Investigating the Behavior of Thin-Film Formation over Time as a Function of Precursor Concentration and Gas Residence Time in Nitrogen Dielectric Barrier Discharge

This study aims to establish a correlation between the fragmentation process and the growth mechanisms of a coating deposited on a fluoropolymer. Deposition was carried out using dielectric barrier discharge at atmospheric pressure, employing an oxygen-containing organic precursor in a nitrogen environment. The findings reveal that the impact of precursor concentration on the formation of specific functionalities is more significant than the influence of treatment time. The X-ray photoelectron spectroscopy (XPS) results obtained indicate a reduction in the N/O ratio on the coating's surface as the precursor concentration in the discharge increases. Fourier transform infrared spectroscopy (FTIR) analysis, conducted in the spectral range of 1500 cm-1 to 1800 cm-1, confirmed the connection between the chemical properties of plasma-deposited thin films and the concentration of organic precursors in the discharge. Furthermore, the emergence of nitrile moieties (C≡N) in the FTIR spectrum at 2160 cm-1 was noted under specific experimental conditions.

Fragmentation Mechanism in a Nitrogen Dielectric Barrier Discharge Plasma on Fluoropolymer Polymer Films

Due to their chemical inertness and low friction coefficient, fluoropolymers are today widely employed in sectors of activity as diverse and distinct as the textile industry, architectural sector, and medicine. However, their low surface energy results in poor adhesion, for example, when used for a component in a composite device with multiple other materials. Among the techniques used to enhance their adhesion, atmospheric pressure discharges provide a fast and low-cost method with a reduced environmental impact. Although this approach has proven to be efficient, the different chemical and physical processes in the discharge remain not fully understood. In this study, fluoropolymer surfaces were modified using an atmospheric pressure dielectric barrier discharge in a nitrogen and organic precursor environment. To prevent any damage to fluoropolymer surfaces, the dissipated power in the discharges was tuned by applying a duty cycle. Evidence shows that plasma treatment allows for the incorporation of oxygen and nitrogen in the surface resulting in the formation of hydrophilic functionalities such as carbonyl groups both in ketone and amide form, amine, and hydroxyl groups after 180 s of treatment. Overall, the data reveal that the discharge duty cycle has more effect on the oxygen and carbon content in the coating than the precursor concentration. In addition, increasing the precursor concentration limits the molecular fragmentation and nitrogen incorporation into the coating. These experiments enable the building of a better fundamental understanding of the formation mechanism of such chemical moieties at the fluoropolymer surface.

Non-woven textiles for medical implants: mechanical performances improvement

Non-woven textile has been largely used as medical implant material over the last decades, especially for scaffold manufacturing purpose. This material presents a large surface area-to-volume ratio, which promotes adequate interaction with biological tissues. However, its strength is limited due to the lack of cohesion between the fibers. The goal of the present work was to investigate if a non-woven substrate can be reinforced by embroidery stitching towards strength increase. Non-woven samples were produced from both melt-blowing and electro-spinning techniques, reinforced with a stitching yarn and tested regarding several performances: ultimate tensile strength, burst strength and strength loss after fatigue stress. Several stitching parameters were considered: distance between stitches, number of stitch lines (1, 2 or 3) and line geometry (horizontal H, vertical L, cross X). The performance values obtained after reinforcement were compared with values obtained for control samples. Results bring out that reinforcement can increase the strength by up to 50% for a melt-blown mat and by up to 100% for an electro-spun mat with an X reinforcement pattern. However, after cyclic loading, the reinforcement yarn tends to degrade the ES mat in particular. Moreover, increasing the number of stitches tends to fragilize the mats.

Fibrous composite material for textile heart valve design: in vitro assessment

With over 150,000 implantations performed over the world, transcatheter aortic valve replacement (TAVR) has become a surgical technique, which largely competes with open surgery valve replacement for an increasing number of patients. The success of the procedure favors the research toward synthetic valve leaflet materials as an alternative to biological tissues, whose durability remains unknown. In particular, fibrous constructions have recently proven to be durable in vivo over a 6-month period of time in animal sheep models. Exaggerated fibrotic tissue formation remains, however, a critical issue to be addressed. This work investigates the design of a composite fibrous construction combining a woven polyethylene terephthalate (PET) layer and a non-woven PET mat, which are expected to provide, respectively, strength and appropriate topography toward limited fibrotic tissue ingrowth. For this purpose, a specific equipment has been developed to produce non-woven PET mats made from fibers with small diameter. These mats were assembled with woven PET substrates using various assembling techniques in order to obtain hybrid fibrous constructions. The physical and mechanical properties of the obtained materials were assessed and valve samples were manufactured to be tested in vitro for hydrodynamic performances. The results show that the composite fibrous construction is characterized by properties suitable for the valve leaflet function, but the durability of the assembling is however limited under accelerated cyclic loading.

Microbiome changes associated with sustained eradication of Clostridium difficile after single faecal microbiota transplantation in children with and without inflammatory bowel disease

BACKGROUND

Little data are available regarding the effectiveness and associated microbiome changes of faecal microbiota transplantation (FMT) for Clostridium difficile infection (CDI) in children, especially in those with inflammatory bowel disease (IBD) with presumed underlying dysbiosis.

AIM

To investigate C. difficile eradication and microbiome changes with FMT in children with and without IBD.

METHODS

Children with a history of recurrent CDI (≥3 recurrences) underwent FMT via colonoscopy. Stool samples were collected pre-FMT and post-FMT at 2-10 weeks, 10-20 weeks and 6 months. The v4 hypervariable region of the 16S rRNA gene was sequenced. C. difficile toxin B gene polymerase chain reaction was performed.

RESULTS

Eight children underwent FMT for CDI; five had IBD. All had resolution of CDI symptoms. All tested had eradication of C. difficile at 10-20 weeks and 6 months post-FMT. Pre-FMT patient samples had significantly decreased bacterial richness compared with donors (P = 0.01), in those with IBD (P = 0.02) and without IBD (P = 0.01). Post-FMT, bacterial diversity in patients increased. Six months post-FMT, there was no significant difference between bacterial diversity of donors and patients without IBD; however, bacterial diversity in those with IBD returned to pre-FMT baseline. Microbiome composition at 6 months in IBD-negative patients more closely approximated donor composition compared to IBD-positive patients.

CONCLUSIONS

FMT gives sustained C. difficile eradication in children with and without IBD. FMT-restored diversity is sustained in children without IBD. In those with IBD, bacterial diversity returns to pre-FMT baseline by 6 months, suggesting IBD host-related mechanisms modify faecal microbiome diversity.

