In vitro and in vivo studies of PEO-grafted blood-contacting cardiovascular prostheses

Mechanisms of controlled drug release from drug-eluting stents

The clinical importance of drug-eluting stents (DESs) has been demonstrated by their unparalleled success in preventing restenosis after stenting procedures. The magnitude of success is historic despite their short history. The current DESs deliver a single drug aiming to prevent or minimize proliferation of smooth muscle cells. Since the restenosis process involves several different biological responses, the ability to deliver the right drugs at the right times is critical for further development of the second generation of DESs. As the type of drugs that can be delivered from DESs varies, it is imperative to understand the drug delivery mechanisms and the approaches available for drug coating on the stents. The drug delivery mechanisms of current DESs that have been used clinically and under clinical trials are explained.

The adhesive properties of endothelial cells on endovascular stent coated by substrates of poly-l-lysine and fibronectin

Optimizing endothelial cell growth and adhesion on the surface of metallic stents implanted in the vascular system is a fundamental issue in understanding and improving their long-term biocompatibility. The ability of the endothelial cell to attach and adhere to the luminal stent surface as well as the capacity to withstand the significant shear stress associated with blood flow are important determinants. The adhesive characteristics of human umbilical vein endothelial cellsectin (HUVEC) on stent surfaces coated with either Poly-L-Lysine (PLL) or fibron (FN) were compared with uncoated controls. Increasing concentrations of PLL and FN were measured using a micropipette aspiration system. The adhesivenamic properties of HUVECs under static flow conditions were compared to a dy environment on endovascular stents using a parallel-plate-flow chamber. A scanning electron microscope picture was used to measure the number and the adhesive cell ratio as well as the percentage of surface coverage of stent by endothelial cells. The adhesive forces of HUVECs on foreign surfaces coated with PLL and FN were higher compared to uncoated surfaces, and were dependent on incr ing concentrations. These coatings resulted in significant increase of the adhesive force of HUVECs. The influence of substrates on the adhesion of the endothelial cell monolayer under static or dynamic flow conditions was highly significant compared with controls (p<0.01). No significant differences were observed between PLL and FN substrates. Both PLL and FN coated surfaces can significantly increase the adhesion and growth of HUVECs on metallic stent surfaces.

Multiscale analysis of surface thrombosis in vivo in a left ventricular assist system

Thrombosis limits the success of ventricular assist devices as the demand for alternatives to heart transplants is increasing. This study mapped the occurrence of thrombosis in a left ventricular assist system (LVAS) to better understand the biologic response to these devices. Nine calves divided into two groups were implanted with LVAS for 28 to 30 days. One group was anticoagulated, whereas the second group received no long-term anticoagulation. The blood-contacting poly(urethane urea) surfaces of blood sacs in the LVAS were examined for macroscopic thrombi upon retrieval. The sac was partitioned into eight sections and imaged for thrombi by scanning electron microscopy. No difference in thrombosis was observed macroscopically between the groups. Anticoagulation appeared to result in reduction of platelet-like structures, but the presence of fibrin-like structures remained similar between groups. Regional differences correlating with high and low shear stress regions were observed. At the macroscale, fewer thrombi were recorded in the high shear stress ports. At the microscale, features resembling fibrin were observed primarily in the ports and platelet-like features were common in lower shear stress regions. These variations in thrombosis with anticoagulation and location are likely due to varied fluid dynamics within the LVAS blood sac.

The effect of high-density-lipoprotein on thrombus formation on and endothelial cell attachement to biomaterial surfaces

Cardiovascular implants such as vascular grafts fail frequently because they lack genuine blood-compatibility. The blood-contacting surface should simultaneously prevent thrombus formation and promote formation of a confluent endothelial cell layer, to achieve sustained haemostasis. Contact activation and endothelialization are known to be determined by the plasma proteins which adsorb onto virtually all synthetic surfaces almost immediately upon contact with blood. A common approach in blood-compatibility research is, therefore, to use hydrophilic biomaterials, which are sometimes claimed to be "protein-repellent". We report here that, for synthetic polymeric surfaces, hydrophilicity is by no means synonymous to protein-repellency. We discovered that significant amounts of proteins, especially high-density lipoprotein, adsorb to hydrophilic surfaces. Pre-incubation of hydrophilic synthetic surfaces with high-density lipoprotein provides a blood-biomaterial interface, which inhibits thrombin generation and subsequent thrombus formation, and also accommodates overgrowth with a confluent endothelial layer. This approach may open the way to truly functional small-caliber arterial prostheses, and may also be relevant to cardiovascular tissue engineering in which de novo vascular tissues are cultured on or within a biomaterial scaffold.

