Use of a fibrin preparation in the engineering of a vascular graft model

Biofabrication of engineered blood vessels for biomedical applications

To successfully engineer large-sized tissues, establishing vascular structures is essential for providing oxygen, nutrients, growth factors and cells to prevent necrosis at the core of the tissue. The diameter scale of the biofabricated vasculatures should range from 100 to 1,000 µm to support the mm-size tissue while being controllably aligned and spaced within the diffusion limit of oxygen. In this review, insights regarding biofabrication considerations and techniques for engineered blood vessels will be presented. Initially, polymers of natural and synthetic origins can be selected, modified, and combined with each other to support maturation of vascular tissue while also being biocompatible. After they are shaped into scaffold structures by different fabrication techniques, surface properties such as physical topography, stiffness, and surface chemistry play a major role in the endothelialization process after transplantation. Furthermore, biological cues such as growth factors (GFs) and endothelial cells (ECs) can be incorporated into the fabricated structures. As variously reported, fabrication techniques, especially 3D printing by extrusion and 3D printing by photopolymerization, allow the construction of vessels at a high resolution with diameters in the desired range. Strategies to fabricate of stable tubular structures with defined channels will also be discussed. This paper provides an overview of the many advances in blood vessel engineering and combinations of different fabrication techniques up to the present time.

Functional regeneration at the blood-biomaterial interface

The use of cardiovascular implants is commonplace in clinical practice. However, reproducing the key bioactive and adaptive properties of native cardiovascular tissues with an artificial replacement is highly challenging. Exciting new treatment strategies are under development to regenerate (parts of) cardiovascular tissues directly in situ using immunomodulatory biomaterials. Direct exposure to the bloodstream and hemodynamic loads is a particular challenge, given the risk of thrombosis and adverse remodeling that it brings. However, the blood is also a source of (immune) cells and proteins that dominantly contribute to functional tissue regeneration. This review explores the potential of the blood as a source for the complete or partial in situ regeneration of cardiovascular tissues, with a particular focus on the endothelium, being the natural blood-tissue barrier. We pinpoint the current scientific challenges to enable rational engineering and testing of blood-contacting implants to leverage the regenerative potential of the blood.

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.

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.

Improved Repopulation Efficacy of Decellularized Small Diameter Vascular Grafts Utilizing the Cord Blood Platelet Lysate

BACKGROUND

The development of functional bioengineered small-diameter vascular grafts (SDVGs), represents a major challenge of tissue engineering. This study aimed to evaluate the repopulation efficacy of biological vessels, utilizing the cord blood platelet lysate (CBPL).

METHODS

Human umbilical arteries (hUAs, n = 10) were submitted to decellularization. Then, an evaluation of decellularized hUAs, involving histological, biochemical and biomechanical analysis, was performed. Wharton's Jelly (WJ) Mesenchymal Stromal Cells (MSCs) were isolated and characterized for their properties. Then, WJ-MSCs (1.5 × 10⁶ cells) were seeded on decellularized hUAs (n = 5) and cultivated with (Group A) or without the presence of the CBPL, (Group B) for 30 days. Histological analysis involving immunohistochemistry (against Ki67, for determination of cell proliferation) and indirect immunofluorescence (against activated MAP kinase, additional marker for cell growth and proliferation) was performed.

RESULTS

The decellularized hUAs retained their initial vessel's properties, in terms of key-specific proteins, the biochemical and biomechanical characteristics were preserved. The evaluation of the repopulation process indicated a more uniform distribution of WJ-MSCs in group A compared to group B. The repopulated vascular grafts of group B were characterized by greater Ki67 and MAP kinase expression compared to group A.

CONCLUSION

The results of this study indicated that the CBPL may improve the repopulation efficacy, thus bringing the biological SDVGs one step closer to clinical application.

