WO2008152588A2

WO2008152588A2 - Materials for capacitive sensors

Info

Publication number: WO2008152588A2
Application number: PCT/IB2008/052298
Authority: WO
Inventors: Bastiaan Feddes; Lena Gourmelon; Gerardus Rudolph Langereis
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-06-15
Filing date: 2008-06-11
Publication date: 2008-12-18
Also published as: WO2008152588A3

Abstract

A capacitive sensor (200) for sensing electrical fields of a body comprising an electrode, a shield, an insulating separation material separating the electrode and the shield and a housing including associated electronic circuits, wherein the tribo-electric property of the insulating separation material or the material used for the housing (206) substantially matches with that of the skin of the body thereby reducing the generation of static charge on the capacitive sensor is disclosed. The capacitive sensor is useful in all applications where motions are present during measurement of electrophysiological signals from the body such as ECG, EMG, wearables for repetitive strain injury prevention, heart -rate straps, ECG, EMG sensors in chairs or beds.

Description

Materials for capacitive sensors

FIELD OF THE INVENTION:

The subject matter relates to capacitive sensors for sensing body electrical fields present at a location on the surface of the body and more specifically to the materials used for the capacitive sensors.

BACKGROUND OF THE INVENTION: US patent document 20050177038 discloses a bio-electrode for detecting heart signals comprising a dry electrode surface having an elevated resistivity to reduce the effect of polarization noise. Such types of bio-electrodes are susceptible to motion artefacts. These motions generate static charges that can affect proper functioning of the bio -electrodes.

It would be advantageous to have a capacitive sensor that can reduce the generation of static charges. It would also be advantageous to have a method of choosing materials that can reduce the generation of static charges in a capacitive sensor.

SUMMARY OF THE INVENTION:

A capacitive sensor for sensing electrical fields of a body comprising an electrods, a shield, an insulating separation material separating the electrode and the shield, and a housing including associated electronic circuits, wherein the tribo-electric property of the insulating separation material or the material used for the housing substantially matches with that of the skin of the body thereby reducing the generation of static charge on the capacitive sensor is disclosed. A method of manufacturing a capacitive sensor for sensing electrical fields of a body comprising an electrode, a shield, an insulating separation material separating the electrode and the shield, and a housing including associated electronic circuits is disclosed. The method comprises substantially matching the tribo-electric property of the insulating separation material or the material used for the housing with that of the skin of the body and selecting the material based on the outcome of the matching is disclosed. BRIEF DESCRIPTION OF THE DRAWINGS:

These and other aspects, features and advantages will be further described, by way of example only, with reference to the accompanying drawings, in which the same reference numerals indicate identical or similar parts, and in which: Fig. 1 schematically shows a block diagram of an exemplary bipolar capacitive sensor configuration;

Fig. 2a and Fig. 2b show a capacitive sensor configuration according to an embodiment of the subject matter;

Fig. 3 schematically shows a capacitive sensor comprising a copper plate in polyimide housing;

Fig. 4a and Fig. 4b schematically shows results of five measurements of the differential voltages obtained using a pair of exemplary contact less Electro-myogram (EMG) capacitive sensors;

Fig. 5 schematically shows an embodiment of the capacitive sensor comprising a conductive coating on the insulating layer;

Fig. 6 schematically shows the measurement of voltage with polyimide electrodes mounted on the body skin and with polyimide electrodes covered by 50 nanometers of aluminum mounted on the body skin;

Fig. 7 schematically shows an embodiment of the capacitive sensor comprising a coating of textile on the electrode interwoven with conductive wiring, yarns, and mesh; and

Fig. 8 schematically shows an embodiment of the capacitive sensor implemented using tribo-electric materials.

It is noted here that the word sensor in this document refers to the capacitive sensor comprising the whole sensing element, consisting of an electrode (e.g. conductive plate) and a surrounding package. It is further noted here that often electrophysiological measurements are made using a pair of sensors.

Referring now to Fig. 1 , EMG and Electro-cardiograph (ECG) sensors are commonly used in a bipolar setup which means that two sensors are used in combination with a third one (reference or local ground) to limit common mode signals. The bipolar capacitive sensor configuration comprises: 1. sensor

2 differential amplifier

3. analog filtering/anti-aliasing circuits 4. an analog to digital converter

5. a personal computer or data processing or storing device

6. an interface box

The operation of the bipolar capacitive sensor is well known in the art and hence not described in detail. Two sensors are drawn with each having their own impedance converter (the small triangles) on top of them. These impedance converters are as close as possible to the electrode. This is required to limit the picking up of noise from the external environment due to the high impedance of capacitive sensors. The common mode signal is fed back to the body (either capacitive or by galvanic coupling) in order to limit the common mode signal on the sensors. The common mode signal must be controlled, not only to limit the common mode rejection ratio demands of the differential amplifier, but especially to avoid that the electronics clip to one of the power lines.

