Electrochemical stimulation of sciatic nerve of a frog in-vivo using flexible electrodes

Conventional functional electrical stimulation is a currently available treatment method for neurological disorders such as paralysis and spinal cord injury. This conventional electrical stimulation results in unwanted nerve signals and temporary or permanent damage on tissues because of the requirement of high current values. In order to prevent these problems, we propose to reduce the threshold value for stimulation which is the minimum required current value to stimulate the nerve. In this study, stimulation current was applied with microfabricated planar and flexible electrodes to reduce the threshold whereas the conventional functional electrical stimulation is implemented with conventional cable. The planar electrode was fabricated with the standard lithography techniques by coating gold on a glass wafer. On the other hand, in order to fabricate the flexible electrode, we used kapton tape as a substrate. These microfabrication methods are provided by the developments of the nanotechnology and we applied these techniques for biomedical applications. In addition, the threshold value is reduced by modulating the ion concentration automatically during the stimulation. By using this technique, the electrochemical stimulation is applied. Since the stimulation depends on the ion concentration gradient, the action potential is produced easily and threshold for stimulation is reduced. The threshold values for these three electrodes; conventional cable, microfabricated planar electrode and flexible electrode were measured in-vivo along the sciatic nerve of a frog. All the animal experiments were performed in pursuance of the ethical protocols. The sciatic nerve which is integrated with the gastrocnemius muscle, was dissected from the frog. To measure the threshold value, the stimulation current was applied by the electrode, and the resulting force on the gastrocnemius muscle was recorded via force transducer. The threshold values for the three types of electrodes were detected by observing whether there is a muscle contraction or not, as a result of the stimulation current applied. The microfabricated planar electrode reduced the threshold values by 60-65% compared to the conventional functional stimulation due to increase in contact area between the nerve and the electrode. Furthermore, the flexible electrode reduced the threshold values by 80-85% in comparison to the conventional cable. This improvement was achieved by increasing the contact area more, through surrounding the nerve by flexible electrode. Finally, Ca2+ is depleted from the extracellular solution by coating the flexible electrode with Ca2+-ISM (Calcium-ion selective membrane). Since the Ca2+ is a natural blocker of voltage-gated sodium channels, the threshold values is reduced by 15-25% when compared to bare electrode by depleting Ca2+ during the stimulation. In conclusion, since the flexible electrode provides efficient contact area in a small device, it can be implanted into a body. Furthermore, the electrochemical stimulation applied and the threshold is reduced by depleting the Ca2+ ion from the extracellular solution. It is the most harmless way to stimulate the nerve by decreasing the stimulation current applied. Hence, the flexible electrode can be used for implantable neuroprosthetic devices for treatment of neural disease or injury.