Assessment of the Flexural Fatigue Performance of 3D-Printed Foot Orthoses Made from Different Thermoplastic Polyurethanes

Abstract

This research examines the flexural fatigue response of 3D-printed foot orthoses produced from various thermoplastic polyurethane (TPU) filaments, including Filaflex 60A, Filaflex 70A, Filaflex 82A, PolyFlex 90A, and varioShore. To subject the insoles to repeated flexion in the metatarsophalangeal area, specialized equipment was developed. A real-world testing scenario was applied to the Filaflex 82A insole, demonstrating that it can sustain over 1,400,000 steps over several months of normal walking (a cadence of approximately 120 steps per minute). Consequently, the experimental conditions were adjusted to double this pace to obtain pertinent results within a shorter testing timeframe. The insoles were subjected to 250 cycles per minute at constant clamping pressures of 176 kPa in the forefoot region. The objective of the evaluation was to determine if 700,000 testing cycles, equivalent to more than two and a half months of daily walking, would induce any damages in the internal structure (infill failure) or external condition (delamination, cracks) of the insoles. Except for compression marks, particularly notable on the foamed material (varioShore TPU) within the clamping zones of the testing device, none of the tested insoles exhibited any signs of external damage after 700,000 cycles. Moreover, the deformations observed in the insoles were non-permanent and nearly entirely disappeared within a few days of rest. The only insole that displayed deterioration of the infill structure was a TPU 82A insole that had been previously worn and then left on a shelf for approximately one year in uncontrolled conditions before being tested at repeated flexion on the apparatus. Additionally, a fifteen-minute walking test was carried out to assess the comfort of each insole, and it was found that the varioShore model, which had a 20% infill density and was 3D-printed at a temperature of 220 °C, stood out as the most comfortable among the tested insoles.