Originally Published MPMN April 2009

SPECIAL FEATURE: CERAMICS

Ceramics Have Medical Device Makers All Fired Up

Strong, heat resistant, and endowed with fine electrical properties, ceramics are proliferating in medical implantables

Bob Michaels

Where would you turn if you were looking for a material that is resilient and stable? That can withstand the unforgiving environment inside the human body? That acts as an excellent electrical insulator that can be patterned with electrical circuitry? That can be miniaturized and molded with extraordinary intricacy? You would turn to ceramics.

Since the 1970s, the use of ceramic materials in medical devices has mushroomed to include a variety of implant applications, many of which rely on the material’s good electrical insulation and dielectric properties. In addition to their well-known use in orthopedic applications such as femoral heads, ceramics today are employed in feedthroughs, implantable sensors, brachytherapy seeds for cancer treatment, and many other components. Harder than steel, more heat and corrosion resistant than metals or polymers, less dense than most metals, and made from plentiful and inexpensive raw materials, ceramics are well on their way to conquering the medical device industry.
Multitalented Materials

Ceramic materials are finding increasing use in implantable medical devices because of their excellent physical and performance characteristics, notes Keith Ferguson, business development manager at Morgan Technical Ceramics (Hayward, CA; www.morgantechnicalceramics.com), a manufacturer of components made from ceramic, piezoelectric, and dielectric materials. Ferguson adds, “The medical device industry continues to find new and expanded applications for implantables as engineers gain experience developing ceramic products.”

Take feedthroughs, for example. Feedthroughs are ceramic components containing metal pins or small tubes in which the ceramic substrate acts as an electrical insulator, isolating the pins from one another. Used to provide an interface between the medical device and the body, feedthroughs can be inserted into the body to help administer drugs to patients. Or take cochlear implant components. Cochlear implants contain electronic components that are housed in a ceramic casing, whereby the ceramic protects the electronics and also acts to conduct the device’s radio-frequency waves. In the last few years, these applications have undergone a process of increasing customization.
In the medical implantables sector, most companies have traditionally worked with metals and plastics, states Andrew Nield, director of sales and marketing at C5 Medical Werks (Grand Junction, CO; www.c5medicalwerks.com), a manufacturer of ceramic medical device components. But both he and Ferguson agree that ceramics combine several distinct advantages over either plastics or metals in certain applications: inertness, high compressive strength, excellent wear characteristics, stability in high heat, electrical insulation capability, and corrosion resistance.
“Manufacturers will decide to shift to ceramics if they encounter problems with plastic or metal—for example, temperature problems or problems with electrical insulation,” Nield remarks. “You can use a plastic material for insulation. But if you get a little bit of high temperature, if the plastic material is affected by acids in the body, or if a biomaterial attack takes place, that’s when you’re going to want to use ceramics.” Ferguson adds that ceramics demonstrate a range of performance characteristics that make them suitable for specific applications, including strength and electrical insulation in implantable feedthroughs and high-temperature resistance in radio-frequency ablation tools. However, transitioning from the use of materials such as metals or plastics is not easy. “Since processing ceramic components is new for many companies,” notes Nield, “the shift is a complicated problem.”
New Materials

The transition to the use of ceramic-based medical components has been complemented by a trend toward increasing parts customization, which has led to the development of novel materials. In turn, the availability of new materials is encouraging manufacturers to create new custom products, Nield states.

With the growth of customer-specific designs, companies started considering new and different materials. Traditionally, the ceramic material of choice was alumina, or aluminum oxide. With the development of custom parts, yttria-stabilized polycrystalline tetragonal zirconia (YTZP) has become a material of choice. A material that can be used to achieve parts with improved mechanical strength, YTZP is stronger than alumina and provides more flexural strength. Hybrids of zirconia and alumina are also coming into play, two examples of which are zirconia-toughened alumina (ZTA) and alumina toughened zirconia (ATZ). As materials with different properties, zirconia and alumina can be combined in various ways so that the zirconia becomes harder and the alumina achieves more flexural strength.
Nield says that with the development of new materials, “you’re seeing medical companies go from parts off the shelf to custom designs. Medical companies ask C5 to develop a new material, or they will say that they need a material with specific properties.”
Optimizing the development cycle is crucial for manufacturers that employ novel materials, explains Nield. Responding to OEM demands that they come up with new materials, suppliers perform risk analysis, biocompatibility testing, electrical testing, mechanical testing, and other analyses. The advantage of this arrangement is that testing does not have to be performed by each customer separately—an arrangement that is becoming a prominent staple of companies that use ceramics to make medical devices.
Piezoceramics Up the Voltage

