BIFLEX: Optimization and experimental characterization of a flexible underactuated gripper mimicking the human thumb–index pair

BIFLEX is a flexible underactuated gripper inspired by the human thumb–index pair and designed for distal precision pinch and proximal adaptive holding. Building on a previously developed soft-hybrid Fin-Ray finger, this study combines finger-geometry optimization, four-bar kinematic synthesis, and shared single-motor actuation through a four-bar and tendon–pulley transmission. Parametric analyses guide rib orientation and compliant-joint geometry, while coupled finite-element analysis quantifies tendon forces, joint stresses, and crank torque during closure. The simulated maximum tendon force was approximately 2 . 9 N , and maximum compliant-joint stress remained below approximately 3.7 MPa. Grasping experiments covered 23 object types, with ten trials per object and success defined as retention for at least 1 0 s without visible slip. Additional tests evaluated three cylindrical objects at motor commands of 120°, 150°, and 180°. Separate load-holding experiments demonstrated stable loads of 6.5 kg in an inclined single-finger configuration and 8 kg in a vertically inverted two-finger configuration. Cyclic tests without object contact characterized angular hysteresis, repeatability, and out-of-plane motion. An SLS-type angular model described the measured creep response, with fitted amplitudes of 0.336°–0.742°. These results characterize the capabilities and configuration-dependent limitations of a single-motor gripper targeting two taxonomy-inspired grasping modes.