WO2008017921A2

WO2008017921A2 - System and method for acquisition of bioelectric signals by means of surface electrodes

Info

Publication number: WO2008017921A2
Application number: PCT/IB2007/002254
Authority: WO
Inventors: Roberto Merletti
Original assignee: Politecnico Di Torino
Priority date: 2006-08-07
Filing date: 2007-08-06
Publication date: 2008-02-14
Also published as: ITTO20060584A1; WO2008017921A3

Abstract

A system (1) for acquisition of bioelectric signals envisages the use of at least a first detection electrode (2), placed in use on a portion of the skin surface (5) to detect bioelectric signals, and an. electrical connection structure (8) for connecting the first detection electrode (2) - to an electronic device - (9) for processing, of the bioelectric signals. The electrical connection structure.. (8) has a first removable connection element (10), based on magnetic interaction. In particular, the first detection electrode is made of ferromagnetic material, - and the first removable connection element (10) has a magnet designed to magnetically couple with the detection electrode during the removable connection. Both the first detection electrode and the removable connection element are plated with a conductive material such as Ag or AgCl. In one embodiment, the acquisition system is provided with an array (4) • of detection electrodes (2) incorporated into an electrode support (3), and the electrical connection structure has a corresponding array of removable connection elements (10) incorporated into a magnet support (12); each removable connection element is placed, in use, corresponding to a respective detection electrode and is designed to magnetically interact with such detection electrode.

Description

SYSTEM AND METHOD FOR ACQUISITION OF BIOELECTRIC SIGNALS BY MEANS OF SURFACE ELECTRODES

TECHNICAL FIELD The present invention concerns a system and a method for bioelectric signal acquisition by means of surface (or skin) electrodes. Specifically and without loss of generality, the following description will refer to the acquisition of electromyographic (EMG) signals by means of an array of surface electrodes .

BACKGROUND ART

It is well known that, when contracting, skeletal muscles produce bioelectric signals that propagate along muscle fibers from the point of generation (known as innervation zone) to tendon endings. Muscles are made by muscle fibers grouped in functional units called motor units (MU) ; a single MU comprises a motoneuron, its axon, terminal branches and neuromuscular junctions and all the fibers that it innervates. The motoneuron carries the neural pulses along the axon from the spinal cord to the neuromuscular junctions and triggers action potentials that travel along the muscle fibers to the tendons .

The electrical activity resulting from muscle activation (given by the combination of the action potentials propagating along the fibers of many motor units) can be detected on the skin with the surface electromyography (sEMG) technique. Such technique implies the detection of electrical signals on the skin (surface EMG signals) by placing detection sensors comprising electrodes made of electrically conductive material on the portion of skin surface above the muscle or muscles of interest. The electrical signals detected in this way reflect the muscular activity and are sent to an electronic system for conditioning by amplification and filtering. The surface detection of EMG ■ signals has the advantage of being totally non invasive (as opposed to the traditional needle detection) but presents problems related to the low amplitude (due to the filtering effect introduced by the tissues interposed between the muscle and the skin) and to difficulty of proper electrode location in relation to the muscle of interest.

Considering such difficulties, the design of effective detection sensors is very important in surface EMG in order to assure good electrode-skin contact and stable position (even during non isometric contractions) , while preferably allowing easy repositioning for the identification of the optimal detection location by successive attempts.

The increasing use of electrode arrays for the simultaneous acquisition of EMG signals from different points on the skin is also well known. Such detection technique allows to track the propagation of motor unit action potentials along the muscle fibers and map the spatial properties of a muscle, to create high order spatial filters for the EMG signals to increase the resolution and to select (either on line or off line) sections of the electrode array with the best signal quality.

The techniques currently used for the detection of surface EMG signals from a subject adopt pairs or groups of adhesive electrodes that are not repositionable. In particular, electrode arrays using flexible printed circuit techniques for implementing electrodes and connections to the conditioning electronics are often used and are applied to the skin using double adhesive foils.

