Comparison of Covered Laser-cut and Braided Respiratory Stents: From Bench to Pre-Clinical Testing.

Lung cancer patients often suffer from severe airway stenosis, the symptoms of which can be relieved by the implantation of stents. Different respiratory stents are commercially available, but the impact of their mechanical performance on tissue responses is not well understood. Two novel laser-cut and hand-braided nitinol stents, partially covered with polycarbonate urethane, were bench tested and implanted in Rhön sheep for 6 weeks. Bench testing highlighted differences in mechanical behavior: the laser-cut stent showed little foreshortening when crimped to a target diameter of 7.5 mm, whereas the braided stent elongated by more than 50%. Testing also revealed that the laser-cut stent generally exerted higher radial resistive and chronic outward forces than the braided stent, but the latter produced significantly higher radial resistive forces at diameters below 9 mm. No migration was observed for either stent type in vivo. In terms of granulation, most stents exerted a low to medium tissue response with only minimal formation of granulation tissue. We have developed a mechanical and in vivo framework to compare the behavior of different stent designs in a large animal model, providing data, which may be employed to improve current stent designs and to achieve better treatment options for lung cancer patients.

New Large Animal Model for Aortic Aneurysms in the Viscerorenal Segment.

BACKGROUND: Aortic aneurysms in the viscerorenal-segment are nowadays treatable by endovascular means. Previously, new endograft techniques were only tested in healthy animals. We aimed to establish a new large animal model for testing complex endovascular stent techniques preclinically. METHODS: In sheep, four juxtarenal and two type IV thoracoabdominal aortic aneurysms were surgically created via a retroperitoneal approach. Two pieces out of a 10 × 15-cm bovine pericardial patch were sewn with the healthy aorta longitudinally. The viscerorenal segment was clamped, and the aorta was incised longitudinally. Then, the patches were longitudinally sewn together. In the meantime, antegrade flow through the native part of the aorta was already established by tangential clamping. Computed tomography angiography was performed after 4, 8, and 52 wk. RESULTS: Technical success was 100%. The median surgical procedure time was 3 h, the median blood loss was 210 mL, and the viscerorenal-segment clamping time was 2-4 min. The animals started drinking 1 h after arousal from anesthesia. One animal died after 1 wk because of delayed bleeding and another died after 1 y because of aneurysm rupture by a secondary bacterial infection. Four animals survived. The proximal landing zone diameter and the clock position of the vessel were stable over 52 wk. CONCLUSIONS: Surgical creation of an aortic aneurysm in the viscerorenal-segment in sheep was successful, without an ischemia/reperfusion injury. This animal model offers a new platform for evaluating innovative endovascular therapy options in vivo.

[Delayed Occluding Membrane (DOM) - A New Concept to Prevent Spinal Cord Ischaemia During Endovascular Aortic Aneurysm Repair]. / Delayed Occluding Membrane (DOM) ­ ein neuer Ansatz zur Protektion der spinalen Ischämie bei endovaskulärem Aortenrepair.

INTRODUCTION: The risk of spinal cord ischemia is a relevant problem in in fields of open and endovascular thoracoabdominal aortic aneurysm repair (TAAA). Despite all efforts, no therapeutical concept exists, which enables a complete treatment of the TAAA without open branches or fenestrations, and reduces the risk for a spinal cord ischemia (SCI) to the minimum. In this article, we would like to present a new concept based on slow-occluding hydrogel-textile membrane, which could help to reduce the SCI risk during endovascular TAAA repair. CONCEPT: A hydrogel textile membrane is under development, which could be used a functional unit of endovascular stentprosthesis. If in contact with blood, glutathion induces swelling of the induces ongoing swelling of the membrane because of the triggered degradation of the crosslinker. Due to the resulting water uptake of the hydrogel textile membrane and mass increase of the gel, the swelling leads to a stabilization of the membrane. In vitro studies show, that the swelling of the hydrogel textile membrane should lead to a controlled decreasing flow into the aneurysm sac. After a pre-defined period, the membrane is occluded and the aneurysm sac perfusion stops. So, by using the hydrogel textile membrane, a complete treatment of the TAAA can be realized in one procedure without further re-intervention or pre-interventional measures. Furthermore, the risk of a SCI would be minimized. As this treatment concept is under development, only interim results are presented. CONCLUSION: The successful development and usage of a slow-occluding hydrogel textile membrane as a part of endovascular stentprosthesis could help to reduce the risk SCI during endovascular TAAA surgery.

Umbilical cord as human cell source for mitral valve tissue engineering - venous vs. arterial cells.

