FI20216303A1

FI20216303A1 - Liquid detection rfid tag arrangement

Info

Publication number: FI20216303A1
Application number: FI20216303A
Authority: FI
Inventors: Lauri Huhtasalo; Kaj Backfolk
Original assignee: Digital Tags Finland Oy
Priority date: 2020-12-21
Filing date: 2021-12-20
Publication date: 2022-06-22
Also published as: SE2051510A1

Abstract

A liquid detecting RFID tag arrangement (1) is disclosed, comprising a substrate (4, 4’), at least one antenna (2, 5) element having a conductive line arranged on said substrate (4, 4’), and an RFID chip (3) connected to the antenna element(s) (2). At least a part of the substrate (4) forms a liquid expandable part, made of a hydroexpansive material, wherein the conductive line is at least partly arranged on the hydropexpansive material, and wherein the hydroexpansive material is arranged to expand when wetted by a liquid to such an extent that the conductive line will break. A corresponding method of detecting liquid is also disclosed.

Description

LIQUID DETECTION RFID TAG ARRANGEMENT Technical field of the invention The present invention is related to a radio frequency identification (RFID) tag arrangement with capability of detecting the presence of liquid.

The invention further relates to a method for detecting presence of liquid.

Background There are many situations and applications where there is a need to detect the presence of a liquid, such as water.

For example, such detection can be of advantage in many different types of absorbent articles, such as diapers, diapers of pant type, incontinence garments, sanitary napkins, tampon-like products, wound or sore dressings and bed protectors, known today for absorption, retention and isolation of body wastes, such as urine, faeces and blood. Frequent checking of the status of the absorbent article is often a cumbersome, tedious and labor intensive task, and may also disturb the user or wound the user's dignity, whereas prolonged exposure to wetness is not only uncomfortable, but may also e.g. cause painful rashes, urinary tract infections, or aggravate or induce bed sores. It would be a great benefit for all stakeholders if the need for changing the diaper could be identified automatically and remotely.

To this end, it is known to use various types of sensors to detect _ urination or defecation in such products, after absorption onto or into the O absorbent article. Such a sensor may, for example, be based on detection of N wetness, a biological analyte and/or a chemical analyte. When such a sensor S 25 detects an event, such as urination or defecation, a detectable response is I generated. By means of the response, the user, parent, care taker, nursing > personnel, etc. may determine that an event has occurred. Thus, sensors in = absorbent articles for detection of an event may be utilized to easily monitor if = an event has occurred, i.e. to easily register the body waste status in an N 30 absorbent article. It is also known to utilize an RFID tag including various types of sensors, such as an inductor-capacitor resonator, for detection of moisture or liquid in an absorbent article.

However, there are also many other applications and situations where there is a need or desire to detect the occurrence of a liquid in a certain location. In particular, this would be the case where the location is hidden or difficult to see or get access to.

For example, there is often a need to detect presence of water in buildings, to detect the occurrence of leakage and the like in the building structure, such a roof leakage and plumbing leakage.

As another example, such detection would be of advantage to evaluate the water-tightness of various products and structures. For example, cars have compartments which should be watertight after the assembly of the car. However, inspection of their water-tightness can be cumbersome and expensive.

WO 2008/069753 discloses an RFID sensor useable to detect air humidity. The RFID sensor uses two RFID units, one of which deteriorates when exposed to humidity, e.g. by the use of dissolvable ink. Further, KR 2019/0129387 discloses an RFID sensor arranged with a moisture absorption layer comprising an alkali metal chloride. When the moisture absorption layer absorbs liquid, e.g. due to air humidity, the alkaline metal chloride is dissolved and spread, and thereby affects the properties of the antenna. However, both these previously known RFID sensors are costly to produce and cumbersome to use.

S Thus, the presently used detector systems are unfortunately guite N 25 complicated and need much calibration to provide accurate detection results. = The known solutions are also very expensive, resulting in a very limited practical use of such systems, despite the great need for detection in many E areas.

0 There is therefore still a need for an improved RFID tag arrangement G 30 for detection of liquid, and which can provide adequate and reliable detection, 3 and which can be produced to a relatively low cost.

Summary It is therefore an object of the present invention to provide a liquid detection RFID tag arrangement and a method for detection of liquid with such an RFID tag arrangement, which alleviates at least part of the above- discussed problems, and at least partially address one or more of the above- mentioned needs.

This object is obtained by means of a liquid detecting RFID tag arrangement and a liquid detecting method in accordance with the appended claims.

According to a first aspect of the present invention, there is provided a liquid detecting RFID tag arrangement comprising a substrate, at least one antenna element having a conductive line arranged on said substrate, and an RFID chip connected to said antenna element(s), wherein at least a part of the substrate forms a liquid expandable part, made of a hydroexpansive material, wherein said conductive line is at least partly arranged on said hydropexpansive material, and wherein the hydroexpansive material is arranged to expand when wetted by a liquid to such an extent that the conductive line will break.

