Medical Design Briefs - July 2022 - 18

Engineered Extrusion Trends
Typical applications include catheters,
electronic medical
equipment, fluid transfer, drug delivery,
and medical instrumentation.
Geometric transitioning extrusion.
Geometric transitioning, or GeoTransTM
,
allows for silicone tubing to change the
cross section during the extrusion process
to eliminate molding and secondary
bonding steps. Reducing the number of
components and the bonding joints between
them allows for a more hygienic
design.
The GeoTrans technology is applied to
custom applications with precision tolerances.
To facilitate the development cycle
for these highly customized extrusions, it
is fastest to identify one cross section as
critical and allow flexibility of other cross
Platinum-cured silicones can eliminate the need for a secondary post-cure process, which is required
for older peroxide-cured silicone systems. (Credit: Trelleborg Healthcare & Medical)
to seal housing assemblies or in heart
valve repair applications. Being nonsymmetrical,
such cross sections can require
longer development cycles and can be
more difficult to measure while trying to
maintain the extrusion's free state.
Twisted extrusions. This extrusion process
produces continuously twisted tubing
for use in applications in which implanted
power or sensing cables require strain relief
from repeated flexing and bending,
such as with pacemaker leads.
The tube cross section typically consists
of a center lumen and multiple outer lumens,
with the twisting process causing
the outer lumen to become spiraled
around the center lumen. In this structure,
wires fed through the outer lumen
are less prone to dynamic flex failures because
stresses are distributed more evenly
to the multiple wires.
The process does not require the tubing
to have a round outer diameter. And
features, such as grooves, can be incorporated
on the outer surface. Accurate verification
of the spiral pitch requires translucent
tubing and custom measurement
techniques.
Bump tubing. Bump or tapered tubing
is created by stretching the tube at a varying
rate to produce tubing with a larger
diameter at one end and a significantly
smaller diameter at the other end. The
process can be applied to plastics and
elastomers, including silicone. The geometry
is limited by the Poisson's ratio of the
18
material during stretching in the green
state.
Formed extrusions. Extruded tubing
is designed to create complex paths,
such as sinusoid shapes for flex and
strain relief and to fit tortuous anatomy,
or spiral shapes that might be used to
soften contact within the bladder, for example.
Measurement of such geometries
in the extrusion's free state can be challenging
and require custom manufacturing
methods.
Bonded or overmolded stops. Stops
can be bonded or molded onto tubing
to lock it in place and keep it from rotating.
The stops are typically added to
peristaltic pumps for infusion, internal
feeding, laboratory equipment, diagnostic
equipment, and bioprocessing
fluid transfer. The key advantage of
peristaltic pumps is their disposable
pump tubing. Only the tubing's inner
diameter comes into contact with the
fluid; therefore, there is no need to
clean or flush the pump between uses.
Multilumen extrusion. Using this technology,
numerous lumens or channels
are produced as a single extrusion. More
lumens mean more dimensions to control
and measure, including the inner diameters
and the walls between diameters.
High-precision vision systems with custom
programming are often required to
ensure quality. Downstream molding can
incorporate overmolded connectors to
separate the lumen into individual tubes.
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sections. Typical changes include:
* Changing the tubing inner diameter,
outer diameter, or both to create tubing
with two or more different cross
sections.
* Bifurcation or converting a dual lumen
extrusion into two single lumen tubes.
* Inclusion of channels to open internal
fluid paths at locations along the extrusion
length.
* Stopping one of multiple lumens to
avoid the need for backfilling, such as
when the lumen will be used for balloon
inflation.
* Thinning a tube wall for a short section
to drive specific balloon inflation, location,
and shape.
Geometric transitioning single lumen.
This technology, also called off-ratio bump
tubing, creates two distinct tube cross sections
with a near-step transition between
them.
Tubing lengths are available in single
transition (cross section A to B) or double
transition (cross section A to B to A) options.
Applications include custom-end assemblies,
such as accommodation of connectors
or fittings, and peristaltic pumps.
Foam extrusion. Closed-cell silicone
foam offers custom mechanical characteristics
by lowering effective durometer
and providing additional cushion
space. Silicone foam extrusion can be
customized in a variety of sizes and
cross sections with precision tolerances.
Cell pore size and foam density can
also be adjusted. Silicone foams are
made from medical-grade HCR, platinum-cured
silicone.
Measurement of foam materials can be
less precise due to the natural variation of
Medical Design Briefs, July 2022
devices, analytical

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Medical Design Briefs - July 2022

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