BACKGROUND

Conductive elastomers have been used for developing soft electrodes, soft actuators, and soft sensors. Such elastomers are particularly important for biopotential electrodes, which convert motoneuron signals into electrical signals. The electrical signals are then processed and amplified for external device control. To maximize the performance of the biopotential electrodes, high conductivity is required, as well as good compatibility with the human body. However, to increase the conductivity, high volumes of conductive filler loading are usually needed. Such high levels of filler decrease the softness and compressibility' of the elastomer, resulting in discomfort for the user and a reduced elastomer reliability.

SUMMARY

Described herein are conductive elastomeric materials and methods of using the same. A conductive elastomeric material as described herein comprises a polymeric matrix and one or more high aspect-ratio (e.g., one-dimensional (ID)) fillers, wherein the high aspect-ratio filler is present in an amount of 50 wt. % or less based on the weight of the conductive elastomeric material. The polymeric matrix can comprise a thermoset polymer or a thermoplastic polymer.

Optionally, the polymeric matrix comprises a thermoset polymer, and the thermoset polymer is selected from the group consisting of a silicone, a urethane, an acrylate, a methacrylate, a fluoropolymer, an epoxy, a thiol-ene, an unsaturated ester, a phenolic resin, and combinations thereof. In some examples, the thermoset polymer is a silicone (e.g., a condensation crosslinkable silicone, such as a tin-catalyzed condensation crosslinkable silicone including a two-part tin-catalyzed condensation crosslinkable silicone).

Optionally, the polymeric matrix comprises a thermoplastic polymer. The thermoplastic polymer can be selected from the group consisting of styrenic block copolymers, thermoplastic polyurethanes, and thermoplastic silicones. Optionally, the thermoplastic polymer is a styrenic block copolymer and wherein the styrenic block copolymer is selected from the group consisting of a styrene-ethylene/butylene-styrene copolymer (SEBS), a styrene-ethylene/proylene-styrene copolymer (SEPS), a styrene- isoprene-styrene block polymer (SIS), and a styrene-butadiene-styrene block polymer (SBS).

The high-aspect ratio filler can be, for example, a one-dimensional (ID) filler, a two- dimensional (2D) filler, or a mixture thereof. The high-aspect ratio filler for use in the conductive elastomeric materials can be, for example, an inorganic filler. Optionally, the high-aspect ratio filler comprises a conductive filler, such as a carbon-based filler. The carbon-based filler can comprise carbon nanotubes, carbon nanofibers, or combinations thereof. In some examples, a surface of the carbon-based filler is functionalized. Optionally , the surface of the carbon-based filler is functionalized with a hydroxyl group, a carboxylic group, a thiol group, or an amino group.

In some cases, the carbon-based filler comprises carbon nanotubes having a diameter from 1 nm to 100 nm (e.g., from 2 nm to 90 nm, from 5 nm to 50 nm, or from 8 nm to 15 nm). Optionally, the carbon-based filler comprises carbon nanofibers having a diameter from 100 nm to 1000 nm (e.g., from 100 nm to 500 nm or from 130 nm to 200 nm). Optionally, the carbon-based filler comprises a combination of carbon nanotubes and carbon nanofibers. A weight ratio of the carbon nanotubes to the carbon nanofibers can be selected to provide synergistic conductive effects to the materials described herein. In some cases, a weight ratio of the carbon nanotubes to the carbon nanofibers is from 1:0.5 to 1 :8, 1: 1 to 1.5, or from 1:2 to 1:4.

Optionally, the high-aspect ratio filler is present in an amount of 5 wt. % to 50 wt. % based on the weight of the conductive elastomeric material. For example, the high-aspect ratio filler can be present in an amount of 8 wt. % to 25 wt. % based on the weight of the conductive elastomeric material.

The conductive elastomeric materials described herein can optionally comprise one or more additional additives. Optionally, the one or more additional additives is selected from the group consisting of dispersants, plasticizers, surfactants, thixotropic agents, and diluents.

Also described herein is a method of making a conductive elastomeric material as described herein, comprising mixing a polymeric matrix and a high aspect-ratio (e.g., one- dimensional (ID)) filler, wherein the mixing is performed using speed-mixing, internal mixing, ball milling, planetary milling, roll-milling, or an attritor. Optionally, the method can further comprise processing the conductive elastomeric material into a molded product. The processing can be performed using compression molding, injection molding or dispensing, three-dimensional processing, freeform fabrication, or direct write extrusion. The method described herein can further comprise coating one or more surfaces with a polymer or other material, the coating comprising dip-coating, ink-jet printing, slot-die coating, screenprinting, aerosol jetting, electrochemical deposition, or a surface treatment using oxygen plasma, a silane treatment, or a corona surface treatment.

