WO2019125156A1

WO2019125156A1 - Bioelectrical sensing electrode assembly

Info

Publication number: WO2019125156A1
Application number: PCT/NL2018/050860
Authority: WO
Inventors: Petrus Wilhelmus Maria Desain
Original assignee: Mindaffect B.V.
Priority date: 2017-12-20
Filing date: 2018-12-20
Publication date: 2019-06-27
Also published as: NL2020131B1

Abstract

An electrode assembly for bioelectrical measurements, such as EEG measurements. A bipolar electrode arrangement (2) has a centre electrode (4) and a surround electrode (6) positioned electrically isolated from and at a predetermined distance from a boundary of the centre electrode (4). The centre electrode (4) and surround electrode (6) span a sensing area (5) corresponding to a predetermined area of interest for the bioelectrical measurement.

Description

Bioelectrical sensing electrode assembly Field of the invention

The present invention relates to an electrode assembly for taking bioelectrical measurements, such as an electrode assembly for use in electroencephalography (EEG).

Background art

International patent application WO2016/210407 A1 discloses a system and method of taking biopotential measurements, with the ability to perform in high-density sensing applications. The invention is a hierarchical referencing method for the electrodes in the biopotential measurement system that is able to recover potential at each location with respect to a global reference with smaller requirements on ADC resolution, and thus with lower power and area requirements as compared to current systems.

US patent application US201 1/054288 A1 discloses a biomedical sensor that provides at least first and second electrical nodes for connection to medical equipment, such as an EEG registration system. The biomedical sensor includes protruding electrodes that extend from a common base. At least one of the protruding electrodes is coupled to the first electrical node, and at least two of the protruding electrodes are coupled to the second electrical node. The protruding electrodes may be adjustable in length such that each of the plurality of protruding electrodes is adapted to contact a non-planar surface of a subject.

The article by G. Prats-Boluda et al, ‘Active flexible concentric ring electrode for non- invasive surface bioelectrical recordings’, Measurement Science and Technology, 23 (2012) 125703, discloses a modular active sensor made up of concentric ring electrodes printed on a flexible substrate using thick-film technology, in combination with a signal conditioning unit. Summary of the invention

The present invention seeks to provide an improved electrode assembly for taking bioelectrical measurements, wherein the electrode assembly greatly reduces the number and complexity of signals to be processed whilst allowing for accurate measurements with a minimum of noise.

According to the present invention, an electrode assembly of the type defined in the preamble is provided, comprising a bipolar electrode arrangement having a centre electrode and a surround electrode positioned electrically isolated from and at a predetermined distance from a boundary of the centre electrode, wherein the centre electrode and surround electrode span a sensing area corresponding to a predetermined area of interest for the bioelectrical measurement. Furthermore, the bioelectrical measurement is an EEG measurement, and the sensing area spans across a plurality of EEG electrode locations.

The bipolar electrode arrangement can be seen as a single bipolar pair of electrodes comprising the centre electrode and the surround electrode, wherein this single bipolar pair completely spans a predetermined area of interest. The bipolar electrode arrangement not only minimizes the number of signals to be processed but it also reduces the number of physical channels of the electrode assembly to be connected, filtered, amplified etc. to two channels or even to a single bipolar channel. The electrode assembly is therefore of simple design without sacrificing the ability to accurately measure bioelectrical signal across the entire predetermined area of interest.

The EEG electrode locations may be electrode locations of the known 10-20 system or locations corresponding to higher resolution systems such as the Modified Combinatorial Nomenclature MCN system (10-10), or even the 5% system (10-5 system). This embodiment allows for a single bipolar pair of electrodes to be used for sensing electrical signals over a plurality of EEG locations, thereby reducing the number of physical channels to be filtered, amplified etc. to a single channel. This greatly simplifies the analysis of the measured signal.

