**3. Respiratory wearable sensors**

Wearable sensors for respiratory monitoring employ various types of electronic sensors that can be mounted into clothes [23], attached to belts [5, 24], fixed on the skin [3, 7], etc. There are many ways to make wearable devices and some of them are described separately by the type of primary sensor in the following sections.

#### **3.1 Pressure sensors**

We can take advantage of the events of diaphragm contraction (as shown in **Figure 1b**) and relaxation (as illustrated in **Figure 1c**) to create wearable devices based on pressure sensors. As an example, researchers have used an electromechanical film (EMFit) to develop a respiratory rate sensor designed as a belt [24] (as shown in **Figure 2a**). They attached the sensor to the belt so that the expansion of the chest during breathing applies a force to the sensor, and produces a voltage change proportional to this movement. EMfit is a capacitive pressure sensor that has a thin porous polypropylene film structure with a sensitivity of 30–170 pC/N.

Another way to use pressure sensors is to use them directly in contact with the inhaled and exhaled air pressure during breathing. The facemask introduced in [8] measures the respiratory impedance and was targeted to home and clinical applications. The solution consists of two pressure transducers, two low power consumption fans, a field-programmable gate array, and a real-time processing engine. The device is based on the forced oscillation technique (FOT), which is a nonstandardized lung function test. The idea is to use fans to input a periodic sinusoidal air pressure signal and measure the opposite force produced by the respiratory tract. With these data, respiratory resistance and compliance, as shown in Eq. (1), can be calculated and sent via Bluetooth to a smartphone (**Figure 2b**).

The EMFit sensor is less intrusive and performed well in the detection of respiratory rate. However, body movements affect the accuracy of the measurement, so the sensor only worked well for still or moderate moving patients [24]. The facemask sensor also performed well and estimated the respiratory impedance satisfactorily. Nevertheless, it was a prototype and its use was not comfortable [8].

#### **3.2 Acoustic sensors**

As seen in Section 2.4, it is possible to monitor lung sounds using acoustic sensors. Acoustic signals related to breathing are usually obtained with the sensors located close to the nose, mouth, throat, and suprasternal notch [3, 25, 26]. **Figure 3a** shows a wireless microphone that is a portable, cheap, and easy-to-use wearable device positioned next to the nose [3]. The purpose was to measure the respiratory rate in sleep. The microphone is fixed near the nose with a tape, and the signals are sent to a smartphone via wireless communication.

BodyScope was developed to record the sounds produced by the throat region in order to classify them into the following categories [25]: eating, drinking, speaking, laughing, and coughing. The developers modified a wireless headset attaching a microphone and a stethoscope chestpiece to minimize external source audio, as illustrated in **Figure 3b**. The position selected to place the sensor was close to the

**57**

**Figure 2.**

**Figure 3.**

application as a wearable system.

*Breathing Monitoring and Pattern Recognition with Wearable Sensors*

carotid artery region as indicated the preliminary test results. The device sends the audio signals to a computer or smartphone likewise solution shown in **Figure 3a** [3]. **Figure 3c** shows a real-time wheeze detector that consists of a wireless sound acquisition module, a wearable mechanical design and a host system [23]. The sensor module was an omnidirectional condenser microphone and a stethoscope bell. A commercial repository of normal and abnormal lung sounds (referred to as the R.A.L.E lung repository) was used to implement and evaluate a wearable sensor that monitors lung sounds continuously for asthma attack detection [27]. The sensor is a microphone array for pre-filtered acoustic signal acquisition. It is an acoustic resonator array consisting of 13 paddle-shaped piezoelectric cantilevers. The results showed that accessing a repository to test for event detections did not hinder its

*Acoustic devices for respiratory monitoring: (a) a wireless microphone connected to a smartphone application [3]; (b) BodyScope system: Bluetooth headset attached with a microphone and a stethoscope chestpiece [25]; (c) a wireless acquisition module embedded into a wearable mechanical design [23] and placed over the right chest.*

*Wearable pressure sensors: (a) pressure sensor (EMFit) attached to the belt and against the skin: the variations of ribcage volume during respiration compresses the sensor, producing a proportional charge [24]; (b) system developed by [8] for respiratory impedance measurement based on the forced oscillation technique.*

*DOI: http://dx.doi.org/10.5772/intechopen.85460*

*Breathing Monitoring and Pattern Recognition with Wearable Sensors DOI: http://dx.doi.org/10.5772/intechopen.85460*

#### **Figure 2.**

*Wearable Devices - The Big Wave of Innovation*

monitoring have been made.

**3.1 Pressure sensors**

30–170 pC/N.

**3.2 Acoustic sensors**

smartphone via wireless communication.

