SPIDER ATTACHMENT HAIR: MICRO- AND NANOSTRUCTURES TUNED FOR STRONG ADHESION
The mechanism of alignment and, consequently, intimate contact of the micro- and nanostructures of adhesive spider hair to a substrate surface was examined using scanning nanofocus X-ray scattering under force control. The results show that the mechanical properties of the hair structures are well adapted to their specific function on several length scales.
Wandering spiders climb vertically and walk upside down on rough and smooth surfaces using a hierarchically structured attachment system on their feet (Figure 78a). The spiders are assumed to adhere by intermolecular van der Waals forces between their adhesive structures and the substrate. The adhesive elements are highly ordered on the attachment hair (setae) . It has been suggested that, while walking, the spiders apply a shear force on their legs to increase friction.
The attachment and detachment process of single setae was examined in situ using combined scanning nanobeam small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) at the nanofocus extension of beamline ID13. The delicate cuticular structures and their changes at different attachment stages and contact geometries were visualised with high spatial resolution. Simultaneously, the attractive forces in the micro-Newton range between a glass surface and the fresh, chemically untreated attachment hair were recorded using a force-sensing cantilever.
Each single hair is built of a backbone and a multitude of so-called microtrichia branching off downward (Figure 78b). At the tips, the microtrichia are widened and form platelet-shaped contact structures (spatulae) approximately 1 µm wide and 20 nm thin. The arrangement of the microtrichia can be interpreted as an adaptation for effective attachment of the adhesive structures by their exposure for immediate contact with the substrate during the spider s locomotion. Without the need of strongly pushing them onto a surface, the hair sticks well to surfaces easily and generates quite high attachment forces (Figure 78c).
Multiple spatulae in contact with the substrate are responsible for large adhesive forces (Figure 79). Upon contact, the small-angle scattering intensity of the spatulae directly at the interface between the hair and the glass surface strongly increased (Figure 79b), pointing to their contact and alignment with the surface perpendicular to the X-ray beam.
Fig. 78: a) Adult female wandering spider of the species Cupiennius salei. The arrows point to the location of the attachment hair on the tips of the legs. Image courtesy of B. Poerschke.
b) Single attachment hair in side view with its tip adhering to a vertical glass surface. The arrow indicates the direction of pulling to obtain the retraction force curve in (c). c) Force measurement of an
attachment (approach) and detachment (retract) cycle of the hair shown in (b).