**4. Plasma treatment**

#### **4.1. Sputtering/etching**

Sputtering is very often the result of an interaction of the target surface or molecule with ion fluency. When the energy of an ion exceeds a certain value during collision, an atom absorbing this energy can leave its position in the molecule and a vacancy is created (free bond). If the atom that has left still has enough energy, other atoms can be released by following collisions and other vacancies can be created. Some of the atoms can be released from the target material which leads to a process called "physical sputtering."

#### *4.1.1. Physical sputtering*

The key parameters of physical sputtering are the binding energy in the target molecule and also the energy and mass of the impinging ion. The dependency on temperature is weak. The threshold energy is relatively high (in range 10–100 eV) depending on the target/impinging ion combination. The sputtered yield changes with the angle of incident, the substrate material or the roughness of surface. Eroded species consist of atoms or small clusters of target material. Investigation of metallic targets with impinging metallic ions showed that the mechanism of sputtering depends on the ratio of impinging ion mass (M1) and the mass of the atom in the sample (M2) [41]. In the case of M1 < M2, the threshold energy does not depend on the mass M1 or M2. As the mass M2 increases, the probability of reflection of ion increases. The mechanism of sputtering follows the next steps:

**1.** Ion penetrates through the surface layer.

and also lateral organization. The wider distribution leads to a hexagonal organization, and synthetic ceramides with a low ceramide chain distribution leads to an orthorhombic organization [37]. A long-periodic phase is formed in the presence of a certain level of unsaturated ceramides, and a long-periodic phase is associated with fluid domain inside. A saturated

The reduction of free fatty acid (FFA) chains was observed in skin diseases which are known by impaired skin barrier function. The reduction of the fatty acid chain can be caused by an increase in the amount of shorter FFA chains such as with 16 or 18 carbons. A shorter chain allows an increase in vibrations followed by conformational disordering. An increase in the number of unsaturated FFA reduces the packing density of the lipid organization [36]. An analysis of the composition of free fatty acids of human stratum corneum showed a dominant presence of saturated free fatty acids with carbon chain lengths from 16 to 30. The most abundant were FFAs with 24 and 26 carbons (50% of all FFAs). The content of unsaturated FFAs appeared to be around 2% of all FFAs; mostly with 18 carbons and with traces of FFAs with

ceramide EOS-S is important for the stabilization of the orthorhombic phase [38].

16, 17 and 20 carbons; 1% of di-unsaturated FFAs (chain length of 18 carbons) [35].

Cholesterol is situated near the ester bond of ceramides and this position allows the formation of a hydrogen bond between the cholesterol ─OH group and the carbonyl group C═O. The presence of cholesterol has an influence on the formation of the long periodicity phase of ceramides (13 nm). There is a minimal amount of required cholesterol for the formation of the long periodicity phase of ceramides (without cholesterol, long periodicity phase is not formed). Cholesterol does not influence only the presence of long periodicity phase but also the packing density in the long periodicity phase [39]. Cholesterol is very important for cor-

Sputtering is very often the result of an interaction of the target surface or molecule with ion fluency. When the energy of an ion exceeds a certain value during collision, an atom absorbing this energy can leave its position in the molecule and a vacancy is created (free bond). If the atom that has left still has enough energy, other atoms can be released by following collisions and other vacancies can be created. Some of the atoms can be released from the target

The key parameters of physical sputtering are the binding energy in the target molecule and also the energy and mass of the impinging ion. The dependency on temperature is weak. The

material which leads to a process called "physical sputtering."

**3.3. Role of fatty acids**

106 Plasma Medicine - Concepts and Clinical Applications

**3.4. Role of cholesterol**

rect skin barrier function [40].

**4. Plasma treatment**

**4.1. Sputtering/etching**

*4.1.1. Physical sputtering*


If M2 ≥ M1, the threshold energy for sputtering is higher than in the first case. The threshold energy also does not depend on the mass of M1 and M2, and sputtering follows the mechanism:


The threshold is higher in the second case because atoms of the target are sputtered by secondary atoms which were released by ion bombardment. Pure physical sputtering can be achieved mostly in discharges with inert gases. Bombardment of lipids inside the stratum corneum will follow mostly the second mechanism when argon is used and the first one in the case of helium. Physical sputtering is also present in nitrogen, oxygen or air plasma, but chemical sputtering can be much more dominant.

