WO1993000410A1

WO1993000410A1 - Conductive adhesives

Info

Publication number: WO1993000410A1
Application number: PCT/GB1992/001150
Authority: WO
Inventors: Peter Douglas; Stewart Townsend; Hamilton Neil Mcmurray
Original assignee: Clinic-Aid Limited
Priority date: 1991-06-24
Filing date: 1992-06-24
Publication date: 1993-01-07
Also published as: GB2257151A; GB9113617D0

Abstract

A device comprising a non-liquid medium which can be arranged to contact and adhere to a patient's skin. The medium comprises a co-polymer of an acrylic moiety and a polyether, together with a hygroscopic electrolyte capable of conducting electrical signals from the patient's skin. The copolymer can be produced by blending together the polyether, acrylic moiety and electrolytic followed by irradiating the blend to initiate copolymerisation of the polyether and acrylic moiety.

Description

Conductive Adhesives

The present invention relates to devices incorporating conductive adhesives capable of adhering to a patient's skin, and a method of production of such devices.

Currently available conductive media which can be used with skin contact electrocardiograph electrodes are based on materials such as water-based electrolytes or on gelled low molecular weight polymers, such conductive media are preformed prior to contact with the electrodes. All such electrodes suffer from limited shelf-life and a variation of both electrical and adhesive properties when exposed to the atmosphere or in contact with the skin.

We have now developed a conductive gel, and a method of preparation therefor, which gel is suitable for use in skin adhering devices and more specifically skin adhering electrodes, and which alleviates the problems experienced with currently available skin contact electrodes.

According to the present invention there is provided a device capable of adhering : .e skin of a patient, which device comprises a non-fluid medium which can be arranged to contact said patient's skin and adhere thereto, said medium comprising a co¬ polymer of an acrylic moiety and a polyether together with a hygroscopic electrolyte capable of conducting electrical signals from said patient's skin.

Preferably the non-fluid medium is in the form of a gel and the adhesive nature thereof enables the device to be repeatedly removed and repositioned on the skin without substantially losing its skin-adhering properties. Typically the gel is present as a film which has been cast onto a suitable substrate prior to copolymerisation of the acrylic moiety and polyether.

The non-fluid medium is typically hydrated, and contains water in an amount of at least about 15%. Water is present in the medium so as tu substantially prevent "dry- out" of the latter under ambient conditions and is also important in increasing the dielectric constant of the non-fluid medium and hence promoting conductivity by increasing the dissociation of counter-ions which may be paired or aggregated in the non-fluid medium. There is further provided by the present invention an electrode capable of establishing electrical contact with a patient's skin, said electrode comprising conductive means connectable to external monitoring means, and a non-fluid medium which can be arranged to contact said patient's skin and adhere thereto, said medium comprising a co¬ polymer of an acrylic moiety and a polyether together with a hygroscopic electrolyte capable of conducting electrical signals from said patient's skin.

Typically the conductive means comprises a stud of silver chloride or the like. However, in alternative embodiments the conductive means may comprise a nylon mesh or one or more carbon fibre fans coated with silver chloride or the like. Preferably, the conductive means comprises a first end portion embedded within the non-fluid medium and a second end portion protruding therefrom so as to stand proud of the substrate layer.

Preferably the polyether comprises a polyalkyleneglycol, preferably polyethyleneglycol. Polypropylene glycol may be used but an increase in the alkyl chain length results in a decrease of the medium dielectric constant and also a decrease in the ability of the medium to solvate ionic species and to retain its desired water content. ^"Consequently a trend of increasing resistivity is observed with increasing chain length. Preferably the acrylic moiety comprises a polyacrylate such as a polyhydroxyethylacrylate or a polyacrylamide; the acrylic moiety may also comprise trace quantities of a monomer such as a hydroxyethylacrylate. Typically the acrylic moiety and polyether may be present in a ratio ranging from about 1:1 to 1:2.5 by weight.

