Bioinspired functionally graded TPU gyroid structures for intervertebral disc replacement fabricated by multimaterial 3D printing

OBJECTIVE: Degenerative disc disease remains a primary cause of chronic back pain, yet current surgical treatments are often limited by the mechanical mismatch between implants and native intervertebral disc (IVD) tissue. This study investigates additively manufactured functionally graded Gyroid structures based on thermoplastic polyurethane (TPU) as biomimetic IVD replacements, focusing on the influence of geometry and material gradients on mechanical performance. Methods: Gyroid-based constructs were fabricated by multi-material fused filament fabrication using TPU filaments of different stiffness. Their mechanical performance was evaluated under static and dynamic compressive loading. Finite element analysis was performed to investigate stress distribution within the Gyroid architecture, while micro-computed tomography (µCT) was used to assess manufacturing-related defects and structural integrity before and after 700,000 loading cycles. Results: All investigated structures exhibited stable and reproducible mechanical behavior. However, only a single geometric configuration reached the targeted physiological load range of 4000-8000 N, irrespective of the material combination. The incorporation of an additional compliant TPU phase did not significantly alter the global force-deformation response compared with two-material configurations, indicating that structural geometry predominantly governs the compressive mechanical behavior. Dynamic testing demonstrated consistent damping capacities of approximately 20% across all configurations together with stable stiffness, storage modulus, and loss modulus during fatigue loading. Finite element analysis revealed homogeneous load distribution with localized stress concentrations at the curved Gyroid junctions. Quantitative µCT analysis showed no measurable increase in micro-porosity, void fraction, or interfacial defects after 600,000 loading cycles. Conclusion: The findings demonstrate that the mechanical performance of TPU-based Gyroid IVD constructs is primarily governed by architectural design rather than material gradients under compressive loading. The combined experimental, numerical, and µCT analyses confirm the mechanical robustness and structural stability of the proposed functionally graded Gyroid structures, highlighting architecture-driven design as a promising strategy for mechanically compatible intervertebral disc replacements. .