Tissue-engineered Vascular Grafts Research Articles

Suture retention strength of P(LLA-CL) tissue-engineered vascular grafts.

The suture retention strength of artificial vascular grafts is a key mechanical property that affects the function of the grafts. Different from the conventional method of testing the suture retention strength, this study simulated the actual suturing state of vascular scaffolds approximately and investigated the effect of graft type, graft wall thickness, and number of stitches on the suture retention strength of poly(l-lactide-co-ε-caprolactone) P(LLA-CL) vascular grafts. The results showed that the P(LLA-CL) tissue-engineered vascular scaffolds have excellent suture performance and, when the graft wall thickness is greater than 0.24 mm, the suture retention strength will meet the vascular transplantation standard. Under the same conditions, the greater the number of stitches, the more uniform the force of the anastomosis and then the greater the suture retention strength. This experiment provides a more comprehensive and accurate suture retention strength value for the application of P(LLA-CL) tissue engineered vascular grafts, which helps to guide the further optimization of tissue-engineered vascular grafts to meet specific mechanical performance requirements.

3D Printing Applied to Tissue Engineered Vascular Grafts

The broad clinical use of synthetic vascular grafts for vascular diseases is limited by their thrombogenicity and low patency rate, especially for vessels with a diameter inferior to 6 mm. Alternatives such as tissue-engineered vascular grafts (TEVGs), have gained increasing interest. Among the different manufacturing approaches, 3D bioprinting presents numerous advantages and enables the fabrication of multi-scale, multi-material, and multicellular tissues with heterogeneous and functional intrinsic structures. Extrusion-, inkjet- and light-based 3D printing techniques have been used for the fabrication of TEVG out of hydrogels, cells, and/or solid polymers. This review discusses the state-of-the-art research on the use of 3D printing for TEVG with a focus on the biomaterials and deposition methods.

Understanding the endothelial – smooth muscle – fibroblastic cells interactions on a tissue-engineered vascular graft

There is still a pressing need to develop small-diameter vascular vessels for vascular reconstructive procedures. Tissue Engineering offers the prospect of being able to meet this medical demand, as it allows the development of structurally complex blood vessels substitutes 1. Accordingly, the ultimate aim of this work is to develop small diameter vascular substitutes based on layering multiple cell types. Co-culture systems of human endothelial-smooth muscle cells and fibroblastic-smooth muscle cells were initially established. These co-cultures were then assembled to develop a tri-culture system, which mimics the structural organization of a blood vessel. Electrospun nanofibrous meshes were used as culturing substrates, which restrict cell migration although enabling biochemical communication 2. All the established culture systems presented viable and proliferive cell populations over time. Interestingly, the tri-culture system presented protein synthesis values much higher than the co-cultures, mostly of collagen. On the immunofluorescence micrographs were observed the maintenance of cell type-specific proteins expression, even in the presence of another cell type. Quantification of Growth Factors (GFs) on conditioned media of the co- and tri-culture systems demonstrated a synergistic interplay between Vascular Endothelial GF (VEGF) and basic Fibroblast GF (Bfgf). The VEGF was mainly expressed by smooth muscle cells, which leads to increasing levels in the co- and tri-culture systems. A similar trend is observed for Bfgf, expectedly produced by the fibroblastic cells. By its side, the platelet derived GF levels remain unaltered among conditions. This study demonstrated the fundamental importance of the intercellular crosstalk between endothelial, smooth muscle and fibroblastic cells. It reinforces the potential of a tri-culture system in the development of tissue engineered blood vessel substitutes.