In Vivo Cartilage Regeneration with Cell-Seeded Natural Biomaterial Scaffold Implants: 15-Year Study

Articular cartilage can be easily damaged from human's daily activities, leading to inflammation and to osteoarthritis, a situation that can diminish the patients' quality of life. For larger cartilage defects, scaffolds are employed to provide cells the appropriate three-dimensional environment to proliferate and differentiate into healthy cartilage tissue. Natural biomaterials used as scaffolds, attract researchers' interest because of their relative nontoxic nature, their abundance as natural products, their easy combination with other materials, and the relative easiness to establish Marketing Authorization. The last 15 years were chosen to review, document, and elucidate the developments on cell-seeded natural biomaterials for articular cartilage treatment in vivo. The parameters of the experimental designs and their results were all documented and presented. Considerations about the newly formed cartilage and the treatment of cartilage defects were discussed, along with difficulties arising when applying natural materials, research limitations, and tissue engineering approaches for hyaline cartilage regeneration.

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.

Future Perspectives in Small-Diameter Vascular Graft Engineering

The increased demands of small-diameter vascular grafts (SDVGs) globally has forced the scientific society to explore alternative strategies utilizing the tissue engineering approaches. Cardiovascular disease (CVD) comprises one of the most lethal groups of non-communicable disorders worldwide. It has been estimated that in Europe, the healthcare cost for the administration of CVD is more than 169 billion €. Common manifestations involve the narrowing or occlusion of blood vessels. The replacement of damaged vessels with autologous grafts represents one of the applied therapeutic approaches in CVD. However, significant drawbacks are accompanying the above procedure; therefore, the exploration of alternative vessel sources must be performed. Engineered SDVGs can be produced through the utilization of non-degradable/degradable and naturally derived materials. Decellularized vessels represent also an alternative valuable source for the development of SDVGs. In this review, a great number of SDVG engineering approaches will be highlighted. Importantly, the state-of-the-art methodologies, which are currently employed, will be comprehensively presented. A discussion summarizing the key marks and the future perspectives of SDVG engineering will be included in this review. Taking into consideration the increased number of patients with CVD, SDVG engineering may assist significantly in cardiovascular reconstructive surgery and, therefore, the overall improvement of patients' life.

Dental stem cells: The role of biomaterials and scaffolds in developing novel therapeutic strategies

Dental stem cells (DSCs) are self-renewable cells that can be obtained easily from dental tissues, and are a desirable source of autologous stem cells. The use of DSCs for stem cell transplantation therapeutic approaches is attractive due to their simple isolation, high plasticity, immunomodulatory properties, and multipotential abilities. Using appropriate scaffolds loaded with favorable biomolecules, such as growth factors, and cytokines, can improve the proliferation, differentiation, migration, and functional capacity of DSCs and can optimize the cellular morphology to build tissue constructs for specific purposes. An enormous variety of scaffolds have been used for tissue engineering with DSCs. Of these, the scaffolds that particularly mimic tissue-specific micromilieu and loaded with biomolecules favorably regulate angiogenesis, cell-matrix interactions, degradation of extracellular matrix, organized matrix formation, and the mineralization abilities of DSCs in both in vitro and in vivo conditions. DSCs represent a promising cell source for tissue engineering, especially for tooth, bone, and neural tissue restoration. The purpose of the present review is to summarize the current developments in the major scaffolding approaches as crucial guidelines for tissue engineering using DSCs and compare their effects in tissue and organ regeneration.

Progressive Reinvention or Destination Lost? Half a Century of Cardiovascular Tissue Engineering

