Non-invasive continuous blood pressure monitoring

Abstract

A non-invasive continuous blood pressure measurement device 200 comprising two wearable sensors 204 close to the patient’s skin 108. These may be held in place by a wearable sleeve 202. The sensors 20

Description

FIELD This disclosure relates to a device and method for monitoring blood pressure value of a subject. In particular, the disclosure relates to a non-invasive continuous blood pressure monitoring device and method. BACKGROUND Continuous monitoring of blood pressure has remained a challenge for centuries. Hypertension, often referred to as high blood pressure, is a common medical condition that affects approximately one billion people worldwide, with a significant portion remaining undiagnosed and untreated. Hypertension may lead to an elevated blood pressure level in the arterial system. The high blood pressure may be a significant risk factor for cardiovascular diseases, such as angina, heart attack, stroke, or chronic kidney diseases. Early detection and continuous monitoring of blood pressure is crucial for managing hypertension and preventing associated complications. Previously, traditional systems and methods of measuring blood pressure involved invasive techniques. Early measurements exploited the use of an inflatable cuff, a stethoscope and a sphygmomanometer. The cuff would be placed around the upper arm of an individual and inflated, while a trained professional listens to a brachial artery with a stethoscope. Blood pressure devices utilising an inflatable cuff and digital measurement sensors evolved from traditional systems. As technology advanced, non-invasive methods like volume clamping were introduced. However, these devices require that an inflatable cuff is placed on a person, which may be cumbersome and restrict a person’s movement throughout their daily activities. In some cases, the cuff is connected to a measurement device that is not sufficiently mobile for a person to use continuously, offering limited support to help treat hypertension. There is accordingly scope for improvement. The preceding discussion of the background is intended only to facilitate an understanding of the present disclosure. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY In accordance with an aspect of the disclosure there is provided a device for non-invasive continuous blood pressure monitoring comprising: at least one sensor which includes a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced; the device further including a measurement processor configured to measure a change in the frequency response of the resonator. The device may include at least two sensors, wherein the at least two sensors may be arranged along a portion of a wearable member, such that the sensors are aligned at multiple waypoints adjacent to an artery of a person and along a path of the artery in response to the wearable member being affixed or fastened to the subject. The device may include a wearable member with at least one sensor and the measurement processor, configured to affix or fasten the device to the subject, such that when affixed or fastened to the subject, the sensor may be positioned with the superstrate material positioned towards the skin and is deformable or displaceable in response to a movement of an area of the skin. The measurement processor may be configured to measure a change in the frequency response of each resonator for each sensor independently. The superstrate material may be a compressible structure filled with a fluid. The compressible bag may be deformable by a biasing force, and wherein the bag may be compressible in a direction generally normal to the skin of the subject and expandable in a direction generally transverse to the skin of a subject in response to the biasing force. The biasing force may be a force applied by the skin of a person in response to an artery under the skin expanding during a cardiac cycle, pushing the skin towards the superstrate material. The superstrate material may have a higher permittivity value than biological tissue. The resonator may be a planar Fano resonator. The frequency response of the resonator may be an asymmetric frequency response. The measurement processor may include a pulse waveform generator configured to generate a pulse waveform in response to a measurement of a change in the frequency response of the resonator. The frequency response change may be directly proportional to the deformation or displacement. In accordance with an aspect of the disclosure there is provided a method for estimating a blood pressure value for non-invasive continuous blood pressure monitoring, comprising: measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator; generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor; determining a time delay between the pulse waveforms of each sensor; determining a pulse transit time (PTT) from the time delay; and, estimating a blood pressure value from the PTT. Measuring a change in frequency response at each sensor may include filtering out a noise measurement of each sensor. Measuring a change of the frequency response may include continuously measuring the frequency response of the resonator. The estimated blood pressure value may be directly proportional to the time delay between pulse waveforms of each sensor. The method may include calibrating each sensor to determine a normalised frequency response without deformation or displacement of the superstrate material relative to the resonator. The method may include continuously estimating the blood pressure value from a plurality of measurements of the frequency response, to form a set of blood pressure values. The method may include processing the set of blood pressure values to determine a hypertension of the subject. The method may include sending an alert signal to a user of the device or a healthcare provider in response to the hypertension being determined. In accordance with a further aspect of the disclosure there is provided a system for non-invasive continuous blood pressure monitoring, the system comprising: a non-transitory computer-readable storage medium; and one or more processors coupled to the non-transitory computer-readable storage medium, wherein the non-transitory computer-readable storage medium comprises program instructions that, when executed on the one or more processors, cause the system to perform operations comprising: measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator; generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor; determining a time delay between the pulse waveforms of each sensor; determining a pulse transit time (PTT) from the time delay; and, estimating a blood pressure value from the PTT. In accordance with a further aspect of the disclosure there is provided a system for non-invasive continuous blood pressure monitoring, the system including a memory for storing computer-readable program code and a processor for executing the computer-readable program code, the system comprising: a measuring component for measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator; a pulse waveform generating component for generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor; a time delay determining component for determining a time delay between the pulse waveforms of each sensor; a pulse transit time determining component for determining a pulse transit time (PTT) from the time delay; and, a blood pressure estimating component for estimating a blood pressure value from the PTT. In accordance with a further