Abstract
A wearable EEG monitor for continuously monitoring the EEG of a user through capacitive coupling to an ear canal of a user includes an ear insert (1) for positioning within the human ear canal, having at least two capacitive electrodes (16) for recording a signal. The electrodes are coated with a dielectricum for electrical insulation. The electrodes are connected to an amplifier (17). The amplifier has an input impedance matched to the impedance of the electrodes. The invention further provides a method of monitoring brain waves.
Claims
1. A method of monitoring brain waves comprising: arranging a capacitive electrode coated with a dielectricum in an ear insert, placing the ear insert within an ear canal of a human subject, arranging at least one reference electrode coated with a dielectricum in contact with the head of the subject, providing a processor, connecting the capacitive and reference electrodes to the processor, where the connection of the capacitive electrode to said processor comprises a signal wire and a shield, and the potential of the shield is kept close to the potential of the signal wire, and wherein a sample-and-hold circuit holds a potential of the shield close to a potential of the signal wire, and using the processor to record and process the signals from the electrodes thereby monitoring the brain waves.
2. The method according to claim 1, wherein said step of arranging said capacitive electrode comprises arranging two of said capacitive electrodes in said ear insert.
3. The method according to claim 1, wherein said connecting step comprises connecting said capacitive electrode through said signal wire and shield to an amplifier having an input impedance matched to an impedance of said capacitive electrode.
4. The method according to claim 1, wherein said processor is configured to measure an EEG response represented by said signal from said capacitive electrode.
5. The method according to claim 3, wherein the amplifier is located within the ear insert.
6. The method according to claim 3, wherein the amplifier is an Auto-Zero-Amplifier.
7. The method according to claim 3, wherein the amplifier is a chopper-stabilized amplifier.
8. The method according to claim 3, wherein the amplifier has an input characteristic of a high pass filter with a corner frequency at 1 Hz.
9. The method according to claim 1, further comprising the step of testing said capacitive coupling using a test circuit having a signal generator for generating a test signal having a frequency outside a frequency range of signals intended to be measured by said capacitive electrode.
10. The method according to claim 1, wherein said processor is located at an in-ear device.
11. The method according to claim 10, wherein said in-ear device comprises a behind-the-ear device.
12. The method according to claim 11, wherein behind-the-ear device further comprises a power supply.
13. The method according to claim 1, wherein the ear insert is made of a flexible material that adapts to the shape of the ear canal.
14. The method according to claim 13, wherein said capacitive electrode is fixed to an inner non-flexible part, which is covered with an outer flexible part of said ear insert, which conforms to the shape of the ear canal.
15. The method according to claim 1, wherein multiple said capacitive electrodes are distributed evenly over a circumference of the ear insert.
16. The method according to claim 1, wherein said capacitive electrode has a double curvature shape.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention will now be described in more detail in respect to the drawings, where
(2) FIG. 1 is an ear insert with capacitive electrodes for picking up an EEG response of the user.
(3) FIG. 2 is an ear insert according to FIG. 1, further comprising a BTE device.
(4) FIG. 3 is an ear insert having a preshaped inner tube comprising electrodes and a soft outer part.
(5) FIG. 4 is an ear insert comprising several flanges with capacitive electrodes.
(6) FIG. 5a is an electrode for use in an ear insert.
(7) FIG. 5b is another electrode for use in an ear insert.
(8) FIG. 5c is an ear insert comprising an electrode as shown in FIG. 5a or 5b.
(9) FIG. 6a is a diagram of the operational amplifier and the capacitive electrode for picking up an EEG response.
(10) FIG. 6b is an equivalence diagram of the amplifier in FIG. 6a, determining the lower cut-off frequency.
(11) FIG. 7 is an electrode with an operational amplifier and an analogue-to-digital converter.
(12) FIG. 8 is a graph depicting the 1/f noise for a traditional operational amplifier versus an Auto Zero Amplifier, and the acceptable noise level at 100 nV/√Hz.
(13) FIG. 9 is an Auto Zero Amplifier (AZA) suitable for amplification of EEG signals in a system according to an embodiment of the invention.
