Optimizing 3D printing and sintering processes for HA/TCP Triply Periodic Minimal Surfaces for bone tissue engineering

Calcium-phosphate based ceramic materials, such as hydroxyapatite (HA) and beta-tri calcium phosphate, have long been investigated for application in bone tissue engineering, due to their similarity to the mineral phase of the tissue. One specific application that has recently been developed is the use of 3D printing to create HA and TCP-based triply periodic minimal surfaces (TPMSs), to exploit their porosity and excellent mechanical properties. Masked stereolithography (mSLA) 3D printing is a very favourable technique to manufacture such structures, by polymerizing a UV-sensitive resin loaded with ceramic powder: the obtained green bodies are subsequently sintered to remove the resin and obtain dense ceramic scaffolds. In this work the rheology, curing depth of three different resin-powder mixtures (using the same resin and containing either HA powder, TCP powder, or a mixture containing 70% TCP and 30% HA by weight which is sintered in biphasic calcium phosphate (BCP)) is investigated to optimize printing conditions. The results are then validated by analyzing printing accuracy at various exposure times. Through this process it was possible to optimize the exposure time of the suspensions from 2.0s to 1.8s for TCP and 1.6s for HA and BCP, achieving lower printing times and higher dimensional accuracy. The printed TPMS samples were then sintered using two-step sintering, a technique to achieve good densification while avoiding grain growth, which is detrimental to the mechanical properties. The mechanical properties of the scaffolds are analyzed through uniaxial compression tests, and their chemical characteristics are investigated through X-ray diffraction: the obtained results are then compared to those obtained by conventional sintering. Results show that 2SS as applied in this work is not beneficial for mechanical properties, but increases phase stability in the sintering process: this provides a useful basis for further investigations of thermal treatments to manufacture effective artificial bone grafts