#### *4.1.1.1. Argon bombardment/argon plasma*

Argon is one of the simplest media for treatment of biological material. Excited states of Ar (Ar\*), metastable states (Arm) and Ar ions (Ar+ , Ar2 + ) [42] can be produced in any electrical discharges. Ionization energy of Ar+ and Ar2 + is 15.8 and 15.5 eV, respectively. Argon metastable states have a relatively long lifetime equal to 38 s [43] in vacuum. However, this time is reduced in atmospheric pressure. The energy difference between the ground state and the metastable state of Ar is 11.55 eV, which can be released by collision. A molecular dynamics simulation demonstrated the sputtering of a lipid-like material by argon ions (**Figure 5A** and **B**). The sputtering threshold energy was between 10 and 20 eV. The yield of sputtered particles increases to four carbon atoms per ion impacting the surface at an energy of 50 eV and increases more slowly up to 10 at 100 eV [44]. If we suppose a similar behavior of lipids and polymers during bombardment, argon ion bombardment of the polymers can lead to decreasing of side chains of the polymer, volatile CH4 , CO, CO2 , HCOOCH3 and H2 , and disordered polymer structure [45]. The sputtering of polymer surface by Ar ions leads to graphite-like

**Figure 5.** Molecular dynamic simulation. A. Lipid-like surface (black—C, gray—H, white—O). B. Lipid-like surface after bombardment by argon ions. [44] C. Virgin polymethylmethacrylate (composed of C—black, H—gray and O—White) D. Polymethylmethacrylate after bombardment by argon ions (damaged, carbon rich layer is formed on surface) [48].

*4.1.1.2. Helium bombardment/helium plasma*

coming from surrounding atmospheric air [47].

ing is much more effective, sometimes 1 or 2 orders.

*4.1.2. Chemical sputtering*

Helium is an inert gas with light atomic mass, which means that the sputtering of any surface is lower in comparison with argon. All atoms in the lipid matrix of the stratum corneum are heavier than helium, so atoms are sputtered mostly by the first mechanism described in Section 4.1.1. Investigation of sputtering of polymers by high energy (1 keV) He and Ar ions demonstrated that light atoms such as hydrogen are sputtered at first, followed by heavier carbon and oxygen [50]. On the other hand, the penetration depth increases with decreasing atomic mass, which means that an atomic mass of helium allows penetration deeper inside the surface than argon [51]. However, helium plasma treatment of crystalline carbon with low energy ions (ion temperature ~0.1 eV) demonstrated mass loss and disorder in carbon structure [52]. Helium plasma-treated polycarbonate containing C, O, H atoms did not change the number of C and O atoms, but a change of structure by breaking carbonate groups was observed [53]. Helium is effective in increasing oxygen and also nitrogen functional groups

**Figure 6.** Cross section of pig skin before atmospheric plasma irradiation, the stratum corneum thickness: 18.09 ± 1.64 μm (left—control), and after atmospheric plasma irradiation (right), the stratum corneum thickness: 13.40 ± 1.46 [11, 49].

Microplasma Drug Delivery

109

http://dx.doi.org/10.5772/intechopen.73498

Unlike physical sputtering, chemical sputtering (sometimes known as reactive etching) has no or a very low threshold. Chemical sputtering varies strongly with surface temperature and it is highly selective depending on the target/impinging particle combination. The impinging particle also involves neutral reactive species which help to increase the number of sputtered atoms. Eroded species consist of molecules involving impinging atoms and target. The number of sputtered atoms increases by decreasing the energy of bonds and also by decreasing the molecular weights of the atoms in the molecule. The rate of reactions on the surface depends on its structure. If the surface is damaged by sputtering, the reaction rate can increase by orders of magnitude. Atoms react with free bonds after ion bombardment. Chemical sputter-

structure, decreasing the rate of sputtering [46]. If the surface consists of too much of O and H, a thin C-rich layer is not created because released O and H atoms can cooperate in surface sputtering and react with free carbon bonds [44]. Comparison of the lipid-like surface and polymethylmethacrylate in **Figure 5A** and **B** shows forming of carbon-like surface in the case of polymer, but this surface is not formed in the case of the lipid-like surface. Etching of skin surface was confirmed by measuring the thickness of the stratum corneum in the skin cross section (**Figure 6**) [11]. The first layer of the stratum corneum was etched after 5 min of irradiation. If argon plasma treatment is realized in atmospheric air, air particles can participate in treatment with processes other than physical sputtering. Chemical sputtering or surface functionalizing can start to be effective. In this case, plasma treatment causes wettability and hydrophilicity of surfaces by increasing the number of functional groups such as oxygen or nitrogen. Argon plasma treatment is able to increase oxygen functional groups on polymer surface if argon plasma discharge is working on atmospheric air [47]. Molecular dynamics (MD) simulation of bombardment of polymers showed that the main products of etching are CO or H2 , polymer units and Cx Hy fragments during transient sputtering. After the formation of a damaged layer, the products of sputtering are H, H<sup>2</sup> , CO, and Cn. Oxygen leaves polymers more difficult than hydrogen because oxygen is larger and less mobile, and that is the reason why oxygen is concentrated under a damaged layer with strong C─O─C bonds and the surface of polymer is created in the layer of amorphous carbon. This process is not valid for all polymers. If polymers compose of many H or O atoms, amorphous layer of carbon is not formed because released H and O atom can etch carbon atoms [48].

