**2. Synthesis, processing, and main properties of polymer coatings**

#### **2.1 Synthesis and processing**

The synthesis and processing of nanocomposite polymer coatings usually have at least two separated stages: the dispersion of nanofillers into the monomers, prepolymers, or polymers and the coating manufacture.

There are numerous different techniques to disperse the carbon nanofiller into monomers and polymers. In fact, there are numerous articles and reviews published. For this reason, in this chapter, they are only mentioned. It is wellknown that the improvement of properties, in special mechanical ones, on nanocomposites is strongly dependent on the dispersion quality together with the polymer-nanofiller interface, which relies on the chemical and physical interaction between functionalized nanofillers and polymer matrix. Good dispersion of nanoparticles is critical to achieve high-performance nanocomposite. The most common **processing techniques of nanocomposites** can be organized on three ways: direct mixing, in situ polymerization in the presence of nanoparticles, and solution mixing. One more processing way is the in situ synthesis of particles, which is usually based on in situ sol–gel process inside polymers, but it is only used for inorganic nanofillers.

#### *Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

Carbon nanofillers must be dispersed on polymer or prepolymers depending on the polymer nature. The dispersion of nanofillers on thermoplastic polymers often carries out during the polymer manufacturing process, as extrusion or calandering. However, the nanofiller dispersion on rubber and thermosetting polymers is usually carried out in a previous step of curing process into monomers or prepolymers. In this last case, different dispersion techniques can be also applied, based on the application of mechanical forces or an electric or magnetic field.

As it was mentioned above, the second step consists on manufacturing the own coating, applying different common **processing techniques of coatings**. Cold spray process is commonly used for processing polymer nanocomposite coatings, avoiding the thermal deterioration of substrate. Dispersion, emulsion, and latex in situ polymerizations are other applied manufacturing processes.

#### **2.2 Properties of nanocomposite polymer coatings**

Graphitic nanofillers are often used to improve the **mechanical properties** of polymer coatings. The poor tribological performance of polymer coatings can be improved by adequately addition of graphitic nanofillers into the matrix because graphite is a solid lubricant. Polymer coating containing graphene can present excellent tribological properties, with low friction coefficient and reduced wear rate [7]. The increment of graphene content gradually decreases both friction coefficient and wear rate of composite coating. Under high temperature, graphene-reinforced thermosetting coatings show better friction reduction and wear resistance than neat coating. The values of these properties are enhanced by the increase of graphene content. Meanwhile, the friction coefficient and the wear rate of the graphene/ composite coatings do not show a clear tendency with the increase of temperature. This behavior could be explained by the formation of a transfer film on the surface, which suppresses the huge heat and contact pressure [7]. CNT/polymer coatings can induce anti-friction, wear-proof, and self-lubrication performance [8], reducing the friction and improving the wear resistance. However, numerous factors affect their tribological behavior, such as the composition and properties of sliding pairs, such as their surface roughness and main mechanical properties (hardness, stiffness, and fracture toughness) and the sliding parameters, such as load, speed, temperature, and lubrication state, among others. This behavior is explained by the different involved mechanisms: bridge crack of CNT and lock the propagation of cracks, lubricant effect by dislodgement of individual graphene layers, strengthening of reinforced polymer matrix and dissipation of heat, and reducing the temperature induced wear [8]. It is worthy to note that there is an optimum carbon nanoparticle content to achieve the best tribological properties. However, this value depends on many factors such as aspect ratio of nanofiller, the dispersion degree and orientation of nanofillers, and the interactions with polymer matrix at interfaces.

The incorporation of carbon nanoparticles into polymer composites also increases their hardness. Increasing nanofiller content leads to improvement of hardness; however, the slope of the curve is reduced as the amount of graphitic nanofiller increases, which is attributed to agglomerations in the composite coating [9].

One of the most important applications of polymers reinforced with graphitic nanofillers is as **anticorrosive coatings**. The anticorrosive coatings can be classified in accordance to the protection mechanism against corrosion [10]: barrier protection, cathodic protection, anodic passivation, electrolytic inhibition, and active corrosion inhibition.

have extraordinary electrical and thermal conductivity and a unique combination of mechanical properties with great stiffness and high toughness [1–6]. They are composed of carbon, exhibiting low toxicity and environmental friendliness. For all these reasons, they are considered as multifunctional fillers of polymer matrix. In fact, polymer nanocomposites reinforced with carbon nanoparticles usually present enhanced mechanical, electrical, and thermal properties together with new perfor-

They can act as strain sensors due to their piezoresistive behavior, varying the electrical resistance of composite induced by the deformation of the electrical network formed by graphitic nanofillers. On the other hand, the nanofillers can be used as actuators, for example, as self-heater due to Joule's heating or as chemical absorbents. In this case, the matrix is a neat stimulus-responsive polymer, while the

carbon nanofillers provide the stimuli to induce the polymer response.

**2. Synthesis, processing, and main properties of polymer coatings**

least two separated stages: the dispersion of nanofillers into the monomers,

monomers and polymers. In fact, there are numerous articles and reviews published. For this reason, in this chapter, they are only mentioned. It is wellknown that the improvement of properties, in special mechanical ones, on nanocomposites is strongly dependent on the dispersion quality together with the polymer-nanofiller interface, which relies on the chemical and physical interaction between functionalized nanofillers and polymer matrix. Good dispersion of nanoparticles is critical to achieve high-performance nanocomposite. The most common **processing techniques of nanocomposites** can be organized on three ways: direct mixing, in situ polymerization in the presence of nanoparticles, and solution mixing. One more processing way is the in situ synthesis of particles, which is usually based on in situ sol–gel process inside polymers, but it is only used for

prepolymers, or polymers and the coating manufacture.

The synthesis and processing of nanocomposite polymer coatings usually have at

There are numerous different techniques to disperse the carbon nanofiller into

mance as smart materials.

*Summary of smart coatings with carbon nanoparticles.*

*21st Century Surface Science - a Handbook*

**Figure 1.**

**2.1 Synthesis and processing**

inorganic nanofillers.

**208**

Graphitic nanoparticles enhances the **barrier properties** of polymer coating due to the "torturous path effect" and "nano-barrier wall effect," which strongly depends on exfoliation, dispersion, and orientation degree of nanofiller, their aspect ratio, the polymer-nanofiller interface, and the crystallinity of thermoplastic polymer or cross-linking degree of thermosetting resins. The presence of nanofillers constructs tortuous paths, decreasing the diffusion coefficient. The orientation of graphene and their high surface area forms a zigzag diffusion pathway hindering the diffusion of corrosion species. In addition, their excellent electrical conductivity prevents the electrons form the cathodic site by providing an alternative path [11]. The functionalization of graphitic nanofillers with polar groups [12] enhances the ionic resistance of coating by the creation of negative charge on the graphitic nanofillers surface when exposed to alkaline and neutral environment, preventing the diffusion of chlorine and hydroxyl anions.

associated to the reduction of the electrical conductivity of neat nanofillers due to the partial breakage of some C-C structure during the functionalization and the surrounding isolating polymer layer formed over the functionalized nanofillers,

Nanoreinforced coatings are being studied to improve the **flame retardancy** (FR) of flammable substrates [20]. The addition of graphene usually reduces the total heat release (THR) because they reduce the release of deleterious gas during thermal decomposition, arising from the radical trapping and layered hindering effect. CNT also enhances the FR behavior due to the strengthening of carbonized layers [21]. Also, they act as excellent physical barrier, reducing significantly the

Among other functionalities, the concept of structural health monitoring (SHM) is of great interest in polymer coatings. It is based in an online inspection of the damage extent. In this context, a proper SHM technique must accomplish the following four levels, established by Rytter et al. [22]: (1) detection, (2) localization, (3) quantification of the damage, and (4) the estimation of remaining life, also

Nowadays, there are a lot of different SHM techniques such as lamb waves, fiber optics, and acoustic emission, among others. However, they usually involve complex mathematical and statistical tools and do not often give an overall information of the health of the structure [23, 24]. Therefore, the development of SHM tech-

As commented before, carbon nanoparticles present unique mechanical and, especially, electrical properties in comparison to other materials [25]. Therefore, their addition into an insulating media promotes the creation of electrical networks. This fact induces an enhancement of the electrical conductivity of several orders of

Here, it is important, firstly, to define the concept of **percolation threshold**. It is the critical volume fraction of nanoparticles in which an efficient electrical network is formed, allowing the current flow. It depends on several factors mainly related to the geometry of the nanofiller (including their 0D, 1D, or 2D nature) as well as their dispersion state, that is, their distribution inside the nanocomposite. The determination of the percolation threshold is a crucial factor that determines the minimum

Furthermore, the influence of the different parameters of the nanoparticle network in the percolation threshold has been widely studied in the last years. Li et al. [28] proposed a simple analytical model correlating the geometry, aspect ratio, and

magnitude, becoming the polymer coating electrically conductive [26, 27].

which hinders the direct contacts between electrical nanoparticles.

