WO2022096081A1

WO2022096081A1 - Wound dressing

Info

Publication number: WO2022096081A1
Application number: PCT/EP2020/080822
Authority: WO
Inventors: Andrew Mills
Original assignee: Clonallon Laboratories Limited
Priority date: 2020-11-03
Filing date: 2020-11-03
Publication date: 2022-05-12

Abstract

The present invention relates to a dressing for monitoring a wound and for displaying whether the wound is infected, wherein the dressing is for placement onto a wound and comprises a colourimetric sensor adapted to change colour in response to a predetermined amount of analyte emitted from the wound.

Description

WOUND DRESSING

The present invention relates to a wound site monitor, in particular to a dressing for placement on a wound dressing and to a wound site monitoring method.

Background to the Invention

Chronic wounds are wounds that do not heal in a typical, predictable manner. Treatment of chronic wounds costs the UK alone billions of dollars per annum. Thus, treatment of chronic wounds places a considerable burden on a country’s health services. It is estimated that 1- 2% of the US population will be affected by a chronic wound during their lifetime.

Whether a dressed wound is infected or not may not always be apparent unless the dressing is removed and a key factor in the treatment of a chronic wound is the need to frequently monitor the wound site, which typically requires the removal of the wound dressing for a visual inspection, coupled with the taking of samples before redressing the wound. A swab of the wound is usually taken and submitted to microbiologists for culture. This is a time consuming and costly process. Furthermore, dressing removal can result in contamination of or further trauma to an otherwise infection free wound.

It is a therefore an object of the present invention to provide a wound dressing which allows for clinical assessment of the underlying wound to be self-evident, making the process much more efficient.

Summary of the Invention

The present invention relates to a dressing for placement on a wound dressing to allow monitor of the wound site without the need for removal of the dressing.

Accordingly, a first aspect of the present invention is a dressing for monitoring a wound and for displaying whether the wound is infected, wherein the dressing is for placement onto a wound and comprises a colourimetric sensor adapted to change colour in response to a predetermined amount of analyte emitted from the wound. By placing the dressing according to the invention onto a wound, the clinical assessment of the underlying wound becomes self-evident and the process of wound site monitoring is much more efficient by allowing a rapid assessment of a wound without the need to physically remove the wound dressing and thereby potentially infecting an uninfected wound.

As the dressing is for placement onto a wound, no direct contact with the wound or wound exudate is required in order to display whether or not the wound is infected. The dressing allows detection of the presence of an analyte as a result of infection in the absence of any physical signs and is thus useful in the management of IV-line infections as signs of infections at cannulation sites are usually absent.

In a preferred embodiment, the dressing comprises a transparent polyurethane film, particularly preferably wherein the colourimetric sensor is chemically welded onto the polyurethane film. This allows the analyte emitted from the wound to diffuse through the polyurethane film and into the colourimetric sensor and allow the colourimetric sensor to change colour with increasing analyte concentrations generated from an infected wound.

The dressing is preferably in the form of an adhesive plaster.

The dressing may be of any suitable size larger than the wound on which it is to be placed. For example, the dressing may measure about 7.2cm in length and about 2.5cm in width. However, such measurements are not be considered limiting.

In a preferred embodiment, the wound analyte is selected from among one or more of the lists consisting of carbon dioxide (CO2), a volatile amine such as cadaverine and putrescine, ammonia, volatile sulphides, oxygen and moisture.

In a preferred embodiment, the required predetermined amount of analyte emitted from the wound to ensure colour change of the colourimetric sensor is from about 0.04% w/w.

In a preferred embodiment, the dressing comprises a plurality of colourimetric sensors, each independently tuned to monitor a specific wound analyte above the wound dressing and change colour in relation to the analyte being monitored. Preferably, the colourimetric sensor is a reversible colour indicator which responds to the presence of one or more analytes associated with an underlying wound infection by colour change.

In a preferred embodiment, the colourimetric sensor is a plastic film comprising pigment particles with a hydrophilic silica core and a dye coating, particularly preferably wherein the colourimetric sensor is extruded between two monolayers of filament.

In a preferred embodiment, the dye coating comprises a dye selected from among xylenol blue (XB), thymol blue (TB), meta-cresol purple (m-CP), phenolphthalein, bromophenol blue (BPB), bromochlorophenol blue (BCPB) and chlorophenol red (CPR).