Electrochemical fabrication of a biomimetic elastin-containing bi-layered scaffold for vascular tissue engineering

Biomimetic tissue-engineered vascular grafts (TEVGs) have immense potential to replace diseased small-diameter arteries (<4 mm) for the treatment of cardiovascular diseases. However, biomimetic approaches developed thus far only partially recapitulate the physicochemical properties of the native vessel. While it is feasible to fabricate scaffolds that are compositionally similar to native vessels (collagen and insoluble elastic matrix) using freeze-drying, these scaffolds do not mimic the aligned topography of collagen and elastic fibers found in native vessels. Extrusion-based scaffolds exhibit anisotropic collagen orientation but these scaffolds are compositionally dissimilar (cannot incorporate insoluble elastic matrix). In this study, an electrochemical fabrication technique was employed to develop a biomimetic elastin-containing bi-layered collagen scaffold which is compositionally and structurally similar to native vessels and the effect of insoluble elastin incorporation on scaffold mechanics and smooth muscle cell (SMC) response was investigated. Further, the functionality of human umbilical vein endothelial cells (HUVECs) on the scaffold lumen surface was assessed via immunofluorescence. Results showed that incorporation of insoluble elastin maintained the overall collagen alignment within electrochemically aligned collagen (ELAC) fibers and this underlying aligned topography can direct cellular orientation. Ring test results showed that circumferential orientation of ELAC fibers significantly improved scaffold mechanics. Real-time PCR revealed that the expression of α-smooth muscle actin (Acta2) and myosin heavy chain (MyhII) was significantly higher on elastin containing scaffolds suggesting that the presence of insoluble elastin can promote contractility in SMCs. Further, mechanical properties of the scaffolds significantly improved post-culture indicating the presence of a mature cell-synthesized and remodeled matrix. Finally, HUVECs expressed functional markers on collagen lumen scaffolds. In conclusion, electrochemical fabrication is a viable method for the generation of a functional biomimetic TEVG with the potential to be used in bypass surgery.

Decellularized Carotid Artery Functions as an Arteriovenous Graft

BackgroundProsthetic arteriovenous grafts (AVG) continue to have a high rate of failure in clinical use, yet there is continued clinical demand for them. However, there is no small animal model of AVG to test novel tissue-engineered vascular grafts. We established a new rat arteriovenous graft model to compare the healing of decellularized carotid artery (CA) to autologous CA. Materials and methodsThe infrarenal vena cava and aorta of Wistar rats were exposed and dissected free below renal artery. A longitudinal 1 mm venotomy and arteriotomy were made on the anterior walls. The conduit was either autologous CA or heterologous decellularized CA; a conduit was sewn to the inferior vena cava and aorta in end-to-side fashion. Rats were sacrificed on postoperative day 21 for examination. ResultsAll rats survived without heart failure. Conduits had 100% patency rate (day 21) in both the control and decellularized CA groups (n = 6). Both control and decellularized CA showed similar rates of reendothelialization, inflammatory cell infiltration, and cell turnover. The outflow vein beyond the autologous or decellularized conduits showed similar neointimal thickness and cell turnover. ConclusionsDecellularized CA may be a viable tissue engineering graft for use as an arteriovenous graft for dialysis access. The rat aorta-vena cava graft is a useful model to test new materials including tissue-engineered grafts for use as AVG.

Design, Preparation, and Performance of a Novel Bilayer Tissue-Engineered Small-Diameter Vascular Graft.

In clinical practice, the need for small-diameter vascular grafts continues to increase. Decellularized xenografts are commonly used for vascular reconstructive procedures. Here, porcine coronary arteries are decellularized, which destroys the extracellular matrix structure, leading to the decrease of vascular strength and the increase of vascular permeability. A bilayer tissue-engineered vascular graft (BTEV) is fabricated by electrospinning poly(l-lactide-co-carprolactone)/gelatin outside of the decellularized vessels and functionalized by immobilizing heparin, which increases the biomechanical strength and anticoagulant activity of decellularized vessels. The biosafety and efficacy of the heparin-modified BTEVs (HBTEVs) are verified by implanting in rat models. HBTEVs remain patent and display no expansion or aneurism. After 4 weeks of implantation, a cell monolayer in the internal surface and a dense middle layer have formed, and the mechanical properties of regenerated vessels are similar to those of rat abdominal aorta. Therefore, HBTEVs can be used for rapid remodeling of small-diameter blood vessels.

