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Taotao Zhan, Haiyang Zou, Hengfei Zhang, Peng He, Zhanlei Liu, Junshuai Chen, Maogang He, Ying Zhang, Zhong Lin Wang. “Smart liquid-piston based triboelectric nanogenerator sensor for real-time monitoring of fluid status,” Nano Energy 111 (2023) 108419. DOI: 10.1016/j.nanoen.2023.108419