Scientists discover how bird feathers resist tearing

Chinese scientists have discovered a sophisticated mechanism in bird feathers that enhances tear resistance, overturning a centuries-old explanation of how bird feathers work. The newly discovered cascaded slide-lock system is composed of flexible hooklets, a slide rail, and spines at the end of the slide rail as terminating structures, said researchers from the Chinese Academy of Sciences.

The finding, published in the journal PNAS, demonstrates that the high durability of bird feathers against tears derives from their cascaded slide-lock system, not from the "hook-groove system" proposed centuries ago. Bird feathers have aroused tremendous attention for their superdurability against tears during long flights through wind and even bushes.

Although feathers may inevitably be unzipped, the separated feather vanes can be repaired easily by bill stroking, which shows the strong advantage of feathered wings over the membrane wings of bats and butterflies. However, the mechanism underlying bird feather superdurability against tears had previously been unclear. Since the first rough model of feather structures in 1665 was drafted, many efforts have been made to explore the structure and function of feathers.

Microscale hooks and grooves have been observed and illustrated using optical and electron microscopy. Unfortunately, to date, the superdurability of feathers against tears has remained linked to the interlocking hook-and-groove model, which ignored the fine structures in feathers and could not adequately explain their superdurability.

Researchers recently made a deep observation of the 3D fine structures and the entire unzipping process of feathers by using microscopy with a micro/nano manipulating system and 3D X-ray microscopy.
They observed a repairable cascaded slide-lock system comprising hooklets, and a slide rail with spines at the end as terminating structures.

They also clearly discerned the function of each part. The hooklets can slide along the slide rail in reverse when affected by external forces. The sliding hooklet can be locked by the spine at the ends of barbules when larger pulling forces are applied and slide even farther away due to the unzipping of the interlocking structure with large deformation of the barbules. This system not only enhances the separation force of adjacent barbs, but also prevents damage to the barbs during separation.

The separation force of adjacent barbs can be maintained above 80 per cent of the initial value even after 1,000 cycles of separation and repair. This cascaded slide-lock system of feathers ensures their superdurability and high self-repair capability, thus helping birds survive in hostile environments. These findings also provide insight into the design of smart textiles and flexible devices.