Fabrication of drug-eluting covered stents with micropores and differential coating of heparin and FK506

Facilitation of adhesion and spreading of endothelial cells on silicone oxide-coated dacron material by microwave-excited low-pressure plasma

Objectives

Autologous transplants are still the means of choice for bypass surgery. In addition to good tolerability, there is a reduced thrombogenicity and fewer neointima hyperplasia compared to artificial materials. However, since viable transplants are limited, attempts are being made to improve existing artificial vascular prosthesis material. Next to the reduction of thrombogenicity, a rapid endothelialization of the vascular graft should reduce intimal hyperplasia and thus prevent stenoses. The effect of newly developed silicon oxide coatings on the growth of endothelial cells was therefore the goal of this work in a cell culture study.

Methods

A woven, uncoated polyethylene terephthalate (PET) vessel prosthesis was used. The coating process was carried out in a low-pressure plasma reactor in a multi-step process. After preparation of the vacuum chamber hexamethyldisiloxane (HDMSO) with oxygen was evaporated using argon plasma. By this an approx. 1 nm thin adhesion promoter layer was separated from plasma and HMDSO. The silicone oxide barrier layer was applied to the PET vessel samples. The carbon content of the layer could be selectively altered by changing the HMDSO oxygen flow ratio, resulting in coatings of 100 nm, 500 nm, and 1,000 nm. In addition, two different oxygen-to-HMDSO ratios were used. To achieve a carbon coating as low as possible, the ratio was set to 200:1. A carbon-rich layer was obtained with the 1:1 setting. The various coatings were then examined for their surface texture by scanning electron microscopy (SEM) as well as by cell culture experiments for cell viability and growth using EA.hy 926 cells.

Results

SEM showed no changes in the surface morphology; however a layer thickness of 1,000 nm showed peeled off coating areas. Alamar blue assays showed a significantly higher metabolic activity (p=0.026) for the coating 500 nm, ratio 200:1 compared to untreated control samples and a significantly lower metabolic activity (p=0.037) of the coating 500 nm, ratio 1:1 compared to the coating 500 nm, ratio 200:1. This underlines the apparent tendency of the 1:1 coating to inhibit the metabolic activity of the cells, while the 200:1 coating increases the activity. Fluorescence microscopy after calcein acetoxymethyl ester (AM) staining showed no significant difference between the different coatings and the uncoated PET material. However, a tendency of the increased surface growth on the coating 500 nm, ratio 200:1, is shown. The coatings with the ratio 1:1 tend to be less densely covered.

Conclusions

The results of this work indicate a great potential in the silicon coating of vascular prosthesis material. The plasma coating can be carried out easy and gently. Cell culture experiments demonstrated a tendency towards better growth of the cells on the 200:1 ratio coating and a poorer growth on the carbon-rich coating 1:1 compared to the uncoated material. The coating with silicon oxide with a thickness of 500 nm and an oxygen-HMDSO ratio of 200:1, a particularly low-carbon layer, appears to be a coating, which should therefore be further investigated for its effects on thrombogenicity and intimal hyperplasia.

Cardiovascular Stents: A Review of Past, Current, and Emerging Devices

One of the leading causes of morbidity and mortality worldwide is coronary artery disease, a condition characterized by the narrowing of the artery due to plaque deposits. The standard of care for treating this disease is the introduction of a stent at the lesion site. This life-saving tubular device ensures vessel support, keeping the blood-flow path open so that the cardiac muscle receives its vital nutrients and oxygen supply. Several generations of stents have been iteratively developed towards improving patient outcomes and diminishing adverse side effects following the implanting procedure. Moving from bare-metal stents to drug-eluting stents, and recently reaching bioresorbable stents, this research field is under continuous development. To keep up with how stent technology has advanced in the past few decades, this paper reviews the evolution of these devices, focusing on how they can be further optimized towards creating an ideal vascular scaffold.

