What is reinforced tubing? At first glance, it looks like any another piece of tubing used to manufacture catheters and similar medical devices. Reinforced tubing is frequently used for support devices that provide access or a delivery conduit for another device. Some applications include vascular access sheaths, stent-placement shafts, endoscopes, and more recently MRI-compatible catheters. By adding reinforcing to the wall of plastic tubing it can handle increased internal pressures, provide kink resistance, column strength, and increase torque transmission compared to non-reinforced tubing. Designing reinforced tubing should start with a description of the desired output performance. This can be as simple as stating in general terms that the product needs to have good column strength, some torque transmission, and decent flexibility. Or it may be more specific such as 100 atm burst pressure, 1/2-in. minimum bend radii, 1.75 in-lbs of torque, or less than 2% linear stretch at 5 pounds of force.

How to reinforce

There are three basic types of reinforcement used for medical tubing: braiding, spiral or coil, and linear members. With braiding, the braid angle and percent coverage are important specifics, as are the size, shape, and tensile strength of the reinforcing material. Braid angle is measured from the longitudinal axis of the tube. This means that a 30° braid angle is closer to parallel with the axis than it is perpendicular. Changing the braid angle changes the flexibility and torque response of the tube. Typically, a lower angle creates a stiffer tube that can deliver more torque and reduce stretching, while a higher angle creates a more flexible kink-resistant tube with somewhat lower torque transmission. Braiding machines are available that can automatically toggle between several braid rates during the run. This creates a product with different braid flexibility between the proximal and distal sections of a shaft without the expense or risk of a molded joint. Higher percent coverage can also add to kink resistance, torque transmission, and pressure resistance. However, if the coverage is too high it interferes with layer bonding which in turn defeats the performance advantages of the reinforcing material.

Spiral reinforcement allows for high (almost perpendicular) angles. High angles take advantage of the reinforcing wire's tight helical configuration using its hoop strength to provide good kink and crush resistance. However, a high-angle spiral design provides almost no torque transmission and will not prevent linear stretching of the tube. When using spiral reinforcements, important characteristics include tensile strength, material, size, and cross section of the reinforcing element, durometer of plastic compounds, and wall thickness. Continuous spiral reinforced tube manufacturing is more limited in availability than continuous braid reinforced tube. This is because of cost and availability of specialized equipment required to manufacture this type of reinforced tubing.

Linear reinforcement provides excellent stretch resistance but limits flexibility depending on the number and location of reinforcing members. It is also possible to combine braided or spiral reinforcing with linear reinforcing elements to produce a hybrid design. Reinforcement material, tensile strength, size, and placement of the elements are critical aspects with linear reinforcing.

The reinforcing material lends a lot to the performance of the tube. High-tensile stainless steel is common and round wire is the most common. Flat stainless steel wire is an alternative material when there is little wall thickness to work with. Other materials, such as Aramid fiber or polymer monofilaments, can dramatically increase the burst pressure of plastic tubing for many applications. Plastic compounds also play a significant role. It's no surprise that lower-durometer materials make more flexible tubing and high-durometer materials make them stiffer. It's also possible to layer different durometer materials with certain reinforcing materials for specific performance characteristics.

Manufacturing reinforced tubing

There are primarily two ways to manufacture thermoplastic reinforced tubing. The first is continuous-layer processing and the second is called component reflow. Continuous-layer processing uses sequential extrusion and reinforcing steps using long lengths of material. Special extrusion equipment and processing conditions bond the extrusion layers through the reinforcing component. Typical runs are 2,000 to 20,000 feet. Generally, just one material is extruded during each run, although some manufacturers have equipment to extrude different materials intermittently during a single run.

The reflow method produces one unit at a time. An operator takes a pre-made, cut length extrusion and braid components, and layers them by hand onto a solid metal mandrel. Heat-shrink tubing slides over the assembled parts and the whole unit is baked in an oven. The heat shrink applies circumferential compression to the polymer layers while transferring heat to the lower melt temperature materials on the mandrel, laminating them together. The advantage of this method is that it combines several different materials longitudinally and “reflows” them together. Also, the reinforcing member can be started and stopped in discrete positions along the shaft. Because this method involves a large amount of handwork, the cost per unit is significantly higher than parts made with the continuous-layer extrusion process.

There are three key concepts to bear in mind when designing reinforced tubing. The first is form follows function. This is not a new idea. The required performance will drive the design. The second is that a reinforced tube has a composite structure. The polymer layers and reinforcing materials are formed into one structure that will exhibit different performance characteristics from the individual materials. To a general degree, the output performance of the composite cross section is predictable. However, when colors and radiopaque fillers are used, the basic performance of the plastic changes making performance predictions more difficult. The third concept is that it is possible to achieve the same performance characteristics using a number of different design combinations. The style and design of the reinforcement and the thicknesses of the polymer layers can be varied for specific performance characteristics. This is particularly advantageous when cost or material availability are potential constraints to successfully completing a project.