FIELD OF THE INVENTION

The present invention relates to the field of pH sensing. In particular, the present invention relates to hydrogel-based pH sensors that are flexible, and optionally printable, for incorporation in medical devices, wearable technologies, and food packaging. However, it will be appreciated that the invention is not limited to these particular applications.

BACKGROUND OF THE INVENTION

The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

Amongst numerous sensors integrated in our modem lifestyle, pH sensors are one of the most common form. pH sensors are used in clinics, laboratories, and industry since many biological and chemical reaction mechanisms are pH dependent. pH is considered a fundamental environmental signal bearing much information about the surrounding environment. Representing the proton activity of the solution, pH can be directly linked to other environmental signals such as the level of CO2 in aqueous solutions or the activity of certain biological species. Traditionally, pH is measured electrochemically or spectrometrically.

Conventional electrolytic pH sensors for measuring the pH level of a liquid are well known. Such pH sensors are made of rigid metallic reference electrodes and glass membranes and require multi-step manufacturing. However, the presence of glass and the metallic reference electrode make the sensors fragile and rigid and unsuitable for applications in which flexibility is critical (such as in wearable healthcare devices, including wound dressings, and food packaging). Indeed, the stiffness of materials used in conventional sensors, such as metals, silicones, metallic oxides, and hard plastics is of the order of 10 0 ⁷ kPa. These sensors also cannot be directly used in wet environments because of the type of materials used in their construction and their sensing mechanism. To address this shortfall, sensors are required to be fully encapsulated for applications where water is present. Furthermore, as nature and biological species are mostly soft, wet, and delicate, with the moduli of most living organisms ranging between 10 and IQ ³ kPa, existing commercial sensors are inherently incompatible with biological systems.

Flexible pH sensors have been developed using electroactive polymeric (EAP) materials such as polyaniline or polypyrrole. The sensing mechanis of EAP pH sensors is based on the change that occurs in the electrical properties of the EAPs in response to changing pH. Generally, the EAP pH sensors are made of three conductive electrodes on a flexible film, where the EAP material is eiectrochemically grown on one of the electrodes. An input signal is sent to the electrodes and the output signal is read back. The output signal is sensitive to the electrical properties of the EAP While the EAP pH sensors are more flexible than glass-based pH sensors, they are still quite rigid for biomedical applications or food packaging. Their multi-step and complex fabrication process also limits their application.

A few flexible pH sensors have also been developed based on ion-sensitive flexible electrodes in which the change in the potential of a working electrode (active sensor) against a reference electrode is used as a measure of pH. Early examples of such ion-sensitive sensors were made by embedding hydrogen ionophore species in the plasticised poly(vinyl chloride) (PVC) laminated on a polyimide (PI) substrate. While the researchers were able to successfully demonstrate ionic sensitivity of these sensors, delamination of PVC from the substrate compromised the performance of the first generation of ion-sensitive sensors. Later, screen printing was utilised to deposit ruthenium oxide films as a proton sensitive layer on polyester films to fabricate flexible pH sensors. Flexible pH sensors have also been made by depositing iridium oxide on a PI substrate. The potential application of such sensors in measuring pH in biological environments has been demonstrated for a live pig's oesophagus, human's and rabbit's hearts. Flexible pH sensors based on nanomaterials, such as carbon nanotubes, tungsten trioxide nanoparticles, and zinc oxide microwires have been developed with improved performance and stability. Despite these technological advancements, the stiffness of materials used in these examples is considerably higher than that of human tissue, which limits their practical application.

Aqueous dispersions of poly(3, 4-ethyl enedioxy thiophene) (PEDOT) doped with negatively charged poly(styrenesulfonate) (PSS) are stable processable materials used for over two decades to produce conductive polymer films. As a hole conductor material, PEDOT:PSS films offer high electrical conductivity but are extremely fragile. As such, PEDOT:PSS has been used in combination with other polymers to make various polymer- based conductive composites. The electronic characteristics of PEDOT:PSS is a combination of hole transportation and ionic conduction and is sensitive to pH. However, PEDOT:PSS films are brittle and readily re-disperse in water, limiting their usefulness.

It is an object of the present invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.

The present invention relates to pH sensing hydrogel materials that are flexible, stretchable, stable in aqueous environments, and in preferred embodiments, are printable. The hydrogel materials comprise a conducting polymer, a negatively charged polymer, and a flexible polymer matrix.

According to a first aspect of the present invention, there is provided a hydrogel for pH sensing, comprising poly(3,4~ethylenedioxythiophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”) dispersed in a polymer matrix (PM), wherein the PEDOT:PSS and PM are present in a ratio such that the hydrogel is stable in aqueous solution.

The following features may be used in the first aspect in isolation or in any suitable combination.

In one embodiment, the hydrogel is stable in aqueous solution if the hydrogel has a swelling ratio measured after 24 hours (Q?) of within about 20% of a swelling ratio of the same hydrogel measured after 12 hours (Q _r ) as calculated using the formula |(Q _{f -} Qr)/Qr|x l00%.

In another embodiment, the hydrogel is stable in aqueous solution if the hydrogel has a swelling ratio measured after 3 days (Q ) of within about 10% of a swelling ratio of the same hydrogel measured after 1 day (Q ) as calculated using the formula |(Q? - Q ^, )/Q.· 100%.

In another embodiment, the hydrogel is stable in aqueous solution if the hydrogel has a swelling ratio measured after 14 days (Qfl of within about 1% of a swelling ratio of the same hydrogel measured after 7 days (Qr) as calculated using the formula ;(Q/ - QrVQrj X 100%.

Preferably the ratio of PEDOT to PSS is 1 : 1. Preferably the PM is a hydrophilic polymer. In one embodiment, the PM is a physically cross-linkable polymer. In another embodiment, the PM is a hydrophilic polyurethane.

In one embodiment, the PM is formed from the polymerisation of a polyether polyol monomer and a diisocyanate monomer. Preferably the polyether polyol is selected from the group consisting of polyethylene glycol, polypropylene glycol and butylene glycol. Preferably the polyol is poly(ethylene glycol) (PEG) of formula H0[CH CH20] H where n is between 1 and 70. Preferably the di isocyanate is of formula [(0=)C=N-R-N=C(=0)] where R is a hydrophobic aliphatic group. In one embodiment, R is an optionally substituted C8-C20 aliphatic group. In another embodiment, R comprises two optionally substituted C5-7 cyclic alkyl groups. Preferably the two cyclic groups are linked by an optionally substituted Ci-Ce linear or branched alkyl group. In one embodiment, the hydrophobic aliphatic group R is selected from the group consisting of:

, and

Preferably the PM is a polymer of Formula I:

wherein n is between 1 and 100, and in is between 10 and 1000

In one embodiment, the polymer of Formula 1 has n of between 1 and 30 and an m of between 100 and 200

Preferably the PM has an average molecular weight of between about 80 and 300 kDa. Preferably the PM has a swelling ratio of between about 1.5 and 3.5 after 48 hours. Preferably the PM is soluble in a mixture of water and ethanol. Preferably the mixture is between 99: 1 and 50:50 ethanol: water.

In one embodiment, the PEDOT:PSS solids fraction (%) is between 0.1 and 30%

Preferably the hydrogel has an electrical resistance that varies linearly or substantially linearly with pH.

Preferably the hydrogel has a swelling ratio of between about 1.5 and 5 after 48 hours.

According to a second aspect of the present invention, there is provided a hydrogel composition comprising: poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”); a polymer matrix (PM) as defined above and a solvent, wherein the PEDOT PSS and PM are present in the composition in a ratio such that the composition dries to form a hydrogel that is stable in aqueous solution.

The following features may be used in the second aspect in isolation or in any suitable combination.

Preferably the composition is in the form of a printable ink. Preferably composition has a viscosity of between about 100 and 10000 cP for 3D gel printing, or a viscosity of between about 10 and 15 cP for inkjet printing, at a temperature of between 20 and 40 °C.

According to a third aspect of the present invention, there is provided method for producing a pH sensing hydrogel, the method comprising: combining po!y(3,4- ethyl enedioxy thiophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”) with a polymer matrix (PM) in a suitable solvent to form a mixture; and, allowing the mixture to dry; wherein the PEDOT:PSS and PM are present in the mixture in a ratio such that the dried mixture forms a hydrogel that is stable in aqueous solution.

The following features may be used in the third aspect in isolation or in any suitable combination .

In one embodiment, the hydrogel is stable in aqueous solution if the hydrogel has a swelling ratio measured after 14 days (Qo) of within about 20% of a swelling ratio of the same hydrogel measured after 7 days (Q,) as calculated using the formula |(Q _{f -}

Preferably the solvent is a mixture of two different and mutually miscible solvents. Preferably the solvent is a mixture of water and an alcohol selected from the group consi sting of ethanol, methanol, «-propanol, /-propanol, and /-butanol, or a mixture of water and acetone, or a mixture of water and acetonitrile. In one embodiment, the solvent is a mixture of water and ethanol. Preferably the solvent is a mixture of water and ethanol, wherein the mixture is between 99: 1 and 90: 10 ethanol: ater.

Preferably the PM is dissolved in the solvent prior to combining with the PEDOT:PSS. Preferably the there is no post-drying doping step.

In one embodiment, the method consists essentially of:

combining poly(3,4-ethylenedioxythiophene) doped with po!y(styrenesulfonate) (“PEDOT:PSS”) with a polymer matrix (PM) in a suitable solvent to form a mixture; and,

allowing the mixture to dry;

wherein the PEDOT:PSS and PM are present in the mixture in a ratio such that the dried mixture forms a hydrogel that is stable in aqueous solution.

According to a fourth aspect of the present invention, there is provided a method for producing a pH sensor, the method comprising applying a hydrogel according to the first aspect above to a substrate.

According to a fifth aspect of the present invention, there is provided a method for producing a pH sensor, the method comprising applying a hydrogel composition according to the second aspect above to a substrate.

Preferably the substrate is flexible. Preferably the substrate is selected from the group consisting of: a polymer film, a fabric, or a paper product. Preferably the substrate is a polymer film, and the polymer film comprises the same polymer matrix (PM) as the hydrogel. Preferably the substrate is a polymer film, and the polymer film consists of the same polymer matrix (PM) as the hydrogel.

The present invention contemplates a pH sensor produced by the method of the fourth or fifth aspects.

In some embodiments of the fourth or fifth aspects, the substrate is a paper product or fabric, and the paper product or fabric is impregnated with the same polymer matrix (PM) as the hydrogel.

Preferably the step of applying comprises printing the composition onto a substrate as defined above. Preferably a single layer of hydrogel composition is printed onto the substrate. In other embodiments, two or more overlapping or overlying layers of hydrogel composition are printed onto the substrate. Preferably the printed composition has a thickness of between about 100 and about 500 pm.

According to a sixth aspect of the present invention, there is provided use of a hydrogel according to the first aspect above as a pH sensor.

According to a seventh aspect of the present invention, there is provided a hydrogel according to the first aspect above when used as a pH sensor.

According to an eighth aspect of the present invention, there is provided a pH sensor compri sing a hydrogel according to the first aspect above.

According to a ninth aspect of the present invention, there is provided A pH sensor comprising a hydrogel according to the first aspect on a substrate as defined herein.

Preferably the pH sensor is incorporated into a wearable item such as a watch, or incorporated into food packaging, beverage packaging, or a wound dressing.

According to a tenth aspect of the present invention, there is provided a device comprising a pH sensor as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now' be described, by way of example only, with reference to the following figures, wherein:

Figure 1 shows the swelling behaviour of the PEDOT:PSS/PU hydrogels in Milli-Q w'ater over two months.

Figure 2 show _' s the correlation between swelling ratio and n for a hydrogel comprising a polymer of Formula I. Figure 3 shows the tensile properties of PEDOT:PSS/PU hydrogels.

Figure 4 shows the mechanical performance of PEDOT:PSS/PU hydrogels under cyclic loading.

Figure 5 shows the electrical properties of 1.25 % PEDOT:PSS/PU films (Sample D). The surface resistivity of dry PEDOT:PSS/PU films (a) and their corresponding hydrogel (inset) as a function of PEDOT:PSS ratio with respect to P!J. The resistance of conductive hydrogel films remained unchanged with twisting and bending (b) with Ro being the undeformed resistance. The resistance of hydrogel films changed with elongation following Pouillefs law and rubber elasticity (c). The resistance of hydrogel films subject to cyclic elongations (50%, 2 Hz) remained unchanged up to 400 cycles (d). Ro ~2.1 x 10 ^' k W.

