A sensor for detecting low levels of carbon dioxide

Abstract

Certain aspects of the disclosure provide a metabolic sensor comprising a gas permeable indicator layer configured to change hue in response to carbon dioxide presence; a blocking layer configured to block carbon dioxide release; and a fluid absorption layer configured to absorb liquid.

Description

A SENSOR FOR DETECTING LOW LEVELS OF CARBON DIOXIDECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 696,430 filed on September 19, 2024 the entire contents of which are hereby incorporated by reference.TECHNICAL FIELD

[0002] Aspects of the present disclosure relate to systems, methods, and devices for carbon dioxide (CO2) detection and monitoring.BACKGROUND

[0003] Carbon dioxide (CO2) is a naturally occurring gas that is produced by animals in the process of cellular respiration and utilized by plants for photosynthesis. Various CO2 sensors have been developed for monitoring CO2 levels in numerous applications, including medical, fitness and activity, industrial, and environmental monitoring.

[0004] In medical scenarios, CO2 levels in a patient’s blood may be used to assess the patient’s respiratory function. Specifically, the partial pressure of CO2 (pCCh) in the blood indicates how well the lungs are ventilating CO2 from the patient. For example, during exercise up to 100 liters of air are exchanged per minute in the human lungs, about 3 liters being oxygen (O2) and CO2. At rest, about 300 mL of O2 and CO2 are exchanged. High pCCh may indicate the lungs are not clearing enough CO2. CO2 levels may also be used in management of respiratory failure, metabolic imbalance, and to evaluate the effectiveness of ventilation therapy. Generally, the rate at which O2 and CO2 produced indicates the rate of energy used by the body, e.g., caloric expenditure. Typical CO2 sensors are part of blood gas analyzers that provide real-time measurements of pCCh, along with other measurements such as blood oxygen levels and pH.

[0005] CO2 sensors may also be used in fitness and activity monitoring to assess metabolism by analyzing CO2 levels in a person’s breath. A user breathes into a CO2 sensor device which measures the concentration of CO2 in the exhaled breath. The concentration ofCO2 may be used to calculate metabolic rate and energy expenditure. Then, fitness and activity levels may be determined.

[0006] CO2 sensors may also be used to detect microbial growth. For example, release of CO2 may be used to indicate levels of microbial growth, as CO2 is released by microbes as they grow. Microbial growth may then be detected in various locations, such as medical devices. As another example, CO2 production may be used to indicate soil respiration.

[0007] Prior CO2 sensors, however, are typically too large to be used outside of a medical or exercise setting. Most CO2 sensors measure CO2 at the level of 1,000 to 40,000 parts per million (ppm), which is suitable for high output applications such as breath analyses or microbial growth, but are inappropriate for slower CO2 production such as is produced in daily activity. Thus, there is a need for a metabolic sensor that can be worn by a user to monitor metabolic activity at lower levels such as below 1000 ppm.SUMMARY

[0008] Provided herein according to certain aspects is a metabolic sensor, comprising: a gas permeable indicator layer configured to change hue in response to carbon dioxide presence; a blocking layer configured to block carbon dioxide release; a fluid absorption layer configured to absorb liquid; and at least one adhesive layer configured to adhere to an environment of the sensor.

[0009] Certain aspects provide a method of preparing a metabolic sensor, comprising: acidifying gas permeable filter paper; mixing a silicone matrix comprising silicone components and a pH indicator; applying the silicone matrix to the gas permeable filter paper to form a gas permeable indicator layer; drying the gas permeable indicator layer; combining the gas permeable indicator layer with a blocking layer configured to block carbon dioxide release; and applying a fluid absorption layer opposite the blocking layer on the gas permeable indicator layer.

[0010] Certain aspects provide for a method of using a metabolic sensor, comprising: applying a metabolic sensor to a portion of a user’s body; obtaining a first reading of theindicator layer of the metabolic sensor at a first time; obtaining a second reading of the indicator layer of the metabolic sensor at a second time; and determining, based on a difference between the first reading and the second reading, an amount of carbon dioxide (CO2) release.

[0011] The following description and the related drawings set forth in detail certain illustrative features of one or more aspects.DESCRIPTION OF THE DRAWINGS

[0012] The appended figures depict certain aspects and are therefore not to be considered limiting of the scope of this disclosure.

[0013] FIG. 1 depicts an example CO2 sensor according to one aspect as provided herein.

[0014] FIGS. 2A-2D depict examplary pH indicators that may be incorporated into a CO2 sensor according to one aspect as provided herein.

[0015] FIG. 3 depicts an examplary method of preparing a CO2 sensor according to one aspect as provided herein.

[0016] FIG. 4 depicts an exemplary method of using a CO2 sensor according to one aspect as provided herein.

[0017] FIG. 5 depicts exemplary data obtained using the CO2 sensor according to one aspect as provided herein.

[0018] To facilitate understanding, identical reference numerals have been used, where possible, to designate elements that are common to the drawings. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.DETAILED DESCRIPTION

[0019] Aspects of the present disclosure provide systems, methods, and devices for carbon dioxide (CO2) detection and monitoring, as well as uses thereof.

[0020] There are many types of CO2 sensors including optical and electrochemical. One optical sensor includes a non-dispersive infrared (NDIR) sensor. An NDIR sensor uses an infrared (IR) source which is absorbed proportional to the number of CO2 molecules in the sample. Electrochemical sensors monitor a change in electrical current as CO2 molecules interact with a chemical solution.

[0021] Photoacoustic sensors utilize sound waves generated as light is absorbed by CO2 molecules. Metal-oxide semiconductor sensors detect CO2 molecules by measuring changes in the electrical resistance of a metal-oxide material.

[0022] Colorimetric and fluorometric CO2 sensors utilize color or fluorescence changes in chemical indicators to indicate CO2 levels. Because CO2 diffuses from higher concentrations to lower concentrations until equilibrium is realized, CO2 moves from a testing substance (e.g., blood, breath, sweat, bacterial liquid culture media, etc.) to the sensor for testing. Some sensors, for example, use a gas permeable but liquid impermeable membrane to separate the CO2, and react the CO2 with water (H2O) to form carbonic acid (H2CO3), thereby lowering the pH. A pH indicator is used and reacts as the pH changes, triggering a color or fluorescence change which reflects the amount of CO2 present in the sample.

