Flexible and reduced-size neural probe using a photosensitive polyimide

Abstract

The understanding of the human brain function is one of the main challenges of modern science, as it comprises the study of an extremely complex organ. The majority of neural interfaces to study the brain rely on non-flexible and relatively thick structures, which do not adapt to the plastic nature of the brain, damaging the tissue and compromising data acquisition [1, 2]. This research presents the fabrication of a flexible, reduced size, and single shaft neural probe, integrating microelectrodes. The main relevant aspect was the use of a flexible and photosensitive substrate of polyimide, where the metallic tracks, microelectrodes, and pads were patterned. Additionally, the neural probe passivation was performed using the same polyimide. This flexible device will adapt to the subtle movements of the brain, which can improve brain signals recording. The ultimate goal is to extend this fabrication process to the implementation of a single device with microelectrodes and also a waveguide for light delivery in neural cells. The neural probe flexibility and reduced dimensions can also allow its future applicability in studies using brain organoids (mini brains), which are a novel approach in neuroscience that avoids experiments in animals [3,4]. Briefly, the neural probe fabrication process consisted of three stages: (i) photolithography to pattern the body of the probe; (ii) photolithography and thin-films deposition to pattern the tracks, microelectrodes, and pads; and (iii) photolithography to passivate the probe. Firstly, polyimide is spin-coated on a sacrificial Si substrate. Then, the polyimide was exposed to UV light, followed by a development process. The polyimide was completely cured in an oven, defining the body of the probe (its geometry). Secondly, a photoresist is spin coated, followed by its exposition to UV light and its development, forming patterns on the body of the probe. Then, thin films of Ti, Al, and Pt were deposited by e beam and DC Sputtering, and after a lift-off process (photoresist removing) the metallic tracks, microelectrodes, and pads were patterned on the body of the probe. Thirdly, the neural probe was passivated with the same polyimide, through a similar photolithography process, leaving only the microelectrodes and pads exposed. Figure 1 shows the fabricated neural probe with 7.5 mm length, 105 µm width, and 8 µm thickness. The probe has 9 Pt microelectrodes (∼ 9 × 9 µm2) and the metal tracks width is 5 µm (pitch of 5 µm).