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MATERIALS
Ted Kucklick
Co-Founder and Chief Technical Officer
Cannuflow, Inc.
Excerpted from the Summer 2007 issue and used with the
permission of Knowledge Enterprises, Inc.
Copyright © 2007 Pelican Magazines and Knowledge
Enterprises, Inc.
Plastics for Medical Devices: a Primer
At the prominent orthopaedic industry conference of the year,
the major companies display their wares, most of which are made
of finely-crafted and polished metal. When it comes to strength,
durability and availability, metal is the logical choice for many
orthopaedic surgical devices and implants.
Metals that are suitable for use in the human body are relatively
few in number: stainless steel, cobalt-chrome, titanium, and
more recently nickel-titanium (Nitinol) alloys, for example.
These are normally formed by casting, machining, grinding and
polishing, and newer methods such as metal injection molding
and rapid prototyping/direct manufacturing methods (laser sintering and electron beam melting).
Plastics are polymers. Polymers are chain molecules. “Poly”
means many and “mer” means an individual unit. Examples of
mono-mers are styrene, methyl, carbonate, etc. Poly-mer is many
units (mers) together, e.g. polyester. In chemistry there is also an
oligimer. “Oligi” means “a few” (like an olig-archy). So, an
oligimer is an arrangement of a few units, monomer, one unit,
and polymer, many units.
Every plastic exists in two basic types: thermoplastics and thermosets. Thermoplastics are materials that may be re-melted. The
polymers are formed of long, unconnected strings that can move
when heated, flow into a new shape and assume a new shape
when cooled, sort of like boiling a pot of spaghetti and freezing
it. The length of the spaghetti strand is analogous to its molecular weight, or how many monomers are linked together to form
the polymer. Shorter strands (like styrene) melt and flow easily.
High molecular weight materials are stronger and flow with more
difficulty. Thermoplastics are usually heated and molded.
Thermosets are the product of an irreversible chemical reaction.
In this process, the polymer chains cross-link like a fishnet, and
the plastic sets up—sort of like a hard-boiled egg. Once it is set
in a shape, it is permanent. Thermosets are usually mixed from
two ingredients and cast into shape. Vulcanization of rubber is a
cross-linking process that results in a thermoset.
In contrast to the relatively smaller number of metals used in
medical devices and implants, plastics come in a bewildering
array with myriad properties and strange trade names and
acronyms. New varieties are invented all the time. Plastics may
be as soft as skin or hard as steel. They can have a range of colors, alloys, fillers and modifiers. How does one begin to sort
through the available choices to find those plastics that solve the
design problem at hand? It may be helpful to screen material
candidates with the following questions.
What does it need to do?
This question has as many facets as there are plastic parts. Does it
need to be hard or soft? Does it need to stretch? Be heat-resistant? Be a good electrical insulator? Does it need to resist particular chemicals? Will it have short or long term contact with the
human body? Here are some plastics that fall into these various
material property categories:
Soft plastics and elastomers
These plastics range in hardness from about 5 Shore A (very soft,
almost gel-like) to about 65 Shore D (very rigid). Some materials
in this category are latex, nitrile, urethanes, silicones, SBRs
(styrene-butadiene rubber), TPEs (thermoplastic elastomers),
TPVs (thermoplastic vulcanizate, a thermoset) and PVC
(polyvinylchloride).
Common uses for elastomers in medical devices are extruded
flexible fluid tubing (usually PVC), gloves (latex or nitrile), soft
injection molded shapes (TPEs), cast molded shapes (silicone
rubber), extruded catheter tubing (polyurethanes), low-pressure
balloons (PVC, latex) and seals (silicone, SBRs).
Rigid commodity plastics
Some of the most common plastics used in molded medical
devices are ABS (acrylonitrile butadiene-styrene) acrylic,
polypropylene, HDPE (high density polyethylene) nylons and
polycarbonate. These materials exist in a very wide range of properties and performance specifications. Another type of rigid plastic is PET (polyethylene terphthalate), commonly used in sheet
form for thermoformed medical device trays and packaging.
Polyester is used as a film in products like angioplasty balloons.
