WO2022173346A1

WO2022173346A1 - Body electrode for electrophysiological signals monitoring and recording

Info

Publication number: WO2022173346A1
Application number: PCT/SE2022/050112
Authority: WO
Inventors: Krister SJÖBERG; Göran Sjöberg
Original assignee: Piotrode Medical Ab
Priority date: 2021-02-10
Filing date: 2022-02-02
Publication date: 2022-08-18
Also published as: SE2150147A1; SE544506C2

Abstract

The present invention relates to a body electrode (100) for recording of electrophysiological signals from a body. The body electrode comprises a flexible lid (108, 208, 308, 808) covering a transducer compartment (109) with a transducer element (107, 807). The flexible lid (108, 208, 308, 808) is moveable from a first position defining a first volume of the transducer compartment (109) to a second position defining a second volume. The body electrode further comprises an air evacuation arrangement (111, 711, 811) arranged to evacuate air from the transducer compartment (109) upon the flexible lid moving from the first to the second position.

Description

BODY ELECTRODE FOR ELECTROPHYSIOLOGICAL SIGNALS MONITORING AND RECORDING

Field of the invention

The present invention relates to a body electrode for recording of electrophysiological signals from a body.

Background of the invention

Electrodes applied on the skin surface of a subject, e.g. a human, can be used to record electrophysiological signals produced e.g. by the heart, i.e. an electrocardiogram (ECG), by the brain, i.e. an electroencephalogram (EEG), by the eyes, i.e. an electroretinogram (ERG) and/or an electrooculogram (EOG). The quality of such a recording is limited by the performance of the used electrodes, i.e. the body electrodes. The body electrodes may be subject for different disturbances that in turn give rise to disturbances in the output of the recorded electrophysiological signals. One such disturbance is motion induced disturbances causing motion induced disturbances in the recorded signal or signals. Motion induced disturbances may be caused by movements, deformations, and/or vibrations of the skin adjacent to the electrode, or of a combination of these. Such artifacts may increase in magnitude when the subject moves e.g. during ambulatory monitoring. The origin of the disturbances can also be movements of the electrode itself caused by external forces on the electrode from surrounding objects and from the electrode lead. Motion induced disturbances may impair a recording of electrophysiological signals, as they can create signal amplitudes that can be many times the amplitude of the electrophysiological signal to be recorded or that the disturbances themselves may be interpreted as electrophysiological signals. Additionally, the signals from motion induced artifacts can be in the frequency range overlapping the frequency range of the electrophysiological signals which makes it difficult to use conventional soft- and hardware filters to remove the artifacts from the recording.

Prior solutions to reduce motion artifacts include to have a collar provided with the body electrode and have it attached to the skin by an adhesive. This partially prevent movements of the body electrode from longitudinal forces on the body electrode or from stretches of the skin. Further, such a collar could be flexible to be able to follow the skin. An alternative to the adhesive is to use vacuum to attach the body electrode. A design that is often used today is to have the actual body electrode conductive skin contacting area enclosed by a cup and thus protected in the bottom of the cup, together with a flexible collar adhering to the skin.

Webster in IEEE Transactions on Biomedical Engineering, vol BME-13, issue 12, 1984 discloses a floating Ag/AgCl electrode that has reduced motion artifacts caused by the electrode.

US 3989035 discloses a body electrode wherein the transducer compartment of the electrode is stabilized with a flexible membrane attached to a flexible ring. The design reduces the motion artifacts on the transducer compartment (receiving and transmitting portion).

US 2005124902 discloses a body electrode with a dry electrode which is stabilized with a stiff outer ring. The electric stability of the transducer compartment is further stabilized with sharp objects penetrating the upper layers of the skin which stabilizes the electrical properties of the recorded signal.

Despite the prior art there is still a need for an improved body electrode that are at least partly prevented from being influenced by external forces longitudinal i.e. in the plane of the skin as well as forces perpendicular to the skin, and for a body electrode that reduces the effect of displacement and / or deformation of the conductive area at the interface between the skin and body electrode.

Summary

Although advances have been made in providing body electrodes that are less sensitive to disturbances resulting in for example motion artifacts in electrocardiograms there is still a need for improvements, especially if the electrocardiograms are to be automatically evaluated or evaluated by other persons than an experienced cardiologist.

This is achieved by the body electrode as defined in claim 1.

According to one aspect of the invention a body electrode for electrophysiological signal monitoring is provided. The body electrode is during use arranged to be attached to the skin of a subject, and the body electrode has a skin facing surface arranged to, during use, be in contact with the skin and a free surface arranged to be in contact with the ambient air. The body electrode comprises:

-a collar comprising a transducer compartment open towards the skin facing surface; -a transducer element at least partly arranged in the transducer compartment; -a connector in electrical contact with the transducer element;

-a flexible lid covering the transducer compartment and arranged so that the transducer compartment is open to the skin facing surface, and wherein the flexible lid is fixed to the collar and arranged to seal the transducer compartment on the side of the free surface of the body electrode; and wherein

-the flexible lid has a first position defining a first volume of the transducer compartment and the flexible lid being movable into the transducer compartment in the direction towards the skin facing surface to a second position defining a second volume of the transducer compartment, the second volume of the transducer compartment being smaller than the first volume and wherein the air evacuation arrangement is arranged to let air out from the transducer compartment when the flexible lid moves from the first to the second position

The body electrode is further provided with at least one air evacuation arrangement comprising a channel, the channel having an inlet in the transducer compartment and extending to an outlet in connection with an air receiving receptacle.

According to one aspect of the invention the body electrode comprises retaining means arranged to retain the flexible lid at the second position after the flexible lid has been moved by an external force from the first position to the second position.

According to one embodiment the channel is a surface channel provided in the skin facing surface of the collar and the surface channel extending in a radial direction from the transducer compartment to the free surface on the outer perimeter of the collar.

According to one aspect of the invention the air evacuation arrangement comprises a valve arranged in connection with the channel and the valve controlling the air flow in the channel.

According to one embodiment the valve is a membrane valve comprise a thin membrane arranged on and partly attached to the free surface and covering the outlet and arranged so that a portion of the thin membrane is not attached to free surface and may raise from the free surface to open the valve at an increase in pressure in the transducer compartment.

According to one embodiment the valve is provided by the channel having a first relaxed and collapsed state in which the channel is closed and a second temporarily open state in which the channel is open, and wherein the channel goes from the first relaxed and collapse state to the open state upon increasing pressure in the transducer compartment. The channel may be a punctured structure in the collar, wherein the punctured structure has been provided by penetrating the collar with a needle, the needle having a diameter of 0.2 - 1 mm. A suitable material for providing this type of valve is a polyvinyl chloride foam having a density of approximately 200 kg/ m3.

According to one embodiment the air receiving receptacle is a space closed from the ambient air and formed by an expanding membrane provided on the free surface and covering the outlet and forming an airtight seal to the channel, wherein the expanding membrane is arranged to expand upon the increase of pressure in the transducer compartment, thereby the volume of the air receiving receptacle is increased. The expanding membrane may cover a major portion of the free surface of the body electrode.

According to one embodiment the valve is a check valve.

According to one aspect of the invention the flexible lid is a bi-stable lid having a convex stable position and a concave stable position with regards to the skin facing surface, wherein the convex stable position constitutes the first position of the flexible lid defining the first volume of the transducer compartment and the concave stable position constitute the second position of the flexible lid defining the second volume of the transducer compartment. The bi-stable lid may comprise a ring-formed stabilizing outer part.

According to one aspect of the invention the body electrode further comprises a retaining member arranged to retain the flexible lid in the second position.

According to one embodiment the retaining member is a bi-stable dome-shaped cover having a stable convex position and a stable concave position with regards to the skin facing surface, wherein at least at the concave stable position the bi-stable dome shaped cover is in contact with the flexible lid and defines the second position of the flexible lid. The bi-stable dome-shaped cover may comprise a ring-formed stabilizing outer part.

According to one embodiment the retaining member is a protrusion on the wall of the transducer compartment, the protrusion arranged to engage with a part of the flexible lid being in the second position. The protrusion may form a circumferential surface facing the free surface of the body electrode, and wherein the circumferential surface is at least partly provided with an adhesive, the adhesive arranged to contact and retain the flexible lid in the second position.

According to one aspect of the invention the body electrode comprises an outer lid movable in the direction towards the skin facing surface, the outer lid comprising a rigid part and a compressible part which is arranged on the side of the rigid part facing the skin facing surface and on its opposite side attached to the flexible lid, and wherein the rigid part extends at least partly a distance from the compressible part in a radial direction, and wherein the outer lid is arranged to have a first position relating to the first position of the flexible lid, wherein the compressible part is uncompressed and wherein there is a gap between the rigid part of the outer lid and the collar, and wherein the outer lid is arranged to have a second position relating to the second position of the flexible lid, wherein the rigid part of the outer lid abuts the collar and the compressible part is compressed. According to one embodiment the collar on a surface facing the outer lid and/or the rigid part of the outer lid on the surface facing the collar is provided with an adhesive so that the outer lid may be retained in its second position and thereby the flexible lid in its second position.

