WO2017101977A1

WO2017101977A1 - An eeg monitor with an implantable part

Info

Publication number: WO2017101977A1
Application number: PCT/EP2015/079714
Authority: WO
Inventors: Rasmus Stig Jensen; Richard Topholm; Rasmus Elsborg Madsen; Erik Skov CHRISTENSEN
Original assignee: T&W Engineering A/S
Priority date: 2015-12-15
Filing date: 2015-12-15
Publication date: 2017-06-22

Abstract

An EEG monitor (2) having an implantable part (3) with EEG electrodes (7) and a non- implantable external part (5) comprising a power supply (25). The implantable part is adapted for arrangement at the head of a person, and is wirelessly connected to the external part through an inductive link (6) for transfer of EEG related data. The inductive link comprises an internal coil (10) connected with the implantable part (3) and an external coil (11) connected with the external part (5). The internal coil extends in a first plane and the external coil is adapted to be arranged such that it extends in a second plane, where an angle between the first and the second plane is less than 45 degrees when the EEG monitor is in use. An area defined by the internal coil (10) is larger than an area defined by the external coil (11), and at least 25 % of the area spanned by the external coil overlaps the area spanned by the internal coil.

Description

An EEG monitor with an implantable part

The present invention relates to an EEG monitor with an implantable part having at least two EEG electrodes and a non-implantable external part having a power supply. The EEG monitor is adapted for arrangement at the head of a person. The implantable part is wirelessly connected to the external part through an inductive link for transfer of EEG related data. The inductive link comprises an internal coil connected with the implantable part and an external coil connected with the external part. Electroencephalogram (EEG) signals are electrical signals generated by a person's brain activity. In recent years, EEG monitoring systems, which may be carried or worn continuously by a person to be monitored, have been devised. A goal is to have personal wearable EEG monitors, which can be carried over an extended interval of time, e.g. several months or years.

Such EEG monitors may be applied for purposes of surveillance of a condition of the person and for providing some kind of alarm or information in case predetermined conditions are met. The monitor may also be applied for collection of data for further analysis, e.g. for diagnostic purposes or for research use. An example of an application is for surveillance of persons having diabetes. An EEG monitor may also be part of another apparatus.

In such an EEG monitor, it is important to have a reliable detection of the EEG signal for an extended interval of time. This can be achieved by having the EEG electrodes implanted subcutaneously at the head of a person. This necessitates that the EEG signal, or features of the EEG signal, is transferred to an external device. This can advantageously be done through an inductive link, which means that a coil preferably should be arranged at the skin surface or close to the skin surface, i.e. within few millimeters from the place where an implanted coil is arranged below the skin surface.

Wearing this external coil at the head will often be of inconvenience to the person to be monitored. This is also the case when the external coil is arranged in a hearing aid like housing behind the ear, because the size of the coil will influence the size of the housing.

There has been a prejudice that the implanted coil should be as small as possible with regards to facilitating an easy implantation of the implant. However, it has now been found that there is no significant difference concerning the implantation process, and that the long-term stability in detecting a stable and reliable EEG signal is often the most important factor. The problem has therefore been to find an EEG monitor with this long-term stability in the detection of an EEG signal. Since it has also been found that the comfort for the person in wearing the external part has significant influence on this long-term stability, the size of the external part of the EEG monitor is a very important parameter. Furthermore, it has now been found that the smaller the external part is, the smaller is the risk that the person to be monitored loses patience and removes the external device.

Therefore, a solution to the above problem has been found by an EEG monitor where the internal coil extends in a first plane and the external coil is adapted to be arranged such that it extends in a second plane, where an angle between the first and the second plane is less than 45 degrees when the EEG monitor is in use. Further, an area defined or spanned by the internal coil is larger than an area defined or spanned by the external coil, and where the area spanned by the external coil overlaps, when projected to the first plane, the area spanned by the internal coil.

One advantage of the solution is that the external part can be made clearly smaller without significantly compromising the quality of the inductive link. The features of having a maximum angle between the plane of the internal coil and the plane of the external coil, as well as of arranging the area spanned by the external coil to overlap the area spanned by the internal coil, have been found to ensure a good mutual inductance and thereby a high coupling coefficient of the inductive link. It should be noted that this overlap of spanned areas of the two coils is defined when projecting the area spanned by the external coil to the first plane into which the internal coil extends. The projection is done by moving each point of the area spanned by the external coil along a line perpendicular to the first plane, and place this point on the first plane.