Fourier transform infrared absorption spectroscopy characterization of gaseous atmospheric pressure plasmas with 2 mm spatial resolution

This paper describes an optical setup built to record Fourier transform infrared (FTIR) absorption spectra in an atmospheric pressure plasma with a spatial resolution of 2 mm. The overall system consisted of three basic parts: (1) optical components located within the FTIR sample compartment, making it possible to define the size of the infrared beam (2 mm × 2 mm over a path length of 50 mm) imaged at the site of the plasma by (2) an optical interface positioned between the spectrometer and the plasma reactor. Once through the plasma region, (3) a retro-reflector module, located behind the plasma reactor, redirected the infrared beam coincident to the incident path up to a 45° beamsplitter to reflect the beam toward a narrow-band mercury-cadmium-telluride detector. The antireflective plasma-coating experiments performed with ammonia and silane demonstrated that it was possible to quantify 42 and 2 ppm of these species in argon, respectively. In the case of ammonia, this was approximately three times less than this gas concentration typically used in plasma coating experiments while the silane limit of quantification was 35 times lower. Moreover, 70% of the incoming infrared radiation was focused within a 2 mm width at the site of the plasma, in reasonable agreement with the expected spatial resolution. The possibility of reaching this spatial resolution thus enabled us to measure the gaseous precursor consumption as a function of their residence time in the plasma.

Micropatterning with aerosols: Application for biomaterials

Adhesion and proliferation behaviors of bovine aortic endothelial cells (BAECs) were investigated on surfaces micropatterned with peptides using a novel approach. This micropatterning technique allows modification of macroscopic three-dimensional (3D) biomaterials surfaces and exploits the semi-random properties of aerosols and the principles of liquid atomization. The possibility to control cell behaviors on polytetrafluoroethylene (PTFE) surfaces tailored with this micropatterning approach was evaluated. CGRGDS and CWQPPRARI peptides were selected for their adhesive, migration and spreading properties. Culture of BAECs on patterned PTFE showed the possibility of modulating cell behaviors. The study showed that CGRGDS spots with a diameter of 10+/-2 microm over a background of CWQPPRARI peptides was the most effective combination to enhance endothelialization of PTFE. This micropatterning technique is innovative, easily adaptable, simple, and rapid for covering large 3D areas.

Development of an optimized electrochemical process for subsequent coating of 316 stainless steel for stent applications

Metallic endovascular stents are used as medical devices to scaffold biological lumen, most often diseased arteries, after balloon angioplasty. They are commonly made of 316L stainless steel or Nitinol, two alloys containing nickel, an element classified as potentially toxic and carcinogenic by the International Agency for Research on Cancer. Although they are largely implanted, the long-term safety of such metallic elements is still controversial, since the corrosion processes may lead to the release of several metallic ions, including nickel ions in diverse oxidation states. To avoid metallic ion release in the body, the strategy behind this work was to develop a process aiming the complete isolation of the stainless steel device from the body fluids by a thin, cohesive and strongly adherent coating of RF-plasma-polymerized fluoropolymer. Nevertheless, prior to the polymer film deposition, an essential aspect was the development of a pre-treatment for the metallic substrate, based on the electrochemical polishing process, aiming the removal of any fragile interlayer, including the native oxide layer and the carbon contaminated layer, in order to obtain a smooth, defect-free surface to optimize the adhesion of the plasma-deposited thin film. In this work, the optimized parameters for electropolishing, such as the duration and the temperature of the electrolysis, and the complementary acid dipping were presented and accurately discussed. Their effects on roughness as well as on the evolution of surface topography were investigated by Atomic Force Microscopy, stylus profilometry and Scanning Electron Microscopy. The modifications induced on the surface atomic concentrations were studied by X-ray Photoelectron Spectroscopy. The improvements in terms of the surface morphology after the pre-treatment were also emphasized, as well as the influence of the original stainless steel surface finish.

Improving arterial prosthesis neo-endothelialization: application of a proactive VEGF construct onto PTFE surfaces

The formation of a confluent endothelium on expanded polytetrafluoroethylene (PTFE) vascular prostheses has never been observed. This lack of endothelialization is known to be one of the main reasons leading to the development of thromboses and/or intimal hyperplasia. In this context, several efforts were put forward to promote endothelial cell coverage on the internal surface of synthetic vascular prostheses. The goal of the present study was to immobilize the vascular endothelial growth factor (VEGF) onto Teflon PTFE surfaces to generate a proactive polymer construct favoring interaction with endothelial cells. An ammonia plasma treatment was first used to graft amino groups on PTFE films. Subsequent reactions were performed to covalently bind human serum albumin (HSA) on the polymer surface and to load this protein with negative charges, which allows adsorbtion of VEGF onto HSA via strong electrostatic interactions. X-ray photoelectron spectroscopy (XPS) experiments along with surface derivatization strategies were performed between each synthesis step to ascertain the occurrence of the various molecules surface immobilization. Finally, the electrostatic binding of VEGF to the negatively charged HSA matrix was performed and validated by ELISA. Endothelial cell adhesion and migration experiments were carried out to validate the potential of this VEGF-containing biological construct to act as a proactive media toward the development of endothelial cells.

Understanding the biodegradation of polyurethanes: from classical implants to tissue engineering materials

After almost half a century of use in the health field, polyurethanes (PUs) remain one of the most popular group of biomaterials applied for medical devices. Their popularity has been sustained as a direct result of their segmented block copolymeric character, which endows them with a wide range of versatility in terms of tailoring their physical properties, blood and tissue compatibility, and more recently their biodegradation character. While they became recognized in the 1970s and 1980s as the blood contacting material of choice in a wide range of cardiovascular devices their application in long-term implants fell under scrutiny with the failure of pacemaker leads and breast implant coatings containing PUs in the late 1980s. During the next decade PUs became extensively researched for their relative sensitivity to biodegradation and the desire to further understand the biological mechanisms for in vivo biodegradation. The advent of molecular biology into mainstream biomedical engineering permitted the probing of molecular pathways leading to the biodegradation of these materials. Knowledge gained throughout the 1990s has not only yielded novel PUs that contribute to the enhancement of biostability for in vivo long-term applications, but has also been translated to form a new class of bioresorbable materials with all the versatility of PUs in terms of physical properties but now with a more integrative nature in terms of biocompatibility. The current review will briefly survey the literature, which initially identified the problem of PU degradation in vivo and the subsequent studies that have led to the field's further understanding of the biological processes mediating the breakdown. An overview of research emerging on PUs sought for use in combination (drug + polymer) products and tissue regeneration applications will then be presented.

Denatured collagen as support for a FGF-2 delivery system: physicochemical characterizations and in vitro release kinetics and bioactivity

Collagen-based materials have scaffold properties to support bioactive molecules such as growth factor (GF). Gelatin, a denatured collagen, may have also some potential to interact with GF. An alternative process to denature collagen using trifluoroacetic acid (TFA) was investigated. Physicochemical characterization (XPS, DSC, isoelectric point, water uptake) of TFA-denatured collagen was comparable to regular gelatin, except a significant hydrophilicity and a pH sensitivity. FGF-2 was mixed with either regular gelatin or TFA-denatured collagen, then incorporated to a collagen sponge. Autoradiography revealed a relatively homogenous distribution of radiolabeled FGF-2 within the sponge. In vitro release kinetic of radiolabeled FGF-2 was investigated as well as the bioactivity of FGF-2 towards endothelial cell growth. The mixture was also sorbed to hydrogels made of ethylene vinyl acetate co-polymer and poly(2-hydroxyethyl methacrylate), and to cell culture insert membranes as control. Release of FGF-2 from collagen was progressive in the presence of TFA-denatured collagen, and cell growth was stimulated (significant peak at 8 and 10 days) by TFA-denatured collagen and FGF-2 eluted particularly from collagen sponges. Whereas control hydrogels, and those with regular gelatin showed a early stimulation of cell growth (1-5 days). Thus, the combination of both FGF-2 and an acid-denatured collagen in collagen sponges allows to sustain in vitro endothelial cell activity.

