Field of the Invention

The present invention relates to a solvent-sensing circuit configured to detect the presence of a solvent, and a method of detecting the presence of a solvent using the solvent-sensing circuit.

Background

Water leaks are a global problem. For example, across England and Wales each year around 3 billion litres of water are lost from leaky pipes. Even worse, if the water leak is present within a building and left undetected, damage may be caused before the leak can be fixed. There is a need for a water detector that at least partially solves these problems.

Summary of the Invention

According to a first aspect of the present invention, there is provided a sensing circuit, configured to detect the presence of a solvent, the sensing circuit comprising: a solvent-sensitive electrical connection comprising at least two electrically conductive elements, and a solvent-sensitive element arranged between and in electrical connection with the at least two electrically conductive elements, and wherein, the solvent-sensitive element is configured to at least partially dissolve in the presence of the solvent in a liquid state to cause the resistance of the solvent-sensitive element between said at least two electrically conductive elements to change.

Optionally, the solvent-sensitive element comprises any one or more of:

• a solvent-soluble polymer; • a plurality of electrically conductive particles (e.g., any one or more of: copper, silver, and graphite) dispersed within the solvent-soluble polymer; and

• a metallic, electrically conductive film arranged on at least one side of the solvent-soluble polymer, or within said polymer, and in electrical contact with said conductive elements.

The solvent-soluble polymer may have an electrical conductivity greater than 10 S/m (be considered an electrical conductor) or less than 10 S/m (be considered an electrical insulator)..

The solvent soluble polymer is configured to at least partially dissolve in the presence of liquid solvent, such that:

• a higher resistance path, comprising air, liquid solvent and/or dissolved solvent soluble polymer (i.e., an electrical discontinuity) develops between said conductive elements, resulting in an increase in resistance between said conductive elements, if the separation between the conductive elements is greater than 5mm; or

• a lower resistance path, comprising conductive particles, develops between the conductive elements in the sensing circuit, if the separation between the conductive elements is no greater than 5mm.

In some examples, the separation between the conductive elements is no greater than 1 cm.

The solvent may be water and the solvent-soluble polymer is a water-soluble polymer. For example, any one or blend of: polyethyloxyazoline, polyvinyl pyrrolidone, polyvinyl alcohol, polyacrylamide, polyglycols and polyacrylic acid, polylactic acid (PLA), polyvinylidene fluoride (PVDF), polyethylene glycol, polyacrylamides, polyacrylic acid copolymer or polyvinyl alcohol.

Alternatively, or in addition: • the solvent may be acetone and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, polyamide, polybutylene terephthalate, polyethylene terephthalate, and polycarbonate;

• the solvent may be benzene, or xylene and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, polyimide, polyethylene terephthalate, and poly(a-methly styrene);

• the solvent may be chlorobenzene and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, polybutylene terephthalate, or polycarbonate;

• the solvent may be chloroform and the solvent-soluble polymer any one or blend of: polyoxymethylene, polyimide, or polyether ether ketone;

• the solvent may be citric acid and the solvent-soluble polymer polyoxymethylene;

• the solvent may be ethylene chloride and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, or polyoxymethylene;

• the solvent may be formic acid and the solvent-soluble polymer polyoxymethylene;

• the solvent may be chlorine or hydrochloric acid and the solvent-soluble polymer any one or blend of: polyoxymethylene, polyamide, polybutylene terephthalate, or polyethylene terephthalate;

• the solvent may be hydrogen peroxide and the solvent-soluble polymer any one or blend of: polyamide, or polyoxymethylene;

• the solvent may be methanol or ethanol and the solvent-soluble polymer any one or blend of: polycarbonate, or methyl and ethyl-derivative polymers;

• the solvent may be nitric acid and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, polyamide, or polybutylene terephthalate;

• the solvent may be nitrobenzene and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, or polycarbonate;

• the solvent may be sulphur dioxide and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, or polycarbonate; • the solvent may be sulphuric acid and the solvent-soluble polymer any one or blend of: polyoxymethylene, acrylonitrile butadiene styrene, polyethylene terephthalate, or polyamide; and

• the solvent may be toluene and the solvent-soluble polymer any one or blend of: acrylonitrile butadiene styrene, polycarbonate, or polysulfone.

The sensing circuit may comprise a plurality of solvent-sensitive electrical connections arranged in series, parallel or in a matrix. A matrix is an arrangement comprising m rows and n columns, wherein m and n are positive integers. In the matrix arrangement, each row is electrically connected to each column via a respective solvent-sensitive electrical connection and the sensing circuit is configured to provide power to each respective solvent-sensitive electrical connection independently using said matrix arrangement. In some examples, there may be more than one solvent-sensitive electrical connection electrically connecting each row to each column.