Further described herein is a molded product, comprising a conductive elastomeric material as described herein. Optionally, the molded product comprises a cylindrical shape having a diameter from 1 mm to 10 mm (e.g., from 3 mm to 7 mm). Optionally, the molded product is a flat shape, a microneedle, a fuzzy structure, a dome shape, a hollow structure, or a cone shape. In some cases, at least one surface of the molded product is surface roughened. The roughened surface is achieved, in some examples, through etching (e.g., reactive-ion etching) or diamond grounding. Optionally, at least one surface of the molded product is surface coated with a coating material. The coating material can optionally be selected from the group consisting of poly (3, 4-ethylenedioxy thiophene) polysty rene sulfonate (PEDOT:PSS), a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, a polymer binder, polyaniline, polypyrrole, silver nanowires (AgNW), gold nanowires (AuNW), liquid metal, and gold.

The molded product as described herein can exhibit a bulk conductivity of 10 Ohm- cm or lower. Optionally, the tensile strength of the molded product is 1 MPa or greater (e.g., 5 MPa or greater). In some cases, the modulus of the molded product is 5 MPa or lower (e.g., 1 MPa or lower). Optionally, the hardness of the molded product is 90 Shore A or lower (e.g., 50 Shore A or lower). The skin contact impedance of the molded product on the skin of a subject can be 1 MOhms or lower (e.g., 0.5 MOhms or lower) with a geometric contact area of 120 mm ² . In some cases, the molded product comprises an electrode (e.g., a biopotential electrode).

Further described herein is a wearable device comprising a molded product as described herein. Optionally, the wearable device can be a wristband or a monolithic conductive band. The wearable device can collect biopotential signals.

The details of one or more embodiments are set forth in the drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. IB are pictures of the conductive elastomers formed into microneedle and fuzzy surface electrodes. FIG. 1C is a picture of the conductive elastomers formed into special geometries.

FIG. 2 is a graph showing the average skin impedance results for the tested electrodes on a subject’s forearm. The tested electrodes include soft electrodes containing 5 wt. % aligned multi-walled carbon nanotubes (MWCNTs) (first, leftmost bar); 5 wt. % aligned MWCNTs and 5 wt. % carbon nanofibers (second bar); 5 wt. % aligned MWCNTs and 10 wt. % carbon nanofibers (third bar); and gold electrodes for comparative purposes (fourth bar).

FIG. 3 is a graph showing the average skin impedance on a subject’s wrist for both gold and soft silicone electrodes as described herein.

DETAILED DESCRIPTION

Described herein are novel conductive elastomeric materials, including a polymeric matrix loaded with one or more high aspect-ratio (e.g., one dimensional (ID)) fillers in an appropriate amount to achieve a desirable balance of conductivity and stretchability. In some cases, combinations of high aspect-ratio fillers, such as ID fillers, are included in the matrix, surprisingly resulting in a synergic enhancement of the bulk conductivity. In addition, the skin-impedance of the conductive materials described herein is desirably lower than the skinimpedance of commercial conductive materials (e.g., commercially available silicones, having high filler loadings).

The elastomeric materials described herein have been successfully processed and shaped into materials such as soft electrodes (e g., biopotential electrodes, including electromyography (EMG) electrodes, electrocardiogram (ECG) electrodes, electroencephalogram (EEG) electrodes, and the like) and additionally placed within wearable devices to effectively capture and collect signals. Notably, the soft electrodes prepared from the conductive elastomeric materials described herein perform similarly to, or more effectively than, gold coated brass electrodes typically used in biopotential devices.

Specifically, the conductive elastomeric materials described herein include a polymeric matrix and one or more high aspect-ratio (e.g., one-dimensional (ID)) fillers. The high aspect-ratio filler loading is lower than that of commercially used elastomeric matenals, such as 50 wt. % or less based on the weight of the conductive elastomeric material. Even with the lower filler loading, the materials herein, including the high aspect-ratio fillers and in some cases, a special combination of high aspect-ratio fillers (e.g., combinations of ID fillers) that synergistically operate to enhance the performance of the materials, achieve the desired conductivity and stretchability. Such properties enhance user comfort when the materials are incorporated, for instance, into a w earable material or device.

