**3.1 Digital ECG design**

Signals produced by bioelectric phenomenon are small potential values and due to this, sophisticated amplifiers are required so as to easily obtain signal values (Vidal & Pavesi, 2004).

Against a physiologic backdrop, these ionic signals are transmitted at a fast-rate without synaptic delay in both direction directed by the electric synapse transmission model. This electric potential is later transformed in a mechanical signal as of using calcium ion that comes from extracellular condition which is also useful for cooking calcium that is released from the internal section of cardiac cells provoking a massive cardiac muscle like a sincitio or functional unit (Clusin, 2008). In this sense, the main finality of an amplifier is to increment the measurable level of the gotten signal by electrodes, avoiding any kind of interference. The capacitive interference of the patient body, electrical fields of electric installations, and other environment electronic devices are examples of interference or noise. (Proakis & Manolakis, 2007) indicate that the quantification can be done using single pole configurations or bipolar. In the single pole quantification, difference between a signal and a common base is measured whereas the bipolar mode measures the difference of two voltage sources (two electrodes) with respect to a common base where any interference voltage generated at the quantification point appears at the amplifier input as common-mode interference signals. Figure 5 illustrates this phenomenon in a bipolar quantification.

Fig. 5. Common-Mode Interferences in a bipolar quantification

A strong source noise which interferes on the ECG signal is the capacitive interference of the patient body. This interference voltage is coupled to the ECG signal reaching values of 2.4 V approximately. A value which is very higher than the ECG signals value range (0.02 mV to 5 mV). In addition to this interference, the capacitive interference due to the equipment or device used to measure the ECG signal which is produced by the equipment power supply. Another noise source is the denominated inductive interference that is caused by the electric net which produces variable in time magnetic fields inducing extra voltages on the next of patient electrodes (Townsend, 2001).

156 Applications of Digital Signal Processing

(Cuesta, 2001; Vidal et al., 2008; Vidal & Gatica, 2010) where the most important part corresponds to the amplifying module because of a bioelectrical signal that represents a low potential, and sophisticated amplifiers are required for obtaining and recording it (Vidal &

The following sections present experiences building a device for getting the ECG signal, and

Signals produced by bioelectric phenomenon are small potential values and due to this, sophisticated amplifiers are required so as to easily obtain signal values (Vidal & Pavesi,

Against a physiologic backdrop, these ionic signals are transmitted at a fast-rate without synaptic delay in both direction directed by the electric synapse transmission model. This electric potential is later transformed in a mechanical signal as of using calcium ion that comes from extracellular condition which is also useful for cooking calcium that is released from the internal section of cardiac cells provoking a massive cardiac muscle like a sincitio or functional unit (Clusin, 2008). In this sense, the main finality of an amplifier is to increment the measurable level of the gotten signal by electrodes, avoiding any kind of interference. The capacitive interference of the patient body, electrical fields of electric installations, and other environment electronic devices are examples of interference or noise. (Proakis & Manolakis, 2007) indicate that the quantification can be done using single pole configurations or bipolar. In the single pole quantification, difference between a signal and a common base is measured whereas the bipolar mode measures the difference of two voltage sources (two electrodes) with respect to a common base where any interference voltage generated at the quantification point appears at the amplifier input as common-mode

interference signals. Figure 5 illustrates this phenomenon in a bipolar quantification.

A strong source noise which interferes on the ECG signal is the capacitive interference of the patient body. This interference voltage is coupled to the ECG signal reaching values of 2.4 V approximately. A value which is very higher than the ECG signals value range (0.02 mV to 5 mV). In addition to this interference, the capacitive interference due to the equipment or device used to measure the ECG signal which is produced by the equipment power supply. Another noise source is the denominated inductive interference that is caused by the electric net which produces variable in time magnetic fields inducing extra voltages on the next of

Fig. 5. Common-Mode Interferences in a bipolar quantification

patient electrodes (Townsend, 2001).

Pavesi, 2004; Vidal et al., 2008; Vidal & Gatica, 2010).

works related to processing ECG signal.

**3.1 Digital ECG design**

2004).

For these reasons, common mode rejection ratio (CMRR) rate is a desirable characteristic of an amplifier working on differential mode. On a day today practice, a problem denominated contact impedance disbalance appears (Townsend, 2001) that is produced when there are different interfaces impedances between the skin and electrodes in a form that the commonmode potential is higher in one of the two voltage sources. Therefore, part of the commonmode voltage is worked as differential voltage and amplified according to the amplifier gain. This occasionally produces saturation on the next amplifying module stage, if the amplification module were composed by more stages. This voltage, which is generally continuous, can be eliminated using a simple high-pass filter. Hence, the output voltage of the differential amplifier would consist of 3 components (Townsend, 2001; Vidal & Pavesi, 2004):

(Wells & Crampton, 2006) indicate that weak signals require an amplification of 1000 at least to produce adequate signal levels for future works on it. (Vidal & Pavesi, 2004) used an instrument amplifier model INA131 which presents a fixed CMRR of 100, and according to the associated datasheet it is adequate for biomedical instrumentation. The analog to digital conversion stage (A/D conversion) is always done when the signal is amplified. The electronic schemes of a digital electrocardiographic device according to (Vidal & Gatica, 2010) are presented on figures 6 and 7, respectively. (Vidal & Pavesi, 2004; Vidal & Gatica, 2010) use the TLC1541 A/D converter. It is necessary to indicate that both electronic items, INA131 and TLC1541, are less expensive.

Fig. 6. ECG Signal Amplifying Module Circuit

A DSP Practical Application: Working on ECG Signal 159

(Vidal & Pavesi, 2004; Vidal & Gatica, 2010) worked on the digital filters application to eliminate noise on an ECG signal, and the use of algorithms for QRS complex detecting. Following subsections describe digital filters to work on the ECG signal, and present the

To work the ECG signal it is necessary to apply digital filters which helps to diminish the noise present on it. One of the most useful filters is Lynn's filters (Goldschlager, 1989) and there are previous works where Lynn's filters are successfully applied to processing ECG signal (Thakor et al., 1984; Kohler et al., 2002; Ahlstrom & Tompkins, 1985). These filters present desirable properties of real-time filters like lineal phase and integer coefficients. There are low-pass and high-pass Lynn's filters versions which are described as follows.

Lynn's filters described in (Ahlstrom & Tompkins, 1985) and used on ECG signal processing in (Pan & Tompkins, 1985; Hamilton & Tompkins, 1986), represent a simple and effective form of applying low-pass filter on ECG signals. These filters obey the next transfer

> (1 ) (1 2 ) ( ) (1 ) (1 2 ) *z zz H z*

*yn yn yn xn xn xn* [ ] 2 [ 1] [ 2] [ ] 2 [ ] [ 2 ]

D

This filter can be implemented by means of the following differences equation:

*z zz*

2 2 1 2 1 2

D

 D

D D

(2)

(1)

Fig. 9. Triangle Signal obtained by the A/D Change Module

Fig. 10. ECG Signal obtained by the A/D Change Module

main principles of a QRS detector algorithm (Vidal et al., 2008).

**4. ECG signal processing**

**4.1 Digital filters for ECG signal**

**4.1.1 Low-pass filter**

function:

Fig. 7. Data Acquisition Module Circuit

#### **3.2 Acquiring and processing ECG signal**

The acquisition data stage has a hardware part composed by the A/D converter, and a software part which is in charge of directing the A/D converter work. Any programming language allowing low level hardware instruction is usable. (Vidal & Pavesi, 2004) and (Vidal & Gatica, 2010) describe the use of C and Visual Basic programming languages for getting and processing the ECG signal. According to these works, the routine written in C language is used to direct the A/D converter functioning using non-standard functions to access the personal computer ports. The obtained quantity of samples is stored in a binary file which is rescued by the Visual Basic programming language routine to processing (applying filters and QRS detection algorithms) and showing the signal. Showing the signal at the computer is done "offline" from the generated file with the ECG signal samples. As (Vidal & Gatica, 2010) highlights using current high level programming languages would be possible to build a showing graphics routine. Using lineal interpolation it is possible to get high level graphic results. Even though the Nyquist's sample theorem indicates that a signal can be rebuild using an ideal interpolation method (Lindner, 2009; Proakis & Manolakis 2007), by means of lineal interpolation, and through this it is possible to get good results for low frequency signals like ECG. It is possible to build a universal graphics generator for getting signals (Vidal & Pavesi, 2004; Vidal & Gatica, 2010). Figures 8 and 9 present a universal graphics generator for a sine curve signal and a triangle signal, respectively. These signals are low frequency signals (2 Hz) generated by a function or electrical waves generator with some acquisition deformities (high negative values are not considered). Figure 10 shows a pure ECG signal got by means of an implemented ECG system (Vidal & Gatica, 2010).

