Patent Publication Number: US-2006000272-A1

Title: Thermal flow sensor having an asymmetric design

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a thermal flow sensor. More particularly, the present invention relates to a thermal flow sensor that can be used to monitor the flow of cerebrospinal fluid (CSF) within a shunt.  
      2. Discussion of Related Art  
      Hydrocephalus is a condition caused by an abnormal accumulation of CSF in cavities inside the brain. If not treated properly, hydrocephalus can cause severe disablements in children and adults, and can even cause death. If cerebrospinal fluid accumulates, the ventricles become enlarged and the pressure inside the brain increases. Hydrocephalus is a severe degenerative condition that occurs in children from birth on. Hydrocephalus is presumably caused by a complex interaction between genetic and environmental factors. A person can also acquire hydrocephalus later in life, which can be due to, for example, spina bifida, brain hemorrhage, meningitis, skull trauma, tumors and cysts. Hydrocephalus occurs in newborns with a frequency of approximately 1 out of 5,000-10,000. There is currently no known prevention or cure for hydrocephalus. The most effective treatment so far is the surgical implantation of a shunt behind the ear. A shunt is a flexible tube that is inserted into the ventricular system of the brain to divert the cerebro fluid to other regions of the body. However, shunts frequently malfunction, leading to infections which can cause severe complications for the patient (e.g., delayed development, learning disabilities).  
      According to some estimates, up to 50% of patients who receive a shunt, will have the shunt malfunction at some time during his or her lifetime. Most shunt malfunctions are due to a blocked catheter and an incorrectly adjusted shunt valve.  
      The present inventors believe that the occurrence of complications due to a shunt malfunction can be detected easier by using a miniaturized implantable flow sensor, in accordance with the present invention, that has been developed for monitoring CSF flow. The sensor employs temperature sensors and a heater that do not contact the CSF, yet measures the CSF flow and can therefore be implanted so as to last for an extended period of time (e.g., greater than 10 years). In particular, when a shunt valve is implanted in children, a malfunction of the implant can be effectively detected by use of an additional implanted sensor.  
      The thermal flow sensor in accordance with the present invention represents a significant advance in the treatment of hydrocephalus in patients and also represents an additional step towards the development of a closed-loop control system, which can continuously optimize the flow rate in the patient&#39;s shunt valve.  
      In addition, the thermal flow sensor of the present invention provides physicians with novel, previously unattainable information about the formation and drainage of cerebro spinal fluid (CSF).  
     SUMMARY OF THE INVENTION  
      In accordance with a currently preferred exemplary embodiment, the present invention involves athermal flow sensor having a first substrate having a first side and a second opposite side. A second substrate has a first side and a second opposite side. The first substrate is connected to the second substrate such that the second side of the first substrate abuts the first side of the second substrate. A third substrate has a first side and a second opposite side. The third substrate is connected to the second substrate such that the second side of the second substrate abuts the first side of the third substrate. The second substrate has a groove formed therein so as to form a conduit bounded by the second substrate and the second side of the first substrate and the first side of the third substrate. The conduit has a fluid flow direction. A heater is disposed on the first side of the first substrate opposed to the conduit. A first temperature sensor is disposed on the first side of the first substrate opposed to the conduit and at a first predetermined distance from the heater in a direction opposite to the fluid flow direction. A second temperature sensor is disposed on the first side of the first substrate opposed to the conduit and at a second predetermined distance from the heater in a direction opposite to the fluid flow direction. The second predetermined distance is greater than the first predetermined direction.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components, and wherein:  
       FIG. 1  is a perspective view of the thermal flow sensor in accordance with the presentinvention;  
       FIG. 2  is a cross-sectional schematic view taken along line  2 - 2  of  FIG. 1  and looking in the direction of the arrows;  
       FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 2  and looking in the direction of the arrows;  
       FIG. 4A  is a cross-sectional view similar to  FIG. 2  showing the thermal flow sensor having only two substrates with the groove formed in the second substrate;  
       FIG. 4B  is a cross-sectional view similar to  FIG. 2  showing the thermal flow sensor having only two substrates with the groove formed in the first substrate;  
       FIG. 4C  is a cross-sectional view similar to  FIG. 2  showing the thermal flow sensor having only two substrates with the groove formed in both the first and second substrate;  
       FIG. 5  is a cross-sectional view similar to  FIG. 2  showing the thermal flow sensor having only one substrate with the groove formed therein;  
       FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 5  and looking in the direction of the arrows;  
       FIG. 7  is an enlarged partial perspective view of the first substrate and the heater and two temperature sensors mounted on the upper surface of the first substrate;  
       FIG. 8A  is a partial cross-sectional view of the thermal flow sensor showing the recesses in the first side of the first substrate;  
       FIG. 8B  is a partial cross-sectional view of the thermal flow sensor showing the recess in the second side of the first substrate;  
       FIG. 8C  is a partial cross-sectional view of the thermal flow sensor showing one of the recesses on the first side of the first substrate and the other recess on the second side of the first substrate;  
       FIG. 8D  is a partial cross-sectional view of the thermal flow sensor showing the recesses on the first side of the first substrate and on the second side of the first substrate;  
       FIG. 9A  is a cross-sectional view of the thermal flow sensor showing an asymmetric design of the temperature sensors upstream from the heater;  
       FIG. 9B  is a cross-sectional view of the thermal flow sensor showing an asymmetric design of the temperature sensors downstream from the heater;  
       FIG. 10A  is a cross-sectional view of the thermal flow sensor showing an asymmetric design of the temperature sensors upstream from the heater and within the conduit;  
       FIG. 10B  is a cross-sectional view of the thermal flow sensor showing an asymmetric design of the temperature sensors downstream from the heater and within the conduit;  
       FIG. 11  is a perspective view showing the thermal flow sensor being incorporated within a shunt; and  
       FIG. 12  is a schematic plan view of the first or second side of the first substrate showing the heater and temperature sensors.  
