Biomedical Engineering Reference
In-Depth Information
Fig. 13.8
A photograph of a metallic stent
on at many levels, including development of stent materials and nonpharmacological
coatings, and technologies to deliver large biopharmaceuticals, especially from
cardiovascular stents [
436
,
437
].
The technical, preclinical, and clinical requirements for placing, securing and
monitoring the stent in its place had all to be taken into consideration for selec-
tion of material and design characteristics. The first self-expanding stents were
reported in 1986 by the name Wallstent
R
. Cobalt-chromium alloys have been used
because their high elastic modulus enabled the process of self-expansion of the
structure. Other alloys such as the L605 (Co-20Cr-15W-10Ni), MP35N (Co-20Cr-
35Ni-10Mo) have entered the scene, through the balloon-expanding process. Their
higher strength enabled the construction of thinner struts, which cause less trauma
and therefore reduce the restenosis rate. They are radioopaque allowing X-ray mon-
itoring and not at least, have improved corrosion resistance. In addition to these
alloys and to stainless steel (316L), many efforts to reduce the strut thickness and
improve the radiopacity by adding, for example, platina are in progress. Other alter-
natives are the use of composite structures: a marketed example is the TriMaxx
R
**
stent which is comprised of a thin layer of tantalum sandwiched between two layers
of 316L stainless steel. Furthermore, knitted wire and helical wound wire structures
of tantalum have been made into stents, and niobium, as well as nickel-free steels
have also been tried.
Mechanical characteristics of present day materials set a limit on further reduc-
tion of struts' thickness. The continuous need for improvement of the bare metallic
stents forces researchers to deal with the surface properties to improve the compat-
ibility with the vascular environment and to decrease restenosis of the lumen. This
cannot be reduced below a certain limit. In an effort to reduce metal ion release,
especially nickel and molybdenum, employment of carbon coatings was investi-
gated. Diamond-like carbon (DLC), pyrolytic carbon, fluorine-doped DLC, even
silicon carbide have been utilized as coatings. Some authors by-passed the prob-
lem of coating adherence by ion implantation of carbon. While short-term effects
showed an improvement, i.e., lower restenosis rates compared to bare stents, longer
term results were not encouraging [
436
]. Other coatings include titanium-nitride-
oxide coating (physical vapor deposition of titanium in an oxygen-nitrogen gas