In a further advantageous embodiment, the surround electrode is a ring shaped electrode with a non-conductive part, so that a loop current through the surround electrode is prevented. The surround electrode may be formed as an open ring, thus wherein the surround electrode is a ring shaped electrode having two opposing ends with a non-conductive gap there between. In specific embodiments the non-conductive gap may be filled with non-conductive material to prevent loop currents, but it may also be implemented as an air gap.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which

Figure 1 shows a plan view of a bipolar electrode arrangement according to an embodiment of the present invention;

Figure 2 shows a schematic view of an electrode assembly according to an embodiment of the present invention;

Figure 3 shows a cross section of an electrode assembly according to an embodiment of the present invention;

Figure 4 shows a three dimensional view of an electrode assembly according to an embodiment of the present invention; and

Figure 5 shows a three dimensional view of an electrode assembly in the form of a headset according to an embodiment of the present invention.

Detailed description of embodiments

Figure 1 shows a cross section of a bipolar electrode arrangement 2 according to an embodiment of the present invention, and Figure 2 shows a schematic view of an electrode assembly 1 comprising the bipolar electrode arrangement 2 according to an embodiment of the present invention. The electrode assembly 1 as depicted is configured to take bioelectrical measurements such as electroencephalography (EEG) measurements and comprises a bipolar electrode arrangement 2 in the form of a centre electrode 4 and a surround electrode 6 positioned electrically isolated from and at a predetermined distance S from a boundary of the centre electrode 4. The centre electrode 4 and surround electrode 6 are configured and shaped to span a sensing area that corresponds to a predetermined area of interest for taking bioelectrical measurement. So given a predetermined area across which bioelectrical measurements are to be taken, the centre electrode 4 and surround electrode 6 are configured to span at least a sensing area as large as the predetermined area. The predetermined area may, for example, comprise an area of a person’s skin or scalp.

According to the present invention, the bipolar electrode arrangement 2 can be seen as a single bipolar pair of electrodes comprising the centre electrode 4 and the surround electrode 6. This single bipolar pair 4, 6 may then be seen as a single transducer for taking measurements, wherein this transducer is configured to completely span the predetermined area of interest. As a result, there is no need to utilize a large number of separate bipolar pair of electrodes placed across the predetermined area as in prior art sensing arrangements. Furthermore, having a single bipolar pair of electrodes 4, 6 that allows the predetermined area to be measured as a whole, reduces the number of physical channels of the electrode assembly 1 to be connected, amplified, filtered etc. to a single channel. In exemplary embodiment the single bipolar pair of the centre electrode 4 and the surround electrode 6 may span a sensing area with an average diameter of about 2 cm to 10 cm (or even larger if necessary).

Known bioelectrical measurement techniques often utilize a large number of bipolar electrode pairs arranged across an area of interest (e.g. the entire skull area for EEG measurements), wherein each pair of electrodes is connected to a (differential) amplifier providing an associated measurement channel. Signals from these various channels will then need to be amplified, filtered, weighted etc. for further analysis. The electrode assembly 1 of present invention on the other hand uses a single bipolar pair of electrodes 4, 6, which merely provide a single physical channel to simplify the signal analysis.

Furthermore, since with the present invention embodiments there is no need to determine whether singular electrode pairs merely provide noise signals, the electrode assembly 1 of the present invention is less susceptible to noise and as such allows for reliable and high quality bioelectrical measurements.

As shown in Figure 2, the electrode assembly 1 comprises the bipolar electrode arrangement 2 as a single bipolar pair of the centre and surround electrodes 4, 6, which are connected to an amplifier 12. In an embodiment, the amplifier 12 may be a differential amplifier 12 connected to the centre electrode 4 and the surround electrode 6. The amplifier 12 provides a single channel signal C to a processing unit 14 which is adapted to analyse signals as provided by the amplifier 12 through the channel signal C. From Figure 2 it is immediately evident that in contrast to known bioelectrical measurement systems, the electrode assembly 1 of the present invention exhibits a considerably less complex design as there is only a single channel signal C to be processed.

In an advantageous embodiment, the bioelectrical measurement for which the electrode assembly 1 is operatively used is an EEG measurement. For example, EEG measurements with regard to Broad-Band Visually Evoked Potentials (BBVEP), Code-modulated Visual Evoked Potential (cVEP) and Steady State Visually Evoked Potential (SSVEP) are possible. It has been found that the electrical EEG signal activity for these type of measurements are mainly present in a restricted number of the regular EEG electrode channels, grouped in a specific area of interest associated with visual processing in the brain. Furthermore, measurements related to EEG sensorimotor measurements can be performed with the electrode assembly 1 as well, using the specific area of interest associated with sensorimotor signal processing in the brain. Since the bipolar electrode arrangement 2 just provides a single channel yet allows a sufficiently large predetermined area to be spanned, this provides a clear advantage over known EEG measurement systems.

