Patent Application
20190029654
Sweat and saliva sensing technologies have enormous potential for applications ranging from athletics, to neonatology, to pharmacological monitoring, to personal digital health, to name a few applications. Sweat and saliva contain many of the same biomarkers, chemicals, or solutes that are carried in blood and can provide significant information enabling one to diagnose ailments, health status, toxins, performance, and other physiological attributes even in advance of any physical sign. Furthermore, sweat or saliva itself, the action of sweating or salivating, and other parameters, attributes, solutes, or features on, near, or beneath the skin or oral mucosa can be measured to further reveal physiological information. Moreover, sensing sweat and saliva is relatively non-invasive compared to other biofluids.
However, some solutes such as molecules (e.g., 100's to 1000's Da) and proteins (e.g., 10,000's Da) are dilute in sweat and saliva compared to their concentrations in plasma, often due to the effects of the larger size or a lack of lipophilicity on filtration. Accessing and analyzing such large molecules in sweat or saliva has proven to be difficult. Such analytes can be accessed through microneedles for analysis, but microneedles are invasive. Such analytes can also be accessed through electroporation of the stratum corneum, but electroporation of the stratum corneum can cause a pathway for infection and can include pain or discomfort. Therefore, improved methods of sophisticated and effective integration and application of sweat stimulation, sweat or saliva collection, and sweat or saliva sensing are needed to address one or more of these drawbacks.
Embodiments of the disclosed invention provide a sweat or saliva sensor device capable of high performance sweat stimulation, electroporation, and/or sweat or saliva sensing at the same site. Elements of the disclosed invention may be used in combination or in some cases individually.
In an embodiment, a method of collecting and sensing a biofluid with enhanced concentration of analytes due to electroporation is provided. The method comprises electroporating biofluid glands that are generating a biofluid and specifically sensing at least one analyte in said biofluid, the at least one analyte having a molecular weight greater than 50 Da.
In another embodiment, a method of collecting and sensing saliva with enhanced concentration of analytes due to electroporation is provided. The method comprises electroporating saliva glands and specifically sensing at least one analyte in said saliva.
In another embodiment, a device wearable on a user's skin for receiving an advective flow of a biofluid, wherein the biofluid is one of sweat, saliva, and tears, is provided. The device comprises at least one of a biofluid stimulation component, a biofluid sensor specific to an analyte, or a biofluid collection element, at least one electroporation electrode for enhancing concentration of at least one analyte in the biofluid having a molecular weight of greater than 50 Da, a counter electrode, and an electroporation waveform generator configured to cause the electroporation electrode to generate and direct a plurality of electroporation pulses into the skin.
The objects and advantages of the disclosed invention will be further appreciated in light of the following detailed descriptions and drawings in which:
The definitions below are provided in the context of sweat and sweat glands, but also apply to saliva, salivary glands, tears, and tear ducts in the context of the disclosed invention.
As used herein, “chronological assurance” means using a sweat sensor device to measure a sweat analyte so that the measurement reflects the analyte's concentration in a fresh sweat sample as it emerges from skin. By contrast, a sweat analyte measurement lacking chronological assurance may reflect the analyte's concentration in a sweat sample consisting of fresh sweat mixed with older sweat. Determining chronological assurance may consider how a particular measurement is affected by potential contamination with previously generated sweat, previously generated solutes, other fluid, or other contamination sources.
As used herein, “continuous monitoring” means the capability of a device to provide at least one measurement of sweat determined by a continuous or multiple collection and sensing of that measurement or to provide a plurality of measurements of sweat over time.
As used herein, “determined” may encompass more specific meanings including but not limited to: something that is predetermined before use of a device; something that is determined during use of a device; and something that could be a combination of determinations made before and during use of a device.
As used herein, “sweat sampling rate” is the effective rate at which new sweat or sweat solutes, originating from the sweat gland or from skin or tissue, reaches a sensor that measures a property of sweat or its solutes. Sweat sampling rate, in some cases, can be far more complex than just sweat generation rate. Sweat sampling rate directly determines or is a contributing factor in determining the chronological assurance. Times and rates are inversely proportional (rates having at least partial units of 1/seconds), therefore a short or small time required to refill a sweat volume can also be said to have a fast or high sweat sampling rate. The inverse of sweat sampling rate (1/s) could also be interpreted as a “sweat sampling interval” (s). Sweat sampling rates or intervals are not necessarily regular, discrete, periodic, discontinuous, or subject to other limitations. Like chronological assurance, sweat sampling rate may also include a determination of the effect of potential contamination with previously generated sweat, previously generated solutes, other fluid, or other measurement contamination sources for the measurement(s). Sweat sampling rate can also be in whole or in part determined from solute generation, transport, advective transport of fluid, diffusion transport of solutes, or other factors that will impact the rate at which new sweat or sweat solutes reach a sensor and/or are altered by older sweat or solutes or other contamination sources. Sensor response times may also affect sampling rate.
