Patent Application
20230404458
The present disclosure relates to a graphene based dry electrode for electrophysiological readings, in particular for use with EEG, EKG, EMG, and EOG systems.
A number of medical conditions can be diagnosed and monitored by obtaining and interpreting electrophysiological readings such as the electrical activity of various organs and locations. Examples include electroencephalography (EEG), electrocardiography (EKG), and electromyography (EMG) which measure the electrical activity of the brain, heart and skeletal muscles respectively. These techniques are non-invasive, which is to say that the electrical activity may be measured by placing electrodes in predetermined locations on the skin, without the need to make any incisions or otherwise harm the patient. For example, in EEG, a set of electrodes are placed on the scalp of the patient, sometimes in a net or cap, in EKG (also sometimes abbreviated as ECG), electrodes are placed on the limbs and torso of the patient, and in EMG, electrodes are placed around the muscle of interest. Similar equipment may also be used to perform electrooculography (EOG) which measures the corneo-retinal standing potential to track eye movements and/or responses to certain stimuli. This involves the placement of electrodes on the skin around the eye of a patient.
EEG also has applications in the creation of brain-computer interfaces (BCI), where an external device is controlled by the detection and translation of electrical signals produced by the brain of a user. In particular, BCIs enable the external device to receive and respond to the electrical signals produced by the action potential fired by groups of cells in the brain and the transduction of ion currents moving through brain tissue. EEG is particularly favoured for these applications due to its relative low cost and non-invasive nature, which is both safer for the user and easier than invasive reporting methods.
The electrodes used for non-invasive EEG applications can be divided into two broad categories: wet electrodes and dry electrodes. Wet electrodes, most commonly Ag/AgCl based electrodes, rely on an electrolytic gel being applied between the electrode and the skin of the patient. The electrolytic gel hydrates the uppermost layer of the skin and allows a conductive bridge between the electrode sensor and the ion currents in the brain tissue. Wet electrodes give low noise and low electrode-skin impedance, however they also have a number of disadvantages. The abrasive gel used is greasy and often uncomfortable for the user, the gel requires application and clean up between uses, and the conductive properties degrade as the gel dries out.
Dry electrodes, on the other hand, do not involve the use of a gel or other intermediate conductor between the skin and electrode, and may be placed directly on the skin. This makes dry electrodes easier to use, however present dry electrodes typically have a much higher impedance, and typically require more complex designs in order to account for this. This makes manufacture more difficult and potentially more costly as a result. Examples of dry electrode materials include conductive polymer foams made of urethane materials and spring-loaded pin electrodes, which use the spring to push against the skin and maintain contact.
Accordingly, there exists a need to provide for a dry electrode with a lower impedance and relatively easy manufacturability.
Graphene, a two dimensional form of graphite, has gained attention as a potential material for sensors for electrophysiological readings owing to its excellent mechanical strength and high electrical conductivity, as well as being chemically stable. Graphene has also been found to be biocompatible in a range of applications and can be cytotoxic against bacteria. Graphene is difficult, however to manufacture into the required geometries for integration into an electrode or other biosensor.
The present disclosure seeks to provide a graphene based dry electrode suitable for applications such as EEG which has equivalent or superior performance compared to existing dry electrodes.
According to a first aspect, there is provided a process for forming a dry electrode for measuring electrophysiological readings, comprising: epitaxially growing a silicon carbide film on a doped silicon substrate; depositing at least two metals on a surface of the silicon carbide film, the at least two metals including at least one first metal and at least one second metal; heating the at least two metals, silicon carbide film, and substrate to cause the at least one first metal to react with silicon of the silicon carbide film to form carbon and at least one stable silicide, and the corresponding solubilities of the carbon in the at least one stable silicide and in the at least one second metal are sufficiently low that the carbon produced by the silicide reaction forms a graphene layer on the silicon carbide film; removing the at least one stable silicide and unreacted at least first and second metals to produce a structure having a doped silicon substrate and a silicon carbide film with a surface layer of graphene; repeatedly contacting the surface layer of graphene with an electrolyte solution to condition the graphene surface prior to use.
In some embodiments, the composition of the electrolyte solution is similar to human sweat.
In some embodiments, the electrolyte solution is phosphate buffered saline solution
In some embodiments, the concentration of the phosphate buffered saline solution is 0.01M.