Development of long in vivo tissue-engineered “Biotube” vascular grafts

In-body tissue architecture (iBTA), a cell-free, in vivo tissue engineering technology that can produce autologous implantable tissues of the desired shape by subcutaneously embedding specially designed molds, was used to develop long tubular collagenous tissues called Biotubes. Spiral molds for long Biotubes were assembled with an outer pipe-shaped spiral shell and an inner spiral mandrel, and embedded into subcutaneous pouches of beagle dogs or goats for 1 or 2 months. Tubular collagenous tissues were formed at the space between the shell and the mandrel of the mold. Depending on the spiral turn number in the mold, Biotubes of 25 cm or 50 cm (internal diameter 4 mm or 5 mm) were prepared with nearly homogeneous mechanical and histological properties over their entire length. Biotubes stored in 70% ethanol were allogenically implanted into beagle dogs or goats to evaluate their in vivo performance. The 25-cm Biotubes functioned as arterial grafts with no need for luminal modification or mechanical support, and demonstrated vascular reconstruction within 3 months after implantation into dogs. The 50-cm Biotubes functioned as arteriovenous shunt grafts in the neck region of goats without thrombus formation and vascular deformation for 1 month. Thus, the world's longest tissue-engineered vascular grafts with small diameter could be developed using iBTA.

Fabrication of a bilayer scaffold for small diameter vascular applications.

One of the greatest challenges plaguing cardiovascular tissue engineering has been the development of a compliant vascular graft. In this work, we report the development of a synthetic vascular graft with compliance similar to native arteries at physiological pressures. A bilayer scaffold was fabricated from a solid polymeric lumen made from poly(1,8 octanediol-co-citrate) (POC) and a microfibrous medial layer composed of type I collagen, elastin, and POC. Mechanical analysis revealed dynamic compliance, ~6.9% within 1% of native vessels, 5.9%. The burst pressure was an order of magnitude lower than native vessels (~400 mmHg vs. ~3000 mmHg) but was above physiological pressure ranges. Biocompatibility studies indicated the scaffold posed no acute cytotoxic risk to relevant cell types and supported the proliferation of vascular smooth muscle cells. In addition, upon exposure of the scaffold to whole blood, there was no statistically significant hemolysis, <2%. Overall this is a promising material system and scaffold to develop a biodegradable tissue-engineered vascular graft. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2850-2862, 2018.

Multivalent biomaterial platform to control the distinct arterial venous differentiation of pluripotent stem cells

Vascular endothelial cells (ECs) differentiated from pluripotent stem cells have enormous potential to be used in a variety of therapeutic areas such as tissue engineering of vascular grafts and re-vascularization of ischemic tissues. To date, various protocols have been developed to differentiate stem cells toward vascular ECs. However, current methods are still not sufficient to drive the distinct arterial venous differentiation. Therefore, developing refined method of arterial-venous differentiation is critically needed to address this gap. Here, we developed a biomaterial platform to mimic multivalent ephrin-B2/EphB4 signaling and investigated its role in the early arterial and venous specification of pluripotent stem cells. Our results show immobilized ephrinB2 or EphB4 on hydrogel substrates have a distinct effect on arterial venous differentiation by regulating several arterial venous markers. When in combination with Wnt pathway agonist or BMP4 signaling, the ephrin-B2/EphB4 biomaterial platform can create diverging EC progenitor populations, demonstrating differential gene expression pattern across a wide range of arterial and venous markers, as well as phenotypic markers such as anti-thrombotic, pro-atherogenic and osteogenic genes, that are consistent with the in vivo expression patterns of arterial and venous ECs. Importantly, this distinct EC progenitor population cannot be achieved by current methods of applying soluble factors or hemodynamic stimuli alone, illustrating that fine-tuning of developmental signals using the biomaterial platform offers a new approach to better control the arterial venous differentiation of stem cells.

Endothelialization mechanisms in vascular grafts.

Despite the wide variety of tissue-engineered vascular grafts that are currently being developed, autologous vessels, such as the saphenous vein, are still the gold standard grafts for surgical treatment of vascular disease. Recently developed technologies have shown promising results in preclinical studies, but they still do not overcome the issues that native vessels present, and only a few have made the transition into clinical use. The endothelial lining is a key aspect for the success or failure of the grafts, especially on smaller diameter grafts (<5mm). However, during the design and evaluation of the grafts, the mechanisms for the formation of this layer are not commonly examined. Therefore, a significant amount of established research might not be relevant to the clinical context, due to important differences that exist between the vascular regeneration mechanisms found in animal models and humans. This article reviews current knowledge about endothelialization mechanisms that have been so far identified: in vitro seeding, transanastomotic growth, transmural infiltration, and fallout endothelialization. Emphasis is placed on the models used for study of theses mechanisms and their effects on the development of tissue-engineering vascular conduits.

