Devices for integrated indirect sweat stimulation and sensing

A sweat sensing device includes at least one sweat generation unit capable of initiating sudomotor axon reflex (SAR) sweating in an indirect stimulation region and at least one analysis unit capable of sensing a physiological parameter of sweat, collecting a sweat sample, or a combination thereof. The at least one analysis unit is located above the indirect stimulation region when the sweat sensing device is placed on skin.

BACKGROUND OF THE INVENTION

Historically, partitioning of biomarkers from blood to sweat has been demonstrated in great detail. As more of these biomarkers emerge, sweat appears to be a convincing media for continuous and spontaneous health monitoring. However, ‘clinical’ techniques of sample collection, purification and analysis have restricted growth in the sweat-sensing area because of the cost and time associated with these techniques. With the advent of miniaturized sensors, however, many of these issues can be alleviated. Still, a large task has largely been left unexplored for compact sweat sensing technologies: sample extraction and collection.

Common techniques for sweat stimulation and analysis involve sweat stimulation in a region from a sweat generating unit10, followed by removal of this unit10from the skin12, cleaning of the skin12and reapplication of a sensing unit14or collection device16, as shown inFIGS. 1A-1C. Often, the sensing unit14has integrated communication protocol to alert the user. This communication method could be via wireless or wired connections. For example, cystic fibrosis testing often involves this technique as demonstrated by ELITechGroup® in their Macroduct® and Nanoduct® products. This technique is problematic because it requires a two-step process that is inconvenient, non-continuous, and where reproducibility could be difficult.

Further, contamination from the stimulation reservoir and sensor region is unavoidable as they share the same area. In reference toFIGS. 1A-1C, the sweat generation unit10has potential to alter the state of the skin12, whether that includes excessive hydration, thermal heating, irritation, pharmacological side-effects, or another side effect. This adds a confounding factor to sensing sweat analytes when the sensing unit14or collection device16is placed on the skin.

Attempts to reduce contamination have previously been made. For example, a technique includes utilizing an isolating membrane between sweat stimulation mechanisms and the sensors and sensing sites. However, such techniques utilizing isolating membranes (or similar techniques) may only partially or temporarily separate sweat stimulation mechanisms, such as an electric field and/or chemicals, from the sensors and sensing sites. In the instance of isolating membranes, these also have the drawback of increasing the dead volume between a sensor and the skin12, which reduces temporal resolution. Furthermore, horizontal iontophoretic driving of an iontophoretic chemical, such as pilocarpine, may be used. However, this will again subject the sensor, sweat, and skin to an electric field and/or contamination. For many biomarkers and sensors, such interference could reduce performance of a sweat sensing device, in some cases making sensing impossible. There is an increasing need to provide improved sweat sensing techniques and devices that address one or more of the above drawbacks.

SUMMARY OF THE INVENTION

Embodiments of the present invention rely in part on the premise that sudomotor axon reflex (SAR) sweating can be utilized by a sweat inducing and sensing device for sweat analysis. SAR sweating can potentially be initiated by a variety of mechanisms: thermal, direct-electrical, chemical, occlusion, and others. In this setting, direct-electrical refers to a biophysical phenomenon where sweating is initiated by the flow of electron and ion current without the aid of a chemical active compound. Additionally, embodiments of the present invention can greatly reduce contamination and improve chronological sampling of sweat. Furthermore, the embodiments of the present invention described below have the ability to use a variety of sensing techniques which greatly improves the impact and variety of applications for such a device.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the present invention described herein improve greatly on prior simultaneous stimulation and collection of sweat sample methods, such as that presented inFIG. 1.