Preferably, xylenol blue (XB) is used to detect CO2. Alternatively, or additionally, one or more of bromophenol blue (BPB), bromochlorophenol blue (BCPB) and chlorophenol red (CPR) are used to detect putrescince vapour.

In a preferred embodiment, the plastic film is low density polyethylene (LDPE).

Brief Description of the drawings

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

Figure 1 shows a cross-section view of the interaction of a preferred dressing according to the invention with a wound tissue;

Figure 2 shows a schematic diagram of the key features of a CO2 colourimetric sensor for use in the dressing of Figure 1 ;

Figure 3a shows a plot of the measured absorbance at 621 nm of TB vs. % CO2 in a dry gas stream;

Figure 3b shows a plot of the measured absorbance nB (RGB) of TB vs. % CO2 in a dry gas stream; Figure 4 shows a schematic of an experimental setup for a CO2 colourimetric sensor for use in the dressing of Figure 1 ;

Figure 5 shows the change in colour of the CO2 colourimetric sensor in Figure 4;

Figure 6 shows FTIR calibration for CO2 in a custom-made FT-IR gas cell (internal volume 73 mL) with injections of known volumes of CO2.

Figure 7 shows an experimental setup for an alternative CO2 colourimetric sensor for use in the dressing of Figure 1 ;

Figure 8a shows the change in colour of the CO2 colourimetric sensor in Figure 6;

Figure 8b shows the exploded view of Figure 8a over the region that CO2 absorbs strongly; and

Figure 9 shows a plot of nB (RGB) vs. % CO2 for the experimental setup of Figure 6.

Detailed Description

Various embodiments of the present invention will be described in detail with reference to the drawings, where like reference numeral represent like parts and assemblies throughout the several views.

Referring to the drawings, Figure 1 shows a preferred dressing 100 according to the invention for monitoring a wound and for displaying whether the wound is infected. As shown, dressing 100 is for placement onto a wound W and comprises colourimetric sensor 20 adapted to change colour in response to a predetermined amount of analyte A emitted from wound W.

Dressing 100 is in the form of an adhesive plaster and comprises a transparent polyurethane film 10, wherein colourimetric sensor 20 is chemically welded onto film 10.

Colourimetric sensor 20 a reversible colour indicator which responds to the presence of one or more analytes associated with an underlying wound infection by colour change and is made of LDPE film 30 embedded with pigment particles 31 extruded between two monolayers of filament. Particles 31 have a silica core 32 and are coated with dye 33 selected from among xylenol blue (XB), thymol blue (TB), meta-cresol purple (m-CP), phenolphthalein, bromophenol blue (BPB), bromochlorophenol blue (BCPB) and chlorophenol red (CPR).

CO2 colourimetric sensor

The CO2 colourimetric sensors (examples 1 , 2 and 3) tested are as colourimetric sensor 20 described above, with dye 33 being TB, m-CP and XB, respectively. The key features of a CO2 colourimetric sensor are shown in Figure 2.

The colour change is reversible and entirely dependent on the local concentration of CO2. The ratio of protonated dye (DH) to deprotonated dye (D-) embedded in the polymer core is dependent on the percentage CO2 in the headspace.

The transformation of the dye from the deprotonated to protonated form due to the presence of CO2 may be measured according to equation 1 :

R = [DH]/[D-] = ax% CO₂ (1) where [DH] is the concentration of the protonated form of the dye;

[D-] is the concentration of the deprotonated form of the dye; a is a proportionality constant (unit: %'¹) which provides a measure of the sensitivity of the CC>2-sensitive optical sensor under test; and

R is a measure of the transformation of the dye from the deprotonated to protonated form due to the presence of CO2.

By sensitivity is meant the percentage of CO2 required to change a colourimetric sensor by 50%, i.e. S = For example, for a blue to yellow indicator such as CO2 colourimetric sensor example 1 , the S= % value corresponds to the level of CO2 necessary to turn the initially blue film to green (i.e. 50% blue pigment and 50% yellow pigment).

