Originally Published MDDI January
2005
Cover Story
Finding Tube Materials that Make the Grade
Top tubing makers discuss material options, new developments, and how to make
the right choice for your application.
William Leventon
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| Materials
courtesy of Colorite Polymers, a Tekni-Plex Co. (Ridgefield, NJ); Eastman Chemical Co. (Kingsport, TN); Putnam Plastics Corp. (Dayville, CT); Natvar, a Tekni-Plex Co. (City of Industry, CA); and B. Braun OEM/Industrial Division (Bethlehem, PA). |
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.”
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| Different materials present various opportunities for customization. From left to right, shown are etch, tip, drilled, and drawdown tubes from Zeus Inc. |
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
 |
| Heat-shrinkable
tubing can be molded onto various sizes as demonstrated on this endo clamp. Photo courtesy of Zeus Inc. |
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.”
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| Vesta Inc. makes silicone rubber tubing for draining applications. |
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
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| The Hygenic Corp. offers an array of both rubber and nonrubber tubing to meet market demand. |
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.
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| Vesta
chooses silicone rubber for its versatility and biocompatibility. |
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
