Surface microarchitectural design in biomedical applications: in vivo analysis of tissue ingrowth in excimer laser-directed micropored scaffold for cardiovascular tissue engineering

Percutaneous coronary intervention: balloons, stents and scaffolds

In this review, major achievements in the field of percutaneous coronary interventions are delineated with particular focus on Germany's contribution. The review deals with important developments, including the first heart catheterization and coronary angiography, first coronary balloon angioplasty and refinement of the technique, coronary stenting and optimization of adjunctive antithrombotic treatment, drug-eluting stents and balloons, as well as bioresorbable polymeric and metallic drug-eluting scaffolds.

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).

Investigating surface topology and cyclic-RGD peptide functionalization on vascular endothelialization

The advantages of endothelialization of a stent surface in comparison with the bare metal and drug-eluting stents used today include reduced late-stent restenosis and in-stent thrombosis. In this article, we study the effect of surface topology and functionalization of tantalum (Ta) with cyclic-(arginine-glycine-aspartic acid-d-phenylalanine-lysine) (cRGDfK) on the attachment, spreading, and growth of vascular endothelial cells. Self-assembled nanodimpling on Ta surfaces was performed using a one-step electropolishing technique. Next, cRGDfK was covalently bonded onto the surface using silane chemistry. Our results suggest that nanotexturing alone was sufficient to enhance cell spreading, but the combination of a nanodimpled surfaces along with the cRGDfK peptide may produce a better endothelialization coating on the surface in terms of higher cell density, better cell spreading, and more cell-cell interactions, when compared to using cRGDfK peptide functionalization alone or nanotexturing alone. We believe that future research should look into how to implement both modifications (topographic and chemical modifications) to optimize the stent surface for endothelialization.

Anisotropic Porous Biodegradable Scaffolds for Musculoskeletal Tissue Engineering

It has been generally accepted that tissue engineered constructs should closely resemble the in-vivo mechanical and structural properties of the tissues they are intended to replace. However, most scaffolds produced so far were isotropic porous scaffolds with non-characterized mechanical properties, different from those of the native healthy tissue. Tissues that are formed into these scaffolds are initially formed in the isotropic porous structure and since most tissues have significant anisotropic extracellular matrix components and concomitant mechanical properties, the formed tissues have no structural and functional relationships with the native tissues. The complete regeneration of tissues requires a second differentiation step after resorption of the isotropic scaffold. It is doubtful if the required plasticity for this remains present in already final differentiated tissue. It would be much more efficacious if the newly formed tissues in the scaffold could differentiate directly into the anisotropic organization of the native tissues. Therefore, anisotropic scaffolds that enable such a direct differentiation might be extremely helpful to realize this goal. Up to now, anisotropic scaffolds have been fabricated using modified conventional techniques, solid free-form fabrication techniques, and a few alternative methods. In this review we present the current status and discuss the procedures that are currently being used for anisotropic scaffold fabrication.

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.

A unique device for controlled electrospinning

The purpose of this research was to develop a system for controlled electrospinning of fibro-porous scaffolds for tissue engineering applications and to use this system to assess mesh architecture sensitivity to manufacturing parameters. The intent was to achieve scaffolds with well-controlled fiber diameters and inter-fiber spacing. To accomplish these objectives, a custom, closed-loop controlled, electrospinning system was built. The system was unique in that it had a collection surface that was independent of the electrodes. The system allowed independent manipulation and analysis of a number of manufacturing parameters: distance between the electrodes, distance from the nozzle to the collection surface, applied voltage, temperature of the melt, collection surface dielectric strength, and collection surface area. Morphological analysis of fabricated meshes showed that all test parameters significantly affected fiber diameter and inter-fiber spacing. Further, contrary to what is generally accepted in the electrospinning literature, voltage and temperature (inversely related to viscosity) were not the most significant parameters. Features of the collection surface, including dielectric strength and surface area, were more significant. This dominance is, in part, a reflection of the unique electrospinning system used. The collection surface, which was not connected to either of the electrodes, substantially altered the electric field between the electrodes. Using the developed controlled electrospinning system, thermoplastic polyurethane meshes with fiber diameters ranging from 5 to 18 microm with variability less than 1.8% were made; inter-fiber spacing ranged from 4 to 90 microm with variability less than 20.2%. The system has potential use in biomedical applications where meshes with controlled fiber diameter and inter-fiber spacing are of interest.

