How insect-inspired microphones are revolutionizing our hearing

3D-printing technology has enabled bio-inspired microphones that mimic the remarkable hearing abilities of insects for environmental monitoring and medical device applications.
Sade Agard
(left to right) Conceptual image of a futuristic hearing aid; Large moth held in hand.
(left to right) Conceptual image of a futuristic hearing aid; Large moth held in hand.

Vladyslav Otsiatsia /Istimages   

  • Insect-inspired microphones hold immense potential for revolutionizing sound capture and auditory understanding.
  • By mimicking the remarkable hearing abilities of insects, scientists are developing miniature, bio-inspired microphones using 3D-printing technology.
  • These innovations offer practical applications across various fields, including environmental monitoring and medical devices.

In the world of insects, where a multitude of fascinating adaptations exist, their hearing abilities often go unnoticed. Yet, hidden within the insects' tiny bodies are unique auditory systems that rival those of larger, more complex organisms. 

Considering the hearing abilities of the parasitic fly, Ormia ochracea, for example, whose sense of directional hearing is second to none in the animal kingdom. Each of its ears— which are unusually connected - has two tiny membranes attached to a jointed stalk.

The stalk acts as a directional amplifier. This adaptation allows the fly to accurately pinpoint the location of its prey by detecting the subtle differences in sound intensity and timing between its two ears.

By exploring the mechanical principles behind such auditory adaptations and using cutting-edge 3D-printing technology, engineers and scientists are already unlocking the potential to revolutionize sound capture. They're developing miniature, bio-inspired microphones. 

How insect-inspired microphones are revolutionizing our hearing
Acroia grisella is a prototype microphone based on the fly.

To delve into the progress taking place in this field, Interesting Engineering (IE) interviewed Dr Andrew Reid, a lecturer, and chancellor's fellow at the University of Strathclyde in the U.K.

He recently developed insect-inspired microphones that autonomously collect acoustic data.

Small but mighty: what are insect-inspired microphones?

"Insect-inspired microphones are low-power, low-data systems," Reid said.

Reid's team draws inspiration from insect ears in several ways. For instance, using 3D-printing technology, they create custom materials that imitate insect membranes on a chemical and structural level.

These synthetic membranes serve as highly sensitive and efficient acoustic sensors. 3D printing is essential because traditional silicon-based approaches to bio-inspired microphones lack the necessary flexibility and customization.

How insect-inspired microphones are revolutionizing our hearing
A CT image of the Acroia grisella ear based on a prototype (from separate work).

"You use the mechanical response of carefully graded membranes to direct only the signal you're interested in into a point on the membrane where you can measure it," he said.

"And then you measure it only when the signal is present (as opposed to a microphone, where you record continuously across a broad frequency range)."

Reid emphasizes that in nature, nerve cells produce spike trains—a sequence or pattern of electrical impulses— that indicate the presence of a signal without encoding additional information.

However, complex details such as frequency composition and signal location can be extracted by mechanically filtering and processing information in the membrane.

"This is how an animal like the Lesser Wax Moth can track down a mating call with an eardrum less than a millimeter across and only four neurons," he says. 

3D-printed membranes using digital light processing

The membranes are 3D printed using a process called digital light processing, Reid told IE. "The resins we use are polymerized by ultraviolet light, which we can selectively cure by displaying an image on the resin surface using a UV source and some projection optics."

"When we vary the light intensity across these images, we change the rate of the polymerization reaction, which leads to variations in thickness, compliance, and, recently, porosity," he revealed. 

By mimicking the properties of insect ears, particularly the directional abilities of insects like the parasitoid fly Ormia ochracea and the Lesser wax moth (Achroia grisella), Reid's team can detect sound source information. They gather this by measuring the signal's amplitude at specific points on the membrane.

How insect-inspired microphones are revolutionizing our hearing
One of the key sources of inspiration for designing miniature bio-inspired microphones is the displacement of the wax moth Acroia Grisella's tympanal membrane.

"Most of the original work was done by biologists, who were primarily interested in insect communication and behavior for its own sake," he said. [They then] needed some engineers to understand the mechanics of what they were seeing."

Reid emphasized that engineers have increasingly explored practical applications as their understanding of these systems has advanced.

Additionally, the lecturer drew attention to the challenge of replicating insect capabilities, as digital signal processing can often achieve similar outcomes with minimal power and space requirements.

"The opportunities to use these systems effectively have only come with the maturation of several technologies, such as microscale additive manufacturing and neuromorphic programming, leading to ultra-low power dedicated sensors, which are ideal for 'fit-and-forget' data-gathering networks," he explained. 

IE prompted Reid with the question of how his insect-inspired microphones might be expected to find practical applications in the real world.

Revolutionizing the way people hear and beyond

"One example we often give, which is in development now, is a cochlear implant. Current cochlear implants will stimulate up to 16 frequency bands, which is limited by the number of electrodes rather than the microphone," Reid said. 

He clarified that when using digital signal processing he focuses on dividing the sound signal into different frequency ranges. This process involves recording the signal, converting it from analog to digital, and performing a fast Fourier transform

Simply put, the Fourier transform is a mathematical technique that allows us to break down a complex signal into its individual frequency components. It's like separating the ingredients of a mixed fruit juice to identify the specific fruits used. 

Fourier transform is commonly used in fields like audio processing, image analysis, and signal processing to analyze and manipulate signals more meaningfully.

Reid noted this transformation introduces a delay between recording the signal and stimulating the auditory nerve.

"This delay depends on the bandwidth and the resolution you want to capture (you need to record a longer time period to get frequencies up to 20 kHz than you would 8 kHz)," he clarified. 

The chancellor's fellow elaborated that a "variable delay" is required, posing challenges for cochlear implant users. He emphasized that by performing the frequency decomposition mechanically, the delay can be significantly reduced and made more consistent.  

"If you can perform the frequency decomposition mechanically, you can greatly reduce the delay and make it more consistent," he said. 

By implementing this improvement, cochlear implant users could experience significant benefits in coping with the processing delays commonly associated with traditional methods. Reid specifically highlighted Hemideima, an Australian company he has collaborated with, that is actively involved in this advancement.

In conclusion, the field of insect-inspired microphones holds immense potential for revolutionizing sound capture and enhancing our understanding of auditory systems. 

By exploring intricate mechanisms and using 3D-printing technology, scientists like Dr Andrew Reid are paving the way for the development of miniature, bio-inspired microphones. These innovations not only mimic the remarkable hearing abilities found in insects but also offer practical applications in various fields. From environmental monitoring to medical devices, the possibilities are vast. 

With researchers continuing to push the boundaries, the future holds the promise of even more remarkable breakthroughs in sound capture and the integration of bio-inspired technology into our everyday lives.

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