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Plasma, a mix of ionized gas molecules and free electrons, is often referred to as the fourth state of matter, and it’s nothing new. Welding arcs, fluorescent lights, and plasma televisions use it. But plasmas can also prepare surfaces for bonding and kill bacteria on delicate living tissues.
Controlling the energy in plasmas is the key to making them safe for patients, according to Dr. Alexander Fridman, director of the A.J. Drexel Plasma Institute at Drexel University. In fact, researchers at the Plasma Institute and Drexel’s medical school have generated plasmas that can kill bacteria on living tissue without damaging the underlying cells. So bacteria that resist or are inaccessible to traditional heat sterilization and antibiotics can still be eradicated by plasma.
The medical profession has known for years that reactive ions generated by plasma, especially those referred to as reactive oxygen species (ROS), can damage or kill cells. In cancer treatment, targeted radiation beams generate ROS inside the body that kill cancer cells.
The work at Drexel uses lowerpower sources that kill surface bacteria but leave mammalian tissue untouched. Hot or high-powered plasmas, such as those for TiG welding, are 10,000 to 24,000°K, but Fridman’s method keeps plasmas between 300 and 1,000°K.
Bacteria’s smaller size, higher surface-to-volume ratio, and lack of surface defenses make them more susceptible to damage by plasma and ROS. Energies around 1 J/cm2 kill bacteria, but leave mammalian cells untouched, according to Fridman.
“Energies below 1 J/cm2 are absolutely safe for mammalian cells,” he says. “Energies between 1 and 10 J/cm2 produce some biological changes in mammalian cells, and above 10 J/cm2 there are enough changes that cells self-destruct. Immediate physical damage starts at 30 to 40 J/cm2.”
Plasmas for medical settings must be uniform in addition to having low energy. To be useful, they must also remain uniform in air at atmospheric pressure. The voltages Fridman’s group uses, 30 to 40 kV, are enough to overcome the dielectric constant of the air and break it down into plasma.
Voltage generates lightning between two electrodes in Fridman’s setup, which are separated by an air gap. By covering one electrode with a strong dielectric, researchers keep the charge from following a direct path between electrodes. Lacking a specific path, the discharge spreads uniformly throughout the intervening space.
Like lightning, such discharges are extremely brief, about 100 nsec. Fridman’s group switches the electrodes’ polarity at about 100 kHz to keep uniform discharges going. Then pulsing the voltage with a modified square wave at about 100 nsec/pulse keeps the plasma from heating.
To take the setup from the lab to the clinic, Fridman’s group uses a single electrode coated with the specially developed dielectric. The voltage differential forms between that electrode and the tissue to be treated, so plasma is attracted to the tissue.
In addition to killing surface bacteria, plasma shows promise for healing wounds. Plasma energies up to 10 J/cm2 stimulate cells to produce more growth factors, which researchers think could prompt stubborn wounds to heal more quickly. Some work with healing wounds in pigs has shown promising results, Fridman says.
Researchers are also looking at treating Crohn’s disease and other types of colitis—poorly understood conditions where unknown factors trigger inflammatory responses in the intestines. Plasma-generated ROS have eased flare-ups in the colons of mice.
Fridman says the next step for both treatments is FDA approval of human trials. Some work has been done with human subjects in other countries, notably Russia and Germany, but Fridman couldn’t say when human trials could start in the U.S.
Plasma preparation
Although they are not nearly as delicate as living tissue, bonding surfaces also benefit from cool plasmas. Surface-preparation plasmas can destroy organic matter on bonding surfaces, just as Fridman’s medical plasmas can. ROS chemically activate surfaces to prepare them for bonding. Plasmas can also enable the deposition of polymerizable precursors to form nanometer-thickness coatings.
Unlike medical plasmas which use a voltage differential to attract plasma to the treatment surface, surface-preparation plasmas rely on compressed air to propel plasmas toward surfaces at about 200 m/sec. The most common configuration—a plasma jet— has an electrode and air inlet at the back of a jet housing. The electrode uses 480 V at 15 A to create plasma in air at atmospheric pressure. Compressed air at about 90 psi and 2 cfm blows the plasma from the electrode toward the jet’s nozzle. When it exits the nozzle, the plasma stream is about 260°C.
