Katharina Stapelmann from North Carolina State University discusses the unique capabilities of low-temperature plasmas for addressing some of the biggest industry challenges.
Many of the outstanding engineering achievements of the last century are taken for granted today, e.g., computers, the internet, etc. The National Academy of Engineering has identified 14 game-changing goals to address new Grand Challenges of Engineering to improve our lives.
Research in science and engineering is laying the foundation for new incredible achievements for the next century. Plasma science and engineering is uniquely well-fit to address many challenges. Plasma, a partly ionised gas called the fourth state of matter, is a “hidden gem” often invisible to the public.
Advances in microelectronics would not have been possible without plasma science. Small features on chips with high-aspect ratios can only be achieved by plasma etching, bringing us smartphones, tablets, small and potent computers, and more.
The grand challenge of ‘Make Solar Energy Economical’ can be addressed by improving these processes.
Plasmas can be roughly divided into “thermal” and “non-thermal” (low-temperature). Researchers worldwide are addressing plasma science and engineering questions to tackle the grand challenge of ‘‘Providing Energy From Fusion‘, one of the more prominent examples of thermal plasmas.
In my research group, we are focusing on low-temperature plasmas and their applications in life sciences to address some of the identified grand challenges:
Managing the nitrogen cycle
The grass looks always greener after a thunderstorm. This is due to the rain and lightning breaking down nitrogen in the air into a form that is more readily available to plants.
Lightning, also a form of plasma, provides high-energetic electrons capable of breaking bonds and dissociating molecules. Nitrogen and oxygen in the air are dissociated and form molecules such as nitrate (NO3–), which the plants can easily absorb and use as a nitrogen source.1
Recent research efforts have focused on creating electrical discharges like miniature lightning to create nitrogen-based fertiliser more efficiently and decentralised. Currently, the majority of nitrogen fertiliser is produced via the Haber-Bosch process. This process requires high temperatures and pressures only obtainable on an industrial scale. While the Haber-Bosch process is still the most efficient process to produce nitrogen-based fertiliser, much of the fertiliser’s nitrogen content is lost via volatilisation, denitrification, and leaching, impacting the environment and groundwater quality.2
Transportation and storage risks add costs and restrict access to fertiliser. To maintain food production and feed the growing population on this planet, more sustainable and better accessible alternatives need to be explored for nitrogen fertiliser production.
Plasma science for nitrogen fixation has already been used in the early 20th century in a process called Birkeland-Eyde.3 This process utilised a thermal arc plasma in the presence of water to create nitrogen (NO3–) fertiliser. The Birkeland-Eyde process was abandoned in favour of the Haber-Bosch process, as this one had shown a higher energy efficiency.
With recent advances in non-thermal plasmas under atmospheric pressure conditions, plasma-based nitrogen fixation has come into focus again. A research field called ‘Plasma Agriculture’4 emerged. Numerous research groups worldwide are investigating plasma-based nitrogen fixation with various designs to enhance energy efficiency.
What most designs have in common is that plasma is created in the air in contact with water to produce NO3– or NH3 and fix it in the treated water. Non-thermal plasmas can be operated intermittently, allowing on-site fertiliser production on demand, potentially even using renewable energy sources. Beyond nitrogen fixation, the plasma-treated water can contain H2O2 and other reactive species that could benefit plant growth.
In hydroponic systems, for example, plasma-treated water can be used as a nitrogen source, a disinfectant, and also for algae removal.2 An enhanced resilience towards abiotic and biotic stressors has been observed as well.1,2 What effects can be observed on the plants and their ecosystem depends on the composition of the plasma-treated water.
Different parameters affect the dissociation and formation of reactive species, e.g., voltage amplitude and frequency, deposited power, feed gas and humidity, etc.2 For efficient fertiliser production, the species produced in the plasma need to be captured in the water, so a high surface-to-volume ratio is desirable for practical transport.
Different approaches have been tested, e.g., igniting plasma in bubbles submerged in water5-8. Here, basic research had to be performed to understand how plasma ignites in bubbles submerged in water. Different voltage polarities and pulse widths, from nanoseconds to microseconds excitation, were employed to study the breakdown mechanisms in water with varying conductivity, providing fundamental insights valuable for designing efficient plasma devices for nitrogen fixation. Another approach is to have air flowing through a plasma and then bubble it into a liquid column for nitrogen fixation9.
Computational investigations of the species production as a function of deposited power and resulting gas temperature have shown voltage parameters (amplitude and frequency) that can maximise nitrate formation. Detailed analysis and optimisation of gas residence time in the plasma, bubble residence time in the liquid, and optimum surface-to-volume ratios for the bubbles helped design an effective lab-scale prototype.
