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The Igaki-Tamai Stent.
The Igaki-Tamai Stent.

Biodegradable stentsare made of biodegradable or absorbable materials, which have good histocompatibility and biodegradability. When implanting a degradable stent at a site of a narrowed blood vessel, it can effectively dilate the blood vessel at the early stage and be degraded gradually, in which the degradation products can be eliminated from the body by metabolism or absorbed by the body for utilization without affecting long-term vascular functions.

The majority of degradable polymer stents are the poly-l-lactic acid (PLLA) polymer stents.PLLA is a thermoplastic aliphatic polymer, which can be degraded to lactic acid by its own catalytic hydrolysis and eventually enter the tricarboxylic acid (TCA) cycle to be metabolized into water and carbon dioxide.

PLLA enhances the radial support strength by combining semi-crystalline polymers.The amorphous polymers are combined, which allow the coated drug to be uniformly dispersed for a predetermined period of time and the stent to be uniformly degraded.The duration of the degradation process generally ranges from 2 to 4 years.

In principle, PLLA needs to be 2.4 times thicker than a conventional metal stent to provide the same radial support strength as the metal stent does.The radial support strength of the Abbott stent proves insufficient as its retraction rate being 16.6%.

The Igaki-Tamai Stent

The Igaki-Tamai stent is the first biodegradable non-drug eluting stent, which has a stent skeleton of 170 μm, made of PLLA for clinical evaluation in the human body.In clinical follow-up, it was found that this stent can be completely degraded in 36 months. The Igaki-Tamai stent is thermoplastic.It requires an 8F sheath in order to be delivered to a specific position, adopting balloon dilatation and heating by contrast media to 80°C, and follows self-dilatation due to the effect of body temperature within 30 minutes. Nishio et al. reported to treat 50 patients with the Igaki-Tamai stent (a total of 63 lesions using 83 stents). After 10 years of follow-up, it was found that the PLLA stent was completely absorbed in 3 years. The target lesion recurrence (TLR) rate was 28% after 10 years.

The Abbott Stent

Abbott Absorb GT1 biodegradable vascular stent is degraded over time.The Abbott stent is the most widely studied degradable drug-eluting stent. The first-generation Abbott stent (BVS 1.0) is made of PLLA polymers, which has a stent skeleton thickness of approximately 150 μm and a surface coating with everolimus. A multicenter human trial consisting of 30 patients evaluated the effect of BVS 1.0 (a single stent of 3.0mm×12.0mm or 3.0 mm×18.0mm) in coronary artery with angina symptoms or silent ischemia. In the 6-month follow-up, it was found that the late lumen loss (LLL) of a stent covered vascular segment reached 0.44 mm (11.8%). The radial support strength of this stent proved insufficient as there wassignificant retraction after implantation. The rate of total major adverse cardiac events (MACE) was 3.4% during the five-year follow-up, including a death occurred due to simple non-Q wave myocardial infarction in the first 6 months.

The second-generation Abbott stent (BVS 1.1) was designed and modified accordingly based on BVS 1.0 to enhance the radial support strengthand increase the stent covering area, making the delivery system easier to handle and enabling storage at room temperature.

The Reva Stent

The ReZolve Stent.The REVA stent is a poly-iodinated tyrosine alkyl carbonate stent developed by REVA Medical, Inc.,in the United States, which can be degraded into water, carbon dioxide, ethanol and iodinated tyrosine alkyl in vivo. The degradation products have almost no toxic side effects to human body and can be absorbed or excreted by the body.

The REVA stent features a unique design of “sliding and locking”, which provides sufficient radial support strengthafter placement, and theradiopaque metal marker on both ends also helps follow-ups at the late stage. This stent can be completely degraded between 18 to 24 months while the degradation rate can be changed as needed. Animal experiments showed that after implanting the REVA stent into the porcine coronary artery for 55 months, the degree of narrowness at the stent segment was significantly less than that immediately after surgery, and the lumen was significantly expanded.

It is a balloon dilatation stent, which can cover 55% of the arterial wall when fully expanded. Pollman et al., conducted the first clinical trial of this stent in the human body, in whicha high incidence of clinical events (TLR=67%) was reported mainly due to insufficient expansion of the stent in the blood vessels.

As a result, the Reva stent was redesigned to produce a polymer with higher strength, sirolimus surface coating and new sliding and spiral locking features—the new generation of ReZolve stent. The clinical evaluation of this new stent started in December 2011, with results of 100% success rate in clinical surgery andzero MACE events at 3 months. However, the sheath delivery mechanism of the stent restricted its use to small blood vessels and blood vessels with a larger curvature. A new ReZolve 2 stent (6F) was then introduced with a sheathless delivery system. The polymer itself was strengthened to increase the radial support strength by 30%. The ReZolve 2 stent was used in a global multicenter trial of RESTORE II, in which the recruitment of the trial started in 2013.

The IDEAL Stent

Biodegradable vascular stents.The IDEAL stent provides mechanical support strength through the polymer backbone chain of polylactic acid anhydrite and salicylic acid sebacic acid trimer, in which the salicylic acid-adipate salicylic acid trimer acts as the carrier and sirolimus coating. Additional coatings of salicylic acid were found to reduce the inflammatory responseof 36in animal studies, which may surpass the conventional polymers. Jabara et al., demonstrated the safety of the first-generation IDEAL stent in the Whisper test (n=11) with no early retraction. However, it may due to the insufficient dosage of sirolimus and short duration of drug release, resulting in significant intimal growth of the stentcovered segmental blood vessels.

The second-generation IDEAL stent increases the dosage of coated drugs, reducesthe rate of drug release and enables a greater compatibility. The specific clinical data of this stent are still under test.

Additionally, there are other degradable polylactic acid (PLA)stents, such as the Amaranth stent (Amaranth Medical, US), the Acute stent (OrbusNeich, US), the MeRes stent (Meril, India) and FADES stent (Zorion Medical, US). Many of these stents are currently in the preclinical stage and thereforeonly briefly introduced.

Prospect

The use of biodegradable materials in stents is revolutionary.

However, the degradation time of degradable stents lacks a definite standard. If the degradation time is too short, retraction may occur with agreatly increased probability of stenosis. If the degradation time is too long, theremay be excessive intimal hyperplasia in the stent with anincreased possibility of stent thrombosis. The degradable drug-eluting stentcan inhibit intimal hyperplasia; however, the rate and duration of drug release and the rate of stent degradation require a balance, or thepromotion of degradable stents may still be limited.

When the degradable stent is completely absorbed, relative to the permanent metal stent, it eliminates the persistent stimulation of the vessel wall.Whether it can truly restore vascular integrity and vasomotor function and whether multiple stent implantation is allowed will be one of the directions for future studies.

Additionally, the positioning of the degradable stent after implantation may be a problem due to the difficulty of stent positioning and inconvenience of postoperative imaging follow-up, as the degradable polymer stent being radiolucent with no clear marker. Whether the presence of the marker can affect stent stability requires further verification.

As science and technology develop, these problems will be properly solved,making degradable stents have greater applicationsin vascular interventional therapy.

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