By Abishek Umashankar - I Msc (AuD), and Prashanth Prabhu, PhD

Neuralink, launched by Elon Musk in 2016, came to the public lime of light with its goal to advance artificial intelligence (AI) in addressing neural conditions and disorders.^1,2 The implant is based on a brain-machine interface (BMI) technology, a neural lace technology that acts as a communication bridge between the brain and a wired device. This technology is poised to help those with sensory and motor disorders and other neurological conditions with AI.^3,4 While research into BMI is still limited, and Neuralink promises to deliver a scalable, high-bandwidth system.⁵

Neuralink has three necessary components: a link, neural threads, and a charger. The link is a sealed implanted device that stimulates and transmits neural signals. The neural threads are small and flexible threads that contain many biocompatible thin film materials, which have polyamide as a substrate encapsulating a gold thin film trace. The threads, which include electrodes for detecting incoming signals, are thinner than a human hair (17 µm), measuring a thickness of 4-6 µm and a length of 20 µm. An array contains 96 threads, which have 32 independent electrodes. One array includes a total of 3,072 electrodes, which makes the transfer of high-volume of data possible.^4-6 The charger can wirelessly recharge the internal battery.

Due to the insertion complications, robotics-based insertion is used for the link, ensuring the accurate insertion of the electrodes.^5,7 This process uses a needle that penetrates the meninges and brain tissue. It is driven by a linear motor, allowing variable insertion speeds and rapid retraction acceleration that is approximately 30,000 mm/s².⁸ The robotic features auto insertion mode and can insert up to six threads per minute. In an experiment conducted in rodents, researchers achieved a  90 percent successful insertion of the implants with maximum internal channel responses.⁵

The link will decode neuronal spikes with computer algorithms' help in real-time virtual control in individuals with spinal cord lesions. The link was initially made to augment individuals with quadriplegia by assisting them with motor movements. However, the organization also claims that the Link can enable an individual to perceive broad frequencies of sounds with high amplitudes, thereby acting as an auditory implant.

OTHER AUDITORY IMPLANTS

In the field of hearing technology, a variety of implants such as middle ear implants,¹¹ cochlear implants,¹² auditory brainstem implants,¹³ auditory midbrain implants,¹⁴ and auditory cortical implants¹⁵ are either in clinical use or in the research phase. A cochlear implant is an electronic device that can restore partial hearing to a hearing-impaired individual who has a hearing loss of severe to profound via electrical stimulation to the residual auditory nerve. The implant contains an external component that houses the microphone, processor, radio frequency transmitter, and an internal component, including a receiver-stimulator and an electrode array with different electrodes. it is these electrodes that send electrical information to the auditory nerve fibers. The implant uses coding strategies to improvise the auditory perception in an individual with complete deafness.¹⁶ The totally implantable cochlear implant device was a long-term goal by the cochlear implant industries to allow recipients to access an invisible hearing without the device's external component. The technology is still under research.¹⁷ Another implant that augments the hearing in deaf individuals is the auditory brainstem implant. The device contains an external component similar to the cochlear implant that houses the microphone, processor, and transmitter. However, the internal Implant consists of an electrode array surgically placed at the dorsal and ventral cochlear nucleus.¹³ The auditory midbrain implant is a type of auditory implant where the electrode array is placed at the level of the inferior colliculus, which is the alternative place to fix a prosthesis apart from the cochlear nucleus. The inferior colliculus is more preferred because it is a core structure in the midbrain with a change in its conductivity property that is tonotopically organized.¹⁸ An auditory cortical implant consists of a titanium-based microfabricated microelectrode that aims to stimulate the auditory thalamus and auditory cortex in a minimally invasive manner.¹⁵ Currently, cochlear implants and auditory brainstem implants are the only devices used in human populations with FDA approval. Midbrain and cortical implants are still under research.

LIMITATIONS OF THESE IMPLANTS

Even though these Implants aim to partially restore the hearing, these devices have several limitations based on the individual's perception. For example, in cochlear implants, even though they use sophisticated technology and coding strategies, implanted individuals still possess slow temporal dynamics and poor spatial resolution.¹⁹ Individuals implanted with brainstem implants possess low resolution due to the lost functionality from the auditory nerve fibers and have impairment in identifying high-frequency information and pitch perception. Even though an ABI restores audibility, its performance is comparatively poorer than cochlear implant individuals.²⁰ It is because there is complete coverage of electrodes in a cochlear implant, but in a brainstem implant, the electrodes partially cover the cochlear nucleus, and the coding strategies used for a Brainstem Implant are CI-based strategies.²¹ Midbrain and cortical implants have even more limitations because the function and fine-tuning got from the lower brainstem are missed out, and even these Implants do not have complete coverage of their electrodes. The higher the centers the Implant is done, the lower the performance gets.^15,21 Cortical implants also possess other threats like vascular damage and nerve tissue damage due to its titanium-based microfabricated electrode at the cortical placement level. Moreover, titanium might allow shorting between neurons contacting the exposed underside of the device. For chronically implanted devices, there are chances of a generation of micromotion induced inflammation that could result in degradation of sound quality.¹⁵

CAN NEURALINK BE A GOOD IMPLANT?

Neuralink initially aimed to restore limb movements in quadriplegic individuals and other tasks such as memory, hearing, vision, stroke, and depression. The work of neuroscientists in developing this Implant is commendable as they utilize current generation and future generation technology to invent it and function it. However, when it comes to Neuralink as an auditory implant, several limitations can occur. As discussed above, the brainstem implants, midbrain implants, and cortical implants have limitations concerning an individual's perception. Similarly, Neuralink will act as a cortical implant and will face a similar limitation as in an auditory cortical implant. Maximally, it can provide a certain range of audibility but cannot enable an individual to perceive a broad frequency range and perceive music through personal listening devices. Another limitation of Implanting using Neuralink technology is concerning the age of implantation. It has been proven that there is the effectiveness of early implantation in children with Prelingual deafness due to the mechanism of Brain plasticity during the initial three years of life.^22,23 Neuralink must have a compatible and safe device to Implant children who are three years and younger as most of the brain implanted devices do not have candidacy criteria that are three years and younger. Only recently, children 12 months and older with Neural deafness can undergo auditory brainstem implantation.²⁰ It would also be cumbersome for Neuralink to get an FDA approval as neither midbrain nor cortical Implant has received FDA approvals. Although Neuralink can restore other features, the device cannot be a successful Bionic Implant for hearing as long-run research hasn't established a successful cortical implant for hearing.

​ABOUT THE AUTHORS: Mr. Abishek Umashankar - II Msc (AuD), is a post-graduate audiology student at All India Institute of Speech and Hearing Mysuru, where Dr. Prashanth Prabhu is an assistant professor in the Department of Audiology.

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