A Magnetically Assembled High‐Aspect‐Ratio Needle Electrode for Recording Neuronal Activity

Abstract

Microelectrodes, which penetrate the brain tissue and are located near neurons, enable the electrical recording of neuronal signals at a high spatiotemporal resolution. Such high spatiotemporal signal quality of the neuronal activity is necessary to understand neuronal networks and the working of brain. [1] Owing to the small size and high density of neurons in tissues, needle-like microelectrode devices are commonly used in neuronal recordings; such devices have made significant contributions in the fields of neuroscience and brain-machine interfaces. [2][3][4][5][6][7] To further develop such microelectrode devices, a microelectrode should i) have a fine needle diameter (<30 µm) to reduce damage to tissues. Several groups have demonstrated chronic recordings using >30 µm diameter wire-electrodes (e.g., four-month recordings of 50 µm diameter wires using rats [8] and five-year recordings of 30-50 µm diameter wire arrays using monkeys [9] ). Further requirements for a microelectrode device include ii) a long needle (e.g., ≈1 mm for rodents [10,11] and ≈2 mm for macaque cortices [12] ) and iii) multiple electrodes with a high density to achieve high spatial recordings (<100 µm in pitch).To meet these requirements, we proposed an assembly technique for vertical microwires on a substrate using the magnetic force. Figure 1a1-a4 shows the principle of the proposed assembly. A magnetic wire is placed on a substrate that comprises a through hole [ Figure 1a1]. The wire is then magnetized by applying a magnetic force [ Figure 1a2]. The wire is magnetized as an n-pole; one end is repelled from the substrate and the other is attracted to the substrate [ Figure 1a3]. After sliding the magnet under the through hole, the wire falls into the through hole [ Figure 1a4]. This principle enables the magnetic assembly of microelectrodes for neuronal recording. Characteristics of both a fine electrode diameter and a long electrode are achieved by using microscale diameter wires and a photo lithographybased etching process. In addition, these wires are assembled into through holes in the platform of the device substrate, allowing an electrical connection from the back side of the substrate [ Figure 1b1]. [13,14] In this study, we report this magnetic assembly using a nickel (Ni) wire as the electrode and a silicon (Si) substrate as the device's platform. We used Ni wires with a 25 µm diameter Microelectrode devices, which enable the detection of neuronal signals in brain tissues, have made significant contributions in the field of neuroscience and the brain-machine interfaces. To further develop such microelectrode devices, the following requirements must be met: i) a fine needle's diameter (<30 µm) to reduce damage to tissues; ii) a long needle (e.g., ≈1 mm for rodents and ≈2 mm for macaques); and iii) multiple electrodes to achieve high spatial recording (<100 µm in pitch). In order to meet these requirements, this study herein reports an assembly technique for high-aspect-ratio microneedles, which employs a magnet. The assembly is...