Its biocompatibility, flexibility, and durability have made extruded silicone the top choice for tubing in many medical devices. Of course, designing devices that will be attached to or be lodged within the body is not the easiest of tasks, even given the positive qualities of silicone.

Today, silicone tubes are being enhanced with a variety of reinforcements and coatings that help manufacturers meet the market's escalating demand for smaller, more durable devices. This article explores a number of such enhancements, with an emphasis on kink-resistant tubing.

Spiral Design Adds Kink Resistance

Silicone is a soft material, and extrudable silicones are in the Shore- A durometer scale. This makes them prone to kinking, especially in small dimensions. As designs call for smaller device footprints and tubes as a result become smaller, tube diameters reach the point where, without reinforcement, they easily bend or collapse, halting the flow of fluid.

The solution is to embed monofilaments, either in a spiral or braided design, within the wall of the tube to add radial strength, making extruded silicone tubes highly resistant to kinking and compression. Today, small silicone tubes (from 1/8 to 3/16 in.) are available with a variety of reinforcements.

Smaller and smaller tubing sizes are now called for in medical devices. Kink-resistant tubing in 1/8 to 3/16 in. dimensions are available with reinforced embedded monofilaments.

Monofilaments in a tight spiral design are ideal for adding kink resistance to small diameter tubes. The reinforcement adds enough radial strength so that tubing can be bent almost in half without affecting fluid flow. Because the monofilaments are embedded in the tube wall, they do not affect the inner diameter of the tube. This configuration is ideal for creating a small, highly flexible tube that will not kink or collapse, even when required to conform around anatomical features.

However, plastic reinforcements are sensitive to heat, so they cannot be used in devices that will be subjected to high temperatures, such as seen in autoclave sterilization. The silicone itself is highly heat resistant, but the autoclave's high temperature would degrade the mechanical performance of the plastic embedded in the wall. In addition to plastic, spiral reinforcements can be manufactured from nylon and stainless steel, both of which are more heat resistant than plastic.

Kink-resistant tubes can be used in a variety of medical devices, including catheters, pacemakers, and penile implants.

Braids Prevent Burst Tubes

Some medical applications require tubes that are slightly less flexible than those that can be produced by spiral reinforcement but which are stronger. Rather than kink resistance, the aim is to produce a tube that won't expand, burst, or collapse under pressure. The solution is a braided monofilament, which can be constructed from a variety of plastics and metals, including polyethylene, nylon, and stainless steel.

The flexibility of a braided monofilament depends on the number of crossed pieces in a given area. This is typically measured in per-inch crosses, or PICs. The higher the PICs, the tighter the braid; the tighter the braid, the less flexible the tube but the stronger it will be.

Burst-resistant tubes are currently being used in a variety of medical applications that require liquid (e.g., a drug or a flushing agent) to be delivered under pressure.

Reinforcers Extend Durability

Reinforced tubing can also be designed to increase wear resistance. Materials such as polyester and polyethylene can be placed inside the silicone wall in a spiral or braid configuration for tubes to be used inside the body.

An application example is where excess tubing is coiled up and placed behind a pacemaker in the chest cavity. This may rub against the pacemaker due to arm/shoulder movement and, over time, this can cause the outer layer of silicone to break down. A smooth, durable reinforcement placed inside the silicone makes the tube considerably more wear resistant, extending the length of time the device can function inside the body before surgical replacement.

Wear-resistant tubes can be used in a large number of implants such as heart pumps and cochlear devices.

Designing with Mandrels in Mind

Unlike thermoplastics, which must be melted to an exact point before extrusion and then cooled quickly to set the shape, high-consistency rubber/elastomer (HCR/E) silicones are a thermoset material that rely on a chemical crosslinking process. HCEs have considerable green strength, making them an ideal material for extrusion, but once crosslinked with heat, they will not remelt.

However, the reinforcement process adds a layer of complexity to tubing manufacture. The tube wall must be built on a mandrel while the spiral or braid is being created. The mandrel is removed once the reinforcement is complete.

Removing the mandrel from the tube can be difficult for long lengths of tubing, limiting the length of certain types of reinforced tubing. When long lengths are specified, one solution is to deliver the tube with the mandrel still inside. The customer cuts the tube to length, then removes the mandrel.

The size of the inner diameter (ID) is also a factor: larger tube IDs require a larger mandrel. As the mandrel increases in size, the surface area's contact with the silicone enlarges significantly, increasing friction and making the mandrel more difficult to remove. Thus, the use of reinforcement in silicone tubes is currently limited to under 1/8 in. in diameter.

Thicker walls can also have a negative impact on removing the mandrel because they may have a tighter grip on the mandrel, increasing the friction coefficient between the silicone wall and the mandrel.

Another design consideration is the temperature threshold of the reinforcement material. Metals such as nitinol and stainless steel can be exposed to high heat without harm but not all plastics can. It would be more difficult to manufacture a tube using a low-melting-point plastic: temperatures that would successfully cure the silicone could compromise the reinforcement.

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