Advanced Metallic and Polymeric Coatings for Neural Interfacing: Structures, Properties and Tissue Responses

Abstract

Neural electrodes are essential for nerve signal recording, neurostimulation, neuroprosthetics and neuroregeneration, which are critical for the advancement of brain science and the establishment of the next-generation brain–electronic interface, central nerve system therapeutics and artificial intelligence. However, the existing neural electrodes suffer from drawbacks such as foreign body responses, low sensitivity and limited functionalities. In order to overcome the drawbacks, efforts have been made to create new constructions and configurations of neural electrodes from soft materials, but it is also more practical and economic to improve the functionalities of the existing neural electrodes via surface coatings. In this article, recently reported surface coatings for neural electrodes are carefully categorized and analyzed. The coatings are classified into different categories based on their chemical compositions, i.e., metals, metal oxides, carbons, conducting polymers and hydrogels. The characteristic microstructures, electrochemical properties and fabrication methods of the coatings are comprehensively presented, and their structure–property correlations are discussed. Special focus is given to the biocompatibilities of the coatings, including their foreign-body response, cell affinity, and long-term stability during implantation. This review article can provide useful and sophisticated insights into the functional design, material selection and structural configuration for the next-generation multifunctional coatings of neural electrodes.

High‐Aspect‐Ratio Nanoelectrodes Enable Long‐Term Recordings of Neuronal Signals with Subthreshold Resolution

“…The values presented below are average impedance with standard deviations measured at 1 kHz, calculated for 10 electrodes. As previously reported, [28,29,34] the nanocavity reduces the electrode impedance compared to the planar electrode (387.9 ± 50.7 vs 699.9 ± 54.1 kΩ), similar to the impedance reduction from PtB [36][37][38] and TiN deposition. [39] Then, we demonstrate that the presence of nanostraws on nanocavities and flat electrodes yields similar values (323.5 ± 73.6 and 668.2 ± 41.5 kΩ, respectively) to without nanostraws.…”

In Vivo Penetrating Microelectrodes for Brain Electrophysiology

“…Flexible Parylene implants led to a neuronal loss of only 12–17% around the implantation site compared to rigid silicon microelectrodes (40%, significant neurodegeneration) four weeks after surgery. Alternatively, it is possible to use rigid silicon microelectrodes with various low-Young-modulus coatings [ 173 , 174 ], for example, hydrogel [ 78 , 114 ]. It is also worth noting that reducing the size of silicon microelectrodes to 3–5 µm significantly improves the parameters of the Young modulus [ 98 , 169 ].…”

The effects of process parameters on polydopamine coatings employed in tissue engineering applications

“…Compared to other coating substances, including proteins ( Gennadios, 2002 ), particular peptide sequences ( Pagel and Beck-Sickinger, 2017 ), ceramics ( Montazerian et al, 2022 ), and metals ( Yin et al, 2021 ), polydopamine (PDA), a melanin-like mussel-inspired coating polymer ( Ghorbani et al, 2022 ), possesses a variety of desirable properties, such as outstanding adhesion features ( Ghalandari et al, 2021 ; Feinberg and Hanks, 2022 ), extraordinary hydrophilicity ( Li et al, 2018 ), biodegradability ( Zhang and King, 2020 ), uniform shape ( Saeed et al, 2021 ), biocompatibility ( Zeng et al, 2020 ), and thermal stability ( Yim et al, 2022 ). Furthermore, its biological properties, including enhanced cellular proliferation, improved bioactivity, free-radical scavenging activities, metal ion chelating capacity, and anti-bacterial capability, originate from its catecholamine and hydroxyl functional groups ( Yu et al, 2017 ; Ghorbani et al, 2019b , 2019a ; Deng et al, 2021 ).…”