WO2022090764A1

WO2022090764A1 - Wearable epidermal sweat sensor

Info

Publication number: WO2022090764A1
Application number: PCT/IB2020/060042
Authority: WO
Inventors: Zhibo Chen
Original assignee: Zhibo Chen
Priority date: 2020-10-27
Filing date: 2020-10-27
Publication date: 2022-05-05

Abstract

Examples of the present disclosure include a wearable epidermal sweat sensor (1) comprising a flexible printed circuit board including a plurality of islands (10, 20, 30) on which electronic components are mounted and a plurality of stretchable connecting lines (15,25) connecting the islands (10, 20, 30). The islands (10, 20, 30) and electronic components may include a first island (10) on which a sweat sensing electrode (12) is mounted; a second island (20) on which an analogue front end module (22) is mounted and a third island (30) on which a processing unit (32) is mounted.

Description

WEARABLE EPIDERMAL SWEAT SENSOR

BACKGROUND

Measuring characteristics of a person’s sweat can provide valuable bio-information. For example, electrolyte imbalance in sweat may indicate progression of diseases and act as an early warning for cardiovascular conditions, myocardial infarction and acute or chronic renal failure. The characteristics of sweat may be measured non-invasively by use of electrodes which need not penetrate the skin.

SUMMARY

A first aspect of the present disclosure provides a wearable epidermal sweat sensor comprising a flexible printed circuit board including a plurality of islands on which electronic components are mounted and a plurality of stretchable connecting lines connecting the islands. The islands and electronic components include at least: a first island on which a sweat sensing electrode is mounted; a second island on which an analogue front end module is mounted for converting analogue signals from the sweat sensing electrode into digital signals; and a third island on which a processing unit is mounted, the processing unit configured for processing digital signals received from the analogue front end module.

A second aspect of the present disclosure provides a wearable epidermal sweat sensor comprising a flexible printed circuit board; a plurality of sweat sensing electrodes, a waveform generator and processing circuitry mounted to the flexible printed circuit board. The waveform generator is to inject an excitation signal having a frequency to at least one of the sweat sensing electrodes. The processing circuitry is configured to measure an admittance of sweat based on a response of at least one of the plurality of sweat sensing electrodes to the excitation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present disclosure will be explained below with reference to the accompanying drawings, in which:-

Fig. 1 shows an example of a sweat sensor including a plurality of islands according to the present disclosure;

Fig. 2A shows another example of a sweat sensor including a plurality of islands according to the present disclosure;

Fig. 2B is a schematic diagramming showing an example of functional blocks of a sweat sensor according to the present disclosure; Fig. 3 shows an example of a sweat sensor according to the present disclosure comprising a flexible printed circuit board sandwiched between a first elastomer layer and a second elastomer layer;

Fig. 4A is a photograph of an example sweat sensor according to the present disclosure on the arm of a subject;

Fig. 4B is a photograph of an example sweat sensor according to the present disclosure which is being twisted;

Fig. 4C is a photograph of an example sweat sensor according to the present disclosure undergoing lateral deformation;

Fig. 4D is a photograph of an example sweat sensor according to the present disclosure being removed from the arm of a subject;

Fig. 5A is a photograph of an example sweat sensor according to the present disclosure when the sweat sensor is not stretched;

Fig. 5B is a photograph of an example sweat sensor according to the present disclosure when the sweat sensor is stretched;

Fig. 5C is a photograph of an example sweat sensor according to the present disclosure when the sweat sensor is stretched further;

Fig. 5D is a close up of photograph of a stretchable connecting line of the sweat sensor of Fig. 5C;

Fig. 6A is a graph showing admittance against strain for an example sweat sensor according to the present disclosure;

Fig. 6B is a graph showing normalised admittance against strain for an example sweat sensor according to the present disclosure;

Fig. 6C is a graph showing the actual concentration of NaCI in a solution against concentration of NaCI in the solution as calculated by an example sweat sensor according to the present disclosure;

Fig. 7A is a circuit diagram of an example sweat sensing electrode and analogue front end according to the present disclosure;

Fig. 7B is a circuit diagram of another example sweat sensing electrode and analogue front end according to the present disclosure;

Fig. 7C is a circuit diagram of another example sweat sensing electrode and analogue front end according to the present disclosure; Fig. 7D is a circuit diagram of another example sweat sensing electrode and analogue front end according to the present disclosure;

Fig. 8A is a diagram showing a structure of an example sweat sensing electrode according to the present disclosure;

Fig. 8B is a diagram showing a structure of another example sweat sensing electrode according to the present disclosure;

Fig. 8C is a diagram showing a structure of an example sweat sensing electrode according to the present disclosure;

Fig. 8D is a diagram showing a structure of an example sweat sensing electrode according to the present disclosure;

Fig. 8E is a diagram showing a structure of an example sweat sensing electrode according to the present disclosure; and

Fig. 9 is a schematic diagram showing another example of a sweat sensor according to the present disclosure.

DETAILED DESCRIPTION

Various examples of the disclosure are discussed below. While specific implementations are discussed, it should be understood that this is done for illustrative purposes and variations with other components and configurations may be used without departing from the scope of the disclosure as defined by appended claims. In the context of the present disclosure the terms “a” and “an” refer to one or more of a particular element.

It would be desirable to provide a wearable sweat sensor which is capable of measuring a characteristic of the wearer’s sweat. However, there are various challenges involved in designing such a device. Skin is a flexible surface, which changes shape as a person moves their body. Such stresses and strains may damage sensitive electronic components, cause inconsistences in measurements due to deformation of the circuitry and/or measurement errors due to loss of contact with the skin.