Hereby, a relatively simple RFID tag can be used to detect liquid.

When the hydroexpandable part is wetted, i.e. when it comes into contact with a liquid, such as water or urine, it will expand, and will thereby break a conductive line of the at least one antenna element, and will thereby affect the output signal from the RFID tag arrangement. Such an RFID tag arrangement S can be produced to a very low cost, since it may use a standard RFID chip, a N 25 relatively simple antenna, and a low-cost substrate. At the same time, the = detection becomes very accurate and reliable.

Compared to previously known solutions, the present RFID tag can be E produced to a cost which is less than at least 1/100 of the cost of such 0 previously known solutions, and most probably less than 1/1000 of this cost.

G 30 This makes the new RFID tag arrangement useable for many products and 3 applications where it was previously not practically and economically feasible to use liquid detection. Further, the low cost makes it possible to use the RFID tag arrangement as a disposable, for single time use.

In addition, the new RFID tag arrangement may be essentially entirely biodegradable, and consequently environmentally friendly.

In one line of embodiments, the whole substrate can be made of a hydroexpansive material, and form said liquid expandable part, which makes production easier and even more cost-effective.

In such embodiments, all antenna elements will be broken when the liquid expandable part is wetted. Thus, a response signal from the RFID tag arrangement, e.g. containing only the ID of the RFID tag, will become much weaker, and with a very limited reading range, which can be used as an indication that there is, or has been, liquid present, and that the liquid expandable part has been expanded. Thus, the signal from the RFID tag arrangement will change from a normal signal, with full signal strength and full reading distance, to a signal with a limited signal strength and limited reading distance upon wetting.

In another embodiment, the RFID tag arrangement may comprise at least two antenna elements connected to the RFID chip, wherein one of said antenna elements is arranged on the liquid expandable part and one of the antenna elements is arranged on a second, non-liquid expandable or less liquid expandable, part.

The second part may e.g. be of a different material, connected to the liquid expandable part. However, it may also be of the same hydroexpansive material, but protected from becoming wet by a protective layer, or in other ways being arranged in such a way that it is not exposed to the liquid.

= In such embodiments, one of the antennas will become inoperative N 25 when the liquid expandable part expands, whereas the other antenna will still = be operative. To this end, the RFID reader may detect that there is no liquid when signals from both antennas are received at full strength, and that there E is, or has been, liguid present when signals are received from only one 0 antenna at full strength, and from the other with very limited strength. Thus, G 30 the signal from the RFID tag arrangement will here generally change from a 3 normal signal to another signal upon wetting.

The two antennas may e.g. be connected to different ports/pads on the RFID chip, and consequently send out different signals. For example, the antenna which will still be fully operative after wetting may be used to send out the ID of the RFID tag arrangement, whereas the antenna which will become to some extent inoperative during wetting may send out a signal that no wetting has occurred. When the signal indicative of that no wetting has 5 occurred is decreased, or even ceases, this may be interpreted by the reader that wetting has in fact occurred.

However, alternatively, the two antennas may be connected to the same port, and be discernible by e.g. different polarizations or the like.

This solution, with two antennas, is somewhat more complex than the first discussed embodiments. On the other hand, this solution is more robust and reliable, since it can here by ensured that the RFID tag is still in operation, even after wetting, and that adequate contact between the RFID tag and the reader has been established even from the same reading distance.

In yet another embodiment, the RFID tag arrangement may further comprise a detection loop connected to the RFID chip, the detection loop being at least partly arranged on said liquid expandable part. The detection loop may e.g. be arranged as a conductive line leading between two ports/pads on the RFID chip. Prior to wetting, the detection loop will remain intact, and provide a connection between the two ports. The RFID chip may use this to determine that no wetting has occurred, and may include this information in responses sent to the reader. As soon as the water expandable part has expanded to a certain degree, the detection loop will break, and = there will no longer be any connection between the ports. This may then be N 25 interpreted by the RFID chip as an indication that wetting has occurred, and = this information may then be included in the information sent in response to the RFID reader.

E Thus, the signal from the RFID tag arrangement will here change from 0 a first signal, confirming no wetting, to a different second signal, confirming G 30 wetting, after contact with a liquid.

3 The RFID tag arrangement is preferably arranged to continue to be operational when wetted by the liquid, but with a lowered signal strength and with a reduced reading distance. This makes it possible to detect that the

RFID tag arrangement has been in contact with a liquid, but also to ensure that the RFID tag is still operational and has not failed for other reasons. Hereby, a detection of the presence of a liquid can be ensured with even higher certainty.

