MSCA-COFUND-CLEAR-Doc - PhD Position #CD22-52: High Density GRAphene Neuronal Implant based on Laser Induced Graphene (HD-Grani)

Dysfunctions of the central nervous system are a major economic and social issue since they affect a growing number of people because of the aging population and longer life expectancy. These may be traumatic (eg, accidents, vascular lesions), psychological (eg, autism, depression, anorexia, bipolar disorder), neurodegenerative (eg, Parkinson, Alzheimer, Huntington), or related to tumors (eg, glioblastoma, medulloblastomas, neuromas). Today, many therapeutic and basic research advances occur at the interface of multiple disciplines ranging from biology (biochemistry, molecular and cellular biology, genetics), materials (biomaterials, new interfaces for the living), mathematics and computer science (modeling, signal processing), electronics (new instrumentation), physics and chemistry. Neuroscience approaches of neurotechnological prostheses and brain-machine interfaces (BMIs) aim to interface large neural ensembles using electrode arrays to access their functional dynamics.

Implantable neuroprosthetic devices offer the promise of restoring neurological functions to disabled individuals. Tests demonstrated that an array of microelectrodes implanted in cortex allows to record activity of the brain and to induce a movement on prosthetic limbs or electrical stimulations restore some visual sensations. For these applications, the lifetime and stability of the electrodes are critical features for the reliable operation of any implantable neuronal device. With around 150 000 neurons by square millimeter for human, it’s also necessary to have high-density implants with small electrodes to cover a large surface of the cortex to have access of neuronal code.

The goal of this thesis is to address one major challenge: - reduce the size of the electrodes to be equivalent to the neuron’s size (10 μm) without degradation of noise and consequently increase the electrode density for a fine mapping of the cortex, while targeting high biological compatibility.

Typical electrodes consist of flat gold and platinum surfaces. When the size of the electrode is reduced, its impedance increases and the rise on thermal noise hinders the ability to correctly record single neuronal signals. To overcome this limit, electrodes are functionalized with Black Pt or PEDOT in order to increase their surface are and thus decrease the impedance. Even if these electrodes give good results on in-vitro and in-vivo research, the degradation at long term prohibits their use for brain implants in humans.

To solve this issue, several teams explored the application of graphene on implant technology. This atomically thin material composed essentially of a monolayer of carbon, offers a good interface with neurons. There are however limitations in the usage of graphene, namely its integration with the production methods for the other parts of implants, requiring a transfer onto target substrates with direct synthesis being a challenge, and the intrinsic flat nature of a graphene film that would show the same problems for standard gold small electrode. The goal of this thesis will be to overcome this limit by using a new approach on the synthesis of graphene electrodes for implants, by introducing Laser Induced Graphene (LIG), a graphene-based highly porous structure obtained by direct laser pyrolysis of polymers. Some preliminary tests showed that is possible to create µm-sized electrodes directly onto polyimide, the same polymer used to currently produce soft implants, making this approach directly integrable into the common production process. Here with this solution, it will be possible to develop graphene implants with a complex structure and not limited by the number of electrode (256 electrodes and more) with a small pitch between electrodes.