MXene/TPU Composite Film for Humidity Sensing and Human Respiration Monitoring

Human respiration reflects abundant physiological information and could enable non‐invasive monitoring, providing information about various biological parameters such as respiration rate and depth. The rapidly growing field of humidity sensors bring forward the requirement for good performance. In this work, by virtue of good hydrophilicity and conductivity, a MXene/thermoplastic polyurethane (TPU) composite film is prepared by coating MXene nanosheets on chitosan‐modified TPU electrospun nanofibers via electrostatic interactions, for fabricating a humidity sensor. Based on the principle that the tunnel resistance changing with water molecules influences the distance of MXene nanosheets, the MXene/TPU humidity sensor exhibits fast response (12 s), wide humidity response range (11–94% RH), low hysteresis (<7% RH), and excellent repeatability. The humidity sensor can be assembled with a face mask for distinguishing different human respiration patterns and accurately monitoring respiratory signals during different physical activities, suggesting its promising applications in the fields of respiratory monitoring.


Introduction
Multifunctional electronic devices have sparked tremendous research interests due to their great potential applications in human body motion, [1] healthcare monitoring, [2] human−machine DOI: 10.1002/adsr.202300014interactions, [3] electronic skins, [4] etc.Among these electronic devices, wearable electronic devices play an essential role in monitoring various biological parameters, such as perspiration, [5] blood pressure, [6] heart rate, [7] oxygen saturation (SpO 2 ), [8] etc., which can provide valuable physiological information for healthcare monitoring and disease diagnosis.Among them, respiratory monitoring can provide helpful information as a reference for daily health observation.Especially, the global pandemic of COVID-19 poses a huge challenge in public health, respiratory monitoring can not only addresses highly contagious respiratory diseases, but also guides the physical activity after recovery from respiratory disease. [9]Humidity sensing is a significant non-invasive strategy for respiratory monitoring because expiratory airflow is warmer containing higher humidity than inspiratory air. [10]Thus, variations in humidity of respiratory process can be used for indicating the respiratory rate. [11]Specifically, humidity sensors can clearly distinguish the inhalation/exhalation time and the depth of each respiratory cycle by detecting the change of relative humidity (RH) during the inhale and exhale processes. [12]In addition, respiratory monitoring based on RH of the airflow is barely affected by the environmental factors, such as temperature, sound noise, and so on, which guarantees the data reliability. [13]So far, many efforts have been made to overcome challenges involved in the trade-offs among sensitivity, response speed, stability, etc. [14] Humidity sensors can be fabricated by introducing sensing materials into flexible substrates. [11,13,15]Considering the demand of flexibility, permeability, biocompatibility, and comfortability in wearable devices, a wide range of porous materials can be used as flexible substrates, such as fabrics, [16] cellulose paper, [17] and polymers. [18]The thermoplastic polyurethane (TPU) electrospun nanofibers are common porous materials for the flexible substrates, [19,20] by virtue of their large specific surface area, high porosity, and good flexibility.Meanwhile, the sensing materials play a critical role in the sensing performance of humidity sensors.2D materials (such as graphene, [21] graphene oxide, [22] carbon nanotubes, [23] etc.) have been used for humidity sensing, however, the inherent hydrophobic characteristics of graphene led to significantly prolonged response times, lasting up to several hundred seconds, or minor normalized responses. [11,18,24,25]Although graphene oxide exhibited enhanced response, its electrical conductivity was compromised, resulting in slow response speed. [26]In the context of metal oxide semiconductors, the dominant sensing mechanism is contingent upon the adsorption of water molecules and their influence on surface conductivity. [27,28]31] Moreover, the conductivity of MXene is greater in the plane direction as compared to perpendicular to it. [29]In case of the intercalation of water molecules within the interlayer of MXene, the low electrical conductivity of water, along with an increase in the layer-to-layer repeating distance, leads to an increase in the resistance of MXene. [32][35][36][37][38][39][40][41] For instance, Zhao et al. reported the MXene-based smart fabric by depositing MXene nanosheets onto cellulose fabrics, which showed highly sensitive humidity response. [15]Wang et al. developed a flexible humidity sensor based on electrospinned poly(vinyl alcohol)/MXene nanofibers film with self-powering by the piezoelectric nanogenerator.Their sensor exhibited the good performance of a large response, fast response/recovery time, low hysteresis, and excellent repeatability. [39]An et al. prepared MXene/Polyelectrolyte multilayers by using layer-by-layer assembly for humidity sensing.However, the weak affinity between MXene and polymer fibers because the hydrophilic groups of MXene is incompatible to hydrophobic nature of polymers, will result in the exfoliation of sensing materials during the deformation process, which restricts the stability of humidity sensors. [11]Although the surface modification can tune MXene surface groups and increase its compatibility with hydrophobic polymers, [42] the process of modification involves complex reactions that may affect its conductivity.Therefore, it is still a major challenge to incorporate MXene sheets with polymer fibers while maintaining their sensing performance.
