Abstract:
A device for detection or measurement of a carbohydrate analyte in fluid comprises:
       an optical sensor comprising components of an assay for carbohydrate analyte, the readout of which is a detectable or measurable optical signal, and a light guide having a distal portion optically coupled to the assay components and a proximal portion; and   a reader for interrogating the optical sensor, the reader comprising an assay interrogating system including a lens; and   an interface portion forming part of at least one of the optical sensor and the reader, the interface portion being capable of removably constraining the proximal portion of the light guide and the lens of the assay interrogating system in an optically coupled arrangement.       
 
     The device may be combined with an insulin-infusion system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under Section 119( e ) from U.S. Provisional Application Ser. No. 61/895,783, filed Oct. 25, 2013, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to devices for detection and measurement of carbohydrate analytes e.g. glucose. Further aspects of the invention relate to components of such devices; to systems including such devices including closed-loop insulin-infusion systems; and to methods of making and using such devices, components and systems. 
       BACKGROUND 
       [0003]    The pancreas of a normal healthy person produces and releases insulin into the blood stream in response to elevated blood plasma glucose levels. Beta cells (β-cells), which reside in the pancreas, produce and secrete the insulin into the blood stream, as it is needed. If β-cells become incapacitated or produce insufficient quantities of insulin, then insulin must be provided to the body from another source. 
         [0004]    Traditionally, since insulin cannot be taken orally, insulin has been injected with a syringe. More recently, the use of infusion pump therapy has been increasing, especially for delivering insulin for diabetics. For example, external infusion pumps are worn on a belt, in a pocket, or the like, and deliver insulin into the body via an infusion tube with a percutaneous needle or a cannula placed in the subcutaneous tissue. Physicians have recognized that continuous infusion provides greater control of a diabetic&#39;s condition, and are increasingly prescribing it for patients. 
         [0005]    Infusion pump devices and systems are relatively well-known in the medical arts for use in delivering or dispensing a prescribed medication, such as insulin, to a patient. In one form, such devices comprise a relatively compact pump housing adapted to receive a syringe or reservoir carrying a prescribed medication for administration to the patient through infusion tubing and an associated catheter or infusion set. Programmable controls can operate the infusion pump continuously or at periodic intervals to obtain a closely controlled and accurate delivery of the medication over an extended period of time. Such infusion pumps are used to administer insulin and other medications. 
         [0006]    There is a baseline insulin need for each body which, in diabetic individuals, may generally be maintained by administration of a basal amount of insulin to the patient on a continual, or continuous, basis using infusion pumps. However, when additional glucose (i.e., beyond the basal level) appears in a diabetic individual&#39;s body, such as, for example, when the individual consumes a meal, the amount and timing of the insulin to be administered must be determined so as to adequately account for the additional glucose while, at the same time, avoiding infusion of too much insulin. Typically, a bolus amount of insulin is administered to compensate for meals (i.e., meal bolus). It is common for diabetics to determine the amount of insulin that they may need to cover an anticipated meal based on carbohydrate content of the meal. 
         [0007]    Over the years, a variety of glucose sensors have been developed for use in obtaining an indication of blood glucose levels in a diabetic patient. Such readings are useful in monitoring and/or adjusting a treatment regimen which typically includes the regular administration of insulin to the patient. 
         [0008]    It has been observed that the concentration of analytes in subcutaneous or interstitial fluid correlates with the concentration of said analytes in the blood, and consequently there have been several reports of the use of glucose sensors which are sited in a subcutaneous location. Such sensors may pass through the skin or may be remotely interrogated. Sensors which pass through the skin may include a base component which remains attached to the user&#39;s body, and a removable reader component used to obtain a reading from the sensor. 
         [0009]    Several types of technology are available, with two of the most common and developed being electrochemical sensing and optical sensing. These types of sensor may be combined in an orthogonally redundant system as described in WO2013/036943. 
         [0010]    Small and flexible electrochemical sensors, for example those constructed in accordance with thin film mask techniques, can be used to obtain periodic readings over an extended period of time. 
         [0011]    Mansouri and Schultz (Biotechnology 1984; 2: pp. 885-890), Meadows and Schultz (Anal. Chim. Acta. 1993 280: pp. 21-30) and U.S. Pat. No. 4,344,438 all describe devices for the in situ monitoring of low molecular weight compounds in the blood by optical means. These devices are designed to be inserted into a blood vessel or placed subcutaneously with optical fiber connections to an external light source and an external detector. 
         [0012]    One form of optical sensing makes use of a proximity-based signal generating/modulating moiety pair (discussed in U.S. Pat. No. 6,232,120), which is typically an energy transfer donor-acceptor pair (comprising an energy donor moiety and an energy acceptor moiety). The energy donor moiety is photoluminescent (usually fluorescent). 
         [0013]    In such methods, an energy transfer donor-acceptor pair is brought into contact with the sample (such as subcutaneous fluid) to be analyzed. The sample is then illuminated and the resultant emission detected. One moiety of the donor-acceptor pair is bound to a receptor carrier (for example a carbohydrate binding molecule), while the other moiety of the donor-acceptor pair (bound to a ligand carrier, for example a carbohydrate analog) and any analyte (for example carbohydrate) present compete for binding sites on the receptor carrier. Energy transfer occurs between the donors and the acceptors when they are brought together. 
         [0014]    An example of such donor-acceptor energy transfer is fluorescence resonance energy transfer (Förster resonance energy transfer, FRET), which is non-radiative transfer of the excited-state energy from the initially excited donor (D) to an acceptor (A). 
         [0015]    An important characteristic of FRET is that it occurs over distances comparable to the dimensions of biological macromolecules. The distance at which FRET is 50% efficient, called the Förster distance, is typically in the range of 20-60 Å. Förster distances ranging from 20 to 90 Å are convenient for competitive binding studies. 
         [0016]    Energy transfer produces a detectable lifetime change (reduction) of the fluorescence of the energy donor moiety. Also, a proportion of the fluorescent signal emitted by the energy donor moiety is quenched. 
         [0017]    The lifetime change is reduced or even eliminated by the competitive binding of the analyte. Thus, by measuring the apparent luminescence lifetime, for example, by phase-modulation fluorometry or time-resolved fluorometry (see Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, 1983, Chapter 3), the amount of analyte in the sample can be determined. The intensity decay time and phase angles of the donor are expected to increase with increasing analyte concentration. Thus, the FRET mechanism permits interrogation of the equilibrium state optically by illuminating the assay and measuring either the lifetime of the excited state (“lifetime interrogation”), and/or the intensity of the emitted fluorescence from the donor fluorophore (“intensity interrogation”). The latter approach is preferred, as it exposes the assay to 25 times less light than with the lifetime interrogation. 
