Scientists develop rapid bacteria killer

Scientists in Australia have developed a copper material that kills deadly bacteria two minutes after contact and could prove effective in destroying SARS-COV-2, the virus that causes COVID-19.

A team from RMIT University in Melbourne, Australia, working in collaboration with CSIRO, the science research agency of the Australia government, reported in the January 2022 edition of the scientific journal, "Biomaterials," that the copper innovation kills bacteria more than 100 times faster and more effectively than standard copper, and could help combat the growing threat of antibiotic-resistant superbugs.

Imminent threat

Pathogenic bacterial infection poses an increasingly severe threat to human health, compounded by the inevitable dramatic rise in antimicrobial resistance.

Currently, more than 700,000 deaths per year worldwide are attributed to antimicrobial-resistant infections, with fatalities expected to swell to 10 million per year by 2050 if the threat is not curtailed. This is particularly serious for bacterial infections, which account for about 90% of all healthcare infections.

"It is imperative that an effective and affordable answer to antibiotic-resistant infection is available," the researchers wrote. "The World Health Organization has heightened the threat, saying, 'New antibiotics alone will not be sufficient to mitigate the threat of antimicrobial resistance. Their development should go hand in hand with infection prevention and control activities ...'"

Clues in nature

During the past decade, researchers observed that a variety of insect and reptile species, such as cicada, dragonfly, and gecko, possess nanostructured wings or skins that can kill many types of bacteria.

These natural designs feature evenly spaced tiny pillars, hemispheres, spikes, etc. They are about 10–100 times smaller than most bacteria and can act to "stretch" the cell envelope of adhered bacteria. This ultimately leads to the tearing and subsequent leakage of intracellular materials essential for the survival of the bacteria.

Inspired by this discovery, researchers worldwide have explored the fabrication of different types of biocompatible nanostructured surfaces that can kill bacteria, including black silicon, polymethylmethacrylate (PMMA), titanium, and gold.

However, there is a high survival rate on each of these biomimetic surfaces even after 24 hours of exposure. Moreover, research has shown that certain types of bacteria exhibit higher resistance to the mechanical strain on them caused by the nanostructured surfaces.

The studies have shown that biomimetic surface design alone is not enough to efficiently kill harmful bacteria. They also remind researchers that emphasis may need to be placed on exploiting material surface chemistry to achieve an adequate combined antimicrobial effect, especially through nano-surface chemistry designed and built from the substrate material of choice.

Seeking copper solutions

Copper has long been used to fight different strains of bacteria, including the commonly found staphylococcus aureus (golden staph), because the ions released from the metal's surface are toxic to bacterial cells.

Copper is used in self-sanitizing surfaces to eliminate various pathogenic microbes in applications ranging from water purification to sterilization of wounds. While the exact mechanisms are not yet fully understood, it is believed that the cytotoxicity of copper arises from both the direct oxidative damage of copper ions as well as the generation of destructive reactive oxygen.

Clinical trials have confirmed the superior antimicrobial performance of copper surfaces to stainless steel and plastic surfaces, including against antibiotic-resistant strains.

Furthermore, evidence of copper being capable of killing pathogenic viruses such as coronavirus 2 (SARS-CoV-2 or HCoV-19), coronavirus 229E, and norovirus adds further striking attributes.

But the process using standard copper is relatively slow, and despite extensive research in recent years, scientists have made little progress in finding a way to speed it up. Thus, copper, historically, has been excluded as a rapid bactericidal material.

Since bacteria can survive on common copper surfaces for hours, significant efforts have focused on engineering copper surfaces to enhance the release of copper ions or the preferred copper surface nano-topography.

As a result, various copper alloy nanoparticles have been investigated as possible alternatives, the researchers wrote. Some materials exhibit superior bactericidal potency to bulk copper surfaces but are still insufficient as an immediate antimicrobial approach (e.g., a reduction of 98% of golden staph bacteria after two hours). Further, applications of these materials are restricted to additives, and thus, not practical for deployment in most physical situations.

In another attempt, copper-coated nanostructured bulk silicon wafers showed only marginally improved bacteria-killing potency versus standard copper.

Apart from the costly and less green fabrication process, the thin copper coating created can be easily worn away, inhibiting its long-term performance.

Shortly after the RMIT University team published its findings, a team of scientists from the University of Waterloo, Ontario, published a report on the increased virucidal properties of thin-film copper or copper compound coatings. More information on the Waterloo research group's findings can be read at Nano copper takes the fight to coronavirus in the current edition of Metal Tech News.

More copper in fight

To date, no bulk, highly potent antimicrobial copper surfaces with controllable nanopatterns have been developed, perhaps due to the technical challenges in engineering them.

"A standard copper surface will kill about 97% of golden staph within four hours," said Ma Qian, Ph.D., a distinguished professor at RMIT University, in announcing the team's breakthrough.

According to the Australian study's lead author, Jackson Leigh Smith, Ph.D., the copper material's unique porous structure is key to its effectiveness as a rapid bacteria killer.

The researchers used a special copper mold-casting process to make the new material, arranging copper and manganese atoms into specific formations. The manganese atoms were then removed from the alloy using a cheap and scalable chemical "de-alloying" process, leaving pure copper full of tiny microscale and nanoscale cavities in its surface.

"Our copper is composed of comb-like microscale cavities and within each tooth of that comb structure are much smaller nanoscale cavities; it has a massive active surface area," Smith explained.

"The pattern also makes the surface super-hydrophilic, or water-loving, so that water lies on it as a flat film rather than as droplets. The hydrophilic effect means bacterial cells struggle to hold their form as they are stretched by the surface nanostructure, while the porous pattern allows copper ions to release faster."

Smith conceived the ideas for the precursor alloy design and antimicrobial application and performed all experimental and analytical procedures in the study.

"Incredibly, when we placed golden staph bacteria on our specially designed copper surface, it destroyed more than 99.99% of the cells in just two minutes. So not only is it more effective, it's 120 times faster," said Qian, who, with two other researchers, T. Song and D. Liang, conceived the idea of fabricating the bulk nanoporous copper structures by chemical de-alloying.

"Our copper structure has shown itself to be remarkably potent for such a common material," he observed.

The scientists envision a wide range of applications for the new material, including antimicrobial door handles and other high-touch surfaces in schools, hospitals, homes, and public transport, as well as filters in antimicrobial respirators, air ventilation systems, and face masks.

They are now looking to investigate the new material's enhanced copper's effectiveness against SARS-CoV-2, the virus that causes COVID-19, including assessing 3D printed samples.

Other studies suggest copper may be highly effective against the virus, leading the U.S. Environmental Protection Agency to officially approve copper surfaces for antiviral uses in 2021.