Embodiments of the present invention take advantage of a biological response referred to as the sudomotor axon reflex (“SAR”). This mechanism acts on the premise that innervation of sweat glands occurs as a result of peripheral functionality of sudomotor units (i.e., the body will stimulate a series of sweat glands directly underneath the stimulation region, “direct stimulation”, but will also generate sweat from sweat glands outside of this region, “indirect stimulation”). For example, in the case of chemical sweat stimulation, the sweat stimulant acts on the neural receptors surrounding the sweat glands to elicit a sweating response. The chemical stimulant can act on two primary receptors at the base of the sweat gland: muscarinic or nicotinic receptors. SAR response is typically observed with chemicals that interact strongly with nicotinic receptors at the base of the sweat gland. For example, pilocarpine acts weakly on nicotinic receptors and is therefore a poor chemical stimulant for SAR response. However, nicotine acts strongly on nicotinic receptors and is therefore an attractive stimulant for SAR response. Furthermore, there are chemical stimulants, such as acetylcholine, that act strongly on both muscarinic and nicotinic receptors, which can be leveraged to produce a SAR response. It should be recognized that, although not named, multiple other chemical stimulants are capable of causing a SAR response and are useful in embodiments of the present invention. Although there has largely been limited research in this area of sweat stimulation, it is also hypothesized that thermal, direct-electrical, occlusion, and other sweat stimulant techniques will produce a similar antidromic responses as chemical stimulants. Consequently, one can stimulate sweat glands in a region within close proximity of a sensor array to generate sweat directly underneath the stimulation region (“direct stimulation”) and directly underneath the sensors (“indirect stimulation”). Typically, the “spreading” of SAR induced sweating, the distance from the edge of direct stimulation to the decay of indirect stimulation, is limited on the order of several tens of millimeters (e.g., up to about 30 mm). The degree of a SAR response, in the case of chemical stimulation, depends largely on the amount and type of sweat stimulant which is delivered to a given location.

In reference toFIG. 2, a device20according to an embodiment of the present invention may have a structure with a sweat generation unit22and at least one of two analysis units: a collection unit24and a sensing unit26. The sweat generation unit22is capable of directly initiating sweat in a direct stimulation region and initiating SAR sweating in indirect stimulation regions under the collection unit24and the sensing unit26. In one embodiment, the sweat generation unit22includes a chemical stimulant, such as acetylcholine, methacholine, nicotine, carbachol, or another chemical that is capable of initiating SAR sweat. The collection unit24and the sensing unit26are located separately from the sweat generation unit22along the skin12. In one embodiment, the analysis units may be spaced apart from the direct stimulation region by a distance of about 0.1 mm to about 30 mm. The SAR-initiated sweat is then collected and sensed by the collection unit24and the sensing unit26, respectively. As shown, embodiments of the present invention promote a single-step process, which is an advantage over previous devices where sweat generation and sweat analysis were performed in multiple steps. Further, continuous and multiple occurrence SAR-initiated sweating for the purpose of sweat analysis is possible. For example, sweat may be sensed or collected in one instance in time, continuously for a length of time, or a combination thereof.

Optionally, each of these regions (sweat generation, sweat sensing, and sweat collecting) may be separated by an isolation layer28as shown inFIG. 2. This isolation layer28could take many forms or materials such as an adhesive or rubber with the purpose of electrically and/or fluidically isolating regions of the device. The isolation layer28serves to improve the reliability of the device and helps assure that fluid will not mix between regions or create unwanted electrical connections (e.g., “electrical shorts”). Furthermore, since this isolation layer28is preferably water-insoluble, common adhesives will not only provide electric and fluidic isolation but will also aid in keeping the device on the skin12. Furthermore, in some cases, the isolation layer28protects the sensing unit26from voltages or currents applied to the sweat generation unit22.

The term “sweat generation unit”, including other denotative or connotative phrases, as used herein, captures a plurality of sweat stimulation methods that are capable of initiating SAR sweating. For example, a sweat generation unit may involve one or more of chemical, thermal, direct-electrical, or other suitable mechanisms which stimulates the generation of sweat and are not specifically described. The most common technique for sweat stimulation is a chemical technique referred to as iontophoresis. This involves electric-field driven movement of a sweat stimulant drug into the skin surface, ultimately reaching the secretory coil of the sweat gland, to initiate sweating. It should be recognized that, although iontophoresis is the most common chemical technique, electroporation, injection or microneedles delivery, passive diffusion of a sweat stimulant from a drug reservoir, which may be improved by a diffusion enhancer (e.g., propylene glycol) applied to the skin prior to device application or incorporated directly into the stimulation unit itself, or other techniques are also possible routes of chemical delivery of a sweat stimulant in embodiments of the present invention. Utilizing such a design according to the present invention will greatly reduce contamination between a stimulation reservoir (e.g., in the sweat generation unit22) and collection and/or the sensor region (e.g., relating to the collection unit24and the sensing unit26).

