From implantable devices to sharps products, surgical tools to x-ray equipment, metal continues to be a mainstay of the medical industry. Despite significant growth in the use of polymers as an effective replacement material, metal remains a key factor in medical advances.

Advances in metallurgy and metalworking are critical to development of many advanced devices. Development of titanium alloys and high-carbon, cast cobalt chrome have enabled the manufacture of successful implantable devices because their properties are well suited for these demanding uses. For example, in addition to durability, other properties that make titanium a good choice for aerospace applications also make it quite suitable for surgical implants—principally that it is chemically inert and does not react with various bodily fluids.

Looking at the projected growth in the market for biocompatible materials, the metals segment is expected to increase from $162,500,000 in 2003 to $212,800,000 by 2008, according to Business Communications Company, Inc.

Not surprisingly, medical advances have been closely linked to progress in metalworking processes. High-performance machining and molding methods have enabled the development of improved surgical instruments and other diagnostic and therapeutic devices.

Machining

Electrochemical Machining Means Greater Precision

Electrochemical machining technology has benefited the semiconductor and biotechnology industries for several years. Just recently, medical device manufacturers also have discovered that electrochemical machining can offer advantages that CNC machines and other mechanical grinding technologies cannot.

"A trend in the medical device industry is toward smaller and smaller devices, which coincides very well with electrochemical etching, pointing, and polishing technology," says John O'Brien, vice president of Point Technologies (Boulder, CO).

The technology is used to etch small-diameter wire or tubing composed of tungsten-rhenium, nitinol, titanium, stainless steel, and numerous other metals common to the medical manufacturing industry. During this process, small amounts of metal are removed electrochemically in a controlled, precise way.

The process delivers maximum precision and tolerances of ±1 µm to treated materials, O'Brien notes. Tube and wire diameters can be reduced to 0.0005 in. or less. The surface of the material is smoothed, removing grind marks and burrs, which can act as stress concentrators.

Parabolic-taper guidewires, with convex or concave taper rates, can be made in lengths ranging from 0.025 to 12.0 in. The company can produce wires of any length, with diameters of 0.001 in. to 0.100 in.

"We also can reduce solid wire points to submicron sharpness," O'Brien explains. Electrochemical machining is thus used to sharpen coils and electrodes like those used in fetal scalp monitoring and cardiac rhythm management. The highly polished tips allow for low eschar buildup and ease cleaning, while the precise sharpness ensures pinpoint accuracy for penetrating single cells.

"As devices continue to get smaller, they are more and more difficult to produce mechanically. Technologies like electrochemical machining can help take devices to the next level," O'Brien concludes.

Catheter Tubing

New Design Combines Best of Traditional Tubes

For years medical device manufacturers have struggled with various materials challenges when making catheter tubes. Tube structure plays an essential role in catheter and guidewire devices. But typically, manufacturers have used plastic nonbraid or braid tubing material, or metal hypo tubing. The problem is none of these tubes encompass all of the qualities a designer might desire.

Plastic nonbraid tube offers excellent flexibility, but its torqueability and pushability aren't strong. Plastic braid tube provides good flexibility but little torqueability and pushability. Metal hypo tubes have excellent pushability and good torque but little flexibility and kink resistance.

In June, Asahi Intecc Co. Ltd. (Newport Beach, CA) introduced ACTONE, a stainless steel, cable-like tube containing several fine wires. The tubing combines the best qualities of the three types of catheter tubes previously available. ACTONE offers excellent flexibility, kink resistance, and torqueability as well as good pushability, a higher break load, and lower elongation.

"ACTONE stainless steel cable is a very unique product. It was developed to meet manufacturer requirements for pushability, flexibility, torqueability, and other qualities that metal hypo tube and conventional plastic tube could not," says Yoshi Terai, director of marketing for Asahi Intecc. "The ACTONE NiTi is much more flexible than NiTi hypo tube. It allows us to insert tortuous vessels where flexibility is very important. It also allows for memory shaping."

The number of filar used determines how stiff a tube is. Therefore, Asahi Intecc varies the number of filar used from 8 to 18 to meet manufacturers' specifications. A larger number of filar provides for a softer tube. Fewer filar create a stiffer one without changing the inside or outside diameter of ACTONE.

Tubing Materials

Increasing Alloy Choices, Properties for Designers

A nontraditional metal-forming reduction technique, formerly used for hard-to-draw alloys specified for nuclear applications, is now giving medical device designers a wider choice of alloys and properties.

In the past, more exotic alloys like titanium, cobalt-based metals, rhenium, tungsten, tantalum, and iridium weren't specified for implantable devices. That's because such alloys were difficult and costly to manufacture through traditional tube-drawing techniques. "This new approach for high-end medical tubing enables designers to use the specific properties of these harder-to-work alloys to provide final attributes like thinner walls, smaller OD (outside diameter) sizes, improved radiopassivity, and superior fatigue resistance," says Jeff Warden, vice president of marketing for Superior Tube (Collegeville, PA). "It also significantly reduces the number of steps required in the process, which may provide a higher material yield and cost advantages."