According to one aspect of the invention the body electrode comprises a sealing rim provided on the skin facing surface of the collar and enclosing the transducer compartment, the sealing rim extending radially at least over a portion of the skin facing surface of the collar. According to one embodiment the sealing rim extends radially inwards a distance into the transducer compartment.

According to one aspect of the invention the body electrode comprises a separator provided in the transducer compartment. The separator is arranged to separate an electrolyte medium from the inlet of the air evacuation arrangement and is permeable to air. According to one embodiment the separator is ring-shaped and extends below the skin facing surface of the collar. According to one embodiment a sealing rim is provided on the skin facing surface of the collar and enclosing the transducer compartment, the sealing rim extending radially at least over a portion of the skin facing surface of the collar and at least over a portion of the skin facing surface of the separator.

Thanks to the invention a body electrode with less sensitivity to disturbances, in particular disturbances caused by motion effects may be provided. Thereby so-called motion artifacts may be reduced in the results from an ECG-measurement, for example making the interpretation of an electrocardiogram easier and more reliable.

One advantage of the body electrode afforded by the invention is that means for evacuating air from the transducer compartment of a body electrode may be provided with a very limited re-design of the basic structure of existing body electrodes, making the novel design fast and relatively cost effective to implement. The means for evacuating air according to the invention, does for example not require re-design of connectors or other parts, hence the body electrode according to the invention may be conveniently and economically introduced in existing measurement systems.

In certain embodiments an under-pressure may be achieved in the transducer compartment as compared to the surrounding ambient air, which, may have positive effects on the stability of the measurements.

It is an advantage of the invention that the air evacuation arrangements and / or the arrangement utilizing a retaining member and/or the stabilizing arrangements may be combined with body electrodes of different types, including but not limited to wet type electrodes, solid type electrodes and dry electrodes.

It is advantage with the invention that an increased pressure on the skin surface at the interface with the body electrode may be achieved, which may be beneficial in reducing and/or stabilizing the electric potential and impedance in the skin and skin/ electrolyte interface resulting in a higher signal to noise ratio.

It may be advantageous in certain applications that the electrolyte in the first upper position may be held a distance above the skin surface. This minimizes the risk that the electrolyte is smeared out on the skin upon applying the body electrode to the skin, thereby takes away some of the errors associated with the human error /handling of the body electrode which prior art electrodes are suffering from. A less trained person may hereby achieve the same result as a highly experienced person.

An advantage is that the strength / aggressiveness in the adhesive glue coated on the body electrode’s skin facing surface can be reduced while avoiding leakage an advantage compared to prior art electrodes. Yet another advantage is the improved long-term stability of the body electrode so that a body electrode according to the invention can be worn for a longer period of time without the risk that gel escapes the transducer compartment and negatively affects the adhesion of the body electrode so that it impairs the recording and in worst case falls off. This is made possible thanks to the sealing rim and the fact that no or at least very little air is trapped in the transducer compartment which may otherwise spread out on the skin facing surface if pressure is applied unintentionally on the body electrode for instance when the patient is lying in bed and the mattress is pushing on the body electrode.

A further advantage with the sealing rim is that the area of the skin contact electrolyte is well defined and located centrally in relation to the rim of the collar.

Yet another advantage is that the lid/ cover may be provided in a stiff material and therefore even further stabilizes recorded electrophysiological signal by mechanically stabilizing the structural integrity of the transducer compartment of the body electrode and in particular the electrolyte/ skin area from deformation and relative movement once the air is evacuated from the transducer compartment.

In the following, the invention will be described in more detail, by way of example only, with regard to non-limiting embodiments thereof, reference being made to the accompanying drawings. Description of drawings

Figures la-g are schematic illustrations of embodiments of the body electrode according to the invention wherein a-d and g) are cross-sectional views through the centre of the body electrode, and e-f) are elevated views;

Figures 2a-b are schematic illustrations of embodiments of the body electrode according to the invention in a cross-sectional view through the centre of the body electrode;

Figures 3a-b are schematic illustrations of embodiments of the body electrode according to the invention in a cross-sectional view through the centre of the body electrode;

Figures 4a-g are schematic illustrations of embodiments of the body electrode according to the invention in a cross-sectional view through the centre of the body electrode;

Figures 5a-b are schematic illustrations of embodiments of the body electrode according to the invention in a cross-sectional view through the centre of the body electrode;

Figure 6 is a schematic illustration of an embodiment of the body electrode according to the invention in a cross-sectional view through the centre of the body electrode;

Figure 7a-e are schematic illustrations of embodiments of the body electrode according to the invention in cross-sectional views through the centre of the body electrode (a-d) and in a view from below (e);

Figure 8a-b are schematic illustrations of embodiments of the body electrode according to the invention in a cross-sectional view through the centre of the body electrode;

Figure 9a-c are schematic illustrations of embodiments of the body electrode according to the invention, wherein a) and c) are cross-sectional views through the centre of the body electrode, and b) is a view from below;

Figure lOa-b are schematic illustrations of embodiments of the body electrode according to the invention in an elevated view; and Figure lla-b are electrocardiograms recorded with a) prior art body electrodes and b) with body electrodes according to the invention.

Detailed description

Terms such as ’’top”, “bottom”, “upper”, “lower”, “below”, “above” etc. are used merely with reference to the geometry of the embodiment of the invention shown in the drawings and/or during normal operation of the electrode and are not intended to limit the invention in any manner.

The function of a body electrode is to receive and record electrophysiological signals from a body. The signals are recorded by a transducer element and transmitted to various medical instruments. A transducer element is in contact with the skin of a body via an electrolyte medium. Body electrodes are commonly categorized according to at least three different types “wet types”, “dry types” and “solid types”. Body electrodes that are so called “wet types’ comprises a viscoelastic gel as the electrolyte medium. Typically for wet type body electrodes the electrolyte medium is a gel comprised in a supporting structure, for example a sponge-like material. “Dry type” electrodes do not comprise an electrolyte medium as fabricated but instead the electrolyte medium is formed by sweat, moisture or an electrolyte (gel, spray, etc.) that is applied directly before use. There is also a type of body electrodes that comprises a solid electrolyte such as a hydrogel (crosslinked polymer matrix), such body electrodes are called ‘solid types’. In the following the body electrode is primarily described and depicted as a “wet type” with the electrolyte medium comprised in a supporting structure. However, all described features are relevant, if not explicitly otherwise stated, for also other types of body electrodes for example, but not limited to the dry types and solid types of body electrodes.

The transducer compartment and in particular the area where the electrolyte medium is in contact with the skin is sensitive to mechanical disturbances. In order to reduce disturbances in the recorded electrocardiogram the transducer compartment and particularly the area where the electrolyte medium is in contact with the skin, should be protected from disturbances, especially disturbances at the interface between the electrolyte medium and the skin, herein also referred to skin/ electrolyte interface. Disturbances may cause changes to the potential at the skin/ electrolyte interface which may result in disturbances in the recorded electrocardiogram, wherein “potential” refers to an electric potential and/or an electrochemical potential, typically measurable with a common voltmeter and/or oscilloscope. Disturbances may be reduced and / or stabilized using a body electrode according to the invention. Disturbances may originate from mechanical movement both longitudinal, i.e. in the plane of the skin, disturbances, and perpendicular, i.e. perpendicular to the skin plane, disturbances e.g. disturbances caused by clothes, movements in the skin etc.

Without being bound by theory, one factor that appears to have importance for the disturbances is the existence of air bubbles and electrolyte voids in the electrolyte medium. Particularly for the wet type body electrodes air bubbles may be comprised in the part of the body electrode that during use comprises the electrolyte medium, i.e. the transducer compartment, and/or in the electrolyte medium. When such a body electrode moves for example during movement of the person the body electrode is attached to, the air bubbles in the transducer compartment may move and cause changes in the electric potential which in turn causes disturbances in the recorded electrophysiological signal. The air bubbles may be present in the transducer compartment, for example in the gel, prior to use or/ and air may become trapped in the transducer compartment when the body electrode is attached to the skin.

In body electrodes of dry or solid type, areas, referred to as electrolyte voids, where little or no electrolyte is present in the interface with skin may occur. Electrolyte voids may for example be areas where the electrolyte in form of thin films of fluid is not present or at least substantially reduced. When such a body electrode is attached to the body the electrolyte voids tend to move around as the body electrode moves and hence cause changes in the electric potential which in turn causes disturbances in the recorded electrophysiological signal.

