Significance Bird feathers have aroused tremendous attention for their contributions to the unique flight capability of birds against wind and even through bushes. Many studies have attempted to explore the mechanism underlying feathers’ superdurability. However, it is not yet clear why feathers are so superdurable. In this study, we discovered and characterized the sophisticated cascaded slide-lock system of bird feathers, which is composed of flexible hooklets, a slide rail, and spines at the end of the slide rail as terminating structures. This finding demonstrates that the superdurability of bird feathers against tears originates from their cascaded slide-lock system, not from the “hook–groove system” proposed centuries ago.

Abstract 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. However, the mechanism underlying bird feathers’ superdurability against tears remains unclear. Here, we reveal that the superdurability of bird feathers arises from their repairable cascaded slide-lock system, which is composed of hooklets, a slide rail, and spines at the end of the slide rail as terminating structures. Microscopy with a micronano manipulating system and 3D X-ray microscopy provided high-level visibility into the 3D fine structures and the entire unzipping process of feathers. The hooklets can slide along the slide rail reversibly when suffering external forces, and the sliding hooklet can be locked by the spine at the ends of barbules when larger pulling forces are applied and even slide farther away due to the unzipping of the interlocking structure with large deformation of the barbules. The elongation before separation of adjacent barbs can reach up to 270%, and the separation force can be maintained above 80% of the initial value even after 1,000 cycles of separating and repairing. These results prove that the cascaded slide-lock system ensures the superdurability of bird feathers against tears.

After a long or tumultuous flight through bushes, birds are often found preening their disordered feathers with their bill, which is considered to repair separated feather vanes (1). To understand the superdurability of feathers against tears, we examined their separation and repair by hand (Fig. 1). With one hand holding the rachis of a feather, the other hand pulled the vanes at the direction from the tip to the calamus (Fig. 1A). The feather vane was stretched and separated along the veins (barbs) in a moment (Fig. 1B, red arrow), while the separated feather vane can be repaired (red dashed line) by stroking lightly with fingers along the direction from the calamus to the tip (Fig. 1C). Remarkably, when pulled at another position (Fig. 1D), the vane was likely to be separated, not along the red dashed line shown in Fig. 1D but along a new one (Fig. 1E, blue arrow), and the separated feather could be repaired again (Fig. 1F, blue dashed line). This separation and repair process can be performed repeatedly, indicating that feathers have a high self-repairing capability.

Fig. 1. Diagram of separation and repair of a feather by hand. (A and B) The feather vane can be stretched and separated along the barbs (red arrow) by pulling on it. (C and D) Light stroking with fingers can easily repair the separated feather (red dashed line). (D–F) When pulled again, the vane may be separated at another location (blue arrow) and can be repaired again (blue dashed line). This separation-repair process can be continually repeated.

The fascinating structural features of bird feathers are closely related to the evolution, courtship, and taxonomy of birds and the unique optical and mechanical properties of feathers, which have attracted tremendous attention over the past centuries. Since Hooke (2) drafted the first rough model of feather structures in 1665, many efforts have been made to explore the structure and function of feathers. Microscale hooks and grooves (3⇓–5) and their functions have been observed and illustrated with optical and electron microscopy (1, 6⇓⇓⇓⇓–11). Unfortunately, to date, the superdurability of feathers against tears has remained linked to the interlocking hook-and-groove model, which cannot yet explain that superdurability adequately. Here, we carried out comprehensive and detailed observations to explore the superdurability of bird feathers and have demonstrated that feathers form a repairable cascaded slide-lock system. The 3D fine structures and the entire unzipping process of feathers were observed via 3D X-ray microscopy and microscopy with a micronano manipulating system, and the feathers’ self-repairing capability and superdurability against tears afforded by the cascaded slide-lock system were verified under applied separation forces. These results indicate an important step toward understanding the feather interlocking system, and the cascaded slide-lock system offers insight into the design of smart textiles and flexible devices.

Materials and Methods Cleaning of the Feathers. Before characterization, the feathers were cleaned with water and alcohol successively by an ultrasonic method. Then, the cleaned feathers were air-dried at room temperature. Scanning Electron Microscopy. The samples were sputtered with a thin layer of platinum. Field-emission scanning electron microscopy (JSM-7500F; JEOL) was performed to investigate the morphology of feathers at an acceleration voltage of 5.0 kV. X-Ray Tomography. To observe the spatial site distribution of the micro/nanostructures on adjacent barbs, an X-ray microscopy (Xradia 510 Versa system, Carl Zeiss) was used (performed by Chunjie Cao at Carl Zeiss, Inc.) at a resolution of 0.8 μm with an X-ray source voltage and power of 50 kV and 4 W, respectively. Mechanical Measurement. The separation force of adjacent barbs was measured by sticking one side (∼6 mm wide) of the closed two adjacent barbs to the pan of a sensitive balance (with an accuracy of 0.01 mg, Mettler Toledo XSl05 DualRange) and lifting the other side with a single-axis push-and-pull device at a pulling speed of 30 mm/min. Weight loss was recorded in real time; multiplied by gravitational acceleration, the weight loss was exactly the separation force.

Acknowledgments This research was supported by the National Natural Science Foundation of China Grants 21425314 and 21421061; the Top-Notch Young Talents Program of China; the National Research Fund for Fundamental Key Projects Grant 2012CB933800; the Key Research Program of the Chinese Academy of Sciences Grant KJZD-EW-M03; and the 111 Project Grant B14009.

Footnotes Author contributions: F.Z., L.J., and S.W. designed research; F.Z. performed research; F.Z., L.J., and S.W. analyzed data; and F.Z. and S.W. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission. J.A. is a guest editor invited by the Editorial Board.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1808293115/-/DCSupplemental.