Haptic feedback is often employed in medical simulators, with the goal to improve user interaction and the training outcome. One option for providing touch sensations is using passive haptics, by including actual physical mock-ups of the anatomical objects in the simulated scene. While this approach has advantages, the usual one-to-one mapping between virtual and physical objects is a fundamental drawback, especially when invasive scene alterations are to be performed, such as cutting or

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... g. In this work we propose to alleviate this situation by modifying the mock surgical instruments used for interaction. Twisted string actuation is employed to display variable stiffness while indenting an anatomical model. Quantitative experiments characterizing the performance of a testbed are reported and a prototype system for a surgical bone drill is introduced. Results show that the setup is capable of providing different stiffness augmentations, representing varying bone densities.