In an exemplary embodiment, the sensing area of the bipolar electrode arrangement 2 may span across a plurality of EEG electrode locations. In this embodiment the EEG electrode locations may be electrode locations of the known 10-20 system or electrode locations corresponding to even higher resolution systems such as the Modified Combinatorial Nomenclature MCN system (10-10), or even the 5% system (10-5 system). Therefore, this embodiment allows the bipolar electrode arrangement 2 to simultaneously span a plurality of EEG locations so that the number of physical channels to be amplified, filtered etc. is reduced. For example, the centre and surround electrodes 4, 6 may simultaneously span both the 01 and 02 positions for measuring visual responses. Alternatively, the centre and surround electrodes 4, 6 may simultaneously span the C3 and C4 positions for measuring motor responses. The sensing area of the bipolar arrangement 2 may have a circular shape, and the centre electrode 4 is then a disc shaped electrode around which the surround electrode 6 and optional ground electrode 8 are positioned in a circular configuration as well. Alternatively, the sensing area may have a different shape, for which the shape of the centre electrode 4 is then adapted, and around which the surround and ground electrode 6, 8 may be positioned. This may even result in an embodiment with two centre electrodes 4 (e.g. having a disc shape) around which the surround electrode 6 is formed in an eight-pattern (two touching rings).

Referring to Figure 1 , in an embodiment the surround electrode 6 may be a ring shaped electrode with a non-conductive part 7, so that loop currents through the surround electrode 6 can be avoided. The ring shaped electrode 6 may be seen as an open ring encircling the centre electrode 4 at a predetermined distance S from an outer boundary of the centre electrode 4. In particular, in this embodiment an annular gap of width S is located between the surround electrode 6 and the centre electrode 4. The open ring comprises opposing ends 6a, 6b with a gap there between. In a group of embodiments the gap between the opposing ends 6a, 6b may be formed by an electrically insulating material provided between the opposing ends 6a, 6b to eliminate loop currents, but the gap may also be implemented as an air gap. In alternative embodiments the surround electrode 6 has a ring like shape, e.g. is formed as an oval shape, as a rectangular shape, or as a horseshoe shape.

In an embodiment, the bipolar electrode arrangement 2 of the present invention may further comprise a ground electrode 8 positioned electrically isolated from and at a predetermined distance Tfrom a boundary of the surround electrode 6. The ground electrode 8 is advantageous in allowing to ground the test subject, yielding a better measurement signal (as no large offset voltage can be present). In this case the ground electrode 8 is in conductive contact with the person’s skin or scalp. In addition, or alternatively, the ground electrode 8 may be advantageous for shielding measurements taken by the centre and surround electrode 4, 6 from outside interference, which otherwise might cause noise components in measured signals.

In an exemplary embodiment, the ground electrode 8 may be a closed ring encircling the centre and surround electrode 4, 6 to allow proper grounding and/or maximum shielding from outside interference, which further minimizes noise components in the measures signals. The ground electrode 8 also can aid in defining the sensing area of the electrode arrangement 2 as being limited to the surface area spanned by the centre electrode 4 and surround electrode 6, i.e. within the inner boundary of ground electrode 8.

When used as a shielding element, the ground electrode 8 may even span over the entire electrode arrangement 2, e.g. as a disc-like shaped or even dome shaped electrode.

Making good contact with a predetermined area to be measured by the electrode arrangement 2 improves signal strength and increases signal to noise ratio. To allow good contact and snug engagement to occur, the bipolar electrode arrangement 2 may comprise a contacting or engagement surface with a predetermined shape. That is, in this embodiment the contacting surface of the bipolar electrode arrangement 2 is specifically adapted to provide a contact/engagement surface that provides snug contact with the predetermined area, such as snug contact with a predetermined skin/scalp area.

