Barbara Leukers

The morphology of anisotropic 3D-printed hydroxyapatite scaffolds

Three-dimensional (3D) scaffolds with tailored pores ranging from the nanometer to millimeter scale can support the reconstruction of centimeter-sized osseous defects. Three-dimensional-printing processes permit the voxel-wise fabrication of scaffolds. The present study rests upon 3D-printing with nano-porous hydroxyapatite granulates. The cylindrical design refers to a hollow bone with higher density at the periphery. The millimeter-wide central channel follows the symmetry axis and connects the perpendicularly arranged micro-pores. Synchrotron radiation-based micro computed tomography has served for the non-destructive characterization of the scaffolds. The 3D data treatment is essential, since, for example, the two-dimensional distance maps overestimate the mean distances to the material by 33-50% with respect to the 3D analysis. The scaffolds contain 70% micrometer-wide pores that are interconnected. Using virtual spheres, which might be related to the cells migrating along the pores, the central channel remains accessible through the micro-pores for spheres with a diameter of up to (350+/-35)mum. Registering the tomograms with their 3D-printing matrices has yielded the almost isotropic shrinking of (27+/-2)% owing to the sintering process. This registration also allows comparing the design and tomographic data in a quantitative manner to extract the quality of the fabricated scaffolds. Histological analysis of the scaffolds seeded with osteogenic-stimulated progenitor cells has confirmed the suitability of the 3D-printed scaffolds for potential clinical applications.

In vitro -Osteoclastic Activity Studies on Surfaces of 3D Printed Calcium Phosphate Scaffolds:

Various biomaterials have been developed for the use as bone substitutes for bone defects. To optimize their integration and functionality, they should be adapted to the individual defect. Rapid prototyping is a manufacturing method to tailor materials to the 3D geometry of the defect. Especially 3D printing allows the manufacture of implants, the shape of which can be designed to fit the bone defect using anatomical information obtained from the patient. 3D printing of calcium phosphates, which are well established as bone substitutes, involves a sintering step after gluing the granules together by a binder liquid. In this study, we analyzed if and how these 3D printed calcium phosphate surfaces can be resorbed by osteoclast-like cells. On 3D printed scaffold surfaces consisting of pure HA and β-TCP as well as a biphasic mixture of HA and TCP the osteoclastic cell differentiation was studied. In this regard, cell proliferation, differentiation, and activation were analyzed with the monocytic cell line RAW 264.7. The results show that osteoclast-like cells were able to resorb calcium phosphate surfaces consisting of granules. Furthermore, biphasic calcium phosphate ceramics exhibit, because of their osteoclastic activation ability, the most promising surface properties to serve as 3D printed bone substitute scaffolds.

Bio-mimetic hollow scaffolds for long bone replacement

The tissue engineering focuses on synthesis or regeneration of tissues and organs. The hierarchical structure of nearly all porous scaffolds on the macro, micro- and nanometer scales resembles that of engineering foams dedicated for technical applications, but differ from the complex architecture of long bone. A major obstacle of scaffold architecture in tissue regeneration is the limited cell infiltration as the result of the engineering approaches. The biological cells seeded on the three-dimensional constructs are finally only located on the scaffolds periphery. This paper reports on the successful realization of calcium phosphate scaffolds with an anatomical architecture similar to long bones. Two base materials, namely nano-porous spray-dried hydroxyapatite hollow spheres and tri-calcium phosphate powder, were used to manufacture cylindrically shaped, 3D-printed scaffolds with micro-passages and one central macro-canal following the general architecture of long bones. The macro-canal is built for the surgical placement of nerves or larger blood vessels. The micro-passages allow for cell migration and capillary formation through the entire scaffold. Finally, the nanoporosity is essential for the molecule transport crucial for signaling, any cell nutrition and waste removal.

Image-based analysis of the internal microstructure of bone replacement scaffolds fabricated by 3D printing

Rapid Prototyping and especially the 3D printing, allows generating complex porous ceramic scaffolds directly from powders. Furthermore, these technologies allow manufacturing patient-specific implants of centimeter size with an internal pore network to mimic bony structures including vascularization. Besides the biocompatibility properties of the base material, a high degree of open, interconnected porosity is crucial for the success of the synthetic bone graft. Pores with diameters between 100 and 500 μm are the prerequisite for vascularization to supply the cells with nutrients and oxygen, because simple diffusion transport is ineffective. The quantification of porosity on the macro-, micro-, and nanometer scale using well-established techniques such as Hg-porosimetry and electron microscopy is restricted. Alternatively, we have applied synchrotron-radiation-based micro computed tomography (SRμCT) to determine the porosity with high precision and to validate the macroscopic internal structure of the scaffold. We report on the difficulties in intensity-based segmentation for nanoporous materials but we also elucidate the power of SRμCT in the quantitative analysis of the pores at the different length scales.