Portable device for detecting gas emitted by an object

Abstract

A device for detecting a gas of interest emitted by an object, intended to be placed on an object, the gas of interest absorbing light in a spectral absorption band, the device comprising: a contact face, configured to be placed against the object, and configured to allow passage of the gas of interest emitted by the object; a collection chamber, opening onto the contact face, and delimited by a side wall, the side wall extending around a transverse axis, perpendicular to the contact face; an inlet, forming an air intake, provided through the side wall, configured to admit ambient air into the collection chamber; a photoacoustic gas sensor. Figure 1A.

Description

Title of the invention: Portable device for detecting a gas emitted by an object Technical field

[0001] The technical field of the invention is the measurement of a gas released by the skin of a living being using a compact device. PREVIOUS ART

[0002] Certain respiratory diseases affect gas exchange between blood and expired air. These can also be modified during resuscitation. Blood contains dissolved gases, including oxygen and carbon dioxide, whose respective partial pressures reflect the gas exchanges occurring in the lungs. To assess the amount of CO2 dissolved in the blood, a blood sample can be taken. This is an invasive method, which can be painful and difficult to apply, particularly in neonatology. In addition, it can only be applied on an ad hoc basis. Despite these drawbacks, its reliability has been validated by the medical profession and it constitutes a reference method.

[0003] Another method consists of estimating the CO2 content of the blood non-invasively, by measuring the CO2 diffusing through the tissues, and in particular the skin. This type of transcutaneous analysis was introduced in the 1980s. Transcutaneous analysis allows for continuous monitoring, for example to monitor the immediate effects of therapeutic treatment. It can also be used to determine the times at which precise quantification, by blood sampling, is necessary. It is thus understood that invasive and non-invasive methods can be combined: one is precise and punctual, while the other can be implemented continuously.

[0004] Compact devices for measuring transcutaneous CO2 have already been described. For example, the publication Eletr S "Cutaneous monitoring of systemic PCO2 on patients in the respiratory intensive care unit being weaned from the ventilator", acta Anaesthesiol Scand, vol. 22, n° 68, pp 123-127, 1978.

[0005] In other applications, one may seek to measure other gases, for example NO, which is a marker of cardiac activity, or of certain respiratory pathologies such as asthma.

[0006] Application EP3979911 describes a non-invasive measuring device, worn by a user, for estimating a concentration of a gas of interest emitted transcutaneously, this gas of interest being able for example to be carbon dioxide. In the exemplary embodiment described, the device implements an optical sensor of the gas of interest.

[0007] The inventors propose a device, based on another detection configuration. The device can be adapted to different gases. The device can address several gases simultaneously. Statement of the invention

[0008] A first object of the invention is a device for detecting a gas of interest, intended to be placed on an object, the gas of interest absorbing light in a spectral absorption band, the device comprising: - a contact face, configured to be arranged against the object, and configured to allow passage of the gas of interest, emitted by the object; - a collection chamber, opening onto the contact face, and delimited by a side wall, the side wall extending around a transverse axis, perpendicular to the contact face; - an intake opening, forming an air intake, provided through the side wall, configured to admit ambient air into the collection chamber; - a gas sensor;

[0009] the device being characterized in that it comprises a conduit, extending, along the transverse axis, between the collection chamber and the gas sensor;

[0010] and in that the gas sensor comprises • a measuring chamber, extending to the outlet of the conduit, and intended to receive the gas of interest; • a light source configured to illuminate the measuring chamber, in the absorption spectral band; • an acoustic transducer, connected to the measuring chamber, and configured to measure a pressure variation in the measuring chamber.

[0011] According to one possibility, the conduit extends, parallel to the transverse axis, to a height of between 0.5 mm and 10 mm or between 0.5 mm and 5 mm.

[0012] According to one possibility, the conduit extends, in a plane perpendicular to the transverse axis, with a diameter or a diagonal less than 4 mm.

[0013] The inlet opening may extend, perpendicular to the transverse axis, by a length of between 3 mm and 15 mm.

[0014] The intake opening may extend, at a height, parallel to the transverse axis, less than or equal to 5 mm or 3 mm.

[0015] According to one possibility, the collection chamber extends, in a plane parallel to the contact face, along a diameter or a diagonal less than 45 mm or 20 mm.