Abstract 218: Biofunctional Tissue-engineered Vascular Grafts Induce Vascular Regeneration

Vascular graft has proven to be an effective strategy for the treatment of cardiovascular diseases. However grafts made solely from synthetic polymers cannot satisfy the clinic requirements in terms of low long-term patency, because they only provide a simple structural or physical replacement without the biological functions. In order to address the issue, we aim to fabricate vascular grafts which could mimic native extracellular matrix (ECM) in terms of both physiological structure and bio-functions. Different types of strategies and techniques have been explored to realize this purpose, including graft structure optimization, surface/interface functionalization, and controlled delivery of vascoactive molecules.Up to now, we have successfully fabricated a kind of vascular grafts with dual surface functions of anti-thrombogenicity and rapid endothelialization. Through structural optimization, the problem of limited cellularization in vascular grafts has been effectively resolved, and thus the vascular regeneration as well as the homeostasis has been improved. Furthermore, incorporation of some specific cytokines (such as Dickkopf-3) into the micro/nano- fibrous vascular grafts could enhance the migration vascular progenitor cells (VPCs) into the grafts and further induce the differentiation of them, contributing to endothelialisation and intima/media formation. In addition, vascular grafts loaded with exosomes derived from mesenchymal stem cells have been successfully prepared, and delivered exosomes exhibited immunomodulation function which is very important for the vascular regeneration and inhibition of calcification.

Magnetic Resonance Imaging of Shear Stress and Wall Thickness in Tissue-Engineered Vascular Grafts.

Tissue-engineered vascular grafts (TEVGs) have demonstrated potential for treating congenital heart disease (CHD); however, quantitative imaging for tracking functional and structural remodeling of TEVGs has not been applied. Therefore, we evaluated the potential of magnetic resonance (MR) imaging for assessing TEVG wall shear stress (WSS) and wall thickness in a large animal model. Cell-seeded (n = 3) or unseeded (n = 3) TEVGs were implanted as inferior vena cava interposition grafts in juvenile lambs. Six months following implantation, two-dimensional phase-contrast MR imaging was performed at 3 slice locations (proximal, middle, and distal) to assess normalized WSS (i.e., WSS-to-cross sectional area). T2-weighted MR imaging was performed to assess TEVG wall thickness. Histology was qualitatively assessed, whereas immunohistochemistry was semiquantitatively assessed for smooth muscle cells (αSMA), macrophage lineage cells (CD11b), and matrix metalloproteinase activity (MMP-2 and MMP-9). Picrosirius Red staining was performed to quantify collagen content. TEVG wall thickness was significantly higher for proximal, middle, and distal slices in unseeded versus cell-seeded grafts. Significantly higher WSS values existed for proximal versus distal slice locations for cell-seeded TEVGs, whereas no differences in WSS existed between slices for unseeded TEVGs. Additionally, no differences in WSS existed between cell-seeded and unseeded groups. Both groups demonstrated elastin formation, without vascular calcification. Unseeded TEVGs possessed greater content of smooth muscle cells when compared with cell-seeded TEVGs. No differences in macrophage, MMP activity, or collagen content existed between groups. MR imaging allows for in vivo assessment of functional and anatomical characteristics of TEVGs and may provide a nonionizing approach that is clinically translatable to children undergoing treatment for CHD.

Effects of aligned and random fibers with different diameter on cell behaviors.