[0023] Generally, such sensors may be suitable for detecting CO2 concentrations between 1,000 and 40,000 parts per million (ppm). Such concentrations may be used for breath analysis or microbial growth. However, such sensors are less effective for lower concentrations and / or slower CO2 levels. For example, specifically, prior colorimetric or fluorometric based sensors rely on CO2 equilibrium being reached and thus are ineffective for detecting low concentrations of CO2 or slow diffusion.

[0024] Aspects of the present disclosure provide for a CO2 sensor capable of detecting CO2 at low concentrations. The disclosed CO2 sensor is capable of capturing small amounts of CO2, e.g., low concentrations optionally at or below 1000 ppm. Specifically, the disclosed CO2 sensor is a fluorometric or colorimetric sensor, such that it changes color, tone, or fluorescence as the concentration of CO2 changes. Thereby, CO2 levels can be determined based on one or more changes in color, tone or fluorescence.

[0025] In particular, the CO2 sensor is formed using a gas permeable, yet liquid impermeable material with an acidified pH indicator solution to form a matrix. As CO2 diffuses through the matrix with the acidified pH indicator, the CO2 forms bicarbonates, neutralizing the acidified pH indicator. The acidification of the pH indicator leads to a pH increase and causes the pH indicator to change color, tone, or fluorescence proportionate the amount of CO2 present. The presence of cations, such as calcium and sodium in the solution, lead to formation of salts as more CO2 enters the sensor.

[0026] Further, this process used in the sensors as provided herein is irreversible such that CO2 will remain as bicarbonate and carbonate until the solution is neutralized because the indicator matrix prevents back diffusion (e.g., the CO2 diffusion is unidirectional). Therefore, the pH will continue to rise as more CO2 is absorbed by the sensor. Because the solution is acidic, trace amounts of CO2 will be sequestered and stored as carbonate salts. Beneficially, even small amounts of CO2 absorbed by the sensor will eventually yield a pH change, and corresponding color, tone, or fluorescence change.

[0027] Furthermore, aspects of the sensor described herein include a blocking layer which acts as a physical block to prevent CO2 from escaping into the environment thereby allowing for a cumulative measure of CO2 levels over a desired time.

[0028] Additionally, the CO2 sensor may be utilized in various useful applications. For example, the CO2 sensor may be used for monitoring metabolism. As another example, a CO2 sensor may be useful for monitoring neonates. In another example, the CO2 sensor may be used to monitor wound healing. In yet another example, the CO2 sensor may be used for measuring and monitoring muscle metabolism. Additionally, the CO2 sensor may be used for measuring and monitoring CO2 produced by microorganisms, for example, in vinification, brewing, pharmaceuticals, medical devices, and others. Further, environmental levels of CO2 may be measured and monitored with the CO2 sensor, such as in aquariums, terrariums, and other confined spaces, as well as other environmental levels. Other applications wherein CO2 levels may be monitored are also contemplated herein.

[0029] In one example, the CO2 sensor may be utilized to measure metabolism of a user. Specifically, CO2 is a byproduct of cellular metabolism and is generally removed from the body through exhalation. CO2 levels in exhaled breath and sweat represent the pCCh levels of the blood. This can be used to determine metabolism. The provided CO2 sensors are able to measure CO2 levels on the skin such as by measuring CO2 levels in sweat to measure the metabolism of a user.

[0030] Beneficially, using the CO2 sensor described herein enables convenient and reliable metabolic monitoring of users. For example, compared to activity trackers, the CO2 sensor described herein may be more reliable because of differences between users, types of activity, as well as other factors which may lead to underestimation or overestimation of caloric expenditure. Similarly, many manual calculations rely on a user to estimate their activity and may not be particularly tailored for a user. Furthermore, more accurate methods like indirect calorimetry are cumbersome and difficult to routinely access. Also, indirect colorimetry methods do not continuously or regularly monitor metabolism. Thus, the presently described CO2 sensor may be conveniently and reliably used to monitor metabolism.

[0031] In one specific example, the CO2 sensor may be used to monitor changes in metabolism, such as due to changes in health and fitness, including due to medications. Some medications may change metabolic rates, including resting metabolic rates. Similarly, some medical conditions may change metabolic rates. The CO2 sensor described herein, may, in some aspects, be used to monitor changes in metabolic rates for users by monitoring metabolic rates over time. For example, metabolic rates may be monitored for users with medical conditions or on medications, and medications or other treatments may be adjusted based on changes in metabolic rates.Example CO 2 Sensor

[0032] FIG. 1 depicts an exemplary CO2 sensor 100 according to one aspect as provided herein. The CO2 sensor 100 comprises a gas permeable indicator layer 108 configured to change hue in response to the presence of CO2. In some aspects, the gas permeable indicator layer 108 comprises a pH indicator configured to change hue in response to a change in pH ofthe gas permeable indicator layer 108. For example, in some aspects the gas permeable indicator layer 108 is configured to change hue based on a change in pH of at least 0.5.

[0033] In some aspects, the pH indicator comprises a fluorophore. An illustrative example of a fluorophore is a fluorescein. Another illustrative example of a fluorophore is hydroxypyrene trisulfonic acid (HTPS). In some aspects, the pH indicator comprises pyranine. In some aspects, the pH indicator comprises a color changing dye. A color changing dye is optionally a triarylmethane (optionally phthalein or sulphonphthalein) or azo dye. Illustrative examples of a color changing dye include but are not limited to methyl red and bromocresol green. In some aspects, the pH indicator comprises at least one of crystal violet, cresol red, cresolphthalein, cresol purple, thymol blue, methyl orange - xylene cyanol, bromophenol blue, Congo red, methyl orange, alizarin red S, bromocresol green, dichlorofluorescein, or methyl red. FIG. 2A depicts the pH ranges of various pH indicators. FIG. 2B depicts the structure of bromocresol green (BCG), and indicates a color of bromocresol green at pH 3.8 is yellow, while at pH 5.4, the color is blue. FIG. 2C depicts the structure of methyl orange (MO), and indicates a color of methyl orange at pH 3.1 is red, while at pH 4.4 the color is yellow. FIG. 2D depicts the structure of methyl red (MR), which at pH 4.4 is red and at pH 6.2 is yellow.

