The wandering spider boasts one of the world’s most sensitive vibration detectors: It can pick up the slightest rustling of a leaf from several meters away. Now, scientists have developed similar sensors that can detect simple human speech. The technology could lead to wearable electronics for speech recognition, health monitoring, and more.
The male wandering spider (Cupiennius salei) courts prospective mates by scratching leaves with its mouth and abdomen, which produce minute vibrations. A female spider can pick up the signal with parallel slits on its legs, known as the lyriform organ, which change shape under tiny forces.
Inspired by the spider’s physiology, engineer Mansoo Choi of Seoul National University and colleagues designed a flexible ultrasensitive motion sensor that’s smaller than a fingernail. The sensor consists of a polymer sheet coated with a thin platinum film. Nanoscale cracks resembling the spider’s slit organs break up the metal film into tiny tiles. When the platinum film is completely flat, the cracks are closed and the edges are in contact, allowing electrical currents to run through the entire film. But any tiny stretching of the film widens the cracks significantly. The wider the cracks are, the less edges are in contact, blocking more electrons from passing through the film. By measuring the changes in the platinum film’s resistance, the researchers can detect minute motions, such as vibrations resembling a ladybug flapping its wings, they report online today in Nature.
“The ultrasensitivity … is really what makes the difference,” says Peter Fratzl, a materials scientist at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, who was not involved with the study.
The sensor has a wide range of potential applications, from selective speech recognition to health monitoring, Choi says. When the researchers attached the sensor to a violin playing the wedding tune “Salut d’Amour” by British composer Edward Elgar, for example, the resistance of the device varied in sync with air vibration produced by the instrument, correctly recording every note of the song. When the researchers attached the sensor to 10 people’s necks and asked them to say simple commands such as “go” and “jump,” the device recorded the sounds more accurately than a commercial microphone when there’s ambient noise. The sensor can also detect wrist pulses, opening up the possibility for miniature Fitbits that can monitor vital signs 24/7.
In addition, the researchers integrated the sensor into a microfluidic system made of micrometer-sized channels containing nanoliters of liquids. Medical researchers could potentially use such devices to integrate multiple medical tests on a single “lab on a chip.” The sensor can detect the pressure difference inside and outside a microfluidic channel, which could allow medical researchers to monitor how fast drugs flow in the tubes.
“There are so many vibrations we’d like to monitor,” Fratzl says. But he says a truly practical sensor would need to discriminate between vibration signals while ignoring unwanted noise—a skill that spiders are very good at. Choi says his team plans to address that by changing the density of cracks formed in the platinum films to customize the device’s sensitivity for different applications. A sensor designed for pulse monitoring, for example, doesn’t need to be as sensitive as a device for speech recognition.
The team also plans to replace platinum with cheaper conductors, such as copper and aluminum, to shrink the cost of the sensor for commercialization. Choi says he hopes to produce a practical device within 5 years.