(14) FIG. 10 is an example of the auto-zero amplifier.
DETAILED DESCRIPTION OF THE INVENTION
(15) FIG. 1 shows an ear insert 10, having capacitive electrodes 16. The ear insert comprises a ventilation channel 12, adapted to ensure that the ear insert does not occlude the ear of the user. The ear insert further comprises an electronic module 17 connected to the capacitive electrodes 16 for amplification of the EEG response. Furthermore, it may be beneficial to have a microphone inlet 11, a microphone 13, a loudspeaker 14 and a sound outlet 15 within the ear insert, to ensure that the user is able to hear sounds coming from the surroundings. The microphone and loudspeaker are connected to the electronic module 17. The electronic module may further have means for amplification of the sound.
(16) In some situations, it is beneficial that the ear insert is adapted for communication with other devices, e.g. an ear insert at the contralateral ear or a remote control. The ear insert therefore also comprises an antenna 18 and a radio transceiver (not shown) for wireless communication. The antenna may be used for transmitting EEG recordings made at one ear insert to the contralateral ear insert, for enabling comparing the EEG recording within the contralateral EEG recording. This is beneficial because synchronization of the recorded EEG from different regions of the brain will enhance the statistical results.
(17) FIG. 2 is an ear insert according to FIG. 1, having capacitive electrodes 16 and a sound outlet 15, and further comprising a Behind-The-Ear (BTE) device 24, adapted for accommodating further electronics (not shown), a battery (not shown), and a digital signal processor (not shown). The BTE device 24 is connected to the ear insert via electrical wires 21, however, the two units may also be adapted to communicate wirelessly. In the embodiment shown, there is a further capacitive pad electrode 23 connected via a wire 22, for placement adjacent to the pinna, or at a location farther from the ear. Such external electrode may also be located at the housing of the BTE device 24, to increase the performance, by increasing the distance between the electrodes and by increasing the reliability.
(18) FIG. 3 is an ear insert, having an inner tube 30 preshaped to match the longitudinal shape of the ear canal and comprising capacitive electrodes 16. The inner tube 30 is preshaped such that it fits the bends and curves of the ear canal of the specific user, while having an outer diameter of the inner tube smaller than the diameter of the ear canal. The ear insert further comprises a dielectric outer material 31, softer than the rigid tube, of e.g. silicone, foam or rubber that enables the ear insert to fit tight to the ear canal, without excessive pressure on the skin. The electrodes 16 are mounted on the inner rigid tube, preferably at the fulcrums of the inner tube, i.e. the points of the ear insert, that support the insert the most and where the distance between the electrode and the skin of the user is foreseen to be minimal and hereby most likely to create a close coupling to the skin through the dielectric material 31. The ear insert further comprises an outer part 32 located at the concha of the user, where the electronic module 17 is located. The electrodes are connected (not shown) to the electronic module for amplification and analysis of the EEG response picked up from the user.
(19) FIG. 4 shows a cross section of the ear insert 40 with capacitive electrodes 16. The ear insert comprises a sound channel 41, for ventilation and transmittal of sound to the ear drum of the user. The sound channel 41 is rigid to always allow free passage in and out of the ear canal. The sound channel 41 may vary in diameter, e.g. with bulges 43, to shape the frequency response of the channel 41. The material 42 surrounding the sound channel is flexible so as to conform to the shape of the ear canal of the user. The ear insert has flanges or rims comprising capacitive electrodes 16, the flanges extending outward from the sound channel. Upon insertion of the ear insert into the ear canal, the flanges bend backward towards the outside of the ear canal, whereby the capacitive electrodes 16, including a dielectricum (not shown), are forced against the skin of the ear canal. Ensuring a minimal distance between the capacitive electrode 16 and the skin optimizes the capacitive coupling between the electrode 16 and the skin. The flanges may be circumferential or extend outward in several directions perpendicular to the sound channel 41. In the circumferential embodiment, each flange preferably comprises several electrodes spaced apart. The electronics may be located within the ear insert (not shown) or in a behind-the-ear device (not shown).
