Question

Describe the major steps involved in manufacturing heart stents. Include the material properties following each process and give a criteria by which you would assess the success of each process. How would you assess the process design specifically related to the materials aspect of the device.

Answer 1

Answer :

Stent :A stent is a tiny tube that your doctor can insert into a blocked passageway to keep it open. The stent restores the flow of blood or other fluids, depending on where it’s placed.

Stents are made of either metal or plastic. Stent grafts are larger stents used for larger arteries. They may be made of a specialized fabric. Stents can also be coated with medication to help keep a blocked artery from closing.

Steps involved in manufacturing of heart stent and material properties :

1)mechanical properties:

The mechanical properties of the stent govern the decision process. This important property will be in charge of providing the correct longitudinal and radial behavior. The mechanical properties of the stents mainly depend on the material, geometry, and medical application of the stent.

Stent’s mechanical properties are interrelated and sometimes contradictory, requiring careful compromise between geometrical and material aspects.

A) Geometrical aspects

Following the classification done by Stoeckel et al. in their manuscript “A survey of stent designs,” stents can be classified into five categories .

a) Coil:

Most common in nonvascular applications, the coil design allows for retrievability after implantation .These designs are extremely flexible, but their strength is limited, and their low expansion ratio results in high-profile devices.

b) Helical spiral:

These designs are generally promoted for their flexibility. With no or minimal internal connection points, they are very flexible but also lack longitudinal support. As such, they can be subject to elongation or compression during delivery and deployment and, consequently, irregular cell size. With internal connection points, some flexibility is sacrificed in exchange for longitudinal stability and additional control over cell size.

c) Woven:

Woven designs are often used for self-expanding structures. While these designs offer excellent coverage, they typically shorten substantially during expansion. The radial strength of such a woven structure is also highly dependent on axial fixation of its ends.

d) Individual rings:

These are commonly used to support grafts or similar prostheses. This design is not typically used as vascular stents by itself.

e) Sequential rings:

This design is the most common in the market and includes two different categories: (I) closed cell, made of sequential ring construction wherein all internal inflection points of the structural members are connected and

(II) open cells, a stent wherein some or all the internal inflection points of the structural member are not connected by bridging elements

B) Material aspects:

Since the introduction of the first stainless steel devices, the materials used for stents have evolved and diversified rapidly. In the drive to obtain a share of what was becoming a vast and growing market, manufacturers invested heavily in research and development to gain continuous.

2)manufacturing process:

Once we have decided the mechanical properties, we give way to the manufacturing process step. The manufacturing process mainly depends on the materials and stent type chosen.

Five technique has been used to manufacture stents: etching, micro-electro discharge machining, electroforming, die-casting, and, nowadays, laser cutting .

a) Etching:

Etching method is based upon the photolithography process .In this process, the desired mask pattern is first projected on the plain sheet coated with photoresist, which after exposure can be developed and etched for the desired pattern .

b) Micro-EDM:

In micro-EDM, the material removal takes place by electro-erosion due to electric discharge generated between closely spaced electrodes in the presence of a dielectric medium. The shape of the machined feature is the mirror image of electrode .

c) Electroforming:

In this process, electroplating is performed on a mandrel in a given pattern. When the desired thickness has been reached, the mandrel is etched away from the electroformed stent, leaving a free standing structure, a fully functional stent.

d) Die-casting:

This is another technique in which the stent can also be formed by subjecting one or more. The metal may be cast directly in a stent-like form or cast into sheet or tubes from which the inventive stents are produced by using any of the method mentioned here.

e) Laser cutting:

A high energy density laser beam is focused on workpiece surface; the thermal energy is absorbed which heats and transforms the workpiece volume into a molten, vaporized, or chemically changed state that can easily be removed by flow of high pressure assist gas jet Currently, this is the technology in the market.

Different types of lasers have been used in stent manufacture including CO2 lasers, Nd:YAG lasers, fiber lasers, excimer lasers, and ultra-short pulse lasers. Fiber lasers have advantages compared to other laser technologies such as better beam quality, reliability, and process efficiency with lower acquisition cost and maintenance. Laser cutting is a thermal process which results in thermal damage such as heat-affected zone (HAZ), striation, recast layer, microcracks, tensile residual stress, and dross. To overcome the thermal damages, basically the following post-processing techniques are applied: pickling techniques, soft etching, annealing, and electropolishing. All these post-processing techniques raise the manufacture cost and could affect the mechanical properties of stents.

3)Additional properties process :

Finally, the additional properties step. Although it constitutes the final layer of the stent pyramid, additional properties cover a range of modifications to stent designs. This last step indirectly relates to the stent type and its medical application.