Figure 6 shows the pH response of PEDOT:PU hydrogels. The resistance of PEDOT:PSS/PU hydrogels was dependent on pH, stabilising in 3 to 4 minutes after pH change (a), with a linear response to a wide range of pH (b). The pH sensitivity of PEDOT:PSS/PU hydrogels can be attributed to the molecular morphology of the nanoparticles of PEDOT:PSS. The arrow's in (a) and (b) indicate the same data points.

Figure 7 shows the printable PEDQT:PSS/PU inks. Optimised inks exhibited favourable rheological properties for 3D gel printing (a) using CAD models. Printing was executed with a gel extruder on dry PU films (b). After hydration stable PEDOT:PSS/PU tracks were formed on PU hydrogel substrates (c, d). Scale bar in (d) is 10 mm.

Figure 8 shows printed pH sensors. The resistance of conductive tracks was directly affected by print parameters especially the number of printed layers (a). The printed tracks, similar to Figure 7c, exhibited pH sensitivity in both ascending and descending pH regimes (b). Rmin in (a) was the resistance of a one layer dry track, and Ro was the resistance between two legs of the pH sensor in Figure 7c in Milli-Q water.

Figure 9 shows printed PU/PEDOT:PSS inks (crossed pattern) on printed Ag tracks. The scale bar is 10 mm. The substrate is paper infused with PU (of Formula I, where n = 12).

Figure 10 show's the impact of PU/PEDOT:PSS pattern design on electrical response of multi -array sensors printed on gel-infused paper. The linear change in resistivity is shown when resistivity w¾s measured between electrodes 1-2, 1-3, 1-4, 1-5, 1-6, and 1-7 for patterns in Figure 11.

Figure 11 show's the patterns of printed sensors used to measure the data in Figure 10. DEFINITIONS

In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary _' skill in the art to which the invention pertains.

Unless the context clearly requires otherwise, throughout the description and the claims, the words‘comprise’,‘comprising’, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of‘including, but not limited to’.

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term‘about’. It is understood that whether the term‘about’ is used explicitly or not, every' quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including approximations due to the experimental and/or measurement conditions for such given value.

As used herein and unless otherwise indicated, the term“PEDOT:PSS solids fraction (%)” is calculated as: [masspEoonpss / (masspEDOxnss + masspvi)] ^x 100 for a system containing PEDOT:PSS and PM as the only solid components. For example, 3.3 niL of a 5. lw/v% solution of PM and 1.7 mL of a l.lw/v% solution of PEDOT:PSS mixed together has a PEDOUPSS solids % calculated as 1 ( 1 .7 U/i 00)/j( i .7 1. 1/ 100) ( 3.3 5. 1/ 100}; ! 100 = ^: 10% (and therefore a PM solids % of 90%). In instances where additional solids, such as nanoparticles, viscosity modifiers, etc, are present in the hydrogel or hydrogel composition, the PEDOT:PSS solids fraction (%) may still be calculated using this formula to obtain a relative proportion of PEDOT:PSS to PM.

Concentrations of PEDOUPSS and PM expressed as %(w/v) represent masspEDonpss / 100 mL (solvent, composition, etc.) and masspM / 100 mL (solvent, composition etc.), respectively.

Concentrations of PEDOT:PSS and PM expressed as wt% represent masspEDOxass / 100 g (solvent, composition etc.) and masspM/ 100 g (solvent, composition etc.), respectively.

The terms‘predominantly’ and‘substantially’ as used herein shall mean comprising more than 50% by weight, unless otherwise indicated.

The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

The terms‘preferred’ and‘preferably’ refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

The abbreviation “PEDOT:PSS” refers to poly(3,4-ethyienedioxythiophene) (PEDOT) doped with negatively charged poly(styrenesulfonate) (PSS).

The abbreviation“PM” refers to a polymer matrix.

The abbreviation“PU” refers to a polyurethane of Formula I wherein n is between 1 and 30 and an m of between 100 and 200 unless the context clearly indicates otherwise.

The term “hydrogel composition”, and related terms such as “hydrogel ink”, “hydrogel mixture” and“hydrogel solution” as used herein, refer to a mixture of PM, PEDOT:PSS and solvent prior to drying.

The term“hydrogel” and related terms such as“hydrogel sensor” and“pH sensing hydrogel” as used herein refer to a discrete hydrogel formed by drying a hydrogel composition, and unless the context clearly indicates otherwise, has been rehydrated in water.

The term“stable” as used herein in the context“water stable” or“stable in aqueous solution” is taken to mean that the swelling ratio of a hydrogel measured after complete submersion for a certain period of time, t, (Qr) is within about 20% of the swelling ratio of the same hydrogel measured after submersion for a shorter reference time, t=r (Qr) as calculated using the formula |(Q - Qr)/Qr| ^x 100%. In some embodiments, the swelling ratio Q is within about 15%, or within about 10%, or within about 5%, or within about 1%, of the swelling ratio Qr The swelling ratio is preferably measured in distilled water having a pH of 7 For example, in some embodiments, Q measured after 3 hours is within about 20%, or within about 15%, or within about 10%, or within about 5%, or within about 1% of Qr measured after 1 hour. Such gels may be particularly suitable for applications where short-term aqueous submersion of the gels is required. In other embodiments, Q _t measured after 24 hours is within about 20%, or within about 15%, or within about 10%, or within about 5%, or within about 1% of Qr measured after 12 hours. In yet other embodiments, Q measured after 3 days is within about 20%, or within about 15%, or within about 10%, or within about 5%, or within about 1% of Q _r measured after 1 day. In further embodiments, Q _t measured after 14 days is within about 20%, or within about 15%, or within about 10%, or within about 5%, or within about 1% of Qr measured after 7 days. Such gels may be particularly suitable for applications where long-term aqueous submersion of the gels is required.

DETAILED DESCRIPTION

Described herein are pH sensing hydrogels comprising poly(3,4- ethylenedi oxythi ophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”) dispersed in a polymer matrix (PM), wherein the PEDOT:PSS and PM are present in a ratio such that the hydrogels are stable in aqueous solution, i.e, the property that on immersion or exposure to water, the hydrogels do not dissolve, or do not substantially dissolve. The pH sensing hydrogels developed in this invention are highly flexible and stretchable, in some embodiments, having mechanical properties resembling those of soft human tissues. Because of their unique chemical structure, these pH sensing hydrogels are capable of absorbing and retaining rvater and other aqueous solutions, facilitating pH sensing. These pH sensing hydrogels are also printable in certain embodiments and can thus be incorporated in medical devices, wound dressings or in food packaging.

PEDOT:PSS

The hydrogels of the present invention comprise poly(3,4-ethylenedioxythiophene) (PEDOT) doped with negatively charged poly(styrenesu!fonate) (PSS),“PEDOT:PSS”. The PEDOT component provides conductivity, and the PSS component is a negatively charged polymer.

The PEDOT:PSS may be provided in any suitable form. For example, the PEDOT:PSS may be a commercial formulation available from Sigma-Aldrich®, Australia, or Clevios® by Heraeus®. The PEDOT:PSS is preferably highly conductive grade. The PEDOT: PSS may have a ratio of PEDOT to PSS ranging from 1:1 to 1:30, where the 1:1 formulation is more conductive relative to the 1:30 formulation. For example, the PEDOT:PSS may have a ratio of PEDOT to PSS ranging from 1:1 to 1:5, 1:1 to 1:10, 1:5 to 1:15, 1:5 to 1:20, 1:10 to 1:30, or 1:15 to 1:30, or 1:1 to 1:10, or, e.g., of 1:1, 1:2.5, 1:5, 1:7.5, 1:10, 1:12.5, 1:15, 1:17.5, 1:20, 1:225, 1:25, 1:27.5 or 1:30. Preferably, the ratio of PEDOT:PSS is less than 1:30. The PEDOT:PSS is preferably provided as a dispersion in aqueous solution. The aqueous solution preferably has a pH of between 5 and 7, e.g., pH of 7. The PEDOT:PSS aqueous dispersion is preferably devoid of surfactant. The PEDOT:PSS may be provided at any suitable solids concentration. For example, the solids concentration of the PEDQT:PSS dispersion may be between about 1 and about 6 wt%, e.g., between 0.5 and 1.5 wt%, or between 4.5 and 5.5 wt%, or about 1.1 w/w%, 1.5 wt%, 2 wt%, 3 wt%, 4 wt% or 5 wt%.

Alternatively, the PEDOT:PSS may be obtained as dry pellets suitable for redispersion in water at any suitable concentration.

The PEDOT:PSS dispersion may comprise PEDQT:PSS polymer particles having a size of between approximately 20 and 80 nrn.

The hydrogels of the present invention may comprise any suitable solids concentration of PEDOT:PSS. For example, when the hydrogel mixtures of the invention are formulated as inks, the solids concentration of PEDOT:PSS in the hydrogel mixtures (i.e., prior to drying) may be between 0.1% and 1.25%(w/v), for example, between 0.1 and 0.5%(w/v), or between 0.3 and 0.7%(w7 v), or between 0.45 and 0.65%(w/v), or between 0.5 and 0.75%(w/v), or between 0.6 and 0.9%(w/v), or between 0.75 and l%(w/v), or between 0.9 and 1.25%(w7v), e.g , of about 0.1, 0.2, 0.3, 0 4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.8, 0.9, 1.0, 1.05, 1.1, 1.15, 1.2 or 1.25%(w/v). More generally, and when the hydrogel mixtures of the invention are formulated for gel casting, the solids concentration of PEDOT:PSS in the hydrogel mixtures may be any suitable concentration, for example, may be between 0.16% and 70%(w/v). For example, the solids concentration of PEDOT:PSS in the hydrogel mixtures may be between 0.16 and l%(w/v), or between 1 and 5%(w/v), or between 0.16 and 8%(w/v), or between 5 and l0%(w/v), or between 5 and 20%(w/v), or between 10 and 30%(w/v), or between 25 and 5()%(w/v), or between 20 and 60%(w/v), or between 30 and 70%(w/v), or between 50 and 70%(w/v), or between 40 and 60%(w/v), e.g., may be 0.16, 0.2, 0.5, 1, 5, 10, 15, 20, 25, 30, 40, 50, 60, or 70%(w/v).

The swelling ratio and mechanical performance of hydrogels in accordance with the present invention are affected by the concentrations of PEDOT:PSS such that when stored in water over an extended period of time (> 2 months, for example), high concentrations of PEDOT:PSS (relative to polymer matrix) can result in excessive water uptake by the hydrogels. This increased rvater content eventually interrupts the physical crosslinks in the PM network, leading to deterioration of mechanical properties of the hydrogel. Accordingly, the concentration of PED()T:PSS in the hydrogels of the invention is preferably adjusted such that relatively lower concentrations of PEDOT:PSS are present in hydrogels intended for continuous and long-term immersion/submersion in aqueous environments in use compared to hydrogels intended for intermittent immersion/submersion or short-term, single exposure applications.

Polymer matrix (PM)

The hydrogels of the present invention comprise PEDOT:PSS dispersed in a polymer matrix (PM). Although it is known in the prior art to modify annealed PED()T:PSS films to improve their conductivity by doping with surfactants, ionic liquids, graphene oxide, dimethyl sulfoxide (DMSO), or polar solvents such as ethylene glycol, the present inventors have discovered that dispersing PEDOT:PSS in a polymer matrix as described herein prior to drying forms a range of hydrogels that upon rehydration, are stable in aqueous environments and are electrically conductive, with their electrical properties directly affected by pH. The PM/PEDOT:PSS compositions are also printable in certain embodiments. The PM/PED()T:PSS hydrogels of the present invention are thus advantageous in that suitably conductive pH-sensing hydrogels are formed in a single step, without the need for additional post-annealing doping step(s). Further, in contrast to devices of the prior art comprising separate conductive polymer and pH sensing layers, the PEDQT:PSS in the present invention is dispersed in, or distributed throughout, the PM such that the hydrogel is a conductive and pH-sensitive single phase system. “Dispersed” in this context describes the distribution of PEDOT:PSS throughout the PM, in contrast, for example, to being present as a layer on the surface of the PM. The PEDO ^’ EPSS may be distributed in a homogeneous, substantially homogeneous, or non-homogeneous manner in the PM.