aspect of the disclosure there is provided a computer program product for non-invasive continuous blood pressure monitoring, the computer program product comprising a computer-readable medium having stored computer-readable program code for performing the steps of: measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator; generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor; determining a time delay between the pulse waveforms of each sensor; determining a pulse transit time (PTT) from the time delay; and, estimating a blood pressure value from the PTT. Further features provide for the computer-readable medium to be a non-transitory computer- readable medium and for the computer-readable program code to be executable by a processing circuit. Embodiments of the technology will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is an illustration of an example arterial dilation during a cardiac cycle; Figure 2 is an illustration of an example device for non-invasive continuous blood pressure monitoring according to aspects of the present disclosure; Figure 3A is an illustration of an example sensor including a superstrate positioned against a surface of a resonator; Figure 3B is an illustration of an example sensor including a superstrate positioned at a distance above a resonator; Figure 4A is an illustration of an example sensor including a superstrate under deformation by an external force; Figure 4B is an illustration of a side view of an example superstrate in an undeformed and deformed state; Figure 5 is an illustration of an example sensor with a superstrate undergoing deformation due to arterial dilation during a cardiac cycle; Figure 6A is an illustration of an example sensor with an artery in an un-dilated state exhibiting properties of a superstrate; Figure 6B is an illustration of an example sensor with the artery in a dilated state exhibiting properties of a superstrate; Figure 7A is an illustration of an example asymmetric frequency response curve of a resonator; Figure 7B is an illustration of an example change in an asymmetric frequency response curve of a resonator; Figure 8 is an illustration of an example of an array of sensors arranged along a path of an artery undergoing dilation during a cardiac cycle; Figure 9 is an illustration of an example asymmetric frequency response of the array of sensors according to Figure 8; Figure 10 is an illustration of an example graph of two pulse waveforms of two sensors of the device; Figure 11 is an illustration of an example graph of a plurality of sensor versus time measurements; Figure 12 is a flow diagram of an example method for non-invasive continuous blood pressure monitoring according to aspects of the present disclosure; Figure 13 is a block diagram which illustrates a device for non-invasive continuous blood pressure monitoring according to aspects of the present disclosure; and, Figure 14 illustrates an example of a computing device in which various aspects of the disclosure may be implemented. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS A device and method for non-invasive continuous blood pressure monitoring, is disclosed. Hypertension is a medical condition associated with high blood pressure. The ability to continuously monitor a subject’s blood pressure may help to control said subject’s blood pressure, such as by using taking an appropriate medication when a high blood pressure is detected. To continuously monitor a subject’s blood pressure, a measurement device that is non-invasive and compact enough to be on or attached to the subject is required. Although this disclosure is presented to monitor a blood pressure for a human, the device and method may equally be applicable in the veterinary space. The non-invasiveness requirement of the device may ensure that the device does not require any puncturing of a surface of a skin of the subject or any other invasive procedures, require assistance from another individual, or inhibit a subject’s motion or physical abilities to a reasonably degree. The continuous requirement of the device may require that the device constantly and / or consistently measures the blood pressure across a period where the subject is active and / or inactive. This may include the subject measuring their blood pressure throughout normal activities of their day, or during rest periods such as sleep. These requirements may ensure that the device is usable during a day where the subject is active and may need to know how their blood pressure fluctuates. Before describing the device and method, a short description of a cardiac cycle (a heartbeat) of a heart is required. Figure 1 is an illustration of an example arterial dilation (expansion) 100 during the cardiac cycle. The heart repeatedly contracts and expands to pump blood throughout the body. A cardiac cycle is one set of contraction and expansion of the heart. As the heart contracts, blood may be pumped in a forward direction 110 throughout a network of arteries in the body. The contraction of the heart may cause an increase in blood pressure in the arterial network, inducing a flow of blood in the arteries. The increase in blood pressure may cause the arteries to expand (also referred to as “dilate” or “dilation”). In Figure 1, the artery 106 underneath a surface of the skin 108 may expand from an un-dilated state with a first cross-sectional area 112 to a dilated state with a larger cross-sectional area 114. The increase in blood pressure may induce a pressure wave to travel along a length of the arteries as the blood is moved forward by a pressure differential between the high blood pressure in near the heart to the rest of the arterial network, and back to the heart via a network of veins. After contraction, the heart muscle may relax. Blood within the arteries may continue to travel forward 110 along the network of arteries due to the potential energy built up in the dilated arteries, which may slowly contract back into an un-expanded, or un-dilated, state. The contraction may ensure a steady flow of blood in the body closer to the extremities. The device may be device configured to non-invasively measure the blood pressure of the subject by detecting the expansion of the arteries underneath the skin during the cardiac cycle. In particular, the device may detect the expansion of an artery along several waypoints along the artery. The expansion of the artery at the several waypoints may be detected by one or more sensors configured to detect the arterial expansion. The device may be configured such that a distance between each sensor is known. The sensors of the device may be synchronised to a standard time. The device may determine a time difference between when each sensor detects an expansion of the artery at each waypoint. The detection of an expansion of the artery may also be referred to as a detection of a pulse. The sensors on the device may be located at specifically selected locations. The known fixed distance between the sensors may serve as a controlled measurement baseline. The device may be calibrated to establish a patient-specific relationship between the sensor measurements and blood pressure, accounting for individual variations in arterial compliance and other physiological factors. Using the time difference between when each sensor detects a pulse, and using the known distance between the sensors, the velocity of the pressure wave may be determined. Additionally, the method for detecting a blood pressure may include determining a pulse transit time (PTT). The PTT may be a