A polyacrylamide is one of the preferred acrylic moieties largely because of the properties of the acrylamide monomer. Acrylamide monomer radicals are highly reactive and the radical catalysed polymerisation of acrylamide monomers proceeds rapidly under ambient conditions. Furthermore, the polarity of the amide function endows acrylamides with a tendency towards solubility in media having a high dielectric constant. In view of the above properties, acrylamide monomers are ideal for incorporation into formulations for photocured conducting adhesives. The only significant drawback associated with the use of acrylamides is their relative toxicity. Acrylamide itself and its principal derivatives are moderately toxic in single doses, a characteristic which does not, in itself, preclude their use under properly controlled conditions. However, certain members of the acrylamide family, particularly acrylamide itself, exhibit severe and cumulative neurotoxicity. Toxicity of this nature requires that when the monomer is handled industrially it is under conditions which preclude human contact therewith. Furthermore, although polyacrylamides are non-toxic, the possibility of residual monomer persisting subsequent to polymerisation makes neurotoxic acrylamides untenable for use in skin contacting formulations.

However, certain N-substituted acrylamide derivatives are currently available which are non-neurotoxic and which possess moderate to low acute toxicity ratings and as such can be incorporated in skin contact devices. Examples of such acrylamides are N,N- methylene-bis-acrylamide, N,N-dimethyl acrylamide, N,N-diethyl acrylamide, N-tert-butyl acrylamide and diacetone acrylamide.

Homopolymers derived from the latter acrylamide monomers do not typically exhibit intrinsic pressure-senstive adhesive properties unless externally plasticised by liquids or low melting solids of appropriate cohesive energy density. Such liquids or low melting melting point solids include glycerol, propanediol, trimethylol propane, polyethylene glycol and water.

Alternatively, the polyacrylamides can be internally plasticised so as to enhance their chain mobility and hence their adhesive properties. Internal plasticisation can be achieved by copolymerisation of the acrylic monomers with bulky functionalised comonomers. Examples of the latter inclwa polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate and glyceryl monomethacrylate.

Preferably the electrolyte comprises a strongly ionised salt such as a chloride salt which can be dissolved in the non-fluid medium. The chloride salt is highly hygroscopic and typically up to 20% w/w water is retained in the non-fluid medium when it is allowed to equilibrate with air at about 45 % relative humidity. In a first embodiment the salt may be dissolved in the non-fluid medium but separate from the copolymer; examples of such salts include lithium chloride (which is often preferred), calcium chloride, sodium chloride and tetraethylammonium chloride. Alternatively the salt may be incorporated into an ionic monomer which is copolymerisable with either the polyacrylate or polyamide. Examples of such ionic monomers include methacrylatopropyl-trimethyl ammonium chloride, methacrylatoethyl-trimethyl ammonium chloride, methacrylamidopropyl-trimethyl ammonium chloride and vinyl trimethyl ammonium chloride.

Typically the copolymer is a cross-linked adhesive gel; the adhesive gel should not swell significantly in water and only a small fraction of the polyether should be extractable. The cross-linking results in elastic properties which preclude viscous flow and the resulting problem of adhesive residue remaining on the skin. The cross-linking may also render the copolymer insoluble, thereby making it less susceptible to the influence of humidity at the skin surface.

Crosslinking is most readily achieved by copolymerisation with a difunctional monomer during photocuring. For the cured adhesive to retain sufficient tack the difunctional monomer should comprise a small fraction, typically < 1 % by weight, of the total copolymer. Suitable polar difunctional monomers include polyethyleneglycol dimethacrylate, polyethyleneglycol divinyl ether, N,N methylene bis acrylamide and ethylene- glycol dimethacrylate.

Preferably the acrylic moiety and polyether are copolymerised by in situ photopolymerisation which is generally initiated by a free-radical initiator.

It is envisaged that both the skin attachment device and the electrode may further incorporate a pressure-sensitive adhesive for more secure attachment to the skin. Such a pressure-sensitive adhesive would generally be used in long term monitoring or stress testing situations.