In an exemplary embodiment, the bipolar electrode arrangement 2 comprise a contact or engagement surface configured to engage a scalp region associated with the predetermined area. In this embodiment the bipolar electrode arrangement 2 is specifically configured so that the scalp is snugly engaged by the centre and surround electrodes 4, 6 to ensure optimum contact for maximum signal strength and signal to noise ratio. Of course, other embodiments are conceivable wherein the bipolar electrode arrangement 2 comprises a contact or engagement surface adapted to snugly engage various skin areas (e.g. neck, chest, back, arms, legs etc.), such that the surface of the bipolar electrode arrangement 2 tightly envelopes the local skin area.

In an advantageous embodiment, the bipolar electrode arrangement 2 comprises dry electrodes, i.e. a dry centre and dry surround electrodes 4, 6. This embodiment makes the measurement process more convenient as it does not require an additional agent such as a gel for establishing good contact between the bipolar electrode arrangement 2 and the predetermined area. In particular, having a dry bipolar electrode arrangement 2 is advantageous for making good contact over a large skin area, such as large areas of the scalp covering a plurality of EEG electrode locations.

Figure 3 shows a cross section of an electrode assembly 1 according to an embodiment of the present invention. In this embodiment, snug engagement with the shape of the predetermined area may be facilitated by having a bipolar electrode arrangement 2 with multi-contact electrodes 9, wherein the multi-contact electrodes 9 allow good contact for various curvatures of the skin through flexible adaptation of the multi-contact electrodes 9. The multi-contact electrodes 9 are e.g. implemented as multi-finger electrodes.

In an exemplary embodiment the multi-contact electrodes 9 comprises a multi-finger electrode having fingers which are flexible and resilient, so that through (mild) deformation of the multi-finger electrodes 9 snug engagement of the bipolar electrode arrangement 2 with the predetermined area is achieved. Moreover, the multi-finger electrodes 9 regain their original form when the electrode assembly is disengaged, so that new measurements can be taken for other parts of the skin having different curvatures. Note that the multi-finger electrodes 9 also allow flexible and resilient engagement to prevent uncomfortable pressure points in sensitive areas of the skin. In an exemplary embodiment, the multi-finger electrodes 9 are implemented as hollow finger elements, which would allow to control flexibility of the multi-finger electrodes 9 adaptively, e.g. using air pressure control in (one or all) hollow finger elements.

The multi-finger electrodes 9 as depicted in Figure 3 may be seen as a bipolar electrode arrangement 2 having a base portion provided with a plurality of fingers or spikes projecting therefrom. The multi-finger electrodes 9 comprise both the centre electrode 4 as well as the surround electrode 6, both of which are multi-finger electrodes themselves. The multi-finger electrodes 9 may further comprise the ground electrode 8, surrounding the surround electrode 6.

By providing the centre and surround electrodes 4, 6, and optionally the ground electrode 8 as multi finger electrodes 9, allows the bipolar electrode arrangement 2 to be shaped as a comfortable electrode pad for snug engagement with a relatively large predetermined skin area, such as skin areas spanning from e.g. 2 cm to 10 cm in average diameter or even larger.

In an embodiment, the multi-contact (or multi-finger) electrodes 9 comprise a conducting flexible material, such as a conductive rubber or plastic material. This allows adaptation of the bipolar electrode arrangement to the shape of the predetermined area and provide good electrical conductivity. It is also possible that the multi-finger electrodes 9 comprise a sponge like material configured for being wetted to enable good conductivity.

Referring back to the embodiment shown in Figure 1 , the bipolar electrode arrangement 2 comprises non-conductive material in areas outside of the multi-contact (or multi-finger) electrodes 9, so that the centre and surround electrode 4, 6, are properly isolated from one another. As depicted the surround electrode 6 is shaped as a ring encircling the centre electrode 4, wherein both the centre and surround electrodes 4, 6 may be seen as multi-finger electrodes 9 spaced apart by an annular gap of width S. This annular gap may then be provided with a non-conductive material. Note that in an embodiment the ground electrode 8 may also be provided as a ring shaped multi-finger electrode encircling the surround electrode 6. In this embodiment the surround and ground electrode 6, 8 are spaced apart by an annular gap of width T. This annular gap may likewise be provided with a non-conductive material.

