Incorporation of Chitosan in Polyurethanes Based on Modified Castor Oil for Cardiovascular Applications

The increased demand for vascular grafts for the treatment of cardiovascular diseases has led to the search for novel biomaterials that can achieve the properties of the tissue. According to this, the investigation of polyurethanes has been a promising approach to overcome the present limitations. However, some biological properties remain to be overcome, such as thrombogenicity and hemocompatibility, among others. This paper aims to synthesize polyurethanes based on castor oil and castor oil transesterified with triethanolamine (TEA) and pentaerythritol (PE) and with the incorporation of 1% chitosan. Analysis of the wettability, enzymatic degradation, mechanical properties (tensile strength and elongation at break), and thermal stability was performed. Along with the evaluation of the cytotoxicity against mouse fibroblast (L929) and human dermal fibroblast (HDFa) cells, the hemolysis rate and platelet adhesion were determined. The castor-oil-based polyurethanes with and without 1% chitosan posed hydrophobic surfaces and water absorptions of less than 2% and enzymatic degradation below 0.5%. Also, they were thermally stable until 300 °C, with tensile strength like cardiovascular tissues. The synthesized castor oil/chitosan polyurethanes are non-cytotoxic (cell viabilities above 80%) to L929 and HDFa cells and non-thrombogenic and non-hemolytic (less than 2%); therefore, they are suitable for cardiovascular applications.

Epsin Mimetic UPI Peptide Delivery Strategies to Improve Endothelization of Vascular Grafts

Endothelialization of engineered vascular grafts for replacement of small-diameter coronary arteries remains a critical challenge. The ability for an acellular vascular graft to promote endothelial cell (EC) recruitment in the body would be very beneficial. This study investigated epsins as a target since they are involved in internalization of vascular endothelial growth factor receptor 2. Specifically, epsin-mimetic UPI peptides are delivered locally from vascular grafts to block epsin activity and promote endothelialization. The peptide delivery from fibrin coatings allowed for controlled loading and provided a significant improvement in EC attachment, migration, and growth in vitro. The peptides have even more important impacts after grafting into rat abdominal aortae. The peptides prevented graft thrombosis and failure that is observed with a fibrin coating alone. They also modulated the in vivo remodeling. The grafts are able to remodel without the formation of a thick fibrous capsule on the adventitia with the 100 µg mL-1 peptide-loaded condition, and this condition enabled the formation of a functional EC monolayer in the graft lumen after only 1 week. Overall, this study demonstrated that the local delivery of UPI peptides is a promising strategy to improve the performance of vascular grafts.

The Influence of Textile Structure Characteristics on the Performance of Artificial Blood Vessels

Cardiovascular disease is a major threat to human health worldwide, and vascular transplantation surgery is a treatment method for this disease. Often, autologous blood vessels cannot meet the needs of surgery. However, allogeneic blood vessels have limited availability or may cause rejection reactions. Therefore, the development of biocompatible artificial blood vessels is needed to solve the problem of donor shortage. Tubular fabrics prepared by textile structures have flexible compliance, which cannot be matched by other structural blood vessels. Therefore, biomedical artificial blood vessels have been widely studied in recent decades up to the present. This article focuses on reviewing four textile methods used, at present, in the manufacture of artificial blood vessels: knitting, weaving, braiding, and electrospinning. The article mainly introduces the particular effects of different structural characteristics possessed by various textile methods on the production of artificial blood vessels, such as compliance, mechanical properties, and pore size. It was concluded that woven blood vessels possess superior mechanical properties and dimensional stability, while the knitted fabrication method facilitates excellent compliance, elasticity, and porosity of blood vessels. Additionally, the study prominently showcases the ease of rebound and compression of braided tubes, as well as the significant biological benefits of electrospinning. Moreover, moderate porosity and good mechanical strength can be achieved by changing the original structural parameters; increasing the floating warp, enlarging the braiding angle, and reducing the fiber fineness and diameter can achieve greater compliance. Furthermore, physical, chemical, or biological methods can be used to further improve the biocompatibility, antibacterial, anti-inflammatory, and endothelialization of blood vessels, thereby improving their functionality. The aim is to provide some guidance for the further development of artificial blood vessels.