Two analytical methods were used for determining the sensitivity of CO2 colourimetric sensor examples 1 , 2 and 3 at ambient temperature (22°C). The colour change of CO2 colourimetric sensors examples 1 , 2 and 3 was monitored by both UV-vis and by RGB analysis of digital photography. RGB analysis of digital photographs is a much faster and cheaper technique compared to UV-vis spectroscopy and precludes the need for highly trained technical staff thereby allowing out-of-lab application of the technique.

The two methods were correlated, allowing subsequent analysis of the colour change of CO2 colourimetric sensor examples 1 , 2 and 3 by RGB analysis only, confident in the knowledge that it gives the same information as the more sophisticated, and expensive, UV-vis spectrophotometric method.

The sensitivities of CO2 colourimetric sensor examples 1 , 2 and 3 were also analysed at elevated temperatures of 30°C. Colourimetric sensor 23 was identified as an ideal candidate, as it undergoes a striking colour change from blue to yellow (via a strong green transition colour) and at a high sensitivity towards CO2 (S = 1 of 0.143% at 32°C).

All chemicals were purchased from Sigma Aldrich, unless otherwise stated, and in the highest purity available. All solutions were freshly prepared and all aqueous solutions were made up using double distilled, deionised water. All solutions for microbiological CO2 generation were prepared fresh and sterilised in an autoclave prior to use. The gases used were from BOC gases. The LDPE powder (MFI = 20) was supplied by PW Hall UK and the LDPE for film preparation (MFI = 4) was supplied as pellets by Ultrapolymers UK. The SiO2 (Aerosil® 130 hydrophilic fumed silica) was from Evonik (BET surface area = 130 ± 25 m2 g-1).

All digital images were captured using a Canon EOS 700D equipped with an EFS 18-135 mm lens and analysed for their R, G and B components using freely available software (Fiji Imaged). UV-vis spectrophotometric measurements were recorded using an Agilent Technologies Cary 60 UV-vis spectrophotometer.

To a 120 mL jar containing 2 g of Aerosil® 130 hydrophilic fumed silica and 100 mL double distilled, deionised water, 0.2 g of thymol blue (TB), meta-cresol purple (m-CP) or xylenol blue (XB) and 3.1 mL of tetrabutyl ammonium hydroxide (TBAOH, 40% in water) was added. The mixture was sonicated for 10 minutes in an ultrasonic bath to break up the larger particles of silica and dye and the resulting suspension was subsequently vigorously stirred for 2 hours using a magnetic stirrer. The suspension was dried using a Buchi Mini Spray Dryer B-290, giving a dry powder pigment (typical yield, 2.5 - 3 g). The prepared pigments were immediately processed into an extruded polymer film as detailed below.

Extruded plastic indicator

The powder pigment was blended in a 1 :9 ratio with fine LDPE powder until the total volume was of a uniform colour (for each of the polymer films prepared). The 10% pigmented polymer powder was pelletised using a Rondol Microlabs Twinscrew extruder, with operating temperatures of 90, 115, 125, 135 and 125°C for the feed, zones 1-3 and pelletising die, respectively, and a pigment/LDPE mixture feed hopper rate of 41 rpm. The extruder screw speed was 80 rpm and the pelletizer speed was 0.5 m min^-1. The resulting pellets were re-pelletised, subsequently diluted using LDPE pellets to 5% pigmentation and run through the pelletiser a further 2 times to ensure even dispersion of the pigment. The pellets were then extruded into a thin film (ca. 42 ± 2 pm) with 5% pigmentation using the extruder, with operating temperatures of 90°C, increasing to 110°C, 125°C, 135°C and finally 140°C for the feed, zones 1-3 and the sheet die, respectively. The film was cut into 25 mm diameter discs for testing as colourimetric sensors.

Ink film

Ink films were prepared by first preparing a polymer solution of 5% wt. ethyl cellulose. Briefly, 5 g of ethyl cellulose (48% ethoxyl) was dissolved into 100 mL of an 80/20 toluene/ethanol mix. To a 10 mL batch of the resultant polymer solution, 1 mg/mL of pH sensitive dye selected from bromophenol blue (BPB), congo red (CR), bromochlorophenol blue (BCPB), bromocresol green (BCG), cochineal (C), chlorophenol red (CPR) and bromocresol purple (BCP) was added and the mixture was stirred until the dye was dispersed throughout. The resultant ink was cast as a film by drop coating onto a 25 mm diameter PET disc, giving a dried ink film thickness of about 4 pm for testing as a colourimetric sensor.