measurement of the time required for the pressure wave, or pulse, to travel from one point of the body to another. The PTT may be a key characteristic of the cardiovascular system. The blood pressure may be determined directly from the PTT, as the PTT may have a direct correlation with blood pressure. The PTT may be directly determined as the time delay between a pulse being detected at each of two or more sensors. The velocity of the pressure wave may also be used to determine the blood pressure as a pulse wave velocity (PWV) is related to blood pressure. The PWV may be calculated by dividing the known distance between the sensors by the measured PTT. The PWV may be used as there may be a direct correlation with blood pressure due to the fundamental relationship between arterial wall stiffness and blood pressure. Additionally, other physiological parameters may be determined from the detection of cardiac pulses by the sensors. Figure 2 is an illustration of an example device 200 for use in non-invasive continuous blood pressure monitoring. The device 200 may include a wearable member 202. In an example, the wearable member 202 may be a sleeve configured to fit on a wrist of the subject 201. The wearable member 202 may be configured to be non-invasive, allowing the subject 201 to continue to perform their daily activities with minimal disruption. The device 200 may include one or more sensors 204. In an example, the sensors 204 may be configured within the wearable member 202. The sensors 204 may be configured, such that when the device affixed, fastened, or placed on to the subject 201, the sensors 204 are aligned along multiple waypoints along a path of the artery 106 under the skin 108. The sensors may independently detect a pulse at each individual location of the sensor. In an example, the device may include a measurement processor. The measurement processor may be configured within the wearable member 202. In an example, the measurement processor may measure a signal from all sensors of the device. In another example, each sensor may include a measurement processor. The measurement processor may be configured to measure a signal generated by each of the sensors 204. The measurement processor may include a pulse waveform generator. The pulse waveform generator may be configured to generate a pulse waveform in response to measuring a signal from the sensors 204. The signal may be a change in frequency response of a resonator. Figure 3A and Figure 3B are examples of a sensor. The sensor 204 may include a superstrate 302 material, a substrate 306 material, a ground 310, and a resonator 308. The resonator 308 may be within the substrate 306 material. In an example, the superstrate 302 material may be positioned against a surface of the resonator 308 as shown in Figure 3A. In an example, the superstrate 302 material may be positioned at a distance 304 away from the resonator 308 as shown in Figure 3B. The sensor 204 may include a material placed between the superstrate 302 and the resonator 308 to position the superstrate and resonator relative to each other. The sensor 204 may include air between the superstrate 302 and the resonator 308, each being held in position relative to each other by an external component, such as the wearable member 202. In an example, the sensor may be arranged on the wearable member 202 such that the superstrate 302 is positioned towards the skin 108 of the subject 201. Furthermore, the superstrate may be in direct contact with the skin 108. The superstrate 302 may be a transpermittivity superstrate. The superstrate 302 may be made from a compressible material. The superstrate 302 material may be configured to be deformable or displaceable relative to the resonator. Figure 4A is an illustration of an example sensor including a superstrate under deformation. Under the influence of an external force 412, the superstrate may be deformed into a deformed superstrate 402. In a further example, the superstrate may be displaced, while being minimally deformed such that the deformation has a negligible impact on the signal output of the sensor. The deformed or displaced superstrate 402 may move relative to the resonator 308. As will be described later, the deformation or displacement of the superstrate induces a measurable signal in the resonator 308. The external force 412 may be a force acting on the superstrate 402 by the skin 108 of a subject 201. The force may be generated by the expansion of the artery under the skin 108, thereby causing the skin to move in the direction of the superstrate and acting upon the superstrate 302. Figure 4B is an illustration of a side view of an example superstrate in an undeformed and deformed state. The superstrate 302 may have a permittivity value. The superstrate may have a permittivity value larger than biological tissue. The superstrate 302 material may be a compressible structure filled with a fluid 410. The fluid may be an incompressible fluid. The fluid may have a specific permittivity value. The undeformed superstrate 302 may deform into a deformed superstrate 402 by the external force 412. The external force 412 may compress 406 the superstrate in a direction generally normal to the external force. Due to the incompressible property of the fluid, the bag may expand 404 in a direction generally transverse to the external force 412. The external force 412 may be generally parallel to the direction of movement of the skin 108 during an expansion of the artery 106. The resonator 308 may be a structure that exhibits a characteristic electromagnetic frequency response when excited by an incident electromagnetic wave. This response may include specific resonant frequencies where the structure efficiently couples with, reflects, or transmits electromagnetic energy. When the superstrate material displaces or deforms relative to the resonator due to arterial deformation or displacement, it may alter an electromagnetic environment around the resonator, causing measurable shifts in these resonant frequencies. The resonator 308 may have a plurality of material properties defining the resonator 308. The resonator may include an effective permittivity material property. A superstrate 302 material in a vicinity of the resonator 308 may alter the electromagnetic environment by altering the effective permittivity. As the superstrate 302 material moves within the vicinity around the resonator 308, a change of the effective permittivity may change the frequency of the resonator 308. The frequency of the resonator 308 may be a frequency response. The resonator 308 may be a planar Fano resonator. Properties of the planar Fano resonator may include any one or more of: an asymmetric frequency response, a high sensitivity to a change in its immediate electromagnetic environment, and an ability to produce detectable frequency response shifts in response to mechanical deformation or displacement of the adjacent superstrate material. The resonator 308 may be any other type of resonator with similar properties to the planar Fano resonator. A resonator with an asymmetric frequency response may have a high selectivity, such that a frequency response curve of the frequency response has a noticeable sharpness and quality factor. The superstrate material 302,402 may be selected such that the change in the frequency response of the resonator, due to a change in the effective permittivity in the vicinity of the resonator, is magnified. This may improve the accuracy of the sensor to detect a pulse. Figure 5 is an illustration of an example