Abstract:
A method and apparatus is provided for determining a property of an analyte using a sensing layer whose optical response changes with the analyte. The apparatus includes a housing with an optically transparent window for receiving the sensing layer. The window passes optical stimulation to the sensing layer and the optical response from the sensing layer. A stimulating light emitter is coupled to a first face of an optical body monolithically coupled to the window and a light detector is coupled to a second face of the optical body for receiving the response. The optical response changes as the concentration of the analyte changes. Reference molecules included in the sensing layer can provide a calibration signal to a second light detector mounted on a third face of the optical body. The first, second and third faces of the optical body are different and not coplanar.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to sensors for measuring one or more parameters or of a sample fluid, and more particularly relates to implantable sensors for in situ measurement of parameters of body fluids.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known in the art to use electro-optical sensing devices for detecting the presence or concentration of an analyte (i.e., a parameter to be measured) in a liquid or gaseous medium. For example, U.S. Pat. Nos. 6,454,710, 6,330,464, 6,304,766, 6,119,028, 6,081,736, 5,910,661, 5,894,351 and 5,728,422 describe various aspects of this technology including self-contained sensing devices stated to be suitable for use in humans for in-situ detection of various analytes.  
           [0003]    Compact chemical sensors able to detect analytes of interest in connection with human and animal health are of great practical utility, especially arrangements that are implantable. The most common optical chemical sensor configuration uses an LED to provide stimulating light that is optically coupled to an indicator layer in or on which are placed molecules whose fluorescence or absorption is affected by the presence of the analyte, that is, the parameter desired to be measured. A photodetector is likewise optically coupled to the indicator layer for detecting the amount of fluorescence, fluorescence quenching or absorption created by the interaction of the indicator molecules in the presence of the stimulating light and the analyte. In this way, the concentration of sugars, oxygen and other parameters or analytes of interest can be measured in situ. Various configurations and arrangements are used for coupling the stimulating light to the indicator molecules and the optical response thereof back to the photodetectors, as for example, optical fibers or wave guides, or optically transparent encapsulating material.  
           [0004]    However the prior art devices and methods suffer from a number of limitations, as for example, high cost, fragile construction, difficult manufacturing, inefficient optical coupling and limited sensitivity under some circumstances. Accordingly, there continues to be a need for improved sensors, especially implantable sensors that are rugged, highly integrated, easily manufactured and functional for detecting the presence and/or amount of various analytes or parameters in situ in human and animal bodies and that generate little or no adverse reactions within the body. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    An apparatus is provided for detecting or measuring a parameter of interest, e.g., an analyte, in a sample fluid, comprising: a housing chemically resistant to the sample fluid; a sensor element substantially within the housing, the sensor element having an optically transparent body with a first surface exposed from the housing for receiving a sensor layer responsive to the parameter of interest in the sample fluid and for receiving a first signal from the sensing layer in response to optical stimulation thereof; wherein the sensor element comprises a light emitter on a second surface of the body, optically coupled to the first surface for providing said optical stimulation; wherein the sensor element comprises a first detector on a third surface of the body, optically coupled to the first surface for detecting the first signal; and wherein of the first, second and third surfaces of the body, two surfaces are opposed and a remaining surface couples the opposed surfaces. The sensor body is preferably monolithic with a truncated cone shape, the sensor layer being on the base of the cone, the light emitter on the top of the cone and the detectors mounted on inward sloping sides of the cone. In a further embodiment, several sensor elements adapted to measure different analytes are located in a common housing.  
