Unique
Properties Make Polymers Suitable to Medical Tubing
Applications
Mr. Dan Lazas,
Senior Director,
Sales Optimization Medical Components, Tekni-Plex |
Extruded polymer tubes are used
prolifically throughout the medical device industry
from intravenous (IV) lines to neurovascular
catheters. Advancements in polymers, additives and
processing technologies have resulted in medical
tubing precisely tailored to specific applications.
This includes functional attributes spanning
mechanical, thermal, electrical and chemical
performance.
Often, polymers and processing
technologies selected for devices are based on
successful application in similar devices. This is
rational logic for a device designer intent on
reducing time-to-market by avoiding reinvention. The
regulatory filing process also encourages use of
proven technology. For example, the intent of a United
States Food & Drug Administration 510(k) regulatory
filing is to prove a device is as safe and effective
as a device that is currently marketed, known as a
predicate
device. |
Unfortunately, the repeated application of polymer
technologies based on predicate devices can desensitize
engineers to the extraordinary benefits of these materials
for healthcare applications. Revisiting unique polymer
attributes that precisely benefit certain medical devices
can lead to innovative applications in future devices.
Some of these unique material benefits stem from
properties that are not generally desirable in broad
context.
Blood Management
Polyvinyl chloride (PVC) is a material of choice for
tubing used throughout the medical device industry.
Applications include blood management, cardiovascular
pumps, dialysis, and respiratory, suction and drainage
tubes. In many instances PVC has revolutionized patient
care, reduced infections and improved safety. For example,
a blood transfusion system in the middle of the 20th
century was a reusable device, which was large, heavy, and
diffult to sterilize. Modern transfusion sets,
substantially comprised of PVC tubing, are single-use
devices that are economical, light weight and easy to use.
Nevertheless, PVC has become controversial in recent
years. One reason is a common plasticizer, di-ehtylhexyl
phthalate (DEHP), used to convert the inherently rigid
polymer into a flexible product suitable for tubes, bags
and films. The FDA issued an advisory risk in 2002
pertaining to reproductive and developmental concerns
associated with patient exposure to high levels of DEHP.
While there are other plasticizers available for PVC, DEHP
is still commonly used for blood management applications.
In addition to the economic benefits of using DEHP,
studies have shown the plasticizer to be advantageous in
preserving blood cells. For example, a 1998 study1 found
that DEHP inhibits the deterioration of red blood cells
during storage in PVC containers that use this
plasticizer. Hemolysis and microvesicle formation were
also shown to decrease with the presence of DEHP in the
same study.
Therapeutic Delivery
Due to versatility in properties, thermoplastic
polyurethanes (TPUs) are used across a wide range of
medical devices, including wound care, vascular access
devices and IV therapy applications. TPUs are a class of
copolymers, created from three chemical components: polyol,
chain extender and diisocyanate. The elastomeric segment
of the polymer chain is created from polyol and isocyanate,
whereas the strength segment is created from the chain
extender and isocyanate. Polyester, polyether and
polycarbonate polyols can be used in the formulation of
TPUs. Likewise, aromatic or aliphatic isocyanates can be
used. Variation in the type and amount of each component
can change elasticity and a variety of other material
properties.
A particularly unique characteristic of TPUs is their
ability to soften with relatively small changes in
temperature. 2 As such, these materials tend to soften
when placed into the body. The degree of softening is
dependent on the initial hardness and the chemical make-up
of the material. Generally, low durometer polyurethanes
soften up to 50% when placed in 98.6o F (37o C) water.
Hard polyurethanes generally soften to a greater extent.
For central catheters, this property is particularly
useful. Peripherally inserted central catheters (i.e. PICC
lines) are indwelling devices that can remain in a
patients arm vein for weeks or longer to allow for
frequent drug delivery. The softening characteristics of
TPU are ideally suited for maximizing comfort in patients
receiving treatment with central catheters.
Cardiac Intervention
The ability to tailor TPU properties from flexible to
rigid using variations in chemical constituents and
amounts would be seemingly ideal for the flexible catheter
shafts used in percutaneous transluminal coronary
angioplasty (PTCA) procedures. Commonly 2 mm (0.079
inches) in diameter and 100 cm (39 inches) long, these
catheters require a perfect balance between strength and
flexibility to be inserted in the upper leg (i.e. femoral
artery) and reach the aortic sinus. However, changing
properties due to material softening, as is characteristic
of TPUs, would be a disadvantage in these devices.
Like TPUs, polyether block amides (PEBAs) are
thermoplastic elastomers created through copolymerization
of soft and hard block molecular chains. Variation in the
percentage of these components results in physical
properties that range from highly flexible to moderately
rigid. For PEBAs the hard block is a polyamide, and the
resulting polymer is resistant to softening when exposed
to human body temperature and fluids. As such, it retains
physical performance characteristics throughout the
procedure. Also, these copolymers are slightly more rigid
than TPUs yet more flexible than nylon 11 or 12. This
allows for better “pushability” of these catheters by
clinicians while retaining flexibility for vascular
navigation.
Since all PEBAs contain common polyamide constituents,
they can be thermally bonded to each other regardless of
flexibility characteristics. This is particularly useful
in the development of cardiovascular guide catheters that
can require more rigidity at the proximal end, where the
device is controlled by the physician, and more
flexibility at the distal end, for atraumatic navigation
of vascular pathways. Shafts of this nature can be created
by thermally bonding multiple segments of PEBA tubes, with
variation in flexibilities.
Data Sheets : Only Part of the Story
Polymer data sheets contain a host of
material properties that allow medical device engineers
and designers to compare properties of the polymers. For
medical polymers used in device tubing applications, these
properties include density (i.e. specific gravity),
strength, elongation, modulus (i.e. rigidity), hardness
(i.e. durometer) and much more.
However, these properties only begin to
tell the complete story of these polymers. Often, the
distinct attributes of polymers that are not found on data
sheets is the determining factor for why specific medical
polymers are consistently used in distinct medical tubing
applications.
AuBuchon, JP et. el. The Effect of
Plasticizer Di-2-Ethylhexyl Phthalate on Survival of
Stored RBCs. Blood, Vol 71, No 2 (February), 1988: pp
448-452.
Walder, A. Kulkarni, P. Thermoplastic
Polyurethanes as Medical Grade Thermoplastic Elastomer.
Lubrizol Advanced Materials, Wilmington, MA..
|