Scaffolds with open, interconnected pores and appropriate mechanical properties are required to provide mechanical support and to guide the formation and development of new tissue in bone tissue engineering. Since the mechanical properties of the scaffold tend to decrease with increasing porosity, a balance must be sought in order to meet these two conflicting requirements. In this research, open, interconnected pores and mechanical properties of biomedical titanium scaffolds prepared by using the space holder method were characterized. Micro-computed tomography (micro-CT) and permeability analysis were carried out to quantify the porous structures and ascertain the presence of open, interconnected pores in the scaffolds fabricated. Diametral compression (DC) tests were performed to generate stress-strain diagrams that could be used to determine the elastic moduli and yield strengths of the scaffolds. Deformation and failure mechanisms involved in the DC tests of the titanium scaffolds were examined. The results of micro-CT and permeability analyses confirmed the presence of open, interconnected pores in the titanium scaffolds with porosity over a range of 31–61%. Among these scaffolds, a maximum specific surface area could be achieved in the scaffold with a total porosity of 5–55%. DC tests showed that the titanium scaffolds with elastic moduli and yield strengths of 0.64–3.47 GPa and 28.67–80 MPa, respectively, could be achieved. By comprehensive consideration of specific surface area, permeability and mechanical properties, the titanium scaffolds with porosities in a range of 50−55% were recommended to be used in cancellous bone tissue engineering.

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Loss of function and impaired life quality as a result of large bone defects remain a serious problem in the society. Basically, the bone tissue has the capability of healing by itself when fractured. However, impaired healing may occur, leading to delayed union or even non-union when a bone segment is excised above a critical size. In recent years, bone tissue engineering has received increasing attention in the biomedical research community as an alternative approach to bone defect reconstruction. With this approach, damaged bone tissue can be repaired and remodelled with new bone cells at the defect site. For this purpose, a synthetic porous material, namely scaffold, is needed to act as a template to facilitate cellular activities, such as the migration and proliferation of osteoblasts and mesenchymal cells, as well as the transport of nutrients and oxygen required for vascularization during bone tissue development at the defect site. Currently, titanium is considered to be a preferred biomaterial for bone tissue engineering scaffolds owing to its excellent biocompatibility and mechanical properties. So far, the space holder method has been preferably used for the fabrication of titanium scaffolds with high porosity and open, interconnected pores for bone tissue engineering. Despite a large number of studies on the scaffold fabrication with this method, the mechanisms involved and the way to control the porous structure of scaffolds during fabrication have not yet been fully understood.@en

The present research was aimed at gaining an understanding of the porous structure changes from the green body through water leaching and sintering to titanium scaffolds. Micro-computed tomography (micro-CT) was performed to generate 3D models of titanium scaffold preforms containing carbamide space-holding particles and sintered scaffolds containing macro- and micro-pores. The porosity values and structural parameters were determined by means of image analysis. The result showed that the porosity values, macro-pore sizes, connectivity densities and specific surface areas of the titanium scaffolds sintered at 1200 °C for 3 h did not significantly deviate from those of the green structures with various volume fractions of the space holder. Titanium scaffolds with a maximum specific surface area could be produced with an addition of 60–65 vol% carbamide particles to the matrix powder. The connectivity of pores inside the scaffold increased with rising volume fraction of the space holder. The shrinkage of the scaffolds prepared with > 50 vol% carbamide space holder, occurring during sintering, was caused by the reductions of macro-pore sizes and micro-pore sizes as well as the thickness of struts. In conclusion, the final porous structural characteristics of titanium scaffolds could be estimated from those of the green body. @en

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. @en

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.@en