The PM for use in the present invention may be any suitable polymer capable of preventing migration of PEDOT:PSS chains out of the PM. Preferably, the PM polymer is a neutral polymer free of acidic or basic moieties, since acidic and basic moieties in the PM may interfere with pH sensing performance of the hydrogel. Preferably, and particularly for certain food and health applications, the PM is chosen to be non-toxic to humans and/or aquatic systems, and where possible, does not require toxic chemicals for curing and cross- linking. The PM may comprise one type of polymer or copolymer, or may comprise two or more different polymers or copolymers.

Preferably, the PM polymer is a hydrophilic polymer. By‘hydrophilic polymer’, it is meant that the polymer is fully soluble in w'ater, or partially soluble in w'ater, or sparingly soluble in water, e.g., at a concentration of up to 2 wt%, or that the polymer is water swellable such that, when gelled, the polymer network incorporates at least 10% by weight water, or at least 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80% by weight water, or absorbs at least twice its own weight in water, or absorbs at least 4 times, 6 times, 8 times, 10 times, 15 times, 20 times, 30 times, 40 times, 50 times, 60 times, 70 times, 80 times, 90 times, 100 times, 150 times, 200 times, 250 times, 300 times, 350 times, or 400 times its own weight in water.

The PM polymer will generally be a cross-linked hydrophilic polymer, which is particularly suitable for applications where the hydrogel will be submerged in water for extended periods of time. The polymer may be chemically or physically cross-linkable.

Preferably, the PM used herein is a physically cross-linking polymer. Physically- crosslinked polymers may include those cross-linked by entangled chains, hydrogen bonding, hydrophobic interaction and crystallite formation mechanisms. Examples of such polymers include hydrophilic polyurethanes, alginate, chitosan and PVA (polyvinyl alcohol). Alginate chains are cross-linkable in contact with Ca ^2÷ cations. Chitosan is soluble at lo ' pH. Preferably, the physically cross-linkable polymer is a hydrophilic polyurethane. In some embodiments, the physically cross-linkable polymer is not polyvinyl alcohol.

Alternatively, it is contemplated that chemically cross-linkable monomers and pre polymers can be used in the hydrogels of the present invention. Chemically cross-linkable monomers and pre-polymers including those having alkene C=C groups can be used along with a radical initiator system (for thermal curing) or radiation (e.g., UV) to form the polymer. In this scenario, the cross-linkable monomers and/or pre-polymers can be added to the PEDOT:PSS in water, and the mixture applied to a suitable substrate and cured. Any hydrophilic, neutral vinyl-functionalised monomers can be used. For example, monomers such as 2-hydroxyethyl methacrylate (HEMA), 2-hydroxyethy! acrylate and their families, acrylamide and its families, polyfethylene glycol) methacrylate, poly(ethylene glycol) acrylate, and poly(ethylene glycol) diacrylate and their families, etc. Any suitable chemical cross-linking initiator may be used. Preferably, the initiator is a water soluble radical initiator.

It is also contemplated that other hydrophilic prepolymers with reactive end groups, such as hydrophilic epoxides, can be used along with PEDOT:PSS in water with appropriate thermal curing.

One preferred PM is a hydrophilic polyurethane. By‘hydrophilic polyurethane’, it is meant that the polyurethane is capable of up-taking water to have a water content of between 10% to 99%, for example, of between 10 and 40%, or between 30 and 70%, or between 40 and 90%, or between 20 and 95%, or between 50 and 70%, or between 70 and 90%, or between 60 and 99%, e.g , is capable of up-taking water with water content of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 99%. Persons skilled in the art will recognise that polyurethanes are formed from the polymerisation of polyol monomers with isocyanate monomers, where the polyol monomers comprise at least two hydroxyl groups per molecule on average, and the isocyanate monomers comprise at least two isocyanate groups per molecule. Especially preferred hydrophilic polyurethanes for use in the present invention are those formed from the polymerisation of polyether polyol monomers or polyether polyol polymers or copolymers with diisocyanate monomers.

Any suitable polyether polyol monomers/polymers/copolymers may be used. Suitable examples of polyether polyol monomers for the hydrophilic polyurethane include polyethylene glycol, polypropylene glycol and butylene glycol, although other polyalkylene glycols may also be used. In one embodiment, the polyol is poly(ethyiene glycol) (PEG) of formula HOj Cl hCt bO]»H where n is between 1 and 70 In another embodiment, the polyol is polypropylene glycol) (PPG) of formula H0[CH CH(CH3)0] H where n is between 1 and 70. In yet another embodiment, the polyol is polyil, 2-butylene glycol) (PBG) of formula --- 1 fOj CH 'Cl !(( ^' ! PC I h)0 ],:l I where n is between 1 and 70. In each of these polyols, n may be between 1 and 10, or between 5 and 25, or between 20 and 50, or between 1 and 50, or between 40 and 60, or between 50 and 70, or between 60 and 90, or between 35 and 65, e.g., n may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, or 70. Alternatively, the polyol may be a copolymer of two or more different polyether polyol monomers, e.g., the polyol may be a PEG-PPG copolymer, a PEG-PBG copolymer, a PPG- PGB copolymer, a PEG-PPG- PBG copolymer. The copolymer may be any suitable copolymer, e.g., a block, alternating, random or graft copolymer. The average molecular mass of the polyols, for example polyether polyols such as PEG, PPG, PBG, etc. may be up to 6500 Da, or up to 600 Da, or up to 5500 Da, or up to 5000 Da, or up to 4500 Da, or up to 4000 Da, or up to 3500 Da, or up to 3000 Da, or up to 2500 Da, or up to 2000 Da, or up to 1000 Da, or up to 800 Da, or up to 600 Da, or up to 400 Da, or between 100 and 250 Da, or between 150 and 300 Da, or between 175 and 350 Da, or between 300 and 450 Da, or between 500 and 1000 Da, or between 750 and 2000 Da, or between 2000 and 3000 Da, or between 1500 and 2500 Da, or between 2500 and 4000 Da, or between 3000 and 5500 Da, or between 4000 and 6500 Da, or may have an average molecular mass of 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 600, 700, 800, 900, 1000,

1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, or 6500 Da.

The hydrophilic polyurethane may be formed from a diisocyanate of formula [(0=)C=N-R-N=C(=0)] where R is a hydrophobic aliphatic group. Although any suitable R group may be used, R is preferably an optionally substituted C8-C20 aliphatic group. Preferably, R comprises at least one optionally substituted C5-7 cyclic alkyl group. More preferably, R comprises two optionally substituted C5-7 cyclic alkyl groups. Where R comprises two or more C5-7 cyclic alkyl groups, the two or more cyclic groups are preferably linked by an optionally substituted Ci-Ce linear or branched alkyl group.

Where the term“optionally substituted” appears, the optional substitutions may be heteroatoms such as O, N, S in the aliphatic group, or the aliphatic group may be substituted by a halogen such as F, Cl, Br, a hydroxyl group, a cyano group, an alkyl group, an alkene group, an alkoxy group, an ester, an ether, an amine, an amide, a thiol, or an aromatic group, or any combination of two or more of these groups.

In one embodiment, the polyurethane may be formed from a diisocyanate of formula [(0=)C=N-R-N=C(=0)] where R is a C8-C20 aliphatic group, e.g., a Cio-Cn aliphatic group, or a C12-C14 aliphatic group. In another embodiment, R in [(0=)C=N-R-N=C(=0)] is Cs-Cao aliphatic group comprising a C5-7 cyclic alkyl group. In another embodiment, R in [(0=)C=lSi— -R— N=C (= O)] is C8-C20 aliphatic group comprising two C5-7 cyclic alkyl groups linked by a Ci-Ce linear or branched alkyl group. In yet a further embodiment, R in [(0=)C=N-R-N ^: =C(=0)] is C10-C17 aliphatic group comprising two C5-7 cyclic alkyl groups linked by a C1-C3 linear alkyl group.

For example, the hydrophobic aliphatic group R may be selected from any one or more of the following (where half-bonds represent the point of attachment of the R group to N atoms in [(0=)C=N-R-N=C(=0)]):

In one embodiment, the PM is a polymer of Formula I:

wherein n is an integer between 1 and 100, and m is between 10 and 1000. For example, n may be an integer between 1 and 10, or 1 and 25, or 1 and 50, or 1 and 75, or 5 and 15, or 1 and 20, or 1 and 15, or 10 and 20, or 15 and 30, or 25 and 50, or 40 and 75, or 30 and 60, or 50 and 75, or 65 and 80, or 70 and 100, or 50 and 100, or 75 and 90, e.g., n may be

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100. The polymer of Formula I may have an m of between 10 and 1000, or between 10 and 50, or 50 and 100, or 75 and 125, or 70 and 160, or 50 and 150, or 50 and 200, or 100 and 200, or 150 and 250, or 125 and 175, or 175 and 275, or 200 and 300, or 250 and 300, or 200 and 500, or 300 and 750, or 500 and 750, or 400 and 600, or 600 and 900, or 700 and 1000, or 800 and 1000, e.g., m may be 50, 75, 100, 125, 150, 175, 200, 225, 250, 275, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000. In one embodiment, the polymer of Formula I has n between 1 and 25 and an m of between 75 and 200. In one embodiment, the polymer of Formula I has n between 1 and 15 and an m of between 75 and 200. In one embodiment, the polymer of Formula I has n between 5 and 15 and an m of between 75 and 200. In one embodiment, the polymer of Formula I has n between 5 and 15 and an m of between 125 and 175. In one embodiment, the polymer of Formula I has n between 10 and 15, an m of between 130 and 170, and a molecular weight (MW) of between 100 and 135 kDa, e.g , of 120 kDa.

The PM may have an average molecular weight of between about 80 and 300 kDa, e.g., of between 60 and 100, or between 90 and 1 10, or between 1 10 and 130, or between 100 and 130, or between 125 and 150 kDa, or between 100 and 175 kDa, or between 150 and 200 kDa, e.g., an average molecular weight of 60, 70, 80, 90, 100, 1 10, 120, 130, 140, 150, 160, 170, 180, 190 or 200 kDa.

The PM may have a swelling ratio of less than between about 1.5 and 3.5, e.g. of between 1 .5 and 2, or 2.5 and 3, or 2 and 3, or 3 and 3 5, e.g., may have a swelling ratio of about 1.5, 2, 2.5, 3, or 3.5. Advantageously, a PM having these swelling ratios allows the final hydrogel to take up enough water for sensing applications in liquid environments. The swelling ratio, Q, may be calculated for a film composed entirely of PM by taking the dry weight (Wo) and the wet weight (W _g ) each in grams or milligrams and using the formula Q = Wg/Wo. The skilled person will be aware that changes in the nature and chain length of the polyol, changes in the diisocyanate, and the overall polymer molecular weight will affect the swelling ratio, and accordingly, the swelling ratio may be adjusted to be within this range through modification of these components. For example, for a polymer of Formula I, the correlation between n and swelling ratio is shown in Figure 2. More generally, for a compound of Formula I, a larger value for n results in a more hydrophilic hydrogel and larger swelling ratio, whilst larger values for m results in more mechanically stable gels.

The PM may be selected so as to have a number of advantageous mechanical properties. For example, the PM may have any suitable tensile strength when gel cast into films. For example, an annealed, rehydrated PM film (rehydrated for a period of at least 2 months) may have a tensile strength of up to about 2500 kPa, or up to about 2000 kPa, or up to about 1500 kPa, or up to about 1000 kPa, or of greater than about 500 kPa, or greater than 1000 kPa, or greater than 1500 kPa, or of greater than 2000 kPa, or of greater than 2500 kPa, e.g., of between about 500 and 3000 kPa, or between about 500 and 1000, or 750 and 2000, or 1000 and 2500, or 1550 and 3000 kPa, e.g. of about 500, 1000, 1500, 2000, 2500 or 3000 kPa.

The PM may have any suitable Young’s modulus when gel east into films. For example, an annealed, rehydrated PM film (rehydrated for a period of at least 2 months) may have a Young’s modulus of up to about 1500 kPa, or up to about 1250 kPa, or up to about 1000 kPa, or up to about 750 kPa, or of greater than about 500 kPa, or greater than 750 kPa, or greater than 1000 kPa, or of greater than 1250 kPa, or of greater than 1500 kPa, e.g., of between about 500 and 2000 kPa, or between about 500 and 650, or 650 and 800, or 750 and 1000, or 850 and 1250 kPa, e.g. of about 500, 750, 1000, 1250, 1500, 1750 kPa or 2000 kPa.

