Mechanical and biocompatible characterizations of a readily available multilayer vascular graft

Redefining vascular repair: revealing cellular responses on PEUU-gelatin electrospun vascular grafts for endothelialization and immune responses on in vitro models

Tissue-engineered vascular grafts (TEVGs) poised for regenerative applications are central to effective vascular repair, with their efficacy being significantly influenced by scaffold architecture and the strategic distribution of bioactive molecules either embedded within the scaffold or elicited from responsive tissues. Despite substantial advancements over recent decades, a thorough understanding of the critical cellular dynamics for clinical success remains to be fully elucidated. Graft failure, often ascribed to thrombogenesis, intimal hyperplasia, or calcification, is predominantly linked to improperly modulated inflammatory reactions. The orchestrated behavior of repopulating cells is crucial for both initial endothelialization and the subsequent differentiation of vascular wall stem cells into functional phenotypes. This necessitates the TEVG to provide an optimal milieu wherein immune cells can promote early angiogenesis and cell recruitment, all while averting persistent inflammation. In this study, we present an innovative TEVG designed to enhance cellular responses by integrating a physicochemical gradient through a multilayered structure utilizing synthetic (poly (ester urethane urea), PEUU) and natural polymers (Gelatin B), thereby modulating inflammatory reactions. The luminal surface is functionalized with a four-arm polyethylene glycol (P4A) to mitigate thrombogenesis, while the incorporation of adhesive peptides (RGD/SV) fosters the adhesion and maturation of functional endothelial cells. The resultant multilayered TEVG, with a diameter of 3.0 cm and a length of 11 cm, exhibits differential porosity along its layers and mechanical properties commensurate with those of native porcine carotid arteries. Analyses indicate high biocompatibility and low thrombogenicity while enabling luminal endothelialization and functional phenotypic behavior, thus limiting inflammation in in-vitro models. The vascular wall demonstrated low immunogenicity with an initial acute inflammatory phase, transitioning towards a pro-regenerative M2 macrophage-predominant phase. These findings underscore the potential of the designed TEVG in inducing favorable immunomodulatory and pro-regenerative environments, thus holding promise for future clinical applications in vascular tissue engineering.

Development and characterization of Polycaprolactone/chitosan-based scaffolds for tissue engineering of various organs: A review

Research in creating 3D structures mirroring the extracellular matrix (ECM) with accurate environmental cues holds paramount significance in biological applications.Biomaterials that replicate ECM properties-mechanical, physicochemical, and biological-emerge as pivotal tools in mimicking ECM behavior.Incorporating synthetic and natural biomaterials is widely used to produce scaffolds suitable for the intended organs.Polycaprolactone (PCL), a synthetic biomaterial, boasts commendable mechanical properties, albeit with relatively modest biological attributes due to its hydrophobic nature.Chitosan (CTS) exhibits strong biological traits but lacks mechanical resilience for complex tissue regeneration.Notably, both PCL and CTS have demonstrated their application in tissue engineering for diverse types of tissues.Their combination across varying PCL:CTS ratios has increased the likelihood of fabricating scaffolds to address defects in sturdy and pliable tissues.This comprehensive analysis aspires to accentuate their distinct attributes within tissue engineering across different organs.The central focus resides in the role of PCL:CTS-based scaffolds, elucidating their contribution to the evolution of advanced functional 3D frameworks tailored for tissue engineering across diverse organs.Moreover, this discourse delves into the considerations pertinent to each organ.

Biomimetic vascular tissue engineering by decellularized scaffold and concurrent cyclic tensile and shear stresses

Decellularization by chemical approaches has harmful effects on extracellular matrix (ECM) proteins, and damages lots of functional peptides and biomolecules present in the ultrastructure. In this study, we employed a combination of chemical and physical decellularization methods to overcome these disadvantages. The induced osmotic pressure by hypertonic/hypotonic solutions dissociated and removed most of cellular membranes significantly without any detergent or chemical agent. In total, 0.025% trypsin solution was found adequate to remove the remaining debrides, and ultimately 1% Triton X-100 was utilized for final cleansing. In addition, conducting all the decellularization processes at 4 °C yielded an ECM with least damages in the ultrastructure which could be inferred by close mechanical strength and swelling ratio to the native vessel, and high quality and quantity of cell attachment, migration and proliferation which were examined by optical microscopy and scanning electron microscopy (SEM) of the histology samples. Moreover, the obtained biological scaffold (BS) had no cytotoxicity according to the MTT assay, and this scaffold is storable at -20 °C. Employing bioreactor for concurrent cyclic tensile and shear stresses improved the cell migration into pores of the BS and made the cells and the scaffold compact in analogous to native tissue. As opening angle test showed by decellularizing of the blood vessel, the residual stress dropped significantly which revealed the role of cells in the amount of induced stress in the structure. However, intact and healthy ECM explicitly recovered upon recellularization and beat the initial residual stress of the native tissue. The tensile test of the blood vessels in longitudinal and radial directions revealed orthotropic behavior which can be explained by collagen fibers direction in the ECM. Furthermore, by the three regions of the stress-strain curve can be elucidated the roles of cells, elastin and collagen fibers in mechanical behavior of the vascular tissues.