The use of single electrodes individually applied to the skin to implement an array is cumbersome and makes it difficult to implement arrays with equally spaced contact points; the use of flexible printed circuit arrays is better but not without problems, that are related to the difficulty of repositioning the arrays and of assuring good electrical contacts of all the electrodes of the arrays. In addition such array sensors are expensive, reusable for only one or very few times because are easily worn out or damaged.

DISCLOSURE OF INVENTION

The aim of this invention is that of providing a system for bioelectric signal acquisition, in particular for surface EMG signals, that provides an improvement over the available techniques and solves completely or in part, the problems out1ined above .

According to the present invention, a system and a method for bioelectric signal acquisition are provided, as defined in claim 1 and 21, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS For an easier understanding of the present invention preferred embodiments thereof will now be described, purely by way of non-limiting examples, and with reference to the attached drawings , wherein :

- Figure 1 shows a transverse section across a portion of a bioelectric signal acquisition system, according to one embodiment of the invention;

- Figure 2 is a prospective view from above of the system shown in Figure 1 ;

- Figure 3 is a prospective view from below of the system shown in Figure 1; and

- Figure 4 is a transverse section across a variation of the system for bioelectric signal acquisition according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As it will be clarified in the following, one aspect of the present invention envisages the use of a removable connection, in particular based on magnetic interaction, between one or more electrodes for detection of EMG signal (or of any other bioelectric signal) , applied to the skin of the subject, and the electronics for signal conditioning.

In detail, with reference to Figures 1-3, a system 1 for bioelectric signal acquisition comprises a plurality of detection electrodes 2, fixed to and incorporated in an electrode support 3 consisting of a foil of cloth or rubber or other natural or synthetic material. The detection electrodes 2 comprise circular metal eyelets riveted or fixed in any other known way to the electrode support 3; the detection electrodes 2, suitable for electrical signal detection and conduction, are made of ferromagnetic (e.g. ferrous) material, possibly plated with electrically conductive material such as silver (Ag) or silver chloride (AgCl) , in order to assure good contact with low impedance and noise with the skin. Each detection electrode 2 has a central opening or is drilled to define an opening 7 that crosses the whole of its thickness and has, for example, a cylindrical shape. In particular, in this example, the detection electrodes 2 are placed in rows and columns to form a detection array or matrix 4 (Figure 2) and are arranged at fixed and uniform distance from one another.

The electrode support 3 is applied on a portion 5 of the skin of a person (Figure 3 shows a contact surface 3a of the electrode support in contact with the skin) , in such a way that the detection electrodes 2 are in contact with the skin, and is held in place, for example by Velcro or elastic bands or straps 6; such elastic straps 6 are attached to the electrode support 3 and constitute its lateral extensions.

The acquisition system 1 further comprises electrical connection means 8 for the connection of the detection electrodes 2 (and of their support that do not include any electronics) to a corresponding signal conditioning electronics 9 (schematically shown in Figure 2, including for example circuits for amplification and filtering of the detected bioelectric signals) .

In detail, the electrical connection means 8 comprise a plurality of magnetic connection elements 10, carried by a magnet support 12 (which is also made of cloth, rubber, or other natural or synthetic material) and placed so that they match with respective individual detection electrodes 2.

In the configuration shown, the magnetic connection elements 10 are placed along rows and columns so as to form a matrix with grid point distances coinciding with that of the electrode array. Indeed, each magnetic connection element 10 will in use interact with, and adhere by magnetic attraction to, a corresponding detection electrode 2 when magnet support 12 is matched with the electrode support 3. The magnetic connection elements 10 have a shape corresponding to that of the detection electrodes 2, for example circular, and are made with high residual induction materials (such as a samarium- cobalt alloy) in such a way to apply a high attractive force to the detection electrodes 2 made of an iron alloy. In addition, also the magnetic connection elements 10 are plated with conductive materials such as silver or silver chloride to assure good electrical contact with low impedance and noise.