**Figure 6.** Cross section of pig skin before atmospheric plasma irradiation, the stratum corneum thickness: 18.09 ± 1.64 μm (left—control), and after atmospheric plasma irradiation (right), the stratum corneum thickness: 13.40 ± 1.46 [11, 49].

#### *4.1.1.2. Helium bombardment/helium plasma*

Helium is an inert gas with light atomic mass, which means that the sputtering of any surface is lower in comparison with argon. All atoms in the lipid matrix of the stratum corneum are heavier than helium, so atoms are sputtered mostly by the first mechanism described in Section 4.1.1. Investigation of sputtering of polymers by high energy (1 keV) He and Ar ions demonstrated that light atoms such as hydrogen are sputtered at first, followed by heavier carbon and oxygen [50]. On the other hand, the penetration depth increases with decreasing atomic mass, which means that an atomic mass of helium allows penetration deeper inside the surface than argon [51]. However, helium plasma treatment of crystalline carbon with low energy ions (ion temperature ~0.1 eV) demonstrated mass loss and disorder in carbon structure [52]. Helium plasma-treated polycarbonate containing C, O, H atoms did not change the number of C and O atoms, but a change of structure by breaking carbonate groups was observed [53]. Helium is effective in increasing oxygen and also nitrogen functional groups coming from surrounding atmospheric air [47].

#### *4.1.2. Chemical sputtering*

structure, decreasing the rate of sputtering [46]. If the surface consists of too much of O and H, a thin C-rich layer is not created because released O and H atoms can cooperate in surface sputtering and react with free carbon bonds [44]. Comparison of the lipid-like surface and polymethylmethacrylate in **Figure 5A** and **B** shows forming of carbon-like surface in the case of polymer, but this surface is not formed in the case of the lipid-like surface. Etching of skin surface was confirmed by measuring the thickness of the stratum corneum in the skin cross section (**Figure 6**) [11]. The first layer of the stratum corneum was etched after 5 min of irradiation. If argon plasma treatment is realized in atmospheric air, air particles can participate in treatment with processes other than physical sputtering. Chemical sputtering or surface functionalizing can start to be effective. In this case, plasma treatment causes wettability and hydrophilicity of surfaces by increasing the number of functional groups such as oxygen or nitrogen. Argon plasma treatment is able to increase oxygen functional groups on polymer surface if argon plasma discharge is working on atmospheric air [47]. Molecular dynamics (MD) simulation of bombardment of polymers showed that the main products of etching are

**Figure 5.** Molecular dynamic simulation. A. Lipid-like surface (black—C, gray—H, white—O). B. Lipid-like surface after bombardment by argon ions. [44] C. Virgin polymethylmethacrylate (composed of C—black, H—gray and O—White) D. Polymethylmethacrylate after bombardment by argon ions (damaged, carbon rich layer is formed on surface) [48].

mers more difficult than hydrogen because oxygen is larger and less mobile, and that is the reason why oxygen is concentrated under a damaged layer with strong C─O─C bonds and the surface of polymer is created in the layer of amorphous carbon. This process is not valid for all polymers. If polymers compose of many H or O atoms, amorphous layer of carbon is

fragments during transient sputtering. After the formation

, CO, and Cn. Oxygen leaves poly-

CO or H2

, polymer units and Cx

108 Plasma Medicine - Concepts and Clinical Applications

Hy

not formed because released H and O atom can etch carbon atoms [48].

of a damaged layer, the products of sputtering are H, H<sup>2</sup>

Unlike physical sputtering, chemical sputtering (sometimes known as reactive etching) has no or a very low threshold. Chemical sputtering varies strongly with surface temperature and it is highly selective depending on the target/impinging particle combination. The impinging particle also involves neutral reactive species which help to increase the number of sputtered atoms. Eroded species consist of molecules involving impinging atoms and target. The number of sputtered atoms increases by decreasing the energy of bonds and also by decreasing the molecular weights of the atoms in the molecule. The rate of reactions on the surface depends on its structure. If the surface is damaged by sputtering, the reaction rate can increase by orders of magnitude. Atoms react with free bonds after ion bombardment. Chemical sputtering is much more effective, sometimes 1 or 2 orders.