[0034] It is noted that any molecule that will produce a detectable change such as a change in hue or a change in fluorescence property over a test pH range (e.g. 1-11) may be used at a pH indicator in a sensor as provided herein. Preferably, a pH indicator will produce a measurable change in property (e.g. hue) over a pH change of 0.1 or more, optionally 0.5 or more.

[0035] A concentration of a pH indicator in the system is optionally from 0.00001 wt% to 1 wt% relative to the gas permeable / liquid impermeable layer material total or any value or range therebetween. Optionally, a concentration of a pH indicator is from 0.0001 wt% to 0.002 wt% or any value or range therebetween. Optionally, a concentration of a pH indicator is 0.0005 to 0.005 wt%. Other concentrations may be used and are similarly suitable. A concentration of a pH indicator is sufficient such that an output such as a hue change or fluorescence change is detectable either by eye or using an instrument suitable for detecting such output.

[0036] Among the powerful aspects of a sensor as provided herein is that the use of the sensor is independent of the type of pH indicator used, independent of changes in concentration of pH indicator, or changes in manufacturing of the sensor. Each sensor may be calibrated to a resting metabolic rate by using a calibration period whereby the change in output of the pH indicator over the resting calibration period is correlated to a known resting metabolic rate. Thus, additional changes in output of the pH indicator is calibrated to the known change per calorie burned over the known time of the calibration period. This allows the sensor to use any pH indicator and any output parameter and be equally functional in determining total metabolic rate from which subtraction of resting metabolic rate identified from the calibration period allows ready calculation of calories burned from exertion of any source during the test period of use of the sensor.

[0037] Optionally a matrix may be formed from gas permeable components and the pH indicator. The matrix may be applied to acidified gas permeable filter paper to form the gas permeable indicator layer 108. In some aspects, the matrix comprises a silicon matrix. In some aspects, the acidified gas permeable filter paper is submerged in the matrix to form the gas permeable indicator layer 108.

[0038] The acidified gas permeable filter paper may be formed by submerging a gas permeable filter paper in an acid solution. Optionally, the gas permeable filter paper comprises qualitative filter paper. For example, the qualitative filter paper may include a filter pore size of at least about 0.1 micron, at least about 0.5 microns, at least about 1 micron, at least about 2 microns, at least about 3 microns, at least about 4 microns, at least about 5 microns, at least about 6 microns, at least about 7 microns, at least about 8 microns, at least about 9 microns, at least about 10 microns, at least about 20 microns, or more. A flow rate of the qualitative filter paper is optionally medium flow. A medium flow rate may be defined as approximately 50- 100 mL / min. In some aspects, a flow rate of the qualitative filter paper is slow flow, for example, defined as approximately 5-50 mL / min. In some aspects, a flow rate of the qualitative filter paper is fast flow, for example, greater than 100 mL / min.

[0039] A diameter of the gas permeable filter paper may be at least of about 1 millimeter (mm). Optionally, a diameter of a gas permeable filter paper is between about 1 mm to about 100 mm. Optionally, a diameter of a gas permeable filter paper is about 1 mm, about 5 mm, about 10 mm, about 20 mm, about 30 mm, about 40 mm, about 50 mm, about 60 mm, or greater. In some aspects, however, the qualitative filter paper may be of any size, cut, shape or otherwise. For example, a size, cute, or shape of the gas permeable filter paper may depend on an application of the CO2 sensor 100.

[0040] In some aspects, the acid solution utilized to acidify the gas permeable filter paper comprises hydrochloric acid (e.g. 0.075 N). In some aspects, the acid solution comprises one or more of sulfuric acid, nitric acid, phosphoric acid, acetic acid, citric acid, propionic acid, and others, or any combination thereof.

[0041] Following acidification, the gas permeable filter paper is removed from the acid solution and dried optionally at ambient temperature (optionally about 25 °C) in air or in one or more substantially inert gases. The acidified gas permeable filter paper may be air dried. The acidified gas permeable filter paper may be dried for at least about 2 hours, at least about 4 hours, at least about 6 hours, at least about 8 hours, at least about 10 hours, or more. In some aspects, the acidified gas permeable filter paper may be dried until visibly dry, and / or dry to the touch, optionally at or about 10 wt% water or lower, optionally about 4 wt% residual water.

[0042] The CO2 sensor 100 further comprises a fluid absorption layer 110 configured to absorb liquid, optionally sweat. In some aspects, the fluid absorption layer 110 comprises chromatography paper. For example, where the CO2 sensor 100 is used as a skin sensor, the fluid absorption layer 110 is configured to absorb sweat from a user.

[0043] The CO2 sensor 100 additionally comprises a blocking layer 106 configured to block CO2 release. A blocking layer may be made from a polymeric material, optionally an acrylic such as a viscoelastic acrylic material available from 3M Corporation (Maplewood, MN). A blocking layer may be or include any material that is substantially impermeable to gas and liquid. A blocking layer is optionally transparent meaning that a hue change or fluorescentchange from the filter may be observed by a user or monitoring device through the blocking layer.

[0044] In the depicted example in FIG. 1, the fluid absorption layer 110 is applied to a first side of the gas permeable indicator layer 108 and the blocking layer 106 is applied to the gas permeable indicator layer 108 on a second side, opposite to the fluid absorption layer 110.

[0045] Furthermore, the CO2 sensor 100 may comprise one or more adhesive layers configured to adhere the CO2 sensor 100 to the environment for use of the CO2 sensor 100. In the depicted example, the CO2 sensor 100 comprises a first adhesive layer 102 and a second adhesive layer 104, independently or together configured to adhere the CO2 sensor 100 to the skin of a user. In other applications, additional or fewer adhesive layers may be utilized to adhere the CO2 sensor 100 to the environment. In some examples, one or more adhesive layers may be disposed on top of the blocking layer 106, such as on the first side of the gas permeable indicator layer 108. In some examples, one or more adhesive layers may be disposed beneath the fluid absorption layer 110, such as on the second side of the gas permeable indicator layer 108.