           [0006]    A method is provided for determining a property of an analyte using a sensing layer exposed to the analyte whose response to optical stimulation changes in the presence of the analyte, comprising: providing the sensing layer exposed to the analyte on an optically transparent surface of a monolithic optical body in a housing; passing the optical stimulation to the sensing layer from a light emitter coupled to a first face of the monolithic optical body; and passing an optical response from the sensing layer to a first light detector coupled to a second face of the monolithic optical body different from the first face, and wherein the first and second faces are not substantially coplanar. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0008]    [0008]FIG. 1 is a simplified schematic cross-sectional view of an implantable chemical sensor according to the present invention;  
         [0009]    FIGS.  2 - 4  are a simplified schematic side cross-sectional views of the opto-electronic portion of the sensor of FIG. 1, showing further details and according to several embodiments;  
         [0010]    FIGS.  5 A-B through  8 A-B are simplified side views (A) and bottom views (B) of the opto-electronic portion of FIGS.  2 - 4 , showing further details and according to still further embodiments; and  
         [0011]    [0011]FIG. 9 is a simplified schematic cross-sectional view similar to FIG. 1 of an implantable chemical sensor according to a still further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. Persons of skill in the art will understand the general principles of operation of optically stimulated chemical sensors and the sensing layer materials commonly used. Any particular choice of sensing materials and sensing layer structures depends on the particular analytes of interest. This invention is particularly concerned with the efficient coupling of light to and from the sensing layer and providing a particularly rugged, efficient, easily manufactured and compact chemical sensor structure.  
         [0013]    [0013]FIG. 1 is a simplified schematic cross-sectional view of implantable chemical sensor  10  according to the present invention. Chemical sensor  10  comprises hermetic enclosure or housing  12  that is sealable along joint  14 . Conveniently located within housing  12  are optical module  30 , electronic control module  16 , energy source  18  (e.g., one or more batteries), optional transceiver  20 , optional antenna  22  and optional pressure sensor  56  having a sensing diaphragm communicating with ambient  37  through an external wall of housing  12 . Optical module  30  has therein light emitter(s)  40  coupled to control module  16  via leads  51 , light detectors  42 ,  44  coupled to control module  16  via leads  53 ,  55 , respectively, and other components as will be presently described. Control module  16  is coupled to energy source  18  via lead(s)  17  and to transceiver  20  via lead(s)  15 . Transceiver  20  is coupled to antenna  22  via lead(s)  21 . While transceiver  20  and antenna  22  are conveniently provided for communicating to and from sensor  10  they are not essential and externally coupled lead(s)  57  may also be used to provide bi-directional communications with sensor  10 . Further, while it is desirable that instructions or other commands be able to be sent to sensor  10  while in situ using lead(s)  57  or antenna  22 , this is not essential. Sensor  10  can be preprogrammed to merely report measured data after implantation without further instructions from outside.  
         [0014]    Enclosure or housing  12  protects the internal components from the sample fluid. Housing  12  is conveniently opaque although this is not essential. Housing or enclosure  12  is preferably made of a bio-inert material suitable for long-term placement in a human or animal body. Titanium is an example of a suitable material but others materials will also serve. Temperature sensor [T] is also desirably included in control module  16  or elsewhere in housing  12  so as to provide ambient temperature information for adjusting or compensating the various signals processed by control module  16 , but this is not essential. Housing  12  may also be conveniently coated with a layer (not shown) of a chemically inert material such as for example, Teflon® or the like.  
         [0015]    Housing  12  has optically transparent window  13  therein with outer surface  131 . Window  13  is hermetically attached to housing  12  by, for example, laser welding, soldering or gluing or other appropriate sealing and attachment means well known in the art at, depicted herein for example, perimeter joint  34 . Sapphire, quartz, glasses and optically transparent plastics are examples of convenient materials for optical window  13 . Other optically materials may also be used. In the preferred embodiment, it is necessary that window  13  be hermetically sealed to housing  12  so as to preclude ambient leakage into housing  12  via the attachment joint (e.g., joint  34 ) and that window  13  be transparent to the light wavelengths of interest. Window  13  is conveniently joined to or a part of body  38  of optical module  30  at interface  32 . Window  13  and body  38  of optical module  30  may be integral, that is, made of a single piece of material, in which case interface  32  is a hypothetical plane internal to body  38 , or window  13  and body  38  can be joined at real interface  32 , as for example, using optically transparent epoxy or solder glasses or by other means well known in the art. Either arrangement is useful and the choice depends upon convenience of manufacture and overall optical properties. What is important is that the maximum amount of light be coupled from body  38  to and from sensing layer  36  on surface  131 . As used herein, the terms “optical body” and “monolithic optical body” are intended to include both arrangements, e.g., attached or integral windows. Window  13  with outer surface  131  on which sensor layer  36  is supported is shown herein as having flat outer surface  131  in contact with sensor layer  36 , but this is merely for convenience of explanation and not intended to be limiting. Surface  131  of window  13  can have a curved or of any other shape in contact with sensor layer  36  provided that such shape does not interfere with optical coupling between sensor layer  36  and optical module  30 .  
