Effect of grain size and microporosity on the in vivo behaviour of ß-tricalcium phosphate scaffolds.

Defining the most adequate architecture of a bone substitute scaffold is a topic that has received much attention over the last 40 years. However, contradictory results exist on the effect of grain size and microporosity. Therefore, the aim of this study was to determine the effect of these two factors on the in vivo behaviour of ß-tricalcium phosphate (ß-TCP) scaffolds. For that purpose, ß-TCP scaffolds were produced with roughly the same macropore size (≈ 150 µm), and porosity (≈ 80 %), but two levels of microporosity (low: 10 % / high: ≈ 25 %) and grain size (small: 1.3 µm /large: ≈ 3.3 µm). The sample architecture was characterised extensively using materialography, Hg porosimetry, micro-computed tomography (µCT), and nitrogen adsorption. The scaffolds were implanted for 2, 4 and 8 weeks in a cylindrical 5-wall cancellous bone defect in sheep. The histological, histomorphometrical and µCT analysis of the samples revealed that all four scaffold types were almost completely resorbed within 8 weeks and replaced by new bone. Despite the three-fold difference in microporosity and grain size, very few biological differences were observed. The only significant effect at p < 0.01 was a slightly faster resorption rate and soft tissue formation between 4 and 8 weeks of implantation when microporosity was increased. Past and present results suggest that the biological response of this particular defect is not very sensitive towards physico-chemical differences of resorbable bone graft substitutes. As bone formed not only in the macropores but also in the micropores, a closer study at the microscopic and localised effects is necessary.

Theoretical and experimental approach to test the cohesion of calcium phosphate pastes.

Recent studies have revealed that the ability of a calcium phosphate cement paste to harden in a physiological environment without desintegrating into small particles might be a key property to ensure a safe and reliable clinical use of calcium phosphate cements. However, this property called cohesion is not well understood and has not been studied extensively. The goal of the present study was to better understand which factors affect the cohesion of a calcium phosphate paste using the combination of a theoretical and experimental approach. In the theoretical approach, factors expected to influence the paste cohesion such as Van der Waals forces, electrostatic and steric interactions, as well as osmotic effects were listed and discussed. In the experimental approach, a new method to measure the cohesion of a non-setting calcium phosphate paste was presented and used to assess the effects of various factors on this property. The new method allowed a continuous measurement of cohesion and gave reproducible results. The experimental results confirmed the theoretical predictions: an increase of the liquid-to-powder ratio of the paste and of the powder particle size, as well as the addition of citrate ions and in limited cases dissolved xanthan polymer chains reduced the paste cohesion.

New depowdering-friendly designs for three-dimensional printing of calcium phosphate bone substitutes.

Powder-based three-dimensional printing (3DP) is a versatile method that allows creating synthetic calcium phosphate (CaP) scaffolds of complex shapes and structures. However, one major drawback is the difficulty of removing all remnants of loose powder from the printed scaffolds, the so-called depowdering step. In this study, a new design approach was proposed to solve this problem. Specifically, the design of the printed scaffolds consisted of a cage with windows large enough to enable depowdering while still trapping loose fillers placed inside the cage. To demonstrate the potential of this new approach, two filler geometries were used: sandglass and cheese segment. The distance between the fillers was varied and they were either glued to the cage or free to move after successful depowdering. Depowdering efficiency was quantified by microstructural morphometry. The results showed that the use of mobile fillers significantly improved depowdering. Based on this study, large 3DP scaffolds can be realized, which might be a step towards a broader clinical use of 3D printed CaP scaffolds.

Moisture based three-dimensional printing of calcium phosphate structures for scaffold engineering.

Powder based three-dimensional printing (3DP) allows great versatility in material and geometry. These characteristics make 3DP an interesting method for the production of tissue engineering scaffolds. However, 3DP has major limitations, such as limited resolution and accuracy, hence preventing the widespread application of this method within scaffold engineering [corrected].In order to reduce these limitations deeper understanding of the complex interactions between powder, binder and roller during 3DP is needed. In the past a lot of effort has been invested to optimize the powder properties for 3DP for a certain layer thickness. Using a powder optimized for an 88 µm layer thickness, this study systematically quantifies the surface roughness and geometrical accuracy in printed specimens and assesses their variation upon changes of different critical parameters such as the moisture application time (0, 5, 10 and 20s), layer thickness (44 and 88 µm) and the number of specimens printed per batch (6 and 12). A best surface roughness value of 25 µm was measured with a moisture application time (using a custom made moisture application device mounted on a linear stage carrying the print head) of 5s and a layer thickness of 44 µm. Geometrical accuracy was generally higher for the 88 µm thick layer, due to a less critical powder bed stability. Moisture application enabled 3DP of a 44 µm thick layer and improved the accuracy even for a powder initially optimized for 88 µm. Moreover, recycling of the humidified powder was not only possible but, in terms of reactivity, even beneficial. In conclusion, moisture-based 3DP is a promising approach for high resolution 3DP of scaffolds.

Aqueous impregnation of porous beta-tricalcium phosphate scaffolds.

The ability of a porous bone graft substitute to be impregnated with an aqueous solution is of great importance for tissue engineering and in vivo applications. This study presents an impregnation test setup and assesses the effect of various synthesis parameters such as sintering temperature, composition, macroporosity and macropore size on the impregnation properties of porous beta-tricalcium phosphate scaffolds dipped in water. Among those parameters, the macropore size had by far the largest effect; generally, the bigger the macropore size, the lower the saturation level. The results also showed that impregnation was less complete when the samples were fully dipped in water than when they were only partially dipped, owing to the requirement for the system to create air bubbles under water.