Development of sutureless vascular connecting system for easy implantation of small-caliber artificial grafts

A novel sutureless vascular connecting system, an assembly with a delivery rod, an introducing sheath, and a connecting device, was developed for easy implantation of small-caliber vascular grafts less than 2 mm in internal diameter. A microporous stainless tube (length 2 mm, external diameter 1.6 mm, wall thickness 65 microm, pore diameter 400 microm, pore-to-pore distance 500 microm) was designed to serve as a connecting device. The feasibility of the system was tested using two types of preliminary animal experiments. One animal model consisted of graft implantation into the rat abdominal aorta (1.5 mm in diameter). The connecting device was inserted into the proximal and distal ends of the aorta through the introducing sheath by pushing the delivery rod with the connecting device placed over it. Subsequently, the aortic segments were inserted into both ends of model grafts made of segmented polyurethane (1.8 mm in internal diameter) and were fixed with banding silk threads from the exterior. The procedure was completed within 20 min without requiring specialized microsurgery techniques. Blood leakage and obstruction did not occur. The second model consisted of an end-to-end anastomosis between rabbit common carotid arteries (2 mm in diameter), which was performed within several minutes of blood flow interruption. Scanning electron microscopy demonstrated that the luminal surface of the device was fully covered with endothelial cells (ECs) after 1 week as a result of transluminal ingrowth of native ECs through the micropores in the device. This endothelialization may prevent early thrombus-induced occlusion. This simple and "easy-to-learn" technique will promote the development of small-caliber arterial grafts, and furthermore, it may have potential for clinical application.

Synthesis and properties of amphiphilic networks 3: preparation and characterization of block conetworks of poly(butyl methacrylate-block-(2,3 propandiol-1-methacrylate-stat-ethandiol dimethacrylate))

Amphiphilic conetwork polymers were prepared and studied as substrates in the culture of dermal fibroblasts. Both block and random conetworks polymers were produced by radical polymerization of either low-molecular weight monomers or oligomeric macromonomers. The oligomeric macromonomers were prepared by methacrylation of biscarboxy oligo(butyl methacrylates) (OBMA). The latter were synthesized by ozononolysis of poly(butyl methacrylate-co-butadiene) materials. The hydrophilic component was derived from copolymerization with 2,3 propandiol-1-methacrylate and cross-linking was provided by inclusion of ethandiol dimethacrylate (EDMA). None of the synthesized materials showed indications of cytotoxicity to human dermal fibroblasts. All of the block conetworks were highly phase separated and possessed pores on the micron length scale. The equilibrium water contents of the latter could be controlled by addition of EDMA. Block conetworks that did not contain EDMA were highly swollen and had smoother surfaces than those that contained EDMA. The former were poor substrates for cell proliferation (as measured by monitoring DNA content) whilst the latter class gave increasing levels of DNA during culture; an indicator proliferation. The performance of these materials in cell culture was also dependent on the fraction of OBMA in the formulation. Increasing the fractions of BMA, either in the random terpolymer or block networks, system had the effect of increasing both cell proliferation and viability (as measured by the Alamar Blue assay).

Transrenal fixation of aortic stent-grafts: current status and future directions

Aortic stent-graft repair has been widely used in clinical practice for more than a decade, achieving satisfactory results compared to open surgical techniques. Transrenal fixation of stent-grafts is designed to obtain secure fixation of the proximal end of the stent-graft to avoid graft migration and to prevent type I endoleak. Unlike infrarenal deployment of stent-grafts, transrenal fixation takes advantage of the relative stability of the suprarenal aorta as a landing zone for the uncovered struts of the proximal stent. These transostial wires have sparked concern about the patency of the renal arteries, interference with renal blood flow, and effects on renal function. Although short to midterm results with suprarenal stent-grafts have not shown significant changes in renal function, long-term effects of this technique are still not fully understood. This review will explore the current status of transrenal fixation of aortic stent-grafts, potential risks of stent struts relative to the renal ostium, alternative methods to preserve blood flow to the renal arteries, and future directions or developments in stent-graft design to prevent myointimal proliferation around the stent struts.