Acellular Tissue-Engineered Vascular Grafts from Polymers: Methods, Achievements, Characterization, and Challenges

Extensive and permanent damage to the vasculature leading to different pathogenesis calls for developing innovative therapeutics, including drugs, medical devices, and cell therapies. Innovative strategies to engineer bioartificial/biomimetic vessels have been extensively exploited as an effective replacement for vessels that have seriously malfunctioned. However, further studies in polymer chemistry, additive manufacturing, and rapid prototyping are required to generate highly engineered vascular segments that can be effectively integrated into the existing vasculature of patients. One recently developed approach involves designing and fabricating acellular vessel equivalents from novel polymeric materials. This review aims to assess the design criteria, engineering factors, and innovative approaches for the fabrication and characterization of biomimetic macro- and micro-scale vessels. At the same time, the engineering correlation between the physical properties of the polymer and biological functionalities of multiscale acellular vascular segments are thoroughly elucidated. Moreover, several emerging characterization techniques for probing the mechanical properties of tissue-engineered vascular grafts are revealed. Finally, significant challenges to the clinical transformation of the highly promising engineered vessels derived from polymers are identified, and unique perspectives on future research directions are presented.

Development of a decellularized human amniotic membrane-based electrospun vascular graft capable of rapid remodeling for small-diameter vascular applications

The performance of small-diameter vascular grafts adapted to vascular replacement is commonly hindered by stenosis. To address this issue, a graft featuring rapid remodeling with degradation is warranted. In this work, a 1.8-mm-diameter graft was constructed by fabricating a decellularized human amniotic membrane (HAM) with polycaprolactone (PCL)/silk fibroin (SF) around it through electrospinning, namely, an HPS graft, and applied in a rat aortic grafting model for comparison to a decellularized porcine small intestinal submucosa (SIS)-integrated PCL/SF (SPS) graft and an autologous aorta. In vitro studies demonstrated that HAM provided a bioactive milieu for rapid endothelial cell proliferation and resisting fibroblast-induced collagen secretion. PCL/SF provides a biocompatible microenvironment for cellular infiltration with mechanical properties resembling those of the rat aorta. In vivo studies showed that the HPS graft induced functional endothelialization more rapidly, along with less intensive ECM deposition than the SPS graft upon the histologically weaker inflammatory response and foreign body reaction 4 weeks after implantation, and maintained patency by progressively stabilizing the remodeling structure approximating the native counterparts over 24 weeks. The bioengineered graft expands the applicability of allogeneic matrices with degradable electrospun polymers for long-term in situ vascular applications. STATEMENT OF SIGNIFICANCE: An orchestrated remodeling of the vascular graft, featuring rapid endothelialization and resisting extracellular matrix (ECM) deposition on the luminal surface, with a mechanically stable structure, is requisite for long-term vascular patency. Nevertheless, off-the-shelf grafts might not fulfil the criteria under abdominal aortic pressure. Herein, we fabricated a 1.8-mm-diameter vascular graft through the integration of a decellularized human amniotic membrane (HAM) with electrospun polycaprolactone (PCL)/silk fibroin (SF). In a rat aortic grafting model, the graft is capable of rapid endothelialization and resisting collagen deposition and provides a native-like mechanical structure for stabilizing the remodeling process towards that of the native aorta. This bioengineered graft has potential for small-diameter vascular regeneration, and provides advanced strategies to facilitate full-remodeling tissue applications.

