Designs are calling for ever smaller and more capable medical devices, and many are made to last only one operation. Smaller tubes accommodate the trend to less invasive procedures because less traumatic procedures shorten patient recovery times. These trends occasionally make it difficult to use readily available stock tubing. That's OK, because there are companies that specialize in making out-of-the-ordinary tubes for medical equipment.

Take Microspec Inc., for example. The Peterborough, N.H.-based firm (microspecorporation.com) has no catalog of standard products. Instead, the company has carved out a niche by making only custom designs. Such tubing manufacturers are ready to accommodate iterative design methods because, “new product designers often go through several trials before finding what works best for their device,” says Microspec President Diane Fukuda.

Tube features

What often works best is a multilumen tube, one with passages for several tasks. “Lumens often carry fluids, wires for steering a catheter, and devices to grasp tissue for a biopsy. And doctors want to monitor their progress during a surgical procedure, so there could also be a fiber optic line in another lumen,” says Fukuda. So the tube has become a delivery system.

To design a delivery system, manufacturers should start with a drawing that details a cross-section profile, diameters, wall thickness, dimensions, tolerances, and a raw material. Microspec founder Tim Steele makes several design recommendations. For example:

Pay attention to tolerances

“The challenge is to make them tight enough for a reliable design in terms of how the product will be used, yet loose enough to facilitate assembly and avoid unneeded features and costs,” he says.

Select an appropriate material

Many, such as urethanes, thermoplastics, nylon blends, extrude well. Occasionally, a selected material may not be an extrusion grade because of certain needed performance characteristics. In those cases, extrusion may be a problem, so expect to pay more for it. (The accompanying box, A brief guide to tubing materials, offers some guidance for selecting an extrusion-grade plastic.)

Tubing need not be round, nor do its lumens. In addition, the tube need not be restricted to one O.D. For instance, bump or taper extrusions provide a transition from one diameter to another and they work for single and multilumen tubing.

Microextrusions are a growing trend for medical equipment. “The smallest we have ever extruded is about 0.0003-in. inner wall thickness, and this is not a limit,” says Fukuda. “It is simply the smallest that has been asked for.”

Longitudinal stripes on tubing is another possible feature. “They can be useful inside and outside the body. Radiopaque stripes show up on X-rays. When needed, designers call for long stripes, not rings, along the tube. They are usually spaced in a long segment alternating with a clear section. The clear sections are sort of windows that let healthcare staff more easily see fluids in the tube. Such a tube has the benefits of portions that are opaque and clear,” she says. One use is on an IV tube in which you want to see that blood or fluid is flowing through it. Also, if a section of tube was ever severed and came loose in the body, you could locate it by its stripes with an X-ray.

Multilayered tubing, another capability, combines two or three materials in one tube. “For example, say a design calls for a radiopaque tube with stripes, but glassy smooth surfaces on the inside and out. The inner tube would have the smooth I.D. with the radiopaque striping on its O.D. while an extruded outer layer provides a smooth O.D. A third or middle layer might be added for strength.

Although many are round on the outside they need not be so on the inside. “Tubes can have oval ODs or square on the outside and equally irregular on the inside,” says Fukuda.

Secondary operations

“Although simulating a tube's function on a computer is a design task, there are certain physical-property values we can supply to assist,” says Fukuda. “For instance, there is a relationship between burst pressure and tensile strength. If we test for tensile strength and report that figure, designers can correlate the two properties to find burst pressure.”

Tipping is a common secondary operation, often performed by a third party. “We supply the tubing with an unfinished end and they would put the needed shape or tip on it,” says Fukuda. Tips could be for example, cross section reductions over most distances.

Printed information is also useful on tubes. Despite plastics' natural lubricity, they can carry useful information such as size, measurement numbers, 360° marker bands, calibration marks, images, even company logos. “We can print on tubes as small as two French, about 0.26-in. OD and smaller, if printed only on one side,” says John Chapman, president of Isoprint Inc., Portland, Ore. (isoprint.com). “Pad printing works well on small curved surfaces and screen printing on larger ones. We etch the image into a metal plate and mount that to a silicon pad to transfer images. The low density silicon pads conform to part curves, into shallow recesses and grooves, and around corners. The same process is used on golf balls to place an image into its dimples,” he says.

