Analysis of the densification behaviour of titanium/carbamide powder mixtures in the preparation of biomedical titanium scaffolds.

Tissue engineering is a promising approach to the reconstruction of critical size bone defects. In this approach, a porous material, namely a scaffold, is devised as a template to support and guide the formation of new bone cells and the regeneration of bone tissue in the damaged site. Titanium is considered a preferred biomedical material for bone tissue engineering scaffolds. Among a number of techniques that have so far been developed to produce porous-structured titanium, the space holder method has been recognized as a viable one owing to its ability to produce porous scaffolds with desired structural characteristics. In this technique, space holding particles are utilized as a pore former. The fabrication process for titanium scaffolds is composed of a series of processing steps, i.e., (i) mixing of a titanium matrix powder with space holding particles, (ii) compaction of the powder mixture to form a composite compact, (iii) removal of space holding particles from the composite compact and (iv) sintering of the porous titanium matrix. Despite initial success in applying this technique, a number of technological challenges are still present, such as the difficulties in controlling the geometry changes of space holding particles during the compaction process. Obviously, compacting pressure must be optimized in order to prevent space holding particles from distortion, so as to ensure pore sizes and shape as desired for the scaffold product. In addition, the correlations between compaction process parameters and porous structure characteristics must be established to facilitate through-process modeling along the whole chain of the fabrication of bone tissue engineering scaffolds in the near future. In the present research, the behavior of titanium/carbamide powder mixtures during cold compaction was characterized and optimum compacting pressures for the fabrication of titanium scaffolds using the space holder method were derived. In addition, the Heckel equation describing the densification of powder mixtures during compaction was applied to assess its validity in the case of the present powder mixtures composed of two mechanically dissimilar components. A titanium powder with spherical particles and a carbamide powder with cubical particles were used as the matrix and pore former, respectively. Titanium/carbamide powder mixtures were prepared by mixing the powders for 3 h. Granular materials were then compacted with an instrumented powder compaction press. The variation of the load during compaction with the punch displacement was registered. The load-displacement plots were analyzed using the Heckel model for powder compaction and the rule of mixtures. The results showed varied compaction behavior of titanium and carbamide powders as their relative volume fractions changed. Titanium/carbamide powder mixtures exhibited intermediate behaviors of the component powders during compaction. The initial density of the compact was found to be of critical importance, as it determined the at-pressure density of the powder mixture compact. A lower compacting pressures was required for the compaction of a powder mixture with a larger volume fraction of carbamide. In addition, the experimental data could be well fitted into the Heckel equation. Although refining is still needed, the model can be used as a guide for the selection of an optimum compacting pressure in the preparation of titanium scaffolds with the space holder method.