The hairy attachment system of spiders enables these animals to walk upside-down on rough and smooth surfaces and support a multiple of the body weight without the use of glue. These outstanding biological structures including hundreds to thousands of specially designed hairs that are made of composite materials consisting of proteins and chitin fibers. Under attachment, the shape of the contact elements and their self-orientation in parallel with a surface change and lead to the establishment of van der Waals forces between the hair tips and the substrate. However, it is not completely understood how specialized structural features of the finest contact elements of single attachment hairs and their changes guide the attachment process, which makes this natural attachment system superior to artificial systems.

The goal of our study is to gain an in-depth understanding of the working principle of the attachment and detachment processes of single hairs to the surface. Synchrotron-based methods like scanning X-ray nanobeam diffraction in combination with nanotomography are ideal tools to reveal the inner structure of the spider hairs, especially the gradient of the mechanical properties, which is essential for the attachment process. In scanning X-ray nanobeam diffraction, simultaneous measurements of wide angle X-ray scattering (WAXS) and the small angle scattering (SAXS) allowed gaining quantitative in-depth information with 200 nm resolution in 2D (Fig. 1). 3D information with high-resolution (50 nm) nanotomography were obtained using Zernike phase contrast. A single attachment hair is attached and detached from a surface in in situ experiments, which have been performed successfully with both methods.

Here, we demonstrate how the smallest contact elements rearrange under attachment and structural properties, as well as mechanical properties of different hierarchical levels enable the attachment with highest contract area. Our goal is to correlate the findings from both methods to get a full picture of the attachment process of this highly tuned attachment system. We anticipate our study to potentially contribute to the development of optimized artificial dry attachment systems.