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Posted: Jun 10, 2008
Novel antimicrobial coating combines carbon nanotubes and natural materials
(Nanowerk Spotlight) The demand for antimicrobial coatings is growing at a fast pace. In the U.S. alone, the market for these products is expected to grow from $175 million in 2005 to over $550 million in 2012. This market is not only driven by the urgent need of hospitals to reduce infections, although it would appear that they need it most: the U.S. Centers for Disease Control and Prevention (CDC) estimates that the infections acquired in hospitals alone affect approximately 2 million persons annually. In the U.S., between 44,000 and 98,000 people die every year from infections they picked up in hospitals. With a growing concern about the role of contaminated surfaces in the spread of infections such as SARS and MRSA, antimicrobial surfaces have become popular in such areas as consumer products, public spaces such as schools and offices, and public transportation.
While many conventional antibacterial coatings release their antimicrobials over time (either controlled or uncontrolled) and thereby lose their antimicrobial efficiency, researchers have now developed a unique multifunctional biomimetic material comprised of carbon nanotubes, DNA, and lysozyme that has robust mechanical properties and exhibits excellent long-term antimicrobial activity.
"We have developed a technique to produce coatings that combine the antimicrobial properties of lysozyme with the mechanical strength of single-walled carbon nanotubes" Dr. Virginia A. Davis tells Nanowerk. "Unlike many other antimicrobial materials, lysozyme is a natural product that can be found in egg whites and human tears. In fact, it is even used in some commercial products such as mouthwash. However, lysozyme itself is not mechanically robust. Single-walled carbon nanotubes, on the other hand, are the strongest known material, and they also have a low density. By producing a film where the nanotubes are encased in the lysozyme, we have achieved the benefits of both materials."
Davis is an Assistant Professor in the Department of Chemical Engineering at Auburn University in Alabama, and her team developed the coating by using a layer-by-layer (LBL) deposition process. This enabled them to control the coating thickness and nanotube orientation very precisely.
This work was motivated by the strong mutual interests and expertise of a truly cross-disciplinary team. Davis, a former student of Drs. Richard E. Smalley and Matteo Pasquali, has considerable expertise in single-walled carbon nanotubes (SWCNTs), their dispersion and
shear alignment. Her post-doctoral researcher, Dr. Dhriti Nepal, has a strong background in SWCNT-biopolymer dispersion. Dr. Aleksandr Simonian, Professor of the Materials Engineering Program in the Mechanical Engineering Department at Auburn is a well-recognized expert in smart bio-functionalized materials and biosensing, and his graduate student Shankar Balasubramanian has large expertise in biosensors and antimicrobial materials.
This is the first time that the unique antimicrobial properties of lysozyme have been combined with the strength of SWCNTs. Davis' team has demonstrated antimicrobial activity of their coating both through standard assays with Micrococcus lysodeikticus and by the inability for intact Staphylococcus aureus cells to grow on the surfaces.
Davis explains that when silicon substrates with and without the LBL assembled coating were incubated with Staphylococcus aureus for 24 hours at 37 °C (body temperature) and imaged under SEM, significantly more bacteria adhered to the uncoated surface than to the coated surface. In addition, the few bacteria that adhered to the coated substrate underwent severe morphological changes. In contrast, on the uncoated silicon surface the cells remained intact and maintained their cocci structure.
SEM image of samples incubated with Staphylococcus aureus at 37 °C for 24 hours of (c) a clean silicon wafer (control) and (d) LBL assembly at 11th layer (top surface LSZ-SWCNT) arrows indicating damaged cells). The scale bars in (c) and (d) represent 1 µm. (Reprinted with permission from American Chemical Society)
"We also showed that the coating’s antimicrobial activity is solely related to the lysozyme presence and not to SWCNTs" says Davis.
Specific applications for these coatings are all the areas where there could be concerns over the spread of infectuous disease: hospitals, airplanes, trains, subways, schools and offices.
Many current disinfection practices require rigorous cleaning with solvent that must remain wet for a given period of time to insure that any germs on the surface are killed. This surface, in contrast, is inherently antimicrobial – how long it has been wet is not an issue.
Davis points out that this work is preliminary. "The results are very promising, but we need to undertake long term studies of the film's robustness – especially upon exposure to cleaning agents which may be applied. For example, walls, computer keyboards, or airline tray tables may require cleaning for aesthetic reasons such as ink marks. We need to make sure that the coatings hold up to this cleaning as well as we expect."
The team is also looking at processes where the coatings can be applied over a very large area using a process other than the classical
dip-coating used in LBL assembly.
While this novel nanocomposite material has significant promise as antimicrobial coating, it also opens the door to other exciting application areas. "The combination of SWCNTs with DNA, proteins and enzymes enables a range of sensing and smart functionality capabilities" says Davis.