Want to fly over eaves and walls? You need my feet

Every young man has such a martial arts dream - holding a sword, wearing a bamboo hat, flying over eaves and walls, and doing justice! In our real life, there is really such a "hero" who has extraordinary light skills and can achieve "freedom on the wall" in minutes.

It is our protagonist today - the gecko.

01

Tips for Gecko Walking

Gekko geckos can move freely on vertical surfaces, ceilings, and in rock crevices. They are masters of all-space movement. This is mainly because the geckos’ feet have the ability to stick to something . Gekko geckos are large, relatively heavy animals that stick to something.

Photo of a giant gecko in captivity

Scientists have already conducted relatively mature research on its adhesion mechanism. The gecko's motor adhesion organ is a typical multi-level, distributed system - there are five independently distributed toes on each sole, and each toe is distributed with 10-15 arc-shaped flaps, which are covered with millions of micro-nano-scale bristles. The adhesion of the sole mainly comes from the intermolecular force generated by the tiny bristles at the bottom and the substrate surface . These bristles are about 5μm in diameter, which is only one-fiftieth of the diameter of human hair. They are really not as strong as a strand of hair! The generation of this adhesion depends on a multi-level adhesion system (including sole-toe-flap-bristle) and the efficient and precise regulation of this adhesion system.

The multi-layered adhesion organ of geckos

02

See how geckos achieve "sticking" freedom

The giant gecko is the heaviest of all adhesive species. Its control over adhesion ability is not only due to the hierarchical adhesion structure, but also depends on the efficient and fine regulation of the adhesion structure . The sole is the adhesion organ of the giant gecko, which has the structural characteristics of rigid-flexible coupling. The study of the movement behavior of the sole during the adhesion-detachment cycle is the basis for understanding the wide adaptability of adhesion movement. The adhesion-detachment movement cycle of the sole is divided into support and swing phases, among which the support phase is mainly divided into three stages: contact , adhesion and release .

Different stages of the gecko's front and rear paw adhesion-detachment process

Existing studies have shown that the movement posture and layout of the gecko's feet during the adhesion stage are related to the adhesion force (direction and size). For example, the gecko changes the layout of the five toes to face inclined planes with different angles and gravity loading directions, and changes the support posture of the toes during the support stage to cope with substrates of different shapes. In addition, geckos can adapt to the movement needs of different environments by adjusting their movement gait . For example, the adhesion time of the feet is adjusted when moving in different directions on the vertical plane, and the movement speed on substrates with different angles changes significantly. However, the study found that the time it takes for a single toe to complete adhesion remains unchanged.

03

Scientists secretly photographed gecko's feet

The research on the regulation of foot movement behavior in adhesion movement is not perfect. What are the foot movement behaviors in different stages of adhesion-detachment movement? Will the movement behaviors at each stage affect the effect of foot adhesion? These questions are still unclear.

In order to systematically study the movement behavior of the gecko's soles during the adhesion-detachment cycle and understand the adaptive regulation strategies adopted by the gecko's soles in different movement environments, researchers studied the gecko's adhesion-detachment movement behavior on different inclined surfaces (horizontal, 45° inclined and vertical) and in different movement directions (upward and downward). It can be said to be a "straight shot against the foot".

Researchers conduct experiments on giant geckos in the laboratory

During the study, a motion capture system was built to accurately measure the movement behavior of small animals with large flexible organs. The marking points on the trunk, limbs, soles and other organs of the gecko were accurately captured during the climbing movement.

Therefore, the movement behavior of the gecko was recorded and quantified, a series of movement parameters were defined to quantitatively characterize the movement of the soles of the feet, the behavioral patterns of the adhesion-detachment movement of the front and rear soles of the feet were analyzed and summarized, the differences and similarities of the adhesion-detachment movement patterns of the front and rear soles of the feet were compared, and the collaborative relationship between the soles of the feet and limbs was summarized.

The motion behavior-reaction force synchronous precision testing system built by the Bionics Institute of Nanjing University of Aeronautics and Astronautics

04

Follow me in slow motion

The adhesion-detachment movement cycles of the front and rear paws of the gecko remain surprisingly similar in different environments, such as: the percentage of contact time during the contact phase (front paw: 7.42%; rear paw 7.44%) and bending angle (front paw 41°; rear paw 51°), the range of motion of the front paw (yaw 163°; pitch 309°) and the pitch range of the rear paw (164°).

