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Harder-wearing surgical implants

   
 
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Artificial hip joints and other surgical implants make severe demands on the materials from which they are made. Once inside the human body even stainless steel and titanium alloys can corrode, become scratched and sometimes break. This wear and tear means that artificial joints have to be replaced every ten years or less.
Using advanced surface treatment techniques such as glow-discharge ion implantation, the researchers have managed to improve the strength, hardness and corrosion resistance of metal implants.

In spite of the success of surgical implants such as artificial hip joints, the materials used to make them are not always quite up to the job. Even stainless steel and titanium alloys can break under the enormous stresses on load-bearing joints, and in the salty environment of the body they can also corrode. Deposits of inorganic salts can scratch bearing surfaces, making joints stiff and awkward. As a result, the lifetime of an implant is at most ten years, and often less.
Metallurgists and engineers often treat the surfaces of metal parts to make them stronger, harder and more resistant to corrosion. This project has adapted innovative surface treatment techniques to the alloys commonly used in surgery, and has shown that the resulting implants have better properties than those treated using conventional techniques. Despite the medical benefits, however, the response from implant manufacturers and the medical profession has been disappointing perhaps for purely cosmetic reasons.

Getting the best of both worlds

Metal parts subject to wear need to be hard, yet hard metals can be brittle and difficult to work with. One way to overcome this is to make the part from an alloy that is relatively soft and tough, and then to modify its surface using heat or chemicals. This can add hardness, corrosion resistance and other desirable properties without spoiling the toughness of the original metal. Carbon and nitrogen are the two most popular elements added during surface treatment.
Most metal surgical implants are made from one of three groups of alloys: titanium-aluminium-vanadium (TiAlV), titanium-aluminium-iron (TiAlFe) or stainless steel (316L). Titanium alloy implants are currently treated using a process known as chemical vapour deposition to add nitrogen to the metal surface. This gives a handsome gold-coloured finish but the resulting layer of titanium nitride is thick (1-2 micrometres) and brittle. As the temperature changes the nitride layer expands at a different rate from the underlying metal, and this can cause it to crack or peel off.
Once the metal has lost its protective coating, ions such as chloride in the body can corrode the surface and start cracks which can cause the insert to break under repeated loading. There is also the problem of biocompatibility: the body tolerates pure titanium, but damage to the metal surface of stainless steel or cobalt-based alloys currently used can release impurities such as nickel and cobalt, both of which can provoke an immune response. Corrosion also releases dissolved metal into the bloodstream, which is undesirable.
Researchers at the Université Louis Pasteur in France decided that newer forms of surface treatment could increase both the corrosion resistance and the biocompatibility of metal implants. In particular, they decided to try surface treatment techniques that add carbon and nitrogen in their elemental forms (as atoms dissolved in the metal matrix) instead of chemically combined with the metal (as carbides and nitrides). By keeping the treated layer thin and avoiding a sharp junction with the underlying metal, the researchers hoped to come up with a durable surface that would not crack or peel.

Using new techniques

The Université Louis Pasteur researchers started BE-4375 in October 1991 with the help of a large consortium of other specialists. The Institut National Polytechnique de Lorraine in France provided expertise in plasma deposition for treating stainless steel, whilst Eurorad, a commercial arm of the French government-funded PHASE laboratory, contributed its knowledge of ion implantation.
Also involved were two French universities, Ecole Nationale Superieure des Arts et Industries de Strasbourg and the University of Nancy; the University of Porto in Portugal, which ran the corrosion tests; the Technical University of Braunschweig, Germany, which specialises in chemical vapour deposition and has worked with car manufacturer Volkswagen on nitriding gearbox parts; and two German firms: titanium supplier Deutsche Titan and medical device manufacturer Peter Brehm.
The researchers used three groups of techniques. The first of these, ion implantation by glow discharge, is a medium-energy (40-60 keV) process that can penetrate deep into the material. Because it does not require the beam of ions to be separated into different atomic masses, it costs only 20 to 50 per cent as much as conventional ion implantation, yet gives many of the same benefits. Both titanium alloys and stainless steel were treated with nitrogen using this method.
Processes in the second group, also used to add nitrogen, are based on the use of plasma. Plasma diffusion (plasma nitriding, PDT) is a low-energy (0.5-2 keV) process whose effects are intermediate between chemical vapour deposition (CVD) - the conventional nitriding process - and ion implantation. It was used to treat both titanium alloys and, in a low-temperature variant, for stainless steel. Plasma-assisted chemical vapour deposition (PACVD) is a development of CVD in which plasma is used to give results that approach those of ion implantation. For titanium alloys the researchers developed a two-stage process in which PACVD followed PDT.
The third group consists of a single technique, a physical deposition process known as reactive magnetron sputtering. This was used to add a layer of carbon-doped stainless steel to the surface of stainless steel components.

New procedures are promising...

The treated inserts were analysed using metallurgical laboratory techniques and subjected to a series of functional tests. These included assessments of the fatigue resistance of the complete inserts, and the coatings' adhesion, biocompatibility and ability to withstand corrosion and wear.
Ion implantation by glow discharge gave good results for all three alloys. Fatigue strength increased by up to 20 percent, biocompatibility was excellent, and the new treatment improved the corrosion resistance of stainless steel. This treatment, say the researchers, is very promising for surgical applications.
PDT and PDT followed by PACVD worked well for titanium alloys. If the treatment temperature is above 800C the metal loses fatigue strength, but at lower temperatures wear resistance increases without affecting fatigue strength. Plasma-nitrided stainless steel performed poorly in biocompatibility tests and was rejected for surgical uses. It should be useful, however, for other engineering applications.
Sputtered carbon coatings on stainless steel give excellent resistance to wear and corrosion. Because this technique, like ion implantations, is straightforward and gives reproducible results, the researchers predict they are well placed for early commercial application.

...but selling it is harder

Despite the fact that the new treatments could be used to make implants that last for 20 years, commercial interest in the project has been muted. Eurorad has tried to market both services and equipment for the new techniques, but implant manufacturers and hospitals have shown little interest. One of the researchers even suggests that this is for purely cosmetic reasons: the new techniques lack the attractive gold finish produced by conventional nitriding, even though this has been shown to perform less well.
The partners have patented their new techniques, however, and hope that eventually they will see commercial applications. As well as surgical implants these could include dental drills and other tools, and as a tougher replacement for PTFE non-stick coatings on cooking pans.

 

Project Title:  
Comparison of surface modifications by ion implantation and cold plasma assisted treatments as an alternative to other coating techniques

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-4375

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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