[0046] A CO2 sensor 100 may further include a release layer 112 configured to reversibly adhere to a blocking layer, an adhesive layer, or other. A release layer will be removable prior to attaching a CO2 sensor to a surface. A release layer may be in the position as depicted in FIG. 1, or may be attached to blocking layer 106, or both depending on the intended use of the CO2 sensor 100. For example, the release layer 112 may be positioned as in FIG. 1 such as when a sensor is to be applied with the fluid absorption layer affixed to a surface such as a user’s skin or other surface. Alternatively, or in addition, a release layer may be positioned on top of adhesive layer 102 for protection, or for when the upper adhesive layer is to be affixed to a surface such as a surface of a tank or other for monitoring CO2 levels in an environment. A release layer may be made of any suitable material such as polymeric materials, woven materials, or other. Optionally, a release layer may be made from a polyethylene-coated paper or a polymeric film. Other release layer materials are known in the art and are similarly suitable.

[0047] Other configurations may be possible in further examples.Example Method of Preparation of a CO2 Sensor

[0048] FIG. 3 depicts an example method 300 of preparation of a CO2 sensor, such as the CO2 sensor 100 depicted in FIG. 1.

[0049] Initially, method 300 begins at block 302 with acidifying gas permeable filter paper. For example, in some aspects, the gas permeable filter paper may be acidified by submerging the gas permeable filter paper in an acid solution; removing the gas permeable filter paper from the acid solution; and drying the gas permeable filter paper. For example, in some aspects, the qualitative filter paper includes a filter pore size of at least about 0.1 microns, at least about 0.5 microns, at least about 1 micron, at least about 2 microns, at least about 3 microns, at least about 4 microns, at least about 5 microns, at least about 6 microns, at least about 7 microns, at least about 8 microns, at least about 9 microns, at least about 10 microns, at least about 20 microns, or more. In some aspects, a flow rate of the qualitative filter paper is medium flow. For example, in some aspects, a medium flow rate may be defined as approximately 50-100 mL / min. In some aspects, a flow rate of the qualitative filter paper is slow flow, for example, defined as approximately 5-50 mL / min. In some aspects, a flow rate of the qualitative filter paper is fast flow, for example, greater than 100 mL / min.

[0050] In some aspects, a diameter of the gas permeable filter paper is about 60 millimeters; however, in some aspects, the qualitative filter paper may be of any size, cut, shape or otherwise. For example, a size, cut, or shape of the gas permeable filter paper may depend on an application of the CO2 sensor 100.

[0051] In some aspects, the acid solution utilized to acidify the gas permeable filter paper comprises hydrochloric acid. In one example, the acid solution comprises 0.075N hydrochloric acid. In some aspects, the acid solution comprises one or more of sulfuric acid, nitric acid, phosphoric acid, acetic acid, citric acid, propionic acid, and others, or any combination thereof.

[0052] In some aspects, the acidified gas permeable filter paper is air dried for at least about 2 hours, at least about 4 hours, at least about 6 hours, at least about 8 hours, at least about 10 hours, or more. In some aspects, the acidified gas permeable filter paper may be dried untilvisibly dry, and / or dry to the touch, optionally to a water content of about 10 wt% or lower, optionally about 4 wt%.

[0053] Method 300 proceeds to block 304 with mixing a silicone matrix comprising silicone components and a pH indicator. An illustrative example of silicone matrix materials include but are not limited to silicone precursors sold by Smooth-On (Macungie, PA) as product numbers SO #26930 A and SO #26930B. For example, the silicone components may comprise encapsulation rubber components. In some aspects, the silicone components and the pH indicator may be added at a specified ratio, for example, a ratio of silicone components to the pH indicator of about 1 :6, of about 1 :10, of about 1 : 16, of about 1 :20, or of about 1 :26. The pH indicator is optionally combined with the least viscus silicone precursor followed by addition of the other component and polymerization or formation of the pH indicator / silicone material is created.

[0054] Alternative, in some aspects, an epoxy matrix or other matrix may be mixed with a pH indicator to form a water-clear matrix.

[0055] In some examples, the pH indicator may comprise a saline-based pH indicator or indicator mixture. In some aspects, the pH indicator comprises fluorescein. In some aspects, the pH indicator comprises pyranine. In some aspects, the pH indicator comprises methyl red and bromocresol green. In some aspects, the pH indicator comprises at least one of crystal violet, cresol red, cresolphthalein, cresol purple, thymol blue, methyl orange - xylene cyanol, bromophenol blue, Congo red, methyl orange, alizarin red S, bromocresol green, dichlorofluorescein, or methyl red or any combination thereof. FIG. 2A depicts the pH ranges of various pH indicators that may be used according to one or more aspects as provided herein.

[0056] Method 300 then proceeds to block 306 with applying the silicone matrix to the acidified gas permeable filter paper to form a gas permeable indicator layer, such as gas permeable indicator layer 108 in FIG. 1. In some aspects, the acidified gas permeable filter paper is submerged in the silicone matrix to form the gas permeable indicator layer.

[0057] Method 300 proceeds to block 308 with drying the gas permeable indicator layer. In some aspects, the acidified gas permeable filter paper is air dried for at least about 2 hours,at least about 4 hours, at least about 6 hours, at least about 8 hours, at least about 10 hours, at least about 12 hours, at least about 24 hours, at least about 48 hours, or more. In some aspects, the gas permeable indicator layer is dried in a low humidity environment. In some aspects, the gas permeable indicator may be dried until visibly dry, and / or dry to the touch, optionally to a water content of about 10 wt% or lower, optionally about 4 wt%.

[0058] Method 300 then proceeds to block 310 with combining the gas permeable indicator layer with a blocking layer configured to block carbon dioxide release, for example, the blocking layer 106 of FIG. 1.

[0059] Method 300 proceeds to block 312 with applying a fluid absorption layer opposite the blocking layer on the gas permeable indicator layer. In some aspects, the fluid absorption layer comprises chromatography paper. In some examples, the chromatography paper comprises cellulose chromatography paper. The chromatography paper may comprise, in some examples, 3 mm thickness. The chromatography paper may have a flow rate, for example, from about 35-70 mL.