Motion artefacts due to the movement of the sensor (i.e. sensor 1 and sensor 2) with respect to the skin are expected to be mainly caused by charge redistributions. The properties of a single capacitor sufficiently explain the phenomena. A capacitor is described by the relation between the charge q, the capacitance C and the voltage V: q = CV (1)

Differentiation with respect to time yields

^ _{= i =} C^ ₊ V^ (2) dt dt dt which is normally reduced to i = C-dV/dt for fixed capacitors. However, the problems with capacitive surface sensors arise from the non-constant capacitance. The first term C-dV/dt describes the desired behavior: the translation of a surface potential to an electrode current. The second term V-dC/dt describes the origin of the motion artefact because dC/dt is not equal to zero because C changes in time. For a planar capacitor with electrode size A and plate spacing d the fluctuations in C as a result of changes in d can be understood from the basic capacitor equation

C = ε- (3) d

So dC/dt is not equal to zero resulting into a motion current. Prior art approaches aim at measuring C continuously in order to compensate for dC/dt. However, the magnitude of motion artefacts is proportional to V-dC/dt, so the bias voltage V enables the motion artefacts. The inventors have found that static charges are induced by motions. These static charges disturb measurement of the electrophysiological signals from the body and should therefore be avoided.

It is to be noted that the impedance converters (the small triangles in Fig. 1) have a resistor at their input of typically IOOGO to drain (static) charge on the electrode to ground. This trick does not drain away charge from the sensor surfaces, for example the packaging of the electrode, as there is no galvanic contact between electrode and sens or surface. Therefore, it would be advantageous to have a method to avoid static charge on the non conductive parts.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Accordingly, a capacitive sensor (200) for sensing electrical fields of a body comprising an electrode, a shield, an insulating separation material, wherein the electrode and the shield are separated by an insulating separation material and a housing including associated electronic circuits, wherein the tribo-electric property of the insulating separation material or the material used for the housing (206) substantially matches with that of the skin of the body thereby reducing the generation of static charge on the capacitive sensor is disclosed.

Referring now to Fig. 2a, the capacitive sensor 200 includes 1. an electrode 202 (e.g. conductive electrode plate) typically round with a diameter of around 12 mm, but in principle all electrode sizes and shapes with typical dimensions from 1 mm - 100 mm can benefit from the present subject matter 2 a shieM 204 disposed on the electrode for bootstrapping (also known as active shielding which is a technique to put the same potential on the shield as on the electrode to reduce parasitic capacitances) 3. local electronics in a housing 206 (shielded box) as an implementation of the impedance converter, bootstrap electronics and optionally some filtering/signal shaping. It is noted here that the housing containing the associated electronics may form a part of the capacitive sensor or may be present outside (i.e. external) the capacitive sensor connected by wires. Fig. 2b shows the layout of the capacitive sensor 200 with all layers. The housing for the electronics is not shown. 202 is the electrode and 204 is the shield (guard) that is shielding the electrode from picking up noise from the outside. The electrode 202 and the shield 204 are separated with insulating separation material (while the electrode 202 and the shield 204 are also electrically separated from one another).

According to the present subject matter, the insulating separation material and all the packaging material used for the housing 206 are implemented in a tribo-electric safe material (i.e. the materials are tribo -electrically well chosen).

There are two ways to avoid the presence of static charges on the capacitive sensors:

1. Avoid charge generation

2 Stimulate quick discharging It is important to distinguish these two effects because the tendency of a material to get charged has nothing to do with the level of conductivity (or ability to discharge) of the material. Especially the combination of these two, avoiding charge generation and stimulating quick discharging, will solve static charge built up on the sensor skin interface. The static charges are generated by the tribo-electric effect. Triboelectricity is generated due to friction when two materials that have different affinity for charge are rubbed against one another. When a material that is an electron liking substance, e.g. teflon, is rubbed against a material which is -sery tempted to donate electrons, e.g. human hands, high voltage differences can be generated. An example of a tribo-electric series is given in Appendix A.

Referring now to Fig. 3, in the sensors that are currently being used, the copper electrode is surrounded by a polyimide polymer or other insulating layer, making it a capacitive sensor as no conductive (galvanic) contact between skin and electrode plate is possible using this device. The inventors have found that the choice to use polyimide from a tribo-electric point is bad. The human skin is on the extreme positive charge generating side of the tribo-electric series (see appendix A) while polyimide is on the negative side.

