Page 42 - Advances in Biomechanics and Tissue Regeneration

36 3. DESIGN, SIMULATION, AND EXPERIMENTATION OF COLONIC STENTS

3.2 IDEAL MECHANICAL PROPERTIES FOR COLONIC STENTS

Colonic stents present the following characteristics:
• Diameters are two to three times greater than other types of stent (30–35mm vs. 10, 12mm in vascular stents and 15,
18mm in esophageal stents).
• Great opening capacity (a lumen between 15 and 20mm must be obtained, at least).
• Recovery force as uniform as possible, independent of the opening diameter.
Therefore the mechanical performance of colonic stents presents a number of differences compared to the usual low
expansion rate coronary stents, needing high rates of elastic recovery (superelasticity) and enough radial expansion
force in a wide deployment diameter range.
At present, there are two design alternatives: the self-expanding stent, based on the superelastic properties of certain
metallic alloys (e.g., Nitinol), and the balloon-expandable stent, which uses plastic deformations in materials like stain-
less steel 316L, with the help of a balloon, and remains the reference method in vascular applications.
However, for colonic stents, with final diameters in the range 25–30mm and expansion rates higher than 5, self-
expanding stents have become a clear alternative to traditional balloon-expandable deployment.
3.2.1 Mechanical Parameters

The following are relevant aspects concerning large diameter stents, including qualitative evaluation of colorectal
applications, and should be considered to evaluate the performance of different models:

• Expansion rate (Fig. 3.2A): The ratio between the unexpanded and expanded diameter of the stent. A higher value
is better.
• Stent shortening (Fig. 3.2B): Reduction of the overall length of the device during expansion (commonly expressed in
percentage). A lower value is better.

FIG. 3.2 Mechanical parameters of stents [48]: (A) expansion rate; (B) stent shortening; (C) radial compression resistance; (D) buckling resistance;
(E) longitudinal flexibility; (F) perimetral adaptability; (G) longitudinal adaptability.

I. BIOMECHANICS