#### *4.1.2.1. Reactive etching/plasma treatment by gases containing oxygen*

The dominant mechanism of sputtering by oxygen plasma discharge is the bombardment of the sample by ions creating vacancies. Oxygen molecules, ozone or radicals react with these defects and form H2 O, CO or CO2 . When an unsaturated bond is created in argon plasma, this bond cannot react with any chemically active species to form a volatile by-product. The only way to form those carbon-carbons unsaturated bonds is to react with others carbons and form a highly crosslinked material. Physical sputtering of hydrocarbon film by argon ions can be increased by a beam of molecular oxygen, which leads to chemical sputtering. Molecular oxygen alone cannot sputter the surface but can be adsorbed. Bombardment induces reactions between adsorbed molecular oxygen and bonds created by Ar+ bombardment on hydrocarbon film. The sputtered amount of atoms and molecules is proportional to the flux of molecular oxygen and the ratio of O<sup>2</sup> /Ar<sup>+</sup> . An increase of molecular flux increases the number of adsorbed molecules and the number of chemical reactions on the surface [54]. A similar effect can be achieved in Ar/O<sup>2</sup> plasma discharge where except molecular oxygen, other active species such as O3 , radicals O and metastable states O2 (a) and Arm are also present. Unsaturated bonds created by Ar/O<sup>2</sup> plasma treatment lead to crosslinking and oxidation [55]. Pure oxygen post-discharge treatment of hexatriacontane resulted in a mass decrease, which corresponds to etching, but no chemical modifications were observed [56]. In this case, only long living particles can interact with hexatriacontane. Wertheimer et al. [57] showed that oxygen radicals O(3 P) and metastable O(1 D) alone are not very effective in interacting with a polymer surface with saturated aliphatic carbons. When hexatriacontane was treated in N<sup>2</sup> -O2 discharge, grafting with etching compete to each other. The mass of hexatriacontane and the amount of oxygen increases followed by etching, and decreases the amount of oxygen [56]. Joubert et al. showed that molecular oxygen does not participate in plasma surface reaction when they compared discharges of N<sup>2</sup> O plasma and N<sup>2</sup> -O2 plasma with the same concentration of atomic oxygen. They concluded that O2 only provides oxygen radicals [58].

*4.1.2.3. Reactive etching/plasma treatment by gases containing hydrogen*

H2 + and H3 +

after bombardment by Ar+

**4.2. Lipid peroxidation**

superoxide anion (O2

to increase sputtering is using an N<sup>2</sup>

increase the etching rate to a higher value than N<sup>2</sup>

−

the structure of the lipid membrane.

Deprotonation can lead to formation of O2

penetrating deeper in the lipid bilayer than HO2

**Superoxide anion radical** (O2

in hydrophobic environment.

**Hydroperoxyl radical** (HO2

Hydrogen is the lightest atom, so similar to helium, the sputtering effect of hydrogen ions H<sup>+</sup>

discharge which leads to more effective reactive etching. Etching by hydrogen plasma causes incorporation of H and reduction of dangling bonds, unlike oxygen plasma which increases the number of dangling bonds [63]. It was observed that sputtering of the surface with a combination of hydrogen atoms and argon ion enhances sputtering more than their sum. A hydrogen-rich surface is easier to etch than a surface with a deficiency of hydrogen or a more crosslinked carbon network, as the hydrogen atoms can react with free carbon bonds

cial for effective etching. It is the most effective at a ratio 400:1. It means that the atomic flux should be several times higher than the ion flux. If the flux ratio is too low, most of the ioninduced defects recombine before they can be passivated by incident H [64, 65]. Another way

Lipids in the lipid membrane of the stratum corneum do not have to be etched by ions but radicals created in plasma or secondary created in the stratum corneum can degrade lipids by the process called lipid peroxidation. Lipid peroxidation is an oxidative degradation of lipids. The reaction occurs on unsaturated fatty acids. As molecular oxygen is not reactive enough to start oxidation, it must be changed to a more reactive state such as hydroxyl radical (OH),

O2

complexes (ferryl-, perferryl radical) [67]. Reactive oxygen species abstract hydrogen atoms from the methylene group and create a lipid radical. The rate of lipid peroxidation exponentially increases with the number of bis-allylic carbons, because this bond is the weakest in the molecule with no relation to chain length. After abstraction of hydrogen, the lipid radical can react with molecular atmospheric oxygen and form a lipid peroxyl radical. At a low concentration of atmospheric oxygen, lipids can react with each other in the lipid membrane [67]. The lipid peroxyl radical can abstract hydrogen from another lipid to form lipid radical (L•) which can react with atmospheric oxygen and again to form lipid peroxyl radical (LOO•). Oxidized lipids form more rigid domains. LOOH is more polar than fatty acids; it can disrupt

mixture. Ions N<sup>+</sup>

or H2


), hydrogen peroxide (H2

oxyl radical (LOO•), alkoxyl radical (LO•), metastable singlet oxygen (O<sup>2</sup>

is much lower than by heavy ions. Hydrogen atomic radicals can be formed in

ions. The ratio of the flux of hydrogen atoms and Ar<sup>+</sup>

, NH<sup>2</sup> + , NH<sup>3</sup> + , NH<sup>4</sup> + , N<sup>2</sup> H+

plasma alone [66].

), hydroperoxyl radical (HO2

−•) reactivity is low in aqueous environments, but it increases

•) is much more reactive than superoxide anion radicals.

