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Artificial joints

Guaranteed Balance

A new treatment for prostheses results in less wear and tear

In Brazil every year some 14,000 surgeries for the replacement of knee and hip joints with prostheses s are carried out through the public health system. In the end, they are fundamental for recovering movements lost as a result of accidents, degenerative diseases such as arthrosis and arthritis or bone tumors. The search for artificial joints that are more and more similar to the originals have led the researchers from the State University of Campinas (Unicamp) to apply a new process in the treatment of a titanium alloy, used in orthopedic applications, which resulted in a biocompatible application with high resistance to wear and tear. “It’s the covering with the lowest attrition yet obtained for this type of application”, says the researcher Emília Tieko Uzumaki, responsible for the study, presented at and a award -winner in two international congresses that took place in September, one at the Congress of the European Society of Biomaterials in Italy and the other at the Surfaces, Coatings and Nanostructured Materials Congress in Portugal. In the research, carried out during her doctorate thesis in the Mechanical Engineering School of Unicamp supervised by professor Cecília Zavaglia, the titanium alloy was covered with diamond-like carbon , also known as DLC, a biocompatible material that possesses properties of high durability, low friction, resistant to wear and tear and to corrosion. “The titanium alloy is the best biocompatible alloy”, says Emília. Nevertheless, in order to be used in prostheses, it had to pass through a surface treatment in order to resist wear and tear.

In order to apply the DLC a process of immersion in plasma developed by the researcher Carlos Salles Lambert, of the Physics Institute of Unicamp was used, which is in the phase of patenting. This technique of deposition makes use of a hydrocarbon plasma – a gas such as methane or ethylene – in order to involve, completely, the parts to be coated with the diamond-like carbon. This makes possible the uniform coating of large areas – up to 1 m2 or more, depending on the size of the equipment used -,at low cost in comparison with other plasma deposition techniques. “Conventional techniques, besides not having good adherence, don’t permit the carbon film to be deposited in major areas and in complex forms”, says Lambert. They are mainly used for small flat surfaces.

Biocompatible materials
Currently the material most widely used in the metallic configuration of knee and hip prostheses is polyethylene of exceptionally high molecular mass with special characteristics, which acts as a “dampener” and is the best-known polymer for this application. Even at that, it is the limiting factor in the useful lifespan of the prosthesis, which can vary from five to fifteen years. This means that, from time to time, the prostheses have to be substituted totally or partially. For this reason, the search for new biocompatible materials for orthopedic prostheses that have a greater lifespan is not stopping. And company interest in innovative products also continues. The coating of the metallic alloy with DLC diminishes the wear on the prosthesis and, through this, increases its useful lifespan. Up until now, three companies have shown interest in the new material.

All of the tests for evaluating whether or not the DLC coating obtained through plasma immersion can be used for the human body, demanded by Brazilian norms and by the American agency that controls food and medicine, the Food and Drug Administration (FDA),are being carried out at Unicamp. In the Cell Biology Department of the Biology Institute, tests on biocompatibility, in vitro – carried out with cell culture in the laboratory -, were done such as cytotoxicity, adhesion and cellular morphology. The test of biocompatibility, in vivo, with implants in muscular tissue and animal bone, was carried out at the Medical and Experimental Surgery Center with the collaboration of professor William Dias Belangero, from the Orthopedic Department of the university’s Medical Sciences College. “The studies show that the cell response was better for the titanium alloy coated with DLC than for the alloy without the coating”, says Emília. “And as well, the coating gave better resistance to corrosion.”

The DLC films obtained by conventional processes began to be studied at Unicamp’s Physics Institute at the start of the 1980’s. The new technique of deposition, which began to be researched by Lambert during the 1990s, has already been used to deal with parts destined to medical areas, auto manufacture, aerospace, petrochemical and petroleum industry. In the auto sector, gearing for the motor, roller bearings and screws are already being coated with DLC in order to diminish wear. In the aerospace industry, the process was used experimentally with success in parts that go to make up the wings of aircraft. “The treatment improves the surface, increasing the lifespan, without increasing the part’s weight”, says the researcher. In the biomedicine area, heart valves and stents, used during surgery as mechanical devices that impede that the artery returns to close itself off, coated with DLC, are being tested in animals. Preliminary results show that the new process helps to reduce the formation of clots. The possibility of the application of DLC on diverse products caught the attention of various companies that attended the International Nanotechnology Congress, held in July in Sao Paulo. After the congress, some of them went to Unicamp’s Innovation Agency (Inova) to start talks that could result in licensing contracts.

Surgical scalpel
In a joint study, Lambert and Emília studied the real conditions of the use of objects subject to abrasion wear. Industrial knives and surgical scalpels with a DLC coating were tested in a textile factory in Jundiai, in the interior of the state of São Paulo. The company uses the surgical scalpel in various machines for cutting large quantities of cloth and plastic. In each one of them, there are from 100 to 200 scalpels paired up – the actual number depends on the width of the desired cuts -, that have an average lifespan of 20 days, taking as a parameter the most abrasive clothes. With the DLC coating the scalpels lasted 60 days, three times longer than the normal time, representing substantial economy for the company, which uses around 500 of them per month. Industrial blades, similar to discs, could also be used for a period three times as long after having been treated with the plasma. Instead of 30 days they resisted some 90 days.

Of the many applications already tested, one is very close to being put on the market. Three companies from the town of Cordeirópolis, in the interior of Sao Paulo State, are disposed to applying a DLC coating on domestic and industrial utensils, such as pots and pans, frying pans, and trays for cakes and bread, including those used in bakeries, substituting the conventional anti-adherents. The plasma immersion process gives resistance to wear and abrasion to the material, properties that guarantee adherence of the coating even when in contact with iron wool during the cleaning process.