Plasma is a quasi-neutral gas of charged and neutral particles that presents its own collective behavior. In other words, it can be said that the word plasma refers to an ionized gas in which all or a significant part of its atoms have lost one or more electrons and turned into positive ions. In another definition, a highly ionized gas whose number of free electrons is almost equal to the number of positive ions is called plasma.
In general, materials are considered to be in three states: solid, liquid and gas. But in scientific discussions, a fourth state is usually assumed for matter. The natural formation of plasma at high temperatures has given it the title of the fourth state of matter. A very natural example of plasma is fire, so the Sun is an example of a large hot plasma.
It is often said that 99% of the matter in the world is in the plasma state; It means that it is in the form of electrified gas whose atoms are divided into positive ions and negative electrons. Although this estimate may not be very accurate, it is a reasonable estimate of the fact that inside the stars and their atmospheres, gas clouds and mostly hydrogen in the interstellar space are in the form of plasma. Near us, as we leave Earth’s atmosphere, we immediately encounter the plasma that includes the radiation belts and the solar wind.
In everyday life
In our daily life, we encounter some limited samples of plasma. The spark of lightning, the soft glow of the aurora borealis, the gases inside a fluorescent or neon lamp, and the brief ionization seen in the exhaust gases of a rocket. So it can be said that we live in a percentage of the universe where plasma is not found naturally.
Temperature in plasma
Like all gases, the temperature in plasma is determined by the average energies of plasma particles (neutral and charged) and their degree of freedom (translational, rotational, vibrational and related to electrical excitation). Therefore, plasmas show multiple temperatures. In the common method of plasma production in the laboratory, energy is first accumulated in the electric field by electrons during collision, and then it is transferred from electrons to heavy particles. Electrons receive energy from the electric field during the free path and during the collision with heavy particles and lose only a small part of that energy; Because electrons are lighter than ions (the lightest ion is approximately 1800 times heavier than an electron). For this reason, the temperature of electrons in plasma is initially higher than that of heavy particles, and the collision of electrons with heavy particles balances their temperature. Of course, there is not enough time or energy for temperature equilibrium and a cooling mechanism such as low pressure ventilators is required to prevent the gas from heating up.
Plasma is thermally divided into two groups: thermal (with high temperature) and non-thermal (with low temperature). Only non-thermal (low temperature) plasma is used in dentistry. In this system, the temperature is the same as the temperature of the ions and is almost equal to the ambient temperature or at most a few degrees higher. In this system, the amount of ionization is 2-3%.
Ionization and chemical processes in plasma
Ionization and chemical phases in plasma are determined by temperature (and indirectly by electric field). This type of quasi-equilibrium plasma is usually called thermal plasma, which is characterized by solar plasma in nature. Ionization and chemical processes in non-equilibrium plasmas are directly determined by electron temperature and are not sensitive to gas temperature. Non-equilibrium plasma is called non-thermal plasma.
Today, non-thermal plasma has found many applications in medicine. This kind of plasma is called non-thermal because the temperature of plasma and ions is in the range of room temperature, while the temperature of electrons is high. Plasma is used in biology and biomedicine to inactivate bacteria and tissue sterilization, blood coagulation, wound healing, treatment of corneal infections, cancer treatment and dentistry.
Plasma is a new method for whitening (Bleaching) the teeth:
The use of plasma to whiten teeth has been considered. The advantage of this method over the usual methods is that in this method of gel Bleaching It is not used and these are free radicals that can oxidize organic substances and whiten teeth; While in other methods, including the laser application technique, the use of Bleaching gel It is mandatory.
One of the methods that may be used in the usual teeth bleaching methods is the use of a high-intensity plasma arc device that comes with gel. In this method, due to the high concentration of the gel and the high intensity of the device, the possibility of tooth sensitivity is high; But the method discussed in this article is without bleaching gel and with the help of plasma gas. In this method, the effect of teeth whitening by (PMJ) plasma micro jet sent by direct current in atmospheric air pressure of 0.9% saline solution has been investigated.
The plasma device used in this article consists of two coaxial cylinders as electrodes, which are separated by a dielectric layer with a thickness of 0.5 mm.
The internal electrode is powered by a high voltage DC power supply while the internal electrode is used due to safety considerations. The gap or nozzle of the plasma device has a diameter of about 0.8 mm.
