The extrusion of medical tubing can be a relatively “simple” task or it can present a number of challenges; it all depends on the application, materials to be used, etc. This article will focus on single lumen tubing and take a walk through a selection of the options available to the designer from the simplest to the most complex.
Many types of tubing and extrusion are available for medical device manufacturers when designing their latest medical device. As well as the many selection options available, there are also considerations and complexities to go along with each choice that need to be factored into the product design, development, and manufacturing process.
In catheter design, the functional requirements of the application allow the designer to identify the key performance requirements of the medical tubing. Broadly speaking, properties such as flexibility, lubricity, clarity, kink resistance, and the ability to hold tight tolerances are critical criteria in selecting the ideal design for a particular medical application. In more complex applications, column or push strength, torque transfer characteristics, and hoop strength of the tubing may be important considerations.
Single Material Extrusion
In single material extrusion, a deep understanding of the various properties of each material is the major requirement in terms of effective design. Additional requirements of the material, such as the ability to solvent or adhesively bond it, its resistance to high energy radiation (E-beam or Gamma sterilization), its biocompatibility, and its chemical and thermal compatibility with other polymers in terms of over molding or thermal bonding, may become relevant.
For many catheter designs, it becomes clear that one material cannot meet all the key functional requirements so the next logical progression is to co-extrusion. This technique can allow various, even contradictory requirements, to be met by the designer. Radiopacity and clarity requirements can be achieved simultaneously by using stripe designs. Often, these stripe designs involve a number of fully encapsulated stripes symmetrically placed within the tubing as shown in Figure 1. Encapsulating the radiopaque stripe gives the additional benefit of a very smooth outer surface.
Co-extrusion using layer technology can also be a very effective method for creating highly radiopaque tubing sections. High loadings of tungsten powder in a substrate polymer located in the B layer of an ABA or the AB (if wall thickness is constrained) is a very effective method of achieving high radiopacity while maintaining a smooth outer surface.
The layered approach has a number of other potential uses. In applications such as insulin delivery, it’s important that there is no interaction between the tubing material and the drug. A good barrier layer, such as HDPE (or a fluoropolymer), is required. A tube extruded completely from HDPE would kink easily. However, a co-extrusion with a thin HDPE outer layer and a PUR or PEBA outer layer provides a kink resistant solution with good barrier properties.
Dissimilar materials, such as HDPE, can be fused to PUR, PEBA, and Polyamide type materials by using tie layer materials. These extrusions generally are in an ABC layer configuration (Figure 2). Tie layers used for these applications are chemically modified polymers (generally polyolefin based) that can adhere to both materials. They are the chemical key to locking together what can be very different polymers.
Continuous Composite Designs
If the design intent cannot be satisfied by a single material or co-extrusion approach, the tubing cost starts to rise. The next progression is to use braid or coil reinforced composite constructions (Figures 3 & 4). These generally consist of an inner layer, a reinforcement layer, and an outer layer. These composite tubings are best viewed as a design approach that combines the best aspects of both metals and polymers. These complex constructions have unique properties of their own.
Continuously braided constructions are the simplest composites. These can be used to make high pressure extension tubing, rated to pressures up to 1,800 PSI. In these constructions, soft polyurethane inner and outer layers are fused around braided polymer monofilaments. With metal braid reinforcement, the tubing can be used to effectively transfer torque. This allows a catheter to be effectively manipulated or rotated within the body. Metal braiding can also be used to increase tubing tensile strength and to improve its kink resistance.
Spiral or coil reinforcement improves a catheter’s hoop strength and its kink resistance dramatically. If tubing is being used to deliver a device into the vascular system, these can be very important design considerations. A potential design drawback of a coiled construction can be the tensile strength achieved with this design approach.
Composite Designs with Lubricous Liners
Often braided or coil reinforced catheters require a lubricous inner layer. This lubricious inner layer allows the easy passage of devices through the catheter lumen. A range of potential liner options exist. PTFE is considered the best in terms of lubricity. Its drawback, however, is that it cannot be used in applications requiring Gamma or E-beam sterilization.
In these cases, HDPE and ETFE are good options. HDPE, FEP, and ETFE are also good options as liners for continuously extruded catheters.
Composite Designs with Lubricous Liners and Variable Durometer
Many catheter designs generally have a requirement for a variation in stiffness along the length of the catheter. This can be achieved in a number of ways. In very complex designs, the catheter often features a lubricous liner, a reinforcing layer, and a variety of stiffness sections on the outer layer. Generally, the catheter tip is very soft and flexible to reduce trauma and to improve the catheter’s trackability. On the proximal end, a higher level of stiffness is required to improve the catheter’s pushability characteristic.
In these designs, a combination of reinforcement is often used. Coil reinforcement in the distal section (to maximize kink properties), which transitions to a braid reinforced design (improved torque characteristics), is a frequently used design approach.
In this article, the most significant design considerations and options in the area of single lumen medical tubing have been highlighted. There are additional variables, however, that have not been discussed. The range of potential variables to be considered by the designer in the area of catheter design adds to the complexity of the area as a whole. An optimal design requires a strong understanding of both the end use application and a complete understanding of the limits of the available tubing and catheter technologies.