Fabrication and biomechanical characterization of a spider silk reinforced fibrin-based vascular prosthesis

With fibrin-based vascular prostheses, vascular tissue engineering offers a promising approach for the fabrication of biologically active regenerative vascular grafts. As a potentially autologous biomaterial, fibrin exhibits excellent hemo- and biocompatibility. However, the major problem in the use of fibrin constructs in vascular tissue engineering, which has so far prevented their widespread clinical application, is the insufficient biomechanical stability of unprocessed fibrin matrices. In this proof-of-concept study, we investigated to what extent the addition of a spider silk network into the wall structure of fibrin-based vascular prostheses leads to an increase in biomechanical stability and an improvement in the biomimetic elastic behavior of the grafts. For the fabrication of hybrid prostheses composed of fibrin and spider silk, a statically cast tubular fibrin matrix was surrounded with an envelope layer of Trichonephila edulis silk using a custom built coiling machine. The fibrin matrix was then compacted and pressed into the spider silk network by transluminal balloon compression. This manufacturing process resulted in a hybrid prosthesis with a luminal diameter of 4 mm. Biomechanical characterization revealed a significant increase in biomechanical stability of spider silk reinforced grafts compared to exclusively compacted fibrin segments with a mean burst pressure of 362 ± 74 mmHg vs. 213 ± 14 mmHg (p < 0.05). Dynamic elastic behavior of the spider silk reinforced grafts was similar to native arteries. In addition, the coiling with spider silk allowed a significant increase in suture retention strength and resistance to external compression without compromising the endothelialization capacity of the grafts. Thus, spider silk reinforcement using the abluminal coiling technique represents an efficient and reproducible technique to optimize the biomechanical behavior of small-diameter fibrin-based vascular grafts.

Case report: Blood pressure variation during cardiopulmonary bypass in a patient with multiple sclerosis

Introduction and importance

Multiple sclerosis is known to be associated with both sympathetic and parasympathetic cardiovascular autonomic dysregulation. Thus, patients with multiple sclerosis comorbidity represent a potentially challenging patient population in cardiac surgery, especially in on-pump operations. Despite this, very little is known about the hemodynamics during cardiopulmonary bypass and the optimal perfusion strategy for patients with multiple sclerosis undergoing cardiac operations.

Case presentation

In this report, the authors describe a patient with relapsing-remitting multiple sclerosis, who underwent successful triple valve operation for aortic and mitral stenosis and tricuspid valve insufficiency. Distinct blood pressure variations in form of temporary pressure dips were noted during total cardiopulmonary bypass time as well as during the reperfusion period.

Clinical discussion

Pressure variations were not attributable to surgical, pharmacological or perfusion-related manoeuvres. Thus, they most likely represent symptoms of cardiovascular autonomic dysregulation manifesting during cardiopulmonary bypass. In this patient, blood pressure variations terminated spontaneously and remained within an acceptable range without external correction.

Conclusions

When treating patients with multiple sclerosis comorbidity, the potential pressure variability due to cardiovascular autonomic dysregulation should be taken into consideration to avoid increased blood pressure volatility due to overcorrection or undercorrection during cardiopulmonary bypass.

Impact of Intercostal Artery Reinsertion on Neurological Outcome after Thoracoabdominal Aortic Replacement: A 25-Year Single-Center Experience

BACKGROUND

Intercostal artery reinsertion (ICAR) during thoracoabdominal aortic replacement remains controversial. While some groups recommend the reinsertion of as many arteries as possible, others consider the sacrifice of multiple intercostals practicable. This study investigates the impact of intercostal artery reinsertion or sacrifice on neurological outcomes and long-term survival after thoracoabdominal aortic repair.

METHODS

A total of 349 consecutive patients undergoing thoracoabdominal aortic replacement at our institution between 1996 and 2021 were analyzed in a retrospective single-center study. ICAR was performed in 213 patients, while all intercostal arteries were ligated and sacrificed in the remaining cases. The neurological outcome was analyzed regarding temporary and permanent paraplegia or paraparesis.

RESULTS

No statistically significant differences were observed between the ICAR and non ICAR groups regarding the cumulative endpoint of transient and permanent spinal cord-related complications (12.2% vs. 11.8%, p = 0.9). Operation, bypass, and cross-clamp times were significantly longer in the ICAR group. Likewise, prolonged mechanical ventilation was more often necessary in the ICAR group (26.4% vs. 16.9%, p = 0.03). Overall long-term survival was similar in both groups in the Kaplan-Meier analysis.

