U.S. patent application number 17/174263 was filed with the patent office on 2022-08-11 for analyte sensor component.
This patent application is currently assigned to SciLogica Corp.. The applicant listed for this patent is SciLogica Corp.. Invention is credited to Nicholas Paul BARWELL, Barry Colin CRANE, Alasdair Allan MACKENZIE.
Application Number | 20220248985 17/174263 |
Document ID | / |
Family ID | 1000005741107 |
Filed Date | 2022-08-11 |
United States Patent
Application |
20220248985 |
Kind Code |
A1 |
BARWELL; Nicholas Paul ; et
al. |
August 11, 2022 |
Analyte Sensor Component
Abstract
A sensor component for use in a system for measuring
concentration of analytes in fluid in a fluid line comprises one or
more sensing elements having an optical property that varies with
the concentration of the analytes, and engages with the fluid line
such that the sensing elements are exposed to the fluid. The sensor
component comprises a connector connecting to one or more optical
waveguides, and transmits light between the waveguides and the
sensing elements. The sensor component comprises one or more of a
sampling port configured to provide fluidic access to the fluid
line, a data storage medium storing data representing information
about the sensor component, and a reflective element. Where it
comprises a reflective element, the sensor component transmits
light between the waveguides and the reflective element on a
separate optical path from an optical path between the waveguides
and the sensing elements.
Inventors: |
BARWELL; Nicholas Paul;
(Warwickshire, GB) ; CRANE; Barry Colin; (Oxon,
GB) ; MACKENZIE; Alasdair Allan; (Herefordshire,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SciLogica Corp. |
Denver |
CO |
US |
|
|
Assignee: |
SciLogica Corp.
Denver
CO
|
Family ID: |
1000005741107 |
Appl. No.: |
17/174263 |
Filed: |
February 11, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 2560/0223 20130101;
A61B 5/1459 20130101; A61B 2562/0271 20130101; A61B 5/14546
20130101 |
International
Class: |
A61B 5/1459 20060101
A61B005/1459; A61B 5/145 20060101 A61B005/145 |
Claims
1. A sensor component for use in a system for measuring the
concentration of one or more analytes in fluid in a fluid line, the
sensor component comprising: one or more sensing elements having an
optical property that varies with the concentration of the one or
more analytes in the fluid, the sensor component being configured
to engage with the fluid line such that the sensing elements are
exposed to the fluid in the fluid line; a connector configured to
connect to one or more optical waveguides, the sensor component
being configured to transmit light between the one or more optical
waveguides and the one or more sensing elements; and a sampling
port configured to provide fluidic access to the fluid line when
the sensor component is engaged with the fluid line.
2. A sensor component according to claim 1, wherein the sampling
port comprises a one-way valve configured to permit one-way fluid
flow out of the fluid line.
3. A sensor component according to claim 1, wherein the sampling
port comprises a component fitting configured to engage with an
external fitting.
4. A sensor component according to claim 3, wherein the component
fitting comprises a Luer fitting.
5. A sensor component according to claim 3, wherein the sampling
port is configured to open upon engagement of the external fitting
with the component fitting.
6. A sensor component according to claim 1, wherein the sampling
port is self-closing.
7. A sensor component according to claim 1, wherein the sampling
port is at most 20 cm from the one or more sensing elements.
8. A sensor component according to claim 1, wherein the sampling
port comprises a removable cap configured to seal the sampling
port.
9. A sensor component according to claim 1, further comprising a
fluid-permeable support element for supporting the sensing elements
arranged to be between the sensing elements and the fluid in the
fluid line when the sensor component is engaged with the fluid
line.
10. A sensor component according to claim 9, wherein the permeable
support element comprises a mesh.
11. A sensor component according to claim 1, wherein the sensing
elements comprise a membrane permeable to at least one of the
analytes configured to be exposed to the fluid in the fluid line
when the sensor component is engaged with the fluid line.
12.-17. (canceled)
18. A sensor component according to claim 1, wherein the sensor
component is configured to engage with a wall of the fluid
line.
19. A sensor component according to claim 1, wherein the sensor
component further comprises a conduit, the sensing elements being
exposed to fluid in the conduit and the conduit is configured to be
inserted into the fluid line for engagement of the sensor component
with the fluid line.
20. A sensor component according to claim 19, wherein the conduit
is configured to be inserted into the fluid line in an in-line
configuration.
21. A sensor component according to claim 19, wherein the conduit
is configured to be inserted into the fluid line in a shunt
configuration.
22. A sensor component for use in a system for measuring the
concentration of one or more analytes in fluid in a fluid line, the
sensor component comprising: one or more sensing elements having an
optical property that varies with the concentration of the one or
more analytes in the fluid, the sensor component being configured
to engage with the fluid line such that the sensing elements are
exposed to the fluid in the fluid line when the sensor component is
engaged with the fluid line; a connector configured to connect to
one or more optical waveguides, the sensor component being
configured to transmit light between the one or more optical
waveguides and the one or more sensing elements; a reflective
element, the sensor component being configured also to transmit the
light between the one or more optical waveguides and the reflective
element on a separate optical path from an optical path between the
one or more optical waveguides and the one or more sensing
elements.
23. A sensor component according to claim 22, wherein the
reflective element is concave.
24. A sensor component according to claim 22, wherein the
reflective element is planar.
25. A sensor component according to claim 22, wherein the
reflective element is configured to reflect at least 10% of light
incident thereon back to the one or more waveguides.
26.-28. (canceled)
29. A sensor component according to claim 22, wherein the sensor
component comprises two or more sensing elements.
30. A sensor component according to claim 29, further comprising a
light-absorbing element positioned between the sensing
elements.
31. A sensor component according to claim 22, wherein the connector
comprises a transparent optical element in respect of the or each
sensing element configured to transfer light between the sensing
element and the one or more optical waveguides.
32. A sensor component according to claim 31, wherein the
transparent optical element comprises a waveguide.
33. A sensor component according to claim 22, further comprising a
temperature sensor arranged to sense the temperature of the one or
more sensing elements.
34. A sensor component according to claim 33, wherein the
temperature-sensitive element is a thermistor or a
thermocouple.
35.-38. (canceled)
39. A replaceable sensor component for use in a system for
measuring the concentration of one or more analytes in fluid in a
fluid line, the sensor component comprising: one or more sensing
elements having an optical property that varies with the
concentration of the one or more analytes in the fluid, the sensor
component being configured to engage with the fluid line such that
the sensing elements are exposed to the fluid in the fluid line
when the sensor component is engaged with the fluid line; a
connector configured to connect to one or more optical waveguides,
the sensor component being configured to transmit light between the
one or more optical waveguides and the one or more sensing
elements; and a data storage medium configured to store data
representing information about the sensor component.
40. A replaceable sensor component according to claim 39, further
comprising an interface circuit configured to transmit signals
between the data storage medium and the system.
