Active, Flexible Circuit Based on Indium-gallium-zinc-oxide Thin-film Transistors to Improve the Spatiotemporal Resolution of Neural Implants

Neural interfaces have become an important tool in studying and treating neural disorders and diseases. Due to the great interest in understanding the behavior of the nervous system, neural interfaces have been evolving toward flexible and small devices with a high density of electrodes to enhance selectivity, electrical performance, and chronic implantation. However, these characteristics are still limited by the harsh environment of the body, the size of the electrodes, and the number of individual wires that connect every single electrode to a pulse generator or application-specific integrated circuit. The integration of soft and flexible polymers as mechanical substrates minimizes the harsh environmental effects of the body on the neural device and increases its useful lifetime. Furthermore, the use of active electronic circuits based on thin-film transistor technology has demonstrated the potential to increase the number of electrodes without significantly increasing the metallic interconnections. However, meeting the electrical performance requirements for neural stimulation remains a difficult challenge. In this work, we explore the use of a two-transistor, one-electrode (2T1E) circuit based on Indium Gallium Zinc Oxide (InGaZnO4 or IGZO) thin-film transistors (TFTs) using a softening polymer as a mechanical substrate for the development of a high-count electrode array for neural stimulation. The design, microfabrication, and electrical performance of the 2T1E circuit and the IGZO TFTs are presented. Additionally, the electrical resilience, stability, and frequency response of the active device are discussed and evaluated. Moreover, electrochemical measurements are presented to determine the potential of the 2T1E circuit to achieve the electrical performance required for neural stimulation. In this work, the ability to control the On/Off state of an electrode and send biphasic stimulation pulses through the IGZO channel was demonstrated. Overall, this work can pave the way for a new generation of high-channel-count electrode arrays with a high spatiotemporal resolution to enable new paradigms for neural stimulation in harsh, aggressive biological environments.