Sensor Calibration

To measure the sensitivity of the colourimetric sensor towards CO2, each colourimetric sensor independently was subjected to a constant stream of gas with a known concentration of CO2, achieved by blending high CO2 air mix (containing 1 % or 5% CO2) with argon using an aluminium/glass gas blender (model IL 60714, Cole Parmer, UK) in different ratios, maintaining a total flow rate of 100 mL min^-1.

The gases were passed through a gas cell containing a 25 mm disc of the colourimetric sensor under test, until the colour of the colourimetric sensor, monitored by UV-vis spectroscopy (Cary 60 UV-vis spectrophotometer, Agilent Technologies) and digital photography, stabilised.

The samples were tested at 30°C (32°C for colourimetric sensor example 3) by purging gases into a custom cell (internal volume 73 mL, not shown) for 10 minutes. The cell was then sealed and placed in an oven at 30°C (32°C for colourimetric sensor 23) for 1 hour before a digital image was taken for subsequent analysis.

The colour change of the colourimetric sensor was monitored by both UV-vis and by RGB analysis of digital photography.

Each digital photographic image of the colourimetric sensor was separated into its R, G and B colour components for each pixel, which each having a value of 0 to 255. The colour component that showed the most striking change (blue, i.e. B, in the case of TB and m-CP) was selected as the monitored colour. In order to eliminate any variation in lighting, which could potentially distort the observed colour, the B component was normalised, with respect to the other component values, using equation 3 to give nB (RGB), used here as a replacement for absorbance, as a direct measure of the concentration of the deprotonated form of the dye, D^_: nB (RGB) = — — (3)

Correlating UV-vis to RGB data for sensitivity measurements at 22°C

TB film colourimetric sensor example 1 was exposed to different levels of CO2 and in each instance both its absorbance (at 621 nm, due to Dj and its photographic image, were recorded. Using this data, the plots in Figures 3a and 3b were constructed, namely a plot of absorbance (at 621 nm, i.e. Figure 3a) and normalised blue, nB (RGB), Figure 3b, vs. % CO2 in a dry gas stream. The absorbance data illustrated in the main plot in figure 3a was used to generate the insert plot of R vs % CO2, based on equation 1 above and the following expression: _ Abs₀-Abs ,4.

Abs- Abs_inf ' ' where Ab_so and Absmf is the absorbance of the colourimetric sensor exposed to 0 and 100% CO2, respectively.

The normalised RGB data illustrated in the main plot in figure 3b was also used to generate the insert plot of R vs % CO2, based on equation 1 above and the following expression:

R ₌ nB₀—nB nB- nB_inf

Each inset diagram reveals a good straight line. The % CO2 sensitivity (S = 2) of colourimetric sensor example 1 was calculated by taking the reciprocal of the gradient. For the thymol blue sensor tested, the calculated sensitivity, S = 2, was found to be very similar for both analytical methods, i.e., 0.105 and 0.109% using UV-vis spectrophotometry and digital photography, respectively, at 22°C.

This close correlation in calculated sensitivity values for colourimetric sensor example 1 means that the two analysis methods are directly correlated, i.e. absorbance nB. Digital photography can therefore be used as the method of analysis of the films, as it is a faster, cheaper and easier method of analysis than UV-vis spectroscopy and offers great potential for application outside of a laboratory environment using mobile phone technology.

The correlation experiment was also repeated for m-CP film colourimetric sensor 22, the results of which are summarised in Table 1 , below and once again show that the absorbance is proportional to nB and so simple colour analysis of photographic images can be used as a substitute for UV-vis spectroscopy.

Table 1 - sensitivity testing of extruded polymer film CO2 colourimetric sensor examples 1 and 2 at 22°C UV-vis data Digital image data

Thymol Blue 9.54 0.105 9.11 0.109

Film sensitivity measurements at 30°C

In light of the fact that any wound monitoring will be normally at a temperature above room temperature and typically at 30°C, the sensitivities of colourimetric sensor examples 1 and 2 were assessed at this elevated temperature. Table 2, below, shows the results for calculated sensitivities, S = 14, for colourimetric sensor examples 1 and 2 at 30°C, analysed using the digital photography method.

