Cholesterol-derivatized polyurethane: characterization and endothelial cell adhesion

Sustainable cycloaliphatic polyurethanes: from synthesis to applications

Polyurethanes (PUs) are a versatile and major polymer family, mainly produced via polyaddition between polyols and polyisocyanates. A large variety of fossil-based building blocks is commonly used to develop a wide range of macromolecular architectures with specific properties. Due to environmental concerns, legislation, rarefaction of some petrol fractions and price fluctuation, sustainable feedstocks are attracting significant attention, e.g., plastic waste and biobased resources from biomass. Consequently, various sustainable building blocks are available to develop new renewable macromolecular architectures such as aromatics, linear aliphatics and cycloaliphatics. Meanwhile, the relationship between the chemical structures of these building blocks and properties of the final PUs can be determined. For instance, aromatic building blocks are remarkable to endow materials with rigidity, hydrophobicity, fire resistance, chemical and thermal stability, whereas acyclic aliphatics endow them with oxidation and UV light resistance, flexibility and transparency. Cycloaliphatics are very interesting as they combine most of the advantages of linear aliphatic and aromatic compounds. This original and unique review presents a comprehensive overview of the synthesis of sustainable cycloaliphatic PUs using various renewable products such as biobased terpenes, carbohydrates, fatty acids and cholesterol and/or plastic waste. Herein, we summarize the chemical modification of the main sustainable cycloaliphatic feedstocks, synthesis of PUs using these building blocks and their corresponding properties and subsequently present their major applications in hot-topic fields, including building, transportation, packaging and biomedicine.

Effect of PEG grafting density on surface properties of polyurethane substrata and the viability of osteoblast and fibroblast cells

The surface of Tecoflex SG-80A Polyurethane (PU) films was modified by grafting polyethylene glycol (PEG) chains at three different molar amounts (0.05, 0.10, and 0.15 mmol). The resulting substrata were characterized by FTIR-ATR, TGA, AFM, SEM and contact angle to assess the surface modifications occurred during the grafting reactions. Osteoblasts and fibroblasts were cultured with PU extracts for 24 h, and their cell viability and morphology were evaluated by CellTiterBlue assay, Crystal Violet staining and Live/Dead assay. FTIR and TGA results indicated that PEG chains were successfully grafted onto PU surfaces, specifically in the hard segment of PU forming allophanate groups as the PEG grafting density increased. SEM and AFM images suggest that PU substrata were partially covered by PEG, increasing the dispersive and basic components of the PU surface energy. It was found that extracts from PEG-grafted polyurethanes increased the osteoblast viability, although fibroblasts viability remained constant regardless PEG grafting density; in spite of this both cells presented a more spread morphology at the lower PEG grafting density. Our results showed that surface energy of PU substrata can be tuned by PEG grafting density; also, the PEG leached tends to increase the pH of culture medium which leads to a higher viability of osteoblasts; nevertheless, PEG grafting density should be optimized to promote a healthy cell morphology as alterations in its morphology were detected at higher concentrations. Graphical abstract.

Exposure of Smaller and Oxidized Graphene on Polyurethane Surface Improves its Antimicrobial Performance

Catheter-related infections are a common worldwide health problem, highlighting the need for antimicrobial catheters. Here, antibacterial potential of graphene nanoplatelets (GNP) incorporated in the commonly used polymer for catheter manufacture-polyurethane (PU)-is investigated. Two strategies are explored: melt-blending, producing a composite, and dip coating, where a composite layer is deposited on top of PU. GNP with different lateral sizes and oxidation degrees-GNP-M5, GNP-M15, GNP-M5ox, GNP-M15ox-are applied in both strategies, and the antimicrobial potential towards Staphylococcus epidermidis of GNP dispersions and GNP-containing PU evaluated. As dispersions, oxidized and smaller GNP powders (GNP-M5ox) inhibit 74% bacteria growth at 128 µg/mL. As surfaces, GNP exposure strongly impacts their antimicrobial profile: GNP absence at the surface of composites yields no significant effects on bacteria, while by varying GNP: PU ratio and GNP concentration, coatings enhance GNP exposure, depicting an antimicrobial profile. Oxidized GNP-containing coatings induce higher antibacterial effect than non-oxidized forms, particularly with smaller GNPox, where a homogeneous layer of fused platelets is formed on PU, leading to 70% reduction in bacterial adhesion and 70% bacterial death. This pioneering work unravels how to turn a polymer clinically used to produce catheters into an antimicrobial surface, crucial to reducing risk of infection associated with catheterization.

