Abstract
A graphene transistor system for measuring electrophysiological signals uses flexible epicortical and intracortical arrays of graphene solution-gated field-effect transistors (gSGFETs) to record infraslow signals alongside signals in the typical local field potential bandwidth. The graphene transistor system includes a processing unit, and at least one graphene transistor (gSGFET) a tunable voltage source connected to the drain and source terminals of the transistor (gSGFET), and at least one filter configured to acquire and split the signal from the transistor into at least a low frequency band signal and high frequency band signal, which are amplifiable with a gain value.
Claims
1. A graphene transistor system comprising: a. a processing unit, b. at least one graphene transistor (gSGFET) comprising graphene as channel material contacted by two terminals, c. a tunable voltage source connected to drain and source terminals of the graphene transistor and d. at least one filter configured to acquire and split the signal from the graphene transistor into at least a low frequency band signal and a high frequency band signal, which are amplified with a gain value.
2. The graphene transistor system of claim 1 wherein the filter is configured to generate one of: a. a low-pass filtered band with a frequency set between 0Hz and 0.16 Hz, and b. a band-filtered band with a frequency comprised between 0.16 Hz and10 kHz.
3. The graphene transistor system of claim 2 wherein the low-pass filter (LPF) and the band-pass filter (BPF) have different gains of 10.sup.4 and 10.sup.6, respectively.
4. A method for measuring electrophysiological signals, using the graphene transistor system of claim 1, the method comprising: a. splitting an input signal into a low frequency and a high frequency signal with the at least one filter, b. merging the low frequency signal and high frequency signal weighted by corresponding gain, and c. transforming the merged signal into a voltage signal according to an intrinsic gain of the graphene transistor.
5. The method according to claim 4, wherein a gain value of amplification is different for each signal.
6. The method according to claim 4 wherein the transforming of the voltage signal is carried out by interpolation using a graphene transistor transfer curve I.sub.ds-V.sub.ds.
7. The method according to claim 6, wherein the graphene transistor transfer curve I.sub.ds-V.sub.ds is generated with a fixed drain-source voltage (V_ds).
8. A graphene transistor system comprising: a processing unit, at least one graphene transistor (gSGFET) comprising graphene as channel material contacted by two terminals, a tunable voltage source connected to drain and source terminals of the graphene transistor, and at least one filter configured to split a signal from the graphene transistor into a low frequency band signal and a high frequency band signal which are amplifiable with a gain value.
9. The graphene transistor system of claim 9 wherein the at least one filter is configured to generate one of: a. a low-pass filtered band with a frequency set between 0 Hz and 0.16 Hz, and b. a band-filtered band with a frequency comprised between 0.16 Hz and10 kHz.
10. The graphene transistor system of claim 9 wherein the low-pass filter (LPF) and the band-pass filter (BPF) have different gains of 10.sup.4 and 10.sup.6, respectively.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] To complement the description being made and in order to aid towards a better understanding of the characteristics of the invention, in accordance with a preferred example of practical embodiment thereof, a set of drawings is attached as an integral part of said description wherein, with illustrative and non-limiting character, the following has been represented:
[0025] FIGS. 1A-1G.—Show: a representation of flexible graphene solution-gated field-effect transistor array technology and characterization. FIG. 1A: A schematic of a graphene transistor polarized in common gate mode. FIG. 1B: Optical microscope images of the active area of the 4×4 gSGFET array and the 15 channel intracortical array. FIG. 1C: a photograph of the neural probe. FIG. 1D: Steady-state characterization of a 100×50-μm2 gSGFET array in 10 mM phosphate buffered saline (PBS) and with a drain-source voltage bias (Vds) of 50 mV; a graph showing gSGFET transfer curves, drain-source current (Ids) vs gate-source voltage (Vgs), together with the mean (dark curves) and standard deviation (lighter curves). Boxplot inset shows charge neutrality point dispersion (center line, median; box limits, upper and lower quartiles). FIG. 1E: A graph for the leakage current (Igs) of all gSGFETs in the array. FIG. 1F: A graph for the transfer curve (blue squares and line) and its first derivative (transconductance (gm), black line) of a gSGFET. FIG. 1G: A graph for the frequency response of the transconductance at two different points of the transfer curve FIG. 1E: Vgs lower than the CNP (green), where gm is negative resulting in a signal inversion (180° phase); and Vgs higher than the CNP (orange), where gm is positive and thus results in no inversion (0° phase). Independently of the branch of the transfer curve where a gSGFET is polarized, the module of gm is similar to the steady-state value for a wide bandwidth (≈0-1 kHz).