In practice, it has been found that the coils can be considered to extend in primarily one respective plane, i.e. they are close to being considered as two-dimensional. This is because an outer diameter of e.g. a circle formed by the windings is considerably larger than the thickness of the band in which the windings are placed.

In an embodiment of the EEG monitor, the area defined by the internal coil is a least 20 % larger, preferably 40 % larger, than the area defined by said external coil. This size difference in the areas of the coils has been found to provide an easy and good alignment of the coils.

In an embodiment of the EEG monitor, a center axis of the external coil is to be arranged to pass the area defined or spanned by the internal coil when using the monitor. This will in practice result in a better coupling in the inductive link.

In a further embodiment of the EEG monitor, the external coil is arranged in a housing comprising the external part, which housing is adapted to be arranged behind an ear of the person. This will make the external part easy to wear and easy to arrange close to an internal coil, which has been implanted behind the ear, thereby achieving a good alignment.

In a further embodiment of the EEG monitor, the inductive link is adapted for transfer of power from the external part to the implantable part. Thereby, there will be no need for having a power supply in the implantable part, which is important whenever the monitor is to be applied for long term monitoring, e.g. several months or longer.

In a further embodiment of the EEG monitor, the first plane in which the internal coil extends is parallel to or substantially parallel to a plane of the adjacent skin barrier when the implantable part is arranged for use. This means that a good coupling coefficient can be achieved by aligning the external coil in parallel to or approximately in parallel to the skin surface.

In a further embodiment of the EEG monitor, a layer of ferrite is arranged parallel to or substantially parallel to the plane of one of, or both of, the internal coil and/or the external coil, on the side of the coil(s) facing away from the other coil in the inductive link. This can give a higher coupling coefficient and may reduce eddy-current losses in surrounding metal components in the external part or the implantable part. In a second aspect, the invention concerns a device for warning a person about an upcoming seizure based on analysis of the EEG signal of the person. This device comprises an EEG monitor as described above. The seizure could be caused by diabetes because of hypoglycemia, or it could be caused by epilepsy. In a third aspect, the invention concerns a method for monitoring an EEG signal of a person comprising the steps of

applying an implantable part comprising at least two EEG electrodes, disclaiming the implantation process of the implantable part,

arranging a non-implantable external part comprising a power supply at the head of a person,

connecting the implantable part wirelessly to the external part through an inductive link for transfer of EEG related data from the implantable part to the external part, the inductive link comprising an internal coil connected with the implantable part and an external coil connected with the external part,

- arranging the internal coil to extend in a first plane,

arranging the external coil such that it extends in a second plane, where an angle between the first and the second plane is less than 45 degrees, such as less than 30 degrees, such as less than 15 degrees, when monitoring the EEG signal, adapting or designing the inductive link such that an area defined by the internal coil is larger than an area defined or spanned by the external coil, and arranging the area spanned by the external coil, when projected to the first plane, to overlap the area spanned by the internal coil. In practice, an attempt will be made to achieve the smallest possible angle between the first and the second plane.

Embodiments of the invention will now be explained in further detail with reference to the figures.

Figure 1 illustrates an EEG monitor with an implantable part mounted at the head of a person.

Figure 2 illustrates an example of an EEG monitor with an inductive link.

Figure 3 illustrates a simple outline of two coils of an inductive link.

Figure 4 illustrates an angle between the respective planes of the two coils in figure 3.

Figure 5 illustrates the two coils of figure 3 where the plane of view has been tilted 90 degrees.

Figure 6 illustrates the two coils of figure 3 with a layer of ferrite arranged.

Figure 7 illustrates the two coils of figure 3 with an enclosing ferrite structure.

Figure 8 illustrates a housing for an external part on an EEG monitor.

Figure 1 shows the head 1 of a person provided with an EEG monitor 2. The person being monitored is wearing an implantable EEG sensor part 3 having at least two electrodes 7 (see figure 2) along a thin cable 4. The implantable part 3 is typically adapted for subcutaneous implantation. Further, the person wears an external part 5 with a non-implantable housing comprising the EEG signal processor. These two parts 3, 5 are adapted to be in wireless communication through the skin of the person by an inductive link 6. This inductive link comprises an internal coil 10 arranged in or connected with the implantable part 3 and an external coil 11 connected with or arranged in the external part 5.

