Towards Neural Interface: Research On The Electrical Modulation Of Peripheral And Central Nervous System And The Related Thermal Effect

The neural interface,on one hand,collects neural information through recording electrodes to decode and control the external device or analyze the internal principle of cognitive psychology and behavior.On the other hand,through the stimulation electrode,the encoded information is transmitted back to the nervous system to form sensory feedback or to modulate the neural activity.The application of the neural interface can provide a new means for the analysis and treatment of a variety of brain diseases,which is of great social significance,bringing the hope of re-standing and walking to some patients with lower limb paralysis caused by spinal cord injury,thereby improving their quality of life.For direct contact between the electronic system and the biological tissue in the neural interface,the neural electrodes should have good biocompatibility and mechanical flexibility to meet the long-term work requirements in the biological environment.In addition,the electric current is an important modulation approach to the nervous system.However,the research and evaluation measures vary,depending on the targeting neural systems,depth of intrusion into neural tissues,and experimental models,ranging from in vitro/ex vivo cell culture to in vivo animal studies.This thesis mainly focuses on the manufacture of flexible microelectrodes applied in the neural interfaces and the electrical modulation of several neural systems.Flexible electrodes were designed to investigate the effects of invasive and contact electrical modulation for both peripheral nerves stimulation and the ex vivo experiments that collect neural signals from the externally stretched and cultured dorsal root ganglia.Moreover,taking human as research object,by establishing numerical models of the spinal cord trunk and head tissues,the effect of multiple factors on non-invasive electrical modulation of both brain and spinal cord nervous systems was systematically simulated and analyzed.The results obtained provide important guidance for future clinical studies on the selection and use of electrodes and the settings of stimulation parameters.Firstly,in this thesis,the microelectronic processing flow of neural microelectrode array was designed based on the flexible material SU-8.A 24-channel microelectrode array was fabricated and the test platforms were built to analyze the performance of the electrode.The electrode tip that is in contact with the biological tissue has a specially designed stent structure.The size and shape of the electrode are matched with the requirements for the electrical signal transmission tested in the neuron axon stretching experiments.Meanwhile,by utilizing current mature technique of flexible printing circuits,another polyimide-based flexible electrode was designed,and the electrodes returned from the foundry were also tested.For the electrical modulation on the rat peripheral nerve system,a sciatic nerve electrical stimulation system with polyimide flexible electrodes was designed.The electroencephalogram signals were collected over rat's skull before and after stimulation to analyze how stimulation influence the activity of brain neural system.This research aimed to explore the feasibility of modulating brain activity through peripheral nerve electrical stimulation,which provides new ideas and solutions without invoking invasive brain stimulation.In addition,for the stretch growth of rat's dorsal root ganglion in vitro,the SU-8/polyimide-based flexible electrode arrays were specially designed to extend neuronal axons for better adhesion and growth of neurons onto the electrodes.On the other hand,the SU-8/polyimide film of the electrode array provided conditions for applying electric fields and collecting neurophysiological signals.The reliability of the electrodes and the platform were evaluated by comparing the electrical properties of the electrodes before and after surface modification,and characterizing the electrodes' performance after the neuron-axon stretching experiment.For the electrical modulation of spinal cord nervous system,the finite element analysis method was used on the electric field and coupled with biothermal conduction.This thesis systematically analyzed the influence of multiple electrode parameters,physiological parameters,and stimulation settings on the electrical and thermal field distribution of the components of the trunk model during the non-invasive spinal cord stimulation.The efficiency and thermal safety of transcutaneous spinal cord electrical stimulation were evaluated.Moreover,in this thesis,a variety of geometric models with different structures were constructed for the human trunk to simulate the influence of individual differences on the transcutaneous spinal cord electrical stimulation-induced temperature changes of various tissue components.The results derived provides suggestions and references for the design and implementation of clinical trials.In the electrical modulation of the brain nervous system,this thesis took human as the research object,and a realistic human head model with four tissue types was established by edge recognition and reconstruction of human medical image data.Based on this head model,a series of electrode models were designed to fit the scalp surface.The finite element analysis method was used to investigate the effect of the electrode-related factors,such as the electrode size,montage,spacing,displacement and geometrical mismatch between the electrolyte and the electrode.Moreover,by coupling the stimulation electric field and the biothermal conduction physics field,the effects of transcranial direct current stimulation on the temperature changes of various tissues were examined for different electrodes and stimulation parameters,and the thermal safety of this neural modulation method was evaluated.