WO2021249850A1

WO2021249850A1 - Wearable device

Info

Publication number: WO2021249850A1
Application number: PCT/EP2021/064854
Authority: WO
Inventors: Rosario Lizio; Michael Huth; Kristina KLINKER; David Sandbach; Jacob Skinner
Original assignee: Evonik Operations Gmbh
Priority date: 2020-06-09
Filing date: 2021-06-02
Publication date: 2021-12-16

Abstract

The present invention relates to a wearable device (10) comprising a main body (11) and at least one strap (12) attached to the main body adapted to acquire physiological data of a subject, the main body comprising at least two photoplethysmographic (PPG) sensors (20) with a defined distance between both PPG sensors (20) in the direction of the blood flow, each PPG sensor comprising at least one light source and at least one photodetector. The wearable device comprises at least one flexible part (30) or at least a ball joint between the strap (12) and the main body (11) to allow a rotational movement of the main body (11) relative to the strap (12) and evenly distribute pressure between the PPG sensors (20) and the skin of the subject.

Description

Wearable device

The present invention relates to an improved wrist attachment for dual PPG channel blood oximetry. Therefore, the invention provides a wearable device, which allows dual PPG measurements and provides two PPG signals with a signal quality, which allows analysis of the PPG signals for several health parameters.

Optical sensors for blood oximetry (photoplethysmography) yield optimum quality data when positioned close to the skin surface, protected from interference from ambient light and critically, with the skin/sensor interface pressure within an optimum range or window. For normal, single sensor systems, this can be set by tightening or loosening the watch strap until the sensor works as desired.

For the analysis of several cardiovascular parameters, the determination of the pulse transit time (PTT) is essential. Pulse transit time (PTT) is the time that a pulse wave takes to travel between two different arterial points, and may be useful in estimating cardiovascular parameters, such as blood pressure. Typically, the PTT is initiated by the R-wave of the ECG and the finger is used as the second arterial location. The arrival of the pulse at this location is usually detected by the use of a PPG sensor. However, it is also possible to determine PTT using two PPG sensors, which need to be located at a certain distance form one another in the direction of blood flow, when the device is worn by a user to allow precise measurement.

The use of PPT for analyzing cardiovascular parameters has been described in the state of the art, such as in US 2015/0148663 A1 proposing a photoplethysmographic measurement apparatus, a photoplethysmographic measurement method, and an apparatus for measuring a biosignal. The photoplethysmographic measurement apparatus includes a probe, a light emitter comprising a nonelectrical light source and disposed at one end of the probe, the light emitter configured to illuminate a measurement part, and a light receiver disposed at another end of the probe and configured to detect light reflected or transmitted by the illuminated measurement part.

In WO 2014/022906 A1 a system is provided that continuously monitors cardiovascular health using an electrocardiography (ECG) source synchronized to an optical (PPG) source, without requiring invasive techniques or ongoing, large-scale external scanning procedures. The system includes an ECG signal source with electrodes contacting the skin, which generates a first set of information, and a mobile device having a camera which acts as a PPG signal source that generates a second set of information. Together with the mobile device's processor, configured to receive and process the first and second sets of information, from which the time differential of the heart beat pulmonary pressure wave can be calculated, continuous data related to cardiovascular health markers such as arterial stiffness can be determined. Variations of the ECG source may include a chest strap, a plug-in adaptor for the mobile device, or electrodes built into the mobile device. US 2013/324859 A1 discloses a method for providing information for diagnosing arterial stiffness noninvasively using PPG. The method of the invention for assessing arterial stiffness comprises: a user information input step, characteristic point extraction step, and arterial stiffness assessment step. In particular the arterial stiffness assessment step includes the result of performing multiple linear regression analysis using the baPWV (branchial-ankle pulse wave velocity) value. PPG segmentation is conducted with the help of the PPG second derivative and the PPG pulses need to be classified to remove corrupted PPG pulses. The additional cardiovascular features, such as augmentation index and vascular age index are directly estimated from the characteristic points of the second derivative waveform. Moreover, the second derivative is used to find the position in the PPG signal of some pivotal points.

