*3.7.3 Application of cold plasma in the fight against Covid-19 pandemic*

First, many researchers have considered using cold plasma to enhance surface decontamination procedures. All of them have been based on their power of microbial decontamination of materials, surfaces proven for a very long time. As example,Bekeschus et al. published a paper [58] recommending the use of cold plasma in the disinfection of contaminated surfaces, liquids... They do, however, advise caution when using it on human tissues in order to minimize its negative effects on the body. Indeed, it is known that the viral load of SARS-CoV-2 is channeled initially from the mouth then the throat before reaching the lungs. Given that the plasma generates the formation of ozone (O3) and nitrogen oxide (NOx). These two gases are essential for the inactivation of pathogens, but they are toxic to the lungs if they accumulate in high quantities. Therefore, it was necessary to be careful about the triggering of toxicological reactions produced by the gas in the plasma.

Otherwise, since SARS-CoV-2 has shown its ability to stabilize for hours on different types of surfaces such as metals, plastics and cardboard. This paralyzes the efforts to destroy transmission chains. For this purpose, Chen and his coworkers [59] at the University of California have reported excellent results on their work conducted on the inactivation of coronavirus Sars-Cov-2 using cold atmospheric plasma by targeting surfaces of leather, plastics and some metals.. They used an atmospheric plasma gas fed with argon. The characteristics of the atmospheric pressure plasma Jet (APPJ) (**Figure 2**) device used are as follows:


Thus, they exposed surfaces contaminated by SARS-CoV-2 to cold argon and helium gases. Then compared to surfaces not exposed to gases [59]. The findings were so promising: they observed that the treatment with argon gas inactivated all the viruses for the different surfaces within a period of less than 180 seconds.

#### **4. Ultrasound technology: a promising alternative for decontamination**

Since its appearance, SARS-CoV-2 has gained a consensus among virologists on its very specific properties in relation to its high capacity for mutation and its speed of propagation. As a result, scientists have always sought to improve the efficiency of methods of disinfecting surfaces in order to decontaminate them from suspensions carrying the virus. From this perspective, ultrasound can represent an effective physical method. Indeed, the mechanical action of ultrasound on the suspensions of contaminated surfaces will be able to clean them while avoiding the side effects and dangers associated with the use of disinfection chemicals.

#### **4.1 Principle of ultrasonic disinfection**

Widely used in the medical field, Ultrasounds are mechanical sound waves, which translate the propagation of acoustic energy in the form of pressure waves. Their frequency range exceeds that of the frequencies of audible sound waves (above 16 kHz). The acoustic intensity I represents the flow of the acoustic power Ps through a surface A. Considering that the pressure amplitude is denoted by "*p*", the different parameters characterizing the propagation of an ultrasonic one are linked by the equation:

$$I = \frac{P\_s}{A} = \frac{p^2}{\rho c} = \frac{p^2}{Z} \tag{7}$$

Where ρ is the density of the medium, ε is the amplitude of the ultrasound, ω is the angular speed (ω = 2πf where f is the frequency), is the speed of sound. The equation can be reduced to the following form:

$$\mathbf{I} = \mathfrak{e}^2 \mathfrak{o}^2 \mathbf{Z} \tag{8}$$

With Z is the acoustic impedance defined by the product ρ � c. During their propagation through different interfaces (air / water for example), ultrasound can undergo either reflections, attenuations or even diffusions. An attenuation coefficient is thus introduced to describe the effect of this passage on the characteristics of the wave transmitted by an interface. For example, for ultrasounds of frequency 20 kHz, the coefficient of their attenuation through a distance of 24 cm is equal to 2. 10<sup>7</sup> cm�<sup>1</sup> . since the difference in impedance is very slight between water and biological cells (approximately 5%), the transmission of ultrasound through biological cells is fluid. This perfectly explains their great use in diagnostic and therapeutic ultrasound [60].

