Self-assembly enabled printable asymmetric self-insulated stretchable conductor

Abstract

Embodiments relate to a printable composite material and conductive structures formed from said material via printing and heating. The printable composite material may include at least one liquid metal, at least one conductive polymer, and at least one non-conductive polymer. The conductive structures may have an asymmetric configuration, such as a conductive first surface and an insulated second surface. The conductive structures may further be highly stretchable and have tissue-like mechanical properties.

Description

SELF-ASSEMBLY ENABLED PRINTABLE ASYMMETRIC SELF-INSULATEDSTRETCHABLE CONDUCTORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application No. 63 / 627,299, filed on January 31, 2024. The entirety of this provisional patent application is incorporated by reference herein.FIELD

[0002] Embodiments relate to a printable composite material and conductive structures formed from said material via printing and heating. The conductive structures may have an asymmetric configuration, such as a conductive first surface and an insulated second surface. The conductive structures may further be highly stretchable and have tissue-like mechanical properties.BACKGROUND

[0003] The field of soft and stretchable electronics has attracted considerable interest due to its capacity for wide application in various domains, including soft robotics, skin-integrated electronics, and biomedical devices. The rapid emergence of soft and flexible conductors holds significant importance for the advancement of next-generation electronic devices. These conductors facilitate the seamless integration of electronics in diverse contexts, including on- body, on-skin, and biomedical implants. Despite progress in stretchable conductors, significant challenges persist in simultaneously achieving high electrical conductivity, stretchability, and tissue-like mechanical properties.SUMMARY

[0004] Hydrogel-based stretchable conductors have attracted considerable attention due to their tissue-like mechanical properties, such as a kPa-level modulus that closely resembles that of natural tissue. While these conductors mechanically mimic native organs or tissues, their electrical conductivity is usually low (<1 S cm"1) compared to metal-based alternatives. Low conductivity may lead to issues such as increased power consumption, signal decay, or elevated heating in applications of fabricated soft electronics, especially when applied to electronics with relatively small features.

[0005] On the other hand, gallium-based liquid metals (LM) possess exceptional mechanical, electrical, and thermal properties along with negligible toxicity, making them potential candidates for developing soft and stretchable conductors. Embedding LM particles in different polymer matrixes has been reported for the preparation of stretchable conductors. However, oxide layers form on the surface of LM particles within the LM-polymer composite network, thereby causing discontinuity in the conduction pathway of the LM-polymer composite and significantly diminishing their electrical conductivity. To achieve considerable electrical conductivity (>0.1 S cm-1) of LM-polymer composites, secondary activation procedures such as stretching, compressing, shear friction, localized pressure, mechanical sintering, sound or laser activation have to be applied. The necessity for secondary processing methods in LM-polymer composites introduces additional complexity and potential challenges in the fabrication process. In addition, another problem is the leakage of LM -based conductors, which could cause a shortage of circuits and instant failure of fabricated devices.

[0006] Soft and stretchable conductors with high electrical conductivity and tissue-like mechanical properties are crucial for both on-skin and implantable electronic devices. Liquid metal-based conductors hold great promise due to their metallic conductivity and minimal stiffness. However, as previously noted, the surface oxidation of liquid metal particles in polymeric matrices poses a challenge in forming a continuous pathway for highly conductive elastic composites.

[0007] We have therefore developed a printable composite material based on a liquid metal and a conducting polymer that undergoes a self-assembly process after printing, achieving high conductivity in a bottom surface while maintaining an insulated top surface, high stretchability, and a modulus akin to human skin tissue. In particular, the material can achieve high conductivity without the need for secondary activation procedures that are required in other LM- based conductors, which may be advantageous for large-area, low-cost, and high-throughput fabrication. Moreover, the insulated top surface solves the problem of leakage issues experienced by other LM-based conductors. The printable material may be 3D printed to fabricate sensors, such as skin-interfaced strain sensors.

[0008] In an exemplary embodiment, a printable composite material comprises at least one non- conductive polymer and at least one liquid metal that is included within the at least one non- conductive polymer to stabilize the liquid metal; and at least one conductive polymer.

[0009] In some embodiments, the conductive polymer is selected from the group consisting of poly(3,4-ethylenedioxythiophene), polystyrene sulfonate, polypyrrole, polyacetylene, polyaniline, polythiophene, and mixtures thereof.

[0010] In some embodiments, the non-conductive polymer is hydrophilic polyurethane, poly lactic acid, poly(lactic-co-glycolic acid), and mixtures thereof.

[0011] In some embodiments, the liquid metal is selected from the group consisting of gallium, indium, tin, mercury, bismuth, francium, cesium, and mixtures thereof.

[0012] In an exemplary embodiment, a conductor is formed from a printable composite material comprising at least one non-conductive polymer and at least one liquid metal that is included within the at least one non-conductive polymer to stabilize the liquid metal; and at least one conductive polymer.

[0013] In some embodiments, the conductor comprises a first surface and a second surface opposite of the second surface, wherein the first surface is conductive, and the second surface is simultaneously insulated.

[0014] In some embodiments, the first surface is conductive without a secondary activation procedure.

[0015] In some embodiments, the conductor has an average ultimate strain of at least 800%.

