Medical Design Briefs - February 2024 - 56

Design Briefs
SCS Parylenes have been proven to withstand typical temperatures used in the balloon attachment process using today's most commonly used materials. (Credit: Specialty
Coating Systems)
The chart most often referred to for
the Parylenes can be seen in Figure 1
and plots the failure points as the log of
time versus temperature. Failure points
are defined as a 50 percent loss in tensile
strength, the resulting curve being based
on an Arrhenius extrapolation. Temperature
data from this chart provides
estimated thermal endurances for Parylenes
N, C, and D.
Since the balloon/catheter sealing process
is significantly shorter than 24 hours, a
more applicable method of determining
suitability was required. Thermogravimetric
analysis (TGA) allows for a more reliable
predictor of short-term thermal endurance
and has since become the
standard for evaluating thermal behavior
across many markets, and additionally informs
us of the impact of the presence of
oxygen on the material behavior. TGA
analysis captures mass as a function of temperature
under a chosen environment.
Heating rates can be tailored to the application
and are typically conducted at 1 degree
K per minute, or 10 degrees K per
minute. Analysis using the higher heating
rate was examined for the purposes of a
fast-heating application such as the thermal
fusion of materials in the catheter
forming process.
The example TGA results of Parylene N
can be used to gain insight regarding suitability,
and confirms why Parylene has become
a coating material of choice for medical
mandrels. Figures 2 and 3 illustrate the
56
Parylene N response to increasing temperatures
in air, and in nitrogen respectively.
The value known as T5% is a measurement
used in materials evaluations to
describe the onset of thermal decomposition
and is defined as the temperature corresponding
to a mass loss of 5 percent.
While not specifically considered a failure
point, it provides an indication of the onset
of thermal degradation.
When the environment is air (Figure 2),
the T5% value for Parylene N is 327 °C. In
a nitrogen environment (substantially oxygen
free) the thermal stability of Parylene
N improves further. This improvement
can be seen in Figure 3, which illustrates a
T5% value of 480 °C.
Looking only at the short-term temperature
capabilities of Parylene N shows
indications that it can easily endure temperatures
well above processing temperatures
for PEBAX, polyurethane, and PET
as well as other newer materials being developed.
In a typical catheter manufacturing
process, the configuration of the
equipment and the materials to be fused
provides some degree of confinement
that limits the exposure to air/oxygen
during the process. This results in an environment
simulated by a point somewhere
between the TGA tests in air and in
nitrogen, placing the T5% value between
327° and 480 °C. This provides further
assurance of the thermal capabilities of
Parylene N in the catheter manufacturing
process, specifically as a coating on
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the mandrel wires which must be easily
removed after the sealing process has
been completed.
With Parylene's successful track record
and well characterized thermal behavior,
those manufacturers currently
utilizing PTFE-coated
mandrels may
choose to avoid potential PFAS concerns
in the future by turning to Parylene.
As a vacuum-deposited longchain
polymerized
coating,
Parylene
can provide the critical release capabilities
in an ultra- thin coating. Its conformal
nature facilitates the tight dimensional
tolerances that are often required.
Because it is not a liquid coating, it does
not require curing, is not constrained by
liquid physics, and is not subject to
shrinkage and/or shedding. Parylene
can also uniformly coat complex custom
wire shapes and configurations that are
becoming more common in today's advanced
catheter technologies.
References
1. " Per- and polyfluoroalkyl substances (PFAS), "
ECHA European Chemicals Agency, https://
echa.europa.eu/hot-topics/perfluoroalkylchemicals-pfas
2.
Kirk-Othmer Encyclopedia of Chemical Technology,
John Wiley & Sons, Inc., Jan. 26, 2001.
This article was written by Carla
Gillespie, Director of Marketing Communications,
Specialty Coating Systems.
For more information, contact
cgillespie@scscoatings.com or visit
www.scscoatings.com.
Medical Design Briefs, February 2024
https://echa.europa.eu/hot-topics/perfluoroalkyl-chemicaleu/hot-topics/perfluoroalkyl-chemicals-pfas http://www.scscoatings.com http://www.medicaldesignbriefs.com

Medical Design Briefs - February 2024

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