Novel porous aortic elastin and collagen scaffolds for tissue engineering

Decellularized vascular matrices are used as scaffolds in cardiovascular tissue engineering because they retain their natural biological composition and three-dimensional (3-D) architecture suitable for cell adhesion and proliferation. However, cell infiltration and subsequent repopulation of these scaffolds was shown to be unsatisfactory due to their dense collagen and elastic fiber networks. In an attempt to create more porous structures for cell repopulation, we selectively removed matrix components from decellularized porcine aorta to obtain two types of scaffolds, namely elastin and collagen scaffolds. Histology and scanning electron microscopy examination of the two scaffolds revealed a well-oriented porous decellularized structure that maintained natural architecture of the aorta. Quantitative DNA analysis confirmed that both scaffolds were completely decellularized. Stress-strain analysis demonstrated adequate mechanical properties for both elastin and collagen scaffolds. In vitro enzyme digestion of the scaffolds suggested that they were highly biodegradable. Furthermore, the biodegradability of collagen scaffolds could be controlled by crosslinking with carbodiimides. Cell culture studies showed that fibroblasts adhered to and proliferated on the scaffold surfaces with excellent cell viability. Fibroblasts infiltrated about 120 microm into elastin scaffolds and about 40 microm into collagen scaffolds after 4 weeks of rotary cell culture. These results indicated that our novel aortic elastin and collagen matrices have the potential to serve as scaffolds for cardiovascular tissue engineering.

Poly(2-hydroxyethyl methacrylate)-based slabs as a mouse embryonic stem cell support

Poly(2-hydroxyethyl methacrylate) (PHEMA) crosslinked with ethylene dimethacrylate (EDMA) or N,O-dimethacryloylhydroxylamine (DMHA) was obtained in the form of slabs by bulk radical polymerization. Two porosity-inducing methods were investigated, phase separation using a low-molecular-weight porogen and a salt-leaching technique using NaCl and saccharose. Compared with the phase separation, the salt-leaching created open porous structures with voids of the size and shape of crystallites. To address its potentials in the context of stem cell therapies, undifferentiated mouse embryonic stem cells D3 (ES D3 cells) were seeded on the slabs and analyzed for the ability to grow on different types of non-degradable and/or degradable porous PHEMA hydrogels. The cells were able to proliferate only on PHEMA crosslinked with EDMA or 2 wt% DMHA. In order to assess the effect of gelatin, which is routinely used for ES cell cultures, PHEMA slabs were soaked in gelatin solutions and compared the number of cells on gelatin-treated and untreated slabs 4 days after cell seeding. Surprisingly, the number of cells was only slightly higher on gelatin-treated slabs.

The influence of physical stent parameters upon restenosis

In this paper we examine whether the structure, geometry and dimensions of coronary stents influence the occurrence of restenosis. Whilst many consider these parameters to be less important since the advent of drug-eluting stents, this view reveals a poor appreciation of the technological development of stents over the last 18 years. Early 'slotted tube' stents were completely inflexible and posed major problems for delivery; and early 'coil' stents had poor radial strength, allowing considerable tissue prolapse. Nowadays, we are used to greatly improved physical stent parameters, which provide better deliverability, visibility, procedural success and scaffolding performance. Many of these physical parameters also impact upon restenosis, even in the current era of drug-eluting stent. In this paper we examine the contribution of mode of expansion (self vs. balloon-expandable), design (coil vs. tube), length and width to restenosis. We also consider the more subtle influence of advanced slotted tube vs. modular design, percent metal coverage, strut thickness, strut shape, surface smoothness and alloy composition.

Estrogen release from metallic stent surface for the prevention of restenosis

For the prevention of coronary restenosis, estrogen was coupled onto a metallic stent and in vitro release of estrogen was investigated. Estrogen was introduced to the metal surface using a hydrolysable covalent bond for local sustained delivery of drug as follows: (i) the stainless steel (SS) surface was activated with silane by plasma polymerization, (ii) the activated surface (SS-Si surface) was treated with acrylic acid by plasma polymerization (SS-Si-AAc surface), and (iii) 17beta-estradiol (E2) was covalently linked to the carboxyl group on that surface (SS-Si-AAc-E2 surface). The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, and water contact angle measurement. The amount of E2 was measured by UV-visible spectrophotometry and high performance liquid chromatography (HPLC). The in vitro release profile of E2 demonstrated sustained release of E2 in aqueous buffer. In summary, a novel method of immobilizing estrogen onto a metallic stent surface using plasma polymerization has been developed. The obtained results attest to the usefulness of the estrogen-releasing stent for preventing restenosis.

