Chalmers University of Technology
Gothenberg, Sweden

Operations for surgical implants, such as hip and knee replacements or dental implants, have increased in recent years. However, in such procedures, there is always a risk of bacterial infection. In the worst-case scenario, this can cause the implant to not attach to the skeleton, meaning it must be removed.

The first use of vertical graphene to kill bacte- ria. (Credit: Johan Bodell)

Bacteria travel around in fluids, such as blood, looking for a surface to cling onto. Once in place, they start to grow and propagate, forming a protective layer, known as a biofilm. Researchers at Chalmers have now shown that a layer of vertical graphene flakes forms a protective surface that makes it impossible for bacteria to attach. Instead, bacteria are sliced apart by the sharp graphene flakes and killed. Coating implants with a layer of graphene flakes can therefore help protect the patient against infection, eliminate the need for antibiotic treatment, and reduce the risk of implant rejection. The osseointegration — the process by which the bone structure grows to attach the implant — is not disturbed. In fact, the graphene has been shown to benefit the bone cells.

The biological applications of graphene did not begin to materialize until a few years ago. The researchers saw conflicting results in earlier studies. Some showed that graphene damaged the bacteria, others that they were not affected.

Graphene research demands high class clean- rooms (Credit: Johan Bodell)

“We discovered that the key parameter is to orient the graphene vertically. If it is horizontal, the bacteria are not harmed,” says Ivan Mijakovic, a professor in the department of biology and biological engineering. The sharp flakes do not damage human cells. The reason is simple: one bacterium is 1 μm — one thousandth of a millimeter — in diameter, while a human cell is 25 μm. So, what constitutes a deadly knife attack for a bacterium, is therefore only a tiny scratch for a human cell. Good bacteria are also killed by the graphene. But that’s not a problem, as the effect is localized and the balance of microflora in the body remains undisturbed.

“We want to prevent bacteria from creating an infection. Otherwise, you may need antibiotics, which could disrupt the balance of normal bacteria and also enhance the risk of antimicrobial resistance by pathogens,” says Santosh Pandit, a postdoc in biology and biological engineering.

Vertical flakes of graphene are not a new invention, having existed for a few years. But the Chalmers research teams are the first to use the vertical graphene in this way. The next step for the research team will be to test the graphene flakes further, by coating implant surfaces and studying the effect on animal cells.

The graphene spikes are only one atomic layer thick so that they slice and kill bacteria but only scratch human cells.

Chalmers cooperated with Wellspect Healthcare, a company that makes catheters and other medical instruments, in this research. The projects are a part of the national strategic innovation program SIO Grafen, supported by the Swedish government agencies Vinnova (Sweden’s innovation agency), the Swedish Energy Agency, and the Swedish Research Council Formas. The research results are published in Advanced Materials Interfaces in the paper, “Vertically Aligned Graphene Coating is Bactericidal and Prevents the Formation of Bacterial Biofilms.”

The Making of Vertical Graphene

Graphene is made of carbon atoms. It is only a single atomic layer thick, and therefore the world's thinnest material. Graphene is made in flakes or films. It is 200 times stronger than steel and has very good conductivity thanks to its rapid electron mobility. Graphene is also extremely sensitive to molecules, which allows it to be used in sensors.

Graphene can be made by chemical vapor deposition. The method is used to create a thin surface coating on a sample. The sample is placed in a vacuum chamber and heated to a high temperature at the same time as three gases — usually hydrogen, methane, and argon — are released into the chamber. The high heat causes gas molecules to react with each other, and a thin layer of carbon atoms is created. To produce vertical graphene forms, a process known as plasma-enhanced chemical vapor deposition is used. Then, an electric field — a plasma — is applied over the sample, which causes the gas to be ionized near the surface. With the plasma, the layer of carbon grows vertically from the surface.

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