as well as the flexibility needed for ideal
performance.
Why Increase the Number of Layers?
Most innovation is market driven and
this is no exception. As medical device
companies developed new products,
increased their performance expectations
(barrier, UV, bonding), and/or wanted to
pursue cost cutting alternatives, it became
obvious that traditional monolayers and
some two- and three-layer structures
would not meet their objectives.
As an example, let’s look at drugs
delivered at higher pressures, such as contrast media. Traditional three-layer tubes
can adequately handle up to 1,200 pounds
per square inch (psi). Drugs requiring
pressures in excess of that (such as 1,800
psi), typically require a braided tube,
which is significantly more expensive.
Medical device companies challenged
manufacturers to come up with a solution
that could not only prevent burst from elevated pressure, but also could potentially
drive out cost by eliminating the braid
in the tube’s construction. The way to
address that was to develop coextrusions
with even more layers.
Technological Advances
Before these four- and five-layer struc-
tures could be commercially viable,
work needed to be done on the extruder
crossheads so that they were capable of
producing concentric layers for more
than three layers. This requires precise
pressure distribution across the extruder
platen to deliver symmetrical tube layers.
Additionally, the material selected for
the various layers needed to be compatible so that the fluid path is functional, safe
and able to deliver appropriate shelf life.
Further, the layers could not delaminate
during sterilization.
The specific materials used in these
four- and five-layer structures are dependent on the application. However, here’s
an example of what a five-layer material
sandwich could be made from. PE inner
layer for inert fluid path delivery/ethylene
vinyl acetate (adhesive)/copolyester for
barrier and burst strength)/EVA/polyvi-nyl chloride (outer).
A colorant could be added to the
middle copolyester layer, as well as a
color-coded stripe in the outer layer.
Another structure type could include
nylon for additional kink resistance and
polyurethane for flexibility.
Applications that Could Benefit from
More Coex Layers
In addition to medications requiring
higher psi for delivery, there are several
other drug categories that could benefit
from the new technologies. These could
include highly corrosive drugs such as
DMSO, which need additional protection
in the fluid path.
Economics
As in every business decision, purchasing
decisions for tubing are also driven by
economics. In considering these high-er-layer coextrusions, the objective was to
make them more cost-effective than their
predecessors by eliminating or minimizing higher price per pound polymers and
substituting multiple, less expensive layers
that would still deliver desired performance.
Let’s use a typical monolayer polyurethane tube, with an inner diameter
of .125 and an outer diameter of .188,
as an example. With polyurethane at
approximately $10 per pound, you would
typically expect to pay around .106 cents
per foot for that structure.
Now let’s compare the same tube using
a multilayer approach. Polyurethane
could be used for the inner and outer wall
(.006 each), plus two or three additional
materials (costing between $2 and $4 dollars per pound) to make up the rest of the
wall thickness (which in total is .0315).
The cost per foot of the multilayer
structure is reduced to .055, resulting in
savings of more than 50 percent, with no
sacrifice in performance.
The specific composition of the .0315
tube wall could potentially be:
• .006 polyurethane inner most layer
• .010 EVA (for adhesion)
• .0035 copolyester (for strength)
• .006 Nylon (barrier)
• .006 polyurethane outer most layer
Other materials that could be considered for the multilayer structure include
polyvinyl chloride and acrylate resins.
Conclusion
For medical device manufacturers looking
for ways to retain or even expand performance capabilities while driving costs
out of their structures, these four- and
five layer structures should be explored.
This approach is particularly valuable for
those currently using expensive monolayer tubing materials and even some
two- and three-layer alternatives. It is
also of interest for companies looking for
alternatives to expensive braided tubes
for higher-pressure delivery systems. MDT
Tubing is examined for clarity, concentricity and cohesion.