The US 2017/0238818 A1 describes a method for measuring blood pressure including illuminating by one PPG sensor included in an electronic device, the skin of a user and measuring a PPG signal based on an illumination absorption by the skin. The method also includes extracting a plurality of parameters from the PPG signal, wherein the parameters may comprise PPG features, heart rate variability (HRV) features, and non-linear features. US 2017/0238819 A1 discloses a system, method, and device for monitoring physiological characteristics of a user includes a wearable monitoring device including one or more LEDs configured to emit light toward a user’s skin tissue and two or more sensors laterally disposed along a longitudinal axis of an extremity of a user. However, the problem that the sensors need to be evenly pressed against the skin surface of the user to enable processable PPG signals, which can be used to analyse physiological parameters.

With two sensors at different longitudinally aligned locations on the wrist it was found by experiment that, using a conventional watch body and strap construction (as described in

US 2017/0238819 A1) the sensor closest to the elbow (PPG sensor 1) of the wearer is typically pressed harder onto the skin surface than the sensor (PPG sensor 2) nearest to the wrist. The effect is more pronounced on wearers with arms having a more pronounced conical shape (having a larger cross-sectional circumference under PPG sensor 1 than PPG sensor 2).

This uneven pressure can often adversely affect the quality of the data measured from one or other sensor and limits the ability to obtain an effective comparison. Therefore, it was an objective of the present invention to provide a wearable device, suitable for dual PPG measurements, which enable the measurement with both PPG sensors in an acceptable signal quality, which allows the analysis of the PPG signals for further health related parameters, such as blood pressure, blood flow and arterial health. Both PPG sensors shall be included within one wearable device for easier wearability for the user.

Therefore, the invention provides a wearable device, which allows dual PPG measurements and provides two PPG signals with a signal quality, which allows analysis of the PPG signals for several health parameters. The invention provides a wearable device comprising a main body (11) and at least one strap (12) attached to the main body adapted to acquire physiological data of a subject, the main body comprising at least two photoplethysmographic (PPG) sensors (20) with a defined distance between both PPG sensors (20) in the direction of the blood flow, each PPG sensor comprising at least one light source and at least one photodetector.

The wearable device comprises at least one flexible part (30) or at least a ball joint between the strap (12) and the main body (11) to allow a rotational movement of the main body (11) relative to the strap (12) and evenly distribute pressure between the PPG sensors (20) and the skin of the subject.

To be able to determine PTT, the two PPG sensors need to be positioned with a defined distance in the direction of the blood flow, when the device is worn by a user. The main blood flow is along the length of the arm in direction of the wrist/hand. Regarding a conventional wearable device, the direction of the blood flow is perpendicular to the direction of the strap (which is closed around the wrist of the user).

The flexible part or ball joint is located between the strap and the main body and thereby allows a rotational movement of the main body relative to the strap. In a specific embodiment, the wearable device comprises one or two straps.

Essentially, the present invention provides an enhanced wearable device with specific means for distributing pressure relatively evenly between both PPG sensors and the skin surface. Therefore, the wearable device comprises either at least one flexible part or at least a ball joint to evenly distribute pressure between the PPG sensors and the skin of the subject. The ball joint allows rotational movement between the parts of the wearable device.

In a preferred embodiment, the PPG sensor comprises at least one green light source and comprises a sampling frequency of preferably 512 Hz.

In an alternative configuration of the present invention, the flexible part or ball joint is located between the first PPG sensor and the second PPG sensor and thereby allows a rotational movement between the two PPG sensors.

In a specific embodiment, the wearable device comprises two PPG sensors and further comprises a bioimpedance sensor. The bioimpedance sensor can allow continuous surveillance of blood glucose level and is relevant in pre-diabetic health assessment. Taking into consideration the blood glucose level of the user, specific nutritional recommendations can be given.

For the analysis of the pulse transit time a certain lateral distance between the two PPG sensors is needed. Therefore, in a preferred configuration, the lateral distance between the two PPG sensors is at least 2 cm, more preferably at least 3 cm, most preferably at least 5 cm. This lateral distance corresponds to the defined distance according to the present invention, meaning that both PPG sensors are positioned behind each other in the direction of the blood flow, when the device is worn by a user.

In a specific configuration, the wearable device comprises two flexible parts or two ball joints between the strap and the main body at two opposite positions of the main body.

