Soft actuation of origami-inspired reconfigurable structures through shape memory ionic polymers

Origami, the ancient art of paper folding, has recently attracted the attention of many researchers not only due to its aesthetically pleasant design, but more importantly because of the reconfigurability which makes it an ideal platform for developing deployable metamaterials. If actuated through shape memory ionic polymers (SMIPs), two-dimensional lightweight sheets can be folded by exploiting predefined crease patterns. A series of multiphysical stimuli can be therefore adopted to reconfigure two-dimensional or three-dimensional origami architectures, leading to tunable mechanical properties. To this aim, SMIPs have to be programmed to be stretched, bent or twisted to allow the origami-inspired structures switching among different spatial configurations. In this investigation, we first describe how to move from an ad-hoc Helmholtz free-energy function for coupled thermo-electro-chemo-mechanical problems and, through the Coleman-Noll procedure, obtain constitutive equations for SMIPs. A total Lagrangian weak formulation of the local conservation laws is next adopted for a finite element implementation leading to a UEL (user-defined element) for Abaqus standard. The multiphysics constitutive model needs to be calibrated through comparison with data collected in an experimental campaign, carried out to test the coupled thermo-mechanical properties and the shape memory effects of SMIP sheets. Numerical examples of SMIP-driven reconfiguration of origami-inspired lightweight structures are finally provided, specifically discussing the geometry of the soft actuation to attain folding and unfolding, and possibly to avoid instability during deployment.