The function of the pad secretion in insects

The sticky pads on the feet of arthropods outperform man-made adhesives in several aspects: they can be controlled on short time-scales, survive thousands of usage cycles without a decline in performance, work on rough and wet surfaces, and regain functionality after contamination. Understanding the structure-function relationship of these pads hence promises to reveal effective strategies for the design of technical adhesives with novel properties. A hallmark of arthropod adhesives is that they are "wet" - the contact between the soft footpads and a surface is mediated by nano-metre thin films of an oily secretion. Despite its discovery almost 200 years ago, the function of this fluid is still unclear, and recent experiments have suggested that the adhesive performance of the pads may be dominated by the properties of the solid footpad itself, reigniting the debate about the fluid's function - why do insects secrete this fluid? Does it serve as a protective layer against wear, or does it prevent dehydration of the pad itself, which would result in an increased pad stiffness, and hence reduced adaptability to rough surfaces? Others have argued that the fluid is crucial for the control of attachment forces: it may serve as a "release layer" which ensures that fast voluntary detachment can be performed with ease, but that strong attachment occurs when the fluid is depleted from the contact zone during the fast sliding events that accompany forced detachment. In this project, we address these hypotheses by quantifying the relevant physical properties of both the footpads and the adhesive secretion in separate experiments: the viscoelastic properties of the attachment pads will be quantified using indentation and state-of-the-art viscoelastic modelling, while the viscosity and surface tension of the pad secretion will be measured with a combination of dewetting experiments and contact angle measurements. The obtained information will then be used to construct and inform mathematical models which will allow us to predict the pads' adhesive performance. By comparing these theoretical predictions to experimental data on the rate-dependence of the adhesion of the biological adhesive pads in the absence and presence of the pad secretion, we will be able to separate the contribution of both the fluid and the pad viscoelasticity to the observed adhesive performance, and thereby increase our understanding of the functional significance of the pad secretion. Our study will hence not only help to solve a centuries-old puzzle, provide new fundamental insights into the mechanical coupling between soft solids and thin liquid films, but importantly yield results relevant for the design of novel man-made adhesives which successfully replicate some of the most outstanding performance characteristics of dynamic biological adhesives.

Dynamic biological adhesives, such as the footpads of insects, promise to provide inspiration for the design of novel synthetic adhesives. However, despite several ingenious attempts to produce powerful bio-inspired adhesives, the performance of the biological "originals" has not been remotely matched. One reason for this discrepancy is a lack of understanding of the key structure-function relationships, arising from the small number of studies investigating the fundamental properties of the biological adhesives themselves. In this project, we will address one of the hallmarks of insect adhesives - the presence of a contact-mediating fluid. The function of this fluid is still unclear, and will be investigated by quantifying (i) the rate-dependence of adhesion of single attachment pads in the presence and absence of the pad secretion, (ii) the viscoelastic properties of the pad cuticle via indentation and state-of-the-art viscoelastic modelling and (iii) the surface tension and viscosity of the secretion itself, using a combination of contact angle measurements and dewetting experiments on surfaces with varying surface energy. These results will be combined with analytical modelling of the rate-dependence of these "wet" and soft adhesives, in order to quantify the relative contributions of the secretion and the soft footpads to the adhesive performance, and to clarify the functional significance of the pad secretion in insects.

The potential economic impact of powerful and effective bio-inspired adhesives is significant, and includes diverse applications in small-scale assembly processes, medical applications and climbing robots. As a consequence, many groups worldwide are currently attempting to mimic animal adhesive organs, but these efforts are often limited by a lack of understanding of the basic functional principles of the structures to be reconstructed. Insects are arguably the most diverse potential source of inspiration, but the presence of a secretion, allegedly crucial for adhesive performance, has held some researchers back, as technical adhesives which leave residues are undesirable. By clarifying the function of this pad secretion we seek to provide new insights into the functional design of insect adhesives, which will be relevant for the improvement of technical adhesives through novel, bio-inspired approaches. The insights gained from this project may not only help to design bio-inspired adhesives, but also reveal novel strategies for the development of insect-repellent coatings for pest control. Previous work has indicated that selective absorption of the pad secretion can reduce the attachment performance of insects, suggesting that non-toxic insect-repellent coatings are feasible. Understanding the functional significance of the secretion will allow a more targeted design of such coatings, and the basic research conducted in this project will push this important field through Technology Readiness Level 1. It is anticipated that following the success of this project, translational grant funding such as from the BBSRC Follow-on Fund or Innovate UK will be sought to develop bio-inspired adhesives either directly through this group or in conjunction with industry partners. To this end, the communication channels of industry bodies relevant to the area will be targeted, for example the Advanced Materials Leadership Council and the Society for Adhesion and Adhesives (part of IOM3).

In addition to reports targeted at academic and industrial partners, this work will be shared with the general public via internet, social and print media (blog posts, tweets, etc), as well as via dedicated outreach efforts in schools; the PI has considerable experience with outreach, as indicated by the high average altmetric score of the majority of the previous publications (within 5% of the top scores), coverage of his work by major news outlets (BBC, The Daily Mail), interviews with high-profile podcasts such as "The Naked Scientist" and consulting work for an upcoming BBC documentary on the importance of body size.