[0016] In some embodiments, the conductor has a Young’s modulus of 1.4 MPa.

[0017] In an exemplary embodiment, a sensor comprises a conductor formed from a printable composite material comprising at least one non-conductive polymer and at least one liquid metal that is included within the at least one non-conductive polymer to stabilize the liquid metal; and at least one conductive polymer.

[0018] In an exemplary embodiment, a method of forming a printable composite material comprises combining at least one liquid metal and a solution comprising at least one non-conductive polymer to form a first mixture; and combining the first mixture and at least one conductive polymer.

[0019] In some embodiments, the solution further comprises an organic solvent.

[0020] In some embodiments, prior to combining the first mixture and at least one conductive polymer, the method further comprises agitating the first mixture to form liquid metal particles.

[0021] In some embodiments, the at least one conductive polymer is provided in an aqueous solution.

[0022] In some embodiments, the aqueous solution comprises a buffer.

[0023] In an exemplary embodiment, a method of forming a conductor comprises printing a composite material onto a substrate to form a structure, the composite material comprising at least one non-conductive polymer and at least one liquid metal that is included within the at least one non-conductive polymer to stabilize the liquid metal, and at least one conductive polymer; and heating the structure to form the conductor.

[0024] In some embodiments, the conductor comprises a first surface and a second surface opposite of the second surface, wherein the first surface is conductive, and the second surface is simultaneously insulated.

[0025] In some embodiments, the first surface is a surface in contact with the substrate during printing.

[0026] Further features, aspects, objects, advantages, and possible applications of the present invention will become apparent from a study of the exemplary embodiments and examples described below, in combination with the Figures, and the appended claims.BRIEF DESCRIPTION OF THE FIGURES

[0027] The above and other objects, aspects, features, advantages, and possible applications of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings. It should be understood that like reference numbers used in the drawings may identify like components.

[0028] FIG. 1 is a schematic illustration of an embodiment of a printed structure formed from an exemplary printable composite material.

[0029] FIG. 2 is a schematic block diagram illustrating an exemplary embodiment of a system for transmitting data collected by a printed conductor / sensor.

[0030] FIG. 3 is a schematic illustration of an exemplary method of forming a printable composite material and a resulting structure via printing and heating.

[0031] FIG. 4 shows photographs of an exemplary printable composite material in a syringe, on a glass substrate, and printed to form a structure.

[0032] FIG. 5 is a schematic illustration of the asymmetric properties of an exemplary printed structure. The top surface of the structure may exhibit electrical insulating properties while the bottom surface of the structure may demonstrate excellent electrical conductivity. Zoom-in images are scanning electron microscope (SEM) images of top and bottom surfaces of the printed structure (scale bars are 50 pm).

[0033] FIG. 6 is a schematic illustration of the 3D printability of the composite material.

[0034] FIG. 7 is a photograph of 3D printing of the composite material on a glass substrate (scale bare is 10 mm).

[0035] FIG. 8 is a photograph of a resulting structure printed from the composite material shown in FIG. 7 (scale bare is 10 mm).

[0036] FIG. 9 shows photographs demonstrating the insulating properties of the top surface of an exemplary printed structure, and highly conductive properties of the bottom surface of the exemplary printed structure. A light-emitting diode (LED) connected to the top surface was unable to light up, while an LED connected to the bottom surface lit up.

[0037] FIG. 10 is a graph demonstrating the electrical conductivity of the bottom surface of an exemplary printed structure, the top surface of the exemplary printed structure, and a direct mixture of hydrophilic polyurethane with PEDOT:PSS.

[0038] FIG. 11 is a graph demonstrating the conductivity of the bottom surface of an exemplary printed structure and the top surface of the exemplary printed structure with an upside-down setup during baking.

[0039] FIG. 12 is a graph showing a high-resolution X-ray photoelectron spectroscopy (XPS) spectrum of Ga3d6 on the bottom surface of an exemplary printed structure.

[0040] FIG. 13 is a graph showing a high-resolution XPS spectrum of Ga3d6 on the top surface of an exemplary printed structure.

[0041] FIG. 14 is a graph showing a high-resolution XPS survey spectrum of the bottom surface of an exemplary printed structure.

[0042] FIG. 15 is a graph showing a high-resolution XPS survey spectrum of the top surface of an exemplary printed structure.

[0043] FIG. 16 is a graph showing a high-resolution XPS survey spectrum of Ga2p3 on the bottom surface of an exemplary printed structure.

[0044] FIG. 17 is a graph showing a high-resolution XPS survey spectrum of Ga2p3 on the top surface of an exemplary printed structure.

[0045] FIG. 18 is a field emission scanning electron microscopy (FESEM) image showing surface morphology of the top surface of an exemplary printed structure.

[0046] FIG. 19 is energy dispersive X-ray spectroscopy (EDS) mapping showing elemental distribution of Ga, In and S on the top surface of an exemplary printed structure.

[0047] FIG. 20 is a FESEM image showing surface morphology of the bottom surface of an exemplary printed structure.

[0048] FIG. 21 is EDS mapping showing elemental distribution of Ga, In and S on the top surface of an exemplary printed structure.

