MEMBRANE SEALING FOR A PHYSIOLOGICAL SENSOR

Abstract

The invention relates to physiological sensor for measurement of carbon dioxide and a method of securing a carbon dioxide permeable membrane of the physiological sensor. The physiological sensor comprising a closed chamber containing a sensor liquid and being bounded, at least partially, by a carbon dioxide permeable membrane (12), at least two electrodes (10) provided within the chamber in contact with the sensor liquid, a support structure (23) for supporting the membrane (12); and at least one filament 28) wound around the support structure (23) and on top of the membrane (12) for securing the gas-permeable membrane (12) to the support structure (23).

Claims

1. A physiological sensor for measurement of carbon dioxide, comprising: a closed chamber containing a sensor liquid and being bounded, at least partially, by a carbon dioxide permeable membrane; at least two electrodes provided within the chamber in contact with the sensor liquid; a support structure for supporting the membrane; and at least one filament wound around the support structure and on top of the membrane for securing the gas-permeable membrane to the support structure.

2. The physiological sensor according to claim 1, wherein the at least one filament is formed from one or more of: low-density polyethylene (LDPE), high-density polyethylene (HDPE), ultra-high-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), nylon, aramid, or stainless steel.

3. The physiological sensor according to claim 1, wherein the at least one filament has been secured to one or more of: itself, the support structure and the carbon dioxide permeable membrane, by a fixation medium, or by welding of the filament.

4. The physiological sensor according to claim 1, wherein the at least one filament comprises a first filament wound around the support structure on a proximal side of the closed chamber, and a second filament wound around the support structure on a distal side of the closed chamber.

5. A method of securing a carbon dioxide permeable membrane of a physiological sensor for measurement of carbon dioxide, the method comprising: applying the carbon dioxide permeable membrane over a support structure so as to define a closed chamber of the physiological sensor; and winding at least one filament around the support structure on top of the membrane so as to secure the membrane to the support structure.

6. The method according to claim 5, wherein the at least one filament is formed from one or more of: low-density polyethylene (LDPE), high-density polyethylene (HDPE), ultra-high-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), nylon, aramid, or stainless steel.

7. The method according to claim 5, further comprising: securing the at least one filament using a fixation medium to one or more of: itself, the support structure and the carbon dioxide permeable membrane; or securing the at least one filament by welding of the at least one filament to one or more of: itself, the support structure and the carbon dioxide permeable membrane.

8. The method according to claim 5, wherein the winding comprises: winding a first filament around the support structure on a proximal side of the closed chamber, and winding a second filament around the support structure on a distal side of the closed chamber.

9. The method according to claim 5, wherein the winding is performed whilst the support structure is submerged in a sensor fluid.

Description

[0071] FIG. 1 is a schematic diagram of a sensing system incorporating a pCO.sub.2 sensor;

[0072] FIG. 2 is a schematic diagram illustrating the measurement principle for the pCO.sub.2 sensor in the system of FIG. 1;

[0073] FIG. 3 is a partially cutaway view of a first pCO.sub.2 sensor;

[0074] FIG. 4 is a cross-sectional view along line A-A of FIG. 3;

[0075] FIG. 4a is a magnified view of the detail indicated by the circle in FIG. 4;

[0076] FIG. 5 is a view of the first pCO.sub.2 sensor with the membrane removed;

[0077] FIG. 6 illustrates a method of securing the membrane to the first pCO.sub.2 sensor;

[0078] FIG. 7 is a view of a wound filament securing the membrane to the first pCO.sub.2 sensor; and

[0079] FIG. 8 is a perspective view of a second pCO.sub.2 sensor.

[0080] A pCO.sub.2 sensing system comprises a disposable sensor unit 1, an electronic surface unit 2, and a monitor unit 3, as shown in FIG. 1. The disposable sensor unit 1 is delivered packaged and sterilised. It comprises a membrane-protected conductometric pCO.sub.2 sensor 4 with a diameter of less than 1 millimetre, and a temperature sensor 5 integrated in the disposable sensor unit 1. Wires 6 connect the pCO.sub.2 sensor 4 and temperature sensor 5 electrically by means of a connector to the electronic surface unit 2.

[0081] The electronic surface unit 2 sends and receives signals to and from the sensor unit 1. It performs signal processing and transmits the conditioned signal to the monitor unit 3.

[0082] The monitor unit 3 is based on a medical grade PC 7 with a USB interface 18 and Labview software (available from National Instruments Corporation of Austin, Tex.).

[0083] The pCO.sub.2 sensor 4 is used for measurements of the level (partial pressure) of CO.sub.2 (pCO.sub.2) in a fluid, according to the measurement principle illustrated in FIG. 2. A measurement chamber comprises two small cavities 9 with an electrode 10 positioned in each. The two cavities 9 are connected by one or more passageways 11 enclosed by a semi-permeable membrane 12, i.e. a membrane that only allows transport of CO.sub.2 in and out of the volume of the sensor 4. The whole volume is filled with a sensor liquid.