In this paper, by employing MXene as the sensing material, we converted the TPU electrospun nanofibers into multifunctional MXene/TPU-based humidity sensors by modifying the surface of TPU fibers with chitosan (CS).The negative charge on the surface of MXene sheets interacted with the positive charge of CS, resulting in the surface of TPU fibers coating with a MXene sensing layer.With increasing humidity, water molecules intercalate into the MXene/TPU multilayer, leading to an increase in the sheet-to-sheet distance, thereby increasing the tunneling resistance between MXene sheets.With decreasing humidity, the reverse process occurs, then decreasing the tunneling resistance between MXene sheets.Based on this sensing mechanism, we demonstrate that our humidity sensor has fast response, wide humidity response range, low hysteresis, and excellent repeatability.We also assembled MXene/TPU-based humidity sensor with a face mask, which can distinguish different degrees of breathing and continuously record respiratory signals accurately during different physical activities.Such MXene/TPU-based humidity sensors exhibit potential applications in the field of respiratory monitoring.

Fabrication and Characterization of the MXene/TPU Humidity Sensor
The MXene/TPU humidity sensor was fabricated by coating MXene nanosheets on the chitosan-modified TPU mats.The whole process can be divided into three steps: i) preparation of Ti 3 C 2 T x MXene nanosheets, ii) preparation and modification of electrospun TPU mats, and iii) fabrication of MXene/TPU humidity sensors, which are schematically illustrated in Figure 1.The successful synthesis of MXene nanosheets is crucial for humidity sensors.Firstly, the MXene nanosheets were prepared by etching the Al atom layers from Ti 3 AlC 2 (MAX) powders with LiF/HCl according to the previous reported method. [43]he scanning electron microscopy (SEM) images of nacrelike layered stacking of Ti 3 C 2 T x after the LiF/HCl etching (Figure 2b,c) confirm successful exfoliation of Ti 3 AlC 2 powders, where the layers of Ti 3 C 2 T x are clearly separated from each other compared to the unreacted powders (Figure 2a) which show the compact layered structure.Further sonication resulted in abundant monolayers MXene nanosheets with lateral sizes ranging from a few micrometers to several hundreds of nanometers according to Transmission Electron Microscopy (TEM) images in Figure 2d.The selected area electron diffraction (SAED) pattern (inset in Figure 2d) indicates the hexagonal symmetrical pattern of the plane originating from the crystal structure of Ti 3 C 2 T x . [44]The X-Ray diffraction (XRD) spectrum of as-prepared MXene is shown in Figure 4a.[46][47] The energy dispersive spectrum (EDS) element mappings (Figure 2e) of a MXene nanosheet confirm the Ti, C, O, and F elements uniformly distributed.The thickness of single MXene sheet was investigated by atomic force microscope (AFM).From the height cutaway view (Figure 2f), the thickness is about 1.48 nm, which indicates that MXene sheet is monolayer. [48]or the second step, the TPU mats were prepared by electrospinning technology in a strictly controlled environment (the temperature was controlled at 30 °C and the relative humidity was settled at 50%).The SEM images in