Dry adhesives can reattach to surfaces due to the reversible bond made by Van der Waals forces. These adhesives can therefore be used as gripping surface that has a high frictional load capacity, independent of the grasping force. In many grippers, the adhesive surface is often pressed into contact with another surface (’substrate’) in an open and closing motion. Since, generally, substrates have non-flat shapes, the adhesive has to be pressed into more directions at once to make full contact. An additional part to the adhesive system is needed here to transform the closing motion to a multi-directional preload on the adhesive surface. To realize this, a passive soft material behind the adhesive is added in this study.
Design objectives for such a material (’backing’) were formulated and different types of bio-inspired backing concepts (solid, sponge, and inflatable) were fabricated. To gain insight in the suitability of these backing concepts with regard to some of the objectives, minimal required preload and minimal residual stresses to avoid detaching forces, two things are measured. Firstly, backing softness was measured as the compression stress-strain characteristic of the backing. Secondly, the preload contact stress distribution of backings was qualitatively measured.
The adhesive was a thin planar adhesive material reinforced with a planar mesh. One sponge backing type and one inflatable backing type were selected as practical backings to fabricate an adhesive system with. An experiment to measure frictional performance was done with these systems whereby backing softness was varied. These cuboid adhesive systems were pressed onto a cylindrical substrate and, after removal of the preload, loaded in the direction of the reinforcement while measuring frictional load capacity and contact area.
For both these two adhesive systems types, experiments showed that an increased backing softness caused an even or greater contact area throughout the whole loading cycle. The linear correlation coefficient, between rest phase contact area and maximum load capacity was 0.96, and at the end of the load phase, between the ’slide’ contact area and ’slide’ load capacity was 0.99.
With an inflatable or sponge backing design it is possible to make a softer backing compared to a solid design made by the same material. Only the sponge backing type distributed the preload relatively even at low and high compression, owing it to its stress plateau in its compression stress-strain characteristic. Although the inflatable has an equal pressure internally and also shows such a plateau, it was found that its contact stress is not even, due to the effect of its outer hull.
Concluding, the addition of a soft backing to help make and keep contact with a general shaped substrate, and thereby increasing load capacity, promises a new design paradigm in synthetic dry adhesives. Furthermore, the results indicates functional relevance of the presence of a relatively large and soft volume between the bones and the adhesive surface of the fingers/toe pads of geckos, tree frogs and humans.

Objective: this paper aims to evaluate the adhesive and frictional properties of the keratinised epithelium on the adhesive pads of tree frogs. Modeling methods: two modeling methods have been developed. One of these methods involves the implementation of discrete fibres in a relatively compliant material matrix while the other method involves an anisotropic hyperelastic material model developed by Holzapfel et al. (HGO model). The adhesional and frictional behaviour of an epithelial cell is evaluated for the contact forces at the interface between the adhesive pads and the interface. These forces are dependent on the tree frog behaviour, which consists of proximal pulling on the limbs and adjustments in the body posture and the position of the limbs. Modeling results: a higher fibre density and fibre-matrix bonding is found to increase adhesive performance. An increase in the fibre-matrix stiffness ratio is found to be beneficial for adhesional performance, while an optimum value for this ratio is found for the frictional performance. The modeling results show that the proximal pulling on the limbs by the tree frog without positional adjustment has no significant effect on the adhesive and frictional performance. An adjustment in the body posture and the position of the limbs, however, is found to significantly increase the adhesive and frictional performance. Experimental methods: samples that mimic the tree frog epithelial composite structure are fabricated. These samples consist of a polydimethylsiloxane (PDMS) material matrix in which acrylonitril-butadieen-styreen (ABS) fibres are embedded. Experiments are performed to measure the frictional and adhesive performance of these samples. The experimental results are used to confirm the modelled results on the influence of the fibre-matrix stiffness ratio and the fibre density. Experimental results: for adhesion, the model results are in agreement with the experimental results. For the frictional response, the agreement between the model results and experimental results is less strong.

The study of adhesion mechanisms in animals such as the gecko has helped mankind with the understanding and development of novel adhesives. The adhesion mechanism of the tree frog is not fully understood, it is a wet adhesion mechanism which works differently than the dry adhesion mechanisms of animals, such as the gecko, studied previously. The wet adhesion mechanism of the tree frog may be based on the capillary force present in liquids. Researching the adhesion mechanisms of the tree frog could help develop new capillary based
adhesives. Although there are several capillary measurement methods and devices used to study the interactions between small volumes of liquids and surfaces, they would require the frog to be immobilized for accurate measurements, which constitutes as harm. This thesis begins the design and development of a system to measure the capillary force created by tree
frogs without harming them. The selected concept functions with the use of a pressure sensor connected to the capillary pressure source with capillary channels. This concept allows for a precision of 12Pa determined by the pressure sensor and a responsiveness of 30 ms determined by fluidic circuit while allowing the frog to roam free around the system input. The components were tested to verify their functionality. A controlled pressure source is used to verify the calibration of the sensor, the time constant and hydraulic capacitance; which are
the parameters that determine the accuracy and responsiveness of the system. The sensor was successfully calibrated to measure pressure linearly up to 10 kPa but the time constant and hydraulic capacitance were experimentally measured to be three orders of magnitude larger than the estimated parameters of the components. In order to solve the disparity between the experimental and theoretical values, different tubing and connections were suggested to reduce the hydraulic capacitance and a method verifying the presence of air bubbles was advised. A Sesille drop experiment was performed on the pressure sensor in order to support the principle of measurement, the results suggests the principle of measurement is valid but the issues surrounding the fluidic circuit needs to be resolved in order to complete the system.