In the world of engineering, the optimization of prosthetics and robotics is at the forefront of research. However, many designs are faced with the same problem – poor stability, especially when it comes to rough or sloped surfaces. This prevents amputees from being able to enjoy outdoor activities such as hiking and rock climbing, and traversing robots from being able to perform complex search and rescue. So, researchers have gotten creative and have decided to look into nature. Naturally, mountain goats became a prime source of inspiration due to their ability to seemingly defy gravity when scaling mountain tops. How do they do this? To answer this question, plenty of research has been conducted to look into things like goat anatomy, joint angles, centers of mass, ground reaction forces, and more.
How do squirrels escape dogs, scavenge for food, and climb their way around trees? The answer is simple: squirrel parkour. With precise calculations and the physical ability to maneuver and adjust, squirrels can effortlessly leap from tree branch to tree branch without falling.
A study done by Nathaniel Hunt et al. tested squirrels on different activities where gap distance, flexibility of the launching bar, and gap height were varied. The squirrels were observed, and results showed a consistent trade-off scenario between branch flexibility and leap distance. When on high-flexibility branches, squirrels jumped closer to the branch connection point. This led to large gap lengths, up to three times the squirrel’s body length. The opposite was true for low-flexibility branches. Squirrels are able to determine branch flexibility and what launch point is necessary for them to safely make leaps to neighboring branches.
In terms of safety, throughout the course of the experiments, the researchers observed that not a single squirrel fell. They were able to successfully adjust to varying branch flexibility, height, and leap distance. The squirrels were able to accomplish this feature by performing a “parkour” move. The squirrels jumped off the branch toward a nearby wall and sprang off of it before landing. The squirrels utilized this parkour move to adjust their landing speeds. Not only does this demonstrate the adaptability of squirrels, but also their ability to adjust their bodies while jumping.
An important concept to consider when looking at mechanics of squirrels jumping is the force and speed at which they take off. Grégoire Boulinguez-Ambroise et al. completed a study to examine the effects of different branch sizes on squirrel take-off velocity and the displacement of their center of mass. Results showed that there was a difference between squirrels jumping upwards off of a flat plate, and off of a branchlike object. When jumping off the branchlike poles, squirrels prioritized shifting their center of mass to jump, whereas on the flat plates, priority was given to the produced force. These results imply that while on branches, squirrels try to maximize balance before jumping. This intuitively makes sense as you would want to be as balanced as you could before making a leap!
Figure 1. Data showing an increase in take-off velocity with a decrease in branchlike diameter as well as data showing an increase in jump height with a decrease in branchlike diameter. Image from Grégoire Boulinguez-Ambroise et al.
Interesting results, shown in Figure 1, showed that squirrels jumped the highest while jumping from the smaller diameter branch, and jumped the lowest when jumping from flat ground. Putting this together with the previously mentioned idea of squirrels shifting their center of mass, squirrels are able to adjust their approach to jumping depending on branch sizes.
Figure 2. Force components between a squirrel foot and a pole substrate before vertical jumping. Image from Grégoire Boulinguez-Ambroise et al.
Based on the size of the branch, squirrels have to adjust their feet position. For smaller branches, squirrels must place the middle of their feet closer to the sides of the branch. Figure 2 shows the forces acting between the feet and the branch. Varying branch sizes will change the angles and size of the forces, and the squirrel will have to adjust accordingly. In the study mentioned above, results showed that the feet placement on the varying diameter sizes did not have a significant impact on the measured take-off velocity. This implies how squirrels can adjust to different sized branches and still maintain high quality jumping performance.
Diving into how squirrels jump, they use their hind limbs to push themselves off of an object. Richard Essner completed a study on squirrel launch kinematics. Results showed that for leaping, squirrels act as small-bodied primates and rely less on their knees than their ankles. Squirrels also use their tails to help with balance.
While squirrels may seem like cute animals who only care about adding to their nut collection, there is a lot more than meets the eye when it comes to their movement between and through trees. The nature of squirrels to be able to apply the parkour move, change foot placement, determine the flexibility of branches, and adjust take-off velocity depending on each individual branch they leap from are just a few of the amazing features that squirrels can accomplish.
As a child, did you ever watch with awe as Spider-Man climbed lofty buildings in New York City? This super-power has captivated audiences for decades because it seems impossible that a tiny fingertip could provide such great strength. However, the powerful and sticky toes of the tree frog bring this super-power to reality. Their toes have round pads that can cling to surfaces, both wet and dry, due to biomechanical forces. What causes their toes to be so sticky?
