The brain–machine interface (BMI) or direct neural interface (DNI) has been intensively investigated for patients with physical function disorders. These interfaces are direct communication pathways between the brain and an external device. Devices for BMI restore physical function in patients after refractory nerve injury. A general technique for transmitting information from a machine to a living body is electrical stimulation with metal electrodes. For transmitting more information, it is necessary to increase the electrode density by miniaturizing the size of the stimulation electrode. However, such miniaturization may deteriorate the performance of the maximum injectable charge without causing tissue damage, especially for charge injection in vivo. To address this issue, we have proposed an implantable stimulation electrode with a hydrophilic gel coating and fabricated platinum (Pt) electrodes covered with a nanocomposite (NC) gel. The performance of the Pt electrode with an NC gel coating is confirmed to be equal to that of electrodes without a gel in vitro. If the charge injection capacity of the electrode covered with the NC gel maintains this value via the absorption of extracellular fluid and the existence of sufficient ions and water around the electrode surface, the performance of the electrode in a living body could increase. The results of the in vitro evaluation of the fabricated electrode show good performance and suggest the enhancement of the in vivo performance.