Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
  • A61B5/14542—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue for measuring blood gases
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
  • A61B5/1455—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters
  • A61B5/14551—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters for measuring blood gases
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
  • A61B5/1491—Heated applicators
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
  • A61B5/6801—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be attached to or worn on the body surface
  • A61B5/6802—Sensor mounted on worn items
  • A61B5/681—Wristwatch-type devices
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B2560/00—Constructional details of operational features of apparatus; Accessories for medical measuring apparatus
  • A61B2560/02—Operational features
  • A61B2560/0223—Operational features of calibration, e.g. protocols for calibrating sensors
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
  • A61B2562/16—Details of sensor housings or probes; Details of structural supports for sensors
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
  • A61B5/1468—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using chemical or electrochemical methods, e.g. by polarographic means

Definitions

• This disclosure relates to techniques for measuring in a non-invasive manner transcutaneous carbon dioxide of a person and more in particular to a wearable device and a related method for non-invasive measuring the partial pressure of transcutaneous CO2 of a person.
• Telemedicine is an area that includes interactive medicine, data collection and sharing between specialists, and remote patient monitoring. This field is constantly growing and there is an increasing interest in the sector also because of the recent COVID- 19 pandemic, which has further encouraged countries to act in the sector. Telemedicine minimizes costs and ensures that you may keep your health under control even when you are unable to go to the hospital or to your doctor.
• main support tools to make telemedicine and, in particular, remote monitoring possible we find wearable devices. They are devices that may be attached to the skin, such as patches or smart fabrics, or worn, like watches, belts, or socks. These are equipped with specific sensors for the detection of particular biological variables of clinical interest. The collected data are then made available to the user and the doctor.
• CO2 carbon dioxide
• PaC02 physiological arterial partial pressure of CO2
• bicarbonates 24 mEq/L
• Formula 2.4 shows the reversible acid-base reaction in blood and any change in the amount of species involved shifts the reaction towards the right or the left.
• Transcutaneous CO2 monitoring is an alternative measurement which may be performed non-invasively by means of electrochemical sensors (Stow-Severinghaus-type PCO2 sensors).
• electrochemical sensors Small-Severinghaus-type PCO2 sensors
• carbon dioxide electrodes produced by private companies such as SenTec. This technology requires specialized personnel and a hospital facility.
• continuous remembranization and calibration are other critical aspects of the apparatus.
• Optical sensors already used for the analysis of carbon dioxide in EtCO2 monitoring, have been used for transcutaneous CO2 monitoring in scientific literature but no device for the analysis of the partial pressure of carbon dioxide has been commercialized.
• Both electrochemical sensors and optical sensors evaluate the transcutaneous pressure of carbon dioxide (PtCO2).
• PtCO2 transcutaneous pressure of carbon dioxide
• the mechanism through which the partial pressure of carbon dioxide is made available outside the skin must be found in the permeability to these gases to the two most superficial layers of the skin itself, which are stratum corneum and epidermis. In the deepest layer, dermis, there is a dense network of capillaries organized in predominantly vertical structures of about 0.2 - 0.4 mm.
• Transcutaneous CO2 measurement methods require the heating of the skin to arterialize the measurement area and increase the diffusion capabilities through the skin.
• An optical approach may be used, exploiting near infrared light.
• the optical method exploits the spectroscopy approach, which is defined as the study of the interaction between matter and electromagnetic radiation as a function of the wavelength or frequency of the radiation.
• the perfect range of wavelength is the near-infrared range, centred at 4.25 pm, because it is invisible to the human eye and safe for human beings because it has low energy.
• chemical compounds that have two or more different atoms may absorb infrared light, so oxygen is not involved in this process but other species such as nitrous oxide can.
• each compounds absorb infrared at a specific wavelength so if 4.25 pm is used, only carbon dioxide may absorb it.
• the milestone of spectroscopy is the Lambert-Beer Law, which basically relates the attenuation of light to the properties of the material through which the light is traveling.
• the Lambert-Beer law states that the number of infrared rays absorbed is proportional to the concentration of the infrared absorbing substance so the more CO2 is present, the more infrared light is absorbed.
• the system is composed by three main elements: an infrared source which emits wavelengths within a narrow band around 4.25 pm, a sample chamber and a detector. There is one side effect called “collision broadening”. In air and body diffused gasses, there are also oxygen and nitrous oxide in addition to CO2.
• EtCO2 systems measure the amount of nitrous oxide and oxygen present and use this information to correct for errors due to collision broadening.
• the output signal of Cozir CO2 in ppm is converted in mmHg by using the calibration plot.
• the wristband has a lag of 5 minutes with respect to EtCO2.
• This wearable device is equipped with a sealing O-ring and a hydrophobic membrane made of PDMS in order to protect the CO2 sensor from the water vapor diffused by the skin. Indeed, water molecules interfere NDIR sensors because water has a high extinction coefficient at wavelengths near 4.26 pm.