Fig. 8. Sine Signal obtained by the A/D Change Module

158 Applications of Digital Signal Processing

The acquisition data stage has a hardware part composed by the A/D converter, and a software part which is in charge of directing the A/D converter work. Any programming language allowing low level hardware instruction is usable. (Vidal & Pavesi, 2004) and (Vidal & Gatica, 2010) describe the use of C and Visual Basic programming languages for getting and processing the ECG signal. According to these works, the routine written in C language is used to direct the A/D converter functioning using non-standard functions to access the personal computer ports. The obtained quantity of samples is stored in a binary file which is rescued by the Visual Basic programming language routine to processing (applying filters and QRS detection algorithms) and showing the signal. Showing the signal at the computer is done "offline" from the generated file with the ECG signal samples. As (Vidal & Gatica, 2010) highlights using current high level programming languages would be possible to build a showing graphics routine. Using lineal interpolation it is possible to get high level graphic results. Even though the Nyquist's sample theorem indicates that a signal can be rebuild using an ideal interpolation method (Lindner, 2009; Proakis & Manolakis 2007), by means of lineal interpolation, and through this it is possible to get good results for low frequency signals like ECG. It is possible to build a universal graphics generator for getting signals (Vidal & Pavesi, 2004; Vidal & Gatica, 2010). Figures 8 and 9 present a universal graphics generator for a sine curve signal and a triangle signal, respectively. These signals are low frequency signals (2 Hz) generated by a function or electrical waves generator with some acquisition deformities (high negative values are not considered). Figure 10 shows a pure ECG signal got by means of an

Fig. 7. Data Acquisition Module Circuit

**3.2 Acquiring and processing ECG signal**

implemented ECG system (Vidal & Gatica, 2010).

Fig. 8. Sine Signal obtained by the A/D Change Module

Fig. 9. Triangle Signal obtained by the A/D Change Module

Fig. 10. ECG Signal obtained by the A/D Change Module

#### **4. ECG signal processing**

(Vidal & Pavesi, 2004; Vidal & Gatica, 2010) worked on the digital filters application to eliminate noise on an ECG signal, and the use of algorithms for QRS complex detecting. Following subsections describe digital filters to work on the ECG signal, and present the main principles of a QRS detector algorithm (Vidal et al., 2008).

#### **4.1 Digital filters for ECG signal**

To work the ECG signal it is necessary to apply digital filters which helps to diminish the noise present on it. One of the most useful filters is Lynn's filters (Goldschlager, 1989) and there are previous works where Lynn's filters are successfully applied to processing ECG signal (Thakor et al., 1984; Kohler et al., 2002; Ahlstrom & Tompkins, 1985). These filters present desirable properties of real-time filters like lineal phase and integer coefficients. There are low-pass and high-pass Lynn's filters versions which are described as follows.

#### **4.1.1 Low-pass filter**

Lynn's filters described in (Ahlstrom & Tompkins, 1985) and used on ECG signal processing in (Pan & Tompkins, 1985; Hamilton & Tompkins, 1986), represent a simple and effective form of applying low-pass filter on ECG signals. These filters obey the next transfer function:

$$H(z) = \frac{(1 - z^{-\alpha})^2}{\left(1 - z^{-1}\right)^2} = \frac{\left(1 - 2z^{-\alpha} + z^{-2\alpha}\right)}{\left(1 - 2z^{-1} + z^{-2}\right)}\tag{1}$$

This filter can be implemented by means of the following differences equation:

$$y[n] = 2y[n-1] - y[n-2] + x[n] - 2x[n-a] + x[n-2a] \tag{2}$$

A DSP Practical Application: Working on ECG Signal 161

Like a low-pass Lynn's filters, there are high-pass Lynn's filters which are described in (Ahlstrom & Tompkins, 1985) and applied to ECG signal processing on (Pan & Tompkins, 1985; Hamilton & Tompkins, 1986). These filters are designed using an all-pass filter and resting over it a low-pass filter, and the result is a high-pass filter (Vidal & Pavesi, 2004). However for an effective design,

> 1

*z*

D

(4)

*z*

1 ( ) 1

*jsen*

 DZ  DZ

 Z

2 2

 Z

 Z

> Z

 Z DZ (5)

cos cos cos cos

 DZ

The High-Pass Lynn's filter starts using the following low-pass filter transfer equation:

*H z*

DZ

Z

cos

 DZ

 Z

> DZ

 Z

¨ © cos

22 2 2 2

Z

> D

*sen sen sen j sen sen*

cos cos

*sen j*

 Z

ª§ · § ·º Ǭ ¸ ¨ ¸» © ¹ © ¹¼

Z

2 22 22 22 22

*sen*

 Z

· § · ¸ ¨ ¸ ¹ © ¹ § · § · ¨ ¸ ¨ ¸ © ¹ © ¹

2

Z

 DZ

> ZZ

cos

§ · ¨ ¸ © ¹ § · ¨ ¸ © ¹

cos

*sen sen j sen j*

 DZ

22 2

cos ( 1) ( 1) 22 2

§ · § ·§ · ¨ ¸ ¨ ¸¨ ¸ © ¹ © ¹© ¹

2

Z

*sen*

Fig. 13. Amplitude Response of Low-Pass Lynn's Filter for ǂ=5

low-pass filter and all-pass filter must be in phase (Smith, 1999).

Amplitude and phase responses are got by:

Z  2

Z

DZ

DZ

 DZ

DZ

DZ

2

*sen j sen*

Z

*sen j sen*

 DZ

 DZ

> Z

22 2

*sen sen j*

*sen sen j*

ZZ

22 2

§

*sen jsen*

D

 Z

*sen sen j*

ZZ

2 2 cos 2 22 2 2 cos 2 22

1 1 cos ( ) 1 1 cos

Z

DZ

DZ

*j j e jsen H*

**4.1.2 High pass filters**

The amplitude answer of this filter is calculated as follows:

$$H(\alpha) = \frac{1 - 2\cos\alpha\alpha + \cos 2\alpha\alpha + j(2\sin\alpha\alpha - \sin 2\alpha\alpha)}{1 - 2\cos\alpha + \cos 2\alpha + j(2\sin\alpha\alpha - \sin 2\alpha)} = $$

$$\frac{|\cos\alpha\alpha - 1|}{|\cos\alpha - 1|} = \frac{\sin^2\left(\frac{\alpha}{2}\alpha\right)}{\sin^2\left(\frac{\alpha}{2}\right)}\tag{3}$$

For a sample frequency of 430 Hz, possible ǂ values and associated cut frequency (-3 dB.) are shown in Table 2. Figures 11, 12, and 13 show associated amplitude response for these filters.

Table 2. Cut Frequencies of Low-Pass Lynn Filter

Fig. 11. Amplitude Response of Low-Pass Lynn's Filter for ǂ=3

Fig. 12. Amplitude Response of Low-Pass Lynn's Filter for ǂ=4

Fig. 13. Amplitude Response of Low-Pass Lynn's Filter for ǂ=5

#### **4.1.2 High pass filters**

160 Applications of Digital Signal Processing

1 2 cos cos2 (2 2 ) ( ) 1 2 cos cos2 (2 2 )

 Z

D

§ ·

Z

 DZ

*j sen sen*

ZZ

 DZ  DZ

(3)

2

*sen*

*sen*

¨ ¸ © ¹ §· ¨ ¸

*j sen sen H*

DZ

Z

2

2

© ¹

**ǂ Value Cut Frequency**  3 48 Hz 4 35 Hz 12 11.46 Hz

For a sample frequency of 430 Hz, possible ǂ values and associated cut frequency (-3 dB.) are shown in Table 2. Figures 11, 12, and 13 show associated amplitude response for these

Z

The amplitude answer of this filter is calculated as follows:

Z

Table 2. Cut Frequencies of Low-Pass Lynn Filter

Fig. 11. Amplitude Response of Low-Pass Lynn's Filter for ǂ=3

Fig. 12. Amplitude Response of Low-Pass Lynn's Filter for ǂ=4

filters.

cos 1 2 cos 1

DZ

Z

> Like a low-pass Lynn's filters, there are high-pass Lynn's filters which are described in (Ahlstrom & Tompkins, 1985) and applied to ECG signal processing on (Pan & Tompkins, 1985; Hamilton & Tompkins, 1986). These filters are designed using an all-pass filter and resting over it a low-pass filter, and the result is a high-pass filter (Vidal & Pavesi, 2004). However for an effective design, low-pass filter and all-pass filter must be in phase (Smith, 1999).

 Low-Pass / High-Pass Lynn's Filter Amplitude Response - Cut Frequency 0.5 Hz

Fig. 16. Low-Pass / High-Pass Lynn's Filter Amplitude Response - Cut Frequency 5 Hz

Lynn's filter application there is a group delay of 160 samples.

Fig. 18. Filtered ECG Signal Using Low-Pass 35 Hz Lynn's Filter

Fig. 17. Pure ECG Signal

Figures 17, 18, 19, 20 and 21 present signals registered by an implement ECG device using Figure 4 and 5 circuits (Vidal & Gatica, 2010). Figure 15 shows a pure signal ECG without applying filters to delete noise. Figure 18 shows the 35 Hz low-pass Lynn's filter application on the Figure 17 signal. Figure 18 presents the application of a 48 Hz low-pass filter application over the Figure 17 signal. In Figures 20 and 21 the application of 0.2 and 0.5 high-pass Lynn's filters respectively on the Figure 17 signal is shown. It is important to be aware of the group delay effect on the ECG signal after the 0.2 Hz high-pass Lynn's filter application, 423 samples in this case (around 1 second). Likewise, for the 0.5 Hz high-pass

Finally, amplitude and phase responses are showed on Eq. 6 and Eq. 7, respectively.

$$\left| H(\alpha) \right| = \left| \frac{\operatorname{sen}\frac{\alpha \alpha}{2}}{\operatorname{sen}\frac{\alpha}{2}} \right| \tag{6}$$

$$\Theta(\alpha) = -\frac{\alpha}{2}(\alpha - 1) \tag{7}$$

The filter's group delay is ( 1) /2 D , and the associated gain for ǚ=0 is ǂ determined evaluating |H (ǚ=0)|.