Polygonal epithelial cells of a tree frog toe pad with channels and micro-pillars. Modified from Kappl, Kaveh, & Barnes, Bioinspiration and Biomimetics 2016
The round toe pads of the tree frog are composed of four layers of surface cells known as epithelial cells. Epithelial cells line all surfaces of the body, including the external surfaces and internal cavities of the body, such as the inner surfaces of organs. The epithelial cells of the toe pad are shaped like hexagons and columnar. Columnar cells have a greater height than width and resemble small pillars. These microscopic pillars are separated from each other by channels filled with mucous. This unique structure creates a phenomenon known as wet adhesion.
The word “adhesion” likely prompts thoughts of everyday adhesives like glue and tape. But, what is wet adhesion? The molecules in liquids are attracted to each other and to the surface surrounding the liquid. In a small container, these attractions cause what are known as capillary forces. When a straw is placed in water, liquid climbs the side of the straw due to these forces. Wet adhesion is the attachment of two materials due to the presence of a liquid with capillary forces. A good example of wet adhesion is when a piece of plastic that is wet gets stuck to a glass table.
Pillar channels filled with mucous create a layer of liquid between the pad and the surface. Modified from Zhang et al., Biosurface and Biotribology 2022
In the toe pads, the mucous secreted into pillar channels creates a layer of liquid between the pad and the external surface, which encourages capillary forces. The liquid layer has also been found to encourage viscous forces. Viscous forces are caused by the attraction between liquid molecules. The attractive forces resist the horizontal movement of liquid molecules when a force is applied tangent to the liquid’s surface. Capillary and viscous forces allow tree frogs to adhere to surfaces at steep angles without sliding off.
Recent research has also shown that frictional forces play a role in toe pad adhesion. The stiffness of the surface of toe pads is not uniform. The polygonal epithelial cells at the surface of the pad have a higher stiffness than the material below. The flexibility of the material below the surface allows for better contour of the pad to a surface. The enhanced interaction with the surface imperfections causes frictional forces to resist sliding and encourage adhesion.
Photo by Pexels on Freerange Stock
The adhesive toe pads of tree frogs have inspired several engineering applications, including wet adhesive materials. Scientists are pursuing the fabrication of artificial materials that can mimic the characteristic micropillars of tree frog epithelial cells. For example, typical surgical tools, such as forceps, may struggle to effectively grasp softer tissues or increase the risk of tissue damage. Wet adhesive surgical graspers may provide a less abrasive method of holding soft tissues throughout a procedure. Tree frog-inspired wet adhesives may also be used to create sensors and monitors that can easily adhere to a patient’s skin. The study of wet adhesives has the potential to greatly benefit the medical field.
Rock climbing may just be the greatest exercise activity. It’s a holistic workout with an infectiously supportive community that involves plenty of problem solving and a good understanding of body movement and biomechanics. If you ever find yourself on the climbing wall, you will inevitably encounter a sequence of moves that seemingly proves to be too difficult or complicated. Whether you’re a veteran or a beginner, below are just a few of the countless biomechanical techniques and tips to keep in mind when coordinating and executing your attempts; they could be the difference between plateauing and finally topping out on that elusive route.
Scientists have studied plants for centuries. Charles Darwin in particular had studied climbing plants for years in an attempt to discover the secrets behind how they climb structures. Despite the years of study, very little is known about the mechanics that allow plants to climb. If the methods can be discovered, scientist might be able to mimic the power and flexibility that climbing plants have in man made structures, or be able to design structures that can better withstand the destruction plants tend to inflict upon buildings and roads.
Have you ever thought about what it would be like to walk on walls? If you’ve ever watched a Spiderman movie or watched a gecko maneuver around its habitat, you probably have. While geckos don’t fight crime, their climbing ability is as fantastic as that of any superhero. Geckos have one of the most unique climbing adaptations of any animal, and scientists are examining the source of this ability to see if human technology could one day achieve something similar.
In the rock climbing world, there is not much that people fear more than the sound of a “pop” coming from their fingers. That sound means months of rehab and can keep you off the rock for up to six months. But what exactly is happening when you hear that dreaded sound? The fingers are so small, how can one injury to the fingers be so devastating? Let’s dive in.