**3. Smart nanocomposite coatings: Self-sensing**

niques is now gaining a great deal of attention.

**3.1 Fundamentals of SHM with carbon nanoparticles**

content of nanofiller that is needed for electrical applications.

*Schematics of the four SHM levels established by Rytter et al.*

peak heat release rate (PHRR).

*Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

known as *prognosis* (**Figure 2**).

**Figure 2.**

**211**

In the last years, the development of **superhydrophobic** surfaces is being extensively researched. Superhydrophobic coatings have a wide range of applications in textile, automotive parts, construction, agriculture, optical, and maritime industry. It is well established that three factors are needed to create a superhydrophobic surface: low surface energy, microscaled roughness, and nanoand microscaled hierarchical surfaces [13]. The hierarchical multiscale surface can be achieved in coatings with high CNT content, forming CNT agglomerates at the microscale and CNT themselves at nanoscale. Polyvinylidine fluoride (PVDF) are commonly used as hydrophobic polymer coating. Water contact angle increases from 105° for neat PVDF to 170° with very high content of CNT [14]. Similar enhancement of hydrophobicity is reached by the addition of graphene nanoplatelets [15]. The superhydrophobicity is usually requested together with other functionalities, such as self-cleaning, anti-icing, and deicing, which will be addressed further on.

The incorporation of graphene into polymers allows increasing the **thermal stability** of polymer composite [16]. Higher specific area and aspect ratio of carbon nanofillers induce higher stability [17]. This enhancement is also accompanied with an important increase of the low thermal conductivity of polymers. Very high enhancements have been reported, from 0.2 W/mK for neat epoxy coating to 20 W/mK for composite with 30% graphene [16]. Graphene is more effective nanofiller than CNT in order to enhance the thermal conductivity of polymer composites. The thermal conductivity of graphene is attributed to phonons and electrons [18], justifying their excellent thermal conductivity (2000 W/mK). For example, copper is considered a good thermal conductor, whose conductivity is 400 W/mK. Here, the contribution of phonons is limited to 1–2% of total. The addition of nonfunctionalized graphene can induce an enhancement of 50% of thermal conductivity. However, when graphene is modified with functional groups, which is able to form covalent and non-covalent bonds with the polymer matrix, the increment can reach 100%. Moreover, a higher 300% of thermal conductivity increment is reached when graphene is modified with a titanium-coupling agent [19]. The researchers [18, 19] explained it by the reduction of interfacial thermal resistance between graphene and polymer due to the surface modification. For the same reason, higher dispersion degree of graphene into polymer matrix also enhances the thermal conductivity of composite.

It is well-known that the **electrical conductivity** is also increased by the addition of graphitic nanofillers. Higher specific area and higher aspect ratio of nanofillers decrease the nanoparticles which lead to reach an important enhancement of electrical conductivity, from 108 to 1010 S/m for the isolating polymers to 0.01–10 S/m for nanocomposites. Here, the functionalization of nanofillers usually implies a low enhancement of electrical conductivity of composites. This is

#### *Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

Graphitic nanoparticles enhances the **barrier properties** of polymer coating due

to the "torturous path effect" and "nano-barrier wall effect," which strongly depends on exfoliation, dispersion, and orientation degree of nanofiller, their aspect ratio, the polymer-nanofiller interface, and the crystallinity of thermoplastic polymer or cross-linking degree of thermosetting resins. The presence of nanofillers constructs tortuous paths, decreasing the diffusion coefficient. The orientation of graphene and their high surface area forms a zigzag diffusion pathway hindering the diffusion of corrosion species. In addition, their excellent electrical conductivity prevents the electrons form the cathodic site by providing an alternative path [11]. The functionalization of graphitic nanofillers with polar groups [12] enhances the ionic resistance of coating by the creation of negative charge on the graphitic nanofillers surface when exposed to alkaline and neutral environment, preventing

In the last years, the development of **superhydrophobic** surfaces is being extensively researched. Superhydrophobic coatings have a wide range of applications in textile, automotive parts, construction, agriculture, optical, and maritime

superhydrophobic surface: low surface energy, microscaled roughness, and nanoand microscaled hierarchical surfaces [13]. The hierarchical multiscale surface can be achieved in coatings with high CNT content, forming CNT agglomerates at the microscale and CNT themselves at nanoscale. Polyvinylidine fluoride (PVDF) are commonly used as hydrophobic polymer coating. Water contact angle increases from 105° for neat PVDF to 170° with very high content of CNT [14]. Similar enhancement of hydrophobicity is reached by the addition of graphene

nanoplatelets [15]. The superhydrophobicity is usually requested together with other functionalities, such as self-cleaning, anti-icing, and deicing, which will be

The incorporation of graphene into polymers allows increasing the **thermal stability** of polymer composite [16]. Higher specific area and aspect ratio of carbon nanofillers induce higher stability [17]. This enhancement is also accompanied with an important increase of the low thermal conductivity of polymers. Very high enhancements have been reported, from 0.2 W/mK for neat epoxy coating to 20 W/mK for composite with 30% graphene [16]. Graphene is more effective nanofiller than CNT in order to enhance the thermal conductivity of polymer composites. The thermal conductivity of graphene is attributed to phonons and electrons [18], justifying their excellent thermal conductivity (2000 W/mK). For example, copper is considered a good thermal conductor, whose conductivity is 400 W/mK. Here, the contribution of phonons is limited to 1–2% of total. The addition of nonfunctionalized graphene can induce an enhancement of 50% of thermal conductivity. However, when graphene is modified with functional groups, which is able to form covalent and non-covalent bonds with the polymer matrix, the increment can reach 100%. Moreover, a higher 300% of thermal conductivity increment is reached when graphene is modified with a titanium-coupling agent [19]. The researchers [18, 19] explained it by the reduction of interfacial thermal resistance between graphene and polymer due to the surface modification. For the same reason, higher dispersion degree of graphene into polymer matrix also

It is well-known that the **electrical conductivity** is also increased by the addition of graphitic nanofillers. Higher specific area and higher aspect ratio of nanofillers decrease the nanoparticles which lead to reach an important enhancement of electrical conductivity, from 108 to 1010 S/m for the isolating polymers to 0.01–10 S/m for nanocomposites. Here, the functionalization of nanofillers usually

implies a low enhancement of electrical conductivity of composites. This is

industry. It is well established that three factors are needed to create a

the diffusion of chlorine and hydroxyl anions.

*21st Century Surface Science - a Handbook*

enhances the thermal conductivity of composite.

addressed further on.

**210**

associated to the reduction of the electrical conductivity of neat nanofillers due to the partial breakage of some C-C structure during the functionalization and the surrounding isolating polymer layer formed over the functionalized nanofillers, which hinders the direct contacts between electrical nanoparticles.

Nanoreinforced coatings are being studied to improve the **flame retardancy** (FR) of flammable substrates [20]. The addition of graphene usually reduces the total heat release (THR) because they reduce the release of deleterious gas during thermal decomposition, arising from the radical trapping and layered hindering effect. CNT also enhances the FR behavior due to the strengthening of carbonized layers [21]. Also, they act as excellent physical barrier, reducing significantly the peak heat release rate (PHRR).

### **3. Smart nanocomposite coatings: Self-sensing**

Among other functionalities, the concept of structural health monitoring (SHM) is of great interest in polymer coatings. It is based in an online inspection of the damage extent. In this context, a proper SHM technique must accomplish the following four levels, established by Rytter et al. [22]: (1) detection, (2) localization, (3) quantification of the damage, and (4) the estimation of remaining life, also known as *prognosis* (**Figure 2**).

Nowadays, there are a lot of different SHM techniques such as lamb waves, fiber optics, and acoustic emission, among others. However, they usually involve complex mathematical and statistical tools and do not often give an overall information of the health of the structure [23, 24]. Therefore, the development of SHM techniques is now gaining a great deal of attention.

