WO2022189814A1

WO2022189814A1 - Triboelectric respiration monitoring sensor and a face mask comprising such a triboelectric respiration monitoring sensor

Info

Publication number: WO2022189814A1
Application number: PCT/HU2022/050021
Authority: WO
Inventors: Haijun He; Jian Guo; DR. Kolos MOLNÁR
Original assignee: Budapesti Műszaki és Gazdaságtudományi Egyetem
Priority date: 2021-03-09
Filing date: 2022-03-08
Publication date: 2022-09-15
Also published as: HUP2100102A1

Abstract

The present invention relates to a triboelectric respiration monitoring sensor (10), comprising first and second flat electrodes (12, 14) arranged opposite each other and at a fixed distance from each other, in said first electrode (12) one or more air permeable openings (12a) are formed, and a triboelectric nanofiber layer (44) is arranged on the side of the second electrode (14) facing the first electrode (12), and a flexible triboelectric nanofiber membrane (42) is attached to the first electrode (12) in a manner that allows the membrane (42) to deflect in the direction of the electrodes (12, 14) and contact the nanofiber layer (44). The invention further relates to a face mask (100) comprising a triboelectric respiration monitoring sensor (10) according to the invention.

Description

Triboelectric respiration monitoring sensor and a face mask comprising such a triboelectric respiration monitoring sensor

The present invention relates to a triboelectric respiration monitoring sensor.

The invention further relates to a face mask comprising a triboelectric respiration monitoring sensor according to the invention.

With the outbreak of coronavirus disease (COVID-19), wearing a face mask has become a daily routine around the world. Especially for those infected by COVID-19, they must wear a face mask all the time, even at the hospital. Treatment of patients with coronavirus involves continuous monitoring of respiration. The various parameters of respiration carry vital information that can be used to monitor the progression of the disease and the general health condition of the patient. Notwithstanding the above, respiratory characteristics (e.g., respiratory rate) are parameters vital to human health in addition to other vital parameters, such as heart rate and blood pressure, so their measurement could be important in athletes, for example.

There are several solutions in the prior art for monitoring respiration, which can be classified into two major groups; i.e. into contact and non-contact monitoring systems. Regarding the contact respiratory monitoring systems, they have to directly contact the subject’s body in order to measure e.g. the sound of respiration, the humidity and temperature of the airflow of exhalation and inhalation, the movement of the chest and abdomen, or the transcutaneous C02, etc. (K. Nepal, E. Biegeleisen, T. Ning, in The IEEE 28th Annual Northeast Bioengineering Conference, Philadelphia, PA, USA, USA, 277).

The biggest drawback of contact devices is that they can be uncomfortable to wear, especially for chronic patients. In contrast, non-contact respiration monitors perform mainly radar-based respiration rate monitoring, optical-based respiration rate monitoring, and temperature-sensing and imaging- based respiration rate monitoring that do not require direct contact with the patient’s body. The undoubted advantage of these systems is that their use does not involve inconvenience, but their disadvantage is that they can be sensitive to the patient's body position and that they are mostly complicated and expensive devices. Moreover, each of the above contact and non-contact devices requires some kind of power source, the continuous provision of which makes the use of the device more difficult. A further disadvantage of the known devices is that they can measure only a part of the respiratory parameters, for example only the number of breaths per minute. In addition, however, there are several other respiratory parameters (e.g., exhalation and inhalation time, or respiratory intensity) that may also be useful in assessing a patient's health status. Knowledge of these additional respiratory parameters may be necessary when analyzing the severity of certain diseases, such as chronic obstructive pulmonary disease (COPD), or cystic fibrosis, or even when assessing the respiratory status of infants.

In recent years, there has been considerable interest in the development of wearables suitable for health monitoring, with high comfort in health monitoring. An important aspect of these devices is the compact and uninterrupted provision of electrical power. As is known, materials with piezoelectric and triboelectric properties have been successfully used to power wearable sensors and wearable electronics (K. Dong, J. Deng, Y. Zi, YC Wang, C. Xu, H. Zou, W. Ding, Y. Dai, B Gu, B. Sun, ZL Wang, Adv Mater 2017, 29, ZL Wang, AC Wang, Mater Today 2019, 30, 34, J. Wang, C. Wu, Y. Dai, Z. Zhao, A. Wang , T. Zhang, ZL Wang, Nat Commun 2017, 8, 88.). The so-called Piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) can take advantage of the triboelectric and piezoelectric properties of nanostructured materials (e.g. nanofiber polymers) to convert mechanical energy into electrical energy, thereby acting as a power source, replacing conventional batteries. It is also known that these nanofiber materials can filter out even very small particles (e.g. pathogens) with good efficiency, therefore they are also used in various air filters, such as face masks (MG Haijun He, Balazs llles, Kolos Molnar, Int J Bioprint 2020, 6, 278).