In some embodiments, the electrolyte is human sweat.
In some embodiments, the corresponding solubility of carbon in the at least one second metal is lower than the corresponding solubility of carbon in the at least one stable silicide.
In some embodiments, the at least one first metal is nickel, and the at least one second metal is copper.
In some embodiments, the heating step is performed in an inert gas atmosphere.
In some embodiments, the heating step is performed under vacuum.
In some embodiments, wherein the vacuum has a pressure of about 10−3 to 10−5 mbar.
In some embodiments, the heating step is carried out above 800° C.
In some embodiments, the heating step is carried out at about 1100° C.
In some embodiments, the step of contacting the surface layer of graphene with the electrolyte solution is repeated more than three times.
In some embodiments, the step of contacting the surface layer of graphene is repeated between three and ten times.
According to a second aspect, there is provided a dry electrode for measuring electrophysiological readings, comprising: a doped silicon substrate; a silicon carbide film on the substrate; a graphene surface on the silicon carbide film; wherein the graphene surface has undergone a conditioning step of repeatedly contacting the surface layer of graphene with an electrolyte solution prior to use.
According to a third aspect, there is provided a dry electrode for measuring electrophysiological readings, comprising: a doped silicon substrate; a silicon carbide film on the substrate; a graphene surface on the silicon carbide film; wherein the graphene surface has undergone a functionalisation and/or intercalation process to increase the amount of oxygen functional groups present.
In some embodiments, the oxygen functional groups include C—OH and COOH.
In some embodiments, the functionalisation and/or intercalation process occurs substantially at the grain boundaries of the graphene surface.
In some embodiments, the substrate is attached to a metal pin button in a manner that enables electrical communication between the graphene surface and the metal pin button through the substrate.
In some embodiments, the substrate is attached to the metal pin button by a carbon tape.
In some embodiments, the electrode is produced by a process according to the first aspect.
According to a fourth aspect, there is provided a system for measuring electrophysiological readings comprising at least one electrode according the second or third aspect.
In some embodiments, the system is an EEG, EKG, EMG or EOG machine.
Other aspects, features, and advantages will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, which are a part of this disclosure and which illustrate, by way of example, principles of the inventions disclosed.
The present disclosure will become better understood from the following detailed description of various non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:
In the following description, the electrodes will be described in reference to sensors electrodes for EEG applications for ease of understanding. It will be understood, however, that similar electrodes may be used for other applications where electrical activity is non-invasively measured through the skin, such as but not limited to ECG, EMG and EOG applications. Where appropriate, these measurements of electrical activity through the skin will be referred to as ‘electrophysiological readings.’
It will be understood that throughout this specification, this term relates to measurements taken at a relatively large scale and in a non-invasive manner by attachment of electrodes to the skin of a patient in a predetermined location chosen based off the organ or tissue of interest.
The inventors have previously created a method for producing graphene layers on silicon carbide which is described in US20160230304. In brief, the method involves depositing at least two metals onto a surface of SiC, and then heating the SiC and metal layers to cause at least one of the metals to react with the silicon in the SiC to form stable silicides as well as carbon, the metals being chosen so that their solubilities are low enough for the carbon to form a graphene layer between the silicide and the remaining SiC.
The inventors have used this technique to fabricate graphene based electrodes which are suitable for EEG and other electrophysiological reading applications. The inventors have found that in order to obtain suitable impedance, the graphene surface must undergo a conditioning step which can be carried out by repeated exposure to skin and air. The inventors have found that this conditioning step improves the impedance performance of the sensors, enabling their use in applications such as EEG, EKG, EMG and EOG.
The present disclosure will become better understood from the following example of a non-limiting embodiment.
The silicon substrate has been highly doped in order to increase the conductivity of the substrate and maintain conductivity from the graphene surface 4 through the silicon carbide film 3, substrate 2 and electrical contact 5, where it can be communicated to an analyser through wiring or other standard methods. Known methods of doping silicon, such as with nitrogen or phosphorous may be used to increase the conductivity of the silicon substrate. In embodiments where the silicon substrate has not been sufficiently doped, a metallic layer or other electrical contact may extend around or through the substrate from the graphene surface to allow electrical communication with an analyser.