In a further preferred embodiment, the axis of rotation runs across the main body, between the two flexible parts or ball joints and orthogonally bisecting the line between two PPG sensors.

Based on the PPG signals, several cardiovascular physiological parameters, related to arterial stiffness can be analyzed in detail.

Relation between cardiovascular parameters and arterial stiffness

With increasing age, the blood vessels usually become stiffer compared to those of a young person. This phenomenon occurs primarily because elastin in blood vessels’ walls deteriorates and is replaced by collagen, which is less flexible. The increased stiffness causes the blood to travel faster through the vessels, therefore arterial stiffness is strongly correlated to the pulse wave velocity PWV. If a person’s arterial stiffness is higher than the normal value for their age, this is a determinant of hypertension, i.e. increased systolic and diastolic blood pressure. As mentioned above, hypertension is an increasingly large problem, thus arterial stiffness is of interest as well. Since increased arterial stiffness can be detected before hypertension occurs, this allows to start treatment or behavioral changes early, possibly avoiding hypertension. It is also well known that atherosclerotic plaques and aneurysms involve changes in vessel wall properties and therefore their stiffness (M. McGarry et al., “In vivo repeatability of the pulse wave inverse problem in human carotid arteries”, J. of biomechanics, vol. 64, pp. 136-144, 2017). Also in this case, an accurate arterial stiffness measurement, in particular its variation, would improve diagnosis and monitoring of the connected diseases. Various cardiovascular parameters can be analyzed to gain information about a person’s cardiovascular health.

Augmentation index (Alx) is a cardiovascular parameter that is usually obtained from a pressure pulse wave and can be measured at a large artery with a device that uses an inflatable cuff. In contrast, the PPG sensor is unable to measure pressure and only detects volume changes in very small arteries and arterioles. It provides an indirect measure of arterial stiffness and further provides information about the pressure wave reflection by the peripheral circulatory system. The Augmentation Index measure was transposed from the Blood Pressure Pulse Wave Analysis to the PPG signal, assuming that one is able to obtain information about the arterial stiffness analyzing the PPG waveform. Just like arterial stiffness, the augmentation index increases with age and can be used to estimate the risk of suffering from a cardiovascular disease in the future.

Vascular age index (Aqlx) is a cardiovascular parameter that gives information on the age condition of the arteries, compared to some normal threshold for a healthy population. It can be determined with devices that uses an inflatable cuff. In the literature the Aglx as given from the second derivative of the PPG pulse wave form. The vascular age is mainly influenced by a genetic predisposition and by the lifestyle. The estimate of this parameter is based on the pressure wave velocity through the vascular tree. In healthy subjects, it should be lower than the chronological age. In hypertensive subjects, it is significantly higher than the chronological age (Lozinsky, Arterial Hypertension, vol. 19, n. 4, pp. 174-178, 2015).

Pulse wave velocity (PWV) describes the velocity of blood that travels through a person’s arteries and is used as a measure of arterial stiffness. PWV is defined as the speed at which the pressure wave propagates through the cardiovascular tree. The PWV assessment provides information about the elastic properties of the arterial system. The most precise devices to measure PWV perform a carotid-femoral measurement. For this measurement, one tonometer is placed at the carotid artery which is located at the neck and a second tonometer is placed at the femoral artery at the upper leg. Those tonometers measure the pressure pulse waves of the arteries. From the time difference between the signals and the distance between the tonometers, PWV can be calculated. A more convenient way to estimate the PWV is by using two PPG sensors at a known distance or one PPG sensor and an electrocardiogram (ECG) and to calculate PWV from the time difference between the signals. Although it is more difficult to assess, the pulse transit time (PTT) provides a better measure for monitoring. This parameter would allow estimating the aortic PWV (the aorta is the reference point to measure the PWV in the literature). PWV can also be measured with only one blood pressure cuff. This technique is used by the “Mobil-OGraph PWA” which is a clinical device by I.E.M. GmbH that has been used as a reference device in the experimental setup.