#### **4.2 Uses of ultrasound in wastewater disinfection**

First, ultrasound was used to disinfect wastewater. The process of ultrasonic disinfection mainly relies on cavitation. Indeed, cavitation is a kind of concentration of energy in well-localized areas in a fluid. This cavitation leads to the creation of very extreme physical conditions (temperatures between 1726.85°C and 4726.85°C, pressures between 1800 atm and 3000 atm) [60]. These conditions cause the appearance of effects directly related to disinfection.

The first is a sonochemical effect which results in the destruction of chemical bonds in water. Thus, several types of free radicals are formed. The second effect is the sonoluminescence effect, which characterizes the emission of photons by excitation of gases.

*Coronavirus Disinfection Physical Methods DOI: http://dx.doi.org/10.5772/intechopen.99091*

When the collapse of the water bubbles is produced in the vicinity of a solid surface, a jet of particles will be emitted with a high velocity (up to 300 m / S), thus causing very strong mechanical effects such as the wave acoustic shock, sound emission … damage to this surface by these different physical effects contributes to disinfection [61]. According to Gibson et al. [60], the contribution of sonoluminescence and sonochemical effects to disinfection is very negligible in comparison with the mechanical and thermal effects.

#### **4.3 Factors influencing droplet cavitation**

Knowing that wastewater contains many types of particles, their interactions with ultrasound do not occur in the same way. Which can alter the cavitation process. For this, several factors must be taken into consideration. The most important of these is the nucleation of the droplets. This nucleation can be affected by the surface tension of liquid S. In fact, in a vapor pressure liquid, the critical pressure necessary to increase the bubble radius of radius R is expressed by the following equation:

$$P\_{cr} = P\_v - 2\frac{\mathcal{S}}{R} \tag{9}$$

Moreover, for a droplet deposited on a liquid surface, the surface tension also depends on the contact angle of this droplet with the surface, which generally varies between 0 (hydrophobic substances) and 180° (hydrophilic substances):

$$P\_{cr} = P\_v - 2\frac{\mathcal{S}\sin\theta}{R} \tag{10}$$

From an energetic point of view, the cavitation process can be altered by failure in one of the energy conversion steps. According to Löning et al. [62], the energy conversion process follows the following Scheme:

$$E\_{EL} \rightarrow E\_{HF} \rightarrow E\_{TH} \rightarrow E\_{CAV} \rightarrow E\_{DOS} \rightarrow E\_{EFF}$$

Where EEL is the input of electrical energy, EHF is the energy of ultrasound, ETH is the power of input into the fluid, ECAV is the energy of droplet cavitation, EDOS is the energy determined by dosimetry, and EEFF is the energy expended on a specific effect.

#### **4.4 Mechanical effects of ultrasound**

Gibson et al. [60] have summarized the main conclusions in relation to the mechanical effects of ultrasound in the form of a few points:


#### **4.5 Surfactants (detergents) as main actors for disinfection**

From a structural standpoint, the SARS-CoV-2 virus is made up of a viral wall layer that is composed of a lipoprotein envelope that wraps RNA in its interior (**Figure 3**).

To kill the virus, material is required to damage the inside of the envelope. It cannot be destroyed only by water, and therefore needs another ingredient: alcohol or surfactant as proposed by WHO [63].

Surfactants are amphiphilic molecules, composed of a polar part (hydrophilic) and another non polar part (hydrophobic) (**Figure 4**).

The hydrophilic–lipophilic balance (HLB) was introduced to measure the predominance of each of these two characters. According to Davies et al. [65, 66], its value can be determined from the following relation:

$$HLB = \sum \text{hydrophilic groups} - \sum \text{hydrophilic groups} + 7 \tag{11}$$

This chemical structure gives surfactants a double affinity, sometimes to polar compounds and sometimes to nonpolar compounds (**Figure 5**). From a physical point of view, surfactants act as agents to attenuate the surface tension between two immiscible phases, promoting the dispersion of one into the other.