The RFID tag arrangement is preferably arranged to reduce the reading distance for the RFID tag by at least a factor 10 when wetted by the liquid. This would ensure that reading under normal conditions would not be possible when the RFID tag arrangement has been exposed to a liquid, but that reading would still be possible if the settings and/or reading distances are changed. The RFID tag arrangement is preferably configured for operation at a frequency within the ultrahigh frequency range (UHF), such as in the range of 860-960 MHz. The RFID chips may e.g. be a high performance and low-cost IC chip, such as the commercially available NXP UCode 8, NXP G2iL+ and Impinj Monza 4, the latter being provided with pads for two separate connections, making it suitable for arrangements with two separate antennas, or an antenna and a separate detection loop. The antenna and the antenna parts may have various shapes and dimensions, as is per se known in the art. For example, the antenna may be a dipole antenna, with antenna parts e.g. extending in a generally linear direction, or extending in a non-linear way, such as in a meandering form or the like. The parts may also be folded or curved, thereby extending in two or = more directions. However, other antenna designs are also feasible, such as N 25 patch antennas, monopole antennas, and the like. = The RFID tag arrangement may be either passive, i.e. powered by a reader’s electromagnetic field, or active, i.e. powered by an onboard battery. E In a preferred embodiment, the RFID tag arrangement is passive. 0 The antennas may be made of any material, as long as the material is G 30 conductive. The antennas may be made by the same material, but may 3 alternatively be made of different materials. For example, the antenna may be formed by aluminum, but other metals, such as silver, and alloys may also be used. Forming of the antenna on the substrate can be made in various ways,

as is per se known in the art, such as by printing with conductive ink, such as silver ink, by first providing a conductive layer on the substrate and subsequently removing or forming this conductive layer into the desired shape, e.g. by means of grinding, cutting or the like. Other dry processes may also be used, such as laminating with a foil and then die-cutting, mechanically or with laser, grinding or with Physical Vapour Deposition (PVD).

Since the substrate material significantly absorbs water, it is preferred that the process of forming the antenna on the substrate does not involve wetting, such as in a chemical (liquid) etching process.

In particular, the ink may comprise a material having a relatively low characteristic melting point. The solvent can be evaporated by means of heating at an elevated temperature, by use of photonic curing, or the like.

In one embodiment, the forming of conductive material in a pattern comprise: transferring a conductive material in a pattern corresponding to said electrically conductive pattern to a surface of the substrate; and heating the conductive material to a temperature exceeding a characteristic melting temperature of the conductive material.

The conductive material is preferably in the form of electrically conductive solid particles. The transferring of conductive material to the substrate surface may e.g. comprise direct printing of electrically conductive particles as a part of a compound that contains, in addition of the electrically conductive solid particles, a fluid or gelatinous substance. However, the electrically conductive solid particles may also be in the form of dry powder.

= Further, an adhesive area may be created on the surface of the substrate N 25 prior to transfer of the particles.

= The transfer of the conductive particles and the curing and solidification may in particular be made in the way disclosed in one or several of WO E 2013/113995, WO 2009/135985, WO 2008/006941 and WO 2016/189446, all 0 of said documents hereby being incorporated in their entirety by reference.

G 30 Curing may be effected by heating, or by a combination of heat and 3 pressure. In case both heat and pressure are used, the curing may be referred to as sintering.

The formation of the antenna element(s) is preferably made by a dry process, thereby not to negatively affect the liquid expandable part of the substrate.

The hydroexpansive material is arranged to expand significantly after a relatively short time when immersed in water, or in other ways wetted. At the same time, the substrate should have a surface and rigidity suitable for use as a substrate for an RFID tag.

Thus, the hydroexpansive material forms a substrate, or part of a substrate, which swells, and preferably also distorts, when exposed to water or other liquid, as the substrate absorbs the liquid.

The hydroexpansive material is preferably arranged to expand in at least one direction with at least 1%, and preferably at least 1.5% and most preferably at least 2% when wetted with a liquid, and in particular water.

The expansion may be determined experimentally in various ways.

Preferably, the test may be conducted by providing a sheet of the hydroexpansive material, and arrange one or more pair of marks on the sheet. Each pair of marks are separated by a predetermined distance, and may e.g. be arranged 200 mm apart. Thereafter, the sheet is soaked in water for a certain time period, such as 30 seconds, at 20 degrees C. Thereafter, the distance between the one or more pair of marks is measured, and the percentage increase in distance is calculated.

Preferably, the hydroexpansive material is further arranged to expand to different extents in different directions when wetted with the liquid. Thus, S the material hereby expands non-uniformly when exposed to liguid.

N 25 Preferably, the hydroexpansive material expands at least 1.5 times more in = the direction of greatest expansion than in the direction of lowest expansion, and preferably at least 2 times, and more preferably at least 2.5 times, and E most preferably at least 3 times. Since RFID antennas are planar 0 components, mainly in X and Y orientation on the substrate, a significantly G 30 greater expansion in one direction compared to another direction promotes 3 breaking of the antenna circuitry.

