Applying dry adhesives to the real world [electronic resource]
Keywords: Mechanical Engineering.

The field of gecko-inspired adhesives is a relatively young one. It was born out of the discovery in 2002 of the mechanism of adhesion in the gecko, namely van der Waals forces. It is the microscopic features on the toes of the geckos that are responsible for this adhesion, both the setal stalks and, adorning the setae, the nanoscale spatulae. These features allow adhesion to nearly every surface, rough or smooth, due to their compliance and ability to make intimate contact with the surface. The setae and spatulae are made of beta keratin, which doesn't attract dirt, leading to reusability and even self-cleaning properties. The features only adhere to a surface when loaded in shear, allowing the adhesive to be turned on and off. The time constant of the features is very small, allowing some geckos to take over a dozen steps in a second. Over the past decade, countless research articles have been published describing adhesives that mimic some of the behaviors of gecko adhesive. Most are reusable, some are able to be turned "on" or "off, " others show basic self-cleaning abilities. But all of this research focuses exclusively on mimicking the micro- and nanoscale features of the gecko without considering a question critical to making these technologies useful in the real world: How should gecko adhesives be implemented to efficiently support real-world loads? This is the guiding question of the following work. Because natural gecko adhesives already function well in the real world, there is much to learn with regard to this question from the gecko at a scale slightly larger than that which has been focused upon to date. At the millimeter and centimeter scale, there are lamellae, onto which the seta are affixed, each supported by a number of tendons. Roughly 20 lamellae are on each toe, and 5 toes comprise a foot. The first and fifth toes work in opposition, allowing the gecko to turns its adhesive on by pulling v these toes together, or by pulling two opposing feet together. This work explores these larger features of the gecko's anatomy to help answer the guiding question. In this work, two design principles are developed to efficiently support real-world loads with gecko adhesives. These principles inform the design of an ankle built around a tile loaded by a tendon that is directed at the center of pressure of the adhesive (the tendon-loaded tile concept). The ankle allows robots as large as 4 kg with payload to climb vertical glass walls, and the adhesive on the device is shown to achieve nearly identical performance to a small sample of adhesive tested in a carefully aligned setup. Building upon the design principles and the tendon-loaded tile concept, a family of designs provide solutions to the guiding question for a broad set of real-world loading situations. A single tendon-loaded tile has a maximum adhesive size and therefore load: if it becomes too large it no longer can match the slight non-flatness seen in real- world surfaces. Therefore, to expand the capabilities of the tendon-loaded tile to large areas, a new method of load-sharing among many small tendon-loaded tiles is described. Very little drop off in adhesive performance is shown, even as the adhesive area increases by four orders of magnitude. A device implemented with such load- sharing allows a human to scale a vertical glass surface with a hand-sized area of gecko-inspired adhesive. During climbing, the tendon-loaded tile is lifted from the surface between steps. However, for very small "micro"-robots this requires mechanical complexity that be- comes difficult to incorporate at a small scale. Therefore, to expand the implemen- tation of adhesives to a small scale, a highly anisotropic version of the adhesive is introduced that creates 200 times more adhesion in one shear direction than the other. A 20 mg climber with two tendon-loaded tiles, each equipped with anisotropic adhe- sive and connected by a single actuator, shows the ability to climb at a scale orders of magnitude smaller than any previous climber. In the above climbing applications, the load is directed mostly along the surface in the tangential direction. In order to support the real-world loading situations where the load is directed perpendicularly from the surface, a pair of tendon-loaded tiles are vi loaded in opposition, inspired by the opposed toes and feet of the gecko. A device with opposed tiles loaded by a common tendon shows the ability to grasp smooth surfaces while requiring almost no pressing force to initiate the grasp and almost no pulling force to release the grasp when desired. This opposed adhesive device is modified for rapid attachment and shown to allow a flying micro air vehicle to perch on a smooth vertical surface. All of the above solutions based on the tendon-loaded tile work on nominally flat surfaces. In order to expand the implementation of adhesives to non-flat surfaces, the rigid section of the tendon-loaded tile is replaced with a thin film cast directly with adhesive. A device is shown grasping a large variety of surface textures and shapes, and a model is presented describing the relationship between surface curvature and maximum adhesion.