Once completely characterized the low-pass filter, designing the high-pass filter is an easy task using the following transfer function:

$$H(z) = z^{\frac{-(a-1)}{2}} - \left(\frac{1-z^{-a}}{1-z^{-1}}\right) / a = \frac{-1/\left.a+z^{\frac{-(a-1)}{2}} - z^{-\frac{-(a-1)}{2}-1} + z^{-a}/a}{1-z^{-1}}\right) \tag{8}$$

This filter can be implemented directly by the following difference equation:

$$y[n] = y[n-1] - x[n] / \left| a + x \right[n - \frac{(a-1)}{2}] - x \left[ n - \frac{(a-1)}{2} - 1 \right] + x[n-a] / \left| a \right] \tag{9}$$

Getting amplitude response for this filter is mathematically complex. Nevertheless, theoretically this filter must have the same cut frequency of the subjacent low-pass filter in inverse order. Furthermore, the values of phase response and group delay of the high-pass filter are the equal to the same parameters for the low-pass filter (Smith, 1999).

For a cut frequency of 430 Hz, ǂ values and associated cut frequency (-3 dB.) are shown on Table 3.

Table 3. Cut Frequencies of High-Pass Lynn Filter

Figures 14, 15 and 16 show the low-pass filter amplitude response which give an idea of the amplitude response of the associated high-pass filter because the cut frequencies are the same.

Fig. 14. Low-Pass / High-Pass Lynn's Filter Amplitude Response - Cut Frequency 0.2 Hz

162 Applications of Digital Signal Processing

2 ( )

( ) ( 1) 2 Z

Once completely characterized the low-pass filter, designing the high-pass filter is an easy

( 1) ( 1) 1 ( 1) 2 2

( 1) ( 1) [ ] [ 1] [ ]/ 1 [ ]/ 2 2 *yn yn xn x n x n x n* D

Getting amplitude response for this filter is mathematically complex. Nevertheless, theoretically this filter must have the same cut frequency of the subjacent low-pass filter in inverse order. Furthermore, the values of phase response and group delay of the high-pass

For a cut frequency of 430 Hz, ǂ values and associated cut frequency (-3 dB.) are shown on

Figures 14, 15 and 16 show the low-pass filter amplitude response which give an idea of the amplitude response of the associated high-pass filter because the cut frequencies are the same.

Fig. 14. Low-Pass / High-Pass Lynn's Filter Amplitude Response - Cut Frequency 0.2 Hz

**Valor de ǂ Frecuencia de Corte**  850 0.2 Hz. 320 0.5 Hz. 35 5 Hz.

ª ºª º « »« »

1 1 1 1 / / ( ) / 1 1 *z zz z Hz z*

D D

*z z*

 § · ¨ ¸

*sen*

*sen*

 D 2

Z

(6)

(7)

, and the associated gain for ǚ=0 is ǂ determined

 D

¬ ¼¬ ¼ (9)

© ¹ (8)

 D  D

D

D D

DZ

Finally, amplitude and phase responses are showed on Eq. 6 and Eq. 7, respectively.

Z

 4 Z

*H*

The filter's group delay is ( 1) /2

task using the following transfer function:

2

Table 3. Cut Frequencies of High-Pass Lynn Filter

D

evaluating |H (ǚ=0)|.

Table 3.

D

D

D

This filter can be implemented directly by the following difference equation:

filter are the equal to the same parameters for the low-pass filter (Smith, 1999).

D

Fig. 15. Low-Pass / High-Pass Lynn's Filter Amplitude Response - Cut Frequency 0.5 Hz

Fig. 16. Low-Pass / High-Pass Lynn's Filter Amplitude Response - Cut Frequency 5 Hz

Figures 17, 18, 19, 20 and 21 present signals registered by an implement ECG device using Figure 4 and 5 circuits (Vidal & Gatica, 2010). Figure 15 shows a pure signal ECG without applying filters to delete noise. Figure 18 shows the 35 Hz low-pass Lynn's filter application on the Figure 17 signal. Figure 18 presents the application of a 48 Hz low-pass filter application over the Figure 17 signal. In Figures 20 and 21 the application of 0.2 and 0.5 high-pass Lynn's filters respectively on the Figure 17 signal is shown. It is important to be aware of the group delay effect on the ECG signal after the 0.2 Hz high-pass Lynn's filter application, 423 samples in this case (around 1 second). Likewise, for the 0.5 Hz high-pass Lynn's filter application there is a group delay of 160 samples.

Fig. 17. Pure ECG Signal

Fig. 18. Filtered ECG Signal Using Low-Pass 35 Hz Lynn's Filter

A DSP Practical Application: Working on ECG Signal 165

In the literature there are several algorithmic approaches for detecting QRS complexes of

The implementation of incremental improvements to a classical algorithm to detect QRS complexes was realized in an experiment as mentioned in (Vidal et al., 2008; Vidal & Gatica, 2010) which in its original form do not have a great performance. The first improvement based on the first derivative is proposed and analyzed in (Friese at al., 1990). The second improvement is based on the use of nonlinear transformations proposed in (Pan & Tompkins, 1985) and analyzed in (Suppappola & Ying, 1994; Hamilton & Tompkins, 1986). The third is proposed and analyzed in (Vidal & Pavesi, 2004; Vidal et al., 2008), as an extension and improvement of that is presented in (Friesen et al., 1994) using characteristics of the algorithm proposed in (Pan & Tompkins, 1985). It should be noted that the three algorithmic improvements recently mentioned, used classical techniques of DSP (Digital Signal Processing). It is noteworthy to indicate that the second improvement proposed in (Pan & Tompkins, 1985) is of great performance in the accurate detection of QRS complexes,

To test the algorithms that work on ECG signal, it is not necessary to implement a data acquisition system. There are specialized databases with ECG records for analyzing the performance of any algorithm to work with ECG signals (Cuesta, 2001; Vidal & Pavesi, 2004). One of the most important is the MIT DB BIH (database of arrhythmias at

In Tables 4, 5, 6 and 7, respectively, are the results obtained with the application of incremental improvements made to the first algorithm for detecting QRS complexes in some records at MIT DB BIH. A good level of performance reached in the final version of algorithm of detection of QRS complexes implemented in this work could be appreciated,

R. 1118 - S. 1 2278 2278 79676 0 3497,63% R. 118 - S. 2 2278 2278 77216 0 3389,64% R. 108 – S. 1<PHONE_NUMBER> 0 1589,50% R. 108 – S. 2<PHONE_NUMBER>9 0 3078,11% Table 4. Results obtained with the Holsinger Algorithm in its Original version, for some of

R. 1118 - S. 1 2278 1558 874 720 69,97% R. 118 - S. 2 2278 1650 798 628 62,60% R. 108 – S. 1 562 346 246 216 82,20% R. 108 – S. 2 562 490 182 72 45,20% Table 5. Results obtained with the Holsinger Algorithm in its Modified version 1, for some

**False Positives (PF)**

**False Positives (PF)**

**False Negatives (NF)**

**False Negatives (NF)**

**(PF + NF) / NL**

**(PF + NF) / NL**

ECG signal with pre-filtering of the signal (Thakor et al., 1984)

for even the modern technology are not able to provide better results.

**True Positives (PV)**

**True Positives (PV)**

Massachusetts Institute of Technology,) (MIT DB, 2008).

(Table 7), compared to its original version (Table 4)

**Pulses Heart (NL)**

**Pulses Heart (NL)**

**Signal**

**Signal**

the MIT Database records.

of the MIT Database records.

Fig. 19. Filtered ECG Signal Using Low-Pass 48 Hz Lynn's Filter

Fig. 20. Filtered ECG Signal Using High-Pass 0.2 Hz Lynn's Filter

Fig. 21. Filtered ECG Signal Using High-Pass 0.5 Hz Lynn's Filter

The filters application allows improving the ECG signal quality in a remarkable manner. Figure 22 shows the application of a low-pass Lynn's filter of 48 Hz and a high-pass Lynn's filter of 0.5 Hz.

Fig. 22. Filtered ECG Signal Using a Low-Pass 48 Hz Lynn's Filter and a High-Pass 0.5 Hz Lynn's Filter

#### **4.2 QRS detection algorithm on ECG signal**

Within the automatic detection waveform of the ECG signal, it is important to detect QRS complex (Cuesta, 2001; Vidal & Pavesi, 2004). This is the dominant feature of the ECG signal. The QRS complex marks the beginning of the contraction of the left ventricle, so the detection of this event has many clinical applications (Vidal et al., 2008; Townsend, 2001).

164 Applications of Digital Signal Processing

Fig. 19. Filtered ECG Signal Using Low-Pass 48 Hz Lynn's Filter

Fig. 20. Filtered ECG Signal Using High-Pass 0.2 Hz Lynn's Filter

Fig. 21. Filtered ECG Signal Using High-Pass 0.5 Hz Lynn's Filter

filter of 0.5 Hz.