41. (canceled)
42. A replaceable sensor component according to claim 39, wherein
the information comprises one or more characteristics of the one or
more sensing elements.
43. A replaceable sensor component according to claim 42, wherein
the information comprises calibration information about the
variation of the optical property of the sensing elements with the
concentration of the one or more analytes and/or the temperature of
the sensing elements.
44. A replaceable sensor component according to claim 39, wherein
the information comprises one or more characteristics of a
calibration fluid used for calibration of the replaceable sensor
component.
45. A replaceable sensor component according to claim 44, wherein
the information comprises a variation of the pH of the calibration
fluid with temperature.
46. A replaceable sensor component according to claim 39, wherein
the information comprises an identifier of a patient with whom the
replaceable sensor component is associated.
47. A replaceable sensor component according to claim 46, wherein
the data storage medium is configured to receive the identifier
during initialisation of the system, and store the identifier such
that the replaceable sensor component is permanently associated
with the patient.
48. A replaceable sensor component according to claim 39, wherein
the information comprises an identifier of the system with which
the replaceable sensor component is associated.
49. A replaceable sensor component according to claim 39, wherein
the information comprises one or more of: an indication of prior
use of the replaceable sensor component; an indication of whether
damage has occurred to the replaceable sensor component; a use-by
date after which the replaceable sensor component should not be
used; an in-use lifetime of the replaceable sensor component; a
length of time for which the replaceable sensor component has been
used; a unique identifier of the replaceable sensor component; a
date of manufacture of the replaceable sensor component; a time at
which the replaceable sensor component was last calibrated; and an
indication of the number and/or type of errors that have occurred
during use of the replaceable sensor component.
50.-62. (canceled)
Description
FIELD
[0001] The present invention relates to sensor components used to
measure the concentration of analytes in a fluid, specifically
using an optical property of a sensing element.
BACKGROUND
[0002] It is desirable in many areas to be able to determine the
concentration of an analyte in a fluid which may contain a mixture
of several different substances. For example, in clinical settings
it is important to be able to accurately determine the
concentration of oxygen in a patient's blood in real time to detect
and prevent hypoxia. Examples of clinical settings where monitoring
of blood analytes is important include cardiopulmonary Bypass
(CPB), extracorporeal membrane oxygenation (ECMO), and continuous
renal replacement therapies (CRRT).
[0003] CPB technology allows cardiac surgical procedures to be
performed in a motionless bloodless surgical field. Modern CPB
machines have systems for monitoring pressures, temperature, oxygen
saturation, haemoglobin as well as bubble detectors and reservoir
low level detection alarms. Cardiopulmonary bypass patients are
subject to physiological variations in in their blood gas levels
due to oxygenation, electrolyte changes, and fluid shifts that
occur during CPB. Potential adverse patient outcomes due to these
variations may include hypoxia and hyperoxia, hypocapnia and
hypercapnia, and acid base alterations. The American Society of
Extracorporeal Technology-Standards and Guidelines for Perfusion
Practice, Appendix D, May 2017, proposes that PO.sub.2, PCO.sub.2,
pH, SO2, potassium, ionised calcium, sodium, lactate, glucose and
haemoglobin/haematocrit be measured frequently or continuously.
Other useful parameters can be calculated from these measured blood
gas data--base excess, bicarbonate, oxygen delivery, oxygen
consumption and oxygen extraction ratio.
[0004] ECMO has more recently evolved from CPB and is applicable to
patients who are hypoxaemic despite maximal conventional
ventilatory support, who have significant ventilator-induced lung
injury, or who are in cardiogenic shock. The equipment set up is
similar to that of CPB but is generally used for very long periods
of time (e.g. 30 days is not uncommon) and usually resides within
the Intensive Care Unit. CRRT are hemodialysis treatments that are
provided as a continuous 24 hour per day therapy and is appropriate
for patients suffering acute renal failure (ARF) that are
hemodynamically unstable. Intermittent hemodialysis is appropriate
for patients that have chronic renal impairment.
[0005] One known type of sensor employed in these settings uses a
luminescent compound, for example a fluorescent organic dye, with a
luminescence lifetime which depends on the concentration of the
analyte. By exciting the luminescent compound and measuring its
luminescence lifetime, the concentration of the analyte can be
determined. This type of system has the advantage that it can be
operated continuously, so does not require taking regular samples,
for example of blood, for analysis or other similarly inconvenient
procedures. The clinical utility of continuous real time monitoring
is particularly important when monitoring clinically critical,
rapidly changing analytes in seriously ill patients. The
information provided enables the clinician to respond by titrating
administered therapies. Thus, continuous real time data can be
considered as providing feedback to therapy.
[0006] With intermittent testing a clinician's view of the status
of the patient at a point where a new blood sample is to be taken
is governed by the analyte value given by the previous sample. Gaps
in the knowledge of a patient's status between blood samples (known
as blind intervals) can be a risk to the patient. These gaps can be
eliminated by using a continuous sensor. Continuous monitoring
should be able to provide accurate measurement of a given analyte
over prolonged periods of time, provide direction to the movement
of analyte concentration (indicating trending) and in doing so
provide continuous, minute by minute, patient status information to
the clinician. This aids the provision of effective therapy and
eliminates blind intervals, thereby reducing risk to the
patient.
SUMMARY
[0007] To provide these benefits an important requirement for a
continuous sensor is to maintain accuracy over prolonged periods of
time. Inaccuracies can be due to continuous drift of the sensor
reading or more sudden changes, for example due to damage to the
sensor. These inaccuracies are of critical importance since they
may not be readily differentiated from true physiological changes
in the concentration of the analyte. Hence, the clinician may be
provided with erroneous information that incorrectly influences the
administered therapy, thereby putting the patient at risk. This is
less of a concern for intermittent monitoring, where a blood sample
is taken and introduced to a sensor such as a blood gas analyser.
This is because the sensor is calibrated just prior to each
measurement, so although the sensor is likely to drift relatively
quickly, it is of no consequence in terms of accuracy since there
is little time to drift between calibration and measurement.
[0008] The first aspect of the present invention is concerned with
providing a sensor component for continuous monitoring that is more
accurate and less prone to drift.
[0009] According to a first aspect of the invention, there is
provided a sensor component for use in a system for measuring the
concentration of one or more analytes in fluid in a fluid line, the
sensor component comprising one or more sensing elements having an
optical property that varies with the concentration of the one or
more analytes in the fluid, the sensor component being configured
to engage with the fluid line such that the sensing elements are
exposed to the fluid in the fluid line, a connector configured to
connect to one or more optical waveguides, the sensor component
being configured to transmit light between the one or more optical
waveguides and the one or more sensing elements, and a sampling
port configured to provide fluidic access to the fluid line when
the sensor component is engaged with the fluid line.