The electrical connection means 8 may further include a flexible printed circuit 13, as part of the magnet support 12 and providing a number of connecting paths associating independently each magnetic connection element 10 to a pin of an electric connector 14 that will interface with the signal conditioning electronics 9 by means of a multipolar signal cable 15. The magnetic connection elements 10 may be electrically connected to such flexible printed circuits and connecting paths by glue or solder connection (for example by means of a conductive adhesive or resin) . The electrical connector 14 may also be carried by the magnet support 12 (for example it may be incorporated in such support) , or it may be a separate element connected to the printed circuit 13 by means of a further signal cable.

In alternative to the above flexible printed circuit, the electrical connection means 8 may include a plurality of electrical conductors 16 (as shown in Figure 4) for the electrical connection of the magnetic connection elements 10 to the electrical connector 14; such electrical conductors may be incorporated in the magnet support 12 or separated from such support. The electrical conductors 16 may be soldered or glued (e.g. with conductive resin) to the magnetic connection elements 10.

The operation of the acquisition system 1 envisages that the detection electrodes 2 are affixed on a portion of skin above one or more muscles to be examined. For this purpose the electrode support 3 is applied over the muscle by means of the elastic bands 6 connected by the Velcro straps. As is well known, the contact between the detection electrodes 2 and the skin may be dry (i.e. determined by the simple reciprocal contact) or wet (i.e. with interposed conductive material suitable to decrease the contact resistance) . In the second case the opening 7 of each eyelet of the detection electrodes 2 is filled with conductive gel (or other conductive material of any density) to improve the contact between the eyelet and the skin (in particular in presence of hair that cause problems with the dry contact) . The filling takes place by means of an automatic syringe or dispenser in order to control the precise amount of material delivered. After this operation, the magnet support 12 is applied above the electrode support 3 so that by magnetic attraction each magnetic connection element 10 connects with and adheres to a corresponding detection electrode 2. The bioelectric signals are detected and transferred to the signal conditioning electronics 9 by means of the printed paths or conductors, electrical connector 14 and multipolar signal cable 15.

The signal conditioning electronics 9 might provide the option of displaying the plot of the detected signals in real time in order to assess their quality and the quality of the contact provided by the various detection electrodes 2. In case of poor signal quality or contact, the acquisition system 1 allows the easy removal of the magnet support 12 from the electrode support 3 , to add additional conductive gel in the eyelets of the detection electrodes showing poor contact, or reposition the whole electrode support 3 in order to find a better detection position on the skin. The magnet support 12 can then be reapplied on the electrode support 3 reestablishing the magnetic connection.

An overview of the acquisition system according to the present invention provides indications of its advantages.

In particular, the removable magnetic connection system allows a simple way to connect and disconnect (both electrically and mechanically) the detection electrodes and the corresponding support (that are passive, do not include any electronics), and a connecting cable interfacing with the signal conditioning electronics. Easy disconnection of the magnetic connection elements from the electrodes allows repositioning of the detection electrodes and modification of the contact quality, to improve the quality of detection of the bioelectric signals. The detection electrodes 2 are easily repositionable on the skin in order to identify an optimal detection position.

Differently from available detection systems, the elements composing the system according to the invention (namely, detection, electrodes 2, electrode support 3, magnetic connection elements 10 and magnet support 12) can be reused many times for signal detection, because they are not damaged or worn out by use. This property, combined with the simple construction, makes the acquisition system cheaper; moreover, both the electrode support 3 and the , magnet support 12 can be separately washed and sterilized in antibacterial solution. The electrode support can also be washed in a washing machine; this allows use of the system by different individuals without hygienic risks.

It is clear that the system described can undergo changes and variations that will not depart from the scope of the present invention, as defined in the attached claims.

In particular, Figure 4 shows a possible variation of the acquisition system 1 described above. In this case the electrical connection means 8 do not include the magnet support 12, and the magnetic connection elements 10 are therefore free and independent from each other and individually connectable to respective detection electrodes 2 in configurations that can be selected by the user. The electrical connection means 8 comprise a plurality of electrical conductors 16, each connected to its magnetic connection element 10 (by soldering or by conductive glue) and collected in the electric connector 14 (here not shown) . This configuration allows action (for example disconnection) oneach of the magnetic connection elements 10 (which might show an unreliable connection with the skin) , for example in order to modify the quantity of the conductive gel, as previously described.