We have recognized that nanofiber materials with triboelectric properties can be used to create a respiration monitoring sensor that can be integrated into a face mask, which generates the electrical power required for its operation and therefore does not need a separate power source and also functions as an effective air filter.

We have also recognized that with a proper arrangement of nanofiber materials and the sensor, multiple respiratory parameters can be measured simultaneously, particularly, for example, respiratory intensity.

We have also recognized that such a sensor, due to its simple design, can be manufactured relatively inexpensively, thus making available to a wider masses a smart face mask that allows easy breathing, excellent filtration efficiency, simultaneous measurement of multiple breathing parameters without an external power source.

It is an object of the present invention to provide a respiration monitoring sensor which is free from the disadvantages of the prior art. In particular, it is an object of the present invention to provide a respiration monitoring sensor that provides effective air filtration and measures several parameters of a patient's respiration, including respiratory intensity, at the same time.

It is a further object of the present invention to provide a face mask comprising such a respiration monitoring sensor.

The present invention relates to a respiration monitoring sensor based on the triboelectric properties of nanofiber materials, which sensor has two opposing electrodes fixed to each other and two triboelectric nanofiber layers arranged between them, one of which is provided as a flexible membrane moved by breathing and which is permeable to air at the same time. The essence of the solution is that the flexible membrane is deformed by respiration and thus comes into contact with and then moves away from the other nanofiber layer, generating a triboelectric voltage. Since the size of the contact surface between the nanofiber layers (and thus the triboelectric voltage generated) is determined by the intensity of exhalation and inhalation, the sensor according to the invention can be used to measure not only respiration rate and duration of exhalation or inhalation, but also respiration intensity.

According to the invention, the object is achieved by a triboelectric respiration monitoring sensor according to claim 1.

According to the invention, the object is further achieved by a face mask according to claim 13, which comprises a triboelectric respiration monitoring sensor according to the invention.

Some preferred embodiments of the invention are defined in the subclaims.

Preferred embodiments of the invention are disclosed in the dependent claims.

Further details of the invention are described in the accompanying drawings. In the accompanying drawings,

Figure 1 is a schematic side sectional view of a first exemplary embodiment of a respiration monitoring sensor according to the invention;

Figure 2 is a schematic side sectional view of a second exemplary embodiment of a respiration monitoring sensor according to the invention;

Figure 3 is a schematic side sectional view of a third exemplary embodiment of a respiration monitoring sensor according to the invention;

Figure 4 is a schematic side sectional view of a fourth exemplary embodiment of a respiration monitoring sensor according to the invention;

Figure 5 is a schematic side sectional view of a fifth exemplary embodiment of a respiration monitoring sensor according to the invention;

Figure 6 is a schematic exploded view of an exemplary embodiment of a respiration monitoring sensor according to the invention;

Figure 7a is a schematic view of the respiration monitoring sensor of Figure 1 showing its state at the end of inhalation and the beginning of exhalation;

Figure 7b is a schematic view of the respiration monitoring sensor of Figure 1 showing its exemplary state during exhalation;

Figure 7c is a schematic view of the respiration monitoring sensor of Figure 1 showing its state at the end of exhalation and the beginning of inhalation;

Figure 7d is a schematic view of the respiration monitoring sensor of Figure 1 showing its exemplary state during inhalation;

Figure 8a is a schematic view of the respiration monitoring sensor of Figure 1 showing its state at the end of inhalation and the beginning of exhalation at a lower exhalation intensity;

Figure 8b is a schematic view of the respiration monitoring sensor of Figure 1 showing its exemplary state during exhalation at a lower exhalation intensity; Figure 8c is a schematic view of the respiration monitoring sensor of Figure 1 showing its state at the end of exhalation and the beginning of inhalation at a lower exhalation intensity;

Figure 8d is a schematic view of the respiration monitoring sensor of Figure 1 showing its exemplary state during inhalation at a lower exhalation intensity;

Figure 9 is a diagram illustrating the time dependence of the voltage measured between the electrodes in the states of the respiration monitoring sensor according to the invention shown in Figures 7a-7d;

Figure 10 is a diagram illustrating the time dependence of the voltage measured between the electrodes in the states of the respiration monitoring sensor according to the invention shown in Figures 8a-8d;

Figure 11 is a schematic view of an exemplary embodiment of a face mask according to the invention.

