Electrospinning Machine| Electrospun Lignin/ZnO Nanofibrous Membranes for Self-Powered Ultrasensitive Flexible Airflow Sensor and Wearable Device

Airflow sensors hold significant application value in fields such as environmental monitoring, biomedical applications, aerospace, motion sensing systems, and wearable devices. Traditional airflow sensors primarily rely on mechanical deformation induced by piezoresistive, capacitive, optical, or magnetic effects, or utilize thermal exchange between heating elements and airflow to achieve signal output through resistance changes. However, conventional sensors commonly suffer from issues such as structural rigidity, bulky size, and dependence on external power supply, which limit their development in flexible, portable, and self-powered sensing applications.

In particular, flexible sensors enhance conformability and comfort, enabling stable and continuous signal monitoring, making them widely suitable for wearable devices. Therefore, developing flexible airflow sensors with self-powering capabilities has become an urgent need to expand application scenarios and improve overall performance.

Water evaporation-induced electricity generation (WEIGs) achieves stable voltage and current output through ion-selective migration in electric double layers driven by water evaporation and has been widely applied in small electronic devices and energy harvesting. Since power generation performance is limited by surface water evaporation rates, developing novel airflow sensing methods based on this principle is of great significance.

Electrospun nanofiber membranes combine high specific surface area, high porosity, and excellent flexibility, making them suitable for WEIGs functional materials. Although PAN (polyacrylonitrile) exhibits good mechanical properties, it lacks charge carriers.

Inspired by water evaporation-induced electricity generation, Professor Kai Zhang's team at the University of Göttingen, Germany, pioneered the application of this principle in the field of sensing. Using electrospinning technology, they successfully fabricated a superhydrophilic lignin/ZnO nanofibrous membrane and incorporated a layer of ZnO nanoparticles on its surface. This structure can generate at least 100 mV of voltage, enabling self-powered signal transmission.

The nanofibrous membrane exhibits exceptional sensitivity to airflow variations, functioning as an ultrasensitive flexible airflow sensor. The sensor demonstrates outstanding performance, including a rapid response time (0.65 s), a broad detection range (with a lower detection limit of 0.25 m/s and an upper limit of 3 m/s), and highly precise airflow velocity measurement. Additionally, the device can serve as a wearable sensor for sweat monitoring, motion detection, and respiration monitoring (capable of accurately detecting breathing frequency, intensity, and speech patterns).

This self-powered, ultrasensitive, and flexible lignin/ZnO airflow sensor provides novel technological insights and application potential for the development of smart textiles and wearable electronics. The research findings were published in Advanced Materials under the title "Electrospun Lignin/ZnO Nanofibrous Membranes for Self-Powered Ultrasensitive Flexible Airflow Sensor and Wearable Device." The first author of the paper is Yifei Zhan from the Institute of Wood Technology and Wood-Based Composites at the University of Göttingen, with Professor Kai Zhang from the same university serving as the corresponding author.

1. Based on the water evaporation-induced electricity generation (WEIGs) mechanism, this study designed and constructed a novel self-powered ultrasensitive flexible airflow sensor. Using electrospinning technology, lignin/polyacrylonitrile nanofibrous membranes (LP-NF) were fabricated, followed by in-situ growth of ZnO nanoparticles on their surface through solvothermal reactions to obtain ZnO-modified nanofibrous membranes (LP-ZnO-NF).

2. Calcium chloride (CaCl₂) was introduced at one end of the LP-ZnO-NF membrane. Leveraging its excellent hygroscopic properties, the membrane maintains localized moisture under ambient humidity, successfully eliminating dependence on external liquid water sources and significantly expanding the sensor's application potential in complex environments.

3. This self-powered flexible airflow sensor can not only monitor the wearer's sweat secretion, respiratory frequency, and intensity changes in real-time but also sensitively detect human movements and environmental airflow variations. It integrates multiple functions including respiration monitoring, motion state detection, and speech pattern recognition, demonstrating broad application prospects in smart wearables, health monitoring, and human-machine interaction fields.