Polymeric Matrix

As noted above, the elastomeric materials described herein include a polymeric matrix. The polymeric matrix can be prepared from a thermoset polymer, a thermoplastic polymer, or a combination of a thermoset polymer and a thermoplastic polymer. As known to those of skill in the art, thermoplastic polymers are capable of melting and reflowing and are soluble in solvents. Thermoset polymers, after they are cured or crosslinked, are not soluble in solvents and will not reflow when heated. As further detailed herein, both types of polymers are suitable for use as the polymer matrix.

Thermoset polymers include polymer materials in which chemical reactions, including cross-linking, occur while the resins are being molded. The appearance, chemical properties, and physical properties of the final product are changed, and the product is generally resistant to further applications of heat. Optionally, the thermoset polymer for use in the elastomeric materials described herein can be a silicone, a urethane, an acrylate, a methacrylate, a fluoropolymer, an epoxy, a thiol-ene, an unsaturated ester, a phenolic resin, or any suitable combination of these.

By way of example, the thermoset polymer can be a silicone polymer is that crosslinkable by condensation. The polymer can be crosslinked using, for example, a tin- catalyzed condensation reaction, such as a two-part tin-catalyzed condensation crosslinking reaction.

Optionally, the silicone resin used as the matrix can be prepared by using a 2-part tin- catalyzed condensation crosslinkable silicone depicted in Scheme 1 below.

Scheme 1:

In Scheme 1, the R group and R' group in Part B can each independently be an alkyl or an aryl. As used herein, alkyl refers to straight- and branched-chain monovalent substituents. Ranges of these groups useful with the compounds and methods described herein include C1-C20 alkyl, Ci-Cls alkyl, C1-C16 alkyl, C1-C14 alkyl, C1-C12 alkyl, C1-C10 alkyl, Ci-Cs alkyl, Ci-Ce alkyl, and C1-C4 alkyl. Examples include methyl, ethyl, propyl, butyl, isobutyl, octyl, and the like. Aryl molecules include, for example, cyclic hydrocarbons that incorporate one or more planar sets of, typically, six carbon atoms that are connected by delocalized electrons numbering the same as if they consisted of alternating single and double covalent bonds. An example of an ary l molecule is benzene (i.e., a phenyl group).

Part A and Part B can be combined in an appropriate amount to initiate a condensation reaction. Specifically, the terminal hydroxyl groups of Part A can be placed in conditions to initiate a nucleophilic attack of the silicates/silanes of Part B. The nucleophilic attack results in a replacement of the R-O- group in the Part B reactants, forming the crosslinked structure and producing alcohol byproducts (R-OH), including, for example, methanol, ethanol, or propanol depending on the R group present in Part B. Other suitable polymer synthesis methods can be used, as determined by one of skill in the art using the guidance of the present disclosure.

In some cases, the polymeric matrix is or includes a thermoplastic polymer. Thermoplastic polymers are polymers that soften or become plastic when they are heated. The process of heating and cooling such polymers can be carried out repeatedly without affecting any appreciable change in the properties of the polymers. After thermoplastic polymers are synthesized, they can be dissolved in a solvent and applied to surfaces. Additionally, these polymers can be heated, causing them to melt flow and generally develop strong adhesive bonds to a substrate.

Optionally, the thermoplastic polymer can be styrenic block copolymers. By way of example, suitable styrenic block copolymers can be, for example, a styrene- ethylene/butylene-styrene copolymer (SEBS), a styrene-ethylene/proylene-styrene copolymer (SEPS), a styrene-isoprene-styrene block polymer (SIS), or a styrene-butadiene-styrene block polymer (SBS). In some cases, the thermoplastic polymer can be thermoplastic polyurethanes or thermoplastic silicones. Other thermoplastic polymers, as known in the art, can also be used as the polymeric matrix in the materials described herein.

High Aspect-Ratio Fillers

The conductive elastomeric materials described herein also include one or more high aspect-ratio fillers, such as one dimensional (ID) fillers and/or two-dimensional (2D) fillers. In some cases, the high aspect-ratio includes a ID filler. Optionally, the high aspect-ratio filler can include an inorganic filler and can be a conductive filler. Optionally, the high aspect-ratio filler can be a carbon-based filler, such as one or more of carbon nanotubes or carbon nanofibers. One or more surfaces of the filler can be functionalized with a functional group, such as a hydroxyl group (-OH), a carboxylic group (-C(O)O-), a thiol group (-SH), or an amino group (-NH2).

