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Silicone Solutions for Lubrication in Medical Applications
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Nathan Wolfe
Technical Sales
NuSil Technology
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For over fifty years silicones have
been used extensively in the design and manufacture of
medical devices intended for short and long-term human
implantation. Silicone’s overwhelming success in the
healthcare arena is not only due to its proven
bio-inertness but also silicone chemistry’s dynamic
nature, which yields a diversity of raw materials
including fluids, gels, adhesives, dispersions, and
elastomers. These two assets explain why many
therapies rely heavily on silicones and why some
medical devices are fabricated entirely of several
types of silicone.
Even so, the success of many
medical devices depends upon minimizing friction at
the interfaces between various components and between
those components and human tissue. Whether due to
critical need for bio-inertness, wetability,
lubricity, or all three, in many cases a silicone
coating or lubricant is the only viable solution to
reduce this friction. This article details these
silicone solutions and delineates the relative
benefits of each. |
First, and most simply, there are nonfunctional silicone
polymers. Silicone polymers are chains comprised of
repeating Si-O units. While there are no carbon-carbon
double bonds on the polymer backbone, the pendant groups
coming off the backbone do contain carbon, making it
appropriate to describe silicone polymers as
rgano-polysiloxanes.
During the polymerization process, there are two main
factors that are controlled: the pendant groups and the
degree of polymerization. The non-functional radical /
pendant groups (R) coming off the polymer backbone may be
methyl, phenyl or methyltrifluoropropyl. Phenyl fluids are
rarely used as lubricants due to their large bulky nature.
Generally, a material is referred to as a “polymer” if a
molecule contains only one type of pendant group and a
“copolymer” if a molecule contains a combination of
pendant groups. Furthermore, all silicone polymers can be
synthesized to a very specific degree of polymerization.
The degree of polymerization dictates the average
molecular weight which in turn governs the viscosity. A
silicone polymer may possess a viscosity close to that of
water (20 cP) or so high that it’s a solid (in the
millions of cP).
Regardless of chemistry, all polymers will have varying
levels of success as a lubricant or a hydrophobic coating
on a variety of substrates including molded silicone
parts, metal, glass, and many plastics. Methods of
applying the polymer include dipping, spraying or wiping.
If a very thin film is desired, these silicone fluids may
be further diluted down as far as 1 – 5% silicone solids
in a compatible solvent.
Methyl polymers may be dispersed in nonpolar organic
solvents, whereas fluoro polymers (and copolymers) may be
dispersed in chlorinated hydrocarbons and key tones, and
to a lesser extent, aromatic hydrocarbons, mineral
spirits, and isopropyl alcohol. For ease of use and
minimal processing, some medical device manufacturers
select polymers predispersed down to specified percent
solids content.
When attempting to coat and lubricate a molded silicone
part, it is important to be mindful of the chemistry of
the molded part versus that of the lubricant. At the time
of application, if more than a few hours of lubrication is
needed it is important to select a fluoro-polymer or a
copolymer. Otherwise the silicone fluid will diffuse into
the elastomer, both swelling the molded component and
depleting the fluid’s surface thereby reducing or
eliminating all lubricating characteristics. Since a
fluid’s rate of diffusion into a silicone elastomer
decreases as the fluid’s molecular weight increases, the
higher-viscosity fluids lubricate a silicone elastomeric
surface for a slightly longer period than the
lower-viscosity fluids.
It’s also worth noting that on temperature resistant
materials such as glass, ceramic, and metal, silicone
polymers and copolymers can be converted to highly durable
hydrophobic films by heating the treated surfaces.
Beyond polymers (predispersed and otherwise) some
dispersed silicone formulations are designed to cure at
ambient conditions. These formulations yield a very
minimal crosslink density such that they result in a
sticky yet slippery finish, not unlike the finish of the
above-described polymers.
Unlike polymers, dispersed silicone formulations minimally
bond to the substrate they coat. This feature makes these
products ideal for lubricating cutting edges, needle
Cannula, etc. These formulations are one-part dispersions
that devolatilize, cure at ambient conditions, and may be
applied by dipping or wiping. When working with these
formulations, it is important to remember that they tend
to be moisture-sensitive. Consequently, if adjustment to
the percent solids or viscosity is needed, it is important
to use a moisture free organic solvent.
Technological advances have resulted in some newer
formulations. Specifically there is now some silicone
elastomers designed for molding that self-lubricate. With
this formulation, as with all of the above, there is some
potential for the uncontrolled migration of the lubricious
component. This introduces the most recent breakthrough in
the evolving technology of silicone coatings: heat-curable
silicone dispersions that covalently bond to their
silicone substrate and result in a dry yet slippery
finish.
The development of formulations such as these came as the
result of two industry goals: design a coating for molded
or extruded silicone parts that overcomes their inherent
blocking characteristic and achieve this without the
potential for the migration of any formulary component.
Because of hydrogen bonding, cured silicones tend to
exhibit an affinity for themselves and other surfaces such
that they want to stick rather than slide. This presents
obvious problems in a host of applications where a molded
or extruded silicone part must move or slide with a
modicum of friction. By spraying or dipping these
friction-diminishing silicone coatings on a substrate one
can expect to see a dramatic reduction in the coefficient
of friction of that part or device.
These formulations have enjoyed an enthusiastic reception
in the healthcare industry, not simply for their
unprecedented performance relative to friction reduction
and regulatory concerns, but also because they achieve
these critical performance goals with negligible impact on
the mechanical properties of the silicone substrates they
coat. It may be said that the coatings mimic the
mechanical properties of the elastomers they coat.
Therefore, a silicone device that must bend, twist,
elongate, etc. may be relied upon to do this coated the
same as if it were uncoated, and without worrying about
cracking, flaking or peeling.
Clearly there are many silicone solutions for devices
requiring safe and effective lubrication between various
materials. The above-described silicone polymers and
low-crosslink density coatings are well-established
performers that routinely deliver on these critical
criteria in a host of applications. Additionally, in light
of the just-described state-of-the-art “dry finish” cured
coatings it is clear that the body of silicone solutions
will only continue to grow.
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