Gecko inspired adhesives : permanent practical manufacturing, new materials, and applications
Keywords: Mechanical Engineering.

Although gecko-inspired dry adhesives have been an active research topic for almost 20 years, there remain few practical applications of the technology. One reason behind this discrepancy is that the adhesives remain difficult to produce and are primarily fabricated essentially by hand, using exacting microfabrication techniques. The one dry adhesive that is readily available today is based on a comparatively simple geometry consisting of a dense array of microscopic pillars with slightly flared tips. This geometry lends itself to the creation of durable molds for high volume production, and fast molding processes. In contrast, directional adhesives, which have some practical advantages including the ability to control the amount of adhesion by varying the shear load, remain essentially an artisanal product, requiring roughly a full day to produce one 8x12cm sample of material. Moreover, the molds used to create these samples last only a few cycles. The motivating hypothesis behind this thesis is that if directional adhesives could be manufactured at high volume and low cost, they, like velcro, could find a wide range of new applications, thereby spurring further refinement and differentiation of the technology. The first contribution of this thesis is to present a method for creating durable metal molds that accurately reproduce the microscopic geometry and surface finish of soft wax and epoxy molds used previously. While metal molds are used for items like vinyl records that also involve microscopic features, the challenge with directional adhesives is that the features are very deep, sharp, and overhanging, so that most micromachining and lithographic methods are precluded. The replication process involves several steps to create a multi-material all-metal daughter mold from a micromachined wax original. We show that the metal molds survive many demolding cycles without degradation and without a need for mold-release to prevent sticking. Adhesives cast from the metal molds are indistinguishable from those cast from original wax molds. An additional benefit to having a metal mold is that it permits molding under elevated temperature and pressure. This greatly accelerates the fabrication process. It also permits the use of elastomers that require hot compression molding but are much tougher than those cast at room temperature and pressure. We show that the adhesive properties of one such material are equivalent to those obtained previously using ambient pressure casting in a soft mold. An additional topic explored in this thesis is the creation of metal molds that capture special features obtained by a post-treatment or ``inking'' process performed on an adhesive sample. The inking process can increase adhesion but has rarely been used because it requires very precise surface alignment and is slow, and must be performed individually on each sample. A metal mold that captures the inking geometry makes it possible to obtain the benefits of this process without the attendant time and labor. A related topic explored is the potential to apply a coating to cast adhesives that improves their surface finish and ease of cleaning. We demonstrate a thin durable coating that has this effect and noticeably increases adhesion on surfaces with microscopic roughness (e.g. painted interior surfaces). These extensions are examples of secondary processes by which directional adhesives can be refined and specialized for different applications. The final main contribution of this thesis is to explore an example of a novel application that could benefit from the availability of durable, low-cost directional adhesives. Most applications to date have been specialized (e.g. climbing robots). Here we explore directional adhesives for power transmission and specifically for a belt-driven continuously variable ratio (CVT) transmission. Unlike conventional CVTs based on Coulomb friction, a transmission using adhesives has the potential to transmit large torques with very low normal force, which in turn permits a lightweight design. The success of this application hinges on efficient power transmission. We show that the efficiency of a smooth belt drive using directional adhesives is comparable to that for a smooth belt with friction. The small energy lost in attaching and peeling from a roller is compensated by the reduced belt tension and lower friction in the system. This example suggests that, like velcro, directional gecko-inspired adhesives might find many new applications if it becomes practical to manufacture them at low cost and high volume