[0049] FIG. 22 shows cross-sectional FESEM image and EDS images of an exemplary printed structure demonstrating elemental distribution of Ga, In and S.

[0050] FIG. 23 is an EDS spectrum showing intensities of different components in the top surface of an exemplary printed structure.

[0051] FIG. 24 is an EDS spectrum showing intensities of different components in the bottom surface of an exemplary printed structure.

[0052] FIG. 25 shows photographs demonstrating an exemplary structure printed in a dog bone shape at 0% strain and 500% strain. Photographs of hand stretching of an exemplary structure printed in a rectangular shape is also shown.

[0053] FIG. 26 is a graph showing ultimate strain of an exemplary printed structure and a direct mixture of PEDOT:PSS with liquid metal (PEDOT-LM).

[0054] FIG. 27 is a graph showing a stress-strain curve for an exemplary printed structure.

[0055] FIG. 28 is a graph showing electrical conductivity versus tensile cycles for multi-cycle tensile stretch of an exemplary printed structure.

[0056] FIG. 29 is a graph including plots for normalized electrical resistance (normalized against resistance at non-deformed state, R / Ro, left axis) and conductivity (right axis) versus engineering strain of an exemplary printed structure.

[0057] FIG. 30 is a graph showing a viscosity vs. shear rate profile of an exemplary printable composite material and directly mixed PEDOT-LM ink.

[0058] FIG. 31 is a photograph of a hanging filament demonstration to exhibit continuous filament and uniform extrusion of an exemplary printable composite material.

[0059] FIG. 32 is a photograph of an exemplary printing process for a composite material with a 100pm nozzle.

[0060] FIG. 33 shows photographs of various 3D printed shapes (e.g., hexagon, star, happy smiley, wow smiley, circle, and half-moon).

[0061] FIG. 34 shows sequential snapshots for 3D printing of a 45 layered rectangular structure.

[0062] FIG. 35 shows a layer-by-layer 3D printing of an exemplary diverse complex structure.

[0063] FIG. 36 shows a layer-by-layer 3D printing of an exemplary diverse complex structure.

[0064] FIG. 37 is a schematic illustration of a wearable device formed from an exemplary printable composite material.

[0065] FIG. 38 shows photographs of an exemplary wearable device at 0% and 250% strain.

[0066] FIG. 39 is electromyography (EMG) signal recorded using an exemplary wearable device.

[0067] FIG. 40 is a graph demonstrating an exemplary wearable strain sensor identifying a wide range of various bending angles of the finger.

[0068] FIG. 41 is a graph demonstrating an exemplary wearable strain sensor being over multiple bending cycles.DETAILED DESCRIPTION OF THE INVENTION

[0069] The following description is of exemplary embodiments presently contemplated for carrying out the present invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles and features of the present invention. The scope of the present invention should be determined with reference to the claims.

[0070] Embodiments relate to printable composite materials including at least one liquid metal, at least one non-conductive polymer, and at least one conductive polymer. The printable composite material may be used in a three-dimensional (3D) printer to create a printed structure, and the at least one liquid metal and the at least one conductive polymer may work in conjunction to create a conductive network within the printed structure.

[0071] As used herein, the term “liquid metal” refers to a metal that is in a liquid state at room temperature. A liquid metal may be selected from the group consisting of gallium, indium, tin, mercury, bismuth, francium, cesium, and mixtures thereof. The material may include any number of different liquid metal(s), such as one or more liquid metals, or a plurality of liquid metals. In preferred embodiments, the liquid metal(s) utilized in the material are biocompatible. The liquid metal may be present in the material in any suitable amount, such as between 8-80%, or between 8-10%, of the total volume.

[0072] In some embodiments, the liquid metal(s) are present in the material in particulate form (e.g., as particles). For example, liquid metal can be processed so that the liquid metal is adjusted into a particulate form for inclusion in the material.

[0073] The non-conductive polymer(s) may be selected from the group consisting of hydrophilic polyurethane (HPU), poly lactic acid (PLA), poly(lactic-co-glycolic acid) (PLGA), and mixtures thereof. Other embodiments may utilize another type of non-conductive polymeric material as well. The material may include any number of different non-conductive polymer(s), such as one or more non-conductive polymers, or a plurality of non-conductive polymers. In preferred embodiments, the non-conductive polymer(s) are biocompatible. The non-conductive polymer(s) may be present in the material in any suitable amount, such as between 17-22 wt%, or about 20 wt%. based on the total weight of the material.

[0074] As seen in FIG. 1, in some embodiments, the non-conductive polymer(s) may form a matrix, and the liquid metal(s) may be embedded and / or dispersed throughout the matrix.

[0075] The conductive polymer(s) may be selected from the group consisting of poly(3,4- ethylenedioxythiophene) (PEDOT), polystyrene sulfonate (PSS), polypyrrole (PPy), polyacetylene, polyaniline (PANI), polythiophene (PTh), and mixtures thereof. Other embodiments may utilize another type of conductive polymeric material as well. The material may include any number of different conductive polymer(s), such as one or more conductive polymers, or a plurality of conductive polymers. In preferred embodiments, the conductive polymer(s) are biocompatible. The conductive polymer(s) may be present in the material in any suitable amount, such as between 5-8 wt%, or about 6 wt%, based on the total weight of the material. It is contemplated that weight percentages greater than 8% may clog the nozzle during printing.