Warden cited an example of a manufacturer that previously received 20 feet of material for a certain price using traditional tube-drawing technology. Superior Tube, using its nontraditional technique, was able to provide 150 feet of material for the same cost.

"Most people think of metal tubing as being drawn through a die and getting smaller and thinner," Warden says. "We use a different technology. We start and finish with a tube, but what happens in between is an innovative, proprietary process."

That process also expands the range of properties medical designers can specify. "We can dial in required properties like tensile and yield strengths, elongation, grain size, and surface finishes more cost effectively and easily than can be done with traditional tube-making processes," Warden notes.

Surface finishes below 10 Ra, dimensional tolerances of 0.0005 in. or better for both the OD and wall, mechanical properties, and fatigue resistance can all be specified with great detail, he adds. Superior Tube can work with tubing as small as 0.030-in. OD and smaller, and with wall thicknesses well below 0.010 in. Superior Tube says its new technology is most applicable to any permanent implantable device that uses tubing, including stents, spinal implants, trauma nails, and orthopedic screws.

Metalworking

New Tools for Metal Fabrication

Medical manufacturing has quite often benefited from developments that were initiated within other industries—advanced materials and electronics concepts from space and military research programs, for example. Now an Ohio firm has a novel metalworking method that may provide new capabilities for medical manufacturers.

The technology, from Superb Industries Inc. (Sugarcreek, OH), is essentially a method for precisely splitting thin sections of metal material. Says Superb president John Miller, "Splitting, I think, accurately describes it. It's a mechanical process that was developed for the telecommunications industry, which allows us to actually split a sheet of material into two either equal or disproportional members. For example, we've taken material down to 0.008 in. thick and split it into two sections that are 0.004 in. thick." He adds that the method can be applied to a number of different types of metal, including stainless steel, titanium brass, and aluminum. He speculates that the method might also be applied successfully to plastics.

Miller suggests that there are no techniques available that are comparable with the firm's Met-Split process. He says, "I'm not aware of any existing technologies, actually. And we've got four patents on it, and part of the patent search is to make sure that there's not another process." Miller adds, however, that "there is another process called skiving that fuses material but does not actually split it."

Although the Met-Split technology was originally developed for telecommunications to make connectors of different sorts and sizes, medical uses are now being explored. Miller says that interest has been expressed from a number of sectors, including stent manufacturers who believe there are potential applications. "We really feel that in medical there will be a lot of different uses," adds Miller.

He speculates that other promising applications could include use in catheters, endoscopy, and minimally invasive surgery (MIS), emphasizing the potential for creating microtools for MIS. "We could do some things there," he says. "We're really in the initial stages of exploring what applications we may find in the medical end of things."

Characterizing the potential benefits of Met-Split, Miller says, "The advantage really is that you can take one piece of metal and, instead of trying to take pieces to form a two-part construction, you can take one piece and split a portion of it. This means that you have better structural integrity. For example, if you're talking about a tweezer at the of an endoscopic tool, instead of making two pieces that are held together with a pin, we could make it out of one piece. And hence you don't have the danger of the thing coming apart."

Miller concludes by saying that the firm is currently working on new capabilities for the Met-Split system. He explains, "We have another patent pending currently that will allow us to not just split it in two, but split it into quadrants of four."

Ceramic to Metal

Ancient Solutions for Modern Challenges

Use of ceramics can be traced back for more than 10,000 years. But many advanced medical devices have been developed by combining these ancient materials with modern metals. Combining metal and ceramic materials has enabled manufacturers to develop new classes of innovative devices and components. Morgan Advanced Ceramics (MAC; New Bedford, MA) has developed ceramic and ceramic-to-metal products for medical applications, including feedthroughs for cochlear, pacemaker, and other implants; surgical instruments; x-ray tube components; and hip joints.

According to Keith Ferguson, MAC medical marketing manager, "Ceramic-to-metal brazing allows a robust, hermetic seal. Other sealing technologies such as glass seals, glue, or mechanical fastening cannot provide the same level of assurance in a harsh environment such as in the body."

Ferguson explains that pacemakers, defibrillators, and cochlear implants are among the devices that can benefit from the use of ceramic-to-metal materials. He adds that the technology can benefit "any device that needs to be implanted in the body and requires a biocompatible leak-tight seal that will last for years. High-power vacuum tubes for x-ray equipment also require a sturdy seal that will last for many scans; the medical x-ray market is moving toward higher power and away from glass."

The ceramic-to-metal sealing technology compares favorably with conventional techniques, such as glass-to-metal sealing. Says Ferguson, "Glass is less expensive to process, but typically less robust. Many advances have been made in recent years to make ceramic-to-metal components cost competitive with glass. Still, most ceramic assemblies are used where the cost of failure is prohibitive."

Ferguson says that among the new areas of development at the company is "a new sealing technology involving Active Braze Alloy (ABA), developed at our sister site, Wesgo Metals in Hayward, California. It has allowed us to take out cost and processing steps for our customers." The ABA alloys provide a breakthrough single-step approach for joining ceramics to metals by eliminating the need for prior metallization of the ceramic surface, according to the firm. A typical use is the production of hermetic joints for vacuum brazing applications.

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