Diamond Material Pave the way for the Next Generation of Neuronal Implants

Brain Computer Interface (BCI) offers a way to restore neuronal dysfunction due to degenerative diseases or accidents. Thus, it becomes possible to restore vision with retinal implant or to offer a way for tetraplegic to control a robot by thinking with electrodes implanted in the cortex and recently to do a digital bridge for paraplegic patient to stand-up and walk. But today these systems are limited by their insufficient stability. Neuronal implants require to operation during 10 years of lifetime, and collect data 24/7 and for 10 hours or more every day. But after several months of implantation, some modification can appear such as swelling of the passivation polymers thus inducing current leakage, or degradation of the electrode material.<br/><br/>With the last development of diamond growth process, it is now possible to integrate this material in micro and nano fabrication process as a standard material to produce sensors or neuronal implants Diamond properties are very attractive for medical application. Diamond is a biocompatible material and it has no native oxide, so there is no degradation when the structure is placed in water or harsh environment. Diamond is also a high density material, so there are no species that can migrate inside diamond In addition, conductive diamond, boron doped diamond (BBD) has a wide electrochemistry window that allow to give possibility to inject more current compare to classical metallic electrode.<br/><br/>We have developed a specific process to achieve a very thin full diamond implant combine conductive diamond for the electrode and intrinsic diamond as protective layer. With this structure we take advantages and strength of conductive and intrinsic diamond to achieve an hermetic implant. To validate our full diamond implant, we have done electrical characterizations impedance Spectroscopy and Cyclic Voltammetry. In-vivo experiments have been conducted on several set-ups to record Visual Evoked Potential and Auditory Evoked Potential, assessing the proper functionality of our implants.<br/><br/>To demonstrate their stability, we developed an accelerate long life set -up. The devices placed on PBS solution at a temperature of 77°c. Follow Arrhenius law, at this temperature acceleration factor is around 16, that means for one month in solution, is equivalent to one year and 4 months. This set up is composed of hotplate, and aluminum piece where we placed a hermetic containers with Phosphate Bovine Solution (PBS) and samples. To control integrity of samples an impedance meter is used to record periodically impedance. This type of set-up is also answer at ethical issue and reduce number of animal experimentation.<br/><br/>With this ageing setup, we compared InterDigited Electrodes (IDE) composed of metallic IDE covered with either intrinsic diamond or polymer like SU8. After 29 weeks (around 9 years), diamond is more stable with a variation of 3 % at 1 kHz (frequency of interest for neurons recording), compared to SU8 with 50 % of variation.<br/><br/>Therefore, intrinsic and doped diamond are very attractive for the next generation of neuronal implants.