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In recent years, cochlear implants have emerged as a groundbreaking solution for restoring hearing to individuals with hearing impairment. However, for those whose cochlear nerves are damaged to a degree that disqualifies them from standard cochlear implants, auditory brainstem implants (ABIs) have offered a glimmer of hope. Yet, traditional ABIs often consist of rigid materials, limiting their effectiveness by failing to establish optimal contact with surrounding tissue. This has resulted in many electrodes being disabled to avoid negative side effects like dizziness and facial twitching, leaving patients with only a vague perception of sound and restricted speech clarity.
Fortunately, researchers at the École Polytechnique Fédérale de Lausanne (EPFL) have introduced an innovative solution: a soft, thin-film ABI. Utilizing micrometer-scale platinum electrodes encased in a flexible silicone matrix, this advanced device measures just a few hundred micrometers in thickness. The development, detailed in the journal Nature Biomedical Engineering, promises improved tissue interaction, potentially minimizing off-target nerve activation and reducing adverse reactions.
“Creating a soft implant that effectively adapts to the brainstem’s unique environment marks a significant advancement in addressing hearing loss for individuals unable to utilize cochlear implants. Our trials in macaques have shown great potential for clinical application, which could lead to a more nuanced and accurate auditory experience,” stated Stéphanie P. Lacour, head of the Laboratory for Soft Bioelectronic Interfaces at EPFL.
Exploring “prosthetic hearing” through complex tasks
In addition to developing the implant, the researchers conducted extensive behavioral studies on macaques with normal hearing capabilities. This approach enabled them to assess the ability of these animals to differentiate patterns of electrical stimulation in a manner analogous to natural sound perception.
“Creating a functional implant is merely one aspect; the other is training these animals to behaviorally demonstrate their hearing capabilities,” remarked Emilie Revol, co-first author of the study and former PhD candidate at EPFL. She rigorously trained the macaques to engage in an auditory discrimination challenge, where the monkeys learned to manipulate a lever to indicate whether successive tones were identical or distinct.
“We gradually introduced stimulation from the soft ABI, starting with a combination of acoustic tones, helping the monkeys transition between natural and artificial hearing. Ultimately, we aimed to determine if the animals could perceive subtle shifts between different electrode pairs when only using the soft ABI. Our findings indicate that they responded to these stimuli similarly to how they would react to actual sounds,” added Revol.
Advantages of a soft array
“Our primary objective was to utilize soft bioelectronic interfaces to enhance the alignment of electrodes with the surrounding tissue,” explained Alix Trouillet, a former postdoctoral researcher at EPFL and co-first author of the research. “If the implant can conform to the brainstem’s natural curvature, we can lower the required stimulation levels while retaining more active electrodes for superior sound resolution.”
Traditional ABIs typically rest on the dorsal surface of the cochlear nucleus, which has a complex geometry with a radius of approximately 3 mm. The use of rigid electrodes often creates air gaps, leading to unintended electrical dispersion and nerve stimulation. In contrast, the flexible design developed by the EPFL team can easily adapt to the contours of the tissue.
Furthermore, the adaptable nature of the soft array’s microfabrication allows for adjustments based on varied anatomical structures. “The possibilities offered by microlithography are vast,” Trouillet continued. “We can envision designs with increased electrode counts or alternative configurations that enhance frequency-specific responses. The current prototype includes 11 electrodes, but future designs could expand this significantly.”
Enhanced comfort and minimized side effects
A pivotal aspect of the macaque study was the lack of observable adverse effects from the new device. The researchers noted that, within the tested range of electrical stimulation, the animals displayed no signs of discomfort or muscle twitches—issues often experienced by human ABI recipients. “The monkey consistently engaged the lever to activate stimulation on its own, indicating a comfortable interaction with the prosthetic input,” explained Revol. “If the stimulation were unpleasant, the animal would likely have ceased the behavior.”
Pathway to clinical application
While these initial results are encouraging, bringing a soft ABI to market will necessitate further research and navigating regulatory frameworks. “One immediate step could involve intraoperative testing during human ABI surgeries,” noted Lacour, pointing out that their clinical partners in Boston perform ABI surgeries for patients suffering from severe cochlear nerve damage. “Our soft array could be temporarily inserted before the standard implant to assess how effectively we alleviate undesirable nerve activation.”
Additionally, ensuring that all components of the device meet medical-grade standards and demonstrate long-term reliability is crucial for patient safety. However, the researchers are optimistic, bolstered by the rigorous assessments the device has already survived. “Our implant remained stable within the animal for several months without any noticeable electrode migration,” Trouillet observed. “This represents a significant advancement, particularly given the tendency for conventional ABIs to migrate over time.”
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