sensor with a superstrate undergoing deformation due to arterial dilation during the cardiac cycle. Figure 5 is a combination of Figure 1 and Figure 4A, illustrating the orientation of the sensor 204 in relation to the artery 106. The sensor may be arranged with the superstrate positioned towards to skin 108. The skin may displace towards the sensor when the artery expands to a dilated position 114, and external force 412 is applied to the superstrate 402. The external force may deform the superstrate 402 material. The deformation of the superstrate 402 materials induces a change in the effective permittivity of the resonator. The change in effective permittivity changes the frequency response of the resonator. The change in frequency response is measurable by the measurement processor, allowing for the pulse to be detected from the measurement. In a further example, the sensor 204 may not require a specific superstrate material. Biological tissue, such as the artery 106 and / or skin 108 of the subject 201 may function as a superstrate material for the purpose of changing the effective permittivity in the vicinity of the resonator. Figure 6A is an illustration of an example sensor with the artery exhibiting properties of a superstrate, in an un-dilated state. The effect on the resonator is similar to an undeformed or undisplaced superstrate. Figure 6B is an illustration of an example sensor with the artery exhibiting properties of a superstrate, in a dilated state. The artery 106, due to a higher blood pressure in the artery, expands to a dilated 114 state. The biological tissue of the artery 106 and / or skin 108 displaces towards the resonator 308, varying the effective permittivity in the vicinity of the resonator. Figure 7A is an illustration of an example asymmetric frequency response curve of the resonator 308. The asymmetric frequency response curve, being a first asymmetric frequency response curve 701 depicts the frequency response of the resonator when the superstrate is undeformed or un-displaced relative to the resonator, such as when no external force is acting upon the superstrate material. The curve 701 may include a first normalised frequency response 702 (f / fO). The first normalised frequency response 702 may occur at a point of maximum magnitude of the response curve 701. The first normalised frequency response 702 may also be referred to as the resonant frequency point of the sensor. The high selectivity of the frequency response curve may be observable by the rapid decrease in magnitude of the curve for a small change in normalised frequency away from the peak 710. Figure 7B is an illustration of an example change in the asymmetric frequency response curve of the resonator. When the superstrate is acted upon by an external force, the deformation of the superstrate may change the effective permittivity in the vicinity near the resonator. The change in effective permittivity may induce a change in the asymmetric frequency response curve, forming a second asymmetric frequency response curve 703. The change in the asymmetric frequency response curve may change the normalised frequency response. The curve 703 may include a second normalised frequency response 704 (f1 / fO). The normalised frequency response changes from the first response 702 to the second response 704. The difference 705 between the two responses can be determined from the change in the frequency responses 702,704. The measurement processor of the device may be configured to measure the difference between the frequency responses 702,704. The measurement processor may be configured to measure the change in frequency response for each sensor independently. The change in frequency response may be directly proportional to the deformation or displacement. The larger the degree of deformation of the superstrate, or the larger the displacement of the superstrate, the larger the change in the asymmetric frequency response curve may be. The first asymmetric frequency response curve 701 may be an example response curve for the sensor of Figure 3B with an undeformed and / or un-displaced superstrate, or for an un-dilated artery according to Figure 6A. The second asymmetric frequency response curve 703 may be an example response curve for the sensor of Figure 4A with a deformed and / or displaced superstrate, or for a dilated artery according to Figure 6B. The frequency response curve may change linearly along the horizontal axis for a linear increase in the external force 412. The effective permittivity in the vicinity of the resonator may be influenced by both the superstrate material and the biological tissue of the subject. The deformation and displacement of both the superstrate and biological tissue may all influence the effective permittivity, and therefore, the frequency response curve of the resonator. The fluid filled superstrates may have a higher permittivity value than a dielectric superstrate. The larger permittivity value may function as a multiplying factor of the superstrate. The multiplying factor may amplify the change in effective permittivity in the vicinity of the resonator for even minor deformation or displacement of the resonator. The superstrate comprising the fluid filled bag may have a larger change in frequency response when compared to a sensor without the fluid filled bag, due to the larger multiplying factor. In an example, the superstrate may be replaced by a dielectric superstrate. For the sensor 204 with a dielectric superstrate or without any superstate at all, the expansion of the artery or the proximity of blood to the resonator may only change the asymmetric frequency response curve by an insignificant amount, such that change may be indistinguishable from sensor noise. An example of an array of sensors 204, arranged at several waypoints along a path of an artery 106 undergoing dilation during a current cardiac cycle, is shown in Figure 8. Figure 8 represents a snapshot in time during a cardiac cycle, wherein a first section 820 of the artery may have already undergone expansion and has since contracted, a second section 822 which may be at a peak expansion, and a third section 824 which may yet still undergo expansion during the cardiac cycle. During a cardiac cycle, blood is pumped by an increase in blood pressure, from the heart to the rest of a subject’s body. In Figure 8, blood flows in a forward 110 direction. The artery 106 expands when the blood pressure increases, after which is the artery contracts as the pressure subsides. The example arrangement includes three sensors including three resonators 801,802,803 with corresponding superstrate material 805,806,807. The superstrates 805,807 are undeformed as the artery is in a normal relaxed state, whereas the superstrate 806 is deformed. The outer edge of the artery in an undeformed state is shown by an undeformed line 810. The outer edge of the artery in a deformed state is shown by a deformed line 811. The deformation of the deformed superstrate 806 is shown by the thinner superstrate aligning with the deformed line 811. An example graph of asymmetric frequency response curves for sensors arranged along the path of an artery undergoing expansion is shown in Figure 9. The example graph corresponds to the arrangement of Figure 8. Figure 9 shows: a first curve 901 with corresponding first frequency response 902, a second curve 903 with corresponding second frequency response 904, and a third curve 905 with corresponding third frequency response 906. As the pressure wave travels along the length of an artery