         [0016]    Optical module  30  has optically transparent body  38  coupled to or integral with surface  131  on which is located sensing layer  36 . Body  38  is conveniently formed of the same class of optically transparent materials as discussed in connection with window  13 . Sensing layer  36  is exposed to ambient fluid or gases  37  whose composition or properties are desired to be analyzed. Body  38  of optical module  30  couples light energy  41  to sensing layer  36  from light source  40 , e.g., one or more LED&#39;s. Light energy  41  is generated by light source(s)  40  and conveniently passes through input filter  50  before entering optical body  38 . Input filter  50  is useful for selecting from the light emitted by light source(s)  40 , just the wavelengths desired to excite sensing layer  36 . While optical filter  50  is desirable it is not essential. Light source(s)  40  may be a single larger area LED or other emitter or an array of smaller emitters, such as for example are shown in FIGS. 14. Either arrangement is suitable. While LEDs are a convenient light source and are preferred, other light sources can be used, as for example and not intended to be limiting, electro-luminescent or gas discharge or other types of light sources well known in the art. As used herein, the term “LED” is intended to include these and other types of light sources. While filter  50  is shown as being a single unit, this is not essential and not intended to be limiting. For example, filter  50  may be different over different LEDs making up light source  40  or may have multiple pass-bands so that multiple selected wavelengths emitted from the same or different LEDs are selectively coupled to optical body  38 . Thus, LEDs  40  and/or filter  50  need not be restricted to a single wavelength.  
         [0017]    Body  38  of module  30  also couples light energy  43 ,  45  from sensing layer  36  to various photo detectors  42 ,  44 . Light energy  43 ,  45  may originate from fluorescence generated in sensing layer  36  by light energy  41  or be a portion of light energy  41  being reflected or scattered back from layer  36 . Filter  52  is located between photodetector(s)  42  and optical body  38  and filter  54  is conveniently located between photodetector(s)  44  and optical body  38 . Filters  52 ,  54  conveniently attenuate extraneous light so that photodetectors  42 ,  44  detect and measure only specific desired wavelengths of light  43 ,  45  respectively. While filters  52 ,  54  are desirable they are not essential. Filters  52 ,  54  should have pass-bands matched to the wavelengths desired to be detected by photodetectors  42 ,  44 , respectively. Photodetectors  42 ,  44  can comprise a single large area photocell or an array of smaller photocells; either arrangement is useful. In FIGS.  1 - 4 , the arrangement using multiple photocells  42 ,  44  is illustrated, but this is not essential. As used herein, the words “photodetector” and “photodetectors” or “photodetector(s)” are intended to include either arrangement.  
         [0018]    While filter-photodetector pairs  42 ,  52  and  44 ,  54  are ordinarily designed to each detect a particular wavelength, this is not essential and the filter-photodetector pairs can be made up of several filter-photodetector elements capable of detecting different wavelengths. For example, under circumstances where it is desired to independently monitor the output of sensor layer  36  at multiple wavelengths, filter-photodetector pair  42 ,  52  may have a first segment that detects a first wavelength and a second segment that independently detects a second wavelength, thus permitting the intensity of the light of the two wavelengths to be compared. Filter-photodetector pair  44 ,  54  may be similarly arranged to have multiple independent elements. The present invention and the appended claims are intended to include such variations.  
