Biocompatibility of the Vascugraft: evaluation of a novel polyester urethane vascular substitute by an organotypic culture technique

Biodegradable, Self-Reinforcing Vascular Grafts for In Situ Tissue Engineering Approaches

Clinically available small-diameter synthetic vascular grafts (SDVGs) have unsatisfactory patency rates due to impaired graft healing. Therefore, autologous implants are still the gold standard for small vessel replacement. Bioresorbable SDVGs may be an alternative, but many polymers have inadequate biomechanical properties that lead to graft failure. To overcome these limitations, a new biodegradable SDVG is developed to ensure safe use until adequate new tissue is formed. SDVGs are electrospun using a polymer blend composed of thermoplastic polyurethane (TPU) and a new self-reinforcing TP(U-urea) (TPUU). Biocompatibility is tested in vitro by cell seeding and hemocompatibility tests. In vivo performance is evaluated in rats over a period for up to six months. Autologous rat aortic implants serve as a control group. Scanning electron microscopy, micro-computed tomography (µCT), histology, and gene expression analyses are applied. TPU/TPUU grafts show significant improvement of biomechanical properties after water incubation and exhibit excellent cyto- and hemocompatibility. All grafts remain patent, and biomechanical properties are sufficient despite wall thinning. No inflammation, aneurysms, intimal hyperplasia, or thrombus formation are observed. Evaluation of graft healing shows similar gene expression profiles of TPU/TPUU and autologous conduits. These new biodegradable, self-reinforcing SDVGs may be promising candidates for clinical use in the future.

Biobased polyurethanes for biomedical applications

Polyurethanes (PUs) are a major family of polymers displaying a wide spectrum of physico-chemical, mechanical and structural properties for a large range of fields. They have shown suitable for biomedical applications and are used in this domain since decades. The current variety of biomass available has extended the diversity of starting materials for the elaboration of new biobased macromolecular architectures, allowing the development of biobased PUs with advanced properties such as controlled biotic and abiotic degradation. In this frame, new tunable biomedical devices have been successfully designed. PU structures with precise tissue biomimicking can be obtained and are adequate for adhesion, proliferation and differentiation of many cell's types. Moreover, new smart shape-memory PUs with adjustable shape-recovery properties have demonstrated promising results for biomedical applications such as wound healing. The fossil-based starting materials substitution for biomedical implants is slowly improving, nonetheless better renewable contents need to be achieved for most PUs to obtain biobased certifications. After a presentation of some PU generalities and an understanding of a biomaterial structure-biocompatibility relationship, recent developments of biobased PUs for non-implantable devices as well as short- and long-term implants are described in detail in this review and compared to more conventional PU structures.

Evaluation of Fibrin-Based Interpenetrating Polymer Networks as Potential Biomaterials for Tissue Engineering

Interpenetrating polymer networks (IPNs) have gained great attention for a number of biomedical applications due to their improved properties compared to individual components alone. In this study, we investigated the capacity of newly-developed naturally-derived IPNs as potential biomaterials for tissue engineering. These IPNs combine the biologic properties of a fibrous fibrin network polymerized at the nanoscale and the mechanical stability of polyethylene oxide (PEO). First, we assessed their cytotoxicity in vitro on L929 fibroblasts. We further evaluated their biocompatibility ex vivo with a chick embryo organotypic culture model. Subcutaneous implantations of the matrices were subsequently conducted on nude mice to investigate their biocompatibility in vivo. Our preliminary data highlighted that our biomaterials were non-cytotoxic (viability above 90%). The organotypic culture showed that the IPN matrices induced higher cell adhesion (across all the explanted organ tissues) and migration (skin, intestine) than the control groups, suggesting the advantages of using a biomimetic, yet mechanically-reinforced IPN-based matrix. We observed no major inflammatory response up to 12 weeks post implantation. All together, these data suggest that these fibrin-based IPNs are promising biomaterials for tissue engineering.