Fig. 1 is a schematic diagram of a wearable epidermal sweat sensor 1 according to an example of the present disclosure. By wearable it is meant that the sweat sensor 1 may be worn on the skin of a subject whose sweat is to be measured. For instance, the wearable epidermal sweat sensor may take the form of an arm band or a plaster which is to be adhered to the skin. The sweat sensor 1 includes a flexible printed circuit board including a plurality of islands 10, 20, 30 on which electronic components are mounted and a plurality of stretchable connecting lines 15, 25 connecting the islands. The flexible printed circuit board may comprise a flexible insulating substrate and conductive lines on or embedded in the substrate. For instance, the substrate may be an elastomer such as, but not limited to, silicone, rubber or thermoplastic polyurethane etc. The conductive lines may include conductive pads and may for instance comprise a metal, such as but not limited to copper, gold etc, or a conductive polymer.

An island is a part of the flexible printed circuit board which is separated from other parts of the flexible printed circuit board by gaps in which there is no flexible circuit board material. The sweat sensor thus comprises a plurality of islands: 10, 20, 30 of flexible printed circuit board material, wherein each island is connected to one or more of the other islands by stretchable connecting lines 15, 25 formed of flexible printed circuit board material, but otherwise there are gaps between the various islands. This is in contrast to a rectangular or circular sheet of flexible printed circuit board material supporting a plurality of components, in which there are no gaps such that the entire sheet forms a single “island”.

The flexible printed circuit board according to the present disclosure may for example comprise one or more layers of conductive material and one or more layers of insulating material and may be designed with an island structure, such that gaps between the islands 10, 20, 30 and the connecting lines 15, 25 do not include the conductive material or the insulating material.

The multiple island structure shown in Fig. 1 increases the flexibility of the sweat sensor. The sweat sensor includes electronic components for performing functions of the sensor. The electronic components may for example include electrodes, amplifiers, filters, resistors, capacitors, excitation signal generators, processors, wireless I/O devices, displays modules and indicators such as LEDs etc.

Electronic components which are part of a similar function group may be grouped together on a same island. Such functional grouping of the electronic components helps to reduce electromagnetic interference between the electronic components.

In the example shown in Fig. 1, the first island 10 is an island on which at least one sweat sensing electrode 12 is mounted. The second island 20 is an island on which an analogue front end module 22 is mounted. The analogue front end module 22 may be configured for converting analogue signals received from the sweat sensing electrode 12 into digital signals. The third island 30 is an island on which a processing unit 32 is mounted. The processing unit 32 may be configured for processing digital signals received from the analogue front end module 22. By placing the electrode 12, analogue front end module 22 and processing unit 32 on different islands, interference between these parts may be reduced or avoided. Thus it can be seen from the above arrangement, that the electronic components are grouped into functional groups and each island supports one or more electronic components belonging to a same functional group. In some examples at least some of the islands have a plurality of electronic components mounted thereon.

Electrical signals may be passed between the islands via the stretchable connecting lines. Thus the stretchable connecting lines perform the dual functions of facilitating communication between electronic components on the different islands and absorbing stress or strain when the wearable sweat sensor is stretched or deformed, thereby protecting the sensitive electronic components on each island. In this way the wearable sweat sensor may be able to conform to the surface of the skin in manner which allows for consistent sweat measurements over a period of time, even as the user moves. Furthermore, as the sweat sensor is flexible and adaptable to the skin, it may be more comfortable for a user to wear. This arrangement also makes it possible to use off-the shelf components, such as microprocessors, analogue to digital converters, signal generators, clocks etc. to build the functional groups on some or all of the island; as even if the off-the shelf components are themselves relatively rigid, the flexibility may be provided by the stretchable connecting lines linking the islands together.

Some islands may have different types of electronic component mounted thereon. For instance, the analogue front end module 22 in Fig. 1 may comprise a plurality of electronic components, such as a filters, resistors, capacitors, an analogue to digital converter and/or a signal generator. Likewise the processing unit 32 may comprise a processor, such as a microprocessor, together with one or more supporting components such as resistors, capacitors, a memory etc. By placing electronic components belong to different functional groups on different islands, interference between the functional groups is reduced or avoided. For example, a signal generator for generating an analogue excitation signal might interfere with the processing unit, but this can be reduced or avoided by placing these components on different islands. At the same time, by combining multiple related electronic components on the same island, external electrical interference from components in different functional groups may be reduced.

Fig. 2A shows another example of a wearable epidermal sweat sensor 100 according to the present disclosure. In the example of Fig. 2A there are six islands and each island supports electronic components for performing a respective function.

The first island 110 is an island on which at least one sweat sensing electrode 112 is mounted. The second island 120 is an island on which an analogue front end module 122 is mounted. The analogue front end module 122 may be configured for converting analogue signals received from the sweat sensing electrode 112 into digital signals. The third island 130 is an island on which a processing unit 132 is mounted. The processing unit 132 may be configured for processing digital signals received from the analogue front end module 122. Thus the first, second and third islands are similar to the islands in the example of Fig. 1.

The fourth island 140 is an island on which a signal pre-conditioning module 142 for preconditioning an analogue signal from the electrode is mounted. For example, the signal preconditioning module may include one or more amplifiers for amplifying an analogue signal received from a sweat sensing electrode or amplifying an analogue signal injected to a sweat sensing electrode and/or one or more filters for filtering noised from the signals received from the electrodes.

The fourth island 140 may be positioned between the first island 110 and the second island 120 and connected to the first island by a first stretchable connecting line 115 and connected to the second island by a second stretchable connecting line 125. In this way analogue electrical signals may be passed from the sweat sensing electrodes 112 to the analogue front end module 122 via the pre-conditioning module 142 and vice versa.

