The invention relates generally to compositions made from microemulsions and to methods of preparing and using such compositions.
Modern medicine employs many medical procedures where electrical signals or currents are received from or delivered to a patient""s body. The interface between medical equipment used in these procedures and the skin of the patient is usually includes a biomedical electrode. Such an electrode typically includes a conductor connected electrically to the equipment and a conductive medium adhered to or otherwise in contact with the patient""s skin.
Therapeutic procedures and devices that make use of biomedical electrodes include transcutaneous electronic nerve stimulation (TENS) devices for pain management; neuromuscular stimulation (NMS) techniques for treating conditions such as scoliosis; defibrillation electrodes for dispensing electrical energy to a chest cavity to defibrillate the heart; and dispersive electrodes to receive electrical energy dispensed into an incision made during electrosurgery.
Diagnostic procedures that make use of biomedical electrodes include electrocardiograms (ECGs) for monitoring heart activity and diagnosing heart abnormalities.
For each diagnostic, therapeutic, or electrosurgical procedure, at least one biomedical electrode having an ionically conductive medium containing an electrolyte is adhered to or otherwise placed in contact with mammalian skin at a location of interest and also electrically connected to electrical diagnostic, therapeutic, or electrosurgical equipment. A critical component of the biomedical electrode is the conductive medium serving as the interface between mammalian skin and diagnostic, therapeutic, or electrosurgical equipment.
A long understood difficulty in formulating these conductive media is the need to balance conductivity with patient skin adhesion. The best formulations for providing conductivity are typically hydrophilic and need water or an aqueous ionic system to provide required ionic conductivity between the skin and the electrical diagnostic, therapeutic, or electrosurgical instrumentation. However, the best formulations for providing skin adhesion are typically hydrophobic in nature. The art has typically balanced these properties in a particular electrode in a way that optimizes neither skin adhesion nor conductivity.
One method for bypassing this tradeoff is to provide a conductive pressure sensitive adhesive having bicontinuous hydrophilic and hydrophobic regimes. Such adhesives can be formed by way of a microemulsion, and the making and use of such adhesives are taught in coassigned U.S. Pat. Nos. 5,670,557; 5,674,561; 5,779,632; 5,853,750; and 5,952,398, which are hereby incorporated by reference.
While these adhesives have generally acceptable properties after compounding, they have at least one drawback during the process of preparation. Because of the low viscosity of the microemulsion solutions (e.g., less than 50 centipoise), it is not easy to dispense them in a thick, uniform coating on an electrode backing. The hydrophilic and hydrophobic portions of the finished bicontinuous adhesive composition are formed simultaneously when polymerization is initiated in both phases of a microemulsion. Before the polymerization, the composition is inconveniently thin for conventional coating techniques; after polymerization the composition is inconveniently viscous.
This is particularly disadvantageous for biomedical electrodes, where a thick, uniform coating is desired. The thick coating provides proper adhesion and prevents electrodes from rapid dry-out. When bicontinuous adhesives are used for wound dressing, the thick coating provides absorbing capacity. When used for drug delivery, the thick coating is important for drug loading.
In practice, the difficulty is resolved by inducing a partial polymerization of the microemulsion, just enough to bring the viscosity of the composition up to a level appropriate for convenient coating by, e.g., a conventional knife coater. A final polymerization step is induced in the coated material, resulting in the finished bicontinuous adhesive. The process is analogous to that discussed in coassigned U.S. Pat. No. 5,773,485 which is hereby incorporated by reference. Though cumbersome, such a process forms a uniform, thick coating of a pressure sensitive adhesive that simultaneously has good conductivity and good skin adhesion.
However, the extra steps involved in arranging for the first, partial polymerization are burdensome and increase the cost of the finished electrode to the medical consumer. This shortcoming cannot readily be achieved by the indiscriminate addition of thickening agents, as such compounds generally tend to destabilize the microemulsion during coating.
The present invention provides a method for making bicontinuous, conductive, pressure sensitive adhesives (PSAs) that have convenient coating viscosities and that require only a single polymerization step. This is accomplished by the use of a class of thickening agents that is compatible with the microemulsions and does not damage the properties of the final adhesives.
More particularly, in one aspect, the invention provides polymerizable compositions comprising: (a) an aqueous microemulsion comprising one or more hydrophoblic monomers, one or more hydrophilic and/or amphiphilic monomers, one or more initiators; and (b) at least one thickening agent comprising a polymer or copolymer of acrylic acid.
In another aspect, the invention provides methods of forming a polymerized microemulsion pressure sensitive adhesive composition in contact with a substrate, the methods generally comprising the steps of:
(1) providing an aqueous microemulsion comprising one or more hydrophoblic monomers, one or more hydrophilic and/or amphiphilic monomers and one or more initiators;
(2) combining the aqueous microemulsion with at least one thickening agent comprising a polymer or copolymer of acrylic acid;
(3) coating the thickened microemulsion onto the substrate; and
(4) irradiating the microemulsion in order to form the pressure sensitive adhesive composition in contact with the substrate.
In still another aspect, the invention provides methods of forming a polymerized microemulsion pressure sensitive adhesive composition in contact with a substrate, the methods generally comprising the steps of:
(1) mixing hydrophilic monomer(s) and/or amphiphilic monomer(s) in a weight percent ratio of from about 100/0 to about 0/100 to form a first mixture;
(2) mixing hydrophobic monomer(s), having a glass transition temperature suitable for forming a hydrophobic pressure sensitive adhesive, into the first mixture in a weight percent ratio of from about 80/20 to about 10/90 hydrophobic monomers/first mixture to form a second mixture;
(3) mixing surfactant(s) into the second mixture in a weight percent ratio of from about 5/95 to about 30/70 surfactant/second mixture to form a third mixture;
(4) mixing initiator(s) into the third mixture in a weight percent ratio of from about 0.01/99.99 to about 2/98 initiator/third mixture to form a fourth mixture,
(5) independently, mixing water and water-soluble or water-dispersible additives together in a weight percent ratio of from about 100/0 to about 80/20 to form an aqueous mixture;
(6) mixing the aqueous mixture and the fourth mixture together in a weight percent ratio of from 5/95 to about 50/50 aqueous mixture/fourth mixture to form a microemulsion;
(7) mixing the microemulsion with a thickening agent comprising a polymer or copolymer of acrylic acid monomer together in a weight ratio of from about 0.5/99.5 to about 5/95 to form a thickened microemulsion;
(8) coating the thickened microemulsion onto the substrate; and
(9) irradiating the microemulsion in order to form the pressure sensitive adhesive composition in contact with the substrate.
In still other aspects, the invention provides pressure sensitive adhesives, biomedical electrodes, medical skin coverings and pharmaceutical delivery devices made from the above-described compositions and according to the above-described methods.