ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE)
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Spirometry air flow measurement using PVDF Film
Manisha R.Mhetre, H.K.Abhyankar Department of Instrumentation Engineering,
Vishwakarma Institute of Technology, Pune - 411037, Maharashtra, India Email: [email protected]
Abstract—Now a days due to air pollution (air born pollutants) respiratory disorders such as Asthma, Chronic obstructive pulmonary disease(COPD),ling cancer are increasing. Due to lack of awareness about this and not having routine checkups facility in small clinics, diseased condition come to know in when it become risky. There is a need to have cost effective and simple measuring device to be available for routine checkups of respiratory system.
Among different sensors used for exhaled air flow measurement PVDF (Polyvinylidene Fluoride) film is used for experimentation with a advantage of having voltage generation without supply with good accuracy.
Experimentation is carried out to investigate the sensitivity and range of voltage generation from exhalation using the piezoelectric sensor through pipes of different diameter and with different locations of film in pipe from mouth.
PVDF (Polyvinylidene Chloride) film is also tested for its pyroelectric effect, CO2 change effect and air volume measurement as there is an increase in temperature and carbon dioxide level of human exhalation blow than atmospheric temperature and carbon dioxide levels. The prototype was developed and tested for detection of air flow rate and volume of different subjects. The results of these experiments are presented in this paper.
Keywords—- Peak expiratory flow (PEF), Asthma , piezoelectric sensor, PVDF film, Exhalation flow measurements
I. INTRODUCTION
Human lung system is the purification centre of the body where deoxygenated blood rich in CO2 from cardiovascular system is purified in tiny air sac called alveoli which is unit functional part of bean shaped lung system and abundant in number .Actual diffusion of oxygen and carbon dioxide is carried out due to partial pressure difference of these gases present in air sac and RBC present in blood. Resistance to inhaled air flow from nasal cavity through trachea and bronchioles to air sac is provided which increases the temperature of the exhaled air. Amount of air and the rate of the exhaled air decide the healthy condition of the respiratory system.
The Peak Expiratory Flow (PEF) is a person‟s maximum speed of expiration. Peak flow readings are higher when patients are well and lower when the airways are constricted. Spirometry (meaning the measuring of breath) is the most common of the Pulmonary Function Tests (PFTs), in which the measurement of the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled is carried out. Spirometery is an important
assessing conditions such as asthma, pulmonary fibrosis, cystic fibrosis and COPD (Chronic Obstructive Pulmonary Disease) and its severity. Spirometry test is performed using a device called SPIROMETER which measures different lung volumes and air flow rate. There are different method for air flow measurement in different types of spirometer viz Turbine type, differential pressure type, bellow type, Ultrasonic etc, each having some advantages and disadvantages.
There are some Challenges in exhaled air flow measurements using Spirometer: (i) very low air force and pressure in mbar is exerted from mouth for its measurement (ii) complex Signal conditioning required as very low amplitude signals (in milli or micro volt range) available (iii) less Span of time of exhalation blow ( 4 to 5 sec only) to capture the signal by the sensor. Many sensors are tested in spirometer to detect proper air flow measurement in the above limitations.
In this paper a new approach is reported and tested for human exhaled air flow measurement using Polyvinylidene fluoride (PVDF) [1]. Piezoelectricity is the ability of the material to produce voltage whenever it is mechanically strained /stressed. PVDF is used for many biomedical applications because of its piezoelectric and pyroelectric properties [2]. The pyroelectricic property of PVDF is used to detect sleep apnea [5] and to monitor a respiration rate [6].
Experiments to investigate the voltage generation by human exhalation using the piezoelectric sensor were undertaken. Piezoelectric sensor was tested for maximum voltage output by human exhalation. Its properties are checked in a Lab for suitability of the sensor for exhaled air flow measurement. After this testing , a prototype is developed with proper signal conditioning which is tested for measurement of exhalation of different subjects. The results of these experiments are presented. The aim of our present work is to investigate the use of the PVDF based air flow sensor as a diagnostic tool to evaluate the exhaled air flow.
II. PIEZOELECTRIC SENSOR
A Greek word „Piezo‟ means pressure electricity.
Piezoelectricity is the creation of an electric charge in a material when subjected to an applied stress. This
atomic arrangement of the material. The charge is created by a slight deformation of a material when subjected to an external stress, which causes a slight variation in the bond lengths between cations and anions, Certain crystals show piezoelectric effect as well as other Composites such as polycrystalline Lead Zirconate Titanate based ferroelectric ceramic materials after being subjected to a certain process to make them piezoelectric materials. When they are subjected to a mechanical strain they become electrically polarized and the degree of polarization is proportional to the applied strain. The opposite effect is also possible: when they are subjected to an external electrical field they are deformed [3].
