Patent Application
20220003684
The present disclosure relates to a colorimetric three-layer system composed of a substrate, a metal or metal alloy thin film, and a stimulus-responsive polymer layer that detects changes in environmental conditions brought about by physical, chemical, or biological stimuli and is useful in colorimetric sensors.
In contrast to chemical dyes (chemical colors), structural colorations are widely found in nature such as birds, butterflies, insects, and marine organisms, where colors originate from micro- or nanostructures instead of chemical structures. One main advantage of structural colors is that they are not easily degraded by environmental conditions such as ultraviolet light, heat, oxygen, and moisture. This is because the structural color arises from a physical structure of non-dyes, which is much more stable than a chemical dye structure. Structural coloration arises from the physical interaction of light with micro- or nanostructures via a variety of optical mechanisms, including thin-film interference, multilayer interference, diffraction gratings, photonic crystals, and scattering. Compared with pigmentary coloration, structural coloration is not only more resistant towards the color degradation caused by environmental conditions, but also easily tunable via changes in structural parameters or refractive index. The bioinspired stimuli-responsive structural coloration offers a wide range of promising applications in medical diagnostics, advanced packaging, environmental and building monitoring, adaptive camouflage, intelligent coatings and textiles, and anti-counterfeiting.
Although structural coloration based on thin film interference is well known, research on stimuli-responsive thin film interference has been mainly limited to materials such as inorganic materials, reflectin proteins, multilayers of polyelectrolytes, and hydrogels, which are typically deposited on nontransparent substrates such as silicon wafer. Compared with other substrates such as glass, single crystalline silicon wafer is relatively expensive to produce and has limited area size.
Compared with most inorganic materials, polymer-based materials have many advantages such as low cost, flexibility, good processability, excellent corrosion resistance, and lightweight. Moreover, the new class of smart polymers can sense their environment (e.g. humidity, temperature, chemicals, biomolecules, light, or mechanical forces), and change the shape, volume, or thickness accordingly. For many potential applications, low-cost substrates other than monocrystalline silicon wafer are highly desirable. For example, the glass substrate can be used for applications where large-area structural coloration is required. In addition, transparent glass substrate is required for smart window-related applications. However, thin films of polymers with appropriate thickness that are directly deposited on glass generally do not exhibit visible structural colors (
While remarkable progress has been made in the field of responsive structural coloration based on photonic crystals and multilayer interference, how to make high-quality responsive structural coloration systems on large scale at low cost still remains a challenge. Thin-film interference is the simplest structural coloration mechanism, which is responsible for the colorful, iridescent reflections that can be seen in oil films on water, and soap bubbles. Owing to its design simplicity, which does not require multilayers of materials with alternative refractive indices or micro- and nanostructures, thin film interference represents a promising solution towards scalable and affordable manufacturing of high-quality responsive structural coloration systems.
The Internet of Things (IoT) is a network of broadly defined devices that are used to collect, exchange, and process information, which enables a wide range of transformative applications, such as environmental monitoring, smart home, wearable health-monitoring electronics, and smart farming. One of the critical challenges that significantly limits the implementation and growth of the IoT is exponentially growing power demand by the vast network of electronic devices. For instance, state-of-the-art sensors use electronics to actively monitor the environment for the infrequent target stimulus, consuming power continuously while waiting for the specific signal. Such active electronic sensors not only have high energy footprints, but also have limited sensor lifetime because sensors are always in the working state. Therefore, developing energy efficient sensors are essential to fully realize the potential of the IoT.
One approach is to use photovoltaics to harvest solar energy to power the sensors. Such self-powered sensors can be used as wearable sensors for the precise and continuous monitoring of biological signals. Another approach is to use triboelectric nanogenerators to harvest mechanical energy from the environment to power the sensors. Since both solar and environmental mechanical energy sources are intermittent by nature and they are not always available for conversion into electricity, energy storage devices such as batteries are generally required to ensure the sensor performance.
Colorimetric chemical sensors convert a chemical input signal into an optical output signal. One main advantage of colorimetric sensors is their self-reporting feature that autonomously exhibits a color change upon exposure to a target stimulus without using external power sources, which make them good candidates for IoT applications.
Disclosed herein is a new scalable and affordable platform technology for fabrication of polymer-based, stimuli-responsive interference colored films. The material system is composed of a polymer layer deposited on a metal-coated substrate. The thickness of the polymer layer determines the reflected color, whereas the thickness of the metal thin film controls the intensity of the reflected color. A full spectrum of bright interference colors can be generated on both rigid and soft substrates such as low-cost glass and soft silicone elastomer (e.g. poly dimethylsiloxane (PDMS)) through a facile fabrication method. Moreover, the interference colored films can exhibit fast and reversible color changes in response to various external stimuli. The sensing function can be achieved by choosing suitable polymer structures that can interact with specific external stimuli. Such affordable, scalable polymer-based, responsive interference coloration (RIC) could enable colorimetric sensing of various environmental stimuli (e.g. humidity, temperature, chemicals, biomolecules, light, or mechanical forces), which could enable a broad range of commercial applications.
In one aspect, the invention provides a responsive interference coloration system comprising: (a) a substrate having a first surface; (b) a continuous thin film of a metal or metal alloy on at least a portion of the first surface of the substrate, wherein the thin film has a thickness configured to filter electromagnetic radiation; and (c) a polymer layer coated on the thin film, wherein the polymer of the polymer layer is a stimulus-responsive polymer.
In another aspect, the invention provides an article of manufacture, such as a sensor, comprising the system of the invention.
Another aspect of the invention provides a method of manufacturing the article comprising (a) depositing a metal or metal alloy on at least a portion of a first surface of a substrate, the metal or metal alloy being deposited as a thin film with a thickness configured to filter electromagnetic radiation; and (b) coating a stimulus-responsive polymer on the thin film to form a polymer layer.
Another aspect of the invention provides a method of detecting a change in an environmental condition comprising (a) contacting the article of the invention with a physical, chemical, or biological stimulus; and (b) detecting a change in color and/or shape of the article.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing of the present invention. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. The materials, methods, and examples disclosed herein are illustrative only and not intended to be limiting.
