Patent Description:
Wearable and touch technology is becoming more evident and widely available on the world market, with increasing technologies and products being adapted into wearables and touch sensitive devices. One particular technological area of increasing interest is in the acquisition of biometric information through biometric sensing. Obtaining this information allows technology to enhance further as it can be implemented into a range of functional applications such as improved security, personalised authentication, personal health monitoring and stress detection, to name a few.

The acquiring and monitoring of biometric electrical signals may be of value in its own right, for example as a mean of monitoring underlying physiological conditions or processes. Additionally, it may have value as a form of biometric sensing for other purposes such as for identification/ authentication. Biometric sensing may be superior over other more common forms of personal physical detection, such as fingerprint recognition, iris response and voice activation. It is particularly favourable in security and authentication systems, as it is more difficult to replicate an individual's natural characteristics, such as their heartbeat, muscle activity and sweat gland response.

The ability to sense characteristic biometric information associated with an individual's heartbeat may be of particular interest in this regard. The electrical signals from the heart provide a trace of data points which are unique to each individual and cannot be replicated, even when the heart is subjected to different stresses, i.e. when an individual exercises.

The most common way to measure a subject's heartbeat is to measure the electrical signals using an electrocardiograph, which records the electric potential changes in the heart over time. A longer recording of such electrical activity is known as an electrocardiogram or ECG. This ECG measurement is recorded using a pair (or more) of electrodes which measure the changes in electric potential between the points of contact of the electrodes. In clinical situations the ECG electrodes make contact with a subject's skin by predominantly using adhesive gelled electrodes, and are positioned at various points across the heart in order to achieve an accurate reading. The change in electric potential is strongly correlated with heart and muscle activity as the heartbeat of an individual is stimulated through electrical impulses. The basic elements of a single heart beat are: a P wave generated when the right and left atria of the heart are depolarized; a QRS complex reflecting the depolarization of the right and left ventricles; and a T wave corresponding to the ventricular repolarization. Existing methodologies attempt to characterise an individual by these different elements and their respective sizes, shapes and positions.

Biometric electrical signals associated with an individual's heartbeat may be particularly characteristic of an individual, and examples are discussed herein in which such characteristic biometric electrical signals are acquired and monitored. However, the skilled person will not infer by such examples that the invention need be limited to any particular class of biometric signal characteristic of any particular physiological activity, but will understand that any biometric signal capable of characterising an individual may find application in the context of the invention.

A useful means for acquiring and monitoring of biometric electrical signals from a subject is a skin contact device, such as a wearable. Many biometric sensing devices have been adapted to measure the natural characteristics of a user while also being small enough to be enabled as a wearable device or affixed in other contact-type devices. However, several issues still arise when seeking to acquire biometric electrical signals from a subject using a skin contact device, such as a wearable, particularly where the use of the device is intended in day-to-day application in non-clinical environments.

Such issues may be considered for example by looking at the problems that might arise when measuring an individual's ECG through a wearable or contact device. The clinical method of measuring ECG is not suitable in such day-to-day application in non-clinical environments. For example, with contact devices for domestic application and/or for use in or comprising part of consumer products, such as smart watches, wearable bands, steering wheels, joysticks, etc. These applications typically require the use of dry electrodes and are often limited to using only two electrodes, due to limited space on small sized consumer products, however if the product is not space limited a larger plurality of electrodes may be used. In addition, the amplifiers used in clinical devices such as clinical ECGs exhibit high voltage rails which allow for greater offset potentials to be tolerated. This is impractical for small battery-operated equipment such as smart watches, or other wearable consumer devices.

Similar problems can be anticipated in relation to the use of a skin contact device, such as a wearable, when seeking to acquire various other biometric electrical signals from a subject where the skin contact device is intended in day-to-day application in non-clinical environments, and for example in domestic applications or for use in consumer products, such as smart watches, wearable bands, steering wheels, joysticks, etc..

