Development Of Small-diameter Vascular Grafts Research Articles

Nonlinear Elasticity of Blood Vessels and Vascular Grafts.

The transplantation of vascular grafts has emerged as a prevailing approach to address vascular disorders. However, the development of small-diameter vascular grafts is still in progress, as they serve in a more complicated mechanical environment than their counterparts with larger diameters. The biocompatibility and functional characteristics of small-diameter vascular grafts have been well developed; however, mismatch in mechanical properties between the vascular grafts and native arteries has not been accomplished, which might facilitate the long-term patency of small-diameter vascular grafts. From a point of view in mechanics, mimicking the nonlinear elastic mechanical behavior exhibited by natural blood vessels might be the state-of-the-art in designing vascular grafts. This review centers on elucidating the nonlinear elastic behavior of natural blood vessels and vascular grafts. The biological functionality and limitations associated with as-reported vascular grafts are meticulously reviewed and the future trajectory for fabricating biomimetic small-diameter grafts is discussed. This review might provide a different insight from the traditional design and fabrication of artificial vascular grafts.

Characterization and promotion of endothelialization of Bombyx mori silk fibroin functionalized with REDV peptide

In the development of small-diameter vascular grafts, it is crucial to achieve early-stage endothelialization to prevent thrombus formation and intimal hyperplasia. Silk fibroin (SF) from Bombyx mori is commonly used for such grafts. However, there is a need to expedite endothelialization post-implantation. In this study, we functionalized SF with Arg-Glu-Asp-Val (REDV) (SF + REDV) using cyanuric chloride to enhance endothelialization. The immobilization of REDV onto SF was confirmed and the amount of immobilized REDV could be calculated by 1H NMR. Furthermore, the conformational changes in Tyr, Ser, and Ala residues in [3-13C]Tyr- and [3-13C]Ser-SF due to REDV immobilization were monitored using 13C solid-state NMR. The REDV immobilized onto the SF film was found to be exposed on the film's surface, as confirmed by biotin-avidin system. Cell culture experiments, including adhesiveness, proliferation, and extensibility, were conducted using normal human umbilical vein endothelial cells (HUVEC) and normal human aortic smooth muscle cells (HAoSMC) on both SF and SF + REDV films to evaluate the impact of REDV on endothelialization. The results indicated a trend towards promoting HUVEC proliferation while inhibiting HAoSMC proliferation. Therefore, these findings suggest that SF + REDV may be more suitable than SF alone for coating small-diameter SF knitted tubes made of SF threads.

Vascular Grafts: Technology Success/Technology Failure.

Vascular prostheses (grafts) are widely used for hemodialysis blood access, trauma repair, aneurism repair, and cardiovascular reconstruction. However, smaller-diameter (≤4 mm) grafts that would be valuable for many reconstructions have not been achieved to date, although hundreds of papers on small-diameter vascular grafts have been published. This perspective article presents a hypothesis that may open new research avenues for the development of small-diameter vascular grafts. A historical review of the vascular graft literature and specific types of vascular grafts is presented focusing on observations important to the hypothesis to be presented. Considerations in critically reviewing the vascular graft literature are discussed. The hypothesis that perhaps the "biocompatible biomaterials" comprising our vascular grafts-biomaterials that generate dense, nonvascularized collagenous capsules upon implantation-may not be all that biocompatible is presented. Examples of materials that heal with tissue reconstruction and vascularity, in contrast to the fibrotic encapsulation, are offered. Such prohealing materials may lead the way to a new generation of vascular grafts suitable for small-diameter reconstructions.

Biological properties of self-assembled nanofibers of elastin-like block polypeptides for tissue-engineered vascular grafts: platelet inhibition, endothelial cell activation and smooth muscle cell maintenance.

Strategic materials design is essential for the development of small-diameter, tissue-engineered vascular grafts. Self-assembled nanofibers of elastin-like polypeptides represent promising vascular graft components as they replicate the organized elastin structure of native blood vessels. Further, the bioactivity of nanofibers can be modified by the addition of functional peptide motifs. In the present study, we describe the development of a novel nanofiber-forming elastin-like polypeptide (ELP) with an arginine-glutamic acid-aspartic acid-valine (REDV) sequence. The biological characteristics of the REDV-modified ELP nanofibers relevant to applications in vascular grafting were compared to ELP without ligands for integrin, ELP with arginine-glycine-aspartic acid (RGD) sequence, collagen and cell culture glass. Among them, REDV-modified ELP nanofibers met the preferred biological properties for vascular graft materials, i.e. (i) inhibition of platelet adhesion and activation, (ii) endothelial cell adhesion and proliferation and (iii) maintenance of smooth muscle cells in a contractile phenotype to prevent cell overgrowth. The results indicate that REDV-modified ELP nanofibers represent promising candidates for the further development of small-diameter vascular grafts.