Synthesis of polyester urethane urea and fabrication of elastomeric nanofibrous scaffolds for myocardial regeneration

Fabrication of bioactive scaffolds is one of the most promising strategies to reconstruct the infarcted myocardium. In this study, we synthesized polyester urethane urea (PEUU), further blended it with gelatin and fabricated PEUU/G nanofibrous scaffolds. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) and X-ray diffraction were used for the characterization of the synthesized PEUU and properties of nanofibrous scaffolds were evaluated using scanning electron microscopy (SEM), ATR-FTIR, contact angle measurement, biodegradation test, tensile strength analysis and dynamic mechanical analysis (DMA). In vitro biocompatibility studies were performed using cardiomyocytes. DMA analysis showed that the scaffolds could be reshaped with cyclic deformations and might remain stable in the frequencies of the physiological activity of the heart. On the whole, our study suggests that aligned PEUU/G 70:30 nanofibrous scaffolds meet the required specifications for cardiac tissue engineering and could be used as a promising construct for myocardial regeneration.

Tissue engineering vascular grafts a fortiori: looking back and going forward

INTRODUCTION

Cardiovascular diseases such as coronary heart disease often necessitate the surgical repair using conduits. Although autografts still remain the gold standard, the inconvenience of harvesting and/or insufficient availability in patients with atherosclerotic disease has given impetus to look into alternative sources for vascular grafts.

AREAS COVERED

There are four main techniques to produce tissue-engineered vascular grafts (TEVGs): i) biodegradable synthetic scaffolds; ii) gel-based scaffolds; iii) decellularised scaffolds and iv) self-assembled cell-sheet-based techniques. The first three techniques can be grouped together as scaffold-guided approach as it involves the use of a construct to function as a supportive framework for the vascular graft. The most significant advantages of TEVGs are that it possesses the ability to grow, remodel and respond to environmental factors. Cell sources for TEVGs include mature somatic cells, stem cells, adult progenitor cells and pluripotent stem cells.

EXPERT OPINION

TEVG holds great promise with advances in nanotechnology, coupled with important refinements in tissue engineering and decellularisation techniques. This will undoubtedly be an important milestone for cardiovascular medicine when it is eventually translated to clinical use.

The effect of CD47 modified polymer surfaces on inflammatory cell attachment and activation

CD47 is a transmembrane protein that is a marker of "self". CD47 binding to its cognate receptor in leukocytes and macrophages, signal-regulatory protein alpha (SIRPα), causes inhibition of inflammatory cell attachment. We hypothesized that immobilization of recombinant CD47 on polymeric surfaces would reduce inflammation. Recombinant CD47 was appended to polyvinyl chloride (PVC) or polyurethane (PU) surfaces via photoactivation chemistry. Cell culture studies showed that CD47 immobilization significantly reduced human neutrophil (HL-60) and human monocyte derived macrophage (MDM) (THP-1) attachment to PVC and PU respectively. A neutralizing antibody, directed against SIRPα, inhibited THP-1 and HL-60 binding to PU and PVC surfaces respectively. This antibody also increased the level of SIRPα tyrosine phosphorylation, thereby indicating a direct role for SIRPα mediated signaling in preventing inflammatory cell attachment. Studies using human blood in an ex vivo flow-loop showed that CD47 modified PVC tubing significantly reduced cell binding and neutrophil activation compared to unmodified tubing or poly-2-methoxy-ethylacrylate (PMEA) coated tubing. In ten-week rat subdermal implants, CD47 functionalized PU films showed a significant reduction in markers of MDM mediated oxidative degradation compared to unmodified PU. In conclusion, CD47 functionalized surfaces can resist inflammatory cell interactions both in vitro and in vivo.