[0084] The sensor liquid comprises an electrolyte solution or a substantially ion-free liquid, such as deionised water. The sensing liquid may optionally comprise a non-ionic excipient so as to prevent egress of the sensor liquid across the membrane 12, without affecting the electrical characteristics of the sensor liquid. The excipient may comprise, for example between 2% and 40% propylene glycol.

[0085] The conductivity in the sensor liquid depends upon the pCO.sub.2, and by measuring the conductivity between the electrodes 10 in the volume, information about pCO.sub.2 may be extracted.

[0086] FIGS. 3 to 7 illustrate a first embodiment of the pCO.sub.2 sensor 4. The pCO.sub.2 sensor 4 comprises an injection moulded plastics support 23, which is substantially cylindrical and surrounded by the semipermeable membrane 12. The support 23 has a tip 24 at its distal end and a body portion 25 which extends proximally from the tip 24. On the body portion 25 are mounted, such as by gluing, two gold electrodes 10. The electrodes 10 extend longitudinally along opposed sides of the body portion 25 and are received in respective recesses in the body portion 25.

[0087] Between the tip 24 and the body portion 25, a sealing surface 26 is provided for securing the membrane 12, as will be discussed in greater detail later. A corresponding sealing surface 26 is provided at the proximal end of the body portion 25. A pair of stop ribs 29 are provided on proximal and distal sides, respectively, of each of the sealing surfaces 26. Furthermore, the sealing surfaces 26 of the support 23 may be made selectively hydrophobic in order to avoid the formation of a water film into which ions may bleed.

[0088] The body portion 25 of the support 23 is provided with a plurality of ribs 27, which are formed with a rounded profile. The ribs 27 provide mechanical support to the membrane 12 and also define the fluid passageways 11 required for the pCO.sub.2 sensor 4 to function effectively. Between each electrode 10 and the fluid passageways 11 formed between the ribs 27 is provided a reservoir 9 formed by the recess in which the electrode 10 is located. The reservoir 9 provides a region of relatively low current density around the electrodes 10 in order to reduce electropolarisation effects.

[0089] As shown in FIGS. 6 and 7, during manufacture, the membrane 12 is fixed onto the support 23, while immersed in the sensor liquid, so that the chamber bounded by the membrane 12, the electrodes 10, and the ribs 27 is completely filled with the sensor liquid. Thus, this chamber forms a pCO.sub.2 sensor 4 as shown schematically in FIG. 2.

[0090] Optionally, the membrane 12 can be fixed first onto the support 23 at one end (e.g. the proximal end) whilst out of the sensor liquid. Then, the membrane 12 can be fixed onto the support 23 at the other end (e.g. the distal end) whilst that end is submerged in the sensor liquid. Thus, the liquid filling happens in a separate step in between the two fixing steps.

[0091] The membrane 12 is secured to the support 23 using high-strength filament 28 made from a biocompatible and medically approved material, such as used for non-absorbable sutures or ligatures. Exemplary materials may include low-density polyethylene (LDPE), high-density polyethylene (HDPE), ultra-high-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), nylon, aramid, etc. Further materials may include organic materials, or metal materials, such as stainless steel. The filament 28 may be single stranded or multi-stranded.

[0092] Filament 28 is wound around each of the sealing surfaces 26 of the support 23 on top of membrane 12, such that the filament 28 circumscribes the central axis of the pCO.sub.2 sensor 4. This compresses the membrane 12 against the respective sealing surfaces 26 and allows for hermetically sealed closure between membrane 12 and support 23. Winding of the filament can start and/or stop on the support 23 or on the membrane 12. The stop ribs 29 on either side of the sealing surfaces 26 have a greater circumference than the sealing surfaces 26 and act to prevent proximal or distal movement of the membrane 12 where it is compressed by the wound filament 28 with respect to the support 23.

[0093] After completion of winding, the wound filament 28 may be covered in a suitable fixation medium, such as an adhesive, glue or epoxy, a UV curable epoxy, etc. Fixation can also be achieved by melting, fusing etc. of the end of the filament 28 to itself or to the membrane 12 or the support 23 by heat, light, chemical reaction etc.

[0094] Fixation or fusing of the filament 28 can also take place during winding by the same means (heating, light, chemicals) e.g. by friction.

[0095] Using a filament 28 wound on top of the membrane 12 around the pCO.sub.2 sensor 4, allows for a gentle, even tension to be applied to the membrane 12 at all points around the pCO.sub.2 sensor 4, whilst still allowing for small imperfections in the support 23 or membrane 12.

