Current trends in medical extrusion can be summed up by considering that function of the extruded part is becoming more specific, less invasive, safer, and qualitatively statistically verifiable.

Having been directly involved in the medical extrusion industry since the mid 1970's, I've observed that the fundamental elements of change have remained constant over the years. The basics I am referring to include: 1) increasing specialization in the function of the extrusion; 2) tolerancing that continues to grow closer and closer; 3) increasing demand for new and better performing polymer materials; 4) heightened levels of testing and inspection of the extrusion; 5) disposability; 6) shorter delivery times; and 7) more technically trained personnel.

Increasing specialization of the extruded part

Looking at the requests for quotes that come across our sales desk, I don't recognize anything as standard and almost nothing that would be considered multifunctional. Procedures are specific and so are the related extrusions and the demand for custom extrusions has been growing since I entered the industry over thirty-five years ago and is growing faster with every passing month. As the extruded parts become more specialized so does the extrusion technology required to produce those parts challenging the custom extruder's expertise, agility and resources. Some of these technologies include multi-lumen extrusion, co-extrusion, miniature and micro-extrusions, over-extrusions, tapered extrusions, braid reinforced extrusions as well as untested concepts and it is common for new designs to integrate several of the above mentioned extrusion technologies into a single extruded part. An important facet in the movement towards specialization in extruded tubing is the trend to miniaturize the extruded tubes not just to be less invasive, but because function requires miniaturization to access body areas too small for larger tubes such as the eye, the brain or a neonate.

Miniature challenges

There is much more to making a miniature tube than extruding with the usual tools and drawing the hot melted plastic, commonly referred to as extrudate, down further than usual by either increasing the haul-off speed or slowing the extruder drive. Although this can work in some cases it is generally not the way to go as lofty draw down ratios are detrimental to the physical and mechanical characteristics of the polymer. Probably the most important decision a device engineer needs to consider when designing a miniature tube is the selection of a suitable raw material. After that other issues include keeping the extruded part design as simple as possible, how to verify quality, post extrusion processing, and selecting someone to extrude the part.

Raw material selection

Whether one is considering an engineering polymer or a TPE (thermoplastic elastomer), this selection is fundamental in determining everything about the extrusion process and its design. For micro-extrusions there is no formula or rule for determining which resin to select. What is important is a material's melt strength or ability to be drawn out and hold its form. This type of information is not textbook information as the published melt flow index only describes flow and not how well the extrudate holds its form as it is drawn down. Experience is the best resource in material selection, and it pays to listen to the person who has been extruding the tubing. However, it is not unusual to work with draw down ratios as high as 40:1 and higher depending on the complexity of the design. Ordinarily a 40:1 draw down ratio would be considered far outside an acceptable draw down range for most thermoplastics. However, the mulit-lumen tube shown on this page has a draw down ratio on the edge of the material's physical capabilities because of our toolmaker's ability to make miniature extrusion tips and dies durable enough withstand the stresses of extrusion.

Materials used successfully

As mentioned earlier, the ideal materials for micro-extrusions have superior melt strength and draw down characteristics. To get the optimal draw down from a polymer, it is best to work with an unfilled resin. That is, no radiopacifiers such as barium should be in the resin, as the filler is detrimental to draw down as well as the surface quality. Materials available for medical extrusion are somewhat limited but many of them are well suited to micro-extrusions, with polyamides leading the way. Although polyurethanes draw down well, they do not have the tensile characteristics of the polyamides and the PEBAs and become a second choice where strength is critical. Among the high heat engineering polymers, PEEK has been used successfully by many extruders, but it is expensive and a laborious cleanup for the technician running the extruder.

Tube design and keeping it simple

In micro-tubing, more so than larger custom tubing, function tends to be very specific simply because the tube is so small. Additionally, in designing a tube the design engineer should keep in mind that simplicity of design is a key to producing a micro-tube successfully.

Micro-tubing can be extruded in most of the same varieties as larger tubing, including single and multi-lumen tubing, co-extruded tubing (striped and layered walls), tapered (bump) tubing, and over-extrusions. Although the tube configurations may be similar, the extrusion process differs greatly and often it is the extruder technician's ingenuity that makes the difference in developing a repeatable process recipe for a tube.

For single lumen tubes, selection of extrusion dies and tips is done much the same as with larger tubing. The same size extruders as those used for larger tubes are often-used. However once we move away from straight single lumen tubing most of the dynamics of micro-extrusion change require special attention, special extrusion machinery, and special extrusion tooling (extrusion crossheads, dies and tips).

All the major extruder manufacturers that have committed to serving the medical extrusion industry have developed downsized extruders capable of the low outputs necessary in extruding many smaller medical tubes. These machines sometimes are just smaller versions of larger extruders and do work well. Also available are the micro-extruders with screw diameters as small as .250in., which I have seen extruding at tradeshow. These micro-extruders are very useful in addressing issues of the polymer's residence time in the extruder. Polymers will degrade once melted so it is important to pump the polymer through the extruder efficiently, maintaining consistent back pressures, slow enough to have manageable haul-off speeds, but fast enough not to degrade the resin. For the extrusion technician this process window can be frustrating to find.

The extrusion dies and tips reflect the profile of the tube. For multi-lumen and co-extruded micro-tubing making these tools can be the biggest challenge in the job. Even with the best extrusion tool designers several versions of an extrusion tool must be made before the tool works and the tube is deemed acceptable. Therefore, having in house capability to fabricate one's own tools is important to short lead times.

Summation

There is much more to extruding reliable micro-tubing than there is for larger standard sized tubing. Designing the tube requires an awareness of a new set of dynamics. Several attempts to “get it right” or find what works may be necessary. Finally, the development of a micro-tube requires close cooperation between the designer and the extruder.