Fabrication of endothelial progenitor cell (EPC)-seeded intravascular stent devices and in vitro endothelialization on hybrid vascular tissue

Rapid re-endothelialization at an atherosclerotic lesion after balloon inflation or stent deployment may be essential for reducing or preventing local thrombus formation and restenosis. In order to prevent these complications via enhanced rapid re-endothelialization, we fabricated two types of endothelial progenitor cell (EPC)-seeded intravascular stent devices. One was a photocured gelatin-coated metallic stent, and the other was a microporous thin segmented polyurethane (SPU) film-covered stent on which photocured gelatin was coated. Both devices were seeded with ex vivo expanded EPCs obtained from canine peripheral blood. Seeded EPCs formed confluent monolayers onto surfaces of both photocured gelatin-coated stent struts and SPU film, and a majority of cells remained on surfaces of stents after stent expansion. The EPC-seeded stent was expanded in a tubular hybrid vascular medial tissue composed of vascular smooth muscle cells and collagen as an arterial media mimic. After 7-day culture, EPCs, which migrated from the stent struts, proliferated and endothelialized the luminal surfaces of the hybrid vascular medial tissue. This in vitro pilot study prior to in vivo experiments suggests that on-stent cell delivery of EPCs may be novel therapeutic devices for re-endothelialization or endothelium lining or paving at an atherosclerotic arterial wall, resulting in the prevention of on-stent thrombus formation and in-stent restenosis, as well as the rapid formation of normal tissue architecture.

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

To reduce in-stent restenosis rates, we developed a novel drug-eluting covered stent with a microporous elastometric covered film, in which its luminal surface was flat and immobilized with heparin for anticoagulation and its outer surface immobilized with FK506 to prevent neointimal hyperplasia. One month after implantation into the bilateral common carotid arteries, all stented arteries were patent and the luminal surfaces were fully covered with a confluent of endothelial cells irrespective of the drug immobilization. In the control group, which consisted of covered stents without drug immobilization, intensive inflammatory cells adjacent to the stents and neointimal hyperplasia, indicating vascular injury, were observed. In contrast, in the developed drug-eluting stents, only a few inflammatory cells around the stent strut and covered film were observed, and there was no significant neointimal thickening.

Fabrication of micropored elastomeric film-covered stents and acute-phase performances

To prevent thrombus formation in the acute phase and restenosis in the subacute to chronic phase after stenting of atherosclerotic arteries, we developed a covered stent with a micropored elastomeric film, the blood-contacting surface of which was coated with a photocured gelatin layer immobilized with heparin. Segmented polyurethane (SPU) film (30 microm in wall thickness) as a cover material was multiply micropored by excimer laser-directed microprocessing (pore diameter, 30 microm; interpore distance, 125 microm). An aqueous mixed solution of benzophenone-derivatized gelatin and heparin was coated on the micropored SPU film. Upon ultraviolet light irradiation, a thin layer of a gelatin gel immobilized with heparin was formed and simultaneously fixed on the SPU film. The fully covered stents were assembled by wrapping a balloon-expandable stent with gelatin/heparin gel-layered SPU film and subsequently suturing and then gluing. To assess the validity of this covered stent in vivo, "half-covered" stents, in which half at the distal side was covered with the gel-layered SPU film, was implanted in rabbit common carotid arteries (about 3 mm in diameter). After 3 months of implantation, all the half-covered stents (n = 7) were patent. Regardless of the covered or noncovered region of the stents, the entire luminal surface of the stents was fully endothelialized and a thin neointimal tissue was formed. The potential advantages of a covered stent as designed above are discussed.

Device-directed therapeutic drug delivery systems

To increase the therapeutic effectiveness of device-directed drug delivery systems for diseased cardiovascular tissues and cancerous tissues, new devices and new functional biomaterials were devised to meet the requirements as listed below: drug-infusible balloon catheter, drug-releasable and covered stents, and in situ hydrogelation on and in cancerous tissues. New therapeutic strategies based on these devices were discussed.