         [0018]    The FRET mechanism offers several advantages. First, FRET fluorescence lifetime measurements are generally insensitive to the relative position of the sensor and the reader unit as long as they are within optical reach of each other, and are also insensitive to changes in the environment, which helps make the system virtually calibration free. Second, FRET is considered very sensitive if the appropriate donor-acceptor ratio and suitable donor-acceptor geometry are obtained. These principles have been used in glucose sensing by energy transfer. WO91/09312 describes a subcutaneous method and device that employs an affinity assay based on glucose (incorporating an energy transfer donor-acceptor pair) that is interrogated remotely by optical means. Commonly-assigned WO97/19188, WO00/02048, WO02/30275, WO03/006992, WO03/072172, WO05/059037, WO05/064318, WO05/110207, WO06/010604, WO06/061207, WO06/061208, WO07/065653, WO09/024521 and WO09/024522 each describe developments of such methods and devices. 
         [0019]    The above-described optical sensor technology offers several advantages over other available technologies. Optical sensors perform well in both the dermis and the subcutaneous region, which allows the optical sensor to maintain functionality even as the sensor is partially explanted, providing the patient with a measurement until the patient is able to replace the sensor. Due to the non-consuming and stable nature of the assay, the measurement technique is insensitive to bio-fouling. As such, it offers the possibility of one single point calibration throughout the entire lifetime of the sensor. Furthermore, the assay typically contains a reference dye, which remains stable with changing glucose concentrations, but is affected by many non-glucose induced changes. Therefore, it serves as a sensor diagnostic tool for the optical sensor, indicating when the integrity of the membrane has been compromised or the optical connection is misaligned. 
         [0020]    Electrochemical sensors as described above have been applied in a telemetered characteristic monitor system as described, e.g., in commonly-assigned U.S. Pat. No. 6,809,653. 
         [0021]    A characteristic monitoring system of the type described above is of practical use only after it has been calibrated based on the unique characteristics of the individual user. Accordingly, the user is required to calibrate the sensor externally. More specifically, a diabetic patient is required to utilize a finger-stick blood glucose meter reading an average of two to four times per day for the duration that the characteristic monitor system is used. Each time, blood is drawn from the user&#39;s finger and analyzed by the blood glucose meter to provide a real-time blood sugar level for the user. The user then inputs this data into the glucose monitor as the user&#39;s current blood sugar level which is used to calibrate the glucose monitoring system. 
         [0022]    Such external calibrations, however, are disadvantageous for various reasons. For example, blood glucose meters include inherent margins of error and only provide discrete readings at one point in time per use. Moreover, even if completely accurate, blood glucose meters are cumbersome to use (e.g., one should not operate an automobile and take a finger stick meter reading at the same time) and are also susceptible to improper use. Furthermore, there is a cost, not to mention pain and discomfort, associated with each application of the finger stick. Thus, finger stick replacement remains a goal for the next generation of glucose monitoring systems. 
         [0023]    As sensor technology improves, there is greater desire to use the sensor values to control the infusion of insulin in a closed-loop system (i.e., an artificial pancreas system). Specifically, a closed-loop system for diabetes includes a glucose sensor and an insulin infusion pump attached to the patient, wherein the delivery of insulin is automatically administered by the controller of the infusion pump-rather than by the user/patient-based on the sensor&#39;s glucose value readings. The benefits of a closed-loop system are several-fold, including tighter glycemic control during the night when the majority of hypoglycemic events Occur. 
         [0024]    An accurate and reliable sensor has long been identified as a necessity for closed-loop realization. Glucose sensor technology has been evolving in an effort to meet the accuracy required for finger stick replacement and the reliability needed for consistent closed-loop functionality. 
         [0025]    The inventors have found that performance of optical sensors which include base and reader components is very sensitive to the alignment of optical components in these components. 
       SUMMARY OF INVENTION 
       [0026]    In a first aspect, the invention provides a device for the detection or measurement of a carbohydrate analyte in fluid comprising:
       an optical sensor comprising components of an assay for carbohydrate analyte, the readout of which is a detectable or measurable optical signal, and a light guide having a distal portion optically coupled to the assay components and a proximal portion; and   a reader for interrogating the optical sensor, the reader comprising an assay interrogating system including a lens; and   an interface portion forming part of at least one of the optical sensor and the reader, the interface portion being capable of removably constraining the proximal portion of the light guide and the lens of the assay interrogating system in an optically coupled arrangement.       
 
         [0030]    Preferably, the device is suitable for use in vivo. In preferred embodiments, the proximal portion of the light guide is disposed externally to a body of a user and the distal portion of the light guide and the assay components are placed internally in the user&#39;s body. 
         [0031]    Preferably, the proximal portion of the light guide includes a proximal end of the light guide. 
         [0032]    Preferably, when the proximal portion of the light guide and the lens are constrained in the optically coupled arrangement, the proximal end of the light guide is within the optical axis tolerance and/or the focal plane tolerance of the lens. The optical axis tolerance and focal plane tolerance acceptable for good performance are dependent on the fiber diameter, lens parameters and light source intensity. 
         [0033]    Typically, the lens optical axis is parallel to the light guide optical axis as measured at the proximal portion of the light guide. Optical simulations show that a deviation of 8° from parallel allows light transmission of approximately 90% of the maximum light transmission which occurs when the optical axes are parallel. Thus, a deviation of the angles of the optical axes of up to 8° from parallel is acceptable. 
         [0034]    The focal plane tolerance is discussed herein in terms of axes x and y, parallel to the focal plane of the lens and centered on the optical axis of the lens. During use of preferred embodiments of the device, axis x is preferably generally parallel to the skin of the user, and axis y is preferably generally perpendicular to the skin of the user. 
         [0035]    The optical axis tolerance is discussed herein in terms of axis z, normal to the focal plane of the lens and centered on the focal plane of the lens. During use of preferred embodiments of the device, axis z is preferably generally parallel to the skin of the user. 
         [0036]    Axes x, y and z, and the focal plane, focal point and optical axis of the lens are indicated in  FIGS. 7 and 8 . The point where x, y and z are 0 corresponds to the intersection between the optical axis of the light guide and the proximal end of the light guide being exactly at the focal point of the lens i.e. optimal optical coupling. 
         [0037]    Preferably, the relative position of the light guide and lens is constrained within a tolerance range of ±10 to 200 μm in each of the x, y and z directions. 
         [0038]    More preferably, the relative position of the light guide and lens is constrained within a tolerance range of less than or equal to ±125 μm, preferably less than or equal to ±50 μm, in the x and y directions (that is, in preferred embodiments, the optical axis of the light guide is aligned so that it is displaced by a maximum of at most 50 μm in any transverse direction from the optical axis of the lens). 
         [0039]    Preferably, the relative position of the light guide and lens is constrained within a tolerance range of less than or equal to ±125 μm, preferably less than or equal to ±70 μm, in the z direction (that is, in preferred embodiments, the position of the proximal portion of the light guide is constrained so that it is displaced by a maximum of at most 70 μm from the focal point along the optical axis of the lens). Experiments indicate however that a tolerance of 250 μm may be acceptable in the z direction. 
         [0040]    Preferably, the device further comprises a detachable connection between the optical sensor and the reader, the detachable connection being capable of further constraining the proximal portion of the light guide and the lens of the assay interrogating system within the optically coupled arrangement. Thus, the interface portion may constrain the relative position of the light guide and lens in the x, y and z directions, but more preferably the interface portion constrains the relative position of the light guide and lens in the x and y directions, with the relative position in the z direction being constrained by a separate connection of the optical sensor and reader as discussed in more detail below. 