As will be appreciated from the above, in the example of Fig. 2A, a pre-conditioning module 142 and an analogue front end module 122 are provided on separate islands. In this way components of the pre-conditioning module 142 are less likely to interfere with components of the analogue front end module 122. For instance any interference between an amplifier of the pre-conditioning module 142 and an analogue to digital converter of the pre-conditioning module 142 may be avoided or reduced. Further, if the analogue front end module 122 includes a signal generator for generating an analogue waveform and/or an amplifier, any interference between such components and components of the pre-conditioning module 142 may be avoided or reduced.

The third island 130 including the processing unit 132 may be connected to the second island 120 including the analogue front end module 122 by a third connecting line 135. In this way the processing unit 132 signals may be passed between the analogue front end module 122 and the processing unit 132.

The sweat sensor may include one or more islands on which an output module and/or an input module is mounted. For example, the third island 130, which hosts the processing unit 132, may be connected to one or more further islands which host input modules or output modules. The input and/or output modules may include digital I/O data ports, wireless communication interfaces such as Bluetooth modules or Wifi modules, devices for outputting a physical signal such as a display, light emitting diodes (LEDs) a speaker or microphone, a touch panel etc. In the example of Fig. 2A, there is a fifth island 150 on which an output module 152 in the form of a wireless interface 152 is mounted. In other examples, the output module 152 may be a digital I/O port for attachment to device such as a wireless communication interface. The fifth island may be connected to the third island 130 by a fourth stretchable connecting line 145. A sixth island 160 has a plurality of LEDs 162 mounted thereto. Thus the sixth island can output visual signals to a user of the sweat sensor. The sixth island may be connected to the third island 130 by a fifth stretchable connecting line 155.

It is to be understood that Figs. 1 and 2A are merely examples of possible arrangements of electronic components and islands. It would be possible to have a different number of islands and/or to have the electronic components arranged or split differently, while still remaining within the scope of the present disclosure. For instance, the various electronic components of the analogue front end module 12 or 122 may be split between a plurality of different islands. For instance, the analogue front end may include an excitation signal generator and an analogue to digital converter hosted on the same island or hosted on two different islands. In some examples, the analogue front end module and signal pre-conditioning module may be hosted on the same island. The islands may include other components not mentioned above. For instance, a temperature sensor. In one example, a temperature sensor may be mounted to the first island in addition to the sweat sensing electrode(s).

In general it is expected that the sweat sensor may have between three and ten islands. For instance, in addition to one or more islands hosting the sweat sensing electrode(s), the sweat sensor may have at least two other islands hosting electronic components relating to the analogue front end, signal conditioning, processing and I/O functionality of the sweat sensor. However if there are more than ten islands, the potential for electrical signal interference becomes greater, as there will be many connecting lines between the various islands.

Fig. 2B is a schematic diagram of the functional blocks of an epidermal sweat sensor 200 according to one example of the present disclosure. This schematic diagram may correspond to Fig. 1, Fig. 2A or other designs with a different number of islands.

The epidermal sweat sensor 200 includes one or more sweat sensing electrodes 212, a signal pre-conditioning module 240, a temperature sensor 270, an analogue front end module 220, a processing unit 230, a wireless output module 250 and a display module 260. The sweat sensing electrodes sense an electrical characteristic of sweat in proximity to the electrodes. The temperature sensor may sense a temperature of the sweat sensor and/or the temperature of the skin surface of a person wearing the sweat sensor. For instance, the temperature sensor may be a thermocouple or a resistance temperature detector etc. The pre-conditioning module 240 may receive an analogue electrical signal from the electrodes and/or the temperature sensor and condition the analogue signal before sending the analogue signal to the analogue front end module 220. The analogue front end module 220 may process the analogue signal to put it in a good condition for sending to the processing unit 230. For instance the analogue front end module 220 may include an analogue to digital converter for converting the analogue signal to a digital signal. The analogue front end may generate an analogue signal for delivery to the sweat sensing electrodes and/or measure an analogue signal received by the sweat sensing electrodes, such as a voltage or electrical current flowing between two sweat sensing electrodes. In examples where the sweat sensor is to measure an admittance of the sweat, the analogue front end module 220 may include an excitation signal generator for generating a waveform for injection into one or more sweat sensing electrodes.

The analogue front end module 220 outputs a digital signal to the processing unit 230. The processing unit 230 may for example include a microcontroller or other type of processor. The processing unit 230 may be configured for processing digital signals received from the analogue front end module 220. For instance, the processing unit 230 may determine a measured characteristic of the sweat based on the signal received from the analogue front end module 220. The measured characteristic may be an electrical characteristic of the sweat, such as impedance, resistance, conductance, admittance etc., or may be a quality of the sweat, such as a concentration of a sweat electrolyte. The processing unit may determine a quality of the sweat, such as a concentration of a sweat electrolyte, based on an electrical characteristic of the sweat. The processing unit may control operation of the sweat sensor and may, for example, store data relating to the measured characteristics or qualities of the sweat and and/ or output data via one or more output modules.

In some examples the processing unit 230 may determine a value for a measured characteristic of the sweat based on the signal received from the analogue front end 220 and the processing unit may be configured to compensate the measured characteristic based on the temperature measured by the temperature sensor 270. For example, the processing unit may be configured to determine a value for a measured characteristic of the sweat based on the signal received from the analogue front end and to compensate the measured characteristic based on the temperature measured by the temperature sensor. The measured characteristic whose value is determined by the processing unit and compensated based on the temperature, may be an electrical characteristic such as admittance, conductance, impedance, resistance, current passing through the sweat or potential difference between two sweat sensing electrodes etc., or a quality of the sweat, such as a concentration of a sweat analyte.

