A novel sensible smart mask using micro thermal-electric energy conversion elements

With the improvement of people’s living standards, more and more people begin to pay attention to their own health status. The mode of monitoring human physiological indicators is gradually shifting to a family-based and miniaturized development model that focuses on prevention. Respiratory rate is an important parameter that reflects the health status of the human body. It is an important diagnostic basis for doctors in clinical prevention and diagnosis of related diseases. Therefore, it is necessary to develop a portable monitoring instrument that can dynamically monitor the human respiratory frequency index in real time. Since most of the existing wearable respiratory frequency detection devices have some disadvantages such as low accuracy, high cost and difficulty in promotion, this paper proposes a new type of smart mask based on thermoelectric effect to monitor human respiratory frequency in real time. The respirator system has the advantages of small size, light weight and suitable for face. Compared with other heavy breathing devices, it has obvious advantages. The Seebeck effect of thermoelectric energy conversion element (TECE) can be used to reflect the respiratory frequency of human body under different respiratory conditions. Experiments show that the sensor has high accuracy and can accurately reflect even small changes in respiratory frequency.


Introduction
Respiratory rate is one of the important vital signs that is able to respond to the health condition of human body [1] .Real-time detection of respiratory rate is a guarantee of life safety for various respiratory disorders such as asthma, and a portable respiratory rate detection device can also be of great use in home detection for patients with respiratory diseases.Therefore, developing a wearable device that can measure respiration rate in real time is critical.Wearable respiratory rate detection devices applied at this stage, usually in the form of wristbands or by integrating sensors on clothing to detect respiratory rate through thoracic undulation while breathing [2,3] , suffer from low detection accuracy, high cost and are not easy to be generalized.This research elaborated the design and functions of the developed smart mask which can measure respiratory rate in real time.In this paper, thermoelectric generator is used as the sensor to detect respiratory rate.Its principle is to convert the heat generated by respiration into electrical signal to reflect respiratory rate.The periodic variation of respiration is employed as the signal source of the

Thermoelectric effect of TECE
The Seebeck effect, which occurs when a temperature difference is applied between two coupled conductors or semiconductors, is the basis for the thermal-electric energy conversion elements (TECE) as a respiratory sensor.When different temperatures are applied to both ends of TECE, a voltage is generated as a result of the temperature difference [4] .TECE outputs voltage by measuring the temperature differential between the temperature of exhaled gas and the ambient temperature.Because breathing will lead to periodic changes in the temperature inside the mask, and the ambient temperature is relatively constant, the output voltage of TECE will also change periodically.
To obtain effective thermoelectric characteristics, Seebeck coefficient are usually processed, that is, they are integrated on the temperature gradient and normalized on the temperature gradient.After normalization, Seebeck coefficient can be regarded as a constant.Therefore, TECE has good linearity when used as a temperature sensor.As shown in Figure 1, the peak output voltage is 27.6 mV at an external temperature of 18℃, an expiratory temperature of 32℃, and a respiratory cycle of 3.0s.The peak value of the simulation value is 29.326 mV, with an error of 6.25%.This part of the error comes from contact resistance, thermal radiation, heat loss at the cold end of the TECE, etc.

Circuit Setup
Based on its excellent thermoelectric effect, TECE can accomplish many functions such as thermoelectric generation, signal sensing, etc.Its output signal direction is based on the temperature gradient direction on both sides, and the output direction can be reversed by exchanging the temperature gradient direction.In the application of the module on the mask, it is possible to default that the exhaled gas temperature is always higher than the ambient temperature, in which case an obvious DC signal can be generated.
The system block diagram of the smart mask is shown in Figure 2a, and the required system components are shown in Figure 2b.In the overall system of the smart mask, the TECE is placed in the position of the respirator valve of the mask, and the display is placed outside the mask to show the number of breaths, as shown in Figure 3.The screen is connected with the TECE on the Arduino Nano core board, which is used to process signals and transmit data to the screen.In addition, lithium batteries power the entire system.When the human breathes once, a temperature difference is generated inside and outside the TECE, and the output voltage at both ends of the TECE is applied to the core board, which processes the signals and transmits the data to the screen.

TECE module testing on masks
Under the test condition at room temperature of 17.5 ℃ , when the breathing period is 1.9s, the maximum voltage output by the TECE is 26.13mV.To obtain effective amplitude data, perform mean removal on the obtained signal, as shown in Figure 4.When the respiratory cycle was 1.9s, the signal amplitude was 18.44mv, when the respiratory cycle was 1.4s, the amplitude was 13.456mv, when the respiratory cycle was 0.69s, the amplitude was 9.416mv, when the respiratory cycle was 0.47s, the signal amplitude was 6.07mv.According to the experiment, the TECE energy exchange unit can produce different amplitude signals and accurately reflect the respiratory frequency under different conditions of human breathing.

Integration of smart masks
Aiming at the application of TECE in breathing, this paper establishes a new type smart mask system that can work autonomously and has a friendly interactive interface, schematic diagram of wearing a mask as shown in Figure 5.The mask has the advantages of light weight, small size, convenient display, and low energy consumption.The system can judge the breathing condition of the mask wearer without the help of other auxiliary tools.The system integrates an Arduino Nano core board, a 0.91 inch screen, a TECE module and other components, has a very small size and weight, is well combined with the mask, and has a good human-computer interface.

Conclusions
In this paper, a new type smart mask based on thermoelectric effect is presented to detect human respiratory rate.Screen that visually reflect information such as the number of breaths and the frequency of breathing have been widely used in the current epidemic.In addition, the respirator system proposed in this paper has the advantages of small size, light weight and facial fit.It has obvious advantages over other heavy respiratory devices.The experimental results show that when a person wears a mask, the system can get the number and frequency of breathing well and display on the screen.

Figure 1 .
Figure 1.Comparison of experimental and simulated values of TECE masks at 18℃

Figure 2 .Figure 3 .
Figure 2. TEG based respiratory rate detection system: (a) System Diagram; (b) Diagram of each part of the system.