Tissue-engineered vascular grafts show promise as a treatment for patients with diseased or damaged arteries. The formation of a confluent endothelium lumen on scaffold is one of the key issues in vascular tissue engineering. The endothelial layer is composed of endothelial cells (ECs) aligned along the long axis of the vessel. The contact guidance effect is believed to fundamentally impact cell morphology, alignment, and behavior. By tuning the scaffold topography where the EC cells reside, the cell adhesion, shape, and other behaviors might be regulated. To this end, poly(ε-caprolactone) was electrospun to obtain different topographies by varying fiber diameter and orientation. The morphology and alignment of EC cells were investigated by analyzing fluorescence staining images. The results show that nano-sized fibers significantly enhance cell adhesion and proliferation due to the comparable geometric feature size of the fiber diameter. ECs align along the fiber oriented direction for both microfibers and nanofibers. It shows a better alignment of ECs on micro-aligned fibers than on nano-aligned fibers, which is due to the active size range of the contact guidance effect. These results indicate that scaffolds with ordered topographies might favor the formation of a confluent endothelium for vascular tissue engineering.

Magnetic targeting of smooth muscle cells in vitro using a magnetic bacterial cellulose to improve cell retention in tissue-engineering vascular grafts

Tissue-engineered vascular grafts (TEVG) use biologically-active cells with or without supporting scaffolds to achieve tissue remodeling and regrowth of injured blood vessels. However, this process may take several weeks because the high hemodynamic shear stress at the damaged site causes cellular denudation and impairs tissue regrowth. We hypothesize that a material with magnetic properties can provide the force required to speed up re-endothelization at the vascular defect by facilitating high cell density coverage, especially during the first 24 h after implantation. To test our hypothesis, we designed a magnetic bacterial cellulose (MBC) to locally target cells in vitro under a pulsatile fluid flow (0.514 dynes cm−2). This strategy can potentially increase cell homing at TEVG, without the need of blood cessation. The MBC was synthesized by an in situ precipitation method of Fe3+ and Fe2+ iron salts into bacterial cellulose (BC) pellicles to form Fe3O4 nanoparticles along the BC’s fibrils, followed by the application of dextran coating to protect the embedded nanoparticles from oxidation. The iron salt concentration used in the synthesis of the MBC was tuned to balance the magnetic properties and cytocompatibility of the magnetic hydrogel. Our results showed a satisfactory MBC magnetization of up to 10 emu/g, which is above the value considered relevant for tissue engineering applications (0.05 emu/g). The MBC captured magnetically-functionalized cells under dynamic flow conditions in vitro. MBC magnetic properties and cytocompatibility indicated a dependence on the initial iron oxide nanoparticle concentration. Statement of SignificanceMagnetic hydrogels represent a new class of functional materials with great potential in TVEG because they offer a platform to (1) release drugs on demand, (2) speed up tissue regrowth, and (3) provide mechanical cues to cells by its deformability capabilities. Here, we showed that a magnetic hydrogel, the MBC, was able to capture and retain magnetically-functionalized smooth muscle cells under pulsatile flow conditions in vitro. A magnetic hydrogel with this feature can be used to obtain high-density cell coverage on sites that are aggressive for cell survival such as the luminal face of vascular grafts, whereas simultaneously can support the formation of a biologically-active cell layer that protects the material from restenosis and inflammation.

An exploratory study on the preparation and evaluation of a “same-day” adipose stem cell–based tissue-engineered vascular graft

ObjectiveTissue-engineered vascular grafts containing adipose-derived mesenchymal stem cells offer an alternative to small-diameter vascular grafts currently used in cardiac and lower-extremity revascularization procedures. Adipose-derived, mesenchymal stem cell–infused, tissue-engineered vascular grafts have been shown to promote remodeling and vascular homeostasis in vivo and offer a possible treatment solution for those with cardiovascular disease. Unfortunately, the time needed to cultivate adipose-derived mesenchymal stem cells remains a large hurdle for tissue-engineered vascular grafts as a treatment option. The purpose of this study was to determine if stromal vascular fraction (known to contain progenitor cells) seeded tissue-engineered vascular grafts would remain patent in vivo and remodel, allowing for a “same-day” process for tissue-engineered vascular graft fabrication and implantation. MethodsStromal vascular fraction, obtained from adult human adipose tissue, was seeded within 4 hours after acquisition from the patient onto poly(ester urethane)urea bilayered scaffolds using a customized rotational vacuum seeding device. Constructs were then surgically implanted as abdominal aortic interposition grafts in Lewis rats. ResultsFindings revealed patency in 5 of 7 implanted scaffolds at 8 weeks, along with neotissue formation and remodeling occurring in patent tissue-engineered vascular grafts. Patency was documented using angiography and gross inspection, and remodeling and vascular components were detected using immunofluorescent chemistry. ConclusionsA “same-day” cell-seeded, tissue-engineered vascular graft can remain patent after implantation in vivo, with neotissue formation and remodeling occurring by 8 weeks.