In some examples, the sensing circuit further comprises a metal-oxide semiconductor (MOS) device, electrically connected to one of the plurality of solvent-sensitive electrical connections. The MOS device is, however, arranged on a different substrate to the solvent-sensitive electrical connection. The sourcedrain current of the MOS device is configured to change responsive to the resistance change of said solvent-sensitive element. Optionally, the different substrate (where the MOS device resides) does not dissolve (i.e. , it is insoluble) in the solvent. In some examples, the sensing circuit comprises a MOS device electrically connected to each of the plurality of solvent-sensitive electrical connections.

In some examples, a capping is arranged around the solvent-sensitive element, which is configured to protect said element from scratch damage, the capping comprising a solvent-permeable material or comprising a mesh to allow solvent ingress to the solvent-sensitive element. Each solvent-sensitive electrical connection may be fabricated on a substrate. For example, a flexible polymer, which is sufficiently compliant to be applied onto an object (e.g., a building component) having a non-planar surface. Alternatively, a rigid polymer, e.g., a PCB module, which can be replaceably connected and disconnected from the sensing circuit. In yet other examples, the building component itself is the substrate. The solvent-sensitive electrical connections may be fabricated onto different substrates. For example, a first sub-set of electrical connections may be fabricated onto a PCB module, a second sub-set onto a building component, and a third-subset onto a flexible polymer.

Each solvent-sensitive electrical connection may be fabricated onto a polymer substrate at uniform intervals using a roll-to-roll process.

In some examples, all or some of the solvent-sensitive electrical connections include a wicking layer in contact with the respective solvent-sensitive element. The wicking layer is adapted to transport solvent from the vicinity of the solventsensitive element to it, thereby increasing the sensing range of the solventsensitive electrical connection.

According to a second aspect of the present invention, there is provided a method of detecting the presence of a solvent using the sensing circuit described above, the method comprising: determining that the solvent is present conditional upon the resistance of the solvent-sensitive electrical connection changing by a magnitude greater than a predetermined threshold.

The sensing circuit may comprise a plurality of solvent-sensitive electrical connections, and said determining step is applied to each solvent-sensitive electrical connection independently.

In some examples, the method further comprises, responsive to said determining step, any one or more of the following steps: sounding an alarm; switching on a light element; and transmitting a RF signal from a RF antenna to a remote device.

If the solvent is liquid water, the predetermined threshold is 2 Kilo Ohms.

If the solvent is water vapour, the predetermined threshold is 0.5 Kilo Ohms.

Brief Description of the Drawinqs

Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a diagram of solvent-sensitive electrical connection;

Figure 2A to 2E are diagrams of structures for the solvent-sensitive electrical connection;

Figure 3A and 3B are diagrams of structures for the solvent-sensitive electrical connection;

Figure 4A and 4B are diagrams of a sensing circuit;

Figure 5 is a diagram of a matrix arrangement of solvent-sensitive electrical connections in a sensing circuit;

Figure 6 is a diagram of a sensing circuit; and

Figure 7 is a diagram of a solvent-sensitive electrical connection with a wicking layer.

Detailed Description of Preferred Embodiments

Referring to Figure 1 , a solvent-sensitive electrical connection 100 is shown. The solvent-sensitive electrical connection comprises: a solvent-sensitive element 102 disposed between at least two electrically conductive elements 104 and in electrical contact with those electrically conductive elements 104. In the presence of solvent, the solvent-sensitive element 102 undergoes a transformation (e.g., dissolution), which results in a measurable change of electrical resistance across the conductive elements 104. The change in resistance may be either an increase or decrease, depending on the material composition of the solvent-sensitive element 102. By monitoring the magnitude of the change in resistance, it can be determined, for example, whether an amount of liquid or solvent indicative of a leak is present at the electrical connection 100, or whether the vapour concentration of the solvent at the electrical connection 100 reaches a threshold indicative of solvent saturation. The electrical connection 100 may be a sensing element within a detector or sensor.

Water as a solvent

Figures 2A to 2E show example structural arrangements 200 for the solventsensitive element 102, in the absence of solvent (left) and in the presence of solvent (right). In the examples described herein, the solvent is liquid water or water vapour. The solvent-sensitive electrical connection 100 may be considered a water-sensitive electrical connection.

In Figure 2A, the water-sensitive element 202 comprises a water soluble, electrically conductive polymer 204. The polymer 204 also acts as a binder to adhere the water-sensitive element 202 to the electrically conductive elements 104 of the electrical connection 100. In these examples, the separation between the electrical elements 104 is no larger than 1cm. The volume of water-sensitive element 202 may be less than 1 cm ³ .

An electrically conductive polymer refers to any polymer having a conductivity greater than 10S/m. The electrical conductivity of polyacetylene, for example, doped with Br2, I2, CI2 and AsFs (p-type) increases from around 10’ ³ S/m up to around 10 ⁵ S/m with increasing dopant concentration. In the absence of water in contact with the water-sensitive element 202, the electrical connection 100 comprising a water soluble, electrically conductive polymer 204 has a relatively high electrical conductivity. The measurable resistance (e.g., voltage drop for given current) across the connection 100 is therefore relatively low in the absence of water. Typical resistances are of the order of 10 Ohms for a spacing of 5mm between conductive elements 104.