In the example of Fig. 2A, the sweat sensor further includes a wireless communication module 250, such as a Bluetooth or wifi module for outputting data to an external device and/or receiving instructions from an external device. The wireless communication module 250 has a wired connection to the processing unit 230. In some examples, the processing unit 230 may be configured to output data to and/or receive instructions from a smart phone or computer app. In this case communication between the wearable sweat sensor and the smart phone, computer, a server or cloud service hosting the app may be via the wireless communication module 250.

In the example of Fig. 2A, the processing unit is connected to a display module 260 comprising a plurality of LED indicators. By controlling the LED indicators the processing unit can communicate the sweat sensor operating status and/or other information to a user. In other examples the display module could be a display screen or touch panel or an audio signal generator.

Fig. 3 shows an example of an epidermal sweat sensor 300 according to the present disclosure in which the flexible printed circuit board 330 and the electronic components 341-345 are sandwiched between a first elastomer layer 310 and a second elastomer layer 320.

The flexible printed circuit board layer 330 includes a plurality of islands and stretchable connecting lines connecting the islands as described in the examples above. The number and configuration of islands shown in Fig. 3 just an example and there may be more or fewer islands in the same or different configurations, as discussed in the variations above in relation to Figs. 1 and 2A. The electronic components 341-345 may be mounted on the upper side, lower side or both sides of the flexible printed circuit board and are to perform the functions discussed above in the examples above including but not limited to Fig. 2B.

The flexible printed circuit board and stretchable connecting lines 330 may include insulating and electrically conductive layers and may be formed of any of the materials mentioned in the examples above. For instance, the insulating layers may comprise silicone, rubber, thermoplastic polyurethane etc, while the conductive layers may comprise a metal such as gold or copper, or a conductive polymer. In the example shown in Fig. 3, the flexible printed circuit board is a two-sided circuit board comprising a layer of conductive material 332, 336 on each side of a layer of insulating material 334. However, in other examples there may be multiple layers of conductive material and multiple layers of insulating material. In still other examples, the flexible printed circuit board may be a one-sided circuit board. The flexible printed circuit board layer 330 may have a thickness of less than 1 mm. This helps to keep the layer flexible so that it easily accommodates movement of the skin.

The first elastomer layer and the second elastomer layer may for example be formed of materials such as, but not limited to, EcoFlex or Polydimethylsiloxane (PDMS). In some examples the first and second elastomer layers comprise the same materials. When the sweat sensor is worn, the first elastomer layer faces away from the skin, while the second elastomer layer may contact the skin of the wearer. The second elastomer layer may be patterned with a micro-structure to enhance skin breathability.

The first sweat sensor may be assembled by first forming the flexible printed circuit board layer including the islands and the connecting lines. In one example the islands and connecting lines may be formed from a sheet of flexible printed circuit board by cutting away, etching or otherwise removing portions which are not an island or connecting line. The electronic components 341-345 may then be attached to the appropriate islands by soldering, adhering or otherwise. Electronic components may be attached to a first side of the islands facing the upper elastomer layer 310, to a second side of the islands facing the lower elastomer layer or both. Where electronic components are attached to both sides of an island, the island may be made smaller, which may increase the resilience and flexibility of the sweat sensor. In one example, the electrode and the temperature sensor (if present) are attached to the second side of the printed flexible circuit board so as to be near the skin when the sweat sensor is worn, but the other electronic components are attached to the first side so as not to irritate the skin. Electrical components on the first and second sides of the flexible printed circuit board may be connected by vias extending through the islands. The elastomer layers 310, 320 may be adhered to the flexible printed circuit board layer 330. For instance, the flexible printed circuit board may be adhered to the second elastomer layer 320 and the first elastomer layer 310 may then be placed on top and adhered to the second elastomer layer using adhesive or heat sealing. The elastomer layers may help to protect the electronic components from damage and the flexible printed circuit board from being accidentally torn or abraded and make it easier to wear the sweat sensor.

The elastomer layers 310, 320 may be very flexible. The flexible printed circuit board 330 may also be flexible, but the electronic components 341-345 mounted to the islands may be relatively rigid. Therefore in order to better support the electronic components and protect the electronic components from damage, the sweat sensor may further comprise a spacer layer for enhancing the rigidity of the island. For instance, each island may include an additional spacer layer for enhancing rigidity of the island compared to the stretchable connecting lines. For instance, the spacer layer may be formed of the same material as the insulating layer of the flexible circuit board or may be formed of a different material. As a result of the spacer layer the islands may have an enhanced thickness compared to the stretchable connecting lines.

In some examples, the total area occupied by the stretchable connecting lines may be greater than 5% of the total area occupied by the islands. In this way the stretchable connecting lines are reasonably robust and less susceptible to wear and tear. In order to allow the sweat sensor to adapt to the contours of the skin and any movement of the skin, the islands comprise less than 50% of the total area of the sweat sensor. In this way, as the islands are only a small part of the total area of the sweat sensor, the stretchable connecting lines may absorb the stress and strain associated with stretching, bending or twisting of the sweat sensor. The total area of the sweat sensor may be defined by the area covered by the elastomer layers, when the elastomer layers are continuous sheets of material having a regular shape, e.g. rectangle as in Fig. 3, circle or the like.

