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

Leaflet Tissue Generation from Microfibrous Heart Valve Leaflet Scaffolds with Native Characteristics

Mechanical and bioprosthetic valves that are currently applied for replacing diseased heart valves are not fully efficient. Heart valve tissue engineering may solve the issues faced by the prosthetic valves in heart valve replacement. The leaflets of native heart valves have a trilayered structure with layer-specific orientations; thus, it is imperative to develop functional leaflet tissue constructs with a native trilayered, oriented structure. Its key solution is to develop leaflet scaffolds with a native morphology and structure. In this study, microfibrous leaflet scaffolds with a native trilayered and oriented structure were developed in an electrospinning system. The scaffolds were implanted for 3 months in rats subcutaneously to study the scaffold efficiencies in generating functional tissue-engineered leaflet constructs. These in vivo tissue-engineered leaflet constructs had a trilayered, oriented structure similar to native leaflets. The tensile properties of constructs indicated that they were able to endure the hydrodynamic load of the native heart valve. Collagen, glycosaminoglycans, and elastin─the predominant extracellular matrix components of native leaflets─were found sufficiently in the leaflet tissue constructs. The residing cells in the leaflet tissue constructs showed vimentin and α-smooth muscle actin expression, i.e., the constructs were in a growing state. Thus, the trilayered, oriented fibrous leaflet scaffolds produced in this study could be useful to develop heart valve scaffolds for successful heart valve replacements.

Elastin-like recombinamer-covered stents: Towards a fully biocompatible and non-thrombogenic device for cardiovascular diseases

We explored the use of recently developed gels obtained by the catalyst free click reaction of elastin-like recombinamers (ELRs) to fabricate a new class of covered stents. The approach consists in embedding bare metal stents in the ELR gels by injection molding, followed by endothelialization under dynamic pressure and flow conditions in a bioreactor. The mechanical properties of the gels could be easily tuned by choosing the adequate concentration of the ELR components and their biofunctionality could be tailored by inserting specific sequences (RGD and REDV). The ELR-covered stents exhibited mechanical stability under high flow conditions and could undergo crimping and deployment without damage. The presence of RGD in the ELR used to cover the stent supported full endothelialization in less than 2weeks in vitro. Minimal platelet adhesion and fibrin adsorption were detected after exposure to blood, as shown by immunostaining and scanning electron microscopy. These results prove the potential of this approach towards a new and more effective generation of covered stents which exclude the atherosclerotic plaque from the blood stream and have high biocompatibility, physiological hemocompatibility and reduced response of the immune system.

Ureteral stent-associated complications--where we are and where we are going

Ureteral stents are one of the most commonly used devices in the treatment of benign and malignant urological diseases. However, they are associated with common complications including encrustation, infection, pain and discomfort caused by ureteral tissue irritation and possibly irregular peristalsis. In addition, stent migration and failure due to external compression by malignancies or restenosis occur, albeit less frequently. As these complications restrict optimal stent function, including maintenance of adequate urine drainage and alleviation of hydronephrosis, novel stent materials and designs are required. In recent years, progress has been made in the development of drug-eluting expandable metal stents and biodegradable stents. New engineering technologies are being investigated to provide stents with increased biocompatibility, decreased susceptibility to encrustation and improved drug-elution characteristics. These novel stent characteristics might help eliminate some of the common complications associated with ureteral stenting and will be an important step towards understanding the behaviour of stents within the urinary tract.

Development of tissue-engineered self-expandable aortic stent grafts (Bio stent grafts) using in-body tissue architecture technology in beagles

In this study, we aimed to describe the development of tissue-engineered self-expandable aortic stent grafts (Bio stent graft) using in-body tissue architecture technology in beagles and to determine its mechanical and histological properties. The preparation mold was assembled by insertion of an acryl rod (outer diameter, 8.6 mm; length, 40 mm) into a self-expanding nitinol stent (internal diameter, 9.0 mm; length, 35 mm). The molds (n = 6) were embedded into the subcutaneous pouches of three beagles for 4 weeks. After harvesting and removing each rod, the excessive fragile tissue connected around the molds was trimmed, and thus tubular autologous connective tissues with the stent were obtained for use as Bio stent grafts (outer diameter, approximately 9.3 mm in all molds). The stent strut was completely surrounded by the dense collagenous membrane (thickness, ∼150 µm). The Bio stent graft luminal surface was extremely flat and smooth. The graft wall of the Bio stent graft possessed an elastic modulus that was almost two times higher than that of the native beagle abdominal aorta. This Bio stent graft is expected to exhibit excellent biocompatibility after being implanted in the aorta, which may reduce the risk of type 1 endoleaks or migration.