         [0019]    Stimulating light source  40  is coupled to control module  16  by leads  51 . Photodetector  42  is coupled to control module  16  by leads  53  and photodetector  44  is coupled to control module  16  by leads  55 . The space within housing  12  surrounding and separating components  16 ,  18 ,  20 ,  22 ,  30  and the various interconnecting leads or buses is conveniently filled with an inert plastic filler or potting material  58  so as to provide ruggedness, mechanical shock resistance, improved hermeticity and electrical insulation. An aspect of the present invention is that filler material  58  need not be optically transparent thereby offering a wider range of material choices and the opportunity to select more favorable mechanical, electrical and optical properties than available in prior art devices where the internal filler or potting material is used as an optical wave guide or optical pathway from the light emitters to the sensing layer and back to the optical detectors.  
         [0020]    In operation, stimulating light source  40  operating under the control of electronic module  16  generates light  41  that passes through filter  50  and optical body  38  to sensing layer  36 . Sensing layer  36  contains various indicator molecules  60  that, for example, fluoresce when illuminated by light  41 . Indicator molecules  60  are sensitive to the analyte desired to be detected in ambient fluids or gases  37 . In a typical situation, if the desired analyte is present in ambient  37 , the fluorescence, e.g., light  43  emitted by indicator molecules  60 , increases or decreases proportional to the concentration of the analyte to which indicator molecules are sensitive. Such indicator molecules are well known in the art and their choice depends upon ambient  37  and the particular analyte desired to be detected. Fluorescent light  43  passes through incoming filter  52  and is measured by photodetector  42 . Filter  52  is chosen to pass the wavelength of the fluorescence emitted by indicator molecules  60  and attenuate other wavelengths, e.g., the wavelengths of light  41 ,  45 .  
         [0021]    In order to increase the measurement accuracy and compensate for difference in temperature and pressure it is desirable to provide a reference signal to which the analyte sensitive signals registered by photodetector  42  may be compared. This is desirably accomplished by including reference indicator molecules  62  in sensing layer  36 . Such reference indicator molecules are well known in the art. Reference indicator molecules  62  are chosen, for example, to fluoresce at a different wavelength than indicator molecules  60  and to be insensitive to the analyte. Such molecules are well known in the art. Reference molecules  62 , for example, emit fluorescent light  45  that is coupled though optical body  38  and filter  54  to photodetector  44 . Filter  54  desirably attenuates other wavelengths, e.g., of light  41 ,  43  so that photodetector  44  measures substantially only the output of reference indicator molecules  62 . The output of photodetector  44  is coupled via leads  55  to electronic control module  16  where the reference signal received by photodetector  44  is used to compensate the analyte sensitive signal from photodetector  42  for changes in conditions.  
         [0022]    Control module  16  conveniently uses transmitter or transceiver  20  to send the test results to antenna  22  where they are broadcast to a receiver outside the body. Persons of skill in the art will understand how to choose transmitter or transceiver frequencies to accomplish this. Similarly, control module  16  can receive program instructions from outside the body via antenna  22  and transceiver  20 . In this way, the operation of control module  16  can be externally programmed to, for example, conduct tests at different times or with different repetition rates or different light stimulation levels and other variations. Alternatively or in combination, externally coupled wires  57  may be used to input instructions to sensor module  10  and receive data therefrom. Either arrangement is suitable.  
         [0023]    FIGS.  2 - 4  are simplified schematic side cross-sectional views of optical module  30  of sensor  10  of FIG. 1, showing further details and according to several embodiments  302 ,  303 ,  304 . Like reference numbers identify like elements. FIGS.  2 - 4  differ only in the composition and arrangement of sensing layer  36 . Other aspects of modules  302 ,  303 ,  304  are the same is in module  30  of FIG. 1.  