The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that do not preclude the possibility of additional acts or structures. The singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. The present disclosure also contemplates other embodiments “comprising,” “consisting of” and “consisting essentially of,” the embodiments or elements presented herein, whether explicitly set forth or not.
The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” The term “about” may refer to plus or minus 10% of the indicated number. For example, “about 10%” may indicate a range of 9% to 11%, and “about 1” may mean from 0.9-1.1. Other meanings of “about” may be apparent from the context, such as rounding off, so, for example “about 1” may also mean from 0.5 to 1.4.
For the recitation of numeric ranges herein, each intervening number there between with the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the number 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are explicitly contemplated.
The system of the invention generally relates to a metal or metal alloy thin film deposited or coated on a substrate surface and further overlaid with a polymer layer of a stimulus-responsive polymer. The thin film functions as an optical filter that reflects sufficient incident light (i.e., electromagnetic radiation) for constructive interference, while simultaneously filtering out unwanted wavelengths of light. The thin film has a thickness configured to filter electromagnetic radiation, such as visible light, ultraviolet (UV) light, and infrared (IR) light. The thin film thickness determines the intensity of reflected light color for visible light. The stimulus-responsive polymer changes properties (e.g., dimensions) in response to changes in environmental conditions, which manifests as a change in observable color from incident visible light.
The thin metal or metal alloy film may be deposited on the substrate by physical or chemical deposition techniques. Physical deposition or physical vapor deposition techniques include evaporation and sputtering techniques. For example, evaporation may be vacuum thermal evaporation, electron beam evaporation, laser beam evaporation, arc evaporation, molecular beam epitaxy, or ion plating evaporation. Sputtering may be direct current sputtering or radio frequency sputtering. Chemical deposition techniques include sol-gel, chemical bath, spray pyrolysis, plating, and chemical vapor deposition. The plating may be electroplating or electroless deposition.
The thin metal or metal alloy film is a continuous film over at least a portion of the first surface of the substrate. The continuous film is thus distinguished from metallic paint coatings that are characterized by metal flakes powder dispersed throughout the coating.
The thin metal or metal alloy film may be composed of various types of metals or metal alloys selected from aluminum, iridium, silver, nichrome, copper, titanium, chromium, nickel, palladium, zinc, iron, carbon, gallium, indium, silicon, germanium, tin, selenium, or tellurium, or a combination. Preferred metals include iridium, silver, aluminum, copper, iron, zinc, titanium. A suitable alloy is nichrome.
The metal or metal alloy film generally may have a thickness between about 0.5 to about 15 nm. The thickness may be 0.5 to 15 nm, 0.5 to 14 nm, 0.5 to 13 nm, 0.5 to 12 nm, 0.5 toll nm, 0.5 to 10 nm, 0.5 to 9 nm, 0.5 to 8 nm, 0.5 to 7 nm, 0.5 to 6 nm, 0.5 to 5 nm, 0.5 to 4 nm, 0.5 to 3 nm, 0.5 to 2 nm, 0.5 to 1 nm, 1 to 15 nm, 1 to 14 nm, 1 to 13 nm, 1 to 12 nm, 1 to 11 nm, 1 to 10 nm, 1 to 9 nm, 1 to 8 nm, 1 to 7 nm, 1 to 6 nm, 1 to 5 nm, 1 to 4 nm, 1 to 3 nm, 1 to 2 nm, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, or about 10 nm. Any of the metals or metal alloys described herein for the thin film may be used in any thickness.
The polymer layer may be coated on the metal or metal alloy thin film using any suitable polymer coating technique such as spin-coating, dip-coating, spraying, plasma coating, thermal coating, inkjet printing, or chemical vapor deposition. The polymer layer is deposited with a thickness of 5 to 800 nm under ambient and equilibrated conditions. The polymer layer may have a thickness of 700 to 800 nm, 600 to 800 nm, 500 to 800 nm, 400 to 800 nm, 300 to 800 nm, 200 to 800 nm, 100 to 800 nm, 50 to 800 nm, 5 to 10 nm, 5 to 20 nm, 5 to 30 nm, 5 to 40 nm, 5 to 50 nm, 50 to 100 nm, 100 to 200 nm, 200 to 300 nm, 300 to 400 nm, 400 to 500 nm, 500 to 600 nm, or 600 to 700 nm.
The stimulus-responsive polymer (also known as a smart polymer) is any polymer that is responsive to one or more of a physical, chemical, or biological stimulus. A stimulus-responsive polymer changes properties in response to a stimulus from the surrounding environment. Changes in properties include a thickness change, a change in refractive index, a change in shape, or a change of other physical or chemical properties of the polymer layer. Physical stimuli include, for example, heating, cooling, electromagnetic radiation (e.g. UV, visible, IR), an electrical signal, a magnetic signal, or mechanical force (e.g., pressure, vibration such as an acoustic signal). Mechanical forces include stretching, bending, pressing, vibrating, etc. Chemical stimuli include, for example, chemical substances or mixtures of chemical substances. Chemical substances include elements and chemical compounds (e.g., salts, molecules including biomolecules). Chemical substances may be in the form of gas, liquid, solid, or chemical substances dissolved in a solvent. Dissolved chemical substances may be cations, anions, molecules, or biomolecules. A particular cation is H+, the measurement of which in aqueous solution is pH (i.e., the chemical stimulus is pH). Gases include any vapors such as water vapor (i.e., humidity) or solvent vapors, such as vapors of the organic solvents described below. Liquids include water, non-aqueous solvents (e.g., organic solvents such as hydrocarbons (e.g., pentane, hexane), halogenated hydrocarbons (e.g., chloroform, carbon tetrachloride, dichloromethane), alcohols (e.g., methanol, ethanol), ethers (e.g., diethyl ether, tetrahydrofuran), esters (e.g., ethyl acetate), ketones (e.g., acetone, 2-butanone), dimethylsulfoxide, dimethylformamide, N-methylpyrrolidone), or mixtures thereof. A chemical stimulus may be a redox stimulus. Biological stimuli include, for example, glucose or an enzyme. Stimulus-responsive polymers include those described in Cohen Stuart et al., Nature Materials (2010) 9, 101-113; Wei et al., Polym. Chem. (2017) 8, 127; and Ganesh et al., RSC Adv. (2014) 4, 53352, which are incorporated herein by reference.