−• at a pH of 4.8. Weak reactivity of O<sup>2</sup>

•. The poor reactivity and relatively long

,

111

ions is cru-

Microplasma Drug Delivery

http://dx.doi.org/10.5772/intechopen.73498

 and H3 +

•), lipid per-

−• allows

(a)), or iron-oxygen

#### *4.1.2.2. Reactive etching/plasma treatment by gases containing nitrogen*

Nitrogen is a very stable molecule and its concentration of atomic radicals is low in comparison with oxygen. On the other hand, nitrogen creates a lot of metastable states. It was observed that even so nitrogen states coming from nitrogen discharge can cause chemical sputtering [59]. Bombardment of carbon film by N<sup>2</sup> + ions with an energy under threshold of physical sputtering demonstrated the formation of CN radicals. If these radicals are formed on the surface, it creates HCN and OHCN after reaction with hydrogen and water. If the radical is formed in sublayers, C2 N2 molecule is created [60]. In the case of nitrogen plasma of hydrocarbon film, we can also observe erosion of the surface and formation CN radicals. However, an admixture of methane can cause deposition of Cx Hy groups. Erosion will compete with deposition and CN, CNH and C<sup>2</sup> N2 groups can be incorporated into the film [61, 62].

#### *4.1.2.3. Reactive etching/plasma treatment by gases containing hydrogen*

Hydrogen is the lightest atom, so similar to helium, the sputtering effect of hydrogen ions H<sup>+</sup> , H2 + and H3 + is much lower than by heavy ions. Hydrogen atomic radicals can be formed in discharge which leads to more effective reactive etching. Etching by hydrogen plasma causes incorporation of H and reduction of dangling bonds, unlike oxygen plasma which increases the number of dangling bonds [63]. It was observed that sputtering of the surface with a combination of hydrogen atoms and argon ion enhances sputtering more than their sum. A hydrogen-rich surface is easier to etch than a surface with a deficiency of hydrogen or a more crosslinked carbon network, as the hydrogen atoms can react with free carbon bonds after bombardment by Ar+ ions. The ratio of the flux of hydrogen atoms and Ar<sup>+</sup> ions is crucial for effective etching. It is the most effective at a ratio 400:1. It means that the atomic flux should be several times higher than the ion flux. If the flux ratio is too low, most of the ioninduced defects recombine before they can be passivated by incident H [64, 65]. Another way to increase sputtering is using an N<sup>2</sup> -H2 mixture. Ions N<sup>+</sup> , NH<sup>2</sup> + , NH<sup>3</sup> + , NH<sup>4</sup> + , N<sup>2</sup> H+ and H3 + increase the etching rate to a higher value than N<sup>2</sup> or H2 plasma alone [66].

#### **4.2. Lipid peroxidation**

*4.1.2.1. Reactive etching/plasma treatment by gases containing oxygen*

O, CO or CO2

the flux of molecular oxygen and the ratio of O<sup>2</sup>

[54]. A similar effect can be achieved in Ar/O<sup>2</sup>

also present. Unsaturated bonds created by Ar/O<sup>2</sup>

plasma surface reaction when they compared discharges of N<sup>2</sup>

*4.1.2.2. Reactive etching/plasma treatment by gases containing nitrogen*

sputtering [59]. Bombardment of carbon film by N<sup>2</sup>

will compete with deposition and CN, CNH and C<sup>2</sup>

If the radical is formed in sublayers, C2

with the same concentration of atomic oxygen. They concluded that O2

oxygen, other active species such as O3

[57] showed that oxygen radicals O(3


was treated in N<sup>2</sup>

radicals [58].

film [61, 62].

defects and form H2

110 Plasma Medicine - Concepts and Clinical Applications

The dominant mechanism of sputtering by oxygen plasma discharge is the bombardment of the sample by ions creating vacancies. Oxygen molecules, ozone or radicals react with these

this bond cannot react with any chemically active species to form a volatile by-product. The only way to form those carbon-carbons unsaturated bonds is to react with others carbons and form a highly crosslinked material. Physical sputtering of hydrocarbon film by argon ions can be increased by a beam of molecular oxygen, which leads to chemical sputtering. Molecular oxygen alone cannot sputter the surface but can be adsorbed. Bombardment

ment on hydrocarbon film. The sputtered amount of atoms and molecules is proportional to

the number of adsorbed molecules and the number of chemical reactions on the surface

oxidation [55]. Pure oxygen post-discharge treatment of hexatriacontane resulted in a mass decrease, which corresponds to etching, but no chemical modifications were observed [56]. In this case, only long living particles can interact with hexatriacontane. Wertheimer et al.

