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

2020 
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 sp3 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 flexible, sensitive, stable neural probe consisting of boron-doped polycrystalline diamond (BDD) microelectrodes has been developed, whereby the BDD growth surface is used as the electrode site for neurophysiological and neurochemical sensing. Among carbon materials, BDD has found widespread use in neurotransmitter detection, but the hardness of diamond has impeded application in neural implants. A team headed by Wen Li at Michigan State University, USA, succeeded in developing a process that allows the growth side of the BDD thin film to be used as the sensing surface: they transferred the BDD patterns from a solid silicon substrate onto a flexible polymer substrate. The authors validated their BDD microelectrode technology for neural recordings both in vitro and in vivo, and they believe it has excellent potential for research into various brain disorders, such as Parkinson’s disease.
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