Next-Generation Stent Manufacturing

Predicting the future of coronary artery disease treatment is not for the faint of heart. With dozens of vascular stent designs, materials, anti-proliferative drugs, and elution techniques, there is no shortage of potential solutions. What is more, different combinations of solutions face a gauntlet of clinical trials and pre-market approvals.

That notwithstanding, the fact remains that in 2010, approximately 700,000 U.S. patients had one or more coronary stents implanted, according to the American Heart Association. The majority of those were metal structures (cobalt-chromium, Nitinol, stainless steel) with reservoirs or coatings of an anti-proliferative drug, a compound that inhibits the restenosis process (narrowing of a vessel). However, the January CE-approval of Abbott’s bioabsorbable vascular stent, ABSORB, and its positive one-year safety and efficacy results in the United States may be indications that polymers will soon have a much larger role in the medical device sector than they already do.

While metal stents are extremely efficacious and have been the standard of care for coronary artery disease, stents made of polymeric, bioabsorbable materials promise significant patient benefits. Most importantly, they dissolve away after providing their life-saving vessel support role (typically within 12 months). With no stent in place, the vessel can resume its natural vasomotion (constriction and dilation), there is no danger of late stent thrombosis (clotting), and there is, generally, no need for long-term anti-platelet therapy. What is more, polymeric stents, even when in situ, do not interfere with the ability to image the chest with CT or MRI technologies, nor the ability of surgeons to go back in to vessels and do further surgical interventions, if necessary.

There is only one catch, but it is a big one. It is exceedingly difficult to machine these new materials with traditional tools. Polyglycolic acid (PGA); polylactic, co-glycolic acid (PLGA); polylactic acid (PLA); and polycaprolactone (PCL) all have low melting and glass transition temperatures. Mechanical processing tears them up, and long pulse lasers melt or otherwise compromise the integrity of the materials.

Enter Raydiance’s ultrafast laser technology coupled with Rofin’s precision workstation engineering. The integration of these two technologies in the StarCut Tube Femto has become an enabling solution for the manufacturing of these next generation stents. Given the intricate and precise designs of these polymeric devices – strut widths are often 150µm wide and 150µm thick – not only is the athermal ablation of the laser key, but so, too, is precise synchronization of the motion control with the laser pulse delivery. Achievement of this is through seamless integration of the laser operating system software with the StarCut Tube workstation. Once a machining design is uploaded to the workstation, the operator chooses the appropriate laser parameters (pulse energy, repetition rate, process gas) – essentially a recipe for a given material type, thickness, and part cycle time – and the workstation takes it from there. It is a fully automated process. Typically, as-machined bioabsorbable stents move from the workstation, for wiping with a lint-free cloth and then for blow-drying with nitrogen. No significant post-processing steps are necessary. As can be seen in the images on the to the left, the StarCut Tube Femto produces high quality, heat-affected zone-free (HAZ) ablation of the traditionally difficult to machine PLLA. For demonstration purposes, machining of a generic stent pattern was into this 3.175mm outer diameter tube. The wall thickness is approximately 214µ.
 

Certainly, as bioabsorbable stents begin to enter the marketplace over the next few years, their metal counterparts will continue to evolve as well. Dimensions will be smaller, designs more intricate, materials more varied. These innovations will place further demands on manufacturing capabilities. Again, the StarCut Tube Femto will provide a ready solution.

To date, application engineers at Rofin and Raydiance have shown that stents made of Nitinol, stainless steel, cobalt-chromium, even platinum and gold are machinable to precise customer design specifications without introducing HAZ. This has dramatic return on investment implications, particularly considering elimination of multiple post-processing steps with the athermal StarCut Tube Femto processing. What is more is the very fact that machining parts without thermal effects means that unexplored areas of stent design become available to manufacturers. The potential to create a whole new generation of metal stents – smaller, with micro-struts for neurovascular applications, for example, or made of innovative alloys or composite materials – will be wide open.
 

Rofin-Lasag Lasers
Buffalo Grove, IL
rofin-inc.com

Raydiance
Petaluma, CA
raydiance.com