[0010] The sampling port provided as part of the sensor component
means that, either during initial setup of the sensor component or
if it has been ascertained that readings from one or more of the
sensing elements do drift, leading to inaccuracy over a period of
time, then a sample of the fluid can be taken from the local port.
The sample can be analysed by an approved analyser and the value
used to adjust the readings from the sensing elements to compensate
for the inaccuracy. Including the sampling port as part of the
sensor component also has the advantage that fluid samples will be
taken in physical proximity to the sensing elements, such that the
concentration of analyte in the fluid sample will reflect the
concentration at the sensing elements as accurately as
possible.
[0011] In some embodiments, the sampling port comprises a one-way
valve configured to permit one-way fluid flow out of the fluid
line. Using a one-way valve prevents the sampling port being used
to introduce substances into the fluid line, which may not be
appropriate at the location of the sensor component, or may cause
erroneous readings from the sensor component by locally affecting
the concentration of an analyte.
[0012] In some embodiments, the sampling port comprises a component
fitting configured to engage with an external fitting. Using a
component fitting allows the sampling port to be reliably and
securely engaged by an external fitting on a device used to take
samples. This can prevent possible contamination or other errors
during sampling. In some embodiments, the component fitting
comprises a Luer fitting. This is a commonly available type of
fitting in clinical settings.
[0013] In some embodiments, the sampling port is configured to open
upon engagement of the external fitting with the component fitting.
This reduces the number of operations required to acquire a sample,
thereby improving ease of use.
[0014] In some embodiments, the sampling port is self-closing. This
also improves ease of use by reducing the operations required by a
user, and reduces the chance of contamination if the sampling port
is not promptly and correctly closed after taking a sample.
[0015] In some embodiments, the sampling port is at most 20 cm from
the one or more sensing elements. Physical proximity to the sensing
elements ensures that the sample taken using the sampling port has
analyte concentrations indicative of those at the sensing elements
at the time of sampling. This helps to improve the accuracy of
calibration performed using the sample.
[0016] In some embodiments, the sampling port comprises a removable
cap configured to seal the sampling port. This allows the port to
be more securely sealed and protected from possible damage,
particularly during extended periods of non-use.
[0017] A second aspect of the present invention is also concerned
with providing a sensor component for continuous monitoring that is
more accurate and less prone to drift.
[0018] According to the second aspect of the invention, there is
provided a sensor component for use in a system for measuring the
concentration of one or more analytes in fluid in a fluid line, the
sensor component comprising one or more sensing elements having an
optical property that varies with the concentration of the one or
more analytes in the fluid, the sensor component being configured
to engage with the fluid line such that the sensing elements are
exposed to the fluid in the fluid line when the sensor component is
engaged with the fluid line, a connector configured to connect to
one or more optical waveguides, the sensor component being
configured to transmit light between the one or more optical
waveguides and the one or more sensing elements, a reflective
element, the sensor component being configured also to transmit the
light between the one or more optical waveguides and the reflective
element on a separate optical path from an optical path between the
one or more optical waveguides and the one or more sensing
elements.
[0019] Providing the reflective element allows for light to be
transmitted along an optical path that does not contain the sensing
elements. This provides an additional measurement that can be
compared to the measurements obtained from optical paths that do
contain the sensing elements. In turn, such comparison ensures that
any inaccuracies generated by factors such as small mechanical
shifts in optical interfaces or the generation of light losses
through minor damage to the one or more optical waveguides can be
distinguished from changes in transmission that are due to changes
in the concentrations of analytes in the fluid. This allows an
assessment of whether the accuracy of the measurements of analyte
concentration are likely to have deteriorated far enough that
remedial action, such as recalibration or replacement of the sensor
component, are necessary.
[0020] In some embodiments, the reflective element is concave. A
concave reflector can concentrate light and cause it to be
transmitted more effectively back to the optical waveguide after
reflection.
[0021] In some embodiments, the reflective element is planar. A
planar reflective element may be more straightforward to
manufacture and assemble than a concave reflector.
[0022] In some embodiments, the reflective element is configured to
reflect at least 10% of light incident thereon back to the one or
more waveguides. This provides a minimum level of reflectivity such
that signal from the reflective element can reliably be
detected.
[0023] Another important requirement for sensor components,
particularly where they are replaceable or disposable, is that
individual sensor components and associated information are not
confused with one another. Sensor components may have properties
that vary between batches, may be associated with particular
equipment or patients, and have various other associated metadata.
Keeping track of such information is important to provide accurate
information and ensure patient safety, but may also be difficult
where multiple individual sensor components are stored or in use in
the same environment.
[0024] A third aspect of the present invention is concerned with
providing a replaceable sensor component for continuous monitoring
that is accurate, and which allows information to be easily and
robustly associated with the sensor component.
[0025] According to the third aspect of the invention, there is
provided a replaceable sensor component for use in a system for
measuring the concentration of one or more analytes in fluid in a
fluid line, the sensor component comprising one or more sensing
elements having an optical property that varies with the
concentration of the one or more analytes in the fluid, the sensor
component being configured to engage with the fluid line such that
the sensing elements are exposed to the fluid in the fluid line
when the sensor component is engaged with the fluid line, a
connector configured to connect to one or more optical waveguides,
the sensor component being configured to transmit light between the
one or more optical waveguides and the one or more sensing
elements, and a data storage medium configured to store data
representing information about the sensor component.
[0026] Storing information about the sensor component on a data
storage medium is advantageous when the sensor component is
replaceable, e.g. disposable and/or single use. This ensures that
information about parameters specific to each sensor component is
intrinsically linked to the particular sensor component, thereby
reducing the likelihood of errors caused by confusion between
different sensor components.
[0027] In some embodiments, the replaceable sensor component
further comprises an interface circuit configured to transmit
signals between the data storage medium and the system. This can
simplify the retrieval or storage of information in the data
storage medium. In some embodiments, the interface circuit is
configured to transmit the signals wirelessly. This can improve
ease of use by eliminating the need for physical connection to the
data storage medium.
[0028] In some embodiments, the information comprises one or more
characteristics of the one or more sensing elements. The
characteristics of the sensing elements may vary between
manufacturing batches, so storing their characteristics on the data
storage medium ensures accurate characteristics for each sensor
component are easily available.
[0029] In some embodiments, the information comprises calibration
information about the variation of the optical property of the
sensing elements with one or both of the concentration of the one
or more analytes and the temperature of the sensing elements. In
this case, storing the information on the data storage medium of
the sensor component improves the accuracy of determinations of the
concentration of the analytes by ensuring calibration information
specific to each sensor component is available.
[0030] In some embodiments, the information comprises one or more
characteristics of a calibration fluid used for calibration of the
replaceable sensor component. This simplifies the calibration
process when particular calibration fluids are used as
characteristics do not have to be provided by the user. In some
embodiments, the information comprises a variation of the pH of the
calibration fluid with temperature. This allows the accuracy of
calibration to be further improved depending on the conditions at
the time of calibration.