Suitable classes of polymers include polyvinylpyrrolidone, polyvinylpolypyrrolidone, fluoropolymers, polycarbonate, polystyrene, polyethylene, polypropylene, polyurethane, polyvinyl chloride, polyacrylonitrile, polytetrafluoroethylene, polychlorotrifluoroethylene, phenol-formaldehyde resin, para-aramid, poly(methyl methacrylate), parylene, polyethylene terephthalate, polychloroprene, polyamide, epoxy resins, polyimide, poly-p-phenylene-2,6-benzobisoxazole, polysiloxanes, polyphosphazene, polyarylsulfones, polybutylene, polybutylene terephthalate, polyetheretherketone, polyetherimide, polyetherketoneketone, perfluoroalkoxy resin, polymethyl pentene, poly(p-phenylene), polyethyleneoxide, polyphenylene ether, polyphenylene oxide, polyphenylene sulfide, polyphenylene sulfide sulfone, polyvinyl alcohol, polyvinylidene chloride, polyvinylidene fluoride, polyvinyl fluoride, poly(lactic acid), polyisoprene, styrene-butadiene rubber, poly(vinyl acetate), polyacetal, polycarbosilanes, polysilazanes, polyhydroxyalkanoates, polycyclodextrins, polybutylene succinate, polycaprolactone, polyanhydrides, cellulose acetates, nitrocellulose, vitrimers, ferrocene-based polymers, hydrogels, organogels, block copolymers, poly(ionic liquid)s, radical polymers, sol-gel precursors, supramolecular polymers, polydopamine, polyamines, covalent organic frameworks, metal-organic frameworks, fluorescent polymers, and their derivatives and composites, or a combination thereof.
Other polymer classes include conjugated polymers and their derivatives and composites: polythiophenes, polyanilines, polyacetylenes, polypyrroles, poly(phenylene vinylene)s, polyparaphenylenes, poly(phenyleneethynylene)s, polyfluorenes.
Other polymer classes include natural or bio-polymers, and their derivatives and composites: glucomannan. cellulose, nanocellulose, lignin, starch, polysaccharides, chitin, chitosan, gelatin, collagen, keratin, silk, enzymes, DNAs, RNAs, polypeptides, proteins, antibodies, lipids.
Other polymer classes include shape-memory polymers, shape-changing polymers, or stimuli-responsive polymers, and their derivatives and composites: Nafion (tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer), liquid crystalline polymers, liquid crystalline elastomers, azopolymers (polymers that contain azo group), thermo-responsive polymers, photo-responsive polymers, electroactive polymers, magneto-responsive polymers, bio-responsive polymers, chemical-responsive polymers, mechano-responsive polymers, redox-responsive polymers, water-responsive polymers, pH-responsive polymers.
Other polymer classes include ionomers and their derivatives and composites. Ionomers include copolymers of ethylene and acrylic and/or methacrylic acid (Surlyn, Nucrel, Primacor, Eltex, Optema) and perfluorinated sulfonic acid ionomers such as tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer (Nafion) Optema.
Other polymer classes include carbon materials and nanocarbon materials: carbon nanotubes, graphene, graphene oxide, fullerenes, diamond, nanodiamond, diamondoids, carbon black, asphalt, graphyne.
Other polymer classes include 2D nanomaterials: boron nitride, C3N4, transition metal dichalcogenides (e.g. MoS2, WS2, WTe2, TiSe2), transition metal carbides (e.g. Mo2C, W2C, WC, TaC, NbC), transition metal oxides, nitrides, phosphides, and arsenides of III A group metals, chalcogenides of IV A group metals, chalcogenides of V A group metals, MXenes.
Other polymer classes include perovskite-structured materials.
The stimuli-responsive polymer may be a derivative or composite of the polymers described above, or a combination of the polymers, and/or their derivatives, and/or composites.
The polymer used in the polymer layer may be a cross-linked polymer.
A preferred group of stimuli-responsive polymers includes polyvinylpyrrolidone, polyvinylpolypyrrolidone, poly dimethylsiloxane, polycarbonate, polystyrene, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer, starch, and glucomannan, and their derivatives and composites, or a combination thereof.
The polymer layer may be transparent or substantially transparent.
The polymer layer may be comprised of polymers, polymer composites, or a combination of different polymers and/or polymer composites.
Any of the stimulus-responsive polymers described herein may be combined with any of the metal or metal alloy thin films and substrates described herein. Any combination of dimensions of the thin film and polymer layer may be used depending on the particular application.
Any substrate material may be generally used with the system disclosed herein. The substrate may be rigid (e.g., glass) or flexible (e.g., an elastomer such as PDMS or rubber). Classes of substrate materials include glass, metal, ceramic, wood, paper, stone, brick, concrete, cement, composite, polymers, or combinations thereof. When a polymer is used as a substrate, it may be a stimulus-responsive polymer, such as polydimethylsiloxane.
In an embodiment the substrate is comprised of a flexible substrate such as silicone elastomer or related materials, rubber or related materials, paper or related materials, or other polymers and polymer composites.
The ability to use substrates such as glass and PDMS allows for a humidity-sensing window, and a self-reporting, self-acting sensor that does not consume external power. Such transparent devices with coupled complementary colors on opposite sides are also desirable for applications such as wearable sensors, where the color change at the on-body side can be transduced into the color change on the opposite side of the film.
The system described herein also provides for patterns of interference coloration on nanoscale, microscale, macroscale, or multiscales by patterning of polymer and/or metal on a substrate using various techniques including but not limited to ink-jet printing, stencil lithography, photolithography, e-beam lithography, soft-lithography, mask-based spraying, mask-based dip-coating, mask-based plasma coating, mask-based thermal coating, mask-based chemical vapor deposition, mask-based sputter coating, patterned electroless plating.
In a representative combination, the substrate is glass, a polymer, or paper; the thin metal film is composed of aluminum; and the stimulus-responsive polymer is polyvinylpolypyrrolidone, starch, or glucomannan.
The system of the invention may be incorporated into various articles of manufacture such as a window or various devices, such as a colorimetric sensor.
