New Application Fields of High-Voltage Electrostatic Flocking—Sensors

Sensors (Part 1) — Airflow Sensors
The sensing principle of existing airflow sensors is primarily based on measuring mechanical deformation induced by external airflow. [81–83] This is characterized by changes in the electrical resistance of piezoresistive materials [13,14,37] or voltage changes in piezoelectric materials. [84,85] To sensitively detect weak airflow, the sensor must amplify the mechanical deformation response caused by subtle airflow variations to obtain a measurable output signal.

Due to their high aspect ratio, hair-like structures are highly sensitive to minute forces, capable of converting tiny displacements at the ciliary contact points into changes in electrical resistance. Inspired by spider hairs, researchers have employed various manufacturing techniques such as in-situ growth, 3D printing, and electrohydrodynamic inkjet printing to develop airflow sensors with hair-like structures. Among these, electrostatic flocking technology has demonstrated unique value in this field due to its low cost, large-scale production capability, and ease of operation. Compared to traditional bulky sensors, those fabricated using electrostatic flocking technology are not only lighter in weight but also exhibit significant performance improvements.

Shen et al. developed an airflow sensor using a two-dimensional planar array of 319-micrometer-long carbon fibers (Figure a below). This sensor specializes in airflow detection, featuring an ultra-low detection limit (0.053 m/s), a broad frequency response range (0.053–2.66 m/s), multi-angle response characteristics (0°–90°), and a fast response time (1.7 s). Similarly, Luo et al. fabricated a fiber-based airflow sensor by flocking 3 mm carbon fibers onto a one-dimensional surface of PVA fibers. [14] This sensor generates differentiated responses based on varying airflow speeds (Figure b below), demonstrating a rapid response time (0.103 s), a low detection limit (0.068 m/s), and a wide detection range (0.068–16 m/s). Beyond basic airflow response functionality, this sensor also possesses capabilities for sound recognition and motion monitoring.

Additionally, by employing an electrospinning process to coat conductive carbon fibers onto a polyurethane substrate, a highly sensitive flexible dual-response sensor for airflow and strain was successfully developed. [37] This sensor can simultaneously detect airflow and strain signals, exhibiting high sensitivity (157.5% for airflow speeds ranging from 4 to 16 m/s), a fast response time of only 37 ms, and performance significantly superior to pure airflow sensors and previously reported airflow sensors. [37]

The differences in response time and detection range between Shen's sensor and Luo's sensor may stem from factors such as fiber density, length, or the one-dimensional/two-dimensional/three-dimensional morphology of the substrate. In the future, exploring the use of fibers softer and more deformable than carbon fibers as the flocking layer may yield more sensitive deformation responses and optimize the sensor's reaction time.

The application fields of electrostatic flocking airflow sensors are extensive, including respiratory monitoring, sound detection, motion detection, and micro-force monitoring. Whether it is the微弱力 generated by airflow, vibrations caused by sound waves during speech, or minute forces during motion, all can disturb the conductive flocked fibers, causing them to separate or cross, thereby altering the sensor's resistance and triggering changes in the output signal value.

a) Manufacturing process diagram of the SCFN airflow sensor. Reprinted with permission. [13] Copyright ©2022 Wiley-VCH GmbH.
b) Two sets of cyclic sensing curves of the sensor under different airflow speeds. Reprinted with permission. [14] Copyright ©2022 Royal Society of Chemistry.

References
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