Electrode for long-term recording

Abstract

The present invention relates to an electrode comprising a reservoir containing a liquid and an electrode material in contact with the reservoir and configured to absorb the liquid. The liquid in the reservoir is configured such that the electrode maintains a small contact resistance with the skin of the subject over a long period of time. The invention also relates to a method for producing an electrode and to a method for continuously hydrating an electrode.

Description

Technical Field

[0001] The present invention relates to an electrode, a biosensor, a method for manufacturing an electrode, and a method for continuously hydrating an electrode. Background Art

[0002] Measuring the electrical activity of organs is useful in many medical and research applications.

[0003] Wearable electrodes, such as cup electrodes, are used to record the electrical activity of organs. For example, the electrical activity of the brain, heart, and muscles can be monitored using electroencephalography (EEG), electrocardiography (ECG), and electromyography (EMG), respectively. Each of these techniques involves placing electrodes on a patient's skin near the organ of interest and measuring and recording the changes in the electrical potential detected by the electrodes over time.

[0004] In these applications, it is desirable to achieve stable readings over extended periods of time.

[0005] Ag / AgCl electrodes are commonly used as wearable electrodes for electrophysiological sensing applications such as EEG, ECG, and EMG. The use of these Ag / AgCl electrodes may involve placing a hydrogel between the electrode material and the patient's skin to maintain sufficient contact between the electrode and the skin. However, the hydrogel tends to dry out over time, which causes the electrode-skin contact to degrade, thereby reducing the measurement quality over time. Therefore, if such electrodes are to be used for long-term measurements, the hydrogel supplied at the electrode-skin interface needs to be replenished at various times during use to maintain contact between the electrode and the skin, for example, by applying the hydrogel directly to the electrode-skin interface via a manual device such as a syringe.

[0006] There are Ag / AgCl electrodes that have a "gel cavity" to provide hydrogel to the electrode. A commercial example is BioSemi's FLAT active electrode, which is an Ag / AgCl electrode that includes a gel cavity to reduce motion artifacts and gel drying. Another example is Biopac's EL250 series of reusable Ag-AgCl electrodes, which have an electrolyte gel cavity to reduce artifacts caused by electrolyte / electrode movement and minimize electrolyte dissipation / drying during long-term recordings.

[0007] Efforts have also been made to avoid the problem of hydrogel drying out by producing dry electrodes that have sufficient adhesion to the skin during use. Zhang et al. have published the fabrication of dry "PWS electrodes" formed from films made of the conductive polymer PEDOT:PSS mixed with the materials WPU and D-sorbitol to improve stretchability and adhesion (Zhang, Lei et al., "Fully organic compliant dry electrodes self-adhesive to skin for long-term motion-robust epidermal biopotential monitoring", Nature Communications, vol. 11, 114683, 17 September 2020, doi: 10.1038 / s41467-020-18503-8). In addition, Yuk et al. have published the fabrication of PEDOT:PSS electrodes by 3D printing (Yuk, H., Lu, B., Lin, S. et al., 3D printing of conducting polymers, Nature Communications 11, 1604 (2020), doi: 10.1038 / s41467-020-15316-7).

[0008] There is a continuing need to improve the long-term stability of electrode readings. In particular, there is a need to improve the stability of electrode readings when used over a period of, for example, several days. Furthermore, there is a need to devise an efficient and reliable method for manufacturing electrodes that produces stable and high-quality results over a long period of time. Summary of the Invention

[0009] The present invention has been devised in view of the above-mentioned problems.

[0010] A first aspect of the present invention is an electrode comprising: a reservoir containing a liquid; and an electrode material in contact with the reservoir and configured to absorb the liquid, wherein the liquid in the reservoir is configured to maintain an electrode-skin contact impedance of less than 100 kΩ for a period of at least 4 days at a frequency of approximately 10 Hz.

[0011] By being in contact with a reservoir containing a liquid, the electrode material continuously absorbs the liquid during use. Thus, the absorptive electrode material can maintain a low contact impedance with the subject's skin ("electrode-skin contact impedance") over an extended period of time by absorbing liquid from the reservoir. Maintaining a low electrode-skin contact impedance when the electrode is used over an extended period of time enables the electrode to produce consistent, high-quality, and high-precision readings with minimal noise over that period of time. The composition of the liquid in the reservoir is such that the electrode can exhibit an electrode-skin contact impedance of less than 100 kΩ at a frequency of approximately 10 Hz over a period of at least 4 days. This represents a significant improvement over known dry and gel-assisted electrodes, which suffer from a reduction in measurement quality over shorter periods of time due to higher electrode-skin impedance or electrode-skin impedance that tends to increase more rapidly over time.

[0012] The period of at least 4 days can be a continuous period of time. More specifically, the period of at least 4 days can be a continuous period of time during which the electrode is in continuous or periodic contact with the subject's skin. During the period of at least 4 days, whenever the electrode is in contact with the subject's skin, the electrode-skin contact impedance is less than 100 kΩ at a frequency of approximately 10 Hz. The start of the period of time in which the electrode-skin contact impedance remains less than this value can be the time when the electrode material of the electrode including the reservoir containing the liquid becomes exposed. For example, when the electrode is provided with liquid in the reservoir and a seal or cover is provided on the surface of the electrode material to be in contact with the subject's skin, the start of the period of time can be the moment when the seal or cover is removed from the electrode material. Similarly, when the electrode is provided with uncovered electrode material and there is no liquid in the reservoir, the start of the period of time can be the moment when the liquid is introduced into the reservoir.

[0013] The time period of at least 4 days during which the electrode-skin impedance is maintained at less than 100 kΩ at a frequency of about 10 Hz can be a time period during which no additional liquid is used to refill or top up the reservoir. That is, the electrode-skin contact impedance can be maintained at less than 100 kΩ over a time period of at least 4 days solely by virtue of the liquid present in the reservoir at the start of the time period. Refilling or topping up the reservoir with liquid can be considered to reset the time period. Similarly, the time period of at least 4 days can be a time period during which no additional liquid is applied to the electrode material by other means (e.g., directly to the electrode-skin interface). Therefore, the construction of the reservoir and the composition of the liquid in the reservoir can be such that a defined electrode-skin impedance is maintained over a defined time period without adding additional liquid during the time period.

[0014] The frequency of about 10 Hz that achieves the electrode-skin contact impedance (ie, the AC frequency) may be, for example, 10 Hz ± 1 Hz, or exactly 10 Hz. Impedance may be measured by impedance spectroscopy within a frequency range where 10 Hz may be the minimum.

[0015] The liquid contained in the reservoir may be an electrolyte.The electrode material in contact with the reservoir may be an absorbent solid material.

[0016] Preferably, the liquid in the reservoir is constituted to maintain the electrode-skin contact impedance less than 100 kΩ for a period of at least 5 days, preferably at least 10 days, preferably at least 20 days, preferably at least 30 days.

[0017] When the liquid is configured to maintain the electrode-skin contact impedance less than 100 kΩ for a longer period of time, the quality of the readings from the electrodes is maintained at a higher level for a longer period of time.

[0018] Preferably, the electrode-skin contact impedance of the electrode (wherein the liquid in the reservoir is configured to maintain the electrode-skin contact impedance for at least 10 days at a frequency of approximately 10 Hz (e.g., 10 Hz)) is less than 90 kΩ, preferably less than 80 kΩ, preferably less than 70 kΩ, preferably less than 60 kΩ of electrode-skin contact impedance.

[0019] When the liquid was configured to maintain the electrode-skin contact impedance below a smaller kΩ value for at least 4 days, readings from the electrodes were maintained with even higher quality over that time period.

[0020] With respect to the kΩ value (wherein the electrode-skin contact impedance is maintained below the kΩ value) and the time period (wherein the contact impedance is maintained during the time period), any combination of kΩ values ​​and time periods disclosed herein is contemplated. In a most preferred example, the liquid is configured to maintain the electrode-skin contact impedance as low as possible for as long as possible, for example, to maintain the electrode-skin contact impedance below 60 kΩ for at least 30 days.

[0021] In applications requiring long-term continuous monitoring, preferably the liquid in the reservoir is configured to maintain the electrode-skin contact impedance below the stated kΩ value (e.g., less than 100 kΩ) at a frequency of approximately 10 Hz for a period of at least the stated number of days (e.g., at least 4 days), during which period the electrode is in continuous contact with the subject's skin.

[0022] That is to say, the electrode of this aspect can be configured so that when the electrode is in continuous contact with the skin of the experimenter in a long continuous period of time, the electrode-skin contact impedance remains small throughout the period of time. The continuous period of time can be at least 4 days, or preferably, can be longer (e.g., at least 5 days, at least 10 days, at least 20 days, or at least 30 days). In this period of time, the electrode-skin contact impedance can remain less than 100kΩ, or preferably, can remain less than a smaller value (e.g., remain less than 90kΩ, remain less than 80kΩ, remain less than 70kΩ, or remain less than 60kΩ). In applications where it is necessary to continuously monitor the electrical activity of the experimenter for a long time, it is advantageous to continuously maintain a smaller electrode-skin contact impedance during the period of continuous use because the measurement quality remains high throughout the period of time.

[0023] In applications requiring long-term periodic monitoring, it is preferred that the liquid in the reservoir is configured to maintain the electrode-skin contact impedance below the kΩ value (e.g., less than 100 kΩ) at a frequency of about 10 Hz for a period of at least the number of days (e.g., at least 4 days), during which the electrode is periodically in contact with the subject's skin. In this case, the time period and impedance value can be any of the time periods and impedance values ​​defined herein.

