Bernstein member involved: Michael Brecht

To the point

• Sense of touch despite thick elephant skin: Researchers have discovered that the hairs on elephants’ trunks are responsible for their extraordinary sense of touch.
• Special material properties: Elephant sensory hairs have a stiff base and a soft tip, which enables them to precisely feel objects and recognize where contact is made. These properties are similar to the whiskers of cats and differ from the completely stiff sensory hairs of rats and mice.
• Interdisciplinary collaboration: The research team at the Department of Haptic Intelligence at the Max Planck Institute for Intelligent Systems was joined by researchers from the fields of neuroscience and materials science.
• Applications in robotics: The findings will be used in the development of robot-assisted sensor technologies that mimic the stiffness gradient of elephant tactile hairs.

A new study from an interdisciplinary German research collaboration, led by the Haptic Intelligence Department at the Max Planck Institute for Intelligent Systems, reveals the secret to the gentle dexterity of the elephant trunk. The 1000 whiskers that cover the elephant trunk have unusual material properties that highlight where contact happens along each whisker, giving elephants an amazing sense of touch that compensates for their thick skin and poor eyesight.

Variable stiffness of a tactile hair

Recently published in Science with the title “Functional gradients facilitate tactile sensing in elephant whiskers”, this research found that the whiskers of elephants and domestic cats have stiff bases that transition to soft rubber-like tips, different from the uniformly stiff whiskers of rats and mice. Known as a functional gradient, this stiff-to-soft transition allows elephants and cats to brush past objects with ease, helps prevent whisker breakage, and provides unique contact encoding along the whisker’s length. The researchers think this unusual stiffness gradient helps elephants know precisely where contact occurs along each of their 1000 trunk whiskers so they can perform feats like picking up a tortilla chip without breaking it or precisely grabbing a peanut. The research team is looking to invent new robotic sensing technologies inspired by the functional gradients they discovered in elephant and cat whiskers.

The research was led by a postdoctoral researcher, Andrew K. Schulz, and Katherine J. Kuchenbecker from the Haptic Intelligence Department at MPI-IS. They worked with neuroscientists from the Humboldt University of Berlin and materials scientists from the University of Stuttgart. Schulz, the study’s lead author and an Alexander von Humboldt postdoctoral fellow, discussed the start of the project, “I came to Germany as an elephant biomechanics expert who wanted to learn about robotics and sensing. My mentor, Prof. Kuchenbecker, is an expert on haptics and tactile robotics, so a natural bridge was for us to work together on touch sensing through the lens of elephant whiskers.” Schulz and his colleagues used an array of biological, materials science, and engineering techniques to image and characterize 5-cm-long whiskers from elephants and cats down to the length scale of one nanometer, which is 1 billionth of a meter.

Analysis of natural properties

The interdisciplinary team examined elephant trunk whiskers to understand how they are shaped (geometry), how porous they are (porosity), and how soft they are (material stiffness). They initially expected elephant whiskers to be similar to the tapered whiskers of mice and rats, which have a circular cross-section, are solid throughout, and have approximately uniform stiffness. Micro-CT allowed the researchers to measure the 3D shape of several whiskers and showed that elephant whiskers are thick and blade-like, with a flattened cross-section, a hollow base, and several long internal channels that resemble the structure of sheep horns and horse hooves. This porous architecture reduces the whisker’s mass and provides impact resistance, allowing elephants to eat hundreds of kilograms of food every day without worrying about damage to their whiskers, which never grow back.

Nanoindentation of both elephant and cat whiskers was performed with a diamond cube indenter the size of a single cell that cyclically pushed into the whisker walls. Indentation performed at the base and the tip of the elephant and cat whiskers showed a transition from a stiff, plastic-like base to a soft, rubber-like tip that could not be permanently indented, a property known as resilience. The team also compared these whiskers to elephant body hair. Schulz said, “The hairs on the head, body, and tail of Asian elephants are stiff from base to tip, which is what we were expecting when we found the surprising stiffness gradient of elephant trunk whiskers.” While exciting, this discovery initially stumped the team as they were not sure how changing stiffness along a whisker would affect touch sensing.

An imitation elephant hair from a 3D printer

To try to figure out why, Schulz worked with colleagues at MPI-IS to 3D print a scaled-up whisker with a stiff, dark base and a soft, transparent tip. Having this physical “whisker wand” prototype helped the researchers develop their intuition for what an elephant trunk feels through its whiskers. Schulz left the wand with his mentor after a meeting, and a few days later…Eureka! Kuchenbecker carried the wand in her hand as she walked through the halls of the Institute, gently hitting the columns and railings. She recounted, “I noticed that tapping the railing with different parts of the whisker wand felt distinct – soft and gentle at the tip, and sharp and strong at the base. I didn’t need to look to know where the contact was happening; I could just feel it.”

To test their hypothesis from the 3D-printed whisker wand, the researchers developed a computational modeling toolkit to assess how the unique geometry, porosity, and stiffness gradients they had measured affect how a whisker responds to contact. The simulations showed that the transition from a stiff base to a soft tip does indeed make it easier to feel where something is touching along the whisker, allowing the elephant to react appropriately and carefully manipulate even delicate objects, such as tortilla chips. Schulz said, “It’s pretty amazing! The stiffness gradient provides a map to allow elephants to detect where contact occurs along each whisker. This property helps them know how close or how far their trunk is from an object…all baked into the geometry, porosity, and stiffness of the whisker. Engineers call this natural phenomenon embodied intelligence.” Excitingly, domestic cat whiskers show the same kind of stiffness gradient.

Transfer from nature to robotics

This discovery excites Schulz and Kuchenbecker, who are working to apply these insights from nature to applications in robotics and intelligent systems. “Bio-inspired sensors that have an artificial elephant-like stiffness gradient could give precise information with little computational cost purely by intelligent material design,” Schulz said. Dr. Lena V. Kaufmann, a co-author of the study and a neuroscience expert at the Humboldt University of Berlin, is excited about the connections to neuroscience: “Our findings contribute to our understanding of the tactile perception of these fascinating animals and open up exciting opportunities to further study the relation of whisker material properties and neuronal computation.” Kuchenbecker reflects back on the entire project, “I’m so proud of what we were able to figure out by working together across disciplines. Andrew pulled together an amazing team of engineers, materials scientists, and neuroscientists from five different research groups and led us on an exhilarating three-year-long journey to discover the secrets behind the powerful elephant’s gentle sense of touch.”