Mechanical and rheological behavior of robocast biphasic calcium phosphate cement reinforced with graphene nanoplatelets.

In this study, a biphasic calcium phosphate cement (BCPC) was fabricated, combining the biocompatibility of hydroxyapatite (HA) and the biodegradability of β-tricalcium phosphate (β-TCP), and processed by robocasting, an extrusion-based technique, for bone tissue engineering applications. Despite its well-known biological performance, this material is brittle - mainly used as bone graft substitutes due to its limited mechanical strength. To overcome this limitation, graphene nanoplatelets (GNPs) were incorporated into the cement as a reinforcing filler. Their addition not only enhanced the mechanical strength but also improved the rheological behavior of BCPC pastes, resulting in better printability during the robocasting process. In particular, the incorporation of GNPs reduced the elastic modulus of the pastes, facilitating extrusion. Bars with and without microporosity were 3D-printed and subsequently evaluated by flexural testing. The flexural strength of microporous BCPC samples increased from 4.2 MPa (0% GNP) to 5.9 MPa (2% GNP), representing a 40% improvement. Although dense samples also exhibited higher strength with GNP addition, the difference was not statistically significant. Distinct stress-strain curve profiles obtained during bending tests revealed the influence of the printed architecture on the mechanical response of the printed parts. The flexural modulus followed the opposite trend to the flexural strength, showing a significant increase in dense parts containing GNP, while no substantial change was observed in the microporous counterparts. Overall, all measured mechanical properties fell within the range reported for trabecular bone substitution applications.