[0016] The collection chamber may extend, parallel to the transverse axis, to a height of less than 30 mm.

[0017] The collection chamber may comprise a dehydrating agent.

[0018] Another subject of the invention is a method for estimating a concentration of a gas of interest emitted by an object using a device according to the first aspect of the invention, the device being applied such that the contact face is arranged facing the object, the method comprising an admission of ambient air through at least one admission opening of the device, arranged in the side wall and an estimation of the concentration of gas of interest in the measuring chamber. The gas of interest can be chosen from NO or CO2.

[0019] The object may be a human or animal user, the device being disposed against a skin of the user, the gas of interest being a transcutaneous gas. FIGURES

[0020] Figures 1A and 1B show diagrammatically two configurations of the device.

[0021] [Fig. IC] shows an example of a photoacoustic gas sensor.

[0022] [Fig.2] shows a configuration of the device that has been modeled.

[0023] [Fig.3A] shows the modeling of the flow velocity in a cross section of the conduit.

[0024] [Fig.3B] shows the modeling of the flow velocity in a section of the inlet opening, made through the side face of the device, the height of the inlet opening being 3 mm.

[0025] [Fig.3C] is equivalent to [Fig.3B], the height of the intake opening being 1 mm.

[0026] [Fig.4] represents an estimate of the flow rate as a function of two dimensions Xi and X 5-

[0027] Figures 5A and 5B show models of the flow in the collection chamber with respectively one inlet opening and two symmetrical inlet openings. PRESENTATION OF SPECIAL EMBODIMENTS

[0028] Figures 1A and 1B are general views of examples of device 1 according to the invention. The device 1 is intended to be placed in contact with, or at a short distance from, an object. By short distance, we mean a distance typically less than 1 cm. The object may be, but is not limited to, a human or an animal, or a plant. The device is intended to carry out a measurement of a concentration of a transcutaneous gas of interest, that is to say emitted through an external surface of the object. When the object is a human or an animal, the external surface of the object is delimited by skin.

[0029] In the example described below, in a non-limiting manner, the gas of interest is nitrogen monoxide (NO), but the invention can be applied to any other gas capable of being emitted through the skin of a living user, for example CO2.

[0030] The device comprises a collection chamber 10. The collection chamber 10 extends between a proximal end 10; and a distal end 10o. The proximal end 10; is intended to be placed in contact or near-contact with the user's skin.

[0031] The proximal end 10 of the collection chamber 10 forms a contact face 11. The contact face 11 is preferably closed by a porous membrane 12, for example a perfluorinated membrane of the Naflon type (registered trademark). The porous membrane is designed to be impermeable to liquids and permeable to gases. It can be replaced by a plate having openings small enough to block a liquid and allow the gas of interest to pass through.

[0032] The contact face 11 is substantially planar, in the sense that it extends parallel to an XY plane, certain portions being able to be inclined relative to the XY plane.

[0033] The proximal end 10; and the distal end 10o are connected to each other by a lateral face 13, extending around a transverse axis Z, perpendicular to the plane XY.

[0034] The function of the collection chamber 10 is to collect a transcutaneous gas of interest for which a concentration is to be determined. In order to facilitate the transport of the gas of interest to a gas sensor 30, the collection chamber comprises one or more inlet openings 15, intended to admit ambient air. Ambient air refers to the air extending around the device 1. The collection chamber may comprise a dehydrating agent (desiccant), for example silica gel, so as to absorb water coming for example from sweat. The dehydrating agent may be arranged in the form of a layer covering the contact face 11 and / or the transverse wall 13.

[0035] The collection chamber 10 is arranged in contact with the user's skin. As a result, inside the collection chamber, the temperature is higher than the ambient air temperature, which will be assumed subsequently. Under the effect of a temperature gradient between the collection chamber 10 and the ambient air, a thermal convection current is formed. The lateral openings 15 allow the appearance of a chimney effect, which constitutes a "motor" allowing propagation of the CO2 through the collection chamber 10, by convection. As a result, the gas of interest and the ambient air admitted into the collection chamber form a gas mixture moving away from the contact face 11 and propagating towards the gas sensor 30. It is assumed that the device is configured such that the gas sensor is located above the collection chamber 10.Ambient air admitted into the collection chamber 10 through the openings 15 acts as a carrier gas and transports the transcutaneous gas through the collection chamber 10, to the distal end 10o of the collection chamber. The contact face 11 forms a hot inlet of the device.