Around 2% of the population in developed nations are affected by mitral valve disease and available valvular replacements are not designed for the atrioventricular position. Recently our group developed the first tissue-engineered heart valve (TEHV) specifically designed for the mitral position - the TexMi valve. The valve recapitulates the main components of the native valve, i.e. annulus, asymmetric leaflets and the crucial chordae tendineae. In the present study, we evaluated the human umbilical cord as a clinically applicable cell source for the TexMi valve. Valves produced with cells isolated from human umbilical cord veins (HUVs) and human umbilical cord arteries (HUAs) were conditioned for 21 days in custom-made bioreactors and evaluated in terms of extracellular matrix (ECM) composition and mechanical properties. In addition, static cell-laden fibrin discs were molded to investigate cell-mediated tissue contraction and differences in ECM production. HUA and HUV cells were able to deliver functional valves with a rich ECM composed mainly of collagen. Particularly noteworthy was the synthesis of elastin, which has been observed rarely in TEHV. The elastin synthesis was significantly higher in TexMi valves produced with HUV cells and therefore the HUV is considered to be the preferred cell source.

PulmoStent: In Vitro to In Vivo Evaluation of a Tissue Engineered Endobronchial Stent.

Currently, there is no optimal treatment available for end stage tumour patients with airway stenosis. The PulmoStent concept aims on overcoming current hurdles in airway stenting by combining a nitinol stent with a nutrient-permeable membrane, which prevents tumour ingrowth. Respiratory epithelial cells can be seeded onto the cover to restore mucociliary clearance. In this study, a novel hand-braided dog bone stent was developed, covered with a polycarbonate urethane nonwoven and mechanically tested. Design and manufacturing of stent and cover were improved in an iterative process according to predefined requirements for permeability and mechanical properties and finally tested in a proof of concept animal study in sheep for up to 24 weeks. In each animal two stents were implanted, one of which was cell-seeded by endoscopic spraying in situ. We demonstrated the suitability of this membrane for our concept by glucose transport testing and in vitro culture of respiratory epithelial cells. In the animal study, no migration occurred in any of the twelve stents. There was only mild granulation tissue formation and tissue reaction; no severe mucus plugging was observed. Thus, the PulmoStent concept might be a step forward for palliative treatment of airway stenosis with a biohybrid stent device.

Tissue-engineered heart valve with a tubular leaflet design for minimally invasive transcatheter implantation.

Transcatheter aortic valve implantation of (nonviable) bioprosthetic valves has been proven a valid alternative to conventional surgical implantation in patients at high or prohibitive mortality risk. In this study we present the in vitro proof-of-principle of a newly developed tissue-engineered heart valve for minimally invasive implantation, with the ultimate aim of adding the unique advantages of a living tissue with regeneration capabilities to the continuously developing transcatheter technologies. The tube-in-stent is a fibrin-based tissue-engineered valve with a tubular leaflet design. It consists of a tubular construct sewn into a self-expandable nitinol stent at three commissural attachment points and along a circumferential line so that it forms three coaptating leaflets by collapsing under diastolic back pressure. The tubular constructs were molded with fibrin and human umbilical vein cells. After 3 weeks of conditioning in a bioreactor, the valves were fully functional with unobstructed opening (systolic phase) and complete closure (diastolic phase). Tissue analysis showed a homogeneous cell distribution throughout the valve's thickness and deposition of collagen types I and III oriented along the longitudinal direction. Immunohistochemical staining against CD31 and scanning electron microscopy revealed a confluent endothelial cell layer on the surface of the valves. After harvesting, the valves underwent crimping for 20 min to simulate the catheter-based delivery. This procedure did not affect the valvular functionality in terms of orifice area during systole and complete closure during diastole. No influence on the extracellular matrix organization, as assessed by immunohistochemistry, nor on the mechanical properties was observed. These results show the potential of combining tissue engineering and minimally invasive implantation technology to obtain a living heart valve with a simple and robust tubular design for transcatheter delivery. The effect of the in vivo remodeling on the functionality of the tube-in-stent valve remains to be tested.

USPIO-labeled textile materials for non-invasive MR imaging of tissue-engineered vascular grafts.