Having dissimilar materials in close contact therefore easily induces charge donation/acceptance. The static charges thus generated cause malfunctioning of the capacitive sensor. The more because the polymer surface has poor discharging characteristics, this tribo-electric charging has to be avoided (i.e. charge that is not generated also does not need to be dissipated).

Referring now to Fig. 4a and Fig. 4b, an experiment was carried out to prove that the assumption of static charge induced motion artefacts is valid. The vertical axis is the differential voltage and the horizontal axis is the time in seconds. Fig. 4a and Fig. 4b differ only in the vertical scaling of the graphs (1Ox). A sensor consisting of a copper electrode in a polyimide package was used. The sensor was tested with:

1. three different brands of conductive gel applied in between sensor and skin

2 no gel applied 3. vaseline applied (vaseline has a very high resistance)

The sensor was put on the biceps of a human arm. The human arm was held at rest most of the time. At t=20 seconds the human arm was shaken once. Between t=40 seconds and t=60 seconds the muscle was contracted (visible in most of the graphs of Fig. 4B). At t=80 second again the human arm was shaken once. Between t=100 and 120 seconds the human arm was shaken continuously. When no gel is applied, the continuous shaking of the human arm generated a lot of surface charge due to the rubbing of skin and the polyimide (being at very different positions on the triboelectric scale). Discharging still went relatively quick as the skin made a galvanic contact with the polyimide of the sensor. In case of the conductive gels (three brands were tested, all fairly similar in conductive properties) almost no static charge was generated and even if this happened, the discharge was efficient due to the conductive gel. Vaseline is a very poor conductor. On the one hand, not so much charge was generated during the human arm movement (either due to the absence of the direct skin-polyimide contact or due to better lubricated rubbing, thanks to the vaseline). On the other hand, the discharging from the polyimide to the skin was extremely poor. This can be observed from the fact that charge that is trapped on the polyimide vaseline interface starts to oscillate with the noisy 50 Hz electrical signal from the surroundings (electrical power outlets, etc.) without having the possibility to discharge.

The experiment proves that static charges play an important role in motion artefacts. It shows that both charge generation should be avoided and charge dissipation should be promoted to avoid motion artefacts.

In an embodiment, the capacitive sensor comprises a conducting coating disposed substantially around the insulating separation material such that the conducting coating is substantially close to the skin of the body so as to facilitate quick discharge of the static charges generated in the capacitive sensor-to-skin interface. Sometimes it can happen that from production point of view it is convenient to use insulating material that has poor tribo -electric properties. In such a case one solution is to apply a coating that has better properties, after the production process (this coating can even be in the form of textile that is fixed to the insulating material because as soon as it is fixed (glued for example) no relative movement will occur and no static charges will be generated). Referring now to Fig. 5, discharging can be promoted by applying a metal or other conductive coating on top of the capacitive sensor. This metal layer is not in galvanic contact with the capacitive plate (electrode) meant for picking up the body electrical fields. On first sight this idea appears counter intuitive, especially because the contact less sensors were mentioned to no longer need a conductive/galvanic contact between skin and sensor. However, in this case the electrical contact to the skin can be of low quality, different than in case of real galvanic measurements where a stable interface is needed.

Referring now to Fig. 6, 602 relates to the voltage measurements of the polyimide electrode mounted on the skin and 604 relates to the voltage measurements of the polyimide electrode covered by an aluminum mounted on the skin. In the experiment, the normal and aluminum covered sensors were mounted on a human arm in exactly identical way. The arm was shaken at t = 10 sec, t = 20 sec and continuously between t = 30 and t = 40 second and some final shaking was done at t = 50 sec. The DC component represented by 604 (aluminum covered electrode) is about 0, because no net static charges are present (they were dissipated very efficiently) and upon shaking the human arm almost no effect could be seen in the sensor output. On the other hand, 602 show big effects due to static charge.

The experiment proves that static charging problems can largely be reduced by solutions disclosed in the present patent application. Additionally, the embodiment that involves a metallic layer on the contact less sensor is very easy to implement (in future production) and is highly effective. It is to be noted that the conductive coating on the sensor is not in contact with the electrode plate of the sensor. It is also to be noted that applying conductive coating on a contact less sensor is very uncommon (not practiced before) and at first sight, completely counterintuitive.

Further, the problems with static charging upon movement are less in the case of the aluminum covered sensor because: i. the charge generation of static electricity due to rubbing is lower (aluminum on skin is less problematic than polyimide on skin) ii. the charge dissipation is very easy because of the conducting coating.