As used herein, “sweat stimulation” is the direct or indirect causing of sweat generation by any external stimulus such as chemical, heat, optical, electrical current, or other methods, with the external stimulus being applied for the purpose of stimulating sweat. One example of sweat stimulation is the administration of a sweat stimulant such as pilocarpine, acetylcholine, methacholine, carbachol, bethanechol, or other suitable chemical stimulant by iontophoresis, diffusion, injection, ingestion, or other suitable means. Some sweat stimulants are effective for a period of minutes, hours, or more. Generally, longer lasting sweat stimulation methods minimize mechanical re-arrangement of components during use. Sweat stimulation may also include sudo-motor axon reflex sweating, where the stimulation site and sweat generation site are not the same but in close in proximity and physiologically linked in the sweat response.
As used herein, a “sweat stimulating component” or “sweat stimulation component” is any component or material that is capable of locally stimulating sweat to a rate greater than the natural local rate if such stimulation were not applied locally to the body.
As used herein, a “sweat sensing component” or “sweat sensor component” is any component or material that is capable of sensing sweat, a solute in sweat, a property of sweat, a property of skin due to sweat, or any other thing to be sensed that is in relation to sweat or causes of sweat. Sweat sensing components can include, for example one or multiple sensors such as, potentiometric, amperometric, impedance, optical, mechanical, or other types known by those skilled in the art. A sweat sensing component may also include supporting materials or features for additional purposes, with non-limiting examples including local-buffering of sensor electronic signals or additional components for sweat management such as microfluidic materials.
As used herein, “sweat generation rate” is the rate at which sweat is generated by the sweat glands themselves. Sweat generation rate is typically measured by the flow rate from each gland in nL/min/gland. In some cases, the measurement is then multiplied by the number of sweat glands from which the sweat is being sampled.
As used herein, “measured” can imply an exact or precise quantitative measurement and can include broader meanings such as, for example, measuring a relative amount of change of something. Measured can also imply a binary measurement, such as ‘yes’ or ‘no’ type measurements.
As used herein, “sweat sampling events” represents the number of sweat samples per a given unit of time that are able to be measured and produce a physiologically meaningful measurement of sweat. These events could be for a continuous flow of sweat and equivalent to sweat sampling rate. These events could be for a discontinuous flow of sweat, for example the number of times the sweat volume or sweat generation rate are adequate to make a proper sweat measurement. For example, if a person needed to measure cortisol three times per day, then the sweat flow rate would need to be adequate to provide a useful sweat cortisol measurement at least three times in the day, and other times during the day could be greater or lower than that adequate sweat flow rate.
As used herein, “sweat volume” is the fluidic volume in a space that can be defined multiple ways. Sweat volume may be the volume that exists between a sensor and the point of generation of sweat. Sweat volume can include the volume that can be occupied by sweat between: the sampling site on the skin and a sensor on the skin where the sensor has no intervening layers, materials, or components between it and the skin; or the sampling site on the skin and a sensor on the skin where there are one or more layers, materials, or components between the sensor and the sampling site on the skin.
As used herein, “solute generation rate” is simply the rate at which solutes move from the body or other sources into sweat. “Solute sampling rate” includes the rate at which these solutes reach one or more sensors.
As used herein, “microfluidic components” are channels in polymer, textiles, paper, or other components known in the art for guiding movement of a fluid or at least partial containment of a fluid.
As used herein, “state void of sweat” is where a space or material or surface that can be wetted, filled, or partially filled by sweat is in a state where it is entirely or substantially (e.g., greater than 50%) dry or void of sweat.
As used herein, “advective transport” is a transport mechanism of a substance or conserved property by a fluid due to the fluid's bulk motion.
As used herein, “diffusion” is the net movement of a substance from a region of high concentration to a region of low concentration. This is also referred to as the movement of a substance down a concentration gradient.
As used herein, “convection” is the concerted, collective movement of groups or aggregates of molecules within fluids and rheids, either through advection or through diffusion or a combination of both.
