Finding Tube Materials that Make the Grade

Originally Published MDDI January 2005

William Leventon

January 1, 2005

14 Min Read
Finding Tube Materials that Make the Grade

Medical tubing comes in a daunting array of materials. These materials have unpronounceable names and are often indistinguishable to the untrained eye. Many have been used for years in the medical device market, but there are many new compounds with varying characteristics. All of these materials exhibit different properties, different strengths, and different shortcomings. They also have different prices.

How do medical device manufacturers sift through the multitude of material choices and find the one that's right for them? In this article, experts from eight firms that specialize in tubing present an evaluation of common tube materials. These experts discuss new material-related developments. They also offer advice on material selection and explain how firms like theirs can make the process easier.

Popular PVC

In the medical industry, the process of selecting a tubing material often begins and ends with polyvinyl chloride (PVC), according to Richard Brooks, vice president of sales and marketing for Bunzl Extrusion Massachusetts (Northborough, MA). Brooks cites several reasons for the primacy of PVC in medical tubing. For one thing, it's one of the least expensive tubing materials on the market. In addition, he says, PVC grades run from flexible to firm, so it can be used to make tubes ranging from rubbery to rigid.

Then there's bonding, which can be troublesome with some tubing materials. But not with PVC, which “bonds like nothing else,” Brooks notes. “You have to bond a tube to a connector, and PVC can bond with just about every plastic molded component out there. That's a tremendous benefit.”

PVC is popular for other reasons as well. Brooks praises its clarity, which lets people see the fluid or gas that is going through the tube. In addition, he says, the material holds up well to chemical exposure and a wide range of temperatures and sterilization methods.

PVC's qualities are on display in various types of medical equipment. Brooks says the material is especially well suited for tubes in high-volume disposable medical devices. For example, the tubing in an intravenous fluid administration set “has to be cheap and as clear as possible so you can see the drug going through it,” he says. “PVC is just the thing for that application.”

PVC is also a good choice for tubing that's part of fluid suction and irrigation systems used during surgery, says Joe Datka, medical process engineer for Teel Plastics Inc., a tubing manufacturer in Baraboo, WI. According to Datka, these systems might consist of a small handheld device hooked up to 20 feet or more of inexpensive PVC tubing.

But PVC isn't suitable for everything. For example, PVC is not a good fit for high-pressure applications. “PVC balloons tend to be soft,” notes Mike Badera, president of Precision Extrusion Inc. (Glens Falls, NY). “As you add pressure, they just keep expanding until they burst.”

Another drawback to PVC is its tackiness, which can slow down assembly operations. However, some manufacturers have found ways to amend such disadvantages. Bunzl can apply a frost finish to the outside of PVC tubing that makes it smooth to the touch. The finish reduces the clarity of the tubing, but users can still see through it, Brooks says.

Perhaps the most serious concerns about PVC relate to some of the plasticizers used to soften it. For example, Brooks points to worries that phthalate plasticizers, which have long been suspected carcinogens, may leach out of the material. But he maintains that no human deaths, injuries, or illnesses have ever been directly linked to the leaching or migration of plasticizers from PVC.

The Rest of the Field

Device manufacturers reluctant to use PVC have many other options. At Bunzl, Brooks believes that polyethylene is the second-most popular choice for medical tubing. Like PVC, he says, polyethylene is chemical resistant, but it weighs about one-third less than PVC. In addition, Badera notes, the material is relatively inexpensive and easy to mold, so people often put molded fittings on polyethylene tubes.

According to Brooks, polyethylene tubes are very strong packaging vessels. The material also features low friction, so catheters “glide” out of tubular polyethylene packaging, adds Jenny Hovde, Teel's medical business development manger.

Polyethylene “is just like wax,” says Brooks, whose company makes high-density polyethylene (HDPE) tubing for guidewire dispenser coils. Guidewires slide easily through lubricious HDPE tubes, notes Duane Dunn, president of Dunn Industries Inc. The Manchester, NH–based company makes thermoplastic tubing for the medical industry.

On the downside, Brooks says, polyethylene isn't as flexible as PVC and doesn't solvent-bond to many plastics. It also looks frosted rather than clear, he adds, making it difficult to see fluid flowing through polyethylene tubes.

Alternatively, polyurethanes are clear like PVC and just as flexible, Brooks says. What's more, he adds, they're more durable and heat resistant than PVC. And they contain no problematic plasticizers, notes Geary Havran, president of NDH Medical Inc. (St. Petersburg, FL), which extrudes custom tubing.

Havran points to a number of published studies that attest to the safety and biocompatibility of polyurethane. And Datka explains, “It's one of the few materials that'll pass the biological testing required for an implantable device.”

Polyurethane also delivers the strength needed for thin-wall and high-pressure tubing applications. “And it won't kink—even if you tie it into a pretzel,” Brooks says.