Physico-mechanical and biological evaluation of heparin/VEGF-loaded electrospun polycaprolactone/decellularized rat aorta extracellular matrix for small-diameter vascular grafts

Although the continuous development of small-diameter vascular grafts (SDVGs) (D < 5 mm) continues, most vascular grafts are made from synthetic polymers, which lead to serious complications from arteriosclerosis, thrombosis, and vascular ischemia. Here, to address these shortcomings, we combine synthetic polymers with natural decellularized small-diameter vessels and loaded with growth factor. We fabricated vascular grafts by electrospinning polycaprolactone (PCL) to decellularized rat aorta matrix (ECM) followed by heparin and vascular endothelial growth factor (VEGF) loading. In- vitro studies showed that PCL/ECM/VEGF vascular grafts, showed excellent hemocompatibility and biocompatibility properties. The vascular grafts implanted into the rat aorta revealed that the PCL/ECM/VEGF grafts promotes endothelial cells and smooth-muscle cells infiltration with a rate of FLK-1, ICAM1, and a-SMA distribution higher than that of the PCL and PCL/ECM vascular grafts at 2 weeks and 4 weeks after implantation. The PCL/ECM/VEGF vascular graft should be considered for potential small-diameter vascular grafts in clinical fields.

The Regulatory Effect of Braided Silk Fiber Skeletons with Differential Porosities on In Vivo Vascular Tissue Regeneration and Long-Term Patency.

The development of small-diameter vascular grafts that can meet the long-term patency required for implementation in clinical practice presents a key challenge to the research field. Although techniques such as the braiding of scaffolds can offer a tunable platform for fabricating vascular grafts, the effects of braided silk fiber skeletons on the porosity, remodeling, and patency in vivo have not been thoroughly investigated. Here, we used finite element analysis of simulated deformation and compliance to design vascular grafts comprised of braided silk fiber skeletons with three different degrees of porosity. Following the synthesis of low-, medium-, and high-porosity silk fiber skeletons, we coated them with hemocompatible sulfated silk fibroin sponges and then evaluated the mechanical and biological functions of the resultant silk tubes with different porosities. Our data showed that high-porosity grafts exhibited higher elastic moduli and compliance but lower suture retention strength, which contrasted with low-porosity grafts. Medium-porosity grafts offered a favorable balance of mechanical properties. Short-term in vivo implantation in rats indicated that porosity served as an effective means to regulate blood leakage, cell infiltration, and neointima formation. High-porosity grafts were susceptible to blood leakage, while low-porosity grafts hindered graft cellularization and tended to induce intimal hyperplasia. Medium-porosity grafts closely mimicked the biomechanical behaviors of native blood vessels and facilitated vascular smooth muscle layer regeneration and polarization of infiltrated macrophages to the M2 phenotype. Due to their superior performance and lack of occlusion, the medium-porosity vascular grafts were evaluated in long-term (24-months) in vivo implantation. The medium-porosity grafts regenerated the vascular smooth muscle cell layers and collagen extracellular matrix, which were circumferentially aligned and resembled the native artery. Furthermore, the formed neoarteries pulsed synchronously with the adjacent native artery and demonstrated contractile function. Overall, our study underscores the importance of braided silk fiber skeleton porosity on long-term vascular graft performance and will help to guide the design of next-generation vascular grafts.

Considerations in the Development of Small-Diameter Vascular Graft as an Alternative for Bypass and Reconstructive Surgeries: A Review.