Figure 3a,c show the morphology of the electrospun TPU nanofibers, which exhibit continuous, homogeneous, and smooth fibrous structures.The cut TPU mats were treated with 3 mol L −1 HCL and then soaked in a mixture solution consisting of 0.5 mol L −1 HCl and 0.28 mg mL −1 chitosan to modify TPU mats for 4 h.After soaking, the chitosan-modified TPU (CS-TPU) mats were washed with DI water to remove the impurity.Comparing with pristine TPU fibers, the CS-TPU fibers exhibit a slight roughness in the morphology (Figure 3c and Figure S1d, Supporting Information).The contact angles of water on pristine TPU, HCl treated TPU and CS-TPU mats are 111.01°,95.73°, and 116.27°(insets in Figure 3a, Figure S1a,b, Supporting Information), respectively, further indicates the successful modification of chitosan on TPU fibers.Then, the CS-TPU mats was soaked in MXene colloidal solution overnight at low temperature.With abundant positively charged chitosan, the CS-TPU can absorb MXene nanosheets by electrostatic interaction to form MXene/TPU composite fibers. [49]From the SEM images of MXene/TPU fibers (Figure 3b,d and Figure S2, Supporting Information), it can be clearly seen that MXene nanosheets were abundantly wrapped on the CS-TPU fibers, with the MXene/TPU mats surface becomes more hydrophilic (contact angles of water change from 111.01°on pristine TPU mats to 92.71°on MXene/TPU mats, insets in Figure 3a,d).Comparing with pristine TPU fibers (Figure S3, Supporting Information), the EDS elemental mappings of MXene/TPU fibers (Figure 3e) illustrate the C, O, F, Ti, and N elements uniformly distribute on the fibers.
XRD, FT-IR, Raman, and XPS analysis were performed for studying the chemical characteristics of MXene/TPU composite films.Figure 4a shows the XRD spectra of TPU, MXene, and MXene/TPU composite films.The broad diffraction peak located at center 2 ≈ 20°(range from 15°to 30°) derives from the short-range of regular ordered structure of the hard and soft domains and the existence of the amorphous phase disordered structure of the TPU. [50]The MXene/TPU composite film shows both characteristic diffraction peaks of MXene and TPU components.Furthermore, in the MXene/TPU XRD pattern, the broad peak derives from TPU becomes weaker and broader, and the specific (002) peak (6.2°) derives from MXene moves to left comparing with the peak of pure MXene (6.7°) indicating that expanded interlayer spacing between MXene nanosheets due to the combination of TPU fibers. [11,51,52]As shown in the FT-IR spectra of TPU (Figure 4b), the peaks at 3331 and 1529 cm −1 are assigned to the stretching and bending vibration of secondary amine.The peaks at 2955 and 1077 cm −1 are assigned to the stretching vibration of methylene and ether group from the soft segment of TPU, respectively.Amino and ester group in the urethane linkage (-H-N-COO-) were assigned to peaks at 511 and 1728 cm −1 , respectively. [51,53]Comparing to TPU, the vibration peaks of amino and carbonyl groups of MXene/TPU composite show slight blue-shifting (511-508 cm −1 and 1728-1726 cm −1 , respectively), indicating the amino and carbonyl groups of TPU fibers probably bonding with functional groups from MXene by interface interaction and hydrogen bond, resulting in effective load transfer during