Thermally Switchable Auxetic Cell Structures for Adaptive Morphing Surfaces: Design, Simulation, and Demonstrator Validation

This paper introduces a morphing surface concept based on thermally switchable auxetic unit cells for adaptive aerodynamic applications. The structure combines a flexible membrane made of thermoplastic polyurethane (TPU) with a Polyethylene Terephthalate Glycol-modified (PETG)-based auxetic lattice, embedded with resistive heating wires using the Wire Encapsulating Additive Manufacturing (WEAM) process. Upon heating, the stiffness of the auxetic cells is significantly reduced, enabling pneumatic deformation. Once cooled, the structure retains its shape due to the thermomechanical properties of the materials. Finite element simulations and mechanical testing confirm that geometric parameters, particularly arm width, strongly influence stiffness. Switching factors of up to 32 were achieved. A demonstrator featuring 60 active cells validated the concept, demonstrating reversible shape changes and programmable deformation without the need for mechanical actuators. The system offers a scalable and energy-efficient solution for morphing surfaces in automotive, aerospace, and soft robotics applications.