Lynn's Filter

**4.2 QRS detection algorithm on ECG signal**

The filters application allows improving the ECG signal quality in a remarkable manner. Figure 22 shows the application of a low-pass Lynn's filter of 48 Hz and a high-pass Lynn's

Fig. 22. Filtered ECG Signal Using a Low-Pass 48 Hz Lynn's Filter and a High-Pass 0.5 Hz

Within the automatic detection waveform of the ECG signal, it is important to detect QRS complex (Cuesta, 2001; Vidal & Pavesi, 2004). This is the dominant feature of the ECG signal. The QRS complex marks the beginning of the contraction of the left ventricle, so the detection of this event has many clinical applications (Vidal et al., 2008; Townsend, 2001).

In the literature there are several algorithmic approaches for detecting QRS complexes of ECG signal with pre-filtering of the signal (Thakor et al., 1984)

The implementation of incremental improvements to a classical algorithm to detect QRS complexes was realized in an experiment as mentioned in (Vidal et al., 2008; Vidal & Gatica, 2010) which in its original form do not have a great performance. The first improvement based on the first derivative is proposed and analyzed in (Friese at al., 1990). The second improvement is based on the use of nonlinear transformations proposed in (Pan & Tompkins, 1985) and analyzed in (Suppappola & Ying, 1994; Hamilton & Tompkins, 1986). The third is proposed and analyzed in (Vidal & Pavesi, 2004; Vidal et al., 2008), as an extension and improvement of that is presented in (Friesen et al., 1994) using characteristics of the algorithm proposed in (Pan & Tompkins, 1985). It should be noted that the three algorithmic improvements recently mentioned, used classical techniques of DSP (Digital Signal Processing). It is noteworthy to indicate that the second improvement proposed in (Pan & Tompkins, 1985) is of great performance in the accurate detection of QRS complexes, for even the modern technology are not able to provide better results.

To test the algorithms that work on ECG signal, it is not necessary to implement a data acquisition system. There are specialized databases with ECG records for analyzing the performance of any algorithm to work with ECG signals (Cuesta, 2001; Vidal & Pavesi, 2004). One of the most important is the MIT DB BIH (database of arrhythmias at Massachusetts Institute of Technology,) (MIT DB, 2008).

In Tables 4, 5, 6 and 7, respectively, are the results obtained with the application of incremental improvements made to the first algorithm for detecting QRS complexes in some records at MIT DB BIH. A good level of performance reached in the final version of algorithm of detection of QRS complexes implemented in this work could be appreciated, (Table 7), compared to its original version (Table 4)

Table 4. Results obtained with the Holsinger Algorithm in its Original version, for some of the MIT Database records.

Table 5. Results obtained with the Holsinger Algorithm in its Modified version 1, for some of the MIT Database records.

A DSP Practical Application: Working on ECG Signal 167

x Apply wavelets in the design and implementation of filtering algorithms and detector

x Analyze other techniques for detection of parameters like, fuzzy logic, genetic

x Make use of information technologies, such as a database in order to obtain relevant

Finally, this work is a good demonstration of the potential applications of Hardware - Software, especially in the field of biotechnology. The quantity and quality of the possible future works show the validity of the affirmation in academic and professional aspects. In addition to the likely use of this work in medical settings, it also gives account of the scope

To Dr. David Cuesta of the Universidad Politécnica de Valencia for his valuable contributions and excellent disposition to the authors of this work; to cardiologist Dr. Patricio Maragaño, director of the Regional Hospital of Talca's Cardiology department, for his clinical assessment and technical recommendations for the development of the

Ahlstrom, M. L.; Tompkins, W. J. (1985). Digital Filters for Real-Time ECG Signal

Clusin, W. T. (2008). Mechanisms of calcium transient and action potential alternans in

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Vol.32, No.9, (March 2007), pp. 708-713, ISSN 0018-9294

*Volume 294, No 1*, (October 2007), H1-H10, Maryland, USA.

13th edition, ISBN<PHONE_NUMBER>-510, Connecticut, USA

Vol.31, No.3, (March 2007), pp. 1157-1165, ISSN 0018-9294

February 2002), pp. 42-57, ISSN 0739-5175

Processing Using Microprocessors, *IEEE Transaction on Biomedical Engineering,*

cardiac cells and tissues. *American Journal of Physiology, Heart and Circle Physiology,*

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of waveforms.

**6. Acknowledgment**

**7. References**

algorithmic procedures undertaken.

0018-9294

ISSN 0018-9294

approaches and neural networks.

information of the patients and their pathologies.

of works such as ECG digital, which are practically limitless.

Table 6. Results obtained with the Holsinger Algorithm Modified Version 2, for some of the MIT Database records

Table 7. Results obtained with the Holsinger Algorithm Modified Version 3, for some of the MIT Database records


*Article* **Microwave-Assisted Synthesis, Characterization and Tribological Properties of a g-C3N4/MoS2 Nanocomposite for Low Friction Coatings**

**Mukul Saxena 1, Anuj Kumar Sharma 1,\*, Ashish Kumar Srivastava 2, Rabesh Kumar Singh 1, Amit Rai Dixit 3, Akash Nag 4 and Sergej Hloch 5**

**Abstract:** This study explores the tribological performance of microwave-assisted synthesized *g-C3N4/*MoS2 coatings. The two-dimensional transition metal dichalcogenide (TMD) nanosheet is getting prominence in the study of tribology due to its layered structure. The graphitic carbon nitride (g-C3N4) nanosheet was made using the calcination method and its nanocomposite with molybdenum disulfide (MoS2) was produced using a microwave-assisted method. The structure and morphology of the samples were characterized by some well-known methods, and tribological properties were studied by a pin-on-disc (POD) apparatus. Morphological analysis revealed that graphitic carbon nitride and molybdenum disulfide coexisted, and the layer structured MoS2 was well dispersed on graphitic carbon nitride nanosheets. BET analysis was used to determine the pore volume and specific surface area of the synthesized materials. The inclusion of MoS2 nanoparticles caused the composite's pore volume and specific surface area to decrease. The reduction in g-C3N4 pore volume and specific surface area confirmed that the pores of calcinated graphitic carbon nitride were filled with MoS2 nanoparticles. The tribological property of g-C3N4/MoS2 nanocomposite was systematically investigated under different factors such as applied loads (5N to 15N), sliding speed (500 to 1000 mm/s) and material composition (uncoated, MoS2-coated, 9 wt.% of g-C3N4 and 20 wt.% of g-C3N4 in the composite). The optimal composite material ratio was taken 9%, by weight of g-C3N4 in the g-C3N4/MoS2 composite for a variety of levels of loads and sliding speeds. The results indicates that the incorporation of g-C3N4 in nanocomposites could reduce friction and improve wear life, which were better than the results with single MoS2. This study demonstrates a solution to broaden the possible uses of g-C3N4 and MoS2-based materials in the field of tribology.

**Keywords:** microwave synthesis; transition metal dichalcogenide; graphitic carbon nitride; coatings; characterization; tribology; wear; steel substrate

#### **1. Introduction**

Friction and wear are the primary causes of failure in many mechanical engineering components used in design. Much energy is used to overcome the resistance to motion caused by friction. Friction causes heat and wear, which in turn can cause material fatigue, noise emissions, surface deterioration, mechanical energy losses, and shortened service life of the components [1]. The cost of fitting, machinery and maintenance because of wear and tear and frictional faults affects the economy of a company. In passenger vehicles, around one third of the gasoline is utilized to overcome friction and wear [2].

**Citation:** Saxena, M.; Sharma, A.K.; Srivastava, A.K.; Singh, R.K.; Dixit, A.R.; Nag, A.; Hloch, S. Microwave-Assisted Synthesis, Characterization and Tribological Properties of a g-C3N4/MoS2 Nanocomposite for Low Friction Coatings. *Coatings* **2022**, *12*, 1840. https://doi.org/10.3390/ coatings12121840

Academic Editors: Cecilia Bartuli and Ajay Vikram Singh

Received: 27 October 2022 Accepted: 25 November 2022 Published: 28 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

One major way to reduce energy consumption is by improving the tribological characteristics of the mating surfaces. The requirements for enhanced lubricants are becoming more and more demanding as a result of their qualities, which include the ability to be used across a wider temperature range, larger loads, faster speeds, and increased endurance and service life [3]. To satisfy the needs of heavy machinery in severe working environments, it is essential to create novel and efficient friction-resistant plus high-bearing lubricant additives. Producing novel lubricants that meet changing requirements in many key domains, including as transportation, industry, and defense, is one of the major scientific problems of the twenty-first century [4].

Rosentsveig et al. [5] suggested that the use of nanoparticles in lubricants might improve their tribological characteristics, significantly lowering the coefficient of kinetic friction in working equipment. Numerous nanoparticle classes have been experimentally explored as potential lubricants [6]. Several nanoparticles are made up of two-dimensional structures that are adjacent to one another and are held together by weak Van der Waal forces. These structures are responsible for lowering the shear strength of the system as well as generating lubricating or sliding effects on the adjoining layer structure [7,8].