One such system, Openair from Plasmatreat, Mississauga, Ont., treats a 25-mm swath from 10 to 20 mm away. The plasma contains high-energy molecular fragments traveling at high speeds. These particles generate an effect similar to grit blasting or blasting with other media. Because fragments are much smaller than media particles, the resulting surface treatment is more uniform.
The ROS in the plasma destroy impurities on bonding surfaces. The beam is electrically neutral, so it is safe for electronics, plastics, metals, and large areas that can build up dangerous static. During the treatment process, the plasma beam raises the temperature of the surface 20°C.
ROS also add functional groups to treated surfaces, changing their surface energy. Higher surface energies mean liquids can more easily wet out the surface. This is particularly helpful when adhesives must evenly coat an entire bonding surface.
Because Openair plasma operates at atmospheric pressure, users aren’t tied to dedicated treatment chambers as needed for low-pressure plasma treatment. Openair units have been added to in-line surface-treatment systems and robots that automate treatment of large areas.
Large-scale robot-assisted plasma surface treatment helps shipbuilders seal the cargo holds of liquefied natural-gas (LNG) tankers. Earlier tankers carried LNG in spherical tanks on the ships’ decks. The newer ships, 300-m long with 42-m beams, carry the payload in large storage tanks inside their hulls.
The hulls’ steel would be embrittled by contact with the –163°C liquid, so the tanks must be lined with two layers of insulation and two barrier layers. The inner barrier layer is low-coefficient-of-thermal- expansion Invar steel. The intermediate barrier between the Invar and hull consists of 1 × 3 m fiberglass-aluminum-sandwich panels.
Because the intermediate barrier’s integrity is critical, shipbuilders must seal joints between sandwich panels with 30-mm-wide flexible strips bonded with two-part epoxy. Plasma treatment raises surface tension from 32 to 72 mN/m and helps the epoxy wet out for a uniform bond.
Each tanker’s 153,500 m3 of cargo volume requires about 40 km of sealing strips. To speed surface treatment, shipbuilders install a network of horizontal rails throughout the cargo hold. Twenty robot-controlled rotating plasma jets travel the rails at 6 m/min, treating the bonding surfaces. With the automation, 300 workers laboring around the clock can complete about 9% of the bonding each week.
Hydrogen helper
Plasmas have also made inroads in turning waste into fuel. Plasma-assisted gasification (PAG) has been used for several years to turn feedstocks like chopped tires and mixed plastics into usable fuel. PAG relies on high-temperature plasma and requires significant energy.
Cool and intermediate-temperature plasmas, which use less energy, are showing promise for certain types of fuel synthesis. Fridman and his colleagues, for example, are using a plasma called gliding-arc discharge to refine methane and other fuels into hydrogen with less energy.
Gliding-arc discharge plasmas use two electrode wires separated by an air gap that widens as one travels away from the base. Where the voltage across the electrodes exceeds air’s dielectric breakdown potential (at least 30 kV) a plasma arc forms. The arc travels away from the base and lengthens to span the growing gap between the electrodes.
One variation of this setup, called a gliding-arc tornado (GAT) uses a ring electrode and a spiral electrode to form the widening gap. The GAT configuration keeps the arc from upping its current draw as it expands. Because the arc draws less power per unit length, its temperature drops as it lengthens. The space between the electrodes fills with numerous weak arcs until a uniform field of plasma at 2,500 to 4,000°K has formed.
To convert methane to hydrogen, a cylindrical reactor encloses the spiral and ring electrodes. Methane and a carrier gas like air are injected into the reactor to create a reverse vortex flow that maximizes the amount of the plasma’s heat reaching the gas.
Heat breaks carbon-carbon bonds in the methane to create diatomic hydrogen and carbon monoxide molecules if the carrier gas contains oxygen. The carbon monoxide-hydrogen mix is often referred to as syngas. Users have the option of siphoning off pure hydrogen.