[0024] Preferably, the electrode material comprises a conductive polymer. More preferably, the electrode material comprises PEDOT:PSS.

[0025] By forming the electrode material from a conductive polymer, the quality of the electrode reading remains particularly high over a particularly long period of time in use. PEDOT:PSS is a particularly advantageous example of a conductive polymer for use as electrode material.

[0026] The electrode material may include conductive metal particles, such as silver (Ag) particles.

[0027] The electrode material may include natural rubber, such as latex.

[0028] Preferably, the liquid comprises a deep eutectic solvent or an ionic liquid.Most preferably, the liquid comprises a deep eutectic solvent.

[0029] Deep eutectic solvents and ionic liquids are particularly suitable as liquids contained in the reservoir of the electrode of the present invention. Specifically, deep eutectic solvents and ionic liquids have properties that allow them to be particularly well absorbed into the electrode material, thereby maintaining a low electrode-skin contact impedance over an extended period of time. Furthermore, deep eutectic solvents and ionic liquids have a low tendency to dry out over time, which is beneficial in electrode manufacturing.

[0030] Deep eutectic solvents are particularly advantageous as the liquid to be contained in the reservoir of the electrode because they have a particularly low tendency to dry out over time, their viscosity and tendency to be absorbed into the electrode material are particularly favorable, and their cost is relatively low compared to ionic liquids. Therefore, it is particularly preferred that the electrode as described herein have a deep eutectic solvent as the "liquid" contained in the reservoir (or as a substantial component of the liquid).

[0031] Preferably, the electrode further comprises a housing containing the reservoir, the housing being formed from a biocompatible polymer.

[0032] The housing can be formed into a rigid or flexible structure to surround the reservoir on all sides except the side that contacts the electrode material. When using the electrode, the user can pick up the electrode by grasping the housing and place it at the desired location on the subject's skin. The housing is formed from a biocompatible polymer, making it suitable for direct contact with the subject's skin.

[0033] Preferably, the housing includes at least one of: an opening for introducing liquid into the reservoir when the electrode material is in contact with the subject; and an opening for receiving a cable.

[0034] Although the liquid in the reservoir enables the electrode to have a small electrode-skin contact impedance during prolonged use, the eventual depletion of the liquid in the reservoir can cause the electrode-skin contact impedance to increase and result in a decrease in measurement quality after a period of time. In this case, the presence of a housing with an opening, wherein the opening is configured so that liquid can be introduced into the reservoir while the electrode material is in contact with the subject (i.e., during use of the electrode), can enable the reservoir to be refilled with liquid without having to disassemble the electrode or even remove the electrode from the subject's skin. After refilling the reservoir with liquid, the electrode can again produce high-quality measurements for a long period of time. The reservoir can in principle be filled an unlimited number of times, so that the capacity of the reservoir has no limit on the length of time during which the electrode can produce high-quality measurements due to having a small electrode-skin contact impedance.

[0035] An opening in the housing for receiving a cable may facilitate electrical connection of the electrode material to a power source and / or an external computer for processing measurements from the electrodes.

[0036] The electrodes of this aspect can include multiple layers of different electrode materials. That is, the electrode material in contact with the reservoir can be referred to as the "first electrode layer," while the additional layers formed on top of the first electrode layer can be referred to as the "second electrode layer," the "third electrode layer," and so on. As will be described herein, the uniqueness of the different layers can result from the nature of the electrode's manufacturing process. The presence of multiple layers of different electrode materials can enhance the electrode's ability to maintain a low electrode-skin contact impedance over long periods of continuous or periodic use.

[0037] A second aspect of the present invention is a biosensor comprising the electrode of the first aspect and a cable, wherein the cable is in electrical contact with the electrode material.

[0038] The cable can connect the electrode material to a power source and / or an external computer for processing measurements from the electrodes. The biosensor can be a wearable biosensor. The biosensor provides all of the advantages of the electrodes described herein and is therefore a useful instrument for relatively long-term, continuous or semi-continuous measurement of a subject's electrical activity (e.g., EEG, ECG, and EMG).

[0039] A third aspect of the present invention is a method for manufacturing an electrode, comprising: introducing a liquid into a reservoir, the liquid in the reservoir defining a liquid surface; and depositing an electrode material on the liquid surface, wherein the liquid is constructed so that the surface tension of the liquid supports the electrode material on the liquid.

[0040] The method of the third aspect can be used as a method for manufacturing the electrode of the first aspect described above, in which case the "liquid", "reservoir" and "electrode material" of the third aspect can be constructed in any such manner as described above in relation to the first aspect, and any additional features of the electrode described above in relation to the first aspect can be included in the electrode manufactured by the method of the third aspect. Similarly, the electrode manufactured by the method of the third aspect can form the electrode of the biosensor of the second aspect. Therefore, the electrode produced by the method of the third aspect can provide any or all of the advantages discussed herein in relation to the first or second aspects.

[0041] In the method for manufacturing an electrode according to the present invention, the electrode material is deposited on the surface of the liquid in the reservoir and supported on the liquid by the surface tension of the liquid. The liquid can advantageously provide a flat and uniform surface on which a uniform layer of electrode material can be deposited, thereby achieving high accuracy and reliability of readings obtained by the manufactured electrode during use.

[0042] The electrode material can be deposited on the liquid surface in solution form. That is, a solution of the electrode material in a solvent (e.g., water) can be deposited on the liquid surface. For example, when the electrode material is a conductive polymer such as PEDOT:PSS, the conductive polymer solution can be deposited on the liquid surface during electrode fabrication. The electrode material solution is then dried to remove the solvent, providing a solid layer of electrode material ("first electrode layer"). The electrode material solution can be deposited on the liquid surface by drop casting.

[0043] When the electrode material is in the form of a solid layer on the liquid surface (eg, after a deposited electrode material solution has dried), the electrode material may be configured to absorb liquid contained in the reservoir, as described herein in relation to the first aspect.

[0044] Preferably, the method of manufacturing an electrode includes solidifying the electrode material on the surface of the liquid to form a first electrode layer. Solidifying the electrode material on the surface of the liquid after the electrode material is deposited on the liquid surface can achieve or accelerate drying and hardening of the electrode material, thereby forming a first electrode layer suitable for absorbing liquid from a reservoir and applying to the skin of a subject. Because the electrode material is deposited on the liquid surface before solidification, the final solidified first electrode layer has a high degree of uniformity, thereby providing high accuracy and reliability of readings obtained by the electrode. When the electrode material is deposited on the liquid surface in the form of a solution (e.g., a PEDOT:PSS solution), the post-deposition solidification step can remove the solvent to leave a dry, uniform layer of electrode material.

[0045] Preferably, the liquid in the reservoir is configured so that the liquid does not evaporate significantly during any curing step. Curing is performed at a relatively high temperature, so that the liquid near the curing area can usually evaporate at an accelerated rate. The evaporation of the liquid on which the electrode material is deposited can lead to inaccuracies when the electrode layer (e.g., the first electrode layer) is formed because the uniformity of the liquid surface on which the electrode material is supported is destroyed. These inaccuracies when the electrode layer is formed can reduce the quality and accuracy of the final measurement performed by the electrode. By configuring the liquid so that evaporation does not occur to a significant extent, the accuracy of forming the electrode layer can be improved, thereby providing better uniformity and better height control of the electrode layer, so that the quality and accuracy of the measurement performed by the manufactured electrode are improved. For example, the liquid can be configured to have a vapor pressure of less than 1 kPa, more preferably less than 500 Pa, more preferably less than 200 Pa, more preferably less than 100 Pa, more preferably less than 50 Pa, more preferably less than 30 Pa, obtained at a temperature of about 313 K.

[0046] Preferably, the liquid is a deep eutectic solvent or an ionic liquid. These liquids are particularly suitable due to their generally low vapor pressure.

[0047] Preferably, the method of manufacturing an electrode further comprises depositing an additional amount of liquid on the electrode material.

[0048] That is, after the electrode material has been deposited on the surface formed by the liquid in the reservoir, an additional amount of the same liquid can be deposited on top of the electrode material. Advantageously, this can increase the speed and uniformity with which the electrode material is saturated with the liquid. In the case where the electrode material is deposited as a solution on the liquid surface and subsequently dried (e.g., by solidification), depositing an additional amount of liquid on the electrode material can fill in the spaces left by the evaporation of the solvent in which the electrode material is dissolved.

[0049] The additional amount of liquid can be deposited by drop casting. The deposition of the additional amount of liquid can occur after the electrode material solidifies. Additionally or alternatively, the deposition of the additional amount of liquid can occur during the solidification of the electrode material. In the latter case, the time required to manufacture the electrode can be reduced because the time spent solidifying the electrode material is also used to deposit the additional liquid, and the stability of the produced electrode can also be enhanced because the evaporated solvent is immediately replaced and the time when the electrode material layer includes empty space is minimized.

[0050] When the method includes depositing an additional amount of liquid on the electrode material, preferably, the method may further include: solidifying the additional amount of liquid deposited on the electrode material.

[0051] Preferably, the method for manufacturing an electrode comprises: forming a plurality of electrode layers comprising a first electrode layer and a second electrode layer, wherein: the second electrode layer is formed by depositing an additional amount of electrode material on the first electrode layer and solidifying the additional amount of electrode material; and optionally, forming one or more additional electrode layers by sequentially depositing an additional amount of electrode material on the previous electrode layer and solidifying the additional amount of electrode material.

