Developments in ceramic materials and processing technology have been a boon for medical implant applications ranging from ceramic-on-ceramic hip joints to feed-thrus for neurostimulators.
Biocompatibility and resistance to wear are attributes that suit ceramic materials for a wide variety of medical implant applications, ranging from artificial joints to implantable electronic sensors, stimulators, and drug delivery devices. For well over a decade, materials such as alumina, zirconia, and other ceramics have successfully proven their ability to withstand the harsh environment of the human body. But the medical industry's need for longer-lasting and ever smaller - yet more complex - components is driving materials scientists to do even more. Today, materials scientists are employing innovative techniques, including injection molding, engineered coatings, and ceramic-metal assemblies, to extend the benefits of ceramics for a broad range of state-of-the-art medical implant applications.
As new developments in ceramic materials and processing contribute to the evolution of medical implant applications, Morgan Technical Ceramics (MTC), of Fairfield, N.J., is playing a key role as a provider of manufactured parts for the medical industry. Morgan Technical Ceramics, consisting of Fairfield, New Jersey-based Morgan Advanced Ceramics (MAC) and Bedford, Ohio-based Morgan Electro Ceramics (MEC), is a prominent manufacturer of what the company calls "innovative ceramic, glass, precious metal, piezoelectric and dielectric materials."
Ceramics Extend Life of Artificial Joints
Advances in the use of ceramics for artificial joints have received a great deal of attention through the years. The spotlight has intensified even further since Jack Nicklaus, the legendary golfer, received a ceramic-on-ceramic total hip replacement in 1999 in an experimental procedure at New England Baptist Hospital. In 2003, ceramic-on-ceramic hip joints received approval from the United States Food and Drug Administration (FDA).
Ceramic materials have been used for artificial joints since the 1970s, when the first generation of alumina products demonstrated a resistance to wear superior to that of traditional metal and polyethylene materials. Advances in material quality and processing techniques, combined with improved understanding of ceramic design, led to the introduction of second-generation alumina components in the 1980s. These second-generation alumina components offered even better wear performance.
The progressive wear incurred by traditional metal-polyethylene hip systems generates polyethylene particulate debris, which induces osteolysis and the weakening of surrounding bone. This causes loosening of the implant, a primary cause of costly revision operations. Ceramic materials are known to generate significantly less polyethylene debris when used in conjunction with polyethylene acetabular components in bearing couples.
State-of-the-art ceramic-on-ceramic technology, where an alumina femoral head is mated with an alumina acetabular cup, totally eliminates polyethylene debris and reduces wear significantly. A study of MAC's HIP Vitox® ceramic-on-ceramic hip joints demonstrated a wear rate of just 0.032mm3/million cycles. In addition to resolving the problems caused by polyethylene debris, the use of ceramic-on-ceramic hip systems alleviates any concerns over the release of metal ions into the body if a metal-on-metal hip system were used.
This superior wear performance extends the life of artificial joints, giving ceramic-on-ceramic joints a predicted life of well over 20 years. As they serve the needs of increasing numbers of younger patients for whom such surgery is now a viable operation, ceramic-on-ceramic joints are enabling them to continue leading active lifestyles.
Implantable Devices Employ Ceramics for Electrical Insulation, Enclosures
New developments in ceramic technology are playing an equally important role in the evolution of implantable electronic devices. In the forty-five years since the first cardiac pacemaker was successfully implanted in a human patient in the United States, researchers and doctors have created a wide array of implantable electronic devices. Among them are pacemakers, defibrillators, cochlear implants, hearing devices, drug delivery devices, and neurostimulators. Medical device companies are testing neurostimulators that pulse various nerves to treat particular medical conditions. Examples include neurostimulators that pulse the hypoglossal nerve, located in the neck, to treat sleep apnea; the sacral nerve, to treat bowel disorders; and the stomach, to treat obesity. Other neurostimulators that are currently being tested include those that pulse the thalamus to treat epilepsy; the vagus nerve to treat chronic depression, and other regions of the deep brain to treat migraines and obsessive-compulsive disorder.
These devices are increasingly relying on ceramic components, such as feed-thrus that provide a functional interface between the device and body tissue. A feed-thru is a ceramic-to-metal seal assembly that contains metal pins or small tubes that pass through a ceramic component. The pins allow electricity to pass in or out of the implanted device in order to sense what is happening in the body, or to administer an electrical charge when needed. A feed-thru can also be used to administer drugs to the patient. The ceramic substrate of the feed-thru acts as an electrical insulator, isolating the pins from each other. Morgan Technical Ceramics can also make ceramic housing assemblies to enclose the electronics for the device, which can attach to a feed-thru.
Feed-thrus for implanted devices must be hermetic, with a leak-tight seal around each pin. This ensures that bodily fluids do not work their way into the device and destroy the internal electronics, and that chemicals do not inadvertently escape from drug delivery devices. A braze material, typically 99.99% gold, is used to join each metal pin to the ceramic insulator. To ensure that the braze adheres securely, MAC has developed a proprietary process, in which the surface of the ceramic is prepared for brazing by the application of a thin film of biocompatible metal, such as platinum, niobium, or titanium, via physical vapor deposition (PVD).
Developers of improved implantable medical devices continually demand smaller and more complex components. Morgan Technical Ceramics has created, for drug delivery applications, a 1-inch-diameter, ceramic feed-thru that houses 104 separate pins. Voltage passes through each pin, activating different combinations of switches and allowing a greater number, or more complex combinations, of drugs to be administered.
Injection Molding and Coatings
Powder injection molding (PIM) has furthered the pursuit of component miniaturization. This method enables production of intricate features and unusual geometries, most notably for hearing-assist devices, bone screws, and implantable heart pumps. Testing of ceramic injection-molded objects has shown that net-shape, as-molded parts exhibit significantly less variation in flexural strength than green machined parts of the same formulation. The narrower Modulus of Rupture distribution of the PIM parts can be attributed to lower variability in surface finish than that which occurs with a comparable machined surface.
Morgan Advanced Ceramics also offers metal injection molding (MIM) technology, a low-cost alternative to machining, investment casting, and stamping. A MIM machine can typically mold parts in about 10 seconds, as opposed to minutes or even hours through conventional techniques. Metal injection molding applications are ideally suited for high-volume production of intricate components, ranging from laparoscopic instruments to biopsy jaws and dental brackets.
Ceramic-based coatings represent another area of ceramic technical development that is important to medical implant applications. These coatings, which include diamond-like carbon (DLC), provide a biocompatible, sterilization-compatible, non-leaching, and wear-resistant surface for key pivot points and wear surfaces. Such coatings are used to reduce friction, increase surface hardness, and prevent ion transfer from metal implant components.
Driven by the rapidly expanding and evolving market for medical implants, materials scientists and ceramic component manufacturers will, in all likelihood, continue to develop new materials and processes for the smaller, more sophisticated, and longer-lasting implant applications of the future.
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