The aforementioned functional groups can optionally be substituted with one or more groups. As used herein, the term substituted includes the addition of an alkoxy, aryloxy, amino, alkyl, alkenyl, alkynyl, aryl, heteroalkyl, heteroalkenyl, heteroalkynyl, heteroaryl, cycloalkyl, or heterocycloalkyl group to a position attached to the main chain of the alkoxy, aryloxy, amino, alkyl, alkenyl, alkynyl, aryl, heteroalkyl, heteroalkenyl, heteroalkynyl, heteroaryl, cycloalkyl, or heterocycloalkyl, e.g., the replacement of a hydrogen by one of these molecules. Examples of substitution groups include, but are not limited to, hydroxy, halogen (e.g., F, Br, Cl, or I), and carboxyl groups. Conversely, as used herein, the term unsubstituted indicates the alkoxy, aryloxy, amino, alkyl, alkenyl, alkynyl, aryl, heteroalkyl, heteroalkenyl, heteroalkynyl, heteroaryl, cycloalkyl, or heterocycloalkyl has a full complement of hydrogens, i.e., commensurate with its saturation level, with no substitutions, e.g., linear decane (-(CFE^-CEE).

The carbon-based fillers for use in the conductive elastomeric materials can have an appropriate size for the desired use. In some cases, carbon nanotubes for use as the carbonbased fillers can have a diameter from 1 nm to 100 nm (e.g., from 2 nm to 90 nm, from 5 nm to 50 nm, or from 8 nm to 15 nm). The diameter of the carbon nanotubes can be, for example 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, or 100 nm.

Optionally, at least 50% of the carbon nanotubes for use as the carbon-based fillers have a diameter in the indicated range (e.g., from 1 nm to 100 nm, from 2 nm to 90 nm, from 5 nm to 50 nm, or from 8 nm to 15 nm). In some cases, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the carbon nanotubes present as carbon-based fillers in the materials have a diameter in the indicated range (e g., from 1 nm to 100 nm, from 2 nm to 90 nm, from 5 nm to 50 nm, or from 8 nm to 15 nm).

In some cases, the amount of carbon nanotubes for use as the carbon-based fillers can be from 2.5 wt. % to 20 wt. % (e.g., 5 wt. % to 15 wt. %) based on the weight of the conductive elastomeric material. For example, the amount of carbon nanotubes in the elastomeric material can be 2.5 wt. %, 3 wt. %, 4 wt. %, 5 wt. %, 6 wt. %, 7 wt. %. 8 wt. %, 9 wt. %, 10 wt. %, 11 wt. %. 12 wt. %, 13 wt. %, 14 wt. %, 15 wt. %, 16 wt. %, 17 wt. %, 18 wt. %, 19 wt. %, or 20 wt. %.

In some cases, the carbon nanofibers for use as the carbon-based fillers can have a diameter from 100 nm to 1000 nm (e.g., from 100 nm to 500 nm or from 130 nm to 200 nm). The diameter of the carbon nanofibers can be, for example 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm.

Optionally, at least 50% of the carbon nanofibers for use as the carbon-based fillers have a diameter in the indicated range (e.g., from 100 nm to 500 nm or from 130 nm to 200 nm). In some cases, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of the carbon nanofibers present as carbon-based fillers in the materials have a diameter in the indicated range (e.g., from 100 nm to 500 nm or from 130 nm to 200 nm).

In some cases, the amount of carbon nanofibers for use as the carbon-based fillers can be from 2.5 wt. % to 20 wt. % (e.g., 5 wt. % to 15 wt. %) based on the weight of the conductive elastomeric material. For example, the amount of carbon nanofibers in the elastomeric material can be 2.5 wt. %, 3 wt. %, 4 wt. %, 5 wt. %, 6 wt. %, 7 wt. %, 8 wt. %, 9 wt. %, 10 wt. %, 11 wt. %, 12 wt. %, 13 wt. %, 14 wt. %, 15 wt. %, 16 wt. %, 17 wt. %, 18 wt. %, 19 wt. %, or 20 wt. %.

In some cases, the carbon-based filler comprises a combination of carbon nanotubes and carbon nanofibers. As described herein and demonstrated in the examples, the combination of carbon nanotubes and carbon nanofibers in the appropriate amounts can synergistically impact the performance of the elastomeric materials (e.g., by desirably low ering the skin-electrode contact impedance of the materials). By way of example, a weight ratio of the carbon nanotubes to the carbon nanofibers can from 1 :0.5 to 1:8 (e.g., from 1:1 to 1:5, from 1: 1.5 to 1 :5, or from 1:2 to 1 :4). In some cases, the weight ratio ofthe carbon nanotubes to the carbon nanofibers for inclusion in the elastomeric materials can be 1:0.5, 1:0.6, 1 :0.7, 1 :0.8, 1:0.9, 1: 1, 1 : 1.1, 1:1.2, 1: 1.3, 1: 1.4, 1:1.5, 1 : 1.6, 1: 1.7, 1 : 1.8, 1 :1.9, 1:2, 1 :2.5, 1 :3, 1:3.5, 1:4, 1 :4.5, 1 :5, 1:5.5, 1:6, 1 :6.5, 1 :7, 1:7.5, or 1:8.