[0052] That is, a first electrode layer formed by depositing electrode material on a liquid surface as described herein and drying (e.g., curing) the electrode material can be used as a surface on which additional electrode material is deposited to form a second electrode layer. The second electrode layer is formed in a similar manner to the first electrode layer, except that the electrode material used to form the second electrode layer is deposited on the first electrode layer rather than on the liquid surface on which the first electrode layer is formed. Similarly, a third electrode layer can be formed on the second electrode layer, a fourth electrode layer can be formed on the third electrode layer, and so on. The formation of each layer involves depositing electrode material (e.g., in solution form) on the previous layer and curing the electrode material to form a solid electrode layer (the solid electrode layer can serve as a basis for subsequent layers). The formation of each layer after the first layer may also include any additional steps or features described herein in connection with the first electrode layer. The formation of multiple layers of electrode material can enhance the ability of the electrode to maintain a low electrode-skin contact impedance over long periods of continuous or periodic use.

[0053] Preferably, when forming multiple electrode layers, the method further comprises: depositing an additional amount of liquid on the electrode material forming the electrode layer. The additional amount may be proportional to the number of electrode layers. For example, after forming the multiple electrode layers, an additional amount of liquid proportional to the number of electrode layers may be deposited on the top layer. Additionally or alternatively, an additional amount of liquid may be deposited on each electrode layer individually after each electrode layer is formed, prior to forming subsequent electrode layers. Preferably, the deposition of the additional amount of liquid may occur when the electrode material on which the liquid is deposited solidifies. The additional liquid may be deposited by drop casting.

[0054] The advantages of depositing an additional amount of liquid on the electrode material and depositing an additional amount of liquid while the electrode material is curing are discussed herein with respect to the first electrode layer. These advantages also apply to the formation of the second electrode layer and any subsequent electrode layers. Preferably, the method may further comprise curing the additional amount of liquid deposited on the electrode material of the plurality of electrode layers, in which case the length of the curing time and / or the curing temperature may be proportional to the number of electrode layers formed.

[0055] Preferably, the method for manufacturing an electrode further comprises: immersing the solidified electrode material in an immersion liquid for at least 12 hours, more preferably at least 24 hours. The immersion liquid may be water. Alternatively, while the immersion liquid is different from the "liquid" introduced into the reservoir to form the electrode as mentioned herein, the immersion liquid may still have the same composition as the aforementioned "liquid" (e.g., a deep eutectic solvent or an ionic liquid). Further alternatively, the immersion liquid may be a solution different from the aforementioned "liquid."

[0056] Immersing the electrode material in an immersion liquid, such as water, after curing is an additional processing step that can enhance the ability of the electrode to maintain a low electrode-skin contact impedance over extended periods of continuous or periodic use. The step of immersing the electrode material in the immersion liquid occurs after the electrode material has been deposited on the surface of the liquid and cured. When the method includes depositing an additional amount of liquid on the electrode material, preferably, the step of immersing the electrode material in the immersion liquid occurs before depositing the additional amount of liquid on the electrode material. When the method includes forming a plurality of electrode layers, the step of immersing the electrode material in the immersion liquid may occur after the plurality of electrode layers have been formed and cured, and may occur before any (optional) deposition of an additional amount of liquid on the electrode material.

[0057] Preferably, the method of manufacturing the electrode comprises forming a housing by additive manufacturing, the housing housing the reservoir.Most preferably, the housing is formed from a biocompatible polymer.

[0058] The housing can be formed into a rigid or flexible structure to surround the reservoir on all sides except the side in contact with the electrode material. When using the manufactured electrode, the user can pick up the electrode by holding the housing and place the electrode in the desired position on the subject's skin. Preferably, the housing formed by additive manufacturing may include an opening for introducing liquid into the reservoir when the electrode material is in contact with the subject and / or an opening for receiving a cable. As discussed in relation to the first aspect herein, these openings can be advantageous.

[0059] Additive manufacturing is an efficient and precise technology for forming the housing. Therefore, the housing can be easily formed into a shape that facilitates the introduction of liquid and the deposition of electrode material onto the liquid to form the electrodes. Forming the housing from a biocompatible polymer makes it suitable for direct contact with a subject's skin.

[0060] A fourth aspect of the present invention is a method of continuously hydrating an electrode, the method comprising: continuously contacting an absorbent electrode material of the electrode with a liquid, wherein the liquid in a reservoir is configured to maintain an electrode-skin contact impedance of less than 100 kΩ at a frequency of approximately 10 Hz for a period of at least 4 days.

[0061] The electrode continuously hydrated by the method of the fourth aspect may be the electrode of the first aspect, may form part of the biosensor of the second aspect, and may be manufactured by the method of the third aspect. Any combination of features or steps described herein in relation to those aspects also applies to the fourth aspect.

[0062] The electrodes are continuously hydrated by continuously contacting the electrodes with a liquid, which may be any of the liquids described herein in relation to the first, second and third aspects, and thus provide similar advantages. By continuously hydrating the electrodes in this manner, the electrodes are able to maintain a low impedance contact with the subject's skin over an extended period of time, thereby enabling consistent, high-quality readings from the electrodes.

[0063] Any of the above aspects may include features described herein in relation to any other aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 An electrode according to an embodiment of the present invention is depicted.

[0065] Figure 2 Describes the effect of the Figure 1 electrodes.

[0066] Figure 3 Possible configurations of the housing of an electrode according to embodiments are depicted.

[0067] Figure 4 Possible configurations of the housing of an electrode according to embodiments are depicted.

[0068] Figure 5 A reservoir containing a liquid before depositing an electrode material on the liquid in a method of manufacturing an electrode according to an embodiment is depicted.

[0069] Figure 6 A multilayer electrode according to an embodiment is depicted.

[0070] Figure 7 Possible configurations of the housing of the electrode are depicted, as well as the deposition of electrode material on the electrode.

[0071] Figure 8 Experimental impedance data comparing electrodes according to embodiments to commercially available electrodes are depicted over a period of time.

[0072] Figure 9 Depicted are experimental EEG data comparing electrodes according to an embodiment with commercially available electrodes.

[0073] Figure 10 Experimental impedance data comparing electrodes according to embodiments to commercially available electrodes over a range of frequencies is depicted.

[0074] Figure 11 Experimental impedance data comparing electrodes according to embodiments having different amounts of electrode layers to commercially available electrodes over a range of frequencies is depicted.

[0075] Figure 12 Experimental impedance data for electrodes with silver and latex additives according to an embodiment are plotted over a range of frequencies.

[0076] Figure 13a and Figure 13b Describes the formation of Figure 7 Cross section of the housing of the electrode.

[0077] Figure 14 Depicted is an experimental ECG recording performed using electrodes according to an embodiment.

[0078] Figure 15a and Figure 15b Depicted is an experimental EEG recording performed using electrodes according to an embodiment.

[0079] Figure 16 Depicted are time-spectrograms of alpha-band modulation recordings from the same EEG experiment depicted by FIG. 15 . DETAILED DESCRIPTION

[0080] Hereinafter, exemplary embodiments will be described with reference to the accompanying drawings.

[0081] The present invention provides an electrode suitable for use with EEG, ECG, EMG, and similar measurement techniques. The electrode of the present invention can be a cup-shaped electrode or a flat electrode. The electrode is capable of providing consistently high-quality measurements over long periods of time (e.g., several days). "High quality" means that the measurements taken by the electrode are accurate and exhibit low levels of noise.

[0082] The electrodes according to the embodiment are Figure 1 . Electrode 1 includes a reservoir 2 containing a liquid, such as an electrolyte. In contact with the reservoir fluid is electrode material 3. Electrode material 3 is an electrical conductor and is the portion of the electrode configured to contact a subject (e.g., the subject's skin) to measure the subject's electrical activity near the electrode. Electrode material 3 is a rigid (i.e., solid) material that interfaces with the liquid in reservoir 2. Electrode material 3 is configured to absorb the liquid in reservoir 2.

[0083] Figure 2 Shows the usage Figure 1An electrode 1 is provided in which the electrode material 3 contacts the skin 5 of the subject. The portion of the subject's skin 5 to which the electrode 1 is applied may vary based on the type of measurement performed by the electrode. For example, if the electrode 1 is to perform an EEG measurement, the electrode 1 may be placed on the subject's scalp, or if the electrode 1 is to perform an ECG measurement, the electrode 1 may be placed on the subject's chest. If a measurement such as EEG, ECG or EMG is performed on the subject, a plurality of electrodes 1 may be placed on the subject's skin 5 in a desired spatial arrangement. When in use, the electrode 1 is also configured to be connected to a power source and a data processing device such as a computer or data logger. The connection may be achieved by means of one or more cables that connect the electrode 1 to external components (such as a power source and data processing device) and optionally to further components of the electrode 1.

[0084] The composition of the electrode material 3 and the liquid in the reservoir 2 allows the electrode 1 to exhibit low electrode-skin contact impedance for extended periods of time during use due to absorption of the liquid by the electrode material 3. In other words, the liquid in the reservoir 2 ensures continuous hydration of the electrode material, which in turn provides a consistently low electrode-skin contact impedance. The liquid reservoir and the absorbent electrode material enable the electrode to achieve low impedance, capturing, for example, μV-level fluctuations in electrical activity, and in the process, achieve long-term stability.