[0026] FIGS. 2A-2B.—Show an exemplary embodiment of the invention incorporating a gSGFET, the custom electronic circuit and post-processing methodology as also examples of the recorded signals: infraslow, local field potential, and wide-band in vivo gSGFET recordings of neural signals. FIG. 2A: Schematic of the gSGFET recording setup and signal post processing methodology. The custom electronic circuit is used to perform the in vivo characterization (transfer curve) and record the transistor current in the low-pass-filtered (LPF) band and the band-pass-filtered (BPF) band. From the combination of both signals and taking into account the current-to-voltage conversion, the wide-band signal (V.sub.sig) is obtained. FIG. 2B: Electrophysiological recordings obtained with a gSGFET epicortical array during the induction of four CSD events (blue shade). From top to bottom: current LPF signal, current BPF and voltage-converted wide-band signal.
[0027] FIG. 3.—Shows an exemplary embodiment of the custom electronic circuit. a, Schematic of the custom electronic instrumentation which controls the polarization of the gSGFETS (V.sub.gs, V.sub.ds) and amplifies differently the two previously mentioned bands: LPF (≈0-0.16 Hz, gain=10.sup.4) and BPF (0.16 Hz-10 kHz, gain=10.sup.6). We use the custom electronic instrumentation to characterize the steady-state behaviour of the gSGFETs as well as the AC modulation of graphene transistors.
[0028] FIGS. 4A-4C.—Show the calibration procedure of gSGFET current recordings to recover the voltage signal at the gate. FIG. 4A: gSGFET current recordings of a 10 Hz, 0.85 mV-peak sinusoidal gate signal applied through a reference electrode. Graphene transistors are biased at V.sub.ds=50 mV and V.sub.gs=250 mV. FIG. 4B: Transfer curves of the same graphene transistors at V.sub.ds=50 mV. Dotted line indicates the V.sub.gs bias voltage used in FIG. 4A. FIG. 4C: Voltage signal as obtained by interpolation of the current signal in FIG. 4A of each transistor into its corresponding transfer curve and removal of the V.sub.gs offset.
[0029] FIGS. 5A-5B.—Show the mapping cortical spreading depression with graphene transistors. FIG. 5A: Infralow frequency signals recorded by a 4×4, 400 μm grid spacing, gSGFET array (black lines) during the occurrence of a CSD event as illustrated in the top left schematic. The contour plot shows the time delays of the onset of CSD with respect to the mean time illustrating the spatiotemporal course of the CSD. FIG. 5B: Interpolated spatial voltage maps showing the propagation of the same CSD event as measured by the gSGFET array. FIGS. 5A-5B: High pass filtered recordings at 0.1 Hz (red lines in FIG. 5A and bottom spatial voltage maps in FIG. 5B) are included to illustrate the loss of signal information in conventional microelectrode recordings.
[0030] FIGS. 6A-6B.—Depict the depth profile of the infralow-frequency voltage variations induced by cortical spreading depression in a rat cortex. FIG. 6A: Layout of the fabricated 15-channel graphene intracortical probe and ordered local field potential recordings. Infralow-frequency recordings (black lines) during the occurrence of a CSD event. Dashed lines, have been interpolated from nearby transistors. FIG. 6B: Colour maps of the temporal course of the infraslow changes during a CSD event across the depth of a rat cortex. FIGS. 6A-6B: Same signal high-pass filtered at 0.1 Hz (red lines) and their spatio-temporal colour map are included to illustrate the loss of information in conventional microelectrode recordings.
PREFERRED EMBODIMENT OF THE INVENTION
[0031] A first aspect of the invention is aimed to a system for registering electrofysiological infraslow signals like cortical spreading depression (CSD) signals i.e. those with a frequency value below 0.1 Hz; the device comprising a processing unit associated or embedded in the very device, and at least one graphene transistor (gSGFET), preferably an array of graphene transistors, comprising graphene as channel material contacted by source and drain terminals, with a reference as gate terminal. Said graphene transistor is connected to at least a filter like a low pass filter (LPF). The recorded current signal is transformed into a voltage signal using the transistor transfer curve I.sub.ds-V.sub.gs recorded to the start of the recordings).
[0032] In an alternative embodiment of the invention, at least one band pass filter (BPF) is arranged in either a sequential or a cascade arrangement along with the low pass filter (LPF). but both filters (LPF, BPF) being configured so that the respective cutting points have the same value.