In some examples there may be no elastomer layers and the sweat sensor may comprise the flexible printed circuit board layer and the electronic components without an elastomer cover, or the elastomer layers may be cut to the same shape as the flexible printed circuit board layer. In that case the total area of the sweat sensor may be considered to be the area of the smallest regular shape which covers all of the islands and connecting lines and the area occupied by the islands may be less than 50% of this total area of the sweat sensor. In some examples, the area occupied by the islands may be even less, e.g. less than 20% of the total area occupied by the sweat sensor.

In one example, the total area of the sweat sensor is 2cm², the total area occupied by the islands is less than 1cm² (i.e. less than 50% of the total area of the sweat sensor), and the total area occupied by the stretchable connecting lines is greater than 0.05cm² (i.e. more than 5% of the area occupied by the islands).

Fig. 4A is a photograph of a sweat sensor according to the present disclosure worn on the arm of a user. The sweat sensor may for instance be in the form of a plaster which can be adhered to the skin of a user. Fig. 4B is a photograph of a sweat sensor according to the present disclosure being twisted, while Fig. 4C is a photograph of a sweat sensor according to the present disclosure being deformed by stretching. The degree of stretching and twisting shown in Figs. 4B and 4C is greater than that anticipated in normal use and illustrate that the sweat sensor is very flexible, resilient and able to accommodate changes due to movement of the surface of the skin. Fig. 4D is a photograph of a sweat sensor according to the present disclosure being removed from the arm of a user and again shows a large degree of stretching.

As discussed above, the sweat sensor according to the present disclosure is able to accommodate a wide range of stresses and strains due to the stretchable connecting lines between the islands. Stretchable means that the connecting lines may be easily deformed. In this way the stretchable connecting lines may be deformed to absorb stress/strain to conform to contours of skin, when skin moves or putting on or taking off the sweat sensor. In some examples the stretchable connecting lines may be elastically deformable, such that they have a tendency to return to their original shape once the force causing the deformation is removed. The stretchable connecting lines may comprise a meander, for instance as shown in Figs. 1 , 2A, 4A and 5A to 5D. A meander is a wave like shape and may comprise two or more successive curves in opposite directions. Thus in Fig. 5A, the first connecting line 510 has a first curve 512 and a second curve 514 in opposite directions which form a meander. The meander shape allows the stretchable connecting line to be particularly flexible and deform easily as two dimensional strains may be absorbed in a third dimension by twisting of the meander or contraction or expansion of the curves. Accordingly, the effective modulus of elasticity (also referred to as Young’s Modulus) of a connecting line may less than the modulus of elasticity of its constituent materials, due to the capability of the meander to absorb stresses and strains.

Thus the effective modulus of elasticity of an island may be greater than the effective modulus of elasticity of a connecting line. In this way the islands are more rigid and better able to protect the electronic components mounted thereon, while the connecting lines are more flexible and able to deform with movement of the skin, it being understood that the higher the modulus of elasticity the more rigid the material. In one example, the effective modulus of elasticity of the connecting lines is 100kPa, while the effective modulus of elasticity of the islands is 3.5 GPa. However, a wide range of modulus of elasticity is possible. For instance, the islands may have an effective modulus of elasticity between 1GPa and 10GPa, while the connecting lines may have an effective modulus of elasticity between 10KPa and 1GPa.

In other examples, the stretchable connecting lines may not include a meander. However, the stretchable connecting lines may in that case comprise a conductive elastomer or a conductive polymer in order to be stretchable with a relatively low modulus of elasticity e.g. less than 1 GPa.

Figs. 5A to 5D show stretching of a sweat sensor 500 according to an example of the present disclosure. In Fig. 5A the sweat sensor is in an unstretched state. In Fig. 5B the sweat sensor has been stretched in the direction left to right of Fig. 5B. In Fig 5C, the sweat sensor has been stretched further in the left to right direction. The inventors conducted a stress analysis and found that the majority of the strain (i.e. stretching) was accommodated by the meander 530 of the connecting line indicated by the dotted box in Fig. 5C. Fig. 5D shows a close up of view of this connecting line. The inventors found that the majority of the stress cause by the stretching was focused on the edges of the curves 540 and 550 as indicated in Fig 5D. In this way the stress and strain was accommodated by the connecting line and far removed from the electronic components on the islands which could be damaged by such stresses and strains. Furthermore, as the strain was focused on the outermost parts of the curves, the conductive lines 561-563 of the connecting lines and transmission of electrical signals was less affected. It is worth noting that the connecting lines may comprise one conductive line or a plurality of conductive lines for carrying electrical signals between the islands. Where there are a plurality of conductive lines, e.g. as shown in Fig. 5D, each conductive line may carry a different signal, e.g. from different electronic components, different electrodes, or different outputs of an analogue front end module or processing unit of an island.

One or more of the islands may include an electrostatic discharge protection circuit to protect electronic components mounted on the island from electromagnetic interference. Where the connecting lines have a meander shape, the meander shape may generate more electromagnetic interference, so electrostatic discharge protection circuits can help to reduce the electromagnetic interference while still allowing the physical flexibility provided by the meanders. For the same reason, as there are different islands which are in relatively close proximity to each other, but with different functions and because meander lines may further increase the electromagnetic interference, one or more of the islands may comprise at least one of an impedance matching circuit, decoupling circuit, differential circuit, signal delay compensation circuit and a noise reduction circuit to compensate for electromagnetic interference.

Sweat sensors according to the present disclosure may be capable of detecting various electrical characteristics of sweat and corresponding sweat qualities depending on the type of electrodes used. The electrodes may for example be bare metal electrodes or electrodes with a surface treatment for reacting with a particular sweat electrolyte. The electrodes may be positioned on or protrude through a surface of the sweat sensor which faces the skin, such as the second layer 320 in Fig. 3. In this way the electrodes may contact the skin or lie close to the skin, e.g. 1mm from the skin, so that the electrodes may contact epidermal sweat or electrical readings from the electrodes may be affected by the presence and qualities of the epidermal sweat.