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Plastics for Medical Devices: a Primer, continued from page 28
Engineering plastics
Beyond commodity plastics are materials with exceptional performance properties. Engineering plastics can be exceptionally
strong, tough, heat resistant, have superior bearing properties and
so on. A few of these specialty materials are PEEK (polyetheretherketone), which is nearly as hard and strong as metal, is
biocompatible and resists heat to 600°F, and Teflon (polytetrafluoroethelyne), a material that is well-known to be lubricious (slippery) and very heat resistant. Teflon resists nearly all known
chemicals. It was an early candidate for joint bearings, put to use
in acetabular cups by Charnley. However, long-term properties
such as lack of creep resistance made it unsuitable. Delrin (acetal)
is a tough plastic that can be autoclaved and is popular for use in
producing machined shapes. Polysulfone is a high-performance
plastic used to mold products like instrument trays that require
repeated steam sterilization, and is recognized by its clear amber
color. UHMWPE (ultra high molecular weight polyethylene) is
widely used in joint replacement bearings.
Engineering plastics offer high performance; however, they can
have high material prices to match. It is important to properly
specify but not over-specify a material, to keep the cost of goods
of the product as reasonable as possible.
How will it be processed?
The answer to this depends largely on how many units you want
to make. Some of the basic processes are machining, thermoforming, casting, molding or die cutting.
If the part is to be used in a medical device, starting with a USP
Class VI or ISO 10993 medical grade material is good practice.
However, this requirement will eliminate a large number of available plastics. If the part is to be machined, look for a supplier
that is knowledgeable of these requirements and has the material
in the rod, plate or sheet form that you need. Even though the
material is medical grade, you will need to run biocompatibility
tests on the finished device to mirror how it will be used in the
body.
Machining is a good method for manufacturing a smaller number of simpler parts, or one or a few more complex parts. Setup is
relatively quick and inexpensive. Cost per part can be higher relative to higher production methods like molding. Die cutting is
another low-cost method for producing parts from sheet material.
Extrusion can produce a wide variety of tubing. Medical device
extruders have become highly proficient at producing very complex geometries and constructions, such as multiple lumens,
braid reinforced tubes for pushability and torqueability, etc.
Extrusion tooling dies are relatively inexpensive. Extrusions are
typically made into their final part by secondary operations such
as cutting, punching and tipping.
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Thermoforming methods such as vacuum or pressure forming are
useful for making packaging trays or large equipment housings.
Tooling is relatively inexpensive.
Injection molding is a high-production process offering the lowest per-part costs, but also the greatest investment up-front in
tooling. High quality molds can cost from $10K-$15K up to
$25K-$50K for a more complex tool with extra slides to make
side facing features, and $75K and more for very high production tools with multiple cavities for producing many parts at a
time.
Now let’s look at some ways to avoid problems when processing
plastics.
When machining parts that might turn into molded parts, try to
use the same or as similar of a material as possible to the material
with which you will be molding. This can reduce repeated and
expensive mechanical, clinical and biocompatibility testing, or
even worse, running tests with a material that is not available in a
form for higher production methods. Always use medical grade
materials whenever possible.
Use rapid prototyping (RP). For bench testing and pre-clinicals, a
number of RP methods and materials are available with properties very close to production materials. One of these is PC-ISO,
an ISO 10993 polycarbonate material used in Stratasys’ fused
deposition modeling systems. Hapco offers their Steralloy medical grade casting materials for short-run prototype production.
Medical grade materials like these for rapid prototyping are very
recent developments.
The Society of Manufacturing Engineering (SME) sponsors the
RAPID show each year, where one can find the latest developments in rapid prototyping. The SME also has a very active
RTAM (rapid technologies and additive manufacturing) medical
special interest group. This year’s RAPID show featured speakers
on such topics as the use of rapid prototyping in surgical planning and tissue engineering. For more information, see
www.SME.org/RAPID.
What type of contact will it have with the human body?
When using a medical plastic, the amount of biocompatibility
testing has a lot to do with how much contact is expected with
the human body. Table 1 displays various device types and interactions.
As you can see, each level of contact from surface to implant has
its own level of biocompatibility testing requirements. Contact a
qualified testing lab to discuss your particular device and order
the appropriate testing. Labs that perform these services are
Pacific BioLabs (Hercules, California), Nelson Labs (Salt Lake
City, Utah) and NAMSA (Northwood, Ohio).
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Table 1. Medical Devices and Interaction with the Human Body.
Table Courtesy of Pacific BioLabs.
Some companies are serving the medical market by offering plastics with ISO 10993 documented biocompatibility for human
tissue contact of 30 days or fewer (i.e., not an implant). This
again is a recent development. Bayer, for example, offers such
materials in their Makrolon polycarbonate and Lustran ABS
medical grade plastics.
