Feature Article

As the applications for stents expand, contract manufacturers have responded with improvements in manufacturing operations.

By: T. Whelan

In today’s competitive and cost conscious world, healthcare providers are increasingly focused on reducing the cost of medical procedures and improving the level of patient care. The modern vascular stent, the once “crazy” idea conceived in the 1970s, is now a well established, minimally invasive therapy that meets these goals.

The human anatomy presents many challenges to the stent designer. Although basic to the human structure, the vascular system still varies from person to person. The vasculature is often tortuous or impossibly small and as a result some lesions are difficult to access with catheter-based devices. Nevertheless, the need for cost-effective, minimally invasive therapies has persisted and the application of stents has steadily increased over the years to include virtually all areas of the human vasculature.

Expanding stent applications, pressure from physicians, insurance companies and increased competition within the medical device industry have all caused stent manufacturers to respond with improvements in all aspects of design and manufacturing.

Modern manufacturing

Modern stents range in size from the miniature 1 mm neurovascular stent to large 50 mm thoracic stents used in conjunction with synthetic grafts to repair thoracic and aortic aneurysms. To address the myriad design challenges, a number of different materials have been tried over the years. Common materials used in modern stent design include alloys of stainless steel, cobalt-chromium and nickel-titanium. Each have their advantages and disadvantages depending on the application. Stents are typically manufactured from wire or laser-cut from tubing.

In the case of tube-based stents, the desired pattern is laser cut into the tube. Although lasers have greatly improved over the years, the process still typically creates an oxide layer and other imperfections on the surfaces of the tube. This is particularly apparent as the laser beam becomes more diffuse, which happens in applications that require thicker tube walls for more structural strength. Subsequent processing is aimed at eliminating those surface defects.

Traditionally, as much as 30% of a stent’s material was removed during its manufacture to get a polished mirror surface finish. Today, there are designs with targets as low as 5% material removal. The device still meets the same high standards, but less material removal yields fewer variations in the end device.

Chemical and microabrasive blasting processes are standard industry methods for attacking this oxide layer. A chemical process treats all surfaces equally, which can be advantageous in stent designs with small features that can otherwise impair the effectiveness of microabrasive blasting. However, what makes microabrasive blasting advantageous is that it can be manipulated to vary the material removal between different areas and surfaces. It can also be pinpointed to small, localised areas of the stent, which is beneficial in particularly challenging designs. For example, the goal may be to deburr a specific spot on the stent and avoid hitting adjacent surfaces or, more material may need to be removed from the laser cut surfaces and less on the inner and outer diameter surfaces. Those are applications for which microabrasive blasting is particularly well suited.

In basic microabrasive blasting, an operator manipulates the stent inside an enclosed workstation using custom designed spindles or holders and, depending on the size of the stent, utilises custom vision enhancement. The blasting nozzle is pointed to the surface to be abraded and the operator activates the blast with a foot pedal. The air pressure and amount of abrasive metering into the airstream are automatically regulated and kept consistent.

Removing both the oxide layer and remelt requires a precise blasting process; too much abrasion will weaken the joints and cause premature device failure. The process is often controlled by measuring the amount of weight that is removed from the stent in thousandths of a gram.

Microabrasive blasting requires a flow of clean, dry air that is mixed with a pure, uniform, abrasive media. Air also flows through the front of the chamber, around the workpiece and is pulled out of the back of the chamber by an industrial dust collection unit. This keeps the work area clean and maximises operator visibility.

Another advantage of microabrasive blasting is the variety of blasting media that is available. Aluminium oxide and silicon carbide are the industry standard abrasives used in stent surface preparation and are available in a range of media sizes. The grit size of the media, which is always measured in microns and blast pressure, is dependent on the product attributes and specifications.

Polishing is the final operation in stent manufacturing. The industry standard is electropolishing, an electrically driven process, but in recent years chemical processes have also been employed. As with surface preparation, both polishing methods have their advantages and disadvantages. The goal of stent surface preparation, whether chemical or abrasive, and stent surface finishing, whether chemical or electro polished, is to reduce and eliminate surface defects and to produce a device that has improved corrosion resistance and fatigue life.

Looking ahead

What started out as a so-called crazy idea in the 1970s is now a multibillion dollar industry today. As physicians and designers continue to improve on the current generation of stents and tackle new challenges, boundaries are being pushed back. With more tools and options available today, stent designers and manufacturers have the opportunity to develop ever more sophisticated designs. Contract companies that specialise in stents are available to help medical device manufacturers meet new challenges, which results in a higher quality, better performing product brought to market faster and more cost-effectively than ever before.

Tim Whelan works in Technical Sales at Comco Inc., 2151 N. Lincoln Street, Burbank, California CA91504-3344, USA, tel. +1 818 841 5500, e-mail: marketing@comcoinc.com, www.comcoinc.com

Steven Parmelee is President of Process Development at Relucent Solutions LLC, Santa Rosa, California, USA, www.relucentsolutions.com