Intrinsic porosity of calcium phosphate cements and its significance for drug delivery and tissue engineering applications.

One key point in the field of tissue engineering and drug delivery is to provide materials with an adequate porosity. Many events, including nutrient and waste exchange in scaffolds for tissue engineering, as well as the drug-loading capacity and control of the release rate in drug delivery systems, are controlled by the size, shape and distribution of the pores in the material. Calcium phosphate cements (CPCs) possess an intrinsic porosity that is highly suited for these applications, and this porosity can be controlled by modifying some processing parameters. The objective of this work was to characterize and control the intrinsic porosity of alpha-tricalcium phosphate (alpha-TCP) cements, and to investigate its role against adsorption of bovine serum albumin (BSA). Cements with different percentages of open porosity (35-55%) were prepared by modifying the liquid-to-powder ratio. In addition, two different TCP particles were used to yield cements with specific surface areas of approximately 20 and approximately 37m(2)g(-1). Mercury porosimetry analysis on the set cements showed in most cases a bimodal pore size distribution which varied with the processing parameters and affected differently the adsorption and penetration of BSA. The peak occurring at larger pore dimensions controlled the penetration of BSA and was ascribed to the voids generated in between crystal aggregates, while the peak appearing at lower pore sizes was believed to be due to the intercrystallite voids within aggregates. It was found that, at the concentrations studied, the high intrinsic porosity in CPC does not ensure protein penetration unless there is an adequate pore size distribution.