Sample electrodes were created using 3C—SiC films with a thickness of around 500 nm epitaxially grown on highly doped Si(100) substrate by placing them in a cryopump deposition chamber operating with CD Ar+ions and 200 mA current and spluttering a Ni layer of around 10 nm and a Cu layer of around 20 nm onto the film surface. The samples were then annealed by heating to around 1100° C. for one hour under vacuum conditions of around 10−5 mbar. This causes the breaking of Si—C bonds in the SiC and the release of carbon atoms which form a graphene layer on the surface, as well as nickel silicides in the metallic layer. These silicides, as well as any metal residues, were then removed by wet etching with Freckle solution for 9 hours. The conductivity of the surfaces was measured and found to be between ˜3 to 8 kΩ/square.
These electrodes were then mounted on foam covered by a copper tape. The foam was added in order to provide a mechanical pressure on the electrode towards the skin, ensuring effective contact with the skin, while the copper tape acts as an electrical contact for the electrode. The copper tape covering the foam backing may be the same copper tape that forms the electrical contact in the diagram of
The inventors have further refined the electrode design as shown in
The presence of graphene on the electrodes was verified and characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). An SEM image, taken at a magnification of 7.62 kX is shown in
Experiments were then carried out to determine the behaviour of these graphene electrodes compared to dry electrodes made of a conductive polymer foam made of a urethane material, dry spring loaded pin electrodes as well as Ag/AgCl wet electrodes when used in electrophysiologic reading applications.
The first experiment measured the impedance of the sensors on human skin across a range of frequencies. This experiment used an impedance analyser with a three electrode configuration, with the graphene electrode being the working electrode, an Ag/AgCl wet electrode used as the reference electrode, and a gold electrode used as the counter electrode.
The performance of the electrodes compared to presently available EEG electrodes are shown in
To further demonstrate the produced electrode's response to varying signals, and the effect of repeated testing of said electrodes,
It is thus clear that there is a skin conditioning effect that occurs to the electrode as a result of repeated skin/air contact. To investigate the mechanism behind this change in impedance performance between the first and third tests, SEM imagery, EDS analysis, and Raman spectroscopy was carried out on an electrode following the surface conditioning effect in a similar fashion to those shown in
Comparing
Comparing
XPS analysis comparing samples of pristine (prior to testing) epitaxial graphene and samples of epitaxial graphene having undergone the skin conditioning step (following 10 repeated tests) are shown in
Comparing these results, it appears that new oxygen species have emerged on the surface as corroborated by the EDS results, indicating that the graphene has undergone functionalisation during or as a result of the skin conditioning step. X
Looking at the C1s peak deconvolution for the pristine epitaxial graphene surface, there is an estimated 65.1, 25.4, 5.9 and 3.7 at % of C—C (graphenic), Si—C, C—OH (chemisorbed water) and COOH (carboxyl) bonds respectively. After ten repeated tests, a minor decrease of C—C bonds and a relatively high decrease in Si—C bonds was observed. The amount of COOH bonds remained roughly the same following testing, but the at % of C—OH bonds significantly increased. Without wishing to be bound by theory, it is surmised that these results indicate the formation of surface or edge functionalisation with hydroxyl groups on the epitaxial graphene following skin contact. The deconvolution of the O1s peak, as seen in
The increase in C—OH and COOH bonds result in beneficial properties such as improved wettability of the electrodes, reduced contact impedance and a marked enhancement of the double-layer capacitance. Otherwise stated, the increased hydrophilicity (wetting) as a result of the functionalisation of the graphene is thought to lead to better electrolyte (sweat) penetration and ion intercalation, resulting in reduced contact impedance with the skin. Sweat is a body fluid containing a number of ions such as sodium, potassium, calcium, magnesium, chloride and lactate and functions as an electrolyte solution in this situation. Hydroxyl groups present in the sweat solution are thought to form C—OH bonds at the grain boundaries of the epitaxial graphene. Following this, water molecules in sweat are adsorbed on the surface (physisorption) starting from hydroxyls at the grain boundaries, and extend over time to cover partially or fully the grains. This forms a boundary layer which is held by weak van der Waals forces on the graphene surface. This theory is supported by XPS results which show a reduced intensity of C—C and Si—C bonds following repeated testing, indicating that a layer is formed on top of the graphene following skin contact.