A body electrode of the invention comprises an improved way of protecting the area where the electrolyte medium is in contact with the skin from mechanical disturbances, in particular it provides a way of reducing the disturbances caused by air bubbles, and/or electrolyte voids, in the transducer compartment.

A body electrode 100 according to the invention is schematically illustrated in Figures la-e. Figure la shows the body electrode 100 in cross-section through the midpoint. The body electrode 100 is arranged to be attached to the skin 101 of a subject, e.g. a human or an animal. The body electrode 100 has two surfaces: a skin facing surface 102 arranged to be in contact with the skin 101 during use of the body electrode 100, and an opposite free surface 103. The free surface 103 is arranged to be in contact with the ambient air 104 during use of the body electrode 100. The body electrode 100 comprises a collar 105 having at least one transducer compartment 109 which is open at least to the skin facing surface 102. The transducer compartment 109 is arranged to, at least during use of the body electrode 100, accommodate an electrolyte medium 110. As an illustrative example, for wet type body electrodes the electrolyte medium 110 is a gel comprised in a supporting structure, for example a sponge-like material, the supporting material is usually mechanically attached to the lid by for instance ultra-sonic welding technique or similar. A transducer element 107 is at least partly arranged in the transducer compartment 109. The transducer compartment 109 is closed on the free surface 103 by a flexible lid 108. The flexible lid 108 may be an integral part of the collar 105, so that the transducer compartment 109 is a cut-out in the collar 105 on the side of the skin facing surface 102. Alternatively, the flexible lid 108 is a separate member of the body electrode 100 and the flexible lid 108 is fastened to the collar 105 by for example gluing, welding or the like. The dimensions and material of the flexible lid 108 and the transducer compartment 109 are selected so that the flexible lid 108 may be pressed towards the skin facing surface 102 a distance into the transducer compartment 109 by the moderate pressure accomplished by a person pressing a finger on the lid, and thereby reducing the volume of the transducer compartment, this will be referred to as the flexible lid 108 having a first position a first distance from the skin facing surface 102 and a second position a second distance from the skin facing surface 102 and wherein the second distance is shorter than the first distance. Different embodiments of the flexible lid 108 will be described below. The first position of the flexible lid 108 defines a first volume of the transducer compartment 109 and the second position defines a second volume of the transducer compartment 109, wherein the second volume of the transducer compartment 109 being smaller than the first volume.

According to embodiments the first position of the flexible lid 108 can be seen as a relaxed position, i.e. the position the flexible lid 108 will have relative the skin facing surface 102 without experiencing any external force. The second position may be a depressed position under the influence of an external force in the direction towards the skin facing surface 102. In embodiments the flexible lid 108 is arranged to maintain the second position also after the external force has been removed. The flexible lid 108 may be maintained at the second position by retaining means including, but not limited to a separate retaining member, the flexible lid having two different mechanical stable positions or that an under-pressure is created in the transducer compartment 109 due to the pressing action thereby keeping the flexible lid 108 at the second position. Different embodiments of the flexible lid 108 will be further described below.

Typically, dimensions of the flexible lid are an outer diameter of 10 - 30 mm, e.g. 17 mm. A typical force needed for the moderate pressure is in the order of 0.5 - 2 N e.g. 1 N. The typical pressure may result in a temporarily reduction of volume of the transducer compartment 109 in the order of 1-50%. For wet type electrodes and solid type electrodes a volume reduction of 1-10% is typically appropriate. For dry type electrodes a reduction of volume in the order of 25-50% is typically appropriate. According to the invention, the body electrode 100 is provided with at least one air evacuation arrangement 111 which is arranged to provide a fluid communication between the transducer compartment 109 to an air receiving receptacle 11 Id. The air evacuation arrangement 111 is provided with an inlet 111b leading from the transducer compartment 109 and an outlet 111c leading into the air receiving receptacle 11 Id, and a channel 111a connecting the inlet 111b and the outlet 111c. The air evacuation arrangement 111 allows for air exiting the transducer compartment 109 when the transducer compartment 109 goes from its first volume to its second volume relating to the first position of the flexible lid 108 and the second position of the flexible lid 108, respectively. According to the embodiment depicted in Figure la the outlet 11 lc is provided on a portion of the collar facing away from the skin facing surface 102 and the channel 111a has an angel to the skin facing surface 102. In this embodiment the air receiving receptacle 11 Id is the ambient air 104 surrounding the free surface 103 of the body electrode 100 during use.

Illustrated in Figure lb is the flexible lid 108 pressed down by the external force for example the force from a fingertip being pressed towards the skin facing surface 102, in the figure indicated by an arrow, resulting in a reduced volume of the transducer compartment 109. The air evacuation arrangement 111 is arranged to let out air from the transducer compartment 109 upon the increase of pressure that will result from applying a force on the flexible lid 108 and the flexible lid 108 thereby moving from the first to the second position as described with reference to Figure lb. A tactile indication to the user that enough pressure has been applied may be provided by the counter-pressure provided by the electrolyte medium 110 after the air in the conductive department has been evacuated. At that point the user will feel an increased counter-pressure, which serves as an indication to release the pressure on the flexible lid. Figure lb further illustrate an embodiment wherein the channel 111a is essentially parallel to the skin facing surface 102. According to one embodiment, schematically illustrated in Figure lc, the air evacuation arrangement 111 is provided with an inlet 111b in the transducer compartment 109 and an outlet 111c leading into a dedicated air receiving receptacle

I I Id. The dedicated air receiving receptacle 11 Id should comprises at least one part that is highly flexible, so that the volume of the dedicated air receiving receptacle

I I I d may increase approximately with the same volume as the volume of the transducer compartment 109 decreases upon applying pressure on the flexible lid 108, as described with references to Figure lb. This may be described as the expansion ability of the dedicated air receiving receptacle 11 Id at least matching the decrease in volume of the transducer compartment 109 upon applying pressure. If a plurality of air evacuation arrangements 111 and dedicated air receiving receptacles 11 Id are provided the expansion ability of the combined dedicated air receiving receptacles 11 Id should at least match the decrease in volume of the transducer compartment 109. According to one embodiment the dedicated air receiving receptable 11 Id comprises an expanding membrane 11 le is provided and covers the outlet 111c. The expanding membrane 11 le is arranged to provide an airtight seal to the free surface 103. The expanding membrane l l le may be glued or melted to the free surface 103, for example. As depicted in Figure lc, the expanding membrane l l le is chosen to be sufficiently flexible to allow an increase in volume of the dedicated air receiving receptacle 11 Id by bulging out from the free surface 103 upon applying pressure on the flexible lid 108. The outlet 111c may be widened close to the free surface as compared to the channel 11 la to provide sufficiently large surface for the expanding membrane l l le to perform the bulging action. Preferably the diameter of the expanding membrane l l le covering the outlet 11 lc is at least in the order of 2-4mm. The expanding membrane l l le may also cover a larger area of the free surface 103, for example covering a plurality of channel outlets 111c, wherein the individual membranes for each channel outlet 111c may be formed from one piece of material by cuts through the membrane, for example semi-circular cut around a channel outlet 100c. The expanding membrane may be fabricated in a sheet of an elastomer for example natural rubber. Alternatively, the expanding membrane may be provided as a coating on at least a portion of the free surface 103.

According to embodiments the air evacuation arrangement 111 is arranged to be openable at least at some point during the action of applying pressure to the flexible lid 108. Thereby the air which was evacuated from the transducer compartment 109 is not re-entered into the transducer compartment 109 upon releasing the force on the flexible lid 108. A pressure equalization or an under-pressure may hence be achieved in the transducer compartment 109 as compared to the surrounding ambient air 104, which, in addition to reducing the amount of air bubbles and/or electrolyte voids for example in the electrolyte medium 110, may have positive effects on the stability of the measurements. It has further been shown important that, during measurement, the air evacuation arrangement 111 do not provide an electrical path from the free surface 103 into the transducer compartment 109. The air evacuation arrangement 111 should be closeable and kept closed during measurements or permanently closed as in the embodiment described with reference to Figure lc or by other means hinder an electrical path to be formed. The air evacuation arrangement 111 may according to embodiments, schematically illustrated in Figure Id be provided with a valve 11 If that is arranged to, upon increase of pressure in the transducer compartment 109, open and provide for an evacuation of air. The valve 11 If may be provided in the channel 111a at the inlet 111b or at the outlet 111c (illustrated). The valve 11 If may be a check valve with a flow direction out from the transducer compartment 109, for example but not limited to a membrane valve. Embodiments of the valve 11 If will be described below.

