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 moisture and/or liquid. The invention further relates to a method for detecting presence of moisture and/or liquid.

Background

There are many situations and applications where there is a need to detect the presence of moisture and 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 absorbent article. Such a sensor may, for example, be based on detection of wetness, a biological analyte and/or a chemical analyte. When such a sensor detects an event, such as urination or defecation, a detectable response is generated. By means of the response, the user, parent, caretaker, 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 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 moisture or 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 moisture or 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.

The presently used detector systems are unfortunately quite 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 areas.

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

Summary

It is therefore an object of the present invention to provide a liquid and/or moisture detection RFID tag arrangement and a method for detection of moisture and/or 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 and/or moisture detecting RFID tag arrangement and a liquid and/or moisture detecting method in accordance with the appended claims. According to a first aspect of the present invention, there is provided a liquid and/or moisture detecting RFID tag arrangement comprising a substrate, at least one antenna element arranged on said substrate, and an RFID chip connected to said antenna element(s), wherein at least a part of the substrate forms a water degradable part, made of a water degradable material, arranged to dissolve or disintegrate when wetted by a liquid.

Flereby, a relatively simple RFID tag can be used to detect moisture and/or liquid. When the water degradable part is wetted, i.e. when it comes into contact with moisture or a liquid, such as water or urine, it will start to disintegrate or dissolve, and will thereby affect the output signal from the RFID tag arrangement. Such an RFID tag arrangement can be produced to a very low cost, since it may use a standard RFID chip, a 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 produced to a cost which is less than at least 1/100 of the cost of such previously known solutions, and most probably less than 1/1000 of this cost. This makes the new RFID tag arrangement useable for many products and applications where it was previously not practically and economically feasible to use liquid and/or moisture 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 antenna element is at least partly arranged on said water degradable part of the substrate, wherein dissolving or disintegration of the water degradable part makes the antenna element inoperative. In such embodiments, the whole substrate can be made of a water degradable material, and form said water degradable part, which makes production easier and even more cost-effective.

In such embodiments, the antenna will become inoperative as soon as the water degradable part dissolves or disintegrates. Thus, a response signal from the RFID tag arrangement, e.g. containing only the ID of the RFID tag, can be used as an indication that no water or moisture is present, whereas a lack of response signal can be used as an indication that there is, or has been, moisture and/or liquid present, and that the water degradable part has been disintegrated or dissolved. Thus, the signal from the RFID tag arrangement will here change from a normal signal to no signal 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 water degradable part and one of the antenna elements is arranged on a non-water degradable part. The non-water degradable part may e.g. be of a different material, connected to the water degradable part. Flowever, it may also be of the same water degradable 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 or moisture.

In such embodiments, one of the antennas will become inoperative when the water degradable part dissolves/disintegrates, whereas the other antenna will still be operative. To this end, the RFID reader may detect that there is no moisture/liquid when signals from both antennas are received, and that there is moisture/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.

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 operative after wetting may be used to send out the ID of the RFID tag arrangement, whereas the antenna which will become inoperative during wetting may send out a signal that no wetting has occurred. When the signal indicative of that no wetting has occurred ceases, this may be interpreted by the reader that wetting has in fact occurred.

Flowever, 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.

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 water degradable 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 degradable part starts to dissolve/disintegrate, the detection loop will break, and there will no longer be any connection between the ports. This may then be 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.

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 moisture/liquid.

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. NXP UCode 8 has two antenna pads for attaching one antenna, which antenna consists of two antenna conductors. NXP G2il_ also has two antenna pads in accordance with UCode 8 and additionally has two pads for connecting a separate detecting loop. Impinj Monza 4 has four pads for attaching two separate dipole antennas, each antenna consists of two separate antenna conductors.

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 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.

In a preferred embodiment, the RFID tag arrangement is passive.

The antennas may be made of any material, as long as the material is conductive. The antennas may be made by the same material, but may 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.

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 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 2013/113995, WO 2009/135985, WO 2008/006941 and WO 2016/189446, all of said documents hereby being incorporated in their entirety by reference.

Curing may be effected by heating, or by a combination of heat and 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 water degradable part of the substrate.

The water degradable material is arranged to disintegrate after a relatively short time when immersed in water, or in other ways wetted. The water degradable part may e.g. disintegrate similar to ordinary toilet paper. At the same time, the substrate should have a surface and rigidity suitable for use as a substrate for an RFID tag.