[0031] In some embodiments, the information comprises an identifier
of a patient with whom the replaceable sensor component is
associated. This can prevent the sensor component being reused with
multiple patients, that could lead to cross-contamination or other
problems. In some embodiments, the data storage medium is
configured to receive the identifier during initialisation of the
system, and store the identifier such that the replaceable sensor
component is permanently associated with the patient. Permanently
storing the patient identifier further helps to prevent reuse of
the sensor component with multiple patients.
[0032] In some embodiments, the information comprises an identifier
of the system with which the replaceable sensor component is
associated. The behaviour or performance of the sensor component
may vary depending on the system to which it is connected, and so
reusing a sensor component with a different system may cause
calibration to become inaccurate. Storing a system identifier can
help to prevent or identify when this occurs.
[0033] In some embodiments, the information comprises one or more
of an indication of prior use of the replaceable sensor component,
an indication of whether damage has occurred to the replaceable
sensor component, a use-by date after which the replaceable sensor
component should not be used, an in-use lifetime of the replaceable
sensor component, a length of time for which the replaceable sensor
component has been used, a unique identifier of the replaceable
sensor component, a date of manufacture of the replaceable sensor
component, a time at which the replaceable sensor component was
last calibrated, and an indication of the number and/or type of
errors that have occurred during use of the replaceable sensor
component. All of these parameters can be used to assess the
validity of the calibration of the sensor component, and whether
its continued use is advisable, thereby improving the accuracy of
readings obtained and patient safety. Knowledge of some of these
parameters may also be required for regulatory compliance, and so
storing them on the sensor component itself reduces the burden on
the user of recording the information elsewhere.
[0034] The following additional features may be combined with any
of the three aspects of the invention discussed above.
[0035] In some embodiments, the sensor component further comprises
a fluid-permeable support element for supporting the sensing
elements arranged to be between the sensing elements and the fluid
in the fluid line when the sensor component is engaged with the
fluid line. This can protect the sensing elements from mechanical
effects due to, for example, pulsatile flow of fluid through the
fluid line, that could affect their position and thereby the
measurements of their optical property. In some embodiments, the
permeable support element comprises a mesh. This is a particularly
straightforward way of providing a fluid-permeable support element
that is easy to manufacture and assemble.
[0036] In some embodiments, the sensing elements comprise a
membrane permeable to at least one of the analytes configured to be
exposed to the fluid in the fluid line when the sensor component is
engaged with the fluid line. This can provide analyte-specific
permeability such that each sensing element is not exposed to other
components of the fluid that may affect its interaction with the
target analyte.
[0037] In some embodiments, the sensor component comprises two or
more sensing elements. This allows sensing of multiple analytes
simultaneously in a single sensor component, thereby reducing the
number of components required to monitor the concentration of
analytes in the fluid.
[0038] In some embodiments, the sensor component further comprises
a light-absorbing element positioned between the sensing elements.
This prevents cross stimulation of sensing elements due to light
directed to other sensing elements, thereby reducing possible
sources of error.
[0039] In some embodiments, the connector comprises a transparent
optical element in respect of the or each sensing element
configured to transfer light between the sensing element and the
one or more optical waveguides. This can protect the sensing
elements from mechanical or chemical damage when the sensor
component is not connected to the one or more optical waveguides.
In some embodiments, the transparent optical element comprises a
waveguide. This reduces light loss through the transparent optical
component.
[0040] In some embodiments, the sensor component further comprises
a temperature sensor arranged to sense the temperature of the one
or more sensing elements. This can allow for adjustment of
measurements to account for changes in the optical property or its
dependence on the concentration of the analyte as a function of
temperature. In some embodiments, the temperature-sensitive element
is a thermistor or a thermocouple. These are readily available and
well-understood components for measuring temperature.
[0041] In some embodiments, the sensor component is configured to
engage with a wall of the fluid line. This provides a convenient
engagement mechanism, and minimises the size of the sensor
component.
[0042] In some embodiments, the sensor component further comprises
a conduit, the sensing elements being exposed to fluid in the
conduit and the conduit is configured to be inserted into the fluid
line for engagement of the sensor component with the fluid line.
This may be more convenient in some situations according to the
configuration of the fluid line with which the sensor component
engages.
[0043] In some embodiments, the conduit is configured to be
inserted into the fluid line in an in-line configuration. This
minimises disruption to the flow of fluid in the fluid line.
[0044] In some embodiments, the conduit is configured to be
inserted into the fluid line in a shunt configuration. This allows
the sensor component to be attached and removed without
interrupting the flow of fluid in the fluid line.
DRAWINGS
[0045] Embodiments of the present invention will now be described
by way of non-limitative example with reference to the accompanying
drawings, in which:
[0046] FIG. 1 is an isometric view of a sensor component prior to
connection to the one or more waveguides and engagement with the
fluid line;
[0047] FIG. 2 is a cross-sectional view of the sensor component of
FIG. 1 engaged with the fluid line and connected to the one or more
waveguides;
[0048] FIG. 3 is an exploded isometric view of the sensor component
of FIGS. 1 and 2;
[0049] FIG. 4 is an exploded view showing further elements of a
sensor component;
[0050] FIG. 5 is an isometric view of the sensor component of FIGS.
1 to 3 engaged with the fluid line;
[0051] FIG. 6 shows an embodiment of a sensor component comprising
a conduit;
[0052] FIG. 7 shows an exploded view of part of the sensor
component of FIG. 6;
[0053] and
[0054] FIG. 8 shows the sensor component of FIG. 6 engaged with the
fluid line in a shunt configuration.
DETAILED DESCRIPTION
[0055] As mentioned above, the present invention has three aspects,
which all relate to improving the accuracy of measurements using
the sensor component, and aiding in detecting and reducing drift in
the measurements when the sensor component is used continuously for
extended periods of use. The features that differ between the three
aspects are respectively the sampling port, the reflective element,
and the data storage medium. Each of these features will be
discussed in further detail below. In the embodiments discussed
herein, all three of these features are provided simultaneously in
the same sensor component. This provides the maximum benefit from
the combination of all of the features. However, it should be
understood that it is not necessary to provide these three features
in combination, and that it would equally be possible to provide a
sensor component with any one of the three features in isolation,
as represented by the three aspects mentioned above. It would also
be possible to provide a sensor component having any combination of
two of the three features, and doing so would still provide
corresponding benefits and advantages.
[0056] FIG. 1 shows a sensor component 1 for use in a system for
measuring the concentration of one or more analytes in fluid in a
fluid line 3. The system is preferably a system for use in clinical
contexts, for example being part of ECMO, CPB, or CRRT machines as
mentioned above. In such cases, the fluid in the fluid line 3 is
blood of a patient. However, this is not essential, and the sensor
component 1 may also be used in other contexts, for example
monitoring of analyte concentrations in gases. Analytes measured by
the system using the sensor component 1 may include oxygen, carbon
dioxide, hydrogen ions (i.e. pH), potassium, sodium, calcium,
magnesium, ammonia, nitric oxide, or anaesthetic gases.