A subbranch of technical—or engineering—ceramics, electroceramic materials are finding increasing use in advanced medical implants because of their hardness, physical stability, extreme heat resistance, chemical inertness, biocompatibility, and electrical properties. Electroceramics include piezoelectric ceramics such as lead zirconate titanate (PZT), an inert material that can convert mechanical energy into an electrical charge or an electrical charge into mechanical energy. Such materials are used in a range of medical implantable components, including piezoceramic sensors, actuators, motors, micropumps, ultrasonic generators, and valves for precision metering. Applications include IV-fed imaging in conjunction with balloon angioplasty, ophthalmology, blood clot dissolution, and fetal heart monitoring.

The growing use of piezoceramics such as PZT in healthcare products is attributable to the increasing complexity and sensitivity of many medical procedures such as open-heart surgery, thyroidectomies, tracheotomies, brain surgery, and cosmetic surgery, comments Mark Ota, president and CEO of NTK Technologies Inc. (Santa Clara, CA; www.ntktech.com).
Piezoceramics are superior to traditional metal applications for medical devices that require precision functionality, according to Ota. In infusion treatments, for instance, PZT ceramics are used in infusion systems for delivering cancer medications that often require a monitored level of delivery over many hours. “In these applications, the electroceramic can more precisely act as a sensor in detecting the presence of air in a liquid substance,” he says.
“PZT ceramic components have been around for many years, and the material itself has already been thoroughly characterized,” comments Ota. “Electroceramic advances are driven by custom application requirements. In many cases, the advancement of the material is tied to its manufacturability.”
Advanced medical applications using PZT require precision tooling and manufacturing, explains Ota. In addition, the complex medical applications that benefit from the material’s unique properties can result in higher-quality and better-performing products. While economies of scale cannot often be met, they are often not required because of the small-volume nature of such applications.
Despite its indisputable advantages, PZT is not without drawbacks. On the environmental front, remarks Ota, the material offers unique characteristics in part because of its lead content. “Although the risk of exposure to lead in medical instruments is virtually nonexistent,” he says, “the challenge of working with this material remains environmental.”
For some piezoelectric applications, higher-dielectric materials, ceramics with a higher piezoelectric charge constant, and materials that offer broadband performance are preferred, notes Ferguson from Morgan Technical Ceramics. These characteristics, he says, are satisfied by the company’s PZT5K piezoceramic material, which is employed in medical sensors and low-power medical devices. Offering high coupling values and high dielectric properties that make it suitable for thin and small parts, the material is used for sensors that are incorporated into catheter, blood pressure, auditory sound, vascular flow, and medical imaging products.
Another piezoelectric material offered by Morgan is PMN-PT, a single-crystal ceramic with transmitting and receiving capabilities that is used in transducers for a variety of medical ultrasound applications. According to Ferguson, Both PZT5K and PMN-PT materials improve acoustic performance without compromising transducer complexity or acoustic matching techniques.
Still in the experimental stage, another class of materials known as multilayer ceramics is being investigated for their electromechanical properties in pickups and transmitters. These materials may find their way into future auditory devices. Still another group of ceramic materials called piezocomposites break up the cross-coupling seen in other types of piezoceramics and help improve resolution in such applications as medical imaging and Doppler blood flow equipment.
“The use of piezoceramics in the medical market is in its infancy for a variety of new medical applications that are still being explored,” concludes Ota. “An expansion of its use is expected in the future, in line with heightened medical demands from an aging population.”
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