Further, the hydroexpansive material preferably has a limited hygroexpandability, thereby limiting expansion due to atmospheric humidity to such an extent that the conductive line will not break due to naturally occurring air humidity. Preferably, the RFID tag arrangement is arranged to withstand an atmospheric humidity of at least 70%, and preferably at least 80%, without breakage of the conductive line.

As used in the present application, hydroexpansion, hydroexpansive and hydroexpandability refers to an expansion, i.e. a dimensional change, caused by a direct contact with a liquid, and in particular water, i.e. by direct wetting of the material with the liquid. The hydroexpansion may be caused not only by water, but can also be caused by other liquids, such as urine, etc.

Different liquid composition may stimulate different expansion. Hygroexpansion, hygroexpansive and hygroexpandability refers to an expansion due to atmospheric relative humidity, i.e. by contact with moisture in gas form. Humidity refers to the concentration of water vapor present in air, and as used here refers to relative humidity, expressed as a percentage, indicating a present state of absolute humidity relative to a maximum humidity given the same temperature. The relative humidity is dependent on temperature, but even at high temperatures, such as at 30 deg. C or higher, and at very high levels of relative humidity, such as close to 100%, a cubic meter of air contains only a few grams of water.

The hydroexpansive material is preferably a cellulose based material. Cellulose fibers are generally hydroexpansive per se, and are also well suited to provide a difference in expansion in different directions, since the cellulose fibers generally expands more in the width direction than in the length S direction. By having the cellulose fibers arranged generally in one direction, N 25 which would typically be the machine direction, MD, the substrate will have a = greater hydroexpansiveness in the cross-direction, CD, i.e. perpendicular to the MD, than in the MD. Cellulose is a hygroscopic material, and hence E naturally absorbs moisture.

0 In a preferred embodiment, the hydroexpansive material is made of G 30 paper. However, the hydroexpansive material could also be a web like or 3 paper like material. Such materials are commercially available to a relatively low cost.

The hydroexpansive material may further comprise, and preferably in addition to cellulose, material which absorbs water to a great extent. Such materials may be cotton. Thus, paper comprising cotton, such as oil painting paper, may be used as the hydroexpansive material. Further, additives such as silica may be used, which is also highly absorptive. Papers containing silica are per se commercially available e.g. as desiccant paper. An example of such a paper is the Onyx Desiccant Paper, sold by Onyx Specialty Papers, Inc, MA, USA. Such a paper may also be used as the hydroexpansive material for the RFID tag arrangement. Other additives, such as super absorbent polymers (SAP), may also be used.

It is also possible to add such additives, such as SAP, in a separate layer, connected to a cellulose layer, such as a paper layer. In such embodiments, the SAP layer, or any other layer of highly absorptive material, is preferably arranged on an opposite side of the cellulose layer than the antenna.

Many other fillers and additives conventionally used in paper are not hydroexpansive by nature, and hence the hydroexpansive material preferably comprises no such fillers or additives, or only to a limited extent.

Further, the hydroexpansive material could be tailored for various uses by the choice of suitable constituents and processing. Thus, the material could be tailored to expand very rapidly already at a low degree of wetting, to make the RFID tag arrangement sensitive to a very low exposure to liquid, or tailored to expand relatively slowly, and only at a high degree of exposure to = wetting. Thus, the response time, i.e. the time it takes for the material to N 25 expand enough to affect the output signal from the RFID arrangement, could = be tailored to be within seconds from when it starts to be wetted, within minutes, or even longer.

E For example, the water degradable material may be sufficiently 0 disintegrated when immersed in water after a response time in the range of 1 G 30 second — 15 minutes, and preferably in the range of 10 seconds — 10 3 minutes, and more preferably in the range of 1 minute — 5 minutes.

The liquid expandable part of the substrate preferably comprises a single layer of a generally homogeneous material.

However, it is also feasible to use two or more layer, e.g. arranged in a sandwich construction.

How fast the hydroexpansive material will expand can also be controlled by suitable additives.

For example, the surface can be made more dense and compact using a biobased composition, such as starch.

The surface can also be made more hydrophobic by suitable additives.

Such additives will prolong the expansion process.

The liquid expandable part, formed by the hydroexpansive material, is preferably arranged not to disintegrate or dissolve when wetted.

To this end, the material may comprise crosslinked or chemically modified fibers.

For example, in a paper-based substrate, the cellulose fibers may be crosslinked.

Additionally, or alternatively, crosslinked starch may be provided.

Crosslinking of cellulose may e.g. be obtained by by oxidising the cellulose to dialdehydecellulose which can form cross-links with adjacent cellulose molecules.

However, substrates which at least partly disintegrate or dissolve when wetted, at least after a certain time of exposure to the liquid, are also feasible, and may also be used for many applications.