Drug-Loaded, Polyurethane Coated Nitinol Stents for the Controlled Release of Docetaxel for the Treatment of Oesophageal Cancer

For several decades, self-expanding metal stents (SEMSs) have shown significant clinical success in the palliation of obstructive metastatic oesophageal cancer. However, these conventional oesophageal stents can suffer from stent blockage caused by malignant tumour cell growth. To overcome this challenge, there is growing interest in drug-releasing stents that, in addition to palliation, provide a sustained and localized release of anticancer drugs to minimise tumour growth. Therefore, in this study we prepared and evaluated an oesophageal stent-based drug delivery platform to provide the sustained release of docetaxel (DTX) for the treatment of oesophageal cancer-related obstructions. The DTX-loaded oesophageal stents were fabricated via dip-coating of bare nitinol stents with DTX-polyurethane (PU) solutions to provide PU coated stents with DTX loadings of 1.92 and 2.79% w/w. Mechanical testing of the DTX-PU coated stents revealed that an increase in the drug loading resulted in a reduction in the ultimate tensile strength, toughness and Young’s modulus. In vitro release studies showed a sustained release of DTX, with ~80–90% released over a period of 33 days. While the DTX-loaded stents exhibited good stability to gamma radiation sterilisation, UV sterilisation or accelerated storage at elevated temperatures (40 °C) resulted in significant DTX degradation. Cell proliferation, apoptosis and Western blotting assays revealed that the DTX released from the stents had comparable anticancer activity to pure DTX against oesophageal cancer cells (KYSE-30). This research demonstrates that the dip-coating technique can be considered as a promising approach for the fabrication of drug-eluting stents (DESs) for oesophageal cancer treatment.

Development of Microporous Covered Stents: Geometrical Design of the Luminal Surface

To reduce in-stent restenosis rates we have developed newly designed covered stents, in which a stent strut is buried into a microporous elastomeric cover film to provide a physical barrier against tissue ingrowth and a pharmacological reservoir for drug-eluting.

The covered stents were prepared by dip-coating balloon expandable stents mounted on a stainless steel rod in a segmented polyurethane (SPU) solution, and were subsequently subjected to laser-processed microporing (pore diameter, 100 μm; interpore distance, 200 μm). The covered stents, which possessed flat luminal surfaces and micropores that were homogeneously arranged on the whole surface of the covering film, were deployed into the bilateral common carotid arteries of normal New Zealand white rabbits. Angiography after one month of implantation showed all stents were patent with little thrombus formation. The mean thickness of the formed neointimal layers was 292 ± 177 μm (n=8), which was close to the size in non-covered bare stent (231 ± 58 μm, n=7), but markedly decreased (about 2/3) from that in the previously developed wrapping-type covered stents (415 ± 173 μm, P<0.01, n=8).

Direct writing of bio-functional coatings for cardiovascular applications

The surface modification of metallic biomaterials is of critical importance to enhance the biocompatibility of surgical implant materials and devices. This article investigates the use of a direct-write inkjet technique for multilayer coatings of a biodegradable polymer (polyester urethane urea (PEUU)) embedded with an anti-proliferation drug paclitaxel (Taxol). The direct-write inkjet technique provides selective patterning capability for depositing multimaterial coatings on three-dimensional implant devices such as pins, screws, and stents for orthopedic and vascular applications. Drug release profiles were studied to observe the influence of drug loading and coating thickness for obtaining tunable release kinetics. Platelet deposition studies were conducted following ovine blood contact and significant reduction in platelet deposition was observed on the Taxol loaded PEUU substrate compared with the unloaded control. Rat smooth muscle cells were used for cell proliferation studies. Significant reduction in cell growth was observed following the release of anti-proliferative drug from the biopolymer thin film. This research provides a basis for developing anti-proliferative biocompatible coatings for different biomedical device applications.

Natural polyelectrolyte self-assembled multilayers based on collagen and alginate: stability and cytocompatibility

Scientific interest in the self-assembly of collagen composite films has been increasing for their potential application in constructing bioactive materials. Here we report a highly stable and cytocompatible collagen/alginate (COL/ALG) ultrathin film, which was linearly fabricated via a layer-by-layer self-assembled technique. The variation in morphology and thickness of the films in air and in solutions with different pH and ion values were tested by atomic force microscopy. Results showed that the solutions with high pH values or solutions that contained electrolytes would disintegrate the film, while films with that were cross-linked for a long time prevented the dissolution and contributed to stability maintenance of the films. Interestingly, the COL/ALG coating not only improved the adhesion and proliferation of the human periodontal ligament cells, but also modified the morphology and migration of cells on the surface of glass and poly-L-lactic acid (PLA) electrospun scaffolds. In conclusion, the COL/ALG ultrathin films were highly stable and cytocompatible and could be easily fabricated by the cost-effective self-assembled technique presented. The findings of this study have the potential to play an important role in the surface modification of biomaterials.