In the sensor, the thickness of the polymer layer determines the reflected colors and the thickness of the metal layer controls the intensity of the reflected color. The sensor that responds to external stimuli using reflectance of light and/or transmission of light to produce a color change. The sensor may thus couple the reflected color on one side and transmitted color on another side.
The sensor may be used to detect an external stimulus, including but not limited to, water vapor, humidity, temperature, light, chemicals, biomolecules, mechanical force, and organic vapor. Chemicals such as organic vapors include, for example, ethanol, hexane, pentane, trimethylamine, ammonia, trifluoroacetic acid, etc. A colorimetric stimuli-sensing window may sense stimuli such as humidity, temperature, light, gas, volatile organic compounds, etc.
A colorimetric sensor may monitor soil moisture level.
A mechanochromic sensor has applications in strain sensing, finger printing, stretchable electronics, anti-counterfeiting, and soft robotics.
The sensor may be a self-reporting and/or self-acting sensor that functions without external power.
The sensor may be a wearable sensor for health monitoring, where the stimuli-induced color change at the on-body side can be transduced into the color change on the opposite side of the film.
Multiple sensors may be assembled in a sensor array for multi-stimuli sensing.
An aspect of the invention provides a method of manufacturing an article comprising the system described herein, the method comprising (a) depositing a metal or metal alloy on at least a portion of a first surface of a substrate, the metal or metal alloy being deposited as a thin film with a thickness configured to filter visible light; and (b) coating a stimulus-responsive polymer on the thin film to form a polymer layer.
An aspect of the invention provides a method of detecting a change in an environmental condition comprising (a) contacting an article with a physical, chemical, or biological stimulus, wherein the article comprises the system described herein; and (b) detecting a change in color of the article.
Materials. Polyvinylpyrrolidone (PVP) powder was purchased from Alfa Aesar. PC pellets was purchased from Sigma-Aldrich. Polydimethylsiloxane (PDMS) precursors (Sylgard 184) were purchased from Dow Corning, and mixed based on the manufacturer's recommended base to crosslinker ratio of 10:1. PVP solutions in ethanol with PVP loadings from 6 to 9 wt % were prepared and stored at room temperature, PC solutions in chloroform with PC loading of 2 wt %, and PDMS solutions in hexane with PDMS precursors loading of 8 wt % were prepared and stored at room temperature. Ethanol (200 proof) was purchased from Koptec. Pentane was acquired from Sigma-Aldrich. Chloroform and hexane were acquired from Sigma-Aldrich. Nichrome wire was purchased from Ted Pella, Inc. High purity silver wire was purchased from Integrity Beads, Inc. Glass substrates (Micro Slides), were purchased from Corning. Glass microscopic slides were rinsed with acetone and isopropanol and then dried with nitrogen prior to use. 8-10% w/w Nafion alcohol solution was prepared by concentrating 5% w/w stock Nafion Alcohol solution purchased from Alfa Aesar. High purity aluminum wire (diameter: 0.015 inches) was purchased from Ted Pella. Sudan III (1-((4-(phenylazo)-phenyl)azo)-2-naphthalenol) dye was purchased from Allied Chemicals.
Preparation of Metal Layer. Ultrathin film of iridium is deposited on a desired substrate (e.g. glass, PDMS) in a sputter coating system (model K150X, Quorum Emitech) using a high purity iridium target (Ted Pella, Inc.) under a vacuum pressure of 2×10−3 mbar (
Preparation of Responsive Interference Coloration (RIC) Films on Glass Substrates. After ultrathin film of metal was deposited on a glass substrate (2.5 cm×2.5 cm), 0.5 mL of the solution of desired polymer (PVP, PC, PDMS, or Nafion-alcohol) was placed or spin-coated on top of the metal-coated glass substrate. The spin coating was carried out at specific spinning rates (1500-7000 rpm) for 30 seconds using a spin coater (model P6700, Specialty Coating Systems, Inc.). Since the reflected color is controlled by the polymer layer thickness, appropriate spinning rate and concentration of the polymer solution were used to obtain the desired color. The obtained color depends on both concentration and spin-coating speed. The entire process was performed at ambient humidity (45±5 RH %) and room temperature (22±2° C.). RIC color patterns were achieved by patterning of the metal layer with a pre-cut plastic stencil mask during the metal coating, followed by spin coating of the polymer layer.
Preparation of Various Polymer-Metal-Glass Films. After ultrathin film of metal was deposited on a glass substrate (2.5 cm×2.5 cm), ˜ 0.5 mL of the solution of desired polymer (PVP (
Preparation of PVP-Ir-PDMS Films. The PDMS substrates were made by mixing and curing the PDMS precursors at 70° C. overnight or 100° C. for about 3 hours. The fully-cured PDMS film was then cut into small pieces (˜2 cm×2 cm), followed by ultrathin metal layer coating. Subsequently, ˜ 0.4 mL of the PVP solution was placed on the metal-coated PDMS substrate, and then spin-coated at a specific spinning rate for 30 seconds. Since the reflected color is controlled by the polymer layer thickness, appropriate spinning rate and concentration of the PVP solution were used to obtain the desired color.
Preparation of PVP-Ir-PDMS Film. PDMS base and curing agent were mixed at a 10:1 (w/w) ratio. The mixture was cast on silicon wafer and left overnight at room temperature, followed by curing at 80° C. for 4 h. The thickness of the PDMS was maintained at ˜ 750 μm. The fully-cured PDMS film was then cut into small pieces (˜2.5 cm×2.5 cm), followed by deposition of 5 nm ultrathin iridium layer coating in a sputter coating system (model K150X, Quorum Emitech) (
Preparation of PVP-Ir-Dyed PDMS Film. PDMS base and curing agent were mixed at a 10:1 (w/w) ratio. The Sudan III dye solution in toluene was then added to the PDMS precursors at a loading of 1 mg dye per mL of PDMS base, followed by thorough mixing. The mixture was cast on silicon wafer and left overnight at room temperature, followed by curing at 80° C. for 4 h. The thickness of the PDMS was maintained at ˜ 650 μm. The rest of the sample preparation is similar to that of PVP-Ir-PDMS film.