[0085] The liquid in the electrode's reservoir 2 is configured to maintain an electrode-skin contact impedance of less than 100 kΩ at a frequency of approximately 10 Hz for a period of at least 4 days. Preferably, the liquid in the electrode's reservoir 2 is configured to maintain an electrode-skin contact impedance of less than 100 kΩ at a frequency of approximately 10 Hz for a period of at least 4 days when the electrode is in contact with the subject's skin, during which period the electrode 1 is continuously or periodically in contact with the subject's skin 5. This means that over the 4-day period, each time the electrode material is in contact with the subject's skin, the electrode 1 is capable of exhibiting an electrode-skin contact impedance of less than 100 kΩ. This contact may occur continuously over the period, in which case the electrode continuously maintains an electrode-skin contact impedance of less than 100 kΩ for the entire 4-day period. Alternatively, the contact may occur periodically (i.e., intermittently), in which case the electrode continuously maintains an electrode-skin contact impedance of less than 100 kΩ for each complete time interval that the electrode is in contact with the skin (the electrode-skin contact impedance is not defined during time intervals when the electrode is not in contact with the skin). Additionally or alternatively, the liquid is configured to maintain an electrode-skin contact impedance of less than 100 kΩ at a frequency of approximately 10 Hz at any point while the electrode is in contact with the subject's skin after the electrode is provided to the liquid in the reservoir and the electrode material is exposed. For example, the electrode may include a seal (not depicted) to cover the electrode material when not in use, and the liquid in the reservoir may be configured such that, after removal of the seal, the electrode-skin contact impedance is less than 100 kΩ at any time during a 4-day period while the electrode material is in contact with the subject's skin.

[0086] As used herein, a time period "during which the electrode is periodically in contact with the subject's skin" may refer to a time period during which the time intervals of electrode-skin contact span substantially the entire width of the time period. That is, expressing a time period in days (wherein the electrode is periodically in contact with the subject's skin during the time period) means a time period during which electrode-skin contact occurs on the first and last days of the time period. Thus, for example, a 4-day time period in which electrode-skin contact occurs during the time intervals associated with Day 1 and the time intervals associated with Day 2, but no electrode-skin contact occurs at all on Day 3 or Day 4, is not considered a "4-day time period during which the electrode is periodically in contact with the subject's skin."

[0087] Generally, it is desirable to keep the electrode-skin contact impedance of the electrode as low as possible for as long as possible during use. Keeping the electrode-skin contact impedance as low as possible ensures high-quality measurements with low noise, and maintaining the low impedance for as long as possible ensures that high-quality measurements with low noise are always achieved during long-term measurements (continuous or periodic).

[0088] Therefore, the electrode-skin contact impedance maintained by the electrode 1 at approximately 10 Hz is less than 100 kΩ, preferably less than 95 kΩ, more preferably less than 90 kΩ, more preferably less than 85 kΩ, more preferably less than 80 kΩ, more preferably less than 75 kΩ, more preferably less than 70 kΩ, more preferably less than 65 kΩ, or most preferably less than 60 kΩ.

[0089] In addition, the time period for which the electrode-skin contact impedance is maintained when the electrode is in contact with the skin is at least 4 days, preferably at least 5 days, more preferably at least 6 days, more preferably at least 7 days, more preferably at least 8 days, more preferably at least 9 days, more preferably at least 10 days, more preferably at least 11 days, more preferably at least 12 days, more preferably at least 13 days, more preferably at least 14 days, more preferably at least 15 days, more preferably at least 16 days, more preferably at least 17 days, more preferably at least 18 days, more preferably at least 19 days, more preferably at least 20 days, more preferably at least 21 days, more preferably at least 22 days, more preferably at least 23 days, more preferably at least 24 days, more preferably at least 25 days, more preferably at least 26 days, more preferably at least 27 days, more preferably at least 28 days, more preferably at least 29 days, and most preferably at least 30 days. As described herein, the time period can be the time period during which the electrode is in continuous or periodic contact with the subject's skin.

[0090] The electrodes maintain a defined electrode-skin impedance for a defined period of time at a frequency of approximately 10 Hz (ie, an AC frequency). However, the electrodes described herein are useful in applications where the AC frequency is different from 10 Hz (eg, anywhere between 1 Hz and 50 Hz).

[0091] The inventors have investigated various materials used to form each component of the electrode to best achieve the properties discussed herein.

[0092] Regarding the composition of electrode material 3, conductive polymers are a particularly advantageous family of materials. Conductive polymers can have high electrical conductivity, high mechanical strength and flexibility, as well as biocompatibility. Conductive polymers can be absorbent, and in particular, the inventors herein have recognized that conductive polymers are particularly effective at absorbing liquids of the type contained in reservoir 2 of electrode 1 (e.g., deep eutectic solvents and ionic liquids). By forming electrode material 3 so that electrode material 3 comprises a conductive polymer, the quality of electrode readings remains particularly high for an exceptionally long period of time during use, as the liquid in reservoir 2 can be effectively absorbed by electrode material 3 to continuously hydrate electrode material 3, and due to the strength and high electrical conductivity of the conductive polymer, the electrode remains stable and accurate. The inventors have discovered that PEDOT:PSS is a particularly advantageous conductive polymer for achieving these advantages. Therefore, preferably, electrode material 3 comprises PEDOT:PSS.

[0093] Electrode material 3 may also include additional components, as well as the host material (e.g., PEDOT:PSS) that forms electrode material 3. Additional components of electrode material 3 may include conductive metal particles, such as silver particles. Dispersing such particles within electrode material 3 can enhance the electrode's conductivity and improve measurement accuracy. Another additional component of electrode material 3 may be natural rubber, such as latex. Natural rubber can enhance the mechanical strength and flexibility of electrode 1, thereby improving its ability to adhere to the skin.

[0094] Regarding the composition of the liquid in reservoir 2, electrolytes are particularly advantageous. The inventors have discovered that deep eutectic solvents and ionic liquids are particularly advantageous when used as the liquid to be contained in reservoir 2 of electrode 1. Deep eutectic solvents and ionic liquids have useful physical properties, including significantly lower vapor pressures, making them less prone to drying out over time than other types of liquids and gels, particularly those commonly used in conventional electrodes. Because deep eutectic solvents and ionic liquids have a reduced tendency to dry out over time, when contained in reservoir 2 of electrode 1, these liquids can continue to be absorbed into electrode material 3 and provide a beneficially low electrode-skin contact impedance for an extended period before being depleted. In other words, providing a deep eutectic solvent or ionic liquid in reservoir 2 of electrode 1 effectively and continuously hydrates electrode material 3 over an exceptionally long period of time, thereby maintaining a low electrode-skin contact impedance and high-quality readings over an extended period of time. Deep eutectic solvents and ionic liquids have a lesser tendency to evaporate and are also more useful with respect to fabricating the electrode 1 , as discussed further below.

[0095] The inventors have discovered that deep eutectic solvents are particularly suitable for use as the liquid contained in the reservoir 2 of the electrode 1. Compared to ionic liquids, deep eutectic solvents have a particularly low tendency to dry out over time, particularly favorable viscosities and a tendency to be absorbed into the electrode material, as well as relatively low cost. Therefore, the liquid contained in the reservoir 2 of the electrode 1 is particularly advantageously a liquid that is or includes a deep eutectic solvent.

[0096] A preferred arrangement of the electrode 1 is one in which the electrode material 3 comprises a conductive polymer and the liquid in the reservoir 2 comprises a deep eutectic solvent. Most preferred is an arrangement in which the electrode material comprises PEDOT:PSS and the liquid in the reservoir 2 comprises a deep eutectic solvent. The inventors have found that when a PEDOT:PSS layer is used as the electrode material 3 and is in continuous contact with a reservoir of deep eutectic solvent, the deep eutectic solvent is absorbed into the PEDOT:PSS layer and the tendency of the deep eutectic solvent to dry out is minimal, which allows for the continuous hydration of the PEDOT:PSS over long periods of time and the ability of the electrode to consistently exhibit very low contact impedance over these long periods of time. The performance of such an electrode is as follows: Figure 8 and Figure 9 shown.

[0097] Still Figure 1 and Figure 2 The housing 4 of the electrode 1 is shown in FIG. The housing 4 houses the reservoir 2 and is formed of a biocompatible polymer. When present, the housing 4 forms a solid structure around the reservoir so that the reservoir 2 is surrounded by the housing 4 and the electrode material 3. Thus, the housing can enhance user convenience by providing a handle by which the user can place the electrode at a desired location on the subject's skin 5. The housing can have a Figure 1 and Figure 2 The simple structure shown here simply encloses the reservoir in a housing. The housing can be formed by additive manufacturing. The housing can be a rigid structure or can have a degree of flexibility.

[0098] Figure 3 The preferred arrangement of the housing 4 is shown. Figure 3, the housing 4 is shown disassembled into two separate parts - an inner part 41 and an outer part 42. The inner part 41 and the outer part 42 can be separate components that are attached together before use (for example, by inserting the inner part 41 into the outer part 42), or the inner part 41 and the outer part 42 can form an integral component. When assembled together, a gap is left in the outer part 42 below the inner part 41 to form a lower area of ​​the reservoir for the introduction of liquid. The inner part 41 includes one or more through holes 11, which provide a fluid communication path between the aforementioned lower area of ​​the reservoir and the position of the electrode material in the assembled electrode. As Figure 3 As shown, providing through-holes 11 in the inner portion 41 may be useful when manufacturing the electrode, as discussed further below. Figure 7 Shown with Figure 3 A similar arrangement is preferred, wherein the through hole 11 is located in the inner portion 41 of the housing 4 .