#### **3.1 Fundamentals of SHM with carbon nanoparticles**

As commented before, carbon nanoparticles present unique mechanical and, especially, electrical properties in comparison to other materials [25]. Therefore, their addition into an insulating media promotes the creation of electrical networks. This fact induces an enhancement of the electrical conductivity of several orders of magnitude, becoming the polymer coating electrically conductive [26, 27].

Here, it is important, firstly, to define the concept of **percolation threshold**. It is the critical volume fraction of nanoparticles in which an efficient electrical network is formed, allowing the current flow. It depends on several factors mainly related to the geometry of the nanofiller (including their 0D, 1D, or 2D nature) as well as their dispersion state, that is, their distribution inside the nanocomposite. The determination of the percolation threshold is a crucial factor that determines the minimum content of nanofiller that is needed for electrical applications.

Furthermore, the influence of the different parameters of the nanoparticle network in the percolation threshold has been widely studied in the last years. Li et al. [28] proposed a simple analytical model correlating the geometry, aspect ratio, and

**Figure 2.** *Schematics of the four SHM levels established by Rytter et al.*

dispersion state with the value of the percolation threshold. They concluded that the lower the aspect ratio and the higher the degree of agglomeration of nanoparticles, the higher the percolation threshold. In this context, carbon black (CB) reinforced polymers show very high values of percolation threshold [29] due to their low aspect ratio Bauhofer et al. [30] did an extensive review of percolation threshold in carbon nanotube (CNT)-based polymer nanocomposites by analyzing the effect of nanofiller geometry and dispersion technique. It was observed that the most aggressive dispersion procedures, such as ultrasonication, although leading to the most homogeneous distribution of nanoparticles, lead to a very significant breakage of the CNTs. This prevalent reduction of the aspect ratio leads to increasing values of percolation threshold.

higher the tunneling distance at the initial situation, that is, when no strain is applied, the higher will be the electrical resistance variation associated to the variation in the tunneling distance when applying strain, as can be observed in **Figure 3**. Here, it is necessary to define the concept of gauge factor (GF), as the variation

> *GF* ¼ *ΔR=R***0** *ε*

Therefore, in order to achieve the highest GFs, it would be necessary to work with volume fractions of nanofiller near the percolation threshold, as the distance between adjacent nanoparticles will be the highest possible to form an efficient electrical network and, thus, the variation of the tunneling distance will be the highest. This has been observed in both GNP and CNT nanocomposites, where the

However, in this sense, there are significant differences among the different nanoparticles. For example, GNP-based nanocomposites have shown a more accused exponential behavior of the electrical resistance with applied strain [35] than CNT-based ones [36]. It means that the values of GF at low strain levels are

This accused exponential behavior of GNP nanocomposites can be explained accordingly to the different interactions that take place inside the electrical network. In fact, the tunneling area of these 2D particles is, generally, much higher than in the case of CNTs, and it leads to the fact that the value of the interparticle distance can be much higher for an efficient tunneling transport. Therefore, as explained before, the higher the tunneling distance, the more accused the exponential correlation between the electrical resistance and the applied strain will be.

Moreover, there is also a correlation between the exponential behavior and the sensitivity of the nanocomposite, and, thus, GNP-based ones show higher GF values than CNT nanocomposites (from 12–15 to 2–4 at low strain levels, respectively, for

nanocomposites manufactured following similar techniques) [35, 36].

*Variation of the tunneling resistance as a function of the tunneling distance between nanofillers.*

contents near the percolation threshold achieved the highest GFs [34, 35].

much lower than at high strain levels.

*Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

**Figure 3.**

**213**

(2)

of the normalized resistance *ΔR=R***0** divided by the applied strain, *ε*:

However, the effect of dispersion procedure varies depending on the nanofiller. In this case, ultrasonication has proved to be a good dispersion technique for graphene nanoplatelet (GNP)-based nanocomposites. GNPs are formed by several layers of graphene, and it is widely used as reinforcement in polymer nanocomposites because of the lower cost. Here, ultrasonication promotes the exfoliation of the graphene layers [31]. Therefore, these exfoliating mechanisms induce a reduction of the percolation threshold due to an increase of the aspect ratio of the nanofillers. The combination of an ultrasonication stage with three roll milling can promote the creation of GNP nanocomposites with very low percolation threshold due to the combination of exfoliation stretching effects.

The concept of SHM with nanoparticles, therefore, is based in the monitoring of the changes of the electrical network when subjected to strain or damage. However, for a better understanding of these SHM techniques, it is important to know which are the main conducting mechanisms in the electrical network. Here, three different mechanisms can be identified: the intrinsic conductivity of the nanofiller, the contact between adjacent nanoparticles, and the tunneling transport that takes places between two neighboring particles that are not in intimate contact. Among them, the tunneling transport plays a dominant role in the electrical network of the nanocomposite [32]. It is explained because the associated tunneling resistance is several orders of magnitude higher than the intrinsic and contact resistance. Therefore, the variations of the electrical network when subjected to strain or damage will be ruled by the variation of the tunneling distance between nanoparticles.

In this regard, Simmons [33] established a linear-exponential correlation between the tunneling resistance and the interparticle distance, also known as tunneling distance. It means that the higher the separation between neighboring nanoparticles, the higher the electrical resistance is. More specifically, when subjected to strain, there is a variation of the electrical resistance that is correlated to an increase of the tunneling distance between adjacent nanoparticles:

$$R\_{tumel} = \frac{h^2 t}{A e^2 \sqrt{2m\rho}} \exp\left(\frac{4\pi t}{h} \sqrt{2m\Phi}\right) \tag{1}$$

where *Rtunnel* is the tunneling resistance; *t* is the tunneling distance; h is the Planck's constant; m and *e* are the electron mass and charge; *A* is the area in which electrical transport takes places or tunneling area; and *φ* is the height barrier of the matrix.

#### **3.2 Sensitivity of polymer-based nanocomposites**

In this context, the concept of percolation threshold that has been discussed before plays a key role. In fact, the linear-exponential dependence means that the

#### *Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

dispersion state with the value of the percolation threshold. They concluded that the lower the aspect ratio and the higher the degree of agglomeration of nanoparticles, the higher the percolation threshold. In this context, carbon black (CB) reinforced polymers show very high values of percolation threshold [29] due to their low aspect ratio Bauhofer et al. [30] did an extensive review of percolation threshold in carbon nanotube (CNT)-based polymer nanocomposites by analyzing the effect of nanofiller geometry and dispersion technique. It was observed that the most aggressive dispersion procedures, such as ultrasonication, although leading to the most homogeneous distribution of nanoparticles, lead to a very significant breakage of the CNTs. This prevalent reduction of the aspect ratio leads to increasing values

However, the effect of dispersion procedure varies depending on the nanofiller. In this case, ultrasonication has proved to be a good dispersion technique for graphene nanoplatelet (GNP)-based nanocomposites. GNPs are formed by several layers of graphene, and it is widely used as reinforcement in polymer nanocomposites because of the lower cost. Here, ultrasonication promotes the exfoliation of the graphene layers [31]. Therefore, these exfoliating mechanisms induce a reduction of the percolation threshold due to an increase of the aspect ratio of the nanofillers. The combination of an ultrasonication stage with three roll milling can promote the creation of GNP nanocomposites with very low percolation threshold due to the combination of

The concept of SHM with nanoparticles, therefore, is based in the monitoring

of the changes of the electrical network when subjected to strain or damage. However, for a better understanding of these SHM techniques, it is important to know which are the main conducting mechanisms in the electrical network. Here, three different mechanisms can be identified: the intrinsic conductivity of the nanofiller, the contact between adjacent nanoparticles, and the tunneling transport that takes places between two neighboring particles that are not in intimate contact. Among them, the tunneling transport plays a dominant role in the electrical network of the nanocomposite [32]. It is explained because the associated tunneling resistance is several orders of magnitude higher than the intrinsic and contact resistance. Therefore, the variations of the electrical network when subjected to strain or damage will be ruled by the variation of the tunneling

In this regard, Simmons [33] established a linear-exponential correlation between the tunneling resistance and the interparticle distance, also known as

> *t Ae***2** ffiffiffiffiffiffiffiffiffiffi **2***mφ* p *exp*

where *Rtunnel* is the tunneling resistance; *t* is the tunneling distance; h is the Planck's constant; m and *e* are the electron mass and charge; *A* is the area in which electrical transport takes places or tunneling area; and *φ* is the height barrier of the matrix.