during the current cardiac cycle, the artery expands from a relaxed state before contracting back to the relaxed state. As the pressure wave travels in the forward direction 110 in Figure 8, a first superstrate 805 may be in an undeformed state after having already been deformed during the current cardiac cycle when the artery was expanded, but may have since contracted already. A second superstrate 806 may be deformed due to the expansion of the artery 106. A third superstrate 807 may yet to be deformed in the current cardia cycle due to the moving pressure wave traveling in the forward direction 110 towards the third superstrate 807. A third resonator 803 may form part of a sensor together with the third superstrate 807. As there may have been no deformation of the third superstrate within the current cardiac cycle, the third curve 905 and the third frequency response 906 may represent the curve and response of a resonator with an undeformed and yet to be deformed superstrate. A first resonator 801 may have already experienced a change in effective permittivity in its vicinity due to the undeformed, but previously deformed during the current cardiac cycle. The corresponding first curve 901 and first frequency response 902 may be returning to its normal state (such as to that of curve 905 and response 906 from curve 903 and response 904, respectively). A second resonator 802 may experience a change in effective permittivity in its vicinity due to the deformed artery in the current cardiac cycle. The corresponding second curve 903 and second frequency response 904 may be in a highly changed state away from the normal position (such as that of curve 905 and response 906). The degree of deformation of the superstrate 806 may be determined by the degree of change of the frequency response (the change along the ‘Normalised Frequency’ axis). A large difference of the frequency response curve and frequency response of a sensor near an expanded artery is essential for accurate and continuous blood pressure monitoring. The effective permittivity experienced by the resonator may be due to both the deformation and / or displacement of the superstrate material, and the biological tissue of the skin and artery. The degree to which the superstrate and biological tissue may have on the change in frequency response may differ. In an example, the effect of the superstrate on the effective permittivity experienced by the resonator may be orders of magnitude larger than that of the biological tissue. Figure 10 is an illustration of an example of two pulse waveforms, one for each of two sensors 204, plotted on a graph. The graph may represent a magnitude of the deformation of a superstrate material for each sensor versus time. A pulse waveform may be generated for each sensor of the device. Devices with more than two sensors may have multiple pulse waveforms. The pulse waveforms may be generated by the measurement processor. The measurement processor may generate each pulse waveform independently for each sensor. In another example, each sensor may generate the pulse waveform instead of the measurement processor. In an example, the pulse waveforms may be generated in response to a change in the frequency response of the sensor. In another example, the pulse waveform may be continuously generated in response to continuous measuring of the frequency response of the sensor. A first pulse waveform 1010 may be generated for a first sensor. A second pulse waveform 1020 may be generated for a second sensor. A time delay 1030 may be determined from the pulse waveforms. As the sensors may be placed along the path of the same artery, the pulse waveforms may have similar waveforms. The pulse waveforms may be synchronised to a central clock. The second pulse waveform 1020 may appear as a delayed version of the first pulse waveform 1010. A first peak 1040 of the first pulse waveform 1010 and a second peak 1050 of the second pulse waveform 1020 may indicate the moment a maximum expansion of the artery occurred during the cardiac cycle. The time difference between the peaks 1040,1050 may be usable to determine the time delay. The time delay 1030 may be used to determine the PTT. In an example, the PTT may be equal to the time delay. Figure 11 is an illustration of an example graph 1100 of time measurements of when a pulse is detected in each of a plurality of sensors for different blood pressure levels. The graph 1100 may plot the sensors 204 of the device 200 along the horizontal axis, and the time that a change in frequency response is measured at each resonator for each sensor on the vertical axis. The time that each sensor detects a pulse (expansion of the artery), the time can be plot against the vertical axis. Figure 11 depicts a low blood pressure 1110 scenario, a normal blood pressure 1120 scenario, and a high blood pressure 1130 scenario. In the normal blood pressure 1120 scenario, a first sensor S1 detects a pulse at a first time t1, a second sensor S2 detects a pulse at a second time t2, and a third sensor S3 detects a pulse at a third time t3. In an example, as the blood pressure value increases, the PTT decreases. Therefore, the time delay between each sensor decreases A larger blood pressure may enable the blood, and the pressure wave, to move faster within the artery. Therefore, a smaller difference in time between the sensors detecting a pulse (such as line 1130) will equate to a higher blood pressure. The arc 1140 depicts a direction of the slope of the lines 1110,1120,1130 in an increasing blood pressure direction. The relationship between blood pressure and time may be linear for sensors spaced equal distance apart. Therefore, the time difference between t1, t2 and t3 may decrease as the blood pressure increases, resulting in a reduced slope of the lines of 1110,1120,1130. The device 200 described above may implement a method 1200 for non-invasive continuous blood pressure monitoring. An exemplary method for non-invasive continuous blood pressure monitoring is illustrated in the flow diagram of Figure 12. The device, which may include one or more sensors 204, may measure 1202 the frequency response of each sensor 204. The frequency response may change continuously due to small perturbations of the skin. The device may filter 1203 noise in the signal from the sensor or the frequency response. The measurement processors may apply signal processing techniques to distinguish sensor noise from legitimate movement of the skin due to arterial expansion during the cardiac cycle. In response to detecting a change in frequency response, the device may generate 1204 a pulse waveform for each sensor. Generating the pulse waveform may be in response to measuring a change in frequency response of the sensor. Alternatively, the pulse waveform may be continuously generated. The pulse waveform may be an amplitude-over-time signal. The amplitude may be a displacement of the skin of the subject. The device may determine 1206 a time delay between each pulse waveform. Determining 1206 the time delay may include detecting the peaks for each pulse waveform that correspond to individual cardiac cycles. Determining 1206 the time delay may include detecting similar sections of the pulse waveforms that correspond to individual cardiac cycles. The time difference between the peaks or sections of the pulse waveforms may be used to determine the time delay. The device may determine 1208 a pulse transit time (PTT) from the time delay. The PTT may be determined between two sensors. If the device includes more