Engineering Micropatterned Dry Adhesives: From Contact Theory to Handling Applications

Abstract

Reversible adhesion is the key functionality to grip, place, and release objects nondestructively. Inspired by nature, micropatterned dry adhesives are promising candidates for this purpose and have attracted the attention of research groups worldwide. Their enhanced adhesion compared to nonpatterned surfaces is frequently demonstrated. An important conclusion is that the contact mechanics involved is at least as important as the surface energy and chemistry. In this paper, the roles of the contact geometry and mechanical properties are reviewed. With a focus on applications, the effects of substrate roughness and of temperature variations, and the long-term performance of micropatterned adhesives are discussed. The paper provides a link between the current, detailed understanding of micropatterned adhesives and emerging applications. The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201800865.are designated for locomotion over various types of substrates, involving attachment and detachment within milliseconds. [1][2][3][4][5] It is of interest for evolutionary biologists that many foot pads are strikingly similar in morphology: elongated fibrillar structures with aspect ratios ranging from 10 to 80 leading to terminal elements with very different shapes (Figure 1a and Figure 3). [6][7][8][9] For some animals such as the gecko, the adhesive ability of their toe pads can be attributed to van der Waals interactions and, to some extent, to capillary forces. [10][11][12][13][14][15] In the gecko, the fibrillar pads are composed of millions of keratinous hairs (called setae) that branch into even finer terminal elements (spatulae) (Figure 1a). [16,17] Such a hierarchically organized structure results in a soft and compliant surface, which allows easy adaption to roughness at the expense of little strain energy and thus enhances adhesion.More than 1000 reports published in the field of bioinspired dry adhesives over the last two decades reflect a considerable interest in resolving the underlying adhesion mechanisms, e.g., refs. [18][19][20][21]. Application-oriented publications were motivated by a strong interest in creating novel gripper devices and pickand-place systems (Figure 1b), e.g., refs. [18-20] and [22][23][24], climbing robots for terrestrial and extraterrestrial activities, [25][26][27][28][29][30][31][32] new gasket designs in microfluidics, [33,34] or novel solutions for biomedical applications. [35][36][37][38] The present feature article aims to describe the path from fundamental considerations, including a detailed understanding of the relevant contact mechanics, to emerging applications. It is now known that the success of mimicking micropatterned dry adhesives critically depends on the interplay between design parameters, such as dimensions of the structure elements and the terminal tip-shape geometry, and the bulk and surface properties of the materials in contact. How these parameters control the adhesive performance of synthetic...

The role of adhesion in contact mechanics

“…One of the keys to this impressive performance is indeed 'contact splitting'. Hensel et al [74] found that in general, the pull-off force P n for a micropattern with n contacts is related to that for a contact without splitting, P 0 , by P n = n s P 0 (34) where s is called the 'contact splitting efficiency', which for hemispherical tips is equal to 1/2. Further, if one applies fracture mechanics arguments to the detachment of an elastic flat-ended pillar of diameter D p perfectly bonded to a rigid substrate, the energy release rate is found as…”

Switchable Adhesives for Multifunctional Interfaces

“…As these concepts are deeply intermingled with measurement, we attempt to provide some guidance toward "best practices" with respect to adhesion measurement. This review will focus on the ability to change the adhesive state of an interface, rather than increases of adhesion alone (e.g., adhesion enhancement of a patterned substrate over a featureless substrate does not constitute switching), as typically demonstrated with fibrillar features, [17][18][19][20][21][22][23][24] wrinkled surfaces, [25][26][27][28] or other surface features. [17,[29][30][31][32][33][34] We will also exclude explicit discussion of friction in order to avoid complex questions posed by "dynamic" adhesion (such as how slow must an object slide to be considered adhered) and we will exclude direct discussion of cellular adhesion.…”

Sleeved Bending Actuators for Soft Grippers: A Durable Solution for High Force-to-Weight Applications

“…Existing soft grippers rely on various technologies, including granular jamming [1], underactuated cable fingers [2], bending fluidic muscles [3], electroadhesive sheets [4], etc. Among soft gripper technologies, the most interesting with regard to force density are shape-memory alloys (payload-to-weight ratio up to 925 [5]), electroadhesion (ratio up to 54 [4]), geckoadhesion (ratio up to 286 [6]), and fluidic actuators (up to ratios of 68 [7]). These impressive densities are, however, counterbalanced by a low frequency and high hysteresis (shape memory alloys) or high dependence on object surface (adhesion technologies).…”