The invention of methods to produce non-equilibrium plasma at atmospheric pressure liberated plasma technology from the oppressive confines of vacuum systems and paved the way for an explosion of applications in sectors transcending the traditional ones. The non-equilibrium implies a different temperature for the electrons, ions and neutral molecular components of the plasma. Because the neutrals which impart sensible heat are cold, these plasmas have come to be popularly known as ‘ Cold Plasma’ and when they are at atmospheric pressure, Ambient Pressure Cold Plasma (APCP). The’hot’ plasmas are those present in welding arcs, plasma torches or thermonuclear fusion devices.
The global cold plasma market was valued at US$ 2.2 Bn in 2022 and is projected to expand at a CAGR of 12.3% from 2023 to 2031 (1). A wide range of industries such as textile, food, medicine, and printing are using this technology, which is the factor that is driving the market growth (2). The increasing demand for environmentally-friendly processing techniques and high-performance materials is fueling this. Avoidance of hazardous chemicals and toxic waste makes it a safer and more sustainable alternative to traditional chemical processing routes (1).
The Plasma State
A gas situated in a strong electric field gets ionized by the process of electron impact ionization producing a fluid made up of electrons, ions and neutral molecules which we call plasma. The energy gained by the electrons gets transferred to the neutral atoms through collisions, which get heated, ultimately forming a plasma in thermal equilibrium. However, if the electric field is applied in pulses with a pulse duration insufficient to produce thermal equilibrium, we get the cold plasma. Nanosecond kilovolt pulse trains produce such cold plasmas.
Thermal equilibration can also be prevented if the plasma production region is confined to microscopic dimensions. The size of the plasma is critical to maintaining low, often near-room temperature, gas temperatures while producing electrons featuring nonequilibrium energy distributions. The small size enables effective cooling through the plasma surface so that gas heating is reduced even at high power densities.
A variety of plasma devices and applicators have been developed. Large-volume plasma excited by capacitive discharge is common. Hand-held Cold plasma jets have been made with capillary flow. Large panels with microscopic hollow discharge arrays have also been developed.
The presence of energetic electrons and excited radicals renders plasma chemically reactive chemically. The plasma chemical process can be a homogeneous gas-phase reaction with reactions occurring between gas-phase species mediated by inelastic collisions between electrons and molecules or between molecules.
Heterogeneous reactions happen when plasma contacts a solid or liquid medium. In one form of heterogenous reaction, the material is removed from the surface by etching or ablation. In another, material is added to the solid surface in the form of thin films as in plasma polymerization or plasma-enhanced chemical vapour deposition. In the third category, the substrate surface exposed to plasma is changed physically and chemically.
Plasma Sterilization and Decontamination
The ability of plasma to sterilize surfaces by destroying spores in the inner surface of vials was discovered in the 1960s. Advanced Sterilization Products (ASP) conceived the novel idea of using ionized hydrogen peroxide for device sterilization and, in 1987, came out with the STERRAD™ 100 System. The use of the vapour phase and ionized H2O2 rapidly sterilizes medical instruments and materials and leaves no toxic residue.
Plasma in Medicine
Plasma medicine uses APCP to generate controllable amounts of chemically reactive species — reactive Oxygen and Nitrogen species — that react with biological targets like cells and tissues (3). In 25 years, the field has progressed enough to deliver a wealth of applications for actual patients.
Small doses of LTP were found to selectively kill cancer cells without harming healthy ones opening up a research avenue of “plasma oncology” (3). Investigators reported promising results on the killing of various cancer cell lines associated with leukaemia, carcinoma, breast, brain, prostate, and colorectal cancer both within and outside living organisms. These efforts finally led to the US Food & Drug Administration (FDA) giving approval to clinical trials in the USA in 2019 (3).
Dental caries, a major oral disease caused by acid production resulting from bacterial metabolism of sugars, is treated conventionally with mechanical or laser ablation. However, these techniques create vibration, noise, and heat resulting in patient discomfort. APCP is a replacement decontamination treatment to remove non-remineralized tissues with an associated bleaching effect useful for teeth whitening.
A National Health Service report in 2016 claims that chronic wounds were present in 6% of people. By 2022, the market for wound closure products was expected to hit $15 billion (4). The ACP improves cell proliferation, initiates changes in junctional proteins, and synthesises extracellular matrix protein leading to rapid wound healing.
Food Waste Treatment
APCP treatment can decrease the number of vegetative bacteria and endospores, mould fungi and pathogens in municipal waste. ACP-mediated pretreatment of municipal waste can minimise environmental pollution during storage and transport.
APCP may also help to produce useful products from food waste. In the treatment of waste composed of polysaccharides and aromatic polymers, APCP destroys toxic compounds that suppress bioethanol fermentation leading to the enhancement of bioethanol conversion. APCP has successfully removed fermentation inhibitors including hydroxymethylfurfural (HMF), formic acid, and furfural from sugarcane bagasse (5). Pineapple peel waste hydrolysate has been detoxified with APCP to produce bacterial cellulose, which has extensive applications in the biomedical field.