than two sensors, the PTT may be calculated as: an average of the time delay between each of the sensors, a largest time delay between the sensors, a smallest time delay between the sensors, or any other combination of time delay measurement data. The device may estimate 1210 a blood pressure of the subject from the PTT. The blood pressure estimation may be directly proportional to the time delay or the PTT. The blood pressure estimation may be determined using the gradient information, as shown in Figure 11. The device may store 1212 the blood pressure estimations. Storing 1212 the blood pressure estimations may include transmitting the estimations to an external storage device for storage. The device may post-process 1214 the blood pressure estimations to determine a hypertension of the subject. In an example, an external device may be coupled to the device 200. The external device may perform specific functions, such as storing device data or performing post-processing 1214. The method may continuously repeat method steps 1202 to 1214 to continuously measure the blood pressure. The continuous measurements may form a set of blood pressure estimation values. The post-processing 1214 may occur repeatedly with newer blood pressure estimation values from the set of blood pressure estimation values when determining a hypertension. The continuous monitoring of the blood pressure estimations may be usable to detect 1216 hypertension of the subject 201. If a hypertension is detected 1216, an alert signal may be sent 1218 to the subject 201 or to a healthcare provider. The alert signal may be any signal configured to alert the user directly such that they may take steps to treat or combat the hypertension. The alert may be sent to the healthcare provider to track a treatment of the subject 201. The method may include calibrating each sensor to determine the asymmetric frequency response curve and the normalised frequency response of the sensor when the superstrate is undeformed and / or un-displaced relative to the resonator. The calibration may further include measuring the PTT while simultaneously a known calibrated blood pressure measurement device (such as an invasive or cuff-based device) records the blood pressure values. The PTT and calibrated blood pressure readings may be used to correlate PTT and blood pressure. The calibration data may be used to create a conversion function that converts PTT measurements into blood pressure values. Furthermore, the calibration may determine a PWV using the known distance between the sensors. The PWV measurements may be calibrated against the known calibrated blood pressure measurements, in a similar manner to calibrating the PTT values. Various components may be provided for implementing the method described above with reference to Figure 12. Figure 13 is a block diagram which illustrates exemplary components which may be provided by a system for non-invasive continuous blood pressure monitoring. The device 200 may include a processor 1302 for executing the functions of components described below, which may be provided by hardware or by software units executing on the X. The software units may be stored in a memory component 1304 and instructions may be provided to the processor 1302 to carry out the functionality of the described components In some cases, for example in a cloud computing implementation, software units arranged to manage and / or process data on behalf of the device 200 may be provided remotely. Some or all of the components may be provided by a software application downloadable onto and executable on the device 200. The device may include a measuring component 1306 arranged to measure a change in frequency response at each of one or more sensors 204 of the device 200. The measurement component may include the measurement processor. The measuring component 1306 may be arranged to filter the signal from the sensors. The device may include a pulse waveform generating component 1308 arranged to generate a pulse waveform in response to a measurement of a change in frequency response. In an example, the component 1308 may continuously generate a pulse waveform of the signal measured from the sensors. The device may include a time delay determining component 1310 arranged to determine a time delay between the pulse waveforms of each sensor. The component 1310 may arranged to receive the pulse waveform from the generating component 1308. The time delay determining component 1310 may be arranged to extract the peaks of the pulse waveforms or to determine similar sections of the pulse waveforms for determining a time delay. The device may include a pulse transit time determining component 1312 arranged to determine the PTT. The pulse transit time determining component 1312 may be determined from the time delay. The pulse transit time determining component 1312 may obtain the time delay from the time delay determining component 1310. The device may include a blood pressure estimating component 1314 arranged to estimate the blood pressure of the subject. The blood pressure estimating component 1314 may use the PTT to estimate the blood pressure. The device may include a storing component 1315 arrange to store device data. The device data may include the blood pressure estimations, the PTT, the time delay, the frequency response, and the time. The device may include a calibration component 1316 arranged to calibrate the sensors of the device. The calibration component 1316 may obtain, from a different calibrated device, validated blood pressure values. The component 1316 may compare a set of measured frequency response values over the same time period as the validated blood pressure values to generate a mapping between the measured frequency response values and the validated blood pressure values. The device may include a hypertension detecting component 1317 arranged to detect a hypertension in the subject in response to blood pressure values. The hypertension detecting component 1317 may be arranged to incorporate a plurality of signals, including the blood pressure estimation values to determine a hypertension. The device may include an alert generating component 1318 arranged to generate an alert signal in response to a hypertension being detected. In an example, the alert may be an instant alert to gain the attention of the subject. In another example, the alert may be a message provided to the subject of their estimated blood pressure values. The device may include an alert sending component 1320 arranged to alert the subject and / or the healthcare provider in response to a hypertension alert being generated. The device may include a connectivity component 1322 arranged to connect to external devices. The component 1322 may be configured to connect to other device 200, or other types of devices such as a smartphone. The component 1322 may be configured to interact with an application on the external device. Figure 14 illustrates an example of a computing device 1400 in which various aspects of the disclosure may be implemented. The computing device 1400 may be embodied as any form of data processing device including a personal computing device (e.g. laptop or desktop computer), a server computer (which may be self-contained, physically distributed over a number of locations), a client computer, or a communication device, such as a mobile phone (e.g. cellular telephone), satellite phone, tablet computer, personal digital assistant or the like. Different embodiments of the computing device may dictate the inclusion or