However, in the presence of a sufficient amount of liquid water (e.g., a droplet), the water-soluble polymer 204 at least partially dissolves. Dissolution of the water soluble polymer 204 develops an electrical discontinuity 206 (or gap) between conductive elements 104, filled with air, water and, potentially, dissolved water- soluble polymer 204. This discontinuity 206 greatly increases the resistance across the connection 100. The discontinuity 206 is not required to extend completely across conductive elements 104 to result in this increase in resistance. For example, the discontinuity 206 may be present within the watersensitive element 202 if it only partially dissolves. Typical resistances across the conductive elements 104 after dissolution of polymer 204 are in the range of several kilo-ohms, which approximates to the open circuit condition.

In the presence of water vapour at sufficiently low concentrations, the water soluble, electrically conductive polymer 204 may partially dissolve so little that an electrical discontinuity between elements 104 does not develop. However, as water molecules from the vapour are absorbed, the resistance of the polymer 204 increases with increasing water vapour concentration and leads to an increase in resistance across elements 104. This process is reversible. If the concentration of the water vapour external to the polymer 204 decreases, water molecules can be released from the polymer 204, thereby reducing its and the water-sensitive element’s 202 resistance. This reversible change in resistance in the presence of water vapour is particularly useful for monitoring dampness and humidity levels, e.g., on walls, within cavity walls etc. It has also been experimentally observed that the resistance of the watersensitive element 202, following a wet-dry cycle, returns to its original “dry state” resistance (within measurement error). It has also been experimentally observed that, the resistance of the water-sensitive electrical element 202 is proportional to the concentration of water vapour present external to the element 202 - up to a critical concentration. The critical water vapour concentration is the saturation limit (i.e., where condensation occurs).

For these reasons, the measured resistance of a water-sensitive element 202 can be compared against calibrated values (determined under a controlled water vapour environment) in order to estimate the concentration of water vapour present at that electrical connection 100 (e.g., for a particular temperature). The water vapour concentration (and hence the resistance) of the electrical connection 100 can be used as an indicator to assess whether building materials (e.g., stone, wood, brick) proximal to that location are suitable in that environment, whether damage is likely, and/or whether intervention is necessary. An example predetermined threshold of a 500 hundred Ohms may be used to determine that the water vapour concentration has reached a level unsuitable for the building materials.

In some examples, as shown in Figure 2B, the water-sensitive element 202 further comprises a dispersion of electrically conductive particles 208 (e.g., a powder having a median diameter of 10pm to a 1 mm) within the water soluble, electrically conductive polymer 204. As with Figure 2A, the polymer 204 acts as a binder to adhere the water-sensitive element 202 to the electrically conductive elements 104 of the electrical connection 100. In the example shown in Figure 2B however, the polymer also provides a matrix in which conductive particles 208 are mixed or dispersed. Increasing the volume fraction of the electrically conductive particles 208 increases the electrical conductivity of the electrical connection 100 (compared to the conductive polymer 204 without those particles 208). In the absence of water, the water-sensitive element 202 of Figure 2B therefore has a higher electrical conductivity than the water-sensitive element 202 of Figure 2A. As with Figure 2A, the polymer 204 at least partially dissolves in the presence of water, causing an electrical discontinuity 206 to form between conductive elements 104 and a corresponding sharp increase in resistance across the elements 104. In the example shown in Figure 2B, the separation between electrically conductive elements 104 is no less than 5mm.

Reducing the resistance of the water-sensitive element 202 is advantageous because less power is lost through Joule heating and less energy is required to power the electrical connection 100. In that regard, increasing the volume fraction of conductive particles 208 in the water-sensitive element 202 is beneficial. That said, there is an upper bound for the volume fraction of the conductive particles 208 because at sufficiently high-volume fractions (e.g., > 0.70), a percolation path of low resistance between conductive elements 104, comprising these conductive particles 208, may develop in either the dry state, or following dissolution of the polymer 204. Such a low resistance path would adversely affect the sensitivity of the water-sensitive element 202 to detect a change in resistance in the presence of water. In an example, the upper bound for volume fraction of the conductive particles 208 in the water-sensitive element is 0.35, 0.3, 0.25. Dependent on the conductive particle and polymer binder 204, volume fractions of conductive particles 208 larger than 0.35 are also detrimental to the binding efficiency of the particles 208 within the polymer binder 204.

In some examples, as shown in Figure 2C, the water-sensitive element 202 comprises a water-soluble polymer 204, having a conductive thin film 210 disposed on at least one of its surfaces, such that the conductive thin film is in electrical contact with conductive elements 104. The water-soluble polymer 204 acts as a binder to adhere the water-sensitive element 202 to the electrically conductive elements 104 of the electrical connection100. In this example, the polymer 204, may be either electrically insulating or conductive. An advantage of this approach is that conductive particles do not need to be incorporated/dispersed within the polymer 204, which, in practice, is difficult to achieve for all polymers 204.