The concept of tissue engineering evolved long before the phrase was forged, driven by the thromboembolic complications associated with the early total artificial heart programs of the 1960s. Yet more than half a century of dedicated research has not fulfilled the promise of successful broad clinical implementation. A historical account outlines reasons for this scientific impasse. For one, there was a disconnect between distinct eras each characterized by different clinical needs and different advocates. Initiated by the pioneers of cardiac surgery attempting to create neointimas on total artificial hearts, tissue engineering became fashionable when vascular surgeons pursued the endothelialisation of vascular grafts in the late 1970s. A decade later, it were cardiac surgeons again who strived to improve the longevity of tissue heart valves, and lastly, cardiologists entered the fray pursuing myocardial regeneration. Each of these disciplines and eras started with immense enthusiasm but were only remotely aware of the preceding efforts. Over the decades, the growing complexity of cellular and molecular biology as well as polymer sciences have led to surgeons gradually being replaced by scientists as the champions of tissue engineering. Together with a widening chasm between clinical purpose, human pathobiology and laboratory-based solutions, clinical implementation increasingly faded away as the singular endpoint of all strategies. Moreover, a loss of insight into the healing of cardiovascular prostheses in humans resulted in the acceptance of misleading animal models compromising the translation from laboratory to clinical reality. This was most evident in vascular graft healing, where the two main impediments to the in-situ generation of functional tissue in humans remained unheeded–the trans-anastomotic outgrowth stoppage of endothelium and the build-up of an impenetrable surface thrombus. To overcome this dead-lock, research focus needs to shift from a biologically possible tissue regeneration response to one that is feasible at the intended site and in the intended host environment of patients. Equipped with an impressive toolbox of modern biomaterials and deep insight into cues for facilitated healing, reconnecting to the “user needs” of patients would bring one of the most exciting concepts of cardiovascular medicine closer to clinical reality.

Dehydration improves biomechanical strength of bioartificial vascular graft material and allows its long-term storage

Introduction

We have recently reported about a novel technique for the generation of bioartificial vascular grafts based on the use of a compacted fibrin matrix. In this study, we evaluated the effects of a dehydration process on the biomechanical properties of compacted fibrin tubes and whether it allows for their long-term storage.

Materials and methods

Fibrin was precipitated from fresh frozen plasma by means of cryoprecipitation and simultaneously with a thrombin solution applied in a high-speed rotating casting mold. Subsequent dehydration of the fibrin tubes (29/38) was performed in dry air with a dilator inside the tube to prevent the collapse of the lumen. Dehydrated fibrin tubes were stored for six (n=9) and 12 months (n=10) at room temperature. Comparative analysis was done on initially generated and dehydrated fibrin tubes before and after storage to evaluate the effects of the dehydration process and storage on the biomechanical properties and structure of the tubes.

Results

Thirty-eight fibrin tubes were generated by high-speed rotation-molding from 142±3 mg fibrinogen with an inner diameter of 5.8±0.1 mm and a length of 100 mm. A centrifugal force of nearly 900×g compacted applied fibrin, while fluid was pressed out of the matrix and drained from the mold via holes resulting in a 16-fold compaction of the fibrin matrix. Dehydration was characterized by shrinkage of the tubes to a diameter of 3.2±0.2 mm, while the length remained at 100 mm equivalent to a further two-fold compaction. The biomechanical strength of the dehydrated fibrin tubes significantly increased to values comparable to that of native ovine carotid arteries and maintained during the first 6 months of storage. After 12 months of storage, only five of 10 tubes were intact, and only one showed maintained biomechanical strength.

Discussion

Compaction of a fibrin matrix in high-speed rotation-moulding and subsequent dehydration enables for the construction of small-caliber fibrin grafts. Over and above, the dehydration process allows their storage and stockpiling as a prerequisite for clinical use.

Development and in vivo validation of tissue-engineered, small-diameter vascular grafts from decellularized aortae of fetal pigs and canine vascular endothelial cells

Background

Tissue engineering has emerged as a promising alternative for small-diameter vascular grafts. The aim of this study was to determine the feasibility of using decellularized aortae of fetal pigs (DAFPs) to construct tissue-engineered, small-diameter vascular grafts and to test the performance and application of DAFPs as vascular tissue-engineered scaffolds in the canine arterial system.

