Shape Memory Polymers in 4D Printing: Investigating Multi-Material Lattice Structures

This paper evaluates the design and fabrication of a thermoplastic polyurethane (TPU) shape memory polymer (SMP) using fused deposition modeling (FDM). The commercially available SMP filament was used to create parts capable of changing their shape following the application of an external heat stimulus. The characterization of thermal and viscoelastic properties of the SMP TPU revealed a proportional change in shape fixity and recovery with respect to heating and cooling rates, as well as a decreasing softening temperature with increasing shape memory history due to changes in the polymer microstructure. Inspired by the advancements in 3D and 4D printing, we investigated the feasibility of creating multi-material lattice structures using SMP and another thermoplastic with poor adhesion to TPU. A variety of interlocking lattice structures were evaluated by combining SMP with another thermoplastic that have poor adhesion with TPU. The tensile strength and failure modes of the fabricated multi-material parts were compared against homogenous SMP TPU specimens. It was found that the lattice interface failed first at approximately 41% of the ultimate strength of the homogenous part on average. The maximum recorded ultimate strength of the multi-material specimens reached 62% of SMP TPU’s ultimate strength. These characterizations can make 4D printing technology more accessible to common users and make it available for new markets.