In the absence of water, the water-sensitive element 202 has a high electrical conductivity, provided by the conductive thin film 210. However, in the presence of water, the polymer 204 dissolves and the conductive thin film 210 breaks. This develops an electrical discontinuity 206 between electrical elements 104, thereby resulting in a sharp increase in resistance across the connection 100. The conductive thin film is metallic and for example, comprises copper.

In another example, as shown in Figure 2D, the conductive thin film 210 is disposed within the water-soluble polymer 204. The water-soluble polymer 204 still acts as a binder to adhere the water-sensitive element 202 to the electrically conductive elements 104 of the electrical connection 100. The water-soluble polymer 204 provides a protective overcoat for the conductive thin film 210, such that scratching or other physical damage does not affect the thin film 210. The thickness of the overcoat can be tailored, such that the conductive thin film 210 breaks after a predetermined volume of liquid water dissolves the polymer 204 surrounding it. The sensitivity of detecting water is therefore controllable. This property may be desirable if the electrical connection 100 is used to detect a leak (which is associated with a certain threshold amount of leaked solvent). False positives which may result from high humidity conditions etc. may be effectively excluded.

In another example, as shown in Figure 2E, the water-sensitive element 202 comprises, in contrast to the above examples, a water soluble, electrically insulating polymer 212 (i. e. with a conductivity below 10S/m ² ) with a dispersion of electrically conductive particles 208 (e.g., powder having a median diameter of 10pm to a 1 mm) dispersed within the polymer 212. The polymer 212 acts as a binder to adhere the water-sensitive element 202 to the electrically conductive elements 104 of the electrical connection 100 and to provide a matrix in which the conductive particles 208 are mixed or dispersed. In the example shown in Figure 2E the separation between the electrical elements 104 is no larger than 5mm, more preferably no higher than 3mm.

In the example of Figure 2E, in the absence of water in contact with the watersensitive element 202, the electrical connection 100 has relatively low electrical conductivity. The measurable resistance (e.g., voltage drop for given current) across the connection 100 is therefore relatively high in the absence of water. Typical resistances are of the order of several kilo-ohms, for a spacing of 3mm between conductive elements 104.

However, in the presence of a sufficient amount of liquid water (e.g., a droplet), the water soluble, electrically insulating polymer 212 at least partially dissolves. However, unlike the examples described above, where dissolution of the polymer binder causes an electrical discontinuity 206 to develop between conductive elements 104, a low resistance path 214, which extends between the conductive elements 104 and comprising the conductive particles 208, develops. The low resistance path comprises aggregates of the conductive particles 208, which is able to extend across the gap between electrical elements 104 since their separation is small (no higher than 5mm). Dissolution of the electrically insulating polymer 212 therefore results in a sharp decrease in resistance across the connection 100. Typical resistances after dissolution are in the range of several or tens of ohms.

In any of the examples described above, the water-sensitive element 202 may be provided with a capping 302 (e.g., a polymer), as shown in Figure 3A and 3B. The capping 302 protects the solvent-sensitive element 102 from scratch damage.

In the example shown in Figure 3A, the capping 302 comprises a solvent permeable material (e.g., a thin membrane), such that liquid solvent and/or gaseous solvent is able to diffuse or percolate therethrough to reach the watersensitive element 202. In a specific example, the capping 302 is a porous material, such as a textile and around 200pm thick.

In the example shown in Figure 3B, the capping 302 comprises a mesh that provides one or more openings for ingress of the solvent to the water-sensitive element 202. In a specific example, the capping 302 is a polyimide membrane comprising a plurality of holes. The thickness of the polyimide membrane may be around 100pm. The capping material may be chosen such that the wetting angle of the solvent is less than 20 degrees, preferably less than 10 degrees, even more preferably less than 5 degrees, such that liquid solvent is able to pass through the openings. In other examples however, the wetting angle of the solvent on the capping 302 may be greater than 90 degrees. Such a wetting angle prevents ingress of liquid solvent through the openings and allows the electrical connection 100 to monitor for gas solvent concentration (e.g., water vapour concentration) without the issue of the water-sensitive element 202 dissolving in the presence of liquid water.

Example electrically insulating polymers 212 include: acrylics and polyurethanes. Such polymers 212 are typically hydrophobic and insoluble in water. The skilled reader will appreciate that ionic groups and/or hydrophilic chains may be added or substituted within the polymer structure to improve water solubility.

Example water soluble, electrically conductive polymers include: polyethylene glycol, polyacrylamides, polyacrylic acid copolymer, polyvinyl alcohol, or blends thereof. These polymers or polymer blends may be doped and/or conductive particles added to them to enhance conductivity.

Example water soluble polymers include: polyethyloxyazoline, polyvinyl pyrrolidone, polyvinyl alcohol, polyacrylamide, polyglycols and polyacrylic acid, Polylactic acid (PLA), Polyvinylidene fluoride (PVDF), or blends thereof.

