Flexible multi-electrode neural probe using active-matrix design of transistor array

To realize a brain-machine interface, a neural probe is one of key hardware. For the neural probe, a high spatial resolution of an electrode array, high electrical sensitivity, and mechanical flexibility remain challenging and essential issues. In regard to these, implantable multi-electrode neural probe employing active-matrix design with field effect transistors (FETs) can attract attention due to their advantages of a suitable integration density and good signal-amplifying abilities. Therefore, we developed a flexible multi-electrode neural probe employing an active-matrix design based on a two-transistor (2T) scheme for application to a flexible neural signal recording probe. As a proof of concept, 4 × 8 electrode array (32 channels) with TFTs was demonstrated. For the flexible active-matrix design, back-gate indium-gallium-zinc-oxide (IGZO) thin film transistors (TFTs) were fabricated on a polyimide (PI). The TFTs used here can amplify signals and achieve a switching function to drive the active matrix. The probe structure was encapsulated by the same PI material again to achieve flexibility with good reliability, as the sandwich-like structure can induce a neutral plane on the TFT and electrodes. To ensure a high signal-to-noise ratio, the structural and process parameters of the TFT were studied. Among different annealing times and temperatures of the IGZO TFT, the optimized annealing condition of the TFT was found to be 250 °C a 1 hour on a flexible substrate. The width over length of the transistors was optimized to 50 μm/10 μm, and the field-effect mobility was 8.33 cm²/V·s. The on current was 2.28 × 10⁻⁶ A at a low driving voltage of 5 V. The transconductance was 2.16 μS and the threshold voltage (V_th) was 1.6 V. By applying 5 V, the switching TFT was turned on, and a doubly amplified signal (> 2.4 times) could be measured on the drain line. The electrode array probe with the active-matrix design can use for accurate neural recording and herald a new generation of flexible neural probes with high spatial resolutions.