[0060] In some aspects, method 300 further comprises applying at least one adhesive layer configured to adhere to an environment of the sensor. For example, where the CO2 sensor is applied as a patch to skin of the user, the at least one adhesive layer may be configured to adhere the sensor to a skin of the user. As another example, where the CO2 sensor is applied to an environment, the at least one adhesive layer may be configured to adhere the sensor to a portion of the environment.

[0061] Note that FIG. 3 is just one example of a method, and other methods including fewer, additional, or alternative operations are possible consistent with this disclosure.Example Method of Use of a CO2 Sensor

[0062] FIG. 4 depicts an example method 400 of use of a CO2 sensor, such as the CO2 sensor 100 depicted in FIG. 1. In the depicted example, the CO2 sensor may be utilized to measure metabolism, for example, metabolism of humans and animals.

[0063] Initially, method 400 begins at block 402 with applying a metabolic sensor, for example, a CO2 sensor. For example, in some aspects, the metabolic sensor may be applied to a portion of a user’s body. Because CO2 is produced by the epidermis, in the histocytes, and diffuses outwardly through the stratum comeum, a sensor applied to the skin may be used to monitor CO2 released by the skin.

[0064] In some aspects, the metabolic sensor is applied to a neonate.

[0065] In some aspects, the metabolic sensor is applied to a portion of a user’s body comprising low muscle tissue. Because muscle activity will emit more CO2 in the area, the metabolic sensor may be applied to a user’s body with low or no muscle tissue, or a seldom used muscle to avoid locally higher CO2 levels. For example, the palmaris longus is a muscle in the superficial compartment of the anterior forearm, and is seldom used. Another muscle, the lumbar multifidus, is in the lower back and is also seldom used. Such areas may be suitable for application of the metabolic sensor to avoid locally higher CO2 levels. Further, as the fingers do not have muscles, the metabolic sensor may be applied to a finger.

[0066] Method 400 proceeds to block 404 with obtaining a first reading of an indicator layer of the metabolic sensor at a first time. In some aspects, the first reading comprises a measurement of a hue value of the indicator layer. For example, a calorimeter may be used to measure a hue value of the indicator layer. In some aspects, the first reading comprises a measurement of a fluorescence of the indicator layer.

[0067] Method 400 then proceeds to block 406 with obtaining a second reading of the indicator layer of the metabolic sensor at a second time. In some aspects, the second reading comprises a measurement of the hue value of the indicator layer. For example, a calorimeter may be used to measure a hue value of the indicator layer. In some aspects, the second reading comprises a measurement of a fluorescence of the indicator layer.

[0068] In some aspects, a period of time between the first time and the second time comprises about 96 hours, about 48 hours, about 24 hours, about 12 hours, about 6 hours, about 4 hours, about 2 hours, about 1 hour, about a half of an hour, or about a quarter of an hour.

[0069] Method 400 proceeds to block 408 with determining, based on a difference between the first reading and the second reading, an amount of carbon dioxide release. In some aspects, the difference between the first reading and the second reading comprises a hue change. In some aspects, the difference between the first reading and the second reading comprises a fluorescence change.

[0070] A sensor may be worn for any desirable period of time, but is most effective if used up to a maximum total caloric output. Optionally, a maximum total caloric output detected by a sensor is up to 4000 kCals. Thus, if a user of the sensor is highly active, the time the sensor is worn may be short such as only a few hours. If a user is sedentary it may take days to exert 4000 kCals and the sensor can therefore be worn for several days.

[0071] In some aspects, the method further comprises determining, based on the amount of carbon dioxide release, a metabolic rate of the user.

[0072] In some aspects, a period of time between the first time and the second time comprises a sleep period, and the metabolic rate of the user comprises a resting metabolic rate of the user. A resting metabolic rate comprises an amount of energy a user’s body needs to function while at rest, including both the basic functions as well as minimal activity. A resting metabolic rate is used to estimate a user’s basal metabolic rate, that is, expenditures necessary for only the basic functions of a user’s body.

[0073] In some aspects, the metabolic sensor may be calibrated. Calibration of the sensor may comprise correlating a hue change between the first time period and the second time with a user’s resting metabolic rate. For example, a user’s resting metabolic rate may be estimated based on one or more characteristics of the user. In one example, the Harris-Benedict equation may be used to estimate a user’s resting metabolic rate. Specifically, for males:Resting Metabolic Rate RMR)maie= (4.38 * weight in pounds) + (14.55 * height in inches)— (5.08 * age in years) + 260Specifically, for females:

[0074] As another example, a user’s resting metabolic rate may be determined based on an indirect calorimetry breath test. Generally, exhaled CO2 comprises about 4% or 40,000 ppm of CO2 of a person’s breath. Indirect calorimetry may be used to estimate a user’s daily resting metabolic rate. Generally, a user’s resting metabolic rate changes very slowly, for example, less than 1% per year. Therefore, a user’s resting metabolic rate may be determined infrequently, for example, annually, semi-annually, or biannually.

[0075] In some aspects, a period of time between the first time and the second time comprises about 24 hours, and the metabolic rate of the user comprises a total metabolic rate of the user. A total metabolic rate of a user comprise an amount of energy a user’ s body expends in a day, including the energy expending to function at rest, to accomplish the day’s activities, and any additional physical activity, as well as thermogenic metabolism to maintain body temperature. Thus, measurement of CO2 release over a 24 hour period may be equivalent to the user’s total metabolic rate.

[0076] Where a user’s resting metabolic rate is known, an active metabolic rate may be determined. An active metabolic rate comprises the energy expenditures beyond those basic functions and includes physical activity. In some aspects, the method further comprises determining an active metabolic rate based on a difference between the total metabolic rate of the user and a resting metabolic rate of the user.

[0077] In some aspects, the active metabolic rate may be used to determine a number of calories burned during a period of physical activity. For example, where the first reading is done prior to a period of physical activity and the second reading is done after the period of physical activity, the difference between the readings may be used to determine the number of calories burned during the period of physical activity.

[0078] In some aspects, the method further comprises obtaining a third reading of the indicator layer of the metabolic sensor at a third time; and determining, based on a subsequentdifference between the second reading and the third reading, a subsequent amount of carbon dioxide release.

