Flexible, diamond-based microelectrodes fabricated using the diamond growth side for neural sensing

Abstract

Diamond possesses many favorable properties for biochemical sensors, including biocompatibility, chemical inertness, resistance to biofouling, an extremely wide potential window, and low double-layer capacitance. The hardness of diamond, however, has hindered its applications in neural implants due to the mechanical property mismatch between diamond and soft nervous tissues. Here, we present a flexible, diamond-based microelectrode probe consisting of multichannel boron-doped polycrystalline diamond (BDD) microelectrodes on a soft Parylene C substrate. We developed and optimized a wafer-scale fabrication approach that allows the use of the growth side of the BDD thin film as the sensing surface. Compared to the nucleation surface, the BDD growth side exhibited a rougher morphology, a higher sp 3 content, a wider water potential window, and a lower background current. The dopamine (DA) sensing capability of the BDD growth surface electrodes was validated in a 1.0 mM DA solution, which shows better sensitivity and stability than the BDD nucleation surface electrodes. The results of these comparative studies suggest that using the BDD growth surface for making implantable microelectrodes has significant advantages in terms of the sensitivity, selectivity, and stability of a neural implant. Furthermore, we validated the functionality of the BDD growth side electrodes for neural recordings both in vitro and in vivo. The biocompatibility of the microcrystalline diamond film was also assessed in vitro using rat cortical neuron cultures.

A Review: Electrode and Packaging Materials for Neurophysiology Recording Implants

“…The example shows an electrode array made of mechanically and chemically stable, boron-doped polycrystalline diamond (BDD) (upper). Morphological response of rat cortical neurons on the Parylene C and microcrystalline diamond (MCD) substrates (lower) appeared similarly to the control substrate (reprinted with permission from Fan et al, 2020 ). (D) Biodegradability and bioresorbability.…”

“…With large amounts of water in their structures, these materials are highly hydrated to increase the energetic penalty of removing water for protein and microorganism attachment. Engineered antifouling electrode materials, such as sp 3 carbon-enriched, boron-doped polycrystalline diamond (BDD), also show the advantages of improved biocompatibility and reduced biofouling compared to conventional electrode materials (Meijs et al, 2016 ; Fan et al, 2020 ), as shown in Figure 2C . Moreover, nanostructured surfaces with low friction and low surface energies can effectively decrease cell attachment onto the implant surface, and hence, reduce the possibility of biofouling formation (Chapman et al, 2017 ; Boehler et al, 2020 ).…”

“…PA can be conformally deposited by CVD at room temperature and structured by oxygen plasma dry etching or laser. Those low fabrication requirements make PA compatible with many materials (Fan et al, 2020 ) and device designs. As a packaging material, PA has excellent biocompatibility (USP class VI), chemical inertness (De la Oliva et al, 2018 ), low conductivity, low intrinsic stress (Zöpfl et al, 2009 ), low pin-hole density, and conformal coating (Rodger et al, 2008 ).…”

Next-Generation Diamond Electrodes for Neurochemical Sensing: Challenges and Opportunities

“…Figure 2 shows schematically these two typical fabrication methods for making all diamond microelectrode probes and MEAs. To further improve the mechanical flexibility, pre-patterned BDD MEAs can be transferred from a solid substrate onto a soft polymer substrate, such as polynorbornene [ 131 ], polyimide [ 132 ] and parylene C [ 133 , 134 ] to form BDD-on-polymer MEAs with reduced mechanical rigidity ( Figure 1 E [ 131 ] and Figure 1 F [ 133 ]). Recently, BDD paste electrodes suitable for electroanalytical applications have been reported by Kondo et al, offering a cheaper alternative to fabrication of flexible BDD MEAs [ 135 , 136 ] ( Figure 1 G [ 136 ]).…”

“…Surface fouling due to accumulation of oxidized products on the electrode surface further limits electrode selectivity and sensitivity in the long-term. While challenges exist, the inherent robustness, electrochemical stability, wide potential window, and good electrocatalytic activity of highly boron-doped polycrystalline or nanocrystalline diamond electrodes make these materials suitable candidates to detect DA [ 96 , 105 , 133 , 144 , 145 ], epinephrine (EP) [ 146 ], and NEP [ 147 , 148 ]. These electroactive NTs belong to the family of catecholamines (e.g., DA), which are readily oxidized to an ortho-quinone that can be electrochemically detected using pristine BDD electrodes without any surface modification or functionalization.…”