SURGICAL ROBOTICS AND INSTRUMENTATION Remote active magnetic actuation for a single-access surgical robotic manipulator

Purpose Magnetic coupling is one of the few strategies to transmit motion through physical obstacles. This approach can be applied to remotely control and manipulate medical devices/robots inside the human body preventing the need for dedicated incisions [1, 2]. The most straightforward approach to take advantage of magnetic coupling to control a medical device inside the human body consists in simply using a couple of permanent magnets, one embedded in the device and the other operated by the user [3, 4]. A magnetic camera for laparoscopy developed by Ethicon Endo-Surgery, Inc., will soon hit the market, promising to reduce the access trauma of minimally invasive surgery [5]. However, due to the exponential decay of magnetic field strength with distance and to the friction occurring at the interface between the controlled object and biological tissues, reliable and precise motion control is hardy achievable. As represented in Fig. 1 and preliminary described in [6], the approach we propose consists in first coupling the driven and the driving object across the obstacle (i.e. the abdominal wall), and then achieving controlled motion of the driven unit on different planes by physically rearranging in space the external source of magnetic field by robotic control (e.g. in Fig. 2 the rotation of external ‘‘driving’’ magnets about their own axis induces the rotation of the internal ‘‘driven’’ magnets, which can be connected by cable transmission to a mechanism or a joint to provide actuation). This novel actuation concept can be used to achieve multiple-degree-of-freedom high-resolution motion in an ultra-compact size, ultimately allowing the deployment of a complete bimanual modular surgical robotic platform through a single 12-mm incision. Methods The robotic arm we designed, represented without the external cover in Fig. 3, is composed by the following parts: the magnetic linkage, providing anchoring, actuation and a first roll degree of freedom (DoF), a double 4-bar mechanism that provides 2 DoF planar robot (which 3 DoF constitute a robotic anthropomorphic arm), and a distal 3 DoF spherical wrist. Its external diameter is 12 mm, while its total length is 25 cm. It is designed to be introduced through a standard 12-mm trocar and coupled with an external magnetic system trough the abdominal skin of the patient. The external magnetic system provides anchoring, the rotational DoF for the manipulator and transmit actuation for the double 4-bar mechanism. In particular, 2 external DC brushless motors rotate 2 external permanent magnets. These are coupled with two internal permanent magnets, embedded on board the manipulator and free to rotate. These two internal magnets actuate the double 4-bar mechanism by cable transmission. A three DoF wrist, actuated by on board miniaturized motors, completes the 6 DoF manipulator. In the present work we emphasize the magnetic actuation concept and the design and characterization of the double 4-bar mechanism. Considering a 3-cm average abdominal tissue thickness upon insufflation, maximum anchoring and magnetic actuation forces were estimated by finite element analysis. Internal magnets (NdFeB) were selected bearing in mind the 12-mm access port constraint. Then, experimental test benches replicating the modelled configurations were developed in order to assess model predictions. Starting from the magnetic actuation force available, we designed, fabricated and tested the double 4-bar mechanism optimizing workspace, speed of motion, and force and torque at the end effector.