Capillary-based gripping for laparoscopic bowel surgery

Abstract

Lifting an object by capillary forces is mostly done with a single liquid bridge which connects the target object to a probe. In this work the potential of capillary forces for soft-tissue manipulation is investigated; not only a single liquid bridge is used, but multiple bridges as well since capillary forces can be enhanced by contact splitting. Specifically, laparoscopic bowel surgery was chosen, because with current instruments it remains difficult to atraumatically grip soft delicate tissues.

Three 30 by 30 mm grippers were cast from PDMS (1:10 weight ratio curing agent to base) with pillars of diameter 2, 1 and 0.5 mm, named D2, D1 and D05 respectively. The ratio of pillar area to total gripper area is 0.6. The pillar structures and a flat reference sample were wetted with demi water and placed on the substrate (glass and 15 wt.% gelatin as tissue phantom). A preload is applied to attach the gripper to the connector and both are retracted at 0.1 mm/s to measure the adhesion. From the camera footage of the measurements on glass was confirmed for D1 and D2 that capillary forces were measured. The best performing pillar structure was D1 with adhesion values between 1.3 – 3.4 kPa, but it did not outmatch the flat samples’ adhesion: 2.2 – 3.1 kPa. On gelatin, the evidence for measuring capillary forces was not conclusive. The mean adhesion of all geometries on gelatin was 0.8 kPa and there was no clear effect of the different pillar diameters on the measured adhesion values.

From these results it is concluded that capillary forces should not be used as the main method to lift tissue, but rather to complement other gripping methods such as suction.