Blood pressure (BP) denotes the pressure that the blood traveling through a large artery exerts onto its walls. Hypertension is a major risk factor for multiple diseases, such as stroke and end- stage renal disease, and overall mortality. By the year 2025, it is expected that the number of people across the world who are hypertensive will have risen to 1.56 billion. If the condition is detected early and treated properly, the risk of disease can be decreased significantly. Therefore, it is important to measure BP regularly in order to detect abnormal changes. Besides this, a change of lifestyle can often decrease BP and prevent hypertension, provided that a tendency towards it is detected early. Currently, there exist several different approaches to measure BP. The most common device is an inflatable cuff that is placed at the patient’s arm and that applies pressure onto the brachial artery. While this allows an accurate measurement, it is perceived as inconvenient by some patients and it requires a visit to a doctor or the purchase of a device. Other approaches are invasive, such as intravenous cannula that are placed inside an artery. Those are only used in a clinical context, e.g. during a surgery. A PPG signal can be obtained comfortably, continuously and at low cost. Extracting information about BP can serve an important purpose: As it is easy to obtain at home, this could warn a person early and advise them to seek medical advice.

Heart rate variability (HRV) describes the variation in the time interval between heartbeats and is usually calculated from an ECG, as the RR intervals from the ECG are required. Nevertheless, for the HRV analysis, in principle, any signal that allows accurately identifying heartbeats can be used. For this reason, the PPG technology seems to be a valid alternative for conducting an HRV analysis (Pinheiro et al., IEEE Explore Digital Library, 2016). Normally, the HRV is determined from the PPG signal based on determining the locations of the systolic feet.

Other PPG parameters

In addition to the aforementioned parameters, various morphological characteristics of the PPG signal and its derivatives have also been studied.

The Pulse Area is defined as the area under the PPG curve. In a recent study (Usman et al., Acta Scientiarum Technology, vol. 36, n. 1 , pp. 123-128, 2013), a significant difference in this parameter was found in relation to two different levels of diabetes. In conclusion, the authors affirmed that it can be used as a useful parameter in determining arterial stiffness. In the work of Wang et al. (Annual International Conferente of the IEEE Engineering in Medicine and Biology Society, 2009), the area is divided into two sub-areas, A1 and A2, at the dicrotic notch. Based on these two measures, the Inflection Point Ratio was defined as the ratio between the two areas, demonstrating that this ratio can be used as an indicator of total peripheral resistance.

The time AT between the systolic peak and the diastolic peak seems to be linked to the blood vessels elasticity. Millasseau et al. (Clinical Science, vol. 103, n. 4, pp. 371-377, 2002) used this time interval to obtain a new index, the Large Artery Stiffness Index (SI), defined as the ratio between the height of the subject and the time interval between the systolic and diastolic peaks, finding that it decreases with age.

Another measure of the PPG signal temporal trend is the Crest Time (CT). Easy to measure, the CT is the time elapsed between the systolic foot and the systolic peak of a PPG wave. It has been assessed as a valid parameter (together with other measurements deriving from the PPG signal) for a cheap and effective Cardiovascular Disease (CVD) screening technique for use in general clinical practice (Alty et al., IEEE Transactions on biomedical engineering, vol. 54, n. 12, pp. 2268- 2275, 2007).

The CT and the SI can be estimated in a more reliable way using the first derivative of the PPG signal, also known as Velocity Photoplethvsmoqraph (VPG), measuring the time interval between the relative zero-cross.

In a specific configuration, the present invention is referring to a wearable device further comprising a processor, wherein the processor is configured to: receive a first PPG signal from the first PPG sensor and a second PPG signal from the second PPG sensor, determine a pulse-transit-time of the subject based on the generated first and second PPG signals, and calculate one or more physiological parameter of the subject based on the calculated pulse- transit-time and the lateral distance between the first and second PPG sensor.

The physiological parameters is or includes: the vascular age index Aglx, the pulse wave velocity PWV, blood pressure BPdia and BP_sys, augmentation index Alx of the subject.

Those physiological parameters can be determined by analyzing the measured PPG signal in detail. A PPG measurement can provide several parameters and indicators, thanks to which it’s possible to obtain information about the cardiovascular system. Elgendi (Current Cardiology Reviews, 2012, 8, 14-25) describes the use of PPG to estimate the skin blood flow using infrared light. Recent studies emphasize the potential information embedded in the PPG waveform signal and it deserves further attention for its possible applications beyond pulse oximetry and heart-rate calculation. Especially, characteristics of the PPG waveform and its derivatives may serve as a basis for evaluating vascular stiffness and aging indices.