As used herein, a “volume-reduced pathway” is a sweat volume that has been reduced by the addition of a material, device, layer, or other body-foreign substance, which therefore increases the sweat sampling interval for a given sweat generation rate. This term can also be used interchangeably in some cases with a “reduced sweat pathway”, which is a pathway between sweat glands and sensors that is reduced in terms of volume or in terms of surfaces wetted by sweat along the pathway. Volume reduced pathways or reduced sweat pathways include those created by sealing the surface of skin, because skin can absorb or exchange water and solutes in sweat, which could increase the sweat sampling interval and/or cause contamination, which can also alter the accuracy or duration of the sweat sampling interval.
As used herein, “volume reducing component” means any component that reduces the sweat volume. In some cases, the volume reducing component is more than just a volume reducing material, because a volume reducing material by itself may not allow proper device function (e.g., the volume reducing material would need to be isolated from a sensor for which the volume reducing material could damage or degrade, and therefore the volume reducing component may comprise the volume reducing material and at least one additional material or layer to isolate volume reducing material from said sensors).
As used herein, “mechanical co-location” refers to one or more components that can be mechanically moved or arranged in a manner that causes the components to be coupled or de-coupled to a common area of skin (i.e., one or both components are movable relative to the common area of skin), and such that the two or more components during at least one point are carried simultaneously by the device, and such that at least one component is continuously carried by the device during its use. The term “mechanical movement” includes manual movement of device components. For example, a device that places a stimulating component onto skin, removes the stimulating component from skin, and then with a separate device places a sensing component onto skin, does not meet the definition of “mechanical co-location” because neither of these components is always carried by the device, as will be further described in the disclosed invention. For a first example, the definition of “mechanical co-location” would be met by a device that carries a sweat sensing component during use of the device and integrates an iontophoretic sweat stimulating component temporarily, with the stimulating component during stimulation being coupled to at least a common portion of skin to which the sensing component is coupled. For a second example, the definition of “mechanical co-location” would be met by a device that carries an electroporation component and a sensing component during use of the device.
As used herein, “electroporation” refers to electroporation dominantly of the eccrine or apocrine sweat ducts and glands, or other biofluid glands, and excludes electroporation that is dominantly of the stratum corneum. Multiple electroporated pores or pathways are possible, for example pathways through cellular membranes, paracellular pathways (including through tight junctions), or other possible pathways. In one aspect, at least ⅔ of the increase in analyte concentration in sweat may be due to electroporation of the sweat ducts and glands (i.e., the additional analytes do not originate from the skin surface). In another aspect, at least 9/10 of the increase in analyte concentration may be due to electroporation of the sweat ducts and glands (i.e., not from the skin surface). Such values could be validated by testing electroporation with or without sweating, and then testing solute concentrations both with and without active sweating, or estimated by using skin impedance with or without sweating. Such values also could be validated by measuring analytes in sweat with and without one or more methods for reduced sweat volume that can isolate sweat ducts from the skin surface. Electroporation as used herein is dependent on a flow of sweat, and/or a sweat volume filled with sweat to enable a pathway for solutes to reach a sensor. Electroporation therefore excludes electro-osmosis in the absence of sweating, for example, as previously shown for the Glucowatch Biographer product for transdermal glucose monitoring, which is entirely reliant on a continuously applied DC voltage with a current density of about 0.3 mA/cm2 to create both a pathway of fluid and a flux of analyte. The GlucoWatch requires a 2 hour warm-up period to reach a steady state flux of glucose by electro-osmosis. Embodiments described below may allow for to analyte signals that are measured in just a few minutes. Electroporation may use any magnitude, frequency, waveform, polarity, current limit, voltage limit, or other voltage or current features or requirements that satisfy the above definition of electroporation. Electroporation may generally include any phenomena that electrically enhances flux into sweat of analytes that are greater than 50 Da in molar mass, and may include physical pores or pathways created in tissue or cell membranes, or between cells, or may include a destabilization of tissues or membranes that allows increased analyte flux. In some cases, electroporation may also increase the flux of analytes, such as ions, that are less than 50 Da. Thus, when electroporation is applied to increase the flux of analytes that are greater than 50 Da, an additional effect may be the increase in flux of analytes that are less than 50 Da.
As used herein, “electroporation waveform” is any waveform for increasing skin permeability that operates by creating physical pores in, or the destabilization of, skin, tissue, or cellular structures thereby increasing flux into sweat of analytes that are greater than 50 Da in molar mass. This increased skin permeability can last for several minutes. Electroporation may be caused entirely or partially by one or more electroporation waveforms. Often, electroporation waveforms are on the order of, but not limited to, 100's to 1000's V/cm. The actual waveform used may vary depending on the specific application, electrode distances, etc. Electroporation waveforms can be monopolar or bipolar (i.e., negative and/or positive).