         [0024]    In FIG. 2, sensing layer  36  of optical module  302  has analyte indicator molecules  60  and reference indicator molecules  62  dispersed substantially homogeneously in layer  36 . In optical module  303  of FIG. 3, a layered arrangement is used for sensing layer  36  wherein, for example, analyte indicator molecules  60  are dispersed in layer  361  and reference indicator molecules  62  are dispersed in layer  362 . In module  303  of FIG. 3, layer  361  is shown as superposed on layer  362  but this is merely for convenience of explanation. Those of skill in the art will understand that the order of layers maybe interchanged depending on the analyte and the reference indicator, that is, with layer  362  outermost and layer  361  innermost. Either arrangement suffices depending on the analyte and reference involved. In optical module  304  of FIG. 4, the regions of sensing layer  36  containing analyte indicator molecules  60  and reference indicator molecules  62  are laterally distinct. In optical module  304 , analyte indicator molecules  60  are in region  363  and reference indicator molecules  62  are in region  364 , but these regions may be interchanged. Other lateral separation arrangements may also be used, as for example, regions  363  and  364  may be interspersed in alternate stripes or alternate concentric circles or otherwise and FIG. 4 is merely intended to be illustrative of lateral separation and not limited to the particular arrangement shown. Similarly, the arrangements illustrated in FIGS.  2 - 4  may be used simultaneously or in combination in different regions of window  13 .  
         [0025]    FIGS.  5 A-B through  8 A-B are simplified side views (A) and bottom views (B) respectively, of optical module  30  of FIGS.  1 - 4 , showing further details and according to still further embodiments  305 ,  306 ,  307 ,  308 . Like reference numbers are used for like elements. In FIGS.  5 - 8 , the details of sensing layer  36  are omitted and filter layers  50 ,  52 ,  54  are not shown, but this is merely for convenience of explanation and not intended to be limiting. Persons of skill in the art will understand based on the description herein that the variations illustrated in FIGS.  2 - 4  also apply to optical sensing modules  305 - 308  of FIGS.  5 - 8 . FIGS.  5 - 8  illustrate various alternative arrangements  305 ,  306 ,  307 ,  308  of optical sensing module  30  with different location and shapes of photodetector  42  for receiving analyte sensing light signal  43  and photodetector  44  for receiving the reference light signal  45  and photo-emitter  40  for generating stimulating light  41 . For convenience of explanation and not intended to be limiting, in FIGS.  5 - 8 , variations of optical sensing modules  30  are identified by reference numerals  305 ,  306 ,  307 ,  308 , variations of analyte signal detector  42  are identified by reference numerals  425 ,  426 ,  427 ,  428 , variations of reference signal detector  44  are identified by reference numerals  445 ,  446 ,  447 ,  448 , variations of stimulating light emitter  40  are identified by reference numerals  405 ,  406 ,  407 ,  408 , variations of optical body  38  are identified by reference numerals  385 ,  386 ,  387 ,  388  and variations of lower surface  33  of optical body  38  are identified by reference numerals  335 ,  336 ,  337 ,  338 . For simplicity, sensing layer  36  and indicator molecules  60 ,  62  are omitted from FIGS.  5 - 8  but those of skill in the art will understand that such are present on window  13  when optical module  30  is present in housing  12 . The explanations provided earlier about elements  30 ,  33 ,  38 ,  42 ,  44  also apply to the variations identified here.  
         [0026]    [0026]FIG. 5A is a side view and FIG. 5B is a bottom view of optical sensing module  305  showing opposed pairs of photodetectors  425  and opposed pairs of photodetectors  445 . Stimulating light emitter  405  is located on bottom surface  335  of electro-optic module  305  opposite outer surface  131  of window  13  on which sensing layer  36  (not shown) would be located. Optical body  385  has a generally truncated cone shape with the larger diameter upper portion mating with housing  12  and surface  131  for supporting sensing layer  36 . Having the photodetectors arranged in opposing pairs increases the optical coupling through optical body  385  and provides a strong signal.  
         [0027]    [0027]FIG. 6A is a side view and FIG. 6B is a bottom view of optical sensing module  306  showing opposed pairs of photodetectors  426  and opposed pairs of photodetectors  446 . Stimulating light emitter  406  is located on bottom surface  336  of electro-optic module  306  opposite outer surface  131  on which sensing layer  36  (not shown) would be located. Optical body  386  has a generally truncated cone shape with the larger diameter upper portion mating with housing  12  and surface  131  for supporting sensing layer  36 . The lower, cone-shaped portion has two curved sides on which detectors  426  are located and two flat sides  346  on which detectors  426  are located. Having the photodetectors arranged in opposing pairs increases the optical coupling through optical body  386  and provides a strong signal.  