Effects of recipient age, heparin release and allogeneic bone marrow-derived stromal cells on vascular graft remodeling

Small-caliber vascular grafts are used in a wide range of clinical conditions. However, there remains a substantial unfulfilled need for readily-available, synthetic vascular grafts with high long-term patency rate. To fulfill the translational goal for bioengineered vascular grafts, important considerations for the pre-clinical evaluation include the graft design, cell incorporation and selection of an animal model. To assess the three factors, we used vascular grafts consisting of core/shell-structured microfibers of polycaprolactone/gelatin with a thin polycaprolactone overlay. The respective influences of the heparin release mode, animal age, and allogeneic bone marrow-derived stromal cells (MSCs) seeded in the lumen on the graft remodeling were assessed after four-and-half-month implantation on an interposition graft of abdominal aorta model. Except two rats dying from graft-unrelated issues, all other rats (18 out of 20) showed good graft patency upon explantation. The cell phenotype, matrix content and structure in the neotissues around the graft, as well as the flow perfusion through the graft were examined. More grafts in the aged rats showed local narrowing and flow incongruence than the other grafts in young adult rats. Compared to acellular grafts, cellular grafts showed efficient recruitment of vascular cells to form more organized structures with elastin in the vascular wall. Endothelialization and α-smooth muscle actin-positive cells were shown in all four types of vascular grafts. This study revealed the significant effects of MSC and recipient age but not heparin release pattern on graft remodeling. STATEMENT OF SIGNIFICANCE: The vascular graft is a mainstream of surgical intervention to treat vascular diseases. Currently, vascular grafts, particularly small-diameter ones, still show high failure rates. This study has evaluated the respective impacts of heparin release pattern, allogeneic bone marrow-derived stromal cell seeding, and recipient age on the long-term remodeling of vascular grafts. There is a dearth of literature which considers the recipient age as an influencing factor for vascular grafting. However, adults particularly elderly constitute the majority of vascular graft recipients in the "real" clinical environment. While juvenile animals were widely used for graft evaluations, this study involved adult animals. The study outcomes provided important implications regarding graft designs and evaluation approaches.

Bio-composites reinforced with unique coral collagen fibers: Towards biomimetic-based small diameter vascular grafts

Cardiovascular Diseases (CVDs) are the leading cause of death worldwide. Approximately 31% of all global deaths are caused by CVDs, of which 42% are attributable to coronary artery disease (CAD). CAD is characterized by a narrowing of arteries that restricts the normal blood flow. Over time, surgical intervention is required in severe cases of occlusions and includes implantation of autologous vessels. Today synthetic grafts are used successfully as replacements for blood vessels with a diameter larger than 6 mm. However, they often fail as small-diameter blood vessel replacements. This study introduces a new biocomposite material system consisting of unique and long (cm-scale) collagen fibers derived from soft corals embedded within an alginate hydrogel matrix. The new biocomposite layers were used to fabricate grafts, towards developing a new class of tissue-engineered small-diameter blood vessels. These constructs consisted of both circumferentially and longitudinally oriented collagen fibers. The mechanical properties of the grafts were investigated via a new experimental setup constructed in our lab for this purpose, which applied internal pressure levels of 0-300 mmHg. Similar to native coronary arteries, the biocomposite tubes demonstrated a compliance of 4.88 ± 0.99%/100 mmHg for a physiologic pressure range of 80-120 mmHg. Furthermore, a numerical finite element simulation model is proposed to generate the overall mechanical response of the construct. It is composed of axial and circumferential fibers embedded within the continuum alginate elements. Good prediction is demonstrated when compared with the measured pressure-strain response. Moreover, we examined biocompatibility and cell growth on the collagen fibers. Fibroblast cells proliferated during the experiment that lasted for 32 days and showed aligned configuration with the collagen fiber orientation. The novelty of this study is manifested in the use of naturally derived coral-based long collagen fibers for the development of a new class of tissue-engineered grafts. The proposed novel biocomposite graft demonstrated both mechanical and biological compatibility and can be further developed for small-diameter blood-vessel replacement.

