Lizards’ astounding crash-landing ability validated with Soft Robot perching
Aerial soft robotic physical models with an active tail reflex provide experimental validation field discovery of Geckos' superpowers in the rainforest canopy are not entirely down to its unparalleled feet. Versatile tails play just as much a pivotal role.
We position our interdisciplinary research at the interface of engineering, materials science, and biomechanics, specializing on soft active materials, biomimetic mechanisms, and robotics inspired by original biomechanical discovery. We explore the behavioral and morphological adaptations of natural systems to recreate robust multimodal locomotion at our Lab at the Max-Planck Institute for Intelligent Systems.
The Asian flat-tailed gecko (Hemidactylus platyurus) is found in the Indomalayan lowland tropical rainforests, where it lives in the trees. Although it was thought to be non-volant, we recorded it can glide considerable distances from tree to tree to evade predators. When the trees are spaced close together, as geckos take to the air, jumping and landing takes place at the blink of an eye. Moving through the air at high speeds of ~120 body lengths per second at 6m/s (or ~21 Km/h) for 'directed aerial descent' the geckos crash-land unbraked into the neighboring tree. Remarkably, following a dedicated landing maneuver the lizards are still able to stick to the vertical surface without becoming dislodged.
Movie 1: Summary video containing supporting information movies from the paper, modified. Narrated by Ardian Jusufi.
We conducted experiments in the Indonesian-Malaysian rainforest to study the mechanism behind this remarkable landing ability. We performed field research climbing seven-meter-high ladders at high ambient temperatures of up to 38 degrees Celsius and 85 % humidity. Thanks to high-speed cameras, we were able to film the flight and perching behavior of geckos in their natural environment, tracking how they use their tails not only to control gliding flight, but also for perching on a challenging vertical target.
The lizards aim at a target, take off and land at lightning speed and are capable of landing in a confined space.
As can be seen in the video below, the gecko tries to cushion the impact by bending its trunk backwards by more than 100 degrees. During this bending, the front feet lose their grip and only the hind legs remain attached. This backward bending of the trunk dissipates energy by pressing the tail against the tree trunk. Animals that lost their tails could not sufficiently cushion the energy and fell off the trunk. The tail thus acts like a fifth leg, helping the gecko stabilize itself after impact.
But how can one conclusively show that the tail has this stabilizing effect? Going back to the drawing board in the lab we developed a perching soft robot to better comprehend the pressures acting on the animal. This robotic physical model of the gecko's landing behavior serves for experimental validation of the dynamics on impact. To emulate the lizard's compliant body, we built a soft robotic platform with a soft torso, allowing the tail to be detached and reattached. When the front foot of the robot comes into contact with a surface, it bends its tail, similar to the climbing gecko reflexes previously found in the rainforest. A microswitch on the robot's underbelly provided tail actuation input, and gave feedback when the bottom of the head made contact with the wall. The information is processed by a microcontroller on the "shoulder" to immediately activate a motor in the "pelvis" that pulls on a "tendon" to push the tail against the wall to stabilize the robot.
The soft robotic lizard was catapulted at the same velocity that the geckos collided with the tree, onto a felt-lined wall to which the robot's Velcro-covered foot could stick. After collision, we assessed the force exerted on the robot's feet. The longer the robot's tail, the less force was required to lift its hind feet off the ground. The less force applied, the easier it was for the robot to maintain its grip. However, for a robot lacking a tail, the pressures acting on the hind feet become excessive, causing the robot to lose its grip, bounce off, and fall.
Ultimately, we would like to transfer the insights gained from the gliding flight of the gecko to robotics. We have already succeeded in doing this with climbing robots, which have a biologically inspired mechanical tail that, like geckos, gives them more stability when climbing.
This research was done by Rob Siddall, Greg Byrnes, Bob Full, and Ardian Jusufi at the Max-Planck Institute for Intelligent Systems Stuttgart, Siena College, and UC Berkeley.
The paper can be found here: https://www.nature.com/articles/s42003-021-02378-6