The PM may have any suitable strain at breaking point when gel cast into films. For example, an annealed, rehydrated PM film (rehydrated for a period of at least 2 months) may have a strain at breaking point of up to about 10 mm/mm, or up to about 8 mm/mm, or up to about 6 mm/mm, or up to about 4 mm/mm, or greater than about 10 mm/mm, or greater than about 8 mm/mm, or greater than about 6 mm/mm, or greater than about 4 mm/mm, or greater than about 2 mm/mm, e.g., of between about 2 and 10 mm/mm, or between about 2 and 5 mm/mm, or between about 3 and 7 mm/mm, or between about 5 and 8 mm/mm, or between about 7 and 10 mm/mm, e.g. of about 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 mm/mm.

The PM is advantageously soluble in certain solvent(s) and/or their mixtures as described in the following section entitled“Solvent”. For example, the PM may be soluble in a mixture of water and ethanol. The mixture of water and ethanol may comprise any suitable ratio of ethanol: water. For example, the ratio of ethanol: water may be between 99: 1 and 50:50, for example, between 95:5 and 50:50, or between 95:5 and 70:30, or between 80:20 and 60:40, or between 70:30 and 50:50, or the ratio may be 95:5, 90: 10, 85: 15, 80:20, 75:25, 70:30, 65:35, 60:40, 55:45, or 50:50.

The hydrogels of the present invention may comprise any suitable solids concentration of PM For example, when the hydrogel mixtures of the invention are formulated as inks, the solids concentration of the PM in the hydrogel mixture (i.e., prior to drying) may be between 0.05% and 8%(w/v), for example, between 0.05 and 2%(w/v), or between 0.1 and 1%( w/v), or between 0.5 and 1.5%(w/v), or between 1.5 and 3%(w/v), or between 2.5 and 4%(w/v), or between 3.5 and 5%(w/v), or between 1 and 4%(w/V), or between 4% and 6%(w/v), or between 5% and 8%(w/v), e.g., of about 0.05, 0.1 , 0 5, 1 0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5 or 8.0%(w/v). More generally, and when the hydrogel mixtures of the invention are formulated for gel casting, the solids concentration of PM in the hydrogel mixtures may be any suitable concentration, for example, may be between 0.30% and 99.9%(w/v). For example, the solids concentration of PEDOT:PSS in the hydrogel mixtures may be between 99.9 and 99%(w/v), or between 99 and 95%(w/v), or between 99.84 and 92%(w/v), or between 95 and 90%(w/v), or between 95 and 80%(w/v), or between 90 and 70%(w/v), or between 75 and 50%(w/v), or between 80 and 40%(w/v), or between 70 and 30%(w/v), or between 50 and 30%(w/v), or between 60 and 40%(w/v), e.g., may be 99.9, 99.8, 99.5, 99, 95, 90, 85, 80, 75, 70, 60, 50, 40, or 30%(w7v).

The amount of PM in the hydrogels and/or hydrogel mixtures may be adjusted depending on the nature of the PM polymer such that the PM stabilises the PEDOT:PSS enabling formation of a stable hydrogel. Accordingly, the concentration of PM in the hydrogels of the invention is preferably adjusted such that relatively higher concentrations of PM are present in hydrogels intended for continuous and long-term immersion/submersion in aqueous environments in use compared to hydrogels intended for intermittent immersion/submersion or short-term, single-exposure applications.

Solvent (S)

The solvent used to manufacture the hydrogels of the invention advantageously solubilises the PM particles prior to incorporation of PEDOT;PSS. Accordingly, the solvent used in the hydrogel mixture will vary depending on the hydrophobic/hydrophilic balance and composition of the PM.

The solvent as used herein may be a neat (pure) solvent or may be a mixture of two or more different solvents. In certain embodiments, the solvent is a single, pure solvent. Any suitable single solvent may be chosen, e.g., the solvent may be any dipolar or polar solvent, including but not limited to alcohols such as methanol, ethanol, «-propanol, /-propanol, or t- butanol, or may be another solvent such as acetonitrile, acetone, dimethylformamide, dimethylsulfoxide, tetrahydrofuran, diethyl ether, ethyl acetate, etc. Preferably, the solvent has a boiling point of less than 120 °C, or less than 1 10 °C, or less than 100 °C, or less than 90 °C, or less than 80 °C, or less than 70 °C, or less than 60 °C, or less than 50 °C, or less than 40 °C, so as to facilitate purification. Preferably, the solvent is selected to be non-toxic. For example, in some embodiments, the solvent is ethanol. In other embodiments, the solvent is methanol, ethanol, «-propanol, /-propanol, /-butanol, acetonitrile, or acetone. In some embodiments, the solvent is not pure water.

In other embodiments, the solvent as used herein is a mixture of two or more different solvents. Where two or more different solvents are used, preferably the solvents are miscible. However, in some circumstances, one solvent may be sufficiently soluble in another solvent under the conditions used to mix the PM and the PEDOT:PSS.

Preferably, the solvent is a mixture of two different solvents. Any two suitable solvents may be chosen. However, the solvent is preferably a mixture of two different and mutually miscible solvents. For example, the solvent may be a mixture of water and a miscible, protic polar solvent such as ethanol, methanol, «-propanol, /-propanol, or /-butanol, etc. The miscible protic solventwater ratio in the mixtures may be any suitable volume ratio, for example, may be between 95:5 and 50:50, e.g , between 95:5 and 70:30, or between 80:20 and 60:40, or between 70:30 and 50:50, or the ratio may be 95:5, 90: 10, 85: 15, 80:20, 75:25, 70:30, 65:35, 60:40, 55:45, or 50:50. In one embodiment, the solvent is a mixture of water and ethanol. For example, where the hydrogel mixture is to be printed, the solvent may be ethanol and water mixed in a volume ratio of between 70:30 and 50:50, or between 65:35 and 55:45, e.g., of about 70:30, 65:35, 60:40, 55:45 or 50:50. In another embodiment, the solvent is a mixture of water and an alcohol selected from the group consisting of ethanol, methanol, «-propanol, /-propanol, and /-butanol. The solvent may instead be a mixture of water and a miscible, aprotic polar solvent such as acetone or acetonitrile, etc. The miscible aprotie so!ventwater ratio in the mixtures may be any suitable volume ratio, for example, may be between 95:5 and 50:50, e.g., between 95:5 and 70:30, or between 80:20 and 60:40, or between 70:30 and 50:50, or the ratio may be 95:5, 90: 10, 85:15, 80:20, 75:25, 70:30, 65:35, 60:40, 55:45, or 50:50. In one embodiment, the solvent is a mixture of water and acetone or a mixture of water and acetonitrile.

Alternatively, the solvent may comprise three or more different, mutually miscible solvents mixed in any suitable ratio. For example, the solvent may be a mixture of water, ethanol and methanol mixed in any suitable volume ratio, e.g., the water:ethanol:methanol volume ratio may be 95:2.5:2.5, or 90:5:5, 85:7.5:7.5, or 80:10: 10, or 90:8:2, or 85: 10:5, or 80: 15:5, etc.

As noted above, the solvent may comprise two (or more) non-miscible solvents, provided that one solvent is sufficiently soluble in another solvent under the conditions used to mix the PM and the PEDOT:PSS, e.g., temperature, concentration range. For example, the solvent may comprise a mixture of water and ethyl acetate in a volume ratio of water: ethyl acetate of about at least 92:8, or at least 95:5, or at least 98:2 at 20 °C. Other suitable mixtures and concentrations will be readily manufactured by persons skilled in the art. Where the hydrogel mixtures of the invention are to be printed, the solvent may be selected on the basis of the final viscosity of the PM/PEDOT:PSS/S mixture as described in the following section entitled“hydrogel mixture”.

Any suitable volume of solvent may be used in the hydrogel mixture so as to obtain a mixture having a concentration of PEDOT:PSS and PM, and a ratio of PM to PEDOT:PSS, within the ranges discussed herein.

Other Components

The hydrogel mixtures described herein may comprise one or more other components, such as one or more initiators (for chemical cross-linking), preservatives, stabilisers, emulsifiers, thickeners, pigments, dyes, pH adjusting agents, viscosity modifiers, etc.

The hydrogel mixtures described herein may comprise components for adjusting or tuning the conductivity of the hydrogels. For example, such components may include conductive nanoparticles or nanowires comprising graphene or silver.

Hydrogel composition

As described herein, there is provided a hydrogel composition comprising: polyp, 4- ethylenedioxythiophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”); a polymer matrix (PM); and a solvent, wherein the PEDOT:PSS and PM are present in the composition in a ratio such that the composition dries to form a hydrogel that is stable in aqueous solution. The hydrogel compositions described herein may be used to make the hydrogels of the invention.

Accordingly, the PEDOT:PSS, PM and solvent in the hydrogel composition may be as described above in the above sections entitled“PEDOT:PSS”,“Polymer Matrix” and “Solvent”, with the optional addition of other components as described in the above section entitled“Other Components”.

The hydrogel compositions herein may be formulated for gel casting, in which case the resultant hydrogels may act as stand-alone pH sensors (e.g., strips), which can be subsequently incorporated into a pH sensing device. Alternatively, the hydrogel compositions herein may be formulated as printable inks, printable by commercial inkjet/3D printing means, so as to form more complex devices and patterns.

The hydrogel mixtures described herein, when formulated for gel casting, may comprise any suitable solids concentration of PM and PEDOT:PSS. When formulated as inks, the hydrogel mixtures described herein may comprise a solids concentration of PM of between 0.05% and 8% (w/v), and a solids concentration of PEDOT:PSS of between 0.1% and 1.25% (w/v). Accordingly, the overall solids loading of the hydrogel mixtures when formulated as inks may be between about 0.15% and 9.25%(w/v), e.g , between 0.15% and l%(w/v), or 0.9% and 2%(w/v), or 1% and 5%(w/v), or 1% and 2.5%(w/v), or 2% and 5%(w/v), or 3% and 6%(w/v), or 2% and 6%(w/v), or 3% and 8%(w/v), or 5% and 9.25%(w/v), or 7% and 9%(w/v), e.g., may be about 0.15, 0.2, 0.5, 0.75, 1.0, 1.2, 1 4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2 8, 3.0, 3.2, 3.4, 3 6, 3.8, 4.0, 4 2, 4.4, 4.6, 4.8, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, or 9.25%(w/v).

The volume of solvent is advantageously adjusted such that the hydrogel inks have a viscosity suitable for the method of printing selected and suitable for gel printing temperature chosen. By way of example only, for inkjet printing, the hydrogel inks of the present invention may have a viscosity of between 5 and 20 cP as measured using an Anton Paar rheometer operating in cone-plate configuration (2 deg, 15 mm) for printing at a temperature of between 20 and 40 °C, e.g., a viscosity of between 5 and 10, or between 7.5 and 12.5, or between 10 and 12, or between 10 and 15, or between 12.5 and 20 cP for printing at a temperature of between 20 and 40 °C, e.g., a viscosity of 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 cP. For 3D gel extrusion printing, the hydrogel inks of the present invention may have viscosities of between about IQ ² and IQ ⁶ cP as measured using an Anton Paar rheometer operating in cone-plate configuration (2 deg, 15 mm) for printing at a temperature of between 20 and 40 °C, e.g., a viscosity of between IQ ² and 10 ³ cP, or between IQ ² and 10 ⁴ cP, or between IQ ³ and IQ ⁴ cP, or between IQ ⁴ and 10 ⁶ cP, or between 10 ⁵ and 10° cP, e.g., a viscosity of 10 ² , IQ ³ , 10 ⁴ , IQ ⁵ , or 10 ⁶ cP for printing at a temperature of between 20 and 40 °C

Hydrogels

Described herein is a pH sensing hydrogel comprising polype- ethyl enedioxy thiophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”) and a polymer matrix (PM), wherein the PEDQT:PSS and PM are present in a ratio such that the hydrogel is stable in aqueous solution.