In one preferred embodiment, the substrate comprises, or consists of, a cellulose substrate with fibers or fibrils containing high amount of charged groups such as sulphate or carboxylic acid groups, etc., which protonates when wetted.

The base substrate can also contain wetting stimulants such as surfactants or wetting agent or humectants which facilitate the wetting and S hydroexpansion.

N 25 The substrate is preferably relatively thin, e.g. having a grammage = within the range of 20-100 gram per square meter (gsm). The RFID tag arrangement is useable as an integrated part of E products, such as in absorbent articles, or as discrete RFID tag 0 arrangements, arrangeable in locations where there is a need to monitor e 30 moisture and/or liquid, such as in building structures, cars, etc. 3 In accordance with one aspect of the invention, there is provided an absorbent article, such as a diaper, comprising a liquid detecting RFID tag arrangement as discussed in the foregoing.

The absorbent article may be a diaper, but may also be other absorbent articles, such as a diaper of pant type, an incontinence garment, a sanitary napkin, a tampon-like product, a wound or sore dressing, a bed protector, or a similar product.

The absorbent product may be arranged to absorb or receive body discharges from a user. In the absorbent product, the RFID tag arrangement may be placed within or below a layer of super absorbent polymer (SAP) or the like. When the super absorbent polymer is wet, or starts to be wetted, the water degradable part of the RFID tag substrate will be wetted as well.

The RFID tag arrangement may, in a preferred embodiment, be provided in the form of an RFID label, which may be attached to, or integrated with, a structure where it is to be used, such as an absorbent article, a package, a building, or the like. According to another aspect of the invention, there is provided a method for detecting liquid, comprising: providing a liquid detecting RFID tag arrangement comprising a substrate, wherein at least a part of the substrate forms a liquid expandable part, made of a hydroexpansive material, arranged to expand when wetted by a liquid; wirelessly reading the RFID tag with an RFID reader; and detecting an alteration in the signal from the RFID tag due to expansion of the liquid expandable part of the substrate upon exposure of said liquid expandable part to a liquid. = In accordance with these aspects, similar features and advantages as N 25 discussed in the foregoing, in relation to the first aspect, may be obtained. = It will be appreciated that the above-mentioned detailed structures and advantages of the first aspect of the present invention also apply to the further E aspects of the present invention. 0 These and other aspects of the invention will be apparent from and G 30 elucidated with reference to the embodiments described hereinafter.

S

Brief description of the drawings For exemplifying purposes, the invention will be described in closer detail in the following with reference to embodiments thereof illustrated in the attached drawings, wherein: Fig. 1 is a top plan view of an RFID tag arrangement in accordance with a first embodiment; Fig. 2 is a top plan view of an RFID tag arrangement in accordance with a second embodiment; Fig. 3 is a top plan view of an RFID tag arrangement in accordance with a third embodiment; Fig. 4 is a cross-sectional sideview of a substrate in accordance with an embodiment; Fig. 5 is a cross-sectional sideview of a substrate in accordance with another embodiment; Fig. 6 is a cross-sectional sideview of a substrate in accordance with yet another embodiment; Fig. 7 is a schematic illustration of a RFID chip for use in the embodiments of Figs. 2 and 3; and Fig. 8 is a schematic illustration of a system incorporating an RFID tag arrangement.

Detailed description of preferred embodiments In the following detailed description preferred embodiments of the = invention will be described.

However, it is to be understood that features of N 25 the different embodiments are exchangeable between the embodiments and = may be combined in different ways, unless anything else is specifically indicated.

It may also be noted that, for the sake of clarity, the dimensions of E certain components, parts and elements illustrated in the drawings may differ 0 from the corresponding dimensions in real-life implementations of the e 30 invention, such as the thickness of various layers, the relative dimensions of 3 the different antenna parts, etc.

Fig. 1 illustrates an RFID tag arrangement 1 in accordance with an embodiment of the present invention.

The RFID tag arrangement comprises an antenna 2. In the illustrative example, the antenna is designed as a folded dipole, comprising two first radiating dipole elements, interconnected by an interconnecting intermediate feeding part. The feeding part comprises an IC gap (not shown), and an RFID chip 3 arranged over said IC gap, to transmit and receive RF power from the two sides of the antenna.

The dipole antenna may also be designed in many other ways, as is per se known in the art. For example, the antenna and the antenna parts may have various shapes and dimensions. Further, even though the antenna is here a dipole antenna, other antenna types may also be used, such as patch antennas, monopole antennas, and the like.

The antenna is arranged on a substrate 4. The antennas may be made of any material, as long as the material is conductive, such as aluminum, silver, or alloys. For example, it is feasible to use an alloy having a relatively low melting temperature, such as an alloy comprising tin and bismuth.

Forming of the antenna on the substrate can be made in various ways, as is per se known in the art, such as by printing, etc.

