Implantable neural interfaces allow for the formation of connections between the nervous system and external devices. Whether by recording the electrical activity of neurons, or influencing it through electrical stimulation, these devices and hold great potential in both the clinic and basic research. Despite this, existing technologies either lack the electrode resolution to communicate with individual or small populations of neurons or degrade after months of implantation due to the foreign body reaction they trigger in the body. Through the use of state-of-the-art materials and fabrication techniques, my work focuses on the development of implantable devices capable of addressing these issues, enabling long-term high-resolution recording and stimulation throughout the nervous system. Making use of these systems, I am developing closed-loop neural interfaces for the treatment of conditions such as spinal cord injury, using acute and chronic implantation and electrophysiology models.