In the PEDOT;PSS/PM hydrogels of the present invention, the PEDOT PSS colloids are integrated within the PM network, which is held together by hydrogen bonds between the PM polymer chains. At low concentrations of PEDOT:PSS, this crossl inked network remains mostly intact. As the amount of PEDOT:PSS in the PM network gradually increases, the chemical potential of the hydrogel increases proportionally, resulting in higher affinity to uptake more water. This increase in the chemical potential to absorb more water is counter balanced by the PM 's elastic network which acts against the swelling process. The balance between these two opposing phenomena is dismpted when the amount of PEDOT:PSS in the network exceeds a critical value. After this point, the affinity of the network to uptake more water and swell further overcomes the opposing elastic forces imposed by the deformed PM network. As more water molecules enter the hydrogel network, the network gets more diluted, the mechanical properties deteriorate, and eventually the hydrogen bonds dissociate resulting in the dissolution of the whole PEDOT :PSS/PM hydrogel in water. This process is time-dependent, and as such, the impact of water on the stability of the hydrogels is more significant over extended periods of time. Accordingly, the ratio by weight of PM to PEDOT:PSS in the hydrogel compositions may be any suitable ratio such that the hydrogel formed from the mixture is stable in aqueous solution. For example, the ratio by weight of PM to PEDOT: PS S in the hydrogel may be between 20: 1 and 1 :5, e.g., between 20: 1 and 10: 1, or between 15: 1 and 5: 1, or between 10: 1 and 1 : 1, or between 5: 1 and 1 :5, e.g., of 20: 1, 15: 1, 10: 1, 5: 1, 1 : 1 or 5: 1. Expressed another way, the PEDOT :PSS solids fraction (%) of the hydrogel compositions may be any suitable fraction % such that the hydrogel formed from the mixture is stable in aqueous solution. For example, the PEDOT:PSS solids fraction (%) may be between 0.1 and 95%, e.g., between 0.1 and 5%, or between 0.1 and 10%, or between 4 and 12%, or between 5 and 10%, or between 5 and 20%, or between 10 and 30%, or between 25 and 50%, or between 30 and 60%, or between 50 and 80%, or between 60 and 90%, or between 75 and 95%, e.g., may be 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 or 95%. The PM solids fraction (%) of the hydrogel compositions [100 - PEDOT:PSS solids fraction (%)] may be any suitable fraction % such that the hydrogel formed from the mixture is stable in aqueous solution. For example, the PM solids fraction (%) may between 5 and 99.9%, e.g., between 5 and 20%, or between 10 and 30%, or between 20 and 50%, or between 50 and 80%, or between 60 and 95%, or between 75 and 99.9%, or between 70 and 99%, or between 90 and 99.9%, or between 50 and 99.9%, e.g., may be 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98, 99, 99.5 or 99 9%.

The hydrogels may have a swelling ratio of between about 1.5 and 5, e.g. of between 1.5 and 2, or 2.5 and 3, or 1.5 and 3, or 2 and 3, or 3 and 3.5, or 2.5 and 5, or 3 and 4, or 3.5 and 5, e.g., may have a swelling ratio of about 1.5, 2, 2.5, 3, 3.5, 4.0, 4.5, or 5.0. The swelling ratio, Q, may be calculated for a given hydrogel by taking the dry weight (Wo) and the wet weight (W _g ) each in grams or milligrams and using the formula Q ^::: Wg/Wo. The swelling ratio may be measured after 24 h, or after 48 h, or after 7 days, or after 14 days, or after 1 month, or after 6 weeks, or after 2 months of immersion in pure deionised water. Advantageously, a hydrogel having these swelling ratios takes up enough water for sensing applications in liquid environments but not so much water that the hydrogel disintegrates. Preferably, the hydrogels have a swelling ratio of between about 1.5 and 5 after 2 months of immersion in pure deionised water.

The hydrogels of the present invention are preferably stable in water for a period of greater than 7 days, or greater than 14 days, or greater than 4 rveeks, or greater than 2 months. For example, the swelling ratio measured after 2 months (Qi) may be within about 20% of the swelling ratio of the same hydrogel measured after 7 days (Qr) as calculated using the formula |(Q _{f -} Q,·} Q,· 100%. In some embodiments, the swelling ratio measured after 2 months may be within about 15%, or within about 10%, or within about 5%, or within about 1%, of the swelling ratio measured after 7 days. In other examples, the swelling ratio measured after 1 month (Qi) may be within about 20%, or within about 15%, or within about 10%, or within about 5%, or within about 1% of the swelling ratio of the same hydrogel measured after 7 days (Qr). In still further examples, the swelling ratio measured after 7 days (Q?) may be within about 20%, or within about 15%, or within about 10%, or within about 5%, or within about 1% of the swelling ratio of the same hydrogel measured after 1 day (Qr). In yet further examples, the swelling ratio measured after 2 days (Q _f ) may be within about 20%, or within about 15%, or within about 10%, or within about 5%, or within about 1% of the swelling ratio of the same hydrogel measured after 1 day (Q _r )

The hydrogels of the present invention may have an water content of up to about 60%, or up to 70%, or up to 80%, e.g , of between about 20 and 80%, or 40 and 80%, or 60 and 80%, or 60 and 70%, e.g., of about 20, 30, 40, 50, 55, 60, 65, 70, 75, or 80%.

The hydrogels of the present invention may be gel cast into free-standing films. In such embodiments, the dried, rehydrated free-standing film thickness may be between about 30 and 1000 pm. For example, the dried, rehydrated film thickness may be between about 30 and 100pm, or between 100 and 250 pm, or between 200 and 300 pm, or between 250 and 400 pm, or between 300 and 500 pm, or between 100 and 500 pm, or between 400 and 750 pm, or between 500 and 800 pm, or between 750 and 1000 pm, e.g., about 30, 50, 75, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000 pm.

Alternatively, the hydrogels of the present invention may be applied as a thin film coating. In such embodiments, the dried film coating thickness may be up to about 5 mih, or up to about 2.5 pm, up to about 1 pm, or between 0.1 and 5 pm, or between 0.1 and 1 pm, or between 1 and 2.5 pm, or between 2 and 4 pm, or between 3 and 5 pm, or between 0.5 and 1.5 pm, e.g., about 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 or 5.0 pm.

The hydrogels of the present invention may be printed onto substrates. In such embodiments, the ink thickness (one single track height) may be between about 10 and 500 pm. For example, the film thickness may be between about 10 and 50 pm, or between 30 and 75 pm, or between 50 and 100 pm, or between 100 and 250 pm, or between 200 and 300 pm, or between 250 and 400 pm, or between 300 and 500 pm, e.g., about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475 or 500 pm. This thickness may be a wet (hydrated) thickness or the thickness of the hydrogel once dried.

The hydrogels of the present invention respond to a change in pH of their surrounding aqueous environment in a predictable manner. More specifically, the electrical resistance of a hydrogel according to the present invention preferably varies linearly with pH or varies substantially linearly with pH. For example, the electrical resistance of the hydrogels may decrease as the pH is lowered (more acidic) and the electrical resistance of the hydrogels may- increase as the pH is raised (more basic). For example, a regression line through a dataset of R/Ro vs pH for a given hydrogel, where R is the resistance of a hydrogel at a given pH and Ro is the resistance of the hydrogel in deionised water, may have an R-squared value of at least 0.999, or at least 0.99, or at least 0.98, or at least 0.97, or at least 0.96, or at least 0.95, or at least 0.90, or at least 0 80, e.g., an R-squared value of 0.8, 0.85, 0 9, 0.925, 0.95, 0.975, 0 98, 0.99, 0.999, or 1.0. The pH range over which the R-squared value may be as described herein may be any suitable pH range according to the proposed application of the hydrogel, but by way of non-limiting example, may be between pH 1 and 14, or between pH 1 and 7, or between pH 3 and 1 1, or between pH 7 and 14, or between pH 1 and 13, or between pH 6 and 9, or between pH 4 and 10, or between pH 6 and 8, etc. The electrical resistance of a hydrogel according to the present invention preferably varies linearly with pH or varies substantially linearly with pH irrespective of the starting pH The hydrogels herein, including printed hydrogel tracks, may have any suitable electrical resistivity at pH 7. For example, the sheet resistance of free-standing dried hydrogel films as described herein may be between about IQ ² to !0 ⁶ k.Q. sq. When hydrated, the sheet resistance of free-standing hydrogel films as described herein increases to between about Q.l to 10 ⁵ kO/sq. For printing tracks, the resistance of the hydrogels described herein may be in the range of 100 W to 10 Mil, depending on the number of prints and the amount of PEDOT:PSS in the hydrogel, but is preferably in the range of 100 W to 100 kO.

The electrical resistance of the hydrogels preferably stabilises at a given pH within 1 min, or within 2 min, or within 3 min, or within 4 min, or within 5 min, or within 6 min, or within 7 min, or within 10 min, or within 15 min of being exposed to that pH.

The electrical resistance of the hydrogels herein may increase as the hydrogels are stretched following Pouillet’s law. The resistivity of the hydrogels herein may not change with elongation, e.g., the stretching may change the resistivity of the hydrogel by less than 1%, or less than 2%, or less than 5%, or less than 10%, or less than 20%, or less than 25%, or less than 30% relative to an unstretched hydrogel.

The electrical resistance of the hydrogels herein is preferably unaffected by twisting or bending deformations of the hydrogels. In some embodiments, physical deformation of the hydrogel by twisting or bending changes the electrical resistance of the hydrogel by less than 1%, or less than 2%, or less than 5%, or less than 10%, or less than 20%, or less than 25%, or less than 30%. For example, where electrical resistance of undeformed hydrogel is Ro, and the electrical resistance of deformed hydrogel is R, R/Ro may be between about 0.5 and about 1.5, e.g., between 0.5 and 1.0, or betw-een 0.75 and 1.25, or between 1.0 and 1.5, or between 0.9 and 1.1, e.g., may be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 or 1.5. Advantageously, hydrogels having an electrical resistance unaffected by twisting or bending deformation can respond to pH in substantially the same way even when bent or twisted and are thus suited to applications requiring flexibility, e.g., wearable technologies, medical dressings, etc.

The electrical resistance of the hydrogels herein is preferably unaffected when subject to cyclic elongations, e.g., of 50% elongations at a frequency of between 1 and 3 Hz for up to 100, 200, 300, 400 or 500 cycles. In some embodiments, the electrical resistance of the hydrogels during cyclic elongations of 50% at a frequency of 2 Hz for up to 100, 200, 300, 400 or 500 cycles varies by less than 1%, or less than 2%, or less than 5%, or less than 10%, or less than 20%, or less than 25%, or less than 30% from the original electrical resistance of the hydrogel. For example, where electrical resistance of hydrogel prior to elongation is Ro, and the electrical resistance of hydrogel after t elongation cycles R , R.7Ro may be between about 0.5 and about 1.5, e.g., between 0.5 and 1.0, or between 0.75 and 1.25, or between 1.0 and 1.5, or between 0.9 and 1.1, e.g., may be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 or 1.5 when t = up to 50 cycles, up to 100 cycles, up to 150 cycles, up to 200 cycles, up to 250 cycles, up to 300 cycles, up to 350 cycles, up to 400 cycles, up to 450 cycles or up to 500 cycles.

The hydrogels of the present invention may have any suitable tensile strength when gel cast into films. For example, the annealed, rehydrated hydrogels may have a tensile strength of up to about 2500 kPa, or up to about 2000 kPa, or up to about 1500 kPa, or up to about 1000 kPa, or of greater than about 500 kPa, or greater than 1000 kPa, or greater than 1500 kPa, or of greater than 2000 kPa, or of greater than 2500 kPa, e.g., of between about 500 and 3000 kPa, or between about 500 and 1000, or 750 and 2000, or 1000 and 2500, or 1550 and 3000 kPa, e.g. of about 500, 1000, 1500, 2000, 2500 or 3000 kPa. The annealed, rehydrated hydrogels may have a tensile strength that is approximately equal to the tensile strength of a PM film without PEDOT:PSS, such as within ±1%, ±2%, ±5%, ±10%, ±15%, ±20%, ±25%, ±30%, ±40%, or ±50%.

