Insights into the printing parameters and characterization of thermoplastic polyurethane soft triply periodic minimal surface and honeycomb lattices for broadening material extrusion applicability

Lattice structures with triply periodic minimal surfaces (TPMS) built using flexible materials are soft porous solids applicable in various fields, including biomedicine and tissue engineering. Such structures are also relevant for material extrusion additive manufacturing (MEAM), whose wide diffusion is pivotal to fostering their spread. Although design approaches are available to exploit the potential of soft TPMS, there are still manufacturing constraints that lead to practical limits on the shape and size of the structures that can be produced due to the complexities related to printing flexible materials. Besides, the computational models investigating the effect of cell type, the surface-to-volume fraction, and the combination of different periodic surfaces (i.e., graded or hybrid) on the mechanical behavior of these lattices are design aspects still debated. Here, the capabilities of MEAM to produce tailored soft lattice structures are explored by combining a design tool, numerical analyses, and mechanical testing using thermoplastic polyurethane (TPU) as feedstock material. The study addresses design issues, delves into optimum printing parameters, and analyzes a set of numerical parameters, which can be used for designing specific structures with tunable mechanical behavior, useful for healthcare and bioengineering. The printing parameters of three lattices, i.e., schwartz-P, gyroid, and honeycomb, with unit cell sizes spanning from 3 to 12 mm were studied. Their mechanical behavior was investigated using FEM simulations and mechanical testing. Lastly, the printability of graded and hybrid lattices with enhanced bearing-load capabilities have been demonstrated. Altogether, our findings addressed multiple challenges associated with developing soft lattice scaffolds with MEAM that can be used to fabricate innovative-engineered materials with tunable properties.