[0036] The convection effect results from the difference in density p of a fluid in the collection chamber, in the presence of a temperature gradient. The difference in height, along the Z axis, between the hot contact face 11 and the gas sensor, connected to the colder ambient air, promotes the establishment of a depression AP which causes the gas to move.

[0037]

[0038] where: - Pf and Pc are the densities of the cold and hot gas respectively. - h is the height between the hot inlet and the cold outlet of the device, described afterwards ; - £ = 9.81m.s2.

[0039] The device comprises a conduit 20, intended to guide the gas of interest, flowing from the collection chamber 10, to the gas sensor 30. The conduit extends between an inlet opening 20; and an outlet opening 20o. The inlet opening is provided at the distal end 10o of the collection chamber 10.

[0040] The gas sensor 30 is intended to allow quantification of the concentration of the gas of interest. The gas sensor is a photoacoustic sensor. It comprises an intensity-modulated light source 31. The light source 31 is preferably a laser source, the emission length of which is located in a spectral absorption band of the gas of interest. The gas of interest is contained in a measuring chamber 32, at the outlet of the conduit 20. The measuring chamber 32 is connected to an acoustic transducer 33, called a measuring transducer, for example of the microphone type. The intensity modulation of the light source 31 is preferably carried out at a frequency that is a multiple of a resonance frequency of the acoustic transducer 33. In a manner known in the field of photoacoustic detection, the absorption of light by the gas of interest generates heating, the latter being repeated periodically, according to the period of the pulses of the light source.The acoustic transducer 33 is configured to detect pressure variations inside the measuring chamber 32, under the effect of the increase in temperature resulting from the modulations of the light intensity. The measuring chamber is connected to an outlet 35, connected to the ambient air, forming the cold outlet of the device.

[0041] It is preferable that the temperature gradient, between the skin and the outlet 35, is greater than 10°C. The transverse axis Z is preferably vertical, to within 10° or 20°. When the ambient air is colder than the skin, the sensor is arranged above the collection chamber. When the ambient air is warmer than the skin, the device is reversed compared to the representation in Figures 1A and 1B.

[0042] The gas sensor 30 may be a Helmholtz resonator type sensor. The operating principle of such a sensor is described in WO03 / 083455. An advantageous configuration of a Helmholtz differential resonator is described in EP2930506. Such a configuration is described in connection with [Fig.lC].

[0043] The device comprises a processing unit 40, configured to control the gas sensor and process the information resulting from the latter. The processing and calculation unit may comprise calculation means, for example a microprocessor, embedded in the device or located remotely from the latter, being connected by a wired or wireless link.

[0044] The use of a gas sensor based on photoacoustic detection has certain advantages over detection based on the attenuation of an infrared optical beam. The response of the sensor is linear with the concentration, whereas it takes an exponential form when an optical sensor is used. In addition, an optical sensor requires a measuring chamber whose dimensions are controlled, as well as certain environmental parameters, in particular humidity. When the concentration to be measured is low, it is preferable for the dimensions of the measuring chamber to be large, so as to increase the optical path, so as to obtain a measurable attenuation. It is usual to increase the optical path by implementing reflections on mirrors.However, such mirrors cannot cover a wide spectral band in the infrared: thus, a sensor based on optical attenuation can only address a limited number of gases. An advantage of photoacoustic detection is that several gases can be measured almost simultaneously, for example NO, CO2 or C3H6O.

[0045] When addressing multiple gases, the number of light sources depends on the absorption wavelengths of each gas. QCL (Quantum Cascade Laser) laser sources can be used, which makes it possible to address a wavelength range varying between 4 pm and 15 pm, within an adjustable range of ± 10 nm for each laser.

[0046] With the same QCL laser light source, CO and N2O can be measured, whose absorption peaks are respectively centered at the wavelengths of 4.46 pm and 4.61 pm. It is estimated that these two gases can be measured with a detection limit close to 1 ppm.

