Spiders hunt in utter darkness. What they lack in visual acuity, these arachnids make up for with tiny slits in their exoskeletons—cracks that flutter in response to vibrations from their external environments. The spider’s nervous system then filters the most important vibrations from other distracting noises. It is nature’s perfect sensor.
Now scientists are taking inspiration from the natural world to make their own artificial version of a spider sensor. In a paper published in the December 11 issue of Nature a team of researchers from South Korea describe their new mechanical sensor. (Scientific American is part of Nature Publishing Group). They built it by embedding cracks in a platinum sheet to mimic exoskeleton slits and say this sensor could provide an unprecedented level of sensitivity, and even have medical applications in the future. “The sensor can be used as an ultrasensitive strain sensor or a physiological monitoring sensor,” says Mansoo Choi, a mechanical engineer at Seoul National University in South Korea and lead author of the study. “It can be used to monitor heart function in detail and it could further be developed to detect minute physical vibrations that a disabled person could make to identify their intentions, such as speaking.”
Nanotech, meet Spidey sense
Choi and his team started by consulting experts in spider physiology. They learned that the slit organs—also called lyriform organs, due to their resemblance to a lyre—are embedded in the exoskeleton near the spider’s leg joint.
The researchers set out to mimic the ability of the slit organs to sense vibrations from the environment by using synthetic materials. At the core of their sensor is a superthin platinum sheet only 20 nanometers thick riddled with tiny, parallel cracks. (A nanometer is one billionth of a meter, or about 40 billionths of an inch.) Subtle vibrations—caused by sound waves or a heartbeat, for instance—will either widen or compress these cracks, which changes their electrical resistance. “This means that even a very small displacement could vary the electrical resistance greatly,” Choi says. “If you measure electrical resistance variation, you can detect even very small displacement variations with high precision.”
The result is a sensor with unprecedented sensitivity. Relative sensitivities are typically measured by their gauge factors—the ratio of change in electrical resistance to mechanical strain. Most sensors have a gauge factor of less than 10. Choi’s sensor has a gauge factor of over 2,000.
Toward wearable sensors
One of the virtues of nanotechnology is that almost any substance becomes pliable if you make it thin enough. Because it is superthin, the platinum film in Choi’s sensor is able to flex with the body while maintaining its high sensitivity—qualities that open up prospects for wearable applications. “The sensor can be attached on curved surfaces such as skin and clothes to detect external force variations with time, which can monitor physiological signal changes,” Choi says.
Because the sensor can distinguish the unique vibrations of certain words, Choi and his team attached the sensor to participants’ necks and demonstrated that they could play a basic computer game using voice commands. When worn on the wrist, the sensor could also gather detailed heart-rate information. “We feel that it can be used in medical applications since it can be attached on human skin with high sensitivity,” Choi adds.
The next step for Choi and his team will be to continue developing the sensor so that it can eventually be commercialized. Choi estimates that his team will need another three to five years before their sensor is ready for market. In the meantime Choi plans look for measures to lower the price tag. “We may need to replace expensive platinum with cheaper, high-conductive metals such as copper or aluminum in order to further reduce the costs,” he says.
Distinguishing signals from noise
Other scientists caution that although Choi’s sensor is extremely sensitive it will not necessarily discriminate seamlessly between different sounds. The spider’s natural sensor not only captures the smallest vibrations, it also weeds out essential signals from superfluous noise. “The spider uses frequency filters to get rid of as much noise as possible,” says Peter Fratzl, a physicist and biomaterials expert at the Max Planck Institute of Colloids and Interfaces in Germany. “If you would record every sound that you hear, you would go crazy—we have to be very discriminatory in what we take in, and ignore the noise. Of course, this is not something that is very easy for a sensor.”
In a related paper published in the same issue of Nature Fratzl acknowledges the sensor’s impressive level of sensitivity but calls for a more discriminating device that can better distinguish the most important vibrations from distracting noise. “It’s a great achievement in terms of its fantastic sensitivity,” Fratzl says, “but I am claiming that the spider’s [natural] sensor still has lots of interesting secrets to look into.”