When compared to other nanomaterials, 2D nanomaterials have a greater specific surface area for absorbing onto the surface of a substrate, hence reducing, or eliminating, the amount of friction that occurs between the contact surfaces [9]. Because of its chemical and physical stability in a coating, molybdenum-disulfide is currently found as a promising two dimensional transition metal chalcogenide. The materials are resistant to most acids, chemically balanced and irradiation-resistant. The rate at which the lubricant works is regulated by the crystalline lamella structure of the lubricant, in which the sulfur lamella is connected by a weak Van der Waals affinity, resulting in lower fiction [10]. The layers of molybdenum disulfide efficiently slide against each other and align parallel to the relative motion during sliding, resulting in the lubricating effect. The strong ionic connection between sulfur and molybdenum atoms, on the other hand, makes the lamellar very resistant to asperity penetration [11]. However, pure molybdenum disulfide quickly absorbs the moisture in any surrounding environment that is humid and can be oxidized in an environment that contains either atomic or molecular oxygen [12]. This results in a rapid increase in the coefficient of friction and a reduction in the lifespan of the frictional surface. Most of the time, the coefficient of friction increases to a value greater than 0.2 because of the oxidation of molybdenum disulfide [13,14]. Because of these factors, its practical application is restricted and limited. The super-lubricity of molybdenum disulfide can be enhanced at the molecular or atomic scale under appropriate ordering of structure or orientations in a humid free dry setting, or can be used in vacuum environment [15].

MoS2 is often mixed with other materials, such as polyurethane and graphene, as well as metals such as titanium [16], aluminum [17], copper [18], and chromium [19,20] to circumvent these restrictions and acquire its superior frictional resistance, mechanical, and thermal qualities. It is presently recognized as a major issue to improve the wear life and coefficient of friction of molybdenum sulfide for its use as a solid lubricant in different fields of application. It is now recognized as a substantial problem to extend the wear life of MoS2 for its application as a solid lubricant in different field while keeping the coefficient of friction at a low level [21]. However, the expensive cost of the metal additions in MoS2 coatings restricts their utilization. Theoretically, materials that are not metals, such as layered graphitic carbon nitride, which has the characteristics of being affordable, easily accessible, adequately stable, and environmentally acceptable, can replace metal components [22,23].

Currently, a variety of disciplines make use of polymer semiconductor graphitic carbon nitride (CN), which exhibits weak Van Der Waals forces between its layers and tris-triazine units [22,24]. CN is frequently added to lubricating oil to improve friction performance. For example, the bonding of CN with octadecylamine resulted in the creation of an efficient boundary coating on the friction surface, which increased the material's resistance to wear [25]. Duan et al. [26] adopted CN as a base oil additive because it considerably improves the wear resistance of thermoset polyimide. Zhu et al., created g-C3N4/poly vinylidene difluoride (PVDF) composites and discovered that the g-C3N4 filler was advantageous in reducing composite wear loss [27].

Several preliminary approaches for the synthesis of MoS2 nanoparticles have been devised, including, high-temperature sulfurization [28], CVD (chemical vapor deposition) [29], laser ablation, hydrothermal process [30], and sometimes even thermal reduction [31]. MoS2 nanoparticles were synthesized with higher yields using hydrothermal and microwave (MW) synthesis processes. The hydrothermal approach is widely employed because to the ease of access to processing equipment, although it suffers from insufficient uniform heating. However, during MW-assisted synthesis, the compounds can be rapidly heated, which results in a more consistent temperature ramp in comparison to the conventional oven based hydrothermal process. Aside from that, the Teflon lined vessel that is used for the reaction is translucent to microwaves, which ensures that the heating is continuous and sustained throughout the reaction vessels. Furthermore, the microwave has advantages over traditional approaches in terms of speedy and accelerated heating, high-temperature uniformity, and selective heating compared to traditional methods [32]. The reactions are essentially dependent on the capacity of their precursors, particularly solvents, to effectively use microwave energy. Because of its homogeneous heating, low energy consumption, greater yield, and quicker synthesis time, the microwave synthesis process outperforms the hydrothermal technique.

In certain articles [33–35], the MoS2 nanosheets are synthesized using microwave technology, which takes less than 30 min, as opposed to conventional heating in the oven, which takes around 24 h. Both MoS2 and CN can be used as lubricants, however little research has examined the use of graphitic carbon nitride as an additive in MoS2 to create solid nanocomposite lubricants. Because MoS2 and CN have atomic layers, they may readily move from one to the next when combined, enhancing the friction capabilities. Even though sophisticated microwave-assisted synthesis of MoS2 and its composite with g-C3N4 haven't been reported in the literature much, and their composites made via microwave heating for the field of tribology haven't been published yet, this study shows that it is possible to make these materials.

In this study, a one-step microwave-assisted (MW) process was used to synthesize the g-C3N4/MoS2 nanocomposite, and its tribological properties as an anti-friction coating were examined. The details of synthesis, characterization, and tribological property of the g-C3N4/MoS2 nanocomposite are described. For tribological analysis, it is important to find the endurance of the coating, and for this purpose three factors at different levels were considered. These factors include the applied load, sliding speed, and composition. Due to the three factors, with three levels of sliding speed, applied load and four levels of composition, the design contained 36 different possible combinations to perform experimental work.

#### **2. Materials and Methods**

For the synthesis of the material, a microwave reactor (Model Monowave450, Anton-Paar, CAS-Lucknow, India) was used, and a sample drying oven (Model HTF-T-1600, Tempsens, CAS-Lucknow, India) was used for the drying process. XRD (Ultima IV with Copper Kα radiation, current 150 mA, and potential 40 kV (wavelength = 1.5418 Å), Rigaku, IIT ISM, Dhanbad, India) was used in examine the phase composition of the developed samples in the range 10–70 ◦C at 4 ◦C steps. A FTIR spectrometer (Alpha II, BRUKER, CAS-Lucknow, India) was used to analyse the FTIR spectra of the sample materials. The microstructures were observed using a FESEM (Gemini500, Zeiss equipped with EDS, CAS-Lucknow, India). BET analysis was performed on Belsorp (mini X) to determine the pore volume, mean pore diameter, and specific surface area of synthesized materials in the relative pressure range (*P* to *P*0) of 0 to 1. The friction pairs and worn surface of the coated steel substrate were characterized using Stereo microscope (Olympus, SZX10, CAS-Lucknow, India).

The analytical grade reagents used in this investigation were sodium molybdate dihydrate (Na2MoO4·H2O), thioacetamide (CH3CSNH2), absolute ethanol, and urea (NH2·CO·NH2). All of the reagents employed were of analytical quality and did not require further purification. First, graphitic carbon nitride powder was prepared by a solid reaction. Urea (20 g) taken into a 50 mL crucible and dried at about 80 ◦C. After the crucible had been allowed to dry, it was covered with a lid, then coated in several layers of aluminum foil, and finally put in a muffle furnace. After that, it underwent calcination at 550 ◦C for four hours at a rate of 3 ◦C per minute. Following the lowering of the temperature to room temperature, the g-C3N4 powder was prepared using the grinding technique. After being cooled to room temperature (~22 ◦C), the g-C3N4 powder was produced by grinding. The conversion of urea (NH2·CO·NH2) into g-C3N4 in the process was around 4.5 percent, as calculated from Equation (1) and presented in Table 1.

$$\begin{array}{c} \textbf{Yield\\_percent} = \left(\frac{m\_t - m\_c}{m\_u}\right) \textbf{\underline{\times}} \textbf{\underline{100}}\\ \cdot \quad . \qquad . \qquad . \qquad . \qquad . \qquad . \qquad . \qquad . \end{array} \tag{1}$$

where, *mt* = mass of obtained catalyst, *mc* = mass of crucible, *mu* = initial urea mass.

**Table 1.** Yield percent calculation.

g-C3N4, sodium molybdate dehydrate and thioacetamide were mixed to a 100 mL beaker containing 70 mL deionized (DI) water in a molar ratio 1:0.89:1.91 and 1:2.33:5.01. This mixture was then subjected to a sonication treatment for three hours, during which time the reagents were mixed thoroughly in the distilled water, and then magnetic stirring for 1 h at room temperature. A schematic of the process is shown in Figure 1.

The solution was transferred into a 30 mL vial and put into microwave reactor for the synthesis. The heat ramp for the microwave-assisted synthesis is shown in Figure 2.

Following the completion of the reaction, the mixture was allowed to reach room temperature by means of a flow of compressed air. Next, filtration was carried out, and g-C3N4/MoS2 powder was collected. After that, the powder was dried in a hot air oven at a temperature of 80 degrees Celsius for two hours. For comparison, the same process was used to create MoS2 without the inclusion of g-C3N4. In order to prepare the surface of steel substrate for coating, acetone was used to clean it, and then it was ultrasonically stirred for one hour. Composition of the substrate was calculated from EDX analysis and listed in Table 2.