The body electrode 100 further comprises a connector 112 at the free surface 103, which is schematically illustrated in elevated views in Figures le-f and in cross- section in Figure lg. The connector 112 is in electrical contact with the transducer element 107, for example via a lead 113 as in Figures 8a-b. The connector 112 can be covered by a connector cover 112a. The connector 112 may be formed by an eyelet/ snap design wherein the eyelet or connector cover 112a is snapped onto the connector 112, so that the connector 112 is covered by a connector cover 112 forming a lead connector receptacle. During use of the body electrode 100, the transducer element 107 is arranged to transmit signals from the skin 101 to the connector 112, via the electrolyte medium 110. The electrolyte medium 110 may be in the form of a gel, a liquid, a liquid gel, a hydrogel, sweat, moisture etc. depending on the type of body electrode as discussed previously. Embodiments of the dry type electrode is further described below with reference to Figures 8 a-b.

The connector 112 may be arranged at a distance from the transducer compartment 109 as illustrated in an elevated view in Figure le. In embodiments with the connector 112 provided a distance from the transducer compartment 109 the transducer element 107 may be attached to the collar 105 and connected to a lead 113 also illustrated in Figure 8 a-b which connects to the connector 112. Alternatively, in another embodiment the connector 112 is placed centrally at the body electrode 100, forming part of the free surface 103, as illustrated in an elevated view in Figure If and in cross section in Figure lg. In this embodiment the transducer element 107 is typically attached to the flexible lid 108 and a part of the transducer element 107 may extend and connect directly to the connector 112. The centrally placed connector 112 may have the advantage of allowing for in the same action connecting to the connector with a corresponding connecting part 115 of a for example cable lead 114 and providing the downward force on the flexible lid 108. In the following the embodiments of the invention will be, for the reason of clarity, shown without the connector 112. However, apart from in cases obviously inappropriate, the connector may be provided either essentially centrally over or at a distance from the transducer compartment 109.

The transducer element 107 may be fabricated in a conducting material for example a metal or a conducting polymer, e.g. acrylonitrile butadiene styrene (ABS) filled with carbon powder. The conducting material may further be coated with Ag/AgCl at least at the surface of the transducer element 107 that faces the skin facing surface 102.

The collar 105 may be used to attach the body electrode 100 to the skin 101. The collar 105 may be fabricated in a closed cell foam in a polymeric material, such as polyethylene (PE), polypropylene (PP), or polyvinyl chloride (PVC) for example or it may be fabricated in an elastomer. Examples of such materials include styrenic block co-polymers (TPS, TPS-s), thermoplastic polyolefmelastomers (TPO, TPE-o), and thermoplastic vulcanizates (TPV, TPE-v).

The collar 105 may be provided with an adhesive or an adhesive layer to fix the body electrode 100 to the skin 101 of the user during use. Alternatively, the body electrode may be fixed to the skin by a strap or similar arrangement that holds the body electrode in place by mechanical means.

A sealing member is provided which is attached to the collar 105 or provided in close proximity to the collar 105. The sealing member may also be provided as an integral part of the collar 105. The sealing member encloses the transducer compartment 109 so that during use, the electrolyte medium 110 will not, at least not in any considerable amount, leak out from the transducer compartment 109 and spread between the skin facing surface 102 and the skin 101 of the user. According to the embodiment depicted in Figure la, the sealing member is a sealing rim 120 provided on the skin facing surface 102 of the collar 105 and with an inner diameter that is the same, or according to one embodiment, a few millimetres smaller than the diameter of the transducer compartment 109 at the skin facing surface 102. The sealing rim 120 may be fabricated in a sheet of elastomer or PE, with a thickness of 0.1-1 mm, on the skin facing surface it may be coated with an adhesive so that it adheres to the skin. The sealing rim 120 prevents electrolyte medium 110 from leaking out especially when the force from the finger is applied to the transducer compartment 109. Especially with a sealing rim 120 extending a distance into the transducer compartment 109 the strength/ aggressiveness in the adhesive glue coated on the body electrode’s skin facing surface 102 can be reduced while avoiding leakage. Yet another advantage is the improved long-term stability of the body electrode so that a body electrode according to the invention can be worn for a longer period of time without the risk that gel escapes the transducer compartment and negatively affects the adhesion of the body electrode so that it impairs the recording and in worst case falls off. This is made possible thanks to the sealing rim and the fact that no or at least very little air is trapped in the transducer compartment which may otherwise spread out on the skin facing surface if pressure is applied unintentionally on the body electrode, for instance when the patient is lying in bed and the mattress is pushing on the body electrode.

According to embodiments of the invention, schematically illustrated in Figures 2a- b the flexible lid 108 of the body electrode 100 is a suspended membrane 208 arranged in the transducer compartment 109 of the collar 105 so that it extends radially in the transducer compartment 109 forming part of the free surface 103. Alternatively, the suspended membrane 208 is arranged in between the free surface 103 and the transducer compartment 109, covering the transducer compartment 109. The suspended membrane 208 is fixed to the collar 105.

According to one embodiment the suspended membrane 208 is arranged to move almost freely in the direction perpendicular to the skin facing surface 102 , and hence the skin 101. The suspended membrane 208 may be fabricated in a material having a high flexibility such as for example an elastomer having a thickness of 0.01-1 mm. Examples of such materials include styrenic block co-polymers (TPS, TPS-s), thermoplastic polyoleiinelastomers (TPO, TPE-o), and thermoplastic vulcanizates (TPV, TPE-v). Pressure can be applied to the suspended membrane 208, for example by a fingertip, at which it can change position from the relaxed position illustrated in Figure 2a to the second position illustrated in Figure 2b. According to one embodiment the suspended membrane 208 comprises two parts: a central part 208a surrounded by an outer flexible part 208b, wherein the central part 208a typically is more rigid than the outer flexible part 208b. The flexible part 208b may comprise grooves or joints 208c to enable movements. To have a central part 208a that is more rigid than the surround part may facilitate an even distribution of pressure upon the downward pressing action of a user.

According to one embodiment of the invention, schematically illustrated in Figures 3a-b that each show a cross-section through the mid-point of a body electrode 100, the flexible lid is a bistable lid 308 which is arranged to switch between two stable positions, a first position or convex position (figure 3a) and a second position or concave position (figure 3b). Convex/ concave is with respect to the skin facing surface 102. The switch of the bistable lid 308 from the convex position to the concave position is initiated by a moderate force on the outside of the bistable lid 308 for example by the pressure of a fingertip. This can be described as a “snap-action” from an initial stable upper position, the first position to a stable lower position, the second position, of the flexible lid 108. According to one embodiment, the electrolyte medium 110 in form of a hydrogel or a gel contained in a sponge-like material is fixed to the bistable lid, when the bistable lid is in convex position, the first position, the surface of the electrolyte medium 110, or the bottom surface of a dry electrode, facing the skin is positioned a distance from the skin facing surface 102 and thereby protected in the transducer compartment. When the bistable lid changes position to the concave position, the second position, by a user pressing with a gentle force from e.g. a fingertip on the bistable lid, the electrolyte medium 110 engages (contacts) the skin. This feature lessens the risk of having the conductive medium smeared out underneath and between the skin contacting surface 102 of the body electrode and the skin and thereby loosing adhesion of the body electrode during the application procedure of the body electrode which may impair the body electrode to record a non- distorted signal.

The bistable lid 308 may be in the shape of a dome having a central part 308a and an outer part 308b which attaches to the collar 105. According to this embodiment only the central part 308a undergoes the snapping action, i.e. the transformation between the two stable positions while the ring-formed outer part 308b is essentially fixed. The ringed-formed outer part 308b further forms a stabilizing member to the body electrode 100, mechanically stabilizing the transducer compartment. The bistable lid 308 may be fabricated in a stiff material such as a polymeric material or a mixture of polymeric materials. Examples of polymeric materials include polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polyamide (Nylon), polycarbonate (PC), polyethylene (PE), polypropylene (PP), and acrylonitrile butadiene styrene (ABS).

According to embodiments of the invention the flexible lid 108 is provided with, or arranged to interact with, a separate retaining member. The retaining member is arranged to hold the flexible lid 108 in the second position, the second position, achieved by the external force pressing the flexible lid 108 towards the skin facing surface 102. The retaining member is combined with the air evacuation arrangement 111 to provide a measurement stabilizing effect, the effect is achieved by excess air being evacuated from the transducer compartment 109 by the pressing action on the flexible lid 108 and by the flexible lid 108 being retained in a lower position.

According to embodiments of the invention the retaining member interacts with the flexible lid 108 through a mechanical interaction wherein a mechanical locking occurs of the flexible lid 108 in the second position. According to embodiments of the invention the retaining member interacts with the flexible lid 108 through adhesion, wherein the flexible lid 108 is maintained at the second position by adhesion to the retaining member.