Most dry electrodes are made from noble metal materials, such as silver, gold, copper, etc, as they have little or no polarisation effects when in contact with skin, reducing the offset potential at the skin/electrode junction. Other highly capacitive materials can be used but produce suboptimal data acquisition performance due to the high polarisation effects at the skin/electrode junction when a DC current passes through the electrode system. This current is required in order to detect when the user makes contact with the electrodes. These materials may be chosen to satisfy aesthetic requirements of the consumer product, such as the need for coloured or textured finishes. To overcome the large and unstable potential offset of these conducting materials alternative methods of operation using AC-coupling to the electrodes are known. The AC-coupling necessitates the use of AC current for lead on and off detection; however this requires more power as the method is much more complex than that using DC current.

Power drain is likely to be a much more critical factor in relation to a device which is intended in day-to-day domestic applications or for use in consumer products, such as smart watches, wearable bands, steering wheels, joysticks, etc. Such a device may be actively acquiring data only intermittently and spend long periods idle waiting for a skin to electrode contact to be made. The effect may be greatest where the power source is a compact battery. The use of AC current for lead on detection in such cases would therefore necessitate much higher battery drain and is likely to be of limited practicability in many such devices.

Therefore, there is a need to reduce power consumption and in particular battery power consumption in wearable and/or portable devices while maintaining a low differential potential offset at the amplifier input, in order to utilise a wider range of conducting materials.

The invention is directed at the provision of a device, system and method for acquiring and monitoring of biometric electrical signals from an individual subject, and in particular of biometric electrical signals tending to be characteristic of the subject.

The invention is in particular directed at the provision of a skin contact device, such as a wearable, and to a system and method incorporating the same, for acquiring and monitoring of biometric electrical signals in non-clinical domestic environments.

The invention is directed at the provision of a device, system and method that facilitates the use of alternative electrode materials as electrodes for dry skin contact in intended use.

Causevic et al. , in <CIT>, discloses a system and method for neurological evaluation. In particular, a device and method for acquiring and processing a patient's brain electrical activity.

Baxi et al. , in <CIT>, discloses a user's physiological context sensing method and apparatus.

<FIG> provides example applications of a <NUM>-electrode ECG detection and acquisition device as described throughout this document as a preferred embodiment. <FIG> illustrates that the device can be implemented in consumer products, for example, but not limited to, (a) a steering wheel <NUM> and (b) a smart watch <NUM>. The electrodes <NUM> in <FIG> are positioned on either side of the steering wheel, i.e. separately spaced locations, to enable <NUM>-point skin contact, namely a user's left and right hand side. The electrodes <NUM> are also constructed and shaped to cover a large portion of the left and right sides of the steering wheel providing an increased contact area. Similarly, the electrodes <NUM> in <FIG> are positioned in spaced locations in order to enable <NUM>-point skin contact, that being the front <NUM> and rear <NUM> faces of the smart watch <NUM>. This enables a wrist contact with one electrode and an opposing finger contact with the other electrode. Again, the electrodes <NUM> are constructed and shaped to fit on either side of the smart watch while providing accessible contact areas. Electrodes can be constructed in various shapes and sizes to fit the consumer product, with each electrode having its own shape and size if necessary, such as illustrated by the different sized front and rear electrodes in <FIG>.

The embodiments are discussed by way of example with reference to obtaining biometric electrical signals associated with an individual's heartbeat, and in particular ECG traces. Such traces are of course valuable in their own right as an indicator of cardiovascular physiology, but may also be particularly characteristic of an individual. Of course, the same principles could be applied to any biometric signal capable of characterising an individual and/ or providing an indicator of a physiological condition or process.

ECG acquisition can be performed when a user has made contact with both <NUM> electrodes, measuring the signal across the electrodes via the user. In relation to the steering wheel <NUM> the measurement can be taken when both hands are in contact with the electrodes <NUM> on either side of the wheel, and for the smart watch <NUM> the acquisition can be made when the wrist is in contact with the rear electrode and a finger makes contact with the front electrode. Other configurations may be apparent depending on the placement of the electrodes in the receiving product.