**Table 2.** Composition of steel substrate analyzed by EDX spectroscopy.

**Figure 1.** Synthesis of material.

**Figure 2.** Heat ramp for microwave synthesis.

The MoS2 powder and g-C3N4/MoS2 nanocomposite were dissolved separately and dispersed into a medium size beaker that contained 50 mL of ethanol. The solution was coated on the substrate by a spin coater after 30 min of ultra-sonication. Spin coating is an easy and efficient technique for coating nanomaterial on a flat surface. The coating solution spreads out over the substrate at low rotational speeds, and coated films are created at high rotational speeds. The substrate was first mounted on the spin chuck. The substrate was held in place by turning on the vacuum line. To coat the sample material, a predetermined volume of substrate was dispensed onto it with a disposable pipette. The substrate was spun after the coating solution was applied, and the lid was then put on the spin coated. After the solution was entirely suspended, the MoS2 and g-C3N4/MoS2 nanomaterial suspension were deposited on the substrate.

A pin-on-disc (POD) Tester (Model: TR-20LE PHM-400, DUCOM Instruments) was employed for the friction studies on the coating. Figure 3 is a schematic illustration of this POD tester. The counterpart pin of diameter 8 mm was made out of AISI304 stainless steel pin. All of the studies were conducted in an environment that had a relative humidity (RH) of around 30% ± 2, and the surrounding temperature was approximately 24 degrees Celsius.

The design of experiment (DOE) was created with the help of MINITAB-19, and for the purpose of conducting an analysis, three factors at their different levels were considered. These factors include the applied load, sliding speed, and composition. Table 3 presents the parameters used as inputs in the investigation.

**Table 3.** Parameters utilized as inputs for the investigation.

Due to three factors with three levels of sliding speed, applied load and four levels of composition, the design contained 36 different possible combinations to perform experimental work. The real time data of the coefficient of friction were continually recorded in software. To rule out the possibility of inadvertent mistakes, every experiment was carried out three times, and the average of the results was then determined.

#### **3. Results and Discussion**

X-ray diffraction and Fourier transform infrared spectroscopy were used, respectively, to investigate the crystal structures and functional groups of synthesized materials (pure MoS2, graphitic carbon nitride, wt.% 9 and 20 g-C3N4/MoS2 composite). Diffraction peaks for g-C3N4 were found at 2θ = 13.0◦ and 27.5◦, as shown in Figure 4 (JCPDS#87-1526). These values correspond to (100) and (002) crystallographic planes, respectively. These two peaks are indicative of conjugated aromatic systems with interlayer stacking [27]. The peaks at 2θ = 14◦, 32.7◦, 35.871◦, and 58.3◦, respectively, are attributable to the (002), (100), (102), and (110) planes of the hexagonal MoS2 (JCPDS#37-1492) [36,37].

**Figure 4.** XRD pattern of MoS2, g-C3N4 and their composites.

FTIR results of synthesized materials are shown in Figure 5. The peaks at 1237.4 and 808 cm−1 in the FTIR spectrum are responsible for the aromatic ring structure and the breathing mode of a triazine ring of g-C3N4, respectively [22]. The extension and respiration of C=N and C-C heterocyclic structure in g-C3N4 results in the peaks at 1407.1 and 1638.9 cm−1 [26,38]. Furthermore, the peak at wavenumber 3432 cm−1 is attributable to stretching vibration due to -NH residual in the C-N ring as well as -OH vibration in the H2O that was absorbed on the surface [22,26,38].

The characteristic peaks of graphitic carbon nitride and molybdenum disulfide present in the FTIR pattern of g-C3N4/MoS2 suggests that MoS2 has no effect on the functional group for g-C3N4 in nanocomposite while microwave-assisted synthesis. The results of all of the characterizations point to the conclusion that the nano-composite of g-C3N4/MoS2 that was successfully synthesized by the microwave-assisted method consisted of two distinct structures of g-C3N4 and MoS2. FESEM was used to examine the surface morphology and microstructures of the samples. Figure 6 depicts FESEM images of the samples.

It can be seen from Figure 6 that the flower-like structure of molybdenum disulfide assembled in several nanosheets, while a smooth and layered structure of graphitic carbon nitride is seen in Figure 7.

Figure 8 shows that MoS2 and g-C3N4 in the developed composite are bundled together, showing that the components of composite were mixed successfully and may provide the lubrication property of the coating.

**Figure 5.** FTIR pattern of MoS2, g-C3N4 and their composites.

**Figure 6.** FESEM images of MoS2 at magnifications of 400 nm (**A**) and 100 (**B**).

**Figure 7.** FESEM image for g-C3N4 at magnifications of 400 nm (**A**) and 100 (**B**).

**Figure 8.** FESEM images 9 wt.% g-C3N4/MoS2 at magnifications of 400 nm (**A**) and 100 (**B**).

It can be seen from Figure 9 that on increasing the weight percentage of graphitic carbon nitride in the composite, MoS2 predominates.

**Figure 9.** FESEM images 20 wt.% g-C3N4/MoS2 at magnifications of 400 nm (**A**) and 100 (**B**).

**Figure 10.** (**a**) SEM image; (**b**) live map; (**c**) element overlay; (**d**) elemental analysis; element (**e**) molybdenum, Mo; (**f**) sulfur, S; (**g**) carbon, C and (**h**) nitrogen, N of the g-C3N4/MoS2 composite.

An EDX detector was used to identify distributed elements in the composite. Carbon, nitrogen, molybdenum, and sulfur are among of the components that make up the

EDX elemental analysis results for graphitic carbon nitride, molybdenum disulfide and their composites are summarized in Table 4.

**Table 4.** Elemental composition of synthesized materials from EDX elemental analysis.

The nitrogen adsorption/desorption analysis determines the specific area of surface (*as*) of each sample (MoS2, g-C3N4, and their composite), as shown in Figure 11. The hysteresis curve loops of the samples were recorded and found to be the H3-IV type.

The specific surface areas of g-C3N4, MoS2 and g-C3N4/MoS2 were approximately 78.94, 1.46, and 25.71 m2g−1. The addition of stacked MoS2 nanoparticles may result in a reduction in the BET specific surface areas of g-C3N4/MoS2. The pore volumes of g-C3N4, MoS2 and g-C3N4/MoS2 were approximately 0.142, 0.072 and 0.021 cm3g−1. In this instance, the inclusion of MoS2 nanoparticles caused the composite's pore volume to decrease. The reduction in g-C3N4 pore volume confirms that the pores of calcinated graphitic carbon nitride were filled with MoS2 nanoparticles [39], so a range up to 15 wt.% was employed for the friction studies on the coating.

A pin-on-disc (POD) wear test was employed for the tribological investigation of the coating. The testing parameters for tribological investigations were selected as per the specifications of the machine available, and three loads of 5, 10 and 15 N with sliding speeds as 1000, 750 and 500 mm/s were used. The revolutions per minute required for the analysis were calculated from the equation *S* = *π*.*d*. *N* 60 , where *S* = sliding speed, *d* = track diameter and *N* = required revolutions per minute. The total track length covered by the pin that slid against disc was calculated from the equation *L* = (*π*.*d*).*N*.*T*, where *L* = sliding speed, *d* = track diameter, *N* = required revolutions per minute and *T* = test duration. Sliding speeds of 1000, 750 and 500 mm/s covered track lengths 600,000 mm, 450,000 mm and 300,000 mm, respectively which were required for effectively analyzing the effect of the coating and its tribological aspects for ultra-long wear life.

**Figure 11.** Nitrogen adsorption-desorption isotherm curves of (**A**) g-C3N4, (**B**) MoS2, (**C**) g-C3N4/MoS2 (9 wt.%) and (**D**) g-C3N4/MoS2 (20 wt.%).

Figure 12 depicts the wear of substrates coated with g-C3N4, MoS2, and g-C3N4/MoS2 coatings in various ratios under loads ranging from 5N to 15N and sliding speeds ranging from 500 to 1000 mm/s as per the design of experiment done with the help of MINITAB-19, as depicted in Table 3. Maximum wear depth was recorded approximately 900 μm for uncoated steel substrate disc for a test duration of 10 min, but was reduced to approximately 500 μm for the MoS2-coated disc (Figure 12B). The MoS2-coated disc restrict the friction and sliding forces for a short duration as compared to the g-C3N4/MoS2 coatings. The wear depth for coated disc dropped to approximately 66% and 50%, respectively at 9 wt.% (Figure 12C) and 20 wt.% (Figure 12D) of graphitic carbon nitride compared to the uncoated steel disc. Figure 12 shows that the optimal composite material ratio was proven to be weight percent 9 of g-C3N4 in the g-C3N4/MoS2 composite for a variety of levels of loads and sliding speeds. The figure also indicates that the friction lifetime might be increased while minimizing wear. Figure 12 also reveals a higher rate of wear for pure steel substrates, which may be due to the direct contact that occurs in between the respective pins and the steel substrates. Substrate coated with only MoS2 had lower wear than uncoated steel substrates but a shorter wear-life since MoS2 is heavily impacted by the environment. Due to the reciprocating type shear stress, the materials with g-C3N4 (9 wt.% and 20 wt.%) in MoS2 were compacted during the running-in phase, generating a dense layer on the coated surface, as shown in Figure 12c,d.

**Figure 12.** Wear depth vs. Time (**A**) for uncoated substrate; (**B**) for pure MoS2-coated substrate; (**C**) for 9 wt.% g-C3N4/MoS2 (**D**) 20 wt.% g-C3N4/MoS2, at different sliding speeds and loadings.