As used herein, “chaser waveform” is a waveform that may be applied before, during (by waveform superposition), or after an electroporation waveform or between a plurality of electroporation waveforms. Electroporation therefore may be caused partially by one or more chaser waveforms. Chaser waveforms have the purpose of enhancing flux of analytes into sweat that are greater than 50 Da in molar mass through the physical pores caused by an electroporation waveform. Chaser waveforms often have, but are not necessarily limited to, voltages on tissues or cells that are lower than those used for electroporation waveforms. Chaser waveforms may be on the order of, but not limited to, 10's to 100's V/cm. The actual waveform used may vary depending on the specific application, electrode distances, etc. Chaser waveforms can be monopolar or bipolar.
Embodiments of the disclosed invention are directed to biofluid sensing devices capable of high performance biofluid sensing whereby greater solute access is enabled by electroporation. Although various embodiments described below are described as being specific to sweat or saliva, such description may apply to sweat, saliva, and tears even if not explicitly mentioned. Although the discussion for sweat focuses on eccrine sweat glands, it should be recognized that the description may apply to other biofluid gland types, such as apocrine glands, salivary glands, or tear glands. Large analytes may be extracted in sweat or saliva with greater concentrations where electroporation is performed in a manner that is more targeted, precise, and limited only to live tissue lining the eccrine sweat glands or salivary glands in the mouth. For tears, electroporation electrodes or sensors could be provided anywhere on or near the surface of the cornea, the canaliculi, the lacrimal sac, or the lacrimal duct. Such an approach is safe and effectively non-invasive because an outward flow of sweat or saliva prevents infection, and live cells can repair any damage or regenerate quickly (e.g., a minute or less) if the electroporation is adequately controlled. When electroporation can be made more selective to just the live cells lining the sweat ducts or salivary ducts/glands, safer and more repeatable/reliable biomarker extraction and sensing mechanisms can be enabled. Furthermore, specific to sweat, repeatability, reliability, and safety can all be improved if the sweat glands are actively sweating (e.g., filled with salty sweat, which is electrically conductive).
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The first substrate 110 is configured to (1) hold the sensing component 120 against the skin 12 when the second substrate 115 is not inserted (
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In one embodiment, the electrode 290 is electrically grounded (e.g., using the same electrical ground used by the sensors 220, 222 (not shown)), and the voltage for electroporation is applied by a larger counter electrode (e.g., the electrode 195) located elsewhere on the skin 12. In other words, the electrode is referenced to the same voltage potential as said at least one biofluid sensor. As a result, the sensors 220, 222 do not experience the electroporation voltage that could interfere with or damage the sensors 220, 222. Additionally or alternatively, electronics in the device 100 (not shown) could also use lock-in-amplifying, shielding, or filtering methods to eliminate electrical noise imposed on the sensors 220, 222 by the electroporation voltages.
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An exemplary method of using the device 100 is now described. First, sweat stimulation is induced as shown in
It should be recognized that the electroporation voltage may vary based on the intended application. In an embodiment, a 1 to 3 V bipolar electroporation voltage could be applied for 1 ms duration at a frequency of repeated pulses or voltage magnitude needed to maintain a certain degree of electroporation. In an embodiment, electroporation is intermittently applied at voltages of 500 mV or greater. If the sweat ducts are targeted for electroporation, a voltage of about 2 to 4 V may be required to penetrate the double cell lining of the sweat ducts. A similar case may exist for salivary glands. At 2 to 4 V, electroporation can enhance partitioning of solutes from the targeted live cells and tissue by one or more orders of magnitude. However, where the stratum corneum of the skin or oral mucosa is the primary target, an electroporation voltage of about 30 to 100 V or more is typically utilized, which is far more damaging to the surrounding cells. In one embodiment, electroporation voltages could be increased to values that allow greater analyte concentrations and/or greater analyte sizes at which the user could begin to sense the presence of the voltage. Therefore, in such cases, a numbing, anti-inflammatory, or pain-relieving agent (such as Oragel) could be topically or iontophoretically applied before or during placement of the devices.