Fourier transform infrared spectroscopy application to vascular biology: comparative analysis of human internal mammary artery and saphenous vein wall

Saphenous vein (SV) and internal mammary artery (IMA) are used for aorto-coronary bypass grafting. IMA is considered to be the graft of choice for coronary revascularization having a long-term patency compared to SV. The aim of this study is to investigate the structure of vascular wall using a new technical approach. We analysed the chemical composition of vessel wall layers (total lipid, lipid ester and protein) of 25 vascular segments (19 SV and 6 IMA) using Fourier transform infrared spectroscopy (FTIR). FTIR analysis showed that in intima layer lipid ester and protein concentration (expressed as arbitrary units) was significantly higher in SV (lipid ester = 0.020 +/- 0.002; protein = 0.449 +/- 0.022) than in IMA (lipid ester = 0.014 +/- 0.002; protein = 0.342 +/- 0.032). Moreover, the percentage of lipid ester on total lipid was significantly higher in SV (intima = 54.7 +/- 2.9%; media = 78.4 +/- 4.9%; adventitia = 83.9 +/- 8.3%) wall layers compared to IMA ones (intima = 37.3 +/- 4.9%; media = 45.4 +/- 3.8; adventitia = 57.1 +/- 4.8). These data suggest that a different chemical composition of wall layers could also be responsible for the morphological modifications observed in SV after grafting.

Totally implantable artificial hearts and left ventricular assist devices: selecting impermeable polycarbonate urethane to manufacture ventricles

In the development of a new generation of totally implantable artificial hearts and left ventricular assist devices (VADs) for long-term use, the selection of an acceptable material for the fabrication of the ventricles probably represents one of the greatest challenges. Segmented polyether urethanes used to be the material of choice due to their superior flexural performance, acceptable blood compatibility, and ease of processing. However, because they are known to degrade and to be readily permeable to water, they cannot meet the rigorous requirements needed for a new generation of implantable artificial hearts and VADs. Therefore, the objective of the present study was to identify alternative polymeric materials that would be satisfactory for fabricating the ventricles, and in particular, to determine the water permeability through membranes made from four commercial polycarbonate urethanes (Carbothane PC3570A, Chronoflex AR, Corethane 80A, and Corethane 55D) in comparison to those made from two traditional polyether urethanes (Tecoflex EG80A and Tecothane TT-1074A). In addition to determining the rate of water transmission through the six membranes by exposing them to deionized water, saline, and albumin-Krebs solution under pressure and measuring the displacement of liquid by means of a recently developed capillary method, the inherent surface and chemical properties of the six membranes were characterized by SEM, contact angle measurements, FTIR, DSC, and GPC techniques. The results of the study demonstrated that the rate of water transmission through the four polycarbonate urethane membranes was significantly lower than through the two polyether urethanes. In fact the lowest values were recorded with the two Corethane membranes, and the harder type 55D polymer had a lower value (2.7 x 10(-7) g/s cm2) than the softer 80A version (3.3 x 10(-7) g/s cm2). This level of water vapor permeability, which appears to be controlled primarily by a Fickian diffusion mechanism, is between 2 and 4 times lower than that obtained with traditional polyether urethane membranes of equivalent thickness. The superior performance of the polycarbonate urethanes is likely due to the inherently lower chain mobility of the carbonate structure in the soft segment phase. In addition, the study shows that additional impermeability to water vapor can be achieved by selecting a polyurethane polymer with a high hard segment content, an aromatic rather than aliphatic diisocyanate comonomer, and a more hydrophobic surface. The use of a higher molecular weight polyurethane is not necessarily efficacious if the above requirements are not met. As expected by Raoult's Law, the study found that the use of physiological media instead of deionized water further decreases the rate of water vapor transmission. Because none of today's commercial polyurethanes are totally impervious to water vapor transmission, additional work is needed to develop permeable polymers or to apply additional treatments to existing candidates to achieve an acceptable impermeable ventricle material.

Modeling lipid uptake in expanded polytetrafluoroethylene vascular prostheses and its effects on mechanical properties

The radial transport across the wall of expanded polytetrafluoroethylene (ePTFE) arterial prostheses has a significant effect on lipid uptake observed in prostheses implanted in humans, which has been postulated to be one of the causes associated with implant failure. The goal of this study was to stimulate radial transport on a lipidic dispersion across the wall of an ePTFE prosthesis and investigate its effects on the circumferential mechanical properties of the prosthesis. An in vitro model was developed to simulate the lipidic radial transport across the wall. Lipids contained in a phosphatidylcholine dispersion were used as the transported molecules. Lipid concentration profiles were obtained after exposing commercial ePTFE prostheses to various transmural pressure and/or lipidic concentration gradients. Phospholipids gradually accumulated up to the external reinforcing wrap of the prosthesis, which clearly acted as a rigid barrier against lipid infiltration. Tensile tests performed on the virgin samples showed that the wrap was much more rigid than the microporous part of the prosthesis. After the lipid simulation, the rigidity of the wrap decreased with respect to what was observed for the virgin prosthesis. Finally, some clinical implications of this phenomena are discussed.

Lipid uptake across the wall of an expanded polytetrafluoroethylene vascular graft

Previous studies have shown that vascular grafts were prone to inducing an atherosclerosis-like phenomenon, thus possibly jeopardizing their performance. Furthermore, lipid retention, observed in most synthetic arterial prostheses explanted from humans, appears to have an important role in the progression of this atherosclerotic process, therefore hindering the healing process and neo-intima formation of these synthetic conduits. The current study examined lipid concentration profiles across prosthesis membranes exposed to lipid dispersion under various transmural pressures, flow rates, and durations of exposure. It was demonstrated that the lipids rapidly permeated the prosthesis membrane, as lipid advection increased to a maximum, then steadily decreased until the membrane became completely impermeable to the fluid. The concentration of lipids within the grafts was monitored using FT-IR microspectroscopy, then correlated as a function of time in order to evaluate the mass transfer coefficients and lipid saturation concentration. Lipid sorption, as a function of time, was described by a mechanism taking into account two first-order kinetic models. The lipids were first rapidly adsorbed onto the Teflon(R), potentially influenced by the strong affinity of these lipids for the highly hydrophobic polytetrafluoroethylene polymer. This affinity then enhanced the germination of the lipid deposits that filled in the prosthesis wall. For lipid retention as a function of the transmural pressure and flow rate, no clear tendency was established.