[0099] Figure 13a and Figure 13b The cross section shows the Figure 3 and Figure 7 The assembled electrode 1 is similarly arranged to the arrangement schematically shown in FIG. The electrode 1 comprises a housing 4 formed by an inner portion 41 and an outer portion 42. The housing 4 comprises a reservoir 2, which itself contains a liquid as described herein. The reservoir 2 comprises a lower region 21 of the reservoir, Figure 13a and Figure 13b In the depicted orientation, the lower region 21 is located below the interior portion 41 of the housing. The interior portion 41 of the housing includes one or more through-holes 11 in fluid communication with the lower region 21. Thus, in this arrangement, the reservoir 2 containing the liquid includes at least two distinct regions: the lower region 21; and the space within the one or more through-holes 11. As described herein, the electrode material 3 is in contact with the reservoir 2 and is configured to absorb liquid from the reservoir 2. Figure 13a and Figure 13b In the arrangement of , the electrode material 3 is in contact with or in close proximity to the opening ("upper opening") of the through-hole 11, which is located on the side of the interior portion 41 of the housing opposite the lower region 21 of the reservoir 2. Figure 13a As shown, the electrode material layer 3 in the assembled electrode can be in contact with the upper surface of the inner portion 41 of the housing and the portion of the reservoir present at the upper opening of the through-hole 11, so that the electrode material 3 is configured to absorb liquid directly from the through-hole 11. Additionally or alternatively, as Figure 13bAs shown, there may be a spatial separation between the upper opening of the through-hole 11 and the electrode material layer 3, thereby forming a region (e.g., a narrow region) into which the liquid can flow between the upper opening of the through-hole 11 and the electrode material 3, which region thereby forms another part of the reservoir 2, and which region itself is in contact with the electrode material 3. In the latter arrangement, the liquid effectively wets the upper surface of the inner portion 41 of the housing 4, wherein the electrode material 3 is placed on top of the wetted upper surface of the inner portion 41.

[0100] As used herein, the "upper opening" of the through hole 11 refers to the opening of the through hole 11 that is located on the side of the through hole opposite to the lower region 21 of the reservoir. Figure 13a and Figure 13b The depicted orientation is the top opening. Similarly, the "upper surface" of the interior portion 41 of the housing refers to the surface of the interior portion of the housing on which the upper opening of the through-hole is located.

[0101] like Figure 3 、 Figure 7 As depicted in FIG13 , the housing 4 is formed of an inner portion 41 and an outer portion 42, thereby providing advantageous structural rigidity to the assembled electrode 1. Furthermore, the provision of the through-holes 11 allows for advantageous diffusion of liquid throughout the electrode material, thereby providing the various advantages described herein. As described elsewhere herein, due to the support provided by the inner portion 41 of the housing during deposition of the electrode material 3, providing the inner portion 41 with the through-holes 11 during manufacture of the electrode 1 is also advantageous.

[0102] like Figure 3 As shown, the housing 4 includes a first opening 6 for introducing liquid into the reservoir when the electrode material is in contact with the subject. That is, the first opening 6 is positioned towards the side of the housing opposite to the side that contacts the skin of the subject when in use, and provides a channel to the reservoir from the outside. When the electrode is used, the eventual depletion of the liquid in the reservoir can lead to an increase in the electrode-skin contact impedance and a reduction in the quality of the measurement after a period of time. Additional liquid can be introduced directly into the reservoir through the opening 6, so that the liquid can be used to refill the electrode without having to disassemble the electrode or without having to remove the electrode from the subject's skin. After refilling, the electrode material can begin to hydrate continuously again, so that the electrode can exhibit a smaller electrode-skin contact impedance and produce high-quality results again. By providing the first opening 6, the reservoir can in principle be refilled an unlimited number of times, so that the volume capacity of the reservoir is not limited to the length of time, wherein, during this length of time, the electrode can produce high-quality measurements due to having a smaller electrode-skin contact impedance. When the housing 4 includes an internal part 41 and an external part 42 (such as Figure 13a and Figure 13bAs shown), the first opening 6 can specifically be communicated with the aforementioned lower area 21 of the reservoir 2.

[0103] Also like Figure 3 As shown, the housing 4 includes a second opening 7 for receiving a cable. The second opening may provide a safe and robust means for accommodating a cable for connecting the electrode to a power supply and / or a data logger or computer as discussed above.

[0104] Figure 4 Another alternative arrangement of the housing 4 is shown, in which the first opening 6 for introducing the liquid into the reservoir is placed on a flat surface of the housing 4 opposite the side that comes into contact with the skin of the subject during use. The advantages of the first opening 6 and the second opening 7 are as follows: Figure 3 discussed in relation to this. Figure 4 The housing arrangement in FIG. 1 is shown in its fully constructed form rather than in a disassembled form.

[0105] The precise shape of the housing is not particularly limited. From the perspective of ease of manufacture, the housing is formed into a generally cylindrical shape (e.g. Figure 3 、 Figure 4 and Figure 7 However, the housing can also be formed into any other three-dimensional shape.

[0106] Optionally, a cable is attached to the electrode 1 to make electrical contact with the electrode material 3 and form a biosensor for long-term, continuous or semi-continuous measurements in applications such as EEG, ECG or EMG.

[0107] The electrode 1 is manufactured by a method comprising at least the following steps: introducing a liquid into a reservoir 2, the liquid in the reservoir defining a liquid surface; and depositing an electrode material 3 on the liquid surface. The liquid in the reservoir is configured such that surface tension at the liquid surface 8 supports the electrode material 3 on the liquid surface 8.

[0108] Figure 5 A reservoir 2 of liquid is depicted defining a liquid surface 8 on which electrode material may be deposited to form an electrode of the invention (e.g. Figure 1 The electrode 1 is depicted. Figure 5The arrows in FIG indicate the deposition of the electrode material onto the liquid surface 8 . Surface tension at the liquid surface 8 supports the electrode material 3, preventing it from sinking or mixing into the liquid in the reservoir. Instead, the electrode material 3 remains on top of the liquid surface 8 and forms an electrode material layer 3 . In the final electrode 1 , this layer is a solid absorbent layer for the electrode material 3 , configured to absorb the liquid onto which the electrode material 3 is deposited. Advantageously, the liquid surface 8 provides a flat, level, and uniform surface for the deposition of the electrode material 3 . In this way, the electrode material 3 can be deposited uniformly in a highly controlled manner, resulting in a flat and uniform electrode material layer with minimal shape defects. Minimizing shape defects in the final electrode material layer helps ensure the accuracy and quality of measurements obtained from the electrode, with minimal noise.

[0109] In order to form a flat and uniform electrode material layer 3 on top of liquid surface 8, it is desirable to deposit the electrode material 3 on the liquid surface in a fluid form. For example, the electrode material 3 can be dissolved to form a solution of the electrode material 3, which is deposited on liquid surface 8 and subsequently dried to form a solid electrode layer. As an example, when the electrode material 3 is or includes a conductive polymer (e.g., PEDOT:PSS), the conductive polymer can be deposited in a solution, which is then dried to evaporate the solvent and leave a dried electrode material layer 3 that is or includes the conductive polymer. When the electrode material 3 is deposited in a solution, the solvent of the electrode material solution is not particularly limited and can be, for example, water. Depositing the electrode material 3 in a fluid form (e.g., in a solution) allows the fluid electrode material 3 to settle in a flat and uniform layer on top of liquid surface 8 before hardening, resulting in a final solid electrode material layer that is advantageously flat and uniform. When depositing the electrode material 3 in a fluid form, the electrode material 3 can be deposited by drop casting.

[0110] like Figure 5 As depicted, the reservoir 2 may be formed within a housing 4, which may be the same housing 4 as discussed elsewhere herein. For example, the housing 4 may be of Figure 3 、 Figure 4 、 Figure 7 or the form depicted in FIG13 . Figure 7 is an image depicting electrode material deposited in liquid form on the inner housing portion 41, similar to Figure 3 As depicted, the inner housing portion 41 forms part of the entire housing 4. The housing 4 may be designed to facilitate the method of manufacturing the electrode. For example, Figure 3 、 Figure 4 and Figure 7The depicted first opening 6 provides a passage through which liquid can be introduced into the reservoir 2 in the method. The liquid can be forced (e.g., pumped) through the first opening 6 while the housing 4 remains upright so that as the liquid enters the reservoir 2, it settles in the reservoir 2 to form a substantially flat liquid surface 8 (e.g., Figure 5 Regardless of the exact shape of the housing 4, it can preferably be made of a biocompatible polymer by additive manufacturing (e.g., 3D printing). This technique allows for precise adjustment of the housing's shape to optimize the subsequent introduction of the liquid into the reservoir 2 within the housing and the formation of a flat and uniform liquid surface 8 on which the electrode material 3 can be deposited.