Synthesis and characterization of biocompatible, degradable, light-curable, polyurethane-based elastic hydrogels

A series of degradable polyurethane-based light-curable elastic hydrogels were synthesized from polycaprolactone diol, polyethylene glycol (PEG), lysine diisocyanate (LDI), and 2-hydroxyethyl methacrylate (HEMA) through UV light initiated polymerization reaction. LDI was used as hard segment and polycaprolactone (PCL) and/or PEG were used as soft segments. By changing the PCL to PEG ratio during the prepolymer synthesis, polyurethanes with different soft segmental structures, hydrophilicity, and cytophilicity were obtained after light-initiated polymerization. The chemical structures of the synthesized polymers were characterized using differential scanning calorimetry and Fourier transform infrared spectroscopy. Physical properties such as swelling, mechanical properties, and in vitro degradation were evaluated. Materials containing a higher ratio of PEG exhibit higher water absorbance, higher degradation rate in vitro, and lower mechanical strength in the hydrated state. Mouse embryonal carcinoma-derived clonal chondrocytes were used as a model cell type to study the cytocompatibility of the synthesized polymers. Chondrocyte attachment, proliferation rates, and morphologies varied with changes in the PCL/PEG ratio. With a higher PEG ratio, lower cell attachment and proliferation were observed. To improve the cell attachment and proliferation on high PEG content hydrogels, bioactive molecules, such as peptides and proteins, were conjugated or immobilized in the gel matrix during the light-curing process. In this study, a short peptide, Arg-Gly-Asp-Ser, was used as a model biomolecule and incorporated into the gels during the light-curing process and improved cell growth was observed. In summary, the use of PCL/PEG at different ratios, as well as the introduction of HEMA into polyurethane, allows the synthesis of a series of biocompatible elastic hydrogels with tunable physical and cytophilic properties through light-initiated polymerization. This series of materials also allows for controlling cell attachment and growth by incorporating bioactive molecules during the light-curing process.

Endothelialization of small-diameter vascular prostheses

In the field of arterial vascular reconstructions there is an increasing need for functional small-diameter artificial grafts (inner diameter < 6mm). When autologous replacement vessels are not available, for example because of the bad condition of the vascular system in the patient, the surgeon has no other alternative than to implant a synthetic polymer-based vessel. After implantation the initial major problem concerning these vessels is the almost immediate occlusion, due to blood coagulation and platelet deposition, under the relatively low flow conditions. As the search for the perfect bio-inert polymer has not revealed a material with suitable properties for this application, improved performance of small-diameter artificial blood vessels is now being sought in the biological field. The poor blood-compatibility of an artificial vascular graft is not simply because of its coagulation-stimulating or platelet-activating properties, but more due to its inability to actively participate in the prevention of blood coagulation and platelet deposition. As these functions are naturally performed by endothelial cells, the utilization of these cells seems inevitable for the construction of a functional small-diameter artificial blood vessels. This review describes the current status of the use of endothelial cells to improve the performance of artificial vascular prostheses.

Preparation of poly(acrylic acid) modified polyurethane membrane for biomaterial by UV radiation without degassing

Poly(acrylic acid) modified polyurethane (AA/PU) membranes were prepared by UV radiation without degassing. The chemical composition of the AA/PU membrane was studied by IR spectroscopy. In addition to those absorption peaks associated with pure PU, the absorption peak at 2400 cm-1 of poly(AA) was also found. The morphology of AA/PU membrane was studied by optical polarizing microscopy. We also measured the glass transition temperature and the decomposition temperature of the AA/PU membrane by differential scanning calorimetry and thermogravimetric analysis. A significant domain was found in the AA/PU membrane, which resulted in different glass transition temperature and decomposition temperature between AA/PU and pure PU membrane. The effect of AA content on the contact angle and water absorption of the AA/PU membrane was determined. It was found that the water content of AA/PU membrane increased with increasing AA content, whereas the contact angle decreased. By using Kaeble's equation and the contact angle data, the surface free energy of AA/PU membrane was determined. The increase of surface free energy resulted from the increase of the dispersion (gammad) term and polar (gammap) term. In order to evaluate the biocompatibility of these membranes, a cytotoxicity test and a cell adhesion and proliferation assay were conducted in cell culture. Immortal cells and primary lymphocytes were both used in this study. The results showed that these AA/PU membranes exhibited very low cytotoxicity and could support cell adhesion and growth. An animal primary test was also done in this study. It was found that the AA/PU membrane could possibly be employed in the treatment of bowel defect.

Elastomers for biomedical applications

Current topics in elastomers for biomedical applications are reviewed. Elastomeric biomaterials, such as silicones, thermoplastic elastomers, polyolefin and polydiene elastomers, poly(vinyl chloride), natural rubber, heparinized polymers, hydrogels, polypeptides elastomers and others are described. In addition biomedical applications, such as cardiovascular devices, prosthetic devices, general medical care products, transdermal therapeutic systems, orthodontics, and ophthalmology are reviewed as well. Elastomers will find increasing use in medical products, offering biocompatibility, durability, design flexibility, and favorable performance/cost ratios. Elastomers will play a key role in medical technology of the future.

