gms | German Medical Science

Objective: To introduce a high frequency electrical stimulation mechanism tailored to stimulate retinal bipolar cells.

Materials and Methods: The decreasing size of stimulation electrodes results in a higher impedance of the electrode due to the reduced surface area. This makes current-controlled stimulation unsafe as the stimulus current pulse could easily force the electrode potential to exceed the water window. Voltage-controlled stimulation has no control over the amount of charge injected into the tissue because the stimulus current is not controlled. For such a situation, the pulse-density modulated switched-capacitor based stimulation (PDM-SCS) has been engineered. The technique cyclically charges a capacitor to a predetermined voltage that is then discharged into the tissue every cycle. By pre-charging a capacitor to a predefined voltage it can be guaranteed that the electrode potential is maintained within the water window. The amount of charge injected is controlled by the size of the capacitor. The slow wave response of bipolar retinal cells to an electrical stimulus mimics the electrical behavior of a low-pass filter. The proposed PDM-SCS approach utilizes such low-pass behavior by injecting the required stimulus charge in the form of high frequency pulses in relation to the membrane properties. The injected charge and the resolution of the stimulus amplitude are controlled through the capacitor charge voltage and modulator sampling frequency.

Discussion: The PDM-SCS stimulation chip is designed, and is currently being fabricated and prepared for evaluation in clinical investigations.

Acknowledgement: This work was supported by the Deutsche Forschungsgemeinschaft (DFG) funding the project (PAK 468).