Martin Stoiber

Healing characteristics of electrospun polyurethane grafts with various porosities.

Pore size and porosity control the rate and depth of cellular migration in electrospun vascular fabrics and thus have a strong impact on long-term graft success. In this study we investigated the effect of graft porosity on cell migration in vitro and in vivo. Polyurethane (PU) grafts were fabricated by electrospinning as fine-mesh, low-porosity grafts (void fraction (VF) 53%) and coarse-mesh, high-porosity grafts (VF 80%). The fabricated grafts were evaluated in vitro for endothelial cell attachment and proliferation. Prostheses were investigated in a rat model for either 7 days, 1, 3 or 6 months (n=7 per time point) and analyzed after retrieval by biomechanical analysis and various histological techniques. Cell migration was calculated by computer-assisted morphometry. In vitro, fine-pore mesh favored early cell attachment. In vivo, coarse mesh grafts revealed significantly higher cell populations at all time points in all areas of the conduit wall. Biomechanical tests indicated sufficient compliance, tensile and suture retention strength before and after implantation. Increased porosity improves host cell ingrowth and survival in electrospun conduits. These conduits show successful natural host vessel reconstitution without limitation of biomechanical properties.

Electrospun polyurethane vascular grafts: In vitro mechanical behavior and endothelial adhesion molecule expression

Engineered small diameter vascular grafts must closely match mechanical characteristics of native vessels and exhibit stimulus-responsive bioactivity. In this study, mechanical homogeneity of electrospun small diameter polyurethane grafts as well as spontaneous attachment, proliferation, and adhesion molecule expression of endothelial cells (EC) in their presence was studied in vitro. Axial and circumferential tensile strengths were measured and found to be twofold higher in the circumferential direction. EC attachment was easily achieved without precoating the fiber matrix. Stimulation of EC with interleukin-1beta (IL-1beta) led to a statistically significant upregulation of the adhesion molecules E-Selectin, ICAM-1, and VCAM-1. Quantification of adhesion molecule expression by means of energy-dispersive X-ray microanalysis revealed no differences in the stimulatory responses of EC cultured on electrospun polyurethane when compared with cells grown on tissue culture-treated cover slips. Summarizing, highly uniform small diameter polyurethane grafts were fabricated and shown to allow spontaneous EC attachment. The synthetic graft surface neither impaired the endothelial response toward IL-1beta stimulation nor did it adversely affect the regulation of expression of endothelial adhesion molecules.

Electrospun Small‐Diameter Polyurethane Vascular Grafts: Ingrowth and Differentiation of Vascular‐Specific Host Cells

No small-diameter synthetic graft has yet shown comparable performance to autologous vessels. Synthetic conduits fail due to their inherent surface thrombogenicity and the development of intimal hyperplasia. In addressing these shortcomings, electrospinning offers an interesting alternative to other nanostructured, cardiovascular substitutes because of the close match of electrospun materials to the biomechanical and structural properties of native vessels. In this study, we investigated the in vivo behavior of electrospun, small-diameter conduits in a rat model. Vascular grafts composed of polyurethane were fabricated by electrospinning. Prostheses were implanted into the abdominal aorta in 40 rats for either 7 days, 4 weeks, 3 months, or 6 months. Retrieved specimens were evaluated by histology, immunohistochemical staining, confocal laser scanning microscopy, and scanning electron microscopy. At all time points, we found no evidence of foreign body reaction or graft degradation. The overall patency rate of the intravascular implants was 95%. Within 7 days, grafts revealed ingrowth of host cells. CD34+ cells increased significantly from 7 days up to 6 months of implantation (P < 0.05). Myofibroblasts and myocytes showed increasing cell numbers up to 3 months (P < 0.05). Ki67 staining indicated unaltered cell proliferation during the whole follow-up period. Besides biomechanical benefits, electrospun polyurethane grafts exhibit excellent biocompatibility in vivo. Cell immigration and differentiation seems to be promoted by the nanostructured artificial matrix.

Biomechanical Properties of Decellularized Porcine Pulmonary Valve Conduits

Tissue-engineered heart valves constructed from a xenogeneic or allogeneic decellularized matrix might overcome the disadvantages of current heart valve substitutes. One major necessity besides effective decellularization is to preserve the biomechanical properties of the valve. Native and decellularized porcine pulmonary heart valve conduits (PPVCs) (with [n = 10] or without [n = 10] cryopreservation) were compared to cryopreserved human pulmonary valve conduits (n = 7). Samples of the conduit were measured for wall thickness and underwent tensile tests. Elongation measurement was performed with a video extensometer. Decellularized PPVC showed a higher failure force both in longitudinal (+73%; P < 0.01) and transverse (+66%; P < 0.001) direction compared to human homografts. Failure force of the tissue after cryopreservation was still higher in the porcine group (longitudinal: +106%, P < 0.01; transverse: +58%, P < 0.001). In comparison to human homografts, both decellularized and decellularized cryopreserved porcine conduits showed a higher extensibility in longitudinal (decellularized: +61%, P < 0.001; decellularized + cryopreserved: +51%, P < 0.01) and transverse (decellularized: +126%, P < 0.001; decellularized + cryopreserved: +118%, P < 0.001) direction. Again, cryopreservation did not influence the biomechanical properties of the decellularized porcine matrix.