<FIG> depicts a typical circuit for contact detection <NUM> used to detect when a subject or user makes contact with the device, which is often referred to as lead-on detection, and lead-off when contact between skin and electrodes is broken. <FIG> illustrates the electrodes <NUM> at the open end of the circuit in idle or "sleep" mode, awaiting contact from a subject or user <NUM>. Common biometric contact detection circuits feed a direct current (DC) current into one electrode, usually positive <NUM>, and out of the other, usually negative <NUM>, and measures the voltage difference between the <NUM> electrodes. This voltage difference will be compared to an established threshold value, i.e. an open circuit voltage such as Vemf. Usually for human subjects if the corresponding resistance is less than a typical resistance threshold of approximately <NUM> MΩ the subject is considered to be connected to the electrodes. Once a connection is made and contact has been detected the ECG acquisition can begin. Typically, this is carried out using a low-power voltage comparator <NUM>. The lead detection can be continued through the signal acquisition for determining lead-off, i.e. when the subject breaks contact from the electrodes. However, there are limitations in the material choice for the dry electrodes used within a DC circuit, due to polarization effects at the skin/electrode interface. Some materials with thin leaky dielectric layers give rise to large polarisation effects, producing a large voltage offset potential. This can make it difficult to detect and acquire the ECG signal over the threshold of the circuit electronics. The imposed voltage offset of these materials is in addition to the electrochemical half-cell potential produced at the boundary between the ions in the subject's body and the electrode itself. The DC offset can also vary between the <NUM> electrodes according to skin site, pressure, area or if two different materials are used for each electrode. The difference in voltage between the electrodes can be more than <NUM>. 5V in some cases. The offset also varies across different people, due to the effect of differing skin impedance. Therefore, altering the combination of electrode material and electronics may not produce a valid ECG signal for a significant proportion of the population, limiting the choice of electrode material and finish.

Alternatively, the circuit for contact detection can have an alternating current (AC) feed rather than DC as depicted in <FIG>. The electrodes <NUM> and subject <NUM> can be the same as for DC current, but instead a modulated AC current <NUM>/<NUM> is passed across the electrodes. It is necessary to operate the modulated AC current at a value higher than the required ECG bandwidth in order for detection and acquisition to occur. This overcomes the issues that arise with the DC circuit but requires more power. Again, like with the DC circuit, the AC circuit has a corresponding voltage comparator <NUM> to measure the difference in AC voltage for acquiring the ECG signal of the subject, and lead on/off detection can also be continued. However, as the AC driven circuit requires more power than DC small wearable devices have a greatly reduced available battery life.

A high-pass filter can be introduced in the DC circuit to combat the effects of polarisation, when using highly conductive materials with thin leaky dielectric layers for dry electrodes, by bringing the signal into a suitable displayed range of the device. High-pass filters can also account for a differentiating DC offset induced by different materials and different users. The value of the induced offset can be as much as <NUM> V, or more, for consumer devices when dry electrodes are used. The high-pass filter for consumer devices typically has a -3dB roll-off value of <NUM>. Typical values for clinical use ECG devices is normally around <NUM> due to the use of adhesive gelled electrodes. The high-pass filter is able to pass the signal that has a higher frequency than a certain cut-off frequency, such as the value of the induced offset, and attenuates any signal that has a lower frequency than the cut-off frequency. This can be achieved using different circuit designs. For example, the high-pass filter can be implemented using a capacitor <NUM> as shown in <FIG>, or by an integrator in a feedback path <NUM> as shown in <FIG>. Other high-pass filter designs may also be adopted. The capacitor <NUM> introduced in the circuit <NUM> of <FIG> is coupled to the different stages of amplification <NUM> through inter-stage AC coupling <NUM> to remove the DC offset and provide an adjusted amplified signal <NUM> at the output. <FIG> shows the integrator <NUM> in the feedback path <NUM> of the circuit <NUM> for baseline recovery to account for the offset. The example in <FIG> also includes an optional fast baseline recovery <NUM> by detecting any out of range signals. This detects where the amplifier output has saturated, and reduces the integrator time constant until the average signal level is within the viable acquisition range, providing a quicker amplified signal <NUM> at the output. This is disclosed in <CIT>.

