Abstract:
A method of creating a sensor that may include applying a first conductive material on a first portion of a substrate to form a reference electrode and depositing a first mask over the substrate, the first mask having an opening that exposes the reference electrode and a second portion of the substrate. The method may also include depositing a second conductive material into the opening in the first mask, the second conductive material being in direct contact with the reference electrode and depositing a second mask over the second conductive material, the second mask having an opening over the second portion of the substrate, the opening exposing a portion of the second conductive material, which forms a working surface to receive a fluid of interest.

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §119 
       [0001]    The present Application for Patent claims priority to Provisional Application No. 60/777,133 filed Feb. 27, 2006, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The invention relates generally to flex circuit technology. More specifically, the invention relates to using flex circuit technology to create a reference electrode channel. 
       BACKGROUND 
       [0003]    Flex circuits have been used in the micro-electronics industry for many years. In recent years, flex circuits have been used to design microelectrodes for in vivo applications. One flex circuit design involves a laminate of a conductive foil (e.g., copper) on a flexible dielectric substrate (e.g., polyimide). The flex circuit is formed on the conductive foil using masking and photolithography techniques. Flex circuits are desirable due to their low manufacturing cost, ease in design integration, and flexibility in motion applications. 
       SUMMARY 
       [0004]    The invention relates to a method of creating a sensor that may include applying a first conductive material on a first portion of a substrate to form a reference electrode and depositing a first mask over the substrate, the first mask having an opening that exposes the reference electrode and a second portion of the substrate. The method may also include depositing a second conductive material into the opening in the first mask, the second conductive material being in direct contact with the reference electrode and depositing a second mask over the second conductive material, the second mask having an opening over the second portion of the substrate, the opening exposing a portion of the second conductive material, which forms a working surface to receive a fluid of interest. 
         [0005]    The invention relates to a method of creating a sensor that may include applying a first conductive material on a first portion of a substrate to form a reference electrode and a second portion of the substrate to form a working electrode, and depositing a first mask on the substrate, the first mask having an opening that exposes the reference electrode, the working electrode, and an area between the reference electrode and the working electrode. The method may also include depositing a second conductive material on the reference electrode and in the area between the reference electrode and the working electrode and depositing a second mask on the second conductive material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The features, objects, and advantages of the invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, wherein: 
           [0007]      FIG. 1  is a cross-section view of a reference electrode channel that is created using a flex circuit according to an embodiment of the invention. 
           [0008]      FIG. 2  is a top view of a flex circuit according to an embodiment of the invention. 
           [0009]      FIG. 3  is a top view of a mask that is used to cover the flex circuit shown in  FIG. 2  according to an embodiment of the invention. 
           [0010]      FIG. 4  is a top view showing a conductive material deposited into the opening of the mask according to an embodiment of the invention. 
           [0011]      FIG. 5  is a top view of a mask that is used to cover a portion of the conductive material and the mask shown in  FIG. 4  according to an embodiment of the invention. 
           [0012]      FIG. 6  is a flow chart showing a method of creating the reference electrode channel of  FIG. 1  according to an embodiment of the invention. 
           [0013]      FIG. 7  is a cross-section view of a reference electrode channel that is created using a flex circuit according to an embodiment of the invention. 
           [0014]      FIG. 8  is a top view of a flex circuit according to an embodiment of the invention. 
           [0015]      FIG. 9  is a top view of a mask that is used to cover the flex circuit shown in  FIG. 8  according to an embodiment of the invention. 
           [0016]      FIG. 10  is a top view showing a conductive material deposited into the opening of the mask according to an embodiment of the invention. 
           [0017]      FIG. 11  is a top view of a mask that is used to cover the conductive material and the mask shown in  FIG. 10  according to an embodiment of the invention. 
           [0018]      FIG. 12  is a flow chart showing a method of creating the reference electrode channel of  FIG. 7  according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The invention is directed toward using a flex circuit to create a reference electrode channel. The flex circuit has a reference electrode that is masked and imaged onto a substrate. A first mask is deposited on the substrate. The first mask may have an opening that has a first end that exposes a portion of the reference electrode and a second end that exposes a portion of the substrate. The opening forms a reference electrode channel. A conductive material may be deposited into the opening of the first mask. A second mask is deposited on the first mask and the conductive material. The second mask may have an opening that exposes a portion of the conductive material that is over the substrate. 