• NDIR non-dispersive infrared
• NiChrome wire with a resistivity of 2.308 Q/m as heater.
• NTC thermistors with a nominal resistance of 100 KQ were chosen, one to detect the temperature of the skin and one for that of the wire.
• the system was also equipped with a LED and a momentary switch to facilitate user interaction.
• the microcontroller used is
• the microcontroller used is
• the microcontroller used is
• the microcontroller used is
• the microcontroller used is
• the microcontroller used is
• the microcontroller used is
• a fabric cuff was provided to attach the device to the skin.
• a sensitivity to the presence of the skin is highlighted through the comparison between the output values of the CO2 sensor in air and the one’s in contact with the skin.
• the skin temperature has to be increased, ideally up to 42 °C.
• the case had a measurement chamber where the air passes before entering the sensor.
• the sensitivity to the presence of the skin could be detected with this first prototype, shown in detail in Figures 4a, 4b and 4c.
• This first prototype which was functionally similar to the device disclosed in WO 2016/173877 and in the article by Vishal Varun Tipparaju et al., surprisingly was largely inaccurate for no apparent reason.
• a method of measuring in a non-invasive manner a partial pressure of transcutaneous CO2 of a person is also disclosed. This method may be implemented by running a software in a microcontroller of the wearable device of this invention.
• Figure 1 depicts a prior device for measuring partial pressure of CO2 at a skin of a person.
• Figures 2 and 3 depict another prior wearable wristband device for measuring partial pressure of CO2 at a skin of a person, comprising a membrane for separating the person's skin to a chamber for CO2 collecting gases emitted by the skin.
• Figures 4a, 4b and 4c illustrate a first prototype, not according to the present invention, of a device for measuring partial pressure of CO2 at a person's skin.
• Figures 5 and 6 are sectional views of a wearable device of this invention highlighting the channel and the inlet for putting the chamber of the device into fluid communication with ambient air.
• Figures 7 and 8 show the outer case of the second prototype of the wearable device of this invention, highlighting the cavity in which the valve for closing/opening the channel is installed.
• Figure 9 is a picture of the second prototype after having removed a part of the outer case to show how the valve is installed.
• Figure 10 is a view of the outer case of the second prototype of the wearable device of this invention, highlighting a seat in which a temperature sensor is installed around a perimeter of the opening of the chamber, destined to be in contact with a person's skin.
• Figure 11 illustrated the three phases of the method for measuring a partial pressure of CO2 using the wearable device of this invention.
• a wearable device of this invention for non-invasive measurement of the partial pressure of transcutaneous CO2 (PtCO2) of a person will be disclosed referring to the enclosed figures from 5 to 11. It is a self-contained device that comprises:
• a housing (1) for a CO2 sensor defining a chamber (2) for measuring gases, having an opening at one bottom wall configured to be airtight closed by the skin of the person when the wearable device is worn so as CO2 emitted by the person’s skin, within the perimeter of the chamber, enters in the chamber (2) through the opening.
• a CO2 sensor (not shown in the figures), installed into the housing (1) to close the chamber (2) from one top wall opposite to the opening and oriented toward the opening in order to detect CO2 emitted by the person's skin, within the perimeter of the chamber, that closes the opening, configured to generate a signal representative of a CO2 concentration in a portion of the chamber (2) delimited by the CO2 sensor and the person's skin;
• - one heater (not shown in the figures) installed into the housing (1) and configured to warm the portion of the person's skin, within the perimeter of the chamber, that closes the opening of the chamber (2).
• microcontroller functionally coupled with the CO2 sensor for receiving the signal representative of a CO2 concentration, and functionally coupled with the one heater to warm the person's skin, the microcontroller being configured to output a value of CO2 concentration, measured into the chamber, corresponding to the signal representative of a CO2 concentration.
• the chamber (2) is not closed by a membrane, but it is closed by the person's skin in direct contact with the opening of the chamber (2).
• the housing (1) further defines an inlet (3) to the chamber (2) and a channel (4) configured to put into fluid communication the inlet (3) of the chamber (2) with ambient air.
• the housing (1), the CO2 sensor and the inlet (3) are configured so as, when the person’s skin closes the opening of the chamber (2), ambient air outside the wearable device may enter in the chamber (2) only passing throughout the channel (4) and the inlet (3).
• the channel (4) is the only way for putting the chamber (2) into fluid communication with the ambient air.
• the wearable device of this invention comprises a controlled valve installed into the channel, configured to close or open the channel respectively in order to seal or to put in fluid communication to ambient air the chamber when the wearable device is worn by the person.
• This valve is controlled by the microcontroller, which is configured to open/close the controlled valve and to display a measured value of CO2 concentration into the chamber.
• the controlled valve must be opened to allow recirculation of ambient air before a new measure of CO2 is to be started, and to be closed when the CO2 measurement is performed.
• air in the chamber is flushed only when the controlled valve is open; moreover, air remains sealed in the chamber only when said controlled valve is closed.
• a second exemplary prototype according to this invention has been realized.
• the carbon dioxide sensor was replaced with a more compact Cozir-LP NDIR CO2.