[0047] With the same QCL laser light source, NO and CO2 can be measured, the absorption peaks of which are respectively centered at the wavelengths of 5.23pm and 5.26pm. It is estimated that NO and CO2 can be detected according to detection limits of approximately 0.1 ppm and 1000 ppm respectively. These orders of magnitude correspond to the cutaneous emission of humans.

[0048] [Fig.lB] shows another geometry of the device. The collection chamber 10 extends along a cross-section which reduces according to the distance from the contact face 11. By cross-section, we mean a section in a plane PXy parallel to the transverse axis Z. In [Fig.lB], the dimensions are shown: - Xi: side of conduit 20, perpendicular to the transverse axis Z; - X2: height of the duct 20, parallel to the transverse axis Z; - X3: length of an intake opening 15, parallel to the face of contact ; - X4: diameter or largest diagonal of the cross-section of the collection chamber 10, parallel to the contact face. - X5: height of the collection chamber 10, parallel to the transverse axis Z; - X6: height of the intake opening 15, parallel to the transverse axis Z.

[0049] L and l are respectively the length and width of the collection chamber perpendicular to the transverse axis Z.

[0050] The sizing of the device allows for optimal coupling with a photoacoustic gas sensor. It must allow for the collection of a sufficient quantity of gas, which requires a contact face with a large surface area, while allowing for flow of the air + gas mixture of interest in a laminar regime, in a suitable flow rate range.

[0051] Generally speaking, it is advantageous for the dimensions to be: - Xi between 0.5 mm and 5 mm; - X2 between 1 mm and 10 mm; - X3 between 3 mm and 15 mm; - X4 between 5 mm and 20 mm or 30 mm, or even 40 mm or 45 mm, which makes it possible to obtain a contact face of a sufficient size to collect the gas of interest, while being compatible with use worn on a limb, for example on the arm of a human; - X5 between 5 mm and 20 mm; - X6 between 0.5 mm and 3 mm;

[0052] In Figure 1B, the collection chamber 10 has the shape of a truncated pyramid, the base of which is a quadrilateral of length L and width l, extending perpendicular to the transverse axis Z, and the section of which decreases from the proximal end 10; towards the distal end 10o.

[0053] [Fig. IC] shows schematically a gas sensor 30 that can be implemented. Such a sensor is described in EP2930506. The sensor comprises a measuring chamber 32 and a measuring transducer 33 as previously described. It also comprises a reference chamber 32' and a reference transducer 33'. The reference chamber 32' is configured not to be illuminated by the light source 31. The reference transducer is configured to detect pressure variations in the reference chamber 32', the latter not undergoing modulation under the effect of the light pulses from the light source. The transducer 33 generates a measurement signal and the reference transducer 33' generates a reference signal. The measurement and reference signals are processed by the processing unit 40 to estimate a gas concentration.

[0054] In [Fig.2], another geometry of the collection chamber 10 is shown, taking the form of a conical truncated section of revolution, the section of which decreases from the proximal end 10; towards the distal end 10o. In [Fig.2], the dimension X4 corresponds to a diameter. In [Fig.2], a three-dimensional mesh is shown, the coordinates being in mm.

[0055] It is preferable that the section of the collection chamber gradually reduces as one approaches the conduit. This promotes flow by limiting the appearance of turbulence.

[0056] The fluid currents formed at the device have been modeled. The gas of interest is admitted into the collection chamber 10 through the contact face 11 (see arrow FJ). The ambient air is admitted through the inlet opening 15 (or the inlet openings 15) made in the side wall 13 (see arrow F2). The gas mixture comprising the ambient air and the transcutaneous gas of interest propagates towards the conduit 20 (see arrow F3), then flows through the conduit 20 towards the measuring chamber 32 of the gas sensor 30 (see arrow F4).

[0057] The inventors have determined dimensions making it possible to obtain, in the conduit 20, a laminar flow, the flow rate of which is preferably greater than 1 mL / min. Such a flow rate is suitable for a measuring chamber 32 of millimetric dimensions, the volume typically being of the order of 25 pL or 50 pL. A flow rate of 1 mL / min allows the measuring chamber to be filled in less than one second. This makes it possible to obtain a device having a rapid response time. The flow rate is preferably less than 20 mL / min if it is desired to maintain a laminar flow.