         [0028]    [0028]FIG. 7A is a side view and FIG. 7B is a bottom view of optical sensing module  307  showing photodetectors  427  and photodetectors  447  in an opposed arrangement. Stimulating light emitter  407  is located on bottom surface  337  of electro-optic module  307  opposite outer surface  131  of window  13  on which sensing layer  36  (not shown) would be located. Optical body  387  has a generally truncated cone shape with the larger diameter upper portion mating with housing  12  and surface  131  for supporting sensing layer  36 . Having the photodetectors arranged so as to lap substantially around side  347  of the cone shape increases the optical coupling through optical body  387  and provides a strong signal.  
         [0029]    [0029]FIG. 8A is a side view and FIG. 8B is a bottom view of optical sensing module  308  showing opposed pairs of photodetectors  428  and opposed pairs of photodetectors  448 . Stimulating light emitter  408  is located on bottom surface  338  of electro-optic module  308  opposite outer surface  131  of window  13  on which sensing layer  36  (not shown) would be located. Optical body  388  has a generally truncated pyramid shape with the larger upper portion mating with housing  12  and surface  131  for supporting sensing layer  36 , and opposed flat sides  348 ,  348 ′. Having the photodetectors  428 ,  448  arranged in opposing pairs increases the optical coupling through optical body  388  and provides a strong signal. As used herein, the term “truncated cone” is intended to include any of the shapes of body portions  385 - 388  of modules  305 - 308  shown in FIGS.  5 - 8  irrespective as to whether the sides of the truncated cone are curved, flat or a combination thereof.  
         [0030]    [0030]FIG. 9 is a simplified schematic cross-sectional view similar to FIG. 1, of implantable chemical sensor  10 ′ according to a still further embodiment of the present invention. Chemical sensor  10 ′ comprises hermetic enclosure or housing  12 ′ that is sealable along joint  14 ′. Conveniently located within housing  12 ′ are optical modules  30 - 1 ,  30 - 2 ,  30 - 3 , electronic control module  16 ′, energy source  18 ′ (e.g., one or more batteries), optional transceiver  20 ′ and optional antenna  22 ′. Optical modules  30 - 1 ,  30 - 2 ,  30 - 3  have therein light emitter(s)  40 - 1 ,  40 - 2 ,  40 - 3  coupled to control module  16 ′ by leads  51 - 1 ,  51 - 2 ,  51 - 3 , light detectors  42 - 1 ,  42 - 2 ,  42 - 3  and  44 - 1 ,  44 - 2 ,  44 - 3  coupled to control module  16 ′ by leads  53 - 1 ,  53 - 2 ,  53 - 3  and  55 - 1 ,  55 - 2 ,  55 - 3 , respectively, and other components as will be presently described. Control module  16 ′ is coupled to energy source  18 ′ by lead(s)  17 ′ and to transceiver  20 ′ by lead(s)  15 ′. Transceiver  20 ′ is coupled to antenna  22 ′ by lead(s)  21 ′. While transceiver  20 ′ and antenna  22 ′ are conveniently provided for communicating to and from sensor  10 ′ they are not essential and externally coupled lead(s)  57 ′ may also be used. Further, while it is desirable that instructions or other commands be able to be sent to sensor  10 ′ while in situ using lead(s)  57 ′ or antenna  22 ′, this is not essential. Sensor  10 ′ can be preprogrammed to merely report measured data after implantation without further instructions from outside.  
         [0031]    Sensor  10 ′ differs from sensor  10  of FIG. 1 in that sensor  10 ′ has three optical modules  30 - 1 ,  30 - 2 , and  30 - 3  therein, analogous to module  30  of FIG. 1 and modules  302 - 308  of FIGS.  2 - 8 . While three optical modules are shown in FIG. 9, persons of skill in the art will understand that this is merely for convenience of explanation and that any number greater than one may be provided in sensor  10 ′, consistent with the space limitations where sensor  10 ′ is intended to be placed and the number of different analytes desired to be simultaneously detected or measured.  
