# Infill Pattern and Density of 3D-Printed Insoles Alter Energy and Pressure Distribution in Gait
## 1 Introduction  
Diabetes mellitus (DM) is a non-communicable disease which affects the daily life of patients and is associated with poor health outcomes. A common complication of diabetic patients is diabetic foot syndrome which can be developed to a severe complication such as diabetic foot ulcer (DFU). It was reported that the global prevalence of DFU was 6.3%, which was higher in males than in females and higher in patients with type 2 DM than those with type 1 DM [1]. The management of DFU becomes a challenge in the healthcare service and it also leads to a socioeconomic burden [2, 3]. Many strategies have been implementedto manage foot DFU such as glycemic control, pharmacological therapy, topical oxygen therapy, wound dressing and debridement [3; 4]. One effective strategy of preventive management for DFU is the off-loading strategy, which aims to reduce and redistribute weight-bearing load over an area of the foot. Several off-loading devices have been applied to diabetic patients including total contact cast, removable cast walkers, orthotic devices and custom-made shoes and insoles [2; 3; 5]. The effective off-loading of diabetic foot is mandatory for ulcer healing and ulcer recurrence prevention. Additive manufacturing technology has been introduced for prototype fabrication and final product fabrication. It is the process of making an object by building material, layer-by-layer, such as 3D-printing technology. Recently, 3D-printing technology has become popular and is easily available as desktop equipment, especially fused deposition modeling (FDM). Making a diabetic custom-made insole using the FDM method can reduce the fabrication process by many steps compared to conventional insole manufacturing, which is laborious work [6]. Furthermore, conventional insole manufacturing creates harmful particles that can affect the respiratory system and environment [7]. Several studies have fabricated 3D-printed insoles for diabetic feet using the FDM method [6; 8; 9; 10]. Various thermoplastic materials can be printed with the FDM method, e.g., acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polycarbonate (PC), thermoplastic elastomer (TPE) and thermoplastic polyurethane (TPU), etc. When making a 3D-printed insole for diabetic feet, the frequently used material is TPU due to homogeneous, isotropic and soft elastic material [9]. Furthermore, aside from materials, important printing factors include infill pattern and infill density. These two factors can affect printing time, material usage and the mechanical properties and off-loading characteristic of insoles. Several computational and experimental studies have been conducted with different infill structures such as porous structural unit and lattice structures [11; 12; 13; 14; 15]. [PERSON] et al. proposed a 3D-printed insole using a Kelvin lattice structure, which can reduce plantar pressure and promotes more balanced weight distribution [13]. [PERSON] et al. performed a simulation using three lattice meshes of midsole designs including hexagonal, elliptical and circular lattice structures [14]. They showed the highest maximum stress occurring in the midsole with the elliptical lattice structure. Their work concluded that using combination of lattice structure to increase the energy absorption capacity could provide more local comfort to user's requirements. [PERSON] and colleagues reported that the gyroid structure has a high specific energy absorption capability when compared to the solid structure. This characteristic is beneficial to the orthotic insole [11]. They suggested that the infill density parameter should be considered for the selection of the structure for 3D-printed insoles. [PERSON] and colleagues varied the stiffness of 3D-printed footbeds by changing the infill density [16]. They showed that stiffer footbeds were suitable for participants who have a high body mass index. It was also reported that 3D-printed insoles show better comfort improvement compared to prefabricated insole in patients with symptomatic flatfoot [17].