[0111] Figure 7 The arrangement of the housing 4 shown (also in Figure 13a and Figure 13b 1 ) are particularly suitable for methods of manufacturing the electrode 1. These forms of housing include a housing inner portion 41 having one or more through-holes 11, and a housing outer portion 42. The housing inner portion 41 may include, for example, Figure 7 Seven through-holes 11 are depicted, however, the number of through-holes is not particularly limited. During the introduction of liquid into the reservoir, the liquid rises through the through-holes 11. The level of the resulting liquid surface 8 (wherein the electrode material 3 is deposited) depends on the volume of liquid introduced into the reservoir 2 before the electrode material 3 is deposited.

[0112] The amount of liquid introduced is the minimum amount sufficient to fill the lower region 21 of the reservoir below the housing inner portion 41 and to fill the space defined within the through-hole 11 in the housing inner portion 41. If only this minimum amount of liquid is introduced, the final liquid surface 8 is formed at the upper opening of the through-hole 11, flush with the upper surface of the housing inner portion 41. This is Figure 13a The arrangement depicted. The liquid surface 8 can then be formed by a plurality of separate liquid surfaces corresponding to the number of through-holes 11 in the housing interior 41. When the liquid surface 8 is formed at the level of the upper openings of the through-holes, the electrode material 3 is subsequently deposited on a flat surface formed by the liquid surface 8 (the liquid surface 8 may be a plurality of liquid surfaces corresponding to the plurality of through-holes) and the upper surface of the housing interior 41 flush with the openings of the aforementioned through-holes. The electrode material 3 is then supported by the surface tension of the housing interior 41 and the liquid surface 8.

[0113] In contrast, the amount of liquid introduced into the reservoir 2 may be sufficient to form a liquid surface 8 above the upper opening of the through-hole 11, so that there is a spatial separation between the housing interior portion 41 and the eventually formed electrode material layer 3. That is, the introduced liquid may wet the upper surface of the housing interior portion 41 so that the electrode material 3 is deposited on this wetted surface. This effectively corresponds to the liquid surface 8 being positioned slightly above the upper opening of the through-hole 11, which is Figure 13b In this case, the electrode material 3 rests on the liquid surface 8 separated from the upper surface of the housing interior 41 by a certain distance (e.g., a short distance), and is supported by the surface tension of the liquid. In this case, the housing interior 41 still plays a considerable role in supporting the electrode material in place.

[0114] The position of the final liquid surface 8 relative to the opening of the through hole is not particularly limited, and the degree of contact between the formed electrode material layer 3 and the inner portion 41 of the housing is not particularly limited, as long as the surface tension of the liquid surface 8 acts to support the electrode material 3 on the liquid surface 8 to a certain extent during the method of manufacturing the electrode. Figure 13a and Figure 13b The arrangements shown may be somewhere between the arrangements shown, for example, an arrangement in which the electrode material 3 contacts some portion of the upper surface of the housing interior portion 41 but is spatially separated from the housing near the opening of the through-hole 11. In any arrangement, the surface tension of the liquid surface 8 advantageously prevents the electrode material 3 from penetrating into the through-hole 11 and preventing the electrode material from mixing with the bulk liquid in the reservoir 2. Forming the electrode layer 3 on or near the housing interior portion 41 may be advantageous when forming one or more subsequent electrode layers as described herein (e.g., reference to FIG. Figure 6 ) before.

[0115] When the electrode material 3 is deposited on the surface of the liquid in a form that requires subsequent drying to form a solid electrode material layer 3 (e.g., deposited in the form of a solution), an additional step can be included in the method of manufacturing the electrode 1 to accelerate the drying. The step of solidifying the electrode material 3 on the surface of the liquid can effectively produce a solid electrode layer by evaporating any solvent present in the electrode material 3 in the form in which the electrode material 3 is deposited. For example, an aqueous solution of a conductive polymer can be solidified to evaporate all or most of the water in the aqueous solution so as to leave a dry conductive polymer layer that is then configured to absorb liquid in the reservoir 2 in contact with it. Therefore, preferably, the method of manufacturing the electrode 1 includes solidifying the electrode material 3 after the electrode material 3 is deposited in a fluid form (e.g., in the form of a liquid solution) on top of the liquid surface 8 and the electrode material 3 is supported on top of the liquid surface 8 by the surface tension of the liquid in the reservoir. This method produces a highly uniform electrode that also has the advantages described herein due to the subsequent absorption of liquid in the reservoir 2 by the electrode material 3.

[0116] It is desirable that the liquid in the reservoir itself not evaporate to any significant extent during the drying and hardening of the electrode material 3 after it has been deposited on the liquid surface 8. That is, the method for manufacturing the electrode may require evaporating the solvent in which the electrode material 3 has been deposited on the liquid surface 8 while preventing significant evaporation of the liquid in the reservoir 2. It is desirable to prevent significant evaporation of the liquid in the reservoir 2 because, if the liquid in the reservoir 2 evaporates significantly during manufacturing, the uniformity of the final electrode material layer 3 may be compromised. For example, evaporation of the liquid in the reservoir 2 may disrupt the uniformity of the liquid surface 8 on which the electrode material is supported. If the liquid in the reservoir 2 evaporates to some extent while the electrode material 3 is still soft or fluid, rather than completely rigid, the electrode material 3 may seep into pores or cavities in the reservoir 2. In this case, the liquid is no longer present before or during the hardening of the electrode material 3, and the desired uniform (e.g., flat) shape of the electrode material layer may not be achieved. Evaporation of the liquid in the reservoir 2 may also cause the height of the liquid surface 8 to change after the electrode material 3 is deposited, causing the electrode layer to not form at the desired height. The liquid surface 8 is intended to be formed flush with the upper opening of the through hole 11 (see Figure 13a ), evaporation of the liquid may cause the electrode material to enter the through-holes 11 before hardening, resulting in an undesirable, non-uniform shape of the final electrode material layer. These defects in the height and / or form of the electrode material layer caused by substantial evaporation of the liquid in the reservoir 2 during manufacturing may, in turn, result in a reduction in the quality or accuracy of measurements made by the electrode 1 when the electrode 1 is used after manufacturing.

[0117] It is particularly important to design the electrode 1 and the manufacturing method for producing the electrode 1 so as to prevent substantial evaporation of the liquid in the reservoir 2 when, as discussed above, the method involves a heat treatment step (e.g., curing) to enable the electrode material 3 to dry or to accelerate the drying of the electrode material 3 to form a solid layer. In this case, for example, if the liquid in the reservoir 2 is composed of the same composition as the solvent used to dissolve the electrode material 3, then substantial evaporation of the liquid from the reservoir 2 is unavoidable. In contrast, it is desirable that the liquid forming the liquid surface 8 in the reservoir 2 is composed so that the liquid does not evaporate significantly during the drying (e.g., curing) of the electrode material. This can be achieved by selecting a suitable liquid to be introduced into the reservoir 2 in the initial steps of the method. Therefore, preferably, the liquid in the reservoir 2 is composed so that the liquid does not evaporate significantly during the drying (e.g., curing) of the electrode material.

[0118] The tendency of the liquid not to evaporate significantly from the reservoir 2 during drying (e.g., curing) of the electrode material 3 can be defined based on the vapor pressure of the liquid. For example, the liquid introduced into the reservoir 2 can be configured to have a vapor pressure of less than 1 kPa, more preferably less than 900 Pa, more preferably less than 800 Pa, more preferably less than 700 Pa, more preferably less than 600 Pa, more preferably less than 500 Pa, more preferably less than 400 Pa, more preferably less than 300 Pa, more preferably less than 200 Pa, more preferably less than 100 Pa, more preferably less than 90 Pa, more preferably less than 80 Pa, more preferably less than 70 Pa, more preferably less than 60 Pa, more preferably less than 50 Pa, more preferably less than 40 Pa, and most preferably less than 30 Pa, with the vapor pressure being defined at a temperature of approximately 313 K. By comparison, the vapor pressure of water at this temperature is approximately 7.4 kPa, indicating a much higher tendency to evaporate.

[0119] Regarding the liquid to be introduced into the reservoir 2 and subsequently deposited with the electrode material, the inventors have determined that deep eutectic solvents and ionic liquids are particularly suitable because they generally have a lower tendency to evaporate than other liquids. As discussed herein, the lower vapor pressures of deep eutectic solvents and ionic liquids make them well-suited as liquids to be contained in the reservoir 2 of the electrode because, during use of the electrode 1, these liquids can continuously hydrate the electrode material 3 over an extended period by being absorbed into the electrode material 3, thereby providing the electrode 1 with a lower electrode-skin contact impedance over an extended period. The inventors have determined that these types of liquids also offer significant advantages during the manufacture of such electrodes. Specifically, the lower tendency of deep eutectic solvents and ionic liquids to evaporate, for example during the step of curing the electrode material 3, improves the final uniformity of the electrode material layer and the precision with which the electrode material layer can be formed. Furthermore, it has been found that deep eutectic solvents and ionic liquids form a liquid surface 8 having a significant surface tension, which is particularly suitable for depositing electrode material thereon. Therefore, deep eutectic solvents and ionic liquids have many advantages because they not only improve the performance of the electrode by continuously hydrating the electrode material 3 over a long period of time, but also greatly facilitate the fabrication of a uniform and reliable electrode material layer.