dynamic stretching. [51]However, the peaks of MXene do not appear in the spectra, probably because of weak sensitivity of MXene and small loading amount of MXene on TPU fibers. [54]In the Raman spectra (Figure 4c), typical characteristic peaks of MXene also appear in the spectra of TPU/MXene composite (peaks at 200 cm −1 belong to the out-ofplane (A 1g ) vibration of Ti, O, and C atoms, peaks at 722 cm −1 belong to another A 1g vibration of C atoms, peaks at around 388 and 607 cm −1 belong to in-plane (E g ) vibrations of surface groups [55] ).Meanwhile, the peaks derive from TPU unsurprisingly retain in the spectrum of TPU/MXene composite (Figure 4d, peak at 1732 cm −1 belongs to (C = O) from ester group in the urethane linkage, peak at 1445 cm −1 belongs to (CH 2 ) from the soft segment, peak at 1311 cm −1 belongs to urethane amide [56] ).XPS is a powerful tool to study surface chemical compositions of materials.From the XPS survey spectra (Figure 4e), all the typical peaks Ti 2p (455.6 eV), Ti 2s (562.8 eV), and F 1s (684.8 eV) derived from MXene are observed from the XPS spectrum of MXene/TPU composite.The Ti 2p high-resolution XPS spectrum (Figure 4f) shows that the signals at 455.0 and 461.0 eV (marked as C-Ti-T x 2p 3/2 and C-Ti-T x 2p 1/2 , respectively) correspond to Ti─C bonds, and the signals at 458.7 and 464.7 eV (marked as TiO 2 2p 3/2 and TiO 2 2p 1/2 , respectively) correspond to Ti─O bonds. [45,57]The C 1s high-resolution XPS spectrum of MXene (Figure S4, Supporting Information) shows core level peaks located at 281.0, 284.8, and 288.8 eV corresponding to C─Ti, C─C, and C─O bonds.After interacting with TPU, the core level peaks of C 1s, O 1s, and F 1s high-resolution XPS spectrum of MXene/TPU (Figure S5, Supporting Information) have changed, which may cause by the combination of MXene and TPU fibers.The above analyses verify the successful preparation of MXene/TPU composite films which can be used as the humidity sensing materials.Finally, we assembled the Ag interdigital electrodes on the MXene/TPU composite films by screen printing, and connected with external circuit through copper wires.

Analysis of the Humidity Sensing Mechanism
The proposed humidity sensing mechanism of the MXene/TPU sensor is schematically illustrated in Figure 5.The total resistance of MXene/TPU composite film (R flim ) depends on the intrinsic resistance of MXene nanosheets (R 1 , R 2 , and R 3 ) and tunneling resistance at the junctions (R t1 and R t2 ).This is represented schematically in Figure 5 and can be expressed with Equation (1): Tunneling resistance at the junctions involves the intercalation of water molecules at the interface between MXene nanosheets, which increases the MXene interlayer distance, resulting in an alteration from ohmic-type to ohmic/capacitive-type behavior that increases resistance at junctions. [22,29,31,32,58]Thus, the humidity sensing mechanism of our MXene/TPU sensor can be explained as follows: in case of the increase of humidity, the sheet-to-sheet distance of MXene increases, causing the increase of the tunneling resistance at the junctions (right in Figure 5).Once the humidity decreases, the interlayer distance of MXene nanosheets decreases, resulting in the recovery of total resistance (left in Figure 5).

