Received: 16 June 2022 Accepted: 21 June 2022 DOI: 10.1002/ctm2.968 EDITORIAL The advantages of mapping slow brain potentials using DC-coupled graphene micro-transistors: Clinical and translational applications There is growing interest in examining oscillations and the two dimensional material graphene7 to implement a brain signals outside traditional EEG bands (0.3–80 Hz), as local transduction of neural signals.8 gSGFETs consist of these regimes contain useful electrographic biomarkers for a graphene sheet in direct contact with the brain tissue the diagnosis, monitoring and prognosis of neurological connected by two metal tracks to the recording electron- disorders and injuries.1 These include high gamma (80– ics (Figure 1A and B). The neural signal transduction 200 Hz), ripples and high-frequency oscillations (HFOs) is produced when brain electrical activity modulates the (200–500 Hz), as well as infraslow oscillations (<0.1 Hz) graphene conductivity and therefore also the current flow- and ultraslow potential shifts (UPS). In particular, UPS ing through the transistor when a constant electric field have remained poorly explored in clinical settings with is applied. The high electrochemical stability of graphene the notable exception of the Co-Operative Studies on permits a stable gain across a wide-range of frequencies, Brain Injury Depolarizations (COSBID) consortium,2 and enabling DC-coupled recordings with the same fidelity specialist epilepsy surgical centres.3 This is due to the asso- as liquid-filled glass micropipettes with Ag/AgCl wires9 ciated technical difficulties recording such slow potentials – the current gold standard for DC-coupled recordings that require DC-coupled amplifiers and highly stable elec- while overcoming their spatial sampling limitations and trodes. However, UPS include clinically relevant events allowing high-density mapping.9 including preseizure DC shifts (1–3 mV), and large (tens of millivolt) spreading depolarisations (SD) which are thought to play an important role in brain injury and con- 2 PRECLINICAL VALIDATION OF tribute to the pathophysiology associated with migraine GRAPHENE-BASED with aura, stroke and epilepsy.4 Therefore, the ability to MICRO-TRANSISTORS DEMONSTRATE record and map a wide range of brain signals, from UPS to THEIR ABILITY TO LOCALISE single units, using the same electrophysiological array will ULTRASLOW BRAIN SIGNALS WITH greatly advance our understanding of brain diseases and HIGH SPATIAL RESOLUTION aid the clinical management of patients with diverse neu- rological disorders and injuries. Therefore, development of The ability to accurately map slow potential changes across improved electrophysiological devices capable of detecting large areas of the cortex will have both preclinical and and mapping wide bandwidth signals with high-fidelity clinical impact. Graphene gSGFETs, flexible, multichannel and spatial resolution is warranted.5,6 surface grids (see Figure 1A) have been used to electro- graphically detect cortical spreading depolarisations (CSD) in vivo (see Figure 1D) either in anesthetised rats9 or in 1 WHAT ARE GRAPHENE awake head-fixed mice.10 Mapping of CSD initiation and MICRO-TRANSISTORS – HOW DO THEY propagation permitted quantification of waveform param- WORK? eters and associated neuronal activity suppression across multiple frequency bands. Spatially localised differences Solution-gated field effect graphene micro-transistors in CSD waveform could be observed despite individual (gSGFETs) take profit from the field effect property of transistors being close to each other (400 μm). This is This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2022 The Authors. Clinical and Translational Medicine published by John Wiley & Sons Australia, Ltd on behalf of Shanghai Institute of Clinical Bioinformatics. Clin. Transl. Med. 2022;12:e968. wileyonlinelibrary.com/journal/ctm2 1 of 4 https://doi.org/10.1002/ctm2.968 2 of 4 EDITORIAL F I G U R E 1 Recording DC-coupled brain potentials with graphene micro-transistors. (A) Graphene micro-transistors can be embedded in surface grids for electrocorticography recordings (right) or in penetrating probes for iEEG (left). (B) Schematic cross-section of a graphene micro-transistor showing