Preparation of PVPP-Metal-Glass and PVPP-Metal-PDMS Films. Heating of a PVP thin film on various substrates (Ir, nichrome, Al, PDMS, etc.) at 200° C., followed by rinsing in deionized (DI) water to remove any unreacted PVP residue, leads to thermal crosslinking of PVP to form more stable PVPP, which is insoluble in common solvents.
Preparation of UV-Crosslinked Starch-Ir-Glass Films. First, a DMSO solution of starch with 1% of sodium benzoate as a UV sensitizer was used to make a starch-Ir-glass film. Then, UV irradiation of the resulting film in the air, followed by rinsing in water and DMSO, respectively, produces the UV-crosslinked starch-Ir-glass film (
Preparation of UV-Crosslinked PS-Ir-Glass Films. First, a toluene solution of PS was used to make a PS-Ir-glass film. Then, UV irradiation of the resulting film under N2, followed by rinsing in toluene, produces the UV-crosslinked PS-Ir-glass film. To make a color pattern, a mask was used to allow localized UV irradiation of the PS-Ir-glass film, followed by rinsing in toluene. Controlling UV irradiation time at different locations leads to formation of a color pattern (
Sample Characterizations. The reflection spectra were acquired using a fiber optic spectrometer (USB2000+, Ocean Optics). The incident light was perpendicular to the plane of the film. The transmission and absorption spectra of the samples were recorded with a Cary 5000 UV-Vis-NIR spectrophotometer. Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 field emission scanning electron microscope. The average polymer layer thickness was determined from SEM measurements of 50 location points of the cross section of the polymer layer. Thickness of the substrates were measured with a Mitutoyo Digital Micrometer. Unless otherwise stated, all sample characterization was carried out at ambient humidity (45±5 RH %) and room temperature (22±2° C.).
Stimuli Response Measurements. The static response measurement of the RIC films to different humidity levels was carried out in a home-built humidity-control chamber based on literature (Steele et al., IEEE Sens. J 2008, 8, 1422-1428; Hawkeye and Brett, Adv. Funct. Mater. 2011, 21, 3652-3658). The RH level of the chamber was varied between 20% and 80% by controlling the relative flow rates of dry and wet N2 gas. Under each humidity condition, the film was kept for 2 hours to ensure fully equilibrated state. The chamber's RH level was monitored with a commercial humidity meter (AcuRite 01083), calibrated with standard salt solutions (Table 1) (Greenspan, J. Res. Natl. Bur. Stand. Sec. A 1977, 81A, 89-96). The chemical vapors (i.e. water and pentane vapors) for sensing experiments were generated by a commercial ultrasonic humidifier (Essential Oil Diffusor, Radha Beauty Co.), and then applied to the samples through a rubber tubing with a small plastic tip (e.g. pipette tip) at the end. The dynamic reflection spectra were acquired continuously using a fiber optic spectrometer (USB2000+, Ocean Optics) with the interval time of 10 ms. Thermal response experiment was performed on a hot plate, and temperature of the RIC film during the experiment was measured with a non-contact infrared thermometer (MICRO-EPSILON thermoMETER LS), which was found to be in good agreement (within +2° C.) with a traditional thermometer.
J. Res. Natl. Bur. Stand. Sec. A 1977, 81A, 89-96).
Estimating the Thickness of the PVP Layer Using the Coefficient of Hygroscopic Expansion of PVP. To verify whether the humidity sensing mechanism of the PVP-Ir-glass film is due to the change in thickness of the PVP layer, we calculated the reflection peak position at each increased static RH level using the expected thickness of the PVP layer at corresponding static RH level. The expected thickness of the PVP layer was estimated using the coefficient of hygroscopic expansion of PVP, with respect to the original thickness of the PVP layer measured by SEM. The volumetric change induced by water absorption in PVP can be estimated by Equation 1 (Zhang and Webb, Opt. Lett. 2014, 39, 3026):
Where β is the coefficient of hygroscopic expansion, f is the fraction of the water that contributes to an increase in the PVP volume (Vogt et al., Polymer 2005, 46, 1635), ρW is the density of water, and W is the water uptake of PVP at 25° C. at specific RH level. As shown in the literature, the water absorption of PVP increases with relative humidity in a non-linear trend (Prudic et al., Eur. J Pharm. Biopharm. 2015, 94, 352). Hence, hygroscopic strain (εh) of PVP should be obtained at various RH levels according to Equation 2 (Stellrecht et al., Exp. Techniques 2003, 27, 40).
εh=β·W (2)
The hygroscopic strain of PVP determines expected thickness (D) of PVP at each static humidity level with respect to an initial thickness (do) according to Equation 3.
The resulting thickness (d) can be used to predict the expected reflection peak position of PVP using the equation for the condition for constructive thin-film interference as described in the main text.
Verification of Humidity Sensing Mechanism. To determine the reflection peak wavelength of the RIC film at different static RH levels, the sample was placed inside a homemade, transparent humidity chamber. The RH level of the chamber was varied between 20% and 70% by controlling the relative flow rates of dry and wet N2 gas, and it was monitored with the calibrated commercial humidity meter. Under each humidity condition, the static reflection spectrum was recorded using a fiber optic spectrometer (USB2000+, Ocean Optics) after the film reached equilibrium state. To verify whether the humidity sensing mechanism of the PVP-Ir-glass film is due to the change in thickness of the PVP layer, the reflection peak position was calculated at each increased static RH level using the expected thickness of the PVP layer at corresponding static RH level. The expected thickness of the PVP layer was estimated using the coefficient of hygroscopic expansion of PVP, with respect to the original thickness of the PVP layer measured by SEM. The details can be found in supporting information. Comparison of the observed and calculated reflection peak positions at each static RH level was then used to determine whether the observed reflection wavelength change is caused by change in thickness of the PVP layer.
Humidity Cycle Test. To investigate the long-term stability of the RIC films, both PVPP-Ir-glass and PVPP-nichrome-glass films were subjected to 50 cycles of localized exposure to water vapor in the same region. The dynamic reflection spectra (θ=0°) were acquired continuously using a fiber optic spectrometer (USB2000+, Ocean Optics) with the interval time of 10 ms during the cycle #1, cycle #25, and cycle #50 of the humidity sensing experiments.