(20) FIG. 5a shows an electrode 50 having a base plate 51 and an electrode salient 52 protruding out from the base plate 51. The electrode 50 substantially has the shape of a bowler.
(21) FIG. 5b shows an elongate electrode 53, where an electrode salient 54 is protruding out from the body of the electrode. Both the electrode salient and the electrode body have an elongate shape. The advantage of the elongated electrode over the electrode of FIG. 5a, is that the elongated electrode has a larger contact area, where it is in contact with the skin and therefore has a higher capacitance than the bowler hat type electrode of FIG. 5a.
(22) FIG. 5c shows a cross section of an ear insert comprising an electrode according to FIG. 5a or 5b. The ear insert is made of an elastic material, which is able to conform to the shape of the ear canal, but still being resilient enough to create a sufficient force, to press the electrode 50 out against the skin of the ear canal. The electrode is extending out through the ear insert. The pick-up salient of the electrode, whether it is a bowler electrode or an elongate electrode, is penetrating the material 55 of the ear insert, so as to extend out through the outer circumference of the ear insert, optimizing the distance to the skin of the ear canal.
(23) FIG. 6a shows two capacitive electrodes CE1 and CE2, an EEG generator 61 and an operational amplifier block 59. The operational amplifier block 59 amplifies the voltage difference between the two electrodes CE1 and CE2. By grounding one terminal of the amplifier, the amplifier is connected as a single ended amplifier and the equivalent capacitance of the electrode C may be computed as the serial capacitance of the two electrodes CE1 and CE2 when placed on the skin of the user,
(24)
(25) FIG. 6b is an electrical diagram of an amplifier connected to a capacitor C1, which is an equivalent of the two electrodes CE1 and CE2 in series. The amplifier is part of the electronics module 17 of FIG. 1. The diagram comprises an EEG generator 61 which equivalents the EEG response of the user, the EEG response being coupled capacitively via the capacitor C1, wherein the one electrode plate is the skin of the user and the other electrode plate is the electrode plate of the capacitive electrode. Between the electrode plate and the skin of the user is a dielectricum, making the electrode couple capacitively to the skin. The block 65 is the electrical circuit for amplification of the signal generated by the EEG generator 61. The capacitor C2 in parallel with the electrode is approximately 1/10 of the capacitance of the electrode C1, hereby creating a voltage divider of one to ten between the two capacitors C1 and C2, in this way, approximately 10% of the voltage from the EEG potential is distributed across the capacitor C1, whereby 90% is available for the operational amplifier 60. Reference 59 designates the amplifier block, comprising the parasitic components C2 and R and the amplifier 60. C2 and R may be chosen appropriately when designing the amplifier block 59.
(26) The sizes of the electrode plates are limited due to the physical size of the ear canal and hereby the surface of the ear insert, consequently the capacitance of the electrode is limited, due to the small electrode capacitance. The impedance of the amplifier should be kept high. The frequency characteristic of the matching circuit should present a high-pass filter having a cut off frequency of approximately 1 Hz.
(27) The operational amplifier block 59 is a suitable low noise amplifier and is connected to each side of the EEG generator 61, i.e. an electrode pair to amplify the difference in potential between the two electrodes 16 (ref. FIG. 1). In order to test the electrode 16, generator 62 can be enabled, generating a test signal at e.g. 30 Hz. A 30 Hz signal is above typical EEG signals of 1 to 10 Hz, and a 30 Hz test signal is therefore easily recognizable outside the EEG signal range. The response to the test signal will give a clear indication of how well the electrodes couple to the skin. The capacitance of C1 depends on the actual sizes of the electrodes and the distances to the skin, i.e. the coupling will vary from user to user and from day to day, e.g. one day the distance between the electrode and the skin may be 0.4 mm, and the next day, the distance may be 0.3 mm. The size and shape of the ear canal changes when moving the jaw, e.g. by chewing, but will in most cases be in the range of 1 to 10 pF. A suitable parallel capacitor is then 1/10 of C1 i.e. 100 to 1000 fF.