Methods

DAFPs were prepared by continuous enzymatic digestion. Canine vascular endothelial cells (ECs) were seeded onto DAFPs in vitro and then the vascular grafts were cultured in a custom-designed vascular bioreactor system for 7 days of dynamic culture following 3 days of static culture. The grafts were then transplanted into the common carotid artery of the same seven dogs from which ECs had been derived (two grafts were prepared for each dog with one as a backup; therefore, a total of 14 tissue-engineered blood vessels were prepared). At 1, 3, and 6 months post-transplantation, ultrasonography and contrast-enhanced computed tomography (CT) were used to check the patency of the grafts. Additionally, vascular grafts were sampled for histological and electron microscopic examination.

Results

Tissue-engineered, small-diameter vascular grafts can be successfully constructed using DAFPs and canine vascular ECs. Ultrasonographic and CT test results confirmed that implanted vascular grafts displayed good patency with no obvious thrombi. Six months after implantation, the grafts had been remodeled and exhibited a similar structure to normal arteries. Immunohistochemical staining showed that cells had evenly infiltrated the tunica media and were identified as muscular fibroblasts. Scanning electron microscopy showed that the graft possessed a complete cell layer, and the internal cells of the graft were confirmed to be ECs by transmission electron microscopy.

Conclusions

Tissue-engineered, small-diameter vascular grafts constructed using DAFPs and canine vascular ECs can be successfully transplanted to replace the canine common carotid artery. This investigation potentially paves the way for solving a problem of considerable clinical need, i.e., the requirement for small-diameter vascular grafts.

Bioreducible, hydrolytically degradable and targeting polymers for gene delivery

Recently, synthetic gene carriers have been intensively developed owing to their promising application in gene therapy and considered as a suitable alternative to viral vectors because of several benefits. But cationic polymers still face some problems like low transfection efficiency, cytotoxicity, and poor cell recognition and internalization. The emerging engineered and smart polymers can respond to some changes in the biological environment like pH change, ionic strength change and redox potential, which is beneficial for cellular uptake. Redox-sensitive disulfide based and hydrolytically degradable cationic polymers serve as gene carriers with excellent transfection efficiency and good biocompatibility owing to degradation in the cytoplasm. Additionally, biodegradable polymeric micelles with cell-targeting function are recently emerging gene carriers, especially for the transfection of endothelial cells. In this review, some strategies for gene carriers based on these bioreducible and hydrolytically degradable polymers will be illustrated.

Current advances and future perspectives in extrusion-based bioprinting

Extrusion-based bioprinting (EBB) is a rapidly growing technology that has made substantial progress during the last decade. It has great versatility in printing various biologics, including cells, tissues, tissue constructs, organ modules and microfluidic devices, in applications from basic research and pharmaceutics to clinics. Despite the great benefits and flexibility in printing a wide range of bioinks, including tissue spheroids, tissue strands, cell pellets, decellularized matrix components, micro-carriers and cell-laden hydrogels, the technology currently faces several limitations and challenges. These include impediments to organ fabrication, the limited resolution of printed features, the need for advanced bioprinting solutions to transition the technology bench to bedside, the necessity of new bioink development for rapid, safe and sustainable delivery of cells in a biomimetically organized microenvironment, and regulatory concerns to transform the technology into a product. This paper, presenting a first-time comprehensive review of EBB, discusses the current advancements in EBB technology and highlights future directions to transform the technology to generate viable end products for tissue engineering and regenerative medicine.

Fibrin matrices: The versatile therapeutic delivery systems

Fibrin sealants, that have been employed for over a century by surgeons to stop post surgery bleeding, are finding novel applications in the controlled delivery of antibiotics and several other therapeutics. Fibrinogen can be easily purified from blood plasma and converted by thrombolysis to fibrin that undergoes spontaneous aggregation to form insoluble clot. During the gelling, fibrin can be formulated into films, clots, threads, microbeads, nanoconstructs and nanoparticles. Whole plasma clots in the form of beads and microparticles can also be prepared by activating endogenous thrombin, for possible drug delivery. Fibrin formulations offer remarkable scope for controlling the porosity as well as in vivo degradability and hence the release of the associated therapeutics. Binding/covalent-linking of therapeutics to the fibrin matrix, crosslinking of the matrix with bifunctional reagents and coentrapment of protease inhibitors have been successful in regulating both in vitro and in vivo release of the therapeutics. The release rates can also be remarkably lowered by preentrapment of therapeutics in insoluble particles like liposomes or by anchoring them to the matrix via molecules that bind them as well as fibrin.