In this context, the concept of percolation threshold that has been discussed before plays a key role. In fact, the linear-exponential dependence means that the

**4***πt h*

ffiffiffiffiffiffiffiffiffiffi **2***mΦ* p � �

(1)

tunneling distance. It means that the higher the separation between neighboring nanoparticles, the higher the electrical resistance is. More specifically, when subjected to strain, there is a variation of the electrical resistance that is correlated to an increase of the tunneling distance between

*Rtunnel* ¼ *h***2**

**3.2 Sensitivity of polymer-based nanocomposites**

of percolation threshold.

*21st Century Surface Science - a Handbook*

exfoliation stretching effects.

distance between nanoparticles.

adjacent nanoparticles:

**212**

higher the tunneling distance at the initial situation, that is, when no strain is applied, the higher will be the electrical resistance variation associated to the variation in the tunneling distance when applying strain, as can be observed in **Figure 3**.

Here, it is necessary to define the concept of gauge factor (GF), as the variation of the normalized resistance *ΔR=R***0** divided by the applied strain, *ε*:

$$\mathbf{GF} = \frac{\Delta \mathbf{R} / \mathbf{R\_0}}{\mathbf{e}} \tag{2}$$

Therefore, in order to achieve the highest GFs, it would be necessary to work with volume fractions of nanofiller near the percolation threshold, as the distance between adjacent nanoparticles will be the highest possible to form an efficient electrical network and, thus, the variation of the tunneling distance will be the highest. This has been observed in both GNP and CNT nanocomposites, where the contents near the percolation threshold achieved the highest GFs [34, 35].

However, in this sense, there are significant differences among the different nanoparticles. For example, GNP-based nanocomposites have shown a more accused exponential behavior of the electrical resistance with applied strain [35] than CNT-based ones [36]. It means that the values of GF at low strain levels are much lower than at high strain levels.

This accused exponential behavior of GNP nanocomposites can be explained accordingly to the different interactions that take place inside the electrical network. In fact, the tunneling area of these 2D particles is, generally, much higher than in the case of CNTs, and it leads to the fact that the value of the interparticle distance can be much higher for an efficient tunneling transport. Therefore, as explained before, the higher the tunneling distance, the more accused the exponential correlation between the electrical resistance and the applied strain will be.

Moreover, there is also a correlation between the exponential behavior and the sensitivity of the nanocomposite, and, thus, GNP-based ones show higher GF values than CNT nanocomposites (from 12–15 to 2–4 at low strain levels, respectively, for nanocomposites manufactured following similar techniques) [35, 36].

**Figure 3.** *Variation of the tunneling resistance as a function of the tunneling distance between nanofillers.*

#### **3.3 SHM in nanocomposite coatings**

The enormous potential of the nanoparticles for SHM applications has been widely exploited in the development of surface sensors and smart coatings. More specifically, their use as substitutes of strain gages is gaining attention nowadays. much higher. Therefore, the amount of nanofiller necessary for SHM purposes is

Furthermore, apart from flexible devices such as strain gauges, wearable electronics, or human motion sensors, their SHM capabilities in other polymer coating based on thermosetting polymers have been widely demonstrated. More specifically, GNP-based coatings have proved to be very sensitive at low strain levels to both tensile and compressive loading, as well as a good repeatability under cyclic loads [42]. In addition, CNT-based ones have also demonstrated good sensing

Although the interest as strain sensing devices is very significant, their crack sensing capabilities can be even of more interest. Here, the crack detection is based in a sudden breakage of the electrical pathways due to the presence of the crack itself. It will be reflected in a sharp increase of the electrical resistivity of the coating, and, thus, the electrical resistance during the measurement will increase as

In this regard, electrical impedance tomography is gaining a great deal of attention as an SHM technique. It is based in a mapping of the electrical conductivity of a structure based in the electrical resistance measurements on its surface. Therefore, by using this technique, it will be possible to detect, locate, and even quantify

*(a) Schematics of electrode disposition and (b) mapping of variation of the electrical resistance correlated to an*

much higher in polymer coatings.

*Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

properties with a high linearity [43].

**Figure 5.**

**215**

*induced damage [43].*

well, as shown in the schematics of **Figure 4**.

Basically, a strain gauge is a device for indicating the strain level of a structure at the point of attachment. To date, the most used are based in conventional metallic foils in which the strain is obtained from the electrical resistance variation due to the deformation of the metallic foil when subjected to this strain level. GF values of conventional strain gauges are around 2 and usually show a very linear dependence of electrical resistance change with applied strain.

The research in strain-sensing devices with carbon-based nanocomposites is mainly focused in the development of highly stretchable sensors. In this regard, graphene, carbon nanotubes, and other carbon nanoparticles, such as carbon black, have been widely explored, among others. The addition of these nanoparticles to polymers with high strain capabilities such as fluoroelastomers [37], thermoplastic polyurethane [38], or vulcanized silicone [39] has demonstrated excellent sensing capabilities. More specifically, they present enormous potential for human motion sensing or wearable electronics [37] as the GF at high strain levels (>20%) can be in the range of 400–4000 depending on the content and morphology of the carbon nanofiller. Here, a highly accused exponential behavior is observed at higher strain levels due to the prevalence of tunneling mechanisms in the carbon nanoparticle network. In addition, they can be also used as pressure sensors, with excellent sensing capabilities in comparison to others [40] as they are able to detect very small pressure changes due to the strain induced fail that they promote.

Here, it can be stated that the 2D nature of the electrical network in a nanocomposite coating promotes an increase of the percolation threshold when compared to a bulk nanocomposites, where a 3D uniform disposition of the nanofillers is supposed to be [41]. Moreover, the cross-sectional area of the coatings is obviously much lower than 3D nanocomposites, so the electrical resistance is

**Figure 4.** *Schematics of the effect of a crack in the electrical network.*

#### *Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

**3.3 SHM in nanocomposite coatings**

*21st Century Surface Science - a Handbook*

**Figure 4.**

**214**

*Schematics of the effect of a crack in the electrical network.*

of electrical resistance change with applied strain.

The enormous potential of the nanoparticles for SHM applications has been widely exploited in the development of surface sensors and smart coatings. More specifically, their use as substitutes of strain gages is gaining attention nowadays. Basically, a strain gauge is a device for indicating the strain level of a structure at the point of attachment. To date, the most used are based in conventional metallic foils in which the strain is obtained from the electrical resistance variation due to the deformation of the metallic foil when subjected to this strain level. GF values of conventional strain gauges are around 2 and usually show a very linear dependence

The research in strain-sensing devices with carbon-based nanocomposites is mainly focused in the development of highly stretchable sensors. In this regard, graphene, carbon nanotubes, and other carbon nanoparticles, such as carbon black, have been widely explored, among others. The addition of these nanoparticles to polymers with high strain capabilities such as fluoroelastomers [37], thermoplastic polyurethane [38], or vulcanized silicone [39] has demonstrated excellent sensing capabilities. More specifically, they present enormous potential for human motion sensing or wearable electronics [37] as the GF at high strain levels (>20%) can be in the range of 400–4000 depending on the content and morphology of the carbon nanofiller. Here, a highly accused exponential behavior is observed at higher strain levels due to the prevalence of tunneling mechanisms in the carbon nanoparticle network. In addition, they can be also used as pressure sensors, with excellent sensing capabilities in comparison to others [40] as they are able to detect very

small pressure changes due to the strain induced fail that they promote. Here, it can be stated that the 2D nature of the electrical network in a nanocomposite coating promotes an increase of the percolation threshold when compared to a bulk nanocomposites, where a 3D uniform disposition of the

nanofillers is supposed to be [41]. Moreover, the cross-sectional area of the coatings is obviously much lower than 3D nanocomposites, so the electrical resistance is

much higher. Therefore, the amount of nanofiller necessary for SHM purposes is much higher in polymer coatings.

Furthermore, apart from flexible devices such as strain gauges, wearable electronics, or human motion sensors, their SHM capabilities in other polymer coating based on thermosetting polymers have been widely demonstrated. More specifically, GNP-based coatings have proved to be very sensitive at low strain levels to both tensile and compressive loading, as well as a good repeatability under cyclic loads [42]. In addition, CNT-based ones have also demonstrated good sensing properties with a high linearity [43].

Although the interest as strain sensing devices is very significant, their crack sensing capabilities can be even of more interest. Here, the crack detection is based in a sudden breakage of the electrical pathways due to the presence of the crack itself. It will be reflected in a sharp increase of the electrical resistivity of the coating, and, thus, the electrical resistance during the measurement will increase as well, as shown in the schematics of **Figure 4**.

