Objective To develop a permanent magnet-based probe with adjustable magnetic field strength for retraction and controlled release of magnetised tissue during laparoscopic surgery.
Method Probe development was conducted using a computer-aided design (Solidworks) and a finite element method package (COMSOL Multiphysics) for simulation and optimization. Two designs were exploited: Design 1 changes the gap between the magnet and magnetised tissue by moving a magnet along the longitudinal probe axis; Design 2 changes magnetic dipole direction by rotating a magnet mounted at the distal portion of a probe shaft. In both designs, transmission mechanisms are used to actuate the movement of the distal magnet. Ex-vivo porcine bowel was magnetized by injection of 2mL magnetic media (stainless steel SS410 microparticles suspended in glycerol/PBS fluid). A tensiometer (Instron) was used to record magnetic retraction force between the probe and the target magnetized tissue. Previous studies had shown a requirement of 2.5N for bowel retraction.
Results Prototype design 1 consists of a 10mm probe with a 20mm-long 8mm-diameter Neodymium iron boron magnet (NeFeB). This gave a maximal magnetic attraction force of 3.5N when in contact with a magnetized target bowel. The force could be reduced to 0.3N by controlled movement of the magnet, thereby enabling detachment of the probe from the target bowel for repositioning or removal of the retraction. Prototype design 2 consists a 15mm-long 5mm-diameter NeFeB magnet (diametrically magnetised) in a thin shell housing which is hinged to the probe shaft at its distal end. The maximal magnetic attractive force is obtained when the magnetic dipole axis is turned and oriented perpendicular to the surface of the magnetized tissue, producing a retraction force of 3.89N on the magnetized bowel. The magnetic force on the magnetized tissue is reduced by altering the orientation of the magnet. However with this design the minimal retraction force on the magnetized tissue achieved was 1.5N and this required actively lifting of the probe from the tissue for detachment.
Conclusions There are two problems in the construction of magnetic probes: (i) a large magnet is needed to provide sufficient attractive force, and (ii) magnetized tissue can only be released from the magnetic probe when the retraction force exceeds the maximal magnetic attractive force. The present study addressed these problems by exploring two designs: (i) a 10mm probe with axially movable magnet can provide sufficient force for bowel manipulation and with controlled release of the retracted tissue; (ii) although a 5mm-probe with rotational distal magnet can provide equally sufficient force for bowel retraction but with this design, reduction of magnetic force is currently insufficient for release, and further design development is needed.