Design of a 3D printed wheelchair cushion for offloading using functionally graded materials and novel FEM topology optimization

Abstract 3D printed wheelchair cushions made from flexible thermoplastic polyurethane gyroid structures enable highly customized shape and stiffness, offering superior pressure offloading for those at risk of pressure injuries. However, techniques to optimize these cushions have not yet been developed, preventing the realisation of this potential. This work presents the first finite element model representative of the seated buttocks on a 3D printed cushion using a novel non-linear homogenization approach. A lattice optimization approach for optimising the 3-dimensional stiffness distribution across the cushion for reducing maximum contact stress is also presented. Using our approach, the optimized 3D printed cushion had a 42% reduction in maximum contact stress compared with a uniform stiffness cushion and a 31% reduction compared with a traditional contoured foam polyurethane cushion design. The deep soft tissue stress under the ischial tuberosities (sit-bones) also decreased by 39% following optimization. This model demonstrates that 3D printed cushions can benefit the millions of people who use wheelchairs, providing tangible improvements to health outcomes. The homogenization approach for representing variable stiffness gyroid structures may be used in future work to inform the design of 3D printed wheelchair cushions for clinical applications.