Medical Device Link.

Originally Published EMDM September 2003

Special Report

A high-strength bioceramic developed by Metoxit AG can withstand loads that are four times greater than conventional ceramics used in hip implants.

You can always do better, as the saying goes. Based on several recent developments, suppliers of biomaterials have taken this dictum to heart.

Silicone rubber has a long history of medical use, but there is room for improvement, says researcher Judit E. Puskas. She holds a patent on a version of a material containing polyisobutylene that is dramatically less permeable than medical-grade silicone. It also contains fewer residual chemicals, because it does not need to undergo vulcanization to achieve shape-retention properties.

A bioceramic material developed by a Swiss firm reportedly features better mechanical and tribological properties than the ceramics currently used in hip implants. The premature failure of another firm’s line of hip implants spurred Metoxit AG to investigate the use of alumina-toughened zirconia for this application.

In this article, EMDM also reports on an implantable stainless steel that combines strength and ductility and a coating that is said to surpass Teflon and parylene in hardness and adhesion. A recently introduced device that optimizes the wear testing of hip implants is described in the sidebar below.

PIB-Based Material May Offer Alternative to Silicone Rubber

A material containing polyisobutylene (PIB) soon may become available as an alternative to medical-grade silicone. The elastomer is less permeable than silicone, according to researchers, and does not require the use of chemical additives for shape retention.

Silicone rubber is routinely used to make short- and long-term implantable devices. It also has a long history of use in tubing and other external products. However, silicone rubber has relatively poor mechanical properties and is plagued by permeability issues. The PIB-based material does not have these drawbacks, says Judit E. Puskas. She was granted a patent this year for a dendritic version of the material.

“As you know, PIB is used in chewing gum, and it also keeps car tires inflated. Without it, we would have to pump air into our car and bicycle tires every day,” says Puskas. “PIB is at least a hundred times less permeable than any known rubber, and a thousand times less permeable than silicone rubber,” she adds.

Nanotechnology improves the tensile strength and formability of a medical-grade stainless steel.

Unlike silicone rubber, which needs to be vulcanized to achieve a desired shape, Puskas’s material retains its shape without the use of chemicals. The presence of residual chemicals can be problematic, especially in long-term implanted applications, she notes.

Preliminary tests show that her biorubber retains its properties better than silicone rubber under simulated physiological conditions. Tests were conducted in Germany at the Department of Polymer Engineering, University of Bayreuth, in collaboration with Professor Volker Altstädt. His lab has special equipment for fatigue testing, says Puskas. (She is currently a visiting professor at the university under the Mercator programme, which is supported by the Deutsche Forschungsgemeinschaft. Puskas also holds the Bayer chair at Ohio’s University of Akron in the United States.)

The material’s fatigue properties were investigated using an environmental chamber attached to a servohydraulic testing machine filled with simulated body fluid (SBF). Test results showed that the dynamic creep properties of silicone rubber were greatly affected by the presence of SBF, even at 24°C. SBF had much less effect on the new material’s fatigue properties under similar conditions, according to preliminary data.

Puskas and her colleagues have also found that the new rubber may be less prone to calcification than silicone in comparative short-term implantation studies. A new, patented process will allow Puskas to embed the material’s surface with antibiotics and other agents to further improve biocompatibility and to reduce or prevent potential adverse reactions such as infection or calcification.

Puskas coinvented with Joseph P. Kennedy a first-generation PIB-based biomaterial that was patented in 1990. In vitro, and short- and long-term in vivo implant testing showed it to be biocompatible and biostable. Puskas is looking forward to pursuing research into a new generation of PIB materials at the University of Akron.

“As the rubber capital of the word, Akron has a long and illustrious tradition in rubber research. This will be the perfect place to continue my work,” she says. With sufficient funding, adds Puskas, the new biorubber could be on the market in 2 to 5 years.

Bioceramic Sets Benchmark for Bending Strength

A composite material made up of 80% tetragonal zirconia polycrystals (ZrO2-TZP) and 20% alumina (Al2O3) is reported to have mechanical and tribological properties superior to ceramics currently used in hip implants. The alumina-toughened zirconia (ATZ) Bio-Hip, developed by Metoxit AG (Thayngen, Switzerland), has a bending strength of up to 2000 MPa, indicating that it can withstand loads that are four times greater than conventional Al2O3 implants.

Currently used to coat capillary tubing and related products, a material developed by Advanced Materials Components Express may undergo testing for use on implantable products by the end of this year. The conformal coating surpasses Teflon and parylene in terms of hardness and adhesive properties, according to the firm.

Metoxit offers a range of mostly custom ceramic products for dental, orthopaedic, industrial, chemical, and electrical applications. Its expertise in the production of endoprosthetic components combined with its knowledge of industrial applications led to the development of the ATZ Bio-Hip, according to marketing and sales manager Martin Schmidt.

“We have a great deal of experience studying wear-, load-, and temperature-related effects with our industrial products,” says Schmidt. “The failure of a competitor’s hip implants that was in the news not too long ago led us to look at alumina-toughened zirconia as a possible alternative material for the ball heads. In the time-honored tradition of medical product development,” adds Schmidt, “we identified a problem and found a potential solution by using a material that had proven itself in other, nonmedical, applications.