**3. Respiratory wearable sensors**

After this brief overview of the main concepts involving respiratory anatomy and physiology, the next section explains how wearable devices for respiratory

Wearable sensors for respiratory monitoring employ various types of electronic sensors that can be mounted into clothes [23], attached to belts [5, 24], fixed on the skin [3, 7], etc. There are many ways to make wearable devices and some of them are described separately by the type of primary sensor in the following sections.

We can take advantage of the events of diaphragm contraction (as shown in **Figure 1b**) and relaxation (as illustrated in **Figure 1c**) to create wearable devices based on pressure sensors. As an example, researchers have used an electromechanical film (EMFit) to develop a respiratory rate sensor designed as a belt [24] (as shown in **Figure 2a**). They attached the sensor to the belt so that the expansion of the chest during breathing applies a force to the sensor, and produces a voltage change proportional to this movement. EMfit is a capacitive pressure sensor that has a thin porous polypropylene film structure with a sensitivity of

Another way to use pressure sensors is to use them directly in contact with the inhaled and exhaled air pressure during breathing. The facemask introduced in [8] measures the respiratory impedance and was targeted to home and clinical applications. The solution consists of two pressure transducers, two low power consumption fans, a field-programmable gate array, and a real-time processing engine. The device is based on the forced oscillation technique (FOT), which is a nonstandardized lung function test. The idea is to use fans to input a periodic sinusoidal air pressure signal and measure the opposite force produced by the respiratory tract. With these data, respiratory resistance and compliance, as shown in Eq. (1), can be

The EMFit sensor is less intrusive and performed well in the detection of respiratory rate. However, body movements affect the accuracy of the measurement, so the sensor only worked well for still or moderate moving patients [24]. The facemask sensor also performed well and estimated the respiratory impedance satisfactorily.

As seen in Section 2.4, it is possible to monitor lung sounds using acoustic sensors. Acoustic signals related to breathing are usually obtained with the sensors located close to the nose, mouth, throat, and suprasternal notch [3, 25, 26]. **Figure 3a** shows a wireless microphone that is a portable, cheap, and easy-to-use wearable device positioned next to the nose [3]. The purpose was to measure the respiratory rate in sleep. The microphone is fixed near the nose with a tape, and the signals are sent to a

BodyScope was developed to record the sounds produced by the throat region in order to classify them into the following categories [25]: eating, drinking, speaking, laughing, and coughing. The developers modified a wireless headset attaching a microphone and a stethoscope chestpiece to minimize external source audio, as illustrated in **Figure 3b**. The position selected to place the sensor was close to the

calculated and sent via Bluetooth to a smartphone (**Figure 2b**).

Nevertheless, it was a prototype and its use was not comfortable [8].

**56**

*Wearable pressure sensors: (a) pressure sensor (EMFit) attached to the belt and against the skin: the variations of ribcage volume during respiration compresses the sensor, producing a proportional charge [24]; (b) system developed by [8] for respiratory impedance measurement based on the forced oscillation technique.*

#### **Figure 3.**

*Acoustic devices for respiratory monitoring: (a) a wireless microphone connected to a smartphone application [3]; (b) BodyScope system: Bluetooth headset attached with a microphone and a stethoscope chestpiece [25]; (c) a wireless acquisition module embedded into a wearable mechanical design [23] and placed over the right chest.*

carotid artery region as indicated the preliminary test results. The device sends the audio signals to a computer or smartphone likewise solution shown in **Figure 3a** [3].

**Figure 3c** shows a real-time wheeze detector that consists of a wireless sound acquisition module, a wearable mechanical design and a host system [23]. The sensor module was an omnidirectional condenser microphone and a stethoscope bell.

A commercial repository of normal and abnormal lung sounds (referred to as the R.A.L.E lung repository) was used to implement and evaluate a wearable sensor that monitors lung sounds continuously for asthma attack detection [27]. The sensor is a microphone array for pre-filtered acoustic signal acquisition. It is an acoustic resonator array consisting of 13 paddle-shaped piezoelectric cantilevers. The results showed that accessing a repository to test for event detections did not hinder its application as a wearable system.

Acoustic wearable sensors can be very practical. However, some challenges are faced during the project design phase such as determining the optimal sensor position, canceling the acoustic ambient noise and the detection of movement artifacts. Depending on the setting, its use is not possible.

### **3.3 Humidity sensors**

Wearable humidity sensors based on the porous graphene network (a chemical structure capable of detecting moisture) have been tested for breathing analysis [4]. The sensors are capable of sensing the human respiration, apnea, speaking, and whistle rhythm. The sensors are attached to the body with a facemask, as shown in **Figure 4**. The disadvantage of using this sensor is that long time use is also uncomfortable. It still needs some improvements to further commercialization.