Humidity Sensing Performance of the MXene/TPU Humidity Sensor
Based on this humidity sensing mechanism, the humidity sensing performance of the prepared MXene/TPU humidity sensors was studied by using a homemade sensing setup.Figure 6a demonstrates the dynamic response-recovery curves of the hu-midity sensor recorded during humidification-dehumidification cycles by gradually increasing RH from initial 11-94% then gradually decreasing RH back to initial RH.It can be seen that the response increases to a plateau with the humidity increasing, and the highest response was achieved at 94% RH, indicating that more water molecules are adsorbed by the MXene sensing materials when humidity increasing.During dehumidification cycles, water molecules desorb from the interface between MXene nanosheets when humidity decreasing, thus resulting in the change of instantaneous current.Furthermore, the current baseline of the response-recovery curves keeps steady during the test, suggesting that water molecules are barely trapped in the interface of sensing materials during recovery with the balance of adsorption-desorption, resulting a relatively fast recovery speed.For further clarifying the adsorption-desorption characteristics of water molecules in MXene sensing materials, the hysteresis was investigated.For humidity sensors, the hysteresis is defined as H = (R B −R F )/S, where R B and R F represent the response of desorption curve and adsorption curve, respectively.S represents the sensitivity, which can be calculated by the slope of the tangent line of fitting curve at each point. [11,13]As shown in Figure 6b, the hysteresis decreased gradually with the humidity increased, specially, almost near zero at highest 94% RH, verifying that the MXene/TPU humidity sensor has good reversibility during the adsorption-desorption process of water molecules.The exponential fitting curve of the response variations versus RH is shown in Figure 6c.The dependence of normalized response variations on RH is described by fitting equation: Response (%) = 1.2590exp(RH/31.824)+ 1.6248, and the regression coefficient R 2 is 0.994, which suggesting the MXene/TPU humidity sensor accords with exponential response depending on RH, consistent with previously reported MXene-based humidity sensor. [13]Considering the application environment, the continuous response between 11% RH and ambient humidity was investigated.As shown in Figure 6d, the response in ambient humidity is about 4.3%, and the corresponding RH can be calculated by the above exponential fitting equation, which is 24% consistent with measured value.The repeatability of MXene/TPU humidity sensor was tested between 11% and three different RH (33%, 58%, and 84% RH) for five cycles (Figure 6e).The humidity responses have hardly any drifting, indicating good repeatability of the humidity sensor.From Figure 6f which is the  magnified response curves in the red frame region in Figure 6e, the response and recovery times under 33%, 58%, and 84% RH can be read, which are 12-16 and 30-32 s, respectively.[61][62][63][64][65] In order to investigate the long-term stability of the humidity sensor, the electrical response was tested of 100 cycles between 11% and 75% RH (Figure S6, Supporting Information).It was observed that the humidity response drift was not obvious, and the response declined merely by 0.01, suggesting the good repeatability of the humidity sensor.The above humidity sensing performances of this MXene/TPU humidity sensor exhibit fast response, wide humidity response range, low hysteresis, and excellent repeatability.The competitive performances of the developed MXene/TPU humidity sensor compared with other humidity sensors are listed in Table S1 (Supporting Information).

Human Respiration Monitoring
The human body inevitably breathes during daily physical activity.Respiration monitoring can provide various biological parameters such as inhalation/exhalation time and the depth of each respiratory cycle, which can reflect the cardiorespiratory state in real-time.The breathing process includes two periodic stages: air containing O 2 being inhaled into the lungs and CO 2 being exhaled in the air released through the nose or mouth. [66]The entire process from the inhalation to exhalation is known as a respiration cycle.As shown in Figure 5, airflow can be detected for human respiration monitoring since exhaled air is warmer, which has higher humidity and contains more water molecules than inhaled air.