Deetect: A Deep Learning-Based Image Analysis Tool for Quantification of Adherent Cell Populations on Oxygenator Membranes after Extracorporeal Membrane Oxygenation Therapy

The strong interaction of blood with the foreign surface of membrane oxygenators during ECMO therapy leads to adhesion of immune cells on the oxygenator membranes, which can be visualized in the form of image sequences using confocal laser scanning microscopy. The segmentation and quantification of these image sequences is a demanding task, but it is essential to understanding the significance of adhering cells during extracorporeal circulation. The aim of this work was to develop and test a deep learning-supported image processing tool (Deetect), suitable for the analysis of confocal image sequences of cell deposits on oxygenator membranes at certain predilection sites. Deetect was tested using confocal image sequences of stained (DAPI) blood cells that adhered to specific predilection sites (junctional warps and hollow fibers) of a phosphorylcholine-coated polymethylpentene membrane oxygenator after patient support (>24 h). Deetect comprises various functions to overcome difficulties that occur during quantification (segmentation, elimination of artifacts). To evaluate Deetects performance, images were counted and segmented manually as a reference and compared with the analysis by a traditional segmentation approach in Fiji and the newly developed tool. Deetect outperformed conventional segmentation in clustered areas. In sections where cell boundaries were difficult to distinguish visually, previously defined post-processing steps of Deetect were applied, resulting in a more objective approach for the resolution of these areas.

Structural Aspects of Electrospun Scaffolds Intended for Prosthetics of Blood Vessels

Electrospinning is a popular method used to fabricate small-diameter vascular grafts. However, the importance of structural characteristics of the scaffold determining interaction with endothelial cells and their precursors and blood cells is still not exhaustively clear. This review discusses current research on the significance and impact of scaffold architecture (fiber characteristics, porosity, and surface roughness of material) on interactions between cells and blood with the material. In addition, data about the effects of scaffold topography on cellular behaviour (adhesion, proliferation, and migration) are necessary to improve the rational design of electrospun vascular grafts with a long-term perspective.

Potential of poly(alkylene terephthalate)s to control endothelial cell adhesion and viability

Poly(ethylene terephthalate) (PET) is known for its various useful characteristics, including its applicability in cardiovascular applications, more precisely as synthetic bypass grafts for large diameter (≥ 6 mm) blood vessels. Although it is widely used, PET is not an optimal material as it is not interactive with endothelial cells, which is required for bypasses to form a complete endothelium. Therefore, in this study, poly(alkylene terephthalate)s (PATs) have been studied. They were synthesized via a single-step solution polycondensation reaction, which requires mild reaction conditions and avoids the use of a catalyst or additives like heat stabilizers. A homologous series was realized in which the alkyl chain length varied from 5 to 12 methylene groups (n = 5-12). Molar masses up to 28,000 g/mol were obtained, while various odd-even trends were observed with modulated differential scanning calorimetry (mDSC) and rapid heat-cool calorimetry (RHC) to access the thermal properties within the homologous series. The synthesized PATs have been subjected to in vitro cell viability assays using Human Umbilical Vein Endothelial Cells (HUVECs) and Human Dermal Microvascular Endothelial Cells (HDMECs). The results showed that HUVECs adhere and proliferate most pronounced onto PAT_(n=9) surfaces, which could be attributed to the surface roughness and morphology as determined by atomic force microscopy (AFM) (i.e. R_q = 204.7 nm). HDMECs were investigated in the context of small diameter vessels and showed superior adhesion and proliferation after seeding onto PAT_(n=6) substrates. These preliminary results already pave the way towards the use of PAT materials as substrates to support endothelial cell adhesion and growth. Indeed, as superior endothelial cell interactivity compared to PET was observed, time-consuming and costly surface modifications of PET grafts could be avoided by exploiting this novel material class.