Returning to Figure 3, in an embodiment the electrode assembly 1 may further comprise an assembly housing 10 in which the amplifier 12 is accommodated, such as a differential amplifier. This embodiment allows for a compact integrated electrode assembly 1 , wherein the electrode assembly 1 conveniently houses the amplifier 12 which is effectively connected to the centre and surround electrodes 4, 6 and optionally the ground electrode 8. No separate signal amplification hardware is needed for connection to the bipolar electrode assembly 2.

In a further embodiment the electrode assembly 1 may comprise a processing unit 14 connected to a differential amplifier 12 as mentioned earlier, but may also further comprise a communication interface 16 and a power source 17 connected to the processing unit 14. This provides a completely integrated electrode assembly 1 requiring even less external hardware for taking bioelectrical measurements. The housing assembly 10 as mentioned earlier may, in addition, accommodate not only the amplifier 12 but also the processing unit 14, the communication interface 16 as well as the power source 17. In an advantageous embodiment, the power source 17 comprises a battery, so that a fully mobile electrode assembly 1 is obtained that can be used to take bioelectrical measurements at various remote locations requiring a minimum of external hardware. In a further embodiment the power source 17 may comprise a rechargeable battery and a charging unit 18, so that portability of the electrode assembly 1 is improved as recharging is possible whenever required. In an even further embodiment the charging unit 18 may be a wireless recharging unit, allowing the electrode assembly 1 to be conveniently charged without any cables, e.g. using induction.

To exchange data with the electrode assembly 1 , an embodiment is provided wherein the communication interface 16 comprises a wireless data communication interface, e.g. Bluetooth. In this way the portability and usability of the electrode assembly 1 is greatly improved, where, in combination with a rechargeable battery, the electrode assembly 1 can be used as a fully portable device for taking bioelectric measurements.

With reference to Figures 4 and 5, in each of these figures a three dimensional view of an electrode assembly 1 according to an exemplary embodiment of the present invention is depicted that can be used for relatively large sensing areas. For example, Figure 4 depicts an embodiment of the electrode assembly 1 as a substantial round or circular measuring pad comprising a bipolar electrode arrangement 2 provided with multi-finger electrode 9 implementing the centre, surround and optional ground electrodes 4, 6, 8. As mentioned earlier, the multi-finger electrodes 9 may be flexible and resilient to ensure proper and snug engagement with a predetermined skin area and to ensure maximum signal strength and signal to noise ratios.

In an embodiment, the bipolar electrode arrangement 2 may be formed as an integrated part, wherein the centre and surround electrodes 4, 6, and optionally the ground electrode 8, may be manufactured through an additive manufacturing process, for example. This will then allow materials to be deposited with conductive or non-conductivity properties to determine as to whether an electrode or a non-conductive area is obtained. Therefore, for the centre and surround electrodes 4, 6 a conductive material can be deposited whilst a non-conductive material can be deposited to obtain the non-conductive areas, such as depositing non-conductive material in the annular gaps of width S and 7 as depicted in Figure 1 , or in the gap between the opposing ends 6a, 6b of the surround electrode 6. Additionally or alternatively, the conductivity ofthe material used may even vary within areas defined by the centre, surround and/or optional ground electrode 4, 6, 8, allowing adaptation of the sensitivity pattern of each electrode 4, 6, 8. Note that in an embodiment the pad-like electrode assembly 1 as shown in Figure 4 can be provided with the housing assembly 10 for accommodating the (differential) amplifier 12, and optionally as well the processing unit 14, the communication interface 16, the power source/battery 17 as well as the charging unit 18. This pad-like electrode assembly 1 is then fully portable, easy to use and ergonomically configured for making good contact and ensure snug engagement with skin areas for optimal measured signal strength and signal to noise ratios.