In this regard, electrical impedance tomography is gaining a great deal of attention as an SHM technique. It is based in a mapping of the electrical conductivity of a structure based in the electrical resistance measurements on its surface. Therefore, by using this technique, it will be possible to detect, locate, and even quantify

superficial defects by analyzing the changes of its surface resistivity. In this context, their effectiveness has been widely demonstrated in polymer coatings for the detection of superficial cracks [43], where the electrical resistance measurement between adjacent channels can easily detect an artificial damage (**Figure 5**), as well as in sensing skins for spatial pressure mapping, where the strain induced by the applied pressure is monitored [44]. Here, the main issue is correlated to the positioning of the electrodes and the data processing, which usually involves the use of complex mathematical tools. However, the results for SHM applications are very promising and give a new functionality to nanoreinforced polymer coatings.

where *Q* is the heat generated, *i* is the current flow, *R* is the electrical resistance,

The first thing that can be analyzed from Eq. (1) is that higher current intensity would lead to higher heat generated and, consequently, higher contents of carbon nanoparticles will be desired for this purpose in order to increase the temperature

Apart from the heat generated, there is an important fact regarding these percolated electrically conductive networks, which is the homogeneous distribution of heat through the coating. Two important effects must be taken into account for this aspect: (1) thermal conductivity of polymers which is particularly low, thus making heat transfer through the coating more difficult, and (2) homogeneous presence of the carbon nanoparticles through the polymer matrix, which is not always easily

Both CNTs and GNPs show extremely good thermal conductivity individually. Nevertheless, in spite of their similar intrinsic thermal conductivity, the morphology of GNPs makes them more interesting for this purpose, even when compared to SWCNT [50]. Even at the same content of both types of nanoparticles, Zakaria et al. found that the thermal conductivity was higher for GNP nanocomposites than the MWCNT ones. In fact, although for electrical properties, higher contents of GNPs are usually required to meet similar properties to the ones found for MWCNT, in that case, at only 3 wt.% of nanoreinforcement, GNP nanocomposites showed an increase of 126.4% in thermal conductivity while 3 wt.% of MWCNT only increased this property by 60.2% [51]. In fact, experimental values of thermal conductivity are usually lower than those predicted theoretically, and it has been attributed mainly to waviness, dispersion, alignment, interfacial resistance, and contact

Proper exfoliation of GNPs causes an important increment on thermal conductivity related to an increase of the aspect ratio. Chu et al. [53] proposed a model to calculate the thermal conductivity of nanocomposites based on randomly oriented nanoparticles which takes into account geometrical aspects of the nanoparticles (aspect ratio) as well as differences in the intrinsic thermal conductivity of the nanofiller in each direction. In the case of GNPs, these aspects will be strongly related to their exfoliation and dispersion in the polymer matrix. On the other hand, the waviness of the nanoreinforcement may reduce the effective aspect ratio of the nanofillers which lead to propose few layers GNPs as an optimal solution instead of

These self-heating coatings do not require extremely high thermal conductivities, but they should be high enough to ensure good heat transfer through the whole

The formation of aggregates is very common in this type of materials, and this may cause that at very low carbon nanoparticle loadings, some resin areas are free of nanoreinforcement, which leads to nonuniform heating of the samples. In that

individual monolayers that tend to roll up easier during dispersion stage.

surface for the purposes mentioned above.

reached or to reduce the voltage required. Although all common carbon nanostructures can be used for this purpose (carbon black, carbon nanotubes, graphene nanoplatelets, or even graphite flakes), the importance of reaching high intensity values usually gives the best results for CNT-filled materials [47]. In fact, very high CNT amounts can be found in the literature in order to increase the electrical conductivity and, consequently, the current flowing at lower voltages applied. This is the case of the study based on ABS as matrix where CNT was added up to 15 wt.% in order to allow reaching temperatures over 200°C when voltages of only 12 V were applied [48] or the research carried out by Chu et al. where similar results in terms of temperature and voltage at contents of 7.5 wt.% of CNT in PDMS were found [49]. The interest in the use of low voltages is based on the use of

batteries commonly installed in cars and trucks, among others.

and t is the time the current is applied.

*Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

reached.

resistance [52].

**217**


*4.4. Dynamic Schedule of EVs with*/*without Appointments*

Three cases are considered in this investigation:

The arrival and departure times are randomly assigned within the eight timeslots. The users with prior booking need to provide the expected arrival time, departure time, and charging demand. For EVs without booking, the EVs' expected departure time and charging demand are provided when they reach the CS. To keep the same total demand in the three cases, only the arrival and departure times are randomly generated. In each case, the results are obtained using the SFLA and then compared with the PSO and FIFS scheduling algorithms. The dynamic system can determine a charging schedule at the beginning of each timeslot using all the EVs with and without booking, and then the PL can charge arrived vehicles with the use of the schedule for the immediate timeslot. At the beginning of each timeslot, the computational process has to be executed.

It can be seen that the electricity purchasing cost of the PL is reduced when the EVs arrived with reservation. Using FIFS, the charging price for cases 2 and 3 is increased by 3.26% and 3.11%, respectively, compared to the base case. Using SFLA, the charging price for cases 2 and 3 increased by 2.06% and 3.03%, respectively, compared to the base case (case 1). Also, using PSO, the charging price for cases 2 and 3 is increased by 2.70% and 2.89%, respectively, compared with case 1. The cost comparison with and without prior booking is given in Table 13, in which the MG case is not considered. Unexpected arrival and departure are considered for unappointed vehicles, as shown in Tables 14 and 15. Table 16 shows the scheduling when 5/20 vehicles have arrived without a booking, and Table 17 shows the scheduling of 10/20 vehicles have arrived without booking. These two cases are performed to minimize the electricity purchase cost, and the results show that the charging demands can be fulfilled in the three scenarios. Also, the needs are the same for the three cases; hence the cost can be compared.

**Table 13.** Cost comparison of the charging schedule of EVs arriving with/without prior booking.

**Table 14.** Unexpected arrival and departure considered for unappointed vehicles: when 5/20 EVs arrived without an appointment.

**Table 15.** Unexpected arrival and departure considered for unappointed vehicles: when 10/20 EVs arrived without an appointment.

**Table 16.** Unexpected arrival and departure considered for unappointed vehicles: schedule when 5/20 EVs have arrived without an appointment.

**Table 17.** Unexpected arrival and departure considered for unappointed vehicles: schedule when 10/20 EVs have arrived without an appointment.

In Table 13, the comparison shows that the electricity cost is less than that obtained using the FIFS scheme.

FIFS scheme.

Furthermore, when the MG supplies the PL, the charging costs are reduced, as presented in Table 18.  Charging costs without MG consideration: (1) 20/20 EVs arriving with booking; (2) 15/20 EVs arriving with booking; and (3) 10/20 EVs arriving with booking. **Figure 6.** Charging costs without MG consideration: (1) 20/20 EVs arriving with booking; (2) 15/20 EVs arriving with booking; and (3) 10 *Energies* **2020**, *13*, x FOR PEER REVIEW /20 EVs arriving with booking. 21 of 25

**Figure 7.** Charging costs with MG consideration: (1) 20/20 EVs arriving with booking; (2) 15/20 EVs arriving with booking; and (3) 10/20 EVs arriving with booking. **Figure 7.** Charging costs with MG consideration: (1) 20/20 EVs arriving with booking; (2) 15/20 EVs arriving with booking; and (3) 10/20 EVs arriving with booking.

#### **5. Conclusions 5. Conclusions**

reservation.

**6. Future Works**

version of the manuscript.