• a normally open solenoid valve, an antiparallel diode and another MOSFET module were used to power the valve.
• a rechargeable 3.7 V Lithium-ion battery and a latching switch to turn the device on and off were selected. Everything was then soldered on a custom-made Printed Circuit Board.
• a housing was designed in SOLIDWORKS® and a bracelet was realized to attach the device to the person's wrist and to hold the battery.
• the main components used for the second prototype are the following:
• a NiChrome wire used as a heater to warm the skin of the subject with a resistivity of 5.755 Q/m
• the results of the first prototype highlighted the need of an output air channel.
• the chamber connected to the CO2 sensor must be as closed as possible in order to be more sensitive to the presence of the skin.
• a MOSFET module is used to deliver the sufficient amount of current.
• a flyback diode is used for discharging the electrical energy accumulated by the valve during the active phase.
• the diode in antiparallel configuration with respect to the valve protects from damages that may occur.
• the 3-D housing is designed in the Fusion 360 software.
• the chosen material for the 3D-printed case is the DraftGrey material used on a Stratasys PolyJet Printer, mostly because of its resistance to high temperatures.
• the volume of the chamber where CO2 diffuses from the skin to the CO2 sensor had to be as small as possible to create a sensitive device to analyse PtCO2.
• the heater for example a NiChrome wire, for heating the person's skin that closes the opening of the chamber (2) may be located in the groove (6) as shown in Figure 10 around the perimeter of the opening, in contact with the person’s skin and covered by thermal paste.
• the temperature sensor that may be for example a thermistor, may be installed too in the groove 6 in direct contact with the person's skin and the heater.
• thermosensor in case the heater (NiChrome wire) is not in direct contact with the person's skin, as shown in Figure 4a making reference to the first prototype not according to the present invention, there may be a first temperature sensor in direct contact with the person's skin, installed in the housing at the position 7, and a second temperature sensor installed in direct contact with the heater to sense the temperature of the heater itself.
• the wearable device allows to measure in an accurate manner the partial pressure of transcutaneous CO2 and thus to obtain a surrogate of the partial pressure of CO2 in the person's blood. This is done by programming the microcontroller of the wearable device to implement three phases, summarized in Figure 11: the first phase involves the analysis of environmental carbon dioxide; the second is the heating of the skin up to a nominal temperature, that may be for example 38.5 °C, or for a nominal time interval (in case said nominal temperature is not attained at the expiring of the nominal time interval); and a last stage of analysis of the carbon dioxide diffused by the skin.
• the microcontroller commands the controlled valve of the wearable device, for example by pushing a button, so as to cause the CO2 sensor measure an ambient CO2 concentration in the environment, to provide an offset value of CO2 concentration.
• the microcontroller commands, for example by pushing the same button again or a second button of the wearable device, the controlled valve for closing the channel to seal the chamber.
• the NiChrome wire starts heating the person's skin up to the nominal temperature (for example 38.5 °C) and/or for a nominal time interval.
• the CO2 sensor measures a CO2 concentration in the chamber, which is sealed by the person's skin and by the closed valve.
• the ambient CO2 concentration may be measured before wearing the device of this invention, or even after the device has been worn.
• the microcontroller may command the controlled valve of the wearable device to open the channel and to flush the chamber with fresh ambient air from an environment in which the wearable device is located. For example, this may be done when a first measurement has just been carried out and a new measurement is desired. Before carrying out a new measurement, with the wearable device still worn on the person's wrist, the controlled valve may be opened to flush the chamber with ambient air to repeat the above method steps.
• the wearable device may be connected, using wires or in wireless mode, to a smartphone or PC to display the results of measurements collected by the microcontroller of the wearable device.
• the digital CO2 values are read at 1 Hz, with the CO2 sensor in polling mode.
• the signal is considered stable and is indicated by the green LED stopping its blinking.
• the average of the 10 values is read by the measurement system. It may be used to consider the offset in CO2 skin measurement. Now the device may be put on the wrist and, after pressing the button, the system passes in the heating phase.
• the heating process starts by setting the Duty Cycle (DC) of the analog pin connected to the NiChrome wire to 35%. Now the system enters in a loop: the device measures skin and wire temperature (note: the wire is in contact with the skin) with a frequency of 1 Hz. All the specification of this heating process have been determined after several experimental attempts. If the skin/wire temperature reaches 44 °C, the DC is set to 0% to avoid burning of the skin.
• DC Duty Cycle
• the third phase may begin.
• the time needed for this phase is in the order of minutes, according to the subject and to the initial skin temperature.
• the process may be stopped at any moment by pressing the button and the subject may put off the device whenever he wants, so as a prototype of a biomedical device it meets the basic safety conditions.
• PtCO2 monitoring marketed by SenTec and it has been proven able to detect an increase in PtCO2.

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Citations (9)

• 2022
  • 2022-03-22 IT IT102022000005612A patent/IT202200005612A1/it unknown
• 2023
  • 2023-03-22 WO PCT/IB2023/052807 patent/WO2023180948A1/en unknown

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