The purpose of this Erratum is to clarify some of the information presented in the original Letter, especially a misleading statement in the abstract: ''While contractile systems enhance the annihilation dynamics of passive systems, extensile systems act to drive defects apart . . ..'' We stress that this statement referred solely to the pair geometry studied in the paper. In general, however, activity can yield repulsion of a defect-antidefect pair in both extensile and contractile systems, depending on the relative orientation of the two defects. As described in the paper, the flow velocity induced by active backflow vanishes at the core of À1=2 disclinations, while it is proportional to the activity parameter at the core of þ1=2 defects. Its direction depends on the sign of . In extensile systems ( < 0), the active backflow at the core points towards the head of the cometlike þ1=2 defect. In contractile systems ( > 0), the active backflow at the core points towards the tail of the cometlike defect. As a result, activity will induce attraction of a pair of defects with relative orientation as in Fig. 1(a) in contractile systems, and repulsion in extensile systems. Conversely, if the positions of the two defects is interchanged, then activity will induce repulsion in contractile systems, and attraction in extensile systems.

This paper proposes a novel master-slave robotic catheter operating system with force feedback and visual feedback for vascular interventional surgery (VIS). The robotic catheter system has good manoeuvrability, it can transmit the surgeon's skill to insert and rotate the catheter and avoids danger during VIS using force and visual feedback. In addition, it can be used to train unskilled surgeons to perform VIS. We performed a simulation experiment to validate our system using an endovascular evaluator (EVE). The experimental results demonstrated that the stability and response of the system were good. The robotic catheter system is suitable for performing VIS.

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