There is further provided by the present invention a kit comprising a device capable of adhering to the skin or an electrode as hereinbefore described together with a non- adhesive layer to which the non-fluid medium can adhere, such that said non-adhesive layer can be arranged as a cover for a least one exposed face of the non-fluid medium. The non-adhesive layer is provided to facilitate handling of both the skin adhering device and the electrode, because the non-fluid medium tends to be of a tacky constituency. Typically the non-adhesive layer is peeled off the non-fluid medium directly before attachment to the skin.

There is further provided by the present invention a method of preparing a device comprising a non-fluid medium, said non-fluid medium being capable of adhering to the skin of a patient, said method comprising:

(a) blending together a polyether, an acrylic moiety and a hygroscopic electrolyte capable of conducting electrical signals from said patient's skin, and a photopolymerisation initiator; and

(b) irradiating the blend so as to initiate copolymerisation of said polyether and said acrylic moiety.

In step (a) of the above method, the acrylic moiety preferably comprises a monomer such as a hydroxyethylacrylate monomer when it is initially blended with the polyether and electrolyte prior to photopolymerisation of the blend in step (b). Optionally step (a) can further comprise a polymerisation stage of said monomer, which polymerisation is photopolymerisation as described in step (b).

Typically, step (a) of the above method may comprise blending together further ingredients, such as allyl, vinyl, acrylic or methacrylic monomers (a preferred monomer being N-vinylpyrrolidone) or polymers bearing acrylic functionality (such as, but not limited to, polyacrylates).

A non-fluid medium of a device obtained by the above method contains substantially no monomeric components, but essentially only polymers or copolymers derived from the monomers present in step (a) together with a polyether and a hygroscopic electrolyte. In certain embodiments, the blend is poured onto a substrate prior to the copolymerisation of step (b) so as to form a two-layer structure. Where the device is required to be used as an electrode, the substrate preferably has attached therethrough conducting means which can be arranged in use to transmit electrical signals from the non- fluid medium to external monitoring means.

The individual ingredients used in the method are generally as described with reference to the skin-adhering device according to the invention.

The invention will now be further illustrated, with reference to the following examples, which do not limit the scope of the invention in any way.

Example 1

This example is for comparative purposes only and illustrates the properties of known polymer films which are not photopolymerised in situ.

Polymers were dissolved in water to give solutions which were 10% w/w polymer. To these solutions was added sufficient glycerol to provide the desired plasticisation rate in the final film. In each case, sodium chloride was added, to the extent that it constituted 2% w/w of the total solids in the formulation.

Test films were cast onto a glass surface using a stainless steel casting blade, the height of the films being varied to keep the final thickness of the dried film between 100 and 150 microns. Drying was achieved by evaporation of water at 60°C in a vacuum oven.

The tack and resistivity properties of various films were measured and the results are shown in the following Table 1: Table 1

Hydroxypropyl cellulose and polyethylene oxide underwent phase separation with glycerol as plasticiser.

* Tack was not measurable because the adhesive force to the probe was greater than the cohesive force of the liquid. 4- Tack here refers to adhesion to the surround on which the aqueous gel is carried. Water-soluble polymers in the high molecular weight range with desirable adhesive properties tend to have very high viscosities in aqueous solutions. Consequently polymer concentrations in casting solutions could not be increased appreciably beyond 10% w/w without these becoming very difficult to handle. This meant that cast plasticised films had to lose at least 50% w/w during drying, which made the latter a time-consuming process (approximately 20 minutes). Example 2

Hydroxyethylacrylate monomer was blended with a polyethyleneglycol prepolymer with a molecular weight of 600 and an aqueous solution (40 % w/w) of lithium chloride. The photo-initiation system employed was a water solubilised benzophenone. This was incorporated at a level of 1% w/w of total solids. Triethanolamine^*was included in formulations as a synergistic co-initiator at a level of 2% w/w total solids. Photopolymerisation was effected by exposure to radiation from a 1.5kw (input power) medium pressure mercury arc lamp (Hanovia). Polymerisation was generally completed in less than 5 seconds.