CONCLUSION

Omitting ICAR during thoracoabdominal aortic replacement may reduce operation and cross-clamp times and thus minimize the duration of intraoperative spinal cord hypoperfusion.

Incidental Finding of Subannular Perfused Aortic Root Abscess

An 83-year-old female presented with aortic valve stenosis requiring surgery, which was diagnosed with a transthoracic echocardiography three years ago. However, the patient declined the surgery at that time due to personal reasons. Three years later she presented again with signs of dizziness and weakness and progression of the aortic valve stenosis. Cardiac catheterization and a computed tomography scan were performed before the planned surgery. Surprisingly, a huge subannular perfused abscess hole around the aortic root companying a pericardial effusion was revealed. The patient underwent an urgent aortic root replacement with a tissue valve and an aortic ascending replacement without any complications. Intraoperative inspection confirmed an active aortic root and valve endocarditis.

Expanding the Minimally Invasive Approach towards the Ascending Aorta-A Practical Overview of the Currently Available Techniques

Minimally invasive techniques have gained immense importance in cardiovascular surgery. While minimal access strategies for coronary and mitral valve surgery are already widely accepted and often used as standard approaches, the application of minimally invasive techniques is currently expanded towards more complex operations of the ascending aorta as well. In this new and developing field, various techniques have been established and reported ranging from upper hemisternotomy approaches, which allow even extensive operations of the ascending aorta to be performed through a minimally invasive access to sternal sparing thoracotomy strategies, which completely avoid sternal trauma during ascending aorta replacements. All of these techniques place high demands on patient selection, preoperative planning, and practical surgical implementation. Application of these strategies is currently limited to high-volume centers and highly experienced surgeons. This narrative review gives an overview of the currently available techniques with a special focus on the practical execution as well as the advantages and disadvantages of the currently available techniques. The first results demonstrate the practicability and safety of minimally invasive techniques for replacement of the ascending aorta in a well-selected patient population. With success and complication rates comparable to classic full sternotomy, the proof of concept for minimally invasive replacement of the ascending aorta is now achieved.

A functioning Björk-Shiley aortic valve after 36 years without anticoagulation

There is a broad consensus among all clinical guidelines that lifelong oral anticoagulation is mandatory after mechanical valve prosthesis implantation. However, in rare cases, patients do not receive anticoagulation or anticoagulation therapy is withdrawn over time. We present a case of an exceptionally durable Björk-Shiley mechanical aortic valve prosthesis still functioning 49 years after implantation. Remarkably, the patient did not receive any anticoagulation or antiplatelet therapy for the first 36 years after implantation. Despite this, no thromboembolic or valve-related adverse events occurred to date. Upon thorough echocardiographic assessment, excellent valve function with a mean transvalvular gradient of 13 mmHg and no prosthetic valve insufficiency was found. This makes the case presented here one of the longest functioning mechanical valve replacements reported.

Mechanical stimulation induces vasa vasorum capillary alignment in a fibrin-based tunica adventitia

Generation of bioartificial blood vessels with a physiological three-layered wall architecture is a long pursued goal in vascular tissue engineering. While considerable advances have been made to resemble the physiological tunica intima and media morphology and function in bioartificial vessels, only very few studies have targeted the generation of a tunica adventitia including its characteristic vascular network known as the vasa vasorum, which are essential for graft nutrition and integration. In healthy native blood vessels, capillary vasa vasorum are aligned longitudinally to the vessel axis. Thus, inducing longitudinal alignment of capillary tubes to generate a physiological tunica adventitia morphology and function may be advantageous in bioengineered vessels as well. In this study, we investigated the effect of two biomechanical stimulation parameters, longitudinal tension and physiological cyclic stretch, on tube alignment in capillary networks formed by self-assembly of human umbilical vein endothelial cells in tunica adventitia-equivalents of fibrin-based bioartificial blood vessels. Moreover, the effect of changes of the biomechanical environment on network remodeling after initial tube formation was analyzed. Both, longitudinal tension and cyclic stretch by pulsatile perfusion induced physiological capillary tube alignment parallel to the longitudinal vessel axis. This effect was even more pronounced when both biomechanical factors were applied simultaneously, which resulted in alignment of 57.2% ± 5.2% within 5° of the main vessel axis. Opposed to that, random tube orientation was observed in vessels incubated statically. Scanning electron microscopy showed that longitudinal tension also resulted in longitudinal alignment of fibrin fibrils, which may function as a guidance structure for directed capillary tube formation. Moreover, existing microvascular networks showed distinct remodeling in response to addition or withdrawal of mechanical stimulation with corresponding increase or decrease of the degree of alignment. With longitudinal tension and cyclic stretch, we identified two mechanical stimuli that facilitate the generation of a pre-vascularized tunica adventitia-equivalent with physiological tube alignment in bioartificial vascular grafts.