Table 2 - Results for sensitivities of TB and m-CP film colourimetric sensors at 30°C

Digital image data

Thymol Blue 1.34 0.746

A brief comparison of the results in Tables 1 and 2 reveals that the sensitivity of colourimetric sensors 21 , 22 decreases by a factor of 7-10 when colourimetric sensor examples 1 and 2 are used at 30°C, rather than 22°C.

Ex vivo porcine skin burn wound models

All work involving the porcine skin burn wound model was carried out under aseptic conditions, and all appliances were sterilised prior to use. Porcine skin was chosen for its percutaneous similarity to human skin. The porcine skin was ethically sourced from stillborn or miscarried pig foetuses provided directly by local farmers and prepared for harvesting by rinsing with cold water to remove any afterbirth and dirt. The areas marked for collection were shaved using Bic 2 Sensitive Disposable razors and sterilized with a 1 % Virkon solution. The porcine skin was traced by making a superficial cut from the median plane behind the head, down to the shoulder, tracing around the shoulder bone, and continuing towards the posterior hip of the pig, above the nipples. At the posterior hip, the cut turned back, returning to the median plane of the pig. Beginning at the cut around the shoulder, the skin was peeled away from the pig as far as possible while teasing away the connective tissue joining the muscle and epithelial layers. Both flanks of the pig were removed in this fashion before being stored in a freezer until use.

The porcine skin was defrosted when needed, and any subcutaneous fat and connective tissue or muscle was carefully removed from the underside of the skin. Any remaining hair was shaved until the skin was smooth. 3 mm diameter circular burns were created using a sterilised brass heating block which was heated to 100°C and placed onto the skin, then weighed down with a 500 g weight for 20 seconds. A 14 mm cork borer was used to excise circular porcine skin explants with a burn at the centre from the piece of skin. The circular explants were disinfected in 70% ethanol for 20 minutes before being allowed to dry under ambient conditions.

Once dry, the explants are placed into a 6-well microtiter plate containing 500 pL PBS in each well to avoid dehydration of the tissue. The wound site of the explant was inoculated with 10 pL of pseudomonas solution (bacterial count: 108, tested by measuring the absorbance of solution in a 1 cm cell, Abs = 0.25 at 550 nm). The explants were transferred to individual containers (internal volume, 994 cm³) and sealed inside using a Mantle SVR Semi-Automatic Modified Atmosphere Tray Sealer, using air as the gas medium.

Ex vivo porcine skin burn wound model 1

As shown in Figure 4, CO2 colourimetric sensor example 1 was affixed to the underside of lid L of sealed container C so that is was suspended over a piece of pork skin P which either had or had not been inoculated with pseudomonas aeruginosa. Headspace H in container C was kept moist to best simulate a wound.

Sealed container C, containing colourimetric sensor example 1 and the ex vivo porcine skin burn wound explant were placed inside a heating cabinet set at 30°C and the colour change of colourimetric sensor example 1 was monitored as a function of time over a period of 24 hours using digital photography. The decay profile for the normalised blue value (nB (RGB)) is shown in Figure 5, wherein black circles represent pseudomonas aeruginosa inoculated porcine burn wound explant and white circles represent blank, i.e. disinfected, porcine burn wound explant.

During the initial hour of monitoring, an increase in the nB (RGB) value is observed, as CO2 colourimetric sensor example 1 becomes less sensitive to the low ambient level of % CO2 (0.04%) present in the package, and therefore changes from green (t = 0) to blue (t = 1).

After a period of 24 hours, the inoculated pork skin sample produced enough CO2 to completely change colourimetric sensor example 1 from its deprotonated blue to its protonated yellow form. For the explant which was not treated with the bacterial inoculant, colourimetric sensor example 1 retained its blue colour.

Ex vivo porcine skin burn wound model 2

Prior to experimenting with CO2 generation of a bacterially inoculated porcine skin explant, the FT-IR spectrometer and custom gas cell was calibrated for CO2 gas. This was done by purging the IR gas cell with argon and subsequently injecting a known volume of 100% CO2 gas into the chamber, after which an IR spectrum was recorded, in transmission mode. The process was repeated for several known quantities of CO2 (i.e. 0.25, 0.5, 0.75 and 1 mL). The area under the CO2 peak, between 2400 and 2200 cm^-1, was calculated and this was plotted against the CO2 volume injected into the gas cell. With this calibration curve, the volume of CO2 generated by the wound model can be calculated and thus, since the volume of the gas cell is known (73 mL), the % CO2 can be calculated using equation (2), above.