Evaluation of autologous tissue sources for the isolation of endothelial cells and adipose tissue-derived mesenchymal stem cells to pre-vascularize tissue-engineered vascular grafts

Currently used synthetic vascular grafts bear a high infection risk due to insufficient microvascularization of the graft wall disabling the infiltration of immune cells. Tissue-engineered grafts with a functional pre-vascularization thus would be desirable. However, autologous tissue sources for capillary forming cells need to be evaluated. Here, peripheral blood outgrowth endothelial cells (PB-OEC) from 17 healthy donors and pericyte-like mesenchymal stem cells derived from adipose tissue (ASC) of 17 patients scheduled for visceral surgery were characterized and investigated regarding their ability to form capillary-like networks in plasma-derived fibrin gels. To obtain proliferating PB-OEC with endothelial cell-specific properties (CD31-, VE-cadherin-expression, ac-LDL uptake and three-dimensional (3D)-tube formation in fibrin gels) both enrichment of CD34

Fibrin-based tissue engineering: comparison of different methods of autologous fibrinogen isolation

OBJECTIVE

This study is focussed on the optimal method of autologous fibrinogen isolation with regard to the yield and the use as a scaffold material. This is particularly relevant for pediatric patients with strictly limited volumes of blood.

MATERIALS AND METHODS

The following isolation methods were evaluated: cryoprecipitation, ethanol (EtOH) precipitation, ammonium sulfate [(NH(4))(2)SO(4))] precipitation, ammonium sulfate precipitation combined with cryoprecipitation, and polyethylene glycol precipitation combined with cryoprecipitation. Fibrinogen yields were quantified spectrophotometrically and by electrophoretic analyses. To test the influence of the different isolation methods on the microstructure of the fibrin gels, scanning electron microscopy (SEM) was used and the mechanical strength of the cell-free and cell-seeded fibrin gels was tested by burst strength measurements. Cytotoxicity assays were performed to analyze the effect of various fibrinogen isolation methods on proliferation, apoptosis, and necrosis. Tissue development and cell migration were analyzed in all samples using immunohistochemical techniques. The synthesis of collagen as an extracellular matrix component by human umbilical cord artery smooth muscle cells in fibrin gels was measured using hydroxyproline assay.

RESULTS

Compared to cryoprecipitation, all other considered methods were superior in quantitative analyses, with maximum fibrinogen yields of ∼80% of total plasma fibrinogen concentration using ethanol precipitation. SEM imaging demonstrated minor differences in the gel microstructure. Ethanol-precipitated fibrin gels exhibited the best mechanical properties. None of the isolation methods had a cytotoxic effect on the cells. Collagen production was similar in all gels except those from ammonium sulfate precipitation. Histological analysis showed good cell compatibility for ethanol-precipitated gels.

CONCLUSION

The results of the present study demonstrated that ethanol precipitation is a simple and effective method for isolation of fibrinogen and a suitable alternative to cryoprecipitation. This technique allows minimization of the necessary blood volume for fibrinogen isolation, particularly important for pediatric applications, and also has no negative influence on microstructure, mechanical properties, cell proliferation, or tissue development.