The nice mechanism now is that the moment some rubbing occurs, thereby possibly generating static electricity, it automatically also means that there is a galvanic (conducting) contact to the skin. So, the static charges can flow away immediately. When the conductive coating is not touching the skin it has no real effect on the functioning of the capacitive sensor, but when it touches two things happen: i. as long as the sensor is in contact with the skin the conducting can be seen as an extension of the skin, the capacitive electrode remains functional ii. static charges that might form upon rubbing of the capacitive electrode to the skin are immediately dissipated, because rubbing automatically means a conducting contact between sensor surface and body.

In a still further embodiment, the conducting coating comprises silver, gold or any alloy including gold and silver. This might be particularly a good choice for some applications because of the antibacterial property of silver. Further, a benefit in an environment where the presence of body heat and humidity (sweat) might be a good place for bacteria to flourish. Furthermore, the advantage of silver is the high electrical conductivity and good biocompatibility. The use of a thin gold coating might be beneficial, as the electrical conduction as well as the corrosion resistance is very good. The latter is an additional benefit in the humid environment of the human skin.

The conductive materials that are used for quick charge dissipation should not be grounded, like in clean room clothing as this induces a lot of environment electrical noise to the sensor. One way of leading most of the static charges away from the probed measuring region is by extending the conductive part, e.g., the conductive coating in between sensor and skin, away from the sensor. A drawback of this solution might be pickup of additional environmental noise. In a still further embodiment, the insulating separation material is a textile material and the conducting coating comprises conductive wires, meshes, yarns or fabrics and any combination thereof. This is shown in Fig. 7. The conductive polymers applied on the insulating material as wires, mesh or solid material might serve the purpose of promoting rapid discharge of static charges, thereby stimulating proper functioning of the sensor and correct measurement of body electrophysiological signals. The wires should be in the material in such a way that they cannot touch the electrode. The case for clean room clothing is different that the conducting parts around the capacitive sensors should not be grounded, as electrical noise will be picked up which becomes visible on the capacitive electrode as a noise signal. A metal contact will also do, because during movements there will always be a small part of the conducting coating/wires in contact with the skin, serving the purpose of a rapid discharge.

In a further embodiment, the tribo-electric property of the insulating separation material used for the insulating- to-skin interface is substantially close to the tribo- electric property of the skin of the body. Instead of a material with poor tribo-electric properties relative to the skin (e.g. polyimide), a material is chosen that has better tribo- electric properties. Especially the layer of the sensor that is in direct contact to the skin is to be chosen carefully as the static charges that develop between sensor surface and skin cause the problems. It is noted that those parts touching the body need to be of a suitable material, especially the sensor surface touching the skin. Hence, having trbio -electrically matched materials can reduce the generation of static charges.

Referring now to Fig. 8, the insulating separation material that touches the skin of the body is implemented in a tribo-electric material that matches the skin of the body and can reduce the generation of static charges. If no static charge is induced, the DC voltage of the electrode remains low with respect to the human skin and so motion of the electrode does not result into an unintended electrical signal. In terms of equation (2): when limiting the static charge on the electrode and on the sMn, the voltage "V" is minimized and so the term "V dC/dt" is reduced, independent of changes in the capacitance C. Further, the better match in tribo-electric properties avoids generation of static charges and/or high voltages, thereby avoiding malfunctioning of the sensor.

In a still further embodiment, the insulating separation or the material used for the housing is at least one of nylon, wool, silk, cotton, paper, aluminum, and glass. When the sensors are made of one of these materials, the static charges induced when rubbed to the human skin (or textiles) are reduced, and so are motion artefacts. The additional advantage of the first four materials (i.e. nylon, wool, silk and cotton) is that these are natural materials used as textiles. The additional advantage of paper is that it is cheap and it can be printed easily with conductive electrodes. Aluminum is known from aluminum foils and is therefore easy to stack with insulating paper to make sensor configurations. Glass is the best material with respect to the tribo-electric series, but is at first sight the least practical to assemble into a comfortable wearable sensor. However, a very thin layer of glass, for example applied by spin coating, might not decrease comfort while significantly improving performance of a capacitive sensor. Further, the above-mentioned materials are relatively close in tribo- electric properties to the human skin, at least much closer than polymeric materials that are commonly used. The textiles have, besides the advantage of suitable tribo-electric properties, the added advantage of comfort and easy integration in garments.

In a still further embodiment, the insulating separation material and the housing are woven or knitted by interlacing strands of a fabric selected from the group consisting of nylon, wool, silk and cotton. The advantage of weaving is that the sensor and the connecting electrical wires are applied in the production process of the textile, only a change of wire is needed.