In some cases, the total amount of high aspect-ratio filler present in the conductive elastomeric materials can be 50 wt. % or less (e.g., from 5 wt. % to 50 wt. %, from 5 wt. % to 35 wt. % or from 8 wt. % to 15 wt. %) based on the weight of the conductive elastomeric material. For example, the total amount of high aspect-ratio filler present (e.g., the combined amount of all high aspect-ratio filler types, including, for example, the carbon nanotubes and the carbon nanofibers) can be 5 wt. %, 6 wt. %, 7 wt. %, 8 wt. %, 9 wt. %, 10 wt. %, 11 wt. %, 12 wt. %, 13 wt. %, 14 wt. %, 15 wt. %, 16 wt. %, 17 wt. %, 18 wt. %, 19 wt. %, 20 wt.

%, 21 wt. %, 22 wt. %, 23 wt. %, 24 wt. %, 25 wt. %, 26 wt. %, 27 wt. %, 28 wt. %, 29 wt.

%, 30 wt. %, 31 wt. %, 32 wt. %, 33 wt. %, 34 wt. %, 35 wt. %, 36 wt. %, 37 wt. %, 38 wt.

%, 39 wt. %, 40 wt. %, 41 wt. %, 42 wt. %, 43 wt. %, 44 wt. %, 45 wt. %, 46 wt. %, 47 wt.

%, 48 wt. %, 49 wt. %, or 50 wt. %.

Additional Additives

The conductive elastomeric materials described herein can optionally include one or more additional additives. Suitable additives for inclusion in the materials described herein can be, for example, one or more of dispersants, plasticizers, surfactants, thixotropic agents, and diluents. Additional additives for use in the materials describe herein can include hardeners, accelerators, thickeners, humectants, desiccants, fire retardants, electrical insulators, vibration dampeners, thermal insulators, corrosion inhibitors, antioxidants, pigments, dyes, magnetic particles, thermochromic agents (i.e., compounds that can change color with changing temperature), mechanochrormc agents (i.e. , compounds that can change color under mechanical deformation), anti-glare agents, anti-reflective agents, infrared reflective agents, stealth agents, textural agents, fragrances, self-cleaning agents, hydrophobic agents, hydrophilic agents, or any combination thereof.

The additional additives can be present in the materials described herein in an amount of 10 wt. % or less based on the weight of the conductive elastomeric material. For example, one or more additional additives can be included in an amount of 0.01 wt. % to 10 wt. %, 0.1 wt. % to 8 wt. %, 0.5 wt. % to 5 wt. %, or 1 wt. % to 3 wt. % based on the weight of the conductive elastomeric material. In some cases, for example for certain diluents and softeners (e.g., silicone oil and mineral oil), the content of the additive can be up to 30 wt. % (e.g., from 1 wt. % to 30 wt. %, from 5 wt. % to 25 wt. %, or from 10 wt. % to 20 wt. %).

Methods of Making and Resulting Products

Also described herein are methods of producing the conductive elastomeric materials described above. The methods for producing the conductive elastomeric materials as described herein can include a step of mixing a polymeric matrix and a high aspect-ratio (e.g., onedimensional (ID)) filler in the requisite amounts as detailed above. The polymer can be included in the mixture in an amount ranging from about 50 % to about 95 % based on the weight of the mixture. For example, the polymer can be present in the molten polymer or polymer mixture in an amount of about 50 %, about 55 %, about 60 %, about 65 %, about 70 %, about 75 %, about 80 %, about 85 %, about 90 %, or about 95 %, based on the weight of the polymer mixture. The filler and optionally any suitable additive mentioned above can be included in their indicated amounts.

The mixing can be performed using any suitable apparatus for the selected components, along with the selected amounts (e.g., laboratory-scale or process-scale). In some examples, the mixing can be performed using speed-mixing, internal mixing, ball milling, planetary milling, roll-milling, or an attritor.

After the mixing has been performed, the resulting mixture can be further processed into a molded product, such as a soft electrode. The processing can be performed using, for example, compression molding, injection molding or dispensing, three-dimensional processing, freeform fabrication, or direct write extrusion.

