Swiss Researchers Discover Why Implant Coatings Detach

Stress corrosion cracking leads to a slow-growing crack in the metal carbide reaction layer, which is barely 5 nm wide. This, in turn, causes the DLC layer to detach from theCoCRMo implant material, as seen in this electron microscope image. Click to enlarge.

Researchers at the Empa material sciences and technology institute in Dübendorf, Switzerland, have discovered why coatings made of diamond-like carbon (DLC) tend to detach from implants. In the process, they have also developed methods to make the interface between the DLC layer and the metal underneath corrosion-resistant and found a way to predict the lifetime of the implants.

DLC coatings perform remarkably well on a range of products, preventing wear and extending operating life in everything from computer hard drives to razor blades. When medical device manufacturers first performed in vitro tests of DLC-coated implants, the products performed exceptionally well. Just a few years after implantation in human patients, however, DLC coatings would detach from the implant material for no apparent reason.
In a project financed by the Swiss Innovation Promotion Agency (CTI) with medical technology company Synthes and coating company Ionbond AG, Empa researchers discovered the weak link: the reaction layer at the interface of the coating and substrate.

Stress corrosion cracking occurred in the reaction layer, and the mechanical load in conjunction with the penetration of body fluids produced slow-growing cracks, which in turn caused the DLC substrate to detach little by little.

Having diagnosed the cause of the problem, Empa and industry partners Synthes and Ionbond focused on a remedy. The researchers developed a corrosion-resistant intermediate layer at the interface to the DLC layer. The researchers also developed a process that can determine a crack’s growth rate under conditions similar to those experienced in the human body as well as the dissolution rate of the reaction layer in cases of crevice corrosion. “This allows us to calculate the expected operating lifetime of the coated implant in the human body,” says Roland Hauert of Empa’s Nanoscale Materials Science laboratory. Whether or not a DLC-coated implant will fail prematurely in vivo now can be determined during product development.

Tags: diamond-like carbon, DLC, implants Empa