The mechanical characterization of blood vessels and their substitutes in the continuous quest for physiological-relevant performances. A critical review

During the last 50 years, novel biomaterials and tissue engineering techniques have been investigated to produce alternative vascular substitutes that recapitulate the unique elastic mechanical features of blood vessels. A large variation in mechanical characterization, including the test type, protocol, and data analysis, is present in literature which complicates the comparison among studies and prevents the blooming and the advancement of this field. In addition, a limited mechanical assessment of the substitute for the intended application is often provided. In this light, this review presents the mechanical environment of blood vessels, discusses their mechanical behavior responsible for the suited blood flow into the body (non-linearity, anisotropy, hysteresis, and compliance), and compares the mechanical properties reported in literature (obtained with compression, tensile, stress-relaxation, creep, dynamic mechanical analysis, burst pressure, and dynamic compliance tests). This perspective highlights that the mechanical properties extracted through conventional tests are not always suitable indicators of the mechanical performance during the working life of a vascular substitute. The available tests can be then strategically used at different stages of the substitute development, prioritizing the simplicity of the method at early stages, and the physiological pertinence at later stages, following as much as possible ISO standards in the field. A consistent mechanical characterization focused on the behavior to which they will be subdued during real life is one key and missing element in the quest for physiological-like mechanical performance of vascular substitutes.

Vascular Grafts with Tailored Stiffness and a Ligand Environment via Multiarmed Polymer Sheath for Expeditious Regeneration

The bypass graft is the mainstream of surgical intervention to treat vascular diseases. Ideal bypass materials, yet to be developed, require mechanical properties, availability, clinically feasible manufacturing logistics, and bioactivities with precise physicochemical cues defined to guide cell activities for arterial regeneration. Such needs instigated our fabrication of vascular grafts, which consist of coaxial, nanostructured fibers exhibiting a polycaprolactone (PCL) core and a photoclickable, 4-arm thiolated polyethylene glycol-norbornene (PEG-NB) sheath. The graft strength and bioactivity were modulated by the PCL concentration and the peptides (RGD, transforming growth factor β-1 or TGF-β1) conjugated to thiol-ene of PEG-NB, respectively. Structural, physical, and mechanical characterizations demonstrated that the fibrous grafts mimicked the key features of the native extracellular matrix, including a crosslinked fiber network for structural stability, viscoelasticity emulating arteries, hydration property, and high porosity for cell infiltration. Meanwhile, these grafts displayed strength and toughness exceeding or meeting surgical criteria. Furthermore, the grafts with higher PCL concentration (3 vs 1.8%) showed thicker fibers, lower porosity and pore size, and increased elastic and storage moduli. Graft bioactivity was determined by the mesenchymal stem cell (MSC) behaviors on the grafts and arterial regeneration in vivo using interposition grafting. Results showed that the cell adhesion and proliferation increased with the RGD density (25 vs 5 mM). After 1 week implantation, all peptide-functionalized PCL/PEG-NB grafts with or without MSC preseeding, as opposed to PCL grafts, showed expeditious endothelial lining, abundant vascular cell infiltration, and matrix production. Compared to RGD grafts, RGD/TGF-β1 grafts enhanced MSC differentiation into smooth muscle cells in vitro and developed thicker smooth muscle cell layers in vivo. Overall, the versatile porous vascular grafts offer superior properties and tunability for future translation.

HuBiogel incorporated fibro-porous hybrid nanomatrix graft for vascular tissue interfaces

Native extracellular matrix (ECM) possesses the biochemical cues to promote cell survival. However, decellularized, the ECM loses its cell supporting mechanical integrity. We report, here, a novel biohybrid vascular graft of polycaprolactone (PCL), poliglecaprone (PGC) incorporated with human biomatrix as functional materials for vascular tissue interfacing by electrospinning, thus harnessing the biochemical cues from the ECM and the mechanical integrity of the polymer blends. The fabricated fibro-porous tubular small diameter graft (i.d. = 4 mm) from polymer blend was coated with a cocktail of collagenous matrix derived from human placenta called HuBiogel™. The compositional, morphological, and mechanical properties of graft were measured and compared with a non-coated tubular PCL/PGC graft using Fourier Transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). BCA assay was used to calculate the protein content and coating-uniformity throughout the hybrid graft. Mechanical properties such as tensile strength (1.6 MPa), Young's modulus (2.4 MPa), burst pressure (>1900 mmHg), and suture retention strength (2.3 N) of hybrid graft were found to be comparable to native blood vessels. Protein coating has improved the hydrophilicity and the biocompatibility (cell viability and cell-attachment) enhanced with human umbilical vein endothelial cells (HUVECs) seeded in vitro onto the lumen layer of the graft over two weeks. The overall results promise this new biohybrid graft to be a potential candidate for vascular tissue interface and regeneration.