Additionally, the terms “sensing unit” and “sensing mechanism,” including other denotative or connotative phrases, as used herein could include one or more of a plurality of mechanisms for sensing sweat and/or its components or properties including potentiometric, amperometric, conductometric, impedance spectroscopy, skin impedance, galvanic skin response (GSR), or other suitable mechanisms. Similarly, the term “collection unit”, including other denotative or connotative phrases, as used herein, describes a collection method, material, or structure.

The terms “collection and/or sensing unit” and “analysis unit”, including other denotative or connotative phrases, as used herein, describe a unit that is capable of sensing sweat, collecting sweat, or a combination of the two. A sensing unit (e.g., sensing unit26) and/or a collection unit (e.g., collection unit24) may have integrated electronics or controls which monitor physiological parameters, provide feedback to a user or similar function. In an embodiment with both a sensing unit(s) and a collection unit(s) (e.g., device20), the units may function independently of each other or may operate together.

Sweat generation units, collection units, sensing units, and combinations of such units may include a variety of functional aspects such as wired or wireless communications, rigid or flexible structure or other method, material, function, or particular structure not specifically described here. These units may also have intelligent communication between or within each unit via optical, electrical, or similar communication method (not shown).

In another embodiment,FIG. 3shows a device30where a sweat generation unit32is placed in vertical alignment with a collection unit34and a sensing unit36. Although the collection34and/or sensing unit36are shown being located within the sweat generation unit32, another embodiment may include at least one collection34and/or sensing unit36located directly on skin12with the sweat generation unit32placed directly over said components (not shown). Although a minimal amount of direct sweating may possibly be produced underneath the collection unit34or sensing unit36from the sweat generation unit32, indirect (SAR) sweating may be the only suitable mechanism for generating sufficient sweat to sense and/or collect. For example, in the case of chemical stimulation via iontophoresis, the collection unit34and the sensing unit36reduce the effectiveness for direct stimulation directly underneath these units. Essentially, these units block or alter the effectiveness of a stimulation technique that initiates non-SAR sweating. However, in leveraging SAR stimulation, the device30will overcome these limitations by indirectly generating sweat underneath the units34,36. In other words, to generate sufficient sweat under the collection unit34and the sensing unit36, the sweat generation unit32includes a mechanism for initiating SAR sweating.

The construction of device30could simplify device construction and assembly compared to other configurations. For example, in one embodiment, units34,36could be fabricated using standard flexible electronics techniques (such as on PET or Kapton film), and pressed against skin12. The sweat generation unit32could be a gel including a sweat stimulant and a driving electrode (not shown) that is pressed down against units34,36and skin12. Some of the iontophoretic chemical stimulant in sweat generation unit32may find itself between units34and36and skin (similar to as diagramed inFIG. 3), but would be incapable of generating a strong direct stimulation of sweat, and thus this example embodiment would rely on the indirect stimulation of sweat by SAR to cause sweat to be received by units34,36.

A benefit of the vertical alignment between the analysis units (e.g. collection unit34and sensing unit36) and a sweat generation unit (e.g., sweat generation unit32) is an increase in the density of the units. This benefit could be realized in other configurations of the sweat generation and analysis units, such as in the honeycomb formation described below. Depending on the sweat generation mechanism, SAR-initiated sweat may only be able to be collected or sensed up to several millimeters away from the direct stimulation region. Therefore, embodiments of the present invention achieve a greater benefit with a high density of sweat generation and analysis units.

RegardingFIGS. 4A and 4B, in one embodiment, a device40is shown positioned on the skin12. The device40includes a plurality of sweat generation units42and a plurality of collection and/or sensing units44arranged in a honeycomb structure. In one embodiment, the sweat generation units42are iontophoretic in nature and the collection and/or sensing units44include potentiometric sensors. In this configuration, the collection and/or sensing units44are placed spatially so that each unit44is within close proximity of a sweat generation unit42(i.e., the units44are located above indirect stimulation regions). With the hexagonal arrangement of device40, each sensing unit44is surrounded by three sweat generation units42, increasing the probability for a SAR sweating response underneath the sensing units44. The device40further includes an optional isolation material46(best shown inFIG. 4B) that isolates the collection and/or sensing units44from the sweat generation units42, which improves the device integrity and functionality. This isolation material46could take one of many forms and materials as previously described. Those of ordinary skill in the art will recognize that the structure may be configured to be a similar shape other than a hexagonal or a honeycomb structure. As shown inFIG. 4A, the stimulation source (i.e., the sweat generation units42) and the collection and/or sensing units44are separate in horizontal location on skin, with no horizontal overlap. The present invention also contemplates devices where at least partial overlap exists where the benefit of SAR sweating in the present invention is still realized, such as device30presented inFIG. 3.