The RFID chip may take any of a number of forms (including those of the type commonly referred to as a "chip" or a "strap" by one of ordinary skill in the art), including any of a number of possible components and being configured to perform any of a number of possible functions. Preferably, the RFID chip includes an integrated circuit for controlling RF communication and other functions of the RFID tag.

At least a part of the substrate 4 forms a liquid expandable part, made S of a hydroexpansive material, arranged to expand when wetted by a liguid. In N 25 the illustrative example of Fig. 1, the entire substrate is made of such a = hydroexpansive material, and consequently the liquid expandable part is here the entire substrate.

E When the substrate comes into contact with liguid, the substrate will 0 expand, and this will make the antenna 2 at least partly inoperative. Thus, a G 30 response signal from the RFID tag arrangement, when interrogated by an 3 RFID reader, will be sent at full reading distance only when the antenna is fully operative, i.e. when the substrate has not been wetted, and a much reduced, or even no, response signal will be sent when the substrate has been wetted, since the substrate will then be expanded, and the antenna 2 will be broken and at least partly inoperative. Thus, the signal from the RFID tag arrangement 1 will here, upon wetting, change from a normal signal, e.g. comprising the RFID tag ID, to no signal or a much reduced signal, e.g. with a much reduced reading distance. Thus, after expansion, the RFID tag may still work, but with a significantly reduced reading distance, so that the RFID tag may only be read at a very close distance. The reading distance may e.g. be reduced by 10x or more. Hereby, the broken RFID tag can no longer be read with similar RFID reader settings as prior to the wetting, deforming and breaking. However, since the RFID tag is still operational, but only at closer distance, malfunctioning of the RFID tag can be detected.

In the embodiment of Fig. 1, It is also feasible that only part of the substrate forms the liquid expandable part, as long as the antenna 2 is at least partly arranged on the liquid expandable part, and as long as this part of the antenna, which will become at least to some extent inoperative upon wetting, will make the entire antenna at least to some extent inoperative, or at least make the antenna operative in a distinctly different way, making it possible to discern from the response signal that the RFID tag arrangement has been exposed to liquid.

In another embodiment, as illustrated in Fig. 2, the RFID tag arrangement may comprise at least two antennas connected to the RFID chip. A first antenna 2, here similar to the one in the first discussed embodiment, may be arranged on a non-liguid expandable, or less liguid = expandable part 4' of the substrate, whereas a second antenna 5 is arranged N 25 on the liquid expandable part 4 of the substrate.

= As will be discussed in more detail in the following, the non or less liquid expandable part may e.g. be of a different material and connected to E the liguid expandable part. However, it may also be of the same 0 hydroexpansive, but protected from becoming wet by a protective layer, or in G 30 other ways being arranged in such a way that it is not exposed to the liquid.

3 In the embodiment of Fig. 2, the antenna 5 will become at least partly inoperative when the liquid expandable part expands, whereas the other antenna 2 will still be operative. To this end, it may be discerned from response signals from the RFID tag that there is no liquid when signals from both antennas are sent, and that there is liquid present when signals are received from only one antenna.

Thus, the signal from the RFID tag arrangement will here change from a normal signal to another signal upon wetting.

To make it possible to discern whether signals have been sent from both antennas, or only one of them, the two antennas may e.g. be connected to different ports/pads on the RFID chip, and consequently send out different signals.

For example, the antenna 2 which will still be operative after wetting may be used to send out the ID of the RFID tag arrangement, whereas the antenna 5 which will become inoperative during wetting may send out a signal indicating that no wetting has occurred.

When the signal indicative of that no wetting has occurred ceases, this may be interpreted as an indication that wetting has occurred.

As discussed in the foregoing, the antenna arranged on the liquid expandable part may also be operative after wetting, but with a much reduced reading distance, providing a further measure to detect wetting.

The embodiment illustrated in Fig. 3 is similar to the one discussed in relation to Fig. 2. However, in the embodiment of Fig. 3, the RFID tag arrangement 1 comprises a detection loop 5 connected to the RFID chip, instead of the above-discussed second antenna 5. The detection loop is at least partly arranged on the liquid expandable part 4. The detection loop may S e.g. be arranged as a conductive line leading between two ports/pads on the N 25 RFID chip.

Prior to wetting, the detection loop will remain intact, and provide a = connection between the two ports.

The RFID chip may use this to determine that no wetting has occurred, and may include this information in responses E sent to the reader when the RFID tag is interrogated.

As soon as the liguid 0 expandable part starts to expand, the detection loop will break, and there will G 30 no longer be any connection between the ports.

This may then be interpreted 3 by the RFID chip as an indication that wetting has occurred, and this information may then be included in the information sent in response to the RFID reader.

Thus, the signal from the RFID tag arrangement will here change from a first signal, confirming no wetting, to a different second signal, confirming wetting, after contact with liquid.