The external part 5 with a housing comprising the EEG signal processor is here arranged in the region behind the ear of the person of whom the EEG signal is being monitored. Preferably, the housing is arranged at a position behind the ear similar to the position that might be occupied by a behind-the-ear hearing aid. This also facilitates a position close to a suitably placed implanted part, which is important for the inductive link, which is relied on for communication and power transfer through the skin. With the position behind the ear of the housing, means for providing some fixation to the ear is also preferable. For this purpose, part of a wire or sound tubing as might be applied for conveying the sound from a behind-the-ear part of a hearing aid and into the ear canal may be used. Figure 2 shows an example of an EEG monitor in more details. The EEG monitor 2 comprises an external part 5 and an implantable EEG sensor part 3. The implantable part 3, suitable for being subcutaneously positioned behind the ear of a person, comprises subcutaneous EEG electrodes 7, which can be arranged along a thin cable 4 and being connected to an electronic module 8. The number of EEG electrodes 7 is at least two. Often three electrodes or more are preferred. The electronic module 8 often comprises an A/D converter (not shown) and a communication controller (not shown), and a voltage regulator (not shown). The electrodes 7 are connected to the A/D

converter; the communications controller is connected to an internal coil 10 of an inductive link 6.

The external part 5 comprises in this example a signal processor 12 having a controller (not shown) connected to a second coil 11 of the inductive link 6. The signal processor 12 is further connected to a power supply 25, e.g. a battery, and possibly also to a loudspeaker 13 for providing an acoustic signal, e.g. a notification to the user or, in case the EEG monitor is applied for surveillance of a person e.g. in risk of having hypoglycemia or epilepsy, an alarm to the user in the event that an upcoming onset of hypoglycemia or epilepsy is identified. The external part 5 may also comprise a memory 16, e.g. for logging of data, as well as a radio 15 with an antenna 14, for wireless communication with external units. The external part 5 may be adapted for being arranged in one housing, where the external coil may be in this housing or connected to it, e.g. by a wire.

If the EEG monitor is for the main purpose of logging the EEG signal, the memory 16, the power supply 25, electronics for controlling the inductive link and the external coil 11 , could be the only components of the external part. When in use, the external part 5 may be placed behind the ear of a person for whom monitoring of an EEG signal is desired, and in the vicinity of a subcutaneously implantable part 3, which preferably is implanted right below the skin and slightly behind the ear of the user and positioned in such a way that a reliable, electrical EEG signal may be detected by the electrodes 7. The electrodes 7 of the implantable part 3 can as mentioned be arranged in one thin cable 4. The electrodes 7 are then arranged with contact to the tissue in limited areas along this cable 4. Such one cable 4 comprising all electrodes 7 associated with respective conductors may facilitate a simple implantation process. Figure 3 shows in schematic form the two coils 10, 11 of an inductive link. The two coils are here indicated to have a circular or a substantially circular shape. Both the internal coil and the external coil could, however, be provided with other shapes as well. The shape may be selected according to the housing or encapsulation in which the coil is arranged. If the shape is not circular an effective diameter D_eff can be defined:

4A

D_eff =—

Where A is the area contained by the coil and p is the length of the periphery of the coil.

The coil-coupling factor is the number one parameter that defines the energetic efficiency of the inductive link: the better the coil coupling, the higher the link efficiency and hence, the longer the battery life. In the embodiment of figure 3, the internal coil 10 in the implant has a diameter Di and the external coil 11 in the external device has a diameter DE. The optimum relationship between these two diameters in terms of inductive coupling for the case of a size-constrained external coil can be decided by the assistance of the relationships (Ko W.H., S.P. Liang and C.D.F. Fung, "Design of radio-frequency powered coils for implant instruments," Med. Biol. Eng. Comput., vol. 15, pp. 634-640, 1977):

Where s is the distance between the plane of the internal coil 10 and the plane of the external coil 1 1 as indicated in figure 3. When Df is equal to Df + 4s² , then nothing is gained with respect to the coupling factor by making Di larger. However, there might be a benefit in that the coil alignment becomes easier when the internal coil size is increased.

