Snake-like manipulators can navigate and perform manipulation in confined spaces. Their recent implementations in surgical robots attracted a lot of attentions. These slender manipulators usually possess either a hyper-redundant articulated vertebrate structure or a continuum one. Primary design considerations usually converge to a balance between proper workspace and acceptable stiffness. Efforts have hence been constantly made to achieve higher or adjustable stiffness for a manipulator to widen its applications. This paper presents a simple continuum manipulator design with variable stiffness based on redundantly arranged elastic backbones and continuously constrained bending curvature. The design concepts, kinematics, a preliminary formulation for stiffness adjustment, system construction and experimental characterizations are elaborated. The results showed that the manipulator's stiffness can be increased up to 4.71 times of the value without the curvature constraining rod, indicating the efficacy of the proposed idea.
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