Piezoelectric materials (PZT) can be used as medium to convert mechanical energy, usually forces into electrical energy that can be stored and used to generate power. It is a technology of great interest where available power is limited [4]
Voltage generation due to stress is piezo film is represented by the equations 1 and 2 as follows:
S = d · E + s · T (1) D = d · T + ε · E
(2) When a strip of piezoelectric film is stretched it generates electrical signal (charge or voltage between upper and lower electrode surfaces), proportional to the amount of elongation. This is the quasi static condition of the material and its detail mathematical expression is given by equations 3 and 4.
S =d31 / t· V + (1/ Y11 · wt) · F (3) Q = d31.l / t· F + C · V (4) Where S is the effective strain of the device, Q is the electrical charge on the electrodes of the device, F is the force exerted on the device, V is the voltage across the electrodes, Y is Young‟s modulus under constant voltage, d is the general piezoelectric coefficient, C is the capacitance under constant force, l, w, t stand for effective length, width and thickness respectively, and the indices stand for the direction.
Voltage developed by the piezoelectric material depend upon piezoelectric strain constant, d; electro-mechanical coupling coefficient, k; piezoelectric voltage constant, g;
and permittivity of the material, ε.
Different Piezo sensors of different manufacturers are available in market for measurement, fig [1] shows the image of Piezo film (PVDF) used in our study by measurement specialist [7].
Fig 1: DT Series Elements with lead attachment PVDF is a non-reactive, flexible, light weight and a bio- compatible polymer available in various thickness and size and has a strong piezoelectric property. As Piezo film is active in nature it produces voltage upon force application. This unique property enables us to measure very low level exhaled air force measurement in 31 mode. It is also extremely durable, capable of withstanding hundreds of millions of flexing cycles, and shock resistant. Table [1] shows the various parameters of PVDF film, with a emphasize on having large stress constant (g31) for conversion of low air force.
In this experiment, force is applied in 3 direction on Piezo sensor placed in a pipe i.e. exhaled air flow and electrode are attached in 1 direction on the sensor to get the voltage output (Mode 31) is used.
Table 1: Specification sheet for PVDF film by MESAS
The DT Series Piezo film sensor with lead attachment having 28µm thickness in mode 31 and, instrumentation amplifier (AD 620, Analog Devices, USA) with high input impedance to interface with PVDF film having high impedance for amplification is used in this experiment. Shot key diode for rectification of ac generated signal of film is used for testing, as it is having very low forward voltage drop of .2V only.
III. EXPERIMENTAL SETUP
First of all usefulness of PVDF film is tested for its output change due to exhalation. Fig.2 shows ac output response of PVDF film on DSO when a person exhaled in a Spirometery pipe in d31 mode The subject is asked to exhale blow from one end of the pipe and the Piezo film is attached to other end of the pipe. The force of blow makes the movement of film which on the other end generates the voltage due to piezoelectric effect.
For mounting of a PVDF film in a pipe, different pipe materials are studied, as internal roughness of a pipe play an important role in terms of low frictional losses requirement of a fluid i.e. air moving through the pipe.
According to standard, material should be light weight and should have low frictional constant. Among different pipe material, Polyvinyl chloride is selected as it is rigid, easily available, less costly and easy to disinfect. Experiments were carried out by taking human exhalation three times for different persons.
Different position of film in pipe from mouth with different pipe diameter(24mm,32mm, 40mm) and same length (18cm) are tested for maximum voltage generation with fast response (taken according to ATS (American Thoracic Standard for peak flow measurement and spirometer). Human exhaled air blow is also measured with anemometer during each measurement with PVDF film giving out a range of exhaled air flow rate from 0.1 m/sec to 8 m/sec depending upon height weight, age and sex. As a result of these experimentation 40 mm diameter pipe with mounting at above center position and at a distance of 3.5 cm from inlet of pipe is selected which gives good responses of the film.
Next paragraph shows statistics of participating persons with experimentation carried out on them which is approved by the ethical committee and consent taken from them.
Fig 2: Response of human blow on PVDF film
IV. STATISTICS OF PARTICIPATING SUBJECTS
A number of experiments were conducted with participation of 12 Subjects of varying age (22 – 40
ft), in order to record the response of piezoelectric film sensor. The details of the 12 Subjects (3 Females and 9 Males) participated in these experiments are furnished in Fig 3.