Examples of the effective diameters of the coils are that the internal coil 10 could have a diameter in the range 10 - 20 mm, such as 12 - 16 mm, and the external coil 1 1 could have a diameter in the range 4 - 12 mm, such as 7 - 10 mm. The winding pack (i.e. the band in which the windings of each coil is arranged) will often have a thickness of approximately 1 mm. With these typical dimensions and thicknesses, the coils can be approximated as two-dimensional structures that each define a plane (figures 3 and 4). In an embodiment where the power consumption of the EEG monitor is very small and the carrier frequency is in the size range of 1 MHz, each of the internal coil 10 and the external coil 1 1 will often have a number of windings of at least 50, and preferably in the range 100 - 300. The number of windings may be different for the two coils in the inductive link, and in some embodiments, the external coil may have a higher number of windings than the internal coil. The number of windings depends on the amount of power, which needs to be transferred. The more power to be transferred the fewer windings on the coils. The number of windings is also selected based on the carrier frequency at which the inductive link communicates. This frequency will be in the range 100 KHz to 40 MHz when both data and power have to be transferred.

The distance s between the plane of the internal coil 10 and the plane of the external coil 1 1 is often in the range 4 - 12 mm, such as the range 5 - 10 mm, such as the range 5 - 8 mm. Such range includes the skin layer, part of the encapsulation of the implantable part 3 and e.g. part of the housing of the external part 5. The exact value of this distance will vary from person to person. When selecting the diameter of the internal coil the calculation should therefore preferably be based on a distance s from the higher end of the interval.

Often ferrite will be used to improve the coupling in the inductive link. Ferrite can be arranged behind one or both coils. Here behind refers to the side of the coil intended to be in the direction opposite to the other coil in the inductive link. Ferrite can also be arranged in the middle of a coil or surrounding the coil. However, the coil area or diameter used above to calculate the optimal dimensions, is defined by the coil windings, not the ferrite.

Figure 4 shows the situation where the planes of the two coils are not parallel but tilted at the angle a. In order to ensure a high coupling factor between the two coils the angle a should be as small as possible. The angle a should preferably be less than 45 degrees, such as less than 30 degrees, such as less than 20 degrees. The smaller the angle a between the two planes, the better the coupling achieved between the internal and external coils, provided that the two coils overlap laterally.

Figure 5 shows the internal coil 10 and the external coil 11 in a view that has been tilted approximately 90 degrees compared to the view of figure 3 and 4. It is here illustrated that the two coils are not necessarily circular in shape. In this particular example, they are both slightly elliptical, but in practice, they could have other, also more irregular shapes as well.

There is an overlap of spanned areas of the two coils where at least 25%, such as at least 50 %, such as at least 75% of the area spanned by the external coil overlaps, when projected to the first plane (into which the internal coil extends), the area spanned by the internal coil. The overlap is defined when projecting the area spanned by the external coil to the first plane into which the internal coil extends. The projection is done by moving each point of the area spanned by the external coil along a line perpendicular to the first plane, and place this point on the first plane. I.e. the area spanned by the external coil is compressed perpendicularly to the plane of the internal coil. Preferably, a center axis 22 (see figure 4) of the external coil 11 is to be arranged to pass the area defined by the internal coil 10 when the inductive link is arranged at a person and is functioning with the EEG monitor. The center axis may be found as the geometrical center of the external coil 11 , which in practice can be considered as an approximately two dimensional coil. The center axis will extend perpendicular to the plane in which the coil extends. If the coil has a more irregular shape, the point where the center axis passes may be found by locating the center of mass in two dimensions. Alternatively, the center axis may be defined as the axis where the magnetic field lines comes closest to a straight line. For at least a circular shaped coil the axis will be the same independently on the definition.

Figure 6 shows the inductive link where a ferrite layer 17 has been arranged in a plane parallel to or substantially parallel to a plane in which the external coil extends. The ferrite layer 17 is arranged on the side of the external coil 11 facing away from the internal coil 10, and is provided with a diameter corresponding to or slightly larger than the diameter of the external coil.

Figure 7 shows a more enclosing ferrite structure around the two coils of the inductive link. A ferrite layer 17 is arranged behind each coil. This is provided with a central extension 19 of ferrite extending towards the center of the coil. The layer 17 is further provided with an encircling extension 18 surrounding the coil with a periphery larger than that of the coil. Similar structures are shown for both coils, but the structure could be arranged in only the external part or only in the implantable part.