The sweat sensing electrodes may be used to measure an electrical characteristic of the sweat, such as an impedance of the sweat, electrical resistance of the sweat, conductivity of the sweat, admittance of the sweat and/or a current passing through the sweat from a first electrode to a second electrode or a potential difference between a first electrode and second electrode.

In one example the sweat sensor includes an analogue front end comprising a signal generator for generating an excitation frequency to be applied to the sweat sensing electrode and the analogue front end and the processing unit are configured to determine an admittance based on the response to the excitation signal. The excitation frequency may be in the range 1mHz-1 MHz. Fig. 7A shows an example of an arrangement 700A for measuring an electrical characteristic of epidermal sweat. The arrangement includes a pair of sweat sensing electrodes 712A, 712B and an analogue front end 720 comprising a signal generator 722. The signal generator 722 is configured to generate an input signal which may be a DC current or an AC current (i.e. an excitation frequency). The input signal may be injected into the first sweat sensing electrode 712A via a conductive line 715A connecting the first sweat sensing electrode 712A with the analogue front end. The second sweat sensing electrode 712B is connected to the analogue front end by a second conductive line 715B which may detect a signal output from the second sweat sensing electrode. Thus the first electrode 712A acts as a working electrode and the second electrode 712B acts as a counter electrode.

The electrical characteristic of the epidermal sweat, such as conductance or admittance etc., may be determined based on the signal output from the second sweat sensing electrode 712B. For instance, a known current may be injected into the epidermal sweat by the first 712A and the analogue front end 720 may measure a voltage drop between the signal injected into the first electrode 712A by the analogue front end and the signal received from the second electrode 712B by the analogue front end, and/or a current flowing between the first and second electrodes 712A, 712B. Use of an alternating current (AC) input signal with a high frequency may provide a more accurate measurement, as AC helps to reduce electrode polarization and Faradic and double layer capacitance at the electrodes is less at higher frequencies.

Fig. 7B shows another example of an arrangement 700B for measuring an admittance of epidermal sweat. The arrangement includes a pair of sweat sensing electrodes 712A, 712B, an analogue front end 720 including a signal generator 722 and conductive lines 715A, 715B, similar to Fig. 7A, which parts are the same as in Fig. 7A. In addition, the example of Fig. 7B includes a voltmeter 723 for measuring a potential difference between the first sweat sensing electrode 712A and the second sweat sensing electrode 712B. The first sweat sensing electrode 712A may be connected to the voltmeter 123 by a third conductive line 715C and the second sweat sensing electrode 712B may be connected to the voltmeter 123 by a fourth conductive line 715D.

The arrangement of Fig. 7B is similar to that of Fig. 7A, but while Fig. 7A is a two point method, Fig. 7B is a four point method as a pair of electrodes 712A, 712B is used to inject the current into the sample and meanwhile the same pair of electrodes 712A, 712B is used to measure the resulting voltage drop. In principle, because no current flows through the voltmeter 723, the injected current completely flows through the sample and therefore, the contact resistance between each electrode 712A/712B and the respective conducting line 715C/715D is largely counteracted. Thus, in Fig. 7B, an electrical characteristic of the epidermal sweat, such as conductance or admittance etc., may be determined based on the signal output from the second sweat sensing electrode 712B.

Fig. 7C shows an example of an arrangement 700C for measuring a current and potential difference between sweat sensing electrodes. The arrangement includes three sweat sensing electrodes 712A, 712B and 712C and an analogue front end 720 including a signal generator 722 and a voltmeter 723. The first sweat sensing electrode 712A and the second sweat sensing electrode 712B are connected to the signal generator 722 and a ground of the analogue front end 720 in the same way as for the arrangement of Fig. 7A described above. The voltmeter 723 is arranged to measure a potential difference between the first electrode 712A and the third electrode 712C, which is a reference electrode. The voltmeter may for example be connected to the first electrode 712A by the first conductive line 715A and the third electrode 712C by a third conductive line 715C. An electrical characteristic of the epidermal sweat may be determined based on the current signal output from the second electrode 712B, the potential difference between the first electrode 712A and second electrode 712B and/or the potential of the third electrode 712C compared to the first or second electrode.

Fig. 7C, is a three point solution as the first, second and third electrodes, 712A, 712B, 712C act as working, counter and reference electrodes respectively. Three-electrode setups have the advantage that, due to the reference electrode they are able to measure potential changes of the working electrode independently of any changes that occur at the counter electrode. That is they are able to specifically measure a characteristic of the part of the sweat sample between the working and reference electrodes.

Fig. 7D shows an example of an arrangement for measuring a current and/or potential difference between sweat sensing electrodes. The arrangement includes four sweat sensing electrodes 712A, 712B, 712C, 712D and an analogue front end 720 including a signal generator 722 and a voltmeter 723. The first sweat sensing electrode 712A and the second sweat sensing electrode 712B are connected to the signal generator 722 and a ground of the analogue front end 720 in the same way as for the arrangement of Fig. 7A described above. The voltmeter 723 is arranged to measure a potential difference between the third electrode 712C and the fourth electrode 712D; the voltmeter 723 may for example be connected to the third electrode 712C by a third conductive line 715C and the fourth electrode 712D by a fourth conductive line 715D. An electrical characteristic of the epidermal sweat may be determined based on the current signal output from the second electrode 712B and the potential difference between the third electrode 712C and fourth electrode 712D.