Commercial polyurethanes: the potential influence of auxiliary chemicals on the biodegradation process

This investigation elucidates some aspects of auxiliary chemicals on the biodegradation of two commercial polyurethanes (Pellethane and Corethane). The materials were incubated for 28 days with cholesterol esterase and/or with phosphatidylcholine. Extraction studies were carried out on the two materials, using different solvents, chosen on the basis of solvent polarity. FT-IR spectra for the extracted materials indicated the presence of poly(methylene)n oxide moities, silicone oil, bis-ethylene-stearamide, aromatic moities, and alkyd-urea compounds in Pellethane. Corethane materials were shown to contain some fatty acids, hydrocarbon waxes, ester-based species, and chlorinated compounds. Analysis of incubation solutions by high performance liquid chromatography failed to isolate methylene dianiline (MDA) or any of its derivatives from the various polymer incubation solutions. However, a methanol extract of Corethane samples that were incubated for 28 days in cholesterol esterase did show the presence of MDA. The absence of MDA in the Pellethane methanol extracted samples may reflect the differences in surface additives found for this material versus the Corethane. FT-IR/ATR analysis of polymer surfaces exposed to cholesterol esterase/phospholipids mixture showed that there was an increase in the uptake of phospholipids over samples that were incubated in phospholipid dispersion alone. The results of this study show that some of the auxiliary chemicals found in commercial polyurethanes may hinder the specific release of hydrolytic degradation products and delay polymer degradation. However, it should be recognized that the surface layer containing these compounds is susceptible to change following the interaction between the polyurethane-based devices and elements of the host environment (i.e. lipids, enzymes, etc.). Hence, recognition and identification of these changes will ultimately be important in assessing a commercial polymer's blood compatibility characteristics.

A new generation of polyurethane vascular prostheses: rara avis or ignis fatuus?

Three polyurethane (PU) vascular grafts with novel designs were investigated and compared in terms of the microporous structure, reinforcement technology, polymer chemistry, microphase separation, and mechanical properties. The Corvita graft, composed of a poly(carbonate urethane) polymer, displayed a helically wound filament structure with communicating inter-fiber spaces. The reinforced model contained an external PET mesh impregnated with a protein sealant, and displayed good microphase separation, the highest Young's modulus in the longitudinal direction, and the second highest in the radial direction. The Thoratec graft was made of a polyetherurethaneurea with an average micropore size of 15 microns. Silicone was observed on both surfaces of the graft. The Thoratec device displayed a low degree of hydrogen-bonding among the urethane groups and had no well-organized hard-segment domains. Its mechanical strength was superior to that of the Pulse-Tec graft. A solid PU layer underneath the luminal surface precluded any communication between the luminal and adventitial sides. The Pulse-Tec prosthesis was composed of polyetherurethane, with an average micropore size of 28 microns. It offered the highest radial compliance, a high degree of hydrogen-bonding, a narrow molecular weight distribution, and a certain degree of microphase separation. Its tensile strength and hysteresis loss were inferior to those of the other two grafts.

Lipid uptake in synthetic vascular prostheses explanted from humans

Previous in vivo studies in humans and dogs have revealed an atherosclerosis-like phenomenon in which lipid penetration within arterial prosthesis wall was observed. The primary goal of the present study was therefore to investigate the occurrence of this lipid retention in ePTFE prostheses implanted in humans and therefore identify potential risk factors related to this phenomenon. Lipid uptake in 367 ePTFE microporous vascular prostheses explanted from humans was studied using Fourier transform infrared spectroscopy. The assignment of the infrared absorption features clearly revealed the presence of strongly bonded unsaturated fatty acids to the microporous structure of the prostheses. A one-way ANOVA statistical analysis showed that the lipid uptake in the synthetic vascular prostheses depended on the duration of implantation of the prosthesis and on the sex of the patient. A two-way ANOVA showed that a relationship existed between the estimated lipid uptake and the internal diameter of the prosthesis. These results confirm that the lipid uptake phenomenon depends on some clinical factors related either to the patients or to the prostheses' morphological parameters.

In vitro cellular response to polypyrrole-coated woven polyester fabrics: potential benefits of electrical conductivity

Electrically conducting polypyrrole-treated films have recently been shown to influence the morphology and function of mammalian cells in vitro. This type of polymer represents a possible alternative biomaterial for use in vascular implantation. The present study compared the in vitro biocompatibility of the five different polyester woven fabrics having increasing levels of electrical conductivity ranging from 4.5 x 10(4) to 123 omega/square with that of low density polyethylene and polydimethylsiloxane primary reference materials. Biocompatibility was measured in terms of four different types of in vitro cellular response, including (a) an indirect and (b) a direct control organotypic culture assay using endothelial cells, (c) a polymorphonuclear (PMN) cell activation study using flow-cytometric measurements of CD11/CD18 integrin molecule expression, and (d) a semiquantification of interleukin (IL)-6 mRNA expression on monocytes/macrophages using reverse-transcriptase polymerase chain reaction. The organotypic culture study revealed that the fabrics with high levels of conductivity exhibited lower cell migration, proliferation, and viability. The PMN activation study of blood from 10 healthy adult donors demonstrated that the two most conductive fabrics were able to identify the more reactive donors. The levels of IL-6 mRNA expression by monocytes/macrophages decreased as the conductivity level of the fabrics increased. The results of the present study therefore indicate that high levels of conductivity (< 200 omega/square) on polyester fabrics are detrimental to the growth, migration, and viability of endothelial cells; induce elevated PMN activation; and affect the intracellular metabolism of monocytes. They also point to a specific range of conductivity (10(3) < 10(4) omega/square) which is associated with an optimum in vitro cellular response.

A continuous and pulsatile flow circulation system for evaluation of cardiovascular devices

The design of a nonpulsatile and pulsatile system using a centrifugal pump is presented. To induce a pulsatile flow with a centrifugal pump, an independent pneumatically driven unit provided flow patterns over a wide range of frequencies and amplitudes. The pulsatile flow was generated by the axial displacement of a cylinder that periodically compressed the flexible conduit that is connected to the pump. The system can accommodate flow rates up to 6,000 ml/min and transmural pressures up to 500 mm Hg and is capable of maintaining the pressure at a constant value. This circuit produced reproducible pressure waves having a frequency up to 4 Hz. The periodicity of the transmural pressure between 80 and 180 mm Hg was similar to the pressure wave propagation observed in peripheral circulation. Capable of adequately reproducing continuous and pulsatile flow, the apparatus is therefore versatile to allow in vitro evaluation of cardiovascular devices.

Lipid uptake in expanded polytetrafluoroethylene vascular grafts

PURPOSE

The mechanisms of vascular prosthesis failure are reported to be associated, in part, with an atherosclerotic degenerative process that is related to an abnormal lipid infiltration. The lipid uptake in expanded polytetrafluoroethylene (ePTFE) vascular grafts was reproduced in vitro, and the effect of time on the permeability of these prostheses was studied.

METHODS

Water permeability tests were carried out under dynamic flow conditions at various hydrostatic pressures. Lipid uptake was simulated by circulating a phosphatidylcholine suspension inside an expanded Teflon prosthesis under pulsatile or continuous transmural pressure ranging between 80 mm Hg and 180 mm Hg, at a flow rate of 500 mL/min and 2000 mL/min, for a duration ranging from 2 hours to 1 month.