In summary, contact with the skin, or more specifically sweat on the skin, is thought to result in the formation of a semisolid boundary layer on top of the graphene surface and to lead to an improved wetting of the graphene surface and to allow ion intercalation in the graphene. Otherwise stated, it is thought that a thin layer of water molecules forms on the graphene surface through chemisorption and physisorption, facilitated by hydroxyl and carboxyl functional groups at the grain boundaries of epitaxial graphene.
To further characterize the surface conditioning effect, the measured impedance at 50 and 100 Hz for ten repeated tests (without breaking skin contact) were plotted. These results are plotted in
To verify that the skin conditioning step is linked to the sweat of the patient, another similar experiment measuring the impedance at 50 and 100 Hz was conducted where the electrodes were placed on a wet cloth of saline water (0.01 M phosphate buffered saline solution) to model sweat rather than placed on human skin. Similar surface conditioning results were observed as for the skin contact. These results, in
To summarise the theorized mechanism of surface conditioning, it is thought that the graphene surface undergoes functionalisation, in particular the formation of C—OH and COOH bonds at the grain boundaries of the graphene due to repeated contact with the skin and in particular sweat which acts as an electrolyte. It is thought that the mechanism for reducing contact impedance between the skin and electrode involves the formation of a thin layer of electrolyte on the epitaxial graphene surface that forms a semisolid surface type of contact during sensing applications. This may cause the graphene structure to become more electrochemically active, resulting in enhanced skin and electrode interaction and a lower contact impedance which is required for successful EEG sensors. This effect may be simulated by conditioning the graphene surface using an electrolyte solution, preferably one which has similar properties to sweat, such as phosphate buffer solution.
An additional benefit of the aforementioned mechanism is that the graphene sensors do not appear to readily undergo delamination following exposure to sweat as would be expected. Rather, contrary to ordinary understanding, the sweat improves the performance of the present sensors by causing functionalisation at the grain boundaries.
The electrochemical properties of epitaxial graphene and silicon carbide film on highly doped silicon in 0.1 M NaCl electrolyte (to simulate human sweat) were analysed by performing cyclic voltammograms (CV) at potential limits of 0.8 V to 0.0 V against Ag/AgCl electrodes in a three electrode system, at a scan rate of 100 mV/s. The results are shown in
To quantify and compare the transfer impedance between the electrodes and the electrolyte, electrochemical impedance spectroscopy (EIS) measurements were conducted in the 0.01 Hz to 100 kHz frequency range with a signal amplitude of 5 mV. The spectra of SiC film on highly doped silicon 141, epitaxial graphene 142, and the same epitaxial graphene after 100 repeated cycles 143. These curves show that the transfer impedance of the epitaxial graphene improves after 100 cycles, compared to both the initial epitaxial graphene and the SiC film reference. The measured charge transfer impedance (Rct) for SiC on silicon was ˜160Ω, whereas the initial Rct of the epitaxial graphene was ˜60Ω, reducing to ˜20Ω after 100 cycles.
The created graphene electrodes were also tested for their suitability for obtaining EEG by inserting them into a brain interface machine. Specifically, a headset with sensors at the locations Fp1, Fp2, Fz, C3, C4, Pz, O1 and O2 (according to the international 10-20 system) had the forehead sensors Fp 1 and Fp2 swapped with the created graphene electrodes. The impedance read by the headset was around 470 kΩ, compared with 421 kΩ and 300 kΩ for commercial foam based electrodes and commercial spring loaded electrodes respectively. The performance of the electrodes, as measured by impedance, could be improved by providing a gentle pressure or force on the electrode towards the skin, for example by securing the electrode to the skin by means of an elastic head band. Otherwise stated, it was observed that when electrodes were secured to the skin by an elastic head band, a lower contact impedance was measured.
In the foregoing description of certain embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents which operate in a similar manner to accomplish a similar technical purpose.
In this specification, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of”. A corresponding meaning is to be attributed to the corresponding words “comprise”, “comprised” and “comprises” where they appear.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.
In addition, the foregoing describes only some embodiments of the invention(s), and alterations, modifications, additions and/or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive.
Furthermore, invention(s) have described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention(s). Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020903960 | Oct 2020 | AU | national |
| 2021900918 | Mar 2021 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2021/051272 | 10/29/2021 | WO |
