The research leading to these results has received funding from the European Commission’s Seventh Framework Programme, project STIFF-FLOP (Grant No: 287728). A novel probe designed to estimate soft tissue stiffness in MIS I.B. Wanninayake 1 , E.L. Secco 1 , L.D. Seneviratne 1 , K. Althoefer 1 1 The Centre for Robotics Research, King's College London, w.wanninayake@kingston.ac.uk INTRODUCTION Recent advances in medical robotics have enabled surgeons to perform most complex and delicate surgical procedures, such as Mitral valve repair [1] and pulmonary resection [2] that were not possible with laparoscopy in the past. Distinct features offered by these advanced surgical systems include high definition vision, EndoWrist instruments with up to 7 degree of freedom (DoF), motion scaling, as well as natural hand eye alignment at the surgical console [3]. Nevertheless, the lack of tactile and force feedback still remains a major drawback of these robotic assisted surgical systems. With current systems, the surgeon has to rely solely on the visual information to assess the tool tissue interactions during the surgical procedure. This limited force and tactile perception could lead to the application of inadvertent forces and may result in increasing tissue trauma and accidental damages. METHOD One of the most commonly used method to visualize the surgical field in real-time and estimate tool-tissue interactions is by using a depth-sensing camera that assesses the local deformability of the tissue [4]. In addition to this, a number of other methods such as Laser Range Scanning (LRS), stereo vision and optoacoustic imaging are currently available in Minimally Invasive Surgery (MIS) to assess soft tissue deformation and also to track the orientation of surgical tools with respect to the anatomical structures. LRS is a non-contact surface registration method that provides high-resolution 3D images, though its acquisition speed is not usually suitable for real-time images. Stereovision is a technique that can be used in real-time for organ tracking and monitoring temporal motion of deformable tissue surfaces [5], but it is not very effective when the internal organs exhibits curved surfaces or are covered with fluids, as it is the case of a real procedure scenario. A thorough review of all techniques is reported in [6]. In view of the limitations associated with the above- mentioned techniques, a new approach to track tissue deformation and estimate tool tissue interactions is presented in this paper. This method uses Surface Profile Sensors (SPS), sensing elements that stay in contact with the tissue surface to continuously track its profile and estimate the tool motion and its orientation with respect to the tissue surface [7]. Fig. 1. Behavior of the Surface Profile Sensors (  ) and forces acting on the upper wall of the . As illustrated in Fig. 1, the  design has a hollow cylindrical body which is free to move in and out in the axial direction. This floating element operates under a supply of compressed air. The two circular channels    deliver air into the . The  is designed in such a way that it can deliver and discharge air out at the tip. When the probe equipped with the SPS is pressurized, the differential pressure on the upper wall of each  move it outward until a contact with the tissue surface is made. Once the contact is established, air cushion at the tip of  keeps it floating just above the tissue surface. This behavior of the surface profile sensors ensures a continuous tracking of the tissue surface, while maintaining the contact, but without applying excessive forces on it. By combining output from a minimum of three  (see Fig. 2), the system could be easily extended to three dimensional space and be used to track   , namely the distance from the surgical tool to the tissue surface and    the orientation of the tool with respect to the tissue surface, according to the following equations: Fig. 2. (a) Measuring probe clearance and orientation with respect to the tissue surface. (b) Structural geometry of the 3D palpation probe.    (      ) √((    )  (    )  (    )  )  ) (1)      √((    )  (    )  (    )  )  ) (2)