CC Constant current

CS Charging station CSP Charging service provider CV Constant voltage

**Abbreviations**

**Funding:** This research received no external funding.

CCCV Constant current constant voltage

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

A dynamic charging scheduling scheme for charging EVs in a PL is proposed for minimizing the charging costs. First, a conventional FIFS scheduling scheme is performed for the EV charging. Economic scheduling is found using PSO and SFLA. In this regard, the dynamic charging scheme allocates the optimal electric power in each slot for the vehicles that have arrived. Scheduling is undertaken for the vehicles that have arrived at the PL with and without prior booking. This scheme considers the electricity price at each hour and the charging limit of the PL every hour for all the three methods considered. When the electricity price is low, the entire timeslot is fully utilized to charge A dynamic charging scheduling scheme for charging EVs in a PL is proposed for minimizing the charging costs. First, a conventional FIFS scheduling scheme is performed for the EV charging. Economic scheduling is found using PSO and SFLA. In this regard, the dynamic charging scheme allocates the optimal electric power in each slot for the vehicles that have arrived. Scheduling is undertaken for the vehicles that have arrived at the PL with and without prior booking. This scheme

> the EVs effectively. Also, the PL operator provides 100% of the EV user's power within the available time. The results showed that more significant savings could be reached if the EV arrives with a prior

**Author Contributions:** Conceptualization, G.S.F. and V.K.; methodology, G.S.F.; software, S.K.; validation, J.S.A., Z.M.A. and S.H.E.A.A.; formal analysis, G.S.F.; investigation, V.K.; resources, S.K.; data curation, G.S.F..; writing—original draft preparation, S.H.E.A.A.; writing—review and editing, J.S.A. and Z.M.A.; visualization, V.K.; supervision, A.E.-S.; project administration, A.E.-S. All authors have read and agreed to the published

available at the CS may help floating customers who want quick charging.

In this work, the charging demand of each EV is considered as 100%, but this limit can vary, and random charging demand can be considered. Service delay can be regarded to benefit both the user and CS operator. Reserve charging points can be taken into account, and the charging can be undertaken for users with a special tariff based on the CS operator's decision. This kind of option considers the electricity price at each hour and the charging limit of the PL every hour for all the three methods considered. When the electricity price is low, the entire timeslot is fully utilized to charge the EVs effectively. Also, the PL operator provides 100% of the EV user's power within the available time. The results showed that more significant savings could be reached if the EV arrives with a prior reservation.


**Hidayat Hussain**

Hidayat Hussain obtained a Ph.D. degree in Medicinal Chemistry from University of Karachi Pakistan in 2004. He did post-doctorate (from 06-2004 to 09-2007) from University of Paderborn Germany and University of Maine France (from 10-2007 to 09-2009) along with senior scientist (from 01-2009 to 10-2010) at University of Paderborn Germany. Dr. Hidayat was also a Visiting Professor at Scripps Institution of Oceanography, San Diego USA from May 2017 to September 2017. He was also associate professor in University of Nizwa (Oman) from 2011 to 2017. Currently, Dr. Hidayat is associated with Leibniz Institute of Plant Biochemistry Halle, Germany. His research interests include the discovery of medicines from nature as synthesis of bioactive molecules for cancer, diabetes, malaria, and hepatitis, Alzheimer's disease. To date, he has authored and coauthored over 340 international publications with a cumulative impact factor of more than 1000 and given over 40 podium lectures at International Conferences. He published one book, over 10 book chapters and various review articles in International reputed Journals viz., Chemical Reviews and Natural Product Reports. He is a section Editor-in-Cheif of "Current Issues in Molecular Biology"Journal along with member of editorial board of several International Journals and also serving as a referee for over 100 international journals. Moreover he has over 7500 citations with h-index 40. According to the Stanford University database, his name is included in the top two percent of the most-cited scientists in various disciplines for 2020 and 2021.


**5. Discussion**

The study highlights the importance of educational attainment in explaining the duration of youth unemployment. Indeed, the fact that young women are unemployed for long periods of time shows that they have low levels of education compared to young men. Several reasons can be given to explain this result. We can mention: the persistent reluctance in the schooling of the girl child in certain regions of the country; the early exits of the girl child from the educational system for household support, because of the weight of tradition relegating the role of the woman, primarily to household chores or to forced marriage and school pregnancies.

Policies for free schooling, especially for girls, should be evaluated and reoriented so that they can achieve their objectives. This new orientation could concern the complete care of the girl up to secondary school. In addition, the "zero school pregnancy" policy should be accentuated. In the short term, a policy to support women's entrepreneurship through a series of training courses and easy access to finance could be a real lever to reduce unemployment among young girls in Côte d'Ivoire. These policies should also integrate the needs of parents through incentives.


Noble and Precious Metals

Properties, Nanoscale Effects and Applications

*Edited by Mohindar Singh Seehra and Alan D. Bristow*

**NOBLE AND PRECIOUS**

**METALS - PROPERTIES,**

**NANOSCALE EFFECTS**

**AND APPLICATIONS**

Edited by **Mohindar Singh Seehra**

and **Alan D. Bristow**


**2. Marmosets**

One female and four male common marmosets, aged 2 to 4 years old and weighing 285- 368g, were used for this assessment (Kodama et al., 2007). Four of the marmosets were inoculated intranasally with 1ml of EHV-9 virus solution containing 106 plaque-forming units. The other was inoculated with 1 ml of MEM as a negative control. The inoculated animals were humanely euthanatized on Days 3, 4 and 5 following inoculation, respectively, at a point when they were in poor condition or dying from a neuronal disorder. The virus was recovered and identified by polymerase chain reaction (PCR) with the primers targeting the EHV-9 specific region of the EHV-9 gene. The PCR primers used for amplification were 5'-CTGGGTTATAGATTGTCGCCTC-3' and 5'-CCCAGAAAGTATTACACGCGAT-3'. The neutralization test was done using the 50% plaque reduction method with the MDBK cell monolayer.

Equine Herpesvirus 9 (EHV-9) Induced Encephalitis in Nonhuman Primates 149

The lesions then spread to the basal ganglia, except for the globus pallidus, and the limbic lobe, including the hippocampus and cingulate gyrus, basal forebrain and temporal lobe in the animals (Fig. 5). The animal euthanized at 5 dpi had widely distributed lesions in the cerebrum. No abnormalities were observed in the cerebellum or spinal cord. Immunohistochemistry to EHV-9 antibody revealed positive reactions in the nucleus, intranuclear inclusions and the cytoplasm in degenerating neuronal cells, and occasionally in normal-appearing neuronal cells in the marginal zone or surroundings of affected areas in the brain (Fig. 6). Prominent inclusion bodies appearing as the full-inclusion type had the

Fig. 2. Olfactory bulb; Neuronal necrosis with intra-nuclear inclusion bodies, appearing as

Cowdry type A (arrow) or full-inclusion (arrow head) inclusions. HE. Bar=3.5μm.

Fig. 3. Olfactory bulb; Frequent intra-nuclear inclusions. HE.

most intensely positive reaction around the nuclear membrane.

All four of the inoculated animals exhibited various neurological signs and finally collapsed. From the second day post-inoculation (dpi), all four inoculated marmosets began to exhibit signs of anorexia and depression. A decrease in body weight was noted from 3 dpi. At 3 dpi, rectal temperatures in each of the treated animals were approximately 2 to 3 degrees lower than those in the non-treated control animal, although the animals recovered from the hypothermia the following day. From 3 to 5 dpi, the inoculated animals showed varying degrees of neurological signs consisting of salivation (Fig. 1), star gazing, convulsions and finally coma. At 4 and 5 dpi the animals refused food, became severely depressed, and collapsed. One animal was euthanized based on schedule, but two animals were euthanized at 3 dpi due to poor condition, and the last one was euthanized at 5 dpi. The control animal was euthanized at 6 dpi.

Fig. 1. Marked salivation with a large amount of foamy saliva at 4 dpi.

At necropsy, no significant abnormalities were observed in the external or visceral organs in any of the inoculated animals.

Histologically, the affected animals had severe encephalitis characterized by neuronal degeneration and necrosis with intra-nuclear inclusions, which extended from the olfactory bulb to the rhinencephalon and piriform lobe. The nuclei of affected degenerating neuronal cells exhibited severe degrees of chromatolysis or karyorrhexis, eventually resulting in neuronal loss and cellular debris (Fig. 2). The degenerating neuronal cells had frequent intra-nuclear inclusion bodies which were equivalent to herpesvirus in morphology, appearing as full-inclusion or Cowdry type A inclusions (Figs. 2 and 3). The inclusion bodies were particularly frequent in the olfactory bulb, piriform lobe and amygdaloid body in the affected animals. Cerebrum lesions were observed in all layers of the olfactory bulb, the piriform lobe (Fig. 4) and a part of the temporal lobe in an animal euthanized at 3 dpi.