The molded product can have any suitable shape, and can be dictated by the end use of the product. In some cases, the molded product can have a cylindrical shape. Optionally, the cylindrical shape can have a diameter ranging from, for example, 1 mm to 10 mm (3 mm to 7 mm). Other suitable shapes include, for example, a flat shape, a microneedle, a fuzzy structure, a dome shape, a hollow structure, or a cone shape, as shown in Figs. 2A-2F.

In some cases, at least one surface of the molded product can be surface roughened. The surface roughening can be achieved, for example, through any suitable surface roughening method, such as etching (e.g., reactive-ion etching) or grounding (e.g., diamond grounding).

Optionally, the processing can further comprise coating one or more surfaces of the material with a polymer or another material. The coating material can be, for example, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, a polymer binder, polyaniline, polypyrrole, silver nanowires (AgNW), gold nanowires (AuNW), liquid metal, or gold. In some cases, the coating can be performed by using dip-coating, ink-jet printing, slot-die coating, screen-printing, aerosol jetting, electrochemical deposition, or a surface treatment using oxygen plasma, a silane treatment, a corona surface treatment, or any other suitable method.

The material can also be cured, after the mixing step and/or after the processing step. In some cases, the curing can be performed at room temperature for a period of time (e.g., overnight or from 8 hours to 24 hours). As used herein, the meaning of “room temperature” can include a temperature of from about 15 °C to about 30 °C, for example about 15 °C, about 16 °C, about 17 °C, about 18 °C, about 19 °C, about 20 °C, about 21 °C, about 22 °C, about 23 °C, about 24 °C, about 25 °C, about 26 °C, about 27 °C, about 28 °C, about 29 °C, or about 30 °C. In some cases, the curing can be performed at an elevated temperature (e.g., from 35 °C to 250 °C, from 70 °C to 225 °C, or from 100 °C to 200 °C) for a period of time. In some cases, the period of time can be up to 3 hours (e.g., from 30 seconds to 3 hours, from 1 minute to 2.5 hours, from 10 minutes to 2 hours, or from 20 minutes to 60 minutes).

The mixing, processing, and subsequent steps can be tailored to suit the selected polymeric matrix. For example, the silicone-based elastomers and urethane acrylate-based elastomers as described herein can be cured at room temperature overnight (e.g., 8 to 15 hours). Fluoropolymers as described herein can be prepared, for example, by solvent-assisted compounding, molded at an elevated temperature (e.g., 35 °C to 75 °C), and cured at a temperature of 130 °C to 170 °C for 20 minutes to 60 minutes.

Optionally, the molded product exhibits a bulk conductivity of 10 Ohm-cm or lower as determined by ASTM D991 (2020). For example, the molded product can have a bulk conductivity of 10 Ohm-cm or lower, 9 Ohm-cm or lower, 8 Ohm-cm or lower, 7 Ohm-cm or lower, 6 Ohm-cm or lower, 5 Ohm-cm or lower, 4 Ohm-cm or lower, 3 Ohm-cm or lower, 2 Ohm-cm or lower, 1 Ohm-cm or lower, or 0.5 Ohm-cm or lower. In some cases, the bulk conductivity is from 0.5 Ohm-cm to 10 Ohm-cm (e.g., from 1 Ohm-cm to 9 Ohm-cm or 2 Ohm-cm to 7 Ohm-cm).

The tensile strength of the molded products prepared from the conductive elastomeric materials described herein can be 1 MPa or greater (e.g., 5 MPa or greater) as determined by ASTM D624 (2020). For example, the tensile strength of the molded products can be 1 MPa, 1.5 MPa, 2 MPa, 2.5 MPa, 3 MPa, 3.5 MPa, 4 MPa, 4.5 MPa, 5 MPa, 5.5 MPa, 6 MPa, 6.5 MPa, 7 MPa, 7.5 MPa, 8 MPa, 8.5 MPa, 9 MPa, 9.5 MPa, or 10 MPa.

In some cases, the modulus of the molded product is 5 MPa or lower as determined by ASTM D624 (2020). For example, the molded product can have a modulus of 5 MPa or lower, 4 MPa or lower, 3 MPa or lower, 2 MPa or lower, or 1 MPa or lower. In some cases, the modulus is from 0.5 MPa to 5 MPa, 1 MPa to 4 MPa, or 2 MPa to 3 MPa.

