Evidence that gecko setae are coated with an ordered nanometre-thin lipid film

Abstract

The fascinating adhesion of gecko to virtually any material has been related to surface interactions of myriads of spatula at the tips of gecko feet. Surprisingly, the molecular details of the surface chemistry of gecko adhesion are still largely unknown. Lipids have been identified within gecko adhesive pads. However, the location of the lipids, the extent to which spatula are coated with lipids, and how the lipids are structured are still open questions. Lipids can modulate adhesion properties and surface hydrophobicity and may play an important role in adhesion. We have therefore studied the molecular structure of lipids at spatula surfaces using near-edge X-ray absorption fine structure imaging. We provide evidence that a nanometre-thin layer of lipids is present at the spatula surfaces of the tokay gecko (Gekko gecko) and that the lipids form ordered, densely packed layers. Such dense, thin lipid layers can effectively protect the spatula proteins from dehydration by forming a barrier against water evaporation. Lipids can also render surfaces hydrophobic and thereby support the gecko adhesive system by enhancement of hydrophobic-hydrophobic interactions with surfaces.

Keywords: adhesion; biotribology; integument; reptiles; spectro-microscopy; toe pad.

Figure 1.

Gecko adhesive system and experimental geometry. (a) Photograph of a gecko toepad attached to a glass surface. The setal arrays are visible. (b) SEM image of the setal arrays. The terminal spatulae shafts and spatula form the contact with the surface. (c) Spatula in contact with a surface. (d) Experimental geometry. With a probing depth of 5–10 nm, the recorded images are representative of the spatula surface.

Figure 2.

NEXAFS spectra of setae arrays and reference gecko tissue. (a) NEXAFS image of the PEY integrated over the entire nitrogen K-edge. The image shows the different types of tissue glued to the sample holder with copper tape. The image was recorded at a NEXAFS incidence angle of 30°. A full NEXAFS spectrum can be extracted from each pixel within the image. The ROI for the extraction of spectra for the setae array and other tissues for comparison (dorsal scales of the body, dorsal scales of the foot and eyelid scales) are indicated as boxes in black and white. (b) Carbon K-edge NEXAFS spectra were extracted from NEXAFS images of the setae arrays as well as the gecko scale tissue. (c) Nitrogen K-edge NEXAFS spectra extracted from the same ROIs as the carbon spectra. (d,e) NEXAFS C K-edge spectra of adhesive pads before and after lipid removal were extracted from images recorded with 80° (near-normal) and 30° (glancing) X-ray incidence angles. The difference spectra are representative of the lipid coating. (f) Difference spectrum of the difference spectra are shown in (a) and (b). The spectrum shows the angle dependence of the difference spectra in (a) and (b) and provides information about the orientation of lipids. (g) Schematic of a spatula and the lipid orientation determined from the spectrum shown in (f). Flat protein sheets (yellow) are covered and infused with lipids (grey).