“Tube Nuances” – Laser Processing of Stents Versus Hypotubes

Sometimes in the world of motion control, we might oversimplify applications a bit and speak in somewhat sweeping generalities.  For instance, while “cylindrical laser processing” might tend to mean “stent-cutting” to many in the realm of precision automation, experts in the field likely appreciate that this phrase also includes other types of tube processing such as hypotubes, catheters, and guidewires.  These other device classifications require the same generic motion control configuration (a rotary stage on top of a linear stage), but much of the similarity – and therefore what ends up being important for system design – ends there.

The more things change…

Let’s begin by delving into the most noticeable differences between these two sub-classes of applications.

  • Perhaps the most obvious difference is in tube length. While stents are usually ones or tens of millimeters in length, hypotubes are often processed in lengths of several hundred millimeters at a time.  With this difference comes a few challenges.  For instance, the linear stage travel must be understandably longer, and processing lengths are now limited by how much “tube sag” can be accommodated and still maintain proper beam focus and cut quality. Tube-fixing strategies might also differ due to this change.  Instead of a simple bushing to support the far end of a stent under process, a steady rest and perhaps an intermediate support might have to be included for longer tubes and wires.
  • As has been discussed at length, stent motion profiles are complex and involve very high accelerations and direction reversals. In contrast, many guidewire and hypotube cuts are simple slots that can be cut as the workpiece rotates continuously (and in one direction) underneath the static laser beam.  Therefore, instead of very high acceleration being most important, the ability of the rotary stage to achieve and maintain very high peak velocities is paramount.
  • At the inception of laser stent processing, those devices were stainless steel (so-called “bare metal stents” or “BMS”). While stainless steel is still the material of choice for hypotubes, stents have progressed through more and more exotic material choices (nitinol, drug-eluting, and now bioresorbable polymers).  Laser processing parameters (and indeed, laser choice!) are influenced by material type.  Furthermore, processing speeds often must be slowed for more complex materials.  As a result, hypotube processing can often take place at much higher part surface speeds than stent processing.
  • Tolerances (and therefore acceptable acceleration values) for hypotubes are generally more loose than for stents. While stents typically require form errors to be on the micron level, continuous tubes can tolerate cut errors perhaps an order of magnitude greater.  This is one more reason that continuous tubes can be cut at much higher rates than stents.

….the more they stay the same….

Now that we have a good picture of what separates stent cutting from long tube processing, let’s assess how they might be similar.

  • Both sub-applications process tubes (many thanks to “Captain Obvious” for that contribution!). Acknowledging that this is a pretty basic observation, it still is important.  For instance, this infers that the workpiece holders will be the same (typically precision collets, or perhaps a pneumatically actuated 3-jaw chuck).  This means that systems designed for either type of processing could conceivably share tooling with one another.
  • While material choice is often different (as already noted), in some cases BMS are still being made. So laser parameters fine-tuned for one application can sometimes be applied to the other, saving development time.
  • Likewise, some hypotubes may in fact have cut shapes more complex than simple slots. In those cases, stent-cutting experts can put all of their hard-earned lessons to use in making cuts such as “I-beam” and “dog-bone” patterns.  While the larger acceptable error tolerances should still allow for overall faster processing than with true stent profiles, acceleration-profiling tools inherent in many advanced controllers will still need to be brought to bear for these continuous tube cuts, just as for stent cutting.

OK, so what does this mean for me?

Well, it probably means that you should carefully review who your choices are for these high performance systems.  Pick a vendor who can appreciate these differences, who will ask all the right questions, and who has optimized their product offering to be the best at all of them.  For instance, although some purpose-designed systems might be perfect for stent cutting, find out if that same vendor has a companion rotary stage that can provide the much higher peak speeds required for continuous tube processing as well.  Verifying this not only simplifies vendor selection, but leverages all the great controller features you might have learned for stent processing, allowing you to be best-in-class for spiral tube cutting as well.