[0057] The sensor component 1 comprises a black plastic
construction. Plastic can be readily manufactured to the desired
specifications and can also be sterilised for use in clinical
contexts. However, the use of plastic is not essential, and other
suitable materials, for example resin or metal, may be used. The
black colour of the sensor component 1 aids in eliminating optical
cross-talk between sensing elements 5. However, in general the
sensor component 1 may have any colour. Preferably, when used in
blood contacting medical devices, the material of the sensor
component 1 is biocompatible and non-leaching to prevent
contamination of a patient's blood. When used for continuous
monitoring application, consideration should also be given to
potential inadvertent changes to the properties of the sensor
component 1 (in particular the sensing elements 5 and optical
parts) post manufacture during shelf life, particularly if the
eventual calibration prior to use depends upon constants determined
during manufacture. Therefore, materials with chemical and optical
properties that are stable over time are preferred. Care should
also be taken to ensure that extraneous materials generated during
manufacture or sterilization do not negatively impact the drift of
the sensor measurements during use and resulting in
inaccuracies.
[0058] The sensor component 1 is preferably provided to the user
packaged so as to be sterile and hydrated with a buffer/calibration
solution that has a known or predetermined concentration of the
analytes that are to be detected. In the case of some analytes
(such as any of the exemplary analytes mentioned above except for
hydrogen ions/pH) the predetermined concentration may preferably be
zero in some embodiments. When oxygen and/or carbon dioxide are to
be detected, their concentrations can be brought to zero by virtue
of scavenger materials enclosed in the packaging with the sensor
component 1. The buffer/calibration solution provides the first of
two calibration points. In some embodiments, the hydration and
sterility of the blood contacting surface of the sensor component 1
is maintained by an aluminium removable tab.
[0059] The sensor component 1 comprises one or more sensing
elements 5. The sensor components shown herein comprise four
sensing elements, but this is not essential, and other embodiments
may comprise one, two, three, or more than four sensing elements 5.
The sensing elements 5 each comprise a luminescent compound,
preferably a fluorescent compound, more preferably a fluorescent
organic dye. The luminescent compound may be different for
different sensing elements 5, and will depend on the analytes to be
measured. Examples of suitable luminescent compounds include
seminaphtharhodafluor (SNARF), mag-fluo-4, and derivatives thereof.
The sensing element may comprise the luminescent compound suspended
in, dissolved in, or molecularly bonded to a matrix. The matrix may
comprise a polymer, for example PMMA or polystyrene. Alternatively,
the matrix may comprise a sol-gel or hydrogel.
[0060] Fluorescent optical continuous monitoring sensors may suffer
from the photobleaching of the fluorophore of the fluorescent
compound, resulting in an effective loss in the concentration of
the fluorescent compound in the sensing element 5. This can
introduce drift in measurements over time. Photobleaching is
typically the result of a portion of the fluorophore molecules
being excited to a reactive triplet state, which can then react
with materials in the local environment to generate non-fluorescent
molecules. The fluorescent compounds are preferably chosen to be
robust so as to minimize photobleaching of the fluorophores.
Another method to minimise photobleaching is to optimise the
intensity of light used to stimulate the luminescent compounds, and
optimise the work cycle of the incident exciting light. For
instance, if a data point is required every 15 seconds to produce a
continuous trend of analyte concentration, then the light source
may be "turned on" for just 10 milliseconds every 15 seconds, so is
only on for 0.07% of the time. In the example of a 4-hour CPB, the
fluorophore is only excited for a total of 10 seconds.
[0061] The sensing element 5 has an optical property that varies
with the concentration of the one or more analytes in the fluid.
The optical property may be emission or absorption of light. In the
case where the sensing element 5 comprises a luminescent compound,
the optical property may be a luminescence lifetime. The optical
property may be the same for all of the sensing elements 5, or may
differ between sensing elements 5. Various measurement modalities
may be used to minimize drift in sensors. Fluorescent lifetime and
ratiometric modalities are commonly used when available, as these
are less vulnerable to common sources of error that can cause
drift. Ratiometric modalities take two measurements of light from
the luminescent compound, for example at different wavelengths, and
calculate a ratio. However, often straight intensity measurements
methods are the only modalities available, and therefore it is
important that aspects of the design of the sensor component 1 are
chosen to minimize drift and inaccuracies.
[0062] The sensor component 1 is configured to engage with the
fluid line 3 such that the sensing elements 5 are exposed to the
fluid in the fluid line 3. As shown in FIG. 2, the sensor component
1 engages with the fluid line 3 with the sensing element 5 exposed
to the interior of the fluid line 3 such that fluid flowing through
the fluid line 3 past the sensor component 1 will come into contact
with the parts of the sensor component 1 facing the interior of the
fluid line 3.
[0063] As shown in FIG. 3, two of the sensing elements 5 comprise a
membrane 21 permeable to at least one of the analytes configured to
be exposed to the fluid in the fluid line 3 when the sensor
component 1 is engaged with the fluid line 3. Membranes 21 may in
general be provided in respect of any or all of the sensing
elements 5. The membrane 21 is permeable to at least the analyte
sensed by the sensing element 5 in respect of which the membrane 21
is provided. The provision of a membrane 21 can ensure greater
specificity by ensuring that the sensing element 5 is not affected
by interaction with analytes other than the one it is intended to
sense, and preventing interaction of the sensing element 5 with
other components of the fluid that may affect the optical property
of the sensing element 5. For example, the membrane 21 may prevent
large, biological molecules such as proteins, or blood cells from
interacting with the sensing elements 5. The membrane 21 may be a
hydrophobic gas permeable membrane if the analyte is O.sub.2,
CO.sub.2, NO, NH.sub.3, or anaesthetic gases. If the analyte is
soluble in water or blood plasma, the membrane 21 may be a
hydrophilic membrane, for example a hydrogel. Suitable membranes
may include microporous or dialysis membranes.
[0064] As shown in FIG. 4, the sensor component 1 comprises a
fluid-permeable support element 19 for supporting the sensing
elements 5 arranged to be between the sensing elements 5 and the
fluid in the fluid line 3 when the sensor component 1 is engaged
with the fluid line 3. The fluid-permeable support element 19
provides mechanical support and protection to the sensing elements
5. This can be advantageous where the flow of fluid in the fluid
line 3 is highly pulsatile, for example in CPB or ECMO machines,
and has significant pressure fluctuations. Without the
fluid-permeable support element 19, such fluctuations could cause
small movements or deformations of the sensing element 5, changing
the optical path length through the sensing element and affect
measurements of its optical property and introducing error. The
permeable support element 19 preferably comprises a mesh, for
example a stainless steel or plastic mesh. Permeable support
elements 19 are provided for each sensing element 5 in the figures,
but this may not be necessary depending on the mechanical
properties of the individual sensing elements 5.