In order to correlate the specific colour of CO2 colourimetric sensor 23 to a measured % CO2, a porcine skin explant, inoculated with pseudomonas, was placed into the above- mentioned custom IR gas cell equipped with CO2 colourimetric sensor 23 and containing 1 mL of water to achieve a high humidity expected at a wound site. The cell was sealed and placed into a Perkin Elmer Spectrum 1 Fourier Transform Infrared Spectroscopy machine. An IR spectrum of the headspace over the porcine explant was taken every hour, coupled with a digital image of a CO2 sensing ink film prepared as described hereinabove.

The colour change observed in colourimetric sensor 23 was correlated to the actual change in % CO2 in the headspace above the inoculated pig skin which generates CO2 as the wound festers, where the latter was measured using Fourier Transform Infrared (FT-IR) spectroscopy. The porcine skin explant wound model was also carried out using 3 different inoculants (pseudomonas aeruginosa, staphylococcus aureus and escherichia coli) and the colour change of the indicator reported. All experiments were performed at 30- 32°C.

In addition to this work on colourimetric sensor 23, preliminary work on volatile amine ink film colourimetric sensors was carried out, in which a number of volatile amine sensitive dyes were incorporated into an ink film made with the water-insoluble polymer, ethyl cellulose (EC). In this initial study, the ink colourimetric sensor 24 was exposed to putrescine vapour, a known volatile amine marker for an ‘at risk’ wound site produced by the bacterial metabolism of amino acids.

Results - CO2 colourimetric sensor 23

Correlation of colour change and % CO2

As shown in Figure 10, CO2 colourimetric sensor 23 and a single sample of porcine skin explant inoculated with pseudomonas were placed into a 73 mL internal volume IR gas cell with 1 mL of water added to ensure the gas in the cell was 100% humid as would be expected from wound sites.

The cell was sealed to be gas tight and placed in an FT-IR spectrometer. A scan from 4000 to 500 cm^-1 was taken every hour from t = 0 to t = 16 and the CO2 peak (between 2400 and 2200 cm^-1) was monitored as the % CO2 level increased as shown in Figures 11a and 11b, wherein the tip trace shows t = 0 and the bottom trace shows t = 16. From this, a % CO2 can be allocated for a given time, t, and correlated with the change in colour of colourimetric sensor 23.

Digital images of the CO2 sensing plastic film were taken at the same time as the IR scan.

In order to calculate the % CO2 in the headspace, the area under the CO2 peak must first be calculated, by summing the %T values for each wavenumber, cm^-1, between 2400 and 2200. This was then compared to the calibration curve for CO2 (a plot of peak area vs mL of CO2 injected into the cell; see Figure 10) to calculate the volume, in mL, of CO2 generated by the pseudomonas on the explant. From this, for each peak illustrated in figure 2, a % CO2 value can be calculated using the following equation 6: X

— x 100 (6)

3 where x is the volume of CO2 associated with a given peak area for a given time, t. A sharp rise in the % CO2 is observed over the first 10 hours of monitoring, before slowing down, but still generating, over the next 38 hours.

The colour change was also observed in colourimetric sensor 23 at the same time the CO2 concentration in the cell was increasing due to the pork skin wound deterioration and the results are illustrated in Figure 12 which is a plot of nB (RGB) vs. % CO2 (as measured by FT-IR spectroscopy). The equation of the straight line is y = -0.0383x + 0.3285, which can be used to find nB(RGB) at % CO2 = 0. Here we note that the normalised blue colour nB(RGB) when plotted against the measured % CO2 a very good straight line is generated. This shows that the optical response of colourimetric sensor 23 can be directly related to the actual CO2 level in the headspace that is generated by the infected wound.

