Page 44 - Advances in Biomechanics and Tissue Regeneration

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

sliding knots. This stent always operates in elastic mode since the deformation from its work shape is achieved by
pulling each wire from the helicoidal mode to an almost straight line. The high number of wires in the Wallstent pro-
vides a small cell size.
3.2.2.1.2 GIANTURCO STENT (FIG. 3.4)
Rings of zigzag wire segments formed by Z-shaped radial springs, joined by fine nylon or stainless steel wires,
form the structure of this stent. In contrast to the version for the esophagus, which is covered by a double layer of
polyethylene, the version for the colon, the colonic Z-stent, does not present any covering. The stent presents a bell
shape at the extremes of the stents. The cells in the stent are extremely open, due to the high length of the wire
segments.

3.2.2.1.3 SONG AND CHOO-Z-MODIFIED GIANTURCO STENTS (FIG. 3.5)
These are variants of the Gianturco type with a curved Z-shaped radial spring. In the Song model (Fig. 3.5B) the end
rings present a greater diameter than the rest of the rings. This configuration introduces a right-angle discontinuity,
which cannot be supported by fine wires, as in the basic Gianturco stent. In the transition zone, the segments are
embedded in polyurethane. The Choo-Z stent (Fig. 3.5C) introduces certain modifications in this design: the number
of rings in the central region increases and the length of the wires decreases.

3.2.2.2 Self-Expanding Nitinol Stents
3.2.2.2.1 ESOPHACOIL STENT (FIG. 3.6)
This stent presents a spiral that forms a unique ribbon helicoidal spring configuration and originally was applied to
the esophagus. The helix is manufactured from a ribbon with a square transversal section and minimum helix pitch so
that it contacts the turns of the spiral. This structure reduces the space between the turns in the central region and can
be considered a stent with a practically closed cell. In general, the manufacturing process is simple and the design
process does not need any modification, except the section.
3.2.2.2.2 ULTRAFLEX STENT (FIG. 3.7)
The model for the colon is a braided and fabric Nitinol wire structure following the typical pattern used in cardio-
vascular applications, known as the Strecker stent, which is a balloon-expandable operating mode stent. It is based on a

FIG. 3.4 Gianturco: (A) geometrical scheme; (B) real view. (Source: colonic Z-stent, Cook Medical.)

I. BIOMECHANICS