Originally Published MDDI January 2003
From precision device components to novel microtools for surgery, a number of new uses for metal tubing are being explored.
How will future historians view the progress made in healthcare during the last half of the 20th century? Many are likely to see it as a period of progress exemplified by major advances in imaging and IVD technology. Others are likely to see it as the waning years of such barbaric practices as open heart surgery. Splitting open the patient's chest and spreading the rib cage to expose the heart is being replaced by far less invasive methods. Historians may also recognize the critical role that tubing has played in developing these less-traumatic methods, as well as the latest IVD and other medical systems. Advanced tube designs and new materials with a broader range of properties have been key factors in moving treatment closer to the patient and reducing trauma.
Available in a range of types and sizes, medical tubing essentially has become a commodity item in the healthcare industry—with applications in device, pharmaceutical, and clinical laboratory sectors. In addition to extracting samples, and delivering gases and fluids, tubing products enable physicians and surgeons to perform minimally invasive diagnostic and therapeutic procedures.
Plastics are commonly used in the manufacture of catheters, ventilator tubes, and other medical devices in which flexibility is a must. Metal is nevertheless the material of choice in products requiring precise tolerance, resistance to certain solvents or fluids, or other functional characteristics.
From Needles to Microtools
Metal tubing uses are varied—including syringe needles and other sharps, implantable device components, in vitro diagnostic probes, and marker bands for catheters and guidewires. More-sophisticated applications include heat-exchanger tubes for controlling blood temperature during surgery, microtools for surgical use, and stents.
By selecting various materials or processes, manufacturers can give metal tubing distinct characteristics. For example, tubing made from cobalt alloys can be used in the production of rigid or stiff endoscopes for use in certain diagnostic procedures. But a more elastic material, such as nitinol, can be used to produce an endoscope that offers a high degree of flexibility and kink resistance.
Tubing Selection Criteria
Metal tubing clearly has been an enabling technology for medical device innovation, and tubing selection can be critical in controlling production costs, improving quality, and enhancing product capabilities. A number of key factors should be considered when selecting tubing for use in medical equipment. Among these are:
- The specific material properties needed.
- The degree of precision required.
- How the material will interact with or be enhanced by any coatings that may be applied.
- Machining and finishing methods to be used.
Materials Characterization and Precision
Various nitinol compositions and grades of stainless steel are common materials of choice for medical tubing. Gold, titanium, platinum, and various alloy forms are also used. Each material offers strengths and weaknesses that should be assessed for the intended device application. For example, components for use in surgical procedures involving the brain will require a specialized material, such as a stainless-steel alloy that can be machined to precise tolerances yet remain free of minute metallic particles. A manufacturer of hypodermic needles may select a grade of stainless steel that can benefit from burr-free machining methods to significantly speed production times. Some stent makers, however, may take advantage of the shape-memory characteristics and biocompatibility of nitinol to increase product capabilities. Others will choose an alloy that can be coated more effectively than other metals.
The materials selected must offer precise levels of dimensional control and consistency. Such material properties as tensile strength, formability, and biocompatibility must be matched to the desired environment and function. As is the case with other materials used in medical manufacturing, metal tube fabrication requires monitoring and tight control of material characteristics.
Different applications can require different tubing fabrication methods—that is, welded or unwelded products. Unwelded seamless tubing has a number of drawbacks. Among these are higher costs, limitations in available sizes, wider tolerances, and longer delivery times. While tungsten inert gas welding is a fairly common method, some firms use proprietary methods that can enhance tubing characteristics. K-Tube (Poway, CA), for example, uses a laser welding technique that produces smaller heat-affected zones. The result, according to the firm, is that "the weld itself is practically homogenous and can withstand the same burst/pressure requirements as seamless."
The addition of specialty coatings can enhance the functionality of metal tubing–based devices. As is done with polymer guidewires and tubing, coatings can be applied to increase lubricity, reduce the risk of infection, or add other desired characteristics. A good example is the use of coatings to increase the therapeutic value of stents—such as heparin or other drugs to combat restenosis. Material selection needs to take into account how that metal will react to intended coatings.
