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

Implantable brain-machine electrical interfaces promise great progress, both for the understanding of the brain’s function and for compensating or replacing functions that are lost after an accident or a neurodegenerative disease: primarily vision, motor skills, voice synthesis or digital writing.

Although these interfaces are still far from being truly operational in the clinic, for some they still represent the hope of increasing human capabilities with applications that are both sensory (e.g. night vision) and functional (increasing memory or intellectual abilities, for example ). Although many of these applications are still science fiction, such as the transmission of sensations or increasing our intellectual performance, others do not seem out of reach, such as infrared or ultraviolet vision for example.

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Although ethical questions accompany the development of brain-machine interfaces at Neuralink, Elon Musk’s much talked about company, the purpose of our article is to explain their technical operation, their technological challenges and the contrast between the hopes they raise and what they are in able to at the moment. of achieving.

In fact, the current devices are confronted with several technological and conceptual locks. Technical limitations currently limit their use to specific clinical cases where the risks associated with the insertion of an implant are outweighed by the estimation of an immediate or future benefit to the patients. We are thus very far from being able to use these implants in the clinical routine and in everyday life, and what is more for fun applications or to increase human capacities.

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 within academic and industrial laboratories already offer interesting perspectives. However, few academic tools currently offer a fully implemented solution with as many electrodes and as much data as those in 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 everyone to control their smartphone, a video game or, in the long term, to increase their human capacity. For this, it aims for a brain implant technology that records a large number of neurons, which would have no aesthetic impact and would not pose any danger – such technology does not currently exist.

To also read:
Against brain prostheses: when artificial neurons and natural dialogue

If Neuralink’s implant proves to work robustly, and if it gains approval from health authorities for use in humans, it could be a step toward more precisely decoding neural activity, designing clinical neuroprostheses, and understanding previously inaccessible states of the brain.

How does it work? From neural implant to neuroprosthesis

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

The neuroprosthesis or electrical brain-machine interface consists of several parts. Moving from the neural system to an interface usable by humans (such as a computer screen), the components of a neuroprosthesis are as follows: 1) a network of electrodes placed in contact with the neural tissue, 2) a connection system that allows the electrodes to be connected to an electronic system, 3) a communication system that allows sending signals to the electrodes or receiving 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, for example a robotic arm. The implantable part, the “neural implant” strictly speaking, is currently composed of parts 1-2 or 1-2-3.

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

The current goal is to have a neural implant with a large number of recording or stimulating electrodes whose effectiveness is maintained over decades. If, despite more than thirty years of research, this goal has not yet been achieved, it is because there are many major challenges associated with it, in particular:

  • The implantation operation must be as minimally traumatic as possible and, in particular, must not damage the blood vessels in the cortex due to punishment for triggering a significant inflammatory reaction.

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

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

  • Many new electrode materials have been developed to detect the very weak electric fields generated by neurons or to stimulate them, which conventional metals such as platinum did not allow. Today, the performance of electrodes has been significantly improved, thanks in particular to the introduction of porous materials.

  • The implant must maintain the integrity of its electrical performance over time, but the current flexible technologies are sensitive to water in the long term, which affects the lifetime of the implants. This point is one of the major technological locks.

  • In order to move normally outside a laboratory or hospital, the implants must be able to communicate and supply themselves with energy, without wires. However, current radiofrequency signal transmission technologies, when there are many electrodes, generate a local temperature increase that is harmful to neuronal tissue – another major technological hurdle.

Ways to make brain-machine interfaces a reality

In an attempt 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 filaments that carry microelectrodes. The materials used are biocompatible and layers of silicon carbide to ensure the electronic integrity of the implants appear to be under investigation (a concept from research laboratories at the University of Berkeley and also under development in France as part of the SiCNeural project funded by the ANR ). Finally, each filament is connected to an electronic chip, which is used to record neuronal activity or generate electrical impulses for stimulation.

To also read:
The symphony of neurons or the mathematics of the brain

In addition, the company is developing an autonomous robot capable of performing all stages of implant surgery, from trepanation to insertion of implants.

The insertion of flexible implants into the brain is actually not simple and several strategies have been developed by different laboratories, such as the temporary stiffening of the implant using a resorbable polymer, the use of a rigid guide or a robotic “sewing machine” approach , also developed at Berkeley, which passes a needle through a hole in the end of the flexible implant to push the implant into the brain and then only remove the needle. This last method has been taken up by Neuralink, which combines it with a system of cameras that identify the areas of the surface of the cortex that are not or poorly vascularized, where implants can be inserted and at the same time limit microbleeding.

Analyze and transfer data without overheating

Regarding the problem of local heating due to analysis and wireless transmission of data, two technologies had been applied to humans so far.

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

The other technology is from CEA Grenoble’s CLINATEC laboratory, which only collects signals that do not require high digitization precision and only records the information of a maximum of 64 electrodes simultaneously. This laboratory has thus produced the first wireless neural implant with so many channels, and completely integrated under the skin. It is inserted to replace part of the skull bone. Neuralink, for its part, offers a smaller chip, also inserted in the bone of the skull, that processes more than 1,000 pathways, but transmits only certain characteristics of the neural signals that are considered important thanks to built-in algorithms.

Regarding the longevity of the implants, we still have to wait a bit to see if the strategy is effective and allows for a stable interface over several years. Once this limit is exceeded, it will certainly be necessary to tackle the collection of an even greater number of signals. At present, it can be estimated that the Neuralink technology can record up to around 3,000 neurons with its 1,024 electrodes: this is impressive from a state-of-the-art point of view, but very far from sufficient to understand the extent of cerebral signals .

Conceptually, despite very good miniaturization, it will be very difficult to achieve the recording of millions of individual neurons with this technology without the implant and associated connectors taking up too much space in the brain. It may be necessary to devise other concepts to go beyond these limits.

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