An encapsulated fibrin-based bioartificial tissue construct with integrated macrovessels, microchannels and capillary tubes

Facilitating sufficient nutrient and oxygen supply in large-scale bioartificial constructs is a critical step in organ bioengineering. Immediate perfusion not only depends on a dense capillary network, but also requires integrated large-diameter vessels that allow vascular anastomoses during implantation. These requirements set high demands for matrix generation as well as for in vitro cultivation techniques and remain mostly unsolved challenges up until today. Additionally, bioartificial constructs must have sufficient biomechanical stability to withstand mechanical stresses during and after implantation. We developed a bioartificial tissue construct with a fibrin matrix containing human umbilical vein endothelial cells and adipose tissue-derived stem cells facilitating capillary-like network formation. This core matrix was surrounded by a dense acellular fibrin capsule providing biomechanical stability. Two fibrin-based macrovessels were integrated on each side of the construct and interconnected via four 1.2 mm thick microchannels penetrating the cellularized core matrix. After four days of perfusion in a custom-built bioreactor, homogenous capillary-like network formation throughout the core matrix was observed. The fibrin capsule stabilized the core matrix and facilitated the generation of a self-supporting construct. Thus, the encapsulated fibrin tissue construct could provide a universal pre-vascularized matrix for seeding with different cell types in various tissue engineering approaches. This article is protected by copyright. All rights reserved.

Pressure-compacted and spider silk-reinforced fibrin demonstrates sufficient biomechanical stability as cardiac patch in vitro

OBJECTIVE

The generation of bio-/hemocompatible cardiovascular patches with sufficient stability and regenerative potential remains an unmet goal. Thus, the aim of this study was the generation and in vitro biomechanical evaluation of a novel cardiovascular patch composed of pressure-compacted fibrin with embedded spider silk cocoons.

METHODS

Fibrin-based patches were cast in a customized circular mold. One cocoon of Nephila odulis spider silk was embedded per patch during the casting process. After polymerization, the fibrin clot was compacted by 2 kg weight for 30 min resulting in thickness reduction from up to 2 cm to <1 mm. Tensile strength and burst pressure was determined after 0 weeks and 14 weeks of storage. A sewing strength test and a long-term load test were performed using a customized device to exert physiological pulsatile stretching of a silicon surface on which the patch had been sutured.

RESULTS

Fibrin patches resisted supraphysiological pressures of well over 2000 mmHg. Embedding of spider silk increased tensile force 1.8-fold and tensile strength 1.45-fold (p < .001), resulting in a final strength of 1.07 MPa and increased sewing strength. Storage for 14 weeks decreased tensile strength, but not significantly and suturing properties of the spider silk patches were satisfactory. The long-term load test indicated that the patches were stable for 4 weeks although slight reduction in patch material was observed.

CONCLUSION

The combination of compacted fibrin matrices and spider silk cocoons may represent a feasible concept to generate stable and biocompatible cardiovascular patches with regenerative potential.

Establishment of a Modular Hemodynamic Simulator for Accurate In Vitro Simulation of Physiological and Pathological Pressure Waveforms in Native and Bioartificial Blood Vessels

Purpose

In vitro stimulation of native and bioartificial vessels in perfusable systems simulating natural mechanical environments of the human vasculature represents an emerging approach in cardiovascular research. Promising results have been achieved for applications in both regenerative medicine and etiopathogenetic investigations. However, accurate and reliable simulation of the wide variety of physiological and pathological pressure environments observed in different vessels still remains an unmet challenge.

