Medical Design Briefs - February 2024 - 55

Mass vs. Temperature in Air
106
105
104
103
102
10
1
10-1
10 yr
1 yr
D
CN
100
110
120
1 month
1 week
1 day
200
150
100
Temperature °C
Fig. 1 - Useful lifetime of Parylenes N, C, and D as a
function of temperature in air. Failure = 50% loss in
tensile strength.
process is completed, the mandrel is then
withdrawn from the assembly.
These forming-mandrels are typically
fashioned from medical-grade stainless
steel, with highly specialized varieties
made from other exotic alloys including
the shape memory alloy called Nitinol (an
alloy of nickel and titanium). Mandrel
lengths can range from a few inches to
120 in. After the completion of the bonding
process, the mandrel wire must be
withdrawn from the assembly without
damaging the new bond, meaning the
mandrel must not stick to the newly
formed bond. A bare mandrel wire will be
difficult to remove and will damage the
bonds. To combat this issue, the removal
process is facilitated by the use of a permanent
release-coating on the mandrel,
which is generally one of two coatings, Parylene
or polytetrafluoroethylene (PTFE).
Both coating materials have similar
cost and performance and have been
successfully used on catheter forming
mandrels for decades. Concerns, however,
are growing regarding the use of fluorinated
polymers for both direct and indirect
human exposure, with government
regulators taking action to limit and
eliminate their use in various industries.
Collectively called PFAS materials (perand
polyfluoroalkyl substances), PTFE is
a subgroup that makes up a large portion
of the PFAS market.
In early 2023, the European Chemicals
Agency (ECHA) published a comprehensive
PFAS restriction
proposal
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60
10
20
30
40
50
60
70
80
90
25 75 125 175
225
275 325 375 425 475
Temp (°C)
Fig. 2 - TGA analysis of Parylene N in air.
Mass vs. Temperature in Nitrogen
100
110
120
10
20
30
40
50
60
70
80
90
25 75 125 175
225
275 325 375 425 475
Temp (°C)
Fig. 3 - TGA analysis of Parylene N in nitrogen.
that calls for the restriction of around
10,000 per- and polyfluoroalkyl substances.1
Related indications from the U.S.
Environmental Protection Agency (EPA)
suggest that regulatory agencies across
the globe are continuing on a similar
path of reduction and elimination, creating
uncertainty about the future of PTFE
use on medical mandrels.
With respect to Parylene, the most
common variant used on mandrels is Parylene
N, which contains no fluorine or
other halogens. SCS Parylenes have been
proven to withstand typical temperatures
used in the balloon attachment process
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using today's most common materials including
Polyether block amides (PEBAX®),
polyurethane and polyethylene
terephthalate (PET), with process temperatures
typically between 175° and 225
°C. During the attachment process these
temperatures are seen for periods of
time between 20 and 45 seconds.
Service temperature exposure durations
between 24 hours and 10 years are
generally assessed for the use of Parylene
at elevated temperatures, relying on a
traditional thermal evaluation, which
can be found in the Kirk-Othmer Encyclopedia
of Chemical Technology.2
55
525 575 625 675 725
775 825 875
525 575 625 675 725
775 825 875
T 5% = 327° C
Atmosphere: Air
Heating rate: 10 K/min
N Thickness: 0.019 mm
T 5% = 480° C
Atmosphere: Nitrogen
Heating rate: 10 K/min
N Thickness: 0.019 mm
Failure time, h
Mass (%)
Mass (%)
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Medical Design Briefs - February 2024

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Medical Design Briefs - February 2024 - Cover Flap
Medical Design Briefs - February 2024 - Cover Flap1A
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