Plasma, often referred to as the fourth state of matter, is an ionized gas consisting of atoms, molecules, ions, and free-moving electrons, making it electrically conductive. While most plasma exists in extremely hot environments like stars or lightning, a special form known as non-thermal plasma (NTP), or cold plasma, has been engineered for use at or near room temperature. This controlled, low-temperature plasma allows for applications involving sensitive materials, including living tissue.
What Defines Cold Plasma
The distinction between hot plasma and cold plasma lies in thermal non-equilibrium. In hot, or thermal, plasma, all constituent particles—electrons, ions, and neutral atoms—share a single, extremely high temperature, often reaching tens of thousands of degrees Celsius. This equilibrium produces intense heat, as seen in traditional plasma torches. Cold plasma, however, is a partially ionized gas where the electrons are highly energetic while the heavier ions and neutral gas particles remain cool.
The electron temperature in cold plasma can exceed 10,000 Kelvin, providing enough energy to drive chemical reactions and cause ionization. Conversely, the bulk gas temperature, which affects materials placed in the plasma, is maintained below 50 degrees Celsius, often near ambient room temperature. This low overall temperature occurs because light electrons do not efficiently transfer energy to the much heavier neutral and ionic species through collisions. Since reactive chemical species are generated without extreme heat, cold plasma can safely interact with temperature-sensitive materials, such as biological tissues.
Essential Hardware for Plasma Generation
Generating cold plasma requires a specialized system designed to input energy into the electrons without heating the entire gas volume. The necessary hardware begins with a high-voltage, high-frequency power supply, which drives the ionization process. These power sources must deliver voltages typically in the kilovolt (kV) range (5 kV to 20 kV) to overcome the gas’s breakdown voltage.
The alternating current (AC) power must be applied at a high frequency, ranging from tens of kilohertz (kHz) up to the megahertz (MHz) radio frequency (RF) range. This rapid cycling prevents the plasma from transitioning into a continuous, high-current arc, which would become a high-temperature thermal plasma. The gas feed system introduces a precursor gas into the reactor chamber. Inert gases like Argon and Helium are commonly used because their lower ionization energy makes it easier to initiate and sustain the plasma discharge at atmospheric pressure.
Finally, a reactor design incorporating electrodes is necessary to create the strong electric field required for ionization. The arrangement of these electrodes and the reactor shape dictates the resulting plasma geometry. Insulating dielectric materials are often incorporated to limit current flow and stabilize the non-thermal nature of the plasma. This careful control ensures energy is channeled primarily into the electrons, maintaining the cold characteristics of the final plasma.
Common Techniques for Creating Cold Plasma
The two most common methods for generating cold plasma are Dielectric Barrier Discharge (DBD) and Atmospheric Pressure Plasma Jets (APPJ). These techniques use specialized hardware to initiate a controlled electrical breakdown of the gas. DBD involves placing a dielectric material, such as glass or ceramic, between two high-voltage electrodes.
When high AC voltage is applied, the gas between the dielectric surfaces breaks down, creating short-lived electrical discharges called microdischarges. The dielectric layer limits charge accumulation, generating a local opposing electric field that quickly extinguishes the microdischarge within nanoseconds. This rapid ignition and quenching prevents the formation of a continuous, high-current arc, maintaining the plasma in a non-thermal state. The DBD configuration is often used for treating large surface areas because the plasma is distributed broadly.
The Atmospheric Pressure Plasma Jet (APPJ) method feeds a noble gas, such as Helium or Argon, through a small tube or nozzle. High-frequency voltage is applied to an electrode inside or around the tube, creating a localized plasma discharge within the flowing gas stream. The energized gas exits the nozzle as a directed, visible plume of plasma propagating into the surrounding air.
This directed plume allows the plasma to be precisely delivered to a target surface without the target needing to be part of the electrical circuit. The mechanism involves the high electric field accelerating electrons, which collide with gas atoms to create reactive species carried in the gas flow. Because the plasma plume is directed and narrow, APPJs are employed for highly localized and precise treatments.
Where Cold Plasma is Used
The ability of cold plasma to generate highly reactive chemical species without causing thermal damage makes it uniquely suited for a wide range of applications. In the biomedical field, this non-thermal property is leveraged for therapeutic use and sterilization. Cold plasma systems sterilize heat-sensitive medical instruments and implants by inactivating pathogens like bacteria, viruses, and fungi on surfaces.
For direct therapeutic treatments, cold plasma shows promise in wound healing and dermatology. The reactive oxygen and nitrogen species (RONS) generated can stimulate cell proliferation, promote blood coagulation, and disinfect chronic wounds. Research also extends to oncology, investigating the localized application of cold plasma for its potential to selectively destroy cancer cells while minimizing harm to surrounding healthy tissues.
Beyond healthcare, cold plasma is utilized in material science for surface modification and processing. Treating materials like polymers and textiles can dramatically alter their surface properties, such as increasing surface energy to improve adhesion for coatings or glues. This process, known as surface activation, allows for better bonding in manufacturing and fabrication. Cold plasma is also used for ultra-fine cleaning and etching in the semiconductor industry, offering an environmentally conscious alternative to traditional chemical processes.