It should be recognized that the characteristics of the applied electroporation voltage may vary based on the intended application. In an aspect of the disclosed invention, the use of various polarities, frequencies, magnitudes, waveforms, and other methods of altering the electroporation voltage application could enhance electroporation specific to certain types or sizes of analytes. Further, electroporation can target different ratios of enhancement of solutes that may come from intra-cellular or from extra-cellular regions of tissue. For example, extracellular glucose may be a preferred analyte to measure compared to intracellular glucose. Different pulse magnitudes and widths can be used to control the depth of electroporation into the sweat gland or other target location (e.g., see Example 4). For example, it may be preferable to only or primarily electroporate the dermal duct so as to not interfere with the secretory portion of the gland. It should be recognized that the voltage applied to the skin is greater than the voltage that reaches the desired location (e.g., a certain length of the dermal duct). In an embodiment, a safety level is set, such as a maximum frequency of repeated pulses or a maximum voltage magnitude, that is not to be exceeded (e.g., see Example 4) in order to avoid damaging the skin or specific skin structures. The electroporation pulses may be of various durations, depending on the application, and may be for example, less than 10 s, less than 1 s, less than 100 ms, less than 10 ms, less than 1 ms, less than 100 μs, less than 10 μs, or less than 1 μs.
Electroporation may be applied once, intermittently (e.g., AC, pulsed DC, etc.), continuously, or as needed by the electrode 290. In an embodiment of the disclosed invention, electroporation may be applied periodically or on demand, rather than continuously, to reduce stress on the body or to reduce interference with the secretory coil, which can inhibit sweat generation. With respect to sweat inhibition, a continued flow of current into the skin can inhibit sweating (e.g., iontophoresis is commonly used to treat hyperhidrosis). This could negatively impact device applications that require sweat stimulation, especially where the electroporation is more aggressive to obtain a higher concentration or larger-size of analytes into sweat. Similar need to reduce stress could exist in cases where a person is particularly sensitive to, or has some physiological problem with, electroporation. Two exemplary embodiments are now provided.
In an aspect of the disclosed invention, one or more types of waveforms may be applied. As previously described, two such waveforms include an electroporation waveform and a chaser waveform. For example, an electroporation waveform could be applied as a bipolar DC square wave (+/−4 V) with a period of 10's of μs and a total of 8 pulses for each polarity, corresponding to a total of 100 to 200 μs of voltage application. Alternately, a similar waveform with a lower duty cycle could be applied using a positive 10 μs long pulse of 4 V at 0 s, and a −4 V and 10 μs pulse at 0.5 s, and repeating 8 times (i.e., 8 seconds total). The resulting permeabilization of the lining of the eccrine duct can typically last for several minutes. It should be recognized that the effective length of the permeabilization may be shorter or longer depending on the waveform applied, the specific target analyte, or other factors.
Merely because the lining of the eccrine duct is permeabilized by an electroporation waveform does not mean that most or all analytes will easily traverse it. Therefore, in an embodiment, a chaser waveform may be applied to increase the flux of analytes through the permeabilized lining of the eccrine duct. The chaser waveform may be applied after or between electroporation waveforms to continue or enhance entry of analytes into the sweat and to increase the concentration of the analytes in the sweat. This enhancement could be due to iontophoretic effects for charged analytes or for non-charged analytes due to localized flow and related drag forces imparted by charged solutes near the non-charged analytes. The chaser waveform could be AC, DC, pulsed, monopolar, bipolar, or any other suitable type of waveform. For example, because large analytes are highly dilute in sweat, if a first polarity for the chaser waveform pulled in the analyte into the lumen of the sweat duct, a second and opposite polarity could be applied later (milliseconds to seconds to minutes), which would not appreciably remove the large analyte from sweat in the lumen because the large analyte has already diffused away from the pore vicinity or has been advectively transported away by the flow of sweat in the lumen.
As disclosed, an electroporation component may deploy a plurality of different electroporation waveforms and chaser waveforms, and may deploy those various electroporation waveforms and chaser waveforms a plurality of times. In various embodiments, therefore, electroporation and chaser waveforms may be bipolar with equal magnitudes for each polarity, asymmetric in magnitude, or may be monopolar and similar or opposite in polarity or magnitude. Further, because sweat pH and/or salinity could alter the efficacy of the electroporation waveform and/or the chaser waveform, these waveforms could be adjusted as needed. For example, the adjustments to the waveforms may be based on one or more measurements taken by device components in communication with an electroporation component, including sensors providing measurements for analyte concentration, skin impedance, sweat pH, sweat salinity, or other measurements.
