• Braided Catheter Shaft Design
• Fluoropolymer Lined, Braid Reinforced Catheter Shaft Design
• Braid Patterns
• Sterilization

Braided Catheter Shaft Design

Braided catheter shaft designs are typically used for applications that require high torque, burst pressure resistance, pushability, steerability and kink resistance. These designs can be produced in a wide range of sizes from 2F through 24F depending on the application.

A common requirement for this type of shaft is to have an un-reinforced soft stem section welded to the distal end of the catheter. In addition to the un-reinforced stem, certain applications require an atraumatic tip as well to provide protection to the intimal tissue during insertion of the catheter. This "A-Tip™" is welded to the distal end of the stem section.

When designing braid reinforced shafts a variety of materials can be specified for the liner and jacket materials to enhance the properties of the shaft depending on the performance characteristics that are to be achieved, i.e. torque, kink resistance, flexibility, etc.

With respect to dimensions, a braided shaft has three elements, the inner liner, the braid and the outer jacket. These elements must be combined in order to achieve the required dimensions and there are some physical limitations that result. The liner and outer jacket thickness is typically limited to 0.002" (0.05 mm) each although this is determined by the overall diameter of the shaft.

To determine the wall, the inner and outer polymer layer thickness must be added to twice the diameter of the braid wire (the braid crosses itself). For example, a braid wire of 0.001" (0.03 mm) would yield a theoretical wall of 0.006" (0.15 mm) by adding the liner thickness of 0.002" (0.05 mm) to the wire diameter 0.001" (0.03 mm) times 2 or 0.002" (0.05 mm) added to the outer jacket thickness of 0.002" (0.05 mm).

Often there are trade off's required in the design depending on the performance requirements. For example, higher torque can be achieved with a higher *pick count but this will reduce the flexibility of the shaft. Likewise, a larger diameter braid wire can be specified to provide more stiffness and torque but this will impact the minimum wall thickness and flexibility. A flat braid wire will reduce the wall, increase the flexibility but this will also reduce the torque.

As a result it is important when designing a braid reinforced catheter shaft to consider the performance requirements up front to assure that the design meets the needs of the user.

For design assistance, please contact your sales representative who will be happy to assist you. Our engineering staff is also available to guide you through the design process as well.

* The pick count is expressed in picks per inch of length (PPI), which represents the number of times the wire crosses for every inch of shaft length. The higher the PPI the more wire coverage is achieved.

Return to top

Fluoropolymer Lined, Braid Reinforced Catheter Shaft Design

Fluoropolymer lined, braided catheter shaft designs are typically used for applications that require liner lubricity, torque, pushability, steerability and kink resistance. These designs can be produced in a wide range of sizes from 2F through 24F depending on the application.

A common requirement for this type of shaft is to have segments of varying durometer and/or material types along the length of the shaft. These segments will provide a change in the stiffness of the shaft from the proximal to the distal end. In addition, the use of a very low durometer material at the very distal end can provide an atraumatic tip to protect the intimal tissue during insertion of the catheter. All of these segments are fused together during the assembly process to form a continuous outer jacket.

The primary feature of the lined catheter is the inclusion of a PTFE, FEP or other fluoropolymer liner. This liner provides the catheter shaft with a high degree of lubricity on the inner diameter that facilitates the passage of other diagnostic or therapeutic devices through the inner lumen. In addition, fluoropolymers are resistant to many chemicals, which make this construction ideal for some types of drug delivery systems as well.

With respect to dimensions, a lined braided shaft has three elements, the inner liner, the braid and the outer jacket. These elements must be combined in order to achieve the required dimensions and there are some physical limitations that result. The liner can be produced as thin as 0.001" (0.03 mm) and the outer jacket thickness is typically limited to 0.002" (0.05 mm) minimum although these will change depending on the overall diameter of the shaft to be manufactured.

To determine the wall, the inner and outer polymer layer thickness must be added to twice the diameter of the braid wire (the braid crosses itself). For example, a braid wire of 0.001" (0.03 mm) would yield a theoretical wall of 0.005" (0.13 mm) by adding the liner thickness of 0.001" (0.03 mm) to the wire diameter 0.001" (0.03 mm) times 2 or 0.002" (0.05 mm) added to the outer jacket thickness of 0.002" (0.05 mm).

Often there are trade off's required in the design depending on the performance requirements. For example, higher torque can be achieved with a higher *pick count but this will reduce the flexibility of the shaft. Likewise, a larger diameter braid wire can be specified to provide more stiffness and torque but this will impact the minimum wall thickness and flexibility. A flat braid wire will reduce the wall, increase the flexibility but this will also reduce the torque.

As a result it is important when designing a braid reinforced catheter shaft to consider the performance requirements up front to assure that the design meets the needs of the user.

Lined catheter shafts offer a high degree of design flexibility making it easy to include other features into the product such as radiopaque marker bands and reinforcing coils. The marker bands are encapsulated in the wall of the catheter shaft and provide excellent radiographic visualization for the identification of tip or other locations while placed in vitro. The coil reinforcement can be added as a segment after the braid or as the sole reinforcing member in the shaft and this coil provides much better kink resistance than a braided shaft alone.

Using our proprietary process, TFX Medical OEM is also able to provide shafts incorporating multiple lumens, each with it's own fluoropolymer liner. These shafts are commonly used in the construction of articulating tip catheters, providing a lubricious liner for the pull wire(s) channel(s).

For design assistance, please contact your sales representative who will be happy to assist you. Our engineering staff is also available to guide you through the design process as well.

* The pick count is expressed in picks per inch of length (PPI), which represents the number of times the wire crosses for every inch of shaft length. The higher the PPI the more wire coverage is achieved.

Return to top

Braid Patterns

Regular Braid Pattern
This is a common pattern that uses 16 wires in a one under two over two pattern.

Diamond Braid Pattern
This pattern also uses 16 wires but differs from the standard pattern in that it produces a two wire under two over two pattern. This pattern tends to provide better torque and more kink resistance than the standard pattern but also has a slightly higher cost.

Diamond Braid Pattern, Half Load
By utilizing half the number of wires, the diamond pattern can be produced in a one wire under one over one pattern. This pattern provides more torque than the regular diamond pattern but incurs a much higher cost due to the reduced wire load.

For design assistance, please contact your sales representative who will be happy to assist you. Our engineering staff is also available to guide you through the design process as well.

Return to top

Sterilization

There are several methods of sterilization commonly used in the medical field and these include Autoclaving, Ethylene Oxide (EtO), Hydrogen Peroxide, Gamma and e-Beam. Each will have a different effect on the materials used in the construction of the device, therefore, determining the type of sterilization that the device will be subjected to is a critical design issue.

Since Gamma sterilization is being used more frequently due to the reduced post sterile release cycle time, we have provided a table below that shows many common materials and the effects that can be expected when the material is exposed to Gamma sterilization. These are for reference only and the actual suitability for use must be determined as part of the design process.

Radiation Stability of Polymers Used for Medical Applications