It has been nearly 20 years since Autumn and colleagues established the central role of van der Waals intermolecular forces in how geckos stick. Much has been discovered about the structure and function of fibrillar adhesives in geckos and other taxa, and substantial success has been achieved in translating natural models into bioinspired synthetic adhesives. Nevertheless, synthetics still cannot match the multidimensional performance observed in the natural gecko system that is simultaneously robust to dirt and water, resilient over thousands of cycles, and purportedly competent on surfaces that are rough at drastically different length scales. Apparent insensitivity of adhesion to variability in roughness is particularly interesting from both a theoretical and applied perspective. Progress on understanding the extent to which and the basis of how the gecko adhesive system is robust to variation in roughness is impeded by the complexity of quantifying roughness of natural surfaces and a dearth of data on free-ranging gecko substrate use. Here we review the main challenges in characterizing rough surfaces as they relate to collecting relevant estimates of variation in gecko adhesive performance across different substrates in their natural habitats. In response to these challenges, we propose a practical protocol (borrowing from thermal biophysical ecological methods) that will enable researchers to design detailed studies of structure–function relationships of the gecko fibrillar system. Employing such an approach will help provide specific hypotheses about how adhesive pad structure translates into a capacity for robust gecko adhesion across large variation in substrate roughness. Preliminary data we present on this approach suggest its promise in advancing the study of how geckos deal with roughness variation. We argue and outline how such data can help advance development of design parameters to improve bioinspired adhesives based on the gecko fibrillar system.
The roughness and wettability of surfaces exploited by free-ranging geckos can be highly variable and attachment to these substrates is context dependent (e.g., presence or absence of surface water). Although previous studies focus on the effect of these variables on attachment independently, geckos encounter a variety of conditions in their natural environment simultaneously. Here, we measured maximum shear load of geckos in air and when their toes were submerged underwater on substrates that varied in both surface roughness and wettability. Gecko attachment was greater in water than in air on smooth and rough hydrophobic substrates, and attachment to rough hydrophilic substrates did not differ when tested in air or water. Attachment varied considerably with surface roughness and characterization revealed that routine measurements of root mean square height can misrepresent the complexity of roughness, especially when measured with single instruments. We used surface roughness power spectra to characterize substrate surface roughness and examined the relationship between gecko attachment performance across the power spectra. This comparison suggests that roughness wavelengths less than 70 nm predominantly dictate gecko attachment. This study highlights the complexity of attachment in natural conditions and the need for comprehensive surface characterization when studying biological adhesive system performance.
more » « less- NSF-PAR ID:
- 10368676
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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