Medical Design Briefs - May 2021 - 14

HCR Versatility

180° C

160° C

140° C
120° C

Torque

100° C

Time
Fig. 1 - Cure profile for platinum-catalyzed HCR at various temperatures.

Why Manufacturers Choose HCRs
for Fabrication
Silicones offer elastomeric properties
coupled with the biocompatibility that
medical manufacturers seek when specifying materials. However, these applications may also demand more complex
capabilities than can be attained with
thermoset or thermoplastic hydrocarbonbased elastomers. Medical device designers sometimes need materials able to
achieve more specific - and often more
rigorous - performance characteristics
while maintaining design flexibility.
HCRs offer versatility for several reasons, including the following.
Established history of use in medical
applications. HCRs were used in the
hydrocephalus shunt, one of the first
implantable devices. Since then, their

track record of reliability gives established
operations and start-ups alike the confidence that their HCR components can
contribute to regulatory approval. Their
long track record can be especially valuable for manufacturers creating a novel
device, where the use of a well-established
material can help the regulatory process
run smoothly.
Biocompatibility. Because HCRs are
available with varying degrees of regulatory support, depending on the specific
material application, qualified products
can be used in long-term (>29 days)
implantable medical devices as well as
for short-term implantable devices or
external medical applications.
Very high mechanical properties.
When compared with other elastomers,
like liquid silicone rubbers (LSRs), the

high molecular weight of HCRs' polymers combined with the reinforcing
fillers allow for desirable mechanical
properties, like greater elongation, and
a lower stress-strain modulus, both ideal
for balloons.
Green strength. One of HCR's unique
properties is its claylike consistency in
the uncured state. It can be formed into
a dimension and will hold that shape
until it is cured. This green strength is a
key characteristic for medical device
manufacturers because it's ideal for a
variety of fabrication methods, such as
extrusion and calendering.
Cost efficiency. In general, the tooling used to mold HCR components is
less complex and relatively inexpensive
compared with other tooling. This
makes HCRs ideal materials for low production volumes, as well as prototyping
molded parts.
Customization. Experienced silicone
providers can synthesize monomers and
polymers to enable complete customization. For example, an HCR can be customized to have minimal hysteresis
compared to standard HCRs. In other
cases, a device manufacturer may want
to pigment the material with color masterbatches supported with the same regulatory filings and biological testing
support.
Understanding HCR Curing
Systems
Adding to the material's overall versatility, HCRs can be supplied with curing
options that suit specific applications or
processing methods. HCRs can be supplied uncatalyzed, allowing medical
device manufacturers to incorporate
their own preferred catalysts. Uncat-

Platinum-Catalyzed Addition Cure

Peroxide Cure

Thick section cure

Yes

Cure

Accelerated with heat

Heat

Table life

Variable

Indefinite

By-products

Yes (can be corrosive)

Post cure

Optional

Required

Shrinkage (approx., temp.
dependent)

2-4%

3-5%

Susceptible to inhibition

Less

Supplied

Precatalyzed or uncatalyzed

Table 1. Platinum vs. peroxide cure HCR systems.

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