(28) FIG. 7 shows a number of electrodes 16, 75, connected to low noise amplifier block 59 via a wire or lead 71 having a shield 72, for shielding the signal wire against interfering electrical coupling, and analogue-to-digital converter (ADC) 73, converting the recorded EEG into digital signals. The electrode 16 is connected to the low noise amplifier (LNA) block 59 via a shielded cable such as a coax cable comprising a signal line 71 and a shield 72. The amplifier is connected to a reference electrode 75, to permit amplification of the EEG signal relative to the reference signal from reference electrode 75. The amplifier signal is transmitted to an analogue-to-digital converter 73. Several electrodes may be arranged with respective analogue-to-digital converters, the outputs from respective A/D converters being fed to the digital signal processor as channel one, channel two and so forth. The electrode 16 further comprises a dielectric material 74 covering the electrode 16 to ensure a capacitive coupling to the skin of the user. The shield 72 is coupled to the output of the LNA. By connecting the shield to the output of the amplifier, the shield has the same potential as the signal wire and consequently there is no difference of potential between the signal wire and the shield.
(29) FIG. 8 shows the 1/f noise of a traditional operational amplifier 81 and of an Auto Zero Amplifier (AZA, to be explained in context with FIG. 10) 82, together with the acceptable noise level 83 at around
(30)
AZA amplifiers are more suitable than traditional amplifiers, because AZA amplifiers have a lower noise level at low frequencies, where EEG signals occur.
(31) FIG. 9 shows an amplifier arrangement for a wearable EEG monitor comprising an electrode 16 with a dielectric material 31 covering the electrode 16, an AZA amplifier 82 connected to the electrode via a wire 71, having a shield 72. The output from the AZA 82 is fed to an analogue-to-digital converter (ADC) 73 and further fed back to the wire shield 72 via a sample-and-hold (S&H) circuit 91, to generate the potential to the shield 72, whereby the shield obtains the same signal potential as the input at electrode 16 and wire 71. The sample & hold circuit, also known as a follow & hold or track & hold circuit, captures the voltage of the output from the AZA and freezes its value to the output. By matching the two potentials, there is no, or minimal, voltage between the wire and the shield, and the capacitive effect between the signal wire and the shield is hereby minimized. The switches 92 and 93 and clock input 94 are synchronized and controlled by a clock generator (not shown). Reference 95 designates a chip pad for connecting the amplifier, which is part of a more complex chip design, to the electrode.
(32) FIG. 10 shows an example of an Auto-Zero Amplifier. It operates in two phases: a zero phase (S1 and S2 are on) and an amplification phase (S1 and S2 are off). The gain is one and has an equivalent input impedance which is related to the switching frequency of S1 and S2 and the input capacitance of the amplifier. We may choose a switching frequency, f.sub.s=200 Hz. Looking at signals in the range of 1-10 Hz or 1-20 Hz, a 200 Hz switching frequency is adequate according to the Nyquist theorem. Suitable amplifiers can be designed having an input capacitance of C.sub.in=100 fF. This results in:
(33)
(34) The cut off frequency, f.sub.n, of the system can be found according to FIG. 6b. In this figure, R represents the input impedance of the amplifier, C2 is the capacitance of the input pad and various parasitic capacitances, and C1 is the electrode capacitance. Below C2 is set to 500 fF.
(35)
(36) This gives a noise corner frequency of 0.45 Hz, i.e. well below the frequency for recording EEG, usually in the range 1 to 10 Hz.
(37) Due to C1 and C2, the voltage at the input of the low noise amplifier is 93% of the EEG voltage for frequencies above 1 Hz.
(38) The dielectricum ensures that no current is running in the crossover between the skin and the electrode, however it should also be as thin as possible, because the capacitance is inversely proportional to the distance between the electrodes,
(39)
where C is the capacitance, A is the area, ε is the dielectric constant of the dielectric material, and d is the distance between the electrode and the skin. The dielectricum may be chosen among a number of different materials, such as silicon oxide, aluminum oxide, polyamide (nylon), PTFE (polytetrafluoroethylene or Teflon), etc.
