High stiffness auxetic cells with intrinsic heating for thermoforming as potential alternative of polypropylene or polyethylene sheet materials for orthoses.

Conventional custom orthotic devices such as scoliosis braces are typically manufactured from monolithic polyethylene or polypropylene sheets, which limit comfort and functionality. Although advanced manufacturing methods enabling functional integration in polymer components have been developed in recent years, they have not yet been adopted by orthotic manufacturing. This study investigates the feasibility of developing a potential alternative to polyethylene/polypropylene sheets under specific conditions, using 3D-printed auxetic metamaterials made from high-performance thermoplastics, enabling improved adaptability and integrated functionality. Differential scanning calorimetry and tensile testing were performed to characterize the polymer and printed structures. Additionally, a wire encapsulating additive manufacturing process was employed for embedding heating elements, providing intrinsic thermal activation for localized and controlled thermoforming. The results indicate that auxetic structures can reach stiffness values in the range of conventional sheet materials in selected orientations, while offering adaptive stiffness and negative Poisson's ratio behavior during forming. These findings highlight the potential of auxetic metamaterials with integrated heating for customizable, patient-specific orthotic solutions. This study is limited to demonstrating feasibility. Long-term durability and cyclic performance will be addressed in future work once the final cell design is completed.