Waste Water Treatment
Over 20% of water pollution originates in the textile dyeing process, where clean water is used along with a variety of chemicals. The highly toxic wastewater is very harmful to the environment. Underwater APCP has been used to degrade almost 70 % of dissolved textile dyes. APCP produces O3, H2O2, and NO3 which mediates the dye decomposition. Agricultural wastewater laden with pesticide residue is another subject of concern. Large fractions of carbaryl, methiocarb, and aminocarb could be degraded by ACP application (5).
Polymers and films
The application of APCP to modify polymer properties concentrate on celluloses, proteins, and polysaccharides due to their potentially large application in the medical, food, and textile industry (5). APCP can be used to enhance the wettability of the cellulose nanofibre coating layer. Bacterial cellulose, when modified using APCP, improves the water vapour permeability of cellulose by the removal of low-molecule fragments. In polypropylene (PP), APCP etches the surface which decreases the contact angle and increases polar functional groups improving hydrophilicity. The application of ACP on polyethylene terephthalate (PET), cotton, and nylon enhances their surface wettability, reducing the time and temperature required for dye binding.
Carboxymethyl cellulose and collagen can be used to coat cinnamaldehyde on low-density polyethylene (LDPE) to improve the quality of active packaging films. When the polypropylene/LDPE film was coated with nanofibrillated cellulose and nisin using ACP, the film illustrated improved oxygen barrier performance in addition to inhibiting the growth rate of Listeria monocytogenes (5).
Plasma-treated seeds, immersed in water, showed larger radicals or starter roots. An atomic force microscope revealed roughening up of the seed surface increasing water absorptivity. MRI images of treated beans showed more water inside, compared to untreated beans. Exposing the seeds to plasma changes the seed morphology. Genetic effects like gene expression and protein level can also be affected. These physical changes are beneficial to increased germination and accelerated growth. APCP-enhanced seed germination has been conducted on soybeans, mung beans, corn etc. APCP is shown to increase the biologically active ingredient of sprouts. Tomatoes irrigated with Plasma-activated water grow faster.
The increase in agricultural productivity in the 20th century is attributed to the Haber−Bosch process for nitrogen fertilizers. At the end of the 20th century, about 40% of global food production depended on fertilizer inputs. The Haber−Bosch process has a heavy Carbon footprint. The option of using APCP to produce and deliver nitrogen fertilizer in situ in the form of NOx uses air instead of ammonia and consumes less energy than the H-B process.
Plasma-activated water can inactivate microorganisms on the seed surface. Nitrite and nitrate ions produced by APCP provide the nutrients essential for plant growth. Studies on the effects of direct and indirect APCP treatment on eight different seeds found that the seed properties determine the efficacy of plasma treatment.
APCP effectively inactivates various pathogens and spoilage organisms without adversely affecting food quality. It can significantly degrade mycotoxins and pesticides present in agricultural produce. APCP is also used to inactivate endogenous enzymes which are responsible for browning reactions, particularly polyphenol oxidase and peroxidases (6). The microorganisms are etched out or their cells are disrupted electrophoretically. It can meet microbial food safety standards, improve the products’ physical, nutritional, and sensory characteristics, preserve unstable bioactive compounds, and modulate enzyme activities (7).
In printing processes such as UV inkjet digital printing, the end result is largely determined by the way in which ink droplets spread on the substrate (8). Most polymers and metals need an increase in surface energy to achieve a durable and high-quality adhesion. The higher the surface energy, the greater wettability and molecular attraction. The surfaces of many materials such as metals, glass, ceramics, wood and textiles are often very difficult to print on but are receptive to plasma functionalization. Plasma processing functionalizes the surface effect through combines processes of removal of organic contaminants through precision cleaning, creation of surface texturing and deposition of functional chemical groups.
Plasma-mediated pre-treatment for printing and coating has acquired maturity as a standard process to precede digital printing, pad printing, and screen or offset printing. The adhesion of the printing inks and varnishes to the surface is enormously improved by pre-treatment with plasma, which significantly increases the print quality (9).
In India, the cold plasma device and application development have been centred at the Facilitation Centre for Industrial Plasma Technologies (FCIPT), a division of the Institute for Plasma Research, India’s premier research centre leading Thermonuclear Fusion and Plasma Research. They supply cold plasma devices and applicators on a commercial basis. Information about the technologies is available on the website: http://www.plasmaindia.com/newsletter.htm
Plasma technology is unique in that it has given us applications ranging from replicating stars on Earth to producing Carbon-free energy. The plasma process for manufacturing Silicon chips amplifies our computational power. It has transcended its traditional confines of energy and material processing to bring applications relevant to everyday life. Let us hope that this frenetic and relentless progress continues.