As an extension of the pad-like electrode assembly 1 as depicted in Figure 4, Figure 5 shows an exemplary embodiment of the electrode assembly 1 configured as a wearable item which functions as a wearable electrodes support, such as a headband or headset 20. In this embodiment, the bipolar electrode arrangement 2 is likewise provided with multi-finger electrodes 9, wherein the centre and surround electrodes 4, 6, and optionally the ground electrode 8, each comprise multifinger electrodes 9. Furthermore, the electrode assembly 1 may be further provided with releasable straps 22 such that the electrode assembly 1 can be conveniently affixed to a predetermined area of interest, e.g. neck, arm, leg, head etc. In an embodiment, the housing assembly 10 may be provided as a functional item, such as the releasable straps 22.

The electrode assembly 1 of the present invention is likewise suitable for being embedded in a functional item such as a helmet, pillow, headrest etc., wherein the bipolar electrode arrangement 1 provides a large sensing area, e.g. 2 cm to 10 cm or larger, spanned by the centre and surround electrodes 4, 6.

Instead of providing the electrode assembly 1 as a wearable item, it is conceivable that the electrode assembly 1 can be implanted subcutaneously. That is, in an embodiment the centre electrode 4 may be envisaged as being formed by a first ring wire, and wherein the surround electrode 6 is provided as a second ring wire encircling the centre electrode 4. This embodiment of the electrode arrangement 2 can be positioned subcutaneously, e.g. below the scalp, to further increase measured signal strength and signal to noise ratios. This is especially advantageous when the electrode arrangement 2 has to be used during prolonged time periods. As in the embodiments described above, it is also possible to position one or more components of the electrode assembly 1 subcutaneously. Alternatively, a similar electrode configuration with a first and second ring wire may also allow use of the electrode arrangement 2 being positioned externally, and glued onto the skin/hair of the person, e.g. using a collodion type of glue agent. In both embodiments, the centre electrode 4 may also be implemented as a solid conductive disc.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

1 . An electrode assembly for bioelectrical measurements, comprising a bipolar electrode arrangement (2) having a centre electrode (4) and a surround electrode (6) positioned electrically isolated from and at a predetermined distance from a boundary of the centre electrode (4), wherein the centre electrode (4) and surround electrode (6) span a sensing area (5) corresponding to a predetermined area of interest for the bioelectrical measurement, wherein the bioelectrical measurement is an EEG measurement, and wherein the sensing area (5) spans across a plurality of EEG electrode locations.

2. The electrode assembly according to claim 1 , wherein the surround electrode (6) is a ring shaped electrode with a non-conductive part (7).

3. The electrode assembly according to claims 1 or 2, wherein the bipolar electrode arrangement (2) further comprises a ground electrode (8) positioned electrically isolated from and at a predetermined distance from a boundary of the surround electrode (6).

4. The electrode assembly according to any one of claims 1 -3, wherein the bipolar electrode arrangement (2) has a contacting surface with a predetermined shape.

5. The electrode assembly according to any one of claims 1 -4, wherein the bipolar electrode arrangement (2) comprises dry electrodes.

6. The electrode assembly according to any one of claims 1 -5, wherein the bipolar electrode arrangement (2) comprises multi-contact electrodes (9).

7. The electrode assembly according to claim 6, wherein the multi-contact electrodes (9) comprise a conducting flexible material.

8. The electrode assembly according to claim 6 or 7, wherein the bipolar electrode arrangement (2) comprises non-conductive material in areas outside of the multi-contact electrodes (9).

9. The electrode assembly according to any one of claims 1 -8, wherein the bipolar electrode arrangement (2) forms an integrated part.

10. The electrode assembly according to any one of claims 1 -9, further comprising a differential amplifier (12) connected to the centre electrode (4) and the surround electrode (6).

1 1 . The electrode assembly according to claim 10, further comprising an assembly housing (10) in which the electrode assembly (1) and differential amplifier (12) are accommodated.

12. The electrode assembly according to claim 11 , wherein the assembly housing (10) is part of a functional item.

13. The electrode assembly according to any one of claims 10-12, further comprising a processing unit (14) connected to the differential amplifier (12), a power source (15) connected to the processing unit (14), and a communication interface (16).

14. The electrode assembly according to claim 13, wherein the power source (15) comprises a battery (17) and a charging unit (18).

15. The electrode assembly according to claim 14, wherein the communication interface (16) comprises a wireless data communication interface.