[0076] In some embodiments, the non-conductive polymer(s) may form a matrix, and the conductive polymer(s) may be embedded and / or dispersed throughout the matrix (see FIG. 1). Inpreferred embodiments, the liquid metal(s) and conductive polymer(s) may be embedded and / or dispersed throughout the matrix such that they work in conjunction to create a conductive network within the matrix.

[0077] Embodiments further relate to methods of forming the printable composite material.Printable composite materials may be prepared in any appropriate manner. Methods may include combining the above-listed components (e.g., at least one liquid metal, at least one non- conductive polymer, and at least one conductive polymer) to form a printable composite material. In some embodiments, the order of addition of the components may not be particularly limited.

[0078] In an exemplary embodiment, a printable composite material may be formed by first combining the liquid metals and the non-conductive polymers. In some embodiments, the non- conductive polymer(s) may be provided in a solution comprising the non-conductive polymer(s) and a solvent (e.g., ethanol, acetone, chloroform, or any other suitable solvent), and the liquid metal(s) may be subsequently combined with the solution. In some embodiments, the liquid metal(s) and the non-conductive polymers may be agitated after their combination. Such agitation may enable to the liquid metal(s) to be present in the material in their particulate form (e.g. bulk liquid metal turn into small particles (pm in size) due to agitation)

[0079] The conductive polymer(s) may then be added to the combination of liquid metal(s) and non-conductive polymer(s) to form the printable composite material. In some embodiments, the conductive polymer(s) may be provided in an aqueous solution comprising the conductive polymer(s). The aqueous solution / conductive polymer(s) may be acidic, and in such embodiments, the aqueous solution may further comprise a buffer (e.g., phosphate-bufferedsaline (PBS)), or a buffer may be added to the material separate from the aqueous solution, such that the acidic properties of the aqueous solution / conductive polymer(s) and oxidation of the liquid metals are mitigated.

[0080] Embodiments further relate to printed structures formed from printable composite materials. A printed structure may be any 3D structure formed from the printable composite material using a 3D printer. In some embodiments, a printed structure may be a multi-layer part created using a 3D printer. A printed structure may be any shape and / or size.

[0081] In some embodiments, the printed structure may be heated (e.g., 140°C) after printing. The heating step may remove solvents involved in the formation of the printable composite material that may compromise electrical and / or mechanical properties of the structure. As described in further detail below, application of heat may also facilitate the development of a structure with both a conductive pathway and a non-conductive membrane through a selfassembly process.

[0082] As printed structures may have at least a conductive pathway, the printed structures may be configured as a conductor. As used herein, the term “conductor” refers to a structure that allows electricity to flow through it. The conductor may be of any shape and / or size.

[0083] The substrate may be any material on which a structure may be printed. The substrate may include a material such as glass, metals, polymers (e.g., polydimethylsiloxane (PDMS)), etc., though the substrate material is not particularly limited. In preferred embodiments, the substrate may be smooth to help facilitate removal of the printed structure from the substrate.

[0084] In exemplary embodiments, a printed structure has at least one first surface and at least one second surface. A first (bottom) surface may be defined as a surface in contact with thesubstrate during printing. A second (top) surface may be defined as a surface exposed to the environment during printing. In some embodiments, first surfaces may be positioned opposite the second surfaces.

[0085] The printed structure may be asymmetrical such that the first (bottom) surface of the structure is conductive while the second (top) surface of the structure is simultaneously insulated (e.g., non-conductive). In particular, application of heat to the structure after printing may facilitate the development of a conductive pathway on the first surface and a non-conductive membrane on the second surface through a self-assembly process. For example, during the printing process (and subsequent heating), liquid metals in contact with the first surface may adhere to the substrate and spread to form a percolating conductive network, while liquid metals on the second surface may be unable to adhere to any surfaces to minimize their surface energy and maintain their particle morphology. Therefore, the liquid metals may not be able to form continuous conductive pathways on the second surface.

[0086] While conductive networks are formed by liquid metals in the first surface, gaps within the network may be present. The conductive polymers may therefore serve as a conductive bridging component to fill and connect these gaps, thereby enabling a more consistent and resilient electrical conductivity across the first surface of the structure.

[0087] The printed structures may form a conductive network without the need for secondary activation procedures (e.g., stretching, compressing, shear friction, localized pressure, mechanical sintering, sound, or laser activation, etc.). In particular, the printed structures may allow liquid metals to form continuous conductive pathways on the first surface instead of isolated liquid metal particles that can be easily oxidized to form oxide shells that constrains theconductivity of the structure. Printed structures may therefore achieve high conductivity without undergoing secondary activation procedure.

[0088] In some embodiments, printed structures may have a conductivity at its first surface between 1200 S / cm to 2100 S / cm.

[0089] The second surface may be an insulating oxidation layer, or a non-conductive membrane. The terms “insulating layer,” “oxidation layer,” and “non-conductive membrane” are used interchangeably herein.