Color Analysis. The image color analysis was carried out using the Image Color Summarizer software (http://mkweb.bcgsc.ca/color-summarizer/). The pixel color partitioning was used to quantify the relative change in pixels of the initial blue color with mechanical strain in the kirigami systems. The average RGB color cluster values for the whole sample film at different mechanical strains were obtained to quantify the mechanochromic response in the PVP-Ir-Dyed PDMS film.
In this work, the thin polymer layer serves as an interference coloration layer, where the reflected color represents the constructive interference, whereas the transmitted color represents the destructive interference. Without the thin polymer layer, the metal-glass film exhibits only light grayish color (
mλ=2n2d2 cos θ (4)
where λ is the wavelength giving the maximum reflectivity, m is the order of diffraction (a positive integer), d2 and n2 are the thickness and refractive index of the polymer layer, respectively, and θ is the angle of incidence (
(m−½)λ=2n2d2 cos θ (5)
where λ represents the wavelength giving the minimum reflectivity (maximum transmissivity) (Kinoshita et al., Rep. Prog. Phys. 2008, 71, 076401; Sun et al., RSC Adv. 2013, 3, 14862-14889).
The RIC system is composed of three layers: 1) The thin polymer layer that exhibits stimuli-responsive thin film interference coloration; 2) The ultrathin metal layer that acts an optical filter; 3) The substrate layer. The key concept is to use an ultrathin metal layer as an optical filter instead of high refractive index substrate or highly reflective substrate. Such an optical filter layer allows tuning of the degree of transparency, the constructive interference reflection light, and complementary destructive interference transmission light via changing the metal layer thickness (
The simple RIC system has the following distinctive advantages: 1) Versatile polymer layer choice: A wide range of thermoplastics, thermosets, and polymer composites can be used for rational engineering of stimuli-responsivity, stability, etc. 2) Versatile metal layer choice: A variety of metals and metal alloys such as iridium, silver, nichrome, aluminum, etc. can be selected for target applications and manufacturing processes. 3) Versatile substrate choice: The RIC design is applicable to many substrates, including glass and PDMS.
We have found that the thickness of the ultrathin metal layer is crucial to tune the intensity of the reflected light color (
According to Equation 4, the thickness of the polymer layer determines the reflected color wavelength when the viewing angle is fixed (e.g. θ=0°). By tuning the polymer layer thickness via spin coating using appropriate spin speeds and concentrations of polymer solutions, various interference colors including purple, blue, green, yellow, and red can be generated by thin films of various polymers such as PVP, PDMS, and PC on metal-coated glass substrates (
Like other conventional interference films and photonic crystals, our current RIC systems exhibit iridescent reflection colors that depend on the viewing angle (
The calculated reflection peak wavelengths are in fairly good agreement with corresponding experimental reflection spectra (
Compared with most inorganic materials, polymer-based materials have many advantages such as low cost, flexibility, good processability, excellent corrosion resistance, and light-weight. Moreover, stimuli-responsive polymers can sense their environment and change the shape and/or material properties accordingly (Stuart et al., Nat. Mater. 2010, 9, 101-113). In our current colorimetric RIC sensor design, the primary sensing mechanism is based on the stimulus-induced thickness change in the polymer layer, which leads to corresponding color change. In this study, we focus on the proof-of-concept demonstration of real-time, continuous, colorimetric RIC sensors for humidity (
There has been a growing interest in low-cost, real-time humidity sensors for applications in agriculture, manufacturing, food industry, healthcare, and environmental monitoring (Chen and Lu, Sensor Lett. 2005, 3, 274-295). The PVP-Ir-glass colorimetric sensor exhibits excellent sensitivity to relative humidity (RH) change, ranging from purple at 20% RH to blue at 40% RH, green at 50% RH, yellow at 70% RH, and red at 80% RH (
To verify whether the humidity sensing mechanism of the PVP-Ir-glass film is due to the proposed change in thickness of the PVP layer (
To investigate the long-term stability of the RIC films, we have carried out the humidity cycle test for PVPP-Ir-glass and PVPP-nichrome-glass RIC films (
Although the PDMS-Ir-glass system has no response to the humidity change, it exhibits exceptional sensitivity towards organic vapors such as hexane that can swell PDMS. It takes just ˜ 0.23 s for the peak wavelength for the second-order of reflection to undergo 200 nm of shift from the blue-colored to red-colored PDMS film upon exposure to the hexane vapor (
Suitable indoor air humidity levels are important for human health and comfort. The EPA recommends the indoor relative humidity stays between 30% and 50%. If the indoor relative humidity is above 60%, it not only makes occupants feel less comfortable, but also allows mold and mildew to grow, which can cause health problems. On the other hand, if the indoor air is too dry with less than 30% relative humidity, it can cause static electricity problems, sensory irritation of the skin, dry eyes, and dry, sore throat. Low-cost, energy-free, real-time, continuous sensors are highly desirable for monitoring and control of temperature, humidity, occupancy, and indoor air quality in smart residential and commercial buildings (Wolkoff, Int. J. Hyg. Environ. Health 2018, 221, 376-390; Neal Stewart Jr. et al., Science 2018, 361, 229-230).
By using a metal layer of 3 nm thickness and transparent substrate, both good transparency and bright interference coloration can be achieved in RIC sensors (
Air leaks through windows and doors represent significant amount of commercial and residential building energy consumption. Detecting the leaking locations of a leaky window is crucial for sealing the leaks and saving the energy. The transparent humidity-sensing window with the sensing layer facing inside enables energy-free, real-time monitoring of potential window leaks with spatial resolution, because the localized air leak can cause the color change at the leaking spot of the window, due to the difference of outdoor and indoor moisture levels. Furthermore, the transparent humidity-sensing window with the sensing layer facing inside or outside can be used for monitoring of the air humidity inside or outside the car, which can help drivers to prevent the car window from fogging up by timely adjustment of humidity and temperature inside the car.