A promising potential candidate for vascular replacement materials with anti-inflammatory action, good hemocompatibility and endotheliocyte-cytocompatibility: phytic acid-fixed amniotic membrane

Due to its excellent biocompatibility and anti-inflammatory activity, amniotic membrane (AM) has attracted much attention from scholars. However, its clinical application in vascular reconstruction was limited for poor processability, rapid biodegradation, and insufficient hemocompatibility. A naturally extracted substance with good cytocompatibility, phytic acid (PA), which can quickly form strong and stable hydrogen bonds on the tissue surface, was used to crosslink decellularized AM (DAM) to prepare a novel vascular replacement material. The results showed that PA-fixed AM had excellent mechanical strength and resistance to enzymatic degradation as well as appropriate surface hydrophilicity. Among all samples, 2% PA-fixed specimen showed excellent human umbilical vein endothelial cells (HUVECs)-cytocompatibility and hemocompatibility. It could also stimulate the secretion of vascular endothelial growth factor and endothelin-1 from seeded HUVECs, indicating that PA might promote neovascularization after implantation of PA-fixed specimens. Also, 2% PA-fixed specimen could inhibit the secretion of tumor necrosis factor-αfrom co-cultured macrophages, thus might reduce the inflammatory response after sample implantation. Finally, the results ofex vivoblood test andin vivoexperiments confirmed our deduction that PA might promote neovascularization after implantation. All the results indicated that prepared PA-fixed DAM could be considered as a promising small-diameter vascular replacement material.

New Vascular Graft Using the Decellularized Human Chorion Membrane

The increase of both arterial occlusive diseases and coronary heart diseases leads to a higher demand for small-diameter vascular grafts (<6 mm). The gold standard for small-diameter vessel replacement is the use of autologous veins. Nevertheless, up to 30% of these patients need to use vascular grafts. Although synthetic polymers have been successfully used for the replacement of large-diameter vascular grafts (>6 mm), they are associated with thrombosis, intimal hyperplasia, calcification, and chronic inflammation when used as small-diameter vascular grafts. Therefore, natural materials have been studied for this application. In this study, a decellularized human chorion membrane (dHCM) vascular graft with a 3-4 mm diameter was created. Herein, the biocompatibility of dHCM with endothelial cells was demonstrated in vitro and ex ovo. Blood biocompatibility of dHCM was also shown by studying plasma protein adsorption, platelet adhesion and activation, and its hemolytic potential. Furthermore, dHCM antibacterial properties against Staphylococcus aureus were also studied. In summary, the dHCM reticular layer side presented all the needed characteristics to be used in the inner side of a vascular graft. Additionally, the mechanical properties of the dHCM tubular construct were studied, being similar to the ones of the saphenous vein, the gold standard for autologous small-diameter vessel replacement.

Direct photoreactive immobilization of water-soluble phospholipid polymers on substrates in an aqueous environment

The purpose of this study is to achieve a simpler and safer surface modification of substrates using a photoreactive polymer in an aqueous environment. We synthesized water-soluble photoreactive polymers with both phenylazide groups and phosphorylcholine groups, poly(2-methacryloyloxyethyl phosphorylcholine-co-4-methacryl tetra(ethylene glycol)oxycarbonyl-4-phenylazide) (PMEPAz), via reversible addition fragmentation chain transfer polymerization. PMEPAz with different polymerization degrees were synthesized with a well-defined structure. To immobilize PMEPAz on the substrate surface by photoreaction, it is necessary to adsorb the polymer on the substrate surface in an aqueous solution because the phenylazide groups chemically bind to the substrate via a hydrogen abstract reaction. The relationship between the polymer solubilization state in the aqueous solution and the adsorption behavior at the surface was investigated. PMEPAz began to form unstable molecular aggregates at a concentration of 10^-2 mg/mL and formed stable aggregates at 10⁰ mg/mL. At a concentration of 10^-1 mg/mL, unstable molecular aggregates of PMEPAz were formed in the aqueous solution, resulting in the maximization of the amount of adsorbed polymer and effective photoreaction with the substrate. The thickness of the reacted polymer layer on the substrate increased with an increase in the polymerization degree, a uniform polymer layer with a thickness of 3.4 nm was formed when the polymerization degree was 400. After surface modification, the hydrophobic surfaces of the original substrates became hydrophilic. Additionally, fibrinogen adsorption and platelet adhesion were effectively suppressed based on the characteristics of the phosphorylcholine unit.