In a still further embodiment, the insulating separation material and the housing are laminated on a textile substrate by combining thin layers of textile material. The advantage of laminating is that it can be applied after the apparel is finalized. In this way, the textile production process is decoupled from the sensor realization process. This has a logistic advantage in the industry.

In a still further embodiment, the insulating separation material and the housing are embroidered or sewn or embedded on a textile substrate using a thread. The advantage of sewing is that it is an application method which is easily done in difficult places on finalized apparel, for example next to a stitch, button or joint

Further, advantage of integration into textile is that textiles are already on the human body and the measurement system is to be merged with the natural surroundings for unobtrusive measurements. All textile application methods (weaving, embroidery and lamination) are known in the textile industry and appear quite common to the user.

Furthermore, the wiring of the capacitive sensor preferably should have good tribo -electric properties. Choosing sensor wiring with a suitable coating might help reduce static charging problems.

It is noted here that in case of electrophysiological measurements on animals, a different matching of material might be required to match the tribo-electric properties of the fur of the specific animal.

The disclosed capacitive sensor is useful in all applications where motions are present during measurement of electrophysiological signals from the body. Some applications are ECG, EMG, wearables for repetitive strain injury prevention, heart-rate straps, ECG, EMG sensors in chairs or beds. The presence of human hair in such measurement also necessitates the ideas disclosed in the present patent application: avoiding charge formation and facilitating efficient charge dissipation. This is advantageous in performing capacitive EEG measurements on the hair covered environment of the head. Appendix A - Triboelectric series

Positive

Air Human hands, skin

Asbestos

Rabit Fur

Glass

Human Hair Mico

Nylon

Wool

Lead

Cat Fur Silk

Aluminum

Paper

Cotton

Steel Wood

Lucite

Sealing Wax

Amber

Rubber balloon Hard Rubber

Mylar

Nickel

Copper

Silver UV-resist

Brass

Synthetic rubber

Gold, Platinum

Sulfur Acetate, Rayon

Polyester

Celluloid

Polystyrene

Orion, Acryllic Cellophane tape

Polyvinylidene chloride (Saran)

Polyurethane

Polyethylene

Polypropoylene Polyimide (Kapton)

PVC (Vinyl)

KeI- F (PCTFE)

Silicon

Silicone Rubber Teflon

Negative

Claims

CLAIMS:

1. A capacitive sensor (200) for sensing electrical fields of a body comprising: an electrode; a shield; an insulating separation material separating the electrode and the shield; and - a housing including associated electronic circuits, wherein the tribo-electric property of the insulating separation material or the material used for the housing (206) substantially matches with that of the skin of the body thereby reducing the generation of static charge on the capacitive sensor.

2. The capacitive sensor as claimed in claim 1, further comprising: a conducting coating disposed substantially around the insulating separation material such that that the conducting coating is substantially close to the skin of the body so as to facilitate quick discharge of the static charges generated in the capacitive sensor -to- skin interface during operation.

3. The capacitive sensor as claimed in claim 2, wherein the conducting coating comprises silver, gold or any alloy comprising gold and silver.

4. The capacitive sensor as claimed in claim 2, wherein the insulating separation material is a textile material and the conducting coating comprises conductive wires, meshes, yarns or fabrics and any combination thereof.

5. The capacitive sensor as claimed in claim 1 , wherein the tribo-electric property of the insulating separation material used for the insulating -to- skin interface is substantially close to the tribo-electric property of the skin of the body.

6. The capacitive sensor as claimed in claim 5, wherein the insulating separation material or the material used for the housing is at least one of - nylon, - wool, silk, cotton, paper, aluminum, - glass.

7. The capacitive sensor as claimed in any one of the claims 1 - 6, wherein the insulating separation material and the housing are woven or knitted by interlacing strands of a fabric selected from the group consisting of nylon, wool, silk and cotton.

8. The capacitive sensor as claimed in any one of the claims 1 - 6, wherein the insulating separation material and the housing are laminated on a textile substrate by combining thin layers of textile material.

9. The capacitive sensor as claimed in any one of the claims 1 - 6, wherein the insulating separation material and the housing are embroidered or sewn or embedded on a textile substrate using a thread.

10. A method of manufacturing a capacitive sensor (200) for sensing electrical fields of a body comprising an electrode; a shield; an insulating separation material separating the electrode and the shield; and a housing including associated electronic circuits, wherein the method comprises: - substantially matching the tribo-electric property of the insulating separation material or the material used for the housing (206) with that of the skin of the body; and selecting the material based on the outcome of the matching.