[0090] The difference in electrical conductivity between the first and second layers may be attributed to self-assembly of liquid metals. During the printing and heating process, the liquid metals may migrate towards the first surface and form a conductive pathway on the first surface with the help of the interface on the substrate that mitigates the liquid metal surface tension compared to the interface between the air / environment on the second surface of the structure. In contrast, insulating properties on the second surface can be attributed to the formation of an oxidation layer. The liquid metal(s) in the insulating layer may undergo passivation in an oxygen-rich environment, resulting in the development of an oxidation layer, which may lead to the formation of an insulated second surface.

[0091] Printed structures may have a high stretchability. In some embodiments, printed structures may have an average ultimate strain of at least 800%. Printed structures may also have a Young’s modulus similar to natural tissue. In some embodiments, printed structures have a modulus between 1 and 2 MPa, such as about 1.4 MPa, which is similar to that of the native tissues of muscle and skin.

[0092] Embodiments further relate to methods of using printed structures formed from printable composite materials. As previously noted, due to their conductive pathways, printed structures may be configured as conductors.

[0093] In an exemplary embodiment, at least one conductor 100 may be used in a sensor 102. Conductors 100 may facilitate the flow of electrical signals within sensors 102. In some embodiments, at least one conductor 100 may be used as electrodes, which may interact with a surrounding environment to measure parameters.

[0094] In some embodiments, a conductor 100 may be used in wearable and implantable sensors, such that a sensor 102 may be integrated with a subject (e.g., internally and / or externally) for continuous detection and analysis of a parameter. The conductors’ stretchability and mechanical properties make them particularly well-suited for wearable applications, as the inconsistencies between tissues and the conductors are minimized. The conductors may be configured to accommodate any body part and / or contour and properly function while undergoing stresses and / or deformations.

[0095] As used herein, the term “subject” may refer to any biological system to which conductors / sensors can be applied, including without limitation, humans and other animals (e.g., dogs, cats, horses, cows, cattle, etc.).

[0096] In some embodiments, conductors 100 may be used in electromyography (EMG) sensors, electrocardiogram (ECG) sensors, electroencephalogram (EEG) sensors or any other sensor, particularly sensors for bioelectrical applications. For example, the conductor 100 can be utilized in a transceiver element of a sensor 102 for the transmission of sensor data and / or receipt of data from a device communicatively connected to the sensor 102.

[0097] Referring to FIG. 2, the sensor 102 including the conductor 100 can be hardwire connected to an input / output device 104 (e.g., a smart phone, tablet, laptop computer, personal computer, etc.), or the sensor 102 can be communicatively connected to the input / output device 104 via a network connection or wireless connection (e.g., internet connection, wide area network connection, near field communication connection, Bluetooth connection, etc.).

[0098] In some embodiments, the input / output device 104 can be configured to receive data from the sensor 102 / conductor 100 for storage and analysis. In some implementations, the input / output device 104 can be configured as a server or cloud-based service providing device for storage and analysis of the data obtained via the sensori 02 / conductor 100. The data can be communicated to a user via display device, which can be a tablet, smart phone, laptop computer, personal computer, or other type of terminal device. The display device can be effectuated via an application programming interface (API) and / or use of an application stored on the display device. It is contemplated that the input / output device 104 can comprise the display device, or the display device can be a separate device.

[0099] In some embodiments, the sensor 102 can alternatively or subsequently be sent to a central computer device 106 (e.g., a server, an operator workstation, etc.) that can be hardwire connected to the sensor 102 and / or the input / output device 104, or can be communicatively connected to the sensor 102 and / or the input / output 104 device via a network connection and / or a wireless connection. The central computer device 106 can be configured to store, analyze, and / or display data received from the sensor 102 / conductor 100 and / or input / output device 104.

[0100] In some embodiments, the collected data can be continuously streamed to the input / output device 104 and / or the central computer device 106. In other embodiments, thecollected data can be periodically streamed to the input / output device 104 and / or the central computer device 106 (e.g., non-continuously at pre-determined intervals). Embodiments of the sensor 102 / conductor 100 can be configured to provide real time data collection.

[0101] The input / output device 104 and / or the central computer device 106 can be a computer device that can include a processor (Proc.) connected to a non-transitory memory (Mem.) and at least one transceiver (Trcvr) for forming communicative connections with one or more other devices. The at least one transceiver (Trcvr) can include a Bluetooth module and / or other type of transceiver unit (Trcvr). The processor can be hardware (e.g., processor, integrated circuit, central processing unit, microprocessor, core processor, computer device, etc.), configured to perform operations by execution of instructions embodied in algorithms, data processing program logic, artificial intelligence programming, automated reasoning programming, etc. that can be defined by code stored in the memory. The processor can facilitate receipt, processing, and / or storage of readings from the sensor 102 / conductor 102 and / or control transmission of the collected data to input / output device 104 and / or the central computer device 106.

[0102] It should be noted that use of processors herein can include hardware, such as for example any one or combination of a Graphics Processing Unit (GPU), a Field Programmable Gate Array (FPGA), a Central Processing Unit (CPU), a microprocessor, a processor, etc. The processor can include one or more processing or operating modules. A processing or operating module can be a software or firmware operating module configured to implement any of the functions disclosed herein. The processing or operating module can be embodied as software and stored in non-transitory memory; the memory being operatively associated with the processor. Aprocessing module can be embodied running a web application, a desktop application, a console application, etc.