Example conductive particles 208 include: carbon-based particles such as carbon black (e.g., graphite), silver, copper or the like. Silver has excellent conductivity (6.30x10E7) but it is relatively expensive. Copper has an excellent trade-off between moderate cost and a high conductivity (5.96x10E7S/m). Carbons have lower conductivities (3 10xE5 S/m) but are cheap. Example electrically conductive polymers include doped variants of: polyacetylene, polyaniline, polypyrole, polythiophene, poly(para-phenylene), poly(phenylenevinylene) (PPV), and polyfuran).

Of the water-soluble polymers mentioned above, polyethylene glycol, polyacrylamides, polyacrylic acid copolymer, and polyvinyl alcohol have particularly advantageous characteristics in that they are: non-toxic; and have the ability to dry when wet at room temperature in less than an hour, or even less than 15 minutes.

Turning now to Figure 4A and 4B, a sensing circuit 400, comprising a power supply 402 and a plurality of solvent-sensitive electrical connections 100 arranged in parallel or series, is shown. Any of the structural arrangements 200 of the solvent sensitive element 102 in the electrical connections 100 described above may be used in the sensing circuit according to any possible combination. The solvent sensitive element 102 constitutes a sensing element for a particular solvent. The example shown in Figure 4A and 4B shows each electrical connection 100 being different but it will be understood by the skilled reader that each electrical connection 100 may conveniently be of the same type. The power supply may be an on-board power supply (e. g. a battery) or mains power supply, optionally connected via a power adaptor.

If the electrical connections 100 are arranged in series (as shown in Figure 4B), a low resistance across each electrical connection 100 is preferred as then less input power is required to operate the sensing circuit 400, saving energy. Electrical connections 100 comprising copper particles 208 dispersed in an electrically conductive polymer 204 are particularly suitable in this regard.

If the electrical connections 100 are arranged in parallel, a diode may be placed downstream of each electrical connection 100 to protect the sensing element/ electrical connection 100. Optionally, an electronic switch may be provided for each electrical connection 100. Such an arrangement allows selective powering of the electrical connections 100 in any combination. An advantage of this approach is therefore improved controllability: individual solvent-sensitive electrical connections 100 can be assessed to determine whether they are in the presence of their solvent, based on the induced current and applied voltage in the sensing circuit 400.

Turning now to Figure 5, a matrix arrangement 500 of electrically conductive elements 104, comprising three rows and three columns is shown. In general, there may be m rows and n columns in the matrix arrangement 500. The matrix arrangement further comprises: a solvent sensitive electrical connection 100 and a protective diode (downstream of the solvent sensitive electrical connection 100) provided to electrically connect each row with each column. Otherwise, the conductive elements 104 are not electrically connected with one another. The matrix arrangement 500 forms part of a sensing circuit with a power supply (not shown).

In the specific example illustrated, the number of solvent sensitive electrical connections 100 is equal to nine (mxn). The electrical connection 100 that connects row m to column n is denoted (m, n) herein.

The skilled reader will appreciate that m and n may take any integer value (e.g., 1x5, 5x10, 10x5, 20x20). The matrix may therefore define a linear array. The spacing between the rows and columns of the matrix arrangement 500 may be 5cm to 1 m, specifically 10cm to 30cm, more specifically 15 to 20cm. The exact number and spacing between rows and columns is set according to the needs of the environment being monitored.

The matrix arrangement 500 shown in Figure 5 allows selective monitoring of an electrical connection (m, n) via the application of a potential difference across row m and column n. This approach does not require electronic switches which is advantageous. By monitoring each electrical connection 100 in the matrix arrangement, it can be determined whether solvent is present within the area covered by the matrix arrangement 500 and, if applicable, the area and/or location of that solvent. Each solvent-sensitive electrical connection 100 may be monitored periodically (e.g., every hour or once or day). In some examples, if solvent is detected, neighbouring electrical connections 100 to that connection are selected for monitoring to more rapidly determine the area where solvent is present (i.e. , the extent of the leak).

The spatial sensitivity (in the lateral and vertical direction) of determining the presence of the solvent can be improved by reducing the spacing between the rows and columns in the matrix arrangement 500. However, the electrical power required to operate a matrix 500 of a given area increases with decreasing spacing between rows and columns and the total time to monitor all the electrical connection 100 also increases. In practical examples therefore, the matrix 500 may be up to several square metres in area, comprising up to 20 rows and columns, having a spacing of 15cm.

The working principle of selectively monitoring electrical connection (1 , 1 ) to determine whether solvent is present at that location is described below. The skilled reader will understand that the working principle extends to any electrical connection (m, n).

If a potential difference V2 - Vi is applied across column 1 (C1 ) and a potential difference V4- V3 is applied across row 1 (R1 ), then the potential difference across the electrical connection (1 , 1 ) and protective diode is equal to V4-V1. A current respectively flows through column 1 (denoted I21), row 1 (denoted I43) and the electrical connection (denoted I41) from column 1 to row 1 , provided that the magnitude of each respective potential difference is greater than zero. The current measured in row 1 is the sum of I43 and I41.