Figure 1 is a schematic side sectional view of an exemplary embodiment of a triboelectric respiration monitoring sensor 10 according to the present invention. The sensor 10 comprises first and second flat electrodes 12, 14 arranged opposite each other and at a fixed distance from each other. In the context of the present invention, flat electrodes 12, 14 are to be understood as meaning parts made of electrically conductive material, such as metal, which are small in thickness compared to their other dimensions, as is known to the person skilled in the art. The electrodes 12, 14 can be formed, for example, from flat plates made of metal, such as aluminum plates, which are fixed at a distance from each other, preferably parallel to one another, as shown, for example, in Fig. 1. The electrodes 12, 14 preferably have a circular circumference, i.e. they are formed as flat discs arranged concentrically with respect to each other (see Fig. 6). It is noted that, a design other than a circle (e.g. triangular, square, polygonal, etc.) is also conceivable (not shown in the figures), and the electrodes 12, 14 are not necessarily flat, but may be e.g. they also have a curved or convex surface. The thickness of the electrodes 12, 14 is preferably varies from a few tenths to a few mm. In a particularly preferred embodiment, the electrodes 12, 14 are fixed relative to one another so that the distance between their surfaces facing each other is at least 0.1 mm and at most 5 mm. The first electrode 12 according to the invention has one or more air- permeable openings 12a through which air can flow through the electrode 12. The surface of the electrode 12 is thus not contiguous and not continuous. The openings 12a can be formed, for example as perforations shown in Figures 1 to 5, or as penetrations preferably arranged symmetrically at the center of the electrode 12 as shown in Figure 6. In the exemplary embodiments shown in Figures 3 and 4, the second electrode 14 has one or more second air permeable openings 14a, which may be similar in design to the openings 12a, for example.

In the embodiments shown in Figs. 1 to 5 the electrodes 12, 14 are separated by an electrically insulating spacer 20, and the electrodes 12, 14 are attached to the spacer 20. In the case of disc-shaped electrodes 12, 14, the spacer 20 can be, for example, cylindrical. In embodiments where the electrode 14 is provided with one or more openings 14a, the surface of the cylinder can be continuous so that air entering a space 30 between the electrodes 12, 14 through the opening 12a can exit through the opening 14a, and vice versa. The spacer 20 may optionally consist of several pieces, i.e. the spacer 20 does not form a closed surface between the electrodes 12, 14. In this way, in embodiments where the electrode 14 does not include openings 14a, the air entering the opening 12a leaves the space 30 between the electrodes 12, 14 through the parts of the spacer 20 or can flow into the space 30 through that. Note that the attachment of the electrodes 12, 14 to the spacer 20 can be direct (Figures 2 and 4) or, optionally, indirect (see Figures 1 and 3). In the latter case, there can be additional elements between the electrodes 12, 14 and the spacer 20, which will be discussed later. In another possible embodiment shown in Figure 6, the sensor 10 comprises a housing 50 preferably made of an electrically insulating material, e.g. plastic, and the electrodes 12, 14 are fixed in the housing 50. During respiration, the air entering the space 30 through the one or more openings 12a may exit through the one or more openings 14a or, in embodiments without the opening 14a, through one or more third openings 50a formed in the housing 50. Note that the openings 12a, 14a, 50a can function as both an output and an input, depending on the direction of the air flow generated by the respiration.

According to the invention, a triboelectric nanofiber layer 44 is arranged on the side of the second electrode 14 facing the first electrode 12, and a flexible triboelectric nanofiber membrane 42 is arranged between the electrodes 12, 14 in a manner that allows the membrane 42 to deflect in the direction of the electrodes 12, 14 and to contact the nanofiber layer 44. In the context of the present invention, a triboelectric nanofiber material is a nonwoven fabric, typically composed of fibers having a thickness of a few tens or a few hundred nanometers, which has triboelectric properties as known to those skilled in the art. The membrane 42 and the layer 44 are produced in a known manner, preferably by electrospinning, and have a surface weight of 1 to 20 g/m². In a particularly preferred embodiment, the membrane 42 and/or the layer 44 is formed as a piezo- triboelectric hybrid membrane 42 and layer 44, respectively, i.e. the membrane 42 and/or the layer 44 have not only triboelectric but also piezoelectric properties. Examples of such materials with hybrid properties are polyacrylonitrile (PAN) or polyvinylidene fluoride (PVDF). Therefore, in a preferred embodiment, the membrane 42 is made of polyacrylonitrile and the nanofiber layer 44 is made of polyvinylidene fluoride. It is noted that the material of the membrane 42 and the layer 44 may be interchangeable or, optionally, made of other triboelectric or hybrid piezo-triboelectric nanofiber materials, as is known to those skilled in the art.