The arrangement of Fig. 7D is a full four point system with four separate electrodes, in which the first electrode 712A acts as a working electrode, the second electrode 712B acts as the counter electrode, the third electrode 712C acts as the working sense electrode and the fourth electrode 712D acts as the reference electrode. In this arrangement, the electrode-electrolyte interface impedance between the electrodes 712A/B/C/D and their respective connecting lines 715A/B/C/D has no influence on the measurement. Therefore, this setup may make more accurate measures of the electrical characteristics of the sweat sample.

The above arrangements may have an island structure. For instance, the sweat sensing electrodes 712A, 712B, 712C, 712D may be mounted on a first island 710 and the analogue front end 720 may be mounted on a second island. The conductive lines 715A, 715B, 715C, 715D may be part of a first stretchable connecting line connecting the first island 710 with the second island on which the analogue front end 720 is mounted. In other examples some of the conductive lines 715A, 715B, 715C, 715D may be on separate stretchable connecting lines. In some examples, there may be a pre-conditioning module on an island separate from the analogue front end 720 and the sweat sensing electrodes may be electrically connected to the pre-conditioning module and the pre-conditioning module electrically connected to the analogue front end.

As will be appreciated from the above, depending upon the electrical characteristic to be measured and design of the system, there may be one sweat sensing electrode, two sweat sensing electrodes or more than two sweat sensing electrodes. In some examples the sweat sensing electrode is a co-planar electrode, i.e. a structure comprising two or more electrodes in the same plane. By way of example, various co-planar electrode structures are shown in Figs. 8A-8E, but the present disclosure is not limited thereto. Figs. 8A-8C have three electrodes, while Figs. 8D and 8E have four electrodes. In some examples the sweat sensing electrodes may be interdigitated electrodes. An interdigitated electrode is an electrode structure in which at least two co-planar electrodes have interlocking parts, such as inter-laced fingers or interlocking spirals. Figs. 8C and 8D are examples of interdigitated electrodes.

The sweat sensing electrodes may be bare electrodes formed of copper, gold, platinum or another metal or another conductive material such as graphite, or may be electrodes in which the metal or conductive material is coated with a chemical reactant which is to react with the sweat. In some examples each of the electrodes may be formed of the same material, while in other examples each of, or some of, the electrodes may be formed of different materials.

As mentioned above, the sweat sensor may measure an electrical characteristic of the sweat. The digital signal output by the analogue front end to the processing unit may include a value representing an electrical characteristic of the sweat measured by the sweat sensor. As the deformation of the wearable sweat sensor due to stretching, twisting etc. is concentrated on the stretchable connecting lines, the measurement of the electrical characteristic may be relatively constant even when the sweat sensor is deformed. According to experiments carried out by the inventor, with a sweat sensor according to some examples of the present disclosure the measured value representing the electrical characteristic may vary by less than 5% when a strain of 30% or less is applied to the sweat sensor.

The inventor conducted a series of tests on a wearable sweat sensor constructed according to the present disclosure. The wearable sweat sensor used in the tests had a construction similar to that shown in Figs. 2A and 3. Fig. 6A is a graph showing admittance as determined by the sweat sensor against strain applied to the sweat sensor. The experiment used a solution with various concentrations of NaCI, as NaCI is one type of sweat electrolyte which may be measured by a sweat sensor according to the present disclosure. Measurements were taken at 0%, 10%, 20% and 30%. The squares show values of admittance measured for a solution with 500 ppm of NaCI, circles 1000 ppm NaCI, upright triangles 3000 ppm NaCI and upside down triangles 5000 ppm NaCI. As can be seen from Fig 6A, the measured admittance varied only minimally as the strain on the sweat sensor was increased. The variance can be seen more clearly in Fig. 6B, which shows the normalised admittance for one of the concentration levels with the admittance at 0% strain taken as 1. It can be seen that with strain of up to 30% the measured admittance varied by less than 5%. Based on the measured electrical characteristic the processing unit of the sweat sensor may determine a quality of the sweat, such as the electrolyte concentration. Fig. 6C shows the variation NaCI concentration determined by the sweat sensor based on the measured admittance on the y-axis against the actual NaCI concentration of the solution on x-axis. It can be seen that a high level of agreement between the two was reached and the relationship was linear within the range of concentrations tested, indicating relatively accurate measurement of NaCI concentration by the sweat sensor.

Fig. 9 is a functional diagram showing an example structure of a wearable epidermal sweat sensor 900 which measures admittance of sweat. The wearable sweat sensor 900 comprises a flexible printed circuit board 920, a plurality of sweat sensing electrodes 920, a waveform generator 930 and processing circuitry 940, all of which are mounted to the flexible printed circuit board. The waveform generator 930 is configured to inject an excitation signal having a frequency to at least one of the sweat sensing electrodes 920. The processing circuitry 940 is configured to measure an admittance of sweat between sweat sensing electrodes based on a response of at least one of the plurality of sweat sensing electrodes to the excitation signal. The excitation frequency may for example be in the range 1mHz -1MHz. The processing circuitry may comprise a processor, such as a microprocessor, which is configured to determine a concentration of an electrolyte in the sweat based on the measured admittance. As discussed above, admittance has a strong relationship to electrolyte concentration and thus is a relatively reliable indicator of electrolyte concentration. The flexible printed circuit board allows the sweat sensor to adapt to flexing or changing of the skin surface due to movement of the user. In some examples, the printed circuit board may comprise one or more islands. For example, by placing the waveform generator, processing circuitry and sensing electrodes on different islands it may be possible to produce the sweat sensor cheaply by commercially available processing unit and waveform generators to the islands of the flexible printed circuit board, rather than attempting to integrate everything into a single package. Further, by placing the waveform generator, electrodes and processing circuitry on separate islands, electromagnetic interference between them may be reduced.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

For clarity of explanation, in some instances the present technology has been presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software.