Experimental Acute Type B Aortic Dissection: Different Sites of Primary Entry Tears Cause Different Ways of Propagation

BACKGROUND Many dissections seem to also have a retrograde component. The aim of the study was to evaluate different sites of primary entry tears and the propagation of the dissecting membrane, antegrade and retrograde, in an experimental model of acute type B aortic dissection. METHODS The entire thoracic aortic aorta including the supraaortic branches was harvested from 26 adult pigs. An intimal tear of 15 mm was created by contralateral incisions sites 20 mm downstream the origin of the left subclavian artery. In 13 cases the dissection was created at the concavity and in 13 cases at the convexity. The aortic annulus was then sewn into a silicon ring of a driving chamber. The distal aorta was connected to a tubing with adjustable resistance elements. The circulation was driven by the pneumatically driven Vienna heart to mimic aortic flow and pressure. RESULTS Mean circulation time was 64 ± 45 minutes. A mean pressure of 152 ± 43 mm Hg and a mean flow of 4.5 ± 1.0 L/minute were reached. The median antegrade propagation length of the dissecting membrane was 65 mm. The median retrograde propagation length in primary entry tears at the convexity was 20 mm and was stopped by the left subclavian artery. In aortas with the primary entry tear at the concavity, median retrograde propagation length was 21 mm extending up to the ascending aorta in 16%. CONCLUSIONS In this experimental model of acute type B aortic dissection, we confirmed that many type B dissections do also have a retrograde component. At the convexity, this component is stopped by the left subclavian artery as an anatomic barrier. At the concavity, the propagation of the dissecting membrane may extend up to the ascending aorta and may therefore cause retrograde type A dissection. These findings may substantiate clinical need for treatment of type B dissections with a primary entry tear at the concavity.

Mesh Fixation in Laparoscopic Incisional Hernia Repair: Glue Fixation Provides Attachment Strength Similar to Absorbable Tacks but Differs Substantially in Different Meshes

BACKGROUND Laparoscopic ventral hernia repair has gained popularity among minimally invasive surgeons. However, mesh fixation remains a matter of discussion. This study was designed to compare noninvasive fibrin-glue attachment with tack fixation of meshes developed primarily for intra-abdominal use. It was hypothesized that particular mesh structures would substantially influence detachment force. STUDY DESIGN For initial evaluation, specimens of laminated polypropylene/polydioxanone meshes were anchored to porcine abdominal walls by either helical titanium tacks or absorbable tacks in vitro. A universal tensile-testing machine was used to measure tangential detachment forces (TF). For subsequent experiments of glue fixation, polypropylene/polydioxanone mesh and 4 additional meshes with diverse particular mesh structure, ie, polyvinylidene fluoride/polypropylene mesh, a titanium-coated polypropylene mesh, a polyester mesh bonded with a resorbable collagen, and a macroporous condensed PTFE mesh were evaluated. RESULTS TF tests revealed that fibrin-glue attachment was not substantially different from that achieved with absorbable tacks (median TF 7.8 Newton [N], range 1.3 to 15.8 N), but only when certain open porous meshes (polyvinylidene fluoride/polypropylene mesh: median 6.2 N, range 3.4 to 10.3 N; titanium-coated polypropylene mesh: median 5.2 N, range 2.1 to 11.7 N) were used. Meshes coated by an anti-adhesive barrier (polypropylene/polydioxanone mesh: median 3.1 N, range 1.7 to 5.8 N; polyester mesh bonded with a resorbable collagen: median 1.3 N, range 0.5 to 1.9 N), or the condensed PTFE mesh (median 3.1 N, range 2.1 to 7.0 N) provided a significantly lower TF (p < 0.01). CONCLUSIONS Fibrin glue appears to be an appealing noninvasive option for mesh fixation in laparoscopic ventral hernia repair, but only if appropriate meshes are used. Glue can also serve as an adjunct to mechanical fixation to reduce the number of invasive tacks.