         [0020]      FIG. 1  is a cross-section view of a reference electrode channel that is created using a flex circuit according to an embodiment of the invention. The flex circuit  100  may include a substrate  110 , a trace  120 , and a reference electrode  125 . The trace  120  and the reference electrode  125  may be masked and imaged onto the substrate  105 . For example, the trace  120  and the reference electrode  125  may be formed on the substrate  105  using screen printing or ink deposition techniques. The trace  120  and the reference electrode  125  may be made of a carbon, copper, gold, graphite, platinum, silver-silver chloride, rodium, or palladium material. 
         [0021]    A first mask  130  may be applied or deposited over a portion of the substrate  110  and over the trace  120 . The first mask  130  may have an opening  135  that expose a portion of the reference electrode  125  and a portion of the substrate  110 . The opening  135  forms the reference electrode channel. A conductive material  140  is deposited in the opening  135  to cover the exposed portion of the reference electrode  125  and the exposed portion of the substrate  110 . A second mask  150  may be applied or deposited over the first mask  130  and the conductive material  140 . The second mask  150  may have an opening  160  over a portion of the conductive material  140  that is over the substrate  110 . The opening  135  is positioned along a first axis or plane and the opening  160  is positioned along a second axis or plane. The first axis or plane is not coincident with the second axis or plane. Hence, the first axis or plane is vertically and/or horizontally offset from the second axis or plane. 
         [0022]    The opening  160  is the measurement site and allows a fluid of interest (e.g., blood, urine, etc.) to come into contact with the conductive material  140  to complete the measurement circuit with another measuring electrode (not show) in contact with the same fluid. The conductive material  140  stabilizes the reference potential in several ways. The conductive material  140  may provide known silver and chloride ion activity, for example, (in the case of a silver-silver chloride reference design) to maintain a stable potential. The conductive material  140  should offer sufficient diffusion resistance to inhibit loss of desired ions to the fluid of interest, while simultaneously inhibiting migration of unwanted ions toward the active surface of the reference electrode  125 . Spacing the opening  160  a sufficient distance from the reference electrode  125 , as shown in  FIG. 1 , enhances this diffusion resistance. Finally, the conductive material  140  may provide a predictable junction potential at the interface with the fluid of interest which facilitates accurate electrochemical measurements using the reference electrode  125 . 
         [0023]      FIG. 2  is a top view of a flex circuit  100  according to an embodiment of the invention. The trace  120  and the reference electrode  125  may be made of a conductive material such as a silver-silver chloride (Ag/AgCl) material and may be formed on the substrate  110  using photolithography or printing techniques ( 610 ). For example, the trace  120  and the reference electrode  125  may be formed on the substrate  110  using screen printing or ink deposition techniques. The substrate  110  may be a flexible dielectric substrate such as a polyimide. The trace  120  may be used to connect to a measurement device (not shown) such as a potentiostat. The trace  120  is used to measure a potential from the reference electrode  125  using the measurement device. Even though  FIG. 1  shows the flex circuit  100  having one trace  120  and one reference electrode  125 , the flex circuit  100  may have more than one trace and more than one electrode. 
         [0024]      FIG. 3  is a top view of a mask  130  that is used to cover the flex circuit  100  shown in  FIG. 2  according to an embodiment of the invention. The mask  130  may be made of a dielectric material such as a photoimagable epoxy or an ultraviolet curable epoxy material. The mask  130  is deposited over the substrate  110  and has a rectangular opening  135  that has a first end  135   a  that exposes a portion of the reference electrode  125  and a second end  135   b  that exposes a portion of the substrate  110  ( 620 ). The rectangular opening  135  may have a length of between about 0.10-0.20 inches and a width of between about 0.010-0.020 inches. The length-to-width ratio of the rectangular opening  135  may be in the range of between about 4:1 to 12:1. In one embodiment, the mask  130  covers the entire top surface of the flex circuit  100  except for the rectangular opening  135 . The mask  130  may have a thickness of between about 0.005 inches and about 0.02 inches. The first end  135   a  of the opening  135  is positioned directly above the electrode  125  so that the electrode  125  is exposed or visible through the opening  135  of the mask  130 . Lithography techniques may be used to deposit or place the mask  130  on the flex circuit  100 . 