Co-culturing monocytes with smooth muscle cells improves cell distribution within a degradable polyurethane scaffold and reduces inflammatory cytokines

Activated monocytes can promote inflammation or wound repair, depending on the nature of the implant environment. Recent work showed that a degradable, polar-hydrophobic-ionic polyurethane (D-PHI) induced an anti-inflammatory monocyte phenotype. In the current study it is hypothesized that wound-healing phenotype monocytes (activated by D-PHI material chemistry) will promote human vascular smooth muscle cells (hVSMC) to attach and migrate into porous D-PHI scaffolds. hVSMC migration is necessary for hVSMC population of the scaffold and tissue formation to occur, and then, once tissue formation is complete, the monocyte should promote contractile phenotype markers in the hVSMC. hVSMC were cultured for up to 28 days with or without monocytes and analyzed for cell viability, attachment (DNA) and migration. Lysates were analyzed for the hVSMC contractile phenotype markers calponin and α-smooth muscle actin (α-SMA) as well as urokinase plasminogen activator (uPA; pro-migration marker) using immunoblotting analysis. Histological staining showed that hVSMC alone remained around the perimeter of the scaffold, whereas co-culture samples had co-localization of monocytes with hVSMC in the pores, a more even cell distribution throughout the scaffold and increased total cell attachment (P<0.05). Co-culture samples had higher cell numbers and more DNA than the addition of both single cell cultures. The water-soluble tetrazolium-1 data suggested that cells were not dying over the 28 day culture period. Calponin, also linked to cell motility, was maintained up to 28 days in the co-culture and hVSMC alone, whereas α-SMA disappeared after 7 days. Co-cultures on D-PHI showed that monocytes were activated to a wound-healing phenotype (low TNF-α, elevated IL-10), while promoting uPA expression. In summary, this study showed that, by co-culturing monocytes with hVSMC, the latter showed increased total cell attachment and infiltration into the D-PHI scaffold compared with hVSMC alone, suggesting that monocytes may promote hVSMC migration, a condition necessary for ultimately achieving uniform tissue formation in porous scaffolds.

Design and Fabrication of PLGA Sandwiched Cell/Fibrin Constructs for Complex Organ Regeneration

A poly(DL-lactic-co-glycolic acid) (PLGA) sandwich fibrinogen/ adipose stem cell (ADSC) construct was fabricated to generate smooth muscle tissue. The mechanical properties and ADSC compatibility of PLGA, poly(ethylene glycol-1,6-hexamethyl diisocyanate-caprolactone) i.e. polyurethane (PU), gelatin, alginate, and fibrin composites were evaluated for vascular smooth muscle tissue generation. Synthetic PLGA and PU combined with natural gelatin, alginate, and fibrin for strength and cell compatibility were found to be effective. A trilayer construct was designed and built with a microporous inner PLGA layer to provide nutrient, oxygen, and metabolite transfer while the outer PLGA layer with no pores prevented leakage during in vitro culture and in vivo implantation. The fibrin layer suitably accommodated ADSC growth, migration, proliferation, and differentiation inside the construct. This design has the potential for wide use in tissue engineering and complex organ construction.

Tissue-engineered small-caliber vascular graft based on a novel biodegradable composite fibrin-polylactide scaffold

Small-caliber vascular grafts (< or =5 mm) constructed from synthetic materials for coronary bypass or peripheral vascular repair below the knee have poor patency rates, while autologous vessels may not be available for harvesting. The present study aimed to create a completely autologous small-caliber vascular graft by utilizing a bioabsorbable, macroporous poly(L/D)lactide 96/4 [P(L/D)LA 96/4] mesh as a support scaffold system combined with an autologous fibrin cell carrier material. A novel molding device was used to integrate a P(L/D)LA 96/4 mesh in the wall of a fibrin-based vascular graft, which was seeded with arterial smooth muscle cells (SMCs)/fibroblasts and subsequently lined with endothelial cells. The mold was connected to a bioreactor circuit for dynamic mechanical conditioning of the graft over a 21-day period. Graft cell phenotype, proliferation, extracellular matrix (ECM) content, and mechanical strength were analyzed. alpha-SMA-positive SMCs and fibroblasts deposited ECM proteins into the graft wall, with a significant increase in both cell number and collagen content over 21 days. A luminal endothelial cell lining was evidenced by vWf staining, while the grafts exhibited supraphysiological burst pressure (>460 mmHg) after dynamic cultivation. The results of our study demonstrated the successful production of an autologous, biodegradable small-caliber vascular graft in vitro, with remodeling capabilities and supraphysiological mechanical properties after 21 days in culture. The approach may be suitable for a variety of clinical applications, including coronary artery and peripheral artery bypass procedures.