RESULTS

Water permeability tests indicated that under hydrostatic pressures of 180 mm Hg and 300 mm Hg, water percolated through the prosthesis wall after an exposure of 720 minutes and 75 minutes, respectively. After exposing the prostheses to the lipid dispersion under the various flow conditions, the fluid convection through the wall occurred. Preferential convection pathways with a constant periodicity were observed across the length of each prosthesis and were, therefore, associated with regularly spaced perforations depicted in the structure of the devices. Phospholipids gradually agglomerated within the prosthesis wall, allowing a restrictive molecular mobility. Infrared spectroscopy results indicated that the lipid uptake depended on the transmural pressure and time of exposure.

CONCLUSION

The occurrence of the membrane permeability may be associated with the dilatation and plastic deformation of the prosthesis. Lipid uptake occurs in ePTFE grafts after an aggressive kinetic process.

Heterogeneous PHPMA hydrogels for tissue repair and axonal regeneration in the injured spinal cord

A biocompatible heterogeneous hydrogel of poly[N-(2-hydroxypropyl) methacrylamide] (PHPMA) showing an open porous structure, viscoelastic properties similar to the neural tissue and a large surface area available for cell interaction, was evaluated for its ability to promote tissue repair and axonal regeneration in the transected rat spinal cord. After implantation, the polymer hydrogel could correctly bridge the tissue defect, from a permissive interface with the host tissue to favour cell ingrowth, angiogenesis and axonal growth occurred within the microstructure of the network. Within 3 months the polymer implant was invaded by host derived tissue, glial cells, blood vessels and axons penetrated the hydrogel implant. Such polymer hydrogel matrices which show neuroinductive and neuroconductive properties have the potential to repair tissue defects in the central nervous system by promoting the formation of a tissue matrix and axonal growth by replacing the lost of tissue.

Use of supercritical fluid extraction as a method of cleaning anterior cruciate ligament prostheses: in vitro and in vivo validation

The process of supercritical fluid extraction (SFE) using carbon dioxide as the mobile phase is finding increasing numbers of applications in a wide variety of industries for the extraction, separation, and cleaning of materials. This study assessed the usefulness of this approach in removing surface contaminants from a knitted polyester anterior cruciate ligament (ACL) prosthesis before packaging and sterilizing the product during manufacture. The physical, dimensional, and chemical properties of SFE treated compared with commercially scoured control samples were characterized using a number of textile test methods: electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry, and solvent extraction analysis. The biocompatibility of the samples was measured in terms of their ability to generate CD18 integrin expression on activated human polymorphonuclear cells, and their inflammatory response when implanted for up to 30 days in the knee joint of rats. SFE treatment was successful in removing most of the nonpolar contaminants from the ACL prosthesis and reducing the amount of residuals to a commercially acceptable level. However, some nitrogen containing compounds and polar salts were not removed by the SFE process. The results from the biocompatibility tests demonstrated that the cleaner SFE treated prosthesis induced significantly lower CD18 expression than the scoured control fabric, and was also associated with a milder inflammatory response and a more rapid rate of healing during the 30 day animal trial. Another effect of SFE processing was to cause the polyester device to shrink and lose porosity because of yarn contraction and modification of the polymer's microcrystalline structure.

Comparison of the in vivo behavior of polyvinylidene fluoride and polypropylene sutures used in vascular surgery

To find a nonabsorbable suture material that is equivalent to polypropylene in ease of handling and tensile properties, and that has low thrombogenicity and tissue reactivity but improved biostability, some researchers and clinicians see merit in considering the suitability of monofilaments made from polyvinylidene fluoride. The current animal study investigated the relative biocompatibility and biostability of these two suture materials by using them to anastomose a polyester arterial prosthesis in a canine thoracoabdominal bypass model for 10 periods of implantation ranging from 4 hr to 2 years. Biocompatibility was assessed with light and scanning electron microscope examinations of the explanted sutures, and biostability of the cleaned sutures was determined by Fourier transform infrared spectroscopy and scanning electron microscope analysis. The polyvinylidene fluoride and polypropylene sutures were found to have similar handling and healing characteristics. During the first months in vivo, both types of suture experienced a temporary increase in carbonyl group absorption that coincided with the duration of the inflammatory response. After 1 and 2 years in vivo, the explanted polypropylene sutures showed visual evidence of surface stress cracking. This was not found with the explanted polyvinylidene fluoride sutures. These results suggest that polyvinylidene fluoride may be more biostable than polypropylene in the long term.

Lipid concentration profile across the wall of pseudoatherosclerotic synthetic arterial prostheses using FTIR microspectroscopy

FTIR microscopy is a versatile technique successfully used to probe the subcellular chemical composition of atherosclerostic arterial walls. To design new vascular substitutes that resist lipid uptake (the major cause of the phenomenon referred to as atherosclerosis-like), identifying and understanding lipid distribution within the pseudoatherosclerosed arterial prostheses is of prime importance. Until now, the amount of lipids present within arterial prostheses that had been explanted from either animals (during in vivo trials) or humans (after the failure of vascular grafts) or had been submitted to in vitro investigations could only be measured through the use of histological techniques or radioactive labeling methods. We present here a novel method to quantitatively measure the lipid concentration profile within the wall of arterial prostheses by means of Fourier transform infrared microspectroscopy. Essentially, prostheses are fixed in a 1% osmium tetraoxide aqueous solution under vacuum and radially cut with a 5-micron thickness with a microtome. The sections are then placed onto BaF2 windows and observed with a microscope attached to a FTIR spectrometer with a 30 microns x 50 microns sampling area. The lipid concentration profile is obtained by scanning the prosthesis wall from the inner to the outer surface and reporting the corresponding integrated absorbance between 2700 and 3100 cm(-1) against a calibration curve. The application of this technique constitutes the first quantitative measurement of the concentration of biological molecules within the wall of artificial arterial substitute.

Identification and quantification of the crystalline structures of poly(vinylidene fluoride) sutures by wide-angle X-ray scattering and differential scanning calorimetry

The outstanding biocompatibility of the polyvinylidene fluoride (PVDF) monofilament suture together with other desirable characteristics, such as ease of handling and resistance to biodegradation, makes it an attractive alternative monofilament suture material for cardiovascular surgery. However, to achieve a high performance suture, the polymeric raw material must be exposed to different treatments, which lead to different degrees and types of crystallization. Since these crystalline modifications deeply influence the mechanical characteristics and the biostability of the sutures, the authors hereby propose a method of quantifying the different structures of PVDF using wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The commercial devices are achieved by coloring and processing the polymeric raw material. The white and unprocessed 4-0 unswaged suture presents 19% of the alpha phase, 38% of the beta structure, and no gamma form. Coloration increases the amount of the beta phase by 5-9% at the expense of the alpha phase. On the other hand, processing the fibers lead to the conversion of some of the amorphous phase to the gamma structure, the importance of which is 6-7%. Finally, tensile measurements performed on the different PVDF fibers clearly proves that their mechanical characteristics depend on the presence of these crystalline forms in the polymeric structure of PVDF.

