MATERIALS
 |
Silicones are suited for an array of applications including a variety of drug-delivery devices.
|
"Medical device firms now have a much greater choice of materials than even a few years ago," says Howard Freese, business development manager at Allvac (Monroe, NC, USA), a company that produces cobalt, titanium, and specialty steel products for medical device firms. "There are no longer only a few standard materials that are used in most medical applications," he adds. Drawing from an ever-expanding palette of materials, medical device designers must consider which ones will give their products the best possible combination of usability, performance, appearance, and endurance. One result of recent developments in materials science is the ability of medical device OEMs to give the biocompatibility and functionality of medial materials more scrutiny than in the past.
Biocompatibility is dependent on many factors including the idiosyncrasies of an individual's body chemistry. "People still don't fully understand the wide ranging chemical acceptability of the human body," Freese notes. "But, in general, progress is being made in terms of improving the host's acceptance of medical devices." He points out that, despite the recent concerns about drug-eluting stents, advances are being made in developing materials that are well tolerated by most patients, owing largely to cross-pollination of scientific disciplines. The emphasis on biocompatibility, matched with a continual desire to improve the functionality of medical-grade materials, is leading to a number of advances in ceramics, metals, and polymers
Growing interest in ceramics
Ceramic and ceramic-on-metal devices have surged in popularity, largely owing to their biocompatibility and the development of manufacturing techniques that produce stronger, more fracture-resistant ceramics than in the past. And progress continues to be made in this area: "Ceramic materials are still being developed with improved strength, which should continue to reduce concerns over fracture," Freese notes. Because modern weight-bearing ceramics are reasonably fracture resistant, and wear debris associated with ceramic joint implants is less of a problem than for plastics or metals, an increasing number of overweight and relatively young patients needing joint implants are being fitted with ceramic devices. "We anticipate growth in both hip and knee replacements, with knee implants expected to see the greatest expansion over the next years," says Steve Hughes, bioceramic products manager of Morgan Advanced Ceramics (Rugby, Warks, UK).
Though demand for the material is steadily growing on the continent, "the market for ceramics is very country-dependent in Europe," Hughes explains. "For instance, the percentage of joint replacement procedures using ceramic materials is greater than 70% in France, whereas it is less than 15% in the United Kingdom." He believes that about 60% of the artificial joints that are implanted in Europe are ceramic. By comparison, ceramic implants made up about 5% of the US market in 2006. But that market is also growing, Hughes says. "I estimate that the use of ceramic implants in the United States is now at or close to double digits."
The expanding role of metals
Improvement in metals technology also has benefitted the medical field in recent years. "Developments in metallurgy have resulted in continual development of alloys with better mechanical and fatigue properties than we had in the past," Freese explains. As the ability to produce artificial joints from ceramics has improved in recent years, so has metal-on-metal joint manufacturing capability. "In particular, there has been a reduction in wear debris associated with loosening that leads to osteolysis for metallic artificial joints," Freese notes. "And as a result of the improvements in medical alloys, I tend to believe that the total share of metallics is growing for orthopaedic and cardiovascular and dental applications," he adds.
Nitinol has continued to grow in importance because of its ability to add functionality to a range of medical devices. Although generally well tolerated inside the body, nitinol's biocompatibility can be further improved by using surface passivation to form an inert surface layer on the alloy.
Once forming microscopic layers of nitinol is practical, other developments of the shape-memory alloy may follow. According to Mark Polinsky, director of engineering at Memry (Bethel, CT, USA), researchers have worked to form microscopic layers of nitinol that would enable the alloy's use in implantable microswitches that are switched on or off in response to temperature cues. Similarly, superthin nitinol layers also could be used to produce motion required by dispensing and actuation devices such as miniature drug-delivery pumps. Such devices currently require multiple parts to perform pumping operations, but a micropump could be actuated by a single thin piece of nitinol.
Polymer trends
Polymer research has led to the development of hydrogels, biodegradable polymers, and plastics that can bend as a result of temperature changes. But most current medical polymer R&D has the comparatively simple aim of tweaking conventional polymers to suit specific medical demands. "Many medical applications require physical properties that off-the-shelf compounds can't meet," explains Anthony Pagliaro, Saint-Gobain healthcare marketing manager. As a result, companies such as Saint Gobain Performance Plastics (Quentin Fallavier, France), NuSil Technology (Sophia Antipolis, France), and Bayer MaterialScience (Leverkusen, Germany) are increasingly formulating polymers to meet the unique needs of applications ranging from drug-delivery devices to surgical instruments.
Because of the growing number of factors to consider when choosing a medical polymer, many plastics and silicones suppliers are collaborating with med-tech firms. "We are noticing that medical device manufacturers are interested in a close cooperation during project development," notes Bayer medical market manager Markus Krieter. Silicone manufacturer NuSil sees a strong demand for design and manufacturability assistance from its customers, says Brian Reilly, NuSil healthcare materials product director. "As silicone experts, we try and convey the properties of the material as well as what is feasible from a tooling design and manufacturing standpoint."
The market demand for silicone-based drug-delivery systems has grown rapidly, according to Reilly. The reasons for their popularity include the ability of silicone drug-delivery devices to reduce toxicity and to improve a drug's absorption and release profile. Capable of being modified to fit an array of applications, silicones are suitable for drug-delivery systems that use either elastomeric matrix or reservoir designs. The flexibility makes the material appropriate for a variety of transdermal, transmucosal, and implantable drug-delivery systems. The permeation characteristics of silicone systems also can be engineered to suit the requirements of a device.
The popularity of minimally invasive procedures also has had significant impact on medical polymers. In an effort to reduce the trauma associated with some medical procedures, device manufacturers are asking for medical plastics that can be moulded to tight tolerances for use in small and complex surgical instruments, according to Kevin Dunay, Bayer MaterialScience medical market segment leader.
Infection control is another major issue in the healthcare industry driving R&D efforts. "Bayer is developing antimicrobial materials for use in medical devices," Dunay explains. "We are looking at different options such as using coatings and adding antimicrobial additives to the polymer itself, that would prevent the bacteria or fungi from degrading the device." Because many infections are linked to improperly sterilized surgical instruments and medical devices such as implants and catheters, polymers with antibacterial properties could be a powerful weapon against postoperative and hospital infections.
