Intensified cross-linking dramatically improved the mechanical properties of hydroxyapatite and cellulose composites for repairing bone segmental defects

Bone segmental defects (BSDs), common in bone injury, often occur in the long bones of limbs. Biomaterials are essential for bone repair, especially in cases of bone nonunion. The inorganic and organic phases in natural bone are tightly cross-linked, which provided the inspiration for the design of the composite materials for repairing BSDs. At present, natural ingredients are good candidates for such materials. However, their poor mechanical properties compared with natural long bones hindered their applications. Herein, it is crucial to understand the relationship between the intensified cross-linking inside the material and the improvement of mechanical properties. Therefore, we fabricated composite materials comprising hydroxyapatite (HA) and cellulose fibril (CF) with a water-mediated molding method. Cross-linking inside the material was intensified by applying high pressure and designing several cross-linking sites with covalent, ionic, and hydrogen bonds (H-bonds). The compression strength, modulus, and toughness were dramatically improved compared with those of existing comparable materials. For the first time, both the strength (137.2 MPa) and toughness (1.268 kJ m-2) were within the ranges of human cortical bone, with a modulus of 1.6 GPa. Covalent and ionic bonds were strong and able to cross-link HA and CF tightly with phosphorylated sites. Besides, there were numerous sites for H-bonds, and water played an important role in H-bond formation. Ionic-wind-enhanced Raman spectroscopy (IWERS) was firstly utilized to analyze H-bonds of water itself without interference of other components. IWERS revealed the change of water H-bonds with different energies, which showed that water had enhanced H-bond interactions with other components. Intensified cross-linking was able to impart superior mechanical properties to the composite material. The in vitro and in vivo tests showed that the composite material had good biocompatibility and promoted the mobility of rats with tibia segmental defects. We anticipate that with further improvements in mechanical properties by means of intensified cross-linking mechanisms, this material can be applied in clinics. Hydroxyapatite and cellulose composites for bone segmental defect repair exhibit the highest mechanical properties among present natural-source composites due to the intensified cross-linking via covalent, ionic and hydrogen bonds.