Bio-inspired device mimics natural hearing to outshine cochlear implants

Better hearing with fewer energy requirements

Current cochlear implants are no match for natural hearing, often having limited frequency resolution. Users can struggle with differentiating between speech and noise as well as have difficulty enjoying music. Changing these implants to improve frequency resolution can raise power needs and thus, battery and implant sizes. Larger, more visible companion devices and implants can be a source of discomfort and prejudice for users.

Zang and colleagues designed a hearing device that could solve some of these issues. Meant to mimic the natural cochlea, the BSEM is made of urethane rubber, resulting in a flexible, elastic and curved structure. This metamaterial helps concentrate energy and split sound waves to various branches within it. Piezoelectric flakes that produce electrical charge when subjected to mechanical stress — in this case sound waves — are present throughout the divisions of the metamaterial. Copper wires carry these electrical signals to the auditory nerves and onwards, to the brain.

In a series of experiments, the team tested the performance, biocompatibility and applicability of the BSEM. To begin with, they checked if the hearing device could sort various frequencies of sound. The structure was set within a saline solution that replicated the cochlear space, and then exposed to different sound waves.

As the piezoelectric flakes converted the mechanical stress of the sound waves to electrical charges, the researchers recorded the BSEM’s response to sound. The bioinspired mechanism distinguished between sound frequencies 30 Hertz (Hz) apart, meaning it could aid in high-resolution hearing.

“This material can process a wide range of frequencies (150 Hz to 12,000 Hz) and has 168 frequency channels, allowing for better sound recognition and appreciation of music,” said Zang. “Our experiments demonstrated that the BSEM has a high frequency resolution (up to 30 Hz) and can activate the auditory pathway in mice without needing a power source. This means it can potentially work without relying on batteries or other power supplies.”

A promising road ahead

Next, the researchers conducted animal experiments using frog nerves and mice to further investigate the BSEM’s abilities and “to visualize the output voltage of BSEM.” The large size of the frog’s sciatic nerve, which runs through the hind leg, made the experimental setup easy, and its partial similarity to mammalian nerves provided perspective on how BSEM might function in mammals. In response to animal sounds and piano music, the sciatic nerve was evidently stimulated in response to the electrical charges from the BSEM.

Further, the researchers wanted to see if the hearing nerves and brain responded to sounds after BSEM implantation. For this, they tested the potential of the BSEM to bring back hearing. On conducting auditory brainstem response (ABR) measurements, the researchers found that BSEM implantation significantly improved hearing in mice. The ABR test is often used to detect hearing loss in the clinical setting.

“The results demonstrated the BSEM’s ability to provide high-frequency resolution, convert mechanical vibrations into electrical signals, and restore auditory function,” said the study’s first author, Hanchuan Tang, a researcher at Huazhong University of Science and Technology in China.

The researchers cautioned that more research, especially clinical trials, are needed. The BSEM needs to be tested at higher frequencies and evaluated across different people, as individual differences may have an impact on its success. Moreover, “The size of the BSEM needs to be further reduced to make it suitable for implantation in humans,” added Tang. “We have explored smaller configurations in simulations, but there is still work to be done to achieve a practical size for human use.”

Zang added, “The progress made in the BSEM research should encourage continued exploration in this area and related fields, as there are many people worldwide who could benefit from improved hearing solutions.”