Current design strategies for small diameter vascular grafts (<6 mm internal diameter; ID) are focused on mimicking native vascular tissue because the commercially available grafts still fail at small diameters, notably due to development of intimal hyperplasia and thrombosis. To overcome these challenges, various design approaches, material selection, and surface modification strategies have been employed to improve the patency of small-diameter grafts. The purpose of this review is to outline various considerations in the development of small-diameter vascular grafts, including material choice, surface modifications to enhance biocompatibility/endothelialization, and mechanical properties of the graft, that are currently being implanted. Additionally, we have taken into account the general vascular physiology, tissue engineering approaches, and collective achievements of the authors in this area. We reviewed both commercially available synthetic grafts (e-PTFE and PET), elastic polymers such as polyurethane and biodegradable and bioresorbable materials. We included naturally occurring materials by focusing on their potential application in the development of future vascular alternatives. Until now, there are few comprehensive reviews regarding considerations in the design of small-diameter vascular grafts in the literature. Here-in, we have discussed in-depth the various strategies employed to generate engineered vascular graft due to their high demand for vascular surgeries. While some TEVG design strategies have shown greater potential in contrast to autologous or synthetic ePTFE conduits, many are still hindered by high production cost which prevents their widespread adoption. Nonetheless, as tissue engineers continue to develop on their strategies and procedures for improved TEVGs, soon, a reliable engineered graft will be available in the market. Hence, we anticipate a viable TEVG with resorbable property, fabricated via electrospinning approach to hold a greater potential that can overcome the challenges observed in both autologous and allogenic grafts. This is because they can be mechanically tuned, incorporated/surface-functionalized with bioactive molecules and mass-manufactured in a reproducible manner. It is also found that most of the success in engineered vascular graft approaching commercialization is for large vessels rather than small-diameter grafts used as cardiovascular bypass grafts. Consequently, the field of vascular engineering is still available for future innovators that can take up the challenge to create a functional arterial substitute.

Biodegradable and water-responsive shape memory PHA-based polyurethane for tissue engineering

Abstract We describe the preparation, characterization and evaluation of a biodegradable water-triggered shape memory polyurethane for tissue engineering. A general strategy for fabricating a soft and hard segment self-deformed shape memory polyurethane (SMPU) is presented. Owing to easy processability like the use of solution casting and particulate leaching methods, these materials can be used for vascular stents and bone tissue engineering scaffolds. The unique characteristics of water-responsive SMPU enable shape recovery in the physiological environment and eliminate the temperature constraints for biomedical applications that require no external sample heating. In the field of tissue engineering, complicated defects in tissues have put forward a high demand for the construction of substitutes with diverse shapes. The outstanding properties of tough SMPU will enable the development of small-diameter vascular grafts and increase their application in bone tissue engineering for triggered shape morphing.

In Vivo Performance of Decellularized Vascular Grafts: A Review Article.

Due to poor vessel quality in patients with cardiovascular diseases, there has been an increased demand for small-diameter tissue-engineered blood vessels that can be used as replacement grafts in bypass surgery. Decellularization techniques to minimize cellular inflammation have been applied in tissue engineering research for the development of small-diameter vascular grafts. The biocompatibility of allogenic or xenogenic decellularized matrices has been evaluated in vitro and in vivo. Both short-term and long-term preclinical studies are crucial for evaluation of the in vivo performance of decellularized vascular grafts. This review offers insight into the various preclinical studies that have been performed using decellularized vascular grafts. Different strategies, such as surface-modified, recellularized, or hybrid vascular grafts, used to improve neoendothelialization and vascular wall remodeling, are also highlighted. This review provides information on the current status and the future development of decellularized vascular grafts.

Biomaterials and Modifications in the Development of Small-Diameter Vascular Grafts.

There remains a significant clinical need for an alternative to autologous vein grafts in small-diameter applications such as coronary bypass, but no clinically viable, synthetic small-diameter vascular grafts have been developed. While ePTFE and Dacron have long been used for large diameter grafts, it is likely that in small-diameter, low flow conditions, alternative materials and techniques are required, which have shown promising experimental results through enhancing compliance, biocompatibility, and endothelialization of vascular grafts. It is likely that the integration of synthetic materials that possess optimized mechanical properties combined with techniques for improved biocompatibility, such as the use of pure extracellular matrix proteins, will be the impetus for the creation of a new generation of clinically viable, small-diameter vascular substitutes.

