Characterization of Silicone-polycarbonate-urethane/PDMS based Material for Polymeric Heart Valves

INTRODUCTION Worldwide the increasing number of deaths per year caused by heart valve diseases, made valve replacement the most common surgical therapy. The research for polymeric heart valves (PHV) has been proposed to overcome problems such as no physiological flow conditions, calcification and limited durability of the currently available heart valves prostheses (mechanical or biological). Segmented polyurethanes have been utilized in medical devices since years due to their established biocompatibility and excellent mechanical properties, but their tendency to degradation hampered their use in long-term implantation. The aim of this work was to study a thermoplastic copolymer chain of polycarbonate-polyurethane and silicone [polydimethylsiloxane (PDMS)] modified with increasing percentages of extra-chain PDMS, for the development of a novel single-body polymeric trileaflet PHV made by a spray, phase-inversion technique. EXPERIMENTAL METHODS CarboSil® (CS) in grain form was dissolved in THF/DMAc 1:1 (v/v) to obtain a 2% (w/v) solution. CarboSil® solutions containing 10% (CS10) and 30% of PDMS (CS30) were obtained by a reaction under stirring and nitrogen flow for 6 h at 82 °C. For each materials planar patches were obtained by a spray, phase-inversion technique on a rotating cylindrical mandrel [1]. After materials deposition the patches were placed for 1 h in dH2O to allow solvents removal, and then pressed (50 g/cm2) during an heat treatment at 100°C for 90 min. In vitro tests were carried out to evaluate the biocompatibility, hemocompatibility, calcification [2], hydrolytic degradation (ISO 13781: Sorensen buffer, pH = 7.4), oxidative degradation [3], environmental stress cracking (ESC) degradation [4] and mechanical properties (ASTM D1708-02). The material cytotoxicity was studied through extraction method according to ISO-10993-5 on L929 fibroblasts; the hemocompatibility was assessed after 2 h of static blood contact to evaluate platelet adhesion, activation and coagulation parameters. After calcification and degradation tests, samples were investigated by infrared analysis and SEM. Uniaxial static tensile tests were performed until failure on both longitudinal and transverse directions on a computer controlled tensile testing machine (100 N load cell). Ten samples were analysed for each materials. For each samples stress-strain data, ultimate tensile strength (UTS) and ultimate elongation (UE) were calculated. A prototype of a tri-leaflet PHV, whose leaflets had the same thickness (300 ?m) and characteristics of the tested patches, was obtained by the spray technique. A 3D mould housing a stent, made by rapid-prototyping technique, was used to reproduce the morphology of a commercially available biological aortic valve and a 3D counter-mould was used to press/cure the valve prototype. RESULTS AND DISCUSSION All material extracts are devoid of any cytotoxic effects on mouse fibroblasts, since no decrease in cell viability (MTT test) and growth (BrdU proliferation test) was observed in comparison with untreated cells. CS30 material induced a lower in vitro platelets adhesion than the CS one, while the coagulation times were similar for all the tested materials. SEM analysis demonstrated that CS10 and CS30 presented significantly less formation of spots of calcification compared to CS. Infrared analysis demonstrated no significant differences among CS, CS10 e CS30 samples after the hydrolytic degradation test. CS30 samples exhibited less susceptibility to oxidative degradation and to ESC. Uniaxial tensile tests show a decrease of the UTS for the CS10 and CS30 with respect to the CS material of about 32%. No significant differences were found between all materials as far as the UE value. A single-body PHV prototype (Fig.1), with a stent incorporated and kept in place without sutures along the peak of the crown, was successfully obtained by spray, phase-inversion technique with CS30. CONCLUSION This study demonstrated that the combination of the properties of the CarboSil®-PDMS with those of a controlled valve fabrication technique, such as the spray material deposition on a 3D mould, allowed the feasibility of a new low-cost and potentially long-term performing PHV.