Construction of hydroxyapatite bioceramics with biomimetic structures by weak magnetic field-assisted 3D printing

Hydroxyapatite (HA) bioceramics have limited use in load-bearing implants because of their poor mechanical properties. Inspired by the oriented and Bouligand structures in natural organisms with remarkable strength and toughness, this study aims to construct biomimetic HA bioceramics with fine microstructures at the nanoscale and microscale to enhance their mechanical properties. An innovative magnetic field-assisted three-dimensional (3D) printer was developed to create oriented and Bouligand structural HA ceramics under weak magnetic field strengths (58–116 mT). The oriented HA bioceramics demonstrate a compressive strength of 93.4 MPa along the printing direction, which is 2.3 times that of non-oriented HA ceramics. The bending strength in the thickness direction is 2.6 times that of non-oriented HA bioceramics, whereas the fracture toughness of oriented HA bioceramics in the printing direction reaches 17.3 MPa·m^1/2, which is 1.44 times that of their non-oriented counterparts. The presence of HA grains hinders crack propagation along the thickness direction, thereby increasing the fracture toughness. Additionally, the compressive strength, bending strength, and fracture toughness of the Bouligand structural HA bioceramics are 2.6 times, 2.8 times, and 1.2 times those of non-oriented HA bioceramics, respectively, with pseudoplastic deformation observed during compression. The Bouligand structural HA bioceramics achieve a combination of excellent strength and toughness comparable to that of cortical bone. This research establishes the magnetic field-assisted 3D printer as an effective method for balancing strength and toughness in brittle ceramics. Additionally, this work lays a foundation for the 3D printing of biomimetic materials with fine microstructures and tunable mechanical properties.