Nanofiber materials are breathable due to their structure. The degree of air permeability depends on the layer thickness of the material, the distance between the nanofibers and the material quality. The membrane 42 is preferably a few hundredths or tenths of a mm thick and is designed to allow comfortable breathing therethrough. The membrane 42 is also flexible, i.e. it is able to bend and deform under the effect of the air flow generated by the respiration. In embodiments in which the electrode 14 includes an opening 14a, the layer 44 is formed in such a way that exhalation or inhalation can be easily carried out through that. In a particularly preferred embodiment, in addition to the above criteria, the membrane 42 and layer 44 themselves together are capable of filtering out at least 95% to 99% of particles or aerosol droplets larger than 300 nm (equivalent aerodynamic diameter) floating in the air.

In the embodiments shown in Figures 1 and 3, the triboelectric nanofiber layer 44 is attached to the surface of the electrode 14 facing the electrode 12 but optionally, embodiments in which the layer 44 is attached to the spacer 20 are conceivable (see Figures 2 and 4).

The flexible membrane 42 according to the invention is fixed between the electrodes 12, 14 in such a way that the membrane 42 -depending on the direction of the air flow- is able to bend towards the electrode 14 or the electrode 12, respectively, due to the air flow generated during respiration, and thus come into contact with the layer 44 or be separated from it, as can be clearly seen in Figures 7a-7d, for example. In the context of the present description, the deflection of the membrane 42 means that the shape of the membrane 42 changes and a part of the membrane 42 moves towards the electrode 12 or the electrode 14, respectively, during the deformation. That is, only a portion of the membrane 42 is secured. Such fixing can be accomplished, for example, by securing the nanofiber membrane 42 only along its rim so that the center of the membrane 42 can move freely. The edge of the membrane 42 can be attached, for example, directly to the first electrode 12 (see Figure 1) or, if appropriate, to the spacer 20 (see Figure 2), in which case the membrane 42 is indirectly attached to the first electrode 12. In the embodiments shown in Figures 1 to 6, the electrodes 12, 14, the nanofiber layer 44 and the nanofiber membrane 42 have circular shape and are arranged concentrically relative to each other. It is noted that the membrane 42 is flexible, but is preferably form-retaining, i.e., it is able to bend and deform under the action of the air flow, while retaining its shape substantially in the absence of force.

The membrane 42 can come into contact with the nanofiber layer 44 upon bending. In doing so, the membrane 42 and the layer 44 are in contact with each other along a given surface, the size of which depends essentially on the magnitude of the force acting on the membrane 42, i.e. the intensity of the air flow. Figures 7c and 8c show how different intensities of air flows cause the membrane 42 to deflect differently (higher intensities cause correspondingly greater deflections), so that the size of the contact surface between the membrane 42 and the layer 44 will also be different.

In a particularly preferred embodiment, the sensor 10 comprises a digital voltage meter 60 for measuring the electrical voltage between the electrodes 12, 14 and operated by the electrical voltage between the electrodes 12, 14. The digital voltage meter 60 is preferably configured to store and preferably transmit the measured voltage data wirelessly. The voltage meter 60 can be, for example, a microcontroller comprising an analog-to-digital converter, as is known to those skilled in the art.

In a possible embodiment, on a side of the second electrode 14 opposite the first electrode 12, at a fixed distance from the second electrode 14, a third electrode 16 is arranged. The electrode 16 can preferably be formed, for example, in the same way as the electrodes 12, 14, as described above. The sensor 10 comprises a piezoelectric layer 46 enclosed by the second and third electrodes 14, 16. The material of the layer 46 may be, for example, polyacrylonitrile (PAN), or polyvinylidene fluoride (PVDF), or other material with known piezoelectric properties, as is known to those skilled in the art. The layer 46 is optionally formed as a nanofiber layer and, like the layer 44, can be produced, for example, by electrospinning. In this embodiment, the sensor 10 is provided with a second digital voltage meter 62 for measuring the piezoelectric voltage between the second and third electrodes 14, 16. The second digital voltage meter 62, like the voltage meter 60, may also be preferably configured to store and preferably transmit the measured voltage data wirelessly. The voltage meter 62 can be, for example, as a microcontroller with an analog-to-digital converter, as is known to those skilled in the art.