         [0041]    Preferably, the interface portion is a light guide interface portion forming part of the reader as discussed in more detail below. Generally, it is preferred for cost reasons to include constructional features on the reader rather than the optical sensor where possible, as the optical sensor has a shorter useful lifetime. 
         [0042]    However, some or all of the interface portion may form part of the optical sensor. Thus, the optical sensor may include a female portion (for example a collar around the light guide) capable of interacting with a male portion on the reader. For example, a collar around the light guide may interact with a protrusion on the reader. This has the potential advantage of protecting the light guide. 
       Components of Device 
       [0043]    Components of the device include some or all of:
       optical sensor
           base including connectors   light guide   assay compartment   assay components   
           reader
           light guide interface portion   assay interrogating system
               lens   light detector   other optical components including beam splitters, filters and mirrors   optical system housing   lens-retaining insert   
               transmitter and further components   housing including connectors   
               
 
         [0059]    These components are discussed in more detail below. 
       Optical Sensor 
       [0060]    In preferred embodiments, the optical sensor remains in position on/in the user&#39;s body over its lifetime which may for example be up to 7 days. The reader typically remains connected to the sensor throughout the sensor&#39;s lifetime. 
       Base 
       [0061]    Preferably, the proximal portion of the optical sensor light guide (disposed externally to the user&#39;s body) is mounted to a base. 
         [0062]    The base is preferably mounted in use to the user&#39;s skin e.g. using adhesive on its lower surface or overtaping. Taping arrangements used in the ENLITE™ sensor may be applied. 
         [0063]    The base may be formed partially or completely of plastics. 
         [0064]    Preferably, the optical sensor and reader include at least one connection arrangement for mechanical connection, so that the optical sensor and reader can be connected, detached and re-connected. 
         [0065]    More preferably, the optical sensor is adapted to connect to the reader by means of several connection arrangements as explained below. It will be appreciated that where one component of a connection arrangement is described as being part of the optical sensor and another component as being part of the reader, the opposite is also possible. A separate connector component may also be used, for example the further locking component mentioned below. Connection arrangements used in the SOF™ sensor may be applied. 
         [0066]    As part of such a connection arrangement, the optical sensor base preferably includes a projecting portion for engaging a bore of the reader. The projecting portion may include an engaging surface, e.g. one or more O-rings, to facilitate engagement. This may provide a general location for the proximal portion of the light guide, with its specific location being determined by the light guide interface portion discussed below. Preferably the projecting portion locates the proximal end of the light guide within the light guide interface portion of the reader. 
         [0067]    As an additional or alternative part of such a connection arrangement, the optical sensor preferably includes connectors for detachable connection to complementary connectors of the reader. The connectors suitably form part of the base. The connectors of the optical sensor preferably include one or more fasteners or latches, preferably moveable latches, for example flexible clips (e.g. resiliently biased clips), which may interact with fixed connectors of the reader. Preferably, the connectors control the relative position of the light guide and lens in the z direction. 
         [0068]    As an additional or alternative part of such a connection arrangement, the optical sensor and reader may comprise an anti-rotation arrangement (e.g. a keying arrangement) to prevent relative rotation of the optical sensor and reader when mounted to one another. In a preferred embodiment, one or more lugs on the reader engage complementary recesses on the optical sensor base. The lugs/recesses may for example be above and below the projecting portion and/or to each side of the projecting portion. 
         [0069]    Optionally, a further locking component (also referred to herein as a “clip”) inhibits or prevents relative movement of the light guide and lens in the z direction, preferably by occupying space between the optical sensor and reader. The locking component is suitably a separate component and is typically of plastics. 
         [0070]    Relative movement of the lens guide and lens is optionally limited further still by connection of the reader to the optical sensor&#39;s overtaping e.g. via hook and loop fastening, and/or by overtaping of the reader. 
         [0071]    Preferably, the base at least initially includes an injector (also referred to herein as an “insertion device” or “serter”) for positioning the distal end of the light guide in the user&#39;s body. This may be a needle which partially or fully encases the distal portion of the sensor e.g. a needle of C-shaped cross-section as disclosed in WO2013/036493. The insertion device is preferably designed to minimize trauma and maximize patient comfort and consistency of sensor delivery. Insertion devices used in the ENLITE™ sensor may be applied. 
         [0072]    Optionally, the base includes components of an electrochemical sensor. 
       Light Guide 
       [0073]    Preferably, the light guide comprises one or more optical fibers. Preferably, the outer diameter of the light guide (or alternatively of each optical fiber) is in the range of 50 to 600 μm, more preferably in the range of 200 to 300 μm (e.g. 235 to 275 μm). Optical fibers of a low diameter may not capture sufficient light to transmit a good optical signal, whereas optical fibers of a high diameter are potentially painful to the user. 
         [0074]    Preferably, the angle between the longitudinal axes of the proximal and distal ends of the light guide is in the range of 0 to 90°. An angle approaching 90° is preferred for needle insertion and to keep the device height to a minimum (as this allows the distal portion of the light guide to be perpendicular to the user&#39;s skin while the proximal portion is parallel to the user&#39;s skin), but a high angle may cause light guide cladding to crack. An angle of 45° may be used. Preferably, the light guide is flexible. 
         [0075]    Preferably, the proximal end of the light guide includes an end face which is preferably planar and is preferably perpendicular to a longitudinal axis of the light guide. 
         [0076]    The light guide is preferably of plastics. It preferably has a cladding e.g. a cladding of thickness around 10 μm. The light guide should have very low or no absorption of water and other liquids. 
         [0077]    The light guide should transmit light of the wavelengths used for excitation, assay signal and reference with little to no attenuation. 
       Assay Compartment 
       [0078]    Preferably, the assay components are retained in one or more assay compartments. In preferred embodiments an assay compartment is defined by the distal part of the light guide and a material that permits diffusion of the analyte but not the assay components (e.g. an analyte-permeable membrane). Preferred materials are co-polymers having hydrophobic units and hydrophilic units, the hydrophilic units each comprising an ester of polyethylene glycol and a diacid, as disclosed in WO05/1102007. Particularly preferred materials are 1000PEGT80PBT20 disclosed therein and 1000PEGT70PBT30. 
         [0079]    Suitably, the assay compartment is at or close to the distal end of the light guide. The assay compartment may lie wholly or partly within a recess or through hole of the light guide. Examples of such designs include a laser-drilled hole, or a rectangular cavity in the side wall of the light guide. 
       Assay Components 
       [0080]    Preferred assay components are discussed in WO2013/036943. 
         [0081]    In preferred embodiments, the analyte is glucose. 
         [0082]    The assay is preferably a competitive assay. 
         [0083]    Preferably, the assay components include an analyte binding molecule labelled with one of a proximity based signal generating/modulating moiety pair; and an analyte analog capable of competing with the analyte for binding to the analyte binding molecule labelled with the other of the proximity based signal generating/modulating moiety pair. The assay may further include a reference fluorophore which serves, inter alia, as a sensor diagnostic tool. 