Fibrinogen and von Willebrand's factor adsorption are both required for platelet adhesion from sheared suspensions to polyethylene preadsorbed with blood plasma

Previous studies showed that platelet adhesion to biomaterials from static suspensions was greatly increased by the adsorption of even very small amounts (<5 ng/cm2) of fibrinogen (Fg). In this study, the sensitivity of platelet adhesion to Fg was reexamined by measuring platelet adhesion under flow conditions. The role of adsorbed von Willebrand's factor (vWf) was also studied. Polyethylene (PE) tubing was preadsorbed with Fg, vWf, vWf-deficient plasma, and Fg-deficient plasma or serum with added Fg, and Fg adsorption measured with 125I Fg. Platelets in a red blood cell suspension were passed through the tubes at either low (50 s(-1)) or high (500 or 1000 s(-1)) shear rates and adhesion measured with an improved LDH assay. Adhesion from flowing suspensions measured after preadsorption with afibrinogenemic plasma or serum was very low, but increased greatly with addition of Fg. Less than 10 ng/cm2 of adsorbed Fg was enough to greatly enhance platelet adhesion. Adhesion at high shear was also strongly affected by vWf, as platelet adhesion at 500 s(-1) to PE preadsorbed with vWf-deficient plasma decreased by more than tenfold compared to adhesion at 50 s(-1), but platelet adhesion to PE preadsorbed with normal plasma increased about eightfold when shear rate was increased. The results show that very low amounts of adsorbed Fg are able to support platelet adhesion under shear flow. However, adsorbed vWf also appears to play an important cofactor role in platelet adhesion to biomaterials, as its presence greatly augments platelet adhesion under high shear.

Poly(ethylene glycol) enhances cell motility on protein-based poly(ethylene glycol)-polycarbonate substrates: a mechanism for cell-guided ligand remodeling

The regulation of cell motility on ligand-adsorbed poly(ethylene glycol) (PEG)-based polymeric biomaterials is governed by variables that are not well characterized. In this report, we examined keratinocyte migratory responsiveness to PEG-variant tyrosine-derived polycarbonates adsorbed with equivalent levels of the cell adhesion ligand, fibronectin. The equivalently adsorbed ligand adopted differential distributions, confirmed via atomic force microscopy, and the total number of exposed cell-binding domains (CBD), quantified through immunosorbent fluorometry, varied as a function of PEG concentration. Specifically, the CBD exposure was maximized at 4 mol % PEG and diminished at 8 mol % PEG, suggesting, based on our previous work (Tziampazis et al., Biomaterials 2000;21:511-520), that activation of cell adhesion and motility could be potentially promoted through increased CBD exposure at intermediate levels of PEG. This was confirmed through cell migration studies wherein cell speed values increased from 11 to 22 microm/h as the PEG concentration was increased from 0 to 4 mol %. Unexpectedly, however, high cell motility rates were sustained at 8 mol % PEG despite diminished levels of initial CBD exposure beyond 4 mol % PEG, suggesting that factors other than the initial CBD exposure may additionally have a role in activating cell migration at higher levels of PEG. Through studies of direct ligand mobility, cell-ligand-polymer interactions via atomic force microscopy, and CBD variation and integrin receptor roles in ligand remodeling, we offer evidence that cell motility is enhanced by a new mechanism for the regimen of higher PEG concentration: upon cell attachment and spreading, the ligand exhibits greater "slippage" at the polymer interface, and undergoes cell-engendered remodeling, which further activates cell motility, likely through enhanced exposure of hitherto encrypted sites for cell binding and signaling.