The movement angles of the front and rear paws of the gecko when climbing up and down bases with different inclinations

In addition, the movement posture of the adhered sole remains unchanged, which is mainly manifested in that the movement angles of the front and rear soles (including yaw and pitch angles) remain consistent in a single adhesion-detachment cycle, and the yaw angles of the front and rear soles have a tendency to first decrease and then increase.

In addition, there are obvious differences in the movement postures of the forefoot and the hindfoot: the change trend of the forefoot pitch angle is opposite to that of the hindfoot. The forefoot pitch angle first decreases and then increases, while the hindfoot first increases and then decreases; and the movement angle range of the forefoot is larger than that of the hindfoot. This difference is mainly due to the different anatomical structures and functional requirements of the forefoot and the hindfoot .

The research results show that the movement behavior of the gecko's front and rear adhesive paws in the adhesion-detachment cycle is consistent, which reflects the rhythmicity in the paw adhesion-detachment movement cycle. This movement pattern reduces the complexity of movement regulation of the adhesive paws when responding to different environments.

The postures of the gecko's front and back paws at different stages of the adhesion-detachment cycle

The gecko's flexible adhesive soles have the same regularity in the adhesion-detachment movement cycle in different environments, including the swinging posture of the soles in the air and the degree of bending when contacting the base. This is mainly because the soles have the structural characteristics of rigid-flexible coupling and specific adhesion function requirements. In the face of environmental changes, the gecko can adjust the movement of its limbs with multiple degrees of freedom and coordinate the adhesion-detachment movement of its soles to achieve fast and stable climbing in various environments, becoming a master of climbing over eaves and walls.

The "Gecko" has inspired us that for bionic adhesive robots with large flexible soles and high-degree-of-freedom limbs, the motion control of the soles is often universal, and the limbs need to be adaptively adjusted according to changes in the environment . Similarly, can we get inspiration from other magical animals to design interesting and useful inventions? This may be the charm of science.

All pictures in this article are from the Kinetic Mechanics Group of the Bionics Institute, School of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics

References:

[1] Zong, W., Wang, Z., Wang, B., et al. Behaviors in Attachment-Detachment Cycles of Geckos in Response to Inclines and Locomotion Orientations. Asian Herpetological Research, 2022, 13(2): 125-136.

[2] Zong W, Wang Z, Xing Q, et al. The method of multi-camera layout in motion capture system for diverse small animals. Applied Sciences, 2018, 8(9): 1562.

[3] Wang Z, Zong W, Wang B, et al. The Marking Technology in Motion Capture for the Complex Locomotor Behavior of Flexible Small Animals (Gekko gecko). Asian Herpetological Research, 2019, 10(3): 197-210D.

[4] Song, Y., Dai, Z., Wang, Z., et al. Role of multiple, adjustable toes in distributed control shown by sideways wall-running in geckos. Proceedings of the Royal Society B, 2020, 287(1926): 20200123.

[5] Wang Zhouyi, Gu Wenhua, Wu Qiang, et al. Test of the force acting on the toes of geckos on walls and ceilings and study of the morphology of the toes. Science in China: Technological Sciences, 2011, 41(9): 1161-1166.

[6] Wang, ZY, Wang, JT, Aihong, JI, et al. Behavior and dynamics of gecko's locomotion: The effects of moving directions on a vertical surface. Chinese Science Bulletin, 2011, 56(6): 573-583.

[7] Wang, Z., Xing, Q., Wang, W., et al. Contribution of friction and adhesion to the reliable attachment of a gecko to smooth inclines. Friction, 2018, 6(4): 407-419.

[8] Autumn K, Peattie A M. Mechanisms of adhesion in geckos. Integrative and comparative biology, 2002, 42(6): 1081-1090.

Produced by | Science Popularization China

Author: Zong Weijia, Wang Zhouyi Nanjing University of Aeronautics and Astronautics

Review｜Zhao Heling, Editor of "Asian Amphibian and Reptile Studies"

Planning | Mao Ping Librarian of Chengdu Institute of Biology, Chinese Academy of Sciences

Producer｜China Science Expo

Submitted by: Computer Information Network Center, Chinese Academy of Sciences

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