Prevention of polyurethane oxidative degradation with phenolic antioxidants covalently attached to the hard segments: structure-function relationships

Oxidative degradation of the polyurethane elastomeric (PU) components greatly reduces the efficacy of PU-containing cardiovascular devices. Covalently appending the phenol-based antioxidant, 4-substituted 2,6-di-tert-butylphenol (DBP), to PU hard segments effectively reduced oxidative degradation of the PU in vivo and in vitro in prior studies by our group. In these experiments, we analyze the contribution of the tethering molecule to the antioxidant capabilities of the DBP-modified PU. Bromoalkylation chemistry was used to link DBP to the hard segment of the polyether PU, Tecothane, via our original linker (PU-DBP) or variants containing side chains with one (PU-C-DBP) or three (PU-3C-DBP) carbons. Two additional DBP variants were fabricated in which the DBP group was appended to the alkyl chain via an oxygen atom (PU-O-DBP) or an amide linkage in the middle of the tether (PU-NHCO-DBP). All DBP variant films and unmodified control films were subject to oxidative degradation via 15-day immersion in a solution of 20% H(2)O(2) + 0.1M CoCl(2). At the end of the oxidation protocol, films were analyzed for the presence of oxidation-related endpoints via scanning electron microscopy, contact angle measurements, and Fourier transformation infrared spectroscopy (FTIR). All DBP-containing variants resisted oxidation damage significantly better than the unmodified control PU. SEM analysis of oxidized PU-C-DBP and PU-O-DBP showed evidence of surface cracking, consistent with oxidative degradation of the PU surfaces. Similarly, there was a trend in increased ether crosslinking, a marker for oxidative degradation, in PU-C-DBP and PU-NHCO-DBP films. Consistent with these FTIR results, both PU-C-DBP and PU-NHCO-DBP had significant reductions in measured surface hydrophobicity as a result of oxidation. These data show for the first time that the choice of linker molecule significantly affects the efficiency of the linked phenolic antioxidant.

CD47-dependent molecular mechanisms of blood outgrowth endothelial cell attachment on cholesterol-modified polyurethane

We previously showed that blood outgrowth endothelial cells (BOECs) had a high affinity for polyurethane (PU) covalently configured with cholesterol residues (PU-Chol). However, the molecular mechanisms responsible for this enhanced affinity were not determined. CD47, a multifunctional transmembrane glycoprotein involved in cellular attachment, can form a cholesterol-dependent complex with integrin alpha(v)beta(3) and heterotrimeric G proteins. We tested herein the hypothesis that CD47, and the other components of the multi-molecular complex, enhance the attachment of BOECs to PU-Chol. Immunoprecipitation studies, of human and ovine BOECs, demonstrated that CD47 associates with integrin alpha(v) and integrin beta(3) as well as G(alphai-2) protein. The three-fold increase in BOEC attachment to PU-Chol, compared to unmodified PU, was reversed with the addition of blocking antibodies specific for CD47 and integrin alpha(v) and integrin beta(3). Similar results were observed with the addition of methyl-beta-cyclodextrin (MbetaCD), a known disruptor of the CD47 complex as well as of the membrane cholesterol content, to seeded BOEC or PU-Chol films. Reducing CD47 expression, via lentivirus transduced shRNA, decreased BOEC binding to PU-Chol by 50% compared to control groups. These data are the first demonstration of a role for the CD47 cholesterol-dependent signaling complex in BOEC attachment onto synthetic surfaces.

Endothelial delivery of antioxidant enzymes loaded into non-polymeric magnetic nanoparticles