Coaxial PCL/PEG-thiol-ene microfiber with tunable physico-chemical properties for regenerative scaffolds

Tissue regeneration requires scaffolds that exhibit mechanical properties similar to the tissues to be replaced while allowing cell infiltration and extracellular matrix production. Ideally, the scaffolds' porous architecture and physico-chemical properties can be precisely defined to address regenerative needs. We thus developed techniques to produce hybrid fibers coaxially structured with a polycaprolactone core and a 4-arm, polyethylene glycol thiol-norbornene sheath. We assessed the respective effects of crosslink density and sheath polymer size on the scaffold architecture, physical and mechanical properties, as well as cell-scaffold interactions in vitro and in vivo. All scaffolds displayed high elasticity, swelling and strength, mimicking soft tissue properties. Importantly, the thiol-ene hydrogel sheath enabled tunable softness and peptide tethering for cellular activities. With increased photopolymerization, stiffening and reduced swelling of scaffolds were found due to intra- and inter-fiber crosslinking. More polymerized scaffolds also enhanced the cell-scaffold interaction in vitro and induced spontaneous, deep cell infiltration to produce collagen and elastin for tissue regeneration in vivo. The molecular weight of sheath polymer provides an additional mechanism to alter the physical properties and biological activities of scaffolds. Overall, these robust scaffolds with tunable elasticity and regenerative cues offered a versatile and effective platform for tissue regeneration.

Design and analysis of a burst strength device for testing vascular grafts

The design and analysis of a device to measure the burst strength (strength under a state of pure radial internal pressure) and compliance of vascular grafts and flexible pressurized tubes is presented. The device comprises three main sections, viz., a clean air-dry pressure controller, a test specimen holder, and automated software for control and data collection. Air pressure is controlled by means of a valve and a dedicated mechanism allowing reaching up to 120 psi in increments of 1 psi, and recording pressure changes with 0.04 psi resolution. The circumferential strain is determined by measuring the radial displacement of the vascular graft using an optical arrangement capable of determining a maximum radial displacement of 10 mm with 0.02 mm resolution. The instrument provides a low uncertainty in compliance (±0.32%/100 mm Hg^-1) and burst strength measurements. Due to its simplicity, the device can easily be reproduced in other laboratories contributing to a dedicated instrument with high resolution at low cost. The reliability of the apparatus is further confirmed by conducting finite element analysis, elasticity solutions for pressurized cylinders, and testing of small diameter vascular grafts made of a commercial aliphatic polyurethane tested under radial internal pressure.

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.

A method that simulates the actual suture status of a vascular graft provides a more consistent suture retention strength.

Effect of fluid dynamics on decellularization efficacy and mechanical properties of blood vessels

Decellularization of blood vessels is a promising approach to generate native biomaterials for replacement of diseased vessels. The decellularization process affects the mechanical properties of the vascular graft and thus can have a negative impact for in vivo functionality. The aim of this study was to determine how detergents under different fluid dynamics affects decellularization efficacy and mechanical properties of the vascular graft. We applied a protocol utilizing 1% TritonX, 1% Tributyl phosphate (TnBP) and DNase on porcine vena cava. The detergents were applied to the vessels under different conditions; static, agitation and perfusion with 3 different perfusion rates (25, 100 and 400 mL/min). The decellularized grafts were analyzed with histological, immunohistochemical and mechanical tests. We found that decellularization efficacy was equal in all groups, however the luminal ultrastructure of the static group showed remnant cell debris and the 400 mL/min perfusion group showed local damage and tearing of the luminal surface. The mechanical stiffness and maximum tensile strength were not influenced by the detergent application method. In conclusion, our results indicate that agitation or low-velocity perfusion with detergents are preferable methods for blood vessel decellularization.

Coaxially-structured fibres with tailored material properties for vascular graft implant