[0079] In some aspects, one or more additional metabolic rates may be determined based on one or more subsequent readings. For example, based on a difference between the second reading and the third reading, a metabolic rate for the period of time between the second reading and the third reading may be determined.

[0080] In some examples, subsequent readings may be taken at determined time intervals, for example, every two hours, every four hours, etc. In some examples, subsequent readings may be taken based on activity of the user, for example, prior to and after physical activity, after rest, and the like.

[0081] In some aspects, metabolic rates of a user may be used to track a user’s activity and / or health and fitness. For example, an active metabolic rate of a user may be monitored during periods of physical activity to monitor a user’s calorie expenditure. Changes in an active metabolic rate may indicate changes in health and fitness of the user. As another example, a total metabolic rate of a user may be monitored for several periods to monitor a user’s total calorie expenditure. Changes in total metabolic rate may indicate changes in health and fitness, for example, changes due to weight or other chronic diseases.

[0082] Note that FIG. 4 is just one example of a method, and other methods including fewer, additional, or alternative operations are possible consistent with this disclosure.Examples

[0083] Various aspects of the present disclosure are illustrated by the following nonlimiting examples. The examples are for illustrative purposes and are not a limitation on any practice of the present invention. It will be understood that variations and modifications can be made without departing from the spirit and scope of the invention.Example 1:

[0084] A skin metabolic sensor substantially as illustrated in FIG. 1 made with the dyes methyl red and bromocresol green at a ratio of 1 :0.7 respectively was removed from a releaseliner and placed on the skin of a user approximately 2-3 inches above the wrist. Readings of the skin metabolic sensor are taken with a colorimeter, a NIX MINI 3 (Nix Color Sensor, Hamilton, ON L8P 0A1, Canada).

[0085] The skin metabolic sensor is calibrated as follows: The skin sensor was applied two hours before bedtime to allow for equilibration. An initial reading is taken before bedtime and recorded. After waking, a second reading is taken and recorded. A change in hue from the pH indicator is recorded and represents the user’s resting metabolic rate. The known resting metabolic rate from a prior measurement using indirect calorimetry or the Harris-Benedict equation is compared to the calibration hue change to determine calories per unit hue change for the specific sensor used with the specific pH indicator.

[0086] Subsequent readings of hue change of the pH indicator were taken every 2 hours for 3 days total. The hue change of the pH indicator at each time point is plotted versus calories at teach time and represents the total metabolic rate for that time period. A subtraction of the resting metabolic rate (calories / time) from the determined total metabolic output yields the number of calories burned from exertion over that period of time. FIG. 5 depicts the results from each of day 2 (following overnight calibration), day 3, and day 4 for the test individual. For the Slopes: Standard deviation = 2.04 Mean = 36 CV (% Standard Deviation) =5.67. For the Constants: Standard deviation = 267 Mean = -2328.33 CV (%Standard Deviation) =11.47.Example Clauses

[0087] Implementation examples are described in the following numbered clauses:

[0088] Clause 1 : A method of manufacturing a metabolic sensor, comprising: acidifying gas permeable filter paper; mixing a silicone matrix comprising silicone components and a pH indicator; applying the silicone matrix to the gas permeable filter paper to form agas permeable indicator layer; drying the gas permeable indicator layer; combining the gas permeable indicator layer with a blocking layer configured to block carbon dioxide release; and applying a fluid absorption layer opposite the blocking layer on the gas permeable indicator layer.

[0089] Clause 2: The method of clause 1, wherein acidifying the gas permeable filter paper comprises: submerging the gas permeable filter paper in an acid solution; removing the gas permeable filter paper from the acid solution; and drying the gas permeable filter paper.

[0090] Clause 3 : The method of clause 2, wherein the gas permeable filter paper is air dried for at least 6 hours.

[0091] Clause 4: The method of any one of clauses 2-3, wherein the acid solution comprises hydrochloric acid.

[0092] Clause 5: The method of any preceding clause, wherein: a filter pore size of the gas permeable filter paper is at least 2 microns; a flow rate of the gas permeable filter paper is medium flow; and a diameter of the gas permeable filter paper is 60 mm.

[0093] Clause 6: The method of any preceding clause, wherein applying the silicone matrix to the gas permeable filter paper comprises submerging the gas permeable filter paper in the silicone matrix.

[0094] Clause 7: The method of any one of clauses 1-6, wherein the pH indicator comprises fluorescein.

[0095] Clause 8: The method of any one of clauses 1-6, wherein the pH indicator comprises pyranine.

[0096] Clause 9: The method of any one of clauses 1-6, wherein the pH indicator comprises methyl red and bromocresol green.

[0097] Clause 10: The method of any one of clauses 1-6, wherein the pH indicator comprises methyl orange and bromocresol green.

[0098] Clause 11 : The method of any of clauses 1-6, wherein the pH indicator comprises at least one of: crystal violet, cresol red, cresolphthalein, cresol purple, thymol blue, methyl orange - xylene cyanol, bromophenol blue, Congo red, methyl orange, alizarin red S, bromocresol green, dichlorofluorescein, or methyl red.

[0099] Clause 12: The method of any preceding clause, further comprising applying at least one adhesive layer configured to adhere to a skin of a user.

[0100] Clause 13: The method of any preceding clause, wherein the absorption layer comprises chromatography paper.

[0101] Clause 14: A metabolic sensor, comprising: a gas permeable indicator layer configured to change hue in response to carbon dioxide presence; a blocking layer configured to block carbon dioxide release; a fluid absorption layer configured to absorb liquid; and at least one adhesive layer configured to adhere to an environment of the sensor.

[0102] Clause 15: The metabolic sensor of clause 14, wherein the gas permeable indicator layer changes hue based on a change in pH of at least 0.5.

[0103] Clause 16: The metabolic sensor of any one of of clauses 14-15, wherein the gas permeable indicator layer further comprises a pH indicator configured to change hue based on a change in pH of the indicator layer.

[0104] Clause 17: The metabolic sensor of clause 16, wherein the pH indicator comprises fluorescein.

[0105] Clause 18: The metabolic sensor of clause 16, wherein the pH indicator comprises pyranine.