A further variation of the acquisition system (not shown in the -Figures) does not envisage neither the magnet support 12 (as -shown in Figure 4) nor the electrode support 3. In this case, the detection electrodes 2 (or even a single detection - S -

electrode) may be individually placed on the skin and separately connected to the corresponding magnetic connection elements 10.

The magnet support 12 might incorporate on the flexible printed circuit 13 some front-end electronics for preliminary conditioning of the bioelectric signals; this front-end electronics could also be included inside the electric connector 14. It is underlined that incorporating such electronics is achievable from the costs point of view, thanks to the cited reusability of the acquisition system for a large number of acquisitions.

Alternative ways for fixation of the electrode support 3 to the skin are possible. For example the electrode support 3 carrying the array of detection electrodes 2 could be incorporated in or be part of a garment or piece of clothing (an elastic sleeve, t-shirt or trousers) worn by the subject. The detection electrodes 2 would contact the skin and also be accessible from an external surface of the sleeve or garment for connection to the magnetic connection elements 10. Alternatively, an adhesive connection could be used to hold the electrode support 3 in contact with the skin, even in cases of difficult positioning of the elastic straps 6.

The magnetic connection elements 10 may have different shapes, for example a "T" shape with a protrusion (the "T" leg) fitting into the opening 7 of the detection electrodes 2. In this way an increased magnetic interaction and a more stable mechanical coupling may be obtained between the electrode support 3 and the magnet support 12. The detection electrodes 2 may have unequal interelectrode distances, for example depending on the muscle to be investigated (smaller muscles requiring smaller interelectrode distances) . The interelectrode distance could also be variable within the same detection array 4. The magnetic connection elements 10 incorporated on the magnet support 12 could be in number different from the number of the detection electrodes 2, so that more than one magnet support ^' could be used over the same electrode support 3, in a modular fashion.

The detection electrodes 2 might also have a shape without opening 7 with roughnesses^" on the surface in contact with the skin, in such a way to form a plurality of seats accommodating hair and increase the quality of contact (in a per se known manner) . In addition, both the electrode support 3 and the magnet support 12 may incorporate hole, pins and other reference means, to facilitate reciprocal alignment and positioning.

Finally, it is pointed out that that the proposed acquisition system 1 is not limited to the use with EMG signals, but can be applied to any bioelectric skin signal, such as electroencephalogram (EEG), electrocardiogram (ECG), etc. In addition, it is evident that the releasable system of magnetic connection may incorporate any number of detection electrodes 2 (even one) , and such electrodes can be positioned in any arbitrary manner (in a matrix or linear array or any desired manner with or without an electrode support 3) .

Claims

1. A system (1) for acquisition of bioelectric signals comprising: - at least a first detection electrode (2) applied, in use, on a portion of skin (5) to detect said bioelectric signals; and electrical connection means (8) to connect said first detection electrode (2) to an electronic device (9) for processing of said bioelectric signals; characterized in that said electrical connection means (8) comprise a first removable connection element (10), based on magnetic interaction.

2. The system according to claim 1, wherein said first detection electrode (2) comprises ferromagnetic material, and said first removable connection element (10) comprises a magnet configured so as to magnetically couple with said first detection electrode (2), during said removable connection.

3. The system according to claim 1 or 2, wherein said electrical connection means (8) comprise a first conductive path (13, 16) electrically connected to said first removable connection element (10) and to said electronic device (9) ; and said first detection electrode (2) and said first removable connection element (10) are plated with electrically conductive material .

4. The system according to claim 3, wherein said electrical connection means (8) further comprise an electrical connector (14) connected to said first conductive path (13, 16), and to said electronic device (9) by means of a signal cable (15) .

5. The system according to any of the preceding claims, wherein said first detection electrode (2) has a shape with a cavity and defines an internal opening (7) , meant to receive a conductive material, namely a conductive gel, to improve an electrical contact with said portion of skin (5) .