The water degradable material could be tailored for various uses by the choice of suitable constituents and processing. Thus, the material could be tailored to disintegrate very rapidly already at a low degree of wetting, to make the RFID tag arrangement sensitive to a very low degree of moisture or liquid, or tailored to disintegrate relatively slowly, and only at a high degree of wetting. Thus, the response time, i.e. the time it takes for the material to disintegrate 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.

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

By water degradable material is here meant a hydrodegradable material which breaks down or disintegrates, preferably by hydrolysis, when placed in an aqueous environment, such as when being immersed in water.

The water degradable material can e.g. be at least one of a polymer- based material and a cellulose based material.

In one embodiment, the water degradable material comprises at least 30 wt% of microfibrillated cellulose and/or nanocellulose. Additionally, or alternatively, the water degradable material may comprise 5-70 wt% of a dissolvable polymer, and preferably 10-60 wt%, and most preferably 20-50 wt%.

The dissolvable polymers may e.g. be at least one of: low molecular water soluble species, such as dextrins, modified and cold-water soluble starches, modified cellulose derivatives, different types of sugars or hemicelluloses such as xylose, mannose and glucose, and highly hydrophilic compounds, such as low molecular weight polyvinyl alcohol, PEG and glycole, or mixtures thereof.

Additionally, or alternatively, and in particular for a polymer based material, the water degradable material may comprise at least one of: PVOH (poly(vinyl alcohol)), starch or cellulose derivatives such as hydroxypropyl cellulose, carboxymethyl cellulose, methyl cellulose, hydroxypropyl methylcellulose, gelatin, polyvinyl pyrrolidine, pectin, pullulan, or mixtures thereof.

In an embodiment, the water degradable part is made of at least one of: paper and polymer film.

The water degradable 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.

The absorbance of the material is a measure of how much and fast water is absorbed by the material. This may be measured as a Cobb value, and e.g. in accordance with the ISO 535:014 standard. In a test for measuring Cobb values, a water pillar is arranged over the material to be measured for a predetermined time, and the difference in weight before and after wetting constitutes the weight of the absorbed water. Different wetting times may be used, such as 60 seconds (Cobb-60), 120 seconds (Cobb-120), but longer and shorter periods may also be used.

A low Cobb value means that only a small amount of water is absorbed, which generally means that the material maintains its shape and rigidity even after some period of wetting. A higher Cobb value means that a greater amount of water is absorbed. This generally means that the material will dissolve/disintegrate more easily during wetting. Thus, finding a material with an adequate Cobb value is an indication that the material may disintegrate sufficiently fast for the intended need.

The water degradable material may have a Cobb-60 value of at least 70 g/m ² , and preferably at least 100 g/m ² , and more preferably at least 150 g/m ² The Cobb value of the water degradable material may be in the range of 70-250 g/m ² , and preferably in the range 70-200 g/m ² , and most preferably in the range of 80-150 g/m ²

How fast the water degradable material will disintegrate 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. Fillers may also be used, such as mineral fillers like kaolin, china clay, titanium dioxide, gypsum, talc, chalk, ground marble, ground calcium carbonate and precipitated calcium carbonate. How fast the water degradable material will disintegrate can also be controlled by adjustment of the thickness of the substrate material.

The water degradable material may e.g. be a web like material, preferably in roll format, and preferably a paper-like, low-cost material. Suitable materials are per se commercially available from manufacturers such as SmartSolve, UPM Raflatac and Stora Enso. One suitable substrate material is e.g. a laminate available from UPM Raflatac comprising a Dissolvable Paper Plus, adhered to a backing of Honey Glassine 65 by the adhesive RP590.

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 moisture and/or liquid, such as in building structures, cars, etc.

In accordance with one aspect of the invention, there is provided an absorbent article, such as a diaper, comprising a liquid and/or moisture 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.

According to another aspect of the invention, there is provided a method for detecting liquid or moisture, comprising: providing a liquid and/or moisture detecting RFID tag arrangement comprising a substrate, wherein at least a part of the substrate forms a water degradable part, made of a water degradable material, arranged to dissolve or disintegrate 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 dissolving or disintegration of the water degradable part of the substrate upon exposure of said water degradable part to a liquid or moisture.