Readily-available small-diameter arterial grafts require a great combination of materials properties, including high strength, compliance, suturability, blood sealing and anti-thrombogenicity, as well as anti-kinking property for those used in challenging anatomical situations. We have constructed grafts composed of coaxially-structured polycaprolactone (PCL)/gelatin nanofibres, and tailored the material structures to achieve high strength, compliance and kink resistance, as well as excellent water sealing and anti-thrombogenicity. Coaxially-structured fibres in the grafts provided mechanical stability through the core, while flexibility and cell adhesion through the sheath. Results showed that graft compliance increased while strength decreased with the concentration ratio between core and sheath polymers. Compared to pure PCL fibrous surfaces, coaxial PCL/gelatin fibrous surfaces potently inhibited platelet adhesion and activation, providing excellent anti-thrombogenicity. To render sufficient burst strength and suturability, an additional layer of pure PCL was necessary to cap the layer of coaxial PCL/gelatin fibres. The two-layered grafts with the wall thickness comparable to native arteries demonstrated artery-like compliance and kink resistance, properties important to arteries under complex mechanical loading. The in vivo evaluation was performed using the interposition carotid artery graft model in rabbits for three months. Interestingly, results from ultrasonic imaging and histological analysis demonstrated that the two-layered grafts with a thinner outer PCL layer, which possessed higher compliance and kink resistance, showed increased blood flow, minimal lumen reduction and fibrosis. All vascular grafts exhibited patency and induced limited cell infiltration. Together, we presented a facile and useful approach to fabricate vascular grafts with superior graft performances, biomechanical properties, and blood compatibility. Grafts with artery-like compliance and flexibility have demonstrated improved implantation outcomes.

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.

Performance of marrow stromal cell-seeded small-caliber multilayered vascular graft in a senescent sheep model

Failure of small-caliber grafts, used as bypass or reconstructive grafts in cardiovascular treatments, is often caused by thrombosis and stenosis. We have developed a multilayered, compliant graft with an electrospun heparin-encapsulated core and collagen-chitosan shell. Herein, the performances of acellular and cell-seeded grafts were evaluated in adult sheep for preclinical assessment. Allogeneic ovine marrow stroma cells (MSCs) were uniformly attached to the lumen of cell-seeded grafts. Interposition grafts were used for carotid arteries. Four grafts were tested for each type. Upon implantation, all grafts successfully restored perfusion and rhythmically deformed under pulsatile arterial flow. Weekly ultrasonography and Doppler revealed that all grafts remained patent for perfusion during the course of one-month study. No formation of blood clots or other complications were found. The diameter of graft lumen did not vary significantly over the time or with the graft type, while narrowing at anastomosis and significant thickening of graft wall were found in both types of grafts. More significant neotissue formation was found at anastomotic sections of acellular controls compared to cell-seeded grafts. Results from histological and immunofluorescent analyses revealed moderate intimal hyperplasia (IH) at anastomosis. When compared to cell-seeded grafts, acellular controls presented thicker IH composed of α-smooth muscle actin positive cells and ground substances, which correlated with reduced and more disturbing flow. IH was thickest at anastomosis and tapered off to a minimum in the mid-section. Few PECAM-positive cells appeared on cell-seeded grafts but not acellular controls. Additionally, lesser graft thickening was found in cell-seed grafts, which might be associated with the function of stromal cells in altering the fibrotic process during tissue repair. Results suggest that MSCs held the potential to reduce hyperplasia and improve healing in an aged, large animal model for vascular grafting.

Viscoelastic properties of multi-layered cellularized vascular tissues fabricated from collagen gel

Since collagen is one of the major extracellular matrix components in vascular tissues, its use for vascular tissue engineering has several advantages. However, collagen extraction and processing for tissue engineering application alters its structure. As a result, collagen-based vascular constructs show poor mechanical properties compared to native tissues. In this work, multi-layer (single, double, and triple) vascular tissue constructs were engineered from porcine smooth muscle cells (PSMCs) entrapped in collagen gel by concentrically and sequentially layering after compaction of the previous layer(s). The engineered tissues were matured for either 14 or 21 days to allow the collagen gel to remodel before viscoelasticity, compliance, histological, and protein expression studies were conducted. While there was no significant difference upon addition of the different layers on the elastic modulus (p > .05), the viscous modulus of the single layer construct was significantly lower than the double and triple layer constructs (p < .05). Increasing the number of layers of the cellularized collagen construct increased the wall thickness and the viscous modulus of the construct. Furthermore, the cellularized single-layer construct had a relatively high compliance, but the double and triple layer constructs had compliance values comparable to both engineered vessels and native vessels. PSMCs were uniformly distributed throughout the cross-section and expressed the anticipated marker proteins smooth muscle-α actin, calponin, and smooth muscle myosin heavy chain. Taken together, this study demonstrated the viscoelastic responsiveness of multi-layer collagen-gel based vascular tissues.

