Abstract:
A catheter for sensing vertebrate conditions has plural piezoresistive sensors directly electrically coupled to discrete application specific integrated circuits. A mechano-responsive element is in pressure sensitive relationship with the piezoresistive sensor to act as a transfer medium through which vertebrate conditions such as pressure, temperature, water, glucose, and pH are transferred. Each integrated circuit is electrically coupled to a data acquisition system with monolithic circuitry in order to amplify and multiplex signals from multiple sensors.

Description:
FIELD OF THE INVENTION 
     This invention relates to catheters. More specifically, this invention relates to catheters for sensing vertebrate conditions. 
     BACKGROUND OF THE INVENTION 
     Catheters known in the prior art have been used in humans and animals for diagnostic, monitoring and treatment purposes. Such catheters must be small and flexible in order to function without irritating a body part into which they are inserted. Catheters have been used to infuse medications or remove samples of tissue and fluid for analysis. Multi-lumen catheters have been used to infuse medication and remove samples at the same time. 
     If a sample must be removed for purposes of analysis, and taken to a laboratory for subsequent analysis, the delay in performing analysis and transmitting the data back to a doctor sometimes can be fatal to a patient. Catheters have also been used to form hydraulic columns for transmitting pressure readings to an external sensor. In pressure sensing catheters, the hydraulic column may have problems with air bubbles, kinks in the tubing, and blood clots affecting the reliability and accuracy of critical readings. 
     Catheters used for monitoring variations in blood pressure within a blood vessel include those using catheter tip transducers insertable into a blood vessel with the transducer providing direct pressure monitoring by transducing blood pressure at the region of interest. Such catheters have used a semiconductor material constructed and arranged with strain sensing beams for creating a proportionate electrical signal representative of the monitored pressure and transmitting the signal by electrical conductors through the length of the catheter to meters or the like located externally of the monitored body. 
     An example of such a catheter is seen in U.S. Pat. No. 4,274,423. This catheter includes a pressure sensor disposed within the end portion of a catheter. The pressure sensor uses a pressure sensitive diaphragm constructed from block of semiconductor material. The diaphragm is located adjacent a side port in the catheter housing connected to the end of the catheter with the port providing access to the pressure environment. The diaphragm is directly deflected relative to the pressure exerted thereon and the deflection is sensed by one or more strain gauges located within the diaphragm itself. The strain gauges are connected by conductors to a processor circuit located external of the catheter. 
     A concern with such catheters is to provide a transducer which is small, while also being sufficiently responsive to pressure variations to provide meaningful electrical output signals. The semiconductor block with the diaphragm or membrane of the type employed in U.S. Pat. No. 4,274,423, may well have a width on the order of 1.2 mm, which limits the size of the catheter and, hence, its application for use in measuring blood pressure and other conditions within a blood vessel. 
     U.S. Pat. No. 4,722,348, shows a pressure sensing catheter of small size, however, since the catheter body shown has side port pressure inlet apertures which are depressed into the catheter body, this catheter is susceptible to air bubbles being trapped within the pressure apertures, resulting in inaccurate sensor measurements. This catheter is also connected to a detector circuit located outside of the catheter. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a catheter having a piezoresistive pressure sensor directly electrically coupled to an integrated circuit for measuring physiological conditions of a vertebrate. The physiological condition is measured by a pressure change which is directly related to the condition and that pressure is detected by the piezoresistive sensor which transmits an electrical impulse directly to the circuit. 
     In one form of the invention, a catheter with an elongated axis carries a conductive member coupled to plural piezoresistive sensors for measuring physiological conditions of a vertebrate. Each sensor is directly electrically coupled with a bonding pad to a discrete integrated circuit carried internally of the catheter. 
     In another form of the invention, the catheter has a mechano-responsive element in functional relationship with the piezoresistive sensor. The mechano-responsive element acts as a transfer medium between a vertebrate condition and the piezoresistive sensor which is directly electrically coupled to integrated circuit. 