         [0084]    Preferably, the assay components include a carbohydrate binding molecule and a carbohydrate analog, and energy donor and acceptor moieties (also referred to as “the FRET pair”) which provide an optical signal. The energy acceptor moiety has an absorption spectrum overlapping the energy donor moiety&#39;s emission spectrum. More preferably, the energy acceptor moiety is non-fluorescent. 
         [0085]    In preferred embodiments, the assay is a competitive glucose binding affinity assay that includes a glucose receptor (the carbohydrate binding molecule), a glucose analog, a first fluorophore (the energy donor moiety) labeled onto the glucose receptor, and an acceptor dye (the energy acceptor moiety) labeled onto the glucose analog. 
         [0086]    A preferred carbohydrate binding molecule is labelled MBL (mannose binding lectin). Concanavalin A is another carbohydrate binding molecule of interest. 
         [0087]    Preferred carbohydrate analogs are labelled macromolecules bearing carbohydrate or carbohydrate mimetic moieties. Examples include optionally derivatized labelled dextran e.g. 110 kDa dextran; labelled synthetic polymers bearing pendant carbohydrate or carbohydrate mimetic moieties; labelled proteins bearing pendant carbohydrate or carbohydrate mimetic moieties. Such carbohydrate analogs are disclosed for example in WO07/065653 and WO06/061208. 
         [0088]    Preferred energy donor moieties are Alexa Fluor fluorophores, Texas Red, and Cy5. Alexa Fluor 594 (AF594) is particularly preferred. AF647, QSY 21, and AF750 are appropriate for use in conjunction with a laser diode source at 645 nm. 
         [0089]    A preferred energy acceptor moiety is hexamethoxy crystalviolet-1 (HMCV1, a proprietary crystal violet derivative manufactured by Medtronic, Inc.), disclosed in WO05/059037. This is particularly suitable where the carbohydrate analog molecule is dextran. 
         [0090]    A preferred reference fluorophore is Alexa Fluor 700 (AF700). The reference fluorophore is preferably labeled onto Human Serum Albumin (HSA) or another macromolecule which does not bind significantly to the carbohydrate binding molecule. 
         [0091]    In preferred embodiments of the invention, it has been found that a degree of labeling (DOL) with AF594 of about 0.8-1 AF594/CRD and 5 HMCV1 molecules per dextran molecule gives optimal dose-response. 
         [0092]    The assay components may also comprise a protective formulation for radiation sterilization. 
       Reader 
       [0093]    The reader is also referred to as the “recording device” or “Glucose sensor transmitter or recorder” (GST/GSR). 
         [0094]    The reader is used to interrogate the optical sensor, and can be removably physically and optically coupled thereto. 
         [0095]    The reader preferably includes a housing, e.g. of plastics. The reader may be wearable on the body of the user, and may be sized so as to have a volume of no more than 15 cm 3  (e.g. about 11 cm 3 ) and a weight of about 10 g. Preferably, the reader has a life of 2 years or more. The reader may need to be charged periodically e.g. every 15 days. 
         [0096]    The reader preferably includes components of connection arrangements for detachable connection to the optical sensor. This is discussed in more detail above. In preferred embodiments the reader includes: a bore for engaging a projection portion of the optical sensor; fixed connectors for connection to moveable connectors of the optical sensor; anti-rotation lugs for engaging recesses of the optical sensor. Some or all of these features are suitably formed as part of reader housing. 
       Assay Interrogating System 
       [0097]    The reader includes an assay interrogating system which receives an optical signal from the optical sensor via the lens referred to above. The assay interrogating system is preferably an optoelectronic interrogating system. The assay interrogation system may operate via lifetime and/or intensity interrogation of the assay as discussed above. 
         [0098]    The lens is preferably a focusing/converging lens e.g. a biconvex lens. The lens is preferably of plastics. 
         [0099]    The assay interrogating system suitably includes one or more light detectors e.g. photodiodes. In use, an optical signal in the form of light reaches the proximal end of the light guide, and is focussed via the lens and then transmitted to the light detectors. 
         [0100]    Preferably, the assay interrogating system includes an illumination source e.g. an LED or a red laser diode, with the latter enabling a substantial reduction in the size and volume of the reader. AF647, QSY 21, and AF750 may be used in conjunction with a laser diode source at 645 nm. Typically, the illumination source is used to interrogate the assay via the light guide. 
         [0101]    The assay interrogating system may include filters for example in the form of a filter substrate having one or more coatings to effect, e.g., an excitation filter and/or one or more emission filters. 
         [0102]    The assay interrogating system may include further components e.g. beam splitters and/or mirrors. Typically beam splitters are present, but mirrors may not be present. 
         [0103]    Components of the assay interrogation system are preferably included in an optical system housing sub-unit of the reader, which is preferably mounted to the reader housing. 
         [0104]    Components of the assay interrogation system are preferably electrically connected to a printed circuit board assembly (PCBA). The components of the assay interrogating system may be mounted to the PCBA via alignment pins and/or screws, or the components may be electrically connected to the PCBA via flex connectors. 
         [0105]    As an alternative to separate optical components, the assay interrogating system may be manufactured as a wafer-scale stacked planar integrated optical system (SPIOS) and diced into smaller units. A Stacked Planar Integrated Optical System (SPIOS) may be created by fixing one multi-functional filter layer between two injection molded layers of optical components to forms a solid block, which is self-supporting. 
       Light Guide Interface Portion and Lens 
       [0106]    The reader preferably includes a light guide interface portion (also referred to herein as an “interface portion”) of the reader. This is suitably a block of tightly controlled dimensions containing a blind or through opening. The light guide interface portion of the reader is preferably mounted to the optical box e.g. via an interference fit. 
         [0107]    Preferably, the light guide interface portion comprises a female part, e.g. a flared opening, adapted to receive the proximal portion of the light guide. 
         [0108]    The term “flared opening” includes arrangements with a narrow portion proximal to the lens and a wider portion distal to the lens. The proximal portion of the flared opening preferably ends in a light guide alignment channel adapted to accommodate the proximal end of the light guide. Suitably, the alignment channel is a circular cross-section channel of diameter slightly larger (e.g. up to 50 μm larger, more preferably up to 30 μm larger) than the light guide. The flared opening guides the light guide proximal end face into its optical coupling position with the lens, without damaging its surface. 
         [0109]    The flared opening preferably comprises one or more chamfers i.e. continuous curved and/or planar surfaces which are not interrupted by edges, projections or other abrupt discontinuities in shape. 
         [0110]    The flared opening of the light guide interface portion of the reader is preferably partially or completely conical or frusto-conical in shape. The cone half angle (or effective half angle) is preferably in the range of 25 to 43° e.g. around 30°. A half angle which is too high may prevent the light guide from being guided into position correctly. 
