Design and synthesis of a 2 DOF 9-bar spatial mechanism for a prosthetic thumb

In the last two decades, different prosthetic hands have been developed. However, little attention has been given to thumb designing. This paper presents the design and kinematic synthesis of a new 2 DOF 9-bar spatial mechanism that can be used as a prosthetic thumb focused on emulating the 5 movements of human thumb joints. The mechanism replicates the movements of the interphalangeal (IP), metacarpophalangeal (MCP) and carpometacarpal (CMC) joints, with the limitation that only the movements of the CMC joint are independent and the movements of the remaining joints are dependent on CMC movements. All the links of the mechanism are contained within an anthropomorphic shape, and its tip is capable of generating a 3D surface as working envelope. These features are the result of combining the mathematical representation of a deep knowledge of the kinematics and anthropometry of the thumb with techniques of optimal synthesis and spatial mechanisms design.