The premix described above was poured out onto hydrophobic polymer substrates to a thickness of approximately 1mm and was then polymerised. Subsequent to polymerisation the product was found to be a cross-linked adhesive gel. Cross-linking occurred due to trace quantities of ethyleneglycol diacrylate present in the hydroxyacrylate and also, probably, chain grafting. The adhesive was found not to swell significantly in water, and only a fraction of the polyethyleneglycol was found to be extractable.

A prepolymer/monomer film as described above is prepared incorporating different electrolyte quantities and the tack and resistivity properties of each film are shown in the following Table 2:

Table 2

Monomer/Prepolymer Tack Resistivity

Ratio % LiCl T/g cm-² (R/KPohm cm-^Ll

Effect of Salt Concentration 2.0 300.0 26.0 '

1:1.5 4.0 263.0 17.0

8.0 238.0 12.0

16.0 200.0 4.0 Table 3 shows the characteristic properties of various polyacrylate/polyether films:

Table 3

Monomer/Prepolymer Ratio Effect of Polymer Composition ^♦(Ratio of polyacrylate to polyether)

1.1

1:1.5

1.2

1:2.5

1:3

1:3.5 Promeon

Example 3

23mm wide strips of non-fluid media comprising copolymers according to the present invention were applied to the cleaned, dry skin of a researcher and the minimum force required to sustain constant velocity peeling measured.

A replaceability assessment, in which the electrode configurations were peeled from the skin, replaced, re-peeled and the cycle repeated, until the electrode was unusable (in the sense that it did not continue to adhere to the skin or give an acceptable cardiogram) was also conducted.

Th skin-peel and replacement characteristics of various copolymers together with the percentage of electrode configurations remaining on the body for 48 hours are shown in the following Table 4. Table 4

"Peelability" is the number of replacements tolerated; and " % pass rate" is the number passing a 48 hour on person test; PA:PE ratio is the ratio of polyacrylate to polyether and PAM is polyacrylamide. Example 4

An assessment of sensitivity of response of non-fluid media comprising copolymers according to the present invention to cardiac output as a function of area of polymer was conducted by averaging output from an ECG monitor, over 10 typical peak to peak waveforms. The results are shown in Table 5.

Table 5 Polyacrylate/polyether 1:2.5 photo-cured tests on skin (20% LiCl)

Arbitrary Sensitivity O/P from ECG Monitor. Averaged over

Dimensions t pical eak- eak readings on of Polymer

3.7cm x 3.7cm

3.7cm x 2.5cm

2.5cm x 2.9cm

1.7cm x 1.7cm

Measurement Sens

Note:

Noise and base line drift became more predominant with the smallest area. Example 5

The following example illustrates the preparation of a typical formulation for use in an adhesive medium according to the present invention. lOg of N,N dimethyl acrylamide was blended with 0.05g of 1-hydroxy cyclohexyl phenyl ketone (IRGACURE 184), 0.05g of triethylene glycol dimethacrylate, 30g of glycerol, lOg of CaCl₂ and 15g H₂0. The resulting mixture was coated onto an aluminised polyester substrate at a thickness of 0.5mm. Photocuring was effected by exposure, in air, of the coated substrate to radiation from a medium pressure mercury lamp (80 watt cm-¹) over a 5 second period. The result was an extremely tacky pressure-sensitive adhesive having excellent adhesion to the substrate and a volume resistivity of 1400 cm"¹. The adhesive was evenly cured throughout and showed minimal change in weight and conductivity with time when maintained at 40% relative humidity.

Claims

CLAIMS:

1. A device capable of adhering to the skin of a patient, which device comprises a non- liquid medium which can be arranged to contact said patient's skin and adhere thereto, said medium comprising a co-polymer of an acrylic moiety and a polyether together with a hygroscopic electrolyte capable of conducting electrical signals from said patient's skin.