The invention further relates to a face mask 100 comprising the respiration monitoring sensor 10 shown above. In the context of the present invention, the face mask 100 is preferably a mask known per se covering both the user's nose and the mouth, preferably tightly fitting to the user's face, the material being, for example, a multilayer nonwoven fabric or a fabric and the like. The face mask 100 provides a physical barrier to aerosol particles in the air between the user's face and the environment such that the face mask 100 defines an interior space with the user's face.

In a particularly preferred embodiment shown in Figure 11 , the respiration monitoring sensor 10 is disposed in the wall of the face mask 100 such that the air connection between the interior space delimited by the face mask 100 and the environment is provided through the one or more air permeable openings 12a of the first electrode 12 and the nanofiber membrane 42. It is noted that the sensor 10 can be integrated into the face mask 100 in two directions. In the first case, the electrode 12 is arranged from the interior bounded by the face mask 100 and the electrode 14 is arranged from the environment outside the face mask 100. In this way, the membrane 42 comes into contact with the layer 44 during exhalation, as shown in Figures 7a-7d and 8a-8d, respectively. In the second case, the electrode 14 is arranged from the interior space delimited by the face mask 100, and the electrode 12 faces the environment outside the face mask 100. The membrane 42 is then deformed and in contact with the layer 44 by the air flow generated during inhalation.

The operation of the sensor 10 and the face mask 100 according to the present invention will now be described briefly with reference to Figures 7a-7d, 8a- 8d and Figures 9 and 10.

In the initial state of the sensor 10 shown in Figures 7a and 8a the combined electrical charge of the pair formed by the electrode 12 and the membrane 42 and the pair formed by the electrode 14 and the layer 44 is zero for both pairs, so that no voltage difference can be measured between the electrodes 12, 14. Figures 7b, 8b illustrating the exhalation phase show how the air flowing through the openings 12a of the electrode 12 deforms the flexible membrane 42 in the direction of the layer 44, which membrane 42 then comes into contact with the layer 44. In doing so, the size of the contact surface increases, while - due to the triboelectric effect - the PAN membrane 42 loses electrons and begins to become positively charged, while electrons migrate to the PVDF layer 44, so its charge will be negative. As a result of the resulting charge difference, a potential difference is formed between the electrodes 12, 14, the direction and magnitude of which are measured by a voltage meter 60 connected to the electrodes 12, 14 (see Figures 9, 10). By the end of the exhalation phase, the air flow ceases and the membrane 42 assumes the shape shown in Figures 7c, 8c. As can be seen, the lower exhalation intensity shown in Figure 8c will also result in a smaller maximum contact area between the membrane 42 and the layer 44 compared to that shown in Figure 7c. As a result, the maximum voltage measured between the electrodes 12, 14 is lower in the case shown in Fig. 8c (see Figs. 9 and 10). That is, the intensity of exhalation (which is proportional to the magnitude of the pressure change during respiration) can be determined from the measured voltages. In the inspiratory phase shown in Figures 7d, 8d, the resulting air stream passes through the membrane 42 so that it deforms away from the layer 44 towards the electrode 12. As the contact surface gradually decreases, electrons migrate back from the PVDF layer 44 to the PAN 42 membrane, creating a measurable potential difference between the electrodes 12, 14. A voltage has an opposite direction (and opposite sign) is measured between the electrodes 12, 14 compared to that of the exhalation phase, as shown in Figure 9 for Figure 7d and Figure 10 for Figure 8d. At the end of the inhalation phase, the air flow ceases and the membrane 42 returns to the initial state shown in Figures 7a, 8a. Based on the voltage curves shown in Figures 9 and 10, not only the intensity of respiration, but also other respiratory parameters such as the number of breaths (per minute) and the duration of exhalation and inhalation, their ratio, etc. can also be determined as will be apparent to one skilled in the art. It is noted that due to the piezoelectric properties of the PAN and PVDF materials, not only a triboelectric voltage but also a piezoelectric voltage is generated during the contact of the membrane 42 and the layer 44. The piezoelectric effect increases the amount of voltage measurable between the electrodes 12, 14.