[0058] [Fig.3A] shows a spatial distribution of the flow velocity in the cross-section of the conduit 20. [Fig.3A] results from a modeling of the thermo-convection, carried out with the COMSOL Multiphysics™ simulation software. A collection chamber was modeled as shown in [Fig.2]. The dimensions were:

[0059] Xi=2 mm; X2=1 mm; X3=6 mm; X4=20 mm; X5=30 mm; DT=10°C correspond to a maximum output speed of 100 mm / s and a flow rate of 12.9 mL / min.

[0060] The thermal modeling parameters were: - skin temperature: 30°C; - room temperature: 20°C.

[0061] The flow rate leaving the conduit is estimated at 12.2 mL / min. In [Fig.3A], we observe that the flow is laminar and that the maximum flow speed is of the order of 100 mm / s in the central axis of the conduit, the latter corresponding to the axis transverse.

[0062] The simulations have shown that the section of the conduit can be cylindrical or polygonal, without significant impact on the flow rate value, at constant cross-section.

[0063] As previously indicated, the collection chamber 10 comprises a transverse opening 15, so as to allow the admission of ambient air. Several transverse openings 15 can be provided through the side wall 13. When the dimensions of the transverse opening, or the cumulative dimensions of each transverse opening, are too large, air backflow can occur. In Figures 3B and 3C, a spatial distribution of the flow of ambient air through a side opening is shown, the height X6 and length X3 of which are respectively 3 mm, 10 mm ([Fig. 3B]) and 1 mm, 10 mm ([Fig. 3C]). In these two configurations, the cross-section of the duct 20 was square, with side Xi = 1 mm and height X2 = 1 mm. The other dimensions were as described in connection with [Fig. 3A]. In Figures 3B and 3C, the abscissa and ordinate axes correspond respectively to the Y and Z axes shown in [Fig.2].

[0064] In [Fig.3C], the air flow is segmented between an incoming flow (lower part of the figure, marked by the in bracket), and an outgoing flow (upper part of the figure, marked by the out bracket) of low speed. In [Fig.3B], the air flow is only incoming. [Fig.3C] corresponds to a limiting configuration: if the dimensions of the lateral opening are further increased, the outgoing flow increases, which is not desirable. It is estimated that when Xi = 1 mm, i.e. a duct with a cross-section of 1 mm2, when X3 = 10 mm, the height Xi is preferably between 0.5 mm and 3 mm. This corresponds to a section between 5 mm2 and 30 mm2.

[0065] From simulations carried out with the COMSOL Multiphysics™ software, an empirical model made it possible to estimate the flow rate Q, in the conduit 20, as a function of the dimensions Xi X2, X3, X4 and X5, taking into account a temperature variation A T= 10K The model establishes a relationship between the flow rate and the dimensions Xi X2, X3, X4 and X5, according to the expression:

[0066] Q — (1q +a4X4 + a^X^ + + ci^^X^X^-^-ci4^X4X^-^-c!4^X^ -i-G44X4 (2)

[0067] where each term ^ is a real coefficient.

[0068] The most influential coefficients areai5.ai i Thus, expression (2) can be approximated by

[0069] Q « a0 + aiXl + a5X5 + ai5X}X5 + ^^(3)

[0070] with:

[0071] «0 —3.162;«t — 3.76; a5 — 2.03; «15 — 2.09;«n— 1.03

[0072] Figure 4 shows an estimate of the flow rate as a function of Xj and X5 taking into account:

[0073] X2 = 1 mm; = 6mm; X4 = 10mm; X6 = 1mm; At 7' = 10K.

[0074] The triangle in Figure 4 corresponds to X} = 1.4 mm and Xs = 20 mm. The flow rate cor respondent is Q = 2.5 mL / min.

[0075] It is observed that the dimension Xi is preferably between 1 and 3 mm: thus, the diameter of the conduit 20 is preferably of the order of or less than 4 or 5 mm.

[0076] In Figures 5A and 5B, flow models are shown inside the collection chamber 10 considering respectively a single transverse opening 15 ([Fig.5A]) and two transverse openings 15 ([Fig.5B]). The dimensions of each transverse opening were identical in both configurations. In these configurations, the dimensions taken into account were:

[0077] A] = 1.4 mm; X2 = 1mm; X3 = 6mm; X4 = 10mm; = 20mm; X6 = 1mm; At T= 10K.

