Encountered-type Haptic Interface Using MR Fluid

Recently, surgical training techniques that can simulate various cases have become increasingly important in developing advanced treatment methods. Therefore, surgical simulators based on virtual reality technology are attracting interest as next-generation training systems. A novel encountered-type haptic interface for surgical simulators is proposed in this research. This interface has a container of magnetorheological (MR) fluid; when an operator inserts a surgical instrument into the fluid, he feels a resistive force. The advantage of this interface is that an operator can move instruments freely when it is not in contact with the MR fluid and change instruments easily. If the instrument is mounted mechanically on a haptic interface driven by servomotors, it is difficult to change surgical tools. Furthermore, the proposed device does not require a procedure for changing tools and can increase a sense of reality.

Outline of encounter-type haptic interface combining MR fluid and motion table

Concept

Fig. 1 Displaying reeaction force through a surgical instrument mounted on a haptic interface driven by servomotors [2]

In open abdominal surgery or brain surgery, many surgical instruments are used, such as, knives, cutting shears, and clamps. Therefore, a haptic interface should display the reaction force of a soft biological tissue through such surgical instruments. A simple solution to this problem is to mechanically mount an actual instrument on the traditional haptic interface driven by servomotors. However, operators lose a sense of reality when they change the instrument because they must perform a procedure for attaching and detaching the instrument to and from the haptic interface, which is not required in actual surgery. Fig. 1 shows an example of procedures for changing a surgical instrument. As shown in Fig. 1(a), an operator feels a reaction force through a knife, while in Fig. 1(b), an operator loosens a screw and detaches the knife for changing the tool when the need arises. In Fig. 1(c), the operator picks up scissors, after tightening a screw for mounting the scissors on the end effector of the haptic interface (Fig. 1(d)); he feels a reaction force through scissors as shown in Fig. 1(e). The processes in Fig. 1(b) and Fig. 1(d) are not required in actual surgery and the operator loses a sense of reality during these processes.

Fig. 2 Cutting force display using MR fluid [2]

The rheological properties of the MR fluid can be changed in a reversible manner within a short period of time by applying a magnetic flux density. Operators can feel a reaction force when they cut the fluid, as if they are cutting real tissue. The resistive force applied to the surgical instrument is controlled by electromagnetic coils. Since surgical instruments are not constrained by mechanical links, operators can move instruments freely when the instruments are not in contact with MR fluid. In other words, operators can change surgical tools easily. Fig. 2 shows procedures for changing a surgical instrument. In order to change the instrument, the operator need not loosen or tighten a screw; these procedures are similar to those involved in actual surgery.

Cutting MR fluid

Fig. 3  Conceptual scheme for displaying the cutting force of soft tissue  [2]

Fig. 3 shows a conceptual scheme for displaying the cutting force of soft tissues using MR fluid and a servomotor. In general, MR fluids are suspensions of magnetizable particles dispersed into liquids of low permeability. The difference in the permeability creates a strong magnetization of the particles that form chains aligned in the direction of the magnetic field. As shown in Fig. 3(a), by cutting chains of the magnetizable particles, an operator can feel a resistive force through a knife. The magnitude of the resistive force can be controlled by the strength of the magnetic field; this phenomenon can be applied to a haptic interface. The following movie shows a demonstration of the cutting MR fluid.

Fig. 4 Demonstration of cutting MR fluid  [1]　(Movies：[ wmv ][ mpg ])

However, since the elastic region of the fluid is small, it cannot display larger deformations of soft tissue. Details can be found in [1]. In order to eliminate this mismatch between tissues and MR fluid, a container of MR fluid is moved by a motion table driven by servomotors. Fig. 3(b) shows a simplified mechanism of the proposed haptic interface using the MR fluid for displaying the cutting force in soft tissues.

Prototype device

Fig. 5 Prototype device [1]　(Movies：[ wmv ][ mpg ])

As a first step, a simplified two degree of freedom (DOF) prototype device is developed. As shown in Fig. 5, the device consists of a fluid container, electromagnetic coils, and a motion table driven by servomotors. The coils apply magnetic fields to the MR fluid, while the container and coils are mounted on the motion table. It has been demonstrated that this device can cause large deformations of soft tissue by controlling the motion table.

Surgical simulator (ongoing)

Fig. 6 An example of system configuration for surgical simulator  [2]

Fig. 6 shows an example of a system diagram of a surgical training system including the haptic interface. The MR fluid container is driven by a six DOF motion table and the position of the surgical instrument is measured by a motion capture system. Based on the position of the instrument, the cutting force of a virtual tissue is computed by a numerical simulator using the finite element method (FEM). The position of the instrument must be measured by an external sensor because the instrument is not rigidly fixed on the haptic interface. As shown in Fig. 6, a motion capture system measures the position and sends it to FEM-based simulation software. In the simulation software, the collisions between the tissue model and instrument model are detected, and these contact points are set as forced displacements in an FEM solver. By using the FEM solver, reaction forces applied to the contact points and displacements of non-contact points are computed. The
computer graphic model is updated by the displacements of contact and non-contact points. The reaction force is sent to a control PC and the force is displayed by the haptic device. The magnetic field strength and the motion table are controlled by the control PC, thereby controlling the force exerted by the MR fluid.

References

  1. Teppei Tsujita, Manabu Ohara, Kazuya Sase, Atsushi Konno, Masano Nakayama, Koyu Abe and Masaru Uchiyama, Development of a Haptic Interface Using MR Fluid for Displaying Cutting Forces of Soft Tissues, in Proceedings of the2005 IEEE Int. Conf. on Robotics and Automation (ICRA), pp. 1044-1049, 2012. [Xplore] [RG]
  2. Teppei Tsujita, Kazuya Sase, Atsushi Konno, Masano Nakayama, XiaoShuai Chen, Koyu Abe and Masaru Uchiyama, Design and Evaluation of an Encountered-type Haptic Interface Using MR fluid for Surgical Simulators, Advanced Robotics (AR), Taylor & Francis, vol. 27, no. 7, pp. 525-540, April 2, 2013. [AR] [RG]
  3. Takuya Kameyama, Teppei Tsujita, Atsushi Konno, Xin Jiang, Satoko Abiko, and Masaru Uchiyama, Displaying Cutting Force of Soft Tissue Using MR Fluid for Surgical Simulators, in Proceedings of the 2014 IEEE Haptics Symposium (Haptics), pp. 283-288, 2014. [Xplore] [RG]
  4. Hiroki Ohnari, Satoko Abiko, Teppei Tsujita, Encountered-type Visual Haptic Display Using MR Fluid, Asia Haptics 2016, 30A-3, Tsukuba, Japan, 1 December, 2016. [YouTube]