“Bending strength is the crucial issue when it comes to hip implants,” says Schmidt. Almost all clinical incidents of fractured ball heads can be traced to insufficient contact between the tapered portion of the stem and the ceramic cone. This can be caused by the presence of particles, a deformation in the taper, and/or misaligned components, he adds. Simulated testing conducted by Metoxit showed that the load-bearing capability of alumina ball heads decreases by 90% when there is contamination between the metal and ceramic surfaces. This may place the fracture load under 10 kN. Under the same conditions, the ATZ material retained a fracture load of almost 40 kN. Revision surgery, in which an alumina head needs to be replaced but not the stem, may represent a key opportunity for the ATZ Bio-Hip, says Schmidt.

The high fracture strength of the material is linked to zirconia’s transformation to a monoclinic phase when a crack forms. “The monoclinic phase takes a slightly higher volume, and the resulting compression prevents propagation of the crack,” explains Schmidt. “And even if a crack were to form, it is then stopped by an alumina grain.” To achieve the high material strength, Schmidt adds, hot isostatic pressing is required.
ATZ Bio-Hip ball heads are not yet in clinical use. Metoxit has invited orthopaedics firms to test the material (and, in fact, some have accepted the invitation). The biocompatibility of ZrO2 and Al2O3 is well established, notes Schmidt. No other inorganic compounds, such as chromium or strontium, are added to the material, which has passed cytotoxicity and haemolysis tests.

Metoxit provides customers with a one-stop service, overseeing everything from the processing of the raw materials to the fabrication of the finished product. Its sister company Saphirwerk in Brügg, Switzerland, provides grinding and polishing services. Keeping all of the manufacturing under its supervision allows Metoxit to provide its customers with reliable, innovative, and cost-effective solutions, says Schmidt.

Nanotechnology Boosts Properties of Medical-Grade Steel

Nanotechnology techniques have enabled Sandvik Bioline UK (Sheffield, UK) to develop a medical-grade stainless steel that combines exceptional levels of strength with ductility. Supplied in the shape of strips, bars, rods, and tubing, the material has an array of applications for the manufacture of medical instruments and tooling.

By manipulating the material at the molecular level, Sandvik was able to substantially enhance performance, according to global technical manager Stephen Cowen. “Nanometre-sized particles in the Sandvik Bioline 1RK91 material bring a range of unique properties such as tensile strength, corrosion resistance, formability, and so forth,” says Cowen. The particles are formed by means of a heat treatment process. “A novel phenomenon occurs within the material [during heat treatment], which produces nanoscale precipitates of a quasi-crystalline structure,” he explains. “That’s where the strength comes from.”

To facilitate machining, Sandvik Bioline 1RK91 can be supplied in a soft unaged condition. “The mechanical properties and ductility in the unaged condition are more conducive to good machining,” says Cowen. Once a part has been produced, it can be subjected to age hardening. “This gives customers the flexibility to manufacture a very complex part and then to age it to enhance the material’s mechanical properties,” says Cowen.

The combination of properties, surface finish, and sterilizability make this grade of stainless steel suitable for use in the fabrication of torque wrenches, bone drills, surgical needles, and numerous other microsurgical medical and dental devices. Because of its strength, Sandvik Bioline 1RK91 may play a role in the development of thinner, and thus lighter, devices that will cause less tissue damage than traditional instruments.
The material is primarily produced at the company’s plant in Sandviken, Sweden. “Integrated production from melting to final product form allows us to control all of the parameters needed to achieve these unique properties,” notes Cowen.

Sandvik Bioline 1RK91 has undergone in vitro testing. It has no cytotoxic potential and meets global allergy and skin-irritation standards.
For medical wire applications, Sandvik has developed a delivery system to ensure that its customers receive the product in the cleanest possible condition. The wire is wound onto white, nonreturnable spools that are protected by a plastic film and packed in a white cardboard box.

Bondable Coating Achieves Surface Energy of Teflon

A coating material that is currently used with hypodermic and DNA testing needles and in microfluidic applications has a set of properties that may benefit implantable devices. Developed by Advanced Materials Components Express (AMCX; Bellefonte, PA, USA), AMC148-18 reportedly surpasses Teflon and parylene in hardness and adhesive properties. The company hopes to begin testing the material for use on implantable products by the end of this year.

“AMC148-18 has the lowest nonspecific protein-binding surface available,” says director of business development Dona Parrotte. “One of the problems with a nonstick coating such as Teflon is that it doesn’t stick to anything. We have achieved the same surface energy with our material, but made it bondable.” 

The conformal coating can be applied to devices constructed of stainless steel, other metals, and ceramics. It is resistant to most chemicals, solvents, and acids. In addition, AMC148-18 has a refractive index close to that of water, making it optically transparent.
The material is currently used to coat deep, small-bore inner diameters such as capillary tubing. Once the coating has been applied, it cannot be removed without partial destruction of the substrate.

“By the end of the year, we would like to have someone test [the material] for implantable use,” says Parrotte. “Then we can proceed with managing the cost to get US FDA approval. We’re also interested in pursuing collaborations.” 

Melody Lee contributed to this article.

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