Generating vascular conduits: from tissue engineering to three-dimensional bioprinting.

Vascular disease – including coronary artery disease, carotid artery disease, and peripheral vascular disease – is a leading cause of morbidity and mortality worldwide. The standard of care for restoring patency or bypassing occluded vessels involves using autologous grafts, typically the saphenous veins or internal mammary arteries. Yet, many patients who need life- or limb-saving procedures have poor outcomes, and a third of patients who need vascular intervention have multivessel disease and therefore lack appropriate vasculature to harvest autologous grafts from. Given the steady increase in the prevalence of vascular disease, there is great need for grafts with the biological and mechanical properties of native vessels that can be used as vascular conduits. In this review, we present an overview of methods that have been employed to generate suitable vascular conduits, focusing on the advances in tissue engineering methods and current three-dimensional (3D) bioprinting methods. Tissue-engineered vascular grafts have been fabricated using a variety of approaches such as using preexisting scaffolds and acellular organic compounds. We also give an extensive overview of the novel use of 3D bioprinting as means of generating new vascular conduits. Different strategies have been employed in bioprinting, and the use of cell-based inks to create de novo structures offers a promising solution to bridge the gap of paucity of optimal donor grafts. Lastly, we provide a glimpse of our work to create scaffold-free, bioreactor-free, 3D bioprinted vessels from a combination of rat vascular smooth muscle cells and fibroblasts that remain patent and retain the tensile and mechanical strength of native vessels.

Regulation Effects of Biomimetic Hybrid Scaffolds on Vascular Endothelium Remodeling.

The formation of complete and well-functioning endothelium is critical for the success of tissue-engineered vascular grafts yet remaining a fundamental challenge. Endothelium remodeling onto the lumen of tissue-engineered vascular grafts is affected by their topographical, mechanical, and biochemical characteristics. For meeting multiple requirements, composite strategies have recently emerged for fabricating hybrid scaffolds, where the integrated properties are tuned by varying their compositions. However, the underlying principle how the integrated properties of hybrid scaffolds regulate vascular endothelium remodeling remains unclear. To uncover the regulation effects of hybrid scaffolds on vascular endothelium remodeling, we prepared different biomimetic hybrid scaffolds using gelatin methacrylamide (GelMA) and poly-ε-caprolactone (PCL) and then investigated vascular endothelial cell responses on them. GelMA and PCL, respectively, conferred the resulting scaffolds with biomimetic bioactivity and mechanical properties, which were tuned by varying GelMA/PCL mass ratios (3:1, 1:1, or 1:3). On different GelMA/PCL hybrid scaffolds, distinct vascular endothelial cell responses were observed. Firm cell-scaffold/cell-cell interactions were rapidly established on the hybrid scaffolds with the highest mass ratio of bioactive GelMA. However, they were mechanically insufficient as vascular grafts. On the contrary, the scaffolds with the highest mass ratio of PCL showed significantly reinforced mechanical properties but poor biological performance. Between the two extremes, the scaffolds with the same GelMA/PCL mass ratio balanced the pros and cons of two materials. Therefore, they could meet the mechanical requirements of vascular grafts and support the early-stage vascular endothelial cell remodeling by appropriate biological signaling and mechanotransduction. This investigation experimentally proves that scaffold bioactivity is the dominant factor affecting vascular endothelial cell adhesion and remodeling, whereas mechanical properties are crucial factors for the integrity of endothelium. This work offers a universal design strategy for desirable vascular grafts for improved endothelium remodeling.

Angiotensin II receptor I blockade prevents stenosis of tissue engineered vascular grafts.