Antioxidant enzymes have shown promise as a therapy for pathological conditions involving increased production of reactive oxygen species (ROS). However the efficiency of their use for combating oxidative stress is dependent on the ability to achieve therapeutically adequate levels of active enzymes at the site of ROS-mediated injury. Thus, the implementation of antioxidant enzyme therapy requires a strategy enabling both guided delivery to the target site and effective protection of the protein in its active form. To address these requirements we developed magnetically responsive nanoparticles (MNP) formed by precipitation of calcium oleate in the presence of magnetite-based ferrofluid (controlled aggregation/precipitation) as a carrier for magnetically guided delivery of therapeutic proteins. We hypothesized that antioxidant enzymes, catalase and superoxide dismutase (SOD), can be protected from proteolytic inactivation by encapsulation in MNP. We also hypothesized that catalase-loaded MNP applied with a high-gradient magnetic field can rescue endothelial cells from hydrogen peroxide toxicity in culture. To test these hypotheses, a family of enzyme-loaded MNP formulations were prepared and characterized with respect to their magnetic properties, enzyme entrapment yields and protection capacity. SOD- and catalase-loaded MNP were formed with average sizes ranging from 300 to 400 nm, and a protein loading efficiency of 20-33%. MNP were strongly magnetically responsive (magnetic moment at saturation of 14.3 emu/g) in the absence of magnetic remanence, and exhibited a protracted release of their cargo protein in plasma. Catalase stably associated with MNP was protected from proteolysis and retained 20% of its initial enzymatic activity after 24h of exposure to pronase. Under magnetic guidance catalase-loaded MNP were rapidly taken up by cultured endothelial cells providing increased resistance to oxidative stress (62+/-12% cells rescued from hydrogen peroxide induced cell death vs. 10+/-4% under non-magnetic conditions). We conclude that non-polymeric MNP formed using the controlled aggregation/precipitation strategy are a promising carrier for targeted antioxidant enzyme therapy, and in combination with magnetic guidance can be applied to protect endothelial cells from oxidative stress mediated damage. This protective effect of magnetically targeted MNP impregnated with antioxidant enzymes can be highly relevant for the treatment of cardiovascular disease and should be further investigated in animal models.

Cholesterol-modified superporous poly(2-hydroxyethyl methacrylate) scaffolds for tissue engineering

Modifications of poly(2-hydroxyethyl methacrylate) (PHEMA) with cholesterol and laminin have been developed to design scaffolds that promote cell-surface interaction. Cholesterol-modified superporous PHEMA scaffolds have been prepared by the bulk radical copolymerization of 2-hydroxyethyl methacrylate (HEMA), cholesterol methacrylate (CHLMA) and the cross-linking agent ethylene dimethacrylate (EDMA) in the presence of ammonium oxalate crystals to introduce interconnected superpores in the matrix. With the aim of immobilizing laminin (LN), carboxyl groups were also introduced to the scaffold by the copolymerization of the above monomers with 2-[(methoxycarbonyl)methoxy]ethyl methacrylate (MCMEMA). Subsequently, the MCMEMA moiety in the resulting hydrogel was hydrolyzed to [2-(methacryloyloxy)ethoxy]acetic acid (MOEAA), and laminin was immobilized via carbodiimide and N-hydroxysulfosuccinimide chemistry. The attachment, viability and morphology of mesenchymal stem cells (MSCs) were evaluated on both nonporous and superporous laminin-modified as well as laminin-unmodified PHEMA and poly(2-hydroxyethyl methacrylate-co-cholesterol methacrylate) P(HEMA-CHLMA) hydrogels. Neat PHEMA and laminin-modified PHEMA (LN-PHEMA) scaffolds facilitated MSC attachment, but did not support cell spreading and proliferation; the viability of the attached cells decreased with time of cultivation. In contrast, MSCs spread and proliferated on P(HEMA-CHLMA) and LN-P(HEMA-CHLMA) hydrogels.

Polymeric heart valves: new materials, emerging hopes

Heart valve (HV) replacements are among the most widely used cardiovascular devices and are in rising demand. Currently, clinically available devices are restricted to slightly modified mechanical and bioprosthetic valves. Polymeric HVs could represent an attractive alternative to the existing prostheses, merging the superior durability of mechanical valves and the enhanced haemodynamic function of bioprosthetic valves. After early unsatisfactory clinical results, polymeric HVs did not reach commercialization, mainly owing to their limited durability. Recent advances in polymers, nanomaterials and surface modification techniques together with the emergence of novel biomaterials have resulted in improved biocompatibility and biostability. Advances in HV design and fabrication methods could also lead to polymeric HVs that are suitable for long-lasting implantation. Considering all these progresses, it is likely that the new generation of polymeric HVs will find successful long-term clinical applications in future.