[0078] The configurations shown in Figures 5A and 5B lead to comparable flow rates, respectively 2.63 mL / min and 2.81 mL / min. In Figures 5A and 5B, the two color codes are representative of the flow velocity at each side opening and the channel, the unit being mm / s. It is observed that the configuration comprising two openings makes it possible to form a more symmetrical flow inside the collection chamber.

[0079] In the preceding configurations, the temperature variation AT between the contact face and the ambient air was considered equal to 10 K. The inventors modeled the impact of a heating element, arranged at the contact face. For the same configuration (X^ = 1 mm; X2 = 1 mm; X3 = 12 mm; X4 = 15 mm; X5 = 10 mm; X6 = 2 mm), two different temperature variations AT were modeled, respectively 10 K and 25 K.

[0080] A variation AT of 10K is obtained by placing the contact face 11 at the skin, by simple thermal conduction between the skin and the contact face. A variation AT of 25K can be obtained by placing a heating means at the contact face, as described in EP3979911. It has been found that increasing AT from 10K to 25K makes it possible to increase the flow rate Q, in the conduit 20, from 0.65 mL:min to 0.93 mL / min, i.e. an increase of 43%. However, such an increase can also be obtained by modifying the dimensions of the device, for example by increasing Xi or X5, as shown in [Fig.4]. Therefore, the inventors consider that the use of a heating element, at the contact face, is not necessary. Especially since this can modify the transcutaneous emission of the gas of interest. This also makes the device more complex, with the use of a power supply to power the heating element.

Claims

Claims

1. Device (1) for detecting a gas of interest, intended to be placed on an object, the gas of interest absorbing light in a spectral absorption band, the device comprising: - a contact face, configured to be placed against the object, and configured to allow passage of the gas of interest, emitted by the object; - a collection chamber (10), opening onto the contact face, and delimited by a side wall (13), the side wall extending around a transverse axis (Z), perpendicular to the contact face; - an intake opening (15), forming an air intake, arranged through the side wall (13), configured to admit ambient air into the collection chamber (10); - a gas sensor (30); the device being characterized in that it comprises a conduit (20), extending, along the transverse axis, between the collection chamber (10) and the gas sensor;and in that the gas sensor comprises: • a measuring chamber (32), extending to the outlet of the conduit, and intended to receive the gas of interest; • a light source (31) configured to illuminate the measuring chamber, in the absorption spectral band; • an acoustic transducer (33), connected to the measuring chamber, and configured to measure a pressure variation in the measuring chamber.;

2. Device according to claim 1, in which the conduit (20) extends, parallel to the transverse axis, at a height (X2) of between 0.5 mm and 10 mm or between 0.5 mm and 5 mm.

3. Device according to any one of the preceding claims, in which the conduit extends, in a plane perpendicular to the transverse axis, with a diameter or diagonal of less than 4 mm.

4. A device according to any preceding claim, wherein the inlet opening (15) extends perpendicular to the axis transverse, according to a length (X3) between 3 mm and 15 mm.

5. Device according to claim 3 in which the inlet opening extends along a height (X6), parallel to the transverse axis, less than or equal to 5 mm or 3 mm.

6. Device according to any one of claims 1 to 5, in which the collection chamber extends, in a plane parallel to the contact face, along a diameter or a diagonal (X4) less than 45 mm or 20 mm.

7. Device according to any one of the preceding claims, in which the collection chamber extends, parallel to the transverse axis, to a height (X5) of less than 30 mm.

8. A device according to any preceding claim, wherein the collection chamber comprises a dehydrating agent.

9. Method for estimating a concentration of a gas of interest emitted by an object using a device (1) according to any one of the preceding claims, the device being applied such that the contact face (11) is arranged opposite the object, the method comprising an admission of ambient air through at least one admission opening (15) of the device, arranged in the side wall (13) and an estimation of the concentration of gas of interest in the measuring chamber (32).

10. The method of claim 9, wherein the gas of interest is selected from NO or CO2.

11. A method according to claim 9 or claim 10, wherein the object is a human or animal user, the device being disposed against a skin of the user, the gas of interest being a transcutaneous gas.

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