On the physicochemical properties, setting chemical reaction, and in vitro bioactivity of aragonite-chitosan composite cement as a bone substitute

In the field of bone substitution, calcium carbonate cements have been proposed as bioactive mineral cements complementary to calcium phosphate cements for applications requiring a faster resorption rate. We investigated for the first time the setting chemical reaction, physicochemical properties and the in vitro evolution in a simulated body fluid (SBF) of calcium carbonate composite cements including chitosan (1.5 wt%, 2.5 wt%, 3.5 wt%, and 4 wt%) in view of bone applications. We implemented a methodology based on the X-ray diffraction data for the quantitative analysis of the four calcium carbonate phases (aragonite, vaterite, calcite and amorphous calcium carbonate) involved in the composite cement setting which allowed determining the setting chemical mechanism and the underlying effect of chitosan. Interestingly, aragonite was the main phase in all the set composite cements. Whatever the chitosan amount, we showed that the initial Mg-stabilised amorphous calcium carbonate crystallised rapidly into a small amount of magnesium-doped calcite (<21 wt%). However, the vaterite recrystallisation into aragonite was slowed down by the presence of chitosan in a dose-dependent manner. The composite cement formulation including 3.5 wt% of chitosan presented the optimal properties such as a good paste injectability (85%), a reduced initial setting time (45 min), a compressive strength (6.5 MPa) analogous to that of spongy bone, and ductile behaviour. In vitro evolution in the SBF showed that chitosan enhanced the bioactivity of the composite cement which promoted the formation of a biomimetic apatite on its surface after 28 days of immersion. This composite cement is a potential candidate for further investigation as a new injectable and resorbable bone substitute material and could also support a new direction for building material applications.