Auxetic soft muscle actuators for enhanced flexibility in lower wearable rehabilitation robotics

Purpose The field of wearable robotics, which focuses on enhancing human compatibility, lightweight design and mobility, has gained prominence in advancing human rehabilitation. This study explores the potential of auxetic structures, characterized by a negative Poisson's ratio, for use in soft auxiliary muscles. Auxetic materials, which expand perpendicular to the applied force, offer unique mechanical properties beneficial for biomedical applications. Specifically, we investigate thermoplastic polyurethane (TPU) auxetic structures due to their exceptional flexibility, mobility and cost effectiveness, making them suitable for biomedical implants and soft actuators. Design/methodology/approach Using finite element analysis (FEA), we systematically vary design parameters to optimize a soft auxetic muscle structure. Parameters such as Angle, cut width, thickness, length, Footage, Number of cells and Number of columns were analyzed. The study evaluates multiple configurations of re-entrant auxetic unit cells under applied loads to identify the optimal geometry for rehabilitation applications. Findings Our findings reveal that the optimal configuration consists of an auxetic reentrant structure with five vertically stacked unit cells, organized into a column of five columns, a 60-degree angle, a 100% internal through cut, a 2 mm thickness, a 17 mm footage and a 6 mm inner side length. This configuration demonstrated superior deformation characteristics under applied loads, making it ideal for rehabilitation applications. Originality/value The optimized design promises significant advancements in rehabilitation technologies, offering improved flexibility and adaptability in wearable robotic applications. By enhancing the mechanical performance of soft muscle actuators, this study contributes to the development of more effective assistive devices, facilitating better patient outcomes in physical therapy and rehabilitation.