    
    
     DETAILED DESCRIPTION OF THE CURRENTLY PREFERRED EXEMPLARY EMBODIMENT  
      Referring now to FIGS.  1  though  6 , a thermal flow sensor  10  in accordance with the present invention is illustrated. The thermal flow sensor in a currently preferred exemplary embodiment includes a first substrate  12 , a second substrate  14  and a third substrate  16 . First substrate  12  has a first side  18  and a second opposite side  20 . Second substrate  14  has a first side  22  and a second opposite side  24 . Third substrate  16  has a first side  26  and a second opposite side  28 . First substrate  12  is connected to second substrate  14  such that the second side  20  of the first substrate  12  abuts the first side  22  of the second substrate  14 . Third substrate  16  is connected to the second substrate  14  such that the second side  24  of the second substrate  14  abuts the first side  26  of the third substrate. The first substrate is preferably bonded to the second substrate, and the second substrate is preferably bonded to the third substrate. The first and third substrates are preferably made of borosilicate glass, for example PYREX® or BOROFLOAT®. The second substrate is preferably made of silicon.  
      Second substrate  14  has a groove  30  formed therein so as to form a conduit  32  bounded by the second substrate  14  and the second side  20  of the first substrate and the first side  26  of the third substrate, as illustrated in  FIG. 1 . The groove is preferably formed by etching into the silicon second substrate  14 . In one exemplary embodiment, the groove may have a cross-sectional dimension of 380 μm×3000 μm. A heater  34  is disposed on the first side  18  of the first substrate  12  opposed to conduit  32 . A first temperature sensor  36  is disposed on the first side  18  of said first substrate  12  opposed to conduit  32 . A second temperature sensor  38  is also disposed on the first side  18  of the first substrate  12  opposed to conduit  32 . This sensor can detect a temperature difference of approximately 0.005° C. at a flow rate of 300 ml/hr.  
      The temperature sensors and heater are preferably created by metal deposition (e.g., evaporation or sputtering) directly on the first side or second side of the first substrate, which is preferably made of borosilicate glass. These metal deposition processes permit one to deposit thin films of metal on the glass surface within a vacuum chamber. A person skilled in the art will readily understand how to pattern the thin films by lithographic processes. In one exemplary embodiment, the metal thin film is made of several layers (e.g., Chromium (Cr), Platinum (Pt), Titanium (Ti) and Gold (Au)). Chromium or Titanium is preferably used as an adhesion layer since it sticks well to the borosilicate glass. Afterwards, a layer of Pt is deposited on the Cr or Ti so that it may be used as the heater and temperature structures. One may also at the same time as when the heater and temperature sensors are created, create the electrical tracks for the remainder of the electronics on the same substrate. A gold layer is preferably deposited on top of the platinum only in the region where there is no heater or temperature sensor structure and serves as the electrical tracks for the rest of the electronic circuit. However, in the region where there is no heater or temperature sensor, the gold layer could be deposited directly on the adhesion layer of Cr or Ti. The heater works by resistive heating by passing a current therethrough, as shown in  FIG. 12 . The temperature sensors work by having their resistance change due to its ambient temperature, as also shown in  FIG. 12 . In the present invention sensors, the ambient temperature at each temperature sensor is dependent upon among other things, the amount of heat created by the heater, the thickness of the first substrate, and the flow rate of the fluid flowing through the conduit.  