The BioStent: novel concept for a viable stent structure

OBJECTIVES

Percutaneous stenting of occluded peripheral vessels is a well-established technique in clinical practice. Unfortunately, the patency rates of small-caliber vessels after stenting remain unsatisfactory. The aim of the BioStent concept is to overcome in-stent restenosis by excluding the diseased vessel segment entirely from the blood stream, in addition to providing an intact endothelial cell layer.

DESIGN

The concept combines the principles of vascular tissue engineering with a self-expanding stent: casting of the stent within a cellularized fibrin gel structure, followed by bioreactor conditioning, allows complete integration of the stent within engineered tissue.

MATERIALS AND METHODS

Small-caliber BioStents (Ø=6 mm; n=4) were produced by casting a nitinol stent within a thin fibrin/vascular smooth muscle cell (vSMC) mixture, followed by luminal endothelial cell seeding, and conditioning of the BioStent within a bioreactor system. The potential remodeling of the fibrin component into tissue was analyzed using routine histological methods. Scanning electron microscopy was used to assess the luminal endothelial cell coverage following the conditioning phase and crimping of the stent.

RESULTS

The BioStent was shown to be noncytotoxic, with no significant effect on cell proliferation. Gross and microscopic analysis revealed complete integration of the nitinol component within a viable tissue structure. Hematoxylin and eosin staining revealed a homogenous distribution of vSMCs throughout the thickness of the BioStent, while a smooth, confluent luminal endothelial cell lining was evident and not significantly affected by the crimping/release process.

CONCLUSIONS

The BioStent concept is a platform technology offering a novel opportunity to generate a viable, self-expanding stent structure with a functional endothelial cell lining. This platform technology can be transferred to different applications depending on the luminal cell lining required.

Ureteral stents: new ideas, new designs

Ureteral stents represent a minimally invasive alternative to preserve urinary drainage whenever ureteral patency is deteriorated or is under a significant risk to be occluded due to extrinsic or intrinsic etiologies. The ideal stent that would combine perfect long-term efficacy with no stent-related morbidity is still lacking and stent usage is associated with several adverse effects that limit its value as a tool for long-term urinary drainage. Several new ideas on stent design, composition material and stent coating currently under evaluation, foreseen to eliminate the aforementioned drawbacks of ureteral stent usage. In this article we review the currently applied novel ideas and new designs of ureteral stents. Moreover, we evaluate potential future prospects of ureteral stent development adopted mostly by the pioneering cardiovascular stent industry, focusing, however, on the differences between ureteral and endothelial tissue.

Long-term animal implantation study of biotube-autologous small-caliber vascular graft fabricated by in-body tissue architecture

A mold for the preparation of an in-body tissue architecture-induced autologous vascular graft, termed "biotube," was prepared by covering a main silicone rod (outer diameter, 3 mm; length, 30 mm) with two pieces of polyurethane sponge tubes (internal diameter, 3 mm; length, 3 mm) at both ends. The molds were embedded into the dorsal subcutaneous pouch of rabbits (weighing ca. 2 kg) for 2 months. After harvesting the rods with the formed surrounding tissues, the rods were removed to create biotubes impregnated with anastomotic reinforcement cuffs at both ends. The biotubes had homogeneous, thin connective tissue wall (thickness, 76 ± 37 μm) that was primarily composed of collagen and fibroblasts. One biotube was loaded with argatroban and autoimplanted in the carotid artery for 26 months. Neither antiplatelet nor anticoagulant agents were administered, except for an intraoperative heparin injection. Follow-up angiography showed no aneurysm formation, rupturing, or stenosis during implantation. At the end of implantation, the wall thickness of biotube (212 ± 24 μm at the anastomosis portion and 150 ± 14 μm at the midportion) was similar to that of native artery (189 ± 23 μm). The luminal surface was completely covered with endothelial cells on the formed lamina elastica interna-like layer. The regenerated vascular walls comprised multilayered smooth muscle cells and dense collagen fibers with regular circumferential orientation. A remarkable multilayered elastin fiber network was observed near the anastomosis portion. Biotubes could thus be used as small-caliber vascular prostheses that greatly facilitate the healing process and exhibit excellent biocompatibility.