Vascugraft polyurethane arterial prosthesis as femoro-popliteal and femoro-peroneal bypasses in humans: pathological, structural and chemical analyses of four excised grafts

Following positive results obtained in in vitro studies and in vivo implantations in animals, a clinical trial using the Vascugraft polyurethane arterial prosthesis as a below-knee substitute was undertaken in 15 patients. Eight grafts became occluded during the first year, and segments from four of them were explanted and made available for pathological, structural and chemical investigations. The implantation periods ranged from 21 to 358 days. Failures were associated with kinking (one case), possible anastomotic mismatch between the graft and the artery (one case), and poor run-off (two cases). No organized collagenous internal encapsulation was noted; however, endothelial-like cells were observed at the anastomotic site of one graft. No significant structural degradation of the prostheses was observed in those grafts implanted for 21, 38 and 46 days. Some deteriorations in the fibrous structure were observed on the external surface of the prosthesis implanted for 358 days. High-resolution carbon C1s analysis by ESCA demonstrated a 60 to 80% decrease in carbonate content on the surface of all explanted prostheses. Chemical analyses of each polyurethane graft by IR, SEC and DSC revealed no significant chemical changes. The clinical performance of the Vascugraft prosthesis for below-knee implantation proved to be no more impressive than that of expanded polytetrafluorethylene, the currently accepted reference. The decision by B. Braun Melsungen AG to end this program is therefore to be regarded as highly professional.

Cytocompatibility of calf pericardium treated by glutaraldehyde and by the acyl azide methods in an organotypic culture model

Glutaraldehyde (GTA) is used to cross-link collagen-based biomaterials, but these materials are often cytotoxic. In order to overcome this problem, we have proposed the use of the acyl azide methods with either hydrazine or diphenylphosphoryl azide (DPPA) as reagents. In this paper we determine the cytocompatibility of acyl azide- and GTA-treated pericardium in vitro, by an organotypic chick aorta culture technique developed for the evaluation of the propensity of vascular cells (both endothelial and smooth muscle cells) to migrate and grow on the surface of biomaterials. We first examined pericardium stabilization as a function of GTA concentration and time, so that we could minimize residual GTA molecules in the material. Treatment for 72 h with 0.05% GTA was optimal for thermal stabilization of the pericardium with a denaturation temperature (Td) of 86.8 degrees C, providing similar results to treatment with 0.6% GTA for 4 h (Td = 85.1 degrees C). Pericardium treated in this way was, however, poorly cytocompatible with little vascular cell migration and growth when compared with tissues treated by the acyl azide methods. The best results were obtained with 0.5% DPPA; treated tissues showed a high level of cross-linking (Td = 82.4 degrees C) and three-fold increases in cell growth and migration over those in a non-toxic control.

Removing fresh tissue from explanted polyurethane prostheses: which approach facilitates physico-chemical analysis?

Chemical, physical and structural analyses of polymers from explanted vascular prostheses are frequently jeopardized because of incomplete removal of the encroaching host tissue. In this study, microporous polyurethane arterial prostheses implanted as a canine thoraco-abdominal bypass were explanted after 1 and 12 months and were cleaned without fixation using four different digesting enzyme treatments, including collagenase, pancreatin and trypsin alone and collagenase and pancreatin in series, followed by washing in a solution of Triton X-100 detergent. By following this approach all the fresh tissue attached to the external and internal walls of the prostheses was removed with minimal damage to the underlying synthetic polymer. The morphology of the explanted and cleaned polyurethane prostheses could be obtained readily by light and scanning electron microscopy. Surface microporous features and the presence of polyurethane microfibres that had experienced in vivo biodegradation could therefore be identified easily. The surface and bulk physico-chemical properties of the polyurethane polymer were determined by electron spectroscopy for chemical analysis, attenuated total reflectance-Fourier transform infrared spectroscopy and differential scanning calorimetry. It was found that the most successful approach for removing fresh tissue and exposing a clean and uncontaminated polyurethane surface was to incubate the explanted samples first in collagenase followed by digestion in pancreatin. This particular cleaning technique has proved valuable in enabling us to monitor small in vivo changes in the surface chemistry and in the bulk microphase segmented structure of polyurethane biomaterials.

