High strength brushite bioceramics obtained by selective regulation of crystal growth with chiral biomolecules

Abstract

Chirality seems to play a key role in mineralization. Indeed, in biominerals, the biomolecules that guide the formation and organization of inorganic crystals and help construct materials with exceptional mechanical properties, are homochiral. Here, we show that addition of homochiral l-(+)-tartaric acid improved the mechanical properties of brushite bioceramics by decreasing their crystal size, following the classic Hall-Petch strengthening effect; d-(-)-tartaric acid had the opposite effect. Adding l-(+)-Tar increased both the compressive strength (26 MPa) and the fracture toughness (0.3 MPa m^1/2) of brushite bioceramics, by 33% and 62%, respectively, compared to brushite bioceramics without additives. In addition, l-(+)-tartaric acid enabled the fabrication of cements with high powder-to-liquid ratios, reaching a compressive strength and fracture toughness as high as 32.2 MPa and 0.6 MPa m^1/2, respectively, approximately 62% and 268% higher than that of brushite bioceramics prepared without additives, respectively. Characterization of brushite crystals from the macro- to the atomic-level revealed that this regulation is attributable to a stereochemical matching between l-(+)-tartaric acid and the chiral steps of brushite crystals, which results in inhibition of brushite crystallization. These findings provide insight into understanding the role of chirality in mineralization, and how to control the crystallographic structure of bioceramics to achieve high-performance mechanical properties. STATEMENT OF SIGNIFICANCE: Calcium-phosphate cements are promising bone repair materials. However, their suboptimal mechanical properties limit their clinical use. Natural biominerals have remarkable mechanical properties that are the result of controlled size, shape and organization of their inorganic crystals. This is achieved by biomineralization proteins that are homochiral, composed of l- amino acids. Despite the importance of chiral l-biomolecules in biominerals, using homochiral molecules to fabricate bone cements has not been studied yet. In this study, we showed that homochiral l-(+)-tartaric acid can regulate the crystal structure and improve the mechanical properties of a calcium-phosphate cement. Hence, these findings open the door for a new way of designing strong bone cement and highlight the importance of chirality in bioceramics.

Keywords: Brushite ceramic; Chirality; Crystal growth; Mechanical properties; Tartaric acid.