PPG waveform

Based on the different layers in which the light propagates, the PPG waveform comprises two parts: a pulsatile (AC) physiological waveform, attributed to cardiac synchronous changes in the blood volume (in vessels) with each heartbeat, which is superimposed on a slowly varying (DC) component. DC or static signals are determined by static elements of body tissue such as, for instance, epidermis, bones and non-pulsatile blood.

The photoplethysmography signal within a cardiac cycle has a stereotyped waveform. Two phases can be detected: the anacrotic phase and the catacrotic phase. The former is mainly due to the systolic event of the cardiac cycle, the latter is partially caused by the diastolic event and by the reflection of the pressure wave by the peripheral vessels.

Landmark points can be detected within the PPG waveform as shown in fig. 5.1. The systolic foot is defined as the minimum value of the PPG wave during the cardiac cycle. The systolic peak is the maximum point. Both points fall in the anacrotic phase. The diastolic peak is the second maximum. The dicrotic notch is a slight negative inflection between the systolic peak and the diastolic peak; whether this notch is present or not depends on several factors (such as age or measurement site). Both the dicrotic notch and the diastolic peak fall in the catacrotic phase.

To determine the different cardiovascular parameters, the PPG waveform needs to be analysed and different features are extracted from the PPG waveform.

Separation of PPG signal into pulses

In order to analyse each individual PPG waveform in the PPG signal and to reduce the effect of motion artefacts, the PPG signal is not examined as a whole but in sections. According to the present invention the signal is divided into individual pulses, as all features which are extracted from the PPG signal can be derived from one pulse wave. The systolic foot is the most prominent feature of a PPG pulse and can therefore be found most reliably in the PPG signal. Therefore, the PPG signal was chopped into PPG pulses at this systolic foot by finding the minima in the PPG signal. This strategy allows to analyse each pulse individually. If a few pulses are not correctly recognized, this does not have a falsifying effect on the final results for a measurement as the final parameter values are calculated by the median of all individual pulses’ results.

From the PPG pulse wave, the systolic A_sys and diastolic Adia peak amplitudes are estimated (corresponding to x and y in formula 5.1 respectively), as well as their times t_s and td. The determination of Adia in the PPG waveform can be very difficult when the reflected wave is very small and there is no visible diastolic peak in the waveform (see Fig. 5.1).

Feature extraction from signal’s derivatives

Other features are obtained from the signal’s derivatives which are calculated by the differences between adjacent samples. A moving average filter was applied to remove high frequency noise introduced by taking the derivative. To reliably find the characteristic points a to e, an algorithm to find the two most prominent maxima was developed and they were marked as a and e, respectively. The point c is then the most prominent peak between point a and e. Furthermore, point b is the most prominent minimum in the second derivative and point d is the most prominent minimum between points c and e (see Fig. 5.2).

Therefore, in a preferred embodiment of the present invention the characteristic points a, b, c, d, and e are automatically derived from the second derivative of the PPG pulse, wherein a and e are the first and second most prominent maxima in the second derivative, respectively, c is the most prominent peak between the points a and e, b is the most prominent minimum in the second derivative and, d is the most prominent minimum between points c and e.

In a further specific configuration, the wearable device further comprises signal processing means adapted to calculate one or more of the following: the vascular age index Aglx using linear regression based on the characteristic points a, b, c, d, and e, age (page), body height (p_heigm) and median heart rate of the subject, the pulse wave velocity PWV using linear regression based on the time difference between the two PPG pulses (PTT), age (page), body height (p_heigm) and median heart rate estimation of the subject, blood pressure BPdia and BP_sys using linear regression based on time difference between the two PPG pulses (PTT) and median heart rate and optionally the augmentation index Alx, based on the systolic A_sys and diastolic Adia peak amplitudes normalized to 75 heartbeats (Alx@75) and using a linear regression based on the normalized augmentation index Alx.