Furthermore, in a device according to the present invention where the sweat generation method is via direct-electrical methods or iontophoresis, a return electrode can be placed on the periphery of the device so as to maximize the amount of current or drug delivered in the desired location. For example, inFIG. 4A, a return electrode48is positioned around an edge of the device40.

InFIG. 5, a portion of a device50according to an embodiment of the present invention is shown. The device50includes a single large area or a plurality of sweat generation units52, a plurality of collection and/or sensing units54, and an optional isolation material56. The structure utilizes a greater than unity aspect ratio of sensors and/or collection units54, which minimizes the amount of lateral distance required to leverage SAR sweating. Due to the need to be above at least one active sweat gland, and/or due to signal to noise requirements, some collection and or sensing units must have a minimum total area of contact with skin. A greater than unity aspect ratio allows a minimum area to be achieved, while further minimizing the amount of lateral distance required to leverage SAR sweating. In one embodiment, the length/width aspect ratio may be about 2:1 or, alternatively, about 1:2. AlthoughFIG. 5shows a rectangular configuration, those of ordinary skill in the art will recognize that a rectangular configuration is not required and a similarly intended effect can be achieved via a concentric design or similar structure (not shown). For example, other geometries, such as a star shape, may also be included that feature a greater than unity aspect ratio. Where the sweat generation method for the device50is via direct-electrical methods or iontophoresis, a return electrode58can be placed on the periphery of the device50so as to maximize the amount of current or drug delivered in the desired location.

With reference toFIGS. 6A-6D, devices according to various embodiments are shown that include improved sweat fluid management. More particularly, devices60a,60b,60c, and60dare shown including sweat generation unit(s)62, collection and/or sensing unit(s)64, and isolation materials66. An optional wicking material68is used as an additional component that acts as a fluid management feature, which improves performance. The wicking material68may also be used to bring sweat from skin12to a collection and/or sensing unit64, as described below. A device including the wicking material68may have varying configurations. In one embodiment, the wicking material68may act to actively move previously analyzed sweat sample to a disposal region (not shown) thereby increasing effective sweat sampling time resolution. For example, the wicking material68may be included as a through-hole component in the collection and/or sensing unit64(FIG. 6A), at the edge of the collection and/or sensing unit64(FIG. 6B), or in the plane of the collection and/or sensing unit64(FIG. 6C). Further, this wicking material68or similar microfluidic structure could be used to transport sweat from the skin to the collection and/or sensing unit(s)64(specifically shown inFIG. 6D). For ease of fabrication or intended application, one particular structure may be more beneficial than another. For instance, regarding device60a, there may be benefit in wicking sweat to a central location within the collection and/or sensing unit64to reduce likelihood of fluid buildup near the isolation boundary66. Similarly, in device60c, the wicking material68is shown partially extended along the length of the sensor and/or collection unit64, which may prove beneficial where a small volume of sweat is generated underneath the sensor and/or collection unit64before being wicked away. In this instance, the wicking material68acts as a static, volume-limiting pump. Furthermore, a combination of one of the structures described may prove beneficial. In an exemplary embodiment, the wicking material68may be paper or a polymer or microfluidic feature that operates via capillary action. Further, the wicking material68could be segmented utilizing more than one wicking material, such as a combination of paper and polymer or another combination of similar wicking materials.

Prediction of Typical Parameters

Peripheral (indirect) sweating after 5 minutes of iontophoresis is significant (e.g., sweat rates of about 1-3 nL/min/gl) on average to at least 8 mm from the boundary of the direct stimulation region. A sweat stimulant could be delivered iontophoretically or by another method and could constitute a wide range of various sweat stimulants (e.g., pilocarpine, acetylcholine, etc.). Considering that sensors can be fabricated utilizing previously demonstrated technology well below 8 mm in diameter (e.g., 500 μm in diameter), an array of biomarker sensors may be easily placed within the indirect sweating region. Although indirect sweating results in a reduction in sweat rate (e.g., about 60% reduction as compared to the sweat rate in the area being directly stimulated), the reduction of the risk of contamination and potential to reduce drug dosage (as described below) outweigh this downfall.