2. A device according to claim 1, wherein said non-liquid medium contains water in an amount of at least 15 % .

3. A device according to claim 1 or 2, wherein said polyether comprises a polyalkyleneglycol.

4. A device according to claim 3, wherein said polyalkyleneglycol comprises a polyethyleneglycol or polypropyleneglycol.

5. A device according to any of claims 1 to 4, wherein said acrylic moiety comprises a polyacrylate or a polyacrylamide.

6. A device according to claim 5, wherein said polyacrylate comprises polyhydroxyethylacrylate.

7. A device according to claim 5, wherein said polyacrylamide contains repeating units derived from one or more of N,N-methylene-bis-acrylamide, N,N-dimethyl acrylamide, N,N-diethyl acrylamide, N-tert-butyl acrylamide and diacetone acrylamide.

8. A device according to claim 7, wherein said acrylamide is plasticised with one or more of glycerol, propanediol, trimethylol propane, polyethylene glycol and water.

9. A device according to claim 7, wherein said acrylamide is co-polymerised with any of polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate and glyceryl monomethacrylate.

10. A device according to any of claims 1 to 9, wherein said acrylic moiety comprises a monomer.

11. A device according to claim 10, wherein said monomer comprises hydroxyethylacrylate.

12. A device according to any of claims 1 to 11, wherein said acrylic moiety and polyether are present in a ratio ranging from about 1:1 to 1:2.5 by weight.

13. A device according to any of claims 1 to 12, wherein said electrolyte comprises a chloride salt.

14. A device according to claim 13, wherein said chloride salt comprises lithium chloride, calcium chloride, sodium chloride or tetraethylammonium chloride.

15. A device according to claim 13, wherein said chloride salt comprises methacrylatopropyl-trimethyl ammonium chloride, methacrylatoethyl-trimethyl ammonium chloride, methacrylamidopropyl-trimethyl ammonium chloride or vinyl trimethyl ammonium chloride.

16. A device according to any of claims 1 to 15, wherein said non-liquid medium comprises a cross-linked adhesive gel.

17. A device according to claim 16, wherein said cross-linked gel contains a co-polymer of polyethyleneglycol dimethacrylate, polyethyleneglycol divinyl ether, N,N methylene bis acrylamide or ethyleneglycol dimethacrylate.

18. A device according to claim 16 or 17, wherein said acrylic moiety and polyether are copolymerised by in situ photopolymerisation.

19. A device according to any of claims 1 to 18, which comprises a pressure-sensitive adhesive.

20. An electrode capable of establishing electrical contact with a patient's skin, said electrode comprising conductive means connectable to external monitoring means, and a device according to any of claims 1 to 19.

21. An electrode according to claim 20 or 21, wherein said conductive means comprises a stud of silver chloride.

22. An electrode according to claim 20, wherein said conductive means comprises a first end portion embedded within said non-fluid medium and a second end portion protruding therefrom so as to stand proud of said device.

23. A kit comprising a device capable of adhering to the skin according to any of claims 1 to 19, or an electrode according to any of claims 20 to 22, together with a non- adhesive layer to which the non-fluid medium can adhere, such that said non-adhesive layer can be arranged as a cover for at least one exposed face of the non-fluid medium.

24. A method of preparing a device according to any of claims 1 to 19, said method comprising:

25. A method according to claim 24, wherein said acrylic moiety comprises hydroxyethylacrylate monomer.

26. A method according to claim 25, wherein step (a) involves photopolymerisation of said hydroxyethylacrylate monomer.

27. A method according to any of claims 24 to 26, wherein step (a) comprises blending together allyl, vinyl, acrylic or methacrylic monomers or polymers bearing acrylic functionality.

28. A method according to any of claims 24 to 27, wherein the blend of step (a) is poured onto a substrate prior to the copolymerisation of step (b) so as to form a two-layer structure.