Addressing thrombogenicity in vascular graft construction

Thrombosis is a major cause of poor patency in synthetic vascular grafts for small diameter vessel (< 6 mm) bypass. Arteries have a host of structural mechanisms by which they prevent triggering of platelet activation and the clotting cascade. Many of these are present in vascular endothelial cells. These mechanisms act together with perpetual feedback at different levels, providing a constantly fine-tuned non-thrombogenic environment. The arterial wall anatomy also serves to promote thrombosis as a healing mechanism when it has been severely injured. Surface modification of synthetic graft surfaces to attenuate the coagulation cascade has reduced thrombosis levels and improved patency in vitro and in animal models. Success in this endeavor is critically dependent on the methods used to modify the surface. Platelets adhere to positively charged surfaces due to their own negative charge. They also preferentially attach to hydrophobic surfaces. Therefore synthetic graft development is concerned with hydrophilic materials with negative surface charge. However, fibrinogen has both hydrophilic and hydrophobic binding sites-amphiphilic materials reduce its adhesion and subsequent platelet activation. The self-endothelializing synthetic graft is an attractive proposition as a confluent endothelial layer incorporates many of the anti-thrombogenic properties of arteries. Surface modification to promote this has shown good results in animal models. The difficulties experienced in achieving spontaneous endothelialisation in humans have lead to the investigation of pre-implantation in vitro endothelial cell seeding. These approaches ultimately aim to result in novel synthetic grafts which are anti-thrombogenic and hence suitable for coronary and distal infrainguinal bypass.

Prevention of oxidative degradation of polyurethane by covalent attachment of di-tert-butylphenol residues

Polyurethane (PU) components of cardiovascular devices are subjected to oxidation-initiated surface degradation, which leads to cracking and ultimately device failure. In the present study, we investigated a novel bromoalkylation chemical strategy to covalently attach the antioxidant, di-tert-butylphenol (DBP), and/or cholesterol (Chol) to the PU urethane nitrogen groups to hypothetically prevent oxidative degradation. These experiments compared PU, PU-DBP, PU-Chol, and PU-Chol-DBP. A series of comparative oxidative degradation studies involved exposing PU samples (modified and unmodified) to H2O2-CoCl2 for 15 days at 37 degrees C, to cause accelerated oxidative degradation. The extent and effects of degradation were assessed by attenuated total reflectance Fourier transformation infrared spectroscopy (FTIR), scanning electron microscopy (SEM), surface contact angle measurements, and mechanical testing. Both the Chol and DBP modification conferred significant resistance to oxidation related changes compared to unmodified PU per FTIR and SEM results. SEM demonstrated cavitation only in unmodified PU. However, contact angle analysis showed significant oxidation-induced changes only in the Chol-modified PU formulations. Most importantly, uniaxial stress-strain testing revealed that only PU-DBP demonstrated bulk elastomeric properties that were minimally affected by oxidation; PU, PU-Chol, PU-Chol-DBP showed marked deterioration of their stress-strain properties following oxidation. In conclusion, these results demonstrate that derivatizing PU with DBP confers significant resistance to oxidative degradation compared with unmodified PU.

Cholesterol-modified polyurethane valve cusps demonstrate blood outgrowth endothelial cell adhesion post-seeding in vitro and in vivo

BACKGROUND

The clinical and experimental use of polyurethane heart valve prostheses has been compromised by thrombosis and calcified thrombus. This is caused in part by the lack of an intact endothelium on these implant surfaces. We hypothesize that endothelial seeding of a polyurethane heart valve leaflet with autologous sheep blood outgrowth endothelial cells (BOECs) could be achieved with cholesterol-modified polyurethane (PU-Chol) to promote BOEC adhesion, thereby resulting in an intact, shear-resistant endothelium that would promote resistance to thrombosis.

METHODS

Cholesterol-derivatized polyurethane was formulated by bromoalkylation of the urethane nitrogens followed by reactive attachment of mercaptocholesterol. In vitro shear flow studies were carried out comparing BOEC retention on control surfaces versus PU-Chol using forces comparable to those observed in vivo with cardiac valves (75 dyne/cm2). Autologous sheep BOECs were seeded onto PU-Chol before pulmonary leaflet replacement surgery under cardiopulmonary bypass. Studies were terminated at 30 and 90 days followed by retrieval analyses.