Control of surface free energy in titanium doped phosphate based glasses by co-doping with zinc

To significantly improve the biocompatibility of titanium doped phosphate based glasses, codoping with zinc has been attempted. This study investigated the effect of doping a quaternary 15Na(2)O:30CaO:5TiO(2):50P(2)O(5) glass with zinc oxide (1, 3, and 5 mol %) on bulk, structural, surface, and biological properties; the results were compared with glasses free from ZnO and/or TiO(2). ZnO as adjunct to TiO(2) was effective in changing density, interchain bond forces, degradation behavior, and ions released from the degrading glasses. Incorporation of both TiO(2) and ZnO in T5Z1, T5Z3, and T5Z5 glasses reduced the level of Zn(2+) release by two to three orders of magnitude compared with glasses containing ZnO only (Z5). (31)P NMR results for T5Z1, T5Z3, and T5Z5 glasses showed the presence of Q(3) species suggesting that the TiO(2) is acting as a network former, and the phosphate network becomes slightly more connected with increasing ZnO incorporation. Regardless of their relative lower hydrophilicity and surface reactivity compared with the control glass free from TiO(2) and ZnO (T0Z0), these glasses have significantly higher surface reactivity compared with Thermanox. This has been also reflected in the maintenance of >98% viable Osteoblasts, proliferation rate, and expression level of osteoblastic marker genes in a comparable manner to Thermanox and T5 glasses, particularly T5Z1 and T5Z3 glasses. However, T0Z0 and Z5 glasses showed significantly reduced viability compared to Thermanox. Therefore, it can be concluded that ZnO doped titanium phosphate glasses, T5Z1 and T5Z3 in particular, can be promising substrates for bone tissue engineering applications.

Development of a composite degradable/nondegradable tissue-engineered vascular graft

The present study aimed to determine the feasibility of constructing a reinforced autologous vascular graft by combining the advantages of fibrin gel as an autologous cell carrier material with the inherent mechanical strength of an integrated mesh structure. It was hypothesized that the mesh and dynamic culture conditions could be combined to generate mechanically stable and implantable vascular grafts within a shorter cultivation period than traditional methods. A two-step moulding technique was developed to integrate a polyvinylidene fluoride (PVDF) mesh (pore size: 1-2 mm) in the wall of a fibrin-based vascular graft (I.D. 5 mm) seeded with carotid myofibroblasts. The graft was cultured under increasing physiological flow conditions for 2 weeks. Histology, burst strength, and suture retention strength were evaluated. Cell growth and tissue development was excellent within the fibrin gel matrix surrounding the PVDF fibers, and tissue structure demonstrated remarkable similarity to native tissue. The grafts were successfully subjected to physiological flow rates and pressure gradients from the outset, and mechanical properties were enhanced by the mesh structure. Mean suture retention strength of the graft tissue was 6.3 N and the burst strength was 236 mm Hg. Using the vascular composite graft technique, the production of tissue engineered, small-caliber vascular grafts with good mechanical properties within a conditioning period of 14 days is feasible.

Undifferentiated mesenchymal stem cells seeded on a vascular prosthesis contribute to the restoration of a physiologic vascular wall

BACKGROUND

We evaluated the possibility of restoring a physiologic vascular wall using undifferentiated mesenchymal stem cells (MSCs) seeded on a polyurethane vascular prosthesis.

METHODS

Undifferentiated MSCs were seeded on a vascular prosthesis and implanted into Wistar male rats (weight, 350 g) to investigate differentiation into smooth muscle cells and to determine graft endothelialization in vivo.