For the purposes of analyzing the physiological information from the human respiration, we assembled the MXene/TPU humidity sensor with a face mask.As shown in Figure 1, the humidity sensor was embedded in the middle layer of the face mask for avoiding the contamination from the saliva.When the humidity sensor serves as wearable electronic devices, the influence from deformation of sensors should be considered.Initially, the bendable property of the MXene/TPU humidity sensor is essential for practical applications because the sensor is integrated into flexible and wearable face mask.Thus, the resistance variations of the MXene/TPU composite film upon bending deformations were measured.The film was subjected to different bendable states which were described by bending radius (r) and bending angle ().As shown in Figure S7a (Supporting Information), the bending angle increased from 0°, 73°, 100°, 147°, and 180°-210°in which angle the film nearly folded, meanwhile, the bending radius (r) decreased from ∞, 9.06, 5.67, 2.88, and 2.41-1.61mm. Figure S7b (Supporting Information) shows the normalized resistance remained stable at ≈1.0 during the whole dynamic bending test, indicating the as-prepared MXene/TPU composite film featured excellent mechanical stability and flexibility.The reason for this good flexible performance was assigned to MXene nanosheets were abundantly wrapped on the TPU fibers to form conductive networks.Furthermore, in order to explore the influence of the sensor bending on the electrical signal, we tested the variations of normalized current (ΔI/I 0 : where ΔI = I-I 0 , I is the current under lateral strain, I 0 is the current without lateral strain) under different bending angles (Figure S8a, Supporting Information).As the compression distance increases, the compression deformation tends to be saturated, so the current variation tends to be balanced.When under the 30°bending angle (the mask does not exceed this degree of bending normally), the current variation is still within 7%, indicating the humidity sensor is impervious to the influence of the deformation causing by the fluctuation of face mask during respiration.Additionally, the influence of the temperature on the humidity sensor was also be considered.We collected the response of the MXene/TPU-based humidity sensor in the temperature range of 10-50 °C.During the test, corresponding humidity atmospheres were achieved through saturated binary salt solutions and were prepared in scintillating vials with specific humidity values according to Greenspan's least squares equations. [67,68]As shown in Figure S9 (Supporting Information), as the temperature increases, the responses increase from 14% at 10 °C to 30% at 48 °C, even though the humid-ity of atmospheres decreased from 86.8% to 81.2%.This can be explained as the negative temperature coefficient of the MXene nanosheets. [69]The MXene material, which has been subjected to etching by Ti 3 AlC 2 ceramic powder, can be classified as a transition metal carbide exhibiting a negative temperature coefficient behavior typical of metals.Specifically, at elevated temperatures, the charge carrier mobility of the MXene is enhanced. [52]s the conductivity of the MXene/TPU composite film was positively correlated with the temperature, the instantaneous current of the humidity sensor increased, resulting in the increase of the response.In general, exhaled gases are humid and have a certain temperature.According to the Haick et al.'s report, the temperature range of exhaled breath is about 31-35 °C. [70]Considering the service temperature of the humidity sensor which ranges from the ambient temperature (25 °C) to the temperature range of exhaled breath (31-35 °C), the response only increases 1.9%, namely from 23.0% at 25 °C to 24.9% at 35 °C (Figure S9, Supporting Information).Combined with the respiratory monitoring results (Figure 7a-c), it could be deduced that human respiration mainly depended on the humidity sensitivity rather than the temperature sensitivity of the MXene/TPUbased humidity sensor.To sum up, the sensor may be a promising candidate for electronic masks with a humidity-sensing function.
As shown in Figure 7a-c, three representative respiratory patterns were monitored by a single subject wearing the humidity sensor integrated face mask.As mentioned before, exhaled air contains more water molecules than inhaled air.Therefore, when the subject was exhaling, the respiratory response curve was rising, while the volunteer was inhaling, the curve was declining.The respiration rate (numbers of breath per minute) and depth (peak-to-peak amplitude) can be determined from the response curves. [11,71]As expected, normal breath exhibits mellow and clear peaks and valleys, with respiration rate of 14 times min −1 and the lowest respiration depth among three patterns.Deep breath exhibits the deepest respiration depth consequently reduces respiration rate to 10 times min −1 .Rapid breath exhibits dense and acuminate peaks and valleys, corresponding to respiration rate of 31 times min −1 and moderate respiration depth.As shown in Figure 7d-f, the first derivative plots of original response curves eliminate the baseline of the response curve, which only reflect the degree of variations in the respiration curve for helping to expose details that are not intuitive in original curves. [11,13]Next, we continuously monitored the respiratory response of a series of motion states.As shown in Figure 7g, the subject firstly stood for a while, then started walking and running for about 100 s, finally stood again.The respiration rate while running is obviously largest.From the magnified response curves in Figure 7g,h, the respiration depth while running is also largest.These results demonstrate that the MXene/TPU humidity sensor can be used for human respiration monitoring, providing valuable breathing information both at rest and during different motion states.

