Medical Design Technology - September/October 2017

Emphasis On Processing MDTmag.com 10 / September/October 2016

Within the MPE process, two or more lumens can easily be split off a center lumen, or merge two lumens into a single lumen—all in a single continuous extruded tube. The multi-lumen process involves moving dies and mandrels in sync, reducing cross contamination of fluids in the separate lumens.

Advantages of Multiple Profile
Extrusion

• Facilitates the extrusion of balloons of
any length
• Eliminates secondary bonding operations
• Allows seams to be eliminated
• Various types of tubing (single lumen,
multi-lumen, transitional Geo Trans®,
etc.) can be produced, as well as rod,
ribbon, and other non-standard profiles
• Suitable for extruding both elastomers
and foams

Disadvantages of Multiple Profile Extrusion

• Material choices limited to HCRs (high consistency rubber)

• Issues can arise from having to move
dies and mandrels in sync
• Cross contamination of fluids in the
separate lumens can occur

Silicone Dip Molding and Coating

Metal, plastic, fabric, and glass medical components can all be coated with thin silicone films that are then vulcanized to produce a smooth, durable, biocompatible finish. For example, dip molding can be used to create silicone coatings on devices such as needles, cannulas, and syringes to enhance patient comfort.

Dip molding can also be a cost-effective
alternative to silicone molding processes in
instances involving costly metal molds. It is
ideal for rapid prototyping of complex, thin
walled shapes that can later be scaled up
for large-volume commercial production.

The dip molding process involves producing a mandrel in the shape of the final part. Mandrels are usually machined from metal, but can be fabricated from engineered plastics and ceramics. The mandrel is immersed in a vessel containing silicone dispersion and then withdrawn. The mandrel, now coated with a thin liquid silicone film, is set and placed in an oven where the silicone is vulcanized. Following vulcanization, the silicone rubber is stripped from the mandrel, creating the finished product.

Wall thickness can be adjusted by varying the number of dips and by adjusting the percent of solid concentration of the silicone dispersion. Several variables contribute to the quality and reproducibility of each part, including evaluation of mandrel surface finish, immersion, and withdrawal angle and speed, dispersion viscosity and temperature, ambient manufacturing conditions, and vulcanization parameters.

Advantages of Dipping/Coating

• Processes include mandrel-type dipping that produces discrete components (mammary shells, balloons) as well as durable coated surfaces (coated surgical blades, non-slip handles on surgical tools)

• Costly metal molds not required • Ideal for prototyping and short runs

Disadvantages of Dipping/Coating

• Choice of silicone raw materials limited
to solvent-based dispersions or low vis-
cosity materials with marginal physical
properties
• Possible variation over long runs depen-
dent on multiple variables

Calendaring and Sheeting

In high consistency rubber (HCR) calendaring, silicone is fed through multiple rollers to produce a film of uniform thickness.

The film is then transferred onto a carrier sheet and may be cured or left uncured.

Calendared sheeting can be pigmented, produced with a number of surface finishes and used to create vulcanized, un vulcanized, reinforced, and non-reinforced products. The process works particularly well to create mandrel-wrapped hoses and ducts, as well as die-cut seals.

Custom sheeting is used in a variety
of medical devices. Discs punched from
vulcanized elastomeric sheets are key
components in various valve assemblies,
mesh-reinforced sheeting is integrated
as sewing rings on artificial heart valves,
and laminated sheeting containing both
vulcanized and un-vulcanized layers is
used to produce seal tissue expanders and
mammary devices.

Advantages of Calendaring

• Used to manufacture silicone sheeting in
a continuous process
• Reinforcing mesh can be added to
increase strength and tear resistance
• Sheets can be die-cut or punched into
components

Disadvantages of Calendaring

• Raw material choice limited to HCRs

Assembly, Packaging and Sterilization

Post-manufacturing, including complete device assembly, packaging, and sterilization, is a final consideration for silicone processing. Not all suppliers have a full range of capabilities, and many silicone components are part of a larger, more complex device. Others, such as punctual plugs and small joint implants, function as complete medical devices but must be properly sterilized and packaged before shipping.

Conclusion

The ability of silicones to be formulated to attain specific performance, aesthetic, or therapeutic properties makes them ideally suited for many medical devices. However, suppliers must demonstrate a strong understanding of the details involved with both manufacturing silicone rubber parts and complex medical devices. With time-to-market such a critical component in the creation and sale of medical devices, the ability to produce rapid prototypes, quickly reach a final design, and consistently produce and deliver high-quality products, are the keys to success. MDT