A capillary method to measure water transmission through polyurethane membranes

A capillary method has been developed to measure the rate of water transmission through polyurethane membranes prepared for use as ventricles in artificial hearts. The system consisted primarily of a leak-proof sample chamber containing the water, a glass capillary flow meter, and a receiver compartment with continuous dry air ventilation. The capillary flow meter monitored the volume of water loss in the sample chamber. The rate of water transmission through the test membrane was found to be proportional to the water loss in the sample chamber, and dependent on the membrane thickness. For thicknesses from 0.09 mm to 0.34 mm, water vapor transmission rates ranged from 7.53 x 10(-8) to 2.76 x 10(-8) mol/s cm2, respectively. Although the concentration of water vapor in the receiver compartment did affect the rate of water vapor transmission through the membrane, within the pressure range 50-200 mmHg, there was very little effect. These findings suggest that water transmission through a polyurethane membrane is dominated by a diffusion process rather than by bulk convection.

Chemical inactivators as sterilization agents for bovine collagen materials

The use of collagen as a biomedical implant raises safety issues with regard to viruses and prions. Specific chemical agents that inactivate prion infectivity could be applied to collagen implants. The physicochemical changes and the in vitro and in vivo biocompatibility of collagen treated by formic acid (FA), trifluoroacetic acid (TFA), tetrafluorethanol (TFE), and hexafluoroisopropanol (HFIP) were investigated. In addition, the effects of these treatments on nucleic acids incorporated in collagen were analyzed. The molecules of FA and, more important, of TFA remained within collagen. FA, TFA, and HFIP treatments modify the secondary structure of collagen, as shown by Fourier transform infrared spectroscopy, while TFE does not. Differential scanning calorimetry measurements showed a decrease in the denaturation temperature compared to untreated collagen. However, resistance to collagenase was modified only after HFIP treatment. In vitro, cell growth was not impaired; in vivo, implants induced a temporary inflammatory reaction that was prolonged with TFA and HFIP treatments. TFE and FA-treated collagen were thoroughly infiltrated by fibroblasts. On the other hand, FA and TFA resulted in extensive depurination of nucleic acids while HFIP and TFE did so to a lesser degree. Among the investigated chemical scrapie inactivators, FA treatment could offer a safe and biocompatible collagen-derived material for biomedical use.

Chemical stability of polyether urethanes versus polycarbonate urethanes

The relative chemical stability of two commercially available polyurethanes-Pellethane, currently used in biomedical devices, and Corethane, considered as a potential biomaterial-was investigated following aging protocols in hydrolytic and oxidative conditions (HOC, water, hydrogen peroxide, and nitric acid) and in physiological media (PHM, phosphate buffer, lipid dispersion, and bile from human donors). The chemical modifications induced on these polymers were characterized using differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and Fourier transform infrared spectroscopy (FTIR). With the exception of nitric acid, all of the aging media promoted a mild hydrolytic reaction leading to a slight molecular weight loss in both polymers. When aged in water and hydrogen peroxide, Pellethane experienced structural modifications through microdomain phase separation along with an increase of the order within the soft-hard segment domains. The incubation of Pellethane in nitric acid also resulted in an important decrease of the melting temperature of its hard segments with chain scission mechanisms. Moreover, incubation in PHM led to an increase of the order within shorter hard-segment domains. FTIR data revealed the presence of aliphatic amide molecules used as additives on the Pellethane's surface. The incubation of Corethane under the same conditions promoted an almost uniform molecular reorganization through a phase separation between the hard and soft segments as well as an increase of the short-range order within the hard-segment domains. Incubation of this polymer in nitric acid also resulted in a chain scission process that was less pronounced than that measured for the Pellethane samples. Finally, lipid adsorption occurred on the Corethane sample incubated in bile for 120 days. Overall data indicate that polycarbonate urethane presents a greater chemical stability than does polyetherurethane.

In vitro and in vivo studies of a polyester arterial prosthesis with a warp-knitted sharkskin structure

The present study was undertaken to assess the performance of a new knitted and gelatin-sealed polyester vascular graft that is believed to have greater dimensional stability than current commercial devices. Samples of the uncrimped, crimped, and sealed prosthesis were submitted to a series of in vitro and in vivo trials. Four commercial polyester knitted devices were included as controls for the in vitro tests, which included measurements of the textile and yarn structure and physical, chemical, and thermal properties of the graft, such as water permeability, dilatation, suture retention strength, melting point, and crystallinity index. The in vivo evaluation involved implanting the prototype device as a canine thoraco-abdominal bypass for periods ranging from 4 h to 1 year and assessing the biocompatibility, biofunctionality, and biostability of the explanted specimens. The warp-knitted structure of the prototype device has a unique sharkskin stitch that confers a superior dilatation resistance and suture retention strength to the prosthesis. The animal trial demonstrated that the gelatin ensures initial hemostasis without preclotting. The gelatin is bioresorbed during the first 2 weeks of implantation, which generates a temporary, moderate, acute inflammatory response. An external capsule of granulomatous tissue and an internal collagen capsule are formed between the first and third month. Analysis of the textile and physical properties of the explanted prostheses confirmed there was neither dilatation nor significant changes in structure or mechanical performance during implantation, thus confirming the biostability of this new prototype device and opening the way for clinical trials.

Are intraaortic balloons suitable for reuse? A survey study of 112 used intraaortic balloons

To assess the safety of reusing single-use intraaortic balloon devices (IABs), 112 used devices were investigated in terms of physical integrity, gas leakage inspection, mechanical performance, surface chemistry and morphology, and physical stability. These IABs were all used clinically only once, and the duration of the IABs in vivo ranged from 6 to 312 h. Macroscopic examination of the balloons and the outer catheters revealed no obvious change in either shape or color. No discernible abrasions or cracks were observed on the balloons. However, 61% of the balloons were creased, and 40% of the central lumens and 21% of the sheaths showed visible bending flaws. Moreover, 65% of the balloons and 38% of the central lumens were contaminated by visible residual organic debris. The physical integrity of each device was verified in a specially designed leakage-fatigue tester for 72 h. Ninety-seven percent of the devices passed the leakage inspection. Stress-strain testing, differential scanning calorimetry, attenuated total reflection-Fourier transform infrared, and scanning electron microscopy analyses clearly indicated that there were no significant differences in the mechanical properties, bulk material morphology, surface chemistry, and external surface morphology between the used balloons and virgin controls. Although some surface modifications occurred on the internal side of the balloons, the external surfaces of most balloons suffered no trauma. Most of the used IABs examined in this study maintained physical and mechanical properties similar to those of the virgin devices. The chemistry of the balloon material was stable after short-term in vivo use. However, it does not seem possible to establish a rigorous protocol of cleaning, sterilization, and inspection to guarantee a safer reuse of these devices. The presence of residual organic debris that cannot be eliminated results in an imperative preclusion not to reuse the IABs.

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.