         [0111]    However, the opening need not be conical, so that the half angle may vary from the proximal to the distal portion of the flared opening, or around the flared opening. For example, an arrangement with two or more chamfers, e.g. a double chamfer arrangement, may be used, as shown in  FIG. 10 . In the light guide interface portion on the left, a double frusto-conical arrangement is shown with an outer half angle of 45° and an inner half angle of 20°. In the light guide interface portion on the right, a single frustoconical arrangement is shown with a half angle of 30°. However, a single chamfer arrangement may be preferred, because of the lack of an edge which could cause wear as discussed below. 
         [0112]    Preferably, the flared opening of the light guide interface portion has a very smooth surface. This is to avoid wear of the light guide on contact with the surface. Such wear could damage the light guide cladding, resulting in debris which blocks light transmission. 
         [0113]    Smoothness may be measured as Roughness Average. Preferably, the flared opening has a surface of Roughness Average 16 to 32 microinch over a 0.004 inch long roughness cut-off (0.8 to 1.6 μm over a distance of 100 μm). 
         [0114]    The flared opening of the light guide interface portion may be built to a particular smoothness specification or polished to achieve this, as discussed below. 
         [0115]    The assay interrogating system of the reader has optical components including a lens, as discussed above. 
         [0116]    Accurate positioning and angling of the lens are important in ensuring that the relative positioning of the light guide and lens is within the desired tolerance ranges set out above. 
         [0117]    The lens is preferably mounted to the light guide interface portion of the reader. Suitably the light guide interface portion includes a recess in which the lens is positioned such that it can be optically coupled to the light guide. Suitably the recess comprises a retaining lip or other similar arrangement to control the position of the face of the lens adjacent to the optical sensor. 
         [0118]    The lens is preferably held in place within the light guide interface portion using a lens-retaining insert. 
         [0119]    In alternative arrangements, however, the lens and/or lens-retaining insert may be mounted directly to the reader housing, or to the optical system housing, e.g. by means of adhesive or a set screw, or the lens may be integrally formed with one of these components. 
         [0120]    The lens-retaining insert may be held in position by a screw thread arrangement (preferably with at least a count of 3 to 4), by a press fit/interference fit arrangement (e.g. using crushed ribs), by a resilient arrangement (e.g. using a spring) and/or using adhesive. A combination of a screw thread arrangement and adhesive is envisaged. The lens-retaining insert must not interfere with the optical signal. The adhesive should transmit light in the near IR and visible regions, and should not fluoresce, particularly in those regions. 
         [0121]    Preferably, the internal surfaces of the light guide interface portion and/or the optical system housing or reader housing in which the lens is mounted are optically black so as to reduce surface reflections (from signal or stray light) from reaching the light detectors. Suitably, these surfaces absorb wavelengths in the visible range (e.g. 300-800 nm). This may be achieved using coatings, e.g. having surface color and/or associated fine texture. The use of such coatings is particularly important if reflective metals are used in forming the device. Preferably, the internal surfaces are black in color. Black sealing tape may be used. 
         [0122]    Suitable materials for the light guide interface portion include metals, plastics and ceramics. In a preferred embodiment the material used is steel e.g. stainless steel. Alternative materials include aluminum, titanium, KOVAR™, INVAR™ and other alloys used in optical applications, plastics such as polyoxymethylene (POM, DELRIN™), PVC, cyclic olefinic copolymers (COP/COC, TOPAS™, ZEONEX™, ZEONOR™ etc.). KOVAR™ and INVAR™ are preferred choices because of their low coefficients of thermal expansion. 
         [0123]    Preferred plastics are optically black as discussed above. Metals can be oxidized (e.g. aluminum to aluminum hard or soft oxide coating, steel to black oxide and so on) or may have black material deposited or impregnated on the surface to absorb light in the visible or near IR range. 
         [0124]    The lens-retaining insert is formed of steel or stainless steel in preferred embodiments. The materials used for the light guide interface portion and lens-retaining insert should ideally be of the same type to match coefficients of thermal expansion. 
       Further Components of Reader 
       [0125]    The reader is preferably capable of reading, filtering, processing and/or transmitting optical signal values representing analyte concentration values. Preferably, the reader includes instrumentation to convert an optical signal from the optical sensor to an analyte concentration value. The reader may further comprise a transmitter for transmitting detected or measured analyte data. 
       Formation of Components 
       [0126]    As explained above, the dimensions of the components, in particular the interface portion and the lens-retaining insert must be tightly controlled, and a smooth surface on the interface portion is desirable. 
         [0127]    These components are preferably formed by molding or machining. 
         [0128]    Multi-axis machining methods are appropriate to provide the desired tightly controlled dimensional tolerances. Swiss screw turning/machining is the preferred method. Deterministic machining and laser machining may be used. 
         [0129]    For molded parts micro-molding is the preferred method. Preferably, the molding tools are precisely machined tools formed using multi-axis machining methods. 
         [0130]    Preferably, the reader components are cleaned e.g. to remove particulates, debris, dirt, machine oils and/or low surface tension agents. Suitable solvents for cleaning include IPA, hexane, acetone and THF. 
         [0131]    As mentioned above, the flared opening of the light guide interface portion may be built to a particular smoothness specification or polished to achieve this. It is important to use polishing materials which cannot impregnate the light guide interface portion and thereby abrade the light guide. For this reason, diamond polishing is not preferred. Hard wood is a preferred polishing material. 
       Implantation of Sensor 
       [0132]    The distal portion of the sensor is preferably introduced subcutaneously or within the skin of a user e.g. into the dermis or epidermis. Alternatively it may be implanted in and/or through inter-peritoneal or peritoneal tissue. The user is preferably a human. 
         [0133]    Preferably, the distal portion of the sensor is implanted or injected e.g. using a needle which may at least initially form part of the optical sensor as mentioned above. Preferably, the needle does not remain in the skin. 
       Systems Including Device 
       [0134]    Optionally, the device also includes one or more non-optical sensors for the carbohydrate analyte e.g. electrochemical sensors as mentioned above. The electrochemical sensor may include a plurality of electrodes. Respective distal portions of the optical sensor and the non-optical sensor may be co-located within the user&#39;s body, and may be implanted together. 
         [0135]    The device may form part of a system further comprising a hand-held monitor having a display and/or an insulin pump. The system may be a closed-loop system, with predictive diagnostics and minimal requirements for external calibration. The reader may wirelessly transmit analyte values to the hand-held monitor and/or insulin pump. 
       FURTHER ASPECTS OF INVENTION 
       [0136]    In a second aspect, the invention relates to a reader for use in a device as described above, comprising an assay interrogating system including a lens; and a light guide interface portion comprising a flared opening adapted to receive a proximal portion of a light guide. 
         [0137]    In a third aspect, the invention relates to a reader for use in a device as described above, comprising a housing; an assay interrogating system including a lens, and a lens-retaining insert holding the lens in position within the housing. 
         [0138]    In a fourth aspect, the invention relates to a method of detecting or measuring a carbohydrate analyte using a device as described above, comprising detecting or measuring the optical signal readout of the assay components via the light guide using the assay interrogating system of the reader. The method may further comprise initial implantation of the assay components and distal portion of the light guide into the body of a user. 