exclusion of various components or subsystems described below. The computing device 1400 may be suitable for storing and executing computer program code. The various participants and elements in the previously described system diagrams may use any suitable number of subsystems or components of the computing device 1400 to facilitate the functions described herein. The computing device 1400 may include subsystems or components interconnected via a communication infrastructure 1405 (for example, a communications bus, a network, etc.). The computing device 1400 may include one or more processors 1410 and at least one memory component in the form of computer-readable media. The one or more processors 1410 may include one or more of: CPUs, graphical processing units (GPUs), microprocessors, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs) and the like. In some configurations, a number of processors may be provided and may be arranged to carry out calculations simultaneously. In some implementations various subsystems or components of the computing device 1400 may be distributed over a number of physical locations (e.g. in a distributed, cluster or cloud-based computing configuration) and appropriate software units may be arranged to manage and / or process data on behalf of remote devices. The memory components may include system memory 1415, which may include read only memory (ROM) and random access memory (RAM). A basic input / output system (BIOS) may be stored in ROM. System software may be stored in the system memory 1415 including operating system software. The memory components may also include secondary memory 1420. The secondary memory 1420 may include a fixed disk 1421, such as a hard disk drive, and, optionally, one or more storage interfaces 1422 for interfacing with storage components 1423, such as removable storage components (e.g. magnetic tape, optical disk, flash memory drive, external hard drive, removable memory chip, etc.), network attached storage components (e.g. NAS drives), remote storage components (e.g. cloud-based storage) or the like. The computing device 1400 may include an external communications interface 1430 for operation of the computing device 1400 in a networked environment enabling transfer of data between multiple computing devices 1400 and / or the Internet. Data transferred via the external communications interface 1430 may be in the form of signals, which may be electronic, electromagnetic, optical, radio, or other types of signal. The external communications interface 1430 may enable communication of data between the computing device 1400 and other computing devices including servers and external storage facilities. Web services may be accessible by and / or from the computing device 1400 via the communications interface 1430. The external communications interface 1430 may be configured for connection to wireless communication channels (e.g., a cellular telephone network, wireless local area network (e.g. using Wi-Fi™), satellite-phone network, Satellite Internet Network, etc.) and may include an associated wireless transfer element, such as an antenna and associated circuitry. The computer-readable media in the form of the various memory components may provide storage of computer-executable instructions, data structures, program modules, software units and other data. A computer program product may be provided by a computer-readable medium having stored computer-readable program code executable by the central processor 1410. A computer program product may be provided by a non-transient or non-transitory computer-readable medium, or may be provided via a signal or other transient or transitory means via the communications interface 1430. Interconnection via the communication infrastructure 1405 allows the one or more processors 1410 to communicate with each subsystem or component and to control the execution of instructions from the memory components, as well as the exchange of information between subsystems or components. Peripherals (such as printers, scanners, cameras, or the like) and input / output (I / O) devices (such as a mouse, touchpad, keyboard, microphone, touch-sensitive display, input buttons, speakers and the like) may couple to or be integrally formed with the computing device 1400 either directly or via an I / O controller 1435. One or more displays 1445 (which may be touch-sensitive displays) may be coupled to or integrally formed with the computing device 1400 via a display or video adapter 1440. The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the technology to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. Any of the steps, operations, components or processes described herein may be performed or implemented with one or more hardware or software units, alone or in combination with other devices. Components or devices configured or arranged to perform described functions or operations may be so arranged or configured through computer-implemented instructions which implement or carry out the described functions, algorithms, or methods. The computer-implemented instructions may be provided by hardware or software units. In one embodiment, a software unit is implemented with a computer program product comprising a non-transient or non-transitory computer-readable medium containing computer program code, which can be executed by a processor for performing any or all of the steps, operations, or processes described. Software units or functions described in this application may be implemented as computer program code using any suitable computer language such as, for example, Java™, C++, or Perl™ using, for example, conventional or object-oriented techniques. The computer program code may be stored as a series of instructions, or commands on a non-transitory computer-readable medium, such as a random access memory (RAM), a read-only memory (ROM), a magnetic medium such as a hard-drive, or an optical medium such as a CD-ROM. Any such computer-readable medium may also reside on or within a single computational apparatus, and may be present on or within different computational apparatuses within a system or network. Flowchart illustrations and block diagrams of methods, systems, and computer program products according to embodiments are used herein. Each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, may provide functions which may be implemented by computer readable program instructions. In some alternative implementations, the functions identified by the blocks may take place in a different order to that shown in the flowchart illustrations. Some portions of this description describe the examples in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations, such as accompanying flow diagrams, are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. The described operations may be embodied in software, firmware, hardware, or any combinations thereof. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the present disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of any accompanying claims. Finally, throughout the specification and any accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A blood pressure measurement device, comprising:at least one sensor which includes a resonator and a superstrate material provided together with the resonator,wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery,wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced; and,the device further including a measurement processor configured to measure a change in the frequency response of the resonator.

