Page 46 - Advances in Biomechanics and Tissue Regeneration

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

FIG. 3.7 Ultraflex stent: (A) geometrical scheme; (B) real view. (Source: Boston Scientific.)

concatenation of radial springs with multiple arcs and sliding joints between the rings. A bell shape is introduced in
half of the stent by means of a thermal treatment for fixing this expanded form. In a rigorous classification, this stent
belongs to the sliding knots design. Its particular braiding provides flexible longitudinal bands alternating with nar-
row rigid bands.

3.2.2.2.3 CHOO STENT (FIGS. 3.8 AND 3.9)
To take advantage of all the Nitinol power, the stent must have fixed knots. Therefore the loss of flexibility is com-
pensated by the superelasticity. This idea is carried out in cardiovascular stents like the Cook (Fig. 3.8) and Symphony
(Fig. 3.9) stents by braided knots or laser, respectively. The Choo colorectal stent introduces an alternative fixation of
the knots. In a peripheral ring, the knots are crossed by torsion of the wires and in the next ring the knots are free.

3.2.3 Mechanical Behavior

3.2.3.1 Resistance Mechanisms
Each type of stent bases its mechanical behavior on several distinct resistance mechanisms that will be analyzed in
this section.

3.2.3.1.1 HELICOIDAL SPRING
The design parameters corresponding to a coil spring are: helix pitch (h), diameter (D), wire diameter (d) or section
dimensions if not circular, and number of wires (n). Due to the change of stiffness with expansion, a nonlinear inverse
problem must be solved to determine the stiffness of the stent corresponding to every expanded configuration.

I. BIOMECHANICS