As described above, in the absence of the solvent, the electrical connection 100 may either have a relatively low (~ several 10s of Ohms) or very high electrical resistance (open circuit condition), depending on the implementation. An electrical connection 100 having a low resistance induces a measurable current (I41 ) that contributes to the current that can be measured in row 1 . Conversely, if the electrical connection 100 has very high resistance, current cannot flow through it, and I41 is therefore negligible.

In the presence of the solvent, the opposite is true. The resistance of the electrical connection 100 either sharply increases (as described in relation to examples 2A to 2D), such that no current can pass through it, or, the resistance of the electrical connection 100 sharply decreases, such that a current can pass through it (as described in relation to example 2E). The current induced in row 1 in the presence and absence of the solvent therefore differs by a magnitude approximately equal to I41. The difference in current can be detected and hence it can be determined whether solvent is present or absent at that location corresponding to the electrical connection (1 , 1 ) in the matrix arrangement 500. This process can be repeated for each of the other electrical connections (m, n).

Specifically, the current induced in row 1 increases by approximately I41 in the presence of solvent, if the solvent-sensitive electrical connection 100 has a very high resistance in the absence of its solvent and has a comparatively lower resistance in the presence of its solvent. Alternatively, the current induced in row 1 decreases by approximately I41 in the presence of solvent, if the solventsensitive electrical connection 100 has a comparatively low resistance in the absence of its solvent and has a very high resistance in the presence of its solvent.

One or more solvent-sensitive electrical connections 100 and/or conductive elements 104 may be fabricated on various substrates. The conductive elements 104 may be applied to the substrate by any method known to the skilled reader (e.g., deposition, printing etc.). A solvent-sensitive element 202 is then deposited or printed within the gaps between the conductive elements 104 to thereby create the solvent-sensitive electrical connections 100. The solvent-sensitive element 102 may be deposited as a solution. The solution may comprise the solvent-sensitive polymer (which may or may not be electrically conductive) at least partially dissolved in a suitable solvent, and optionally having conductive particles 208 mixed/dispersed therein. It is emphasised that the solvent in the solution may be different to the solvent to which the sensing circuit is supposed to detect. For example, the solvent in the solution may be an alcohol and the solvent to be detected is water.

During deposition, a controlled volume of the solution is deposited onto specific locations on the substrate (i.e., between conductive elements 104) to form the electrical connection. As the solvent present in the solution evaporates, the solvent-sensitive polymer begins to mechanically adhere to the electrical elements 104 to establish the connection 100. More volatile solvents (e.g., alcohols cf. water) for the solution are beneficial in this regard.

Example deposition methods include inkjet printing, screen printing or the like.

Example substrates include: a rigid polymer or composite (e.g. a PCB), a building component (e.g., bricks, plaster board, wall tiles, concrete, waterproof membranes, wood, pipes or the like); or a flexible polymer (e.g., polyethylene terephthalate)). A polymer substrate is considered flexible if it is sufficiently compliant to be applied onto an object having a non-planar surface. It is preferred, although not essential, that the substrate comprises a material that does not dissolve in the presence of the solvent/analyte.

Using the building component as a substrate, or using a flexible substrate is advantageous, because sensing circuits 400 may be applied to objects with non- planar surfaces (e.g., pipes, and in particular, at pipe joints). A thin polymer substrate with a thickness less than 0.3mm may be regarded a flexible substrate. This is particularly beneficial when applying the sensing circuit to water pipes or joints. In general, it is easier to deposit the solvent-sensitive element 102 onto a substrate (e.g., a flexible polymer substrate), which is later applied to a building component using an adhesive, than applying the element 102 directly to a building component.

In some examples, the substrate is subjected to a surface treatment such as polishing prior to application of the solvent-sensitive element (e.g., on a flexible polymer substrate) 102, 202 and/or conductive elements 104 to ensure good adhesion.

In some examples, the rigid or flexible polymer substrate comprises an adhesive backing sheet integrated on the opposing side to the sensing circuit 400. Prior to installation (e.g., on a building component), the backing sheet is peeled off by the user and the substrate with sensing circuit 400 applied (e.g., onto a building component), as appropriate.

In some examples, the flexible polymer substrate, solvent-sensitive electrical connections 100 and/or conductive elements 104 are manufactured using a roll- to-roll (also known as a “reel-to-reel”) process. This processing route has excellent throughput. The sensing elements may be produced on the polymer reel at uniform intervals (e.g., every 10cm or less). Depending on the use scenario, a length of printed sheet can then be cut, as required. An example polymer reel size is 20mm by a kilometre in length. In some examples, different sensing circuit 400 arrangements (e.g., series, parallel, individual) may be printed on a particular printed sheet and then cut to size, as required. For example, a small area may be required for monitoring a pipe joint. A large area may be required for monitoring a roof.