The wearable device may be used for a method for measuring one or more cardiovascular parameters in a subject, by estimating one or more cardiovascular parameters in a subject, the subject having an age and a body height with the following steps:

- determining the age (p_age) and body height (p_height) of the subject, - measuring at least two photoplethysmographic (PPG) signals with at least two PPG sensors at two different positions at the subject,

- separating the PPG signal into PPG pulses, whereby the start point and the end point of the pulse corresponds the systolic foot of the PPG signal,

- determining the heart rate of the subject (pHR) and calculating the median heart rate,

- determining the systolic Asys and diastolic Adia peak amplitudes and their times ts and td,

- calculating the second derivative of the PPG pulse, and determining the characteristic points a, b, c, d, and e from the second derivative of the PPG pulse, wherein a and e are the first and second most prominent maxima in the second derivative, respectively, c is the most prominent peak between the points a and e, b is the most prominent minimum in the second derivative and, d is the most prominent minimum between points c and e,

- determining: a) the vascular age index Aglx using linear regression based on the characteristic points a, b, c, d, and e, age (page), body height (pheight) and median heart rate of the subject, b) the pulse wave velocity PWV using linear regression based on the time difference between the two PPG pulses (PTT), age (page), body height (pheight) and median heart rate estimation of the subject, c) blood pressure BPdia and BPsys using linear regression based on time difference between the two PPG pulses (PTT) and median heart rate and d) optionally the augmentation index Alx, based on the systolic Asys and diastolic Adia peak amplitudes normalized to 75 heartbeats (Alx@75) and using a linear regression based on the normalized augmentation index Alx,

In a preferred configuration, the method further comprises the determination of Crest Time (CT), Stiffness Index (SI) and Pulse Area (PA) of the PPG signal and wherein the cardiovascular parameters are estimated with the following equations: a) vascular age index Aglx:

, wherein Aglx is estimated based on characteristic points a, b, c, d, and e:

Aglx = 45.4 * ^¾~c~d~e + 65.9; b) pulse wave velocity PWV:

c) blood pressure BPdia and BPsys:

BP _sy_s = ko_s + k_lsPTT + k_2s'median(HRy, d) normalized augmentation index Alx@75:

Alx = (x - y)/y by the sum of two exponential, and

AIx@7S = b₀ + b AIx@ 75 , wherein Alx@75 is the augmentation index (Alx) normalized to 75 heartbeats; wherein, p_age is the age and p_heigm is the body height of the subject, median (HR) is the median heart rate, PTT is the time difference between the PPG pulses, A_sys and Adia are magnitudes of the systolic and diastolic peak, respectively, CT is the Crest Time, ST is the Stiffness Index and PA is the Pulse Area of the PPG signal, do to d4, go to g4, lo to Ik , kos to k2s, and bo to bi represent the coefficients of the respective linear regression equation.

In a preferred configuration, the cardiovascular parameters are estimated based on at least 60 PPG pulses, preferably at least 100 PPG pulses, more preferably at least 120 PPG pulses. The estimation of 60 pulses corresponds to measurement time of approximately 1 minute (with 60 pulses per minute). Therefore, the preferred configurations refer to a measurement time of at least 1 minute (60 PPG pulses), preferably at least 1.7 minutes (100 PPG pulses), more preferably at least 2 minutes (120 PPG pulses). By combining the results obtained by every PPG pulse mediated in the measured time, this allows a more reliable estimation. In this way, if there is a corrupted PPG pulse, its effect can be smoothed if the signals are mediated over the measured time. The measurement of PPG pulses over a defined time has the advantage that the single PPG pulses do not need to be classified as it necessary in the state of the art (e.g. such as in US2013/324859A1) and this provides a more efficient algorithm.

The wearable device can be a fitness tracker or a smartwatch.

Examples

A wearable device with two PPG sensors was constructed with different attachment points between the main body and the straps to analyse the effect of the strap attachment on the signal quality of the measured PPG signal. The distance between the two centers of the PPG sensors was approximately 4 cm and both PPG sensors had a sampling frequency of 512Hz.

