Flexible, diamond-based microelectrodes fabricated using the diamond growth side for neural sensing
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-07-12 |
| Journal | Microsystems & Nanoengineering |
| Authors | Bin Fan, Cory A. Rusinek, Cort H. Thompson, Monica B. Setien, Yue Guo |
| Institutions | Michigan State University, Fraunhofer USA |
| Citations | 69 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis analysis summarizes the development of flexible, boron-doped polycrystalline diamond (BDD) microelectrodes optimized for simultaneous neurophysiology and neurochemical sensing.
- Core Innovation: A wafer-scale fabrication and transfer method was developed to expose the BDD growth side as the active sensing surface, differentiating it from previous methods that exposed only the nucleation side.
- Superior Performance: The BDD growth side exhibited significantly improved electrochemical properties compared to the nucleation side, including a wider water potential window and lower background current.
- Reduced Impedance: The growth side electrodes showed a 4-5 times lower 1 kHz impedance (~207.9 kΩ) than the nucleation side (~1123.8 kΩ), attributed to rougher morphology and larger grain size, which is critical for low-noise neural recording.
- Enhanced Kinetics and Stability: The growth side demonstrated faster electron transfer kinetics (ÎEp = 80 mV for Ru(NH3)62+/3+) and high resistance to chemical fouling (no DA absorption after 10 min soak).
- High Sensitivity Sensing: The device achieved highly selective Dopamine (DA) detection in the presence of Ascorbic Acid (AA) using square-wave voltammetry (SWV), yielding a limit of detection (LOD) of 830 nM.
- Biocompatibility and Functionality: The flexible BDD-Parylene C probes supported neuronal growth in vitro and successfully recorded visually induced local field potentials (gamma band, 50-180 Hz) in the primary visual cortex (V1) of a rat in vivo.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| BDD Film Thickness | 5.5 | ”m | Used for comparative study |
| Bulk Resistivity (BDD) | 5 x 10-3 | Ω.cm | Heavily boron-doped film |
| Boron Doping (B/C ratio) | 37,500 | ppm | Achieved high conductivity |
| Diamond Peak (sp3) | ~1305 | cm-1 | Broadened and downshifted due to heavy doping |
| Non-Diamond Peak (sp2) | ~1470 | cm-1 | Present only on nucleation side |
| Average Grain Size (Growth Side) | 0.5 | ”m | Microcrystalline BDD morphology |
| 1 kHz Impedance (Growth Side) | ~207.9 | kΩ | Measured in 0.1 M PBS (pH 7.4) |
| 1 kHz Impedance (Nucleation Side) | ~1123.8 | kΩ | Measured in 0.1 M PBS (pH 7.4) |
| Double-Layer Capacitance (Growth) | ~10 | ”F/cm2 | Lower background noise |
| Double-Layer Capacitance (Nucleation) | ~24 | ”F/cm2 | Higher background noise |
| Redox Peak Separation (ÎEp, Growth) | 80 | mV | Ru(NH3)62+/3+ couple (quasireversible) |
| Redox Peak Separation (ÎEp, Nucleation) | 191 | mV | Ru(NH3)62+/3+ couple (sluggish kinetics) |
| Dopamine (DA) Limit of Detection (LOD) | 830 | nM | Estimated via linear fitting of SWV data |
| Working Electrode (WE) Area | 0.0079 | mm2 | Exposed BDD area |
| Counter Electrode (CE) Area | 0.028 | mm2 | Exposed BDD area |
| Reference Electrode (RE) Area | 0.035 | mm2 | Exposed BDD area |
| Parylene C Thickness (Substrate) | ~15 | ”m | Flexible substrate layer |
Key Methodologies
Section titled âKey MethodologiesâThe flexible BDD probes were fabricated using a wafer-scale transfer process designed to expose the BDD growth side.
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BDD Synthesis and Patterning:
- Silicon wafers were coated with 1 ”m SiO2, scratch seeded, and BDD films were grown using Microwave Plasma-Assisted Chemical Vapor Deposition (MW-PACVD).
- CVD Parameters: Microwave power 8 kW, stage temperature 850 °C, chamber pressure 65 Torr, gas chemistry 1% CH4 in H2 balance.
- Doping: Diborane (B2H6) was added at a B/C ratio of 37,500 ppm.
- Patterning: Aluminum mask was patterned via UV photolithography, followed by Electron Cyclotron Resonance Reactive Ion Etching (RIE) using SF6/Ar/O2 plasma (1000 W microwave, 150 W RF bias).
- Anchor Formation: SiO2 was slightly overetched using Buffered Oxide Etchant (BOE) to undercut the BDD structures, forming Parylene anchors.
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BDD Transfer Process (Growth Side Exposure):
- Substrate Deposition: ~15 ”m Parylene C was deposited onto the patterned BDD film (nucleation side down).
- Backside Etch: Parylene C on the backside of the silicon wafer was removed using O2 RF plasma.
- Release: The silicon substrate was completely etched in 35% KOH at 70 °C to release the BDD-Parylene C film.
- Encapsulation: 10 ”m Parylene C was deposited onto the exposed nucleation side (now facing up) to encapsulate it.
- Carrier Mounting: The film was temporarily glued onto a silicon carrier using photoresist, with the BDD growth side facing upwards.
- Growth Side Opening: Aluminum was deposited and patterned, followed by O2 plasma etching of the top Parylene C layer, and subsequent aluminum removal to expose the BDD growth side electrodes.
- Final Release: The device was released from the carrier by dissolving the sacrificial photoresist in acetone.
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Electrochemical and Neural Validation:
- Electrochemical Characterization: EIS and CV measurements were performed in PBS and KCl solutions using a three-electrode setup (BDD WE, Pt CE, Ag/AgCl RE).
- DA Sensing: Selectivity was tested using SWV in a mixture of DA (5-100 ”M) and 100 ”M AA.
- In Vitro Recording: Flexible probes were tested against cultured rat cortical neurons, simultaneously recording extracellular spikes alongside intracellular patch-clamp activity.
- In Vivo Recording: Probes were stiffened with PEG, implanted into the V1 cortex of an anesthetized rat, and used to record local field potentials induced by blue LED visual stimulation.
Commercial Applications
Section titled âCommercial ApplicationsâThe flexible, high-performance BDD microelectrode technology is highly relevant for advanced neuro-interfacing and biosensing applications, leveraging diamondâs unique material properties.
- Neuroscience Research:
- Simultaneous, long-term monitoring of both electrical neural activity (electrophysiology) and chemical neurotransmitter dynamics (neurochemistry) in vivo.
- High-resolution mapping of dopamine (DA) release and uptake kinetics associated with neurological disorders (e.g., Parkinsonâs disease).
- Implantable Medical Devices:
- Chronic neural implants requiring high biocompatibility, chemical inertness, and resistance to biofouling, enabled by the BDD surface.
- Flexible probes designed to reduce mechanical mismatch with soft nervous tissues, minimizing chronic inflammation and fibrosis.
- Advanced Biosensing:
- Electrochemical sensors requiring a wide potential window and low double-layer capacitance for highly selective detection of electroactive species in complex biological environments.
- Drug Discovery and Therapeutics:
- Real-time monitoring of drug effects on neurotransmitter levels and neural circuit activity.