The choice of biopolymer is crucial to trigger angiogenesis with vascular endothelial growth factor releasing coatings

Bio-based coatings and release systems for pro-angiogenic growth factors are of interest to overcome insufficient vascularization and bio-integration of implants. This study compares different biopolymer-based coatings on polyethylene terephthalate (PET) membranes in terms of coating homogeneity and stability, coating thickness in the swollen state, endothelial cell adhesion, vascular endothelial growth factor (VEGF) release and pro-angiogenic properties. Coatings consisted of carbodiimide cross-linked gelatin type A (GelA), type B (GelB) or albumin (Alb), and heparin (Hep), or they consisted of radically cross-linked gelatin methacryloyl-acetyl (GM5A5) and heparin methacrylate (HepM5). We prepared films with thicknesses of 8-10 µm and found that all coatings were homogeneous after washing. All gelatin-based coatings enhanced the adhesion of primary human endothelial cells compared to the uncoated membrane. The VEGF release was tunable with the loading concentration and dependent on the isoelectric points and hydrophilicities of the biopolymers used for coating: GelA-Hep showed the highest releases, while releases were indistinguishable for GelB-Hep and Alb-Hep, and lowest for GM5A5-HepM5. Interestingly, not only the amount of VEGF released from the coatings determined whether angiogenesis was induced, but a combination of VEGF release, metabolic activity and adhesion of endothelial cells. VEGF releasing GelA-Hep and GelB-Hep coatings induced angiogenesis in a chorioallantoic membrane assay, so that these coatings should be considered for further in vivo testing.

Current status on vascular substitutes

In the last decades, the main evolution in the field of vascular surgery was correlated to the development and introduction of vascular substitutes. Currently, two types of synthetic vascular substitutes have been widely adopted: polyethylene terephthalate and expanded polytetrafluoroethylene. Ideally, they should demonstrate a behavior as close as possible as that of human arteries in terms of mechanical and biological properties. However, no vascular substitute has been found to compare with the patency rates of gold-standard autologous conduits, and major drawbacks of current vascular substitutes remain both thrombogenicity and infectability.

Surface Engineering of Cardiovascular Devices for Improved Hemocompatibility and Rapid Endothelialization

Cardiovascular devices have been widely applied in the clinical treatment of cardiovascular diseases. However, poor hemocompatibility and slow endothelialization on their surface still exist. Numerous surface engineering strategies have mainly sought to modify the device surface through physical, chemical, and biological approaches to improve surface hemocompatibility and endothelialization. The alteration of physical characteristics and pattern topographies brings some hopeful outcomes and plays a notable role in this respect. The chemical and biological approaches can provide potential signs of success in the endothelialization of vascular device surfaces. They usually involve therapeutic drugs, specific peptides, adhesive proteins, antibodies, growth factors and nitric oxide (NO) donors. The gene engineering can enhance the proliferation, growth, and migration of vascular cells, thus boosting the endothelialization. In this review, the surface engineering strategies are highlighted and summarized to improve hemocompatibility and rapid endothelialization on the cardiovascular devices. The potential outlook is also briefly discussed to help guide endothelialization strategies and inspire further innovations. It is hoped that this review can assist with the surface engineering of cardiovascular devices and promote future advancements in this emerging research field.