Polyester Arterial Prostheses

To evaluate recent developments in the design and production of polyester vascular prostheses in eastern Europe, a series of in vitro physical and chemical tests and an in vivo study was performed on three new prototype devices from the Czech Republic and one from Poland. The in vitro results for these four prostheses, referred to as the Ra-1n (warp knitted, uncrimped), Ra-1v (warp knitted, crimped), Mikrofroté (weft knitted, uncrimped), and Dallon (warp knitted, crimped) prostheses, were compared against values for three commercial devices of western origin, namely the Triaxial, the Vasculour II, and the Cooley II grafts. The animal trial involved implanting the four prototype devices as a thoracoabdominal bypass in dogs for eight different periods ranging from 4 hrs to 6 months and undertaking histologic and structural investigations on the retrieved grafts. Because of its poor long-term dimensional stability in vivo, the continued use of a weft knitted structure, like the Mikrofroté prosthesis, is to be deprecated. Conversely, the introduction of a more dimensionally stable warp knitted structure in three prototypes is to be acknowledged. However, the presence of surface contaminants was most likely responsible for the excessive inflammatory reaction generated by all four prostheses during the first month in vivo, which resulted in delayed healing performance. In addition, an unusually high surface carbon-oxygen ratio suggests that the crimping process needs further refinement. Improved cleaning and packaging procedures are essential before these products can complete against existing commercial prostheses of western origin. In conclusion, these new developments illustrate that the technology of warp knitting, which is now spreading worldwide, should be evaluated.

In vivo characterization of a fluoropassivated gelatin-impregnated polyester mesh for hernia repair

The present study was undertaken to evaluate a new prototype mesh that consists of a knitted polyester structure treated with a fluoropolymer and impregnated with gelatin. The Fluoropassiv mesh, as well as two controls, the Surgipro polypropylene mesh and the Gore-Tex expanded polytetrafluoroethylene patch, were used for the repair of experimentally induced abdominal hernias in piglets and followed for scheduled implantation periods of 4, 15, and 60 days. At the sacrifice the mesh and surrounding tissue were excised for histological assessment of the healing sequence, for the identification of changes in hematologic and immunological characteristics, and for the measurement of the mechanical properties. After cleaning to remove the encroaching tissue, the explanted devices were monitored for biostability by infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The present study has demonstrated that the Fluoropassiv mesh provides adequate mechanical strength and compares favorably with the two controls. No exacerbated systemic or in situ hematologic or immunological reactions were observed with either the meshes of the patch material. Histological studies revealed that thick collagenous and vascularized tissue were well anchored to the three biomaterials as early as 15 days after implantation. The degree of tissue penetration differed depending on the device. Chemically, they proved stable over time.

Chemical and morphological analysis of explanted polyurethane vascular prostheses: the challenge of removing fixed adhering tissue

During in vivo experiments to evaluate the biocompatibility and biostability of alternative biomaterials, the ideal protocol for the handling and preservation of the explanted material is often compromised in order to meet the needs of both the pathologist and the materials scientist. Explants surrounded by tissue are often fixed in formalin or glutaraldehyde to facilitate later pathological and histological analysis, but the subsequent removal of such fixed tissue from thermally sensitive and less chemically stable polymers, such as polyurethanes, poses major problems for the materials scientist, who does not wish to modify the chemical, physical or morphological characteristics of the underlying biomaterial. The present study has attempted to find a solution to this problem by exposing virgin specimens of the microporous polyurethane Vascugraft vascular prosthesis to six different cleaning conditions, all known to be effective in removing fixed tissue. These conditions included the use of 20% aqueous potassium hydroxide solution for 48 h at room temperature, 5% sodium bicarbonate solution for 5 min at the boil, and 9, 10, 11 and 12N hydrochloric acid for 48 h at room temperature. The appearance and chemical properties of the virgin and treated specimens were compared using electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, gel permeation chromatography for molecular weight and differential scanning calorimetry techniques. The use of temperatures close to the boil resulted in the formation of a translucent, rubbery material with gross changes in the microporous and microfibrous structure. The strongly acidic and alkaline conditions caused a loss in the surface carbonate group content. In addition, 12N hydrochloric acid reduced the molecular weight and urethane content. Consequently, 9N hydrochloric acid is recommended as the cleaning agent of choice for removing fixed tissue from this type of microporous polyurethane. Control experiments on virgin material should also be included in any cleaning protocol.

In vitro characterization of a fluoropassivated gelatin-impregnated polyester mesh for hernia repair

The surgical management of abdominal hernias requires prosthetic grafting in situations where the defect is too large or the surrounding tissue is not available for repair. Flat patches made of different biomaterials have been used in textile or microporous forms. The present work describes the results of an in vitro study comparing the morphological, mechanical, and chemical characteristics of a new textile prototype, Fluoropassiv, made of polyester fibers treated with a fluoropolymer and impregnated with gelatin to those of seven existing commercial meshes and patches made from polypropylene, polyester, polytetrafluoroethylene (PTFE) yarns, and expanded microporous PTFE graft. The morphological study revealed a diversity of structures having a minimal relative porosity of 70%, high bursting, and suture retention strengths in comparison with natural muscular tissue. Elasticmoduli proved to depend more on the direction of the textile the rigidity was higher for those materials having tight structure, like the Fluoropassiv and the Surgipro meshes (> 30 MPa), whereas those with more open structures, such as the Marlex, Trelex, Lars, Bard Teflon, and GoreTex structures, showed lower elastic modulus (10 mPa). In addition, chemical analyses confirmed no irregularities in the polymers used in all prostheses and demonstrated that the fluoropolymer coating of the Fluoropassiv was uniformly distributed. The innovative aspects in the construction of the knitted fabric Fluoropassiv appears to make it suitable for repairing hernias, and the inclusion of both continuous fluoropolymer surface treatment of polyester fibers and gelatin impregnation appears to improve the healing process.

Selecting valid in vitro biocompatibility tests that predict the in vivo healing response of synthetic vascular prostheses

We have investigated the usefulness of six in vitro biocompatibility tests in predicting the healing performance of polyester vascular prostheses as observed in previous canine in vivo trials. Vascular grafts were evaluated by using (i) a direct contact (DC) assay, (ii) an extract dilution (ED) assay on murine fibroblast cells, (iii) a DC assay on endothelial cells, (iv) a complement activation study, (v) a leucocyte activation study of CD18 integrin subunit expression on human polymorphonuclear cells (PMNs) and (vi) interleukin-2 receptor expression on lymphocytes. Uncleaned polyester grafts had previously been associated with poor healing and gelatin-impregnated polyester grafts with delayed but satisfactory healing, whereas commercially cleaned polyester grafts had demonstrated excellent healing. Lightweight and heavyweight knitted and woven polyester grafts supplied specifically for this project were studied, each with a different surface condition, i.e. commercially available (CP), uncleaned (UP) and impregnated with gelatin (GP). The UP grafts induced fibroblast cytotoxicity according to the ED assay, poor migration and viability of endothelial cells, and an elevated expression of CD18 and interleukin-2 receptor on PMNs and lymphocytes, respectively. In contrast, the CP grafts promoted good endothelial cell growth, no evidence of cytotoxicity and a weaker cell activation, and the GP grafts were found to be non-cytotoxic, to exhibit a good cellular response and to moderate cell activation. The complement activation assay and the DC assay on fibroblasts were found to be less useful and less discriminating. From this, it is concluded that the two cell activation measurements, the DC assay on endothelial cells and ED assay on fibroblasts, are useful in predicting the in vivo healing response of arterial polyester substitutes.

