Design Enhancement of Soft-Rigid Hybrid Structures for Assistive and Collaborative Robots

Robotic manipulation systems face an enduring trade-off: rigid mechanisms deliver precision and force but compromise safety and adaptability, while soft mechanisms offer compliance and safety at the cost of load capacity and controllability. This thesis addresses this trade-off through the design, fabrication, and experimental validation of soft-rigid hybrid structures for two robotics domains: wearable assistive devices for individuals with impaired hand function, and collaborative grippers for human-robot and human-tool interaction, including surgical settings. The first contribution redesigns the Robotic Sixth-Finger, a wearable supernumerary digit assisting chronic stroke patients with grasping. Diagnostic force-body analysis identified two mechanical failure modes in the original design: an unconstrained cantilever inter-modular joint and a tendon path parallel to the contact surface, both limiting grasp stability on cylindrical objects. Four targeted modifications, a ball-and-socket inter-modular joint, a curved tendon-routing path, a flexible proximal appendage, and refined module geometry, were introduced without altering existing actuation or control hardware. Bench-top torque testing showed counter-rotational torque resistance increasing from 0.082 Nm to 0.514 Nm, approximately 627% of the original failure torque, while force-sensitive-resistor instrumentation confirmed grasp effort is now distributed across all modules rather than concentrated at the fingertip. The redesign was further validated through a real-world water-jar-opening task. The second contribution addresses collaborative manipulation through a surgical co-gripper achieving a stable pinch grasp for thin instruments, a capability unattainable with conventional single-tendon under-actuated fingers. A flexor-extensor dual-tendon architecture, actuated via a manual ratchet, achieves controlled hyper-extension of the distal phalanx, while detachable, sterilizable silicone pads improve grip and hygiene. Pull-force testing across seven surgical instruments revealed no correlation between object mass and grip resistance, with surface geometry, evidenced by the highest recorded resistance of 7.1 N on a ridged scalpel handle, dominating grasp stability. Ongoing work extends this platform through two additional mechanisms: linear and circular pulley-based tendon differentials that redistribute actuation between a finger pair to compensate for asynchronous object contact, and a Flow-ScoopGripper, incorporating collapsing folds, a soft embodied constraint, and high-infill TPU fingers, extending grasping capability to liquids, granular media, and fragile objects, validated through qualitative water- and egg-handling demonstrations. Finally, the redesigned Sixth-Finger was evaluated in a pilot study with chronic post-stroke participants under a sensorimotor augmentation paradigm, wherein the device operates as an additional, non-biological degree of freedom rather than a replacement for impaired function. Two control modalities, discrete button actuation and continuous residual-motion control via wearable IMUs and principal component analysis, proved functionally viable, differing in execution style rather than task feasibility, with high completion rates and positive System Usability Scale scores across both participants. Collectively, these contributions demonstrate that targeted mechanical redesign, rather than increased actuation or control complexity, substantially improves grasp stability, force distribution, and task versatility in soft-rigid hybrid manipulators, with validated benefit extending from bench-top mechanics to real-world users.