Immobilization of selenocystamine on TiO2 surfaces for in situ catalytic generation of nitric oxide and potential application in intravascular stents

Immobilization of selenocystamine on TiO(2) film deposited on silicon wafer and 316 stainless steel stents for catalytic generation of nitric oxide was described. Polydopamine was used as the linker for immobilization of selenocystamine to the TiO(2) surface. In vitro stability of the immobilized selenocystamine was investigated and the result shows surface selenium loss occurs mostly in the first four weeks. The selenocystamine immobilized surface possesses glutathione peroxidase (GPx) activity, and the activity increases with the amount of grafted polydopamine. Such selenocystamine immobilized surfaces show the ability of catalytically decomposing endogenous S-nitrosothiols (RSNO), generating NO; thus the surface displays the ability to inhibit collagen-induced platelet acitivation and aggregation. Additionally, smooth muscle cells are inhibited from adhering to the selenocystamine immobilized sample when RSNO is added to the culture media. ELISA analysis reveals that cGMP in both platelets and smooth muscle cells significantly increases with NO release on selenocystamine immobilized samples. Two months in vivo results show that selenocystamine immobilized stents are endothelialized, and show significant anti-proliferation properties, indicating that this is a favorable method for potential application in vascular stents.

Atualização sobre endopróteses vasculares (stents): dos estudos experimentais à prática clínica

Atualmente, o tratamento das doenças vasculares periféricas é uma das áreas da medicina de maior expansão. O número de intervenções vasculares aumenta e os resultados das novas técnicas endovasculares estão muito próximos aos das tradicionais cirurgias vasculares. Embora a angioplastia ofereça bons resultados em curto prazo, o implante de stents procura melhorar o sucesso do procedimento e estender o seu uso a um número maior de pacientes com doença vascular periférica. Entretanto, a sua utilização ainda é controversa. O implante de stents no sistema aorto-ilíaco tem bons resultados; porém, a sua indicação para as lesões femoro-poplíteas ainda é discutida. Além disso, o rápido desenvolvimento de stents e sua escolha para uso no sistema vascular periférico têm sido uma difícil tarefa para o cirurgião endovascular. Muitos fatores influenciam a escolha do stent, e um amplo conhecimento desse material é essencial. Tal escolha depende da avaliação pré-operatória, da localização e das características da lesão e também do uso do stent primário ou seletivo. Nesse trabalho, são realizadas revisão do histórico do desenvolvimento dos stents, desde os estudos experimentais até os ensaios clínicos e também discussão sobre a sua aplicação no tratamento das doenças vasculares periféricas.

In vitro studies of platelet adhesion, activation, and protein adsorption on curcumin-eluting biodegradable stent materials

A major complication of coronary stenting is in-stent restenosis (ISR) due to thrombus formation. We hypothesized that locally released curcumin from coronary stent surface would inhibit ISR due to thrombus formation because of antithrombosis of curcumin. In the present work, curcumin-eluting polylactic acid-co-glycolic acid (PLGA) films were fabricated and their properties in vitro were investigated. The in vitro platelet adhesion and activation, as well as protein adsorption on curcumin-loading PLGA films were investigated to evaluate the blood compatibility of curcumin-eluting films. The structure of curcumin-eluting PLGA film and control was examined by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicating that the peaks of curcumin did not shift in curcumin-eluting films. The results of contact angle and surface free energy indicated that loading curcumin in PLGA would make PLGA become more hydrophilic, which contributed to the increase of polar fraction of surface free energy. With the increase of curcumin in films, platelets adhering to the curcumin-eluting films decreased significantly. The number of activation platelets decreased after incorporating curcumin in PLGA films. Loading curcumin in PLGA film can markedly reduce the fibrinogen adsorption. All results indicated that incorporating curcumin in PLGA film can improve the blood compatibility of PLGA films. It can be used to fabricate drug-eluting stent to prevent thrombosis formation.

Development of in vivo tissue-engineered autologous tissue-covered stents (biocovered stents)

Biocovered stents, which are stents covered with autologous membranous tissues, were developed by applying a novel concept based on in vivo tissue engineering. Balloon-expandable stents crimped on silicone rods as a mold (diameter: 2 mm) were embedded into dorsal subcutaneous pouches in rabbits. After 1 month, the struts of the stents were fully encapsulated with membranous connective tissues formed around the silicone rods. Upon removing the silicone rod, stents covered with tubular connective tissues, in which the struts were completely impregnated, were obtained as biocovered stents. These tissues were composed mainly of collagen and fibroblasts and had a thickness of less than approximately 200 microm with an excellent high burst strength of approximately 1000 mmHg. The luminal surface of the tissues was extremely flat and smooth. The stents could be mounted on balloon catheters with a hand crimping tool and could be expanded by inflation with little damage to the tissues. It is anticipated that these novel stents may greatly enhance early normal vascular reconstruction with high reliability, thereby reducing the rate of in-stent restenosis.

