3D Printing of Hierarchical Gyroid Hydroxyapatite-Akermanite Scaffolds with Improved Compressive Strength

It is necessary for an ideal bioceramic scaffold to have a suitable structure. The structure can affect the mechanical properties of the scaffold (i.e., elastic modulus and compressive strength) and the biological properties of the scaffold (i.e., degradability and cell growth rate). Lattice structure is a kind of periodic porous structure, which has some advantages of light weight and high strength, and is widely used in the preparation of bioceramic scaffolders. For the structure of the scaffold, high porosity and large pore size are important for bone growth, bone integration and promoting good mechanical interlocking between neighboring bones and the scaffold. However, scaffolds with a high porosity often lack mechanical strength. In addition, different parts of the bone have different structural requirements. In this paper, scaffolds with a non-uniform structure or a hierarchical structure were designed, with loose and porous exterior to facilitate cell adhesion, osteogenic differentiation and vascularization as well as relatively dense interior to provide sufficient mechanical support for bone repair.

In this work, composite ceramics scaffolds with 10% akermanite content were prepared by DLP technology. The scaffold had a high porosity outside to promote the growth of bone tissue, and a low porosity inside to withstand external forces. The compressive strength, fracture form, in-vitro degradation performance and bioactivity of graded bioceramic scaffolds were investigated. The models of scaffolds were imported into the DLP printer with a 405 nm light. The samples were printed with the intensity of 8 mJ/cm² and a layer thickness of 50 μm. Finally, the ceramic samples were sintered at 1100 ℃. The degradability of the hierarchical gyroid bioceramic scaffolds was evaluated through immersion in Tris-HCl solution and SBF solution at a ratio of 200 mL/g. The bioactivity of bioceramic was obtained via immersing them in SBF solution for two weeks. The concentrations of calcium, phosphate, silicon, and magnesium ions in the soaking solution were determined by an inductively coupled plasma optical emission spectrometer.

In this work, a hierarchical Gyroid structure HA-AK10 scaffold (sintered at 1100 ℃) with a radial internal porosity of 50% and an external porosity of 70% is prepared, and the influence of structural form on the compressive strength and degradation performance of the scaffold is investigated. The biological activity of the bioceramics in vitro is also verified. The mechanical simulation results show that the stress distribution corresponds to the porosity distribution of the structure, and the low porosity is larger and the overall stress concentration phenomenon does not appear. After soaking in SBF solution, Si—OH is firstly formed on the surface of bioceramics, and then silicon gel layer is produced due to the presence of calcium and silicon ions. The silicon gel layer is dissociated into negatively charged groups under alkaline environment secondary adsorption of calcium ions and phosphate ions, forming amorphous calcium phosphate, and finally amorphous calcium phosphate crystals and adsorption of carbonate ions, forming carbonate hydroxyapatite. This indicates that the composite bioceramics have a good biological activity in-vitro and can provide a good environment for the growth of bone cells. A hierarchical Gyroid ceramic scaffold with a bone geometry is prepared via applying the hierarchical structure to the bone contour scaffold. The maximum load capacity of the hierarchical Gyroid ceramic scaffold is 8 times that of the uniform structure.

The hierarchical structure scaffold designed had good overall compressive performance, good degradation performance, and still maintained a good mechanical stability during degradation. In addition, in-vitro biological experimental results showed that the surface graded composite scaffold could have a good in-vitro biological activity and provide a good environment for bone cells. Compared to the heterosexual structure, the graded scaffold had greater mechanical properties.