[0106] Clause 19: The metabolic sensor of clause 16, wherein the pH indicator comprises methyl red and bromocresol green.

[0107] Clause 20: The metabolic sensor of clause 16, wherein the pH indicator comprises methyl orange and bromocresol green.

[0108] Clause 21 : The metabolic sensor of clause 16, wherein the pH indicator comprises at least one of: crystal violet, cresol red, cresolphthalein, cresol purple, thymol blue, methyl orange - xylene cyanol, bromophenol blue, Congo red, methyl orange, alizarin red S, bromocresol green, dichlorofluorescein, or methyl red.

[0109] Clause 22: The metabolic sensor of any one of clauses 14-21, wherein the indicator layer comprises acidified qualitative filter paper.

[0110] Clause 23 : The metabolic sensor of clause 22, wherein a filter size of the acidified qualitative filter paper is 2 microns; a flow rate of the acidified qualitative filter paper is medium flow; and a diameter of the acidified qualitative filter paper is 60 mm.[OHl] Clause 24: The metabolic sensor of any one of clauses 14-23, wherein the absorption layer comprises chromatography paper.

[0112] Clause 25: A method of using a metabolic sensor comprising: applying the metabolic sensor of any one of clausesl4-24 to a portion of a user’s body; obtaining a first reading of the indicator layer of the metabolic sensor at a first time; obtaining a second reading of the indicator layer of the metabolic sensors at a second time; and determining, based on a difference between the first reading and the second reading, an amount of carbon dioxide release.

[0113] Clause 26: The method of clause 25, wherein each of the first reading and the second reading comprise a measurement of a hue value of the indicator layer.

[0114] Clause 27: The method of clause 26, wherein the difference between the first reading and the second reading comprises a hue change.

[0115] Clause 28: The method of clause 25, wherein each of the first reading and the second reading comprise a measurement of a fluorescence of the indicator layer.

[0116] Clause 29: The method of clause 28, wherein the difference between the first reading and the second reading comprises a fluorescence change.

[0117] Clause 30: The method of any one of clauses 25-29, wherein the metabolic sensor is applied to a neonate.

[0118] Clause 31 : The method any one of clauses 25-30, wherein the metabolic sensor is applied to a portion of a user’s body comprising low muscle tissue.

[0119] Clause 32: The method of any one of clauses 25-31, wherein a period of time between the first time and the second time comprises about 96 hours.

[0120] Clause 33: The method of any one of clauses 25-31, wherein a period of time between the first time and the second time comprises about 48 hours.

[0121] Clause 34: The method of any one of clauses 25-31, wherein a period of time between the first time and the second time comprises at least 2 hours.

[0122] Clause 35: The method of any one of clauses 25-34, further comprising determining, based on the amount of carbon dioxide release, a metabolic rate of the user.

[0123] Clause 36: The method of clause 35, wherein: a period of time between the first time and the second time comprises a sleep period, and the metabolic rate of the user comprises a resting metabolic rate of the user.

[0124] Clause 37: The method of clause 35, wherein: a period of time between the first time and the second time comprises about 24 hours, and the metabolic rate of the user comprises a total metabolic rate of the user.

[0125] Clause 38: The method of clause 37, further comprising determining an active metabolic rate based on a difference between the total metabolic rate of the user and a resting metabolic rate of the user.

[0126] Clause 39: The method of clause 37, further comprising: obtaining a third reading of the indicator layer of the metabolic sensor at a third time; and determining, based on a subsequent difference between the second reading and the third reading, a subsequent amount of carbon dioxide release.Additional Considerations

[0127] The preceding description is provided to enable any person skilled in the art to practice the various embodiments described herein. The examples discussed herein are not limiting of the scope, applicability, or embodiments set forth in the claims. Various modifications to these embodiments will be readily apparent to those skilled in the art, and thegeneric principles defined herein may be applied to other embodiments. For example, changes may be made in the function and arrangement of elements discussed without departing from the scope of the disclosure. Various examples may omit, substitute, or add various procedures or components as appropriate. For instance, the methods described may be performed in an order different from that described, and various steps may be added, omitted, or combined. Also, features described with respect to some examples may be combined in some other examples. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, the scope of the disclosure is intended to cover such an apparatus or method that is practiced using other structure, functionality, or structure and functionality in addition to, or other than, the various aspects of the disclosure set forth herein. It should be understood that any aspect of the disclosure disclosed herein may be embodied by one or more elements of a claim.

[0128] As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects.

[0129] As used herein, a phrase referring to “at least one of’ a list of items refers to any combination of those items, including single members. As an example, “at least one of a, b, or c” is intended to cover a, b, c, a-b, a-c, b-c, and a-b-c, as well as any combination with multiples of the same element (e.g., a-a, a-a-a, a-a-b, a-a-c, a-b-b, a-c-c, b-b, b-b-b, b-b-c, c-c, and c-c-c or any other ordering of a, b, and c). Reference to an element in the singular is not intended to mean only one unless specifically so stated, but rather “one or more.” For example, reference to an element (e.g., “a processor,” “a memory,” etc.), unless otherwise specifically stated, should be understood to refer to one or more elements (e.g., “one or more processors,” “one or more memories,” etc.). The terms “set” and “group” are intended to include one or more elements, and may be used interchangeably with “one or more.” Where reference is made to one or more elements performing functions (e.g., steps of a method), one element may perform all functions, or more than one element may collectively perform the functions. When more than one element collectively performs the functions, each function need not be performed by each of those elements (e.g., different functions may be performed by different elements)and / or each function need not be performed in whole by only one element (e.g., different elements may perform different sub-functions of a function). Similarly, where reference is made to one or more elements configured to cause another element (e.g., an apparatus) to perform functions, one element may be configured to cause the other element to perform all functions, or more than one element may collectively be configured to cause the other element to perform the functions. Unless specifically stated otherwise, the term “some” refers to one or more.

[0130] The methods disclosed herein comprise one or more steps or actions for achieving the methods. The method steps and / or actions may be interchanged with one another without departing from the scope of the claims. In other words, unless a specific order of steps or actions is specified, the order and / or use of specific steps and / or actions may be modified without departing from the scope of the claims. Further, the various operations of methods described above may be performed by any suitable means capable of performing the corresponding functions.

