# Infill Pattern and Density of 3D-Printed Insoles Alter Energy and Pressure Distribution in Gait
## 4 Discussion
The honeycomb infill pattern possibly has good shock absorption characteristics due to its high energy absorption, while the gyroid infill pattern has low energy absorption. Furthermore, our study revealed that increased infill density provided higher energy absorption. Our findings are supported by [PERSON] et al., who reported that gyroid structure showed lower elastic absorbed energy per unit volume than solid structure [11], and [PERSON] et al., who concluded that density is a major factor that influences cushioning energy absorption performance [24]. Materials with high energy absorption and elastic modulus indicate its potential application in dampers, shock absorbers and cushioning [25]. It was found that the specimens taken from the single material insole had larger relative energy dissipation compared to that taken from the dual-material insole, providing higher stress response [26; 27]. However, good energy absorption materials relate to high stiffness, which should be considered when using them as materials for diabetic insoles. Three-dimensional-printed insoles have been proposed in many studies for diabetic foot management in the aspect of off-loading function and plantar pressure reduction. Stiffness optimization of 3D-printed footbets was conducted in fifteen diabetic patients by [PERSON] et al. They found that the optimum stiffness resulted in a maximum percentage of plantar pressure reduction [16]. In our study, subjects who walked with 3D-printed insoles had lower mean plantar pressure in the hindfoot region on both left and right sides than without insoles, whereas they had elevated mean plantar pressure in other regions of the foot. The area under the force-time curve during gait represented the vertical impulse showed similar values between insoles with honeycomb and gyroid infill patterns. However, the test specimen with honeycomb infill pattern showed higher energy dissipation than that with gyroid infill pattern. [PERSON] and [PERSON] used 3D-printed insoles made of kelvin lattice infill to look at the plantar pressure distribution during standing and showed that the insole-clad foot condition had lower average pressure compared to the barefoot condition, similar in both feet [13]. Another study by [PERSON] et al. found that 3D-printed insoles using thermoplastic polyurethane polyester-based polymer with hexagonal infill pattern and 40% infill density had lower average peak pressure than the standard insole [28]. [PERSON] et al. also reported that another filament that they used for 3D-printed insoles showed slightly higher average peak pressure than the standard insoles. This suggests that wearing insoles is beneficial in plantar pressure reduction. This benefit is also supported by [PERSON] and colleagues, who confirmed that 3D-printed personalized metamaterial insoles could reduce plantar pressure in offloading regions [29]. A total contact insole (TCI) with graded stiffness showed a reduction in peak plantar pressure better than a normal TCI, a flat insole with graded stiffness and a normal flat insole during walking, especially during upward acceleration [9]. Our study used 3D-printed flat insoles that might not fit the foot shape of each participant, but they were able to reduce plantar pressure at the heel. In clinical practice, contoured insoles are provided to diabetic patients due to their offloading performance [27, 30]. The material selection for insoles is also important for offloading performance, and soft insole materials usually have better performance than rigid insole materials. Furthermore, the thickness of an insole can influence offloading performance as well as energy absorption. In our study, 3D-printed insoles with 3 mm thickness were tested. This might be a reason why there is no significant difference in plantar pressure reduction and area under the curve of force-time plot when compared to walking without insole.