New approach to dealing with biofilms and biofouling

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2013

A surface coating developed by Duke researchers detaches biofilms produced by bacteria and reduces biofouling.

KEY CONCEPTS
• A new trilayer, laminate surface coating has the potential to detach biofilms produced by bacteria and barnacles from the hulls of marine vessels upon application of an external stimulus such as an electric field.
• In testing, 95% of a biofilm is detached after a voltage was applied over 200 on-off cycles.
• Air pressure applied through the presence of air channels was also used as an external stimulus to enable the coating to detach biofilms.

PAST ARTICLES IN THIS COLUMN have discussed ongoing research to better understand how bacteria adhere to surfaces and form biofilms. This issue is important to users of metalworking fluids because bacterial contamination has been found to decrease operating life and can lead to health and safety concerns for workers monitoring these fluids.

In a previous TLT article, research detailing how a specific bacterium (Caulobacter crescentus) adheres to a system through a reversible, irreversible two-stage process was discussed (1). In the first step, the bacterium will establish a low adherence to a specific surface to determine if it is the right location. Then a stronger polysaccharide- based adhesive is produced and secreted in the second step to enable the bacterium to remain on the surface in an irreversible manner for a long period of time. This natural adhesive has the performance attributes of super glue.

A second problem involving bacteria is their presence on the hull of maritime vessels. Biofouling leads to an increase in the weight of the vessel, a decrease in speed and, as a consequence, an increase in fuel consumption and overall operating cost.

Xuanhe Zhao, assistant professor in the department of mechanical engineering and materials science at Duke University in Durham, N.C., says, “Two approaches are currently used to minimize biofouling. The surface chemistry of the vessel can be modified through the use of, for example, toxic materials that will kill the bacteria. But the problem is that toxic materials used in the coating have a negative impact on the environment.”

Zhao continues, “A second approach is to physically modify the modulus, topography or roughness of the vessel coatings. Some benefits are found to limit the growth of specific microbes but over the long-term, this approach is not very effective.”

Other options to deal with biofilms and biofouling are offered by nature because animals are constantly exposed to this problem. Gabriel P. Lopez, professor of biomedical engineering at Duke, says, “Nature has developed a clever way to minimize biofouling by the development of biological surfaces that clean themselves through dynamic deformation and motion. One obvious example is the presence of cilia in the respiratory tract of humans. Cilia expel foreign particles that can form protective mucous layers on surfaces in the lung.”

Cilia also are used by marine organisms such as mollusks and corals. A third example is the periodic shedding of skin by animals such as lizards and snakes.

If a coating can be prepared that is similar in performance characteristics to what is done in nature, then a technique may become available that can minimize the presence of biofilms and biofouling in such applications as MWF systems and the hulls of maritime vessels. Such a technology has now been developed.

TRILAYER LAMINATE
Zhao and Lopez, along with fellow Duke researchers, have developed a surface coating that constantly deforms as a means to detach biofilms and eliminate biofouling. Zhao says, “We determined that two external stimuli can be used to activate the deformation.” The stimuli used are application of an electric field and the use of pneumatic networks fabricated into the coating.

A trilayer laminate coating was developed that involves the bonding of a silicone elastomer, rigid insulating substrate and a metal foil. Zhao says, “The silicone elastomer is currently used in this type of marine coating application. A rigid insulating substance was used to protect the silicone elastomer from the external stimuli used.”

The researchers applied the trilayer laminate coating onto a surface exposed to artificial seawater suspensions of the bacterium, Cobetia marina. The concentration of the bacterium in the suspension was 7x10⁷ cells per milliliter of fluid. Lopez says, “We chose this bacterium because it has been isolated from marine biofilms and can be cultured to form nice biofilms. Cobetia marina is quite tenacious and forms very sticky biofilms.”

Prior to applying an external stimulus, the bacterium was allowed to form biofilms on the surface coating for a period of four days. Once the electric field is applied and reaches a critical value, the surface coating deforms into a pattern of craters, leading to the removal of the biofilm. Figure 2 illustrates this process.


Figure 2. Deformation of a surface coating prepared with a trilayer laminate by an external stimulus leads to the formation of craters that facilitates the detachment of biofilms and, as a consequence, reduces biofouling. (Courtesy of Duke University)

After applying a voltage over 200 on-off cycles covering a time frame of 10 minutes, over 95% of the biofilm is detached from the surface. Additional experiments were done with the induced electric field on undeformed surfaces to show that this stimulus itself is not responsible for detaching the biofilm.

Preparation of air channels also was conducted to enable air pressure to be used to deform the surface. In this experiment, biofilms of Cobetia marina were grown on the surface in combination with adult barnacles seen in maritime applications. An air pressure of 3 kPa created 23% surface strain and led to the successful detachment of the biofilm. To achieve the same result with the barnacles, a higher pressure of approximately 15 kPa was needed.

The researchers are looking to license this technology. Further details can be obtained by contacting Rob Hallford, Jr., of Duke at rob.hallford@duke.edu. Additional information on this research can be found in a recent article (2) or by contacting Zhao at xuanhe.zhao@duke.edu.

REFERENCES
1. Canter, N. (2012), “Bacterial Adhesion on Surfaces,” TLT, 68 (3), pp. 12-13.
2. Shivapooja, P., Wang. Q., Orihuela, B., Rittschof, D., Lopez, G. and Zhao, X. (2013), “Bioinspired Surfaces with Dynamic Topography for Active Control of Biofouling,” Advanced Materials, DOI: 10.1002/adma.201203374.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.