However, these types of DC circuit modifications can also present problems. The inclusion of high-pass filters, such as the capacitor <NUM> and integrator <NUM> designs presented, can suffer from limited offset potential capability due to the saturation of one or more of the amplification stages. This is especially problematic in the case of small wearable devices where the device is battery operated. This is especially limiting as the typical battery operated electronics for these types of devices operate with a low supply rail voltage, which is typically <NUM> V.

A further example to improve the ECG acquisition following lead-on detection is to AC couple both electrode inputs as shown in <FIG>. This circuit design <NUM> offers improved biopotential signal processing for ECG acquisition as the AC coupling <NUM> is performed at the front-end of the circuitry instead of the inter-stages of the amplification <NUM> as shown by the capacitor <NUM> in <FIG>. The AC coupled capacitors <NUM> and input bias resistors <NUM>/<NUM> at the front-end act as a balanced high-pass filter, removing the induced offset potential. However, this example is costly in order to achieve the required balanced high-pass filter. It requires expensive close-tolerance components to main a good common-mode rejection ratio (CMRR) at <NUM> or <NUM>, which is needed for removing the effects of mains interference and avoiding low-frequency phase distortion in the acquired waveform ECG signal. Also, as the AC coupling <NUM> exists at the front end DC contact detection cannot be used. Therefore, AC current is required to pass through the capacitors during the inactive idle mode to detect a lead-on event (where the subject contacts both active electrodes), which as indicated above uses more battery power as it requires an increased current consumption during this mode.

In the preferred embodiment of <FIG>, the biometric sensing device includes an electronic integrated circuit <NUM> implemented to acquire biometric electrical signals, the acquired biometric signals being at least one of electrocardiogram (ECG), electrodermal activity (EDA), electroencephalograph (EEG) or electromyograph (EMG) signal. It will be realised that the device may be adapted to measure other biometric signals. For this embodiment the signal being detected and acquired is ECG. The circuit <NUM> of the ECG sensing device includes <NUM> dry electrodes <NUM> to make contact between the device and a subject or user <NUM>. The electrodes <NUM> convert ionic current within the body into electrical current. The electrodes <NUM> may comprise of one or more conducting or semiconducting materials, including those with a thin leaky dielectric layer that can be susceptible to polarisation effects. This allows a greater choice of materials and combinations of materials to be used for the dry electrodes, which can have other desirable properties such as varied colour options, or varied textured options, etc, providing enhanced aesthetics when incorporating in a consumer product. These include but are not limited to chromium-plated metals or plastics, coloured nitride coatings applied by PVD methods, stainless steels including stainless steels with passivating coatings, etc..

In a preferred embodiment, the circuit <NUM> includes AC coupled inputs <NUM>, coupling the electrodes <NUM> to the amplifier <NUM> electronics. The amplifier <NUM> may be a differential instrumentation amplifier but other amplifiers may be apparent. The input AC coupling <NUM> prevents any mean DC current flowing through the electrode/skin interface <NUM>, overcoming the potential offset that arises at such interface for highly polarising materials. This includes the current that is necessary to bias the amplifier <NUM> inputs to keep the amplifier electronics operating in a linear mode. The AC coupling also prevents the DC current which is induced in the circuit from possible motion artefacts or disturbances of the ECG baseline signal by movement, such as emf current. As the option to implement the device in a smart watch or other portable wearable is of great importance, there is a requirement to suppress or eliminate these currents from the output signal.