Fig. 1. LP-ZnO-CaCl₂-NF membrane fabrication, working principle, and applications.

The research team systematically characterized the structural, chemical, and morphological properties of the nanofibrous membrane, confirming its outstanding comprehensive performance. Scanning electron microscopy (SEM) images revealed that the unmodified lignin-polyacrylonitrile nanofibers exhibited well-aligned orientation with an average diameter of approximately 346 nm. After ZnO modification, the fiber diameter increased to 703 nm, with clearly visible ZnO nanoparticles uniformly distributed on the fiber surfaces.

Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses further verified the successful modification of ZnO on the nanofiber surfaces, while X-ray diffraction (XRD) patterns confirmed the crystalline structure of ZnO. Water contact angle measurements demonstrated that the incorporation of both ZnO and lignin significantly enhanced the membrane's hydrophilicity, enabling complete water droplet absorption within 1 second - a characteristic that facilitates efficient moisture transport and evaporation.

Tensile and bending tests showed that the ZnO modification substantially improved the membrane's toughness and flexibility, ensuring reliable stability and durability for wearable device applications.

Fig. 2. Structural, chemical, and morphological characterization.

In airflow response performance tests, the nanofibrous membrane generated approximately 105 mV voltage even in static air conditions. The researchers systematically evaluated the sensor's response characteristics by controlling variables including nitrogen flow rate, angle of incidence, and detection position.

The results demonstrated an exceptionally fast response time of 0.65 seconds, with the voltage change showing excellent linear correlation to airflow velocity within the 0.25-3 m/s range, achieving a sensitivity of 1.79% (m/s)⁻¹. Additional tests revealed directional sensitivity, with the strongest response occurring at the top region - attributed to weaker moisture replenishment and more pronounced evaporation changes in this area.

Structural optimization experiments confirmed that aligned nanofiber arrangements delivered faster and stronger responses compared to randomly distributed structures. Long-term stability and cycling reliability tests showed consistent performance during continuous 4-hour operation and 75 on-off cycles without significant degradation, outperforming mainstream airflow sensors developed in recent five years.

Fig. 3. Airflow sensing performance.

In practical application testing, the research team demonstrated the broad application potential of this nanofibrous membrane in multifunctional wearable devices. The sensor's ability to monitor airflow changes generated by passing toy cars enables simple speed measurement functions, featuring self-powering capabilities suitable for environmental monitoring and auxiliary speed detection applications.

By incorporating calcium chloride (CaCl₂) at one end of the membrane to create a gradient hygroscopic structure, the material's performance in high-humidity environments was significantly enhanced while effectively eliminating dependence on liquid water sources. Artificial sweat simulation tests confirmed the membrane's sensitivity to perspiration changes, allowing real-time sweat monitoring. When integrated into clothing, the material could effectively detect nearby human movement, providing safety warnings for visually impaired individuals or in low-visibility environments.

Furthermore, researchers integrated the nanofibrous membrane into face masks, achieving precise monitoring of the wearer's respiratory status. The sensor produced clear, distinguishable response curves during resting, exercise, and normal breathing states, and even demonstrated preliminary speech pattern recognition capability by differentiating vocalization states.

Fig. 4. Applications of LP-ZnO-NF and LP-ZnO-CaCl₂-NF.

This study innovatively integrates water evaporation-induced electricity generation with flexible airflow sensing, successfully developing a novel sensor that combines self-powering capability, ultrahigh sensitivity, wide detection range, and excellent flexibility. Through structural design optimization, the researchers have significantly expanded its application scenarios across various wearable devices.

With further improvements in fabrication processes and functional integration, this class of nanofibrous membranes shows promising potential for widespread applications in smart textiles, health monitoring, and human-machine interaction systems, thereby advancing the continuous development of flexible electronics technology.

Original link: https://doi.org/10.1002/adma.202502211