         [0139]    Preferably, and suitably between implantation and detection or measurement, the method further comprises a step of optically coupling the proximal portion of the light guide to the lens of the assay interrogating system via the interface portion. Preferably, the optical sensor and the reader are also connected via the connection arrangement. 
         [0140]    The method may also include a step of separating the optical sensor and reader such that the proximal portion of the light guide and the lens of the assay interrogating system are no longer coupled. Optionally, the optical sensor is then replaced with a further optical sensor. 
         [0141]    In a fifth aspect, the invention relates to an interface portion as described above. 
         [0142]    All features described in connection with any aspect of the invention can be used with any other aspect of the invention. 
     
    
     
       DRAWINGS 
         [0143]    The invention will be further described with reference to a preferred embodiment, as shown in the drawings in which: 
           [0144]      FIG. 1  shows schematically a preferred embodiment of the device of the invention, including the assay interrogating system. 
           [0145]      FIG. 2  shows the distal portion of the light guide of the optical sensor of  FIG. 1  in more detail. 
           [0146]      FIG. 3   a  is a side view of a needle for housing and deploying the distal portion of the light guide of the optical sensor of  FIG. 2 .  FIG. 3   b  is a perspective view of the connected optical sensor (left) and reader (right) of the preferred embodiment of the invention, showing the position of the needle of  FIG. 3   a  before injection. 
           [0147]      FIG. 4  is a perspective view of the optical sensor (left) and reader (right) of  FIG. 3   b  before connection. 
           [0148]      FIG. 5  is a plan view of the connected optical sensor and reader of  FIG. 3   b , with the upper shell of the reader housing removed. 
           [0149]      FIG. 6  is a cross-sectional view of the connection between the optical sensor and reader of  FIG. 5 . 
           [0150]      FIG. 7  is a cross-sectional view of the lens fiber interface portion of  FIG. 6 , also showing the proximal portion of the optical fiber, the lens and the lens-retaining insert. 
           [0151]      FIG. 8  is a perspective view of the optical sensor of  FIG. 4 , showing an enlarged view of the projecting portion. 
           [0152]      FIG. 9  is a perspective view of the reader of  FIG. 4 . 
           [0153]      FIG. 10  is a cross-sectional view of two lens fiber interface portions alternative to that of  FIG. 7 . 
           [0154]      FIG. 11  is a perspective view of an alternative embodiment of the invention, including a locking component. The locking component is shown before and after engagement with the sensor and reader (top and bottom respectively). 
       
    
    
     DETAILED DESCRIPTION 
       [0155]    In the following description, reference is made to the accompanying drawings which form a part hereof and which illustrate several embodiments of the present invention. It is understood that other embodiments may be utilized and structural and operational changes may be made without departing from the scope of the present invention. 
       DEFINITIONS 
       [0156]    The term “optical axis” in relation to a lens refers to an imaginary line that defines the path along which light propagates through the system. Often but not necessarily this coincides with the mechanical axis and axis of rotational symmetry of the lens. The lens optical axis is shown as  14  in  FIG. 7 . 
         [0157]    The term “optical axis” in relation to a light guide refers to an imaginary line that defines the path along which light propagates through the system. Often but not necessarily this coincides with the longitudinal axis and axis of rotational symmetry of the light guide. Where the light guide is not straight, the optical axis may be defined in terms of a cross-section. If the cross-section is cut such that the shape of the section is the same as the shape of the proximal end, typically the optical axis is a line that travels perpendicular to the cross-sectional face and through its centroid. The light guide optical axis is shown as  16  in  FIG. 7 . 
         [0158]    The term “focal point” in relation to a lens refers to a point on the lens optical axis at which initially collimated rays are brought to a focus, e.g. in air; it is separated from the lens by the focal distance. A lens will typically have front and rear focal points. The focal point referred to herein is generally that on the same side as the light guide. The focal point is shown as  10  in  FIG. 7 . 
         [0159]    The term “focal plane” in relation to a lens refers to a plane perpendicular to the lens optical axis and containing the focal point. This may also be referred to as the “back (or rear) focal plane”. The focal plane is shown as  12  in  FIG. 7 . 
         [0160]    The term “focal plane tolerance” (or transverse tolerance) as used herein refers to the range of positions of the light guide optical axis within the focal plane wherein coupling between the light guide and lens is such that light transmission is at least 80% of the maximum light transmission which occurs when the light guide optical axis is at the focal point of the lens. 
         [0161]    The term “optical axis tolerance” (or axial tolerance) as used herein refers to the range of positions of the proximal end of the light guide along the lens optical axis wherein coupling between the light guide and lens is such that light transmission is at least 80% of the maximum light transmission which occurs when the proximal end of the light guide is at the focal point of the lens. 
         [0162]    In the discussion herein, preferred embodiments of the devices, systems, and methods of the invention are described with reference to glucose as the analyte whose level/concentration in the blood and/or bodily fluids of the user is to be determined. However, this is by way of illustration and not limitation, as the principles, devices, systems, and methods of the present invention may be used for sensing and/or determining the level of a variety of other physiological parameters, agents, characteristics, and/or compositions. 
         [0163]    The preferred embodiment of the device is shown schematically in  FIG. 1 . The device includes an optical sensor  100 , shown in more detail in the following figures. 
       Optical Sensor of Preferred Embodiment 
       [0164]    The optical sensor  100  includes a base  130  ( FIGS. 4 ,  8 ) and an optical fiber  110  (also referred to herein as a “light guide”) having a proximal portion  116  ( FIG. 2 ) mounted to the base  130  as explained in more detail below. 
         [0165]    The optical fiber  110  is formed of plastics having tensile and fatigue properties that ensure robustness. 
         [0166]    The proximal portion  116  of the optical fiber  110  terminates in a proximal end  117  (in the form of a planar face perpendicular to the mechanical/optical axis  16  of the optical fiber  110  ( FIGS. 2 ,  7 ). 
         [0167]    The distal portion  112  of the optical fiber  110  ( FIG. 2 ) is designed for insertion into a user&#39;s body as described in more detail below. A glucose-permeable membrane of PolyActive™ (a biocompatible, biodegradable polymer 1000PEGT70PBT30 from Integra Orthobiologics, Irvine, Calif.) is heat sealed to the fiber&#39;s distal end  115  to form an assay compartment  120  housing assay components  125 . 
         [0168]    The optical sensor base  130  is of polycarbonate. It is generally wide and flat in form, with a planar lower plate  135  ( FIG. 5 ). The lower plate  135  is provided on its lower surface with a contact adhesive sheet (not shown) for mounting to a user&#39;s skin, the adhesive being initially protected by a cover sheet (not shown). The adhesive is capable of strong adhesion for 7 days. 
         [0169]    The base has a front end for connection to the reader  200  as described in more detail below, and a back end ( FIG. 4 ). A projecting portion  140  for insertion into the reader  200  extends from the front end of the optical sensor base  130  parallel to the lower plate  135 ; the projecting portion  140  is generally cylindrical in shape. The projecting portion  140  has two annular recesses, each being provided with an O-ring  145  ( FIGS. 4 ,  6 ,  8 ). 