Although a variety of examples and other information was used to explain aspects within the scope of the appended claims, no limitation of the claims should be implied based on particular features or arrangements in such examples, as one of ordinary skill would be able to use these examples to derive a wide variety of implementations. Further and although some subject matter may have been described in language specific to examples of structural features and/or method steps, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to these described features or acts. For example, such functionality can be distributed differently or performed in components other than those identified herein. Rather, the described features and steps are disclosed as examples of components of systems and methods within the scope of the appended claims.

It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any other of the examples.

Claims

1. A wearable epidermal sweat sensor comprising: a flexible printed circuit board including a plurality of islands on which electronic components are mounted and a plurality of stretchable connecting lines connecting the islands; wherein the islands and electronic components include at least: a first island on which a sweat sensing electrode is mounted; a second island on which an analogue front end module is mounted for converting analogue signals from the sweat sensing electrode into digital signals; and a third island on which a processing unit is mounted, the processing unit configured for processing digital signals received from the analogue front end module.

2. The sweat sensor of claim 1 wherein the islands and electronic components further comprise an island on which a signal pre-conditioning module for pre-conditioning an analogue signal from the electrode is mounted.

3. The sweat sensor according to any of the above claims further comprising at least one of an island on which an output module is mounted and an island on which an input module module is mounted.

4. The sweat sensor according to any one of the above claims wherein at least some of the islands have a plurality of electronic components mounted thereon.

5. The sweat sensor according to claim 4 wherein the electronic components are grouped into functional groups and each island supports one or more electronic components belonging to a same functional group.

6. The sweat sensor of any of the above claims, wherein the flexible printed circuit board comprises at least one layer of conductive material and at least one layer of insulating material and wherein gaps between the islands and the connecting lines do not include the conductive material or the insulating material.

7. The sweat sensor of claim 6 wherein the flexible printed circuit board is a two-sided circuit board comprising a layer of conductive material on each side of a layer of insulating material.

8. The sweat sensor of claim 6 or 7 wherein the islands further comprises a spacer layer for enhancing the rigidity of the island.

9. The sweat sensor of any of the above claims wherein the flexible printed circuit board and the electronic components are sandwiched between a first elastomer layer and a second elastomer layer of the sweat sensor.

10. The sweat sensor of claim 9 wherein the second elastomer layer has a micro-structure to enhance skin breathability.

11. The sweat sensor of any of the above claims wherein each stretchable connecting line comprises a plurality of conductive lines.

12. The sweat sensor of any of the above claims wherein the stretchable connecting lines have a meander shape.

13. The sweat sensor of any of claims 1-11 wherein the stretchable connecting lines comprise a conductive elastomer, a conductive polymer or a metal.

14. The sweat sensor of any of the above claims wherein there are no more than 10 islands.

15. The sweat sensor of any of the above claims wherein the sweat sensing electrode is a co-planar electrode.

16. The sweat sensor of claim 15 wherein there are at least two sweat sensing electrodes and the sweat sensing electrodes are interdigitated electrodes.

17. The sweat sensor of any of the above claims wherein the flexible printed circuit board has a thickness of less than 1 mm.

18. The sweat sensor of any of the above claims wherein the effective modulus of elasticity of the connecting lines is 10GPa or less, while the effective modulus of elasticity of the islands is less than 1 MPa or more.

19. The sweat sensor of any of the above claims wherein the digital signal output by the analogue front end to the processing unit includes a value representing an electrical characteristic of the sweat measured by the sweat sensor and wherein said value varies by less than 5% under a strain of 30% applied to the sweat sensor.

21. The sweat sensor of any of the above claims wherein the analogue front end comprises a signal generator for generating an excitation frequency to be applied to the sweat sensing electrode and the analogue front end and the processing unit are configured to determine an admittance based on the response to the excitation signal.

22. The sweat sensor of claim 21 wherein the excitation frequency is in the range 1 mHz -1 MHz.

23. The sweat sensor of any of the above claims further comprising a temperature sensor and wherein the processing unit is determine a value for a measured characteristic of the sweat based on the signal received from the analogue front end and wherein the processing unit is configured to compensate the measured characteristic based on the temperature measured by the temperature sensor.

24. The sweat sensor of any of claims 1-23 wherein a total area occupied by the stretchable connecting lines is at least 5% of a total area occupied by the islands.

25. The sweat sensor of any of claims 1-24 wherein the islands comprise less than 50% of the total area of the sweat sensor.

26. The sweat sensor of any of claims 1-25 further comprising an electrostatic discharge protection circuit.

27. The sweat sensor of any of claims 1-26 wherein one or more of the islands includes at least one of an impedance matching circuit, decouple circuit, differential circuit, signal delay compensation circuit and a noise reduction circuit to compensate for electromagnetic interference caused by the meaner line.

28. A wearable epidermal sweat sensor comprising: a flexible printed circuit board; a plurality of sweat sensing electrodes, a waveform generator and processing circuitry mounted to the flexible printed circuit board; wherein the waveform generator is to inject an excitation signal having a frequency to at least one of the sweat sensing electrodes; and the processing circuitry is to measure an admittance of sweat based on a response of at least one of the plurality of sweat sensing electrodes to the excitation signal.

29. The sweat sensor of claim 28 wherein the processing circuitry comprises a processor which is to determine a concentration of an electrolyte in the sweat based on the measured admittance.

30. The sweat sensor of claim 28 or 29 further comprising any of the features of claims 1-27.