One-Pot Covalent Grafting of Gelatin on Poly(Vinyl Alcohol) Hydrogel to Enhance Endothelialization and Hemocompatibility for Synthetic Vascular Graft Applications

Cardiovascular diseases remain the leading cause of death worldwide. Patency rates of clinically-utilized small diameter synthetic vascular grafts such as Dacron® and expanded polytetrafluoroethylene (ePTFE) to treat cardiovascular disease are inadequate due to lack of endothelialization. Sodium trimetaphosphate (STMP) crosslinked PVA could be potentially employed as blood-compatible small diameter vascular graft for the treatment of cardiovascular disease. However, PVA severely lacks cell adhesion properties, and the efforts to endothelialize STMP-PVA have been insufficient to produce a functioning endothelium. To this end, we developed a one-pot method to conjugate cell-adhesive protein via hydroxyl-to-amine coupling using carbonyldiimidazole by targeting residual hydroxyl groups on crosslinked STMP-PVA hydrogel. Primary human umbilical vascular endothelial cells (HUVECs) demonstrated significantly improved cells adhesion, viability and spreading on modified PVA. Cells formed a confluent endothelial monolayer, and expressed vinculin focal adhesions, cell-cell junction protein zonula occludens 1 (ZO1), and vascular endothelial cadherin (VE-Cadherin). Extensive characterization of the blood-compatibility was performed on modified PVA hydrogel by examining platelet activation, platelet microparticle formation, platelet CD61 and CD62P expression, and thrombin generation, which showed that the modified PVA was blood-compatible. Additionally, grafts were tested under whole, flowing blood without any anticoagulants in a non-human primate, arteriovenous shunt model. No differences were seen in platelet or fibrin accumulation between the modified-PVA, unmodified PVA or clinical, ePTFE controls. This study presents a significant step in the modification of PVA for the development of next generation in situ endothelialized synthetic vascular grafts.

Co-immobilization of ACH11 antithrombotic peptide and CAG cell-adhesive peptide onto vascular grafts for improved hemocompatibility and endothelialization

Surface modification by conjugating biomolecules has been widely proved to enhance biocompatibility of small-caliber artificial vascular grafts. In this study, we aimed at developing a multifunctional vascular graft that provides not only good hemocompatibility but also in situ rapid endothelialization. Herein, a vascular graft (inner diameter ∼2 mm) was fabricated by electrospinning with poly(lactic acid-co-caprolactone) and gelatin, and then biofunctionalized with antithrombotic peptide with sequence LTFPRIVFVLG (ACH₁₁) and cell adhesion peptide with sequence CAG through adhesive poly(dopamine) coating. We developed this graft with the synergistic properties of low thrombogenicity and rapid endothelialization. The successful grafting of both CAG and ACH₁₁ peptides was confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The surface micromorphology of the modified surfaces was observed by field emission scanning electron microscopy. Our results demonstrated that the multifunctional surface suppressed the denaturation of absorbed fibrinogen, hindered coagulation factor Xa activation, and inhibited platelet adhesion and aggregation. Importantly, this modified surface could selectively enhance endothelial cells adhesion, proliferation and release of nitric oxide. Upon in vivo implantation of 6 weeks, the multifunctional vascular graft showed improved patency and superior vascular endothelialization. Overall, the results effectively demonstrated that the co-immobilization of ACH₁₁ and CAG provided a promising method for the improvement of hemocompatibility and endothelialization of vascular grafts. STATEMENT OF SIGNIFICANCE: Electrospun small-caliber vascular grafts are increasingly used to treat cardiovascular diseases. Despite their success related to their good biodegradation and mechanical strength, they have some drawbacks, such as low hemocompatibility and endothelialization. The single-function ligands are insufficient to modify surface with both good hemocompatibility and rapid endothelialization simultaneously. Therefore, we functionalized electrospun vascular graft by novel antithrombotic peptide and cell-adhesive peptide to construct superior anticoagulation and ECs-selective adhesion surface in present study. The multifunctional vascular grafts benefit for high long-term patency and rapid endothelialization.