Systemic pressure does not directly affect pressure gradient and valve area estimates in aortic stenosis in vitro

AIMS Hypertension is a frequent finding in patients with aortic stenosis (AS). However, controversial data about the influence of systemic blood pressure on the quantification of AS have been published. METHODS AND RESULTS Various models of AS (plates and biological valves) were studied in an in vitro circuit. Valve areas were calculated with the Doppler continuity equation and the Gorlin formula. Systolic systemic pressures were increased from 80 to 200 mmHg while flow rates were maintained constant. In addition, a computational fluid dynamics (CFD) model was constructed to test the effect of systemic pressures on pressure gradient and valve area estimates. When systemic pressure was raised, pressure gradients as well as valve areas did not change (mean difference 3.4 +/- 1.8 mmHg, range 0.4-6.8 mmHg; mean difference 0.01 +/- 0.03 cm(2), range -0.02 to 0.05 cm(2)). By multivariable analysis, neither valve area nor pressure gradient were independently affected by systemic pressure. In addition, CFD analysis revealed no effect of systemic pressure on pressure gradient and valve area. CONCLUSION Our results suggest that blood pressure itself does not directly affect pressure gradients and valve area estimates in AS. Thus, when observed in vivo, these changes are most likely due to afterload-related variations of ejection fraction and, therefore, flow rate.

Electrodynamic control of the nanofiber alignment during electrospinning

A technique is presented to electrospin straight and aligned fibers on a stationary featureless target. Two parallel rotatable plate-like auxiliary electrodes were applied with a time-varying square wave potential. Thereby, the electrospinning jet was periodically deflected between the electrodes, which led to an aligned fiber-deposition. Straight fibers were deposited at a potential difference of 11 kV and a switching frequency of 40 Hz between the auxiliary electrodes. With this setup, freely adjustable orientations can be achieved regardless of the targets design or its movement.

Acquired von Willebrand factor deficiency caused by LVAD is ADAMTS-13 and platelet dependent☆

INTRODUCTION The high shear rates induced by left ventricular assist devices cause acquired von Willebrand disease (aVWD). We hypothesised that an ex vivo model could be established to study whether mechanical shear stress alone causes aVWD or whether this process depends also on the VWF cleavage protein ADAMTS-13 and on platelets. MATERIALS AND METHODS Healthy volunteers and two patients with congenital ADAMTS-13 deficiency donated blood. In vitro closed extracorporeal circuits were established using medically approved left ventricular assist devices (LVAD). VWF multimers were quantified by gel electrophoresis; VWF antigen, ristocetin cofactor activity (VWF:RCo), ADAMTS-13 levels and platelet function were assessed. RESULTS The high shear stress in the extracorporeal circulation rapidly decreased VWF:RCo and thereby the VWF:RCo/VWF:Ag ratio by 47% (p<0.01) to pathologically low values. Concomitantly, high molecular weight multimers (HMWM) decreased: up to 14-15 mers were visible on the gels at baseline, which were reduced by a maximum of 6-7 mers, corresponding to an average 68% lower densitometry signal of HMWM (p<0.001). This was accompanied by marked reduction of aggregation by various agonists (p<0.005). In contrast, the two patients with congenital thrombocytopenic purpura with virtually complete deficiency of ADAMTS-13 activity had only a minimal or no decrease in multimers (p<0.005 vs. healthy controls). Similarly, no or minimal depletion of large multimers occurred, when normal plasma circulated without platelets. CONCLUSION An in vitro model for LVAD associated aVWD demonstrated that ADAMTS-13 and platelets contribute to the depletion of HMWM of VWF.

Extended in vivo evaluation of a miniaturized axial flow pump with a novel inflow cannula for a minimal invasive implantation procedure

BACKGROUND Minimally invasive techniques are desirable to minimize surgical trauma during left ventricular assist device (LVAD) implantation. This is particularly challenging for full-flow support. In this study, a minimally invasive implantation technique was developed for a microaxial rotary pump. The system was evaluated in a chronic sheep model. METHODS A HeartWare MVAD (HeartWare, Miami Lakes, FL) pump (length, 50 mm; diameter, 21 mm; maximum flow, 7-8 liters/min) was combined with a novel inflow cannula, including a new flow-optimized tip. The device was implanted into sheep (range, 60-80 kg, mean, 71.6 ± 6.8 kg) through a right-sided minithoracotomy. The inflow cannula was inserted through the superior pulmonary vein, passing through the left atrium into the left ventricle. Scheduled implant period was 30 days for 8 sheep and 100 days for 3 sheep. Mean support flow was set to half of the nominal cardiac output. RESULTS Six of 8 sheep finished the scheduled 30-day investigation period (one failed due to early non-pump-related post-operative bleeding and one due to prototype controller failure). The 3 sheep scheduled for 100 days reached the study end point. Peak pump flows of up to 6.9 liters/min were achieved. At necropsy, no signs of mitral valve lesions or thrombus formation around the cannula, the tip, or the insertion site were observed, except for valve leaflet erosion in 1 animal, where the cannula had been entangled in the sub-valvular chords due to lack of ultrasound monitoring. CONCLUSIONS The minimally invasive implantation technique using the HeartWare MVAD pump, together with a new cannula, provided excellent results in a chronic animal model.