The hydrogels of the present invention may have any suitable Young’s modulus when gel cast into films. For example, the annealed, rehydrated hydrogels may have a Young’s modulus of up to about 1500 kPa, or up to about 1250 kPa, or up to about 1000 kPa, or up to about 750 kPa, or of greater than about 500 kPa, or greater than 750 kPa, or greater than 1000 kPa, or of greater than 1250 kPa, or of greater than 1500 kPa, e.g., of between about 500 and 2000 kPa, or between about 500 and 650, or 650 and 800, or 750 and 1000, or 850 and 1250 kPa, e.g. of about 500, 750, 1000, 1250, 1500, 1750 kPa or 2000 kPa. The annealed, rehydrated hydrogels may have a Young’s modulus that is approximately equal to the Young’s modulus of a PM film without PEDGT:PSS, such as within ±1%, ±2%, ±5%, ±10%, ±15%, ±20%, ±25%, ±30%, ±40%, or ±50%

The hydrogels of the present invention may have any suitable strain at breaking point when gel cast into films. For example, the annealed, rehydrated hydrogels may have a strain at breaking point of up to about 10 mm/mm, or up to about 8 mm/mm, or up to about 6 mm/mm, or up to about 4 mm/mm, or greater than about 10 mm/mm, or greater than about 8 mm/mm, or greater than about 6 mm/mm, or greater than about 4 mm/mm, or greater than about 2 mm/mm, e.g., of between about 2 and 10 mm/mm, or between about 2 and 5 mm/mm, or between about 3 and 7 mm/mm, or between about 5 and 8 rnm/rnm, or between about 7 and 10 mm/ram, e.g. of about 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 ram/mm. The annealed, rehydrated hydrogels may have a strain at breaking point that is approximately equal to the strain at breaking point of a PM film without PEDOT:PSS, such as within ±1%, ±2%, ±5%, ±10%, ±15%, ±20%, ±25%, ±30%, ±40%, or ±50%.

The mechanical properties of the hydrogels of the present invention may substantially recover after repeated loading/unloading cycles. For example, the maximum stress and Young’s modulus recovery of the hydrogel may be greater than 90% after two cycles, greater than 80% after three cycles, and greater than 70% after four cycles. Without wishing to be bound by theory _' , hydrogen bonding between the PM and PEDOT:PSS may assist the hydrogels in restoring their mechanical performance from cycle to cycle.

As noted above, hydrogel inks of the present invention may be printed onto substrates. In such embodiments, the inks may be formulated for compatibility with the printing method chosen, such as by adjusting the viscosity of the hydrogel inks by varying the amount of solvent and the chemical makeup of the solvent. Viscosity may additionally or alternatively be modified by changing the ratio or fraction (%) of PEDOT:PSS to PM, and/or by addition of a viscosity modifier.

As noted above, the hydrogels of the present invention are thus advantageous in that a suitably conductive pH-sensing hydrogel can be formed in a single step, that is, by combining PEDOT:PSS and a PM in a solvent and allowing it to dry (either by gel casting, coating or printing) without the need for an additional post-drying modification or doping step. Accordingly, in some embodiments, the hydrogels of the present invention are not modified after drying by treatment with surfactants, ionic liquids, graphene oxide, dimethyl sulfoxide (DMSO), or polar solvents such as ethylene glycol.

Methods and Lises

Hydrogels

As described herein, there is provided a method for producing a pH sensing hydrogel, the method comprising: combining poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”) with a polymer matrix (PM) in a suitable solvent to form a mixture; and, allowing the mixture to dry, wherein the PEDOT:PSS and PM are present in the mixture in a ratio such that the dried mixture forms a hydrogel that is stable in aqueous solution. The PEDO EPSS may be provided in the form of an aqueous dispersion. The PM may be dissolved in a solvent, e.g., an ethanol/water mixture. The aqueous dispersion of PEDOT:PSS and the PM dissolved in a solvent may be mixed together, with stirring.

Accordingly, in one embodiment, there is provided a method for producing a pH sensing hydrogel, the method comprising:

providing poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate)

(“PEDOT:PSS”);

providing a solution of polymer matrix (PM) dissolved in a solvent;

combining the PEDOT:PSS and the PM dissolved in solvent, with stirring, to form a mixture; and,

allowing the mixture to dry;

wherein the PEDOT:PSS and PM are present in the mixture in a ratio such that the dried mixture forms a hydrogel that is stable in aqueous solution.

In another embodiment, there is provided a method for producing a pH sensing hydrogel, the method comprising:

providing an aqueous dispersion of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (“PEDOTiPSS”);

providing a solution of polymer matrix (PM) dissolved in a solvent;

combining the aqueous PEDOT:PSS dispersion and the PM dissolved in solvent, with stirring, to form a mixture; and,

allowing the mixture to dry;

wherein the PEDOT:PSS and PM are present in the mixture in a ratio such that the dried mixture forms a hydrogel that is stable in aqueous solution.

In this method, the mixture may applied to a substrate prior to being allowed to dry, e.g., may be printed onto a substrate as described in the following section.

The drying may be effected at any suitable temperature and for any suitable time. For example, the mixture may be dried at a temperature of at least 20 °C, at least 30 °C, at least 40 °C, at least 50 °C, or at least 60 °C, or at least 70 °C, or at least 80 °C, or at least 90 °C, or at least 100 °C, or at least 120 °C, or at least 140 °C, e.g., at a temperature of between about 20 and 50 °C, or between about 30 and 60 °C, or between about 70 and 150 °C, or between 70 and 100 °C, or between 100 and 140 °C, or between 1 10 and 130 °C, e.g., at a temperature of 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, or 140 °C. The drying time may be between 1 hour and 14 days, e.g , between 1 and 5 hours, or between 2 and 10 hours, or between 6 and 12 hours, or between 12 and 24 hours, or between 1 and 2 days, or between 1 and 7 days, or between 3 and 5 days, or between 1 and 2 weeks, or between 6 and 10 days, e.g., may be 5 h, 10 h, 16 h, 24 h, 48 h, 72 h, 4 days, 5 days, 6 days, 7 days, 9 days, 12 days or two weeks. The gel is considered dry when it ceases or substantially ceases to decrease in weight over time. Any suitable drying apparatus may be used, e.g., an oven or other radiant heat source.

As noted above, the hydrogels herein do not require a post-drying doping step in order to function as pH sensors; the dried mixture forms a pH sensing hydrogel. Accordingly, the method for producing a pH sensing hydrogel described herein may advantageously exclude a post-drying doping step. Post-drying doping steps known in the art include addition/electrodeposition of doping compounds such as surfactants, ionic liquids, graphene oxide, dimethyl sulfoxide (DMSO), etc. In such embodiments, there is provided a method for producing a pH sensing hydrogel, the method comprising: combining poly(3,4- ethylenedioxy thiophene) doped with poly(styrenesuJfonate) ( "FEDOT: PS (G) with a polymer matrix (PM) in a suitable solvent to form a mixture; and, allowing the mixture to dry, thereby forming a pH sensing hydrogel; wherein the PEDOTPSS and PM are present in the mixture in a ratio such that the dried mixture forms a hydrogel that is stable in aqueous solution. There is also provided a method for producing a pH sensing hydrogel, the method consisting essentially of: combining poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (“PEDOT:PSS”) with a polymer matrix (PM) in a suitable solvent to form a mixture; and, allowing the mixture to dry; wherein the PEDOTPSS and PM are present in the mixture in a ratio such that the dried mixture forms a hydrogel that is stable in aqueous solution.

Printing pH sensors and devices

The hydrogels of the present invention respond to a change in pH of their surrounding aqueous environment. Accordingly, the present invention provides pH sensors comprising a hydrogel as described herein. The present invention also provides pH sensors comprising a hydrogel as described herein on a substrate, and in particular, on a substrate as defined herein, e.g., as a coating or in a printed pattern or array. The present invention further provides for the use of a hydrogel as described herein as a pH sensor, and still further provides for a hydrogel as described herein when used as a pH sensor.

As also described herein, there is provided a method for producing a pH sensor, the method comprising applying a hydrogel as described herein to a substrate. Further described herein is a method for producing a pH sensor, the method comprising applying a hydrogel composition as described herein to a substrate. Yet further described herein is a pH sensor produced by either of these methods.

The pH sensors described herein may comprise a hydrogel as described herein without a substrate. In such embodiments, the free-form hydrogel may be provided in the form of a strip or tape, which can be exposed to aqueous environments of varying pH. The electrical resistance of the hydrogel will change with a change in pH of the aqueous environment, and the value of electrical resistance correlated with pH to provide a read out of the pH.

However, in many embodiments, the pH sensor will comprise a hydrogel on a substrate. The substrate advantageously structurally supports the hydrogel. The substrate may be any suitable substrate. Advantageously, the substrate is a flexible substrate. Flexible substrates may include fabric or cloth, polymer films, paper products, or thin films of metals or alloys. In some embodiments, the substrate is selected from the group consisting of: a polymer film, a fabric, or a paper product.

In one embodiment, the substrate is a polymer film. Any suitable polymer film can be used, however, examples of suitable polymer films may include hydrogel polymer films such as those comprising hydrophilic polymers discussed in the section above entitled“Polymer Matrix'’, e.g., polyether polyurethanes, although other common polymers such as polyethylene, polypropylene, polyivinyl chloride), polyethylene terephthalate), polyurethane families, elastomeric materials, etc. may be used in some embodiments. The polymer film may be package or packet (or part thereof), e.g., food package or packet. Preferably the polymer film comprises the same PM used in the hydrogel applied to it. In one embodiment, the polymer film consists of the same polymer matrix as the hydrogel.

In other embodiments, the substrate is a paper product. Examples of suitable paper products include copy paper (e.g., 90-150 gsm copy paper), filter papers, pure cellulose papers, paper towel, cardboard, etc. In such cases, the paper product may be impregnated with the same polymer matrix as the hydrogel, or it may have a surface coating comprising or consisting of the same polymer matrix as the hydrogel.

In yet further embodiments, the substrate is a fabric. Examples of suitable fabrics include non -woven fabrics such as geotextile fabrics, diaper covers, filters (air & liquid), medical products, industrial protective apparel, surgical gowns, blankets, upholstery', masks, etc and woven or knitted fabrics such as acrylic cloth, wool, calico, cotton, linen, silk, satin, lace, polyester, nylon, rayon, etc. In such cases, the fabric may be impregnated with the same polymer matrix as the hydrogel or may have a surface coating comprising or consisting of the same polymer matrix as the hydrogel.

Advantageously, a substrate comprising the same PM used in the hydrogel facilitates intimate bonding between a printed or coated hydrogel and the substrate such that the hydrogel and substrate are resistant to separation in use, e.g., when exposed to an aqueous environment. For example, when hydrogel compositions of the invention are applied to substrates comprising the same PM used in the hydrogel (e.g , by printing), the PM on or in the substrate may partially dissolve in the solvent and facilitate integration of the hydrogel with the substrate.

The substrate may have any suitable thickness depending on the intended application. By way of suggestion only, the substrate may have a thickness of between about 100 and about 500 pm, e.g., between about 100 and 250 pm, or between 200 and 300 pm, or between 250 and 400 pm, or between 300 and 500 pm, e.g., about 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475 or 500 pm. In some embodiments, the thickness and type of substrate are chosen to have moduli close to that of living tissues.

The hydrogel composition as described herein may be applied to the substrate by any suitable means. For example, a hydrogel composition may be poured onto an inert substrate such as a glass dish and dried so as to form a hydrogel that can be peeled off or separated from the inert substrate. The hydrogel may then be affixed to a substrate using any suitable fixant, such as glue. Alternatively, the hydrogel composition may be coated or printed on a substrate. Once printed or coated, the hydrogel composition may be dried in situ ready for use.

Accordingly, provided herein is use of a hydrogel composition as described herein as an ink. Also provided is an ink comprising a hydrogel composition as described herein. Further provided is a hydrogel composition as described herein when formulated as an ink. The printing may be effected by any suitable printing apparatus, such as through use of an inkjet printer or 3D printer. Commercial printers such as the EnvisionTEC 3D-Biopl otter® Manufacturing Series (Germany) or a Dimatix Materials Printer DMP-2850 by FujiFilm® may be used.

The hydrogel composition may be printed in any suitable pattern or shape. As the pattern will determine the final resistance of the hydrogel, different resistances may be achieved by application of the composition in different patterns. The compositions of the present invention may thus be printed in a wide variety of patterns to access a variety of different final resistances suited to a range of different applications. The skilled person will be aware of suitable patterns for printing the hydrogel composition. The hydrogel composition may be printed as a single layer. Alternatively, in some embodiments, the composition may be printed in two or more partially or completely overlapping or overlying layers. The present inventors have found that in some embodiments, increasing the number of printed layers increases the conductivity of the hydrogel tracks. The hydrogel composition may thus be printed so as to have any suitable thickness. The skilled person will recognise that varying the size of printing needle, for example, can vary the applied thickness of the composition. For example, in certain embodiments, the hydrogel composition may have a printed thickness of between about 100 and 500 pm, e.g., between about 100 and 250 pm, or between 200 and 300 pm, or between 250 and 400 pm, or between 300 and 500 pm, e.g., about 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475 or 500 pm. Once dried, the printed track thickness may be about 5-10 times thinner than the printed track thickness. By way of non-limiting example, the dried printed track thickness may be between about 10 and 100 pm. For example, the dried film thickness may be between about 10 and 50 pm, or between 30 and 75 pm, or between 50 and 100 pm, e.g., about 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 pm. More generally, pH sensors described herein, whether printed, painted, or in the form of films or strips, may comprise a single layer of hydrogel, or may comprise two or more layers of hydrogel. Accordingly, a variety of thicknesses of hydrogel are contemplated herein for use in pH sensors. By way of non-limiting example, the hydrogel in pH sensors may have a thickness of thickness of between about 100 and 500 pm, e.g., between about 100 and 250 pm, or between 200 and 300 pm, or between 250 and 400 pm, or between 300 and 500 pm, e.g., about 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475 or 500 pm.

