Page 328 - Biodegradable Polyesters

306 12 Biodegradable Polyesters Polymer–Polymer Composites

pressure of 120 mmHg. It is also stated that unhealthy vessels may exert less
pressure on stents. In addition, the authors did not take recoil into account; this
will be discussed later when a case study is reviewed. In a stent design handbook
by Bonsignore, the mean vessel pressure, determined by taking vessel compliance
and blood pressure into account, was determined to be 100 mmHg [23]. The figure
quoted by Venkatraman of 100 kPa (750 mmHg) deviates enormously from those
quoted by Lanzer et al. and Bonsignore and is potentially a typographical error,
considering that no explanation was given as to how the figure was determined.
Finally, the material of which a stent is made must be able to withstand the high
strains and plastic deformation resulting from balloon expansion [20]. Stent and
stent material requirements are summarised in Table 12.1.
A point to stress here is that although there are some requirements outlining
the physical requirements to be met by stents, they are neither all explicit nor
in-depth. For example, the change in the level of support required by a vessel
while it heals is not reported. Furthermore, creep is not addressed by the require-
ments – potentially because permanent stents, which preceded biodegradable
stents, are metallic and creep is not an important consideration for metals
∘
at 37 C. Biodegradable stent technology is still rather undeveloped, which is
evidenced by the majority of stents on the market being permanent and made
of metal alloys. In fact, even permanent stent technology is still being developed
and various medical device firms still produce only permanent stents (Boston
Scientific being an example).
As was mentioned previously during the outline of stent evolution, it is essen-
tial that clots (formed by thrombocytes or “platelets” in blood) do not form on a
stent after implantation. Thus the stent material should have low thrombogenic-
ity [4], that is, a low tendency to attract thrombocytes to its surface. While a stent
degrades, the material must retain enough stiffness and strength in order to pro-
vide support to the healing vessel for 6 months [20, 25, 26]. Furthermore, when
the material begins to disintegrate, the resulting fragments must not be released

Table 12.1 Stent and stent material requirements.

Requirement References
Biocompatibility of stent material and its degradation products [20–22]
Low thrombogenicity to prevent stent thrombosis [4]
The material must retain enough strength and stiffness for the stent to provide [20, 25, 26]
sufficient support for 6 months
Fragments of material must not be released into the bloodstream during [20]
degradation
Stent must be radio-opaque for tracking during delivery [4, 20]
Stent must be able to withstand a minimum crush-pressure but the value [20, 23, 24]
reported ranges from 100 to 750 mmHg
The material must have sufficient flexibility to withstand being bent to get [4, 22]
through tortuous vessels during delivery
The material must deform plastically upon balloon expansion to avoid recoil [20]