Methods

We established a modular hemodynamic simulator (MHS) with interchangeable and modifiable components suitable for the perfusion of native porcine—(i.e. the aorta, brachial and radial arteries and the inferior vena cava) and bioartificial fibrin-based vessels with anatomical site specific pressure curves. Additionally, different pathological pressure waveforms associated with cardiovascular diseases including hyper- and hypotension, tachy- and bradycardia, aortic valve stenosis and insufficiency, heart failure, obstructive cardiomyopathy and arterial stiffening were simulated. Pressure curves, cyclic distension and shear stress were measured for each vessel and compared to ideal clinical pressure waveforms.

Results

The pressure waveforms obtained in the MHS showed high similarity to the ideal anatomical site specific pressure curves of different vessel types. Moreover, the system facilitated accurate emulation of physiological and different pathological pressure conditions in small diameter fibrin-based vessels.

Conclusion

The MHS serves as a variable in vitro platform for accurate emulation of physiological and pathological pressure environments in biological probes. Potential applications of the system include bioartificial vessel maturation in cardiovascular tissue engineering approaches as well as etiopathogenetic investigations of various cardiovascular pathologies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13239-021-00577-0.

Chemically induced hypoxia by dimethyloxalylglycine (DMOG)-loaded nanoporous silica nanoparticles supports endothelial tube formation by sustained VEGF release from adipose tissue-derived stem cells

Inadequate vascularization leading to insufficient oxygen and nutrient supply in deeper layers of bioartificial tissues remains a limitation in current tissue engineering approaches to which pre-vascularization offers a promising solution. Hypoxia triggering pre-vascularization by enhanced vascular endothelial growth factor (VEGF) expression can be induced chemically by dimethyloxalylglycine (DMOG). Nanoporous silica nanoparticles (NPSNPs, or mesoporous silica nanoparticles, MSNs) enable sustained delivery of molecules and potentially release DMOG allowing a durable capillarization of a construct. Here we evaluated the effects of soluble DMOG and DMOG-loaded NPSNPs on VEGF secretion of adipose tissue-derived stem cells (ASC) and on tube formation by human umbilical vein endothelial cells (HUVEC)-ASC co-cultures. Repeated doses of 100 µM and 500 µM soluble DMOG on ASC resulted in 3- to 7-fold increased VEGF levels on day 9 (P < 0.0001). Same doses of DMOG-NPSNPs enhanced VEGF secretion 7.7-fold (P < 0.0001) which could be maintained until day 12 with 500 µM DMOG-NPSNPs. In fibrin-based tube formation assays, 100 µM DMOG-NPSNPs had inhibitory effects whereas 50 µM significantly increased tube length, area and number of junctions transiently for 4 days. Thus, DMOG-NPSNPs supported endothelial tube formation by upregulated VEGF secretion from ASC and thus display a promising tool for pre-vascularization of tissue-engineered constructs. Further studies will evaluate their effect in hydrogels under perfusion.

A 3-Layered Bioartificial Blood Vessel with Physiological Wall Architecture Generated by Mechanical Stimulation

The generation of cellularized bioartificial blood vessels resembling all three layers of the natural vessel wall with physiological morphology and cell alignment is a long pursued goal in vascular tissue engineering. Simultaneous culture of all three layers under physiological mechanical conditions requires highly sophisticated perfusion techniques and still today remains a key challenge. Here, three-layered bioartificial vessels based on fibrin matrices were generated using a stepwise molding technique. Adipose-derived stem cells (ASC) were differentiated to smooth muscle cells (SMC) and integrated in a compacted tubular fibrin matrix to resemble the tunica media. The tunica adventitia-equivalent containing human umbilical vein endothelial cells (HUVEC) and ASC in a low concentration fibrin matrix was molded around it. Luminal seeding with HUVEC resembled the tunica intima. Subsequently, constructs were exposed to physiological mechanical stimulation in a pulsatile bioreactor for 72 h. Compared to statically incubated controls, mechanical stimulation induced physiological cell alignment in each layer: Luminal endothelial cells showed longitudinal alignment, cells in the media-layer were aligned circumferentially and expressed characteristic SMC marker proteins. HUVEC in the adventitia-layer formed longitudinally aligned microvascular tubes resembling vasa vasorum capillaries. Thus, physiologically organized three-layered bioartificial vessels were successfully manufactured by stepwise fibrin molding with subsequent mechanical stimulation.