As noted above, hydrogel inks of the present invention may be formulated for compatibility with the printing method chosen, and/or the printing parameters may be adjusted to optimise ink application. Printing parameters such as the needle size and printing temperature, pressure and speed may be adjusted according to the rheological characteristics of the ink. Methods of adjusting these and other relevant printing parameters will be known to those of skill in the art. By way of example only, a PEDOT:PSS/PU hydrogel ink of composition Sample E in Examples Table 1 comprising 0.1 wt% PEDGT:PSS and 4.6 wt% of a polyurethane of Formula I (having a PEDOT PSS solids fraction (%) of 2.5%) in a mixture of ethanol and water was 3D printed using an EnvisionTEC 3D-Biopl otter® Manufacturing Series (Germany). The ink was loaded in 30 ml plastic barrels fitted with 200 pm straight needles. The plastic barrels then were inserted into the printer's cartridge holder and the temperature of the print head was set to 27 °C. The optimised printing parameters were 0.8 bar and 5-100 mrn/sec for, respectively, print pressure and speed. The pre- and postflows were also optimised to achieve high resolution and high reproducibility. The track size of the printed patterns was -200 pm.

Provided herein is also use of a hydrogel composition as described herein as a coating. Further provided is a coating comprising a hydrogel composition as described herein. Still further provided is a hydrogel composition as described herein when formulated as a coating. The coating may be applied to a substrate using any suitable apparatus, such as a brush, roller, spray, etc. or by dipping. The coating may be applied at any suitable thickness and may comprise a single layer or two or more layers of hydrogel composition.

The present invention further provides for a device comprising a pH sensor as described herein. Devices comprising pH sensors may be fabricated using any suitable techniques known to those of skill in the art. For example, electronic circuits may printed using commercial conductive inks, such as silver or carbon-based inks having any suitable viscosities, e.g., high viscosities of from 5 to iO ⁶ cP, e.g., -10000 cP. The circuit may be designed using commercial software, such as Autodesk Fusion360®. There is also provided use of a device comprising a pH sensor as a pH sensor. Devices and pH sensors described herein may comprise additional components, such as electrodes and wires that enable a complete circuit to be formed for measuring electrical parameters such as electrical resistance for correlation with pH.

The devices and pH sensors described herein may be incorporated into wearable items such as watches, clothing, accessories, etc., or may be incorporated into food packaging, beverage packaging, wound dressings, etc. Other applications may include in water quality monitoring, during food and beverage production, etc. The flexibility of the hydrogels described herein as well as their simplicity of fabrication and versatility in substrate makes them ideal for a variety of diverse applications where pH monitoring is desirable. EXAMPLES

The present invention will now be described with reference to the following examples which should be considered in all respects as illustrative and non-restrictive.

Materials: All materials were used as received. The aqueous PEDQT:PSS solution was purchased from Sigma-Aldrieh, Australia with 1.1 w/w% PEDOT:PSS content (highly conductive grade, no surfactant). The PU was purchased from AdvanSource Biomaterials (USA) Ethanol was the absolute grade from Sigma-Aldrich (Australia), HC1 was purchased from Murk (Australia), and NaOH granules were purchased from Sigma-Aldrich, Australia. Milli-Q water was used to make up all the solutions where needed.

Example 1 - Hydrogel preparation

A PM solution was made by dissolving PM particles in a suitable solvent system. The hydrogels w ^? ere prepared by dropwise addition of various amounts of a 1.1 w/w% PEDOT:PSS aqueous dispersion to this PM solution followed by slow mixing for 48 hours

A range of different hydrogels manufactured according to this method using are shown in Table 1, where PU = a 5 1 w/v% solution of a polyurethane of Formula I in 95:5 EtOH: water solvent.

Table 1. Composition of a range of different hydrogels

The formulations in Table 1 are highly conductive and printable. The samples towards the top of table tend to be more stable but less conductive than the samples at the bottom of the table. In terms of printability, the samples in the middle of the table are more suitable for gel printing and the ones at both extremes are more suitable for inkjet.

Results for a series of hydrogels made using a polyurethane of Formula I, where p 12 and m = -145 (molecular weight -120 kDa) is given in Table 2. The polyurethane (PU) was dissolved in a 95:5 ethanokwater mixture to make a range of PU concentrations (1 - 10 wt%). The PEDOUPSS (provided as a 1.1 wt% water dispersion) was then added to the PU solution in the listed volumetric ratios (e.g., where“2: 1 PU to FEDOT: PS S ^" indicates 2 mL PU and 1 mL PEDOUPSS).

Table 2. Impact of ink composition on hydrogel ink flow properties on a scale of 1* to 5*, where * = water-like and ***** = gel-like.

In Table 1, as a general trend, with PU concentration viscosity increases at higher volume ratios of PU to PEDOUPSS. However around volume ratio 1 : 1, where EtOH: Water is 47.5:52.5, the solution turns to a gel-like system. Beyond this point, as ratio of PEDOT:PSS to PU increases, the amount of water in the system increases as well, with EtOH: water ranging 38:62 (volume ratio 1 : 1.5), 32:68 (volume ratio 1 :2), and 24:76 (volume ratio 1 :3).

Example 2 - Gel solution casting

Hydrogel films were prepared by solution casting the PEDOT:PSS/PU hydrogel mixtures from Example 1 (Samples C, D, E and F) in flat glass petri dishes. The hydrogel mixtures (4.5 mL) were poured into dishes having a 35 mm diameter then placed in a fan- forced oven for one week to remove the solvent. The dry films were submerged in Milli-Q water and the hydrogel films were stored in Milli-Q water for further testing and characterisation. The thickness of the hydrogel films varied with their swelling ratio, ranging from 200 to 300 pm.

Swelling ratio: The swelling ratio of the solution cast hydrogel films, Q, was calculated from the weight of the films before and after drying, W _g and Wo, respectively, using Q ----- W _g /Wo. The hydrogel films were dried at 60 °C for three days.

Mechanical testing: Tensile properties of the solution cast hydrogel films were measured using the lnstron equipped with a 10 N load cell. Mechanical testing was performed on hydrogel strips cut from the hydrogel films (5x20 mm) at the crosshead rate of 10 mm/min. Milli-Q water was constantly applied to the surface of the hydrogels during the course of the test to ensure hydrogels’ stability _{' .} Stress and strain were calculated based on the initial cross- sectional area and length of the samples.

Conductivity measurement: Surface resistivity of the conductive films was measured in hydrated and dry states using a two-electrode configuration in which electrodes were 10 mm apart. pH sensitivity: Different concentrations of acidic and alkaline solutions were prepared by diluting 1 M I iC! and NaOH solutions with Milli-Q water. The pH of these solutions was then measured using a pH meter. To evaluate the pH sensitivity of the conductive hydrogel films and printed sensors (see Example 3 below), samples were removed from Milli-Q water and were submerged in the acidic and alkaline solutions. Every 60 seconds samples were removed from the pH solution, tap dried, and their resistance was measured using a two- electrode configuration.

Results and Discussion

Swelling behaviour and mechanical properties

Swelling ratio, Q, was measured for each of the hydrogel films which were stored in Milli-Q w _' ater for two months, to determine their stability in water over an extended period of time. The results are shown in Figure 1.

As shown in Figure 1, while increasing the solids fraction % of PEDOT:PSS from 0 625% (Sample C) to 5% (Sample D) had negligible effect on the hydrogels' immediate swelling ratio, its effect was paramount after two months. The hydrogel films swelled four and eight fold after two months compared to one week when using samples 2.5% (Sample E) and 5% PEDOT:PSS (Sample F), respectively. In contrast, the hydrogels with lower solids fraction % of PEDOT:PSS (i.e. Sample C 0.625 and Sample D 1.25%) did not exhibit any additional swelling over this two months period.

All hydrogel films with PEDGT:PSS solids fraction % of less than 5% were highly flexible and stretchable. To determine the performance of PEDOT:PSS/PU hydrogels in wet environments, tensile testing was performed on hydrogel films after soaking them in water for two months. We observed that elastic properties of hydrogel were decreased over time by elevating the amount of PEDOT:PSS. For conductive hydrogels with PEDQT:PSS solids fraction % of less than 2.5% the mechanical properties did not change over time. In contrast, tensile properties of hydrogels with 2.5% (Sample E) and 5% (Sample F) PEDOT:PSS solids fraction % significantly declined after two month. For samples with 2.5% solids fraction % PEDOT:PSS (Sample E), strength and modulus dropped by, respectively, 20 and 3.4 fold after two months compared to one week. The hydrogels with PEDOT:PSS solids fraction % of 5% (Sample F) completely dissociated in water after two months. Accordingly, these hydrogels are more suited to applications requiring short-term submersion of the gels.

Figure 3a show's examples of tensile stress-strain curves of Samples C, D, and E PEDOT:PSS/PU hydrogels with PEDOT:PSS solids fraction % of less than 5% after two months in water. As shown in Figure 3b, the Young's modulus of PEDOT:PSS/PU hydrogels Samples C, D, and E did not change by increasing PEDOT:PSS solids fraction % up to 1.25%, after which the Young's modulus dropped sharply by 4 times for samples with 2.5% PEDOT;PSS solids fraction % . Similar trends were observed for tensile strengths and strains at break of conductive hydrogels: the tensile strength and strain at break of hydrogels negligibly changed by addition of up to 1.25% solids fraction % PEDOT:PSS. Increasing the solids fraction % of PEDOT:PSS further results in deterioration of mechanical properties. The PU hydrogel films with no PEDOEPSS (“PIT) (swelling ratio of 2.4) had average modulus, tensile strength, and elongation at break of, respectively 1047±63 kPa, 2Q72± 130 kPa. and 6 8±0.6 The average tensile modulus, tensile strength, and elongation at break of the conductive hydrogels with 0.625 (Sample C) and 1.25% (Sample D) solids fraction % PEDOT PSS were, respectively, 1023±95 kPa and 1158±196 kPa, 2301 ±85 kPa and 1534± 190 kPa, and 7.4± 1.9 and 4.95±0.6. The addition of slightly more PEDOT:PSS to the composition resulted in considerable deterioration of the mechanical properties. For PEDOT:PSS/PU hydrogels with 2.5% (Sample E) solids fraction % PEDOT:PSS, Young's modulus, tensile strength and elongation at break were 30Q±7, 58±5, and 0.35±0.04 In Figure 3, and elsewhere throughout the figures unless otherwise stated,“PU” refers to a PU film, where the PEI is of Formula I (n = 12, m = 145) of approximately 200 - 300 pm thickness prepared by solution casting a 5.1 w7v% PU solution in 95:5 ethanol/water in flat petri dishes. The petri dishes were place in a fan-forced oven at 37 °C for several days to remove the EtO!Twater solvent.

Load cycle

The performance of hydrogels w¾s assessed after multiple loading/unloading cycles to determine the stability of conductive hydrogels under external dynamic forces. The results in Figure 4 demonstrate that the mechanical properties of hydrogels w ^? ere negligibly decreased after each loading/unlading cycle. Unlike conventional hydrogels where the breakage of covalent bonds in the first cycle dramatically reduces the mechanical properties of the hydrogels in consecutive cycles, here a significant recovery in the maximum stress and Young's modulus was observed. Based on the initial maximum stress in the first cycle, the stress recovery in the consecutive cycles was, respectively, 92%, 83%, and 74%. The recovery of Young's modulus between consecutive cycles was 100%, 98%, 80%, and 99%, from cycle one to five, respectively.