P) and metastable O(1

interacting with a polymer surface with saturated aliphatic carbons. When hexatriacontane

hexatriacontane and the amount of oxygen increases followed by etching, and decreases the amount of oxygen [56]. Joubert et al. showed that molecular oxygen does not participate in

Nitrogen is a very stable molecule and its concentration of atomic radicals is low in comparison with oxygen. On the other hand, nitrogen creates a lot of metastable states. It was observed that even so nitrogen states coming from nitrogen discharge can cause chemical

old of physical sputtering demonstrated the formation of CN radicals. If these radicals are formed on the surface, it creates HCN and OHCN after reaction with hydrogen and water.

plasma of hydrocarbon film, we can also observe erosion of the surface and formation CN

N2

radicals. However, an admixture of methane can cause deposition of Cx

/Ar<sup>+</sup>

, radicals O and metastable states O2

discharge, grafting with etching compete to each other. The mass of

+

N2

induces reactions between adsorbed molecular oxygen and bonds created by Ar+

. When an unsaturated bond is created in argon plasma,

. An increase of molecular flux increases

plasma discharge where except molecular

plasma treatment lead to crosslinking and

O plasma and N<sup>2</sup>

ions with an energy under thresh-

Hy

groups can be incorporated into the

molecule is created [60]. In the case of nitrogen

D) alone are not very effective in

bombard-

(a) and Arm are


groups. Erosion

only provides oxygen

plasma

Lipids in the lipid membrane of the stratum corneum do not have to be etched by ions but radicals created in plasma or secondary created in the stratum corneum can degrade lipids by the process called lipid peroxidation. Lipid peroxidation is an oxidative degradation of lipids. The reaction occurs on unsaturated fatty acids. As molecular oxygen is not reactive enough to start oxidation, it must be changed to a more reactive state such as hydroxyl radical (OH), superoxide anion (O2 − ), hydrogen peroxide (H2 O2 ), hydroperoxyl radical (HO2 •), lipid peroxyl radical (LOO•), alkoxyl radical (LO•), metastable singlet oxygen (O<sup>2</sup> (a)), or iron-oxygen complexes (ferryl-, perferryl radical) [67]. Reactive oxygen species abstract hydrogen atoms from the methylene group and create a lipid radical. The rate of lipid peroxidation exponentially increases with the number of bis-allylic carbons, because this bond is the weakest in the molecule with no relation to chain length. After abstraction of hydrogen, the lipid radical can react with molecular atmospheric oxygen and form a lipid peroxyl radical. At a low concentration of atmospheric oxygen, lipids can react with each other in the lipid membrane [67]. The lipid peroxyl radical can abstract hydrogen from another lipid to form lipid radical (L•) which can react with atmospheric oxygen and again to form lipid peroxyl radical (LOO•). Oxidized lipids form more rigid domains. LOOH is more polar than fatty acids; it can disrupt the structure of the lipid membrane.

**Superoxide anion radical** (O2 −•) reactivity is low in aqueous environments, but it increases in hydrophobic environment.

**Hydroperoxyl radical** (HO2 •) is much more reactive than superoxide anion radicals. Deprotonation can lead to formation of O2 −• at a pH of 4.8. Weak reactivity of O<sup>2</sup> −• allows penetrating deeper in the lipid bilayer than HO2 •. The poor reactivity and relatively long half-life of O2 −• allows it to diffuse more effectively from its generation site to targets such as membrane lipid bilayers than HO2 • or other reactive species.

**Hydrogen peroxide** (H2 O2 ) has limited reactivity and can diffuse to target longer time and has potential to create other reactive short-living species near the lipid membrane. Creation of lipid radical can be realized in the presence of iron as a catalyst.

**Hydroxyl radical** (OH•) reactivity is very high and its lifetime is very short; it can react with any molecule in tissue. It can be created from H2 O2 by catalysis of iron or ultrasound in water [67]:

$$\text{Fe(II)}-\text{complex} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe(III)}-\text{complex} + \text{OH}^- + \text{OH} \tag{1}$$

similar results. About 9 and 8 μg/cm<sup>2</sup>

[13], with the permission of the American Vacuum Society).

the skin is needed.

of Cyclosporine A penetrated through the epidermal

Microplasma Drug Delivery

113

http://dx.doi.org/10.5772/intechopen.73498

layer using microplasma or plasma jet, respectively (**Figure 7**). The maximum delivery rate was reached after 3 h of delivery and decreased to its minimal value after 9 h (**Figure 8**)

Although, it was shown that microplasma can enhance the permeability of Cyclosporine A, it is only half of the problem which must be solved before clinical use. The second important question is, whether this penetrated amount is sufficient. This problem can be solved with pharmacokinetics, and it was shown that the treated area of the skin must be equal to 225 cm2 and the concentration of Cyclosporine A should be 400 mg/ml in propylene glycol. If this condition is fulfilled, a therapeutic concentration in blood can be achieved (**Figure 9**). However, the therapeutic concentration is achieved only for up to 10 h and then another treatment of

**Figure 8.** Evolution of the rate of drug delivery through the epidermal layer of the pig skin (four samples were used for microplasma dielectric barrier discharge treatment and five samples for plasma jet treatment) [13] (reproduced from Ref.