A crucial finding was that the process cannot be optimised by increasing NO3– production in the plasma; the process becomes transport-limited, and much of the NO3– is lost. Different NxOy species, however, have larger Henry’s law constants (a number that determines the ratio between gas phase and liquid phase densities) and will decompose to NO3– in the liquid. This opens new avenues for optimising plasma-based nitrogen fixation, as the production of species and their transport into the liquid needs to be considered.
Providing access to clean water
Many concepts discussed above for nitrogen fixation also apply to treating wastewater or contaminated water. One of the significant challenges today is the removal of per and polyfluoroalkyl substances (PFAS) from water. PFAS, also dubbed “forever chemicals”, are difficult to degrade and can be found virtually everywhere in the environment10.
Their thermal and chemical stability combined with their tendency to accumulate (PFAS were found in human blood, milk, urine, tissues, and organs10, made PFAS an increasing concern for human health and wildlife ecology. Conventional methods can only collect longer-chain PFAS, creating waste with the potential of re-introduction to the environment after disposal. The only option available for destruction is currently thermal decomposition in halogen-resistant incinerators11.
Electrochemical oxidation, sonolysis, and low-temperature plasma treatment have become available alternatives recently11. A bench-scale plasma reactor demonstrated the removal of PFOA+PFOS found in landfill leachate to below 70 ng/L (health advisory limit)12.
While the capability of plasma science to remove PFAS has been demonstrated, the mechanisms of action are not known, which hinders targeted optimisation and scale-up of the devices. Much research has been done on oxidising species produced by plasma, but PFAS are not oxidisable. Basic research is needed to explore the roles of non-oxidative species in plasma-based water treatment13. In a recently funded NSF project, we are investigating the individual contributions of non-oxidative species, e.g., photons, electrons, and the H radical.
By carefully designing experiments that allow different levels of contribution from each suspect combined with state-of-the-art diagnostics to quantify each species, we aim to deconvolute the effectors to optimise the process for scale-up and commercialisation.
Engineering better medicine
The reactive oxygen and nitrogen species produced in plasmas in the air are the same that the human body uses for signalling processes and the oxidative burst, an immune response to fight bacteria and infection.
Several approved medical devices are available in Europe to treat chronic wounds. The lab and clinical trials have shown reduced bacterial load in the wound, increased microcirculation, and triggering of the immune response14-16.
Clinical applications are based on empirical observations, and all patients/diseases are treated using the same regimen. The extent of clinical efficacy cannot be evaluated until several days after treatment. To address this, we are working in an interdisciplinary team on an NIH-funded project to explore real-time measurements during plasma treatment for endpoint detection17.
By measuring different potential effectors and comparing their concentrations to wound healing outcomes, we aim to define a “plasma dose” for optimum biological outcomes. Plasma has antimicrobial effects, and this is exploited in another application where plasma is used to treat fungal infections in the eye. Fungal infections are rising due to climate change, with many fungal strains not responding to available treatment options18.
Plasma was shown in vitro and ex vivo to significantly reduce fungal contamination19. In a very early stage, plasma has shown promise as a novel treatment strategy for cancer20–22. Different approaches to explain the antitumor effects of plasma are currently being investigated and discussed among researchers.
More research is needed to safely and effectively apply plasma science for cancer treatment.
The future of plasma science
Plasma science is a versatile and unique tool that can benefit society in various applications. Our work ranges from basic research to understand discharge behaviour to exploring mechanisms of action for optimisation and targeted design of plasma devices, suitable for the application.
With the grand challenges ahead of us, plasma science will play an essential role in the engineering achievements of the 21st century.
Acknowledgements
This work would not have been possible without the contributions of graduate and undergraduate students and PostDocs working in the lab. I want to thank all former and current lab members for their hard work and dedication. This work would also not be possible without funding.
I gratefully acknowledge support for this work from National Science Foundation grants PHY 2107901 and PHY2308857, Department of Energy grants DE-SC0023235 and DE-SC0021329, the National Institute of Health NIH, National Institute of Biomedical Imaging R01EB029705, USDA NIFA 2020-67017-31260, as well as funding from the UNC Lineberger Comprehensive Cancer Center and the support from North Carolina State University’s Game-Changing Research Incentive Program for the Plant Science Initiative GRIP4PSI.
References
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Stapelmann K 2021 Plasma Connection: Plasma Agriculture IEEE Educational Briefs
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