In the preferred embodiment, the circuit <NUM> includes a switching mechanism which enables the AC coupling capacitors <NUM> to be switched in and out of the circuit. This is indicated by the switches bridging the capacitors <NUM>. This configuration enables the device to change between <NUM> modes of operation, namely DC current, idle mode, and AC current, active mode. In DC mode the AC coupling capacitors are switched out of the circuit permitting DC current to flow to the electrodes. This mode of operation sets the device to a low power state and enables DC lead-on detection to occur, whereby the device detects when skin/electrode contact has been made to both electrodes. When the AC coupled capacitors are switched in to the circuit the second mode of operation is enabled and ECG signal acquisition can occur. This active mode of operation permits the signal to be acquired while removing the electrode potential offsets, with the aid of a high-pass filter. In this embodiment the input AC coupling capacitor and the amplifier input bias resistor form a high-pass filter <NUM> at the front end of the circuitry <NUM>. The -3dB breakpoint, or cut-off frequency, of this high-pass filter needs to be below the lower bandwidth of potential offset frequency, such as approximately <NUM> for use in consumer products (~<NUM> for clinical use). The -3dB point is found using the standard formula, fc=<NUM>/2πRC, as shown by the plot <NUM> in <FIG>. To ensure this condition is maintained a high-pass filter <NUM> is incorporated in to the amplifier stages <NUM> of the circuit <NUM>. This high-pass filter can aid in removing the long RC time-constant due to the effects of the coupling capacitors in combination with the input bias resistors of the amplifier instrumentation <NUM>. With this implementation the time-constant is less than <NUM>% of the AC coupling circuit. The high-pass filter is an integrator positioned in the feedback path <NUM> of the circuit for baseline recovery. Incorporating <NUM> high-pass filters, such as the ones disclosed, results in minimal phase difference at frequencies above <NUM>, as can be seen by the amplitude <NUM> and phase response <NUM> plots in <FIG>, respectively. The high-pass filters in this case are a <NUM> CR high-pass filter for the front end input <NUM> and a <NUM> integrator baseline recovery high-pass filter <NUM> at the back-end <NUM> of the circuit. In addition, an out-of-range detection system <NUM> is used for fast baseline recovery, keeping the signal average within the linear range of the instrument.

In the preferred embodiment, for the switching mechanism to operate a bias switching source <NUM> is introduced into the circuit <NUM>. The switching can be activated by either using fixed resistors or a programmable constant current source. In a further configuration a mid-rail bias point is introduced with mode switching occurring both across the AC coupling capacitors <NUM>/<NUM> and at the AC current source <NUM>/<NUM> as illustrated in the circuit <NUM> in <FIG>. When switching between modes of operation, the pair of capacitor switches <NUM> and <NUM> operate together, and the pair of voltage bias switches <NUM> and <NUM> operate together. The sequence in which the detection, switching and acquisition is important.

Claim 1:
A device for acquiring biometric electrical signals, comprising:
a data acquisition module;
two dry electrodes (<NUM>);
two capacitors (<NUM>), each on one side connected to one of the electrodes;
and
two bias resistors, each on one side connected to the other side of said capacitors and the same side of the resistors also being connected to a respective input of a differential amplifier (<NUM>) and the other side of the bias resistors being connected to a bias switching component (<NUM>),
the bias switching component (<NUM>) comprising:
a current supply, the current supply comprising a direct current, DC, supply (<NUM>, <NUM>) and an alternating current, AC, supply (<NUM>, <NUM>); and
a switching mechanism actuatable to switch selectively between the DC supply and the AC supply so that:
in a first mode of operation (<NUM>), the switching mechanism is configured to connect the DC supply to the bias resistors; and
in a second mode of operation (<NUM>), the switching mechanism is configured to connect the AC supply to the bias resistors,
wherein each of the capacitors (<NUM>) is bridged by a switch, which is configured to be closed in the first mode and configured to be open in the second mode,
wherein in the first mode of operation (<NUM>) a direct electrical connection is made between the electrodes (<NUM>) and the data acquisition module,
wherein the second mode of operation (<NUM>) allows AC coupled biometric data acquisition, and
wherein the device is adapted to automatically switch from the first mode of operation (<NUM>) to the second mode of operation (<NUM>) when the device detects skin contact between the electrodes and the skin of a subject in use.