We previously developed a tissue-engineered vascular graft (TEVG) made by seeding autologous cells onto a biodegradable tubular scaffold, in an attempt to create a living vascular graft with growth potential for use in children undergoing congenital heart surgery. Results of our clinical trial showed that the TEVG possesses growth capacity but that its widespread clinical use is not yet advisable due to the high incidence of TEVG stenosis. In animal models, TEVG stenosis is caused by increased monocytic cell recruitment and its classic ("M1") activation. Here, we report on the source and regulation of these monocytes. TEVGs were implanted in wild-type, CCR2 knockout ( Ccr2-/-), splenectomized, and spleen graft recipient mice. We found that bone marrow-derived Ly6C+hi monocytes released from sequestration by the spleen are the source of mononuclear cells infiltrating the TEVG during the acute phase of neovessel formation. Furthermore, short-term administration of losartan (0.6 g/L, 2 wk), an angiotensin II type 1 receptor antagonist, significantly reduced the macrophage populations (Ly6C+/-/F480+) in the scaffolds and improved long-term patency in TEVGs. Notably, the combined effect of bone marrow-derived mononuclear cell seeding with short-term losartan treatment completely prevented the development of TEVG stenosis. Our results provide support for pharmacologic treatment with losartan as a strategy to modulate monocyte infiltration into the grafts and thus prevent TEVG stenosis.-Ruiz-Rosado, J. D. D., Lee, Y.-U., Mahler, N., Yi, T., Robledo-Avila, F., Martinez-Saucedo, D., Lee, A. Y., Shoji, T., Heuer, E., Yates, A. R., Pober, J. S., Shinoka, T., Partida-Sanchez, S., Breuer, C. K. Angiotensin II receptor I blockade prevents stenosis of tissue engineered vascular grafts.

Tissue-engineered submillimeter-diameter vascular grafts for free flap survival in rat model

Vascular grafts for free flap transfers should be of very small diameter and remain patent for approximately three weeks to supply blood until the revascularization from the surrounding tissue is established, with the autologous vein grafts acting as the gold standard. Artificial submillimeter-diameter vascular grafts with clinically useful size of 0.6 mm inner diameter and 5 cm length were prepared and evaluated by replacing the axial artery of free flap in rats. The rat tail artery, selected as a novel bioscaffold material, was decellularized using ultrahigh-hydrostatic pressure (UHP) method and compared with the detergent-based conventional method. To induce rapid endothelialization, the graft lumen was modified with synthesized peptides, having high affinity to the endothelial progenitor cells. The UHP method and peptide modification at 37 °C were found to preserve the extracellular matrix structure well, leading to the stable immobilization of the peptide at the luminal surface. These grafts showed the neointima formation, even at the center position far from the native vessels, remained patent for three weeks, and resulted in the flap survival in the rat free-flap model. The tissue-engineered vascular grafts with functionalized lumen have great future potential as an alternative to autologous vein grafts in free flap transfers.

Triple-Layer Vascular Grafts Fabricated by Combined E-Jet 3D Printing and Electrospinning.

Small-diameter tissue-engineered vascular grafts are urgently needed for clinic arterial substitute. To simulate the structures and functions of natural blood vessels, we designed a novel triple-layer poly(ε-caprolactone) (PCL) fibrous vascular graft by combining E-jet 3D printing and electrospinning techniques. The resultant vascular graft consisted of an interior layer comprising 3D-printed highly aligned strong fibers, a middle layer made by electrospun densely fibers, and an exterior structure composed of mixed fibers fabricated by co-electrospraying. The biocompatible triple-layer graft was used for in vivo implantation, and results demonstrated that the longitudinally-aligned fibers within the lumen of the graft could enhance the proliferation and migration of endothelial cells, while maintained good mechanical properties. The exterior layer provided a pathway that encouraged cells to migrate into the scaffold after implantation. This experimental graft overcame the limitations of conventionally electrospun vascular grafts of inadequate porosity and lowly cell penetration. The unique structure of the triple-layer vascular graft promoted cell growth and infiltration in vivo, thus provided an encouraging substitute for in situ tissue engineering.