RESULTS

Seeded or nonseeded grafts were surgically implanted. Undifferentiated MSCs were first labelled for green fluorescent protein. After 2 weeks in vivo, MSC that were initially self-expanded on the graft in a monolayer were organized in a multicellular layer mimicking media of aortic adjacent wall. They coexpressed green fluorescent protein and smooth muscle proteins that were not present before the in vivo engraftment, indicating that in vivo conditions induced smooth muscle protein maturation. Undifferentiated MSC showed an electrophysiologic profile quite different than mature smooth muscle cells. In both in vitro- and in vivo-differentiated MSCs, adenosine triphosphate, an IP(3)-dependent agonist, induced an increase in calcium similar to that which occurred in mature smooth muscle cells. However, MSCs failed to respond to caffeine, a ryanodine receptor activator, indicating the absence of mature calcium signaling, and finally, contraction was absent. Endothelialization attested by immunohistology and scanning electron microscopy was greater in MSC-seeded grafts that prevent thrombosis.

CONCLUSION

Only partial smooth muscle cell differentiation of MSCs resulted when seeded on vascular grafts, but MSCs spontaneously restore a media-like thick wall. Mesenchymal stem cells have a positive impact on in vivo endothelialization in rats that supports their potential for use in vascular surgery.

CLINICAL RELEVANCE

Thrombosis of vascular prostheses is a major complication of surgery. We showed on rat aorta that mesenchymal stem cells seeded on polyurethane patch restore endothelium. It also induced incomplete smooth muscle differentiation. In the future, stem cell could prevent thrombosis of vascular prostheses.

Autologous Blood Vessels Engineered from Peripheral Blood Sample

OBJECTIVE

Although many efforts have been made to generate small-diameter (< or =5mm) vascular grafts by means of tissue engineering, improvement in patency and functionality still remains a great challenge. It is our hypothesis that to achieve long-term functionality and patency, not only the complete lining with endothelial cells but also full biocompatibility is essential.

DESIGN

The aim was the development of a conduit from a scaffold and endothelial progenitor cells (EPC) separated from peripheral blood of a single donor.

MATERIALS AND METHODS

EPC and a fibrin preparation were separated from porcine peripheral blood. Fibrin segments were generated seeded with EPC and were perfused in a bioreactor in vitro.

RESULTS

From 100ml blood 12-15 cm long fibrin tubes were successfully generated lined with endothelial-like cells. Seeded tubes showed a remarkable elasticity and burst strength up to 90 mm mercury.

CONCLUSIONS

Stable fibrin tubes were successfully generated completely lined with an endothelium-like monolayer from fibrin and EPC, both isolated from the same volume of blood. Although their stability is not those needed for arterial grafting, our results raise the hope, that with distinct improvements in future studies functional autologous vascular grafts could be engineered from the patient's own blood.

Endothelialization and flow conditioning of fibrin-based media-equivalents

It is generally accepted that endothelialization and subsequent development of a functional endothelium are of paramount importance to the success of any bioartificial artery. In this study, we aimed to assess the ability of smooth muscle cell-remodeled, fibrin-based media-equivalents (MEs) to be endothelialized, examine the morphological changes of endothelial cells (ECs) associated with exposure to physiologically-relevant shear stress in a custom-built bioreactor, and determine if adherent ECs are capable of withstanding average physiological shear stresses. It was found that MEs could be readily endothelialized with surface coverages of 98.8 +/- 0.9% after two days, and the ECs expressed von Willebrand factor. Furthermore, EC retention remained high (steady: 96.5 +/- 4.4%, pulsatile: 94.3 +/- 4.3%) under exposure to physiologically relevant shear stresses for 48 h. The results indicate that these MEs are conducive to generating an EC monolayer, with the ECs possessing adhesion strength sufficient to withstand physiological shear stress and maintain a normal phenotype.