Metal tubing used in medical equipment applications generally has specific finishing requirements. Any process used must ensure that a smooth surface texture is provided while allowing control of the precision of both the inside and the outside diameters of the tube.
Methods used to machine metal tubing for medical devices range from custom systems developed for specific fabrication needs to techniques adapted from other industries. Electrochemical machining (ECM), for example, is a noncontact method for shaping metal parts while reducing wear on both tools and machined parts. ECM has been used by jet engine manufacturers to produce complex parts, such as casings and compressor blades. The equipment is used to machine these components from the hardest alloys within tolerances of one-thousandth of an inch.
Among medical manufacturers, ECM has proven equal to a number of machining tasks—yielding high levels of quality and offering tools that can maintain high production levels. Uses range from electrochemical grinding for sharpening trocars to an electrochemical grinding wheel manufactured by Everite Machine Products Co. (Philadelphia) that can produce nearly 260 syringe needles per minute.
Lasers have been used to perform various machining operations in the manufacture of both polymer and metal microdevices for medical applications. Among these are drilling, skiving, stripping, and scribing. Resonetics Inc. (Nashua, NH), for example, uses laser micromachining to produce catheters for various medical procedures. Laser techniques have also been used to create precise microscale holes that have less than a 0.004 in. diameter. Such methods are used to manufacture catheters that incorporate electrical sensors, which can be used to monitor blood oxygenation in premature babies.
Steel and Nitinol Show Their Mettle
The metals and alloys available for use in medical tubing applications encompass commodity materials as well as those that are more innovative and specialized. Examples of the former include many of the stainless steels: nitinol typifies the latter. Each type has applications that are well suited to the materials' individual characteristics.
Stainless steel is commonly used in medical tubing applications—from stethoscopes to cannulae. But stainless steel can also be the metal of choice for manufacturing devices in which the material characteristics are critical. For instance, seamless vacuum-melted 316L-stainless-steel tubing produced by Superior Tube Co. (Collegeville, PA) has been used to produce devices used during brain surgery as well as coronary stents. The metal selected for such applications must be able to be formed and machined with high precision, and the finished part must be particle free.
Dennis Gudgel of Plymouth Tube Co. (Warrenville, IL) emphasizes the need for manufacturers to carefully consider the specifications of a given tubing application to avoid quality problems. He says bright annealed stainless steel is generally specified for medical applications. Gudgel adds, "There are additional finishing processes that can be specified, depending on application requirements. For an additional degree of cleanliness, the bright annealed tube can be thermocouple cleaned." For the highest degree of cleanliness and smoothness, Gudgel recommends electropolishing following cleaning. This process imparts a chromium-enriched interior surface that is advantageous.
No longer considered just a novel springy material, nitinol offers a number of distinct advantages for device designers and manufacturers. This unique metal has already proven useful in such medical applications as catheter guidewires, stents, and microsurgery tools. When properly treated, the material has been shown to have excellent corrosion resistance when compared with stainless steel, and to offer a high level of biocompatibility. For some applications, nitinol has been selected simply because its kink resistance enables manufacturers to tightly coil long metal tubes for efficient packaging.
Nitinol tubing also has been used as shaft material for complex microinstruments, according to T. W. Guerig of Zurich Research Laboratory (Zurich, Switzerland). Such a device, a 0.3-mm "grasper" cut from nitinol tubing, is used to percutaneously retrieve embolized occlusion coils (used to repair aneurysms) or even clots from the brain via an incision in the groin. "Micromachining capabilities continue to grow, and devices will become smaller and procedures less invasive," Guerig says. "These are trends that will continue to provide fuel for nitinol's continued development."
Metal tubing is a key element in the development of a range of innovative medical products. At one time, medical uses encompassed little more than syringe needles, trocars, and rigid endoscopes, but applications engineers now take advantage of the distinctive properties of such alloys as nitinol to fabricate new tools for surgeons and other caregivers. A number of critical factors, however, should be considered to ensure that materials and processes used provide adequate precision and safeguard the bottom line. Among these are material properties, the tubing's surface finish, the degree of precision needed, the coatings used, and the machining required.
Gregg Nighswonger is the senior editor of MD&DI.
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