[0103] The memory (Mem.) can be a non-transitory computer readable memory configured to store data. Embodiments of the memory can include a processor module and other circuitry to allow for the transfer of data to and from the memory, which can include to and from other components of a communication system. This transfer can be via hardwired links or wireless transmission communication links. The communication system can include transceivers, which can be used in combination with switches, receivers, transmitters, routers, gateways, waveguides, etc. to facilitate communications between different devices via a communication approach or protocol for controlled and coordinated signal transmission and processing to any other component or combination of components of the communication system. The transmission can be via a communication link, which can be a wireless type of communication connection and / or a wired type of connection.

[0104] The computer or non-transitory machine-readable medium can be configured to store one or more instructions thereon. The instructions can be in the form of algorithms, program logic, etc. that cause the processor to execute any of the functions disclosed herein.

[0105] The processor can be in communication with other processors of other devices (e.g., additional external device, a computer system, a laptop computer, a desktop computer, etc.). An exemplary other device can be a Bluetooth enabled device, near field communication device, etc. Any of those other devices can include any of the exemplary processors disclosed herein as well as transceivers or other communication devices / circuitry to facilitate transmission and reception of wireless signals or other type of communicative connections.

[0106] Either the input / output device 104 and / or the central computer device 106 can be configured to be connected to other input devices and output devices. Examples of input devices can include a scanner device (e.g., scanner), a microphone, a keyboard, a touch screen, a button, a sensor a detector, or other type of input device. Examples of output devices can include a display, a printer, a speaker, or other type of output device.

[0107] As noted above, once collected data is transmitted to the input / output device 104 and / or the central computer device 106, the data can be analyzed and evaluated to determine an output. The output can be based on an evaluation of the electromyography (EMG) sensor data, electrocardiogram (ECG) data, or other data that may be transmitted via the conductor(s) 100 of the sensor(s) 102. For instance, the evaluation of the data and / or processing of the sensor data can result in a display of data, display of graphs or other output based on the data, the inclusion of data in a graphical user interface (GUI) or other display element to be displayed by an output device or input / output device 104.

[0108] In some embodiments, the data can be processed and evaluated to determine one or more parameters. In some embodiments, the data can be processed using artificial intelligence or machine learning algorithms stored on the input / output device 104 and / or the server central computer device 106. In particular, the input / output device 104 and / or the server central computer device 106 can run a program that uses the collected data along with a module trained via a machine learning process that received the collected sensor data and processes that data to determine an output.

[0109] Embodiments further relate to methods of using printed structures formed from printable composite materials. As previously noted, due to their conductive pathways, printed structures may be configured as conductors.

[0110] In an exemplary embodiment, a printed structure may be used in (or as) a wire. For example, as wires are configured to carry electrical current from one point to another, due to the conductive pathways of the above described printed structures, they may be formed and / or shaped into a wire.

[0111] In some embodiments, the printed structure may be surrounded by or wrapped in an insulating material. Such insulated material may prevent unintended contact between the printed structure and other conductive surfaces, which may cause short circuits and / or electrical shocks.EXAMPLES

[0112] Below are examples of specific embodiments for carrying out the present application. The examples are offered for illustrative purposes only, and are not intended to limit the scope of the present application in any way. Efforts have been made to ensure accuracy with respect to numbers used (e.g., amounts, temperatures, etc.), but some experimental error and deviation should, of course, be allowed for.

[0113] Design and Implementation of Printable Composite Material / PrintedStructure (Conductor): The process of developing a printable asymmetric self-insulated stretchable conductor is illustrated in FIG. 3. The printed conductor may be enabled by a unique combination of a highly conductive component, a soft and robust non-conductive component, and a polymeric conductive component. The initial step in the development process of theprintable composite material involved integrating liquid metals (LM) into an ethanol solution containing hydrophilic polyurethane (HPPU), and the LM underwent a transformation into particles through agitation. Subsequently, an aqueous solution that contains a polymeric conductive component, PEDOT: PSS, was introduced into the matrix for the formation of the printable composite material (FIG. 4). Considering that the acidity of PEDOT:PSS solution in deionized (DI) water could accelerate the oxidation of LM, phosphate-buffered saline (PBS) was employed to mitigate the acidic properties of PEDOT:PSS and minimize the oxidation of LM. Compared to simple mixture of HPPU and liquid metal, where liquid metal particles are not stable as evidenced by the sustaining movement of LM particles within the polymer matrix, the printable composite material demonstrated significantly improved stability without such motion observed.

[0114] The prepared material was then transferred to syringe barrels and printed on glass substrates using a precision 3D printer. Following the printing of the material to fomi sample structures, the printed samples were subject to thermal treatment. The application of heat may facilitate the development of a conductive pathway on the bottom surface and a non-conductive membrane on the top surface through a self-assembly process. During the printing process and subsequent solvent evaporation, the LM particles in contact with the bottom surface adhered to the substrate and spread to form a percolating conductive network, while those on the top surface were unable to adhere to any surfaces to minimize their surface energy and maintains their particle morphology. Therefore, they were not able to form continuous pathways on the top surface (FIGS. 1, 5). The heating process also removed solvents involved in the materialpreparation that could compromise the electrical and mechanical properties of the soft conductor.3D printing process can be employed to fabricate the conductor to various features (FIGS. 6-8).