Silicone elastomers for reduced protein adsorption

Monofunctional poly(ethylene oxide) polymers of molecular weight (MW) 350, 750, and 2000, respectively, were modified with Si(OEt)3 groups. These polymers underwent classic condensation cure with hydroxy-terminated silicone polymers and Si(OEt)4 to give composites with poly(ethylene oxide) (PEO) rich surfaces under aqueous conditions, as shown by contact angle and XPS data. The hydrophobicity of the surfaces was considerably higher in air. The greatest PEO concentration was observed with relatively short chain polymers of MW 350. Silicone polymers bearing short chain PEO chains were also observed to be the most protein rejecting from either buffer (fibrinogen) (90%) or plasma (85%). The silicone/TES-MPEO formulation offers the advantage of a one step/one shot polymerization process that gives materials with a high protein rejection ability than can be cast as films, or molded into complex shapes. Covalently linked PEO films of a variety of chain lengths and total surface coverage can be readily accommodated.

PEO-like plasma polymerized tetraglyme surface interactions with leukocytes and proteins: in vitro and in vivo studies

Polyethylene oxide (PEO) surfaces reduce non-specific protein and cell interactions with implanted biomaterials and may improve their biocompatibility. PEO-like polymerized tetraglyme surfaces were made by glow discharge plasma deposition onto fluorinated ethylene propylene copolymer (FEP) substrates and were shown to adsorb less than 10 ng/cm2 of fibrinogen in vitro. The ability of the polymerized tetraglyme surfaces to resist leukocyte adhesion was studied in vitro and in vivo. Polymerized tetraglyme and FEP were implanted subcutaneously in mice and removed after 1 day or 4 weeks. Histological analysis showed a similar degree of fibrous encapsulation around all of the 4-week implants. Darkly stained wells were present in the fibrous tissues at the tissue-material interface of both FEP and tetraglyme. Scanning electron micrographs showed that in vivo macrophage adhesion to polymerized tetraglyme was much higher than to FEP. After 2-hour contact with heparinized whole blood, polymorphonuclear leukocyte (PMN) adhesion to polymerized tetraglyme was much higher than to FEP, while platelet adhesion to polymerized tetraglyme was lower than to FEP. When PMNs isolated from blood were suspended in 10% autologous plasma, cell adhesion to polymerized tetraglyme was higher than to FEP; however when the cells were suspended in heat inactivated serum, cell adhesion to FEP was higher than to polymerized tetraglyme. The surface chemistry of polymerized tetraglyme did not change after 2-hour blood contact, but displayed nitrogen functional groups after 1-day implantation and became slightly degraded after 4-week implantation. The surface chemistry of FEP did not change significantly after blood contact or implantation. Loosely bound proteins such as fibrinogen on polymerized tetraglyme may contribute to the adhesion of PMNs and macrophages and ultimately to fibrous encapsulation (the foreign body response) around the implants.

Two liquid adsorptive entrapment of a pluronic polymer into the surface of polyaniline films

Pluronic triblock copolymers were entrapped on the surface of polyaniline (PANI) films by first immersing the latter in N-methylpyrrolidinone (NMP) solutions of one of the Pluronics for a short time. This softened the surface of the films and allowed the Pluronic molecules to entangle with PANI segments of the swollen film on the surface. Further, the films were taken out from the NMP solution and dipped into water, which is a nonsolvent for PANI. The rapid surface deswelling of PANI by the water resulted in the entrapment of the Pluronic on its surface, with the hydrophilic blocks toward water and the hydrophobic block imbedded in the PANI films. The modified PANI obtained was examined by X-ray photoelectron spectroscopy, water droplets contact angles, scanning electron microscopy, and wide angle X-ray diffraction. The surface of the Pluronic entrapped PANI films became more hydrophilic than the hydrophobic PANI films and decreased the amount of bovine serum albumin protein adsorbed on them. This means that, by reducing the biofouling, the life of the modified polyaniline film can be extended when the latter is employed as a biosensor.

Improved surface properties of polyaniline films by blending with Pluronic polymers without the modification of the other characteristics

Films of conductive polyaniline and amphiphilic Pluronic (P105) copolymer blends were prepared by dissolving the two polymers in N-methylpyrrolidinone (NMP) followed by a slow solvent evaporation at 55 degrees C. The characteristics of both doped and undoped films were determined by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), water droplet contact angles, differential scanning calorimetry (DSC), thermal gravimetry analysis (TG), wide-angle X-ray diffraction (WAXD), and tensile strength measurements. The surface of the blends became more hydrophilic than that of the hydrophobic PANI film, but the other properties of the blends did not change appreciably for Pluronic content lower than 50 wt%. Compared to PANI films, the more hydrophilic surfaces decreased the amount of bovine serum albumin protein adsorbed. By preventing biofouling, the polyaniline-Pluronic blends can become more useful as biosensors than the polyaniline films.

