Additive manufacturing of Functionally Graded Lattice Structures for personalized Below-Knee prosthetic dampers

Functionally Graded Cellular Structures (FGCSs) have a multitude of applications to a wide range of industries. Utilising the ever-progressing technology of Additive Manufacturing (AM), FGCSs can be applied to control material grading and achieve the desired mechanical properties. The current study explores the design and optimisation of FGCSs for AM, with a focus on improving the compression and impact performance of Below Knee (BK) prosthetic limbs made of thermoplastic polyurethane (TPU). A multiscale research methodology integrating Topology Optimization (TO), Finite Element Analysis (FEA), and Design of Experiments (DoE) was adopted to optimise lattice structures in terms of stiffness and lightweight properties. Two-unit cell designs were considered in the study: Schwarz P gyroid and body-centered cubic (BCC). Response Surface Methodology (RSM) was implemented to analyse the effect of minimum and maximum cell wall thickness, cell size, and unit cell type on the mechanical performance of TPU FGCS structures. The results indicated that a Schwarz P FGCS structure with cell size, minimum and maximum cell wall thickness of 6, 0.9 and 2.8 mm, respectively, could be optimal for a compromise between performance and weight. In this optimized case, stiffness and volume fraction values of 684 N/mm and 0.64 were obtained, respectively. The study also presents a proof-of-concept design for a BK prosthetic damper, highlighting the potential of FGCSs to enhance patient comfort, reduce manufacturing costs, and enable personalised designs through 3D scanning and AM. The obtained results could be a step forward towards the incorporation of AM technologies in prosthetics, offering a pathway to lightweight, cost-effective, and functionally tailored solutions.