Decellularized ovine arteries as small-diameter vascular grafts

Atherosclerosis and its complications still represent the leading cause of death in the developed countries. While autologous blood vessels may be regarded as the best solution for peripheral and coronary bypass, they are unavailable in most patients. Even though tissue engineering techniques are often applied to the development of small-diameter vascular grafts, limiting factors of this approach are represented by the lack of essential extracellular matrix proteins and/or poor biomechanical properties of the scaffolds used. Along these lines, the aim of this study was to develop a decellularization protocol for ovine carotids to be used as suitable small-diameter vascular grafts. Samples were treated either with sodium dodecyl sulphate (SDS) or with Trypsin and Triton X-100; a final nuclease digestion was performed for both protocols. Morphological analyses demonstrate complete removal of nuclei and cellular components in treated vessels, also confirmed by significant reduction in wall thickness and DNA content. Essential extracellular matrix proteins such as collagen, elastin, and fibronectin are well preserved after decellularization. From a mechanical point of view, Trypsin and Triton X-100 treated arteries show elastic modules and compliance comparable to native carotids, whereas the use of SDS makes samples stiffer, with a significant decrease in the compliance mean value and an increase in longitudinal and circumferential Young’s modules. It is demonstrated that the treatment where Trypsin and Triton X-100 are combined guarantees complete decellularization of carotids, with no significant alteration of biomechanical and structural properties, thus preserving a suitable environment for adhesion, proliferation, and migration of cells.

Functionalization of electrospun poly(ε-caprolactone) scaffold with heparin and vascular endothelial growth factors for potential application as vascular grafts

In this study, a heparin-conjugated poly(ε-caprolactone) electrospun fiber was constructed to develop a functional scaffold for controlled release of vascular endothelial growth factors. The immobilization of vascular endothelial growth factor was achieved through affinity binding between heparin and vascular endothelial growth factor molecules. The sustained release of vascular endothelial growth factor from the scaffold was followed for up to 15 days. The endothelial cell adhesion and proliferation assay demonstrated that immobilized vascular endothelial growth factor maintained its activity. The blood compatibility of the scaffold was evaluated by activated partial thromboplastin time, platelet adhesion test, and arteriovenous shunt, and the functionalized scaffold showed improved anticoagulation properties. The biocompatibility was evaluated by subcutaneous implantation. Results showed that this vascular endothelial growth factor–releasing scaffold stimulated neovascularization with minimum immunological rejection compared to the unmodified poly(ε-caprolactone) scaffold. The present study demonstrated a new strategy of building bioactive scaffolds for the development of small-diameter vascular graft.

Development of Small-Diameter Vascular Grafts Based on Silk Fibroin Fibers from Bombyx mori for Vascular Regeneration

In the field of surgical revascularization, the need for functional small-diameter (1.5–4.0 mm in diameter) 1vascular grafts is increasing. Several synthetic biomaterials have been tested for this purpose, but in many cases they cause thrombosis. In this study, we report the development of small-diameter vascular grafts made from silk fibroin fibers from the domestic silkworm Bombyx mori or recombinant silk fibroin fibers from a transgenic silkworm. The vascular grafts were prepared by braiding, flattening and winding the silk fibers twice onto a cylindrical polymer tube followed by coating with an aqueous silk fibroin solution. The grafts, which are 1.5 mm in inner diameter and 10 mm in length, were implanted into rat abdominal aorta. An excellent patency (ca. 85%, n= 27) at 12 months after grafting with wild-type silk fibers was obtained. Endothelial cells and smooth muscle cells migrated into the silk fibroin graft early after implantation, and became organized into an endothelium and a media-like smooth muscle layer.

The influence of protein adsorption on interactions of cultured human endothelial cells with polymers.

A systematic study of the effects of polymer surface properties on the interaction with human endothelial cells (HEC) may lead to the development of small-diameter vascular grafts. HEC, suspended in culture medium containing 20% serum adhered and spread onto moderately wettable polymers such as TCPS (tissue culture polystyrene). Reduced or no adhesion of HEC was observed upon the hydrophobic polymers PETP (polyethyleneterephthalate, Dacron) and FEP (fluoroethylenepropylene copolymer, Teflon). Polymers precoated with the proteins albumin (Alb), high density lipoprotein (HDL), and immunoglobulin G (IgG) inhibited the adhesion of HEC, whereas fibronectin (Fn) coatings promoted cell adhesion. Endothelialization of PETP and FEP only occurred after precoating of these materials with Fn. The adsorption of Fn, Alb, HDL, and IgG from solutions of different serum concentrations onto TCPS, PETP, and FEP was related to the adhesion of HEC. Serum Fn only adsorbed onto TCPS, with the maximum at 0.1% serum concentration. Maximal cell adhesion onto TCPS was also observed after pretreatment with a solution containing 0.1% serum. The cell adhesion inhibiting proteins Alb and HDL preferentially adsorbed at higher serum concentrations. Desorption of these proteins and exchange for, e.g., cellular Fn may result in cell spreading and proliferation of HEC upon TCPS.