The wearable devices were worn by 20 test users and the PPG signals were determined over a time period of 14 days. The following preprocessing steps were applied:

1. Average between the output from the two photodetectors of the same PPG sensors

2. For each averaged signal: a. Normalization with z-score b. Moving average filter to delete the drift c. 4th order Butterworth low pass filter, cut off frequency = 10 Hz

1. Conventional strap attachment (comparative^')

In a first step, a fitness tracker with conventional strap attachments were used, where the wearable device (10) comprised a main body (11) and two straps (12) with conventional strap attachment points, a) view from the top and b) view from the bottom, showing PPG sensor 1 and PPG sensor 2. Each PPG sensor (20) comprises one central LED and two photodiodes, whose readings are combined to provide one PPG trace. The fitness tracker is depicted in Figure 1 and comprises two standard watch hinges, where each strap is terminated with a hinge, allowing pivoting only one axis.

Figure 2 shows PPG signals measured with wearable device with conventional strap attachment (as shown in Figure 1). The graph shows a short sample of data from both PPG sensors after processing.

It could be observed that the fitness tracker did not evenly contact the skin with both PPG sensors, when worn by the user, especially, when the user was moving. The portion of the arm where the wearable device was secured is not cylindrical, but has a conical element to its shape - the upper arm (under sensor 1) has a larger cross sectional area than the wrist (under sensor 2). With a standard watch hinge, allowing rotation about one axis, it was noted that PPG sensor 1 (higher up the arm) was more tightly in contact with the skin than PPG sensor 2, which was loose and able to move relative to the skin surface.

The data from PPG sensor 1 is too noisy to identify the features of the PPG waveform, which are needed for further analysis of physiological parameters. 2. Strap attachment with flexible portions (invention^')

In order to ensure permanent contact to the skin of the user and even contact for both PPG sensors, an improved fitness tracker architecture was used. The wearable device (10) comprises a main body (11) and a flexible portion (30) introduced at both sides between the strap (12) and the main body (11). An overview over the wearable device is shown in Figure 3a) and a detailed view of the attachment points with indication of rotational axis (RA) as dotted lines and the direction of the rotation (R) is shown in Figure 3b).

This figure illustrates a wrist worn device housing a single electronic circuit including two PPG sensors (not shown) exposed at the underside surface of the main body. The main body is the large rectangular volume where a watch body would normally sit. Two flexible silicone straps for attaching the main body to a wearer's wrist are partially shown in this view. The view of the strap in the foreground of the image includes a conventional watch strap attachment mechanism, comprising a metal rod that runs through the end of the strap, having a sprung portion at one end, such that both ends of the rod protrude from the strap into two holes in a strap attachment portion of the main body, to secure the strap and the main body together. A similar attachment portion and flexible portion are positioned on the opposite side of the main body, hidden in this view.

The base of the main body housing, together with the flexible portions and strap attachment portions is formed as one piece from a plastic material, for example Nylon 6, that has some flexibility such that it is deformable under mechanical load. Thin portions of this material are readily deformable compared to thicker portions, thus the flexible portion illustrated is more deformable under mechanical load than the main body, due to its narrow form. The strap attachment portion of the main body is connected through these narrow portions (flexible portion) to the main body.

When the strap is twisted in relation to the main body, rotational force is applied through the flexible portion, resulting in a small twisting deformation of the flexible portion. Similarly, when the strap is pulled in a lateral movement (towards the hand or elbow of the wearer) a bending force is applied to the flexible portion, resulting in a small deformation of the flexible portion, bringing one end of the rod slightly closer to the main body than it's opposite end. Thus, the flexible portion is capable of being deformed by bending and / or by twisting, depending on the direction of the forces applied.

Only small deformations of the two flexible portions are required, in order to make the small angular adjustments of the main body relative to the watch strap that are necessary to provide a good fit, between the underside of the main body and the (non-cylindrical) surface of the wearer's arm. At this range of small deformations, each flexible portion acts in a similar way to a ball joint, facilitating angular movement about its central point, of the strap and main body relative to each other. The two flexible portions on either side of the main body facilitate rotation of the main body relative to the straps, substantially about the rotational axis illustrated.

The optimized fitness tracker provides a wrist attachment with two flexing portions (ball joints), providing pivoting movement of the body incorporating two PPG sensors, such that the pressure between each PPG sensor and the skin surface is more evenly distributed, facilitating improved dual PPG measurement.

Figure 4 shows the PPG signals measured with wearable device with two flexible portions introduced between the straps and the main body (as shown in Figure 3). The graph shows a short sample of data from both PPG sensors after processing.

