Temperature-Driven Modeling of Foamed VarioShoreTPU for the Optimization of Hardness Distribution in Prosthetic Stems

Due to the high rejection rate of prosthetic devices caused by discomfort or poor fit—often linked to rigid or inadequately adapted sockets—this study introduces a method to optimize local hardness in prosthetic components by utilizing the temperature-responsive foaming behavior of the VarioShoreTPU filament. A controlled temperature gradient was applied to evaluate the relationship between print temperature, dimensional expansion, and material hardness, allowing the derivation of a foaming factor (FF) and the corresponding extrusion correction. A custom algorithm was developed to model residual limb geometries, simulating localized compression, and generating STL and G-code files with spatially variable material properties. By differentiating external surfaces from internal core regions and applying distinct temperature and infill settings, it became possible to achieve zone-specific stiffness modulation within a single-material print. The methodology is compatible with standard fused filament fabrication 3D printers and does not require specialized hardware modifications. The experimental results revealed a clear temperature-to-foaming correlation, with FF peaks at 189% at 225°C, while the corresponding extrusion multipliers decreased to 0.53. Hardness testing indicated Shore A variations from top-to-bottom ranging from 5% to 22%, particularly in highly foamed regions. The system demonstrated reliable dimensional accuracy, repeatability, and surface control, thereby supporting the development of custom prosthetic solutions with embedded mechanical gradients. Future improvements will focus on implementing planar temperature variation within single layers to further refine local stiffness control in anatomically adaptive prosthetic designs.