(40) The size of the electrodes is a trade off between the option to fit several electrodes within a confined space, and the capacitance of the electrode, which is proportional to the area, pointing to large electrode sizes. A preferable size is between 5 mm.sup.2 and 100 mm.sup.2. The electrode may be flexible but is preferably preshaped in a double curved shape to best fit the area of the ear, where it is to be placed. The monitoring device has several electrodes, where each one of them may have an individual shape, to best fit that particular area, where it is supposed to fit the user.
(41) The ear insert may have many different shapes, the common goal for all shapes being, to have an ear insert that gives a close fit to the user's skin and is comfortable to wear, meaning that it should occlude the ear as little as possible.
(42) In one embodiment the ear insert has a customized shape for the ear canal of the user. The ear insert is a hollow shell and is made for the specific ear canal according to an imprint of the ear canal. The electrodes may be mounted on the inside or outside of the hollow shell. If mounted on the inside of the shell, the shell itself may be sufficient dielectric to ensure pure capacitive coupling. Furthermore mounting the electrodes on the inside of a shell makes wiring of the electronic easier, than does outside mounting of the electrodes.
(43) In another embodiment, the ear insert comprises a pipe, where the electrodes may be mounted on the inner or outer circumference of the pipe. The pipe is made in different diameters, as to best fit the diameter of the ear canal. The pipe can also be shaped to fit the shape of the ear canal in the longitudinal direction. On the outer circumference the pipe is covered with a soft and flexible material like silicone, foam, rubber or another soft material that ensures a secure and comfortable fit for the user.
(44) In another embodiment, the ear insert is in the form of a stent. Stents have the advantage that they are flexible, in the way that they can be inserted into the ear canal in a contracted state, and then released to form a close fitting ear insert. The stent may be a self-expanding metallic stent, which is covered by a dielectricum and hereby form a capacitive electrode, which can be connected to the amplifier and signal processor.
(45) A particular problem with amplifiers for EEG monitoring devices is the low frequency noise of the amplifier, also known as the 1/f noise. EEG signals are low frequency, i.e. typically 1 to 10 Hz, however the noise of typical amplifiers is very high at low frequencies, i.e. with a noise corner frequency in the range at or above 100 Hz or even between 1 to 2 kHz for high speed amplifiers, making these amplifiers unsuitable as EEG signal amplifier. This problem is usually solved by using large transistors having large capacitors at the input of the operational amplifier, but increasing the capacitor size also increases the power consumption of the amplifier. This is however not an option when the whole system has to be carried at or in the ear, and powered by a small battery as known from the hearing aid industry. To keep the power consumption low, it is proposed to use an auto-zero-amplifier or a chopper-stabilized amplifier to amplify the EEG recording.
(46) The electrodes picking up the EEG response of the user are connected to an amplifier, feeding an analogue-to-digital converter, after which the signal is handled by a Digital Signal Processor (DSP). The connection between the electrode and the amplifier is preferably via a shielded wire such as a coax cable. The shield is floating at the electrode end, while the shield is connected to the output of the amplifier at the amplifier end. By connecting the shield to the amplifier output, the potential of the shield is kept high whereby the capacitive coupling between the signal wire and the shield is minimized. In a preferred embodiment, the shield is connected to the operational amplifier via a “sample & hold” amplifier for generating a voltage potential to the shield.
(47) An ear insert according to the proposed invention may be used for continuous monitoring of EEG signals indicative of a medical seizure like hypoglycemia, epilepsy, or similar conditions. The device is used to foresee a seizure by analyzing the EEG signal by a digital signal processor, and notify the user in case the analyzed signal indicates an impending seizure. The signal processor is continuously evaluating the EEG recording with statistical data analysis and machine learning methods.
(48) The signal processor, power supply, microphone, loudspeaker etc. may be located at the ear insert or at a behind-the-ear (BTE) part. Whether these parts are located at the ear insert or in the BTE part depends on the size and shape of the ear canal i.e. whether the ear insert is large enough for accommodating all components.