the driving voltage (Vds) and the operation point selector voltage (Vgs). Current fluctuations in the drain-source current (Ids) are then converted back to voltage using the transfer characteristics of the transistor (Ids-Vgs curve). (C), (D) Example DC-coupled recordings obtained by gSGFETs. DC-coupled trace (black) and the same trace high-pass filtered > 0.1 Hz (blue). (C) Recording of a seizure followed by a spreading depolarization using an intracortical array.11 (D) Recordings of multiple spreading depolarisations triggered by potassium chloride (KCl) application to the surface of the rat brain.9 relevant, as DC-coupled high-density mapping using gSGFET probes into the somatosensory cortex of a rat localised electrophysiological biomarkers based on SD model of absence epilepsy revealed that spontaneous waveform (amplitude, duration etc.) can be used to infer spike-and-wave discharges (5–9 Hz) were phase-coupled the underlying metabolic condition and health of brain to an infraslow oscillation (ISO < 0.1 Hz) most prominent tissue in a regionally dependent manner.4 in the superficial layers, suggesting that these ISOs open In addition to epicortical arrays, highly flexible pene- susceptibility windows for seizure initiation.11 trating probes composed of gSGFETs were developed (see SDs can be associated with seizures and vice versa. The Figure 1A) and shown to record with high fidelity a wide dynamic relationship, both temporal and spatial, between bandwidth of brain signals, from UPS to HFOs.11 To further these two events is complex and not properly understood. investigate the spatial resolution afforded by multichannel High-density mapping of both fast seizure activity and DC-coupled recordings, current-source-density analysis ultraslow SDs using gSGFETs promise to shed impor- identified sinks and sources of activation through the cor- tant insights into these intriguing and clinically important tical laminae during chemoconvulsant-induced seizures interactions (see Figure 1C for an example recording). Due in awake mice. High-pass filtering the data >0.5 Hz to to their optical transparency, gSGFET arrays are compati- mimic typical AC-coupled recordings failed to report the ble with a range of commonly used imaging modalities and same ionic sinks and sources preceding and during the optogenetics10 making them particularly attractive tools seizure, illustrating the importance of using DC-coupled for preclinical neuroscience research. They also provide a recordings for current source density analysis and to avoid tool that can be used to directly readout in vivo the effi- misinterpretation of the extracellular potential sinks and cacy of pharmacological10 or genetic strategies to suppress sources. A similar layer-specific localisation of ‘active’ DC or modulate UPS and ISA. shifts preceding seizures could be identified.11 Ictal DC shifts can be differentiated into active shifts that precede the seizure and passive shifts that follow the intense neu- 3 POTENTIAL CLINICAL ronal firing at seizure onset.12 Active DC shifts are of APPLICATIONS particular interest for both clinical and basic research, as they provide an electrophysiological biomarker for seizure Clinical translation of graphene micro-transistor intracra- onset zone localisation,13 and they can further provide nial probes could offer advantages over current tech- a quantifiable parameter to investigate the mechanisms nologies currently used in neurocritical care, epilepsy, underlying seizure initiation.14 Chronic implantation of intraoperative monitoring and neuroprostheses. EDITORIAL 3 of 4 3.1 Neurocritical care the clinic certainly faces many regulatory, adoption and business challenges, we envision a bright potential in clin- The COSBID group have extensively demonstrated that ical applications such as epilepsy, traumatic brain injury, SDs occur clinically following neurotrauma and actively stroke or migraine. contribute to lesion size in subarachnoid haemorrhage and in post-traumatic brain injury.2 Application of gSGFET AC K N OW L E D G E M E N T S arrays to detect SDs in these patients might aid clinical R.W. is funded by a Senior Research Fellowship awarded management and provide a direct readout of therapeu- by the Worshipful Company of Pewterers. This work has tic interventions designed to suppress SD initiation and received funding from the European Union’s Horizon 2020 propagation. research and innovation programme under Grant Agree- ment No 881603 (GrapheneCore3). We thank Dámaso Torres for technical assistance making figures. 