In the embodiment shown in Figure 5, the membrane 42 in contact with the electrode 14 causes the electrode 14 to deform, causing the layer 46 to compress. The compressive force generates a piezoelectric voltage between the electrodes 14, 16, which can be detected as a voltage signal by the voltage meter 62. Since the magnitude of the generated piezoelectric signal depends on the compressive force created by the air flow generated during respiration, the intensity of the air flow (pressure difference) and its time evolution can be deduced from the piezoelectric signal thus obtained.

Although in the above examples the membrane 42 has been brought into contact with the layer 44 by the air flow generated during exhalation, the sensor 10 can be mounted in the face mask 100 in the reverse direction. That is, the membrane 42 then comes into contact with the layer 44 upon inhalation and moves away from it upon exhalation. The voltages shown in Figures 9 and 10 for each respiratory phase then change sign accordingly.

It is noted that in the above examples, air passes through the membrane 42 during exhalation and inhalation, therefore it functions as an air filter. That is, the user inhales filtered air and the face mask 100 is left by filtered air. In Figures 3 and 4, air also passes through layer 44, further increasing filtration efficiency. Various modifications to the above disclosed embodiments will be apparent to a person skilled in the art without departing from the scope of protection determined by the attached claims.

Claims

1. A triboelectric respiration monitoring sensor (10), comprising first and second flat electrodes (12, 14) arranged opposite each other and at a fixed distance from each other, in said first electrode (12) one or more air permeable openings (12a) are formed, and a triboelectric nanofiber layer (44) is arranged on the side of the second electrode (14) facing the first electrode (12), and a flexible triboelectric nanofiber membrane (42) is attached to the first electrode (12) allowing the membrane (42) to deflect in the direction of the electrodes (12, 14) and to contact the nanofiber layer (44).

2. The respiration monitoring sensor (10) according to claim 1 , wherein a third electrode (16) is arranged on the side of the second electrode (14) opposite the first electrode (12) at a fixed distance from the second electrode (14), and the sensor (10) comprising a piezoelectric layer (46) enclosed by the second and third electrodes (14, 16) and a second digital voltage meter (62) for measuring the voltage between the second and third electrodes (14, 16).

3. The respiration monitoring sensor (10) according to claim 1 or 2, wherein the nanofiber membrane (42) has a rim and is attached to the first electrode (12) along its rim.

4. The respiration monitoring sensor (10) according to any one of claims 1 to 3, wherein the first and second electrodes (12, 14) are separated by an electrically insulating spacer (20) and the electrodes (12, 14) are attached to the spacer (20).

5. The respiration monitoring sensor (10) according to any one of claims 1 to 4, wherein the electrodes (12, 14, 16), the nanofiber layer (44) and the nanofiber membrane (42) are circular and arranged concentrically.

6. The respiration monitoring sensor (10) according to any one of claims 1 to 5, wherein the nanofiber layer (44) and the nanofiber membrane (42) are produced by electrospinning.

7. The respiration monitoring sensor (10) according to any one of claims 1 to 6, wherein the nanofiber layer (44) and/or the nanofiber membrane (42) is a piezo-triboelectric hybrid layer or membrane, respectively.

8. The respiration monitoring sensor (10) according to any one of claims 1 to 7, wherein the nanofiber membrane (42) is made of polyacrylonitrile and the nanofiber layer (44) is made of polyvinylidene fluoride.

9. The respiration monitoring sensor (10) according to any one of claims 1 to 8, comprising a digital voltage meter (60) adapted to measure and operated by the electrical voltage between the electrodes (12, 14) generated during the deflection of the nanofiber membrane (42).

10. The respiration monitoring sensor (10) according to claim 9, wherein the digital voltage meter (60) is configured to store and preferably transmit the measured voltage data wirelessly.

11. The respiration monitoring sensor (10) according to any one of claims 1 to 10, wherein the electrodes (12, 14, 16) are made of aluminum.

12. The respiration monitoring sensor (10) according to any one of claims 1 to 11 , wherein the distance between the electrodes (12, 14, 16) is at most 3 mm.

13. The respiration monitoring sensor (10) according to any one of claims 1 to 12, wherein the second electrode (14) is provided with one or more air- permeable openings (14a).

14. A face mask (100) comprising a triboelectric respiration monitoring sensor (10) according to any one of claims 1 -13.

15. The face mask (100) according to claim 14, wherein the respiration monitoring sensor (10) is arranged in a wall of the face mask (100) in such a way that the air connection between an interior space delimited by the face mask (100) and the environment is provided through the one or more air permeable openings (12a) of the first electrode (12) and the nanofiber membrane (42).