According to one embodiment, schematically illustrated in a cross-section through the mid-point of a body electrode 100 in Figures 4a-b, the body electrode 100 is provided with a dome-shaped cover 401, the dome-shape cover 401 forming both a protective cover and the retaining member. The dome-shaped cover 401 is arranged to have a first stable convex position, as illustrated in Figure 4a, and a second stable concave position, as illustrated in Figure 4b, wherein concave and convex is in relation to the skin facing surface 102. The dome-shaped cover 401 is arranged to change from its convex position to its concave position by application of pressure at the outside surface of the dome-shaped cover 401, for example by a fingertip, in a similar snap-action as described above. When the dome-shape cover 401 is in the first stable convex position the flexible lid 108 will be in its first position. When in its concave position the dome-shaped cover 401 will interact with the flexible lid 108 to hold the flexible lid 108 in its second position. The flexible lid 108 a suspended membrane 208 with its first position being its relaxed position. The vertical movement of the flexible lid 108 resulting from the dome-shaped cover 401 changing between the convex and concave position may be in the order of 0.1-5 mm.

The “snap action” characterizing the bi-stable lid and the bi-stable cover going from the convex to the concave position provides an audible and / or tactile indication to the user that the body electrode is ready to use.

The dome-shaped cover 401may be in the shape of a dome having a central part 401a and an outer part 401b which attaches to the collar 105. According to this embodiment only the central part 401a undergoes the snapping action, i.e. the transformation between the two stable positions while the ring-formed outer part 401b is essentially fixed. The ringed-formed outer part 401b further forms a stabilizing member to the body electrode 100, mechanically stabilizing the transducer compartment.

The dome-shaped cover 401 may be fabricated in a stiff material such as a polymeric material or a mixture of polymeric materials. Examples of polymeric materials include polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polyamide (Nylon), polycarbonate (PC), polyethylene (PE), polypropylene (PP), and acrylonitrile butadiene styrene (ABS).

According to embodiments the bistable lid 308 or the dome shape cover 401 are arranged to be movable beyond its second position, the concave position, under the action of providing external pressure to the bistable lid 308 or dome-shaped cover 401. After the pressing action, the bistable lid 308 or the dome shaped cover 401 will move slightly upwards to its second position, the stable concave position. An under pressure may hence be achieved in the transducer compartment 109 as compared to the surrounding ambient air 104, which, in addition to reducing the amount of electrolyte voids and air bubbles in the electrolyte medium 110, may fixate the skin inside the transducer compartment and in particular the skin/ electrolyte interface and/or the electrolyte / solid interface. Also, other embodiments of the invention may allow pressing the flexible lid 108 beyond its second position towards the skin facing surface 102 and returning to the second position after releasing the pressure. Thereby an additional amount of air is evacuated from the transducer compartment 109 and an under-pressure is achieved in the transducer compartment 109.

According to embodiments of the invention, schematically illustrated in Figures 4c- e the body electrode 100 is provided with an outer lid 450 that is movable in the direction towards the skin facing surface 102. In the embodiments illustrated in Figures 4c-e the outer lid 450 has a central area 450a second compared to a circumferential section 450b of the outer lid 450 with regards to the skin facing surface 102. The central area 450a of the outer lid 450 is attached to a central portion of the flexible lid, for example and preferably to the rigid central part 208a of the suspended membrane 208. The outer lid 450 is arranged to have a first position, illustrated in Figure 4c, relating to the first position, for example a relaxed position, of the flexible lid 108 wherein there is a gap between the outer lid 450 and the collar 105. In a second position, illustrated in Figure 4d, relating to the second position of the flexible lid 108, the outer lid 450 abuts the collar 105. The changing from the first position to the second position is a result from a user pressing the outer lid 450 downwards towards the skin facing surface 102. According to one embodiment the outer lid 450 has a concave portion 450c and a flat circumferential portion forming a rim 450d as illustrated in Figure 4e. The concave portion 450c has a diameter that is smaller than the diameter of the transducer compartment 109 and the rim 450d extends at least partly radially over the collar 105. As an alternative to the outer lid 450 having a concave form, the outer lid may have a varying thickness with the thickest portion in the central area 450a. A further alternative is the outer lid 450 being provided in the form of a truncated cone instead of the concave portion.

According to embodiments of the invention the collar 105 and/or the outer lid 450 is provided with an adhesive 453 so that the outer lid 450 may be retained in its second position and thereby the flexible lid 108 in its second position. In embodiments wherein the outer lid 450 is provided with a rim 450d, the adhesive 453 may be provided on the upper surface of the collar 105 and/or the lower surface of the rim 450d as illustrated in Figure 4e.

According to one embodiment schematical illustrated in Figures 4f-g the body electrode 100 is provided with an outer lid 451 that is movable in the direction towards the skin facing surface 102. The outer lid 451 comprises a rigid part 451a and a compressible part 451b which is arranged on the side of the rigid part facing the skin facing surface 102. On the opposite side the compressible part 451b is attached to the flexible lid 108, for example and preferably to the rigid central part 208a of the suspended membrane 208. The rigid part 451a extends at least partly a distance from the compressible part 451b in a radial direction. The outer lid 451 is arranged to have a first position, illustrated in Figure 4f, relating to the first position of the flexible lid 108, wherein the compressible part 451b is uncompressed and wherein there is a gap between the rigid part 451a of the outer lid 451 and the collar 105. In a second position, illustrated in Figure 4g, relating to the second position of the flexible lid 108, the rigid part 451a of the outer lid 451 abuts the collar 105 and the compressible part 451b is compressed. The changing from the first position to the second position is a result from a user pressing the outer lid 451 downwards towards the skin facing surface 102. The distance in radial extension between the rigid part 451a and the compressible part 451b is arranged so that the rigid part 451a at least partly extends beyond the compressible part 451b in both the un compressed and the compressed state of the compressible part 451b.

According to embodiments of the invention the collar 105 on a surface facing the outer lid 451 and/or the rigid part 451a of the outer lid 451 on the surface facing the collar 105 is provided with an adhesive 453 so that the outer lid 451 may be retained in its second position and thereby the flexible lid 108 in its second position.

The outer lid 450 and/or the rigid part 451a of the outer lid 451 is preferably fabricated in a stiff material such as a polymeric material or a mixture of polymeric materials. Examples of polymeric materials include polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polyamide (Nylon), polycarbonate (PC) and acrylonitrile butadiene styrene (ABS). The compressible part 451b is preferably fabricated in a plastic foam of open cell type made of polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC).

According to one embodiment of the invention, schematically illustrated in Figure 5a-b a mechanical retaining member 522 is utilized to achieve a similar functionality as with the bistable cover/lid. The mechanical retaining member 522 is arranged to interact with the flexible lid 108 at a predetermined distance from the skin facing surface 102. The flexible lid 108 may be the described suspended membrane 208 comprising a rigid central part 208a surrounded by an outer flexible part 208b. The mechanical retaining member may be provided as a protrusion, for example a rim 522, as depicted, around the inner surface of the transducer compartment 109, wherein the diameter of the opening defined by the rim 522 is slightly smaller than the diameter of the central part 208a of the suspended membrane 208. The suspended membrane 208 is arranged to initially be at the first position, the relaxed position. By applying pressure to the central non-flexible part 208a of the suspended membrane 208 the central non-flexible part 208a is forced to pass the rim 522, Figure 5a and after release of the pressure the rim 522 retains the suspended membrane 208 at the second position, Figure 5b. Alternatively, a number of protrusions, preferably three or more, are provided around the inner wall of the transducer compartment, providing the same retaining function as the continuous rim.

According to one embodiment, schematically illustrated in Figure 6a, the retaining member 622 comprises of protrusion in the form a shoulder 622a formed by the walls of the transducer compartment 109, providing a circumferential surface facing the free surface 103 and an adhesive 622b provided on the circumferential surface. The flexible lid 108 may be the described suspended membrane 208 comprising a rigid central part 208a surrounded by an outer flexible part 208b, being moved towards the skin facing surface 102, will come in contact with, and adhere to the adhesive 622b, maintaining the suspended membrane 208 at its second position. Alternatively, a rim is provided, similar to the rim described with reference to Figures 5a-b, which is provided with adhesive on the surface facing the free surface 103. Alternatively, a number of protrusions are provided around the inner wall of the transducer compartment, each provided with an adhesive.

A tactile indication, similar to the tactile indication given by the compression of the electrolyte medium as described above may be provided by the interaction of the flexible lid with the rim 522, protrusions or the shoulder 622a.

The air evacuation arrangement 111 may be arranged in the collar 105 and/or in the flexible lid 108/the suspended membrane 208.

