Design and experimental validation of ligament-mimetic compliant joints for 3D-printed prosthetic elbows

Abstract

This study presents the design and experimental validation of a prosthetic elbow joint based on compliant mechanisms that mimic the function of elbow ligaments. The proposed design uses crossed flexural beams to enable two modes of motion—flexion–extension and pronation–supination—while reducing the need for discrete mechanical components. Eighteen flexural-beam configurations were evaluated for each motion mode by varying the number of flexural beam elements, beam width, beam thickness, and reinforcement topology. Finite element analysis was used to evaluate maximum equivalent stress, deformation response, and actuation force at maximum deformation. The results showed a nonlinear relationship between rotation angle and both maximum equivalent stress and required actuation force. The flexural-beam analysis also indicated that beam thickness strongly influenced joint stiffness, consistent with the cubic dependence of bending stiffness on thickness. For preliminary validation, six configurations were fabricated using fused deposition modeling (FDM) with PETG and experimentally tested. The highest agreement between experimental and finite element results was observed with R2=0.924. A Pareto-based multi-objective design selection method was then applied to identify the optimal configuration by minimizing equivalent stress while maintaining low actuation force. The selected mechanisms were integrated into a lightweight prototype measuring 87.1 mm in length and weighing 160 g. The prototype achieved the required range of motion for flexion–extension and pronation–supination. However, the 1 kg load-bearing requirement was only partially satisfied, as the device supported the load in full extension but could not maintain it at approximately 90∘ of flexion. Overall, this study establishes mechanics-based design guidelines for a flexible, compact, and FDM-manufacturable prosthetic elbow joint based on compliant mechanisms.