The spacing between the stimulation regions and the collection and/or sensor units may vary depending on the configuration. In one embodiment, an array of stimulation units and analysis units may have a minimum half-pitch (smallest repeating unit) less than one millimeter at an accuracy within tens of micrometers or better.

A sampling effectiveness metric (“SE”) for comparing sweat stimulation devices can be defined by Equation 1:

SE=QΓ
where Q is the sweat rate per minute, and F is the dose of drug stimulant delivered in the case of iontophoretic delivery. A base metric, e.g., SE0, for the case of the stimulation region being the same as the collection region (e.g., as shown inFIGS. 1A-1C) may be used to compare sweat stimulation devices without using actual sweat rate data. The hexagonal device40shown inFIGS. 4A and 4Bhave one stimulation unit42per four total stimulation and collection/sensor units42,44per unit cell and would utilize at least ¼, or 25%, of the stimulation region and, thus, 25% of the dosage Γ0. Similarly, the sensing/collection regions would comprise 75% of the original area, A0. Further, as described above, the hexagonal device40would generate 40% of the original sweat rate, Q0, in the indirect regions. Therefore, in comparison to the case of the same stimulation and collection region, a hexagonal array would provide a decrease in total sweat rate of 70% (i.e., 100%−(40% Q0* 75% A0)=70%), resulting in a sweat rate of 30% of the original sweat rate, Q0, while using 25% of the original dose, Γ0. This calculation completely excludes any sweat generated underneath the stimulation region. Overall however, the SE ratio would increase 20% as a result of the reduced stimulation area (i.e., 0.30 Q0/0.25 Γ0=1.2 SE0). Further, if one is able to collect the sweat generated underneath the stimulation region, the SE could increase 120% (i.e., (0.30 Q0+0.25 Q0)/0.25 Γ0=2.2 SE0). Thus, not only is the contamination reduced with this method, but the sampling efficiency (SE) is increased. It should be noted that this reduction in area and ‘dose’ applies to all methods of stimulation where these nerve fibers are activated. Therefore, thermal, direct-electrical or other methods would also benefit from this unique device structure.

In alternative embodiments, the stimulation region could be much smaller than the collection/sensor region. This is because only a small area would be required to initiate the SAR response of many nearby sweat glands. Furthermore, in the case of iontophoresis, this reduction in the stimulation area would improve the SE, which would allow for less drug delivery (dosage) to achieve the same previously attained SE.

Dead Volume

A metric for comparing the dead volume (“DV”) of such devices can be defined by Equation 2:

DV=VdeadAsensor
where Vdeadis the dead volume between the sensor and skin, and Asensoris the area of the sensor. Comparing to an instance where the stimulation and collection regions are in the same area and the device includes an element to reduce contamination (not shown), such as a membrane, the gap between the sensor and skin could be on the order of 200 μm (e.g., due to roughness of skin, thickness of paper wicking layer, thickness of a drug reservoir, etc.). However, with indirect stimulation, one can greatly improve the contact between the skin and sensor while reducing the contamination from the stimulation. In this regard, the device may be so intimate with the skin that the only source of dead volume would be from the topology of the skin (e.g., about 30 μm). Thus, the reduction in dead volume between the device where the stimulation and collection regions are in the same area and this embodiment would be a reduction of at least 6× (200 μm/30 μm). This holds great significance when one estimates the time to “refresh” the sweat underneath the sensor. If there is 6× less volume to refill underneath the sensor, then for a given flow rate, the time required to refresh the sweat underneath the sensor would also be reduced 6×. This has profound impact on time-resolution capabilities.
Total Figure of Merit (TOT-FOM)

Utilizing the two calculations above for SE and DV and assuming the addition of an isolating membrane between sensor and skin, the effective improvement may be the multiplication of these two values. Therefore, the total improvement between an exemplary previous device described above and an embodiment of the present invention would be an improvement of at least 7.2× with potentially an improvement approaching 13.2×—when sweat rate per unit area, dose per unit area and dead volume per unit area are considered.