Development of self-expandable covered stents

We newly developed self-expandable covered stents by combining two of our original technologies. Of these, the first is the dip-coating covering method that was developed previously for balloon-expandable stents; the other is the newly developed self-expandable Nitinol stents, namely, Sendai stents. The three types of covered stents with the expansion diameter of 4.5, 5.0, or 6.0 mm thus obtained had a laser-processed microporous elastomeric cover film (pore diameter: 100 microm, interpore distance: 250 microm). Although the film was extremely thin (approximately 15 microm), the film could be expanded without causing any damage, the strut was completely embedded within the film, and the luminal surface of the film was smooth and flat. Mechanical properties such as ideal flexibility to follow the shapes of arteries were almost retained even after covering. As appropriate drugs, the blood-contacting inner and tissue-contacting outer surfaces of the film were differentially coated with argatroban for antithrombogenicity or FK506 for anti-inflammation, respectively. The preliminary in vivo study indicated that the covered stents mounted in the delivery catheter were navigated and placed to appropriate position in the arteries, and permissible neointimal thickening after 1-month implantation was observed similarly in noncovered stents.

Coronary stents: a materials perspective

The objective of this review is to describe the suitability of different biomaterials as coronary stents. This review focuses on the following topics: (1) different materials used for stents, (2) surface characteristics that influence stent-biology interactions, (3) the use of polymers in stents, and (4) drug-eluting stents, especially those that are commercially available.

Development of a polymeric matrix metalloproteinase inhibitor as a bioactive stent coating material for prevention of restenosis

Drug-eluting stents have been developed to prevent restenosis derived from excessive growth of smooth muscle cells (SMCs) after stenting. In almost every case, however, less- or non-biocompatible polymers were selected for the platform material for impregnating drugs on the stent strut. Consideration was given principally to the physical properties of the polymers, such as their adhesion to the strut and the drug dispersibility in the polymeric matrix. In this study, we designed a matrix metalloproteinase inhibitor (MMPI)-derivatized hydrophobic polymer (PMMPI) for use as a bioactive material for stent coating. This was a copolymer of n-butylmethacrylate and a vinyl monomer of synthetic MMPI (N-Hydroxy-5-carboxyethylcarbonyloxy-2(S)-methy-4(S)-(4-phenoxybenzoyl)amino-pentanamide: ONO-M11-335) with a molecular weight of about 32,000 and MMPI content of 45 per molecule. The precursor of the MMPI monomer produced significant activity in temporally inhibiting SMC proliferation without any cellular damage. After coating with the PMMPI, adhesion and proliferation of SMCs were manifestly prevented even when a small amount of MMPI was released from the polymer. The MMPI-immobilized surface may thus be effective for inhibiting both adhesion and proliferation of SMCs, which is the first step toward in vivo experimentation. It is very much expected that coating stent struts with PMMPI containing an appropriate combination of impregnated drugs will provide a powerful tool for prevention of restenosis with little cytotoxicity.

Local drug delivery in restenosis injury: thermoresponsive co-polymers as potential drug delivery systems

The success of percutaneous transluminal coronary angioplasty in treatment of acute coronary syndromes has been compromised by the incidence of restenosis. The physical insult of balloon insertion can damage or remove the endothelial monolayer, thereby generating a prothrombotic surface. The resulting inappropriate response to injury can also lead to penetration of inflammatory cells, conversion of the underlying media to a synthetic phenotype, deposition of extracellular matrix, constrictive remodeling, and neointimal hyperplasia. While stent implantation at the time of balloon insertion has offset some of these events, inflammatory responses to the implanted biomaterial (stent) and intimal hyperplasia are still prominent features of the procedure, leading in 20-30% of cases to in-stent restenosis within a year. Systemic delivery of drugs designed to offset in-stent restenosis injury has been largely unsuccessful, which has led to the development of strategies for coating stents with drugs for local delivery. Drug-eluting stents constitute an innovative means of further reducing the incidence of restenosis injury and clinical trials have shown encouraging results. This review focuses on properties of a class of environment-sensitive hydrogels, the N-isopropylacrylamide-based thermoresponsive co-polymers, on their potential roles as stent coatings, on their demonstrated ability to incorporate and release drugs that modify vascular endothelial and smooth muscle cell functions, and on issues that still await clarification, prior to their adoption in a clinical setting.