#### **3.4 Oximetry sensors**

Oximetry is the technique used to measure oxygen saturation. It consists, basically, of a small infrared emitter that illuminates a small portion of the skin and a receiver that measures the light absorption depending on the oxygenated and deoxygenated blood levels [28]. Wearable oximetry sensors can be worn on the wrist, finger, head, earphones, earlobe, thigh, and ankle, and they have been widely commercialized [1] (**Figure 5**).

#### **3.5 Acceleration sensors**

Accelerometers can be used to capture the respiratory movements during inhalation and exhalation events [29]. An adhesive sensor (called BiostampRC®) made of a triaxial accelerometer that can be placed on the chest wall (**Figure 6b**) has been used [29].

Researchers adapted the EMFit-based sensor to evaluate MEMS (microelectro mechanical system) high-resolution capacitive accelerometers for the detection of respiratory rate at the same time [24]. They attached two monoaxial accelerometers to the belt as shown in **Figure 6a**.

A better signal can be obtained depending on the location of the sensor [30, 31], because people may have disorders that affect muscle contraction during breathing [32], as seen in Section 2.3. Accelerometers have found application in many areas, recently, since sensors operate in a wide spectral range and have small dimensions

**59**

undesirable artifacts.

*Location of some oximetry wearable devices [1].*

**Figure 5.**

**3.6 Resistive sensors**

*Breathing Monitoring and Pattern Recognition with Wearable Sensors*

[33, 34]. In spite of that, in the clinical setting, body movement seriously influences them [35]. The sensitivity can be set to measure vibrations with amplitude varying from gross body movements to small artery pulsation [36]. Therefore, likewise applications with acoustic sensors, unwanted artifacts have to be detected in order to prevent taking decisions based on contaminated lung signals [37]. The activation of synchronized functional electrical stimulation should consider these

Another work used a textile sensor to detect talk events based on changes in breathing patterns [10]. The solution consisted of resistive stretch sensors that are made with a conductive material and a polymer mixture. These components were attached to three different belts: upper chest, lower chest, and abdomen as illustrated in **Figure 7a**. The events of thoracic or abdominal expansion and relaxation result in variation in the resistance of the stretch sensor with this sensor configuration. The idea is that the sensor can be directly integrated into the clothing in the future. Piezoresistive sensors can also be used for the production of wearable

devices. **Figure 7b** shows an example in which a smart textile fabric for respiratory rate monitoring was developed using a conductive piezoresistivity-based yarn garment [5]. Movement artifacts are also a problem for this kind of sensor. Researchers are working on improvements to incorporate these sensors in clothes and allow for

activities such as running and cycling in the future [5, 38, 39].

*DOI: http://dx.doi.org/10.5772/intechopen.85460*

**Figure 4.** *Humidity sensor attached to a facemask [4].*

*Breathing Monitoring and Pattern Recognition with Wearable Sensors DOI: http://dx.doi.org/10.5772/intechopen.85460*

*Wearable Devices - The Big Wave of Innovation*

**3.3 Humidity sensors**

**3.4 Oximetry sensors**

commercialized [1] (**Figure 5**).

to the belt as shown in **Figure 6a**.

*Humidity sensor attached to a facemask [4].*

**3.5 Acceleration sensors**

used [29].

Depending on the setting, its use is not possible.

Acoustic wearable sensors can be very practical. However, some challenges are faced during the project design phase such as determining the optimal sensor position, canceling the acoustic ambient noise and the detection of movement artifacts.

Wearable humidity sensors based on the porous graphene network (a chemical structure capable of detecting moisture) have been tested for breathing analysis [4]. The sensors are capable of sensing the human respiration, apnea, speaking, and whistle rhythm. The sensors are attached to the body with a facemask, as shown in **Figure 4**. The disadvantage of using this sensor is that long time use is also uncom-

fortable. It still needs some improvements to further commercialization.

Oximetry is the technique used to measure oxygen saturation. It consists, basically, of a small infrared emitter that illuminates a small portion of the skin and a receiver that measures the light absorption depending on the oxygenated and deoxygenated blood levels [28]. Wearable oximetry sensors can be worn on the wrist, finger, head, earphones, earlobe, thigh, and ankle, and they have been widely

Accelerometers can be used to capture the respiratory movements during inhalation and exhalation events [29]. An adhesive sensor (called BiostampRC®) made of a triaxial accelerometer that can be placed on the chest wall (**Figure 6b**) has been

Researchers adapted the EMFit-based sensor to evaluate MEMS (microelectro mechanical system) high-resolution capacitive accelerometers for the detection of respiratory rate at the same time [24]. They attached two monoaxial accelerometers

A better signal can be obtained depending on the location of the sensor [30, 31], because people may have disorders that affect muscle contraction during breathing [32], as seen in Section 2.3. Accelerometers have found application in many areas, recently, since sensors operate in a wide spectral range and have small dimensions

**58**

**Figure 4.**

[33, 34]. In spite of that, in the clinical setting, body movement seriously influences them [35]. The sensitivity can be set to measure vibrations with amplitude varying from gross body movements to small artery pulsation [36]. Therefore, likewise applications with acoustic sensors, unwanted artifacts have to be detected in order to prevent taking decisions based on contaminated lung signals [37]. The activation of synchronized functional electrical stimulation should consider these undesirable artifacts.