[0065] FIG. 4 also shows that the sensor component 1 further
comprises a light-absorbing element 23 positioned between the
sensing elements 5. This may be provided in any embodiment where
the sensor component 1 comprises two or more sensing elements 5.
The light-absorbing element 23 prevents optical cross-talk between
the sensing element 5 that could introduce errors in measurements
of their optical properties.
[0066] The sensor component 1 comprises a connector 7 configured to
connect to one or more optical waveguides. In the embodiments shown
in the figures, the optical waveguides are comprised by an
opto-electrical interface 9, and the connector 7 comprises a recess
in the sensor component 1 that engages the interface 9. However, in
general the connector 7 may take any suitable form, and may
comprise retention elements such as clips or screws to prevent
movement of the one or more optical waveguides relative to the
connector 7.
[0067] The optical waveguides allow light to be transmitted to and
from one or more light sources elsewhere in the system in which the
sensor component 1 is used. In the embodiments shown in the
figures, light is transmitted along the interface 9 through the
optical waveguides from the one or more light sources. Suitable
light sources include LEDs or laser diodes. The opto-electrical
interface 9 is generally non-disposable, and connects the sensor
component 1 to the system for measuring analyte concentration, for
example a Patient Data Module (PDM). The optical waveguides may
comprise optical fibres or optical fibre bundles to transmit the
excitation light to the sensing elements 5. Light emitted from (or
transmitted through) the sensing elements 5 is also returned via
the optical waveguides in the interface 9 to detectors in the PDM
that detect the intensity of light from the sensing elements 5. The
optical waveguides are butted against the transparent optical
elements 25. The interface 9 also provides a means for electrical
connection to the temperature sensor 27 and data storage medium
15.
[0068] In other embodiments, the sensor component 1 may comprise
the one or more optical waveguides and/or the one or more light
sources and detectors, and the interface 9 may provide only
electrical connection to other parts of the system, or may be
entirely absent. The sensor component 1 is configured to transmit
light between the one or more optical waveguides and the one or
more sensing elements 5, such that the optical property of the
sensing elements 5 can be measured.
[0069] As shown in FIG. 2, the connector 7 comprises a transparent
optical element 25 in respect of the or each sensing element 5
configured to transfer light between the sensing element 5 and the
one or more optical waveguides. This provides protection for the
sensing elements 5 to prevent physical or chemical damage,
particularly when the interface cable 9 is not connected. The
transparent optical element 25 may itself comprise a waveguide to
ensure optimal transmission of light to and from the sensing
elements 5. The transparent optical elements 25 act as optical
windows that interface the optical waveguides with the sensing
elements 5.
[0070] As shown in FIGS. 1 to 3, the sensor component 1 comprises a
temperature sensor 27 arranged to sense the temperature of the one
or more sensing elements 5. The optical property of the sensing
elements 5, and/or its dependence on the concentration of analyte,
may vary depending on the temperature of the sensing element 5.
Therefore, knowing the temperature of the sensing elements 5 can
improve the accuracy of determination of the concentration of
analyte in the fluid. Preferably, the temperature-sensitive element
27 is a thermistor or a thermocouple. The sensor component 1 may
comprise one or more electrical contacts, or contact wells, to
permit electrical connection to the temperature-sensitive element
27 for the purposes of measuring temperature. The temperature
sensor 27 may also be used for measuring blood temperature.
[0071] There are two main options for placing the sensor component
1 into the fluid line 3, which may be an extracorporeal blood line.
The sensor component 1 may be placed into the main fluid line 3
itself, or as a shunt in a line peripheral to the main fluid line
3. The sensor component of FIGS. 1 to 5 is configured to engage
with a wall of the fluid line 3. This embodiment is therefore
engaged with the main fluid line 3.
[0072] In some embodiments, the sensor component 1 further
comprises a conduit 29, the sensing elements 5 being exposed to
fluid in the conduit 29. The conduit 29 may comprise a section of
the same type of tubing as the fluid 3, for example the section of
tubing with which the sensor component 1 engages in FIG. 5. The
conduit 29 may be configured to be inserted into the fluid line 3
for engagement of the sensor component with the fluid line 3. As
mentioned above, the conduit 29 may be configured to be inserted
into the fluid line 3 in an in-line configuration, or in a shunt
configuration (also referred to as a bypass configuration).
[0073] Accurate calibration pre-use is important to achieve
subsequent accurate monitoring. Point of use calibration should
also be made as simple for the user as possible, if not totally
invisible. The sensor component 1 described herein uses a
calibration method requiring two calibration measurements at the
point of use.
[0074] For an embodiment such as shown in FIG. 5, where the sensor
component 1 engages with the main line of the fluid line 3, the
setup process for use of the sensor component 1 may be as
follows.
[0075] Prior to engagement of the sensor component 1 with the fluid
line 3, the sensor component 1 is provided hydrated (with the
sensing element 5 exposed to a buffered solution) and sterile
within a separate pack. The in-line connector 33 with which the
sensor component 1 engages is already integrated into the fluid
line 3, with a protective cap/plug in place covering the aperture
31. Where the sensor component 1 comprise a conduit 29, the in-line
connector 33 may function as the conduit 29. The sensor component 1
is then connected to the interface 9, which provides via the one or
more optical waveguides, the transmitted light at an appropriate
wavelength for measuring the optical property of the respective
sensing elements 5. The one or more optical waveguides also allow
for the return of light back to detectors in the system. The
buffered solution in the packaging of the sensor component 1
preferably contains a known or predetermined concentration of the
analytes to be measured.
[0076] The system automatically measures a first calibration point
for the one or more analytes when the sensor component 1 is
connected to the interface, this process being invisible to the
user, thereby improving ease of use. The hydration/buffer solution,
which may be trapped behind an aluminium foil layer, acts as a
first calibration solution.
[0077] The in-line connector 33 is filled with a priming fluid,
which may have known concentrations of the one or more analytes
different to the concentration in the buffered solution. The
protective cap/plug is removed from the aperture 31 at the same
time as a protective covering is removed from the sensing element
5, and the sensor component 1 is immediately attached to the
in-line connector 33. The priming fluid is displaced by the fluid
(e.g. blood).
[0078] Once the sensor component 1 is placed in-line and the
sensing elements 5 are in contact with the fluid, a fluid sample is
taken adjacent to the sensor component 1 either upstream or
downstream in the fluid line 3. The concentration of the one or
more analytes in the sample are measured on an approved external
analyser such as a blood-gas analyser. The data are fed back into
the system to provide a second calibration point.
[0079] An alternative to the sensor component 1 engaging with the
main line of the fluid line 3 as just described is a shunt system
that by-passes the mainline. In such embodiment, the sensor
component 1 comprises a conduit 29, as shown in FIG. 6, integrated
into one sterile device. The conduit 29 is then configured to be
inserted into the fluid line 3 in a shunt configuration, also
referred to as a shunt configuration. The conduit 29 may be joined
to the rest of the sensor component 1 by any suitable means, for
example by ultrasound welding.
