The onset of the COVID-19 pandemic caused by the spread of the SARS-CoV-2 virus has significantly affected the lives of all of us who work in the tribology and lubrication field. Initially discovered in China late in 2019, COVID-19 has now spread throughout the world impacting every region.

The best scientific evidence indicates that SARS-CoV-2 readily transmits through the air. A recent update from the World Health Organization (WHO)¹ defines two forms of particles that can cause infection: Respiratory droplets that are five to 10 microns in diameter and droplet nuclei or aerosols with diameters less than five microns. The WHO reports that experimental studies conducted under laboratory conditions determined that aerosols containing SARS-CoV-2 can remain in the air for up to three hours.

For those of us in the Northern Hemisphere, the summer has been very hot, and air conditioning has been in high demand. But concern has been present that individuals in an indoor air-conditioned environment might be highly vulnerable to infection.

The U.S. Center for Disease Control and Prevention published an article² written by researchers in China about an outbreak of COVID-19 generated by individuals eating indoors in an air-conditioned restaurant in Guangzhou, China. In this study, the Chinese researchers traced an outbreak of COVID-19 to a family that just arrived from the source of the infection, Wuhan, China, and were eating at the restaurant in late January 2020.

There are no windows at the restaurant, and the only source of air flow is an air conditioning unit. The air inlet and return air inlet for the air conditioning unit are located above the dining tables.

Through contact tracing, the Chinese researchers found that an individual from Wuhan, infected with COVID-19, spread the disease to other families dining at the restaurant. Even though droplets do not stay in the air for long periods of time and do not travel in general distances greater than one meter, the authors of this study hypothesized that strong air flow from the air conditioning unit probably caused COVID-19 to be spread from the infected individual to other individuals eating at nearby tables.

Origin of the project
Zhifeng Ren, M.D. Anderson Chair Professor in the department of physics and director of the Texas Center for Superconductivity at the University of Houston in Houston, Texas, has varying research interests in areas such as high-performance thermoelectric materials and catalysts for water splitting. A previous TLT article³ discussed work his laboratory conducted to produce a new thermoelectric material based on magnesium, tin, germanium and a small amount of antimony that exhibited the desired properties of good electrical conductivity and poor thermal conductivity.

With this background, Ren was contacted in late March 2020 by a medical real estate company located in Houston seeking assistance in determining how to minimize the possibility of infection in a closed air-conditioned environment. Ren says, “The concept we discussed was to develop a filter that will inactivate SARS-CoV-2 present in the air before it returns to the air conditioning systems for reuse. The hope was to find a cost-effective process for purifying the air to make the working/living environment safer.”

Due to SARS-CoV-2 remaining active at ambient temperature, more than just a conventional filter will be required. Ren says, “Research on SARS-CoV-2 has shown that this virus, in a similar manner to other viruses, is vulnerable to inactivation at elevated temperature.”

Ren and his colleagues knew that increasing the air temperature to 70 C will inactivate SARS-CoV-2 in about five minutes. But the researchers realized that a shorter inactivation time was needed for any air conditioning system, which necessitated a higher filter temperature.

A novel approach has been used to produce an air filter that can operate at sufficiently high temperatures to inactivate SARS-CoV-2 in a shorter period of time, under one second.

Designing the filter
In evaluating his options, Ren wanted to work with a metal filter that can withstand the high temperature conditions needed to inactivate the virus. Most air conditioner filters are prepared with either fiberglass or aluminum. Unfortunately, both materials are difficult to heat and contain very large pores (up to one millimeter in diameter) that will not be able to trap and inactivate SARS-CoV-2 particles.

Ren says, “We have been working with nickel foam for another project and felt this metallic material had the potential to be effective at elevated temperatures. Nickel foam exhibits a high degree of porosity with randomly located pores present with diameters ranging 50-500 microns. This structure means that nickel foam displays sufficiently high surface area to utilize van der Waals forces to catch SARS-CoV-2 particles passing through in the air. Nickel foam also is quite flexible and exhibits good mechanical properties.”

The intermolecular attractive forces between uncharged molecules that are known as van der Waals forces will increase with an increase in surface area because more nickel foam is available to interact with molecular structures on the surface of the virus particles as they move through the filter.

Ren indicated that other metal foams, such as copper and titanium, were options, but both had drawbacks. He says, “Copper is not stable and will easily oxidize at elevated temperatures while titanium is more expensive than nickel.”

The researchers decided that 200 C was a high enough temperature to almost instantaneously inactivate the virus. Heat can be either conducted into the nickel foam externally or internally generated, however, this highly porous material does not conduct heat effectively. The researchers turned to internally generating heat as a better option and designed the nickel foam filter to be self-heating with the energy source being an electrical outlet.

Ren says, “Nickel conducts electricity very efficiently, but the metal cannot generate heat well, meaning it has low electrical resistance. This means that just using a flat piece of nickel foam will not effectively inactivate the virus because the temperature will not be high enough. This approach will not be feasible in meeting the requirements for heating, ventilation and air conditioning systems and the U.S. voltage requirement of 110 volts.”

Moving to a folded design provides the researchers with a number of added benefits. Ren says, “With a flat piece of nickel foam, the thickness is only 1.6 millimeters, which is inadequate to trap the much smaller SARS-CoV-2 particles. By using the folded design, we increased the effective distance to capture virus particles by many times. Through control of the bending length of the nickel foam, the temperature can be adjusted as there is a direct relationship between the two parameters. The folded design also minimizes heat loss, which means a smaller voltage can be applied to achieve the desired high filter temperature needed to inactivate the virus.”