In this study, compressed air is used as the working gas and is amplified between the internal electrodes. The stable voltage of microjet plasma is in the range of 400-600 volts with an operating current of 20-35 mA.
For the purpose of this research, 30 normal healthy teeth from caries were selected from the extracted premolars for orthodontic reasons and kept in a 0.1% thymol solution. They were randomly divided into 3 groups: Group A, Group B, and Group C.
In group A, the tooth was subjected to air flow and saline solution for 20 minutes.
In group B, the teeth were subjected to plasma and saline solution for 20 minutes (a flow of A m 30). The tooth was placed at a distance of 10 mm from the external nozzle of the plasma microjet (PMJ) device. During this period, the temperature was measured by a thermal couple in the approximate range of 40 c. No whitening gel was used in this method.
In group c, the teeth were exposed to 35% H2O2 gel at room temperature for 20 minutes. On the surface of group A and B teeth, saline solution was used once every 30 s to prevent dehydration.
As it is clear in the pictures and graphs, the plasma method has brought better results.
Although its main mechanism is not fully understood, reactive oxygen groups (ROS) produced by the plasma system are essential for the plasma teeth whitening process.
It is believed that the formation of perhydroxyl ions (HO2–) and OH is necessary. The effectiveness of PMJ in teeth whitening is due to the production of ROS (reactive oxygen groups) at the tooth-liquid-plasma interface, which is very similar to the mechanism of oxygen production in the teeth-whitening gel interface in traditional treatments.
A study showed that trace amounts of ozone can be found downstream of the PMJ. atomic oxygen and ozone produced at the place of water and plasma interaction; produces O2–, OH. .OH, O2. and other mediators of ROS or active oxygen groups are mixed together.
The improved efficacy of PMJ combined with saline in tooth whitening is due to the following mechanisms:
- (.OH ,O2,.O–2…) ROS caused by water-plasma interactions that directly or indirectly react with pigment molecules on the tooth surface and break long carbon chain bonds, resulting in the whitening effect.
- ROS produced at the liquid-plasma interface (reactive oxygen groups) is easier and more effective than H2O2 gel.
- Despite its absence in plasma and aqueous environment, small amounts of nitric acid have been found. As a result, local pickling of the above acid on the surface of the tooth increases the efficiency of tooth whitening.
PMJ reacts with NacL in saline solution and produces bischlorine ions and radicals. However, because the concentration of NacL in the saline solution is very low (0.9%), only small amounts of chlorine base ions and radicals are produced; Therefore, they do not cause a significant whitening effect.
Enamel surface of treated teeth
The enamel surface of the teeth treated by PMJ and saline solution is slightly rougher compared to the control samples; But they are similar to teeth treated with H2O2 gel. Tooth whitening by H2O2 or carbamide peroxide causes slight changes in enamel surface morphology. that this amount of changes in morphology is acceptable.
In another research of plasma with H2O2 35% in one group and H2O2 35% was used alone in the other group. In the group that used plasma, the color change (ΔE) was significantly improved. In the plasma group, the temperature measured on the surface was about 37 degrees Celsius and no temperature increase was seen.
In another study, the effect of plasma for Non Vital Bleaching of root canal teeth was evaluated. In this research, 20 discolored teeth were bleached with 30% hydrogen peroxide for 30 minutes, and 20 other teeth were treated with 30% hydrogen peroxide for 30 minutes along with plasma.
In both groups, the temperature was measured at 37 degrees; But the amount of bleaching in the plasma group was statistically significantly better than the first group. Although plasma itself produces free radicals, the presence of hydrogen peroxide produces more OH free radicals, which maximizes its effect. It should be noted that helium gas was used in this study.
Although non-thermal plasmas have few known harmful effects on humans, a number of safety concerns should be addressed in future studies; For example, ROS produced in this system has a key role in teeth whitening; But if its dose is not controlled correctly, they can be harmful to oral tissues and respiratory system.
Although plasma has been effective in teeth whitening, more studies are needed to evaluate its durability and stability.
This article is dedicated to Pir and Murshid and Muradam; Dr. Mustafa Chamran, who was not only one of the pioneers of plasma physics in the world; Rather, he was a master of morality and mysticism and he proved his authority over all free people and men of wisdom.
God, thank you for giving us the pleasure of getting to know your parents.
Dr. Kasri Tabari – restorative and cosmetic specialist
Dr. Fariba Azoji–restorative and beauty resident
Mustafa Aqiqi – Plasma engineer