Insect ears are inspiring the design of tiny 3D-printed microphones that could pinpoint a sound’s direction, replacing the much bulkier, energy-hungry gear currently needed for such purposes, researchers say.
The insect ear possesses a thin sheet of tissue, known as the tympanum, that is much like the human eardrum. Sound waves make this membrane vibrate, and the sensory apparatus within the ear converts these vibrations into nerve signals.
Although an insect’s tympanum is typically a millimeter or so wide, insects are capable of feats of hearing that currently require devices much larger in size. For instance, to pinpoint which direction a gunshot came from, the vehicle-mounted Boomerang system from Raytheon depends on a microphone array roughly a half-meter wide. In comparison, the nocturnal moth Achroia grisella can also identify which direction sounds are coming from, and can do so with just one tympanum only about half a millimeter wide. (The moth likely evolved this skill for both detecting predatory bats and ultrasonic mating calls.)
In order to mimic what insect ears can accomplish, scientists at first attempted to copy insect structures with silicon microelectromechanical systems (MEMS). However, the resulting devices lacked the flexibility and the microscopic 3D structural variations seen in real insect ears that help them hear so well, says Andrew Reid, an electrical engineer of the University of Strathclyde, in Glasgow.
Now Reid and his colleagues are experimenting with 3D printing to more faithfully copy insect ears. He detailed his team’s research at the annual meeting of the Acoustical Society of America on 10 May in Chicago. The research builds upon the team’s earlier work to understand how insects have such stellar directional hearing.
The researchers have 3D printed a variety of membranes to copy a range of insect tympana. The base material for these membranes is typically a flexible hydrogel such as polyethylene glycol diacrylate. The membranes also often include a piezoelectric material such as the perovskite oxide crystal known as PMN-PT, which can convert acoustic energy to electric signals, and electrically conductive silver-based compounds, Reid says.
To improve the piezoelectric performance of these synthetic membranes, the scientists have made them more porous, mimicking the porosity at times seen in insect tympana. They dissolve methanol into the 3D-printing resin, and as the resin solidifies, it no longer becomes soluble to methanol. This leads the methanol to separate and form droplets within the resin, which form the basis of the pores.
The microscopic 3D variations in thickness, porosity, density, and pliability in the synthetic membranes help them behave like highly sensitive and efficient acoustic sensors. Their design helps them automatically filter sound in a mechanical manner, which means they do not require the power and computation needs of relatively bulky digital sound processors.
This image shows the displacement of the lesser wax moth membrane, one of the key sources of inspiration for designing miniature, bioinspired microphones.Andrew Reid
Reid suggests that insect-inspired microphones may find applications where a sound sensor is needed to quickly detect specific signals without consuming a lot of energy. Such devices would also require very little in the way of data or hardware.
Moreover, a mechanical way to separate different frequencies of sound with the precision of, for example, the ear of the desert locust Schistocerca gregaria would prove useful for cochlear implants, Reid says. Cochlear implants currently require digital signal processing, which involves receiving a sound, converting it from analog to digital, and processing the digital signal before stimulating the auditory nerve. All those steps result in delays in hearing through the devices. If an implant can instead perform this frequency separation mechanically, “you can greatly reduce the delay,” Reid says.
There’s still a lot that scientists don’t know about how the microscopic structural variations in insects’ tympana help them hear as well as they can. There are competing models about how each of these variations improve hearing, Reid says.
It’s also uncertain as to why exactly porosity improves the piezoelectric performance of the membranes. The way the pores concentrate the rest of the membrane’s material together may help channel acoustic energy to the piezoelectric nanoparticles, Reid says. The pores could also make the membranes more flexible and receptive to sound waves, he adds.
The optics of currently available 3D printers limit the resolution of the features of the synthetic membranes to roughly 200 micrometers, Reid says, adding that improving the optics could lead to resolutions of less than 10 µm. This may further improve the performance of these devices.
“The work that we’ve done to date is still short of a solid proof of concept for practical sensor design,” Reid says.