[0115] Electrical and Mechanical Properties: The successful implementation of the asymmetric self-insulated stretchable conductor may be demonstrated by the observed electrical conductivity and mechanical properties of the fabricated conductor. Initial observations demonstrated that a light-emitting diode (LED) connected to an electrical circuit through the bottom surface of the conductor could successfully be illuminated (FIG. 9). However, the LED connected to the same electrical circuit through the top surface of the conductor was not illuminated (FIG. 9). This outcome clearly demonstrated that the conductor material on the bottom surface possessed the necessary electrical conductivity to complete the circuit and enabled the flow of current, leading to the illumination of the LED. In contrast, the top surface of the conductor failed to conduct any electricity. This observation revealed the significant disparity in electrical conductivity between the two surfaces. Further quantitative characterization demonstrated that the conductor exhibits a significantly higher electrical conductivity (2089 S cm'1) on its bottom surface than its top surface and a direct mixture of HPPU with PEDOT:PSS at the same ratio (FIG. 10).

[0116] The mechanism of the conductor in assembling a percolating network of LM pathways to form a conductive bottom surface and an insulating top surface was then investigated. To determine whether it is the gravity or the interface disparity that caused the apparent asymmetric properties of the conductor, the substrate containing freshly printed material was turned over and baked inside an oven at 140°C. Interestingly, the bottom layer of the resulting sample remained conductive, and the top layer remained insulating (FIG. 11). Thatis, even if the bottom surface was placed in an inverted position, which was averse to the effect of gravity, it was still highly conductive, while the top surface that was placed in a way that was advantageous for gravity to take effect, remains insulating.

[0117] The X-ray photoelectron spectroscopy (XPS) analysis (FIGS. 12-17) conducted on the samples indicates a discernible variation in the chemical composition between the top and bottom surface of the conductor. The X-ray photoelectron spectroscopy (XPS) analysis of the sample reveals the predominant presence of carbon, oxygen, sulfur, and gallium, which can be attributed to the constituents HPPU, PEDOT: PSS, and LM, respectively. The high-resolution Ga3d spectrum (FIG. 12) exhibits distinct peaks at 20.7 and 18.6 eV on the bottom surface of conductor, which can be ascribed to the presence of Ga3+and Ga°. The higher peak observed at 18.6 eV suggests a greater abundance of Ga° compared to Ga3+, indicating a stronger presence of metallic Ga. This can be attributed to the enhanced conductivity of the bottom surface.Conversely, the top surface (FIG. 13) exhibits a discernible peak at 20.4eV, indicating the existence of Ga3+ions. This observation can be attributed to the presence of a gallium oxidation layer. The absence of the Ga metal peak at 18.6eV on the top surface suggests the lack of conductive Ga metal presence. Therefore, we concluded that the conductor allowed liquid metal particles to form continuous pathways on its bottom surface instead of isolated liquid metal particles that can be easily oxidized to form oxide shells that constrains the conductivity of the composite. Therefore, it can achieve significantly higher conductivity without undergoing secondary activation procedure.

[0118] Additionally, SEM images and EDS elemental mapping (FIGS. 18-21) show the surface morphology of the top and bottom surface of the conductor and provide insight into thedistribution and arrangement of LM on the polymer surface. The SEM image (FIG. 18) and EDS elemental mapping (FIG. 19) of the top surface revealed the presence of sporadic pockets of liquid metal on the polymer surface. A continuous network of liquid metal was notably absent on the top surface, contributing to its insulating behavior. In contrast, the bottom surface (FIGS. 20- 21) exhibited networks of LM conductive pathways, resulting in high electrical conductivity on the bottom surface. The asymmetric distribution of LM in the conductor’s matrix was further confirmed by cross-sectional SEM image (FIG. 22) and the elemental analysis of EDS (FIGS. 23-24). Notably, while conductive networks are formed by the liquid metal assemblies in the bottom surface of the conductor, small gaps within the network are visible (Figure S5, Supporting Information). The incorporation of PEDOT:PSS, serving as a conductive bridging component, effectively fills and connects these gaps, thereby guaranteeing a more consistent and resilient electrical conductivity across the entire bottom surface of the conductor. In addition to the superior electrical properties, unique asymmetricity, and self-insulation properties, the conductor also possesses superior mechanical properties. The conductor is highly stretchable (FIG. 25) with an average ultimate strain higher than 800% (FIG. 26). The conductor also has a Young’s modulus of ~1.4 MPa (FIG. 27), which is similar to that of the native tissues of muscle and skin. The conductor remained highly conductive even after undergoing 500 cycles of 25% tensile strain (FIG. 28). Interestingly, we observed that the application of tensile stress resulted in an increase in conductivity. The conductivity increased from 2089 S cm'1to 3873 S cm'1after the first cycle. The increased conductivity can be ascribed to the rupture of the oxide layer present on the liquid metal surface upon stretching at the first cycle. The conductivity kept increasing until 80% strain (FIG. 29).