Design and evaluation of novel blood incubation systems for in vitro hemocompatibility assessment of planar solid surfaces

Success in the development of hemocompatible biomaterials depends on adequate equipment and procedures for standardized analysis of blood-materials interactions in vitro. In view of the limited standard of knowledge on that important aspect, two novel incubation systems were designed, built, and evaluated for the in vitro assessment of the hemocompatibility of planar solid surfaces: A screening setup was introduced for the comparison of up to 12 different samples. A perfusion setup was developed to model the directed blood flow in the vascular system during incubation by a recirculation circuit, allowing the variation of the wall shear rate at the sample surface. The incubation procedures utilized freshly drawn, heparinized whole human blood. Hemocompatibility in terms of selected aspects of coagulation, thrombogenicity, and immune responses was quantified through plasma levels of characteristic molecules (immunoassays), cell counting, and analysis of adherent cells and fibrin formation (scanning electron microscopy), respectively. Prevention of blood-air contact and mechanical stress, constant temperature and blood pH during incubation, and the suitable choice of reference materials were found to be crucial for reliable testing. Considering those requirements, screening and perfusion system both provided sensitive discrimination between a given set of planar solid surfaces. In conclusion, the suggested methods for an in vitro hemocompatibility assessment permit versatile, sensitive, and efficient analysis of important blood-material interactions despite the unavoidable variability of blood characteristics in different experiments.

Protein adsorption, fibroblast activity and antibacterial properties of poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) grafted with chitosan and chitooligosaccharide after immobilized with hyaluronic acid

Poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) membrane was treated with ozone and grafted with acrylic acid. The resulting membranes were further grafted with chitosan (CS) or chitooligosaccharide (COS) via esterification. Afterward hyaluronic acid (HA) was immobilized onto CS- or COS-grafting membranes. The antibacterial activity of CS and COS against Staphylococus aureus, Escherichia coli, and Pseudomonas aeruginosa was preserved after HA immobilization. Among them, CS-grafted PHBV membrane showed higher antibacterial activity than COS-grafted PHBV membrane. In addition, after CS- or COS-grafting, the L929 fibroblasts attachment and protein adsorption were improved, while the cell number was decrease. After immobilizing HA, the cell proliferation was promoted, the protein adsorption was decreased, and the cell attachment was slightly lower than CS- or COS-grafting PHBV.

Enhanced blood compatibility of polymers grafted by sulfonated PEO via a negative cilia concept

In our laboratory sulfonated PEO (PEO-SO(3)) was designed as a "negative cilia model" to investigate a synergistic effect of PEO and negatively charged SO(3) groups. PEO-SO(3) itself exhibited a heparin-like anticoagulant activity of 14% of free heparin. Polyurethane grafted with PEO-SO(3) (PU-PEO-SO(3)) increased the albumin adsorption to a great extent but suppressed other proteins, while PU-PEO decreased the adsorption of all the proteins. The platelet adhesion was decreased on PU-PEO but least on PU-PEO-SO(3) to demonstrate an additional effect of SO(3) groups. The enhanced blood compatibility of PU-PEO-SO(3) in the ex vivo rabbit and in vivo canine implanting tests was confirmed. Furthermore, PU-PEO-SO(3) exhibited an improved biostability and suppressed calcification in addition to the enhanced antithrombogenicity. The in vivo antithrombogenicity and biostability were improved in the order of PU<PU-PEO<PU-PEO-SO(3). The calcium amounts deposited was decreased in the order of PU>PU-PEO>PU-PEO-SO(3) in spite of the possible attraction between negative SO(3) groups and positive calcium ions. The bioprosthetic tissue (BT) was grafted with H(2)N-PEO-SO(3) via glutaraldehyde (GA) residues after conventional GA fixation. BT-PEO-SO(3) also displayed the decreased calcification by in vivo animal models. The application of PEO-SO(3) was extended by designing amphiphilic copolymers containing PEO-SO(3) moiety and hydrophobic long alkyl groups as anchors. The superior effect of PEO-SO(3) groups on thromboresistance compared to PEO was confirmed also in the case of copolymers coated or blended with other polymers and the systems coupled by UV irradiation, photoreaction or gold/sulfur or silane coupling technology, and therefore it might be very useful for the medical devices.