Non-invasive imaging might assist in the clinical translation of tissue-engineered vascular grafts (TEVG). It can e.g. be used to facilitate the implantation of TEVG, to longitudinally monitor their localization and function, and to provide non-invasive and quantitative feedback on their remodeling and resorption. We here incorporated ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles into polyvinylidene fluoride (PVDF)-based textile fibers, and used them to prepare imageable tissue-engineered vascular grafts (iTEVG). The USPIO-labeled scaffold materials were molded with a mixture of fibrin, fibroblasts and smooth muscle cells, and then endothelialized in a bioreactor under physiological flow conditions. The resulting grafts could be sensitively detected using T1-, T2- and T2*-weighted MRI, both during bioreactor cultivation and upon surgical implantation into sheep, in which they were used as an arteriovenous shunt between the carotid artery and the jugular vein. In vivo, the iTEVG were shown to be biocompatible and functional. Post-mortem ex vivo analyses provided evidence for efficient endothelialization and for endogenous neo-vascularization within the biohybrid vessel wall. These findings show that labeling polymer-based textile materials with MR contrast agents is straightforward and safe, and they indicate that such theranostic tissue engineering approaches might be highly useful for improving the production, performance, personalization and translation of biohybrid vascular grafts.

TexMi: development of tissue-engineered textile-reinforced mitral valve prosthesis.

Mitral valve regurgitation together with aortic stenosis is the most common valvular heart disease in Europe and North America. Mechanical and biological prostheses available for mitral valve replacement have significant limitations such as the need of a long-term anticoagulation therapy and failure by calcifications. Both types are unable to remodel, self-repair, and adapt to the changing hemodynamic conditions. Moreover, they are mostly designed for the aortic position and do not reproduce the native annular-ventricular continuity, resulting in suboptimal hemodynamics, limited durability, and gradually decreasing ventricular pumping efficiency. A tissue-engineered heart valve specifically designed for the mitral position has the potential to overcome the limitations of the commercially available substitutes. For this purpose, we developed the TexMi, a living textile-reinforced mitral valve, which recapitulates the key elements of the native one: annulus, asymmetric leaflets (anterior and posterior), and chordae tendineae to maintain the native annular-ventricular continuity. The tissue-engineered valve is based on a composite scaffold consisting of the fibrin gel as a cell carrier and a textile tubular structure with the twofold task of defining the gross three-dimensional (3D) geometry of the valve and conferring mechanical stability. The TexMi valves were molded with ovine umbilical vein cells and stimulated under dynamic conditions for 21 days in a custom-made bioreactor. Histological and immunohistological stainings showed remarkable tissue development with abundant aligned collagen fibers and elastin deposition. No cell-mediated tissue contraction occurred. This study presents the proof-of-principle for the realization of a tissue-engineered mitral valve with a simple and reliable injection molding process readily adaptable to the patient's anatomy and pathological situation by producing a patient-specific rapid prototyped mold.

Tissue-engineered fibrin-based heart valve with a tubular leaflet design.

The general approach in heart valve tissue engineering is to mimic the shape of the native valve in the attempt to recreate the natural haemodynamics. In this article, we report the fabrication of the first tissue-engineered heart valve (TEHV) based on a tubular leaflet design, where the function of the leaflets of semilunar heart valves is performed by a simple tubular construct sutured along a circumferential line at the root and at three single points at the sinotubular junction. The tubular design is a recent development in pericardial (nonviable) bioprostheses, which has attracted interest because of the simplicity of the construction and the reliability of the implantation technique. Here we push the potential of the concept further from the fabrication and material point of view to realize the tube-in-tube valve: an autologous, living HV with remodelling and growing capability, physiological haemocompatibility, simple to construct and fast to implant. We developed two different fabrication/conditioning procedures and produced fibrin-based constructs embedding cells from the ovine umbilical cord artery according to the two different approaches. Tissue formation was confirmed by histology and immunohistology. The design of the tube-in-tube foresees the possibility of using a textile coscaffold (here demonstrated with a warp-knitted mesh) to achieve enhanced mechanical properties in vision of implantation in the aortic position. The tube-in-tube represents an attractive alternative to the conventional design of TEHVs aiming at reproducing the valvular geometry.

Nondestructive monitoring of tissue-engineered constructs.

Abstract Tissue engineering as a multidisciplinary field enables the development of living substitutes to replace, maintain, or restore diseased tissue and organs. Since the term was introduced in medicine in 1987, tissue engineering strategies have experienced significant progress. However, up to now, only a few substitutes were able to overcome the gap from bench to bedside and have been successfully approved for clinical use. Substantial donor variability makes it difficult to predict the quality of tissue-engineered constructs. It is essential to collect sufficient data to ensure that poor or immature constructs are not implanted into patients. The fulfillment of certain quality requirements, such as mechanical and structural properties, is crucial for a successful implantation. There is a clear need for new nondestructive and real-time online monitoring and evaluation methods for tissue-engineered constructs, which are applicable on the biomaterial, tissue, cellular, and subcellular levels. This paper reviews current established nondestructive techniques for implant monitoring including biochemical methods and noninvasive imaging.