It is to be appreciated by the skilled reader that many other variations are also feasible. Thus, the liquid expandable part can be arranged at essentially any location of the substrate, and have essentially any shape and dimensions, as long as expansion of the liquid expandable part can lead to a clearly discernible effect for the RFID tag itself, as in the embodiment with the detection loop, or for the receiver of signals sent out by the RFID tag, as in the above-discussed embodiments of Figs. 1 and 2.

In one embodiment, the substrate may comprise two separate RFID tags, one being arranged on a liquid expandable part, and one on a non, or less, liquid expandable part. Prior to being wetted, both RFID tags will respond to interrogations from an RFID reader. After being wetted, the RFID tag being arranged on the liquid expandable part will stop functioning, and thereafter, the RFID reader will only get a response from the RFID tag arranged on the non, or less, liquid expandable part.

The hydroexpansive material is preferably arranged to expand after a relatively short time when immersed in water, or in other ways wetted. The hydroexpansive material could be tailored for various uses by the choice of suitable constituents and processing. Thus, the material could be tailored to expand very rapidly already at a low degree of wetting, to make the RFID tag arrangement sensitive to a very low degree of liquid, or tailored to expand = relatively slowly, and only at a high degree of wetting. Thus, the response N 25 time, i.e. the time it takes for the material to expand enough to affect the = output signal from the RFID arrangement, could be tailored to be within N seconds from when it starts to be wetted, within minutes, or even longer. E For example, the hydroexpansive material may be sufficiently 0 expanded when immersed in water after a response time in the range of 1 G 30 second — 15 minutes, and preferably in the range of 10 seconds — 10 3 minutes, and more preferably in the range of 1 minute — 5 minutes. The hydroexpansive material can e.g. be a cellulose based material.

The liquid expandable part of the substrate preferably comprises a single layer of a generally homogeneous material. However, it is also feasible to use two or more layer, e.g. arranged in a laminated, sandwiched construction.

As discussed in the foregoing, the substrate can be made entirely by the hydroexpansive material. However, for embodiments where part of the substrate forms a liquid expandable part, and part of the substrate forms a non, or less, liquid expandable part, these parts may be realized in various ways. Some examples of this will be explained in further detail in the following.

In one embodiment, illustrated in Fig. 4, the liquid expandable part 4 and the non, or less, liquid expandable part 4 may be made of different materials, wherein the liquid expandable part 4 is significantly more liquid expandable than the non, or less, liquid expandable part 4’. This could e.g. be accomplished by use of different grades of paper, different additives, different amounts of cellulose, etc.

The parts could be connected together in various ways. In the example illustrated in Fig. 4, the parts are arranged with a small overlap, and connected together at the overlap. The connection can e.g. be made with adhesive.

In another embodiment, illustrated in Fig. 5, the non, or less, liquid expandable part 4’ essentially entirely overlaps the sheet of hydroexpansive material forming the liquid expandable part 4. In this embodiment, the part of = the sheet that extends out from the non, or less, liguid expandable part 4' N 25 forms the liguid expandable part 4. The sheets can here be connected = together with adhesive, or other per se known lamination technologies.

In yet another embodiment, illustrated in Fig. 6, the sheet of E hydroexpansive material is partly enveloped inside a material which is less 0 liquid expandable, or non-liquid expandable. Thus, the enveloped part of the G 30 sheet here forms the non, or less, liquid expandable part 4’, whereas the non- 3 enveloped part forms the liquid expandable part 4.

It is to be appreciated by the skilled reader that other ways of forming parts with distinctly different water/liquid expandability are also feasible.

In embodiments where the RFID chip is connected both to an antenna and to another antenna or detection loop, the RFID chip preferably comprises at least four ports or bumps. Such an RFID chip is schematically illustrated in Fig. 7, where the RFID chip comprises two ports 31, 32 connected to an UHF antenna 2, and two ports 33, 34, connected to another antenna, or, as in the illustrative example, a detection loop 5’.

The RFID tag arrangement is useable as an integrated part of products, such as in absorbent articles, or as discrete RFID tag arrangements, arrangeable in locations where there is a need to monitor liquid, such as in building structures, cars, etc.

In an embodiment illustrated in Fig. 8, the RFID tag arrangement 1 is arranged in an absorbent article 6. In the illustrative example, the absorbent article is a diaper, but it may also be other absorbent articles, such as a diaper of pant type, an incontinence garment, a sanitary napkin, a tampon-like product, a wound or sore dressing, a bed protector, or a similar product.