6. The system according to claim 5, wherein said first detection electrode (2) comprises ferromagnetic material, and said first removable connection element (10) comprises a magnet having a protrusion configured so as to penetrate into said opening (7) .

7. The system according to any of claims 1-4, wherein said first detection electrode (2) has a shape without opening with roughnesses on its surface in contact with the skin.

8. The system according to any of the previous claims, comprising further detection electrodes (2) placed, in use, on said portion of skin (5) for detection of said bioelectric signals, said first and further electrodes being arranged in any desired configuration; and wherein said electrical connection means (8) comprise further removable connection elements (10) based on magnetic interaction, arranged, with said first removable connection element, in such a way to correspond to said desired configuration, and to magnetically interact each with a respective one of said first and further detection electrodes (2) .

9. The system according to claim 8, wherein said first and further detection electrodes (2) , and said first and further removable connection elements (10) are placed in such a way as to form a matrix.

10. The system according to claim 8 or 9, wherein said electrical connection means (8) comprise a first and further conductive paths (13, 16) electrically connected to said first and further removable connection elements (10) , and an electric connector (14) connected to said first and further conductive paths (13, 16), and to said electronic device (9) through a signal cable (15), of a multipolar type.

11. The system according to any of claims 8-10, wherein said first and further detection electrodes (2) are placed in an electrode support (3), configured so as to be fixed above said portion of skin (5) .

12. The system according to claim 11, wherein said electrode support (3) comprises a foil of cloth or rubber.

13. The system according to claim 11 or 12, wherein said first and further electrodes (2) comprise eyelets fixed to, in particular riveted on, said electrode support (3).

14. The system according to any of claims 11-13, wherein said electrode support (3) is a portion of a garment that can be worn over said portion of skin (5) .

15. The system according to any of claims 11-14, wherein said first and further removable connection elements (10) are placed within a magnet support (12), positioned, in use, above said electrode support (3).

16. The system according to claim 15, wherein said electrical connection means (8) comprise a first and further electrical conductors (13, 16) electrically connected to said first and further removable connection elements (10) , and an electric connector (14) connected to said first and further electrical conductors (13, 16), and to said electronic device (9) by means of a multipolar signal cable (15); said first and further electrical conductors being incorporated into said magnet support (12) .

17. The system according to claim 16, wherein said magnet support (3) comprises a printed circuit (13) integrating said first and further electrical conductors (13, 16), and further carries said electric connector (14).

18. The system according to claim 17, wherein said printed circuit (13) , in particular of a flexible type, further integrates front-end electronic circuitry for conditioning of said bioelectric signals.

19. The system according to claim 16 or 17, wherein said electric connector (14) integrates front-end electronic circuitry for conditioning of said bioelectric signals .

20. The system according to any of the preceding claims, wherein said bioelectric signals are surface electromyographic signals, and said first detection electrode (2) is placed on said portion of skin (5) corresponding to a muscle whose contraction generates said surface electromyographic signals.

21. A method for acquisition of bioelectric signals, comprising:

- placing at least a first detection electrode (2) on a portion of skin (5) for detection of said bioelectric signals; and

- connecting electrical connection means (8) to said first detection electrode (2) for its connection to an electronic device (9) for processing of said bioelectric signals; characterized in that said step of connecting comprises removably coupling said electrical connection means (8) to said first detection electrode (2), via magnetic interaction.

22. The method according to claim 21, wherein said first detection electrode (2) has a shape with a cavity and defines an opening (7); and said step of coupling comprises attaching via magnetic interaction a removable connection element (10) to said first detection electrode; further comprising, before said step of coupling, the step of introducing in said opening (7) a conductive material, in particular a conductive gel, to improve an electrical contact with said portion of skin (5) .

23. The method according to claim 22, further comprising, after said step of coupling, the step of checking a quality of said bioelectric signals, and the steps of removing said removable connection element (10), refill said opening (7) with said conductive material, and re-attach said removable connection element (10), whenever the quality of said bioelectric signals does not satisfy given criteria.