All four of the inoculated animals exhibited various neurological signs and finally collapsed. From the second day post-inoculation (dpi), all four inoculated marmosets began to exhibit signs of anorexia and depression. A decrease in body weight was noted from 3 dpi. At 3 dpi, rectal temperatures in each of the treated animals were approximately 2 to 3 degrees lower than those in the non-treated control animal, although the animals recovered from the hypothermia the following day. From 3 to 5 dpi, the inoculated animals showed varying degrees of neurological signs consisting of salivation (Fig. 1), star gazing, convulsions and finally coma. At 4 and 5 dpi the animals refused food, became severely depressed, and collapsed. One animal was euthanized based on schedule, but two animals were euthanized at 3 dpi due to poor condition, and the last one was euthanized at 5 dpi. The control animal

Fig. 1. Marked salivation with a large amount of foamy saliva at 4 dpi.

At necropsy, no significant abnormalities were observed in the external or visceral organs in

Histologically, the affected animals had severe encephalitis characterized by neuronal degeneration and necrosis with intra-nuclear inclusions, which extended from the olfactory bulb to the rhinencephalon and piriform lobe. The nuclei of affected degenerating neuronal cells exhibited severe degrees of chromatolysis or karyorrhexis, eventually resulting in neuronal loss and cellular debris (Fig. 2). The degenerating neuronal cells had frequent intra-nuclear inclusion bodies which were equivalent to herpesvirus in morphology, appearing as full-inclusion or Cowdry type A inclusions (Figs. 2 and 3). The inclusion bodies were particularly frequent in the olfactory bulb, piriform lobe and amygdaloid body in the affected animals. Cerebrum lesions were observed in all layers of the olfactory bulb, the piriform lobe (Fig. 4) and a part of the temporal lobe in an animal euthanized at 3 dpi.

was euthanized at 6 dpi.

any of the inoculated animals.

The lesions then spread to the basal ganglia, except for the globus pallidus, and the limbic lobe, including the hippocampus and cingulate gyrus, basal forebrain and temporal lobe in the animals (Fig. 5). The animal euthanized at 5 dpi had widely distributed lesions in the cerebrum. No abnormalities were observed in the cerebellum or spinal cord. Immunohistochemistry to EHV-9 antibody revealed positive reactions in the nucleus, intranuclear inclusions and the cytoplasm in degenerating neuronal cells, and occasionally in normal-appearing neuronal cells in the marginal zone or surroundings of affected areas in the brain (Fig. 6). Prominent inclusion bodies appearing as the full-inclusion type had the most intensely positive reaction around the nuclear membrane.

Fig. 2. Olfactory bulb; Neuronal necrosis with intra-nuclear inclusion bodies, appearing as Cowdry type A (arrow) or full-inclusion (arrow head) inclusions. HE. Bar=3.5μm.

Fig. 3. Olfactory bulb; Frequent intra-nuclear inclusions. HE.

Equine Herpesvirus 9 (EHV-9) Induced Encephalitis in Nonhuman Primates 151

Fig. 6. Olfactory bulb. Most neuronal cells in all layers of the glomerular, mitral and granule

In the nasal cavities of all of the animals inoculated with EHV-9, there was severe necrotizing rhinitis characterized by severe necrosis of the olfactory epithelium and olfactory glands in the olfactory area (Fig. 7). The marmosets that had been inoculated exhibited more severe desquamation of the olfactory epithelium, as well as degeneration and necrosis at 3 dpi. In addition, occasional olfactory Schwann cells were degenerative or necrotic. Frequent intra-neuronal inclusion bodies were seen in the degenerating epithelial cells in both the olfactory epithelium and olfactory glandular epithelium (Fig. 8). In the respiratory area, sight focal necrosis and ablation were occasionally observed in the mucosa. Immunohistochemically, there were frequent positive reactions in both intra-nuclear inclusion bodies and the cytoplasm of degenerating cells in the olfactory epithelium and olfactory glandular epithelium in all animals inoculated with EHV-9 (Figs. 9, 10). Occasional degenerating mucosal epithelium in the respiratory area and degenerating Schwann cells

Fig. 7. Olfactory area in the nasal cavity; The degenerating epithelial epithelium and olfactory glandular epithelium with frequent intranuclear inclusions. HE. Bar = 8.6 μm

layers are frequently positive to EHV-9 antigen. ABC method. Bar=5.7μm.

Immunostaining for EHV-9 antigen. Frequent positive reactions.

also had positive reactions.

Fig. 4. Piriform lobe; Neuronal degeneration and necrosis with intra-nuclear inclusions.

Fig. 5. Time-course extension of EHV-9 induced encephalitis in the brain.

Fig. 4. Piriform lobe; Neuronal degeneration and necrosis with intra-nuclear inclusions.

Fig. 5. Time-course extension of EHV-9 induced encephalitis in the brain.

Fig. 6. Olfactory bulb. Most neuronal cells in all layers of the glomerular, mitral and granule layers are frequently positive to EHV-9 antigen. ABC method. Bar=5.7μm. Immunostaining for EHV-9 antigen. Frequent positive reactions.

In the nasal cavities of all of the animals inoculated with EHV-9, there was severe necrotizing rhinitis characterized by severe necrosis of the olfactory epithelium and olfactory glands in the olfactory area (Fig. 7). The marmosets that had been inoculated exhibited more severe desquamation of the olfactory epithelium, as well as degeneration and necrosis at 3 dpi. In addition, occasional olfactory Schwann cells were degenerative or necrotic. Frequent intra-neuronal inclusion bodies were seen in the degenerating epithelial cells in both the olfactory epithelium and olfactory glandular epithelium (Fig. 8). In the respiratory area, sight focal necrosis and ablation were occasionally observed in the mucosa. Immunohistochemically, there were frequent positive reactions in both intra-nuclear inclusion bodies and the cytoplasm of degenerating cells in the olfactory epithelium and olfactory glandular epithelium in all animals inoculated with EHV-9 (Figs. 9, 10). Occasional degenerating mucosal epithelium in the respiratory area and degenerating Schwann cells also had positive reactions.

Fig. 7. Olfactory area in the nasal cavity; The degenerating epithelial epithelium and olfactory glandular epithelium with frequent intranuclear inclusions. HE. Bar = 8.6 μm

Equine Herpesvirus 9 (EHV-9) Induced Encephalitis in Nonhuman Primates 153

Fig. 10. Olfactory area in the nasal cavity; Positive reaction is seen at the intra-nuclear inclusion bodies (arrows), nucleus and cytoplasm (arrow head) in the degenerating olfactory glandular epihelial cells. Immunohistochemical stain for EHV-9. ABC method. Bar=3.3μm. EHV-9 was recovered from the olfactory bulb and frontal lobe in all inoculated marmosets. In addition, EHV-9 was isolated from the lung in one marmoset and from the salivary gland and mandible lymph node in one marmoset. The recovered virus was confirmed to be EHV-9 by the PCR method. The EHV-9 virus was not neutralized by 4-times dilution serum in the

EHV-9 demonstrated ready infectivity and rapid progression of the disease in marmosets, which indicates the possibility of EHV-9 infectivity in primates, including humans, as in other domestic animals and rodents. Nasal inoculation of EHV-9 resulted in varying degrees of encephalitis in a wide variety of animals, including rodents, domestic ruminants and companion animals. As with other novel herpesvirus infections such as pseudorabies virus infection in dogs (Yanai et al., 2003a) and B virus infection in humans (Adam and Graham, 1994), EHV-9 could possibly be more virulent in unusual hosts than in the natural host. The pathogenicity of EHV-9 to humans is currently unknown; however, we may be able to assess the virulence of EHV-9 in humans to a certain extent by using nonhuman primates as a model because nonhuman primates have many infectious diseases in common with humans. It was estimated that EHV-9 could possibly be pathogenic to humans if nonhuman primates were infected with it. We used common marmosets for the experimental infection of EHV-9 because they are commonly used for experiments and because they are known to be susceptible to some herpesviruses. Generally, nonhuman primates are highly susceptible to cross-species infectivity by human alpha-herpesvirus, including herpes simplex virus (HSV). In new world monkeys, HSV causes outbreaks characterized by severe ulcerative gingivostomatitis and encephalitis, and these outbreaks are quick to occur in zoo-housed marmosets, with marmosets being thought to be highly susceptible to HSV (Matz-Rensing et al., 2003). On the other hand, simian herpes virus, Herpesvirus B (Cercopithecine herpesvirus 1), has been implicated as the cause of approximately 40 cases of meningoencephalitis affecting persons in direct or indirect contact with laboratory

In marmosets, as in other animals, EHV-9 shows strong neurotropism and causes selective neuronal necrosis. The distribution of lesions and the virus are most similar to those in cats

neutralization test. No virus was isolated from other organs or blood.

macaques (Weigler, 1992).

