Implantation force: Russia has made neuroimplant technology safer

Russian scientists have developed and patented a material for soft electrodes that can stretch by almost 50% without loss of electrical conductivity. Unlike rigid analogues, the new technology will reduce the risk of damage to nerve fibers and, in the long term, extend the service life of implants that help restore mobility and sensitivity after severe injuries. The flexibility and small thickness of the design potentially reduce mechanical irritation of tissues during brain movements, experts said. At the same time, experts emphasized that such electrodes must undergo a full cycle of necessary tests.

New material for flexible neuroimplants

Scientists of the Biomaterials department of the Scientific Center for Genetics and Life Sciences of the Sirius University of Science and Technology have patented a new material for the production of soft electrodes. They can be used in brain and spinal cord implants designed to restore mobility and sensitivity in people after injuries. Unlike rigid electrodes, which can damage nerve tissue and eventually disrupt the work of implants, the new material is able to stretch by almost 50% without losing electrical conductivity. However, it does not injure brain tissue.

Injuries to the brain and spinal cord, as well as neurodegenerative diseases, often lead to severe disability, the scientists said. A person loses the ability to move, feel, and remember new things. One of the most promising approaches to restoring impaired functions is neuroprostheses, systems that provide data exchange between the nervous system and electronic devices. The problem with using such systems in practice is that over time, the quality of the electrode's contact with the nervous tissue is lost. The fact is that the brain is a very soft environment, and hard electrodes, when interacting with it, easily damage the nervous tissue. In response, the body triggers local repair, the contact of the electrode with the nervous tissue is disrupted, and the neuroprosthesis loses its ability to perform its functions.

Soft, elastic electrode systems, which are similar in their mechanical properties to biological tissues, can become a solution to the problem. Scientists have developed and patented a material with this property, an electrically conductive two—layer nanocomposite coating on a flexible substrate.

— With conventional rigid electrodes, the formation of a glial scar around the implant takes about one to three months and leads to a decrease in the quality of the signal being captured. It is important to understand that this technology still requires optimization and further biological testing, but reducing the stiffness of the electrode can increase the active life of neuroimplants in the future," explained Sergey Kravchenko, co-author of the development, senior researcher at the Biomaterials Research Center for Genetics and Life Sciences at Sirius University of Science and Technology.

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The material is a flexible base made of polydimethylsiloxane (PDMS) — silicone polymer, which has long been used in implantable medical devices due to its biocompatibility, softness and elasticity. Silicone itself does not conduct electric current, so its surface is covered with conductive particles. In this case, the scientists used graphene: it has high electrical conductivity and is safe for the body.

The main technological feature of the development is the method of graphene deposition. This usually requires expensive equipment or harsh chemicals. Sirius engineers have proposed an alternative approach: they apply graphene diluted in hexane to partially cured silicone. The solvent softens its top layer slightly, so that the graphene particles sink into the polymer and end up under a thin film of silicone. After evaporation of the solvent, graphene is firmly fixed on the surface of the electrode system.

Such a mount is one of the main advantages of the development, experts noted. Tests have shown that graphene does not peel off by itself and remains in place even if the material is bent or stretched. It covers almost the entire surface of silicone, and the conductive layer is about 25 micrometers thick, which is thinner than a human hair.

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The brain and spinal cord have a complex organization and are highly sensitive to mechanical influences. A hard implant and soft nerve tissue react differently to body movements and natural tissue micro-displacements, Artem Tolmachev, Associate professor of the L.O. Badalyan Department of Neurology, Neurosurgery and Medical Genetics at the Institute of Neuroscience and Neurotechnology (INN) at Pirogov University, commented on the development. As a result, repeated micro-injuries, an inflammatory reaction, and the formation of a glial scar around the electrode may occur during prolonged contact. This can impair the transmission of electrical signals and reduce the efficiency of the device.

— Therefore, the creation of electrodes, the mechanical properties of which are close to the properties of biological tissues, is of great importance for the development of neuroprosthetics. If the material retains electrical conductivity during deformation, this potentially allows it to maintain stable contact with the nervous tissue under conditions of constant mechanical stress. When assessing the safety of a development, it is necessary to take into account not only the mechanical flexibility and electrical conductivity, but also the biocompatibility of the material in the long term," the expert said.

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Graphene has a number of properties that make it a promising material for creating soft electronics, Vsevolod Belousov, Corresponding Member of the Russian Academy of Sciences, director of the Federal Center for Brain and Neurotechnology at the FMBA, Chairman of the scientific Committee of the National Prize in the Field of Future Technologies "Challenge", told Izvestia. Due to its high electrical conductivity, it allows recording the electrical activity of neurons. The flexibility and small thickness of the graphene coating, in turn, potentially reduce mechanical irritation of tissues during brain movements.

— It is possible to produce microelectrode arrays with high spatial density. Colleagues have proposed a new approach to manufacturing such electrodes on a silicone substrate, which potentially solves the problem of the toxicity of nano- and microparticles of graphene, which can form during long—term operation of such electrodes inside the body, as well as the problem of peeling the electrode from the substrate, - said the scientist.

But even if the fresh electrode is completely biocompatible, microcracks, delamination, delamination, surface changes, accumulation of degradation products and chronic activation of microglia may occur over the years of operation. At the next stage, long-term animal studies are needed to assess gliosis, neurodegeneration and functional disorders, Vsevolod Belousov said.

The combination of an elastic polymer base with a conductive graphene layer can be used in mass-produced devices if it is possible to confirm the long-term safety and reliability of such a solution, and the production technology proves to be cost-competitive, said Denis Kuleshov, CEO of Neuroimplants Elvis, Neuronet market expert. If the development of Sirius makes it possible to simplify the production process, reduce its cost and at the same time provide comparable or higher reliability, this will become a serious competitive advantage.