Microbial contamination has proven to reduce the performance of lubricants and also to be a potential health and safety concern to workers. Water-based lubricants such as metalworking fluids are particularly vulnerable because microbes can proliferate due to the presence of water and organic components that are food sources.

Bacteria are microbes that tend to grow on surfaces in metalworking fluid systems. As part of this growth process, bacteria form multicellular structures known as biofilms that can be difficult to remove. In a previous TLT article,¹ researchers found that a bacterium known as Caulobacter crescentus attaches to a surface through a two-stage process. Initially, the bacterium reversibly adheres to a surface but can detach at any time. In a second step, the bacterium secretes a polysaccharide-based adhesive that enables the microbe to more permanently remain on the surface. The adhesive can be synthesized and applied in a matter of seconds.

Some metals have proven to be effective in retarding the growth of bacteria. Rahim Rahimi, assistant professor of materials engineering at Purdue University in West Lafayette, Ind., says, “Copper and silver have demonstrated the most effectiveness of any metals in retarding the growth of bacteria. They function by disrupting the bacterial cell wall causing its rupture in a process known as ‘contact killing.’”

In particular, copper is an attractive metal to use in applications where antimicrobial surfaces are required due to its lower cost compared to silver. Newer types of copper nanoparticles have been developed in an effort to improve antimicrobial performance. The objective is to increase the contact area of the metal with respect to individual bacterium by increasing the ratio of surface area to volume. Rahimi says, “Two problems have been encountered with preparing copper nanoparticles. Their synthesis is complex, and they tend to leach into the environment, which can cause health and environmental issues.”

Preparation of immobilized nanostructures on a copper surface might prove to increase the antimicrobial properties of the metal. Rahimi and his colleagues have now developed a new approach by modifying the surface through use of laser machining.

Laser texturing
The researchers developed a technique known as laser texturing that involved photothermal (pyrolytic) processing of 50-micron sheets of copper foil. The laser operated at a wavelength of 1.06 microns with pulse duration in a nanosecond regime and a repetition frequency of 30 kilohertz.

Rahimi says, “Laser texturing is a simple process where the laser beam imparts nanoroughness on the copper surface enhancing the surface area by an order of magnitude. The process changes the surface appearance from the reddish-brown color characteristic of pure copper to dark brown.”

The laser texturing technique generates a hierarchical micro-nano-dual scale structure. Nanoroughness was established on the copper surface in average spot-size diameters of 100 microns, which meant that multiple sweeps of the laser beam were required to produce laser texturing on a large scale. Figure 3 shows a piece of copper placed on top of a laser-textured copper surface.


Figure 3. A comparison of a reddish-brown colored laser textured surface that improves the antimicrobial properties of copper and a piece of the pure metal is shown. Figure courtesy of Purdue University.

Field emission scanning electron microscopy shows that under low magnification, microscale protuberances and wrinkles are detected with an average width of 100 micrometers. Upon use of higher magnification, uniform mesoporous structures were observed on the copper surface that exhibit an average size of 17.7 +/- 10.8 nanometers.

Elemental composition analysis of the surface of the laser textured copper conducted by energy-dispersive X-ray spectroscopy determined there was a 93.6% increase in the surface oxygen content after the laser treatment. Rahimi says, “The high-temperature laser surface treatment under atmospheric conditions caused significant oxidation of the copper surface. The formation of a mixture of copper (I) and copper (II) oxides was confirmed. This phenomenon increased the hydrophilicity of the copper surface.”

The degree of hydrophilicity was obtained by measurement of the water contact angle. Copper exhibited an average of 101 degrees while laser textured copper displayed a water contact angle of 0 degrees. Rahimi says, “The laser treatment not only increased the hydrophilicity of the copper surface but enabled it to achieve super hydrophilicity, which greatly increases its antimicrobial activity. We believe another reason for this high degree of wettability was the added roughness that was produced on the copper surface during the laser texturing process.”

Rahimi indicates that the superhydrophilicity and the high degree of copper oxide functionality attract bacteria to the copper surface, and then effectively inactivate them. To demonstrate the antimicrobial properties of laser textured copper, this material and copper were treated with bacterial suspensions of 7.1 log₁₀ colony forming units per milliliter of E. coli. After two hours of exposure at ambient temperature, no bacterial colonies were detected on the laser textured copper surface while the concentration on copper was 1.5 log₁₀ colony forming units per milliliter.

Rahimi says, “In the future, we will be changing the experimental conditions used in the laser treatment to determine if that will improve the efficacy of the surface. Other metals such as brass and steel also will be evaluated.”

Due to the current COVID-19 crisis, Rahimi indicated that an analysis of how this technique will work on viruses will be conducted. He says, “The laser technique shows a great deal of potential because it is a non-contact approach that does not need to utilize consumables such as biocides.”

Additional information can be found in a recent article² or by contacting Rahimi at rrahimi@purdue.edu.

REFERENCES
1. Canter, N. (2012), “Bacterial adhesion on surfaces,” TLT, 68 (3), pp. 12-13.
2. Selvamani, V., Zareei, A., Elkashif, A., Maruthamuthu, M., Chittiboyina, S., Delisi, D., Li, Z., Cai, L., Pol, V., Seleem, M. and Rahimi, R. (2020), “Hierarchical Micro/Mesoporous Copper Structure with Enhanced Antimicrobial Property via Laser Surface Texturing,” Advanced Materials Interfaces, 7 (7), 1901890.