Another biocompatibility issue to consider is latex allergies, as
some people are sensitive to the proteins in natural latex rubber.
An alternative material to latex is nitrile, a synthetic version of
latex. Another is the presence of pthalates (DEHP) in PVC. You
will need to see if these issues affect your product if you are planning to use these materials.
How will the parts be put together?
Yet another consideration is how the parts will be joined into a
final device. Adhesives used in medical devices must meet the
same biocompatibility requirements as the plastics themselves,
and biocompatibility testing needs to be done on the finished
device even if the individual materials are USP Class VI or ISO
10993.
Medical device adhesives have evolved and improved significantly
in the past ten years. New versions of moisture cured cyanoacrylates (a.k.a. “Krazy Glue”), UV cured adhesives and now hybrid
light-cured cyanoacrylates as well as epoxies offer a wide range of
solutions to bond nearly any plastic to any other plastic, even
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Plastics for Medical Devices: a Primer, continued from page 31
those that have been very difficult to bond, such as polypropylene, Delrin and even Teflon. Loctite offers a comprehensive
range of medical device adhesives for plastics. Dymax makes
medical grade UV cure adhesives, and Master Bond offers a line
of medical grade epoxy adhesives.
How will it be sterilized?
The final consideration is how your product will be sterilized.
One issue is how plastics fare under different types of sterilization, especially radiation sterilization. Some plastics do well (radiation stable polycarbonate, TPEs ABS) and some don’t do well at
all (Delirin/acetal and Teflon). Delrin becomes brittle and Teflon
is badly damaged by gamma. If you require these materials in
your product, EtO may be an alternative. Radiation affects all
plastics, so check with your supplier to see if the plastic you specify is compatible with the sterilization method you plan to use.
Plastics intended to be autoclaved must of course have heat resistance to allow for this. Candidates for autoclave sterilization are
polysulfone, PEEK and acetal.
How do I find materials?
Many, many good resources are available. The first is your injection or extrusion molder, if you have one. If you are working
with a quality molder that produces parts for the medical industry, they can tell you from their experience which materials can
produce the results you want with the processes you are using.
Look for a molder that is ISO certified. A good place to find vendors that serve the medical industry is the Medical Device and
Manufacturing Show (MD&M) held around the country by
Cannon Communications. The show is a huge supermarket of
vendors of all kinds of services and components. Cannon maintains a web page, www.devicelink.com, where you can search for
vendors that serve the medical device industry.
Another source is the manufacturers and distributors of the plastics. The major plastics manufacturers, Bayer, GE, Cyco, Victrex,
GLS and so on are excellent sources of information. Many maintain a library of helpful and informative case studies that can help
guide your material selection choices and generate new product
ideas.Yet another useful source is MatWeb (www.matweb.com), a
database of all types of materials for manufacturing. It is searchable by trade name, material type and so on. Further, try The
Modern Plastics Encyclopedia Online
(www.modplas.com/worldencyclopedia/search). A number of distributors carry stock shapes of plastics. One that specializes in the
harder to find medical grades of rod and plate for machining is
Modern Plastics; also try K-Mac and Westlake Plastics. Yet
another useful website is www.uhmwpe.org, sponsored by
Ticona/Celanese, which covers in detail the popular orthopaedic
bearing material, UHMWPE.
Finally, check out the library of written materials available to
you. One key book on the subject of biomaterials is Biomaterials
Science by Buddy Ratner. Another one addressing metals is
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Handbook of Materials for Medical Devices, by J.R. Davis (ed.),
ASM International Press. A general coverage of materials used in
manufacturing is found in the Handbook of Materials for Product
Design by Charles A. Harper.
Ted Kucklick works in research and development of innovative medical devices, and is Co-founder and CTO of Cannuflow, Inc., a company dedicated to developing devices for orthopaedic surgery. Ted is
an inventor on several issued and pending patents, and is a member
of the IEEE/EMBS, IDSA, AMI and SMIT professional societies.
He is also the author and editor of The Medical Device R&D
Handbook. Contact Ted at tkucklick@kucklickdesign.com.
Cannuflow, Inc.
1190 Coleman Avenue,
Suite 250
San Jose, CA 95110
408-280-0359 (phone)
408-280-0323 (fax)
www.cannuflow.com