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"Intelligent" implant coatings are designed to combat infections

11/22/2019 8:18 PM

"Intelligent" implant coatings are designed to combat infections

A material that has a toxic effect, especially when there are bacteria around it? Physicists at the University of Augsburg have developed such an "intelligent" coating together with colleagues from Hamburg and Munich. In the future, it could help prevent complications in the healing of endoprostheses. The coating offers other advantages: It is extremely poorly worn, yet so rough that bone cells adhere well. The study was published in the journal Scientific Reports by Nature Publishing Group.

Implantation of artificial hip or knee joints is now a part of daily surgical practice. One of the problems doctors face is infection near the prosthesis. They delay healing and can permanently impair the stability of the link between the endoprosthesis and the bone. In addition, they increase the surgical risk for the patient. "To make matters worse, more and more bacteria are becoming resistant to common antibiotics," explains Dr. Christoph Westerhausen of the Department of Experimental Physics I at the University of Augsburg. "It forces us to look for alternatives."

One such could be the new lining that the research team from Augsburg, Hamburg and Munich has now developed and tested. It is a thin coat of diamond carbon waffle, traded under the abbreviation "DLC" by experts. DLC coatings are extremely solid; Therefore, they have been used for many years when wear and tear must be kept to a minimum. But the clou of the new coating lies elsewhere: "We were specifically contacting it with zinc oxide particles," Westerhausen points out. "Zinc ions are toxic to microorganisms; The dissolution of zinc oxide is also highly dependent on the pH of the solution."

Unfortunately, heavy metal has one major flaw: it can also damage or even kill body cells – of course, an effect that is undesirable in healing the implant. However, as long as zinc oxide is incorporated into the DLC layer, there is no danger. Only when the nanoparticles dissolve in the tissue fluid and zinc thus becomes a free-moving ion does it develop its toxic potential. This happens especially quickly in acidic environments (the effect, by the way, is used in the production of pre-expanded zinc-carbon batteries). "And it was this phenomenon that moved us into the design of our DLC layers," Westerhausen says.
When the wound becomes acidic

Infections often go hand in hand with a decrease in pH. Scientist's idea: It may mean that the carbon coating releases its zinc content, especially in the presence of bacteria – in other words, when it is really necessary. "That's why we first created tiny zinc oxide particles," explains the biophysicist. "Each was less than twenty the size of the bacterium." The researchers then mixed this "heavy metal dust" into a liquid polymer solution and used it to wet their test implants. The thin polymer film was converted to DLC using a pickling process.

The coating thus obtained was tested for their behavior at different pH values. Tissue fluid is normally neutral to minimally alkaline; with inflammation, however, it becomes a little acidic. In fact, under these conditions, zinc oxide dissolves significantly faster: the coating released about thirty percent more zinc ions in the initial phase of release, reducing the pH by one step (which roughly corresponds to the typical value of infection). In an even more acidic environment, the increase is as high as 130 percent. In microbial tests, researchers were also able to show that the coating can effectively inhibit bacteria in their growth – especially at acidic pH. The microbes tested also included those that were resistant to various antibiotics and therefore caused more problems after surgery.

Although tissue cells are damaged by zinc ions. "But they also have an effect, especially at acidic pH and then at infection," emphasizes Dr. Med. Westerhausen. "In such a situation, however, the advantage – that of killing the bacteria – far outweighs the lack of tissue damage." In further tests, scientists now want to change the various parameters of the new coating, such as the amount of zinc ions added. They hope to be able to further optimize the effect. Westerhausen: "We are already seeing a great potential in our DLC coatings to significantly reduce complications in the healing of endoprostheses."

Scientific contact:

Dr. Christoph Westerhausen, Experimental Physics I at the University of Augsburg
Telephone: 0821 598-331,

Original issues:

Sascha Buchegger, Andrej Kamenac, Sven Fuchs, Rudolf Herrmann, Pia Houdek, Christian Gorzelanny, Andreas Obermeier, Stephan Heller, Rainer Burgkart, Bernd Stritzker, Achim Wixforth, and Christoph Westerhausen: Clever antimicrobial efficacy using a pH-sensitive ZnO coating; Scientific reports;

Further information: For publication

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