Direct ink writing of functionally graded bone scaffolds using ceramic foams to mimic natural bone hierarchical porosity and multiple gradients

The heterogeneity of natural bone requires versatile materials to provide performance-matched substitutes for critical-size defective tissue. However, replicating the complex gradients and multi-scale structure of natural bone remains a critical challenge in bone tissue engineering. To overcome this, the work presents a novel approach to print functional gradient bone scaffolds by online mixing zirconia-based ceramics (ZbC) foam and alumina-based ceramics (AbC) foam. After forming, ZbC exhibited a compressive strength of 88.7 MPa and an elastic modulus of 3.9 GPa. In contrast, AbC with 90 % porosity showed only 1/17th of the compressive strength but a 5.5-fold higher elastic modulus. These mechanical properties align well with those of cortical and cancellous bone. AbC, structurally characterized by large pores built by thin filaments and small pores obtained by porogenic agents, provides the necessary channels for the ingrowth of cells and vascular veins. Particularly, by combining ZbC and AbC in a continuous gradient, the scaffold mimics the femur's mechanical gradients, porosity, and connectivity, minimizing stress mismatch at interfaces. Cell adhesion, spread, and proliferation within the scaffold pores further validate its potential. Therefore, this low-cost, multi-scale, and multi-material 3D printing technology offers a promising strategy for the insufficient donor problem of bone defects.