In vitro exposure of a novel polyesterurethane graft to enzymes: a study of the biostability of the Vascugraft arterial prosthesis

The biostability of the Vascugraft arterial prosthesis, a porous synthetic graft made by a novel spinning process from a unique poly(ester urethane) polymer, has been studied by means of an in vitro enzyme incubation technique. Samples of the Vascugraft were exposed to buffered solutions of collagenase and pancreatin, as well as the buffer solutions alone, for periods of up to 100 days at 37 +/- 1 degrees C. On removal and after cleaning, a number of different analytic methods, including X-ray photoelectron spectroscopy for chemical analysis (ESCA), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC), size exclusion chromatography (SEC), scanning electron microscopy (SEM), interference microscopy, moisture content and contact angle measurements, were used to examine the changes in chemical structure and surface morphology of the samples. During incubation in both enzymes the molecular weight of the polyurethane appeared to decrease in the presence of enzyme but increase in the presence of buffer. Further microphase separation in the polyurethane material developed during incubation in buffer solutions. Such changes in microstructure were associated with increased surface hydrophilicity, increased moisture content and a significant improvement in the extent of order and preferred orientation of the hard segment domains within the fibres. In the sampling depth of about 5 nm, both enzymes decreased the carbonate group content at the surface of the prosthesis to as little as 40% of their original values. The results from ATR-FTIR and DSC demonstrated that this phenomenon was limited primarily to the soft segment phase. While the Vascugraft prosthesis did exhibit some limited chemical modifications on exposure to concentrated enzyme solutions, nevertheless such changes were confined to the surface layer of the polyurethane microfibres. The importance and significance of those results will be more adequately determined by in vivo investigation.

Endothelial cell compatibility testing of three different Pellethanes

There is a need for viable small diameter vascular grafts, the luminal surface of which could be seeded by endothelial cells (ECs) to prevent thrombosis. In order to select candidates for EC seeding before implantation, the in vitro cytocompatibility of three different Pellethanes (polyetherurethanes) using human ECs was investigated. The methodology included two stages depending on either direct contact between cells and materials or contact between cells and material extracts, obtained under standardized conditions. By the latter method, we observed a cytotoxic effect on cell growth with 2363-55 D Pellethane extract at a 50% (v/v) concentration in the nutrient medium, likely provoked by leachables and correlated with the lowest levels of tPA, PAI1, and vWF antigens in the supernatants. By the former method, we studied EC attachment and growth. Morphology was studied by classical means and completed by scintigraphy and microautoradiography after 111Indium-labeling of the EC monolayer. Differentiation was determined by the release of vWF antigen and measurement of vWF activity (multimeric organization) after human thrombin stimulation. Despite an inhibition of proliferation for both 55 D and 75 D types (compared to the control), a functional monolayer of ECs was obtained on 75 D. Pellethane 75 D could be the best support for in vitro endothelization.

A novel microporous polyurethane blood conduit: biocompatibility assessment of the UTA arterial prosthesis by an organo-typic culture technique

An organotypic culture assay has been used to assess the biocompatibility and cytotoxicity of an arterial prosthesis developed at the University of Texas-Arlington (the UTA graft) from a structurally modified polyurethane (PU) elastomer (Tecoflex). The cell culture test was applied to the UTA graft after sterilization by ethylene oxide and by gamma radiation in two separate series. First, small specimens of the prosthesis were incubated for 7 days on a semisolid nutrient medium with their luminal surface in direct contact with endothelium explanted from the aorta of chick embryos. Second, the possibility of cytotoxic contaminants being leached from the polyurethane was assessed by immersing the biomaterial in the liquid culture medium for 5 days at 37 degrees C prior to conducting the organo-typic culture assay on a standard control surface. The structure of the UTA polyurethane prosthesis is porous, but the graft wall is impervious because it contains closed (i.e., noncommunicating) pores. In addition, four other vascular prostheses were included in the study for comparison. They were the Hydrophilic Mitrathane PU graft with a similar impervious, closed pore structure, an experimental Hydrophobic Mitrathane PU graft with a fibrous, open pore structure, and the commercial Impra and Reinforced Goretex expanded PTFE grafts. Following 7 days of cell culture, the biocompatibility and cytotoxicity of the various biomaterials were measured in terms of the area of migrating cells, the density of cells surrounding the explants, and the level of cell adhesion. Comparison of the results against control cultures demonstrated that the UTA graft, along with the other four prostheses, does not release cytotoxic extractables. Microscopic observations of its cultured surface indicated that the UTA graft promotes a high density of cell growth over a limited area, similar to the Hydrophilic Mitrathane graft. This level of biocompatibility is considered inferior to that of the two PTFE and the Hydrophobic Mitrathane prostheses, which promote more extensive cell migration, greater cell adhesion, and cell growth in a continuous single layer.