The hardness of the molded product can be 90 Shore A or lower as determined by ASTM D2240-15 (2021). For example, the hardness of the molded product can be 85 Shore A or lower, 80 Shore A or lower, 75 Shore A or lower, 70 Shore A or lower, 65 Shore A or lower, 60 Shore A or lower, 55 Shore A or lower, 50 Shore A or lower, 45 Shore A or lower, 40 Shore A or lower, 35 Shore A or lower, or 30 Shore A or lower. In some instances, when a molded product of an increased hardness is desired, the hardness of molded product can be from greater than 50 Shore A to 100 Shore A. The skin contact impedance of the molded product on the skin of a subject (e.g., on the forearm or wrist of a subject) can be 1 MOhms or lower (e.g., 0.9 MOhms or lower, 0.8 MOhms or lower, 0.7 MOhms or lower, 0.6 MOhms or lower, 0.5 MOhms or lower, 0.4 MOhms or lower, 0.3 MOhms or lower, 0.2 MOhms or lower, or 0.1 MOhms or lower) with a geometric contact area of 120 mm ² . The skin contact impedance can be conducted, for example, on the forearm with a weight positioned on top of the skin (e.g., a 50 g weight) and having an electrode size of 210 mm ² . An Ag/AgCl wet get electrode can be used as the reference electrode.

The molded products described herein can be integrated into a wearable device. Optionally, the wearable devices can be used to collect biopotential signals. Suitable wearable devices include, for example, a wristband. Optionally, the molded product can be a monolithic conductive band, in which the elastomer is molded directly into the band rather than incorporating electrodes into the band. Specifically, electrodes are not needed in the monolithic conductive band since the entirety of the band is actively conductive, thus ensuring high surface area for maximal and effective contact. Beneficially, the monolithic conductive band also minimizes noise which may interfere with signal collection.

Throughout this application, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application.

The examples below are intended to further illustrate certain aspects of the methods and compositions described herein, and are not intended to limit the scope of the claims.

EXAMPLES

Example 1: Fabricated Conductive Silicone Elastomers

Conductive silicone elastomers were fabricated by loading a silicone elastomer with ID carbon nanotubes and/or carbon nanofibers. The silicone resin used as the matrix was prepared by using a 2-part tin-catalyzed condensation crosslinkable silicone, as shown in Scheme 1 (provided above and reproduced below).

Scheme 1:

In Scheme 1, the R group in Part B was selected from a short chain alkyl, such as methyl, ethyl, or propyl, and the R’ group in Part B was selected from a longer chain alkyl or an aryl, such as octyl or phenyl. Part A and Part B were blended together in a 10: 1 ratio to initiate the condensation reaction. Specifically, the terminal hydroxyl groups of Part A were placed in conditions to initiate a nucleophilic attack of the silicates/silanes of Part B. The nucleophilic attack resulted in a replacement of the R-O- group in the Part B reactants, forming the crosslinked structure and producing alcohol byproducts (R-OH), including methanol, ethanol, or propanol depending on the R group present in Part B. Notably, the condensation chemistry performed to prepare the silicate was less sensitive to carbon-based fillers as compared to platinum catalyzed hydrosilylation, for example. The condensation reaction completes at room temperature overnight, or at an elevated temperature of 120 °C for 2 hours. Silicone resins prepared according to this procedure were used to prepare the conductive silicone elastomers, as further detailed below.

Sample 1 was prepared by loading the silicone elastomer with 5 wt. % of aligned multi-walled carbon nanotubes (MWCNTs) having a diameter ranging from 8 to 15 nm. Sample 2 was prepared by preparing a similar loaded silicone elastomer, including 5 wt. % aligned MWCNTs, and further loading 10 wt. % of carbon nanofibers (CNF) having a diameter of 150 nm. The elastomer was cured at room temperature overnight. The mechanical properties of the resulting loaded silicone elastomers were measured, including the tensile strength, Young’s modulus, elongation at break, hardness, and bulk conductivity (Table 1). As shown in Table 1, the ID carbon nanotubes and carbon nanofibers show a synergetic effect to enhance the bulk conductivity. Sample 2, which is the conductive silicone elastomers with 5 wt. % of carbon nanotubes and 10 wt. % of carbon nanofibers, simultaneously achieved a balanced conductivity and stretchability.

Table 1

Example 2: Electrode Fabrication Method

Sample 2, fabricated as detailed above in Example 1, was formed into a soft biopotential electrode. Specifically, compression molding was used to prepare flat surface electrodes. The prepared flat surface electrodes were cylindrical in shape, having a diameter of 6.5 mm and a height of 7 mm.

