A recent study published in Engineering delves into the latest progress in intracortical neural interface technologies for freely moving animals. These interfaces, which establish a connection between the nervous system and external devices, have the potential to revolutionize neuroscience research and clinical medicine.

The researchers, led by Xinxia Cai, Zhaojie Xu and Yirong Wu, analyzed four key technological directions for ideal implantable neural interface devices: higher spatial density, improved biocompatibility, enhanced multimodal detection of electrical/neurotransmitter signals, and more effective neural modulation.

In terms of high spatial density, microelectrode array (MEA) designs have evolved significantly. The Utah array and Michigan array, two classic MEA structures, have been reconfigured with new processes and materials. For example, the Utah graded electrode array achieved a higher channel density by tilting the electrode needles and integrating longitudinal multisite on a needle. The Michigan array, on the other hand, has explored methods like electron beam lithography and dual-layer wiring to increase the number of recording sites. CMOS technology has also enabled the integration of neural electrodes with amplifier circuits, reducing the size of the backend circuit.

Long-term stability of MEAs is crucial but challenged by tissue damage and immune responses. To address this, flexible substrates such as polyimide, parylene, and PDMS are being used. These materials have a lower Young’s modulus, similar to that of brain tissue, reducing the immune response. Surface preparation methods, including electrode coatings and electroplated layers at the electrode sites, are also being employed to enhance the quality and longevity of recorded signals.

Multimodal recording MEAs are another area of focus. These devices can detect both electrophysiological and neurotransmitter signals. Electrochemical methods, such as amperometry and fast-scan cyclic voltammetry, are used to measure neurotransmitter concentrations. Different sensitive layers are constructed for various neurotransmitters, with materials like carbon-based materials, conductive polymers, and enzymes. However, challenges remain in achieving high-resolution and selective detection, as well as integrating the detection circuits.

Bidirectional neural probes that can both record and modulate neural activity are also being developed. Electrical stimulation (ES), optical modulation, and microfluidic delivery are the main modulation methods. ES has limitations in specificity, while optical modulation offers higher cellular specificity. Microfluidic delivery can precisely deliver drugs or chemical molecules to specific brain regions.

These technological advancements in intracortical neural interfaces have wide-ranging applications. They can help researchers better understand neural circuit functions, the mechanisms of neural encoding and decoding, and the pathogenesis of clinical disorders. In the future, they may also contribute to the development of more effective and personalized therapies for neurological diseases, as well as the restoration of motor and sensory functions. However, challenges such as the maturity of flexible CMOS fabrication technologies and the management of thermal and electrical noise still need to be overcome.

The paper “Recent Advances in Intracortical Neural Interfaces for Freely Moving Animals: Technologies and Applications,” authored by Xinxia Cai, Zhaojie Xu, Jingquan Liu, Robert Wang, and Yirong Wu. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.12.012. For more information about the Engineering, follow us on X (https://twitter.com/EngineeringJrnl) & like us on Facebook (https://www.facebook.com/EngineeringJrnl).