Fatigue behavior of 3D-printed solid liquid composite (SLC) orthotic insole: an experimental study

Abstract The absence of standardized protocols for assessing the durability of foot orthoses, combined with limited research on insole material behavior under dynamic loading, presents a significant challenge in evaluating advanced orthotic designs. This study addresses that gap by adapting the ISO 10328:2016 standard—originally developed for prosthetic structural testing—to investigate the fatigue behavior of a novel 3D printed solid-liquid composite (SLC) insole. The SLC insole features a functionally graded lattice architecture designed to reduce peak plantar pressure in individuals at risk of diabetic foot complications. A prosthetic foot was used as an indenter to apply uniaxial compressive stress to the insole, simulating the critical gait phases of heel strike and push-off at constant strain. The insole was subjected to 100,000 loading cycles, equivalent to approximately 4 months of regular walking activity. Scanning electron microscopy (SEM) was used to examine the morphology of the insoles and identify manufacturing defects, surface irregularities, and structural inconsistencies inherent to the 3D printing process. Fatigue testing revealed a progressive decline in energy dissipation and stiffness, indicating material degradation due to cyclic softening. The SLC insole exhibited viscoelastic behavior during testing, characterized by structural rearrangement under load that contributed to temporary impact absorption. However, early onset of plastic deformation and microstructural damage—particularly under heel strike loading—was observed, compromising long-term durability. SEM analysis revealed voids, microcracks, and surface irregularities acting as crack initiation sites, underscoring the influence of print quality and structural consistency on fatigue life. These findings highlight the complex fatigue response of functionally graded SLC insoles and underscore the importance of evaluating mechanical performance under dynamic, physiologically relevant conditions. The adapted testing method and chosen performance measures provide a strong basis for creating standardized ways to assess insole durability. This helps improve the clinical reliability of custom foot orthoses and supports the wider use of advanced, pressure-driven insoles for people at high risk of injury, especially those with diabetic foot complications.