Ray Ledinsky, Teleflex Medical OEM, provides expertise on designing a catheter to achieve optimal performance.
Design
In catheter design, the functional requirements of the application allow the designer to identify performance requirements such as flexibility, lubricity, kink resistance, column or push strength, and torque transfer characteristics. Development of an optimal catheter design requires a strong understanding of catheter technologies to achieve the desired performance characteristics. This article offers a layer-by-layer approach to several design considerations for your device. At its simplest form, a reinforced catheter design is composed of an inner liner layer, a central reinforcement layer, and an outer polymeric overlayer.
Inner layer: Liners
When selecting a liner material, it is essential to consider both the benefits of a material and its design considerations. There are trade-offs with any material. The key is to understand what is critical for the catheter’s application. Here is a reference guide to selected liner materials.
PTFE (polytetrafluoroethylene)
Benefits:
- Best lubricity, lowest coefficient of friction of any fluoropolymer and thermoplastic
- Ideal for multi-durometer design
- Thin walls
- EtO and autoclave sterilization
Design considerations:
- Requires manual assembly
- No Gamma or E-beam sterilisation
- Etching required
FEP (fluorinated ethylene propylene)
Benefits:
- Good lubricity
- Lower coefficient of friction compared to ETFE, HDPE, and Pebax
- More flexible than PTFE
- Single-durometer (continuous process) or multi-durometer
- Gamma, EtO, E-beam, and autoclave sterilisation
Design considerations:
- Single durometer is most efficient when using a continuous process
- Etching required
ETFE (ethylene tetrafluoroethylene)
Benefits:
- Superior tensile strength and stiffness
- Higher lubricity as compared to HDPE and Pebax
- Single-durometer (continuous process) or multi-durometer
- Excellent impact resistance
- Gamma, EtO, E-beam, and autoclave sterilisation
Design considerations:
- Only single- durometer is available when using a continuous process
- Etching required
HDPE (high-density polyethylene)
Benefits:
- Good lubricity
- Good adhesion
- Continuous process
- Gamma, EtO, E-beam, and autoclave sterilisation
Design considerations:
- Not as lubricious as PTFE, FEP, or ETFE
- Only single- durometer is available when using a continuous process
- No etching require
- Adhesion is based on tie-layer technology
- Increased reinforcement options are possible
Polyamides/Pebax
Benefits:
- High material strength
- Good for thin-walled, high- pressure applications
- Single-durometer (continuous process) or multi-durometer
- Gamma, EtO, and autoclave sterilisation
Design considerations:
- Only single-durometer is available when using a continuous process
- Can be manually assembled for multi-durometer process
Central layer: Reinforcement
There are two distinct types of catheter reinforcement: braid and coil. Advanced partners should be able to offer you technologies for increasing the tensile yield of a reinforced catheter. These include the incorporation of longitudinal, reinforcing components in a variety of materials such as flexible, high-tensile, advanced fibres.
Braid reinforcement
Excellent torque control is the primary driver of a braided catheter design. This can be achieved with braid reinforcement. Manipulation of the distal tip of a catheter, by twisting the proximal end, requires good torque transmission with little ‘whip’. A relatively linear response is a desired catheter characteristic for end-use applications.
The wire size, profile, density (PPI), and braid configurations can be engineered to provide a great balance of pushability with good torque control. In some cases, multiple braid layers are necessary to meet the level of manipulation that is required.
- The pitch of the braid pattern can also be modified, by section along the length of the catheter, to vary both the catheter flexibility and hoop strength.
- Materials used in the reinforcement can be metallic or non-metallic. As for metallics, the most popular is stainless steel. Nitinol, due to its kink resistance, is becoming popular in microcatheters.
There are three common braid patterns typically used in catheters. Each pattern produces different levels of torque and kink resistance.
- Regular braid pattern is a common pattern that uses 16 wires in a one-under-two, over-two pattern.
- Diamond braid pattern also uses 16 wires but differs from the regular braid pattern in that it produces a two-under-two, over-two wire pattern. This pattern tends to provide better torque and more kink resistance than the regular braid pattern but also has a slightly higher cost.
- Diamond braid pattern, half load: By utilising half the number of wires, the diamond pattern can be produced in a one-under-one, over-one wire pattern. This pattern provides more torque than the regular diamond pattern but incurs a much higher cost due to the reduced wire load.
Device designers are not limited to these three basic patterns. Several original equipment manufacturers can create custom-engineered braid and coiling variations. Now it is even possible to utilise variable pitch, continuous reinforcement that can vary performance characteristics along the length of the shaft. Also, there are novel technologies for connecting dissimilar sections of the shaft without sacrificing shaft flexibility or performance. Designers can create precise catheter characteristics by combining any number of diameters, reinforcements, and hardnesses.
Coil reinforcement
Outstanding hoop strength, kink resistance, and good pushability are characteristics of coil reinforcement.
- Teleflex Medical OEM can provide discreet, coil-reinforced catheters, on a PTFE liner, using a laid-up assembly process. This is generally limited to a single pitch along the length of the catheter.
- A recent technology is continuous, coil-reinforced assembly that allows the coil pitch to be varied along the length (variable pitch coiling). This allows differing amounts of flexibility and kink-resistance along the catheter shaft.
- A continuous-coil design, using an HDPE liner, is one of the most cost-effective, composite designs available for a catheter.
Often there are trade offs required in the catheter 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 and increase the flexibility, but this will also reduce the torque. As a result, it is important when designing a reinforced catheter shaft to consider the performance requirements up front to assure that the design meets the needs of the user.
Outer layer – polymeric overlayers
This selection of the outer layer material strongly influences tensile yield, stiffness, and catheter pushability. The outer layer may be multiple materials or durometers.
To perform effectively, a reinforced shaft needs strong adhesion between the inner and outer layers. Delamination between these two layers can compromise the catheter’s functional performance. This limits the choice of outer layer materials to polar materials, because they can adhere directly to either an etched surface or a tie layer.
Polar materials are also beneficial for adding a hydrophilic coating on the outer layer, as they provide better adhesion. Lubricity and durability are the key requirements for consistent performance from a hydrophilic coating. Lubricity provides for ease of device insertion and tracking to the treatment site while simultaneously reducing damage to the endothelial layer of the vasculature. The coating’s durability properties are critical to meet the reliability demands of modern catheter applications.
More construction elements for consideration
We’ve examined the three, basic components of catheter design. By incorporating a variety of construction elements, manufacturers can create catheter shafts with unique features.
- A steerable catheter shaft can be produced with multiple steerable wires, enabling clinicians to maneuver the tip precisely in multiple directions. Clinicians must be able to position the tip in the right place so that they can deploy the device properly.
- Multi-durometer segments along the shaft and tip, with varying degrees of softness or hardness, can alter the catheter’s flexibility, bend radius, and deflection angles.
- Balloons, with thin walls and small profiles, can be added to the shaft tip to create innovative catheter dilation and delivery systems.
- Marker bands of high density precious metals—typically tantalum, gold, or platinum — can be positioned along the shaft and used as a guide to distinguish key areas along the length of the catheter. Flexible radiopaque markers are sometimes used in lieu of metal marker bands. Encapsulated with tungsten-filled Pebax®, these markers provide similar radiographic visibility, while being soft and pliable.
A plethora of decisions go into producing high-performance catheters. At each step of the process, these decisions can positively or negatively impact the overall function of the device. In addition, taking a concept from a functional prototype to a marketable catheter requires in-house expertise, and superior design and manufacturing capabilities. Finding the right partner that can deliver along those attributes can go a long way toward a successful product launch.
* 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.