# Infill Pattern and Density of 3D-Printed Insoles Alter Energy and Pressure Distribution in Gait
## 4 Discussion  
This study demonstrated that infill patterns and density influence the mechanical properties of 3D-printed objects in terms of maximum compressive load and energy absorption. The findings of this study suggest that stiffness of 3D-printed object depends on infill pattern and infill density. Walking with insoles, the selected infill pattern (honeycomb and gyroid) with 20% infill density could reduce the plantar pressure at the heel around 10% when compared to walking without insole. However, there was increased plantar pressure in midfoot and forefoot regions when using insoles. This addresses the insole design and printing parameters that need to be considered when making 3D-printed insoles for diabetic patients. There are several materials that are commonly used for insoles such as gel, foam, cork and leather. These materials have properties ranging from good flexibility to good firmness to add an extra cushion, support and control. It has been reported that low- to moderate-stiffness materials are suitable to provide a cushioning function [19; 20]. Soffer materials provide comfortable accommodation, whereas stiffer materials are more durable [21]. [PERSON] et al. pointed out that the correct selection of cushioning stiffness can significantly reduce plantar pressure: stiffer cushioning materials were necessary for those with a higher BMI, and softer cushioning materials were suitable to reduce peak plantar pressure more at the midfoot than other foot regions [16; 22]. They also showed increased infill densities from 10% to 20% provided higher relative stiffness for 3D-printed TPU footbets. Their finding was, like in our study, that increased infill densities obtained a higher maximum compression load, especially for honeycomb and gyroid patterns. [PERSON] and [PERSON] looked at the compression strength of customized shoe insole with different infill patterns and infill densities using 3D printing [23]. Four infill patterns were investigated with 25%, 40% and 60% infill densities: they were triangle, rectilinear, cubic and gyroid. Compressive strength was higher when the infill density increased for all infill patterns. Gyroid and cubic infill patterns had lower compressive strength compared to the other two patterns. Their research has similar results to our study, showing that gyroid and cubic infill patterns had lower  
Figure 6: Center of pressure (COP) trajectories during walking with and without 3D-printed insoles in two participants. The trajectory begins at heel contact and progresses through to the midfoot to the forefoot and toes during walking. A smooth, forward-moving COP trajectory indicates a stable and coordinated gait. maximum compression load compared to the other three patterns. However, in our study, the honeycomb infill pattern provides the highest maximum compression load among five infill patterns, not the triangle infill pattern. [PERSON] and colleagues performed mechanical tests on the 3D-printed specimens using elastic filament and found that hardness, Young's modulus and tensile stress were increased when infill density was increased [6]. Some computational studies looked at the most effective insole infill for plantar pressure and weight distribution. [PERSON] and [PERSON] used finite element analysis to demonstrate that lattice structures affected maximum stress and deformation [12]. Similarly, [PERSON] et al. numerically showed that increasing the number of unit cells enhances the stiffness of the insole model, and the unit cell architecture plays a role in stiffness [13]. Cushioning, shock absorption and durability are important features of diabetic insoles, which help to distribute pressure across the foot, reduce the impact of walking or standing and provide long-lasting support, respectively. Our study showed that TPU showed viscoelastic behavior when under the loading-unloading compression test. The energy absorbed during a loading-unloading cycle is different in each infill pattern and infill density.