Referring back to Figure 5, in some examples, each electrical connection 100 that connects a row to a column in the matrix arrangement 500 is fabricated or printed on a separate PCB module, which can be connected and disconnecting from the sensing circuit, thereby enabling convenient replacement of the electrical connection 100. In an example, the PCB has a footprint of up to 10mm by 10mm. The replaceable PCB module may include solder bumps for connecting to the sensing circuit. This allows replacement of individual electrical connections 100 without having to replace the whole array.

The sensing circuits 400, 500 described above may further comprise a sensor panel having any one or more of the following auxiliary components: an alarm, light element (e.g., LED) and/or an RF antenna, which are activated (i.e., the alarm sounds, the light element is powered on and the RF transmits a signal) if solvent is detected by the sensing circuit 400, 500. In some examples, the sensor panel is configured to activate in response to a resistance of a solvent-sensitive electrical connection 100 changing or being greater than a predetermined threshold (e.g., two kilo ohms for liquid water and a hundred ohms for water vapour) that is indicative of solvent ingress. This applies for Figures 2A to 2D. The reverse is true for Figure 2E. That is, the sensor panel is configured to activate in response to a resistance of the solvent-sensitive electrical connection 100 changing or being less than the predetermined threshold. The sensing panel comprises an electronic controller, which is configured to operate the auxiliary components in response to the predetermined threshold being met. This threshold can be set according to desired sensitivity. For example, the resistance could be set to a relatively low value if the humidity sensing is required, or higher if liquid solvent detection is required.

The RFID of the RF antenna may be used to identify the RF antenna and correspondingly the sensor location. The RF antenna may transmit a signal to a mobile or a laptop or a server (e.g., via WiFi or Bluetooth), which is monitoring the status of the sensing panel. Each sensing circuit 400, 500 may comprise one or more sensing panels. The sensing panels may either be provided on the same substrate as the sensing circuit or a different substrate but electrically connected thereto. Referring to Figure 6, the sensing circuit 400, 500 may include a metal oxide semiconductor (MOS) device 600 electrically coupled to the solvent-sensitive electrical connection 100 described above. The MOS device 600 comprises a metallic gate 602, metal oxide insulator 604, a source 606, a drain 608 and a semiconductor body 610 defining a channel for charge carriers between the source and drain. The working principle of a MOS device is known to the skilled reader. The sensing circuit shown 400, 500 including the MOS device can be applied to any of the sensing circuit configurations described above.

In the presence of the solvent, the resistance of the solvent sensitive element 102 changes, which results in a change in the voltage applied to the gate of the MOS device 600. As the voltage changes across the gate, the current flowing from the source to drain correspondingly changes. A controller may monitor the sourcedrain current in order to determine whether the solvent-sensitive element 102 is in the presence or absence of solvent.

For example, in the presence of its solvent, the solvent-sensitive element 102 may partially dissolve forming an electrical discontinuity, resulting in a greater potential drop across the solvent-sensitive element 102. Hence, the voltage across the gate decreases and the source-drain current changes accordingly. It will be understood by the skilled reader that the resistance of the solvent-sensitive element 102 may decrease in the presence of its solvent (e.g., as in Figure 2E) and the source-drain current will vary in an analogous manner.

This configuration is advantageous for highly oxidising solvents (e.g., those which contain chlorine) because the solvent-sensitive element 102 may be spatially separated from the MOS device. Components of the MOS device (e.g., its substrate) may therefore be protected/encapsulated to prevent damage from that solvent, whilst still enabling solvent detection.

In another example, the sensing circuit 400, 500 includes a metal semiconductor (MES) device, which operates in an analogous manner. Turning to Figure 7, the solvent-sensitive electrical connection 100 and structural arrangements 200 thereof may comprise a wicking layer 702 in contact with the solvent-sensitive element 102 and adapted to transport any solvent (e.g., water) present in the vicinity of the solvent-sensitive element 102 to the solvent-sensitive element 102 through capillary action. The sensing range of the solvent-sensitive electrical connection 100 therefore increases (from the approximate area of the sensing element 102 to the area of the wicking layer 702) and leaks can be detected more reliably. In a specific example, the wicking layer is a strip of cotton or paper having a length of around 20cm and a width of 1 cm.

Other solvents

The examples above are described in relation to water as a solvent. However, other solvents and solvent sensitive elements are also possible. It is noted that some solvent-sensitive elements 102 may at least partially dissolve in more than one type of solvent.

As described above, the solvent-sensitive element 102 comprises a polymer binder 204, 212 (which may or may not be electrically conductive). It will be understood that the polymer 204, 212 in the solvent-sensitive element 102 selected is based on its solubility in the solvent which the sensing circuit 400, 500 is configured to detect.

Table 1 shown below summarises the polymers 204, 212 (and their blends thereof) which are suitable for detecting the presence of particular analyte solvents in a sensing circuit 400, 500.

Solvent-sensitive elements 102 sensitive to alcohols (e.g., ethanol), or aromatic compounds (e.g., benzene, toluene, xylene) would be useful in the alcoholic beverage industry or the petrochemical industry respectively. Table 1

Each of the structural arrangements for the solvent-sensitive element 102 described above in relation to a water-sensitive element 102 applies, regardless of solvent. The details are not repeated here.