According to one embodiment the air evacuation arrangement 111 is arranged in the collar 105 and comprises a channel 111a that is arranged to be arranged parallel with the skin facing surface 102 or at an incline of for example 30-45 degrees, which is illustrated in Figure la. Figure 7a-e illustrate, in a close-up cross sections, different embodiments of the valve 11 If. Illustrated in Figure 7a is a close up according the embodiment described with reference to Figure Id one embodiment comprising a valve 711 of membrane type. A thin membrane 71 lg is provided to cover the outlet 111c and arranged so that at least one portion of the membrane may open up at an increase pressure in the transducer compartment 109. The membrane may be comprised of an elastomer e.g. nitrile rubber (NBR) or hydrogenated nitrile rubbers (HNBR) and of a thickness of 0.1 - 1 mm e.g. 0.5 mm and is for example glued or welded to the free surface 103. Illustrated in Figure 7b-c, is an embodiment wherein the air evacuation arrangement 111 comprises essentially only a channel 711, wherein the channel has been provided with dimensions, which in combination with the material chosen for the collar 105, makes it possible for the channel to act as the valve 11 If. Illustrated in Figure 7b is the channel 111a in a closed or collapsed state. In Figure 7c the increase of the pressure in the transducer compartment 109 due to the flexible lid 108 having been pressed towards the skin facing surface 102, transducer compartment has caused the channel 711a to temporarily open to evacuate air from the transducer compartment 109. According to one embodiment the collar 105 is preferably made in closed cell polyvinyl chloride (PVC) foam having a density of -200 kg/ m³. The channel 111a is a punctured structure, i.e. the channel having been manufactured using a needle having a 0.2 mm - 1 mm diameter that is punctured through the collar 105 without removing any material from the collar 105. Due to the composition of the collar 105 having the elastic properties given by the chosen material, the air evacuation arrangement 111 is closed, when no over pressure is applied to the transducer compartment 109. If pressure is applied, for example by a touch of a finger on the flexible lid 108, the channel 711a will open up and the air evacuation arrangement 111 will let out an amount of air. After the air evacuation the channel 711a will return to its relaxed closed state and the air evacuation arrangement 111 is closed again preventing the ambient air 104 to enter the transducer compartment 109. Alternatively, or in combination, one or more channels 711a in the form of a punctured hole may be provided in the flexible lid 108.

In embodiments when the air evacuation arrangement 111 is arranged in the collar 105, the channel 11 la of air evacuation arrangement 111 is in the order of 2-10 mm long.

According to one embodiment the air evacuation arrangement 111 is arranged on the skin facing surface 102 of the collar 105 which is schematically illustrated in Figures 7d-e, wherein d) is a cross sectional view and e) is a view from below. The air evacuation arrangement 111 comprises according to this embodiment at least one and preferably a plurality of surface channels 721 in the skin facing surface 102 surface of the collar 105 and extends essentially radially from the transducer compartment 109 to the free surface 103 on the outer perimeter of the collar 105. During use the skin surface 101 and/or the sealing rim 120 will form part of the air channel (sealing rim not visible in this cross-sectional view). The surface channel 721 may be realized as a cut out in radial direction from the center, wherein the cut outs may have been made by a sharp blade. Alternatively, the cut-out structure may be formed by grooves that has been provided by hot stamping the collar 105. A further realization of the surface channels 721 is to provide a layer material to the collar 105 and arrange the surface channels in the layer material. Yet a further realization of the surface channels 721 is formed by a fold or dent in the skin facing surface 102 surface of the collar 105. The surface channels 721 may be fabricated in dimensions and with a choice of material of the collar 105 so the same “self-closing” functionality as described with reference to Figures 7b-c is provided. Alternatively, the surface channel 721 has an outlet 721c into a dedicated air receiving receptacle 11 Id or into a valve 11 If as described with reference to Figure lc-d.

Embodiments providing a bistable lid, are schematically illustrated in Figures 8a-b, each show a cross-section through the mid-point of a body electrode 100, the flexible lid is a bistable lid 808 which is arranged to switch between two stable positions, a convex position, the first position, (figure 8a) and a concave position, the second position (figure 8b). Convex/ concave is with respect to the skin facing surface 102. The switch of the bistable lid 808 from the convex position to the concave position is initiated by a moderate force on the outside of the bistable lid 808 for example by the pressure of a fingertip. This can be described as a “snap-action” from an initial stable upper position to a second stable lower position of the bi-stable lid 808. The bistable lid 808 may be in the shape of a dome having a central part 808a and an outer part 808b which attaches to the collar 105.

According to the embodiment illustrated in Figures 8a-b the body electrode 100 comprises a centrally positioned transducer element in the form of a solid sensor 807 comprising solid sensor base portion 807a and an eyelet or sensor cover 807b, the solid sensor base portion 807a has a skin facing surface 807c arranged to be in contact with the skin 101 when the body electrode 100 is in use. The solid sensor 807 is arranged to be fixed to the central part of the bistable lid 808. The solid sensor base portion 807a may have a protruding part that extends through an opening in the central part and anchors it to the bistable lid 808. The solid sensor is held in position by eyelet / snap design wherein the eyelet or sensor cover is snapped onto the solid sensor base portion and thereby locking the solid sensor in position on the central rigid part of the bistable lid 808.

The solid sensor 807 comprises an electron-conducting material, for example a metal or a conducting plastic made of e.g. acrylonitrile butadiene styrene (ABS) filled with carbon powder. The solid sensor 807 may further be coated with Ag/AgCl at least at the skin facing surface 807c that is arranged to be in contact with the skin 101 during use of the body electrode 100. The body electrode 100 further comprises a connector (not shown) arranged at a distance from the solid sensor at the free surface 103, the connector is in electrical contact with the solid sensor 807 via a lead 113 made for example of a thin metal strip with a strain relief 113a. The lead may be soldered to the solid sensor 807, preferably to the sensor cover. The solid sensor base portion 807a may be circular with a diameter of 4 mm up to 25 mm e.g. 8 mm depending on the size and form of the body electrode 100.

The solid sensor 807 engages the skin when the bistable lid 808 changes position from the convex position to the concave position. The skin facing surface 807c of the solid sensor 807 may in the second position be in the same vertical level as the skin facing surface 102 associated with the collar 105 or it may be lower than the skin facing surface 102 of the collar 105. The distance, d, between the skin facing surface 807c of the solid sensor 807 and the skin facing surface 102 of the collar 105 may range from 1 to 5 mm, typically 1-2 mm. A lowered position, i.e., larger distance d, means that the solid sensor 807 is pressed firmly down into the body surface creating a surface pressure on the skin 101 underneath the solid sensor 807. The increased pressure on the solid sensor / skin interface evens out the electrolyte and ensures that the electrolyte is evenly distributed on the surface of the solid sensor hence reducing electrolyte voids and thus stabilizing the measured signal from motion artifacts.

According to one embodiment the body electrode 100 the air evacuation arrangement comprises at least one pressure-controlled valve 811arranged in the bistable lid 808. The pressure-controlled valve 811 should be arranged so that it is in fluid connection with the transducer compartment 109. In one embodiment the pressure-controlled valve 811 comprises a sealing membrane 811a that covers one or more channels or openings 811b in the bistable lid 808, as illustrated in Figures 8a-b. The sealing membrane 811a may comprise a protruding part that extends through an opening in the bistable lid 808 and anchors the sealing membrane 81 la to the bistable lid 808. Alternatively, the sealing membrane 811a may be glued or welded to the flexible lid 808. Figure 8a illustrates the pressure-controlled valve 811 and the sealing membrane 81 la in a closed position, the pressure-controlled valve 811 changes from closed to open position i.e when pressure is applied on the bistable lid 808. The close-up show illustrations of the pressure-controlled valve 811 in higher magnification. The pressure-controlled valve 811 is arranged to be openable during the action of applying pressure to the bistable lid 808. The pressure-controlled valve 811 positioned in the flexible lid or they may also be combined with other types of body electrodes, for example the wet type and the solid type and in particular the here described embodiments.

The air which was evacuated from the transducer compartment 109 is not re-entered into the transducer compartment 109 upon releasing the force on the bistable lid 808 due to the air evacuation valves 811. According to one embodiment the bistable lid 808 is arranged to be movable beyond its concave position, the second position, under the action of providing external pressure to the bistable lid 808. After the pressing action the bistable lid 808 will move slightly upwards to its stable concave position, i.e. the second position. An under-pressure may hence be achieved in the transducer compartment 109 as compared to the surrounding ambient air 104, which, in addition to reducing the amount of electrolyte voids and air bubbles in the electrolyte medium 110, may fixate the skin inside the transducer compartment and in particular the skin/ electrolyte and/or the electrolyte/ solid sensor interfaces which stabilizes the measured signal even further.

According to one embodiment, illustrated in Figure 8a-b, the bi-stable lid 808 comprises an outer part 808b with a mechanically stabilizing structure 808c in the form of a ring which provides mechanical stability to the transducer compartment 109 and to the inner dome-shaped part 808a which has the stable convex position and the stable concave position, as described above. The pressure-controlled valves 811 may preferably be placed in the outer part 808b as to not be blocked during the pressing action, for example by a finger. The bi-stable lid 108 may also be combined with air evacuation arrangements 111 provided in the collar 105. The bistable lid may be fabricated from plastic sheets, typically having a thickness of 0.1 - 1 e.g. 0.2 mm made of plastic material for example polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), or polypropylene (PP) in an vacuum molding/ forming process.

