MEDICAL CABLE OVER-MOLD MATERIALS – PART III of IV : A HIGHLIGHT ON SILICONE (LSR)
For the past couple of months, we have been focused on providing some detailed insights into various options for over-mold. For this month, we are shifting gears into the world of silicone, a material that is very different from the thermoplastics we talked about earlier, yet a great option for medical cable assemblies. Silicone becomes a critical material for medical cable assemblies that are required to withstand a high number of autoclave steam sterilization cycles and is the material of choice for both cable jacket and over-molded strain reliefs. Silicone can also be used in medical device applications where extreme flexibility and a high level of biocompatibility is required. In case you missed the first two parts, you can find more information about thermoplastic elastomers and thermoplastic urethanes on our blog page.
SILICON VS. SILICONE
Silicone – with an “e” on the end – is not the same as the naturally occurring crystalline chemical element silicon. Silicone is a synthetic material and is classified as an elastomeric polymer composed of a variety of elements including silicon, carbon, hydrogen and oxygen. In various forms silicone can be used as an adhesive, lubricant, insulation, and sealant! Silicone is an inert, synthetic polymer that is both heat-resistant and “rubber-like,” two characteristics that are highly beneficial for medical cable assemblies.
LIQUID SILICONE RUBBER INJECTION MOLDING
When manufacturing medical cable assemblies using Silicone as the primary material, it requires an injection mold process which is different than the insert mold processing we talked about with TPU and TPE/TPR. Liquid Injection Molding (LIM) have two main silicone molding processes. First is Liquid Silicone Rubber (LSR), an injection molding of silicone rubber and Fluoro Liquid Silicone Rubber (F-LSR). Of the two, LIM is the most common method used in the manufacture of medical devices.
During the manufacturing process, two components of liquid silicone rubber are pumped through a static mixer by a metering pump. One of the components contains the catalyst which is typically platinum based and is commonly referred to as the “B” component. This is when your preferred colorant can be added before the materials enter the static mixer section. In the static mixer the components are combined and are then transferred to the metering section.
Once the two components are brought together the curing process begins immediately. To prevent premature curing, the mixture is kept chilled until it is injected into the heated mold cavity where vulcanization – curing – takes place.
The process of molding silicone rubber is nearly the opposite of that used to mold thermoplastic elastomers. Silicone starts as a liquid and is transformed into a solid by a catalyst with heat applied during molding. On the other hand, thermoplastic elastomers start as a solid, are liquefied by heat and then returned to a solid state when injected into and cooled by mold tooling.
ADVANTAGES OF SILICONE VS. THERMOPLASTIC ELASTOMERS
Now that we understand the chemistry behind silicone, you’re probably still scratching your head as to when the use of silicone is the right material for your medical cable assembly application versus others. For starters, silicone has excellent biocompatibility characteristic and is incredibly stable over a wide temperature range and offers mechanical resilience. Here are some additional advantages:
- Biologically Inert – Liquid silicone rubber used in medical applications is typically formulated to comply with U.S. Pharmacopoeia (USP) Class VI and ISO 10993 biocompatibility requirements and offers superior compatibility with human tissue and body fluids. These materials are odorless, tasteless and do not support bacterial growth. Silicone rubbers typically do not corrode or stain other materials.
- Non-Reactive – Silicone is considered a non-reactive material and molded or extruded assemblies do not react with most cleaning and disinfecting agents. This characteristic is very beneficial for medical cable assemblies that must be cleaned and disinfected often.
- High Heat Resistance – Molded silicone parts have an operating temperature above the melting temperature of most thermoplastic resins. While medical cable assemblies are not typically subject to high temperatures during use, this characteristic contributes to the ability to withstand the high temperatures of multiple steam sterilization cycles.
- Great Strength – Through vulcanization, the cross-linking of silicone molecules gives silicone rubber products moderate tear resistance. This allows for over-molds to be designed with relatively thin walls and the subsequent molded parts to be very durable.
- Low Viscosity – LSR has much lower viscosity than most thermoplastic resins. This allows mold tooling to be designed with thin walls while still allowing the un-vulcanized liquid material to flow freely around the part that is being over-molded, filling cavities and mechanically “locking-onto” any retention features.
SILICONE CABLE JACKETING
Silicone jacketed cable can be extremely flexible, a desirable characteristic for cables that are manipulated frequently, like during surgical or endoscopic procedures. Depending on the method used to jacket the cabled components, even greater flexibility can be achieved.
Silicone cable is commonly jacketed using both extrusion and tubing methods. In the extrusion process, liquid silicone is injected through a heated die through which a single or multi-conductor cable is moving. The silicone material fully surrounds and conforms to the wire or cable bundle. For many medical device applications, this construction – in which the cabled components are fully encapsulated – is ideal.
For other applications where increased flexibility is advantageous, cabled components may be inserted into a pre-formed silicone tube. When a “tubed-on” jacket is used, the cable bundle is not held in place by the cable jacket, which allows the components to move axially as the cable is flexed. This, in turn yields an increases the overall flexibility of the cable.
CONFORMAL COATING OF SILICONE
Silicone stand-alone tends to feel sticky, and it does not glide easily over surfaces and tends to retain debris. For this reason, silicone jacketed cable often has a conformal coating such as Parylene (Poly-para-Xylylene) is recommended.
Parylene is biologically and chemically inert. It has been approved for human implantable devices. The thermal resistance of Parylene is similar to that of silicone. Parylene is applied by vapor deposition and has a smooth, silky feel. The surface of Parylene coated cable has a dry lubricity feel and that does not wear off.
SILICONE OVER-MOLDING
If your medical cable assembly is required to withstand a high number of steam sterilization cycles, it is also recommended that you incorporate metal shell connectors with the silicone-jacketed cable. Over-molding silicone provides a stronger bond to metal connector components with use of primers and other adhesion promoters, creating a moisture-proof mechanical bond that contributes to sterilization withstand. Typically, the same over-molding captures the silicone jacketed cable and creates a chemical bond between over-molded strain relief and the cable jacket also enhancing sterilization withstand.
SILICONE MOLD TOOL DESIGN
The design of mold tooling for liquid silicone rubber is similar to the design of molds for thermoplastic resins. There are, however, some important differences in the behavior of liquid silicone that need to be considered.
While relatively low viscosity is considered a pro with of liquid silicone rubbers, the fill-time of the mold cavity can be very short. To avoid entrapping air, good mold venting, even vacuum displacement may be required. Silicone will flow and flash into spaces less than 0.001” requiring parting lines and shut-offs to be precisely tooled.
Mold tools used for thermoplastic elastomers are commonly liquid cooled. In contrast, silicone mold tools are heated to accelerate curing. This requires embedded heating elements and a connection to a temperature control unit.
SUMMARY – ADVANTAGES OF SILICONE CABLE ASSEMBLIES
Silicone cable assemblies are characterized by excellent flexibility, durability and the ability to withstand cleaning, disinfection and hundreds of steam autoclave sterilization cycles. These characteristics make silicone a logical material choice for cables used for video endoscopy, surgical power tools, electrosurgical instruments, laparoscopy, ophthalmic lasers and robotic surgery.
Have more questions, or comments about the use of silicone in medical device and cable assembly applications? Be sure to contact us – we’d love to hear your thoughts!
