Detecting temperature is an important function of skin. Snakes can use their skin to track warm-blooded prey, even in the dark. Now a highly sensitive, flexible sensor film could also make this characteristic available for robots and prosthetics.
Whether in factories, the office or the kitchen: Robots continue to encroach on various aspects of our lives. When it comes to safety, that increases requirements for the “man-machine interface” considerably. Which is why developing sensitive robot skins has become a hot topic in robot research. After all, “collisions” can only be avoided if you can “see” your counterpart—as quickly and accurately as possible. Methods for doing so range from image processing using mechanical devices to contact-free sensor solutions.
For example, scientists at the Technical University of Munich (TUM) have been working on artificial skin made of small hexagon plates with infrared, temperature and acceleration sensors. The infrared sensors register when things come close to the robot.
Researchers at ETH Zurich and the California Institute of Technology (Caltech) are pursuing a more natural approach. Their temperature sensor is based on the plant material pectin. Like the snake’s extremely sensitive pit organ that can sense a mammal’s warm body up to a meter away, it can precisely measure temperatures to one hundredth of a degree. That is twice as sensitive as human skin.
“Cyber wood” as a temperature sensor
Discovering the artificial “snake organ” was actually a coincidence. It turns out that the electrical conductivity of cell walls in trees depends on temperature. That is because of the plant material pectin, which can also be used in the kitchen as a gelling agent for puddings and jams.
Measurements that were taken on a type of “cyber wood” made of pectin and carbon nanotubes revealed that the higher the temperature, the more free calcium ions were formed at the contact points between two sugar molecules. Electrical conductivity increases proportionally. That is how the sensor idea was born. All that was missing was the “skin”. The answer: A 20-micrometer-thick film made of simple pectin gel laced with a calcium solution.
Safe human-robot collaboration
Initial testing revealed that the ultrathin transparent film that can be formed into nearly any shape can measure temperatures from 10 to 50 degrees Celsius with a precision of one hundredth of a degree. Supposedly, the “prey” that was used was a teddy bear—fresh from the microwave.
To spatially resolve hot or cold sensations like human skin, researchers attached several electrodes along the long and short sides of a piece of “skin” measuring 25 square centimeters. The resulting grid made it possible to determine the position of temperature changes at specific locations.
The “snake skin” is extremely easy to make and is more robust and less prone to interference than existing flexible temperature sensors equipped with transistors. After improving the computer algorithms used to analyze the electrode signals and improving the electrical contacts, the “snake skin” should be ready for a field trial in robotics or prosthetics.