Stents

A stent is a miniature mesh tube, made of a biocompatible metal, biodegradable metal or polymer, placed inside of a blood vessel (Cardiovascular, neurovascular and peripheral blood vessels) or a natural conduit (Gastrointestinal, urinary and biliary tracts). The stent acts as a scaffold, pushing against the internal walls of the conduit / vessel to open a blocked area and thereby enables natural flow and prevents the vessel from collapsing, narrowing or closing.

Stents differ greatly in their design, dimensions and the optimal material for their fabrication, based on their intended application. The following table enumerates common stent applications and the range of possible dimensions:

Coronary stents

Coronary stents are implanted during minimally invasive PCI procedures treating narrowed coronary arteries, which supply Oxygen to the heart muscle. Stainless steel and cobalt chrome alloys have the plastic properties that enable balloon-expandable stents to withstand the high-pressure present in these small blood vessels. Balloon-expandable stents undergo plastic deformation during balloon-expansion thereby fixing their shape.

In cases of perforation or significant damage to the arterial wall, special stents coated with pericardium tissue can be applied as an emergency measure to "seal" the damaged area and enable blood flow through the artery. 

Neurovascular stents

The two most common types of cerebrovascular disease are ischemic stroke (87%) caused by a blocked or narrowed intracranial blood vessel and hemorrhagic stroke (10%) often caused by a ruptured aneurysm.

Tiny self-expandable stents can be used to re-open blocked brain blood vessels. On the other hand, stent-assisted coil embolization treats intracranial aneurysm by threading a thin wire (embolic coil) through a catheter into the affected area of the brain in order to fill the weakened portion of the vessel.

Balloon-expanded stents require high pressure for their expansion which can damage the thin blood vessels of the brain. Self-expanding stents eliminate the need for high pressure expansion making Nitinol the metal of choice for neurovascular stents.

Carotid stents

A self-expandable Nitinol stent is fitted inside a narrowed carotid artery, which feeds blood to the brain, to improve blood flow and prevent the artery from collapsing or closing after the procedure. Carotid stents need to be elastic because of the perceived risk that outside pressure on a balloon-expandable stent (stainless steel or cobalt chrome) may permanently damage the stent and block the artery.

Peripheral stents

Peripheral stents are implanted in peripheral blood vessels in limbs and internal organs. Peripheral stents are often self-expandable Nitinol stents that can be bare metal, drug coated, or membrane coated.

Biliary stents

Biliary stents are used to treat obstructions in the bile duct. The complex delivery and positioning of biliary stents requires high flexibility to allow maneuverability through loops, curvatures and angulated anatomies.

Biliary stent design takes into account flexibility, migration resistance and radial pressure requirements. STI manufactures the metal stent, typically made of Nitinol, and welds radiopaque markers as needed. A membrane cover is added later to prevent in-growth and ensure free passage through these relatively long (≤ 120 mm) stents.

Stent manufacturing process

Stent manufacturing is an art that requires expertise in many areas, starting from the selection of the optimal raw material, through high-precision laser cutting of complex geometries, to perfect finishing that includes both surface treatments and heat treatments to ensure the highest quality of the desired product.

Raw materials

The first step entails selecting the optimal material for the intended application. Then, the quality of incoming raw materials must be inspected and verified to ensure consistent production at tight tolerances across and within production lots. Metal stents are typically made of implantable grade metals, such as: Stainless Steel (316LVM), Nickel Titanium (Nitinol), Titanium or Cobalt Chromium (CoCr) alloys (L605, MP35N).

Laser machining

Stent fabrication methods include: Wire braiding or knitting, laser sheet cutting, and especially laser tube cutting. Today, stents are being fabricated mostly by laser cutting metal tubes of various diameters, lengths and wall thickness.

Heat treatments

Balloon-expandable stents made of stainless steel and cobalt chrome alloys undergo annealing to: Relieve internal stresses, soften the metal, improve elongation rate, lower the risk of strut breakage, and improve fatigue resistance. Self-expanding stents utilize the elastic properties of Nitinol and require a different process called shape-setting to fix the final shape of the stent and the transition temperature.

Surface treatments

Post laser processing steps include: honing, micro-blasting, pickling, electropolishing, passivation and ultrasonic cleaning. These steps result in a high-quality biocompatible product having a bright, shiny surface, free of defects, and improved corrosion resistance.

Improving x-ray visibility

Advances in imaging technology and stent design lead to smaller delivery systems and thinner stent profiles. Fluoroscopic visibility of smaller stents decreases with their size and therefore requires the integration of radiopaque markers into the stent design. Markers are made of precious metals, such as: Gold, Tantalum and Platinum-Iridium. Laser welding of dissimilar materials introduce galvanic corrosion challenges that must be addressed.

Quality assurance

Visual and dimensional inspections undertaken throughout the manufacturing process, using high-resolution optical microscopes and video inspection systems, ensure that the final product meets the customer’s specifications. STI’s stent manufacturing and assembling processes adhere to ISO 13485:2003 and ISO 9001:2008 quality standards.

Stent kit assembly

Clean room assembly of cardiovascular stent kits and heterologous tissue covered stents, including: biological tissue processing, final product assembly and packaging.

© 2011 STI Laser Industries, Ltd.   ISO 13485:2003, ISO 9001:2008 - Contract manufacturing, assembly and packaging of medical devices.