      A cap  40  is mounted on the first side  18  of the first substrate  12 , thereby forming an interior chamber  42 . Cap  40  is preferably made of PYREX® and is brazed to the first substrate, thereby forming a hermetically sealed interior chamber  42 . When the sensor is used as an implantable medical device, a final parylene layer is applied on the outer surface of the sensor to prevent rejection of the implant by the body. Heater  34 , first temperature sensor  36  and second temperature sensor  38  are disposed within interior chamber  42 . Other electronics  44  are also disposed within chamber  42  and are electrically connected to heater  34 , first temperature sensor  36  and second temperature sensor  38 . One skilled in the art will readily know how to assemble the electronics so that data from the heater and/or sensors can be communicated by telemetry to and from an external control unit. By placing the temperature sensors and the heater on the opposite side of the first substrate from the conduit, the sensors and heater are not in direct contact with the fluid (e.g., CSF) within the conduit. This structure is referred to as an inverted substrate. Thus, the sensor in accordance with the present invention is a biocompatible design, which is favorable for long-term implants such as a hydrocephalus shunt, an infusion pump (e.g. &gt;10 years). The biocompatible packaging of the sensor and the electronics has at least the following advantages: 
          The body fluid comes in contact only with biocompatible glass.     The Ti/Pt sensors, heater and sensor electronics are located on the same substrate, which reduces their manufacturing cost.     The sensor electronics can be drastically miniaturized by employing an ASIC, which can be fabricated by flip-chip technology.        

      In accordance with an alternative embodiment, the thermal flow sensor can comprise of two substrates  12 ′ and  14 ′, with a groove  30 ′ formed within either substrate or both to form a conduit  32 ′ bounded by both substrates, as illustrated in  FIGS. 4   a, b  and  c.  In another alternative embodiment, the thermal flow sensor can comprise of only one substrate  12 ″, as shown in  FIGS. 5 and 6 . Substrate  12 ″ has a first upper side  18 ″ and a second opposite lower side  20 ″, and at least one side edge  46 ″ extending between first upper side  18 ″ and second lower side  20 ″. A conduit  32 ″ is formed within substrate  12 ″. Conduit  12 ″ has an inlet opening  48 ″ and an outlet opening  50 ″. Each of the openings  48 ″,  50 ″ are formed in the at least one edge  46 ″, as shown in  FIG. 3 .  
      To determine the flow rate of a fluid flowing within conduit  32 ,  32 ′,  32 ″, fluid is permitted to flow through the conduit by entering into the inlet opening of the conduit and exiting from the exit opening. The fluid is heated with the heater  34  opposed to and remote from the conduit. In other words, the heater and temperature sensors are not in contact with the fluid flowing within the conduit. The temperature of the fluid is detected with the first temperature sensor disposed on the first side of the body opposed to and remote from the conduit. The temperature of the fluid may also be detected with the second temperature sensor disposed on the first side of the body opposed to and remote from the conduit. In a currently preferred exemplary embodiment, the two temperature sensors are spaced apart by about 2000 μm. The spacing between the temperature sensors is in part dependent upon the flow rate to be measured. Based on the detected temperature(s), the flow rate of the fluid can readily be determined by one skilled in the art. The fluid is preferably CSF, and thermal flow sensor  10  is preferably disposed within shunt  100 , as shown in  FIG. 10 .  
      In designing the sensor in accordance with the present invention, the sensor was optimized through static and dynamic FEM simulations for flow ranges reaching 300 ml/hr, with optimized sensitivity at a flow range of 25 ml/hr, and for rapid step responses of 2 seconds. The normal flow range of CSF is about 25 ml/hr. At a flow range of 25 ml/hr, the sensitivity of the sensor signal is about 140 mV/ml/hr; and for high flow ranges of &gt;270 ml/hr, the sensitivity of the sensor signal is still about 5 mV/ml/hr. The response time of the sensor of about 2 sec. is considerably reduced as compared to about 10 sec. for conventional sensors on a glass substrate. In addition, these conventional sensors can only detect flow rates up to 2-3 ml/hr. The fast step response makes it possible to measure CSF flow even when the patient&#39;s head position changes rapidly (e.g., when arising, or getting up from sleeping, etc.).  
      Referring now to  FIG. 3 , the first, second and third substrate together form a multi-layer body structure that has at least one edge  46  extending between the first side  18  of the first substrate and said second side  28  of the third substrate. Conduit  32  has an inlet opening  48  and an outlet opening  50 , each of which are formed in the at least one edge  46 . In a currently preferred exemplary embodiment, inlet opening  48  and outlet opening  50  are disposed solely in the second substrate  14 . A dicing saw may be used to cut through the three layers to expose the openings in the second substrate. This embodiment is referred to as a streamline packaging because the inlet and outlet openings are in the side edges of the body structure as opposed to the top and/or bottom surface.  