But all these advantages don't come cheap. Polyurethane costs about six times as much as flexible PVC, according to Brooks.

Another drawback is its tackiness, Dunn notes. For tube users looking for a less-sticky alternative that offers similar physical properties, the right choice could be nylon. A very tough material, nylon provides high tensile strength and good kink resistance in thin tube walls, Badera says.

Nylon molecules can be oriented to maximize burst strength and other physical properties, he explains. This makes the material a popular choice for balloons for angioplasty products.

However, nylons aren't as flexible as the urethanes, Dunn notes. Neither do they bond as well. Nylons often require some kind of surface treatment to facilitate bonding, while “urethanes can bond with just about anything,” Badera says.

Like urethanes, nylons are far more expensive than PVC. But price is of secondary importance to some makers of angioplasty and angiography catheters. “They want thinner walls, kink resistance, and physical properties they can't get out of PVC,” Badera says. “So they're willing to pay 10 times as much for the resin.”
Another high-performance option is the group of materials known as fluoropolymers.

Although difficult to process and extrude, fluoropolymers offer a low friction coefficient and high dielectric and tensile strength, says Bob Jennings, vice president of medical sales and marketing for Zeus Inc. Based in Orangeburg, SC, Zeus manufactures tubing made of fluoropolymers and other materials. According to Jennings, fluoropolymers are also lubricious, resistant to chemicals, and completely inert. These properties make them a good choice for numerous catheter-based applications, he says.

Rubber Pros and Cons

Drain tubes have long been made of flexible and inexpensive natural rubber. But concerns about allergic reactions have driven medical device companies away from natural rubber and natural rubber latex, notes Nancy Hunter, product manager at The Hygenic Corp. (Akron, OH), which makes rubber tubing.

According to Hunter, many medical companies have switched to synthetic materials that mimic natural rubber. But the synthetic products are more expensive than natural rubber and have inferior physical properties. So some medical device companies offer two similar products. One is made with natural rubber or natural rubber latex tubing, and a more expensive version is made with synthetic rubber tubing for customers concerned about latex allergies.

The synthetic rubber category includes thermoplastic rubbers (TPRs), which combine vulcanized rubber properties with the processing advantages of conventional thermoplastics. Bunzl makes tubes out of a TPR called Kraton G [manufactured by Kraton Polymers (Houston, TX) and compounded by GLS Corp.(McHenry, IL)], which Brooks calls “the best latex-free alternative today.” Processing difficulties for Kraton exist, and the tube's appearance can be somewhat foggy. Still, Kraton offers the silkiness, flexibility, and stretchiness of latex, but costs much less than latex, according to Brooks.

Kraton is used in chest drainage systems, which include large tubes that used to be made of latex. But since the tubes come in contact with patients, allergy concerns spurred the switch to the nonlatex alternative, Brooks explains.

A more common rubber tubing alternative is silicone, which can handle exposure to high heat and corrosive body chemicals. It also offers “unsurpassed flexibility,” Brooks says.

The main reason medical firms choose silicone is biocompatibility, according to Charles Heide, market development manager for Vesta Inc. (Franklin, WI), which extrudes silicone rubber tubing. Medical device OEMs use Vesta's silicone tubing for catheter and drainage products.

But they pay a high price for it. Implant grades of the material run upwards of $150 a pound, notes Bill Woinowski, Vesta's research and development manager. Bonding for the material is also difficult, he adds, though a number of new products have been introduced for better bonding to nonsilicone substrates.

Other new types of silicone are aimed at peristaltic pump tubing. Developed by Dow Corning, C6 elastomers are designed to offer good resilience, low compression set, and a relatively high modulus of elasticity. “Customers have told us that the materials worked out very well for them,” Woinowski says.

New Developments

One of the most significant new developments in the tubing industry is the push for alternatives to conventional PVC. The demand to solve disposal problems and address concerns about the material's phthalate plasticizers is high. Teel is working with medical device companies on PVC substitutes that can be burned or recycled. Other alternatives will degrade in a landfill. Though the details are confidential, some of these materials are PVC formulations that lack the material's problematic ingredients. Others are entirely different materials with only the PVC properties needed for a particular application, Datka reports.

In recent years, Bunzl has gotten many requests for PVC without diethylhexyl phthalate (DEHP), the material's least expensive and most common phthalate plasticizer. According to Brooks, PVC can be combined with nonphthalate plasticizers to produce materials with capabilities and properties equal to those of conventional PVC. But tubing made with these materials can cost up to 25% more than tubing made with normal PVC. In addition, he says, the substitutes don't extrude as well as normal PVC. As a result, the tubing may be less clear or more blemished than conventional PVC tubing.

