Abstract:
Polyurethane heart valves have been widely studied as possible replacements for mechanical and biological heart valves. To function optimally, these valves have to be free from calcification, require no anticoagulation, function silently and have longevities exceeding those of biological heart valves. The potential widespread use of such valves certainly exists and modern materials and fabrication methods place it within reach. However, polymer heart valves that are available at present are unaffordable for large parts of the population. Therefore, the development of an inexpensive routine production technique for manufacturing of polyurethane heart valves will greatly benefit a very large number of patients in developing and emerging countries. Polyurethane heart valves show favourable physical properties and flow dynamics compared to human heart valves; however, the successful outcome of producing a polyurethane heart valve with the required flexibility, durability and hemodynamic function is often difficult to predict. The design of the mould, the selection of the material and the fabrication method used are key factors that influence the achievement of an acceptable heart valve for use in the human body.
In this study, a repeatable, semi-automated dip moulding process for producing tri-leaflet polyurethane heart valves was developed. An experimental facility was designed and established, as well as an experimental dip-moulding mould, which allowed the selection of an appropriate set of dip-moulding process parameters and mould properties for the routine production of polyurethane valve leaflets with the required physical and mechanical properties. The results obtained from the experimental facility were evaluated with regard to compliance of the relevant properties, namely leaflet thickness, surface topography and mechanical strength, with the requirements of polymer heart valves. Additive manufacturing of Ti6Al4V (ELI) was used for producing the heart valve frame, sewing ring and mould. By applying the experimentally developed dip-moulding process and using the mould assembly, the valve leaflets were moulded directly onto the valve frame. The first prototype polyurethane heart valves were tested in the pulse duplicator machine of the Robert WM Frater Cardiovascular Research Centre of the University of the Free State. The valves displayed good opening and closing performance, mimicking the behaviour of natural heart valves during the pulse duplicator tests. Although these prototype valves did not meet all the minimum specifications of the ISO 5840 standard, it was clearly demonstrated that with further design improvements it would be possible to produce tri-leaflet valves through dip moulding, which would provide an attractive alternative to the tissue and mechanical heart valves currently used.