Though the relative scaling of the traces is not the same, the data from both PPG sensor 1 and PPG sensor 2 are good and features can easily be identified for comparison.

3. Strap attachment with ball joint (invention^')

An alternative embodiment includes two ball joints positioned at the locations of the flexible portions, to facilitate bending and twisting movement between the strap attachment points and the main body, thus bringing both PPG sensors into contact with the wearers skin at the wrist, pressed against the skin of the wearer under similar mechanical pressure. The novel design introduces a pair of ball joints between the strap attachment and the main body, which comprises the PPG sensors, allowing the body to rotate (somewhat) relative to the strap. It could be observed that, when the fitness tracker was worn by a user, the flexible portions ensured that both PPG sensors contacted the skin of the user nearly permanently. The pivot joint also accommodates the non- cylindrical shape of the arm at the wrist region (the lateral axis of the device can sit at an angle relative to the longitudinal line of the arm, rather than parallel to it). It was also noted that each of the two PPG sensors can be seen (and measured) to press against the skin with a similar pressure.

Similar results for PPG measurements could be obtained.

4. Flexible part between the two PPG sensors (invention)

In an alternative embodiment, the flexible portion is located between the two PPG sensors to allow rotational movement between the two PPG sensors. Due to the rotational movement, both PPG sensors can be evenly pressed on the user’s skin to allow precise measurement of both PPG signals.

Claims

1 . Wearable device (10) comprising a main body (11) and at least one strap (12) attached to the main body adapted to acquire physiological data of a subject, the main body comprising at least two photoplethysmographic (PPG) sensors (20) with a defined distance between both PPG sensors (20) in the direction of the blood flow, each PPG sensor comprising at least one light source and at least one photodetector, characterized in that the wearable device comprises at least one flexible part (30) or at least a ball joint between the strap (12) and the main body (11) to allow a rotational movement of the main body (11) relative to the strap (12) and evenly distribute pressure between the PPG sensors (20) and the skin of the subject.

2. Wearable device (10) according to claim 1 , comprising a flexible part or ball joint (30) located between the first PPG sensor and the second PPG sensor (20) and thereby allows a rotational movement between the two PPG sensors (20).

3. Wearable device (10) according to any one of the preceding claims, wherein the lateral distance between the two PPG sensors (20) is at least 2 cm, more preferably at least 3 cm, most preferably at least 5 cm.

4. Wearable device (10) according to any one of the preceding claims, comprising two flexible parts or ball joints (30) between the strap (12) and the main body (11) at two opposite positions of the main body (11).

5. Wearable device (10) according to any one of the preceding claims, wherein the axis of rotation runs across the main body (11), between the two flexible parts or ball joints (30) and orthogonally bisecting the line between two PPG sensors (20).

6. Wearable device (10) according to any one of the preceding claims, further comprising a processor, wherein the processor is configured to: receive a first PPG signal from the first PPG sensor and a second PPG signal from the second PPG sensor, determine a pulse-transit-time of the subject based on the generated first and second PPG signals, and calculate one or more physiological parameter of the subject based on the calculated pulse-transit-time and the lateral distance between the first and second PPG sensor.

7. Wearable device (10) according to any one of the preceding claims, wherein the physiological parameters is or includes

- the vascular age index Aglx,

- the pulse wave velocity PWV, - blood pressure BPdia and BP_sys,

- augmentation index Alx of the subject.

8. Wearable device (10) according to any one of the preceding claims, further comprising signal processing means adapted to calculate one or more of the following: - the vascular age index Aglx using linear regression based on the characteristic points a, b, c, d, and e, age (page), body height (p_heigm) and median heart rate of the subject,

- the pulse wave velocity PWV using linear regression based on the time difference between the two PPG pulses (PTT), age (page), body height (p_heigm) and median heart rate estimation of the subject, - blood pressure BPdia and BP_sys using linear regression based on time difference between the two PPG pulses (PTT) and median heart rate and

- optionally the augmentation index Alx, based on the systolic A_sys and diastolic Adia peak amplitudes normalized to 75 heartbeats (Alx@75) and using a linear regression based on the normalized augmentation index Alx,