One other consideration the researchers need to determine was the temperature of the air after it passed through the heated filter. Too high of a temperature might compromise the efficiency of the air conditioning system.

The researchers conducted an experiment where high purity nitrogen gas was placed approximately 3.5 centimeters from a nickel foam filter heated to 115 C in a room kept at a temperature of approximately 22 C. Measurements were made of the air temperature at various distances on the other side of the filter. The data collected showed that the temperature four centimeters away from the filter was close to room temperature. Based on this result, the researchers concluded that heating the nickel foam air filter to a high temperature will not adversely affect the temperature of the room where the unit is located.

The researchers now assembled a closed device that contained two filters assembled in a parallel fashion. Each filter contained eight pieces of folded nickel foam that are connected electrically in series. This approach provided sufficient resistance to enable regular-gauge electrical wires to be used to power the filter unit. Ren says, “We also made sure the filter was air tight so as not to allow any SARS-CoV-2 particles to bypass it.”

Figure 1 shows an image of the nickel foam air conditioning filter.


Figure 1. The air conditioning filter shown was prepared from porous nickel foam, heated to 200 C, and found to inactivate 99.8% of the virus that causes COVID-19, SARS-CoV-2, in a single pass. Figure courtesy of the University of Houston.

Filter testing
The researchers received assistance from colleagues at the University of Texas Medical Branch, Galveston National Laboratory in Galveston, Texas, to evaluate the filter. Ren says, “We literally placed the prototype in a car and drove it down to Galveston from Houston for the evaluation.”

At this point, the Galveston researchers isolated SARS-CoV-2 virus from human patients, used a cell culture medium to grow enough virus for testing and aerosolized particles in an air flow directed to the prototype nickel foam air filter heated to a temperature of 200 C.

The Galveston researchers used the median tissue culture infectious dose (TCID50) method for determining the effectiveness of the heated air conditioning filter. This method measures the reduction in viral titer, which determines the number of virus particles capable of infecting a host cell.

Testing showed that the concentration of SARS-CoV-2 in the air stream dropped from a 100% level upstream of the heated air conditioning filter to a 0.2% level downstream. This result means that the heated air conditioning filter effectively inactivated 99.8% of the virus in a single pass.

At the same time, the researchers also prepared a similar heated air conditioning filter and evaluated its effectiveness versus Bacillus anthracis, the bacterium that causes the very serious disease, anthrax. Spores of Bacillus anthracis were evaluated in a similar manner to SARS-CoV-2, and the heated air conditioning filter was found to kill 99.9% of the bacterium in a single pass.

One concern that this result showed is that nickel foam is able to inactivate virus particles that are much smaller, minimizing the risk that they might go right through the filter. Ren says, “We recognized that the air conditioning filter had larger pores than some of the virus pores. The nickel foam is highly porous with 95% of its composition containing pores of various sizes arranged in a random manner. In developing folded filters, we prepared a filter structure that forced virus particles through a meandering tunnel. In increasing the filter thickness, the virus particles were faced not only with navigating through the filter but also dealing with the 200 C temperature over a relatively longer period of time. The initial results are very promising in that nearly all of the SARS-CoV-2 particles were inactivated.”

The influence of elevated temperature in inactivating the virus cannot be underestimated. Ren says, “When the experiment was run under identical conditions without the use of temperature, only a very small percentage of the virus particles were inactivated. For anthrax, the log reduction of spores encountering a heated filter is 1.2. In contrast, the log reduction for an unheated filter is 0.19 and for no filter is 0.02.”

From a durability standpoint, Ren believes that the nickel foam air conditioning filter should be able to handle the high temperature conditions over a long period of time without any difficulty. He says, “The current filter design should have a six-month lifetime. We anticipate that continuous improvements should boost the filter life to 12 months.”

Ren advises that a sensor will need to be connected to the filter to ensure it is working properly by measuring the electrical resistance of the nickel foam. When the filter has exceeded its operating life, removal should be routine because the virus particles present will be inactive.”

Future work
The promising results demonstrated by the researchers show the potential for using this type of heated air conditioning filter commercially. Applications can be envisioned for any area where people tend to congregate.

The researchers believe that the highest priority will go to hospitals, other health care facilities and nursing homes where essential workers need to be protected. The heated air conditioning filter has the potential for use in schools and in public transit (airplanes, trains and buses).

From the standpoint of the lubricant industry, this device will be useful in laboratories, manufacturing facilities and office building environments. Ren reports that his partners are looking at the possibility of having a desktop model that would be effective in purifying the air near a specific individual’s working environment.

The heated air conditioning filter also has shown effectiveness against other antigens such as anthrax, which suggests that it can be used against other pathogens now and in the future.

Future work will involve optimization of the temperature of the heated air conditioning filter to find the best possible approach for inactivating closer to 100% of the virus. Additional information can be found in a recent article⁴ or by contacting Ren at zren@uh.edu.

REFERENCES
1. Click here.
2. Click here.
3. Canter, N. (2015), “High-power-factor thermoelectric material,” TLT, 71 (7), pp. 12-13.
4. Luo, Y., Peel, G., Cheema, F., Lawrence, W., Bukreyeva, N., Jinks, C., Peel, J., Peterson, J., Paessler S., Hourani, M. and Ren, Z. (2020), “Catching and killing of airborne SARS-CoV-2 to control spread of COVID-19 by a heated air disinfection system,” Materials Today Physics, 15, Article 100249.