So Bunzl now offers customers another option: PEX-PF, a proprietary polyolefin-based thermoplastic elastomer that doesn't include plasticizers (PF stands for plasticizer free). PEX-PF “has all the properties and characteristics of PVC. But it's not PVC,” Brooks says.

The bad news: it also costs five times more than PVC. But this may not be a problem when it's used to make tubes that are very small and thin. In these cases, Brooks points out that the cost of an expensive material may only amount to 15% of the total cost of a tube. But as tubes get bigger and walls get thicker, he adds, the cost of expensive materials can rise to as much as 50% of the total tube cost, making PEX-PF an impractical option.

Whatever the tubing material in question, there's a good chance it has become more versatile in recent years. Thanks to new technologies and additives, “a lot of materials can do things they couldn't do five years ago,” Dunn says.

Great versatility can be seen in the nylon and polyurethane families, which include versions ranging from soft to rigid. “So, if you like urethanes, you can get the feel you want in urethanes. Or if you like nylons, you can get the feel you want in nylons,” Badera says.

Harder Cases

Despite the wide variety of material choices and the many options within individual material categories, some OEMs still have trouble finding what they want. At NDH Medical, Havran constantly hears from customers who want to free up more space in device designs by reducing tube wall thickness. “To get thinner walls, people are looking for materials with better physical properties,” he says. A search through a polymer database may turn up many materials with the properties needed in an application. “But if you add the requirement that it be an FDA- or USP-approved material, that eliminates a large percentage of the available choices,” he says.

Why? “The problem could be that polymer manufacturers don't want to take the risk of product liability in medical applications. So they won't make the material available for medical applications,” Havran says. “We see that with a number of grades of polyethylene and polypropylene. One would think the material could pass the biocompatibility requirements for a particular device application, but the manufacturer forbids the sale of the material for medical applications.”

So what do you do if you can't find what you're looking for? In some cases, the answer might be to change the molecules of a material. Zeus has developed such techniques to improve the inherent characteristics of nylon. These techniques make changes at the molecular level to significantly enhance properties such as lubricity and tensile strength, according to Jennings. Nylon enhanced in this manner is often used in catheter applications.

Why modify molecules rather than use different material grades or formulations? Some customers want changes that are subtler than those provided by a completely different grade of material, Jennings explains. Molecular enhancement techniques are also useful in cases when customers want changes to additive-free homopolymers. “You can make a molecular adjustment to make the plastic behave a little differently, without it becoming a mishmash of different materials,” he says.

Another possibility for people who can't find the right tubing material is to combine two or more materials in a process called coextrusion. For example, Datka says, a customer might request a multilayer, coextruded tube that consists of a chemical-resistant material surrounded by layers of softer material that will prevent the inside layer from cracking. At Teel, coextrusion processes can produce tubes made up of as many as seven different layers.

According to Havran, the general rule of thumb in coextrusion is to make the different layers out of similar polymers in order to facilitate bonding. In some cases, though, “customers want the different layers to be so different in properties that you end up making them out of very dissimilar materials,” he says. So a coextruded tube might include layers of adhesive to join materials that won't bond together, Hovde says.

Other Considerations

Besides the properties and characteristics of the various options, there are other considerations when choosing a tubing material. These include:

• Coatings. What, if any, coatings are available to improve material characteristics such as lubricity and biocompatibility? Is there a certain type of coating you want to put on the tube? And if so, can that coating be used with the materials you're considering? The answers to these questions may help narrow the field of material options, Havran says.
• Sterilization. Consider the sterilization method that will be used on the tubing. Gamma sterilization can turn some materials brittle or yellow, while autoclave sterilization is too hot for some types of disposable tubing materials, Datka notes.

When asked (and many OEMs don't ask), tubing manufacturers can provide information that may help steer customers to the right material. Sometimes Badera directs them to Web sites or material manufacturers that can answer their technical questions. He can also provide them with samples of tubing that are close to what they're after. Precision Extrusion keeps more than 2000 samples of different tubing products made by the company over the years.

For customers who want more guidance, tubing manufacturers can suggest a suitable material when informed of the product's characteristics and requirements. They can also recommend alternative materials that offer important advantages over the customer's initial choice. “A lot of people are price sensitive,” Dunn says. “So if there are extra labor costs involved in working with a particular material, you can suggest another material that may not be exactly what they want, but makes assembly easier and saves them on labor costs.”

In Jennings' opinion, the key to selecting the best material for a tubing application is for design engineers to work closely with the tube manufacturer in the early stages of the project. Hunter agrees. “A lot of times, medical device manufacturers just want samples and don't necessarily want our input,” she says. “But if they would work with our R&D group from the get-go, we would save them a lot of time. Our people have a lot of knowledge about whether or not an application would work with different materials.”

William Leventon is a frequent contributor to MD&DI. He is based in Somers Point, NJ.

Copyright ©2005 Medical Device & Diagnostic Industry

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