The Structure of Lipids and Keratin in Adhesive Gecko Setae Determined by NEXAFS Imaging
Daniel A. Fischer, Cherno Jaye, Katinka R. Holler, Mette H. Rasmussen, Joe E. Baio, Stanislav N. Gorb, Tobias Weidner
Geckos have the astonishing ability to climb on vertical surfaces due to the adhesive properties of fibrous setae at the tips of their toe pads. While the adhesion mechanism principle, based on van-der-Waals interactions of myriads of spatula located at the outermost end of the setal arrays, has been studied extensively, there are still open questions about the molecular structure of gecko setae. Recently it has been shown that phospholipids are associated with the setal arrays and that geckos leave behind lipid footprints. However, the lipid distribution and mechanism of replenishment are still unclear. The same is true for the backbone of the gecko adhesive system, the keratin fibrils assembled to support the entire setal structure: the structure and alignment of the keratin molecules within the ultrafine spatula tissue, which can support the enormous mechanical strain still remain unknown. We have therefore studied the molecular structure of gecko spatula setae using near-edge X-ray absorption fine structure (NEXAFS) imaging. Nitrogen and carbon K-edge spectra showed that a mere nanometer-thin layer of lipids is present at the spatula surface. A simple diffusion model suggests that the replenishment of the lipid film commences via bulk diffusion of lipids through the spatula shaft. Angle-resolved NEXAFS images provide evidence that keratin structure is aligned with the adhesion direction of the setae, which supports lipids diffusion along the setae. Importantly, alignment of keratin molecules along the shaft together with similar orientation of microfibers may provide additional mechanical stability to the setae and their resistance to wear across different length scales of setal architecture.
, Jaye, C.
, Holler, K.
, Rasmussen, M.
, Baio, J.
, Gorb, S.
and Weidner, T.
The Structure of Lipids and Keratin in Adhesive Gecko Setae Determined by NEXAFS Imaging, The Journal of Physical Chemistry Letters, [online], https://doi.org/10.1021/acs.jpclett.2c00004
(Accessed December 4, 2023)