Drainage of fluid from wet adhesive-substrate interfaces is often mentioned to be an important mechanism in obtaining strong grip. It has been hypothesized that micropatterned adhesives – that contain features (e.g., pillars) separated by channels on their surface – have a better drainage capability compared to unpatterned adhesives. Nevertheless, the fluid flow behavior on micropatterned interfaces has not been analyzed before. In this work, the fluid flow on wet adhesive-substrate interfaces was captured on three focal planes and analyzed in terms of flow direction, flow trajectory and flow velocity. The aspect ratio and spacing ratio of the features on the micropatterned adhesives as well as the stiffness of the substrates were varied. Ultimately, the fluid flow velocity was related to the pull-off and friction forces of the adhesives on the substrates. From the results it appeared that the initiation of drainage through the channels of micropatterned adhesives is likely not the direct cause of changes in flow direction. A straight flow path and a semi-wave flow path are distinguished as the two main flow trajectories of the fluid flow. The flow velocity depends on the flow path width and the separation distance between the adhesive and the substrate: the flow velocity increases with a smaller flow path width, and is higher before than after adhesive-substrate contact was established. The flow velocity did not differ among the substrates, which could be the result of the effects of the stiffness and the wetting capability of the substrates cancelling each other out. Moreover, our results suggest that shear forces increase with a larger channel area and a smaller fluid volume, whereas this is not necessarily the case for pull-off forces. The outcomes of this study can be used to develop surfaces for the generation of strong grip under wet conditions.

For over a decade the use of bio inspired adhesives have been explored to achieve high reversable attachment on a wide range of surfaces. Based on the fibrillar adhesive toepads of geckos, many patterned adhesives have been produced that are able to form and preserve contact with rigid and smooth substrates. But on other types of surfaces, such as soft and deformable substrates, the performance of these micro patterned adhesives diminishes. Alternative methods have to be investigated to create adhesives that optimize the ability to form and preserve contact on these types of substrates. In this study, 3D printing is used to produce a series of adhesives with internal cylindrical porous structures of various geometry. The internal structure modulates the adhesives stiffness in the normal and shearing direction, designed to affect the friction force. Friction forces are measured on glass, soft elastomeric substrates and cloth. On glass, the highest friction forces are generated by the adhesive with the lowest normal stiffness, unaffected by their shear stiffness. On soft substrates, the highest friction forces are achieved with adhesives that combine a low normal stiffness, promoting contact formation, with a high sear stiffness, enhancing contact preservation. The effect of this anisotropic stiffness becomes more pronounced when the stiffness of the substrate decreases, contributing to contact formation. On cloth, generation of friction forces remains challenging, more research is required to create adhesives that excel on these kinds of substrates.

Reversible attachment on soft substrates is useful in a range of applications, including soft robotics and soft-tissue surgical instrumentation. On rigid substrates, the use of micropatterned adhesives has been extensively explored. It has been shown that surface micropatterns provide conformability, thereby enabling the formation and preservation of contact with the substrate. On soft, deformable substrates, on the other hand, surface micropatterns largely lose their functionality. Alternative mechanisms have to be explored to maximize conformability and thus formation and preservation of contact on soft substrates. 3D-printing is used to fabricate adhesives with internal cylindrical pores of various configurations leading to different combinations of high/low normal/shearing stiffness, and shear forces are measured on glass and on soft elastomeric substrates. On the glass substrate, shear forces are highest for the adhesives with the lowest normal stiffness, independently from their shear stiffness. On the soft substrates, the highest shear forces are achieved for the adhesives combining low normal stiffness, enabling contact formation, with high shear stiffness, promoting contact preservation. The beneficial effect of such anisotropic stiffness on shear forces increased with the deformability of the substrate.

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The adhesiveness of biological micropatterned adhesives primarily relies on their geometry (e.g., feature size, architecture) and material properties (e.g., stiffness). Over the last few decades, researchers have been mimicking the geometry and material properties of biological micropatterned adhesives. The performance of these biomimetic micropatterned adhesives is usually tested on hard substrates. Much less is known about the effect of geometry, feature size, and material properties on the performance of micropatterned adhesives when the substrate is deformable. Here, micropatterned adhesives of two stiffness degrees (Young's moduli of 280 and 580 kPa) were fabricated from poly(dimethylsiloxane) (PDMS) and tested on soft poly(vinyl alcohol) (PVA) substrates of two stiffness degrees (12 and 18 kPa), and on hard glass substrates as a reference. An out-of-the-cleanroom colloidal lithographic approach was successfully expanded to fabricate adhesives with two geometries, namely dimples with and without a terminal layer. Dimples without a terminal layer were fabricated on two length scales, namely with sub-microscale and microscale dimple diameters. The cross section of samples with a terminal layer showed voids with a spherical shape, separated by hourglass-shaped walls. These voids penetrate the terminal layer, resulting in an array of holes at the surface. We found that on soft substrates, generally, the size of the dimples did not affect pull-off forces. The positive effects of sub-microscale features on pull-off and friction forces, such as defect control and crack trapping, as reported in the literature for hard substrates, seem to disappear on soft substrates. The dimple geometry with a terminal layer generated significantly higher pull-off forces compared to other geometries, presumably due to interlocking of the soft substrate into the holes of the terminal layer. Pull-off from soft substrates increased with the substrate stiffness for all tested geometries. Friction forces on soft substrates were the highest for microscale dimples without a terminal layer, likely due to interlocking of the soft substrate between the dimples.

