Implantable intracortical microelectrodes can record a neuron’s rapidly changing action potentials (spikes). In vivo neural activity recording methods often have either high temporal or spatial resolution, but not both. There is an increasing need to record more neurons over a longer duration in vivo. However, there remain many challenges to overcome before achieving long-term, stable, high-quality recordings and realizing comprehensive, accurate brain activity analysis. Based on the vision of an idealized implantable microelectrode device, the performance requirements for microelectrodes are divided into four aspects, including recording quality, recording stability, recording throughput, and multifunctionality, which are presented in order of importance. The challenges and current possible solutions for implantable microelectrodes are given from the perspective of each aspect. The current developments in microelectrode technology are analyzed and summarized.

A high-speed steady-state visual evoked potentials (SSVEP)-based brain-computer interface (BCI) system using dry EEG electrodes was demonstrated in this study. The dry electrode was fabricated in our laboratory. It was designed as claw-like structure with a diameter of 14 mm, featuring 8 small fingers of 6 mm length and 2 mm diameter. The structure and elasticity can help the fingers pass through the hair and contact the scalp when the electrode is placed on head. The electrode was capable of recording spontaneous EEG and evoked brain activities such as SSVEP with high signal-to-noise ratio. This study implemented a twelve-class SSVEP-based BCI system with eight electrodes embedded in a headband. Subjects also completed a comfort level questionnaire with the dry electrodes. Using a preprocessing algorithm of filter bank analysis (FBA) and a classification algorithm based on task-related component analysis (TRCA), the average classification accuracy of eleven participants was 93.2% using 1-second-long SSVEPs, leading to an average information transfer rate (ITR) of 92.35 bits/min. All subjects did not report obvious discomfort with the dry electrodes. This result represented the highest communication speed in the dry-electrode based BCI systems. The proposed system could provide a comfortable user experience and a stable control method for developing practical BCIs.

Objective. Low-frequency steady-state visual evoked potential (SSVEP)-based brain–computer interface (BCI) systems with high performance are prone to cause visual discomfort and fatigue. High-frequency SSVEP-based BCI systems can alleviate the discomfort, but always obtain lower performance. This study optimized the spatial properties of a proposed checkerboard-like visual stimulus toward high-performance and user-friendly SSVEP-based BCI systems. Approach. On the one hand, two checkerboard-like stimuli with distinct spatial contrasts (the black- and white-background) were designed to balance the tradeoff between BCI performance and user experience and compared with the traditional flickering stimulus. On the other hand, the impacts of the spatial frequency of the new checkerboard-like stimulus on the flicker perception and the intensity of the elicited SSVEP were clarified. The SSVEP-based BCI systems were implemented based on the checkerboard-like stimuli under low-frequency and high-frequency conditions. The user experience for each stimulation pattern was estimated by questionnaires for subjective evaluation. Main results. The comparison results indicate that the black-background checkerboard-like stimulus with an optimized spatial frequency achieved comparable performance and enhanced visual comfort compared with the flickering stimulus. Furthermore, the online nine-target BCI system using the black-background checkerboard-like stimuli achieved averaged information transfer rates of 124.0 ± 2.3 and 109.0 ± 20.4 bits min ⁻¹ with low-frequency and high-frequency stimulation respectively. Significance. The new checkerboard-like stimuli with optimized properties show superiority of system performance and user experience in implementing SSVEP-based BCI, which will promote its practical applications in communication and control.

The miniscope system is one of the calcium (Ca²⁺) imaging tools with small size and lightweight and can realize the deep-brain Ca²⁺ imaging not confined to the cerebral cortex. Combining Ca²⁺ imaging and electrophysiology recording has been an efficient method for extracting high temporal-spatial resolution signals in the brain. In this study, a particular electrode probe was developed and assembled on the imaging lens to modify the miniscope system. The electrode probe can be tightly integrated into the lens of the miniscope without increasing the volume, weight, and implantation complexity. In vivo tests verified that the proposed modified system has realized the simultaneous recording of Ca²⁺ signals and local field potential (LFP) signal in the hippocampus CA1 region of an adult mouse.

Flexible multichannel electrode arrays (fMEAs) with multiple filaments can be flexibly implanted in various patterns. It is necessary to develop a method for implanting the fMEA in different locations and at various depths based on the recording demands. This study proposed a strategy for reducing the microelectrode volume with integrated packaging. An implantation system was developed specifically for semiautomatic distributed implantation. The feasibility and convenience of the fMEA and implantation platform were verified in rodents. The acute and chronic recording results provied the effectiveness of the packaging and implantation methods. These methods could provide a novel strategy for developing fMEAs with more filaments and recording sites to measure functional interactions across multiple brain regions.