In an aspect of the disclosed invention, concentration ratios of two or more analytes can be measured and can be compared over time instead of, or in addition to, comparing the absolute concentrations of each analyte over time. In that regard, for some analytes, especially the larger sized analytes or more dilute analytes, electroporation will be unable to provide concentrations of a particular analyte that are similar to the concentration of that analyte in plasma, cells, intracellular fluid, or other fluid of interest. The two or more analytes would be chosen from those that have similar enhancement of concentration in sweat due to electroporation (e.g., due to a similar size, charge, hydrophilicity, shape, etc.). Measuring and comparing the changes in a concentration ratio of a reference analyte and the target analyte allows for determination of a condition, disease, or other factor in the body and, in some cases, is preferable to measuring the concentration of the analyte itself directly in blood. Exemplary applications include measuring concentrations of cortisol and dehydroepiandrosterone sulfate (DHEAS) and determining a cortisol/DHEAS ratio, which is of interest for stress monitoring, or determining ratios of two cytokines (e.g., one pro-inflammatory, one anti-inflammatory). In an embodiment, a sensor could be provided for each of IL-1 beta (e.g., sensor 220) and for TNF-alpha (e.g., sensor 222), which have similar molecular weights of 17 kDa and similar effective diffusivities, and their ratios compared over time. Other examples of cytokine ratios or other analyte ratios are possible.
In an aspect of the disclosed invention, devices can be utilized to collect sweat without onboard sensors for sensing the targeted analyte. To that end, the device may only include those features and elements useful for electroporation and for collecting an appropriate sample of sweat (e.g., a sweat collection element), which can be analyzed by a sensor or technique outside the device. For example, a device of the disclosed invention could include a microfluidic component for collecting sweat, such as the wicking components 230, 232 shown in
With reference to
In an aspect of the disclosed invention, electroporation may be applied to more than one location on the skin because electroporation could cause stress on the skin. In an embodiment, a single device (e.g., device 100) could be placed on the skin and moved to a new position on skin so that the device senses sweat and electroporates at more than one location. For example, the elements 219, 290, 232 could be physically moved within a device 100 to alternate locations during use of the device 100. Alternately, a device may include multiple sets of the elements 219, 290, 232 where each set is positioned at a unique location on skin to wick sweat to a common set of sensors 220, 222. In another embodiment, a device could include multiple subcomponents 120, each at a unique location on skin. Applying the electroporation at more than one location on the skin may improve the comfort level for the user.
With reference to
In an aspect of the disclosed invention, the electrode sizes and contact area with skin may be designed to mitigate issues with pain or discomfort caused by electrical current passing into the skin 12. Pain or discomfort caused by electrical current in skin does not scale linearly in terms of the relationship of current density to electrode area. The smaller the electrode area that contacts skin is, generally the larger the current density that can be used without a perception of the current or perception of pain. For example, an electrode of 24 cm2 area generates a tingle at 0.08 mA/cm2, whereas an electrode of 0.64 cm2 generates a tingle at 0.4 mA/cm2 (varies based on location on skin and from person to person). In an aspect of the disclosed invention, due to the reduced sample volumes, the areas of electrical contact with skin for reverse iontophoresis are reduced, as discussed above. Consider for example, sampling biofluid from pre-existing pathways that are sweat ducts with densities of 100 glands/cm2.—The contact areas needed to cover an average of 5, 10, and 50 glands, therefore, would be 0.05 cm2, 0.1 cm2, and 0.5 cm2, respectively. With sweat ducts at densities of 200 glands/cm2, then the contact areas needed to cover an average of 5, 10, and 50 glands would be 0.025 cm2, 0.05 cm2, and 0.25 cm2, respectively. Even fewer glands could be covered, so the above areas of contact may represent upper or lower limits for contact areas for one or more embodiments of the disclosed invention. These areas can be of the electrodes themselves or, in the case of intervening materials or layers between the electrodes and skin, can represent the electrical contact area with skin.
With further reference to
In an aspect of the disclosed invention, a device may be configured to apply electroporation only when it would likely result in the desired increase of the target analyte. In that regard, electroporation would be ineffective to increase the flux of the target analyte if there is no presence or flow of sweat. Similarly, some sweat generation rates could be so high (e.g., several nL/min/gland) that analyte concentrations in sweat are too dilute to be sensed, even with application of electroporation. Therefore, a device could measure an advective flow of sweat from the sweat ducts in a binary yes/no format (e.g., is sweat flowing or not) or measure the magnitude of the flow of sweat (e.g., the sweat generation rate in nL/min/gland). Such a yes/no determination could be made by measuring skin impedance, measuring sweat sodium (Na+) concentration, using a microfluidic flow meter or thermal flow meter, or other suitable methods. This additional flow measurement could allow electroporation to be applied by the device only when it would be useful (i.e., in the presence of sweat or when an adequate or non-excessive sweat flow is occurring). To that end, the flow measurement component may be in electronic communication with the electroporation components of the device. In other words, the electroporation components may be controlled using feedback from the flow measurement component. For example, with reference again to
While the above embodiments are described relative to sweat, embodiments of the disclosed invention are not so limited. Saliva is a biofluid that is similar to sweat and is diluted of larger molecules and proteins. Saliva does not necessarily require stimulation (i.e., it is always flowing), so a device for sensing saliva could use or not use a stimulation method, such as those described above as specified for sweat. With further reference to
With reference to
Saliva monitoring devices could be mechanically less comfortable or ergonomic for longer term use than sweat monitoring devices. However, because saliva is always generating in the mouth, it could be suitable for one-time biomarker analysis. As a result, a collection-only device (similar to that described previously for sweat) may be useful.