[0120] As discussed herein, deep eutectic solvents are particularly suitable as the liquid in reservoir 2 because they have a particularly low tendency to evaporate, particularly favorable viscosity and tendency to be absorbed into the electrode material, and are relatively low cost compared to ionic liquids. Deep eutectic solvents also form a particularly strong liquid surface 8 with surface tension that can stably support the deposition of electrode material (e.g., PEDOT:PSS solution) and prevent the electrode material from penetrating into the liquid reservoir 2. Therefore, a particularly preferred method for manufacturing an electrode 1 includes introducing a deep eutectic solvent into reservoir 2 and subsequently depositing an electrode material 3 (e.g., PEDOT:PSS in solution) onto the liquid surface 8 formed by the deep eutectic solvent. The resulting electrode 1 can have an advantageously uniform layer of electrode material 3 and also provide the various advantages associated with the continuous absorption of the deep eutectic solvent discussed herein.

[0121] After the electrode material 3 has been deposited on the liquid surface 8, it may be advantageous to deposit an additional amount of liquid (i.e., the same liquid introduced into the reservoir at the beginning of the method) on top of the deposited electrode material 3. The additional amount of liquid can be deposited by drop casting to achieve uniform dispersion. The electrode material 3 can be configured to absorb the liquid contained in the reservoir, thereby providing the advantages of reduced electrode-skin impedance discussed herein over a long period of time. In this case, depositing an additional amount of liquid on top of the electrode material 3 can increase the rate at which the entire electrode material layer 3 absorbs the liquid. That is, liquid can be absorbed into the electrode material 3 from the surface facing the reservoir 2 (i.e., absorbing liquid already in the reservoir 2) and from the outwardly facing surface of the reservoir (i.e., absorbing additional liquid deposited on the electrode material 3). This can advantageously improve the uniformity of the liquid saturation of the electrode material 3. In cases where the electrode material 3 is deposited on the liquid surface as a solution and subsequently dried (e.g., by curing), depositing an additional amount of liquid on the electrode material 3 can also fill spaces left by evaporation of the solvent in which the electrode material 3 was initially dissolved. For example, when an aqueous solution of the electrode material PEDOT:PSS has been deposited on the liquid surface 8 and subsequently solidified to dry the PEDOT:PSS, the additionally deposited liquid can replace the evaporated water. This can prevent the resulting electrode 1 from having pores or cavities in the electrode material layer 3, which could otherwise impair the quality of the measurements produced by the electrode 1. For the reasons discussed herein, the liquid in the reservoir 2 and additionally deposited on the electrode material 3 is preferably a deep eutectic solvent or an ionic liquid, most preferably a deep eutectic solvent.

[0122] The deposition of additional liquid on top of the electrode material 3 can occur after the electrode material 3 solidifies. Preferably, the deposition of additional liquid on top of the electrode material 3 occurs during the solidification of the electrode material. The deposition of additional liquid on top of the electrode material 3 can occur during and after the solidification of the electrode material 3. Depositing at least some of the additional amount of liquid on the electrode material 3 during the solidification of the electrode material can reduce the total manufacturing time of the electrode 1. In addition, if the electrode material 3 is deposited on the liquid surface 8 in the form of a solution, by depositing additional liquid on the electrode material 3 as the electrode material 3 solidifies, the solvent evaporated during the solidification can be immediately replaced by the additional liquid. This avoids the formation of voids left by the evaporated solvent for a considerable period of time. Therefore, the stability of the produced electrode 1 can be improved.

[0123] When the method includes depositing an additional liquid on the electrode material 3 , the method may further include a subsequent additional solidification step in which the electrode material 3 to which the additional liquid is added is solidified.

[0124] Figure 1The electrode 1 shown is an electrode according to one embodiment and also manufactured by methods according to other embodiments, which is depicted as having a single electrode material layer 3. However, the electrode 1 is not limited to having only a single electrode material layer 3, and the method of manufacturing the electrode is not limited to producing only a single electrode material layer 3.

[0125] When the electrode material 3 deposited on the liquid surface 8 has dried and hardened, the electrode material 3 may be referred to as a “first electrode layer.” It may be advantageous to provide one or more further layers of electrode material on top of the first electrode layer. Figure 6 An electrode 1 having a plurality of electrode layers is depicted. Figure 6 Three electrode layers are shown: Figure 1 Depicted electrode 1 shows a first electrode layer 3; a second electrode layer 9; and a third electrode layer 10. The electrode 1 can, in principle, be composed of any number of different electrode layers, including two electrode layers, or more than three electrode layers. A second electrode layer, different from the first electrode layer, can be formed by depositing additional electrode material in a fluid form (e.g., in a solution) on top of the already dried electrode material forming the first electrode layer, and then drying the newly deposited electrode material to form the second electrode layer. Drying to form the second electrode layer can be achieved by curing, which can occur after a curing step that facilitates or enables drying of the first electrode layer 3. Additional electrode layers (e.g., the third electrode layer 10 and any subsequent electrode layers) can similarly be formed by depositing and drying (e.g., curing) additional electrode material on top of the previously dried layer. For example, forming an electrode 1 having a total of six electrode layers has been found to be advantageous in maintaining a low electrode-skin contact impedance over an extended period of time.

[0126] Thus, a preferred method for manufacturing an electrode 1 having multiple electrode layers may include: forming a first electrode layer by introducing a liquid into a reservoir 2, depositing an electrode material 3 on a liquid surface 8 formed by the liquid, and subsequently drying (e.g., curing) the electrode material; and forming a second electrode layer 9 by depositing and drying (e.g., curing) an additional amount of electrode material on the first electrode layer. Alternatively, one or more additional electrode layers may be formed by sequentially depositing an additional amount of electrode material on the previous electrode layer and curing the additional amount of electrode material (e.g., a third electrode layer 10 may be formed on the second electrode layer 9).

[0127] In the case of forming multiple electrode layers in the electrode 1, any features or method steps described herein in relation to forming the initial electrode material layer may also be used to form subsequent layers. For example, after forming the multiple electrode layers, an additional amount of the same liquid present in the reservoir may be deposited on the electrode material of the formed layer. The amount of liquid forming this additional amount is not particularly limited. For example, after forming the multiple electrode layers, an additional amount of liquid may be deposited on the top layer in an amount determined based on a proportional relationship with the number of layers (for example, an electrode with four electrode layers may be subjected to twice as much additional liquid deposited on the top layer as an electrode with two electrode layers). Additionally or alternatively, an additional amount of the same liquid present in the reservoir may be deposited on each electrode material layer deposited to form the electrode. As an example, during or after the electrode material deposited on top of the already formed first electrode layer 3 solidifies to form the second electrode layer 9, the additional amount of liquid contained in the reservoir 2 may be deposited (e.g., drop-cast) on top of the electrode material forming the second electrode layer 9. This can provide similar advantages as described above in relation to the first electrode layer 3, in particular filling the voids left by the evaporated solvent to enhance the stability of the second electrode layer 9. Additional liquid can be deposited on each electrode layer, on none of the electrode layers, or on some of the electrode layers. Furthermore, for each electrode layer on which additional liquid is deposited to enhance stability, an additional curing step as described herein in relation to the first electrode layer can be employed.

[0128] The performance of the electrode 1 produced by the method described herein can be further improved by including a processing step involving immersing one or more layers of the electrode material in an immersion liquid, which is preferably water. Other immersion liquids that may be suitable are deep eutectic solvents or ionic liquids (for example, having the same composition as the liquid in the reservoir 2) and various other solutions. The immersion step can be carried out over a period of at least 12 hours, or more preferably at least 24 hours. The inventors have determined that immersing the electrode material in an immersion liquid such as water, particularly after the electrode material has solidified, can improve the long-term quality of the measurements produced by the electrode 1. Immersion in the immersion liquid helps the electrode material to release excess PSS (polystyrene sulfonate), which is insulating and acidic. This improves the conductivity and biocompatibility of the electrode. When as discussed herein and as Figure 6 When multiple layers of electrode material are formed as shown, each immersion step can occur after forming certain layers or each layer individually, or a single immersion step can occur after all layers have been formed. Preferably, the immersion step occurs after only the electrode layer of electrode material is added and allowed to solidify, and after the immersion, additional liquid (i.e., the "liquid" present in the reservoir) is added on top of the electrode layer and the liquid is solidified.

[0129] An exemplary process for forming an electrode is as follows: (1) drop-casting the electrode material to form a layer every 20 to 30 minutes and curing it for one hour after depositing the last layer; (2) immersing the cured layer in water for 24 hours; (3) additionally drop-casting the same liquid as the liquid contained in the reservoir (e.g., drop-casting 100 μL every 10 to 20 minutes) and curing it over a period of time proportional to the number of layers (e.g., approximately 50 μL of liquid per layer, so after forming 6 layers of electrode material, depositing 100 μL of liquid three times, each with a 20-minute interval, and curing the product for 4 hours).

[0130] As described herein, the electrodes 1 and methods of making the same according to various embodiments of the present invention provide various advantages. One aspect of the present invention can be considered to be a process that occurs within the electrode during use, namely a continuous hydration process of the electrode. As described herein, the continuous hydration occurs by contacting the absorbent electrode material with a liquid, the liquid being constituted so that when the electrode is in contact with the skin of the subject, the electrode-skin contact impedance remains less than 100 kΩ at a frequency of approximately 10 Hz (e.g., 10 Hz) for a period of at least 4 days (e.g., during which the electrode is continuously or periodically in contact with the subject). This method of continuously hydrating the electrode may include any of the features or any combination of features discussed herein in relation to other embodiments (e.g., embodiments of the electrode 1 itself or embodiments of the method of making the electrode 1).