         [0025]      FIG. 4  is a top view showing a conductive material  140  deposited into the opening  135  of the mask  130  according to an embodiment of the invention. The conductive material  140  is deposited in the opening  135  to cover and to come into direct contact with the exposed portion of the reference electrode  125  and the exposed portion of the substrate  110  ( 630 ). The conductive material  140  may be a conductive fluid, a conductive solution, a conductive gel, a salt containing gel, a conductive polymer containing potassium chloride (KCl) with a small amount of silver ion (Ag + ), or a material having conductive properties. For the case of a silver-silver chloride reference electrode  125 , addition of a trace of silver nitrate solution to a matrix containing potassium chloride precipitates some amount of silver chloride within the conductive matrix, but maintains a silver ion concentration at a constant amount according to the solubility product of silver chloride, which is 1.56×10 −10 . 
         [0026]      FIG. 5  is a top view of a mask  150  that is used to cover a portion of the conductive material  140  and the mask  130  shown in  FIG. 4  according to an embodiment of the invention. The mask  150  may be made of a dielectric material such as a photoimagable epoxy or an ultraviolet curable epoxy material. The mask  150  has an opening  160  that exposes a portion of the conductive material  140  that forms a working surface to receive a fluid of interest ( 640 ). Lithography techniques may be used to deposit or place the mask  150  on the mask  130  and the conductive material  140 . 
         [0027]      FIG. 7  is a cross-section view of a reference electrode channel that is created using a flex circuit according to an embodiment of the invention. The flex circuit  200  may include a substrate  210 , traces  220  and  230 , a reference electrode  225 , and a working electrode  235 . The traces  220  and  230 , the reference electrode  225 , and the working electrode  235  may be masked and imaged onto the substrate  210 . For example, the traces  220  and  230 , the reference electrode  225 , and the working electrode  235  may be formed on the substrate  210  using screen printing or ink deposition techniques. The traces  220  and  230 , the reference electrode  225 , and the working electrode  235  may be made of a carbon, copper, gold, graphite, platinum, silver-silver chloride, rodium, or palladium material. 
         [0028]    A first mask  240  may be applied or deposited over a portion of the substrate  210  and over the traces  220  and  230 . The first mask  240  may have an opening  250  that expose a portion of the reference electrode  225 , a portion of the working electrode  235 , and a portion of the substrate  210 . The term “channel” (shown as channel  255 ) may be used to refer to the portion between the reference electrode  225  and the working electrode  235 . Hence, the opening  250  may form the reference electrode channel. A conductive material  260  is deposited in the opening  250  to cover and to come into direct contact with the exposed portion of the reference electrode  225  and up to the edge of the exposed portion of the substrate  210 . A second mask  265  may be applied or deposited over the first mask  240  and the conductive material  260 . The second mask  265  may have an opening  270  over a portion of the working electrode  235 . The reference electrode  225  is positioned along a first axis or plane and the working electrode  235  is positioned along a second axis or plane. The first axis or plane is not coincident with the second axis or plane. Hence, the first axis or plane is vertically and/or horizontally offset from the second axis or plane. 
         [0029]    The opening  270  is the measurement site and allows a fluid of interest (e.g., blood, urine, etc.) to come into contact with the working electrode  235  and the conductive material  260  for a more accurate measurement. The conductive material  260  stabilizes the reference potential in several ways. The conductive material  260  may provide known silver and chloride ion activity for example (in the case of a silver-silver chloride reference design) to maintain a stable potential. The conductive material  260  should offer sufficient diffusion resistance to inhibit loss of desired ions to the solution, while simultaneously inhibiting migration of unwanted ions toward the active surface of the reference electrode  225 . Spacing the opening  270  a sufficient distance from the reference electrode  225 , as shown in  FIG. 7 , enhances this diffusion resistance. In addition, the opening  270  communicates directly with the end of the conductive material  260  at a smaller opening  275 . The proximity of the smaller opening  275  to the working electrode  235  makes this embodiment ideal for situations where the solution resistance between the reference electrode and the working electrode needs to be keep at a minimum, such as in the case of a 3-electrode amperometric cell, for example. 