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Embodiments of the disclosed invention apply at least to any type of biofluid sensor device that stimulates and measures biofluid (e.g., sweat, saliva, and tears), solutes within the biofluid, solutes that transfer into the biofluid from skin, a property of or things on the surface of skin, or properties or things beneath the skin. The disclosed invention applies to sweat sensing devices that can take on forms including patches, bands, straps, portions of clothing, wearables, or any suitable mechanism that reliably brings sweat stimulating, sweat collecting, and/or sweat sensing technology into intimate proximity with sweat as it is generated. Some embodiments of the disclosed invention utilize adhesives to hold the device near the skin, but devices could also be held by other mechanisms that hold the device secure against the skin, such as a strap or embedding in a helmet. Certain embodiments of the disclosed invention show sensors as simple individual elements. It is understood that many sensors require two or more electrodes, reference electrodes, or additional supporting technology or features which are not explicitly described in the description herein. Sensors are preferably electrical in nature, but may also include optical, chemical, mechanical, or other known biosensing mechanisms. Sensors can be in duplicate, triplicate, or more, to provide improved data and readings. The above description of various embodiments of the disclosed invention may not include a description of each and every component that may be required for the functioning of the devices depending on the application (e.g., a battery, or a counter electrode for iontophoresis), although it should be recognized that such components are included in the scope of the disclosed invention. For the purpose of brevity and to provide a focus on the inventive aspects described above, such components are not explicitly shown in the diagrams or included in the relevant description.
The following examples are provided to help illustrate the disclosed invention, and are not comprehensive or limiting in any manner.
Vasopressin, also known as antidiuretic hormone, is a neurohypophysial hormone related to hydration. Vasopressin is an analyte that is roughly 1,000 Da in molecular weight and, therefore, can become diluted in sweat compared to smaller lipophilic molecules such as cortisol. A device according to an embodiment of the disclosed invention could constantly apply a mild level of electroporation to increase the concentration of vasopressin extracted in sweat. Consider first the use of periodic electroporation for monitoring dehydration. In such an application, it may be desirable to track water loss through measuring sweat generation rate at an unstimulated sweat sensing site (e.g., by measuring sodium (Na) concentration, skin impedance, and/or using a flow meter) and also through measuring a dehydration biomarker (e.g., vasopressin) every hour. In a case where skin impedance, Na+, urea, and vasopressin are being measured to monitor dehydration, vasopressin might be the only measured analyte where the concentration in sweat would significantly increase due to electroporation. Because the measurements are hourly, there would be no need to continuously electroporate. Therefore, the electroporation—which may be preceded by sweat stimulation if needed—could be applied for 10 minutes each hour, which would be only ⅙ of the total electroporation time compared to continuous electroporation. As a result, the total amount of electroporation is dramatically reduced.
Luteinizing hormone, also known as lutropin and sometimes lutrophin, is a hormone produced by gonadotropic cells in the anterior pituitary gland, and in women is a marker of ovulation. Luteinizing hormone is large at around 30,000 Da. A device according to an embodiment of the disclosed invention could apply a moderate level of electroporation for a short period of time once a day to enhance the extraction of the luteinizing hormone through sweat and, therefore, the sensing of the luteinizing hormone in the generated sweat.
Consider next the use of on-demand electroporation for measuring the luteinizing hormone for fertility monitoring. A new device or a new disposable portion of a device could be applied each day. The device may measure estrogen and progesterone in sweat or some other biomarker, e.g. Cl− concentration, which can be used to indicate the body's thermal set-point and, in turn, impending ovulation. The device could on demand implement electroporation to increase the concentration of luteinizing hormone in sweat. On demand electroporation could be initiated at a set time each day or at an opportune time. For example, in an embodiment, the electroporation could be initiated by the user at an opportune time. As a result, in some cases, electroporation for a user may only occur once or very few times per month. Electroporation could also be implemented automatically based on feedback such as measurements of progesterone, for example.
