Features

Over the past five years, technological advances have enabled product applications for microextrusion to penetrate into the medical OEM arena. Simply speaking, micro now really means micro. Years ago, when an OEM was asking for micro tubing, they typically were looking for ranges between 0.060 and 0.100 in. OD with 2-4 lumens. Now micro range dimensions are down to 0.024 in. OD with up to 6 lumens, including an ID range as low as 0.003 in.

A sampling of microextuded tubing. (Credit: Natvar)

While the medical device sector is looking to miniaturize devices and provide enhanced features, it is also focused on cost savings. Miniaturized components for medical devices allow OEMs to develop a wide range of products from enhanced catheters to precise flow restrictors.

Moving to a microextrusion process from traditional injection molding provides more flexibility to achieve those objectives. For example, should a manufacturer need to modify the dimensions of an injection molded part, a new mold is needed. By contrast, if the manufacturer microextrudes a profile and then needs it to be 0.003 in. bigger, the extruder can be easily adjusted to achieve the new dimension. The micro-extrusion process provides more efficiency and downstream flexibility when compared to injection molding.

Using catheters as an example, being able to get down to 762 μm (or 0.030 in.) OD with wall thicknesses down to 0.003 in., presents as many challenges as it does opportunities. From a challenge standpoint, material is a key factor along with downstream equipment. Running products on a micro level requires materials to be processed at extremely low output levels, thus creating the potential for high shear and material degradation.

In the design of any micro device, the material selection is a critical component that must be considered. Materials that can handle high temperature for long periods of time, such as polytetra-fluoroethylene (PTFE), nylon, and fluorinated ethylene propylene (FEP), are generally favorable candidates because of the low output and long dwell time in the screw and die. Unlike traditional extrusion, which processes a large amount of material quickly, microextrusion processes a small amount of material slowly to get the desired physical and performance attributes.

In addition to the benefits provided by the reduction in size, another plus is to have the ability to build in advanced steerable capabilities, enhanced with multilumen radiopaque features to aid in complex procedures. This type of structure can be produced in multilayer size, with enhanced performance in many areas. Depending on the application and device, you can modify configurations to aid the designers even further.

This opens up a wide range of potential applications in the neurosurgical field along with vascular device applications. As minimally invasive surgical applications are developed, the need for smaller, highly advanced microextrusions will become more prevalent in the medical market. Adding radiopaque stripes to microextrusion can provide additional advantages in many devices.

Enteral feeding tubes are a great example of microextrusion and radiopaque technologies coming together. The positioning of these types of devices are critical to their performance. The radiopaque feature allows clinicians to place the tube in the body at exact locations for maximum effect. These types of microextrusion-driven devices will enhance common procedures such as heart catheterization, stent implantations, and atrial reconstructions above the neck and procedures below the knee where vascular devices have to perform at a high level for a successful outcome.

In other applications, such as critical flow restriction where glass extrusion is used, microextrusion can also be a good alternative. Because of the tight ID tolerances needed to provide a precise flow through a restrictor, glass was typically the first material engineers turned to. Now these tolerances can be held with microextrusion, thus eliminating the need for expensive glass.

Microextruded tubing exiting the extruder. Microextrusion processes a small amount of material slowly. (Credit: Natvar)

In addition to material selection, downstream processing equipment is another major aspect of a successful product introduction. A closed-loop feedback system is highly recommended. It adjusts screw speed to maintain a constant die pressure, thereby minimizing variations in the extruder output.

For example, feedback loops based on the tube OD readings are taken by a laser micrometer. If the OD readings move to the high end of the allowable range, the system automatically adjusts the extrusion process. It then continues to monitor the process and make further adjustments as required independently. With these type of extrusions, ultrasonic equipment should be considered to monitor wall thickness along with OD (to calculate to ID).

With miniaturization comes tight tolerances, which is the next hurdle to overcome. A typical process capability index (CPK) on medical extrusion runs between 1.0 and 1.3 for long-term stability. However, microextrusion ranges should be above 2.0 to provide a stable and repeatable process. This is critical for the device manufacturing process.

If producing a device at sizes mentioned above, the assembly process has to be set up to handle microextrusions along with tight tolerances. If assembling a device using ultrasonic welding, for example, the ultrasonic frequency (kHz) will also have to have a high range and repeatability factor to provide a robust and trustworthy assembly process.

With products at this scale, inspection is as critical off-line as it is online. Vision systems with high optical resolution are required along with special cutting techniques for parts provided at specific lengths. Fortunately, the off-line inspection techniques available have kept up with microextrusion advances. From nitrogen cutting to high-resolution cameras along with specialized measuring optics, the industry is in a good position to provide a quality product with repeatability.

Microextrusion is not just for tubing. It also is a great choice for profile extrusion where injection molding is currently being utilized. Micro gaskets and liners are starting to make their way into medical device platforms more often. In the past, devices that contained corrosive medications may have used a rubber injection molded part to prevent patients from being exposed to medication leakage from a damaged delivery system.

With micro profile extrusion, these shaped gaskets can now be extruded and welded at lower cost than injection molded components. This attribute enables design engineers to create parts more quickly using lower-cost micro profile dies rather than expensive injection molds. In addition to reduced mold costs, OEMs benefit by shortened development timelines.

Micro transition tubing also helps engineers facilitate product design. An example of this is the need to transition from a microcatheter where the lumen is too small for the clinician to insert the guide wire. In most cases, a micro-catheter is attached to an external handheld instrument for inserting a guide wire or some other apparatus such as fiber-optic for light into the device. Microtransition tubing allows for a larger opening at the tip of the instrument where a device can be easily inserted to provide an ease-of-use factor that is very important in the design of most microcatheter applications. For example, a catheter can have a lager distal opening for ease of use (0.060 in. ID) with the proximal end being as small as 0.030 in. OD.

As mentioned previously, downstream equipment for this type of application is critical from alternating puller speeds in conjunction with closed-loop feedback to the drive motors of the extruders. Devices such as these would typically be used in procedures involving the femoral artery, where the insertion point is large but the end use of the catheter is the coronary artery or even further into the heart itself, the anterior descending artery where blood clots can occur.

Other microextrusion tubing applications for devices include procedures for deep venous thrombosis where getting into the lower extremities of the legs is difficult and requires the use of minimally invasive devices with high-speed vascular drills attached.

With the ability to design multilayer microbore in a multilumen design, the tube (or profile) becomes a functional and advantageous part of the overall device. It allows product design engineers to deliver medications, guide wires, fiber-optics, and surgical devices within one microextrusion structure. Advances in microextrusion technology will allow OEMs to push technology and material science to develop the generation of micro devices.

This article was written by Bob Donohue, General Manager, Natvar, a Tekni-Plex business, Wayne, PA. For more information, visit here.