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Gecko-inspired adhesives mimic the external structure of geckos with micropatterned surfaces and the internal structure by fabric reinforcement in soft elastomer adhesive pads. Previous research measured the friction forces of synthetic adhesives, with either an external or internal structure, mainly on hard substrates. Much less is known about the effects on static friction forces on soft substrates of adhesives with a combined external and internal structure and with a contact area beyond a centimetre square. We fabricated 40 by 40 mm adhesive pads (Epad = 2.1 +/- 0.1 MPa) from polydimethylsiloxane (PDMS) elastomer and tested them on two soft PDMS substrates (Esub = 2.6 +/- 0.2 MPa and 1.0 +/- 0.1 MPa). A colloidal lithographic approach was used to fabricate the external structures of the adhesive pads with microscale dimples with and without a terminal layer (TL). The internal structures were fabricated by reinforcement of the adhesive pads with carbon fibre fabric (CFF), which varied in the types of CFF weave and its orientation with respect to the substrate. We found that samples without an external structure generated lower friction on the softer substrates, whereas samples with micropatterned surface generated similar friction between the substrates, presumably due to mechanical interlocking between the external structure and soft substrates. Samples with microscale dimples without TL generated the lowest friction forces among all samples, likely due to limited initial contact with the substrates. Samples with microscale dimples with TL generated similar friction forces as samples without an external structure. Those samples with TL were not able to generate higher friction, due to their fabrication method which restricted the movement of the fibre bundles, hindering the stress redistribution along the sample during the measurements. Samples with an internal structure showed significant higher friction compared to samples without reinforcement, due to a better stress distribution along the samples, but generated similar friction forces among the types of CFF weave.

With growing interest in wet adhesion, animals such as tree frogs are often used as paradigm for creating grippers in wet environments. Tree frogs are capable of attaching to surfaces under several conditions (smooth, rough, dry, wet, and flooded) due to the microstructure of their toe pads. This microstructure consists of pillar-shaped hexagonal cells separated by thin and deep channels. Drainage is an important phenomenon to consider, since too much liquid between the toe pad and surface results in a low adhesive force. Therefore, understanding the relationship between pillar-shaped microstructures and drainage can lead to the creation of adhesive pads that are functional in wet conditions and to other drainage related applications.

Adhesives with a pillar-shaped microstructure were fabricated and their drainage speed was experimentally studied against a glass substrate. Gathering experimental data was accomplished by using Particle Image Velocimetry, and by using fluid velocity as a proxy for drainage. A custom-made set-up was built to accurately control the speed at which the adhesive and the substrate approached each other. Adhesives with various pillar diameters, pillar spacings, and pillar alignments were tested with fluids of various viscosities. It was found that the pillar diameter most likely has a significant effect on drainage, whereas the effect of pillar spacing and the effect of fluid type could not be determined. Pillar alignment did not have a significant effect on drainage.

In future work, it is recommended to complement the findings of this thesis with adhesion and friction measurements to understand how drainage associates to adhesive force.

When designing a new nanostructure or microstructure, one can follow a processing-based manufacturing pathway, in which the structure properties are defined based on the processing capabilities of the fabrication method at hand. Alternatively, a performance-based pathway can be followed, where the envisioned performance is first defined, and then suitable fabrication methods are sought. To support the latter pathway, fabrication methods are here reviewed based on the geometric and material complexity, resolution, total size, geometric and material diversity, and throughput they can achieve, independently from processing capabilities. Ten groups of fabrication methods are identified and compared in terms of these seven moderators. The highest resolution is obtained with electron beam lithography, with feature sizes below 5 nm. The highest geometric complexity is attained with vat photopolymerization. For high throughput, parallel methods, such as photolithography (≈10¹ m² h⁻¹), are needed. This review offers a decision-making tool for identifying which method to use for fabricating a structure with predefined properties.

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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.