Researchers develop smart antimicrobial implant coating
AUGSBURG, Germany: Physicists at the University of Augsburg, in cooperation with scientists from the University Medical Center Hamburg-Eppendorf and the Technical University of Munich, have developed a new coating that releases antimicrobial ions. In the future, it could help prevent complications during the healing of endoprostheses.
Today, the implantation of artificial hip or knee joints is part of everyday surgery. One problem that doctors have to contend with is infections near the prosthesis. These delay healing and can permanently impair the stability of the connection between the endoprosthesis and the bone. In addition, they increase the surgical risk for the patient. “The situation is further complicated by the fact that more and more bacteria are becoming resistant to common antibiotics,” explained Dr Christoph Westerhausen, group leader of the Department of Experimental Physics I at the University of Augsburg. “This forces us to search for alternatives.”
One such alternative could be the novel coating that the research team has developed and tested. It consists of an extremely thin coating made of diamond-like carbon (DLC). These coatings are extremely resistant and have been used for many years to minimise wear and abrasion. However, the special feature of the new coating lies elsewhere: “We dope it specifically with zinc oxide particles,” explained Westerhausen. “Zinc ions are toxic to microorganisms; the dissolution of zinc oxide is also strongly dependent on the pH value of the solution.”
However, the heavy metal has a significant disadvantage. It can damage or even kill body cells. As long as the zinc oxide is embedded in the DLC layer, it does not pose any danger. Only when the nanoparticles dissolve in the tissue fluid and the zinc becomes a freely mobile ion does it unfold its toxic potential. This happens particularly quickly in acidic environments. “And it was precisely this phenomenon that led us to the design of our DLC coatings,” said Westerhausen.
Infections are often accompanied by a decrease in pH value, which led the scientists to believe that the carbon coating releases zinc mainly in the presence of bacteria. “That was why we initially produced tiny zinc oxide particles,” explained Westerhausen. “Each of them was not even one twentieth as thick as a bacterium.” The researchers then stirred this heavy metal dust into a liquid polymer solution and wetted their test implants with it. Then, they converted the thin polymer film into DLC.
They tested the resulting coating for its behavior at different pH values. Normally, the tissue fluid is neutral to minimally alkaline; in case of inflammation, however, it becomes slightly acidic. In fact, the zinc oxide dissolved much faster under these conditions. When the pH value was lowered by one level, which is roughly equivalent to the value during an infection, the coating released around 30% more zinc ions in the initial phase of the release. In an even more acidic environment, the increase was 130%. In microbial tests, the researchers were also able to show that the coating can effectively inhibit the growth of bacteria—especially at an acidic pH value. Among the microbes tested were those that are resistant to various antibiotics and that are more likely to cause problems after surgery.
Tissue cells were also damaged by the zinc ions. “However, the effect also occurs with an acid pH, that is with an infection,” stressed Westerhausen. “In such a situation, however, the advantage—namely killing bacteria—far outweighs the disadvantage of tissue damage.” Now, the scientists want to vary different parameters of the new coating, such as the amount of zinc ions added in further tests. They hope to further optimise the effect. “We already see great potential in our DLC coatings to significantly reduce complications during the healing of endoprostheses,” said Westerhausen.
The study, titled ”Smart antimicrobial efficacy employing pH-sensitive ZnO-doped DLC coatings”, was published on 21 November 2019 in Scientific Reports.