## 4 Conclusions and Further Work  
This research proposed several novelties, with the most notable being the evaluation of the flexural fatigue resistance of 3D-printed insoles manufactured through an additive manufacturing-based material extrusion process using various thermoplastic polyurethane filaments. The focus was the metatarsophalangeal area, being the most mechanically stressed one in cyclic flexion during walking or running. Another novel aspect was the testing of varioShore filament material, based on foaming technology, for the first time in this type of application. Two different designs were examined, while two of the 3D-printed orthoses also underwent real-life wear scenarios. The results demonstrated that each orthosis, regardless of its design and the material used, withstood a minimum of 700,000 cycles in the testing conditions of a dedicated apparatus. Daily use was simulated by testing the insoles for 7000 cycles followed by a rest period, then another 7000 cycles, and so on. No surface damage was observed on the insoles, except for non-permanent deformations, which were more prominent in the varioShore samples. The in-shoe Pedar measurements on insoles with a variable infill density demonstrated a reduction in plantar pressure, highlighting their effectiveness. These same insoles withstood over 1,400,000 steps during daily walking. The primary conclusion drawn from this study was that the available TPU filaments are suitable for insole manufacturing when tested for flexural fatigue resistance. Variations in the materials' shore A hardness resulted in different insole stiffnesses, which affects the plantar pressure and comfort. In this research, the most comfortable among the tested insoles was the varioShore model, which had a 20% infill density and was 3D-printed at a temperature of 220 \({}^{\circ}\)C. Thus, depending on the intended application (e.g., diabetic foot insoles, insoles for plantar fasciitis, insoles for prolonged standing, insoles for comfort, running, or other sports, etc., meaning different variations between the cushion and support), \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **Design** & **SDi** & **SDa** & **SDa** - **SDi** \\ \hline Filaflex 60A & D1 & 1.89 & 2.21 & 0.32 \\ \hline Filaflex 70A & D1 & 1.68 & 1.23 & \(-\)0.45 \\ \hline Filaflex 82A black & D1 & 2.30 & 2.65 & 0.35 \\ \hline Filaflex 82A blue & D1 & 1.61 & 1.85 & 0.24 \\ \hline PolyFlex 90A & D1 & 2.93 & 2.96 & 0.03 \\ \hline Varioshore 190 \({}^{\circ}\)C–220 \({}^{\circ}\)C-20\% & D1 & 1.22 & 1.07 & \(-\)0.15 \\ \hline Varioshore 220 \({}^{\circ}\)C-25\% & D1 & 1.83 & 2.27 & 0.44 \\ \hline PolyFlex 90A & D2 & 2.18 & 2.44 & 0.25 \\ \hline Varioshore 220 \({}^{\circ}\)C-25\% & D2 & 1.80 & 1.36 & \(-\)0.44 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Standard deviation: initial (SDi) and after tests (SDa). a filament can be chosen with the assurance that the insoles will endure at least two months of daily use. Based on the experience of one insole which was worn during summer and then kept on a shelf for around one year and tested for flexural fatigue, which determined an internal damage of the infill structure, further research will assess the influence of different wear factors, as well as aging, on the flexural fatigue resistance of the 3D-printed TPU insoles. Conceptualization, D.P. ([PERSON]); Data curation, M.C.I. and D.P. ([PERSON]); Formal analysis, M.C.I. and R.M. ; Investigation, M.C.I., D.P. ([PERSON]) and D.P. ([PERSON]); Methodology, M.C.I., D.P. ([PERSON]) and D.P. ([PERSON]); Validation, D.P. ([PERSON]) and R.M. ; Writing--original draft, M.C.I. and D.P. ([PERSON]); Writing--review and editing, M.C.I., D.P. ([PERSON]), D.P. ([PERSON]) and R.M. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patients to publish this paper.