3-Tesla magnetic resonance angiographic assessment of a tissue-engineered small-caliber vascular graft implanted in a rat

In the development of small-caliber vascular grafts (diameter; less than 3 mm), animal implantation studies have been mostly performed by using rat abdominal aortas, and their certain patency must evaluate with sacrificing every observation periods, which is both labor-intensive and time-consuming when performing a large number of experiments. This study is the first to demonstrate the application of 3-Tesla contrast-free time-of-flight magnetic resonance angiography (TOF-MRA) in the continuous assessment of the status of a tissue-engineered vascular graft in rat. As a model graft, a single connective tubular tissue (diameter; 1.5 mm), prepared by embedding the silicone rod (diameter; 1.5 mm) into a subcutaneous pouch of a rat for 2 weeks an in vivo tissue-engineering, was used. The graft was implanted in the abdominal aorta (diameter; 1.3 mm) of the rat by end-to-end anastomosis. Repeated TOF-MRA imaging of the graft obtained over a 3-month follow-up period after implantation made it possible to evaluate the patency of the graft, both simply and noninvasively. It also permitted visualization of the connected abdominal aorta and renal and common iliac arteries having smaller caliber (diameter; less than 1 mm). In addition, the degree of the stenosis or aneurysm could also be detected. 3-Tesla MRA allowed the simplified and noninvasive assessment of the status on the vascular graft, including the formation of a stenosis or aneurysm, in the same rat at different times, which will be contributing to enhance the development of tissue-engineered vascular grafts even with small caliber.

Autologous small-caliber "biotube" vascular grafts with argatroban loading: a histomorphological examination after implantation to rabbits

Functional autologous tubular tissues, termed "biotubes," have been developed as small-caliber vascular grafts. Biotubes can be easily and safely constructed in vivo by using a novel concept in regenerative medicine-in body tissue architecture technology, which requires neither clean specialized laboratories nor complex cell management. Biotubes with "anastomotic reinforcement cuffs" were prepared by embedding a silicone rod (diameter, 3 mm; length, 30 mm) as a mold in the dorsal subcutaneous pouches of rabbits. The rod was covered at both ends with 2 pieces of polyurethane sponge tubes (length, 3 mm), and it was removed when the grafts were harvested. These biotubes had homogeneous thin connective tissue walls (thickness: 76 +/- 37 microm) that were primarily composed of collagen and fibroblasts. The resulting cuff-impregnated biotubes were auto-implanted in the carotid arteries for predetermined periods of up to 12 weeks and then morphologically examined. On implantation of the biotubes after argatroban loading, the total patency was 9/11 without any instance of aneurysm formation or rupture. At 12 weeks after implantation, no significant neointimal thickening was observed (170 +/- 30 microm). In addition, minimal thrombus formation was observed on the luminal surfaces, which were completely covered with endothelial cells regularly oriented longitudinally. The regenerated vascular walls comprised multilayered smooth muscle cells and dense collagen fibers with regular circumferential orientation with few elastin fibers and were similar to native arteries. Biotubes with argatroban loading could thus be used as small-caliber vascular prostheses that greatly facilitate healing process and exhibit excellent biocompatibility.

In vitro maturation of "biotube" vascular grafts induced by a 2-day pulsatile flow loading

Autologous vascular tissues with a small diameter, "biotubes," were developed in vivo using a novel concept in regenerative medicine, "in-body tissue architecture technology." The effect of pulsatile flow in vitro was investigated on the structural and functional properties of the biotubes. Silicone rods (diameter, 3.0 mm; length, 35.0 mm), used as molds, were embedded into dorsal subcutaneous spaces of Wister rats. After 4 weeks, the autologous tubular tissues formed around the rods were harvested. Some tissues were incubated for 2 days under pulsatile flow simulating conditions in the human arteries with small caliber (wall shear stress (WSS), 15.5-77.3 dyn/cm(2); circumferential stress (CS), 0.6-4.5 x 10(5) dyn/cm(2)). Upon flow loading, the sparse, randomly oriented collagen fibers in the biotubes became dense and oriented in the regular circumferential direction. Compliances (beta values) of the control (ca. 30) and flow-loaded (ca. 20) biotubes were equivalent to that of the human coronary arteries and femoral arteries, respectively. Further, upon flow loading, the burst pressure significantly increased from ca. 1000 mmHg to ca. 1800 mmHg, along with the alpha-SMA-positive cell ratio. Pulsatile flow loading in vitro for 2 days could induce biotube maturation in terms of collagen structures and mechanical properties.