Conclusion
In conclusion, we have successfully fabricated the MXene/TPU humidity sensor by coating MXene nanosheets on the chitosan-modified TPU electrospun nanofibers via electrostatic interaction.The morphology and chemical characteristics of the MXene/TPU composite film were characterized by SEM, EDS, XRD, FT-IR, Raman, and XPS, confirming the successful combination of MXene sensing materials and TPU substrate.The MXene/TPU humidity sensor exhibits the good performance including fast response, wide humidity response range, low hysteresis, and excellent repeatability.We also assembled this humidity sensor with a face mask for distinguishing different degrees of breathing and accurately monitoring respiratory signals during different physical activities.The humidity sensing mechanism is also explained by the modeling of the tunnel resistance changing with water molecules' influence on the distance of MXene nanosheets.This work provides a new perspective for the development of not only humidity sensors, but also MXene/polymer sensors based upon changes in resistance.
Preparation of Ti3C2Tx MXene Nanosheets: Ti 3 C 2 T x MXene nanosheets were synthesized via previously reported minimally intensive layer delamination (MILD) method.In brief, 1.6 g LiF was dissolved into 20 mL HCl (9 mol L −1 ) with continuous stirring to prepare etching agent, and then 1 g Ti 3 AlC 2 MAX phase was gently added to avoid the side-reactions.The mixture was maintained at 35 °C and continuously stirred for 24 h.After that, the product was washed with deionized water (DI water) for several times until the pH value of the supernatant greater than 6.Then the product was collected by centrifugation and redispersed into DI water before intercalation and the suspension was ultrasonicated for 1 h under Ar protective atmosphere in an ice-bath.Finally, the colloidal solution containing mono-/multi-layer Ti 3 C 2 T x MXene nanosheets can be obtained after centrifugation at 3500 rpm.
Preparation of Electrospun TPU Mats: The DMF and THF (volume ratio: 3:7) were mixed as a solvent to dissolve the TPU particles and 18 wt% TPU was added into the solvent which was then magnetically stirred for 8 h to ensure complete dissolution.The obtained transparent homogeneous solution was filled into injector for the subsequent electrospinning process.The PU fibers network was electrospun for 8 h with a feed rate of 1.0 mL h −1 under 14 kV applied voltage and the distance between spinneret tip and fixed receiving plate was 15 cm.In addition, to improve the quality of electrospun PU nanofiber, the surrounding environment should be strictly controlled, i.e., the temperature was controlled at 30 °C and the relative humidity was settled at 50%.After electrospinning the obtained TPU mats were dried at 60 °C overnight to remove residual solvent.
Fabrication of MXene/TPU Humidity Sensors: The electrospun TPU mats were modified by positively charged chitosan to absorb MXene nanosheets via electrostatic interaction.In detail, the cut TPU mats (10 × 20 mm 2 rectangular samples) were ultrasonic cleaned (5 min) and socked in ethanol (1 h) to remove impurity.After socking in HCl solution (3 mol L −1 ) for 4 h and washing with DI water, the TPU mats were put into a mixture solution consisting of 0.5 mol L −1 HCl and 0.28 mg mL −1 chitosan to modify TPU mats which were washed with DI water after 4 h soak.The modified TPU mats was then socked in MXene colloidal solution overnight at low temperature, and the MXene/TPU sensing layer was successfully prepared after washing with DI water and drying under vacuum at 40 °C.The conductive silver electrode was printed on MXene/TPU sensing layer using an interdigital silk-screen and connected with external circuit through copper wires.
