Introduction removed while the stent stays in

Introduction

One of the causes that leads to
narrowing of blood vessels which leads to restriction of oxygen-rich blood to
the heart is the coronary heart disease (CHD), which is a condition that caused
by atherosclerosis. Atherosclerosis is the chronic growth of plaque deposits
beneath the cells that line the interior surface of blood vessels and lymphatic
vessels -The Endothelium-. The advanced CHD can be treated by re-open the
artery and push back the plaque-laden vessel walls using a mesh-like
cylindrical structures. 1 shanon C These structures can be placed within the
lumen of a tubular structure such as vessels, arteries, veins, the biliary
tree, the urinary tract, the tracheobronchial and the cerebral aqueduct to hold
a dissection in place, preventing closure during spasm, and preventing acute
closure due to thrombosis, these structures are called stents paul j. This
article will focus on stents for vascular applications to maintain an open
passageway through the vessels. The stent is deformable between a contracted
state and deployed one. In a contracted state, the stent is capable to entry an
movement along a corporeal duct to the targeted site, while in the deployed
state the stent will be able to rest against the wall of the conduit.us2009. A percutaneous transluminal coronary angioplasty and
stenting can be done to open the arteries surrounding the heart by using a
balloon catheter inserted into an artery in the groin or the arm to put the
stent into the narrowed coronary artery. Once it is there, the balloon is
inflated, and the stent expands to the size of the artery and holds it open.
The balloon is then deflated and removed while the stent stays in place video
to aseel.

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History

Design
and Fabrication

Generally
a stent can be designed by following varied manufacturing techniques, such as:
laser-based micromaching of thin-wall tubular forms or by microelectrodischarge
machining (kenichi). The following is a method reported by Richter, Aviv and
Yaron for fabricating stents:

The designer
should firstly prepare a drawing of the desired stent pattern on a flat format.
The stent pattern is then cut into a flat sheet of biocompatible metal or polymer,
such as: stainless steel. This can be done by etching or by cutting with a very
fine laser or tool.

The metal pattern is deformed by rolling process to cause its long
sides to meet each other and then edges are joined together by suitable spot
welding. Finally, the stent is polished in order to remove any excess material
that was not removed by cutting process, this can be done mechanically, by
rubbing a polishing stick having diamond dust on its outside or by using an
electropolishing unit.(aviv) (jacob)

The
stent can also be formed by braiding several metal wires made of memory-shape
alloy. One of the shortcoming of known stents is that they exhibit no
adaptability of diameter or shape with regard to different diameter vessels and
opeinings. Thus, some of the embodiments of present invention remedy this
shortcoming by using designing methods, such as: “forming a stent by
simple wire whereof each strand runs helicoidally from one end to the other and
is braided to the other strands” (us2009). Other embodiments address the
flowing of blood between the wires of the stent by coating it using suitable materials,
such as: Polytetrafluoroethylene (PTFE) membrane. .(us2009) . After building
a model several tests must be done to check the biocampatibilty and the
effectiveness of these stents as well as the other properties. For example, the
stent deployment process can be stimulated by performing elasto-plastic finite
element analysis and a subsequent mechanical testing via uniaxial radial
compression for example.  Moreover, the
latter can be used to characterize the radial stiffness of the deployed stents.
Measurements of load-displacement behavior under uniaxial radial compression
can be also performed for a variety of stents orientations. (shannon).  

