Natural Hook-and-loop Fasteners: Anatomy, Mechanical Properties, And Attachment Force Of The Jointed Hooks Of The Galium Aparine Fruit

Diaspores of many flowering plants are adapted to dispersal by mechanical interlocking to animal fur and feathers. In this study, the attachment force of the fruit hooks of Galium aparine plants is measured using a force tester. It ranges from 9.37 to 44.89 mN (x=23.23, SD=8.13, n=50) and depends on the hook size. Using light and scanning electron microscopy, it is found that the hook consists of two parts, a joint-like base and a hooked cone. The latter originates from the outer layer of the pericarp and may be considered as a modified hollow unicellular trichome. Staining of resin-embedded semithin sections with safranin and fast green shows that the hook wall contains cellulose and lignin. To calculate the Young's modulus, a mathematical model describing the effect of the complex hook geometry was applied. The hooks with and without the base are characterised by different values of the elastic moduli: 2.02 GPa and 23.20 GPa, respectively. Experimental data show that jointed hooks are better adapted to resist forces in different directions compared with non-jointed hooks. This effect is not present in many other plant hooks and is also unknown in industrial hook-and-loop fasteners.