Humidity sensing is an important non-invasive strategy for respiratory
monitoring, as the exhaled airflow has a higher temperature and humidity
than the inhaled airflow. Therefore, respiratory frequency can be monitored
by measuring the humidity changes during the breathing process. Humidity
sensors can be prepared by embedding sensing materials into flexible substrates.
Thermoplastic polyurethane (TPU) electrospun nanofibers have the characteristics
of large specific surface area, high porosity, and good flexibility, making them
commonly used as flexible substrate porous materials. MXene has excellent
water sensitivity and conductivity, making it very suitable as a sensing material
in humidity sensing applications.
According to Maims Consulting, a joint research team from the First Affiliated
Hospital of Xi'an Jiaotong University and the School of Advanced Materials and
Nanotechnology of Xi'an University of Electronic Science and Technology recently
published a paper titled "MXene/TPU Composite Film for Humidity Sensing and
Human Response Monitoring" in the Advanced Sensor Research journal. The first
author of this paper is Tianqing Liu from Xi'an University of Electronic Science and
Technology, and the corresponding authors are Chief Physician Zhang Guangjian
from the First Affiliated Hospital of Xi'an Jiaotong University, Professor Wu Weiwei
and Lecturer Du Tao from Xi'an University of Electronic Science and Technology.
This research work utilized the excellent hydrophilicity and conductivity of
MXene to coat MXene nanosheets onto chitosan modified TPU electrospun nanofibers
through electrostatic interactions, and prepared MXene/TPU composite films. Based
on this film, humidity sensors were prepared. Based on the principle that the distance
between MXene nanosheets is affected by changes in water molecule concentration,
thereby changing tunnel resistance, MXene/TPU humidity sensors exhibit many
characteristics such as fast response speed (12 seconds), wide humidity response
range (11% -94% relative humidity (RH)), low hysteresis (<7% RH), and high
repeatability. This humidity sensor can be integrated into a mask to distinguish
different respiratory patterns of the human body and accurately monitor respiratory
frequency signals during different movement states. It has broad application
prospects in the field of respiratory monitoring.
Researchers prepared MXene/TPU humidity sensors by coating MXene nanosheets
on chitosan modified TPU pads. The entire preparation process can be divided into
three steps (as shown in Figure 1):
(1) Preparation of Ti3C2Tx? MXene nanosheets;
(2) Preparation and modification of electrospun TPU pads;
(3) Prepare MXene/TPU humidity sensors.
Figure 1 Schematic diagram of the preparation process of MXene/TPU humidity sensor
Subsequently, the researchers measured and characterized the prepared MXene
sensing material and electrospun TPU pad, as shown in Figures 2 and 3.
Figure 2 Characterization of MXene sensing material
Figure 3 Characterization of electrospun TPU liner
Subsequently, in order to study the chemical properties of MXene/TPU composite
films, researchers used techniques such as X-ray diffraction (XRD), Fourier transform
infrared spectroscopy (FT-IR), Raman spectroscopy (Raman), and X-ray photoelectron
spectroscopy (XPS) to characterize them. The results are shown in Figure 4. These results
validate the successful preparation of MXene/TPU composite films and their potential
as humidity sensitive materials. Next, Ag interdigital electrodes are integrated into
MXene/TPU composite films using screen printing technology, and connected to external
circuits through copper wires.
Figure 4 Chemical characterization of MXene/TPU composite film
The sensing mechanism schematic diagram of the MXene/TPU humidity sensor is
shown in Figure 5. As the humidity increases, the spacing between MXene nanosheets
increases, resulting in an increase in the tunnel resistance at the connection (as shown
in Figure 5, right). When the humidity decreases, the spacing between MXene nanosheets
decreases, causing the total resistance to recover (as shown on the left in Figure 5).
Figure 5 Schematic diagram of the sensing mechanism of MXene/TPU humidity sensor
Based on the above humidity sensing principles, a self-made sensing device was used
to characterize the humidity sensing performance of the newly prepared MXene/TPU
humidity sensor, as shown in Figure 6. The performance of the MXene/TPU humidity sensor
is: a humidity response range of 11% -94% RH, hysteresis of<7% RH, high repeatability, and
a response speed of up to 12 seconds.
Figure 6 Performance characterization of MXene/TPU humidity sensor
Finally, in order to analyze physiological information from human respiration, researchers
integrated the MXene/TPU humidity sensor into the mask, and the relevant test results are
shown in Figure 7. Figures 7a to 7c show the monitoring results of three typical respiratory
patterns in a single subject wearing an integrated humidity sensor mask. The results from
Figures 7d to 7h indicate that the MXene/TPU humidity sensor can be used for human
respiratory monitoring, providing valuable respiratory monitoring information under both
resting and different exercise states.
Figure 7 Test results of respiratory monitoring using a mask with integrated MXene/TPU humidity sensor
In summary, researchers successfully prepared MXene/TPU humidity sensors by coating
MXene nanosheets on chitosan modified TPU electrospun nanofibers using electrostatic
interactions. The morphology and chemical properties of MXene/TPU composite films were
characterized by methods such as SEM, EDS, XRD, FT-IR, Raman, and XPS, confirming the
effective binding of MXene sensing materials to TPU substrates. The MXene/TPU humidity
sensor has good performance such as fast response speed, wide humidity response range,
low hysteresis, and high repeatability. The researchers also integrated the humidity sensor
into the mask to identify different breathing patterns and accurately monitor respiratory
signals under different movement states. This paper explains the principle of humidity sensing
by simulating the effect of changes in water molecule concentration on the spacing of MXene
nanosheets, thereby altering the tunneling resistance. This research work not only provides a
new perspective for the development of humidity sensors, but also provides new ideas for the
development of MXene/polymer sensors based on resistance changes.
This research was funded and supported by Shaanxi Provincial Key Research and
Development Program (2022ZDLSF01-04 and 2020GXLH-Y-012), the Open Fund of the State
Key Laboratory of Solid Lubrication of Lanzhou Institute of Chemical Physics, Chinese Academy
of Sciences (LSL-1905), and the Science Foundation of Shandong Advanced Materials and Green
Manufacturing Laboratory (Yantai).
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