Supplementary Material: "Suture-inspired joints in FDM 3D printing as a combined attachment mechanism"

Reliable attachment between separately manufactured components is a fundamental requirement in engineering design, where the performance of an assembled structure often depends on the effectiveness of its joints. Inspired by cranial sutures of the red deer Cervus elaphus, this study investigates suture-inspired joints as a combined attachment mechanism for 3D-printed plates subjected to out-of-plane bending. Six geometrically distinct sutural structures were designed from female and male red deer cranial sutures and fabricated from polylactic acid (PLA) using fused deposition modeling (FDM) 3D printing. The samples were assembled using three bonding strategies: chemical bonding, mechanical bonding, and combined mechanical-chemical bonding. Their mechanical performance was evaluated through quasi-static three-point bending tests using maximum force and work to failure initiation as the main mechanical parameters. The combined mechanical-chemical bonding strategy resulted in the highest performance across all tested suture geometries, suggesting that mechanical interlocking can complement adhesive bonding and enhance the attachment performance of 3D-printed parts. The results also showed that mechanical interlocking was strongly geometry-dependent, with different sutures leading to different load-bearing capacities, work to failure initiation, and failure modes. To relate suture morphology to bending performance, we compared established geometric complexity metrics, including sinuosity, lobe-based complexity, and fractal-dimension-based descriptors, with a moment-sensitive complexity index introduced in this study. By weighting transverse deviations from the midline, this metric provides an additional geometry descriptor relevant to out-of-plane deformation. Overall, the study suggests that suture-inspired interfaces can influence bending behavior beyond in-plane interlocking and provide a promising structural design strategy for attachment in additive manufacturing.