Consider an embodiment where electroporation is applied to enable a sweat sensing device to measure a large protein that has a negative zeta potential at the pH and salinity of sweat. In an embodiment, a 10 ms DC electroporation pulse of +0.2 to +0.4 V could be applied once every second to the electroporation electrode ( 1/100 duty cycle) to help transport this protein into the sweat duct. The pulse may be applied once a second because, for example, roughly one second is required before this negatively charged protein is able to repopulate its concentration near the permeabilized locations of the eccrine duct. In an embodiment where both negatively and positively charged proteins are targeted for electroporation, the above example pulse could be bipolar in nature (e.g., +/−0.2 to +/−0.4 V, 1/50 duty cycle).
This example shows how different pulse magnitudes and widths can be used to control the depth of electroporation into the sweat gland. For example, it may be preferable to only or primarily electroporate the dermal duct so as to not interfere with the secretory portion of the gland where sweat is generated. A basic electrical model may be used to determine the desired pulse duration to get the applied voltage to the skin to be 80% of its applied level along the entire length of the dermal duct. An RC time constant for the dermal duct can be estimated as 33 Mohm*0.03 nF=0.0009 s, or about 1 ms (for 63.2% change in voltage). The cutoff frequency is therefore 1/(2*Pi*1 ms)=160 Hz. The rise time of the pulse to ensure that 80% of voltage gets to the bottom of the dermal duct is therefore 1.4 ms using the same RC time-constant calculation based on an inverse exponential trend.
This basic electrical model can be further explored. The dermis is mainly open space (collagen) filled with interstitial fluid. If one assumes an average of two layers of cells in the dermal duct lining, there are four lipid bilayers to electroporate. Assuming the full capacitance between the sweat duct and a counter electrode, the actual capacitance due to the presence of many lipid bilayers could be about 4× smaller (e.g., requiring a pulse of about 350 μs, per the calculations above). Also, if much of the field is vertical (i.e., downward into the skin), the capacitance will increase deeper inside the duct. A conventional target for electroporation of stand-alone cells is often 10 μs, but in the context of electroporating sweat glands, the required pulse may be longer in some instances to achieve deeper penetration of electroporation into the sweat gland.
If the voltage is increased, the pulse width decreases non-linearly, so that higher voltages could require less total wattage, reducing electrical stress on the skin. For example, starting with the 350 μs pulse calculated above, the time to charge the bottom of the dermal duct to 10% of the applied voltage is 35 μs, using V=Vapp*(1−e(−tRC). Therefore, if shorter pulses are needed for the electroporation waveform, the applied voltage may be raised by 10×. If the initial electroporation target voltage was about 2 V (for four bilayers), then the increased voltage would be about 20 V to charge the bottom of the duct to 2 V, which is the maximum voltage used for conventional iontophoresis (typical range is 12 to 15 V). Now, assume 8 bipolar pulses of 35 μs each. That would be less than 1 ms of total electrical stress, which is 120,000× less electrical stress than the 2 minutes used for Nanoduct sweat stimulation.
In another example, it may be desirable to only electroporate the first third of the dermal duct (e.g., 0.66 mm deep from the skin surface). Assume a sweat rate of 0.3 nL/min gland, which requires 1 minute for the sweat to traverse this upper third of the dermal duct, and assume the sweat is always within 7.5 μm distance from the duct wall (i.e., very close). In this case, the capacitance goes down by 3×, the resistance goes down by 3×, and the RC time constant is almost 10× lower. Therefore, in this example, the electroporation pulse can be reduced to only 3.5 μs.
As a further example, a plurality of different electroporation pulses could be applied, each pulse having a higher or lower voltage magnitude and a higher or longer pulse duration than the previous pulse. For example, a DC ramp could first be applied for about 200 μs, which enables more of the sweat gland depth to be at equipotential and near the threshold for electroporation compared to the natural state. Then, a higher frequency voltage could be applied with shorter pulsing (e.g., 10's of μs) to cause the electroporation. After this, one or more chaser waveforms could then be applied. In this example, the electroporation pulse would be superimposed on a lower frequency or DC waveform.
While specific embodiments have been described in considerable detail to illustrate the present invention, the description is not intended to restrict or in any way limit the scope of the appended claims to such detail. The various features discussed herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.
This application is the U.S. National Phase Application claiming priority to PCT Application No. PCT/US2017/013453 filed Jan. 13, 2017 which claims priority to U.S. Provisional Application Nos. 62/279,189 filed Jan. 15, 2016 and 62/307,131 filed Mar. 11, 2016, the disclosures of which are hereby incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US17/13453 | 1/13/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62279189 | Jan 2016 | US | |
| 62307131 | Mar 2016 | US |