Electrical conductivity

The electrical conductivity of the hydrogels and their corresponding dried films was measured using a two electrode setup (Figure 5a). Both dry and hydrated films exhibited percolation behaviours in which PEDOTPSS was the conductive phase and PU and Milli-Q water (in the case of hydrogels) were the insulating matrix phase. The percolation behaviour of a conductive-insulator system can be modelled by Equation (1):

Here, s is the electrical conductivity of the system when the volume fraction of the conductive phase (PEDOTPSS in this study) is <p _c . In this case, the percolation threshold is <p, and is defined as the lowest volume fraction of the conductive phase at which a continuous conductive path is formed.

Equation (1) is valid for volume fractions above the percolation threshold, i.e. f > f _€ . The critical exponent t determines the morphology of the conducting network when the percolation threshold was approached. It is important to note that percolation thresholds of PEDOTPSS was nearly 0.016 ± 0 .002 regardless of keeping the sensor in dry or wet conditions. This behaviours was attributed to identical properties of percolation behaviour of the conductive phase, i.e. PEDOT:PSS at both conditions. In this calculation we included water in the hydrogel for estimating f, assuming that both water and PU together formed the insulating phase. As such, the addition of water to the dry PEDOT SS/PU has only diluted the system and has trivial impact on the PEDOTPSS phase percolation in the system. The critical exponent, t, in equation (1) for the dry and hydrated films were 2.31 and 2.98, respectively. The critical exponent provides helpful information on the morphology of the conductive networks that forms when f exceeds <p _c. The conductivity exponent can be expressed with a generic equation such as in Equation (2):

In Equation (2), t2 is the critical exponent for a two-dimensional network, and is given a value of -1.3. d is the Euclidean dimension. According to equation (2), when d ® dc =6 then t 3, which is in agreement with the mean field value of conductivity critical exponent at six dimensions. For diy PEDOTPSS/PU films d is -3.6, while for PEDOTPSS/PU hydrogels d is ~6. The electrical performance of the hydrogel films was independent of bending and twisting as shown in Figure 5b. The electrical stability of the films during bending and twisting is particularly important for flexible sensor applications. Unlike twisting and bending, stretching had a considerable effect on the electrical performance of the films (Figure 5c), where resistance continuously increased as films were stretched. The change in resistance was a power-law function of elongation with exponent ratio of 3/2 as shown in Figure 5c. The 3/2 exponent highlights three important points: (I) the electrical resistance of the conductive hydrogels, R, follows the Pouillefs law: R = p x (l/A), where p is resistivity, / is length and A is cross section; (2) hydrogels follow the rubber elasticity theory in which Poisson's ratio is ~0.5; (3) the resistivity of hydrogels, p, does not change with elongation.

For sensing applications, in addition to bending and twisting it can be important to consider the effect of cyclic deformation of hydrogel films on their electrical properties. To evaluate the impact of dynamic external forces on long-term electrical performance of PEDOT:PSS/PU hydrogels, a conductive hydrogel film with 1.25% solids fraction % PEDOT:PSS was subject to cyclic tensile deformations with an amplitude of 50% elongation and a frequency of 2 Hz. The resistance of this hydrogel film was measured every 100 cycle for 1000 cycles (Figure 5d). During the cyclic testing, the hydrogel film was submerged in Milli-Q water to avoid undesired water loss during the measurements. No considerable change in the electrical resistance of the hydrogels was noticed during the first 400 cycles for deformation. As cyclic deformation continued beyond 400 cycles, the resistance gradually increased and eventually doubled after 800 cycles. No further increase in the resistance as cycling continued to 1000 cycles was observed pH sensing

The performance of the PEDOT:PSS/PU hydrogel films as pH sensors was next investigated by submerging the PEDOT:PSS/PU films in various HC1 and NaOH aqueous solutions with pH ranging from 3 to 13. The pH sensitivity of hydrogels with 0.625 (Sample C) and 1.25% (Sample D) PEDOT:PSS was explored. Overall, the resistance of the conductive hydrogel films decreased in the acidic pHs, while in the basic environment their resistance increased proportional to pH. Regardless of the pH of the solution, the resistance of the films stabilised after three minutes from when pH was changed. As an example, Figure 6a shows the electrical response of a 220 pm thin PEDOT:PSS/PU hydrogel film (PEDOT:PSS solids %: 1.25%) when the media was switched from Milli-Q water to a pH 2 solution. Here, immediately after the film was exposed to the new acidic solution the resistance reduced, reaching to 50% of its original value after 60 seconds. The drop in resistance plateaued after three minutes, reaching to one tenth of its starting value. The collective response of the PEDOT:PSS/PU films with 1.25% (Sample D) solids fraction % PEDGT:PSS to pH after four minutes is shown in Figure 6b in terms of resistance ratio A clear linear trend can be observed in the resistance ratio of these conductive hydrogel films as pH was increased from acidic (pH 2) to basic (pH 13 ). While for the example shown in Figure 6b the starting media was Milli-Q water, we noticed that the electrical response of the PEDOT:PSS/PU films did not depend on the pH of the starting media; although the response time slightly varied depending on the initial pH of the starting solution.

The remarkable effect of pH on the electrical properties of the PEDOT:PSS/PU hydrogel films originates from the ionic interaction between PEDOT and PSS polymer chains. The conductivity of the PEDOT:PSS system is coupled with the ionic states of each of the constituting components, namely PEDOT and PSS. In aqueous solutions, the morphological structure of PEDOT:PSS is similar to the illustration shown in Figure 6c. It is believed that aqueous PEDOT:PSS dispersions contain water swollen colloidal particles in the range of 20-80 nm. These colloidal particles are composed of short chain PEDOT molecules decorating the longer chain PSS polymers. The negatively charged PSS polymer chains act as counter-ions, compensating the positively charged PEDOT segments. The ionic interaction between PEDOT and PSS chains stabilises the system and at the same time p~ dopes the PEDOT. The level of p-doping of PEDOT directly impacts the conductivity of the whole system and is highly dependent on the charge content of the system. Under slightly acidic conditions, PEDOT chains are uniformly distributed along the PSS polymer chains. This optimised distribution of PEDOT chains within the colloidal PEDOT:PSS system ensures the formation of continuous electrical connections between PEDOT segments. When pH shifts from acidic to more alkaline, the homogeneous distribution of PEDOT along the PSS polymer chains is interrupted by negatively charged hydroxyl groups. In more basic conditions, the PEDOT short chains are neutralised by the hydroxyl groups, forming a new hydrophobic phase which is covered by the long chains of PSS. As a result, not only the level of p-doping of the system changes with pH, but also the PEDOT chains are buried inside the insulating PSS phase. Consequently, the conductivity of PEDOT PSS continuously decreases with an increase pH.

Example 3 - Printing Hydrogel Inks

Rheological characterisation

Rheological properties of the hydrogel inks were studied using an Anton Paar rheometer operating in cone-plate configuration (2°, 15 mm). Frequency sweep tests were performed at 0.1% amplitude over the range of 10 ¹ to IQ ² Hz Storage and loss moduli and complex viscosity of the inks were recorded. Solvent trap was used to prevent any solvent loss during the measurements.

Rheological characterisation performed with an Anton Paar rheometer showed that depending on their formulations, inks had different viscoelastic behaviour ranging from viscous liquid to gel-like. The inks with PEDOT:PSS solids fraction % of 1.25 and 2.5% had flow and gelation behaviours suitable for gel extrusion printing.

Gel extrusion printing

Considering the favourable rheological properties and high conductivity of the ink with PEDOT:PSS solids fraction % of 2 5% (Sample E) (Figure 7a), this ink was selected to print various conductive patterns. The printing process was evaluated by first designing computer-aided design (CAD) circuit models using the Autodesk Investigator software. These CAD files were compiled to stereolithography (STL) files which were then used for 3D printing. Printing was executed by utilising an EnvisionTEC 3D-Bioplotter® printer (Figure 7b) following the process outlined below.

PEDOT:PSS/PU hydrogel inks were 3D printed into different conductive patterns using the EnvisionTEC 3 D-Bioplotter® Manufacturing Series (Gennany). Digital models of the conductive circuits were developed with computer-aided design (CAD) software (Autodesk Inventor Pro). The Constructor software (EnvisionTEC) was used to slice the CAD designs into 160 pm 2D layers suitable for additive manufacturing. The conductive inks were loaded in 30 ml plastic barrels fitted with 200 pm straight needles. The plastic barrels then were inserted into the printers cartridge holder and the temperature of the print head was set to 27 ^C 'C. The printing parameters were optimised for each individual ink to achieve reproducible printed patterns with the conductive inks. For the best performing conductive ink with 1.25% PEDOT/PSS (Sample D), the optimised printing parameters were 0.8 bar and 5-100 mm/sec for, respectively, print pressure and speed. The pre- and post-flows were also optimised to achieve high resolution and high reproducibility. The track size of the printed patterns was -200 pm.

Substrate

The substrate on which the conductive patterns were printed was made from the same PU solution that was used in the preparation of the inks. The PU films of approximately 200 - 300 pm thickness were prepared by solution casting a 5.1 w/v% PU solution in flat petri dishes. The petri dishes were place in a fan-forced oven at 37 °C for several days to remove the EtOH:water solvent. The PU films obtained in this process were directly used as the substrates in the printing process with the substrate temperature set to 40 °C.

Sensors

To fabricate the pH sensors, the PEDOT:PSS/PU ink was 3D printed on dry PU films, and then were submerged in Milli-Q water. Approximately five minutes after rehydration, the PU films with printed PEDOT:PSS/PU patterns were released from the plastic substrates and stored in Milli-Q water. Examples of the 30 printed patterns on free standing hydrogel films are shown in Figure 7c, d. The printed hydrogel sensors remained intact in water for over two months where no considerable loss in conductivity was detected during this period. Additionally, no delamination between the printed conductive tracks and the under layer PU was found. The perfect adhesion between the PU layer and the printed tracks is the solvent used in the ink can also partially dissolve the surface of the PU substrate resulting in the permanent integration of the printed tracks on the PU gel.

For the printing conditions used in this Example, the conductivity of the printed tracks increased with the number of printed layers, as shown in Figure 5a. The performance of one of the printed arrays (Figure 4c) as a pH sensor was evaluated by submerging the top-end of the array in various pH solutions and recording the resistance between the end legs. The resistance ratio of this array varied proportional to pH of the solutions with a slight drift depending on the direction of pH change (Fi gure 8b). In Figure 8b, pH of the testing solutions was increased from 2 to 11 and then decreased back from 11 to 2. There was a four minutes interval between each measurement, during which the array was soaked in Milli-Q water. While in the ascending pH regime the resistance ratio was higher than in the descending pH regime, in both cases the change in resistance remained almost linear with the pH.

Multi-array sensors

pH sensors have also been manufactured using other substrates such as paper and hydrogel-infused paper. Figure 9 is a photograph of a sensor printed on a gel infused paper.

The two inks (Ag and PU/PEDOT :PSS) where printed using the EnvisionTEC, and were printed in one-step. Ag was a commercial ink. The seven-legged electrode design is only for demonstration purposes shows pH can be measured along the length of sensor. The only electrical connection between silver electrodes is through PU/PEDOT :PSS lines. By measuring the resistivity change between any pair of electrodes the pH between those two electrodes can be determined. We have proven that the resistivity (in dry state) varies linearly between any two pair of electrodes where one electrode is always #1 (1-2, 1-3, 1-4, 1-5, 1-6, and 1-7). This linear increase in resistivity is expected based on ohm-law as the distance between electrodes increases, but it proves that the PU/'PF.DOTVPSS lines are uniform along the x-axis.

The impact of PU/PEDOT:PSS pattern on conductivity has also been explored. Figure 10 shows the linear change in resistivity when resistivity was measured between electrodes 1- 2, 1-3, 1-4, 1-5, 1-6, and 1-7 for various patterns. The impact of print pattern of the PU/PEDOT:PSS part on resistivity is clear in Figure 1 1.

The hydrogel-infused substrate was made by placing a dilute solution of P!J in EtOFLWater on various papers. The hydrogel-infused paper gained considerably higher mechanical properties and had an increased water absorbance (compared to normal paper). Three different types of paper including normal A4 print papers were explored. A4 print paper resulted in a suitable substrate because of its mechanical integrity in contact with PU solution. Other suitable papers included filter paper. The properties of paper, such as thickness and wettability, impacted the quality of gel-infused paper.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. In particular, features of any one of the various described examples may be provided in any combination in any of the other described examples. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.