[0096] The filament 28 exerts a radial force at each point it contacts the membrane 12, but each wrap is disconnected from previous windings, meaning that the force is distributed along the length of the winding, but accumulates with increased windings as a resulting force on the membrane 12, creating a hermetical seal between the membrane 12 and the support 23.

[0097] The use of a high strength filament to secure the membrane 12 allows for a strong clamping force to be achieved with a very low additional thickness—allowing thicknesses on the micrometer scale.

[0098] It is possible for the pCO.sub.2 sensor 4 to include more than one sensing chamber. For example, two parallel electrodes 10 separated by a longitudinal wall member may be provided on each side of the support 23. A sensing chamber is thereby formed between one electrode 10 on one side of support 23 via the fluid passageways 11 between the ribs 27 on the top of the support 23 to one of the electrodes 10 on the other side of the support 23. A corresponding sensing chamber is provided between the remaining electrodes 10 and the fluid passageways 11 on the bottom of the support 23. An electrode 10 from each of these chambers may be electrically connected to the corresponding electrode from the other chamber, such that the electrical signal from the pCO.sub.2 sensor 4 reflects the conductivity of both chambers.

[0099] The temperature sensor 5 (not shown in FIGS. 3 to 8) takes the form of a thermocouple and may be connected to the end of the support 23 or may be provided within the cabling 6. The temperature sensor 5 is used both for pCO.sub.2 corrective calculations and for the measured tissue temperatures to be displayed on the monitor 3, which is informative for medical diagnosis. The temperature sensor 5 has a minimum measuring range of at least 33-42° C. and a minimum accuracy of +1-0.2° C.

[0100] The cable 6 is electrically and mechanically connected to the electrodes 10 of the pCO.sub.2 sensor 4 and to the temperature sensor 5. The electrodes 10 may be formed as extensions of the conductors of the cable 6. Where the cable 6 and the connection to the support 23 are sufficiently strong, the cable 6 can be used to pull the sensor unit 1 from its position of use. Alternatively, a Kevlar line may be provided, for example incorporated with the cable 6, to provide a strong external mechanical connection.

[0101] The membrane 12 may extend proximally from the support 23 with the cable 6 to form a catheter around the cable 6. Alternatively, a separate catheter may be provided for the cable 6. In this case, the separate catheter may be bonded to the support 23 proximally of the electrodes 10 and the membrane 12.

[0102] The catheter tip with the integrated pCO.sub.2 sensor 4 is placed 0.5-4 cm into organ tissue during surgical procedures to monitor ischemia during a period of up to two weeks. The sensor unit 1 may be used in orthopaedic and reconstructive surgery measuring pCO.sub.2 in muscle and subcutaneous tissue, and in organs such as the liver, kidneys, pancreas, heart muscle, brain and intestines. An insertion tool (not shown) may be used for the placement of the sensor unit 1, and there may be a fixation aid to keep the sensor tip 24 in position.

[0103] The sensor unit 1 has a maximum diameter of 1 mm and the maximum distance from the catheter tip to the sensor element is 2 mm. The pCO.sub.2 sensor 4 has a minimum pCO.sub.2 measuring range of 2-25 kPa, with a minimum detectable pCO.sub.2 difference of 0.2 kPa. The response of the pCO.sub.2 sensor 4 is less than 20 seconds. The maximum allowable measurement current in any area of the fluid chamber is such that j<1 mA/cm.sup.2 while the measuring input voltage is not more than 50 mV RMS.

[0104] The electrodes 10 are gold plated and their total area is approximately 3 mm.sup.2. The measurement frequency f.sub.meas should be higher than 100 Hz. At lower frequencies, polarisation effects in the measurement chamber dominate the measurements. At frequencies above 10 kHz, the low impedance of the capacitances become a significant issue. The measurement resistance R_.sub.measure is in the range of 500 kOhm to 7 MOhm.

[0105] FIG. 8 shows an alternative design for the sensor unit 1 having a capsule membrane 12′ instead of a tubular membrane 12.

[0106] The sensor unit 1 illustrated in FIG. 8 operates on the same principle as the sensor unit 1 illustrated in FIGS. 3 to 7. However, instead of a membrane 12 having an open-ended tubular shape, the membrane 12′ has a tubular shape where the distal end is closed. This design means that it is only necessary to secure the membrane to the support 23 at its proximal end. The membrane 12′ may be secured using a wound filament 28, in the same manner as discussed above.

[0107] Whilst only two designs of the sensor unit 1 have been illustrated, it will be appreciated that many further minor variations in design are possible.

[0108] The sensor unit 1 is electrically connected to an electronic surface unit 2 located on the patient skin by the cable 6, which has a length between 5 cm and 2 metre. The maximum diameter of the cable/catheter is 1 mm and the preferred length of the cable/catheter is about 50 cm. The cable/catheter is soft and flexible so that it does not excessively disturb the neighbouring tissue and organs. The cable/catheter and its connections are also sufficiently robust to withstand the strong pulling forces which may be caused by both normal and “abnormal” use.