In accordance with these aspects, similar features and advantages as 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 aspects of the present invention.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

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. Flowever, it is to be understood that features of 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 certain components, parts and elements illustrated in the drawings may differ from the corresponding dimensions in real-life implementations of the invention, such as the thickness of various layers, the relative dimensions of 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 water degradable part, made of a water degradable material, arranged to dissolve or disintegrate when wetted by a liquid. In the illustrative example of Fig. 1 , the entire substrate is made of such a water degradable material, and consequently the water degradable part is here the entire substrate.

When the substrate comes into contact with moisture or liquid, the substrate will dissolve or disintegrate, and this will make the antenna 2 inoperative. Thus, a response signal from the RFID tag arrangement, when interrogated by an RFID reader, will be sent only when the antenna is operative, i.e. when the substrate has not been wetted, and no response signal will be sent when the substrate has been wetted, since the substrate will then be disintegrated or dissolved, and the antenna 2 will be 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.

In the embodiment of Fig. 1 , It is also feasible that only part of the substrate forms the water degradable part, as long as the antenna 2 is at least partly arranged on the water degradable part, and as long as this part of the antenna, which will become inoperative upon wetting, will make the entire antenna 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 moisture and/or 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-water degradable part 4’ of the substrate, whereas a second antenna 5 is arranged on a water degradable part 4 of the substrate.

As will be discussed in more detail in the following, the non-water degradable part may e.g. be of a different material and connected to the water degradable part. Flowever, it may also be of the same water degradable 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 or moisture.

In the embodiment of Fig. 2, the antenna 5 will become inoperative when the water degradable part dissolves/disintegrates, 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 moisture/liquid when signals from both antennas are sent, and that there is moisture/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.

Flowever, alternatively, the two antennas may be connected to the same port, and be discernible by e.g. different polarizations or the like. The embodiment illustrated in Fig. 3 is similar to the one discussed in relation to Fig. 2. Flowever, 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 water degradable part 4. 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 when the RFID tag is interrogated. As soon as the water degradable part starts to dissolve/disintegrate, the detection loop will break, and there will no longer be any connection between the ports. This may then be 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.

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 moisture/liquid.

It is to be appreciated by the skilled reader that many other variations are also feasible. Thus, the water degradable part can be arranged at essentially any location of the substrate, and have essentially any shape and dimensions, as long as disintegration/dissolving of the water degradable 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 water degradable part, and one on a non-water degradable 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 water degradable part will stop functioning, and thereafter, the RFID reader will only get a response from the RFID tag arranged on the non-water degradable part. The water degradable material is arranged to disintegrate after a relatively short time when immersed in water, or in other ways wetted. The water degradable material could be tailored for various uses by the choice of suitable constituents and processing. Thus, the material could be tailored to disintegrate very rapidly already at a low degree of wetting, to make the RFID tag arrangement sensitive to a very low degree of moisture or liquid, or tailored to disintegrate relatively slowly, and only at a high degree of wetting. Thus, the response time, i.e. the time it takes for the material to disintegrate 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.

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

The water degradable material can e.g. be at least one of a polymer based material and a cellulose based material.

The water degradable part of the substrate preferably comprises a single layer of a generally homogeneous material. Flowever, 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 water degradable material. Flowever, for embodiments where part of the substrate forms a water degradable part, and part of the substrate forms a non-water degradable 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 water degradable part 4 and the non-water degradable part 4’ may be made of different materials, wherein the water degradable part 4 is significantly more water degradable than the non-water degradable part 4’. This could e.g. be accomplished by use of different grades of paper, different additives, different amounts of cellulose or dissolvable polymer, 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-water degradable part 4’ essentially entirely overlaps the sheet of water degradable material forming the water degradable part 4. In this embodiment, the part of the sheet that extends out from the non-water degradable part 4’ forms the water degradable 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 water degradable material is partly enveloped inside a material which is less water degradable, or non-water degradable. Thus, the enveloped part of the sheet here forms the non-water degradable part 4’, whereas the non-enveloped part forms the water degradable part 4.

It is to be appreciated by the skilled reader that other ways of forming parts with distinctly different water degradability 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 UFIF 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 moisture and/or 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 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.

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 alone unit, or be part of a surveillance system or the like. For example, the RFID reader may be connected to a central server 8 by a wired or wireless connection. Based on the responses received from the RFID tag arrangement 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 water degradable 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 disintegration or dissolving of the water degradable 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 claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.