Development of novel ceramic processing techniques for manufacturing of heart valves : investigating the use of Powder Reaction Injection Moulding Engineering (PRIME) for the manufacture of novel, seam-free ceramic heart valves

Past research at Nottingham Trent University led to the design of a new prosthetic heart valve, to he made from alumina (aluminium oxide) because of its excellent wear properties and unequalled bio-compatibility. This paper documents a project performed to meet the challenge of using powder reaction injection moulding and extrusion (PRIME) to manufacture the alumina valve to the necessary high accuracy and finish. Butyl-cyanoacrylate and methyl methacrylate were investigated and compared as binders for the alumina powder. The cyanoacrylate binder was highly reactive, posing problems when mixing and handling feedstock, so inhibition levels were investigated. Catalysing mediums also were investigated, and steam and boiling water were found to he the best; these were subsequently used in moulding investigations. The rheologieal properties of the mixture of cyanoacrylate and alumina were investigated and it was found that the viscosity was within acceptable limits for injection moulding. The feedstock behaved like a power law fluid at the investigated shear rates, and a model was developed and used to predict viscosity at the shear rates experienced within the moulding process. The moulded feedstock was investigated to determine its suitability for machining so that, if necessary, the internal surface of the heart valve could he improved if necessary to achieve precise tolerances. Samples turned exceptionally well using a standard lathe, and the results suggested that green machining of PRIME feedstock would provide many benefits to a production process. For the moulding process, an alloy mould and core were found to best withstand moulding pressures, and by melting them after moulding the heart valve could he released without any damage. A wax skeleton was used within the mould assembly to allow boiling water or steam to pass through it and so enhance polymerisation and venting of the mould. The process that was developed has been patented. In order to understand exactly how the moulding process worked, a computational analysis (CED) was used to determine the pressures inside the mould and highlight possible areas of increased pressure. The CED model that was developed using PHOENICS 3.1 code was validated by analysing Newtonian fluid flow through a perspex heart valve mould and comparing pressure results from the practical and computational investigation. The processes developed in this project have been successfully applied to the manufacture of the alumina heart valve, and are equally suited to the manufacture of other low volume, high quality ceramic products.