In one embodiment, schematically illustrated in cross-section through the midpoint in Figure 9a, the body electrode 100 further comprises a separator 910 provided in the transducer compartment 109 and which is arranged to separate the electrolyte medium 110 from the inlet 111b of the air evacuation arrangement 111. According to one embodiment the separator 910 is ring-shaped and encloses the electrolyte medium 110. Alternatively, the separator 910 comprises separate parts 910b, wherein each part is arranged to cover at least one inlet 11 lb, which is schematically in Figure 9b, a view of the body electrode 100 from below. The separator 910 is made of a material permeable to air, but with significant less permeability to the gel, that typically forms the electrolyte medium 110. Such materials include, but is not limited to dense foams, mesh textiles and semipermeable membrane materials. Example of such materials are; Gore Tex and open cell polyethylene (PE) foam of density 150 kg/m3. The separator 910 may be provided in direct contact with the walls of the transducer compartment and/or the electrolyte medium 110, or alternatively the separator is provided a distance from the inner wall of the transducer compartment 109 and/or the electrolyte medium 110. According to one embodiment a sealing rim 920 is provided which has an outer portion 920a attached to the skin facing portion 102 of the collar 105 and an inner portion 905b attached to the skin facing surface of the separator 910 and thereby provides the same function as the above-described sealing rim 120. The sealing rim 920 may extend a distance radially inwards beyond the separator 910, which as described above provides an increased security against leakage of electrolyte medium. The distance may be in the order of a few millimetres.

In one embodiment, schematically illustrated in cross-section through the midpoint in Figure 9c, the separator 910c is ring-shaped and extends below the skin facing surface 102 of the collar 105. The sealing rim 920 may as describe above extend at least partly over the skin facing surface 102 of the collar 105 and the skin facing surface of the separator 910 and also a distance radially inwards beyond the separator 910. When the body electrode is attached to the skin the part of the sealing rim 920b provided on the separator 910 adheres as a first step to the skin 101 prior to the adhesion of the collar in this way leakage of gel from the transducer compartment is further reduced. The sealing rim prevents electrolyte medium 110 to leak out underneath the collar and thereby preventing it from negatively effecting the adhesive on the collar to adhere to the skin. An adhesive 905 may be provided that covers at least a portion of the skin facing surface of the collar 105 and the skin facing part of the sealing rim and/or ring-shaped separator 910.

The embodiments comprising a separator 910, 910b and 910c may advantageously be combined with all of the described air evacuation arrangements, for example a parallel or longitudinal extending air evacuation arrangement as in Figures 9a-b and an angled air evacuation arrangement as in Figure 9c. In embodiments the body electrode 100 comprises a plurality of air evacuation arrangements 111. Such an embodiment is schematically illustrated in Figures 10a- b showing an elevated view of a body electrode 100. In the embodiment illustrated in Figure 10a the body electrode 100 comprises four air evacuation arrangements 111 arranged on the flexible lid 108. The air evacuation arrangements 111 may be arranged symmetrically around a centre axis, , that passes vertically through the body electrode 100 at the centre of transducer compartment 109. Alternatively, the plurality of air evacuation arrangements 111 are provided with both their inlets 111b and outlets 111c in the collar 105, illustrated in Figure 10b and above described with reference to Figure 1-6. A combination of air evacuation arrangements 111 both in the collar 105 and in the flexible lid 108 may also be provided. It is preferably to provide the body electrode 100 with a plurality of air evacuation arrangement 111 in order to provide a redundancy if one of the air evacuation arrangements 111 malfunction, for example due to blockage, to facilitate air evacuation in all parts of the transducer compartment 109 and ensure that the air evacuation process is fast and smooth.

As previously described, mechanical stability of the body electrodes is of high importance. In particular, the transducer compartment 109 should be mechanically stable enough to keep its form (structural integrity) if subjected to external forces, both longitudinal and perpendicular forces. Several of the described features of the embodiments of the invention do form a stabilizing element or structure to the transducer compartment 109.

In the embodiments described with reference to Figures 4a-g, 5a-b and 6a the flexible lid 108, comprising a rigid central part 208a will in its retained position, i.e. the position of the flexible lid 108 during use of the body electrode 100, will also form a stabilizing structure being engaged with the retaining members. Similarly, in the embodiments described with reference to Figures 4a-g the dome-shaped cover 401 and the outer lid 450, 451 forms stabilizing structure to the body electrode 100 in the second positions that are activated during use of the body electrode 100. Thereby these members provide a dual function of retaining the flexible lid 108 in the second position and ensuring that air is not re-entered into the transducer compartment 109 and to provide mechanical stability to the body electrode 100, which in turn increases the signal stability during the ECG measurement. The structural stability of the body electrode 100 may be further enhanced by providing a stabilizing structure in form of a stabilizing ring on or in proximity to the collar 105. A stabilizing ring may, as described above be provided as a part, the outer part 308b, 808b, 401b of a bistable lid 308, 808 or a dome-shaped cover 401. Alternatively, a stabilizing ring is provided as a separate member provided on or in the collar 105 in proximity to the transducer compartment 105. An oval or circular shape of the stabilizing ring is typically preferable and a certain structural stiffness e.g. by being fabricated in a stiff material such as plastic, metal etc. or in a mixture of material wherein the mixture forms a stiff material. The stabilizing ring may have an inner diameter of 20 mm, an outer diameter of 22 mm, and a height/ thickness of 1 mm and be made of nylon or PET. According to embodiments of the invention the stabilizing ring is an integral part of a bistable lid 308, 808 or the dome shaped cover 401 wherein it reinforces the structural integrity of the transducer compartment 109 even further, as depicted in Figures 3a-b, 4a-b and 8a-b.

The longitudinal stiffness of the stabilizing structure can be measured by applying a 1 Newton tensile force parallel to the stabilizing structure and measure the deformation. An example of a measurement set-up for such a measurement is two clamps that are firmly attached to the outer opposite sides of the stabilizing structure. After attachment, a 1 N tensile force is applied to the clamps. The deformation may be less than 1 mm., i.e. at least a longitudinal stiffness of the stabilizing structure of I N/ mm is desirable.

For a stabilizing structure having any of the forms and dimensions as described above the perpendicular stiffness may be at least 0.5 N/mm. The longitudinal stiffness should at least be 1 N/mm as previously described.

The perpendicular stiffness may be measured by applying a force of one Newton vertically to the stabilizing structure. The perpendicular deformation after such a force should be less than 2 mm, for the stabilizing structure to be able to maintain the structural integrity of the transducer compartment.

The perpendicular stiffness may be measured using the measurement set-up described below. The stabilizing structure under test is placed on top of two support pillars having parallel flat end surfaces of approximately 1 mm. The distance between the support pillars should be maximized with respect to the form and shape of the stabilizing structure under test. The perpendicular force may be applied with an edge shaped straight object having an approximately 1 mm flat surface contacting the stabilizing structure. The force is applied at the center at equal distance to the support pillars.

The stabilizing structure may be fabricated in a polymeric material or a mixture of polymeric materials. Examples of polymeric materials include polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polyamide (Nylon), polycarbonate (PC), polyethylene (PE), polypropylene (PP), and acrylonitrile butadiene styrene (ABS).

The herein described forms, shapes, and sizes are advantageously suitable for a body electrode 100 having a central circular transducer compartment 109 with a diameter of between 10-20 mm. The body electrode 100 may have an oval shape with a length of 20-70 mm, and a width of 20-50 mm. It may have a circular shape with a diameter of 20-70 mm.

The air evacuation arrangement 111, having a valve-like function, may be open to the ambient air 104 when the pressure in the transducer compartment 109 is at 0.05-0.1 bar, or 0.1-0.5 bar, over-pressure as compared to the ambient air 104.

The volume of air that is evacuated via the at least one air evacuation arrangement 111 may be 60-200 mm³, depending on the volume of the transducer compartment 109 and the amount of trapped air.

The effectiveness of the body electrode according to the invention of reducing the effects of mechanical disturbances has been experimentally verified. An example is presented in the electrocardiograms of Figure l la-b. Figure l la-b show two electrocardiogram recordings using body electrodes on a human at close locations using electrocardiogram amplifiers with the same amplifications. Each spike shown on the curves represents disturbances in the signals obtained for the same type of mechanical disturbance in the form of tapping on the skin adjacent to the body electrodes. Figure 11a shows a recording using two body electrodes according to the prior art. Figure l ib shows a recording using two body electrodes 100 according to the invention. The measurement units for both electrocardiograms are the same and are displayed with a vertical arrow representing 1 mV and with a horizontal arrow representing 200 ms. The two body electrodes 100 corresponds to an embodiment with the principal design of the air-evacuation arrangements as described with references to Figures 7c-d, a bistable lid according to Figures 4a-b and a sealing rim. All embodiments and variations described herein can be described unless explicitly stated otherwise. In particular, the different elements described in the illustrative embodiments may be combined in other configurations than here described and depicted such as the various embodiments of the air evacuation arrangements being combined with the various embodiments of the flexible lid, the positioning of the connector etc.