Furthermore, the friction process could exfoliate MoS2 nanosheets into lamella due to the reciprocating type shear stress that arises as a result of the sliding friction between the coating and counterpart pin. Instrument software recorded the mean coefficient of friction (COF) for different combinations of factors and levels as summarized in Table 5.

**Table 5.** Mean coefficient of friction for different combinations of factors.

**Table 5.** *Cont.*

The coating material doesn't peel off instantly, preventing substrate from exposure to the pin directly. Despite the fact that the mean coefficient of friction of g-C3N4/MoS2 is somewhat lower than that of pure MoS2, g-C3N4 exhibits outstanding resistance to wear and may greatly extend the wear life of MoS2. The coefficient of friction was determined to be at its lowest when the load being applied was approximately 15 Newton, which demonstrates that the greater the load, the lower the coefficient of friction. This is due to the fact that the rate of increases in load is higher than that for increase in contact area.

Figure 13 shows a microscopic view of the material wear loss for a nanocompositecoated steel substrate disc. It shows that the amount of wear loss leading to a small ploughing action. With different material composition in the coating, the chance of pinto-disc contact increased. While analyzing the worn surface, some small micro cracks and grooves could be seen in the MoS2-coated substrate (Figure 13a) which may be due to oxidation of molybdenum disulfide at high-speed sliding between pin and substrate disc. The increase in wear loss of the 20 wt.% g-C3N4/MoS2 composite at the applied load caused ploughing on the surface, and deep grooves with large amount of wear debris were found (Figure 13c). It can be seen from Figure 13b that the nanocomposite coating with 9 wt.% g-C3N4/MoS2 shows a lower value of wear loss when compared with MoS2, as well as 20 wt.% of g-C3N4 in the composite. An amount of coating remained on the substrate disc even after long-duration sliding, as can clearly be seen in Figure 13b. A few traces of g-C3N4/MoS2 material within the valleys would continue to provide lubrication, and affirms good adhesion of coating to substrate. Uncoated bare substrate was strongly affected by sliding with the pin due to shear fracture and a comparatively rough surface was found after the wear test. Some wear debris, deep groves, abrasive and ploughing lines can be clearly seen in Figure 13d. Worn surface analysis confirms the addition of g-C3N4 would reduce the wear loss of the MoS2 nanocomposite-coated substrate and increase the wear resistance.

The notable features of g-C3N4/MoS2 coating were excellent wear protection and lifetime of the composite coating. The aforementioned phenomena suggest that MoS2 (solid lubricant) and an additive of g-C3N4 have effective characteristics when used together. g-C3N4/MoS2 composites have a lower coefficient of friction (COF) and a longer wear-life because the proper quantity of addition of MoS2 can greatly reduce the coefficient of friction for g-C3N4, while g-C3N4 can enhance the wear-life of MoS2.

**Figure 13.** Micro-graphs of wear surfaces: (**a**) MoS2; (**b**) 9 wt.% g-C3N4/MoS2; (**c**) 20 wt.% g-C3N4/MoS2; (**d**) uncoated steel substrate.

#### **4. Conclusions**

Calcination of urea was used to make graphitic carbon nitride nanosheets, and a novel microwave-assisted synthesis process was used to make a g-C3N4/MoS2 nanocomposite. Physical and morphological characterizations were performed by XRD, FTIR, FESEM and EDX spectroscopy and showed the well-controlled morphology of MoS2 nanosheets and the structural features of nanocomposites. The tribological characteristics of coated steel substrate were studied using a pin-on-disc (POD) test for different loads and sliding speeds between AISI304 steel pin and substrate.

After analyzing and comparing the results, the following conclusions can be made:

• It was established that the coefficient of friction was at its lowest when the applied load was approximately 15 N, indicating that the higher the load, the lower the coefficient of friction. This is because the rate of increase in the applied load exceeded the rate of increase in contact area.

This study provides a solution to broaden the potential application of graphitic carbon nitride and molybdenum disulfide-based materials in the domain of tribology.

**Author Contributions:** M.S.: Conceptualization, Writing; A.K.S. (Anuj Kumar Sharma): Review and editing; A.K.S. (Ashish Kumar Srivastava): methodology, validation, Visualization, Writing—Review & Editing; R.K.S.: resources; A.R.D.: Supervison; Writing—Review & Editing, A.N.: software, data curation; S.H.: project administration, Writing—Review & Editing, Visualization. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Slovak Research and Development Agency under Contract No. APVV-17-0490.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**3. Discussion**

We observed a SARS-CoV-2 infection presenting with fever, respiratory and gastrointestinal symptoms in a severely immunosuppressed LTX recipient with good evolution during hospitalization on intravenous empiric antibiotics. We did not administer the drug regimen for COVID-19 recommended at that time, which included lopinavir/ritonavir and hydroxychloroquine due to potential drug–drug interactions and absence of deterioration during hospitalization and also because of emerging evidence of the antiviral combination lacking therapeutic benefits [4]. The special features of this SARS-CoV-2 infection in this LTX recipient were: (1) predominant gastrointestinal symptoms at presentation with a diagnosis of norovirus co-infection and fairly mild respiratory symptoms, despite some new and atypical findings on chest imaging that showed regression in the follow-up chest CT 6 weeks later, (2) a moderately elevated pro-inflammatory response (no "cytokine storm" or hyperinflammation) during early infection, probably blunted by immunosuppression, and (3) a delayed increase in sIL-2R, which normalized after admission.

We assume the positive outcome in this case was due to profound immunosuppression, probably because this averted most inflammatory responses, with an initially reduced cytokine storm and a lack of progression of pulmonary infiltrates, both of which are typically observed in SARS-CoV infection of immunocompetent patients [5,6]. The inflammatory immune reaction was limited to a mildly increased IL-6 at presentation and a sIL-2R peak after 1 week, which returned to reference values one week later. The observed inflammation markers were only slightly raised when compared to the reported cytokine data in immunocompetent COVID-19 patients. This modified inflammatory response correlates with the mild clinical course, since cytokines like IL-6 and sIL-2R are described as useful markers to estimate the severity of COVID-19 [7]. The pathogenetical role of IL-6 in COVID-19 manifestations is still not fully understood and there is currently a lack of evidence of the beneficial therapeutic impact of IL-6 inhibitors. The current multicenter randomized controlled trial of tocilizumab (IL-6 receptor blockade, licensed for cytokine release syndrome) in patients with severe pneumonia due to SARS-CoV-2 and elevated IL-6 may shed more light on this topic. The N-protein of SARS-CoV seems responsible for the cytokine dysbalance and for the inflammatory-mediated acute lung injury [8]. The lower inflammatory response relies on glucocorticoids mitigating the N-protein of SARS-CoV-2-induced pulmonary inflammation and modulating the involved cytokines. In the future, the harm and benefit of corticosteroid treatment needs to be carefully considered in patients after SOT with SARS-CoV-2 infection. In immunocompetent patients, the routine administration of high doses of corticosteroids are currently discussed controversially due to concerns that steroids might even exacerbate lung injury by facilitating viral replication in patients with SARS-CoV-2-associated lung injury [9], as previously shown in influenza pneumonia [10]. Another therapeutic approach supports immunosuppressive therapy by aiming to reduce hyperinflammation in particular by using the mechanisms of action observed for tacrolimus in similar settings. It targets the inhibition of viral replication and, accordingly, leads to a reduction of virus titer [11,12]. Anti-inflammatory and anti-cytokine drugs seem to be a promising therapeutic approach in controlling viral damage, assuming that SARS-CoV-2 invades endothelial cells and induces endothelial inflammation, resulting in microvascular dysfunction with a subsequent pro-coagulant state and ischemia in essential organs like the heart, lung, kidney, liver and intestine, which also explains the worse clinical outcome of patients with pre-existing cardiovascular disease [13]. In LTX recipients, respiratory viral infections have been shown to play a major role in acute and chronic allograft dysfunction. Higher sIL-2R levels released from activated B-cells and monocytes have been found in patients suffering from neoplastic, infectious and autoimmune diseases, but also in SOT recipients affected by allograft rejection [14]. Although these results are suggestive of imminent allograft dysfunction in our patient, the most recent results from the follow-up visit 6 weeks after hospital discharge demonstrated a continuously increasing lung function and the chest CT scan showed no evidence of chronic allograft dysfunction, as judged by the lack of detection of air trapping in expiration images. Of course, a longer follow-up of the patient is needed to allow for a final assessment of this aspect, since the onset of complications may be delayed by months. This case of COVID-19 has an atypical pulmonary presentation in the initial chest CT, showing three small nodular consolidations without predominant ground-glass opacities, which is in contrast to the already published cases in the literature (Figure 1a–c). We attribute this to the suppressed immune system and the thereby modified or reduced cytokine storm, which might explain the mild clinical course of the disease, as mentioned

above. The follow-up chest CT showed diminished or completely dissolved consolidations as a sign of recovery from the modified pulmonary involvement of COVID-19 (Figure 1d–f).

In order to understand the disease evolution and to choose the optimal time point in the disease course for initializing therapy, COVID-19 illness was classified recently into Stages I–III, representing increasing grades of severity [15]. Our patient would be placed between early infection (Stage I: mild symptoms, lymphocytopenia, dry cough and diarrhea) and the beginning of the pulmonary phase (Stage II A: abnormal chest imaging).