         [0030]      FIG. 8  is a top view of a flex circuit  100  according to an embodiment of the invention. The traces  220  and  230 , the reference electrode  225  and the working electrode  235  may be made of a conductive material such as a copper material, a platinum material, a silver-silver chloride (Ag/AgCl) material and are formed on the substrate  210  using masking and photolithography techniques ( 1210 ). For example, the traces  220  and  230 , the reference electrode  225 , and the working electrode  235  may be formed on the substrate  210  using screen printing or ink deposition techniques. The substrate  210  may be a flexible dielectric substrate such as a polyimide. The traces  220  and  230  may be used to connect to a measurement device (not shown) such as a potentiostat. The traces  220  and  230  may be used to carry voltage or current from the reference electrode  225  and the working electrode  235  to the measurement device. 
         [0031]      FIG. 9  is a top view of a mask  240  that is used to cover the flex circuit  200  shown in  FIG. 8  according to an embodiment of the invention. The mask  240  may be made of a dielectric material such as a photoimagable epoxy or an ultraviolet curable epoxy material. The mask  240  is deposited over the substrate  210  and has a rectangular opening  250  that has a first end  250   a  that exposes a portion of the reference electrode  225 , a second end  250   b  that exposes a portion of the working electrode  235 , and a channel or an area  255  between the reference electrode  225  and the working electrode  235  that exposes a portion of the substrate  210  ( 1220 ). The rectangular opening  250  may have a length of between about 0.10-0.20 inches and a width of between about 0.010-0.020 inches. The length-to-width ratio of the rectangular opening  250  may be in the range of between about 4:1 to 12:1. In one embodiment, the mask  240  covers the entire top surface of the flex circuit  210  except for the rectangular opening  250 . The mask  240  may have a thickness of between about 0.005 inches and about 0.02 inches. In one embodiment, the first end  250   a  of the opening  250  is positioned directly above the reference electrode  225  so that the reference electrode  225  is exposed or visible through the opening  250  of the mask  240 . In one embodiment, the second end  250   b  of the opening  250  is positioned directly above the working electrode  235  so that the working electrode  235  is exposed or visible through the opening  250  of the mask  240 . Lithography techniques may be used to deposit or place the mask  240  on the flex circuit  200 . 
         [0032]      FIG. 10  is a top view showing a conductive material  260  deposited into the opening  250  of the mask  240  according to an embodiment of the invention. The conductive material  260  is deposited in the opening  250  to cover and to come into direct contact with the exposed portion of the reference electrode  225  and in the area  255  between the reference electrode  225  and the working electrode  235  (i.e., on the exposed portion of the substrate  210 ) ( 1230 ). In one embodiment, a screenable gel or a conductive polymer is applied in the opening  250  to cover and to come into direct contact with the exposed portion of the reference electrode  225  and in the area  255  between the reference electrode  225  and the working electrode  235 . The conductive material  260  may be a conductive fluid, a conductive solution, a conductive gel, a salt containing gel, a conductive polymer containing potassium chloride (KCl) with a small amount of silver ion (Ag + ), or a material having conductive properties. The conductive material  260  may form a salt channel or a reference electrode channel. 
         [0033]      FIG. 11  is a top view of a mask  265  that is used to cover the conductive material  260  and the mask  240  shown in  FIG. 10  according to an embodiment of the invention. The mask  265  may be made of a dielectric material such as a photoimagable epoxy or an ultraviolet curable epoxy material. The mask  265  has an opening  270  that exposes a portion of the working electrode  235  and an edge of the conductive material  260 , which forms a space to receive a fluid of interest. Lithography techniques may be used to deposit or place the mask  265  on the mask  240  and the conductive material  260  ( 1240 ). 
         [0034]    While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.