2. The device as claimed in claim 1, including at least two sensors, wherein the at least two sensors are arranged along a portion of a wearable member, such that the sensors are aligned at multiple waypoints adjacent to an artery of a person and along a path of the artery in response to the wearable member being affixed or fastened to the subject.

3. The device as claimed in claims 1 or 2, including a wearable member with at least one sensor and the measurement processor, configured to affix or fasten the device to the subject, such that when affixed or fastened to the subject, the sensor is positioned with the superstrate material positioned towards the skin and is deformable or displaceable in response to a movement of an area of the skin.

4. The device as claimed in any one of the preceding claims, wherein the measurement processor is configured to measure a change in the frequency response of each resonator for each sensor independently.

5. The device as claimed in any one of the preceding claims, wherein the superstrate material is a compressible structure filled with a fluid.

6. The device as claimed in claim 5, wherein the compressible bag is deformable by a biasing force, and wherein the bag is compressible in a direction generally normal to the skin of the subject and expandable in a direction generally transverse to the skin of a subject in response to the biasing force.

7. The device as claimed in claim 6, wherein the biasing force is a force applied by the skin of a person in response to an artery under the skin expanding during a cardiac cycle, pushing the skin towards the superstrate material.

8. The device as claimed in any of the preceding claims, wherein the superstrate material has a higher permittivity value than biological tissue.

9. The device as claimed in any of the preceding claims, wherein the resonator is a planar Fano resonator.

10. The device as claimed in any of the preceding claims, where the frequency response of the resonator is an asymmetric frequency response.

11. The device as claimed in any of the preceding claims, wherein the measurement processor includes a pulse waveform generator configured to generate a pulse waveform in response to a measurement of a change in the frequency response of the resonator.

12. The device as claimed in any of the preceding claims, wherein the frequency response change is directly proportional to the deformation or displacement.

13. A method for estimating a blood pressure value, comprising:measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator;generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor;determining a time delay between the pulse waveforms of each sensor;determining a pulse transit time (PTT) from the time delay; and, estimating a blood pressure value from the PTT.

14. The method as claimed in claim 13, wherein measuring a change in frequency response at each sensor includes filtering out a noise measurement of each sensor.

15. The method as claimed in claims 13 or 14, wherein measuring a change of the frequency response includes continuously measuring the frequency response of the resonator.

16. The method as claimed in any one of claims 13 to 15, wherein the estimated blood pressure value is directly proportional to the time delay between pulse waveforms of each sensor.

17. The method as claimed in any one of claims 13 to 16, including calibrating each sensor to determine a normalised frequency response without deformation or displacement of the superstrate material relative to the resonator.

18. The method as claimed in any one of claims 13 to 17, including continuously estimating the blood pressure value from a plurality of measurements of the frequency response, to form a set of blood pressure values.

19. The method as claimed in claim 18, including processing the set of blood pressure values to determine a hypertension of the subject.

20. The method as claimed in claim 19, including sending an alert signal to a user of the deviceor a healthcare provider in response to the hypertension being determined.

21. A system for non-invasive continuous blood pressure monitoring, the system comprising: a non-transitory computer-readable storage medium; and one or more processors coupled to the non-transitory computer-readable storage medium, wherein the non-transitory computer-readable storage medium comprises program instructions that, when executed on the one or more processors, cause the system to perform operations comprising:measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed ordisplaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator;generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor;determining a time delay between the pulse waveforms of each sensor;determining a pulse transit time (PTT) from the time delay; and, estimating a blood pressure value from the PTT.

22. A system for non-invasive continuous blood pressure monitoring, the system including a memory for storing computer-readable program code and a processor for executing the computer-readable program code, the system comprising:a measuring component for measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator;a pulse waveform generating component for generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor;a time delay determining component for determining a time delay between the pulse waveforms of each sensor;a pulse transit time determining component for determining a pulse transit time (PTT) from the time delay; and,a blood pressure estimating component for estimating a blood pressure value from the PTT.

23. A computer program product for non-invasive continuous blood pressure monitoring, the computer program product comprising a computer-readable medium having stored computer-readable program code for performing the steps of:measuring, with a blood pressure measurement device comprising at least two sensors, a change of a frequency response at each of the at least two sensors, each sensor including a resonator and a superstrate material provided together with the resonator, wherein the sensor is configured for attachment to a skin of a subject close to10an artery of the subject, and the superstrate material is moveable or deformable relative to the resonator through cardiac pulses that occur in the artery, wherein the resonator has a frequency response which changes when the superstrate material is deformed or displaced, and the device further including a measurement processor configured to measure the change in the frequency response of each resonator;generating a pulse waveform, for each sensor, in response to measuring a change in frequency response at each sensor;determining a time delay between the pulse waveforms of each sensor;determining a pulse transit time (PTT) from the time delay; and, estimating a blood pressure value from the PTT.

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