Characterization: The morphologies of samples were observed by field-emission scanning electron microscope (FE-SEM, FEI Apreo HiVac, 5 kV) and transmission electron microscopy (TEM, JEOL JEM-2100F, 200 kV).Energy dispersive X-ray spectroscopy (EDS) was performed in the same instrument using a silicon drift detector (X-Max N 80, Oxford Instruments, UK) at a beam voltage of 15 kV.AFM were measured by Cypher ES (Asylum Research) in tapping mode.The crystallographic information was examined by X-ray diffraction spectroscopy (XRD, Bruker D8 Advance Xray diffractometer, Cu-K radiation).The chemical composition was determined by XPS using a Thermo Scientific K-Alpha + with a monochromatic Al K radiation, FT-IR spectrometer (Nicolet iS10, Thermo Scientific, USA) and Raman spectrometer (Renishaw-inVia).
Sensing Performance Test: The humidity sensing was performed in a homemade testing system at room temperature (25 °C).The RH was calibrated by OIML R 121.In brief, 100 mL saturated salts solution (i.e., LiCl, MgCl 2 , NaBr, NaCl, KCl and KNO 3 ) were added to 500 mL glass bottle, and the volume above the container was constant humidity (corresponding to 11% RH, 33% RH, 58% RH, 75% RH, 84% RH, and 94% RH for different solution).An electrochemical workstation (CHInstruments, CHI660E, China) was used to measure and record the current signals under a constant bias of 1 V. Before testing, the exact of RH were calibrated by a commercial industrial hygrometer (Testo 610, range 0-100% RH, resolution 0.1% RH, Testo Instruments).The experiments involving the human respiration monitoring of the MXene/TPU-based humidity sensors were performed with the full, informed consent of all participants, who are also authors of the manuscript.In addition, ethics committee approval was not required to perform these experiments.
Statistics Analysis: The sensor response was defined by Equation (2): [52] Reponse (%) = where I is the corresponding instantaneous current at the target humidity, while I 0 is the initial current at 11% RH.The hysteresis of humidity sensor was expressed as Equation (3): [13] where R B and R F represent the electrical response of desorption curve and adsorption curve, respectively.S represents the sensitivity.In Figure 6c, the mean value represents the average responses of the MXene/TPU-based humidity sensors under different humidity testing conditions.Results are depicted as mean values ± standard deviation derived from three independent experiments.

Figure 2 .
Figure 2. Characterization of MXene sensing materials.a) SEM images of Ti 3 AlC 2 (MAX) and b,c) Ti 3 C 2 T x MXene sheets.d) TEM image of the MXene nanosheets and corresponding indexed SAED pattern (inset).e) EDS elemental mappings of MXene nanosheets.f) AFM images of the MXene nanosheets.

Figure 3 .
Figure 3. Different magnified SEM images of a,c) TPU fibers and b,d) MXene/TPU fibers.Insets show the contact angle of water on corresponding mats.e) EDS elemental mappings of C, O, F, Ti, and N of the MXene/TPU fibers.

Figure 5 .
Figure 5. Schematic image describing the proposed humidity sensing mechanism of the MXene/TPU sensor.R 1 , R 2 , and R 3 represent the intrinsic resistance of MXene nanosheets.R t1 and R t2 represent tunneling resistance at the junctions.R flim represents the total resistance of MXene/TPU composite film.

Figure 6 .
Figure 6.a) The response-recovery curves of MXene/TPU humidity sensor under different RH.b) Absorption-desorption curves of humidity sensor (left axis) and corresponding hysteresis (right axis).c) The exponential fitting curve of the response variations versus RH.The data are presented as mean values ± standard deviation (n = 3).d) Continuous response between 11% RH and ambient humidity.e) Repetitive response between 11% RH and three different RH (33%, 58%, and 84% RH) for five cycles.f) Response and recovery times under 33%, 58%, and 84% RH, respectively.

Figure 7 .
Figure 7. a-c) Respiration response curves and d-f) corresponding first derivative plots of normal breath, deep breath, and rapid breath.g) Respiration response curves during continuous different motion states.h-k) Magnified response curves in the black frame region in (e).