Fig.3:Statistics of participating subjects
Fig 4: set up of Amplification circuit for piezo film sensor
As Piezo film output change upon air force is in µV, amplification is necessary for recoding and analysis purposes. Fig 4 shows the experimental set up with charge amplifier used with Piezo film for amplification.The output of the charge amplifier is determined by Q/C. Q is the developed charge on Piezo film and C is the feedback capacitance of the charge amplifier. The output voltage of the charge amplifier depends on the feedback capacitance, not the input capacitance. This indicates that the output voltage of a charge amplifier is independent of the cable capacitance.
The major advantage of a charge amplifier, therefore, can be found when a long cable is used between a Piezo film sensor and electronics. In addition, it also minimizes charge leakage through the stray capacitance around the sensor. So long cables used in the experiment does not contribute to the small voltage generation due to the human exhaled blow.
The response of the piezoelectric sensor to exhalation blow recorded with participation of different Subjects is
shown in Figure 5 and 6 with different exhalation condition viz forceful exhalation and normal breathing which is important for Spirometery development.
Fig. 5: The response of the piezoelectric sensor of exhalation blow
Fig: 6: response of PVDF film
In spirometry it is necessary to have forceful exhalation for the measurement of lung volumes. According to the ATS standard for spirometry testing, experimentation is carried out by forceful initial exhalation and normal breathing effect on PVDF film which is shown in the fig 4. This result demonstrate that,Exhalation at start and during initial blow time (first 1-2 sec of entire spirometry blow) gives a highest peak voltage as compared to the normal breathe measured with micro voltmeter(Agilent make). The output voltage range comes to be 0.2 to 3.0 Volts and it depends on the respiration rate of different subjects which ultimately depend upon of height weight, age and sex. It is more in case of Male participants (3 Volts) as compared to the female counterparts. Also as the age, weight and height of a person is more the output blow of exhalation is of more force giving more output.t
V. TEMPERATURE EFFECT ON PIEZOELECTRIC SENSOR
Fig 7 : Experimental setup for temperature effect As exhaled air is having temperature change of 2 to 3 degrees from inhalation( as air has to be passed from surrounding through nose and travel through small respiratory tract it attains temperature change).While using PVDF sensor for air flow measurement, temperature change effect need to be tested because it may affect the final prototype reading. Fig 7 shows the set up of measurement of this effect. Thermometer is used as a calibrating temperature device and Light bulb with variable intensity level using calibrated variac, changes the temperature. Set up show lamp bank with Spirometry pipe in which PVDF film is placed and observing the voltage on calibrated Agilent make Micrometer. Result of this measurement is shown in Fig.
8.
Fig.8: Temperature effect on piezoelectric sensor The output voltage varied from 35.77 to 96.89 millivolt for temperatures ranging from 31 to 50oC taken for 5 sec as spirometry blow for average person is from 4 to 6 sec.
The maximum output voltage of 96.89 millivolt was recorded for the 50oC temperature.
But as human body temperature range is very small from 36oC to 40oC, it is observed that temperature change is negligible in this range and can be ignored as it will not contribute into the voltage generated due to the air flow.
VI. CALIBRATION OF PIEZOELECTRIC SENSOR
Calibration of PVDF sensor for air volume measurement is important for spirometry and peak flow measurement.
Calibration is carried out with 2.5 L calibration syringe developed in Lab and shown in Fig 9. Different air volumes with different stroke and for different time periods is passed over the film placed in the tube with specified time to get the film output. Volume of syringe is calculated from its dimensions(which we designed and build suing PVC pipe in LAB), known different volumes as 0.5lit,1lit,1.5lit,and 2.5lit volume is passed with varying time stroke from 1 sec to 8 sec to get flow rate as lit/sec as per the requirement of ATS standard and its response is measured on PVDF film and calibration is carried out. Resulting graph is shown in Figure 10. The output voltage increased with increase in airflow volume. Maximum output of 97.98 millivolt was recorded for 2500ml of airflow volume measured for 8 seconds.
Fig 9:PVDF film Calibration set up with calibration syringe with piezo film spirometry pipe
Fig.10:Calibration of Piezoelectric sensor with constant airflow volume in fixed time interval (8 seconds)
From this graph(Fig 10), we can get the equation in terms of voltage output with flow rate change .This equation will help in the design of the prototype , i.e.
sensor responses viz. varying exhalation volumes of different subjects.
VII. CONCLUSION
Wide and varied Experimentation carried out on the PVDF film shows that PVDF film gives out appreciable change in output which after amplification and proper calibration can be used for the detection of lung volumes and capacities. Spirometer and peak flow measuring device can be built up using the film.
The results of the above experiments will have ramifications in the design of prototype.
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