A strategy to optimize the strength and degradation rate of calcium polyphosphates biomaterials

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Hip and knee replacements are increasingly used to replace joints where the cartilage has been destroyed by arthritis. A novel alternative approach to total joint replacement by artificial materials (metals, ceramics or polymers) is tissue engineering where new cartilage tissue is grown in vitro. One of the main problems with this approach is the anchorage of the newly formed cartilage tissue into bone. Our group (Drs. Kandel, Pilliar and Grynpas) has developed a method to form biphasic constructs consisting of tissue engineered cartilage anchored on top of a calcium polyphosphate (CPP) porous solid. A small biphasic construct has been implanted in a sheep knee for up to 9 months at which point the cartilage of the construct has integrated with the surrounding cartilage. Bone has grown into the pores of the CPP solid, which has been shown to be biocompatible and to degrade slowly. It is essential to ensure that this CPP material has sufficient mechanical properties to support a whole joint or to be used as a bone substitute in load bearing applications where only metal cages are currently used. In addition the rate of degradation of the CPP is lower than desired. The mechanism by which the polyphosphate chains of CPP hydrolyze into shorter chains, polyphosphate rings and orthophosphates is still not well understood. The aim of this project is to maximize the mechanical properties of the CPP biomaterial and to understand its mechanisms of degradation. In this proposal we will try and modify the factors that affect the material properties of CPP such as polyphosphate chain length, water content of the solid, amount and type of dopants. CPP glass will be made by calcining Ca phosphate monobasic monohydrate with or without dopants. We will vary both the calcining time and the melt time to modify the chain length. We will use both classical sintering methods and solid freeform fabrication.