The absorbent product may be arranged to absorb or receive body discharges from a user. In the absorbent product, the RFID tag arrangement may be integrated in the absorbent product, and e.g. be placed within or above or below a layer of super absorbent polymer (SAP) or the like. When the super absorbent polymer is wet, or starts to be wetted, the liquid expandable part of the RFID tag substrate will be expanded. Preferably, the SAP layer is arranged so that the liquid must pass through or via the RFID tag arrangement before contacting the SAP or absorbent core. If the structure is a S diaper or absorbent product, the RFID tag arrangement can be arranged N 25 close to, or within, the liquid distribution layer. = Upon use, the RFID tag arrangement 1 may be intermittently and regularly interrogated by an RFID reader 7. The RFID reader may be a stand E alone unit, or be part of a surveillance system or the like. For example, the 0 RFID reader may be connected to a central server 8 by a wired or wireless G 30 connection. Based on the responses received from the RFID tag arrangement 3 it may be discerned, e.g. in one of the various ways discussed in the foregoing, that wetting of the RFID tag arrangement has occurred. Such determination could be made in any one of the RFID chip, the RFID reader and the central server.

The person skilled in the art realizes that the present invention is not limited to the above-described embodiments. For example, the general antenna design may be varied in many ways, as is per se well-known in the art. For example, the dipole elements may be shaped differently than in the above-discussed embodiments, and the feeding loop, etc, may also have other shapes. Other type of antennas may also be used, and the antenna may further be adapted for different operational frequencies. Further, the liquid expandable part may be realized in many different ways, and may also have various shapes, dimensions and locations on the substrate. Further, a change in response signal from the RFID tag arrangement upon expansion of the liquid expandable part can be accomplished in various ways, and for example, as exemplified in the foregoing, by making the main antenna totally or partly inoperative, by making a secondary antenna fully or partly inoperative, by breaking a detection loop, or the like.

Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-described embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word “comprising” does not = exclude the presence of other elements or steps than those listed in the N 25 claim. The word “a” or “an” preceding an element does not exclude the = presence of a plurality of such elements.

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Claims

1. A liquid detecting RFID tag arrangement (1) comprising a substrate (4, 4°), at least one antenna (2, 5) element having a conductive line arranged on said substrate (4, 4’), and an RFID chip (3) connected to said antenna element(s) (2), wherein at least a part of the substrate (4) forms a liquid expandable part, made of a hydroexpansive material, wherein said conductive line is at least partly arranged on said hydropexpansive material, and wherein the hydroexpansive material is arranged to expand when wetted by a liquid to such an extent that the conductive line will break, wherein the RFID tag arrangement (1) comprises at least two antenna elements (2, 5) connected to said RFID chip (3), wherein one of said antenna elements is arranged on said liquid expandable part (4) and one of said antenna elements is arranged on a non-liquid expandable or less liquid expandable part (4) .

2. The RFID tag arrangement of claim 1, wherein the whole substrate (4) is made of the hydroexpansive material, and forms said liguid expandable part.

3. The RFID tag arrangement of claim 1, further comprising a detection loop (5') connected to said RFID chip (3), the detection loop (5') being at least partly arranged on said liguid expandable part.

4. The RFID tag arrangement of any one of the preceding claims, wherein the tag arrangement (1) is configured for operation at a freguency within the range of 860-960 MHz.

S 5. The RFID tag arrangement of any one of the preceding claims, N 25 wherein the RFID tag is arranged to continue to be operational when wetted = by the liquid, but with a lowered signal strength and with a reduced reading distance.

E

6. The RFID tag arrangement of any one of the preceding claims, 0 wherein the RFID tag is arranged to reduce the reading distance for the RFID G 30 tag by at least a factor 10 when wetted by the liquid.

3

7. The RFID tag arrangement of any one of the preceding claims, wherein the hydroexpansive material has a limited hygroexpandability,

thereby limiting expansion due to atmospheric humidity to such an extent that the conductive line will not break due to naturally occurring air humidity.

8. The RFID tag arrangement of any one of the preceding claims, wherein the hydroexpansive material is a cellulose based material.

9. The RFID tag arrangement of any one of the preceding claims, wherein the hydroexpansive material is made of paper.

10. The RFID tag arrangement of any one of the preceding claims, wherein the hydroexpansive material is arranged to expand in at least one direction with at least 1%, and preferably at least 1.5% and most preferably at least 2% when wetted with said liquid.

11. The RFID tag arrangement of any one of the preceding claims, wherein the hydroexpansive material is arranged to expand to different extents in different directions when wetted with said liquid.

12. An absorbent article (6), such as a diaper, comprising a liquid detecting RFID tag arrangement (1) in accordance with any one of the claims 1-11.

13. A method for detecting liquid, comprising: providing a liquid detecting RFID tag arrangement (1) comprising a substrate (4, 4’), wherein at least a part of the substrate forms a liquid expandable part (4), made of a hydroexpansive material, arranged to expand when wetted by a liquid; wirelessly reading the RFID tag with an RFID reader (7); and detecting an alteration in the signal from the RFID tag due to S expansion of the liguid expandable part (4) of the substrate upon exposure of N 25 said liquid expandable part to a liquid.

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