Multiple Fiber-Reinforced Internal Chambers for Thumb-Assist Soft Pneumatic Actuators

Soft actuators designed for finger assistance typically provide adequate support for flexion, but often cannot provide effective support for abduction and thumb opposition. In a previous study, we found that a continuum thumb-assistance actuator with multiple chambers (a middle chamber for flexion and two side chambers for abduction-adduction support) achieved a superior pinch performance than a multiactuator approach owing to the spatial constraints around the thumb joints. However, when pressurized, the chambers of our multichamber actuator interacted with each other, causing interference and deviant deformation, the mechanisms of which remain unknown. This study proposed the use of fiber-reinforced interior chambers to reduce the interchamber interference of semicylindrical actuators with multiple degrees-of-freedom (DoF) and proved their effectiveness. Three thumb actuators with exterior, interior, and interior-exterior hybrid fiber reinforcement windings were designed, prototyped, and compared in terms of range of motion, force output, and an enhanced Kapandji test. The causes and effects of interchamber interference were investigated using Finite Element Analysis (FEA). The FEA results suggest that the interchamber interference that occurs when simultaneously actuating the middle and side chamber can improve thumb opposition. Additionally, independent actuation of the middle and side chambers revealed that middle-to-side interference had a positive effect on thumb flexion when the middle chamber was actuated, whereas side-to-middle interference negatively affected thumb abduction when the side chamber was actuated. Our results indicate that hybrid fiber reinforcement mitigates negative interchamber interference while maintaining good multi-DoF support.