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).