Low-cost, self-reporting, real-time soil moisture sensors with zero power consumption are crucial for precise water management in agriculture, which will help farmers save water and increase yields and the quality of the crop by improved management of soil moisture during critical plant growth stages. The combination of low-cost RIC soil moisture sensors (
The transparent RIC films also make it possible to develop other stimuli-responsive windows by choosing appropriate sensing polymers. For instance, the volatile organic compounds (VOCs) are common indoor pollutants, which may have short- and long-term adverse health effects. We can use RGB-based response patterns of the Nafion-Ir-glass sensor (
In addition to chemical stimuli such as humidity and organic vapor, the RIC system with suitable polymer layer can also respond to physical stimuli such as temperature. Since PDMS has a relatively large linear thermal expansion coefficient (3.0×10−4/° C.) than typical polymers such as PC (6.7×10−5/° C.), the PDMS-Ir-glass sensor shows a detectable color change upon heating from 20° C. to 150° C. (
We have developed a general strategy for powerless self-reporting and self-acting chemical sensors, which can differentiate two different chemical stimuli by transforming one chemical stimulus such as nontoxic water vapor into one type of self-reporting output signal (i.e. color change), whereas transducing another chemical stimulus into two different types of self-reporting output signals (i.e. color change+bending). The bending actuation could be used as the self-acting function such as waking an electric circuit of an alarm system upon the detection of a specific stimulus such as toxic organic vapor (
The bioinspired stimuli-responsive structural coloration has received great interest in the past two decades due to its wide range of promising applications (Kinoshita et al., Rep. Prog. Phys. 2008, 71, 076401; Sun et al., RSC Adv. 2013, 3, 14862-14889; Fenzl et al., Chem. Int. Ed. 2014, 53, 3318-3335 and Angew. Chem. 2014, 126, 3384-3402; Ge and Yin, Angew. Chem. Int. Ed. 2011, 50, 1492-1522 and Angew. Chem. 2011, 123, 1530-1561; Zhao et al., Chem. Soc. Rev. 2012, 41, 3297-3317; Chan et al., Adv. Mater. 2013, 25, 3934-3947; Cai et al., Anal. Chem. 2015, 87, 5013-5025; Phillips et al., Chem. Soc. Rev. 2016, 45, 281-322; Dumanli and Savin, Chem. Soc. Rev. 2016, 45, 6698-6724; Isapour and Lattuada, Adv. Mater. 2018, 30, 1707069). Thin-film interference is the simplest structural coloration mechanism, which is responsible for the colorful, iridescent reflections that can be seen in oil films on water, and soap bubbles (Kinoshita et al., Rep. Prog. Phys. 2008, 71, 076401; Sun et al., RSC Adv. 2013, 3, 14862-14889; Kats and Capasso, Laser Photonics Rev. 2016, 10, 735-749; Kramer et al., Nat. Mater. 2007, 6, 533-538; Phan et al., Adv. Mater. 2013, 25, 5621-5625; Qin et al., Adv. Mater. 2018, 30, 1800468). Thanks to its design simplicity, which does not require multilayers of materials with alternative refractive indices or micro- and nanostructures, thin film interference represents a promising solution towards scalable and affordable manufacturing of high-quality responsive structural coloration systems. However, thin films of polymers with appropriate thickness generally do not exhibit visible structural colors if they are directly deposited on low-cost substrates such as glass (Banisadr et al., ACS Appl. Mater. Interfaces 2019, 11, 7415-7422) and polydimethylsiloxane (PDMS) (
Our previous study was focused on the sensing properties of the glass-based thin film interference films (Banisadr et al., ACS Appl. Mater. Interfaces 2019, 11, 7415-7422). Since the glass substrate is rigid and not responsive to external stimuli by itself, the actuation is impossible in these glass-based films. In current study, we have been successful for the first time in powerless integration of sensing with actuation functions in thin film interference films by using the flexible PDMS substrate, which also acts as the second sensing layer (
The condition for constructive thin-film interference is determined by Equation 4 where is the wavelength giving the maximum reflectivity, m is the order of diffraction (a positive integer), d2 and n2 are the thickness and refractive index of the polymer layer, respectively, and θ is the angle of incidence (
PVP and PDMS show opposite stimuli-responsive properties because of their different chemical structures (
In contrast, when exposed to a pentane vapor, the PDMS layer swells and leads to the bending of the PVP-Ir-PDMS film towards the PVP side (
In addition to chemical stimuli, flexible trilayer thin-film sensors are also responsive to the compressive force and changes the color accordingly (
Owing to the transparency of PDMS, both constructive interference reflected colors and complementary destructive interference transmitted colors across the spectrum can be created simultaneously on opposite sides of the substrate, respectively. The degree of transparency in the interference system can be tuned via changing the thickness of the ultrathin metal film. Such transparent and flexible films with coupled complementary colors on opposite sides are desirable for applications such as wearable sweat sensors, where the color change at the on-body side can be transduced into the color change on the opposite side of the film (
In summary, we have developed a general strategy for powerless self-reporting and self-acting chemical sensors, which is applicable to a broad range of stimuli-responsive polymer materials (Cohen Stuart et al., Nat. Mater. 2010, 9, 101-113). Our simple yet versatile trilayer thin-film transducer system enables integration of sensing with actuation, and allows on-site management of intelligent response and action towards different chemical stimuli. Such new type of chemical sensors not only can remain dormant but always alert while monitoring of the environment without consuming power, but also can initiate autonomous reporting and acting functions when a chemical signal of interest is detected.
Thin films of polymers with appropriate thickness generally do not exhibit visible structural colors if they are directly deposited on low-cost substrates such as glass (Banisadr et al., ACS Appl. Mater. Interfaces 2019, 11, 7415-7422) and polydimethylsiloxane (PDMS). We have discovered recently that, in order to see bright thin-film interference color on glass or PDMS, it is crucial to use an ultrathin metal layer as an optical filter (Banisadr et al., ACS Appl. Mater. Interfaces 2019, 11, 7415-7422). Such an optical filter layer can dramatically enhance the interference color intensity by simultaneously optimizing both the constructive interference reflection light and complementary destructive interference transmission light. In this study, we choose a polyvinylpyrrolidone (PVP)—Ir-PDMS trilayer film as a model material system for mechanochromism, where the PVP layer exhibits the interference color, and the PDMS layer serves as a stretchable substrate (
We first investigate the mechanochromic properties of the PVP-Ir-PDMS film. The condition for constructive thin-film interference is determined by Equation 4 where is the wavelength giving the maximum reflectivity, m is the order of diffraction (a positive integer), d2 and n2 are the thickness and refractive index of the polymer layer, respectively, and θ is the angle of incidence (
Our initial study suggests that solutions are needed to significantly enhance the mechanochromism based on thin film interference. Herein, we report two different implementation approaches of our new strategy for dynamic, reversible mechanochromism: 1) Kirigami approach; 2) Tunable reflectivity shield approach.