Polyvinylidene fluoride (PVDF) as a biomaterial: from polymeric raw material to monofilament vascular suture

This study identified the effects of various manufacturing processes on the crystalline microstructure, mechanical properties, and biocompatibility of a polyvinylidene fluoride (PVDF) suture. To achieve this, changes in the crystalline microstructure and the tensile behavior of PVDF monofilaments were monitored in vitro after different thermal processing, coloration, and sterilization treatments. In addition, the in vivo biocompatibility of the manufactured and sterilized PVDF suture was assessed by using it to anastomose a preclotted polyester vascular prosthesis as a thoracoabdominal bypass in a series of dogs. The tissue response was followed by histologic and scanning electron microscopy over implantation periods ranging from 4 h to 6 months. Differential scanning calorimetry and infrared spectroscopy (FTIR-ATR) showed that thermal processing and the addition of a coloring agent had a direct effect on modifying the crystalline microstructure and hence changing the mechanical properties. For example, thermal processing converted some of the alpha phase into the beta and gamma polymorphs, whereas coloration led only to a major increase in the beta-to-alpha ratio. The tensile properties were found to be optimized when the relative proportion of the beta and gamma phases combined compared to the alpha form gave rise to an FTIR A509/A532 absorption ratio between 4.0 and 4.5. Sterilization was found to cause some modifications to the crystalline microstructure near the surface of the monofilaments, but it did not change their mechanical properties. Pathologic examination of the anastomotic regions after different periods of implantation revealed a minimal cellular response, with no mineralization, intimal hyperplasia, or excessive fibrous tissue reaction. This good biocompatibility, together with other desirable characteristics such as ease of manipulation and satisfactory mechanical strength, makes PVDF an attractive alternative monofilament suture material for cardiovascular surgery.

Polyvinylidene fluoride monofilament sutures: can they be used safely for long-term anastomoses in the thoracic aorta?

Polyvinylidene fluoride (PVDF) represents an attractive alternative to polypropylene as a monofilament vascular suture because of its satisfactory physicochemical properties, it ease of handling, and its good biocompatibility. However, the polymer's ability to remain mechanically and chemically stable when exposed to a mild hydrolytic environment over the long term has yet to be demonstrated. One in vitro study involved the comparison of the long-term relative resistance of PVDF and polypropylene sutures to hydrolysis for a period of 9 years. The PVDF suture showed major molecular rearrangements from the original ratio of three crystalline structures to the single beta crystalline phase. The observation of some surface oxidation and water inhibition did not significantly modify the tensile strength of the PVDF suture, which retained 92.5% of its original value. In contrast, the polypropylene sample did not undergo any recrystallization but was associated with more oxidation byproducts and more water molecules near the surface, which contributed to a 46.6% loss in initial tensile strength. An in vivo study confirmed that PVDF sutures are biocompatible and are able to maintain satisfactory biostability when used to anastomose thoracic aortic allografts for a period of 6 months in the dog. The cellular reaction of fresh allografts as well as the control autografts to PVDF sutures was minimal. In other allografts that had been preserved in a supplemented medium for 1 week prior to implantation, the PVDF sutures healed satisfactorily with the formation of neocollagen and few macrophages surrounding the monofilament. No evidence of instability at the allograft-host artery junction was observed, confirming that the PVDF sutures were able to ensure a secure anastomosis in the thoracic aorta. PVDF sutures have demonstrated superior long-term biostability in vitro and minimal tissue response in vivo. These are two essential requirements when evaluating the use of a suture for vascular surgery in general and thoracic aortic surgery in particular.

The gelweave polyester arterial prosthesis

OBJECTIVE

To determine the effect of the gelatin coating on the efficacy of Gelweave, a new gelatin-sealed woven polyester graft material, as an arterial prosthesis.

DESIGN

In-vitro and in-vivo studies of the prosthesis.

SETTING

A laboratory of experimental surgery in a university teaching institution.

SUBJECTS

After in-vitro testing of the material, eight dogs were subjected to a series of in-vivo tests to evaluate the properties of Gelweave in comparison with its unsealed precursor and a commerically available collagen-coated woven polyester prosthesis.

INTERVENTION

Implantation of the prosthesis as a thoracoabdominal bypass for prescheduled periods ranging from 4 hours to 6 months.

MAIN OUTCOME MEASURES

Physical and chemical properties of the virgin prosthesis compared with the other two prostheses, effects of the gelatin-sealed prosthesis on healing, the hematologic characteristics of the dogs before operation and at sacrifice, microscopic studies, fibrin and platelet uptakes, prostaglandin secretion, and properties of the Gelweave grafts removed at varying periods after implantation.

RESULTS

The gelatin sealant in the Gelweave prosthesis effectively reduced the water permeability of the new prototype to zero. Neither blood loss at implantation nor infection during the postimplantation period was observed. The gelatin impregnation did not cause any adverse response in the dogs and was completely lysed within 2 weeks, thus allowing encapsulation and graft healing to progress satisfactorily. After 2 weeks, the prostacyclin:thromboxane ratio was greater than 1.0, whereas the fibrin and platelet uptakes on the luminal surface of the Gelweave grafts remained low, regardless of the period of implantation. Analysis of the explanted grafts confirmed that this gelatin-sealed prototype prosthesis healed satisfactorily and no adverse biologic response occurred as a result of the gelatin coating. It maintained its biostability during 6 months in situ.

CONCLUSION

The new Gelweave arterial prosthesis is ready for clinical use as a thoracic and abdominal vascular substitute.

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.

Porosity and biological properties of polyethylene glycol-conjugated collagen materials

Collagen-based materials can be designed for use as scaffolds for connective tissue reconstruction. The goal of the present study was to evaluate the behavior of collagen materials as well as cell and tissue reactions after the conjugation of activated polyethylene glycols (PEGs) with collagen. It is known that proteins conjugated with PEGs exhibit a decrease in their biodegradation rate and their immunogenicity. Different concentrations and molecular weights of activated PEGs (PEG-750 and PEG-5000) were conjugated to collagen materials (films or sponges) which were then investigated by collagenase assay, fibroblast cell culture, and subcutaneous implantation. PEG-conjugated collagen sponge degradation by collagenase was delayed in comparison to untreated sponges. In culture, fibroblasts with a normal morphology reached confluency on PEG-conjugated collagen films. In vivo, the porous structure of non-modified sponges collapsed by day 15 with a few observable fibroblasts between the collagen fibers. In PEG-modified collagen sponges, the porous structure remained stable for 30 days. Cell infiltration was particularly enhanced in PEG-750-conjugated collagen sponges. In conclusion, PEGs conjugated onto collagen sponges stabilize the porous structure without deactivating the biological properties of collagen. These porous composite materials could function as a scaffold to organize tissue ingrowth.