         [0170]    A connector in the form of a resiliently biased flexible arm  150  extends along each side of the optical sensor base  130  from its back end to beyond its front end, separated from the body of the optical sensor base  150  by a space ( FIGS. 4 ,  5 ,  8 ). The front end of each arm  150  terminates in a latch  155 , such that there is a latch  155  on either side of the projecting portion  140 . The arms  150  can be flexed inwards towards the projecting portion  140  but will return to their initial positions when released. Grips  160  in the form of ribs are provided on the outer sides of the arms to assist in flexing. 
         [0171]    Above and below the projecting portion  140  of the optical sensor base  130 , the optical sensor base  130  contains recesses  165  ( FIG. 8 ) which form part of an anti-rotation arrangement as described below. 
         [0172]    The optical sensor base  130  contains a through channel  170  ( FIG. 6 ) extending from the back end to the projecting portion  140 . The optical fiber  110  passes through the channel  170  and its proximal end  117  protrudes from a front face of the projecting portion  140  ( FIGS. 4 ,  8 ). The optical fiber  110  is held in place within the optical sensor base  130  by UV-curing adhesive which is back-filled through the channel  170 . The proximal end  117  of the optical fiber  110  is initially covered by a protective cap of vinyl plastics (not shown) which is held in place by the O-rings  145 . 
         [0173]    An insertion device  500  passes through a channel in the optical sensor base  130  at an angle of approximately 45° to the lower plate  135  ( FIGS. 3   a  and  3   b ). The insertion device  500  relies on a disposable, automatically retracting needle  510  of C-shaped cross-section which is designed with the optical sensor base  130  to deliver the distal portion  112  of the optical sensor  100  through the user&#39;s skin at an angle of approximately 45°. 
       Reader of Preferred Embodiment 
       [0174]    The reader  200  ( FIGS. 4 ,  5 ,  6 ,  9 ) comprises a two-part housing of polycarbonate/acrylonitrile butadiene styrene blend (BAYBLEND™) having a base and an upper shell. The lower surface of the reader base is provided with one element of hook and loop fastening (not shown). 
         [0175]    The reader  200  houses the optical system used to interrogate the assay (also referred to herein as the “assay interrogating system”). 
         [0176]    The optical system ( FIGS. 1 ,  5 ) is essentially a modified epi-fluorescence set-up with one light source to excite (i.e. illuminate) the assay and two detectors to detect the fluorescence emitted from the assay and the internal reference, respectively. As noted, the intensity of the emitted fluorescence correlates to the glucose concentration. Here, the measured intensity of the emitted fluorescence is affected by the intensity of the light source and the coupling between the assay and the optical system. 
         [0177]    A driver circuit  1310  modulates a LED  1320  at a low frequency (solely with the purpose of eliminating the 1/f noise and canceling out ambient light) with a wavelength range capable of simultaneously exciting the assay and reference fluorophores. The LED output is filtered using a multilayer dielectrical filter  1330  to select a distinct wavelength region. The filtered LED output is reflected by a first dichroic beam splitter  1340  and focused onto the optical sensor  1300 / 100 , which includes the assay and the reference (also referred to herein as the “reference fluorophore”), by a lens  1350 . The interface between the optical sensor and the lens  1350  is described in more detail below. 
         [0178]    The assay and the reference emit fluorescence. The emitted fluorescence  1301  and the reflected excitation light  1323  are picked up and collimated by the lens  1350 . The first dichroic beam splitter  1340  transmits the fluorescence  1301 . However, it reflects the majority of the back reflected excitation light  1323 . A second beam splitter  1344  reflects the reference fluorescence  1307  at a 90° angle, but it transmits the assay fluorescence  1309 . A first emission filter  1360  with a distinct wavelength region red shifted with respect to, and not overlapping, the pass band of the excitation filter and matching the desired part of the assay fluorescence spectrum then blocks the remaining part of the excitation light and transmits the assay fluorescence. Similarly, a second emission filter  1364  with a distinct wavelength region red shifted with respect to, and not overlapping, the pass band of the excitation filter and matching the desired part of the assay fluorescence blocks the remaining part of the excitation light and transmits the reference fluorescence  1307 . Thus, in effect, only the fluorescence from the assay and the fluorescence from the reference are focused onto their respective photo detectors (also referred to herein as “light detectors”)  1380 ,  1384  using respective lenses  1370 ,  1374 . The ratio between the detected assay fluorescence and the detected reference fluorescence correlates with the glucose concentration in the assay. 
         [0179]    The optical system is mounted within the reader housing in an optically black optical system housing  202  ( FIG. 5 ). Components of the optical system are electrically connected to a printed circuit board assembly  204 . 
         [0180]    The light guide interface portion  220  of the reader  200  is now described. 
         [0181]    The reader housing includes a generally cylindrical bore  205  into which the projecting portion  140  of the optical sensor base  130  is to be inserted ( FIGS. 6 ,  9 ). The inside surface of the bore  205  engages the O-rings  145  of the optical sensor base  130  ( FIG. 6 ). Two fixed connectors in the form of projections  210  complementary to the latches  155  on the optical sensor base  130  are located at the mouth of the reader housing bore  205  ( FIG. 5 ). Above and below the bore  205 , the reader housing is provided with lugs  215  as part of the anti-rotation arrangement ( FIG. 9 ). 
         [0182]    The light guide interface portion  220  of the reader  200  is mounted within the optical system housing  202  at the internal end of the reader housing bore ( FIGS. 5 ,  6 ). This light guide interface portion  220  is shown in  FIG. 7 . 
         [0183]    The light guide interface portion  220  is generally in the form of a stepped cylindrical block having a through channel as described below. The block is formed by machining. At the inner end of the block as defined below, the block wall is notched such that it forms three fingers (not shown). 
         [0184]    From the outer end to the inner end (i.e. moving away from the optical sensor  100  when connected to the reader  200  in use) the channel has a smooth flared opening in the form of a frustoconical portion  225  of half angle 30° with a wide mouth  232 ; a narrow light guide alignment channel  230  of constant diameter extending from the narrow end  234  of the frustoconical part  230 , and a wide recess  235  in which the lens  1350  is mounted, the wide recess  235  being defined by the three fingers of the block wall, which are provided with an internal screw thread. 
         [0185]    The lens  1350  is a biconvex convergent lens of focal length approximately 2 mm. The lens  1350  is mounted with its optical axis  14  parallel to and aligned with the light guide alignment channel  230 . The lens  1350  is held in place by a generally tubular lens-retaining insert  240  of complementary shape to the lens  1350 . The lens-retaining insert  240  has an external screw thread which engages the internal screw thread of the light guide interface portion  220 . The lens-retaining insert  240  is also fixed in position by means of adhesive. The assay photodetector  1380  is positioned within the lens-retaining insert  240  such that light is focused onto it by the lens  1350 . 
         [0186]    The reader  200  houses further components (not all shown) including diagnostics, one or more microprocessors and/or digital signal processors (DSPs), memory, a RF communication chip (using, e.g., 2.4 GHz TelD protocol), and a rechargeable battery  250 . The reader is capable of the conversion of signals received from the sensors to glucose values and of wireless communication, including transmission of the glucose values (or an averaged, weighted, or otherwise modified version thereof) to a monitor, an infusion pump or a display device (not shown). 