Kirigami allows transformation of a flat sheet into a complex 3D shape. Kirigami-based design principles have been exploited very recently to create or enhance material functions without altering material compositions, which enable potential applications such as dynamic solar tracking (Lamoureux et al., Nat. Commun. 2015, 6, 8092), tunable optical transmission windows (Zhang et al., Proc. Natl. Acad. Sci. USA 2015, 112, 11757-11764), stretchable electronics and optoelectronic devices (Shyu et al., Nat. Mater. 2015, 14, 785-789), stretchable triboelectric nanogenerators (Wu et al., ACS Nano 2016, 10, 4652-4659), optical chirality components (Liu et al., Sci. Adv. 2018, 4, eaat4436), and soft actuators (Rafsanjani et al., Sci. Robot. 2018, 3, eaar7555; Oyefusi and Chen, Angew. Chem. 2017, 129, 8362-8365 and Angew. Chem. Int. Ed. 2017, 56, 8250-8253). Although traditional kirigami involves both cutting and folding, recent studies have shown that the cuts alone in a flat sheet are sufficient to form a 3D object via out-of-plane buckling under strain (Lamoureux et al., Nat. Commun. 2015, 6, 8092; Zhang et al., Proc. Natl. Acad. Sci. USA 2015, 112, 11757-11764; Shyu et al., Nat. Mater. 2015, 14, 785-789; Wu et al., ACS Nano 2016, 10, 4652-4659; Liu et al., Sci. Adv. 2018, 4, eaat4436; Rafsanjani et al., Sci. Robot. 2018, 3, eaar7555; Rafsanjani and Bertoldi, Phys. Rev. Lett. 2017, 118, 084301). In our kirigami approach, the synergistic coupling of buckling-induced kirigami (Rafsanjani and Bertoldi, Phys. Rev. Lett. 2017, 118, 084301) and viewing angle-dependent interference color (Banisadr et al., ACS Appl. Mater. Interfaces 2019, 11, 7415-7422) leads to dramatic enhancement in mechanochromism based on thin film interference.
We choose an array of mutually orthogonal cuts in our kirigami design because 3D transformation of such a cut pattern is controlled by the uniaxial tensile direction (Rafsanjani and Bertoldi, Phys. Rev. Lett. 2017, 118, 084301). In our first kirigami structure (kirigami I), the load direction is at 45° to the cuts, whereas in our second kirigami structure (kirigami II), the load direction is at 0°/90° to the cuts (
The strain-induced spatially heterogeneous color change in kirigami can be recorded by a video camera and analyzed by the Image Color Summarizer software. The image color analysis allows quantitative assessment of mechanochromic properties of different kirigami structures by tracking the total sample area of initial blue color at each mechanical strain. Most interestingly, we have observed that the kirigami I shows nonlinear mechanochromic response with highest sensitivity in the region of 13%-17% strain, whereas the kirigami II exhibits nearly linear mechanochromic response until it reaches the plateau around 17% strain (
In our second mechanochromic approach, we use the PVP-Ir bilayer film as a mechanically tunable reflectivity shield to program its interference reflection color intensity and the visibility of the underlying dyed PDMS layer (
The mechanochromic data recorded by a video camera can be quantitatively analyzed by the Image Color Summarizer software, which produces blue (B) and red (R) values that represent blue and red color intensity, respectively, at each mechanical strain (
To investigate the strain-induced metal cracking by SEM, we use the Ir-PDMS film with comparable metal and PDMS layer thickness as a model system to avoid the severe charging from the insulating PVP layer in SEM imaging. We have also observed the strain-induced diminishing intensity of the broad reflection spectra of Ir-PDMS and Ir-Dyed PDMS films, respectively, owing to the metal cracking (
Various interference color patterns such as dots and stripes can be produced by patterning of the ultrathin metal film with different plastic stencil masks on top of the PDMS substrate during the metal deposition (
We calculated the expected shift in reflection peak at different strain in the absence of PVP/metal cracking in the PVP-Ir-PDMS film by using the condition for constructive thin-film interference defined by Equation 4 where λ is the wavelength giving the maximum reflectivity, m is the order of diffraction (a positive integer), d2 and n2 are the thickness and refractive index of the polymer layer, respectively, and θ is the angle of incidence (Kinoshita et al., Rep. Prog. Phys. 2008, 71, 076401; Banisadr et al., ACS Appl. Mater. Interfaces 2019, 11, 7415-7422). The following assumptions were made in our calculation: The PVP layer is continuous without cracking, the volume of the PVP layer is conserved upon uniaxial stretching, and the dead ends are negligible and the PVP-Ir-PDMS film is rectangular both in the unstretched and stretched state. This allows for approximation of the area covered by the PVP film in the stretched and unstretched states as shown in
The initial thickness of the PVP layer in the PVP-Ir-PDMS film at 0% strain can be calculated using Equation 4, where n2 is 1.53 for PVP and A is 467 nm. The thickness of the PVP layer in the absence of PVP/metal cracking at different strains can then be calculated according to Equation 6:
V=Ad
2 (6)
Where V, A, and d2 are the volume, area, and thickness, respectively, of the PVP layer.
The calculated reflection peak wavelength at different strains is then obtained using the calculated thickness of the PVP layer and Equation 4. Since the calculation assumes that there is no PVP/metal cracking upon mechanical stretching, the significant disagreement between the experimental and calculated reflection peak wavelengths of the PVP-Ir-PDMS film upon mechanical stretching (
Since the PVP and ultrathin metal layers are crucial for observed interference reflection color intensity, the PVP/metal cracking upon stretching also causes significant intensity reduction of the reflection peak (
While several embodiments of the present invention have been described and illustrated herein, it is to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed.
This application claims priority to U.S. Provisional Application No. 62/757,288, filed Nov. 8, 2018, which is hereby incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2019/060457 | 11/8/2019 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62757288 | Nov 2018 | US |