Most tubing materials can carry ink. “The only material difficult to print on is Teflon. It's made to resist chemical adhesion and it works well. A few materials need a plasma pretreatment. Plasma gas removes unwanted ions and radicals from the surface and lets ink bond. Plasma changes the surface of the material without changing the material. We use a variety of different inks depending on material surface. And most any color is available,” says Chapman.

Two methods can print 360° bands on tubes, and tube-size decides the method. One prints 180° at a time. The other prints the tube completely in one hit by rolling it to transfer ink the full circumference.

Chapman says most inks are not radiopaque so be sure to specify it when needed. For printing jobs, he says he'd like a drawing with engineering specs, dimensions, and colors.

Coatings, another post production operation, can add biocompatibility, lubricity, or some biologic function to a tube. Although a detailed list of coatings is beyond the scope of this article, one example may be useful. Hydak coatings, from Biocoat Inc., Fort Washington, Pa, (biocoat.com). are based on the biopolymer hyaluronan, which was originally developed to give devices lubricious, durable, biocompatible properties.

A brief guide to tubing materials

Engineers at tubing manufacturer Zeus Inc., Orangeburg, S.C. (zeusinc.com) say this short list of extrudable materials gives an idea of where to start a design.

Fluoropolymers
PTFE: polytetrafluoroethylene	Lowest coefficient of friction A wide working temperature range Maximum working temperature of 500F
FEP: fluorinatedethylenepropylene	More transparent than PTFE Lower gas and vapor permeability than PTFE Maximum working temperature of 400F
PFA: perfluoroalkoxy	Higher continuous service temperature than FEP Melt processable, so it can be processed in longer continuous lengths than PTFE Maximum working temperature of 500F
PVDF: Polyvinylidene Flouride	Excellent chemical resistance High level of purity Maximum working temperature of 235F
THV: Tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride	The most flexible fluoropolymer Highest optical clarity
ETFE: ethylenetetrafluoroethylene	Excellent impact resistance Good resistance to stress cracking
ECTFE: chloro trifluoro ethylene / ethylene copolymer	Ultra-low permeability Superior durability
Other Polymers
PET: polyethylene terepthalate polyester	UV resistant Extremely strong
PEEK: polyetheretherketone	Extremely high tensile strength Maximum working temperature of 482F
Nylon, Nylon 6/6, Nylon 11, Nylon 12, Pebax	Good chemical resistance Wide range of flexibility
HDPE, MDPE, LDPE: polyethylene	Inherent lubricity Excellent chemical resistance

Medical plastics to hit 3.3 million pounds by 2011

The U.S. medical plastics market reached almost 2.7 billion pounds recently and will grow annually at 4.4%, consuming over 3.3 billion pounds by 2011, according to BCC Research Inc., Westley, Mass, (bccresearch.com) in a 2006 report. What's more, commodity thermoplastics dominate the market with a little over 50% of total volume. This sector of the market should consume 1.7 billion pounds by the end of 2011, a growth rate of 4.2%.

Thermoplastic elastomers have the highest growth rate through the forecast period. In 2006, 137 million lb were consumed. By the end of 2011, consumption will reach 181 million lb, a 5.7% growth rate. The report includes an initial treatment of the U.S. medical device industry, with an overview of the current market and definitions, forecasts for the U.S. market through 2011, along with recent and upcoming legislation, regulatory issues, and concerns for the plastics in medical devices.

Platinum cures silicone without odor

Curing silicone hoses with platinum provides a cleaner and odor-free product than other curing agents provide. Developer Parker Page, Ft. Worth, Texas, (parker.com) says its platinum-cured silicone hoses come in seamless, flexible, convoluted, or smoothbore versions with an additional extruded silicone layer over the hose. The top layer protects braided hose from contaminants and ensures safer handling because it constrains braid fray and provides insulation to protect fluids from heat. Also, the firm says the hoses are autoclavable and seamless silicone makes for easy cleaning.

Add features by taking away material

Laser machining is another way to give medical tubing special characteristics. For instance, MicroLumen Inc., Tampa, Fla, (microlumen.com) uses lasers to modify medical tubing for specific tasks. Precision holes can deliver drugs or a reduced OD can improve a bond.

A few services provided by the company include the precise removal of polyimide tubing layers to expose braid or coil wires. Selective OD ablation provides variable flexibility within a given length, or selectively removing layers can aid in secondary processes and assembly.

Reducing an OD surface produces a step-down effect, such as, multiple outside diameters on one shaft. Custom drilled holes, slots, and shapes allow for precise liquid or gas delivery.