### **3.6 Resistive sensors**

Another work used a textile sensor to detect talk events based on changes in breathing patterns [10]. The solution consisted of resistive stretch sensors that are made with a conductive material and a polymer mixture. These components were attached to three different belts: upper chest, lower chest, and abdomen as illustrated in **Figure 7a**. The events of thoracic or abdominal expansion and relaxation result in variation in the resistance of the stretch sensor with this sensor configuration. The idea is that the sensor can be directly integrated into the clothing in the future.

Piezoresistive sensors can also be used for the production of wearable devices. **Figure 7b** shows an example in which a smart textile fabric for respiratory rate monitoring was developed using a conductive piezoresistivity-based yarn garment [5].

Movement artifacts are also a problem for this kind of sensor. Researchers are working on improvements to incorporate these sensors in clothes and allow for activities such as running and cycling in the future [5, 38, 39].

**Figure 6.**

*(a) The 1-axis accelerometers were mounted perpendicularly and parallel relative to the chest plane [24]; (b) the BiostampRC® system.*

**Figure 7.** *(a) System consisting of different belts to monitor chest and/or abdominal breathing [10]; (b) piezoresistive sensor [5].*

**61**

**4.1 Amplification**

*Breathing Monitoring and Pattern Recognition with Wearable Sensors*

*DOI: http://dx.doi.org/10.5772/intechopen.85460*

**3.7 Multimodal sensing platforms**

**Figure 8.**

*Multimodal system [7].*

Low-power multimodal wearable systems for the continuous monitoring of respiratory activity have been developed. **Figure 8** shows a system with a sensing platform that consists of a chest-patch, a wristband, and a handheld spirometer [7]. Its aim is to monitor health and the environment for asthma management. The chest-patch measures electrocardiogram (ECG), skin impedance, photoplethysmography (PPG), movement, and acoustic signals. The spirometer can measure forced expiratory volume in 1 s (FEV1), peak expiratory flow (PEF), and forced expiratory capacity (FVC). The wristband sensors are intended to measure ozone exposure, ambient temperature, relative humidity, PPG, and movement. The idea is to create a system for continuous long-term monitoring of the state of health and

the environmental factors relevant to respiratory problems such as asthma.

movement artifacts other than the pulmonary ones.

**4. Signal processing methods for respiratory signals**

small, high-impedance voltage amplifiers must be used [24].

This brief overview revealed that different sensors can monitor the same respiratory event and there are different ways to apply them. The sensors discussed are not limited to the applications mentioned in this chapter; they can be used in many other applications and combinations. One of the most difficult tasks is to develop a respiratory wearable device that is low cost, low power consuming, and immune to

Some sensor signals have very low amplitude and need to be processed. The sensitivity of the EMFit, for example, is about 2.2–7 mV/mmHg. For signals so

*Breathing Monitoring and Pattern Recognition with Wearable Sensors DOI: http://dx.doi.org/10.5772/intechopen.85460*

**Figure 8.** *Multimodal system [7].*

*Wearable Devices - The Big Wave of Innovation*

**60**

**Figure 7.**

**Figure 6.**

*(b) the BiostampRC® system.*

*sensor [5].*

*(a) System consisting of different belts to monitor chest and/or abdominal breathing [10]; (b) piezoresistive* 

*(a) The 1-axis accelerometers were mounted perpendicularly and parallel relative to the chest plane [24];* 

#### **3.7 Multimodal sensing platforms**

Low-power multimodal wearable systems for the continuous monitoring of respiratory activity have been developed. **Figure 8** shows a system with a sensing platform that consists of a chest-patch, a wristband, and a handheld spirometer [7]. Its aim is to monitor health and the environment for asthma management. The chest-patch measures electrocardiogram (ECG), skin impedance, photoplethysmography (PPG), movement, and acoustic signals. The spirometer can measure forced expiratory volume in 1 s (FEV1), peak expiratory flow (PEF), and forced expiratory capacity (FVC). The wristband sensors are intended to measure ozone exposure, ambient temperature, relative humidity, PPG, and movement. The idea is to create a system for continuous long-term monitoring of the state of health and the environmental factors relevant to respiratory problems such as asthma.

This brief overview revealed that different sensors can monitor the same respiratory event and there are different ways to apply them. The sensors discussed are not limited to the applications mentioned in this chapter; they can be used in many other applications and combinations. One of the most difficult tasks is to develop a respiratory wearable device that is low cost, low power consuming, and immune to movement artifacts other than the pulmonary ones.