         [0187]    The reader includes electrical contacts  245  for interface with a charger (not shown) and with the optical sensor base  130 . 
       Use of Device 
       [0188]    In use, the optical sensor base  130  is mounted to the skin of a user by removal of the cover sheet and application of the adhesive to the skin. The distal portion  112  of the optical fiber  110  of the optical sensor  100 , including the assay compartment  120 , is implanted subcutaneously using insertion device  500 . The needle  510  automatically retracts. The optical sensor base  130  is further secured to the skin by overtaping (not shown) to reduce the risk of the optical sensor  100  being pulled out of the skin accidentally. The overtaping has an element of hook and loop fastening (not shown) complementary to that on the reader base. 
         [0189]    The reader  200  is connected to the optical sensor  100  as follows. 
         [0190]    The projecting portion  140  of the optical sensor base  130  is pushed into the bore  205  of the reader  200 . The positional tolerance of the projecting portion  140  within the bore  205  is approximately ±0.02 inches (±500 μm) in the x and y directions. This locates the proximal end  117  of the optical fiber  115  within the frustoconical portion  225  of the light guide interface portion  220  of the reader  200 . 
         [0191]    The frustoconical portion  225  serves to direct the proximal end  117  of the optical fiber  115  into the light guide alignment channel  230  so that it is constrained in the x and y directions. The positional tolerance of the optical fiber  115  inside the light guide alignment channel  230  is approximately ±10 μm. 
         [0192]    The arms  150  of the optical sensor base  130  are flexed inwards using the grips  160 . When the optical sensor projecting portion  140  is fully inserted into the reader  200 , the arms  150  are released and return to their initial positions, wherein the latches  155  engage the projections  210  of the reader  200  such that the optical sensor  100  and reader  200  cannot be separated by pulling apart. The O-rings  145  of the optical sensor  100  engage the bore  205  of the reader  200  to prevent relative movement of the optical fiber  110  and the lens  1350  of the reader in the z direction. The O-rings  145  provide a radial seal between the optical sensor  100  and the reader  200  to water-resistance standard IPX8. 
         [0193]    When the optical sensor  100  and reader  200  are connected, lugs  215  of the reader  200  engage recesses  165  of the optical sensor  100  in an anti-rotation arrangement, preventing relative rotation of the optical sensor  100  and reader  200 . 
         [0194]    Thus, the proximal end  117  of the optical fiber  110  is constrained at or very close to the focal point  10  of the lens  1350 , with the lens optical axis  14  and optical fiber optical axis  16  aligned. 
         [0195]    Relative motion between the reader  200  and sensor  100  is further prevented by engagement of the complementary hook and loop fastening elements of the reader base and the sensor overtaping. 
         [0196]    The assay interrogating system is interrogated as explained above. As shown in  FIG. 2 , excitation light travels from the proximal end  117  of the optical fiber  115  to the assay components  125 , and the fluorescence response travels back up the optical fiber  110  to the assay interrogating system. Light from the proximal end  117  of the optical fiber  115  is focused by the lens  1350  onto the light detector  1380 . Accurate relative positioning of the proximal end  117  of the optical fiber  110  and the lens  1350  ensures that light is efficiently coupled from the optical fiber  110  into the assay interrogating system. 
         [0197]    To separate the optical sensor  100  from the reader  200 , the optical sensor connector arms  150  are flexed inwards to release the latches  155  from the reader connector projections  210 , after which the reader  200  slides easily away from the optical sensor  100 . 
         [0198]    In an alternative embodiment shown in  FIG. 11 , a further separate locking component  520  is provided to inhibit relative motion between the reader  200  and optical sensor  100 . The locking component  520  is of plastics, and is adapted to engage the upper surface of the optical sensor base  130  and adjacent parts of the shell of the reader  200  when the optical sensor  100  and reader  200  are connected. Thus, the locking component  520  is generally in the form of a plate complementary to the upper surface of the optical sensor base  130 , but with a space  550  at the forward central part. Descending parts  530  of the locking component  520  correspond to the spaces between the sensor body and the flexible arms  150 . Forward wings  540  of the locking component  520  are complementary to the upper surface of adjacent parts of the shell of the reader  200 . A tab  560  extends from each wing  540  into the space  550 . 
         [0199]    The locking component  520  is connected to and disconnected from the connected optical sensor  100  and reader  200  vertically (i.e. in the y direction). The locking component  520  lies generally above the optical sensor base  130 , with forward wings  540  engaging the shell of the reader  200 . Parts  530  engage the spaces defined by the optical sensor base  130 , flexible arms  150  and reader body, preventing the optical sensor base  130  from moving further towards the reader  200 . Tabs  560  occupy positions between the optical sensor  100  and reader  200  to either side of the lugs  215 /recesses  165  of the anti-rotation arrangement described above, preventing the optical sensor base  130  from moving away from the reader  200 . In this way, the locking component  520  prevents relative movement between the optical sensor  100  and reader  200 , especially in the z direction. 
       EXAMPLES 
     Comparative Example 
       [0200]    In an optical sensor with the optical fiber optical axis parallel to the lens optical axis, it was found that if the optical fiber optical axis was not within ±50 μm of the lens optical axis in the x and y directions, the optical signal was reduced by 20 to 50%. Theory suggests that if the optical fiber optical axis was not within ±100 μm of the lens optical axis, the optical signal would be reduced by 80%. 
         [0201]    It was also found that if the proximal end of the optical fiber was not within ±200 μm of the lens focal point in the z direction, the optical signal was reduced by 20% (experimental results) to 30% (theoretical results). The relationship between distance and performance was not linear, and performance dropped off at an increasing rate as the proximal end of the optical fiber was moved away from the focal point. 
         [0202]    Advantages of the preferred embodiments of the invention include:
       the tolerance in the x and y directions permitted by the light guide alignment channel of the light guide interface portion of the reader is very low, whereas the tolerance in the z direction permitted by the connection between the reader and the optical sensor is slightly higher. It was a contribution of the present inventors to realize that higher tolerance in the z direction is acceptable. It was a further contribution of the present inventors to appreciate that different arrangements can be used to constrain movement in the x and y directions and the z direction.   The smooth flared opening of the light guide interface portion assists in insertion of the optical fiber into the very narrow light guide alignment channel. Without this feature it would be difficult to insert the delicate optical fiber into the light guide channel without damaging the optical fiber.   The light guide interface portion is included on the reader rather than the optical sensor. The reader has a longer life than the optical sensor, and so this type of arrangement is more cost-effective.       
 
         [0206]    While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
         [0207]    The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
         [0208]    In this specification, unless expressly otherwise indicated, the word ‘or’ is used in the sense of an operator that returns a true value when either or both of the stated conditions is met, as opposed to the operator ‘exclusive or’ which requires that only one of the conditions is met. The word ‘comprising’ is used in the sense of ‘including’ rather than to mean ‘consisting of’. All prior teachings acknowledged above are hereby incorporated by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge at the date hereof.