Evolution of macro- and micro-pores in the porous structures of biomedical titanium scaffolds during isothermal sintering

Porous structure with an appropriate set of geometrical parameters is of critical importance in the design of titanium scaffolds for bone tissue engineering. The space holder method is generally considered a viable technique for the brication of titanium scaffolds. With this technique, scaffold fabrication is composed of four steps, i.e., (i) powder mixing, (ii) compaction, (iii) removal of space holding particles and (iv) sintering. The resultant porous structure contains both macro-pores and micro-pores that are formed from the space occupied by removed space-holding particles and from the incomplete sintering process, respectively. Controlling macro- and micro-pores to ensure desirable interconnections between macro-pores and micro-pore sizes and volume fraction is a technical challenge. In this study, the effect of sintering time on the evolution of macro- and micro-pores in titanium scaffolds prepared with the space holder method was nvestigated. A spherical titanium powder and rectangular carbamide particles were used as the matrix material and space holder, respectively. The starting powders with a carbamide volume fraction of 50% were first mixed for 3 h by using a tube roller mixer. To prevent the mixture from powder segregation, prior to mixing, a liquid polyvinyl-alcohol (PVA) binder was added to the starting powders. The titanium/carbamide mixture was then uniaxially compacted at a pressure of 250 MPa. To create macro-pores in the scaffold, carbamide particles were removed from the compacted powder mixture through water leaching. Finally, the scaffold preform was sintered at 1200 °C for 15 - 180 min under flowing argon atmosphere. The resultant porous structures were characterized by means of quantitative metallographic analysis. The results showed macro-pores in the porous structures had geometrical parameters quite close to those of space-holding particles. The interconnections between macro-pores were retained from those resulting from the coalescence of space-holding particles during powder compaction and it was relatively insensitive to the sintering time. Micro-pores were formed as a result of neck formation between titanium particles during sintering. With increasing sintering time, micro-pore sizes decreased, accompanied by the shrinkage of the scaffold. In conclusion, the porous structure of titanium scaffolds prepared with the space holder method could be controlled by optimizing sintering time.