How does the Neuralink implant and other brain-machine interfaces work?

Implantable electrical brain-machine interfaces promise major advances, both for understanding how the brain works and for compensating or replacing functions lost after an accident or neurodegenerative disease: primary vision, motor skills, speech synthesis, or digital writing.

While these interfaces are still far from being truly operational in the clinic, for some they still represent the hope of expanding human capabilities, with applications that are both sensory (e.g., night vision) and functional ( e.g. improve memory or intellectual ability). . While many of these uses are still science fiction, such as transmitting sensory input or enhancing our mental abilities, others, such as infrared or ultraviolet vision, do not seem out of reach.

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While ethical issues accompany the development of brain-machine interfaces at Neuralink, Elon Musk’s acclaimed company, our article aims to explore their technical workings, their technological challenges, and the contrast between the hopes they inspire and what they can currently do , explain to achieve.

In fact, the current devices face several technological and conceptual barriers. Technical limitations currently limit their use to certain clinical cases where the risks associated with placing an implant are outweighed by an assessment of immediate or future benefits to patients. So we are a long way from using these implants in clinical routine and everyday life, for fun applications or to increase human performance.

Where are the current implants and in particular the Neuralink implant?

For the medical part and the understanding of the brain, the interfaces that are being developed in academic and industrial laboratories already offer interesting perspectives. But few academic tools currently offer a fully implemented solution with as many electrodes and as much data as that of the Neuralink interface.

This aims to set up an implantable brain-machine interface in one morning, both for the medical field for people with paralysis, but also to allow anyone to control their smartphone, a video game or, in the long term, increase their human capacities . For this, it is aiming for a brain implant technology that captures a large number of neurons that would have no aesthetic impact and would not pose a threat – such a technology does not currently exist.

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If Neuralink’s implant proves to be robust and receives approval from health authorities for use in humans, it could be a step toward more accurately deciphering neural activity, designing clinical neuroprostheses, and understanding previously inaccessible modes of the brain be.

How it works ? From nerve implant to neuroprosthesis

In the literature and in the news we find the terms “brain-machine electrical interface”, “neuroprosthesis” or “neuronal implant” indiscriminately. A “neuroprosthesis” is a type of brain-machine interface that will make it possible to augment or replace a lost function. Just as the nervous system sends or receives information from its environment, neuroprostheses capture information from our environment through artificial systems to send back to the nervous system, or capture information from the nervous system to send back either to itself or to our environment using artificial systems Devices.

The neuroprosthesis or electrical brain-machine interface consists of several parts. From the neural system to an interface usable by humans (like a computer screen), the components of a neuroprosthesis are as follows: 1) a network of electrodes that are placed in contact with the neural tissue, 2) a connection system that enables the electrodes to be connected to a electronic system, 3) a communication system allowing to send signals to the electrodes or to receive the signals collected by the electrodes, 4) a data recording system, 5) a data processing and decoding system, 6) a system for sending Information to one or more effectors, such as a robotic arm. The implantable part, strictly speaking the “neural implant”, is currently made up of parts 1-2 and 1-2-3.

What are the current technological frontiers of brain-machine interfaces?

The current goal is a neural implant with a large number of recording or stimulation electrodes that will remain effective for decades. If this goal has not yet been achieved despite more than thirty years of research, it is because there are many major challenges associated with it, in particular:

  • The implantation operation must be as non-traumatic as possible and, in particular, must not damage the blood microvessels of the cortex, otherwise trigger a significant inflammatory response.

  • The implant must be as thin as possible, even flexible, in order not to cause too much trauma or a rejection reaction in the brain during insertion. In addition, over time, the protective gait generated by the nervous system can prevent communication between the electrodes and the neurons.

  • In order to capture or stimulate as many neurons as possible, it was necessary to develop microfabrication methods on flexible microdevices in order to integrate as many electrodes as possible in a small space. Current electrodes can reach sizes on the order of 5 to 10 microns.

  • Many new electrode materials have been developed to detect or stimulate the very weak electric fields generated by neurons, which was not possible with traditional metals such as platinum. Today, the performance of electrodes has greatly improved, thanks in particular to the introduction of porous materials.

  • The implant must maintain the integrity of its electrical performance over time, but current flexible technologies are sensitive to water over the long term, impacting the lifespan of the implants. This point is one of the most important technological barriers.

  • In order to be able to move normally outside of a laboratory or hospital, the implants must be able to communicate wirelessly and be able to supply themselves with energy. But current high-frequency signal transmission technologies produce a local temperature rise at many electrodes that is harmful to neural tissue – another major technological obstacle.

Ways to make brain-machine interfaces a reality

To solve these problems, the company Neuralink, for example, has designed a network of electrodes to stimulate or record neuronal activity, distributed over several flexible polymer threads carrying microelectrodes. The materials used are biocompatible and layers of silicon carbide to ensure the electronic integrity of the implants seem to be studied (a concept under development by research laboratories at the University of Berkeley and also in France as part of the SiCNeural project funded by the ANR). ). Finally, each filament is connected to an electronic chip that serves to record neural activity or generate electrical impulses for stimulation.

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In addition, the company is developing an autonomous robot capable of performing all phases of implant surgery, from trephination to implant placement.

Indeed, the insertion of flexible implants into the brain is not easy and several strategies have been developed by different laboratories, e.g. also developed at Berkeley, which involves threading a needle through a hole in the end of the flexible implant to push the implant into the brain and then just removing the needle. This last method is taken up by Neuralink, which combines it with a camera system that identifies the areas of the cortical surface that are not or poorly vascularized, where implants can be placed while limiting microbleeds.

Analyze and transfer data without overheating

As for the problem of local heating through the analysis and wireless transmission of data, two technologies have been applied to humans so far.

The first is that of the company BlackRock Neurotech, which moves the signal processing and transmission circuitry over the cranial box. This causes aesthetic problems, but also risks of infection due to the threads that run from the skin to the brain.

The second technology is that of the CLINATEC laboratory of the CEA Grenoble, which only collects signals that do not require high digitization precision and only records the information on a maximum of 64 electrodes simultaneously. This laboratory has thus produced the first wireless neural implant with so many channels that is completely integrated under the skin. It is used to replace part of the skull bone. For its part, Neuralink offers a smaller chip that is also placed in the cranial bone and processes more than 1,000 signaling pathways, but only sends certain characteristics of the neural signals that are deemed important thanks to integrated algorithms.

With regard to the lifespan of the implants, we still have to wait and see whether the strategy will take effect and enable a stable interface over several years. Once this limit is exceeded, the collection of an even larger number of signals must certainly be tackled. Currently, it can be estimated that the Neuralink technology, with its 1,024 electrodes, can capture up to about 3,000 neurons: that’s impressive by today’s standards, but far from sufficient to capture the immensity of the brain’s signals.

Despite very good miniaturization, it will be conceptually very difficult to achieve the acquisition of millions of individual neurons with this technology without the implant and the associated connectors taking up too much space in the brain. Other concepts may need to be developed to overcome these limits.

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