RESULTS

Blood outgrowth endothelial cell seeding of PU-Chol surfaces resulted in an endothelial monolayer that was positive for von Willebrand factor. Polyurethane-cholesterol demonstrated significantly greater BOEC adhesion under 75 dyne/cm2 shear force in vitro than control polyurethane (75.3% +/- 12.3% versus 5.8% +/- 3.9%, respectively; p < 0.001). Sheep pulmonary cusp replacements demonstrated retention of seeded BOECs on PU-Chol leaflets with no significant differences in the extent of cellular density comparing unimplanted specimens with explants. Control explants (nonseeded PU-Chol and nonseeded polyurethane) demonstrated no evidence of endothelial recruitment.

CONCLUSIONS

Polyurethane-cholesterol represents a polyurethane formulation with very high adhesive properties for BOECs under heart valve level shear forces both in vitro and in vivo.

Adenoviral gene vector tethering to nanoparticle surfaces results in receptor-independent cell entry and increased transgene expression

The present studies investigated the hypothesis that affinity immobilization of replication-defective adenoviruses (Ad) on the surfaces of biodegradable nanoparticles (NP) can improve transduction through uncoupling cellular uptake from the coxsackie-adenovirus receptor (CAR). Ad was tethered to the surfaces of polylactide-based NP that were surface-activated using a photoreactive polyallylamine-benzophenone-pyridyldithiocarboxylate polymer, which enabled (via thiol chemistry) the covalent attachment of Ad-binding proteins, either the recombinant D1 domain of CAR or an adenoviral knob-specific monoclonal antibody. Gene transfer by NP-Ad complexes was studied in relation to cellular uptake as a function of cell type and the character of NP-Ad binding. NP-Ad complexes, but not Ad applied with or without control nonimmune IgG-modified NP, significantly increased green fluorescent protein reporter expression in endothelioma and endothelial and arterial smooth muscle cells (SMC) in direct correlation to the extent of NP-Ad internalization. CAR-independent uptake of NP-Ad was confirmed by demonstrating inhibition of free Ad- but not NP-Ad complex-mediated transduction by knob protein. Complexes formulated with an Ad encoding inducible nitric oxide synthase inhibited growth of cultured SMC to a significantly greater extent than those with (GFP)Ad or (NULL)Ad or free vector. It is concluded that Ad-specific affinity tethering to biodegradable NP can significantly increase the level of gene expression via a CAR-independent uptake mechanism.

Site specific gene delivery in the cardiovascular system

Gene therapy holds great promise for treating both genetic and acquired disorders. However, progress toward effective human gene therapy has been thwarted by a number of problems including vector toxicity, poor targeting of diseased tissues, and host immune and inflammatory activity to name but a few of the challenges. Gene therapy for cardiovascular disease has been the subject of many fewer clinical trials than other disorders such as cancer or cystic fibrosis. Nevertheless, the challenges are comparable. The present paper reports a review of investigations related to our hypothesis that site specific cardiovascular gene therapy represents an approach that can lead to both optimizing efficacy and reducing the impact of gene vector-related systemic adverse effects. We report experimental studies demonstrating proof of principle in three areas: gene therapy for heart valve disease, gene delivery stents, and gene therapy to treat cardiac arrhythmias. Heart valve disease is the second most common indication for open heart surgery and is now only treatable by surgical removal or repair of the diseased heart valve. Our investigations demonstrate that gene vectors can be immobilized on the surface of prosthetic heart valve leaflets thereby enabling a therapeutic genetic modification of host cells around the valve annulus and on the leaflet. Other animal studies have shown that vascular stents used to relieve arterial obstruction can also be used as gene delivery systems to provide therapeutic vector constructs that can both locally prevent post stenting reobstruction, known as in-stent restenosis, and treat the underlying vascular disease. Cardiac arrhythmias are the cause of sudden death due to heart disease and affect millions of others on a chronic basis. Our group has successfully investigated in animal studies localized gene therapy using an ion channel mutation to treat atrial arrhythmias.