      Referring now to  FIG. 7 , in accordance with another embodiment of the present invention, a first recess  52  is formed in the first side  18  of the first substrate  12  between heater  34  and the first temperature sensor  36 . As shown, first recess  52  is disposed immediately adjacent to heater  34 . A second recess  54  is formed in the first side  18  of the first substrate  12  between heater  34  and the second temperature sensor  38  (see  FIG. 8A ). As shown, second recess  54  is disposed immediately adjacent to heater  34  on an opposite side of the heater from the first recess. Alternatively, as shown in  FIGS. 8B and 8C , the recesses  52 ,  54  can be formed in the second side of the first substrate  12  or one on one side of the first substrate and the other on the second side of the first substrate, respectively. Recesses  52 ,  54  preferably extend into the first substrate for about half of the thickness of the first substrate. In accordance with another variation of the present invention, the recesses  52 ,  54  can be disposed on the first side of the first substrate and on the second side of the first substrate.  
      The recesses  52 ,  54  are used to help guide the heat generated by heater  34  through the first substrate, as indicated by arrows A, and into conduit  32 . The heat energy absorbed by the fluid is then transferred back through the first substrate, as indicated by arrows B, to the first and second temperature sensors. Because air is not a very good conductor of heat, most, if not effectively all, of the heat generated by the heater travels along the path indicated by arrows A and B. Of course, some heat will travel through the first substrate, but one of skill in the art will readily be able to calibrate the thermal flow sensor in accordance with the present invention to take this factor into account. Depending upon the thickness of the first substrate, how much heat is generated by the heater, the dimension of the recesses, and other factors known to those skilled in the art, one can readily determine the flow rate of the fluid flowing through the conduit. This information can then be transmitted by telemetry to an external control unit (not shown).  
      As in the previous alternative embodiments shown in  FIGS. 4A-5 , the thermal flow sensor having recesses  52 ,  54  can also be comprised of two substrates  12 ′ and  14 ′, as illustrated in  FIGS. 4   a, b  and  c,  or with only one substrate  12 ″, as shown in  FIG. 5 .  
      Referring now to  FIG. 9A , a thermal flow sensor in accordance with yet another embodiment of the present invention is illustrated. In this embodiment, first temperature sensor  36  is disposed on the first side of the first substrate opposed to the conduit and at a first predetermined distance from heater  34  in a direction opposite to the fluid flow direction within the conduit. Second temperature sensor  38  is disposed on the first side of the first substrate opposed to the conduit and at a second predetermined distance from heater  34  in a direction opposite to the fluid flow direction. As illustrated in  FIG. 9A , the second predetermined distance is greater than the first predetermined distance. This embodiment is referred to as an asymmetric sensor design because both temperature sensors are disposed on one side of the heater, as opposed to having the heater being disposed between the two temperature sensors with respect to the flow direction. Referring now to  FIG. 9B , a variation of the embodiment of  FIG. 9  is illustrated. In this variation, the first and second temperature sensors are disposed at a respective first and second predetermined distance from the heater in the fluid flow direction, as opposed to opposite to the fluid flow direction.  
      Referring now to  FIGS. 10A and 10B , another variation of the embodiment of  FIG. 9  is illustrated. In accordance with this variation, the heater and the temperature sensors are disposed within the conduit and, therefore, in contact with the fluid flowing within the conduit. In accordance with this variation the first and second temperature sensors are disposed at a respective first and second predetermined distance from the heater just as in the  FIG. 9A  embodiment opposite to the fluid flow direction as shown in  FIG. 10 , or as in the  FIG. 9B  embodiment in the fluid flow direction, as shown in  FIG. 10 . As in the previous alternative embodiments shown in  FIGS. 4A-5 , the thermal flow sensor, which has the first and second temperature sensors disposed on the sane side of the heater, either opposite to the flow direction or in the flow direction, can also be comprised of two substrates  12 ′ and  14 ′, as illustrated in  FIGS. 4   a, b  and  c,  or with only one substrate  12 ″, as shown in  FIG. 5 .  
      The present inventors have discovered that the asymmetric sensor design can not detect flow below a certain flow rate that will be referred to as the cut-off flow rate. The cut-off flow rate is typically about 1 to 2 ml/hr. To detect flow from 0 ml/hr up to the cut-off rate, one may use a second heater  56 , as illustrated in  FIG. 9A . Heater  56  is disposed between second sensor  38  and first sensor  36  with respect to the flow direction.  
      Having described the presently preferred exemplary embodiment of a thermal flow sensor in accordance with the present invention, it is believed that other modifications, variations and changes will be suggested to those skilled in the art in view of the teachings set forth herein. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale, but that they are merely conceptual in nature. It is, therefore, to be understood that all such modifications, variations, and changes are believed to fall within the scope of the present invention as defined by the appended claims.  
      Every issued patent, pending patent application, publication, journal article, book or any other reference cited herein is each incorporated by reference in their entirety.