[0109] During sterilisation, storage and transport the sensor unit 1 is covered by deionised, sterile and endotoxin-free water to make sure that there is substantially no net loss of water from the sensor reservoir.

[0110] As shown in FIGS. 1 and 2, the electronic surface unit 2 comprises a sine generator 13 which provides a voltage of at least 5 Volts and a current supply of 50 mV, and is powered via a USB interface 18 or by batteries 14. A filter 15 is provided for filtering or averaging the input of the lock-in amplifier 16. A passive filter can be used which reduces the current consumption. A pre-amplifier 17 is combined with a servo mechanism to remove DC current from the signal to reduce electrolysis effects. According to the servo arrangement, the output of the pre-amplifier is fed back to its input via a low pass filter. Thus, only DC components of the output are fed back and cancel any DC current drawn through the pCO.sub.2 sensor 4. In this way, it is ensured that there is no DC current through the pCO.sub.2 sensor 4 which would degrade the electrodes 10. The op-amp used in this stage consumes minimal current and has a large CMMR value. At the same time, the bias current is minimal. A lock-in amplifier 16 amplifies the AC signal from the pCO.sub.2 sensor 4. This may be built with op-amps or using an IC package with at least 1% accuracy for the signal detection at frequencies lower than 1 kHz. A galvanic division 19 such as an optocoupler or a coil coupler is provided to prevent noise transfer from the monitor unit 3 and associated cabling 18. The optocoupler is normally favoured due to the noise signal ratio. A temperature signal amplification and conditioning unit 20 is provided to amplify the signal from the temperature sensor 5. The electronic unit 2 is powered via the USB interface 18. However, optionally, a rechargeable and changeable standard type battery 14 may be used where the battery capacity is sufficient for at least 14 days continuous monitoring. The surface unit 2 is also provided with an on/off indicator LED 21, and a battery status indicator if appropriate. Communication between the surface unit 2 and the monitor 3 is through the USB interface 18. A USB cable between the surface unit 2 and the monitor 2 is light and flexible and at least 1 m long, preferably about 2 m long.

[0111] As shown in FIGS. 1 and 2, an AC current is generated by sine generator 13 and fed to one of the pCO.sub.2 sensor electrodes 10 and to a lock-in amplifier 16. The high-pass signal from the other pCO.sub.2 electrode 10 is passed through a filter 15 to a low noise amplifier 17 and from there to the lock-in amplifier 16 where it is compared to the reference signal generated by the sine generator 13. Out of phase components, i.e. undesired components, of the signal are rejected and the remaining portion of the signal is amplified. The amplified signal is proportional to pCO.sub.2 (or conductance) and is passed on to the monitor 3 for recordal or further manipulation.

[0112] The surface unit 2 may also be electrically connected to a reference electrode (not shown) that is electrically connected to the patient's skin. The signal from the reference electrode can be used to compensate the signals from the sensor unit 1 for the effect of electromagnetic noise generated by the patient.

[0113] A single surface unit 2 may receive signals from several sensor units 1 and provide a multiplexed output to the monitor unit 3.

[0114] The monitor unit 3 comprises a portable PC 7 or similar computing device which can collect signals from a number of different surface units 2 simultaneously. The power supply 22 for the monitor unit 3 is of a medically approved type operating on both 110V and 230V.

[0115] The software functions of the monitor unit 3 may be implemented in Labview, a software package available from National Instruments of Austin, Tex. and capable of handling up to 4 different surface units simultaneously. The software provides the facility for calibration of the sensor(s) with three calibration points and a second order calibration function. The software can be modified to support any other number of calibration points and type of calibration function. The software also has the facility to smooth the signal from the pCO.sub.2 sensor 4 over defined time intervals. It is possible to have at least two alarm levels for the measurement values and two alarm levels for their gradients. The measurement value gradients are calculated for individually defined time intervals. The alarm is both visible and audible. It is possible to stop an alarm indication while keeping the other alarms active. The monitor 3 can log all measured values, parameter settings and alarms throughout a session. With a 30 second logging interval there should be a storage capacity for at least 10 two week sessions on the hard disc. The session log can be saved to a writeable data storage medium in a format readable by Microsoft Excel.

[0116] In summary, a physiological sensing device for the measurement of pCO.sub.2 includes a closed chamber bounded, at least partially, by a carbon dioxide permeable membrane. There are two electrodes within the chamber. The chamber contains a sensor liquid in contact with electrodes and the membrane. The carbon dioxide permeable membrane is secure to a support structure of the physiological sensing device by a filament wound around the support structure and on top of the membrane.