Figure 8 shows a housing 20 comprising the external part 5 and the external coil 11. This housing would typically be provided with a shape and size making it feasible for arrangement behind the ear of a person to be monitored. The external coil 11 is indicated in figure 6 even though it would often be arranged inside the housing 20. The windings of the external coil 11 may be made from a coiled wire or lead, or it can be printed on a circuit board or print on a surface, e.g. an inside surface of the housing 20.

In one embodiment, the external coil 11 is arranged separate from the housing 20, while the external coil is connected to the housing through a wire, and the external coil is arranged at the skin close to the internal coil 10. In that case the external coil may be attached to the skin by an adhesive or by a permanent magnet, while the housing may or may not be adapted for arrangement at or behind the ear. Some initial signal processing components may also be arranged with the external coil at the skin. This could be e.g. the signal processor 12 comprising pre-amplification, a controller for the inductive link etc.

Claims

1. An EEG monitor (2) having an implantable part (3) adapted for arrangement at the head of a person and comprising EEG electrodes (7), and an external part (5) comprising a power supply (25), the implantable part being connected to the external part by an inductive link (6) for transfer of EEG related data from the implantable part (3) to the external part (5), the inductive link comprising an internal coil (10) associated with the implantable part (3) and an external coil (11) associated with the external part (5), wherein an area spanned by the internal coil (10) is larger than an area spanned by the external coil (11), wherein the internal coil extends in a first plane and the external coil extends in a second plane, wherein the internal coil and the external coil are adapted to be arranged in a use position where an angle between the first and the second plane is less than 45 degrees, and wherein at least 25 % of the area spanned by the external coil overlaps, when projected to the first plane, the area spanned by the internal coil.

The EEG monitor according to claim 1 , wherein the area spanned by the internal coil is a least 20 % larger, preferably 40 % larger, than the area spanned by the external coil.

The EEG monitor according to claim 1 , wherein a center axis of the external coil (11) is to be arranged to pass the area spanned by the internal coil (10) when in use.

The EEG monitor according to claim 1, wherein the external coil (11) is arranged in a housing (20) comprising the external part (5), which housing adapted to be arranged behind an ear of the person.

5. The EEG monitor according to claim 1 , wherein the inductive link (6) is

adapted for transfer of power from the external part (5) to the implantable part (3).

6. The EEG monitor according to claim 1 , wherein the first plane in which the internal coil (10) extends is parallel to or substantially parallel to a plane of the adjacent skin barrier of the person being monitored, when the implantable part (3) is arranged for use.

7. The EEG monitor according to claim 1, wherein both the internal coil (10) and the external coil (11) have at least 50 windings each.

The EEG monitor according to claim 1 , wherein a layer of ferrite is arranged parallel or substantially parallel to the plane of one of the internal coil (10) or the external coil (11), on the side of the coil facing away from the other coil in the inductive link (6).

The EEG monitor according to claim 1 , wherein the square of an effective diameter of the internal coil (10) is equal to or substantially equal to the sum of the square of an effective diameter of the external coil (11) and four times the square of a distance between the planes of the two coils (10, 11) along the center axis of the external coil, when in use.

10. The EEG monitor according to claim 1, wherein the external coil (11) is

arranged separate from a housing comprising the external part (5), and the external coil is connected to the external part by a wire connection.

11. A device for warning a person about an upcoming seizure based on analysis of the EEG signal of the person, comprising an EEG monitor (2) according to any one of the claims 1 - 10.

12. A method for monitoring an EEG signal of a person comprising the steps of - applying an implanted implantable part (3) comprising EEG electrodes (7) at the head of the person, disclaiming the implantation process of the implantable part, arranging a non-implantable external part (5) comprising a power supply (25) at the person,

connecting the implantable part (3) to the external part (5) by an inductive link (6) for transfer of EEG related data from the implantable part to the external part, the inductive link having an internal coil (10) associated with the implantable part (3) and an external coil (11) associated with the external part (5), the external coil and the internal coil being adapted such that an area spanned by the internal coil (10) is larger than an area spanned by the external coil (11),

arranging the internal coil (10) to extend in a first plane,

arranging the external coil (11) to extend in a second plane, where an angle between the first and the second plane is less than 45 degrees when monitoring the EEG signal, and

arranging the area spanned by the external coil, when projected to the first plane, to overlap the area spanned by the internal coil.