Advances in tissue engineering of nanocellulose-based scaffolds: A review

Scaffolds based on nanocellulose (NC) have crucial applications in tissue engineering (TE) owing to the biocompatibility, water absorption, water retention, optical transparency, and chemo-mechanical properties. In this review, we summarize the scaffolds based on nanocellulose, including nanocrystalline cellulose and nanofibrillated cellulose. We compare four representative methods to prepare NC-based scaffolds, containing electrospinning, freeze-drying, 3D printing, and solvent casting. We outline the characteristics of scaffolds obtained by different methods. Our focus is on the applications of NC-based scaffolds to repair, improve or replace damaged tissues and organs, including skin, blood vessel, nerve, skeletal muscle, heart, liver, and ophthalmology. NC-based scaffolds are attractive materials for regeneration of different tissues and organs due to the remarkable features. Finally, we propose the challenges and potentials of NC-based TE scaffolds.

Preparation of Small-Diameter Tissue-Engineered Vascular Grafts Electrospun from Heparin End-Capped PCL and Evaluation in a Rabbit Carotid Artery Replacement Model

Aiming to construct small diameter (ID <6 mm) off-the-shelf tissue-engineered vascular grafts, the end-group heparinizd poly(ε-caprolactone) (PCL) is synthesized by a three-step process and then electrospun into an inner layer of double-layer vascular scaffolds (DLVSs) showing a hierarchical double distribution of nano- and microfibers. Afterward, PCL without the end-group heparinization is electrospun into an outer layer. A steady release of grafted heparin and the existence of a glycocalyx structure give the grafts anticoagulation activity and the conjugation of heparin also improves hydrophilicity and accelerates degradation of the scaffolds. The DLVSs are evaluated in six rabbits via a carotid artery interpositional model for a period of three months. All the grafts are patent until explantation, and meanwhile smooth endothelialization and fine revascularization are observed in the grafts. The composition of the outer layer of scaffolds exhibits a significant effect on the aneurysm dilation after implantation. Only one aneurysm dilation is detected at two months and no calcification is formed in the follow-up term. How to prevent aneurysms remains a challenging topic.

Biomimetic strategy towards gelatin coatings on PET. Effect of protocol on coating stability and cell-interactive properties

Gelatin-modified poly(ethylene terephthalate) (PET) surfaces have been previously realized via an intermediate dopamine coating procedure that resulted in surfaces with superior haemocompatibility compared to unfunctionalized PET. The present study addresses the biocompatibility assessment of these coated PET surfaces. In this context, the stability of the gelatin coating upon exposure to physiological conditions and its cell-interactive properties were investigated. The proposed gelatin-dopamine-PET surfaces showed an increased protein coating stability up to 24 days and promoted the attachment and spreading of both endothelial cells (ECs) and smooth muscle cells (SMCs). In parallel, physisorbed gelatin coatings exhibited similar cell-interactive properties, albeit temporarily, as the coating delaminated within 1 week after cell seeding. Furthermore, no or only minimal immunogenic or inflammatory responses were observed during in vitro cytotoxicity and endotoxicity assessment for all gelatin-modified PET surfaces evaluated. Overall, the combined enhanced biocompatibility reported herein together with the previously proven haemocompatibility show the potential of the gelatin-dopamine-PET surfaces to function as vascular graft candidates.

Heparinized Polyurethane Surface Via a One-Step Photografting Method

Traditional methods using coupling chemistry for surface grafting of heparin onto polyurethane (PU) are disadvantageous due to their generally low efficiency. In order to overcome this problem, a quick one-step photografting method is proposed here. Three heparin derivatives incorporating 0.21, 0.58, and 0.88 wt% pendant aryl azide groups were immobilized onto PU surfaces, leading to similar grafting densities of 1.07, 1.17, and 1.13 μg/cm², respectively, yet with increasing densities of anchoring points. The most negatively charged surface and the maximum binding ability towards antithrombin III were found for the heparinized PU with the lowest amount of aryl azide/anchor sites. Furthermore, decreasing the density of anchoring points was found to inhibit platelet adhesion to a larger extent and to prolong plasma recalcification time, prothrombin time, thrombin time, and activated partial thromboplastin time to a larger extent. This was also found to enhance the bioactivity of immobilized heparin from 22.9% for raw heparin to 36.9%. This could be explained by the enhanced molecular mobility of immobilized heparin when it is more loosely anchored to the PU surface, as well as a higher surface charge.