Fig. 8. Olfactory area in the nasal cavity; Frequent intra-neuronal inclusion bodies are seen epithelium and olfactory glandular epithelium. HE.

Fig. 9. Olfactory area in the nasal cavity; Positive reaction is seen in the degenerating epithelial cells in both the olfactory epithelium and olfactory glandular epithelium. Immunohistochemical stain for EHV-9. ABC method. Bar=8.6μm.

Fig. 8. Olfactory area in the nasal cavity; Frequent intra-neuronal inclusion bodies are seen

Fig. 9. Olfactory area in the nasal cavity; Positive reaction is seen in the degenerating epithelial cells in both the olfactory epithelium and olfactory glandular epithelium.

Immunohistochemical stain for EHV-9. ABC method. Bar=8.6μm.

epithelium and olfactory glandular epithelium. HE.

Fig. 10. Olfactory area in the nasal cavity; Positive reaction is seen at the intra-nuclear inclusion bodies (arrows), nucleus and cytoplasm (arrow head) in the degenerating olfactory glandular epihelial cells. Immunohistochemical stain for EHV-9. ABC method. Bar=3.3μm.

EHV-9 was recovered from the olfactory bulb and frontal lobe in all inoculated marmosets. In addition, EHV-9 was isolated from the lung in one marmoset and from the salivary gland and mandible lymph node in one marmoset. The recovered virus was confirmed to be EHV-9 by the PCR method. The EHV-9 virus was not neutralized by 4-times dilution serum in the neutralization test. No virus was isolated from other organs or blood.

EHV-9 demonstrated ready infectivity and rapid progression of the disease in marmosets, which indicates the possibility of EHV-9 infectivity in primates, including humans, as in other domestic animals and rodents. Nasal inoculation of EHV-9 resulted in varying degrees of encephalitis in a wide variety of animals, including rodents, domestic ruminants and companion animals. As with other novel herpesvirus infections such as pseudorabies virus infection in dogs (Yanai et al., 2003a) and B virus infection in humans (Adam and Graham, 1994), EHV-9 could possibly be more virulent in unusual hosts than in the natural host. The pathogenicity of EHV-9 to humans is currently unknown; however, we may be able to assess the virulence of EHV-9 in humans to a certain extent by using nonhuman primates as a model because nonhuman primates have many infectious diseases in common with humans. It was estimated that EHV-9 could possibly be pathogenic to humans if nonhuman primates were infected with it. We used common marmosets for the experimental infection of EHV-9 because they are commonly used for experiments and because they are known to be susceptible to some herpesviruses. Generally, nonhuman primates are highly susceptible to cross-species infectivity by human alpha-herpesvirus, including herpes simplex virus (HSV). In new world monkeys, HSV causes outbreaks characterized by severe ulcerative gingivostomatitis and encephalitis, and these outbreaks are quick to occur in zoo-housed marmosets, with marmosets being thought to be highly susceptible to HSV (Matz-Rensing et al., 2003). On the other hand, simian herpes virus, Herpesvirus B (Cercopithecine herpesvirus 1), has been implicated as the cause of approximately 40 cases of meningoencephalitis affecting persons in direct or indirect contact with laboratory macaques (Weigler, 1992).

In marmosets, as in other animals, EHV-9 shows strong neurotropism and causes selective neuronal necrosis. The distribution of lesions and the virus are most similar to those in cats

Equine Herpesvirus 9 (EHV-9) Induced Encephalitis in Nonhuman Primates 155

organs or tissues, including the nasal cavities or brains of the animals. In the amplification of the gene for gB (ORF33) using specific primers for EHV-9 by PCR, no bands were detected in any of the samples from the right olfactory bulb, right cerebrum or right cerebellum, the blood samples, or the nasal swabs. It was demonstrated that the nasal swabs had no infectivity. The EHV-9 virus was not neutralized by the serum from any of the animals in

The results in this study suggest that EHV-9 may be non-pathogenic for adult cynomolgus monkeys. The etiology for the clinical symptom was not apparent because there were no histopathological changes. However, the symptom may have been a response to the EHV-9

Although lethal encephalitis was induced via the nasal, oral, intraperitoneal and ocular routes of EHV-9 inoculation in hamsters (El-Habashi et al., 2010), the intranasal route may be the most probable one. In addition, EHV-9 replicated in the olfactory epithelium and olfactory glandular epithelium in common marmosets (Kodama et al., 2007). Based on those results, successful infection to the olfactory epithelium might be among the essential factors for following EHV-9 induced encephalitis. The proportion of surface area covered by the olfactory epithelium in macaques may be considerably smaller than that in rodents and dogs (Herkema, 1991). Thus, there may be constitutional barriers to the entrance of the EHV-9 into the olfactory epithelium. However, because it has been suggested that the relative amount of the olfactory epithelium in common marmosets is much closer to that of macaques than that of rats, the constitutional distinction may not be associated with EHV-9

**4. Conclusion: EHV-9 can induce encephalitis in primates, including humans**  In assessments of infectivity of EHV-9 in nonhuman primates, there were different results between new world and old world monkeys. While EHV-9 caused fulminant encephalitis, no infectivity was seen in cynomolgus macaques. It is still uncertain whether macaques have barriers to invasion of EHV-9 at the entrance, and this could apply to humans as well. Thus, it has not been determined that EHV-9 can cross the species barrier between new world and old world monkeys and humans. It is also uncertain whether EHV-9 can infect immunecompromised patients like AIDS patients, infants or elderly people. Ocular transmission of viruses to the CNS has been suspected in many viral diseases such as Cercopithecine herpes virus 1 (B-virus) in humans (CDC, 1999). Fatal infection by B-virus in humans via ocular exposure from biological fluid from macaque monkeys has been reported (CDC, 1999). Further studies using nonhuman primates may be needed to assess the risk to infants from

[1] Adams JH, Graham DI: Virus and other infections. *In*: An Introduction to

[2] Borchers K, Wiik H, Frolich K, Ludwig H, East ML: Antibodies against equine

the Serengeti ecosystem. J Wildl Dis 41: 80-86, 2005

Neuropathology, second edition, pp. 103-132. Churchill Livingstone, Edinburgh,

herpesviruses and equine arteritis virus in Burchell's zebras (Equus burchelli) from

infection induced via the intranasal route (Wako et al., 1999).

the neutralization test.

the ocular route of infection.

**5. References**

1994

inoculation.

(Yanai et al., 2003b). As the most severely affected area was the olfactory bulb, the virus inoculated into the nasal cavity gained access to the olfactory bulb and then progressed along the rhinencephalon over time. In our study, all of the marmosets inoculated intranasally had severe rhinitis with intranuclear inclusions, and EHV-9 antigen was detected in the olfactory epithelium and glands. From these findings, the EHV-9 inoculated into the nasal cavity presumably multiplied in the olfactory mucosa, and then intruded into the nerve sheath connected to the olfactory bulb, going through the foramina in the cribriform plate of the ethmoid bone. As the olfactory epithelium is usually covered by secreta produced in the olfactory glands, the question remains of how the virus infected the epithelial and glandular cells. The inclusion body formation in the glandular epithelium indicated that a large amount of EHV-9 virus had multiplied in the olfactory glands and shed into the nasal cavity. This suggests that, once EHV-9 has infected the nasal mucosa, the marmoset sheds a large enough quantity of virus for transmission to another animal when contact occurs. In addition, a smaller amount of the virus may cause rhinitis with viral propagation once the virus intrudes into the nasal cavity. However, in the present study, it was not clear how the nerve axons were affected by the EHV-9 in the olfactory mucosa, or how the virus in the nerve sheath migrated to the olfactory bulb.

The study demonstrated that marmosets can be infected by a larger dose of EHV-9 by the intranasal route. It suggests the possibility that EHV-9 might be transmitted to other primates, including macaques, great apes and humans. EHV-9 might also be easily transmitted via the nasal route to immuno-compromised animals and patients such as AIDS patients. A recent study showed that Burchell's zebras (*Equus burchelli*) in Tanzania had a high seroprevalence to EHV-9 or to viruses serologically similar to EHV-9 (Borcher et al., 2005), which suggested that EHV-9 is present in zebras permanently residing in East Africa. Also, there has been some possibility of an outbreak of EHV-9 infection as an emerging infection in immuno-compromised animals and patients.