         [0032]    Modules  30 - 1 ,  30 - 2 , and  30 - 3  are hermetically sealed to enclosure  12 ′ and electrically coupled to control module  16 ′. Optical modules  30 - 1 ,  30 - 2 ,  30 - 3  have, respectively, optical bodies  38 - 1 ,  38 - 2 ,  38 - 3  analogous to optical body  38  of sensor  10 , and corresponding light sources  40 - 1 ,  40 - 2 ,  40 - 3  and detectors  42 - 1 ,  42 - 2 ,  42 - 3  and  44 - 1 ,  44 - 2 ,  44 - 3  analogous to light source  40  and detectors  42 ,  44  of sensor  10  of FIG. 1. Connecting leads  51 - 1 ,  51 - 2 ,  51 - 3 ;  53 - 1 ,  53 - 2 ,  53 - 3 ; and  55 - 1 ,  55 - 2 ,  55 - 3  are analogous to leads  51 ,  53 , and  55  of sensor  10  of FIG. 1. For simplicity, separate window(s)  13  and filters  50 ,  52 ,  54  are not shown in sensor elements  30 - 1 ,  30 - 2  and  30 - 3  of FIG. 9 but persons of skill in the art will understand that they can be included as desired. Temperature sensor [T], pressure sensor  56  and light beams  41 ,  43 ,  45  shown in FIG. 1 are omitted from FIG. 9 for clarity but this is not intended to be limiting or imply that such features or elements are not included.  
         [0033]    Optical modules  30 - 1 ,  30 - 2 ,  30 - 3  have, respectively, outer surfaces  131 - 1 ,  131 - 2 ,  131 - 3  analogous to outer surface  131  of FIGS.  1 - 4 , on which are located sensor layers  36 - 1 ,  36 - 2 , and  36 - 3  equivalent to layers  36 ,  361 - 364  shown in FIGS.  1 - 4 . Similarly, indicator molecules and reference molecules  60 ,  62  are omitted from FIG. 9 for simplicity and not intended to be limiting. Optical modules  30 - 1 ,  30 - 2 ,  30 - 3  perform substantially the same function as optical modules  30 ,  302 - 308  of FIGS.  1 - 8  in substantially the same way and the discussion and variations associated with FIGS.  1 - 8  are applicable to modules  30 - 1 ,  30 - 2 ,  30 - 3  and sensor  10 ′ of FIG. 9. In FIG. 9, there is further illustrated in module  30 - 2 , the optional use of non-planar outer surface  131 - 2  supporting sensor layer  36 - 2 .  
         [0034]    Multiple optical modules  30 - 1 ,  30 - 2 ,  30 - 3  in sensor  10 ′ are conveniently used with sensor layers  36 - 1 ,  36 - 2 ,  36 - 3  of different composition, with different sensing and/or reference molecules. The wavelengths emitted by light sources  40 - 1 ,  40 - 2 ,  40 - 3  and the filters used with light sources  40 - 1 ,  40 - 2 ,  40 - 3  and with detectors  42 - 1 ,  42 - 2 ,  42 - 3  and  44 - 1 ,  44 - 2 ,  44 - 3  can be appropriately modified for the different sensing layers intended to respond to different analytes. In this way, different analytes of ambient  37  may be separately monitored at the same time using single implantable sensor  10 ′. Further, sensor molecules and reference molecules may be place on different sensor elements rather than combined on the same sensor element in situations where they might undesirably interact. Thus, the use of multiple optical modules in the same housing can greatly enhance the ability to monitor important biological and/or chemical activity in situ in a human or animal body. As used herein, the word “animal” is intended to include any non-human animate organism on land or in the sea or air. Persons of skill in the art will also understand that while the present invention is particularly useful in connection with living organisms, it may also be used in any situation where in situ measurement or detection of particular analytes is desired and relevant sensor molecules are available.  
         [0035]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.