Sensors inspired by beetle hairs could bring a more nuanced sense of touch to robots and wearable electronics.
A flexible electronic sensor based on two layers of interlocking nanohairs can detect very faint pressures and distinguish shear and twisting forces, just as human skin can. Sheets of the sensor material can detect the gentle steps of a ladybug and can be strapped to the wrist to serve as a heart-rate monitor.
The sensor design, described today in Nature Materials1, was inspired by beetle wing-locking structures, says Kahp-Yang Suh, an engineer at Seoul National University in Korea. When they’re on the ground, a row of hairs on some beetles’ wings locks into an array of hairs on the body. A kind of static attraction, called Van der Waals forces, attracts the hairs to each other, locking the wings in place. In Suh’s sensors, the “hairs” are molded arrays of polymer fibers 100 nanometres in diameter and one micrometer long, coated with metal so that they are electrically conductive. When the sheets are sandwiched together, the nanohairs are attracted to one another and locked in, just like the beetle hairs. The device is then wired so that an electrical current can be applied, and sandwiched in a layer of soft, protective polymer.
When the sensor sheet is pressed, twisted, or brushed, the squishy metal-coated hairs change position, leading to measurable changes in the sensor’s electrical resistance. This nanohair design is sensitive to the gentlest of touches, about five pascals. By analyzing how the resistance changes in response to mechanical stress, and then recovers when the stress is removed, Suh and his colleagues can distinguish between different kinds of mechanical strain: pressure, which comes straight down on the sensor; shear force, a frictional slide along the surface; and torsion, a twisting motion. “We can decouple these three different signals,” says Suh.
Human skin can distinguish between these different kinds of strain, but most artificial sensors cannot. “Sensing shear and torsion is difficult,” says Zhenan Bao, a materials scientist at Stanford University in Palo Alto, California who is also developing flexible strain sensors. Other sensors can only account for the total applied force, but don’t say anything about its direction. The methods for teasing out the nature of the strain from the electrical readings in Suh’s sensors need some work, says Bao, but getting this kind of information from a flexible sensor is unique.
This subtle tactile input would be very useful for robots designed to interact with people, says Matei Ciocarlie, a scientist at Willow Garage, a robotics company in Menlo Park, California. Cameras can provide a lot of information about the environment, but sometimes they get blocked, and they tend to provide information overload. Tactile sensors, in contrast, only collect data at the point of contact and can’t be blocked. However, says Ciocarlie, “Skin has been an overlooked part of robotics,” because it’s such a challenging problem. In addition to being robust, sensitive, and flexible, electronic skin also needs to be made in very large sheets. Suh says it should be simple and inexpensive to make large sheets of his sensor by making large molds.
Electronic skin also needs high spatial resolution. Suh’s group demonstrated an 8-by-5 centimeter sensor network made up of 64 pixels by recording the meandering motion of two ladybugs along its surface. They also recorded a bouncing water droplet and demonstrated that the sensor sheets can be taped to the wrist to measure someone’s pulse. Suh says his group is now talking to a healthcare company about developing heart monitors based on these materials.
Pang, C., et al. Nature Materials. DOI:10.1038/NMAT3380 (2012).
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Bourzac, K. Hairy electronic sensors rival the sensitivity of human skin [back by 4pm pls]. Nature (2012). https://doi.org/10.1038/nature.2012.11081