In addition, the techniques of injection molding and dispensing were used to prepare additional desirable electrode shapes, including a dome surface, a microneedle and fuzzy surface (Fig. 1 A and Fig. IB), and special geometries (Fig. 1C).

Example 3: Electrical Performance (Component Level)

Skin-electrode impedance of the biopotential electrodes made from the conductive elastomers described in Example 2 (i.e., a silicone elastomer including 5 wt. % aligned MWCNT and 10% CNF), with the cylindrical shape of 6.5 mm in diameter and 7 mm in height, were tested using electrochemical impedance spectroscopy. In addition, biopotential electrodes of the same size were prepared from silicone elastomers loaded with 5% aligned MWCNT and silicone elastomers loaded with 5% aligned MWCNT and 5 wt. % CNF. For comparative purposes, commercially available gold pins, which is c360 brass plated with gold (TakeWing, TP015-0101R0A92) were tested. The biopotential electrodes were positioned on the forearm of a subject, and biopotential signals were collected using an electrode-skin impedance system.

The average skin impedance on the subject’s forearm for the tested electrodes is shown in Fig. 2. As shown in Fig. 2, the combination of carbon nanotubes and carbon nanofibers had a synergistic effect in desirably lowering the skin-electrode contact impedance.

Example 4: Electrical Performance (System Level)

Skin-electrode impedance of the biopotential electrodes made from the conductive elastomers described in Example 2, with the cylindrical shape of 6.5 mm in diameter and 7 mm in height, were tested using electrochemical impedance spectroscopy. Briefly, three subjects were fitted with a biopotential wristband to which the soft biopotential electrodes were attached. The biopotential electrodes were positioned at 16 different wrist locations and data were collected on four different days (over the course of a 20 day-period). Biopotential signals were also collected by using gold coated brass electrodes in a wristband. The average skin impedance on the subjects’ wrist, for both the gold and the soft electrodes, is shown in Fig. 3 and Table 2. In this study, the skin-electrode impedance around the wrist is in the range of 0.2-0.5 MOhm.

Table 2

As shown in Table 2 and in Fig. 3, the skin impedance on the wrist was approximately the same for the soft electrodes and the gold electrodes, as the difference as evaluated by the Z score was not statistically significant. Specifically, the Z score of the soft electrode to the Z score of the gold electrode was approximately 1.1 to 1.

Example 5: Biopotential Signals Collected on Bipolar Wristband

Biopotential electrodes were made from the conductive elastomers described in Example 2, with the cylindrical shape of 6.5 mm in diameter and 7 mm in height, and were fitted on a bipolar wristband. For comparative purposes, gold electrodes were also tested and biopotential signals were collected. Filters were applied to remove motion artifacts and a high pass filter was enabled at 40 Hz. Good biopotential signals were collected on both the soft electrodes and gold electrodes on over 10 channels, such that different hand postures could be identified.

Example 6: Fabricated Conductive Elastomers on Other Materials Additional conductive elastomers were fabricated by loading the elastomer with ID carbon nanotubes and/or carbon nanofibers. The elastomers included a silicone elastomer and a urethane acrylate elastomer. The silicone elastomer was prepared as described above in Example 1; see Sample 2 in Table 1. Sample 3, the urethane acrylate sample, was prepared by loading the elastomer w ith 5 wt. % of aligned multi -walled carbon nanotubes (MWCNTs) having a diameter ranging from 8 to 15 nm. The loaded elastomer was cured at room temperature overnight.

The mechanical properties of the resulting loaded elastomers were measured, including the bulk conductivity, hardness, tensile strength, and elongation at break. The feel to touch was also assessed to determine whether the elastomer felt non-tacky (rating 1), slightly tacky (rating 2), or tacky (rating 3). The electrode-skin impedance measurements were also taken at the forearm and the wri st and compared to gold electrodes. See Table 3.

As shown in Table 3, various elastomers can be successfully loaded with conductive ID fillers to achieve desired properties as dictated by the intended use for the elastomer. Table 3

The compositions and methods of the appended claims are not limited in scope by the specific compositions and methods described herein, which are intended as illustrations of a few aspects of the claims and any compositions and methods that are functionally equivalent are within the scope of this disclosure. Various modifications of the compositions and methods in addition to those shown and described herein are intended to fall within the scope of the appended claims. Further, while only certain representative compositions, methods, and aspects of these compositions and methods are specifically described, other compositions and methods are intended to fall within the scope of the appended claims. Thus, a combination of steps, elements, components, or constituents can be explicitly mentioned herein; however, all other combinations of steps, elements, components, and constituents are included, even though not explicitly stated.