Development of an electrospun biomimetic polyurea scaffold suitable for vascular grafting

The optimization of biomechanical and biochemical properties of a vascular graft to render properties relevant to physiological environments is a major challenge today. These critical properties of a vascular graft not only regulate its stability and integrity, but also control invasion of cells for scaffold remodeling permitting its integration with native tissue. In this work, we have synthesized a biomimetic scaffold by electrospinning a blend of a polyurea, poly(serinol hexamethylene urea) (PSHU), and, a polyester, poly-ε-caprolactone (PCL). Mechanical properties of the scaffold were varied by varying polymer blending ratio and electrospinning flow rate. Mechanical characterization revealed that scaffolds with lower PSHU content relative to PCL content resulted in elasticity close to native mammalian arteries. We also found that increasing electrospinning flow rates also increased the elasticity of the matrix. Optimization of elasticity generated scaffolds that enabled vascular smooth muscle cells (SMCs) to adhere, grow and maintain a SMC phenotype. The 30/70 scaffold also underwent slower degradation than scaffolds with higher PSHU content, thereby, providing the best option for in vivo remodeling. Further, Gly-Arg-Gly-Asp-Ser (RGD) covalently conjugated to the polyurea backbone in 30/70 scaffold resulted in significantly increased clotting times. Reducing surface thrombogenicity by the conjugation of RGD is critical to avoiding intimal hyperplasia. Hence, biomechanical and biochemical properties of a vascular graft can be balanced by optimizing synthesis parameters and constituent components. For these reasons, the optimized RGD-conjugated 30/70 scaffold electrospun at 2.5 or 5 mL/h has great potential as a suitable material for vascular grafting applications. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 278-290, 2018.

In Vitro Mechanical Property Evaluation of Chitosan-Based Hydrogels Intended for Vascular Graft Development

Vascular grafts made of synthetic polymers perform poorly in cardiac and peripheral bypass applications. In these applications, chitosan-based materials can be produced and shaped to provide a novel scaffold for vascular tissue engineering. The goal of this study was to evaluate in vitro the mechanical properties of a novel chitosan formulation to assess its potential for this scaffold. Two chitosan-based hydrogel tubes were produced by modulating chitosan concentration. Based on the standard ISO 7198:1998, the hydrogel tubes were characterized in vitro in terms of suture retention strength, tensile strength, compliance, and burst pressure. By increasing chitosan concentration, suture retention value increased to reach 1.1 N; average burst strength and elastic moduli also increased significantly. The compliance seemed to exhibit a low value for chitosan tubes of high concentration. By modulating chitosan concentration, we produced scaffolds with suitable mechanical properties to be implanted in vivo and withstand physiological blood pressures.

Tissue engineered vascular grafts: current state of the field

INTRODUCTION

Conventional synthetic vascular grafts are limited by the inability to remodel, as well as issues of patency at smaller diameters. Tissue-engineered vascular grafts (TEVGs), constructed from biologically active cells and biodegradable scaffolds have the potential to overcome these limitations, and provide growth capacity and self-repair. Areas covered: This article outlines the TEVG design, biodegradable scaffolds, TEVG fabrication methods, cell seeding, drug delivery, strategies to reduce wait times, clinical trials, as well as a 5-year view with expert commentary. Expert commentary: TEVG technology has progressed significantly with advances in scaffold material and design, graft design, cell seeding and drug delivery. Strategies have been put in place to reduce wait times and improve 'off-the-shelf' capability of TEVGs. More recently, clinical trials have been conducted to investigate the clinical applications of TEVGs.

Highly Compliant Vascular Grafts with Gelatin-Sheathed Coaxially Structured Nanofibers

