Mechanical properties of a soft TPMS cellular metamaterial for anatomic applications

Cellular structures, also called Lattices, are structures composed by the periodic repetition of a basic minimal unit called "unit cell" in the space. This unit cell consists of a geometric structure, partially empty, which can be composed of beams, trusses or solids generated by analytic equations. Triply Periodic Minimal Surfaces are a class of periodic surfaces generated from trigonometric functions. When the sheet surface is thickened, the resulting porous geometry exhibits interesting properties, such as low density and good mechanical strength. Moreover, it is possible to tune the mechanical properties by modifying the parameters of the geometry, thanks to a modern approach to design called Field Driven Design. This ability to architect the material properties, combined with the versatility of additive manufacturing, may prove very useful in industrial and medical uses where precise structural and geometrical characteristics are required. The focus of this work was to investigate the stiffness and the crushing behaviour of some sheet-based TPMS structures manufactured with an elastomeric polymer (TPU 95A), characterized by low-stiffness and large elastic strains. The experimental results were compared to the results obtained analytically and through Finite Element simulations, showing a good correlation. The knowledge obtained was used to design and manufacture a therapeutic insole customized on the patient, using data coming from 3d-scans of the foot and baropodometric analysis (pressure distribution of the foot on the ground) to tune the lattice geometry and obtain the required mechanical properties and the most accurate shape. The procedures and the techniques used in the design and fabrication stage were explained both practically and theoretically. To conclude, the proposed methodology allows to design a product tailored around the patient need and to minimize both the manual work and the number of materials, leading to further investigations.