**Figure 9.** Concentration of Cyclosporine A in blood of adults 55, 70 and 85 kg in weight (400 mg/ml of Cycloporine A in propylene glycol). The two peaks at 6 and 8 h come from experimental data of drug flux containing some errors [13]

(reproduced from Ref. [13], with the permission of the American Vacuum Society).

#### **5. Application of plasma discharges in transdermal drug delivery**

DBD plasma discharge in direct contact with skin was used for transdermal delivery in the air [15]. The time of plasma application was set up to 2 min in pulsed mode 1–10 μs and frequency ranged from 50 Hz to 3.5 kHz. The delivery of large molecules such as dextran molecules with a molecular weight 10 kDa can penetrate to a depth of 600 μm within 1 h and larger molecules such as albumin (66 kDa), IgG human immunoglobulin (115 kDa) and SiO<sup>2</sup> nanoparticles with a diameter of 50 nm can penetrate to a depth of 200 μm within 1 h. Even liposomes with a diameter of 100 nm were delivered to a depth of 100 μm after 1 h. Remote microplasma or plasma jet in argon gas were investigated for transdermal delivery of galantamine hydrobromide (368 Da) [14] and Cyclosporine A (1200 Da) [13]. After 3 min of microplasma treatment, the permeability of galantamine hydrobromide increased by a factor 2 after 24 h in comparison with non-treated skin. The permeability of Cyclosporine A was not able to penetrate through the stratum corneum without plasma treatment. Argon plasma and microplasma showed

**Figure 7.** Accumulative amount of penetrated Cyclosporine A through the epidermal layer of the pig skin (four samples were used for microplasma dielectric barrier discharge treatment and five samples for plasma jet treatment) [13] (reproduced from Ref. [13], with the permission of the American Vacuum Society).

similar results. About 9 and 8 μg/cm<sup>2</sup> of Cyclosporine A penetrated through the epidermal layer using microplasma or plasma jet, respectively (**Figure 7**). The maximum delivery rate was reached after 3 h of delivery and decreased to its minimal value after 9 h (**Figure 8**)

half-life of O2

in water [67]:

membrane lipid bilayers than HO2

112 Plasma Medicine - Concepts and Clinical Applications

O2

with any molecule in tissue. It can be created from H2

of lipid radical can be realized in the presence of iron as a catalyst.

**Hydrogen peroxide** (H2

−• allows it to diffuse more effectively from its generation site to targets such as

) has limited reactivity and can diffuse to target longer time and

by catalysis of iron or ultrasound

nanoparticles with

O2

• or other reactive species.

has potential to create other reactive short-living species near the lipid membrane. Creation

**Hydroxyl radical** (OH•) reactivity is very high and its lifetime is very short; it can react

Fe(II) − complex + H2 O2 → Fe(III) − complex + OH<sup>−</sup> + OH (1)

DBD plasma discharge in direct contact with skin was used for transdermal delivery in the air [15]. The time of plasma application was set up to 2 min in pulsed mode 1–10 μs and frequency ranged from 50 Hz to 3.5 kHz. The delivery of large molecules such as dextran molecules with a molecular weight 10 kDa can penetrate to a depth of 600 μm within 1 h and larger molecules

a diameter of 50 nm can penetrate to a depth of 200 μm within 1 h. Even liposomes with a diameter of 100 nm were delivered to a depth of 100 μm after 1 h. Remote microplasma or plasma jet in argon gas were investigated for transdermal delivery of galantamine hydrobromide (368 Da) [14] and Cyclosporine A (1200 Da) [13]. After 3 min of microplasma treatment, the permeability of galantamine hydrobromide increased by a factor 2 after 24 h in comparison with non-treated skin. The permeability of Cyclosporine A was not able to penetrate through the stratum corneum without plasma treatment. Argon plasma and microplasma showed

**Figure 7.** Accumulative amount of penetrated Cyclosporine A through the epidermal layer of the pig skin (four samples were used for microplasma dielectric barrier discharge treatment and five samples for plasma jet treatment) [13]

(reproduced from Ref. [13], with the permission of the American Vacuum Society).

**5. Application of plasma discharges in transdermal drug delivery**

such as albumin (66 kDa), IgG human immunoglobulin (115 kDa) and SiO<sup>2</sup>

Although, it was shown that microplasma can enhance the permeability of Cyclosporine A, it is only half of the problem which must be solved before clinical use. The second important question is, whether this penetrated amount is sufficient. This problem can be solved with pharmacokinetics, and it was shown that the treated area of the skin must be equal to 225 cm2 and the concentration of Cyclosporine A should be 400 mg/ml in propylene glycol. If this condition is fulfilled, a therapeutic concentration in blood can be achieved (**Figure 9**). However, the therapeutic concentration is achieved only for up to 10 h and then another treatment of the skin is needed.

