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.

Background: Soft-tissue grip is a challenge in minimally invasive surgery. Grasping instruments used in clinical practice require high pinch forces in order to generate sufficient grip for manipulating soft tissue without slipping.
In nature, several animals employ adhesion in order to grip on, not only hard, but also soft substrates. Among these animals, geckos and tree frogs are of special interest for engineered gripping systems, because of their high body mass. The toe pads of both animals are soft and characterized by a hierarchical pillar structure ranging from micro to nanoscale. Several research groups have mimicked this structure and demonstrated its potential for adhesive grip. Next to pillars, the toe pads of both animals possess a network of stiff inner fibers, which possibly also contributes to (friction) grip.
Aim: Inspired by the inner fiber network of geckos and tree frogs, the aim of this work was to investigate whether reinforcing a soft pad with stiff fibers increases friction on soft substrates as compared to a fiber-less pad. Our hypothesis was that provided that a soft exterior of such a pad establishes adhesive contact with the substrate, stiff fibers at a direction parallel to the substrate reduce the compliance of the pad in this direction, thereby preserving the established contact and thus increase the peak friction force.
Methods: Three experiments were conducted. In Experiment 1, composites consisting of 3D-printed fibers encapsulated in a polydimethylsiloxane (PDMS) pad were fabricated, and their adhesion and friction forces were measured. In Experiment 2, the friction of composites with stiffer 3D-printed fibers than those in Experiment 1 was measured. Lastly, in Experiment 3, the friction of composites consisting of a carbon fiber fabric encapsulated in PDMS of various stiffness degrees were tested. All experiments were conducted on both hard and soft gelatin substrates of various stiffness degrees, the latter functioning as soft-tissue phantoms.
Results: The results showed that, for all substrate types, adding 3D printed fibers with varying degrees of stiffness to a PDMS pad significantly reduced peak friction force, whereas adding a carbon fiber fabric significantly increased peak friction force compared to a fiber-less PDMS pad. The PDMS stiffness did not have a significant effect on friction force.
Conclusion: The results from this research are promising for developing fiber-reinforced composites for gripping to soft substrates, including minimally invasive grasping instruments for soft-tissue grip.