Objective During laparoscopic surgery tissue retraction is necessary for exposure of the operating field. Currently this is performed manually by grasping the tissue in laparoscopic forceps but could be performed automatically with an appropriate device. This would need to automatically detect when tissue is slipping from its grasp and react appropriately.
Method Polyvinylidene fluoride (PVDF), a piezoelectric polymer, is under investigation for its suitability for use as a slip sensor in this application. The initial proof of concept study investigated the magnitude and repeatability of the slip signal from the PVDF with various forces applied to the tissue. The final grasper design will use this slip signal to modulate the force applied to the tissue with the aim of minimising tissue damage through the use of inappropriate forces.
PVDF sensors were designed and developed in house and potted in silicon which protects the sensor but also permits deformation. Various patterns were made on the silicon to investigate which would provide the best slip signal. The sensors were attached to a parallel occlusion grasper which applies uniform pressure to the surface of the tissue in contrast to conventional graspers which exert a high ‘pinch’ force at the proximal end.
To determine if the signal from the slip sensor is reproducible and useful a pilot study was carried out and then a definitive study. This was done by securing the graspers in a tensiometer (Instron). Ex vivo pig bowel was placed in the graspers and retracted at a rate of 3mm/s until the tissue had fallen from the graspers. Various grasping forces were applied to the tissue. The slip data were recorded and the data from the pilot study used to define the power required in the definitive study.
Preliminary Results Initial results show that slip is easily detected using PVDF sensors attached to parallel occlusion grasper jaws. Thus when applying 4N of force to the tissue a threshold for slip detection can be set at about 5% of the peak output voltage of the PVDF. A slip signal was present for all of the sensor designs tested although patterning the contact surface of the silicon affected the slip signal. The sensor with cylindrical protrusions output the highest voltage in comparison to sensors with a flat surface design and raised ridges (87.5% and 75% respectively.) However we observed that as the applied force increases the output voltage from the sensors decreases, regardless of surface profile.
Discussion The initial data indicates that PVDF is a viable solution for a system to detect and autonomously inhibit slip without applying excessive forces to the tissue. Results showed that setting a threshold of 5% of the peak sensor output would ensure that any slip events are reliably recorded.