[0033] An gSGFETS is a device in which graphene is used as channel material, contacted by two metal leads (source and drain terminals), and is immersed in an electrolyte solution where a reference electrode is used as gate terminal (FIG. 1A). Flexible probes containing arrays of gSGFETs in both epicortical and intracortical designs were produced. In particular, a 4×4 array of 100 μm wide by 50 μm long graphene channels were designed for epicortical recordings while a design consisting of a linear array of 15 graphene channels (80 μm width, 30 μm length) was used for intracortical recordings (FIG. 1B). Both array designs were fabricated on a 10 μm thick polyimide layer coated on a 4-inch silicon wafer. Flexible gSGFET arrays were placed in zero insertion force connectors for interfacing with recording electronics (FIG. 1C). The transfer curve, drain current (I_ds) vs gate-source voltage (V_gs), of all gSGFETs in each array was measured with a fixed drain-source voltage (V_ds). The dispersion of the charge neutrality point (CNP=243.6±6.1 mV), which is the minimum of the transfer curve, indicates the homogeneity of the transistors (FIG. 1D). Importantly, since the V_gs and V_ds bias are shared, the small CNP dispersion allows near-optimal recording performance for all gSGFETs in the same array. FIG. 1E shows the sum of leakage current (I_gs) for all gSGFETs in the array, which is in the nA range throughout the voltage sweep, demonstrating the good insulation of the passivation layer and the negligible reactivity of the graphene. Furthermore, we measured the frequency response of the transconductance (gm) of a gSGFET, which indicates the efficiency of the signal coupling ((∂I_ds)/(∂V_gs)), obtaining constant values in a wide bandwidth including inflalow frequencies (FIGS. 1A-1G). The negative gm for Vgs values lower than the CNP results in an inversion (180° phase) of the signals measured at such bias; for Vgs values higher than the CNP the signal phase is preserved.
[0034] The device of the invention was compared to conventional high-pass filtered recordings, to do so the propagation of cortical spreading depression (CSD) events using a 4×4 epicortical gSGFET array was mapped and then compared with what is observed in conventional high-pass filtered recordings (FIGS. 5A-5B). The recording of the whole CSD event with the gSGFET array reveals that while the onset of the negative shift is similar for all gSGFETs, there is much more variety in the subsequent recovery, with some transistors exhibiting a second negative shift with higher amplitude than the first one. This effect can also be observed in the last frames (corresponding to 80 s and 90 s) of the spatial maps of gSGFET recordings (FIG. 5B) where recovered and still depressed brain areas coexist. Importantly, this information is lost in conventional microelectrode recordings, where only the CSD onset is observed due to the high pass filter in the recording electronics. The following results are referred to a sample of 10 CSDs collected from two different subjects in the somatosensory cortex: we found that the mean duration of CSD events is 47.24±7.65 sand a speed of propagation of 7.68±1.35 mm/min, in agreement with the literature defining CSDs as infraslow brainwaves.
[0035] To further illustrate the potential of the device of the invention and taking advantage of the design versatility offered by this technology, a linear array of 15 gSGFETs spanning the entire depth of the cortex (FIG. 6A) was arranged. From either the ordered recording or the spatiotemporal voltage map (FIG. 6B), it can be seen how CSD occurs in the whole cortex depth. These results highlight the capability of the device of the invention to reveal the rich pattern of infraslow signals in the cortex; in this particular case, a transition from a superficial long depolarization to a shorter one preceded and followed by a hyperpolarization in the deeper layers is clearly observed. The origin of such depth-dependent effect is not well understood and will be the target of further investigations, taking advantage of the demonstrated capability of gSGFET technology to monitor ISA with high spatial resolution.
[0036] In a second aspect of the invention a method for recording infraslow brain signals, being infraslow signals those with a frequency value below 0.1 Hz is provided, Cortical spreading depression (CSD) was chosen to illustrate the capabilities of aspects of the invention to record in a wide bandwidth. Experimentally, two craniotomies were performed over the left hemisphere of isoflurane-anaesthetized Wistar rats: a larger craniotomy over the primary somatosensory cortex, where the epicortical probe was placed, and a smaller one in the frontal cortex, where 5 mM KCI was applied locally to induce CSD (FIG. 2B). A custom electronic circuit allowed us to simultaneously record at two frequency bands: low-pass filtered band (LPF, ≈0-0.16 Hz) and band-pass filtered band (BPF, 0.16 Hz-10 kHz) with different gains (10.sup.4, and 10.sup.6 respectively) to avoid amplifier saturation due to the high-amplitude CSD signal. In a first set of experiments, we recorded the LPF and BPF current signals with an epicortical gSGFET array during the induction of CSD events (FIG. 2C). The graphene transistors were polarized in the hole conduction regime, i.e. V.sub.gs<CNP (negative g.sub.m); therefore, the recorded LPF and BPF current signals are inverted with respect to the voltage signal occurring at the gate. The LPF signal shows the very slow CSD event whereas the BPF signal corresponds to the local field potential, revealing the silencing of activity typical of cortical spreading depression. After summation of the LPF and BPF signals and then transforming the current into a voltage signal (using the transistor transfer curve I.sub.ds-V.sub.gs recorded in vivo prior to the start of the recordings), the wide-band electrophysiological signal can be obtained (see FIGS. 2A, 2C). In each CSD event a small positive shift of 1-2 mV generally precedes the depression, immediately after which a steep negative change (≈−20 mV) can be observed, which slowly recovers during the next minute or so. The CSD-associated silencing of high-frequency activity and its progressive recovery is shown in the voltage wave and spectrogram of FIG. 2D.
