Promising faster recovery times and less postoperative pain than traditional procedures, minimally invasive surgery has revolutionized modern medicine and patient care. The rising demand for endoscopes, delivery systems, catheters, and stents that can be inserted into the body through tiny incisions and navigate the terrain without causing harm has, in turn, also revolutionized the manufacture of metal tubing.

Lending mechanical strength, reliability, and proven efficacy, among other properties, to medical devices, metal has emerged as the material of choice for many minimally invasive tubing applications. But to accommodate the needs and end applications of such small devices, metal tube fabricators have been challenged with achieving unprecedented inside and outside diameters (IDs and ODs) and product features. "Stainless-steel tubing has grown with the medical device industry and has adapted its production technologies to produce products that are smaller, tighter, smoother, and capable of intricate fabrications," says Lance Heft, president and CEO of metal tube fabricator International Tube (Collegeville, PA; www.internationaltube.com). Despite adapting, however, metal tube fabricators acknowledge that small-diameter tubing can present some exceptionally large challenges.

Luck of the Draw

Small-diameter metal medical tube fabrication is typically performed through cold-working operations, which provide dimensional and tolerance control, good surface finishes, refined grain sizes, and high mechanical strength. Under the umbrella of cold working, there are several methods of tube drawing, the most common of which are rod drawing, fixed- or floating-plug drawing, and tube sinking. The former technique entails drawing the tube--featuring a mandrel inside--through a die. Fixed-plug drawing, on the other hand, employs a stationary plug (or mandrel) positioned at the end of the die to support the product's ID; floating-plug drawing forms the tube ID using an unanchored plug or mandrel that floats in the die and is held in by the friction between the tube and plug. Tube sinking, or free tube drawing, does not use any ID tool at all. Instead, it relies on a variety of factors to determine the ID, including ID and OD of the stock tube and OD of the final product.

Of the various techniques, International Tube, for one, champions floating-plug drawing. "Most people do rod drawing of tubing, and that creates waves in the tube, it has straightness issues, and you don't have consistency of wall," Heft opines. "Floating-plug drawing provides the most-consistent type of drawing that there is. Our process allows for the best ID finish and the best grain structure." Poor grain structure can result in holes, cracking, or inconsistencies in the wall thickness, he adds.

Superior Tube Company, Inc. (Collegeville, PA; www.superiortube.com) also performs floating-plug drawing; however, it is equipped to provide other techniques, depending on the application, according to David Schaeffer, medical team metallurgist. Likewise, miniature stainless-steel tube supplier K-Tube Corp. (Poway, CA; www.k-tube.com) offers a choice of application-dependent methods for its customers. It, too, employs floating-plug drawing for small-diameter tubes requiring very smooth surface finishes. However, the company also uses floating-plug drawing coupled with tube sinking or just tube sinking operations, if deemed the best process for a particular tube.

"Each process has its own advantages based upon the end use of the product," notes Terry McCune, company president. "Customers need to consider their need for ID cleanliness, ID surface finish, wall thickness, tolerances, and mechanical and physical characteristics in order for us to determine the best process for their use."

Smooth Moves

Regardless of the technique, metal tube fabrication for the medical device market is generally a labor-intensive process. To achieve the small-diameter tubes required for minimally invasive applications, such as coronary stents and specialty instruments, the part must often undergo multiple passes. "We start with a very large-sized product. And stent tubing is very, very small," Schaeffer explains. "We have to go through in the range of 10 to 15 size-reduction cycles to get from the starting size to the final size."

Small-diameter metal tube forming requires the fabricator to execute the selected drawing process over and over until the specified diameter is obtained. In addition to the actual drawing method, interim steps that include removing lubricant after drawing, annealing, and in-process cutting can also figure into the overall process. "We go through this cycle of drawing, taking the lubricant off, annealing to soften the tubing, and then drawing it again," Schaeffer states. "It's a lengthy and complex process. The smaller you go in size, the more challenging it is to make that product."

Part of the challenge lies in the trend that as diameters decrease in size, precision requirements become increasingly demanding. As medical parts get smaller, medical device design engineers request tighter OD and wall tolerances, as well as good surface finishes. "The appropriate surface finish will benefit the end-user by providing a product that functions as intended, thus preventing or significantly reducing the potential for assembly and/or field failures," says McCune, whose company is capable of achieving surface finishes ranging from better than 8 µin. Rq to more than 100 µin. Rq. She adds that rougher finishes, for example, may be desirable for applications benefitting from friction effects or creating turbulent flow.

An ultrasmooth surface finish, however, is of particular concern to engineers designing implantable devices and products used in minimally invasive applications. For many of these applications, it is imperative that metal tubes are burr-free and have extremely low surface roughness. Likely to come in contact with cells and tissue, the metal tube usually needs to cause minimal disruption in the body. Furthermore, it is often engineered with the intent to maximize flow and to allow for easy passage of other devices through the body of the tube.

Surface roughness in these instances could be disastrous, as International Tube witnessed when a device manufacturer came to it to help solve a serious problem. The tube fabricator reports that a manufacturer approached it to consult on a small-diameter metal tube employed in microsurgery applications to guide the insertion of fiber-optic cameras. Upon receiving several complaints, the manufacturer sought help to correct the delicate fibers of the camera from snagging on the inside of the tube--a problem that was exacerbated in some instances by shocks to the patients induced by this metal-to-metal contact. By producing an ultrasmooth surface finish on the ID of the tube, International Tube was able to dramatically improve the end product, according to Heft.

Material Missteps

It's important for engineers to spec out the ideal surface finish, tolerances, wall thickness, and other characteristics of the desired metal tube for a given application. But they also need to be flexible with their designs to a degree. "It's easy to specify something, but that doesn't mean it's going to work," Heft points out.

The general consensus among metal tube fabricators is that the most common pitfalls for design engineers tend to involve materials. In other words: Just because a metal seems perfect for a tubing application doesn't mean that it is. For instance, the availability of certain alloys and the inability to successfully fabricate some materials in tube form or to the small diameters and tight tolerances specified are factors that engineers frequently overlook, according to Schaeffer. "[The alloy] has to be something that serves their end-use application well but can also be acquired and fabricated into a tube."

Engineers also need to be mindful of the fact that an alloy's mechanical properties are altered when processed into tube form. "For custom-engineered tube products, mechanical properties can change based upon the alloy, chemistry and grain size control, tube production processes, and heat treating," McCune explains. "In other words, the mechanical properties are a function of the total manufacturing process."

Ultimately, the best way to optimize a metal tube for a given application is to simply consult with fabricators early in the process. "If [design engineers] have special straightness, ovality, wall thickness, or consistency requirements, we need to know those things as soon as possible in the discussion process," Schaeffer advises. "The more information that we get about their needs and expectations, the better we are able to produce products that will make them happy."

For additional articles and information on medical tubing, go to devicelink.com/mpmn/tubing

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