This study focused on manufacturing 3D printed conductive re-entrant (RE) midsoles with two slicing directions (horizontal and vertical) and three infill densities. Optimal 3D printing conditions were assessed through analyses of slicing processes, morphology, compressive and electrical properties, electromechanical property, and plantar pressure analysis. The analysis of the RE midsole was further divided into three parts: Meta (MT), Midfoot (MF), and Heel (HL). As results, horizontal direction (HD) layers were stacked horizontally, while vertical direction (VD) layers were deposited vertically, with VD being 1.5 times more rigid than HD. For VD, rigidity decreased in the order of MF > HL > MT, while for HD, it was HL > MF > MT. Both slicing directions showed similar electrical properties, with conductivity improving with higher infill density. The 50% infill density demonstrated the best electrical and electromechanical properties. Plantar pressure analysis revealed that HD provided a wider pressure area and better pressure distribution. Overall, HD midsoles with 50% infill density exhibited softer compressive property and superior electrical property during compression, offering better stability by distributing plantar pressure more effectively.
The midsole is the most important functional part of a shoe, playing essential roles in body stabilization, shock absorption, cushioning, and rebound resilience. In addition, the pressure applied to the sole of the foot can be reduced by distributing the load to the sole of the foot. In order to provide these properties, a certain degree of elasticity and flexibility is required at the core of the shoe. Additionally, this can be achieved by providing cushioning through the hardness of the midsole. Recently, research has been progressing on midsoles using meta-structures and biomimetic structures to provide these functions. Especially, meta-structures are artificially designed structures with properties and functions that natural substances cannot achieve. Unlike traditional materials, the key features of meta-structures are expressed through intentionally designed repetitive structures rather than the properties of their constituent molecules. Auxetic structures among meta-structures are noteworthy for their negative poisson's ratio (NPR), which gives them unique mechanical characteristics. As a result, the arrangement, pattern, and repeating structure of a material play a crucial role in determining its physical properties. Among auxetic structures, the re-entrant (RE) structure exhibits a negative inward angle, resulting in axial compression of the inwardly turned edges, resulting in NPR behavior. This structure offers superior indentation resistance, toughness, and energy absorption compared to other auxetic structures. Therefore, it is highly suitable for shoe midsoles, which require shock absorption against repeated impacts. Recent studies have reported that applying RE structures to midsoles or insoles can reduce pressure and improve comfort, increase the contact area with the sole, and reduce peak pressure. Furthermore, in recent years, electronic footwear has a function of analyzing the user's health signal by integrating a sensor function. It can be manufactured in the form of an insole integrated with a sock and a sensor. The sensor is an elastomeric smart plantar sensing system and can be attached with a capacitive, piezoelectric, force-sensitive resistor, or pressure sensor.
Customization of midsole is an extremely important research field, as the core of a shoe requires a variety of designs depending on the shape and size of the wearer's feet. This makes 3D printing very suitable for customized modeling manufacturing, which can easily represent different physical properties depending on the output conditions. FFF (Fused Filament Fabrication) 3D printing is a method where filament is melted and extruded through a nozzle, then layered to build up the object. This allows for easy printing of models. Additionally, it can express various physical properties through the setting of processing conditions. 3D printing processing conditions, such as infill pattern, infill density, print orientation, layer height, print speed, nozzle temperature, and bed temperature, can be variously configured using a 3D slicing program or depending on the materials. Recently, many studies have been conducted on these output conditions and the performance evaluation of printed objects based on these conditions. In particular, infill conditions determine the internal structure of the printed object and the nozzle's movement path. Once the infill pattern is set, the infill density is chosen, ranging from 0% to 100%. Generally, lower infill density results in softer characteristics, while higher infill density leads to harder characteristics. Additionally, in FFF 3D printing, the printing direction is a crucial factor. The printing direction refers to the way and orientation in which the object is placed on the 3D printer's bed. This determines the manner in which layers are stacked, significantly impacting the structural properties of the printed object.
Recently, in a study developing shoe components using 3D printing, Leung et al. manufactured a heel pad with three re-entrant angles of 60°, 80°, and 90° using SLA 3D printers and flexible resins for use by diabetic mellitus patients. The results of the compression performance assessment show that flexible resin has elasticity and strength over conventional PU foam. And, when compressed to 750 N using various hardness hempisphere, the 80° re-entrant structure showed the most contact points with the compression ball. The contact point increased by more than 15% from 90°. On the other hand, it was confirmed that the lowest pressure value was identified during compression, which could provide optimal cushioning. Chen and Lee had shown the outsole designs with 3-, 4-, and 6-pointed star-shaped patterns and various thicknesses for 5.0, 7.5, and 10.0 mm, which were fabricated with a FDM 3D printer using lightweight TPU filament. The recovery absorption capacity of the prototypes was improved for adding thickness outsoles for n-pointed star-shaped outsoles. The static compressive confirmed with decreased tendency as the thickness increased, indicating the recovery absorption capacity was improved for adding thickness outsoles. In the case of surface pressure evaluation, the LW 3PS-10 presented the largest pressure area and lower pressure force, and was considered a comfort prototype. Therefore, the midsole should be able to disperse pressure exerted on the foot to reduce overall pressure.
Thus, in this research, conductive re-entrant (RE) midsole structure was manufactured according to the FFF 3D printing with various process conditions for checking the optimal 3D printing conditions. For manufacturing the midsole structure, the carbon black/TPU composite filament was used. In our previous research, this filament was confirmed the most suitable for 3D printed conductive structure. And a RE pattern was applied. Also, the 3D printing conditions with various infill densities and printing directions were applied. For infill density, 20, 50, and 80% were applied. For printing direction, two options were used: vertical direction (VD) and horizontal direction (HD). Then, the re-entrant midsole structure was analyzed by slicing image analysis, compressive property, electrical property, electro-mechanical property, and plantar pressure analysis. Additionally, the performance of the RE midsole was assessed by dividing it into three parts: Meta (MT), Midfoot (MF), and Heel (HL). Our previous studies have investigated the electromechanical properties of 3D-printed structures using CB/TPU composite filaments, focusing on both re-entrant auxetic geometries and simple cubic unit cells with varying infill patterns. These researches demonstrated that CB/TPU structures exhibited measurable electrical responses under mechanical deformation and hold potential for sensing applications. Building upon these findings, the present study expands the scope by implementing a full-scale midsole design based on the RE structure, aiming to evaluate not only compressive and electrical properties but also plantar pressure performance under simulated foot conditions. This multi-faceted approach bridges structural, electrical, and application perspectives to explore the feasibility of electrically functional midsoles for wearable applications.