Generally, surfactant molecules are classified according to the properties of their polar part, two main families are distinguished:


**Figure 3.** *Structure of the coronavirus (Sars-CoV2) [63].*

**Figure 4.** *Chemical structure of surfactant molecules.*

#### *Coronavirus Disinfection Physical Methods DOI: http://dx.doi.org/10.5772/intechopen.99091*

A surfactant's detergency strength measures its ability to work on the soil to remove it. Every type of soil, whether fatty, solid, etc., can actually build physical connections with surfactant molecules. These interactions can be either hydrophilic (or else hydrophobic) interactions, or attractive electrostatic interactions. As a result, the detergency mechanism operates according to the different types of loads of dirt on one side and surfactant on the other side. Positive surfactants attract negatively charged soils, which they will partially neutralize. The positive part of the surfactant therefore binds to the negative part of the soil. A positively charged surfactant is interested in negatively charged soils. However, an agent (+) will not have any influence on a soiling (+) since both repel each other [67].

For long time, the soap is known for its very powerful detergent power. For this, since the Covid-19 emergence, the world health organization (WHO) recommended firstly to use it as first weapon against the virus by washing hands several times along the day. Other detergents, such as laundry detergents, are made in synthetics but they are all molecular in the same kind.

Soap is composed of fats, oils, and fatty acids. A hydrophilic polar head and a hydrophobic carbon chain, which have an affinity to organic compounds and consequently to fatty substances, constitute the molecular structure of soap.

When the soap molecules are added to the water, the hydrophobic tails orient towards the air to avoid contact with the water molecules. To bring them into

**Figure 5.** *Classes of surfactant molecules.*

**Figure 6.** *Soap action on aerosol droplets containing SARS-CoV-2.*

contact with the fatty compounds, mechanical action is necessary, in particular rubbing. Once in contact with the fats, the soap molecules surround it on all sides, thus forming spherical micelles (**Figure 6**). In order to decontaminate surfaces containing aerosols carrying SARS-CoV-2. A large concentration of soap molecules must be spread by rubbing the entire surface.

#### **5. Conclusion**

In the fight against a new virological epidemic, the most traditional approach is immune system development, which gives the immune system the ability to identify and attack the virus once it has entered the body. This can only be accomplished by manufacturing vaccines. However, waiting for the vaccine to be produced may cost us the lives of millions of people in a pandemic characterized by a very large spread rate such as covid-19. The use of disinfection methods (along with barrier precautions) remains the most promising way to combat this pandemic.

In this context, we have presented in this chapter the main physical methods used to disinfect contaminated surfaces. Initially, special emphasis was placed on methods based on electromagnetic irradiations, specifically ultraviolet UV radiation and gamma radiation. The required doses, capable of inactivating the virus and used in the production of disinfection devices such as UVC lamps, were presented. The parameters influencing the efficiency of these techniques have been also discussed. Second, we concentrated on the use of conventional disinfection techniques that have already proven effective in the fight against other epidemics, such as disinfection by heating, which relies on the ability of high temperatures to destroy the lipid bonds that comprise the virulent layer of SARS-CoV-2. Particular attention has been paid to the use of ultrasound in the disinfection of contaminated surfaces, this technique which is based on the mechanical action of ultrasonic waves manifested by cavitation and thus producing sonolumiscent and sonochemical effects and also a thermal effect. The principle of disinfection by gas jets of cold plasma was then described. In this regard, we presented bibliographic data demonstrating its efficacy in the decontamination of surfaces contaminated with SARS-CoV-2 in a short period (less than 2 minutes). Finally, it appears critical to discuss the basic chemical compounds used in disinfection chemicals, namely detergents. We have dedicated a section to describing the physical and structural properties of the major detergents.

We believe that, in the absence of an effective medical treatment, the bibliographical review study on various disinfection procedures represents, at this time, the best kits for both medical personnel and policymakers in the fight against this new pandemic.

#### **Acknowledgements**

The authors gratefully acknowledge financial support from the Tunisian Ministry of Education, Research, and Technology.

#### **Conflict of interest**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

*Coronavirus Disinfection Physical Methods DOI: http://dx.doi.org/10.5772/intechopen.99091*