We have developed three types of materials composed of polyurethane-gelatin, polycaprolactone-gelatin, or polylactic acid-gelatin nanofibers by coaxially electrospinning the hydrophobic core and gelatin sheath with a ratio of 1:5 at fixed concentrations. Results from attenuated total reflection-Fourier transformed infrared spectroscopy demonstrated the gelatin coating around nanofibers in all of the materials. Transmission electron microscopy images further displayed the core-sheath structures showing the core-to-sheath thickness ratio varied greatly with the highest ratio found in polyurethane-gelatin nanofibers. Scanning electron microscopy images revealed similar, uniform fibrous structures in all of the materials, which changed with genipin cross-linking due to interfiber interactions. Thermal analyses revealed varied interactions between the hydrophilic sheath and hydrophobic core among the three materials, which likely caused different core-sheath structures, and thus physicomechanical properties. The addition of gelatin around the hydrophobic polymer and their interactions led to the formation of graft scaffolds with tissue-like viscoelasticity, high compliance, excellent swelling capability, and absence of water permeability while maintaining competent tensile modulus, burst pressure, and suture retention. The hydrogel-like characteristics are advantageous for vascular grafting use, because of the capability of bypassing preclotting prior to implantation, retaining vascular fluid volume, and facilitating molecular transport across the graft wall, as shown by coculturing vascular cells sandwiched over a thick-wall scaffold. Varied core-sheath interactions within scaffolding nanofibers led to differences in graft functional properties such as water swelling ratio, compliance, and supporting growth of cocultured vascular cells. The PCL-gelatin scaffold with thick gelatin-sheathed nanofibers demonstrated a more compliant structure, elastic mechanics, and high water swelling property. Our results demonstrate a feasible approach to produce new hybrid, biodegradable nanofibrous scaffold biomaterials with interactive core-sheath structure, good biocompatibility, and tissue-like viscoelasticity, which may reduce potential problems with the use of individual polymers for vascular grafts.

The Tissue-Engineered Vascular Graft-Past, Present, and Future

Cardiovascular disease is the leading cause of death worldwide, with this trend predicted to continue for the foreseeable future. Common disorders are associated with the stenosis or occlusion of blood vessels. The preferred treatment for the long-term revascularization of occluded vessels is surgery utilizing vascular grafts, such as coronary artery bypass grafting and peripheral artery bypass grafting. Currently, autologous vessels such as the saphenous vein and internal thoracic artery represent the gold standard grafts for small-diameter vessels (<6 mm), outperforming synthetic alternatives. However, these vessels are of limited availability, require invasive harvest, and are often unsuitable for use. To address this, the development of a tissue-engineered vascular graft (TEVG) has been rigorously pursued. This article reviews the current state of the art of TEVGs. The various approaches being explored to generate TEVGs are described, including scaffold-based methods (using synthetic and natural polymers), the use of decellularized natural matrices, and tissue self-assembly processes, with the results of various in vivo studies, including clinical trials, highlighted. A discussion of the key areas for further investigation, including graft cell source, mechanical properties, hemodynamics, integration, and assessment in animal models, is then presented.

A combined method for bilayered vascular graft fabrication

Autologous saphenous vein is still the conduit of choice for peripheral by-pass. Synthetic vascular grafts in polyethylene terephthalate and expanded polytetrafluoroethylene are used if vein access cannot be obtained. However they are successfully used to replace large diameter vessels, but they fail in small diameters (<6 mm). In the present study a bilayered synthetic vascular graft was developed. The graft was composed of an inner nanofibrous layer obtained by electrospinning able to host endothelial cells and a highly porous external layer obtained by spray, phase-inversion technique capable to sustain tunica media regeneration. Graft morphology and thickness, fiber size, pore size and layer adhesion strength were assessed. The innovative combination of two different consolidated techniques allowed to manufacture a nanostructured composite graft featuring a homogeneous microporous layer firmly attached on the top of the electrospun layer. By tuning the mechanical properties and the porosity of vascular prostheses, it will be possible to optimize the graft for in situ tissue regeneration while preventing blood leakage.

Endothelialization of electrospun polycaprolactone (PCL) small caliber vascular grafts spun from different polymer blends

Small caliber vascular grafts represent a challenge to material scientists. In contrast to large caliber grafts, prostheses with diameter <6 mm, lead to increased hemodynamic disturbances and thrombogenic complications. Thus, endothelialization of small caliber grafts should create a compatible interface for hemodynamic processes. The purpose of our study was to compare different compositions of electrospun scaffolds with conventional ePTFE grafts with an inner diameter of 4 mm as well as different pre-coatings to create an optimized physiological interface for endothelialization. Polycaprolactone, polylactide, and polyethylenglycol (PCL/PLA and PCL/PLA/PEG) electrospun grafts and ePTFE grafts were pre-coated with blood, gelatine or fibronectin and seeded with endothelial cells from the human term placenta. Best results were obtained with fibronectin-coated PCL/PLA/PEG grafts. Here, the number of attached viable cells was 78-81% higher than on fibronectin pre-treated ePTFE grafts. Cells attached to PCL/PLA/PEG grafts appeared in physiological cobblestone morphology. Viability analysis showed a high cell viability of more than 98%. Fibronectin-coated PCL/PLA/PEG grafts may be a promising improvement to conventionally used ePTFE grafts.