A Micro-Scale Wirelessly Actuated Magnetic Surgical Tool for Minimally Invasive Grasping

Lim, Andrew 1, 2 ; Salmanipour, Sajad 3 ; Forbrigger, Cameron 3 ; Onaizah, Onaizah 3 ; Looi, Thomas 2 ; Drake, James 1, 2 ;  Diller, Eric 3

1. Institute of Biomaterials and Biomedical Engineering, University of Toronto; 2. Center for Image Guided Innovation & Therapeutic Intervention, The Hospital for Sick Children; 3. Department of Mechanical and Industrial Engineering, University of Toronto

Minimally invasive approaches to common surgical techniques have significantly improved patient outcomes leading to less patient trauma. Robotically-assisted surgery is commonly implemented to increase surgical precision and dexterity in fields such as urology and general surgery. However, fields with smaller surgical workspaces, such as neurosurgery and pediatric cases, have yet to see these benefits largely due to the lack of small, yet dexterous tools.

The smallest commercially available tool for surgical robotic systems is 5 mm in diameter. However, endoscopic neurosurgical applications require instruments ranging from 1-2 mm in diameter. Existing tool designs typically rely on cable driven actuation systems. Although this method for actuation is reliable and can generate substantial tool tip forces at larger scales, miniaturization remains a challenge. Frictional inconsistencies in cable driven systems become amplified and part fabrication is incredibly difficult at smaller scales. Magnetic actuation is an effective method for actuating multiple degrees of freedom of complex mechanical mechanisms used in various microrobotic applications. Therefore, a novel magnetically-actuated, cable-less strategy for a 5 mm gripper-wrist tool tip was investigated.

The gripper-wrist design is composed entirely of nitinol, a superelastic material often used for its shape setting properties and ability to resist plastic deformation. A single flexural joint is used to mitigate frictional inconsistencies while simultaneously providing rigidity. The gripper is composed of two parallel gripping digits connected by a semicircular spring. Permanent magnets are fixed to the gripper digits, creating local and global magnetization vectors. The gripper’s orientation and closure are actuated using an 8-coil electromagnetic system that can deliver a controlled external magnetic field. Fabrication of this device was performed in-house using a high-temperature oven at 600°C for 5 minutes to shape set and a laser welder to join the nitinol components. Permanent magnets were fixed using epoxy and left to cure for at least 24 hours.

Actuation of the gripper-wrist was characterized with various field magnitudes up to 104 mT. Full gripper tip closure and a maximum bidirectional wrist deflection of 90° were achieved. A “pick-and-place” demonstration was performed on a miniature tissue mimicking polydimethylsiloxane-based cargo using a custom, time-varying external magnetic field.

These results show that the use of magnetic actuation for surgical robotic tools is feasible for transferring miniature tissue mimicking objects representative of those found in minimally invasive surgery. Further work will involve characterizing tool tip force exertion. Ultimately, the simple design of this gripper lends itself well to further miniaturization down to the sizes necessary for neurosurgical and pediatric applications, enabling the use of robotically-assisted techniques in these fields.