Abstract
The gecko adhesion system fascinates biologists and materials scientists alike for its strong, reversible, glue-free, dry adhesion. Understanding the adhesion system’s performance on various surfaces can give clues as to gecko behaviour, as well as towards designing synthetic adhesive mimics. Geckos encounter a variety of surfaces in their natural habitats; tropical geckos, such as Gekko gecko, encounter hard, rough tree trunks as well as soft, flexible leaves. While gecko adhesion on hard surfaces has been extensively studied, little work has been done on soft surfaces. Here, we investigate for the first time the influence of macroscale and nanoscale substrate modulus on whole animal adhesion on two different substrates (cellulose acetate and polydimethylsiloxane) in air and find that across 5 orders of magnitude in macroscale modulus, there is no change in adhesion. On the nanoscale, however, gecko adhesion is shown to depend on substrate modulus. This suggests that low surface-layer modulus may inhibit the gecko adhesion system, independent of other influencing factors such as macroscale composite modulus and surface energy. Understanding the limits of gecko adhesion is vital for clarifying adhesive mechanisms and in the design of synthetic adhesives for soft substrates (including for biomedical applications and wearable electronics).
Highlights
The gecko adhesion system fascinates biologists and materials scientists alike for its strong, reversible, glue-free, dry adhesion
Both cellulose acetate and PDMS substrates were made with different composite moduli
There was no change in shear adhesion with varying composite modulus for the cellulose acetate substrates, indicating that gecko shear adhesion was not affected by underlying soft layers
Summary
The gecko adhesion system fascinates biologists and materials scientists alike for its strong, reversible, glue-free, dry adhesion. Geckos have been intriguing for both biologists and engineers due to their dry adhesive system, which allows them to stick to smooth and rough surfaces[1,2,3,4], as well as underwater[5,6,7,8,9] Their adhesion system is based on making intimate contact with a surface over a large area, utilizing van der Waals forces as their main adhesion mechanism[10,11]. Biomedical structured synthetics, have been made and tested for soft substrates, an area in which there have been no targeted gecko studies Both unstructured and structured synthetic adhesives have been designed for use on skin and biological tissue[19,20,21]. A high modulus pillar can be terminated with a low modulus tip, allowing for overall strength while maintaining high surface contact[20]
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