Graphene Based Flexible Neural Electrodes

Brain-machine interface can collect brain signals and converts them into commands for connecting the nervous system of the brain to electronics.As a crucial component of brain-machine interface,neural electrodes are required to be flexible,stable,and high-performance.Graphene is a carbon material with high specific surface area,high electrical conductivity and good biocompatibility,hence is potentially applicable to neural electrodes.However,pure graphene as a brain-machine interface material has low charge storage capacity（CSC）and high interfacial impedance.Thus the conductive polymer with high CSC and electrical conductivity can be utilized to enhance the CSC of the graphene electrode while reducing the interfacial impedance between the electrode and nerve tissue.In this work,different neural electrodes are designed based on graphene materials and are investigated.（1）Preparation of graphene fiber electrodes and their applications in insect flight control.Graphene neural electrodes were prepared using wet spinning technique,which has lower density and elastic modulus compared to traditional metal microfilament electrodes.The electrochemical performance of graphene fiber electrodes measured in phosphate buffer show lower impedance and higher charge storage capacity.Graphene fiber electrodes were successfully used to control the flight of insects by implanting in the optic lobe of insects,while the surface morphology and elements of the electrodes did not show significant changes after electronic stimuli,indicating that the graphene fiber electrodes exhibited good chemical stability and electrical stimulation performance in the insects after electrification,reflecting the good potential of graphene fibers as neural electrodes.（2）Graphene fiber electrodes were modified via doping with poly（3,4-ethylenedioxythiophene）-poly（styrenesulfonate）（PEDOT:PSS）.The composite fiber electrode was fabricated by mixing PEDOT:PSS with graphene oxide（GO）in wet-spinning.The results indicate that the maximum doping content of PEDOT:PSS was capped at 30% of GO mass,and the optimal mechanical properties of the fibers can be obtained when the doping content is 20%.Moreover,the addition of PEDOT:PSS reduces the cytotoxicity of the electrode.The impedance of the modified fiber electrode decreased to 42.2% at 1 k Hz,and the CSC increased by 5 times.The modified fiber electrode exhibited lower initiation voltage in flight start-stop experiments,which effectively reduce the interfacial impedance between the electrode and the tissue.（3）Preparation of multichannel graphene-based flexible nerve electrode.Printable aqueous inks were prepared using graphene oxide as a matrix for the preparation of multichannel flexible neural electrodes.The rheological experiments revealed that the concentration of GO has a tight relationship with the viscosity,and the concentration limit of GO is 30% and the optimum concentration range is 7-10%,whereas the pure GO ink tends to fracture during drying after printing.The electrode cracking was effectively overcome by doping with the water-soluble conductive polymer PEDOT:PSS.Finally,a 8-channel flexible neural electrode was fabricated via ink direct writing and transfer technology,and the electrode performed excellent chemical stability and high CSC owing to the superior properties of graphene.