3D printing of damage-tolerant bone scaffolds with bioinspired hierarchical porous structures

Tissue engineering scaffolds with hierarchical porous structures has been a promising strategy for healing large bone defects. The fabrication of multiscale porous structures resembling real bone tissues presents a great challenge. In addition, as the porosity increases, the mechanical strength of scaffolds decreases dramatically, leading to catastrophic failure of scaffolds under external loadings. In the current work, a cryogenic 3D printing technique of fabricating hierarchical porous hydroxyapatite (HA) and polyether ether ketone (PEEK) composite scaffolds was proposed. Both pure HA and HA/PEEK composite scaffolds with porosity ranging from 60% to 80% were successfully fabricated. Submillimeter scale pores (∼300 µm), microscale pores (∼10 µm), and nanopores (∼500 nm) were integrated in the 3D-printed scaffolds. Because of the hierarchically arrangement of pores, the printed HA scaffolds show an unusual long plateau strain during compressions, indicating excellent damage tolerance. The HA/PEEK composite scaffold can withstand compression strains up to 80% without disintegration, showing even better damage resistance. Compared to traditional porous ceramic bone scaffolds, the cryogenic 3D-printed scaffolds show extraordinary specific mechanical strength and damage tolerance by keeping high volume percentage of pores at the same time. The degradation capability of HA nanoparticles in vitro and cellular activity testing of the scaffolds further demonstrates the potential application in bone tissue repair.