Claims

1. A body electrode (100) for electrophysiological signal monitoring, the body electrode (100) during use arranged to be attached to the skin of a subject, wherein the body electrode (100) has a skin facing surface (102) arranged to, during use, be in contact with the skin (101) and a free surface (103) arranged to be in contact with the ambient air (104); the body electrode (100) comprising:

-a collar (105) comprising a transducer compartment (109) open towards the skin facing surface (102);

-a transducer element (107, 807) at least partly arranged in the transducer compartment (109);

-a connector (112) in electrical contact with the transducer element (107, 807);

-a flexible lid (108, 208, 308, 808) covering the transducer compartment (109) and arranged so that the transducer compartment (109) is open to the skin facing surface (102), and wherein the flexible lid (108, 208, 308, 808) fixed to the collar (105) and arranged to seal the transducer compartment (109) on the side of the free surface (103) of the body electrode (100); and wherein the body electrode (100) is characterized in that -the flexible lid (108, 208, 308, 808) has a first position defining a first volume of the transducer compartment (109) and the flexible lid (108, 208, 308, 808) being movable into the transducer compartment (109) in the direction towards the skin facing surface (102) to a second position defining a second volume of the transducer compartment (109), the second volume of the transducer compartment (109) being smaller than the first volume; and by

-at least one air evacuation arrangement (111, 711, 811) comprising a channel (111a, 711a, 721, 811a), the channel (111a, 711a, 721, 811a) having an inlet (111b) in the transducer compartment (109) and extending to an outlet (111c) in connection with an air receiving receptacle (104, 11 Id), wherein the air evacuation arrangement (111; 711; 811) is arranged to let air out from the transducer compartment (109) when the flexible lid (108, 208, 308, 808) moves from the first to the second position.

2. The body electrode (100) according to claim 1, further comprising retaining means arranged to retain the flexible lid (108, 208, 308, 808) at the second position after the flexible lid (108, 208, 308, 808) has been moved by an external force from the first position to the second position.

3. The body electrode (100) according to claim 1 or 2, wherein the air evacuation arrangement (111, 711, 811) comprises a valve (11 If, 811) arranged in connection with the channel (111a, 711a, 721, 811a) and the valve (11 If, 811) controlling the air flow in the channel (111a, 711a, 811a).

4. The body electrode (100) according to claim 1, wherein the channel is a surface channel (721) provided in the skin facing surface (102) of the collar (105) and the surface channel (721) extending in a radial direction from the transducer compartment (109) to the free surface (103) on the outer perimeter of the collar (105).

5. The body electrode (100) according to claim 3, wherein the valve (11 If, 811) is a membrane valve (11 If, 811) comprise a thin membrane (11 lg) arranged on and partly attached to the free surface (103) and covering the outlet (111c) and arranged so that a portion of the thin membrane (11 lg) is not attached to free surface (103) and may raise from the free surface (103) to open the valve ( 111 f , 811) at an increase in pressure in the transducer compartment (109).

6. The body electrode (100) according to claim 3, wherein the valve is provided by the channel (711) having a first relaxed and collapsed state in which the channel (711) is closed and a second temporarily open state in which the channel (711) is open, and wherein the channel (711) goes from the first relaxed and collapse state to the open state upon increasing pressure in the transducer compartment (109).

7. The body electrode (100) according to claim 6, wherein the channel (711) is a punctured structure in the collar (105), wherein the punctured structure has been provided by penetrating the collar (105) with a needle, the needle having a diameter of 0.2 - 1 mm.

8. The body electrode (100) according to claim 6, wherein the collar (105) is a polyvinyl chloride foam having a density of approximately 200 kg/m³.

9. The body electrode (100) according to any of the preceding claims, wherein a dedicated air receiving receptacle (11 Id) provided by a space closed from the ambient air and formed by an expanding membrane (11 le) provided on the free surface (103) and covering the outlet (111c) and forming an airtight seal to the channel (111a, 711a, 811a), wherein the expanding membrane (l l le) is arranged to expand upon the increase of pressure in the transducer compartment (109), thereby the volume of the dedicated air receiving receptacle (11 Id) is increased.

10. The body electrode (100) according to claim 9, wherein the expanding membrane is covering a major portion of the free surface (103) of the body electrode (100).

11. The body electrode (100) according to claim 3, wherein the valve (11 If, 811) is a check valve.

12. The body electrode (100) according to any of claims 2-11, wherein the flexible lid is a bi-stable lid (308, 808) having a stable convex position and a stable concave position, wherein the convex stable position constitutes the first position of the flexible lid defining the first volume of the transducer compartment (109) and the concave stable position constitute the second position of the flexible lid defining the second volume of the transducer compartment (109).

13. The body electrode (100) according to claim 12, wherein the bi-stable lid comprises a ring-formed stabilizing outer part (308b, 808b).

14. The body electrode (100) according to any of claims 2-11, further comprising a retaining member (401, 450, 451, 522, 622), arranged to retain the flexible lid (108, 208, 308, 808) in the second position.

15. The body electrode (100) according to claim 14, wherein the retaining member is a bi-stable dome-shaped cover (401) having a stable convex position and a stable concave position, wherein at least at the concave stable position the bi-stable dome-shaped cover (401) is in contact with the flexible lid (108, 208, 308, 808) and defines the second position of the flexible lid (108, 208, 308, 808).

16. The body electrode (100) according to claim 15, wherein the bi-stable dome shaped cover (401) comprises a ring-formed stabilizing outer part (401b).

17. The body electrode (100) according to claim 14, wherein the retaining member is a protrusion (522) on the wall of the transducer compartment (109), the protrusion arranged to engage with a part of the flexible lid (108, 208, 308, 808) being in the second position.

18. The body electrode (100) according to claim 17, wherein the protrusion provides a circumferential surface (622b) facing the free surface (103) of the body electrode ( 100) , and wherein the circumferential surface is at least partly provided with an adhesive, the adhesive arranged to contact and retain the flexible lid (108, 208, 308, 808) in the second position.

19. The body electrode (100) according to claim 14, further comprising an outer lid (451) movable in the direction towards the skin facing surface (102), the outer lid (451) comprising a rigid part (451a) and a compressible part (451b) which is arranged on the side of the rigid part (451a) facing the skin facing surface (102) and on its opposite side attached to the flexible lid (108, 208, 308, 808), and wherein the rigid part (451a) extends at least partly a distance from the compressible part (451b) in a radial direction, and wherein the outer lid (451) is arranged to have a first position relating to the first position of the flexible lid (108, 208, 308, 808), wherein the compressible part (451b) is uncompressed and wherein there is a gap between the rigid part (451a) of the outer lid (451) and the collar (105), and wherein the outer lid (451) is arranged to have a second position relating to the second position of the flexible lid (108, 208, 308, 808), wherein the rigid part (451a) of the outer lid 451 abuts the collar (105) and the compressible part 451b is compressed.

20. The body electrode (100) according to claim 19, wherein the collar (105) on a surface facing the outer lid (451) and/or the rigid part (451a) of the outer lid (451) on the surface facing the collar (105) is provided with an adhesive (453) so that the outer lid (451) may be retained in its second position and thereby the flexible lid (108, 208, 308, 808) in its second position.

21. The body electrode (100) according to any of the preceding claims, further comprising a sealing rim (120) provided on the skin facing surface (102) of the collar (105) and enclosing the transducer compartment (109), the sealing rim (120) extending radially at least over a portion of the skin facing surface (102) of the collar (105).

22. The body electrode (100) according to claim 21, wherein the sealing rim (120) extends radially inwards a distance into the transducer compartment (109).

23. The body electrode (100) according to any of the preceding claims, further comprising a separator (910) provided in the transducer compartment (109), the separator arranged to separate an electrolyte medium (110) from the inlet (111b) of the air evacuation arrangement (111, 711, 811), the separator (910) being permeable to air.

24. The body electrode (100) according to claim 23, wherein the separator (910) is ring-shaped and extends below the skin facing surface (102) of the collar (105).

25. The body electrode (100) according to any of claims 24-24, further comprising a sealing rim (120) provided on the skin facing surface (102) of the collar (105) and enclosing the transducer compartment (109), the sealing rim (120) extending radially at least over a portion of the skin facing surface (102) of the collar (105) and at least over a portion of the skin facing surface of the separator (910).