[0080] For an embodiment such as shown in FIG. 6, where the sensor
component 1 engages with the fluid line 3 in a shunt configuration,
the setup process for use of the sensor component 1 may be as
follows.
[0081] As for the in-line configuration described above, the sensor
component 1 is provided hydrated and sterile within a separate
pack. However, in this embodiment, the sensor component 1 also
comprises the conduit 29. The sensor component is packaged with
attached shunt tubing 35 and shunt taps 37 which are also sterile.
The main fluid line 3 is provided with taps 39 to receive the shunt
tubing 35. The main fluid line 3 will be sterilised with the
extracorporeal tubing set. As for the in-line configuration, the
sensor component 1 is connected to the interface 9 to measure a
first calibration point.
[0082] Protective caps are then removed from the shunt taps 37, and
the shunt taps 37 connected to the taps 39 in the main fluid line.
The shunt taps 37 are then opened, and fluid flows through the
shunt tubing and through the conduit 29. The sensing elements 5 are
exposed to the fluid in the conduit 29. Finally, as for the in-line
configuration, a sample of fluid is taken and used to provide a
second calibration point.
[0083] A large number of factors can influence the accuracy and
drift in measurements of the optical property of the sensing
elements 5. Where the sensing elements 5 comprise a fluorescent
compound, fluorescence emission F.sub.1 is given by
F.sub.1=I.sub.0(2.303.epsilon.cl).phi.
where:
[0084] I.sub.0 is the intensity of light entering the sensing
element 5. This light originates from the one or more light sources
and is transmitted to the sensing elements 5 via the one or more
optical waveguides;
[0085] .epsilon. is the absorptivity or molar attenuation
coefficient and is constant for a given fluorescent compound. It is
defined as the light absorbed by a 1 molar concentration of the
detecting fluorescent compound with a path length of 1 cm;
[0086] c is the concentration of the absorbing species, which in
this example is the fluorescent compound;
[0087] l is the optical path length between the light source and
detector which contains the fluorescent compound;
[0088] .phi. is the quantum efficiency and is a measure of a change
of energy when a molecule is excited to a high energy level and
then drops to a lower energy with the emission of fluorescence.
[0089] All of these parameters are accounted for and kept constant
during the process of calibration. Changes in their values post
calibration, either continuously or suddenly by damage to the
sensor component 1, will result in inaccuracy in the measurements
of analyte concentration either through gradual drift or a more
sudden change in signal. The design of continuous sensor components
should keep these parameters constant as far as possible. It is
likely that only low-level drift will be encountered after all of
the means of reducing drift have been applied to the sensor
component 1 design. Therefore inaccuracies will only become
significant over long periods of time. This is unlikely to be a
problem for CPD, but is possible for lengthy dialysis and ECMO
treatment.
[0090] As discussed above, to provide a second calibration point, a
sample of fluid is required. The first aspect of the invention
concerns a sampling port 11 as shown in FIGS. 2 and 3. The sampling
port 11 is configured to provide fluidic access to the fluid line 3
when the sensor component 1 is engaged with the fluid line 3, as
shown in FIG. 2.
[0091] The provision of the sampling port 11 integrated into the
sensor component 1 ensures that the fluid sample is taken from the
fluid flowing over the sensing elements 5 close to or as near as
possible to the sensing elements 5, to ensure accuracy of
calibration. In some embodiments, the sampling port 11 is at most
20 cm, preferably at most 10 cm, more preferably at most 5 cm from
the one or more sensing elements 5.
[0092] Proximity of the sampling port 11 to the sensing elements 5
is advantageous because a fluid sample taken distant from the
sensing elements 5 may have analyte concentrations different from
those measured by the sensing elements 5 at the time the fluid
sample is taken. For example, this can occur due to metabolism of
the analyte. This will mean the concentration in the fluid sample
is not representative of the concentration measured by the sensing
elements 5, causing a calibration error and subsequent measurement
errors. Samples may be taken through the sampling port 11 in any
suitable manner, for example using a syringe.
[0093] The sampling port 11 comprises a one-way valve configured to
permit one-way fluid flow out of the fluid line 3. This allows
fluid samples to be taken without compromising sterility of the
fluid line 3 or risking any contamination of the fluid in the fluid
line 3. The sampling port 11 comprises a component fitting
configured to engage with an external fitting. This allows for a
secure connection when taking samples. Specifically, the component
fitting comprises a Luer fitting, such that the sampling port 11 is
a Luer-activated sampling port. While a Luer fitting is preferred,
it is not essential, and other suitable types of component fitting
may also be used.
[0094] The sampling port 11 is configured to open upon engagement
of the external fitting with the component fitting. The sampling
port 11 may further be self-closing. This helps to improve ease of
use for the user, and to assure sterility and eliminate leaks of
fluid from the fluid line 3 through the sampling port 11. The
sampling port 11 further comprises a removable cap 13 configured to
seal the sampling port 11, although this is not essential. The
removable cap 13 can protect the sampling port 11 from damage or
contamination if it is not used for extended periods of time.
[0095] The second aspect of the invention concerns a data storage
medium 15 configured to store data representing information about
the sensor component 1. This is particularly useful where the
sensor component 1 is replaceable. For example, where the sensor
component 1 is designed to be disposable and intended for use only
in a single treatment for a single patient. The data storage medium
15 may comprise a microchip.
[0096] To enable the above-mentioned method of calibration that
requires only two calibration points at the point of use, some of
the properties of the sensing elements 5 and the sensor component 1
may be determined during or after manufacture, before the
replaceable sensor component 1 is supplied to the end user. These
properties travel with the replaceable sensor component 1 stored by
memory in the data storage medium 15. This is particularly useful,
for example, if the detection characteristics of the sensor
component 1 vary between manufacturing batches. The data storage
medium 15, ay also store data to ensure that the replaceable sensor
component 1 is in good working order at the time of calibration and
use, and/or data to ensure compliance with various legal and/or
clinical requirements on the replaceable sensor component 1 and its
use.
[0097] In some embodiments, the sensor component 1 may comprise an
interface circuit configured to transmit signals between the data
storage medium 15 and the system. This allows for data to be
accessed from and written to the data storage medium 15. The
interface circuit may provide for optical and/or electrical
transmission of signals to and from the data storage medium 15. It
may alternatively be configured to transmit the signals wirelessly,
in which case the interface circuit may comprise an antenna. In
some embodiments, the data storage medium 15 may comprise the
interface circuit. In other embodiments, the data storage medium
may not require an interface circuit for data stored to be accessed
or modified, and may merely comprise electrical contacts for an
external connection, for example via the interface 9.