As chlorine is a strong oxidiser, chlorine gas and hydrochloric acid readily react with many polymers, disintegrating them in the process. Polymers such as Acrylonitrile butadiene styrene, polyether ether ketone, polycarbonate, polyethylene and polypropylene have reasonable chemical resistance to chlorine and these polymers are therefore suitable substrate materials.

Use cases

The sensing circuits 400, 500 comprising the solvent-sensitive electrical connections 100 above may be used in a variety of applications where solvent detection is desirable. In general, matrix arrangements as shown in Figure 5 are suitable for covering large areas (e.g., greater than 1 m ² ), whereas, series or parallel arrangements are suitable for smaller areas. A particular use case is the building industry; both during construction and after construction.

Most, if not all, building constructions have a concrete foundation. During pouring of the concrete, voids may develop within the concrete that trap water within the set concrete. As cement is porous, the trapped water can percolate through it, causing damage once the building construction has been completed and at a time where intervention is unpractical and extremely expensive. The sensing circuits 400, 500 of the present disclosure may be applied to the concrete foundation in order to determine whether water is present at a level indicative of a leak from a water void within the concrete. The quality of the cement and hence the foundation may therefore be assessed prior to completion of the building. This saves cost in the event that the concrete is of poor quality. If it is determined that a water void of sufficiently large volume is present within the concrete, the concrete foundation may be replaced. The sensing circuits 400, 500 may also be left in-situ for future monitoring of the concrete foundation.

The skilled reader will appreciate that concrete is used in other phases of building construction and the sensing circuit described in the present disclosure may also be applied to that concrete to monitor its quality and/or water leaks.

Example building constructions include: water tanks; buildings, in particular basements, roofs, and balconies; swimming pools; and concrete-lined storm water channels, or the like.

Water leaks in chimneys are a major concern being exposed to every kind of weather conditions. Leaking chimney is akin to any other leak on roof as well, and a potential cause of water damage or damp problems within the building.

In particular, leaks are prevalent around chimney flashing and the chimney crown. Chimney flashing is a waterproof seal to protect the chimney and roof from water penetration and damage. Flashing is built around the base of the chimney. If not properly built, the flashing can come loose and allow water (rain water) ingress into the chimney walls, eventually causing damage to internal walls in the building. In such an example, the sensing circuit 400, 500 comprising the solventsensitive electrical connections 100 are produced on a flexible substrate, which is appropriately shaped to match the area of the flashing. The flexible substrate is then applied to the flashing area using a retaining element (e.g., adhesive, screws or the like).

The chimney crown is a cement covering on top of the chimney. The reason for a chimney crown is to prevent rain, snow and other wintery elements from getting into the chimney and masonry.

Over time the crown can become damaged, caused from shifts in the structure or shrinkage. The impact of hot and cold weather can cause the crown to expand and contract, causing movements over the years that can lead to crack, which grow over time until they allow water ingress. Such water ingress may damage the lining of the chimney, cause blockages in the flue, and lead to issues with brickwork (e.g., loosening).

In some examples, the solvent-sensitive electrical connections 100 in the sensing circuit 400, 500 are applied under roof tiles, roof flashing and/or, where applicable, on the roof-facing side of a waterproof membrane. As the area is planar and relatively large, matrix arrangements 500 for the sensing circuit may be particularly suitable.

In some examples, the solvent-sensitive electrical connections 100 in the sensing circuit 400, 500 are applied under around window frames. Series or parallel arrangements may be particularly suitable in this case.

In some examples, the solvent-sensitive electrical connections 100 in the sensing circuit 400, 500 are applied beneath flooring (e.g., floorboards, stone slabs etc.). As the area is planar and relatively large, matrix arrangements 500 for the sensing circuit may be particularly suitable.

In some examples, the solvent-sensitive electrical connections 100 in the sensing circuit 400, 500 are applied behind tiles in a bathroom (e.g., on plasterboard), in particular tiles adjacent to a shower. As the area is planar and relatively large, matrix arrangements 500 for the sensing circuit may be particularly suitable.

In some examples, the solvent-sensitive electrical connections 100 in the sensing circuit 400, 500 are applied around pipework. In particular, i) near pipe couplings, elbow joints ii) bathtubs, showers and iii) water-based household appliances. Series or parallel arrangements either applied directly to the pipework, or via a flexible substrate, may be particularly suitable in this case.

In some examples, the solvent-sensitive electrical connections 100 in the sensing circuit 400, 500 are applied behind walls within a building. As the area is planar and relatively large, matrix arrangements 500 for the sensing circuit may be particularly suitable.

Other use cases include detecting the water moisture content in breath, the moisture content (in the air and/or through condensation), exudes from body wounds and urine leakage in nappies.

The invention has been described in detail with reference to the examples. Modifications may, however, be made without departing from the scope of the invention as defined by the claims. Each feature disclosed or illustrated may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.