The porcine skin explant model was repeated using different inoculants on the explant, i.e. pseudomonas aeruginosa, staphylococcus aureus and escherichia coli. All three inoculants tested showed the same characteristic curve, with a blue to yellow colour change via a distinctive green colour. The time taken for 90% of the colour change to occur, tgo, which is a measure of the speed of CO2 generation by the inoculant in the headspace, varied greatly between inoculants, with pseudomonas showing the fastest response. Table 3 below shows inoculant used vs. tgo.

Table 3 -times taken to trip colourimetric sensor 23 (i.e. S = 2) with pseudomonas, staphylococcus and e-coli.

Inoculant t90 (h)

Table 3 shows that the CO2 gas sensor responds to the presence of different bacteria, an unsurprising result considering all of them generate CO2 as a result of their metabolism. The differences in the t₉₀ values are due to the differences in the growth rates of the different bacteria.

Results - volatile amine colourimetric sensor 24

Like the CO2 colourimetric sensor examples 1 , 2 and 3 the volatile amine colourimetric sensors according to the invention operate by adsorption of a wound analyte into the polymer matrix, which alters the pH of the system, giving rise to a striking colour change, in accordance with equation 1 , above. However, where the CO2 colourimetric sensor exhibits its colour change as a result of a lowering of the pH as the concentration of CO2 increases, the volatile amine colourimetric sensor undergoes an increase in pH as the basic volatile amines are generated at a wound site.

A number of pH sensitive dyes were identified that undergo a striking colour change when exposed to basic gas phase analytes. As described hereinabove, the dyes, BPB, CR, BCPB, BCG, C, CPR and BCP were dissolved into a hydrophobic polymer solution of ethyl cellulose in 80/20 toluene/ethanol, before being drop cast onto 25 mm diameter polyethylene terephthalate (PET) discs. After drying, the ink films were exposed to putrescine vapour, and the colour change was recorded by digital photography. From these images, BPB, BCPB and CR were considered ideal candidates for use as a volatile amine indicator ink.

It is to be understood that the invention is not limited to the specific details described herein which are given by way of example only and that various modifications and alterations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS:

1. A dressing for monitoring a wound and for displaying whether the wound is infected, wherein the dressing is for placement onto a wound and comprises a colourimetric sensor adapted to change colour in response to a predetermined amount of analyte emitted from the wound.

2. The wound dressing of claim 1 , wherein the dressing comprises a plurality of colourimetric sensors, each independently tuned to monitor a specific wound analyte above the wound dressing and change colour in relation to the analyte being monitored.

3. The wound dressing of claim 1 or claim 2, wherein the dressing comprises a transparent polyurethane film.

4. The wound dressing of claim 3, wherein the or each colourimetric sensor is chemically welded onto the polyurethane film.

5. The wound dressing of any one of the preceding claims, wherein the dressing is in the form of an adhesive plaster.

6. The wound dressing of any one of the preceding claims, wherein the required predetermined amount of analyte emitted from the wound to ensure colour change of the colourimetric sensor is from about 0.04% w/w.

7. The wound dressing of any one of the preceding claims, wherein the wound analyte is carbon dioxide (CO2).

8. The wound dressing of any one of the preceding claims, wherein the wound analyte is a volatile amine such as cadaverine and putrescine.

9. The wound dressing of any one of the preceding claims, wherein the colourimetric sensor is a plastic film comprising pigment particles with a hydrophilic silica core and a dye coating.

10. The wound dressing of claim 9, wherein the colourimetric sensor is extruded between two monolayers of filament.

11 . The wound dressing of claim 9 or claim 10, wherein the dye coating comprises a dye selected from among xylenol blue (XB), thymol blue (TB), meta-cresol purple (m-CP), phenolphthalein, bromophenol blue (BPB), bromochlorophenol blue (BCPB) and chlorophenol red (CPR).

12. The wound dressing of claim 7, wherein the colourimetric sensor is a plastic film comprising pigment particles with a hydrophilic silica core and a dye coating comprising xylenol blue (XB).

13. The wound dressing of claim 8, wherein the colourimetric sensor is a plastic film comprising pigment particles with a hydrophilic silica core and a dye coating comprising one or more of bromophenol blue (BPB), bromochlorophenol blue (BCPB) and chlorophenol red (CPR) are used to detect putrescince vapour.

14. The wound dressing of any one of claims 9 to 13, wherein the plastic film is low density polyethylene (LDPE).