[0131] Experimental data

[0132] Figure 8 A graph depicting how the electrode-skin contact impedance changes over time at 10 Hz for an electrode 1 of the present invention comprising a single PEDOT:PSS layer as the electrode material 3 and a deep eutectic solvent as the liquid in the reservoir 2, compared to a commercially available gel-assisted Ag / AgCl electrode (reference numeral 13), with the same skin contact area (i.e., the same diameter and shape of the electrode material). Figure 8 The results shown in FIG. 1 were obtained by contacting the electrode 1 of the present invention with the skin of the subject at time intervals associated with day 1, day 6, day 14, day 25, and day 30, respectively, within a total period of 30 days (i.e., a 30-day period during which the electrode was periodically contacted with the skin of the subject). A commercially available Ag / AgCl electrode was contacted with the skin of the subject at time intervals associated with day 1, day 6, and day 14. Figure 8As shown, the stability of the inventive electrode 1, comprising PEDOT:PSS and a deep eutectic solvent, was significantly improved compared to a commercially available electrode. Compared to a commercial Ag / AgCl electrode, the inventive electrode 1 exhibited significantly less impedance deviation within the first 14 days, with the impedance of the commercial Ag / AgCl electrode increasing by more than five-fold during this period. The inventive electrode 1 also continued to exhibit minimal impedance increase over the remainder of the 30-day period. It can be seen that the inventive electrode maintained a low impedance for 30 days after fabrication, while the impedance of the gel-assisted electrode rapidly increased after being exposed to the open air for more than one day.

[0133] Figure 9 Depicted are the EEG signals obtained from the same electrode 1 of the present invention 30 days after manufacture (reference numeral 14) and the EEG signals obtained from the same commercially available gel-assisted Ag / AgCl electrode directly after removing its protective film (reference numeral 15). Figure 9 As shown, the electrode 1 of the present invention still produces a higher quality and lower noise signal than the signal produced by a new commercial electrode even after 30 days from manufacture. The signal-to-noise ratio (SNR) observed for the electrode of the present invention is about 10 dB, while the SNR observed for the commercial electrode is about 7 dB.

[0134] Figure 10 Depicted is a comparison of electrode-skin impedance (reference numeral 12) obtained directly after manufacture of the same electrode 1 of the present invention and electrode-skin impedance (reference numeral 13) obtained directly after manufacture of the same commercially available gel-assisted Ag / AgCl electrode over a range of frequencies. As described herein, the electrodes and methods of the present invention are particularly useful in applications where the AC frequency is between 1 Hz and 50 Hz. Figure 10 It is shown that within this frequency range, even directly after manufacture, the electrodes of the present invention outperform commercially available electrodes in having a smaller electrode-skin impedance when in use.

[0135] Figure 11 The electrode-skin impedance of the electrode 1 of the present invention having multiple electrode layers (reference number 16 = 2 layers; reference number 17 = 6 layers) obtained directly after manufacture is compared with the electrode-skin impedance (reference number 13) obtained directly after manufacture of the same commercially available gel-assisted Ag / AgCl electrode over a frequency range. In the 2-layer electrode and 6-layer electrode of the present invention in this experiment, each electrode material layer included 100 μL of PEDT:PSS, and each electrode of the present invention was also immersed in water for more than 24 hours after manufacture. Figure 11 As shown, the multilayer electrode of the present invention also outperforms commercially available electrodes in terms of having a smaller electrode-skin impedance when used within the frequency range of interest (1 Hz to 50 Hz).

[0136] Figure 12 The electrode-skin impedance of the electrode 1 of the present invention is depicted over a frequency range. The electrode 1 has six PEDOT:PSS electrode layers (each layer includes 100 μL of electrode material) and a deep eutectic solvent reservoir, and has silver particles and latex added to the PEDOT:PSS electrode material. Figure 12 As shown, the additive reduces the impedance to approximately 10 kΩ in the frequency range of interest (1 Hz to 50 Hz).

[0137] Figure 14 Depicted is an ECG recording using the present invention's electrode 1, which features PEDOT:PSS electrode material and a deep eutectic solvent reservoir. Electrode 1 and a reference Ag / AgCl electrode were placed on the patient's chest. In this experiment, the notch filter was set to 50 Hz, and the bandpass filter was set from 0.5 Hz to 120 Hz.

[0138] FIG15 depicts an EEG recording using the electrode 1 of the present invention having a PEDOT:PSS electrode material and a deep eutectic solvent reservoir. Figure 15a As depicted, electrode 1 and reference Ag / AgCl electrode 18 are placed on the patient's forehead. Figure 15b Depicted is an 11-second resting-state EEG recorded using electrode 1 and the associated power spectral density (PSD) plot. The two states (eyes open and eyes closed) are separated by an electrooculography (EOG) event (blink) indicating eye closure. The PSD shows a characteristic alpha peak (power of 10 dB) at 10 Hz in the eyes closed state. In the eyes open state, the associated alpha peak is not obvious.

[0139] Figure 16 Depicted are time-spectrograms of alpha-band modulation recordings from the same EEG experiment represented by FIG. 15 . Figure 16 The alpha waves that appear in the eyes-closed state are shown.

[0140] The devices and methods described herein are applicable to EEG, ECG, EMG, and similar techniques. However, devices having substantially the same arrangement may also be configured to be used as electrodes in a variety of other applications, as will be apparent to those skilled in the art. Any combination of the features and method steps described herein may be performed without departing from the scope of the present invention.

Claims

1. An electrode comprising: a reservoir containing a liquid; as well as an electrode material in contact with the reservoir and configured to absorb the liquid, wherein the liquid in the reservoir is configured to maintain an electrode-skin contact impedance of less than 100 kΩ at a frequency of approximately 10 Hz for a period of at least 4 days.

2. The electrode according to claim 1, wherein: The liquid in the reservoir is configured to maintain an electrode-skin contact impedance of less than 100 kΩ at a frequency of about 10 Hz for a period of at least 5 days, preferably at least 10 days, preferably at least 20 days, preferably at least 30 days; and / or The liquid in the reservoir is configured to maintain electrode-skin contact impedance at a frequency of about 10 Hz of less than 90 kΩ, preferably less than 80 kΩ, preferably less than 70 kΩ, preferably less than 60 kΩ for a period of at least 4 days.

3. The electrode according to claim 1 or claim 2, wherein The electrode material comprises a conductive polymer, preferably wherein The electrode material comprises PEDOT:PSS, and further preferably wherein: The electrode material additionally comprises conductive metal particles, such as silver particles; and / or The electrode material additionally includes natural rubber, such as latex.

4. An electrode according to any one of the preceding claims, wherein The liquid comprises a deep eutectic solvent or an ionic liquid, preferably wherein The liquid includes the deep eutectic solvent.

5. The electrode according to any one of the preceding claims, further comprising a housing for accommodating the reservoir, the housing being formed from a biocompatible polymer, preferably wherein the housing comprises at least one of the following: an opening for introducing the liquid into the reservoir when the electrode material is in contact with a subject; and Opening to receive cables.

6. A biosensor comprising: An electrode according to any one of the preceding claims; as well as A cable is in electrical contact with the electrode material.

7. A method for manufacturing an electrode, the method comprising: introducing a liquid into a reservoir, said liquid in said reservoir defining a liquid surface; as well as depositing electrode material on the surface of the liquid, The liquid is configured such that the surface tension of the liquid supports the electrode material on the liquid.

8. The method according to claim 7, further comprising: solidifying the electrode material on the surface of the liquid to form a first electrode layer, Preferably, The liquid is configured such that the liquid does not significantly evaporate during curing of the electrode material; and / or At a temperature of about 313 K, the liquid has a vapor pressure of less than 1 kPa, more preferably less than 500 Pa, more preferably less than 200 Pa, more preferably less than 100 Pa, more preferably less than 50 Pa, more preferably less than 30 Pa.

9. The method according to claim 7 or claim 8, further comprising: depositing an additional amount of liquid on said electrode material, Preferably, The additional amount of liquid is deposited by drop casting.

10. The method according to claim 9, wherein: depositing the additional amount of liquid on the electrode material during solidification of the electrode material; and / or The method includes solidifying the additional amount of liquid deposited on the electrode material.

11. The method according to claim 8, comprising: A plurality of electrode layers including the first electrode layer and the second electrode layer are formed, wherein: forming the second electrode layer by depositing an additional amount of electrode material on the first electrode layer and curing the additional amount of electrode material; and Optionally, one or more further electrode layers are formed by sequentially depositing additional amounts of electrode material on a previous electrode layer and curing the additional amounts of electrode material.

12. The method according to claim 11, further comprising: depositing an additional amount of liquid on the electrode material forming the electrode layers, the additional amount being proportional to the number of the electrode layers, preferably wherein: depositing the additional amount of liquid while the electrode material solidifies; and / or The additional amount of liquid solidifies after being deposited on the electrode material.

13. The method according to any one of claims 8 to 12, further comprising: The cured electrode material is immersed in an immersion liquid, preferably water, for at least 12 hours.

14. The method according to any one of claims 7 to 13, comprising: A housing is formed by additive manufacturing, the housing housing the reservoir, preferably wherein the housing is formed from a biocompatible polymer.

15. A method for continuously hydrating an electrode, comprising: bringing the absorbent electrode material of the electrode into continuous contact with the liquid, wherein the liquid in the reservoir is configured to maintain an electrode-skin contact impedance of less than 100 kΩ at a frequency of approximately 10 Hz for a period of at least 4 days.

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