Perfusion promotes endothelialized pore formation in high concentration fibrin gels otherwise unsuitable for tube development

Vascularization of tissue engineered implants is crucial for their survival and integration in the recipient's body. Pre-vascularized, fibrin-based implants offer a solution since low concentration fibrin hydrogels (1 mg/mL) have been shown to promote tube formation of endothelial cells in co-culture with adipogenic stem cells. However, higher fibrinogen concentrations (> 20 mg/mL) enabling the fabrication of stable implants are necessary.We here characterized fibrin gels of 1-30 mg/mL for their rheological properties and whether they support tube formation of endothelial cell-adipogenic stem cell co-cultures for up to 7 days. Moreover, 20 mg/mL gels containing preformed channels and endothelial cell-adipogenic stem cell co-culture were perfused continuously in a customized flow chamber with 3.9 dyn/cm² for 12 days and analyzed for capillary formation.Rheology of fibrin gels showed increasing stability proportional to fibrinogen concentration with 20 mg/mL gels having a storage module of 465 Pa. Complex tube networks stable for 7 days were observed at 1-5 mg/mL gels whereas higher concentrations showed initial sprouting only. However, perfusion of 20 mg/mL fibrin gels resulted in endothelialized pore formation in several layers of the gel with endothelial cell-adipogenic stem cell co-culture.Thus, perfusion supports the formation of capillary-like structures in fibrin gels that are too dense for spontaneous tube formation under static conditions. Future studies are necessary to further increase pore density and to investigate proper nutrition of tissue-specific target cells in the scaffold.

Biochemical Myogenic Differentiation of Adipogenic Stem Cells Is Donor Dependent and Requires Sound Characterization

IMPACT STATEMENT

We here showed that even under optimized conditions for biochemical differentiation of adipose-derived stem cells (with respect to a pronounced marker protein expression for a reasonable period of time) it was not possible to obtain functional smooth muscle cells from all donors. Moreover, an underestimated role may play the effect of the scaffold material on smooth muscle cell functionality. Both aspects are crucial for the successful tissue engineering of the vascular medial layer combining autologous cells with a suitable scaffold material and thus should be thoroughly addressed in each individualized therapeutic approach.

Complete Myogenic Differentiation of Adipogenic Stem Cells Requires Both Biochemical and Mechanical Stimulation

Vascular tissue engineering of the middle layer of natural arteries requires contractile smooth muscle cells (SMC) which can be differentiated from adipose-derived mesenchymal stem cells (ASC) by treatment with transforming growth factor-β, sphingosylphosphorylcholine and bone morphogenetic protein-4 (TSB). Since mechanical stimulation may support or replace TSB-driven differentiation, we investigated its effect plus TSB-treatment on SMC orientation and contractile protein expression. Tubular fibrin scaffolds with incorporated ASC or pre-differentiated SMC were exposed to pulsatile perfusion for 10 days with or without TSB. Statically incubated scaffolds served as controls. Pulsatile incubation resulted in collagen-I expression and orientation of either cell type circumferentially around the lumen as shown by alpha smooth muscle actin (αSMA), calponin and smoothelin staining as early, intermediate and late marker proteins. Semi-quantitative Westernblot analyses revealed strongly increased αSMA and calponin expression by either pulsatile (12.48-fold; p < 0.01 and 38.15-fold; p = 0.07) or static incubation plus TSB pre-treatment (8.91-fold; p < 0.05 and 37.69-fold; p < 0.05). In contrast, contractility and smoothelin expression required both mechanical and TSB stimulation since it was 2.57-fold increased (p < 0.05) only by combining pulsatile perfusion and TSB. Moreover, pre-differentiation of ASC prior to pulsatile perfusion was not necessary since it could not further increase the expression level of any marker.

[Complications of transtympanic eustacian tube insufflation (author's transl)]

Serous otitis media is sometimes treated by Eustacian tube catheterization or by insufflation of the middle ear via a myringotomy. Both techniques can cause death as has been described in the literature. Two patients, in whom following middle ear insufflation via a myringotomy air was found intracranially are presented. In one the finding on skull X-ray was coincidental and no symptoms occurred, whereas in the other a paresis of the right foot that lasted for 30 min occurred from the time of the middle ear insufflation for his serous otitis media.