Types of Stents

The first generations of
stents are the bare-metal stents, which are made from wires formed into a
mesh-like cylinder. The wires must be made of biocompatible metals depending on
the site of application, such as: stainless steels, platinum and
platinum-iridium alloys and copper alloy. This type of stents will be
permanently implanted in the body and it has approved to have acute and good
mid-term results(curr). On the other hand, some stents have the disadvantage of
that aggressive ends or metal prongs that may form from the length cutting
process of the stent from an elongated metal mesh tube, these ends may have
damaging consequences because they could impact dangerously the passage of
these stents from the healthy tissue to the target vessels. If these stents
where permanently placed in the coronary or the vessel, the continuous stress
from the beating of the heart would cause these prongs to damage the
surrounding tissues. richard +us2009. So it would be desirable to provide a
stent that has structural integrity and it is flexible and compliant to be
delivered safely to the site of a coronary obstruction. Moreover, by studying
the chronic arterial responses to metal stents with intravascular ultrasound,
it was noticed that metal stents prevent the lumen expansion associated with
late favorable remodeling, also the surgical revascularization could be blocked
by the use of long stents and full-lesion coverage which lead to the extension
of stented segments for several inches. So it is better to avoid the
limitations of chronic implantation by forming these stents from an absorbable
material that is adapted to be absorbed or safely degrade within the
bloodstream of an artery or vein over the lapse of a period of time. .richard
c,antonio,6,7.  These types of stents
are called biodegradable stents.
The rationale behind using these stents has been explained by Colombo and
Karvouni with the allegory of Cincinnatus, a prominent Roman leader  who served his country when he was needed and after
fulfilling his duty  he stepped down and returned
to his simple life as a farmer, ” a biodegradable stent fulfills the
mission and steps away”. The first biodegradable stent was
developed by Clark and Stack at Duke University in the late 1980s.(28 in joao),
a specialized form of polylatic acid (PLA) 
was used for this prototype and then it was implanted in animals. They
must restore blood flow and povide support for a period of time to facilitate
artery healing, and then fail safely and be absorbed in a controllable manner
but the dissolved portions of these stents must be prevented from forming
emboli therefore the biodegradation should occur within the vessel rather than
in the lumen by forming these stents in a way, for example porous or mesh-like
configuration, that would enable the endothelial cells at the angioplasty to
grow into and over the stent paul, richard,joao. The open mesh-like
configuration will allow the stent to be readily entrapped by endothelial
overgrowth. Thus, this will reinforce the vessel wall and because of this
encapsulation the portions of the stents as it is dissolving will not break off
into the blood stream and the stent will not be continuously exposed to the
blood therefore the endothelial overgrowth forms a smooth inner vessel wall, which
minimize the risk of thrombus and hardening of vessel wall.(richard) . But the
design process is significantly more challenging for this type of stents and
there are many challenges associated with material formulation, and material
properties characterization and keeping the artery open while keeping other
design parameters with acceptable ranges. Moreover, the designer needs to
account for changes occurring during processing until the controllable failure since
degradation depends on the environment. Also, the implant is exposed to bear
mechanical loads which increases the biomechanical challenges associated with
the application of biodegradable stents.

These
challenges are obvious when considering the safety and regulatory
issues, also cost issues that cause the market lag of
using biodegradable stents and the non existence of it on the US market for along time despite more than
30 years of development efforts are some reasons behind.(joao)

Stents can cause local thrombosis
which can lead to restinosis. Restinosis is reclosure of a stenosed coronary or
peripheral vessel   that leads to restrict blood flow and it could
be a natural healing reaction to the injury of the arterial wall that is caused
by angioplasty (227,68). It is suggested that several drugs could be supplied
in stents to reduce the occurrence of restinosis such as: antimetabolic agents,
anti-inflammatory agents and other vasoreactive agents such as nitric oxide
releasing agents (27). As a result, a second generation bare-metal stents were invented
that deliver drug to the target vessel. This type of stents consists of three
main components: scaffold that could be metallic or polymeric stents, drug and
a bioabsorbable or biostable polymer. The polymer allows the drug retained on
the stent during expansion of the stent and also elute the drug at a controlled
rate (27,84) By immersing the stent into a solution, which contains a polymer
that was dissolved in a solvent and a therapeutic drug dispersed in the
solvent,  or by spraying the solution
onto the stent this type of stents can be made. The amount of drug on the stent
can be controlled by applying multiple thin coats of the solution which
allowing to dry between coats then the solvent is evaporated.