[0098] The data storage medium 15 may be read-only in respect of
some or all of the information stored by the data storage medium
15. For example the information determined at the time of
manufacture may not be modifiable by the end user. Other types of
information may be modifiable or settable by the end user. The data
storage medium 15 may be configured such that some or all of the
information can be set only once by the end user, and is not
subsequently modifiable.
[0099] The information comprises one or more characteristics of the
one or more sensing elements 5. In particular, the information may
comprise calibration information about the variation of the optical
property of the sensing elements 5 with one or both of the
concentration of the one or more analytes and the temperature of
the sensing elements 5.
[0100] The information may further comprise one or more
characteristics of a calibration fluid used for calibration of the
replaceable sensor component 1. The calibration fluid may comprise
the buffer solution mentioned above. For example, the information
may comprise a variation of the pH of the calibration fluid with
temperature.
[0101] This will improve the accuracy with which the calibration
can be determined from the two calibration points.
[0102] The information may also comprise information concerning the
use of the replaceable sensor component 1. For example, the
information may comprise an identifier of a patient with whom the
replaceable sensor component 1 is associated. This information may
be used to prevent reuse of the sensor component 1. The data
storage medium 15 may be configured to receive the identifier
during initialisation of the system, and store the identifier such
that the replaceable sensor component 1 is permanently associated
with the patient. As mentioned above, this may be achieved by
allowing the information regarding the patient identifier to be set
only once in the data storage medium 15. Similarly, the information
may comprise an identifier of the system with which the replaceable
sensor component 1 is associated. This can be used to prevent reuse
of the sensor component 1. Storing an identifier of the system can
also reduce inaccuracies in the determined concentrations of the
analytes, as the properties of the light sources and detectors used
to measure the optical property of the sensing elements 5 may vary
between systems. As for the patient identifier, the system
identifier may also be set permanently during initialisation of the
system.
[0103] Other information that may be stored by the data storage
medium 15 may include: [0104] an indication of prior use of the
replaceable sensor component 1; [0105] an indication of whether
damage has occurred to the replaceable sensor component 1; [0106] a
use-by date after which the replaceable sensor component 1 should
not be used; [0107] an in-use lifetime of the replaceable sensor
component 1, i.e. a maximum length of time for which the sensor
component 1 should be used, for example 128 hours; [0108] a length
of time for which the replaceable sensor component has been used;
[0109] a unique identifier of the replaceable sensor component;
[0110] a date of manufacture of the replaceable sensor component;
[0111] a time at which the replaceable sensor component was last
calibrated; and [0112] an indication of the number and/or type of
errors that have occurred during use of the replaceable sensor
component.
[0113] The third aspect of the invention concerns a reflective
element 17 as shown in FIGS. 3 and 7. As mentioned above, one of
the parameters which may affect the measurements of the optical
property of the sensing elements 5 is the optical path length
between the light source in the system and the detector that
detects the light after it has passed through the sensing element
5. This optical path length may be affected by small mechanical
shifts of the optical interfaces, for example between the optical
waveguides and the sensing elements. It may also be affected by
damage to one or more of the optical waveguides.
[0114] To reduce the adverse effect of such errors, the sensor
component 1 comprises a reflective element 17. The sensor component
1 is configured also to transmit the light between the one or more
optical waveguides and the reflective element 17 on a separate
optical path from an optical path between the one or more optical
waveguides and the one or more sensing elements 5. This provides a
reference beam, generating a reference signal at the detector,
against which the light transmitted between the optical waveguides
and the one or more sensing elements 5 can be compared. The
reflective element may be, for example, a reflecting mirror.
[0115] The optical path between the one or more optical waveguides
and the reflective element 17 is preferably a replicate of the
optical paths between the one or more optical waveguides and the
sensing elements 5, with the exception that the optical path
between the one or more optical waveguides and the reflective
element 17 does not pass through a sensing element 5. The light
reflected by the reflective element 17 is therefore not altered by
analyte concentration. This means that a ratio can be calculated
between the signals obtained from the sensing elements 5 and the
reference signal during calibration and during continuous
measurements. Thereby, a measurement is obtained in which the
optical path length in the equation above is cancelled out, and the
use of the ratio ensures that any inaccuracies generated by small
mechanical shifts in optical interfaces do not affect the
measurement. Further, if the same light source is used for
measuring the optical property of the sensing element 5 as is used
for the reference beam, the initial intensity I.sub.0 is also
cancelled out in the equation above, and so the generation of light
losses through minor damage to optical fibres (which causes a
change of I.sub.0) will also not affect the measurement.
[0116] The reference beam must be distinguishable from the light
which has passed through the one or more sensing elements 5 in
order that the ratio can be calculated. The reference beam may be
optically distinguishable, e.g. by having a different wavelength to
the light passing through the sensing elements, and/or may be
physically distinguished by travelling along a separate optical
path between the light source, the reflective element 17, and the
detector from the optical path between the light source, sensing
elements 5 and detector. Therefore, if the same light source is
used for measuring the optical property of the sensing element 5 as
is used for the reference beam, the reference beam must be
physically distinguished from the light that passes through the
sensing element 5.
[0117] The use of a ratio can normally only accommodate minor
damage or shifts to optical interfaces post-calibration while still
allowing the sensor component 1 to operate normally. Catastrophic
damage will result in a sudden change in signal (in either or both
of the reference signal or the signal from the sensing elements 5)
which will be recognized by the system as not being due to a
physiological change. In this case, the system will generate a
warning to alert the user that the data is suspect and should be
received with caution.
[0118] It is not essential that the optical path between the one or
more optical waveguides and the reflective element 17 is the same
as the optical paths between the one or more optical waveguides and
the sensing elements 5 to obtain an advantage. For example, where
the same light source is used for measuring the optical property of
the sensing element 5 as is used for the reference beam, a ratio of
the reference signal and the signal from the sensing elements 5 can
still reduce the effect of changes in the initial intensity
I.sub.0. In addition, for a sensor component 1 with multiple
sensing elements 5, the presence of the reference beam makes it
possible to distinguish where a sudden large change in signal from
a sensing element 5 is due to failure of the sensing element 5 or
from other causes.
[0119] In the embodiments shown in the figures, the reflective
element 17 is concave, i.e. a concave reflector. This is preferred
to improve the collection of light reflected by the reflective
element 17. However, it is not essential, and in other embodiments,
the reflective element 17 may be planar.
[0120] The amount of light (i.e. as a proportion of the amount of
light incident on the reflective element 17) reflected by the
reflective element 17 is not important to the function of the
reflective element 17, as long as the proportion reflected is
consistent over time. However, reflection of a larger proportion of
the incident light improves signal to noise at the detector, and
makes the effect of noise on the ratio measurement less
significant. Therefore, it is preferable that the reflective
element 17 is configured to reflect at least 10%, preferably at
least 25%, more preferably at least 50%, of light incident thereon
back to the one or more waveguides.
* * * * *