Biomimetic radial gradient titanium scaffolds via freeze-assisted direct ink writing: Hierarchical architecture and damage tolerance

Reconciling the conflict between high load-bearing capacity and damage tolerance remains a pivotal challenge for porous metallic implants. Herein, hierarchical porous titanium scaffolds featuring biomimetic radial gradients were fabricated via a freeze-assisted direct ink writing (FA-DIW) technique. This approach engineered a continuous "dense-shell/porous-core" architecture comprising printing path-defined macropores, freeze-casting lamellar micropores, and sintering-derived nanopores. Crucially, by integrating theoretical modeling with in situ thermal monitoring, we elucidate that this radial heterogeneity is governed by anisotropic ice nucleation kinetics and the Ostwald ripening mechanism, where the latter was triggered by intrinsic thermal cycling during deposition. Mechanistically, the gradient structure exhibits a decoupled response: it follows a stretching-dominated law in the elastic regime for high stiffness efficiency, yet transitions to a buckling-dominated mode during plastic failure, ensuring predictable safety. Consequently, the scaffolds achieve exceptional biomechanical compatibility, with adjustable elastic moduli (1.20–3.76 GPa) and compressive strengths (77.8–309.3 MPa) that closely match human bone, alongside robust dynamic impact resistance (up to 380 MPa). This study establishes a mechanical foundation for designing next-generation biomimetic implants with tailored stiffness and dynamic reliability.