**Figure 8.** Evolution of the rate of drug delivery through the epidermal layer of the pig skin (four samples were used for microplasma dielectric barrier discharge treatment and five samples for plasma jet treatment) [13] (reproduced from Ref. [13], with the permission of the American Vacuum Society).

**Figure 9.** Concentration of Cyclosporine A in blood of adults 55, 70 and 85 kg in weight (400 mg/ml of Cycloporine A in propylene glycol). The two peaks at 6 and 8 h come from experimental data of drug flux containing some errors [13] (reproduced from Ref. [13], with the permission of the American Vacuum Society).

treatment of the other materials, such as polymers. The thickness of the stratum corneum will be decreased. How effective the interaction of plasma can be, depends on the use of a gas or gas mixture. The presence of ions can be very effective to start the interaction with the skin. Usually, oxygen containing plasma can strongly etch the surface with the help of reactivity of active species with free bonds created by ions. If the result is a fluidic or heavy crosslinked structure, the structure depends on the amount of species which is able to react with the free bonds. On the other hand, helium plasma is not so strong of an etching medium and the result of the interaction can be less destructive. However, plasma treatment can also bring some reactive species close to the lipid membrane which can react with lipids without the help of ions and cause peroxidation of lipids and decrease their barrier function. It was shown that plasma sources can influence the skin and help the penetration of various kinds of molecules. Microplasma treatment of the skin and delivery of the Cyclosporine A showed that delivery

Microplasma Drug Delivery

115

http://dx.doi.org/10.5772/intechopen.73498

We would like to thank to Professor Damon M. Chandler, Mr. Hideto Miyamoto and Dr. Marius G. Blajan of Shizuoka University, for fruitful discussion. This work was supported

[1] Donnelly VM, Kornblit A. Plasma etching: Yesterday, today, and tomorrow. Journal of Vacuum Science and Technology A. 2013;**31**(5):050825. DOI: 10.1116/1.4819316

[2] Proslier T, Klug JA, Becker NC, Elam JW, Pellin MJ. Atomic layer deposition of supercon-

[3] Hegemann D, Brunner H, Oehr Ch. Plasma treatment of polymers for surface and adhesion improvement. Nuclear Instruments and Methods in Physics Research Section B.

[4] Shintani H, Sakudo A, Burke P, McDonnell G. Gas plasma sterilization of microorganisms and mechanisms of action (review). Experimental and Therapeutic Medicine. 2010;

ductors. ECS Transactions. 2011;**41**(2):237. DOI: 10.1149/1.3633673

of a therapeutic amount of drug is possible.

by JSPS KAKENHI Grant Number JP16H04085.

\*Address all correspondence to: shimizu@cjr.shizuoka.ac.jp

2003;**208**:281. DOI: 10.1016/S0168-583X(03)00644-X

**1**:731. DOI: 10.3892/etm.2010.136

Kazuo Shimizu\* and Jaroslav Krištof

Shizuoka University, Hamamatsu, Japan

**Acknowledgements**

**Author details**

**References**

**Figure 10.** (A) Transdermal drug delivery of Cyclosporine A every 24 h. (B) Transdermal drug delivery of Cyclosporine A every 12 h. The weight of the patient was 55 kg in the model. The red line indicates the lowest therapeutic concentration [68].

Repeated treatment by microplasma and application of Cyclosporine A every 12 h/24 h, to a human of 55 kg is shown in **Figure 10**. As it is seen, treatment every 24 h is not sufficient, and it must be realized at least every 12 h [68]

#### **6. Conclusion**

The stratum corneum is the main barrier of the skin. The strength of this barrier is not homogeneous through the whole thickness, but it is stronger in the direction of the living tissues. The lipid matrix is mainly used for drug delivery through the skin. This matrix is composed of lipid molecules which are crucial for barrier properties. If these molecules are peroxided or shortened by breaking bonds, the permeability of the skin can be changed. Lipid molecules are hydrocarbons composed mainly of carbon, hydrogen, oxygen and nitrogen. If plasma is applied on the skin, we suppose that this surface treatment can be compared to treatment of the other materials, such as polymers. The thickness of the stratum corneum will be decreased. How effective the interaction of plasma can be, depends on the use of a gas or gas mixture. The presence of ions can be very effective to start the interaction with the skin. Usually, oxygen containing plasma can strongly etch the surface with the help of reactivity of active species with free bonds created by ions. If the result is a fluidic or heavy crosslinked structure, the structure depends on the amount of species which is able to react with the free bonds. On the other hand, helium plasma is not so strong of an etching medium and the result of the interaction can be less destructive. However, plasma treatment can also bring some reactive species close to the lipid membrane which can react with lipids without the help of ions and cause peroxidation of lipids and decrease their barrier function. It was shown that plasma sources can influence the skin and help the penetration of various kinds of molecules. Microplasma treatment of the skin and delivery of the Cyclosporine A showed that delivery of a therapeutic amount of drug is possible.