Viral diseases can have atypical presentations, as has been observed for the different clinical manifestations of SARS-CoV-2 infection and COVID-19 [2,16,17]. SOT recipients should therefore remain cautious avoiding infection [6,18]. Due to atypical presentations, an infection with SARS-CoV-2 may also lead to delay in diagnosis if there is not a high index of suspicion initially for SARS-CoV-2. A kidney transplant recipient, initially suffering from gastrointestinal symptoms, but then developing respiratory symptoms within 48 h, has recently been described [18].

An important feature was the clinical presentation with vomiting, diarrhea and fever up to 38.9 ◦C. Fever and diarrhea seem to be common initial symptoms in SOT, which has also been reported in SOT recipients under immunosuppressive therapy [19–21]. Until today, SARS-CoV-2-infected patients after SOT are scarce compared to the worldwide incidence of COVID-19 [22–25]. We suspect an underestimated number of SARS-CoV-2 infections in SOT due to atypical clinical presentations thus often not qualifying the patient for SARS-CoV-2 testing in many centers. Moreover, the prudent behavior of LTX recipients and other SOT recipients due to the fear of infectious disease may also explain the low number of COVID-19 cases among these patients.

The stool specimen of our patient was positive for norovirus. Norovirus might increase gastrointestinal wall permeability, resulting in diarrhea and leading to a secondary SARS-CoV-2 infection by fecal–oral transmission [26]. However, this hypothesis cannot be proven, especially since diarrhea is a common manifestation of COVID-19 in immunocompetent and solid organ transplant recipients. Based on the current knowledge we cannot determine the route of infection of this patient since no index patient could be found, and because the patient presented with both gastrointestinal and respiratory symptoms.


*6.2.2 Adult mosquito* Aedes albopictus

*Aedes albopictus* is a type of mosquito that is currently the main vector in various parts of the world. Moreover, these mosquitoes have the ability to transmit various diseases (acting as vectors) from arbovirus to worms such as *Dirofilaria immitis* and vector for 22 arboviruses [105]. *Aedes albopictus* is medium in size (2–10 mm) and the males are smaller than the females. The males can also be distinguished by their more feathered antennae than the females [106]*.* Its abdomen area is covered by black scales. The morphological characteristics of *Aedes albopictus* are slightly different from *Aedes aegypti*. Scutum thorax of this species is characterized with a narrow median-longitudinal white stripe.  The anterior part of the midfemur has longitudinal white stripes and the head of the clypeus without white scales [102].

## *6.2.3 Eggs*

Female *Aedes* mosquitoes usually lay eggs on a substrate that is on the surface of the water either in artificial or natural water containers [107]. *Aedes* eggs are white and soft when laid but later turn black and become hard and increase in size [108, 109]. Eggs of *Aedes aegypti* and *Aedes albopictus* do not form groups, but individually and float on the surface of a wet substrate such as water [110, 111]. *Aedes* eggs can survive dry conditions for months or years [110] and also these eggs can have viability despite being faced with excessive water conditions [112–114]. *Aedes* eggs can withstand extreme conditions because they have a shell or what is called an eggshell that protects the oocyte, egg, embryo from extreme conditions but is still able to exchange enough gas to survive [107]. The difference between *Aedes albopictus* and *Aedes aegypti* lies in the difference of the micropylar collar shape, where *Aedes aegypti*'s eggs have a prominent micropylar collar and in *Aedes albopictus* it is not too striking. *Aedes albopictus* has a large tubercle in the middle so it looks like a smoother, lighter one than in *Aedes aegypti*.

#### **Figure 2.**

*The differences of comb scale on* A. aegypti *dan* A. albopictus *[103, 104].*

#### *6.2.4 Larvae*

*Aedes aegypti*'s larvae usually have an oval head, a thorax and also an abdomen consisting of 9 segments. On the posterior side, there are 4 lobes and also a siphon which functions to help breathing on the water surface. On the surface of the water, *Aedes* larvae will have a hanging position almost forming a vertical direction [102]. *Aedes aegypti* has a specific characteristic that its 8th abdominal segment has comb scales equipped with lateral spines (**Figure 2A**). Furthermore, *Aedes aegypti* larvae also has pectent teeth on the siphon. In addition, *Aedes aegypti* larvae has 5 pairs of hairs on its ventral brush. In *Aedes albopictus* mosquito larvae, the brush scales do not have lateral spines, pecten teeth with two branches while the ventral brush has 4 unpaired hairs (**Figure 2B**).

The life cycle of the *Aedes albopictus* mosquito is highly dependent on ambient temperature and pH of 5.2-7.6 with an optimal pH of about 6.8 and 7.6 in Asia [115]. Research conducted by Satoto, et al. States that the larvae of *Aedes aegypti* and *Aedes albopictus* can be found as much as 61% in flower pots, 15.38% bathtubs, containers, large and small buckets containing 55% water. *Aedes albopictus* larvae are active feeders, which means that they eat various kinds of organic matter in the water.

#### **6.3 Bionomics**

#### *6.3.1 Breeding places*

The breeding places habitats of the two vectors are somewhat different. For *Aedes aegypti*, its preferred place to lay eggs is in a clear water reservoir in the house, which is protected from the sun. Water reservoirs that can hold water for a long time make this habitat easy to breed [116, 117], such as bathtubs in bathrooms (toilets), bathtubs, drinking water reservoirs, buckets, jars, drums, and the like [101, 104, 118]. In Dar es Salaam, it is found in piped water systems due to intermittent water supply and rainwater storage which is used for community needs [119].

#### *Epidemiology of Chikungunya in Indonesia DOI: http://dx.doi.org/10.5772/intechopen.98330*

*Aedes albopictus* prefers to lay eggs in water reservoirs outside the house such as cans, bottles, discarded tires, tree holes, plant grooves, pieces of bamboo, and open coconuts. This shelter is not used for daily household needs. This is in accordance with the nature of *Aedes aegypti* which has a tendency as a house mosquito and *Aedes albopictus* which is an outdoor mosquito [100, 103, 117].

#### *6.3.2 Feeding habit*

The *Aedes aegypti* mosquito is anthropophagic, which means that it prefers to suck human blood in a single gonotrophic cycle [120]. While the *Aedes albopictus* mosquito is a more zoophagic, random bloodsucker [121], it has also been shown to exhibit strong anthropophagic behavior like *Aedes aegypti* [122]. To find their host, mosquitoes are active in the morning, which is around 8 am–10 am and in the afternoon 3 pm–5 pm [123]. Three days after sucking blood, female mosquitoes produce 100–200 eggs depending on the amount of blood sucked. The more blood it sucked, the more eggs will be produced [124].

#### *6.3.3 Resting places*

Places that mosquitoes prefer to rest while waiting to lay eggs are the ones which are dark, humidwith little wind [125]. *Aedes aegypti* prefers dark, damp, and hidden places in the house or building as a place to rest, including in the bedroom, in the bathroom, and in the kitchen. Indoors, a mosquito's preferred resting surface is under the furniture, hanging objects such as clothes and curtains, and walls. These mosquitoes are rarely found outdoors, in plants, or other protected areas. Meanwhile, the *Aedes albopictus* mosquito, known as the Asian tiger mosquito, prefers places outside the house, such as in tree holes, plant grooves, and gardens or forest edge areas [118, 126, 127].

#### *6.3.4 Flight range*

The movement of *Aedes aegypti* mosquitoes from breeding places to prey and rest areas is determined by the ability of mosquitoes to fly. The average flight range of the *Aedes aegypti* mosquito is about 100 m, but in certain circumstances these mosquitoes can fly up to several kilometers in an attempt to find breeding places to lay their eggs. The mosquito *Aedes albopictus* has a flight range of 400-600 m [128].

Several studies have shown that the average mosquito has a flight range (mainly related to migration) between 50 m and 50 km [129]. *Aedes albopictus*, a type of mosquito that breeds in containers, is a very weak flyer (mean maximum 676 m) [130]. The flight ability of mosquitoes is highly dependent on wind assistance as some species can disperse during periods of high winds and energy required to travel great distances. For active flight *Aedes aegypti* and *Aedes albopictus* depend on carbohydrates [131].

#### **7. Prognosis**

A retrospective study of 107 serologically proven cases of chikungunya infection (CHIKV) was conducted. All respondents had contracted the disease at least 3 years before; 87.9% had fully recovered, 3.7% had only occasional stiffness or mild discomfort, 2.8% had residual joint stiffness but no pain, while 5.6% had persistent pain and stiffness and frequent effusions. All patients with persistent joint pain and stiffness had very high antibody titres against the CHIK virus [132]. In some

isolation, CHIKV performed in severe cases showed bleeding manifestations, neurological abnormalities, and heart muscle abnormalities. Sports activities can worsen clinical symptoms such as joint pain, especially in the morning. The knee joint can swell as can the wrist and finger joints [133]. CHIKV infection, both clinical and silent, will provide lifelong immunity, so it is difficult for the same disease to attack the same patient [134]. Most patients recover completely from infection, but in some cases, joint pain can last for months, or even years [135].


**6.9 Cerebral beriberi**

Also known as "Wernicke's Encephalopathy" is a degenerative brain disorder due to the lack of Vit B1(thiamin). It may be the result of alcohol abuse, prolonged vomiting, eating disorders, and dietary deficiencies.