[0131] The following claims are not intended to be limited to the embodiments shown herein, but are to be accorded the full scope consistent with the language of the claims. Within a claim, reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” Unless specifically stated otherwise, the term “some” refers to one or more. No claim element is to be construed under the provisions of 35 U.S.C. §112(f) unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.” All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims.

Claims

WHAT IS CLAIMED IS:

1. A method of manufacturing a metabolic sensor, comprising: acidifying gas permeable filter paper; mixing a silicone matrix comprising silicone components and a pH indicator; applying the silicone matrix to the gas permeable filter paper to form a gas permeable indicator layer; drying the gas permeable indicator layer; combining the gas permeable indicator layer with a blocking layer configured to block carbon dioxide release; and applying a fluid absorption layer opposite the blocking layer on the gas permeable indicator layer.

2. The method of claim 1, wherein acidifying the gas permeable filter paper comprises: submerging the gas permeable filter paper in an acid solution; removing the gas permeable filter paper from the acid solution; and drying the gas permeable filter paper.

3. The method of claim 2, wherein the gas permeable filter paper is air dried for at least 6 hours.

4. The method of claim 2, wherein the acid solution comprises hydrochloric acid.

5. The method of claim 1, wherein: a filter pore size of the gas permeable filter paper is at least 2 microns; a flow rate of the gas permeable filter paper is medium flow; and a diameter of the gas permeable filter paper is 60 mm.

6. The method of claim 1, wherein applying the silicone matrix to the gas permeable filter paper comprises submerging the gas permeable filter paper in the silicone matrix.

7. The method of claim 1, wherein the pH indicator comprises fluorescein.

8. The method of claim 1, wherein the pH indicator comprises pyranine.

9. The method of claim 1, wherein the pH indicator comprises methyl red and bromocresol green.

10. The method of claim 1, wherein the pH indicator comprises methyl orange and bromocresol green.

11. The method of claim 1, wherein the pH indicator comprises at least one of: crystal violet, cresol red, cresolphthalein, cresol purple, thymol blue, methyl orange - xylene cyanol, bromophenol blue, Congo red, methyl orange, alizarin red S, bromocresol green, dichlorofluorescein, or methyl red.

12. The method of claim 1, further comprising applying at least one adhesive layer configured to adhere to a skin of a user.

13. The method of claim 1, wherein the fluid absorption layer comprises chromatography paper.

14. A metabolic sensor, comprising: a gas permeable indicator layer configured to change hue in response to carbon dioxide presence; a blocking layer configured to block carbon dioxide release; a fluid absorption layer configured to absorb liquid; and at least one adhesive layer configured to adhere to an environment of the metabolic sensor.

15. The metabolic sensor of claim 14, wherein the gas permeable indicator layer changes hue based on a change in pH of at least 0.5.

16. The metabolic sensor of claim 14, wherein the gas permeable indicator layer further comprises a pH indicator configured to change hue based on a change in pH of the gas permeable indicator layer.

17. The metabolic sensor of claim 16, wherein the pH indicator comprises fluorescein.

18. The metabolic sensor of claim 17, wherein the pH indicator comprises pyranine.

19. The metabolic sensor of claim 17, wherein the pH indicator comprises methyl red and bromocresol green.

20. The metabolic sensor of claim 17, wherein the pH indicator comprises methyl orange and bromocresol green.

21. The metabolic sensor of claim 17, wherein the pH indicator comprises at least one of: crystal violet, cresol red, cresolphthalein, cresol purple, thymol blue, methyl orange - xylene cyanol, bromophenol blue, Congo red, methyl orange, alizarin red S, bromocresol green, dichlorofluorescein, or methyl red.

22. The metabolic sensor of claim 14, wherein the gas permeable indicator layer comprises acidified qualitative filter paper.

23. The metabolic sensor of claim 22, wherein a filter size of the acidified qualitative filter paper is 2 microns; a flow rate of the acidified qualitative filter paper is medium flow; and a diameter of the acidified qualitative filter paper is 60 mm.

24. The metabolic sensor of claim 14, wherein the fluid absorption layer comprises chromatography paper.

25. A method of using a metabolic sensor comprising:applying the metabolic sensor of any one of claims 14-24 to a portion of a user’s body; obtaining a first reading of the gas permeable indicator layer of the metabolic sensor at a first time; obtaining a second reading of the gas permeable indicator layer of the metabolic sensors at a second time; and determining, based on a difference between the first reading and the second reading, an amount of carbon dioxide release.

26. The method of claim 25, wherein each of the first reading and the second reading comprise a measurement of a hue value of the gas permeable indicator layer.

27. The method of claim 26, wherein the difference between the first reading and the second reading comprises a hue change.

28. The method of claim 25, wherein each of the first reading and the second reading comprise a measurement of a fluorescence of the gas permeable indicator layer.

29. The method of claim 28, wherein the difference between the first reading and the second reading comprises a fluorescence change.

30. The method of claim 25, wherein the metabolic sensor is applied to a neonate.

31. The method of claim 25, wherein the metabolic sensor is applied to a portion of a user’s body comprising low muscle tissue.

32. The method of claim 25, wherein a period of time between the first time and the second time comprises about 96 hours.

33. The method of claim 25, wherein a period of time between the first time and the second time comprises about 48 hours.

34. The method of claim 25, wherein a period of time between the first time and the second time comprises at least 2 hours.

35. The method of claim 25, further comprising determining, based on the amount of carbon dioxide release, a metabolic rate of the user.

36. The method of claim 35, wherein: a period of time between the first time and the second time comprises a sleep period, and the metabolic rate of the user comprises a resting metabolic rate of the user.

37. The method of claim 35, wherein: a period of time between the first time and the second time comprises about 24 hours, and the metabolic rate of the user comprises a total metabolic rate of the user.

38. The method of claim 37, further comprising determining an active metabolic rate based on a difference between the total metabolic rate of the user and a resting metabolic rate of the user.

39. The method of claim 37, further comprising: obtaining a third reading of the gas permeable indicator layer of the metabolic sensor at a third time; and determining, based on a subsequent difference between the second reading and the third reading, a subsequent amount of carbon dioxide release.

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