[0119] Additive Manufacturing Capabilities: The viscoelastic measurement (FIG. 30) demonstrated shear thinning characteristics of the composite material and printability. The hanging filament quantification test (FIG. 31) revealed an extruded filament length of 60 mm for the material, indicating that the material can be extruded uniformly into filaments and used to print multi-layer constructions. The exceptional printability of the composite material facilitates a wide range of advanced 3D printing functionalities, including the capacity to produce intricate structures with high resolution and high aspect ratio (FIGS. 32-36). To showcase the printability of composite material in intricate structures, we conducted experiments involving the printing of various structures such as hexagon, stars, faces, etc. (FIGS. 32-33). We also successfully printed a rectangular shape with multiple layers (FIG. 34), as well as other 3D objects (FIGS. 35-36), demonstrating its ability to be printed to robust 3D structures.

[0120] Human Interfacing Devices: Utilizing the readily applicable nature of the printable composite material and taking advantage of multi-material 3D printing, we further demonstrated swift fabrication of wearable devices using 3D printing techniques (FIG. 37). The 3D printed flexible electronic device enabled by the material can be easily stretched for more than 250% (FIG. 38). The fabricated device can be used to acquire electromyogram (EMG) signals on the skin for tracking muscle activities. Obvious EMG activities were recorded when bicep brachii muscles were undergoing contraction (FIG. 39). Furthermore, the material was applied to fabricate soft and stretchable on-skin strain sensors through 3D printing. The wearable strain sensor was able to identify a wide range of various bending angles of the finger. As shown in FIG. 40, the angle of finger bending had a pronounced effect on the current flow through the device. Notably, the current remained stable when the finger was held in a fixed bending position(FIG. 40). In addition, the strain sensor was able to track the angles of finger bending during rapid finger bending motion (FIG. 41). Moreover, it can accurately and reliably sense finger bending angles over multiple bending cycles (FIG. 41), indicating significant promise for on-skin gesture detection systems.

[0121] It should be understood that the disclosure of a range of values is a disclosure of every numerical value within that range, including the end points. It should also be appreciated that some components, features, and / or configurations may be described in connection with only one particular embodiment, but these same components, features, and / or configurations can be applied or used with many other embodiments and should be considered applicable to the other embodiments, unless stated otherwise or unless such a component, feature, and / or configuration is technically impossible to use with the other embodiment. Thus, the components, features, and / or configurations of the various embodiments can be combined together in any manner and such combinations are expressly contemplated and disclosed by this statement.

[0122] It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible considering the above teachings of the disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein.Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention, which is to be given the full breadth thereof.

[0123] It should be understood that modifications to the embodiments disclosed herein can be made to meet a particular set of design criteria. Therefore, while certain exemplary embodiments of the apparatus and methods of using and making the same disclosed herein have beendiscussed and illustrated, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A printable composite material comprising: at least one non-conductive polymer and at least one liquid metal that is included within the at least one non-conductive polymer to stabilize the liquid metal; and at least one conductive polymer.

2. The printable composite material of claim 1 , wherein the conductive polymer is selected from the group consisting of poly(3,4-ethylenedioxythiophene), polystyrene sulfonate, polypyrrole, polyacetylene, polyaniline, polythiophene, and mixtures thereof.

3. The printable composite material of claim 1, wherein the non-conductive polymer is hydrophilic polyurethane, poly lactic acid, poly(lactic-co-glycolic acid), and mixtures thereof.

4. The printable composite material of claim 1 , wherein the liquid metal is selected from the group consisting of gallium, indium, tin, mercury, bismuth, francium, cesium, and mixtures thereof.

5. A conductor formed from the printable composite material of claim 1.

6. The conductor of claim 5, where the conductor comprises: a first surface and a second surface opposite of the second surface,wherein the first surface is conductive, and the second surface is simultaneously insulated.

7. The conductor of claim 6, wherein the first surface is conductive without a secondary activation procedure.

8. The conductor of claim 6, wherein the conductor has an average ultimate strain of at least 800%.

9. The conductor of claim 6, wherein the conductor has a Young’s modulus of 1.4 MPa.

10. A sensor comprising the conductor of claim 4.

11. A method of forming a printable composite material, the method comprising: combining at least one liquid metal and a solution comprising at least one non-conductive polymer to form a first mixture; and combining the first mixture and at least one conductive polymer.

12. The method of claim 11, wherein the solution further comprises an organic solvent.

13. The method of claim 11, wherein prior to combining the first mixture and at least one conductive polymer, the method further comprises:agitating the first mixture to form liquid metal particles.

14. The method of claim 11, wherein the at least one conductive polymer is provided in an aqueous solution.

15. The method of claim 14, wherein the aqueous solution comprises a buffer.

16. A method of forming a conductor, the method comprising: printing a composite material onto a substrate to form a structure, the composite material comprising: at least one non-conductive polymer and at least one liquid metal that is included within the at least one non-conductive polymer to stabilize the liquid metal, and at least one conductive polymer; and heating the structure to form the conductor.

17. The method of claim 16, wherein the conductor comprises: a first surface and a second surface opposite of the second surface, wherein the first surface is conductive, and the second surface is simultaneously insulated.

18. The method of claim 17, wherein the first surface is a surface in contact with the substrate during printing.

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