3.2 Epilepsy COMPETING INTEREST Favourable epilepsy surgery outcome depends upon accu- J.A.G. and A.G.-B. declare financial interest in INBRAIN rate localisation and resection of the epileptogenic zone. Neuroelectronics. R.C.W and E.M-C. have no competing As long-term seizure freedom after surgery is only around interests. ∼50%,15 improvement in technology that more precisely defines surgical margins is warranted. Of note, the few Rob. C Wykes1,2 studies that have investigated ictal DC shifts report that Eduard Masvidal-Codina3,4 these signals co-localise with seizure onset zones and Anton Guimerà-Brunet4,5 are more spatially restricted than conventional EEG Jose. A Garrido3,6 recordings.3 Therefore, routine inclusion of accurate DC- 1 Department of Clinical & Experimental Epilepsy, UCL coupled recordings to surgical monitoring could result in less extensive, yet more effective surgical resections.16 Queen Square Institute of Neurology, London, United Additionally, due to the ability to fabricate high channel Kingdom 2 Nanomedicine Lab, University of Manchester, Manchester, density probes with small recording sites, the increased spatial resolution and coverage should allow detection of United Kingdom 3 Catalan Institute of Nanoscience andNanotechnology micro-seizures, which may be missed using large inter- spaced electrodes. Beyond presurgical clinical applica- (ICN2), CSIC and The Barcelona Institute of Science and tions, this technology also offers to provide valuable insight Technology (BIST), Campus UAB, Bellaterra, Barcelona, into the involvement of UPS post seizure, including inves- Spain 4 Centro de Investigación Biomédica en Red de tigating the mechanisms underlying postictal generalised EEG suppression (PGES) and sudden unexplained death Bioingeniería, Biomateriales y Nanomedicina in epilepsy (SUDEP).17 (CIBER-BBN), Instituto de Salud Carlos III, Madrid, Spain 5 Institut de Microelectrònica de Barcelona, IMB-CNM (CSIC), Esfera UAB, Bellaterra, Spain 3.3 Intraoperative monitoring and 6 ICREA, Barcelona, Spain neuroprostheses Correspondence gSGFET arrays have been shown to preserve their high- Rob. C Wykes, Department of Clinical & Experimental fidelity DC-coupled recordings when used in multiplexed Epilepsy, UCL Queen Square Institute of Neurology, operation, thus reducing the number of wires and de- London, UK. cluttering the surgical site while retaining high spatial Email: r.wykes@ucl.ac.uk mapping with excellent signal-to-noise.18,19,20 These fea- tures will be beneficial for intraoperative monitoring Anton Guimerà-Brunet, Institut de Microelectrònica de and mapping eloquent cortex. Future applications could Barcelona, IMB-CNM (CSIC), Esfera UAB, Bellaterra, include large-area, high-density brain mapping devices for Spain. neuroprostheses. Email: anton.guimera@imb-cnm.csic.es In summary, graphene micro-transistors already con- tribute to the electrophysiology toolkit in neuroscience Jose. A Garrido, Catalan Institute of Nanoscience and research by providing unique capabilities for wide band- Nanotechnology (ICN2), Campus UAB, Bellaterra, width recordings, in particular high-fidelity mapping of Barcelona, Spain. ultraslow events. While translation of this technology to Email: joseantonio.garrido@icn2.cat 4 of 4 EDITORIAL ORCID 10. Masvidal-Codina E, Smith TM, Rathore D, et al. Characteriza- Rob. C Wykes https://orcid.org/0000-0002-6141-6822 tion of optogenetically-induced cortical spreading depression in awake mice using graphene micro-transistor arrays. J Neural Eng. 2021; 18(5). REFERENCES 11. Bonaccini Calia A, Masvidal-Codina E, Smith TM, et al. Full- 1. Kovac S, Speckmann E-J, Gorji A. Uncensored EEG: the role of bandwidth electrophysiology of seizures and epileptiform activ- DC potentials in neurobiology of the brain. Prog Neurobiol. 2018; ity enabled by flexible graphene microtransistor depth neural 165-167: 51-65. probes. 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