     Various physiological vertebrate conditions, e.g., pressure, pH, glucose, water, and temperature, may be detected by the pressure responsive sensors of the catheter. These conditions are transmitted to a computer based data acquisition system. In a preferred embodiment, a stranded copper wire is connected to an application specific integrated circuit and a piezoresistive sensor coupled therewith internally of the catheter. The piezoresistive sensor is directly electrically coupled to the integrated circuit with a solder bonding pad by low temperature thermal compression bonding. In this preferred embodiment, the sensor has a trough in its top surface that receives therein a mechano-responsive element. The mechano-responsive element comprises a fluid or a gel that is sealed from coming into direct contact with the body by a permeable film which acts as a sheath over the piezoresistive sensor. The mechano-responsive element enables conditions such as pH, glucose, water, and temperature to be detected and the detecting signal to be converted into pressure which is sensed by the piezoresistive sensor and then thereby converted to an electrical impulse which is transmitted to the circuit for ultimate processing by a computer. Each sensing site is connected to other sensing sites with the stranded copper wire. Data received at a sensing site from a condition measured from a vertebrate body is transmitted to the data acquisition system via monolithic circuitry. That is, each circuit amplifies and multiplexes signals from plural sensors using a simple count and select scheme. 
     Other advantages of the invention will become more apparent to those of ordinary skill upon review of the following detailed description of the preferred embodiment taken in conjunction with the accompanying detailed drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a catheter according to the present invention; 
     FIG. 2 is a perspective cutaway view of a single sensing site on the catheter of FIG. 1; 
     FIG. 2A is an alternative embodiment of the sensing site of FIG. 2; 
     FIG. 3 is a perspective cutaway view of the sensing site of FIG. 2A; and 
     FIG. 4 is an exploded perspective view of the sensing site of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As seen in FIG. 1, a catheter  10  has a catheter body  12  with an elongated axis  15  having at least one sensing site  14 . As will be used in the present description, a catheter  10  is generally an elongated tube or device for either measuring a physiological condition, or sampling a fluid or tissue. In the preferred embodiment, the catheter body  12  is made from a physiologically acceptable material, e.g., polyvinyl chlorate or silicone rubber. Such catheter bodies are available from Microvasive, Boston, Mass. 
     Each sensing site  14  is covered by a film  16  that prevents the sensing site  14  from coming into direct contact with a vertebrate body. As will be understood by those skilled in the art, the catheter  10  may be banded with the film  16  covering only the sensing site  14 , or the film  16  may be formed as a sleeve covering the entire catheter body  12 . In the preferred embodiment, the film  16  is made of silicon. In an alternative embodiment (FIG.  2 A), the film  16  is permeable to allow conditions such as pH, glucose, temperature, and water to interact with the sensing site  14 . 
     As seen in FIG. 2, the catheter body  12  defines a lumen  18  in fluid communication with a catheter recess  20  defined in the catheter body  12 . A conductive member  22  is carried in the lumen  18  and is approximately coextensive therewith (FIGS.  3  and  4 ). In the preferred embodiment, the conductive member  22  is stranded copper wire, however, those skilled in the art will understand that other conductive members  22  may be carried by the catheter body  12 . In an alternative embodiment, solid core copper wire is used as the conductive member  22 . In another alternative embodiment, copper traces (not shown) deposited on a non-conductive polymer strip (not shown) are used as the conductive member  22 . The conductive member  22  carries power to and data from a piezoresistive sensor  24  that is acted on by a vertebrate condition. In the preferred embodiment, the sensor  24  is about 1.74 to 1.76 mm long, about 1.74 to 1.76 mm wide, and about 0.24 to 0.26 mm deep. The sensor  24  has a bottom surface  29  and a top surface  26  defining a trough  28  therein. The trough  28  has a trough floor  30  that is deformable according to pressures exerted upon the floor  30  by a mechano-responsive element, i.e., a fluid  32  or a gel  32   a . As is used in the present description, the term “mechano-responsive element” will be understood to mean an element  32 ,  32   a  that has a measurable physical response in proportion to a condition influencing the element  32 ,  32   a.    
     In the preferred embodiment, the sensor  24  is made from piezoresistive material, e.g., silicon and silicon dioxide. The sensor  24  has at least one bonding pad  34  affixed to the bottom surface  29 . In the preferred embodiment, the bonding pad  34  is a eutectic solder having about a 63/37 lead/tin composition. In an alternative embodiment, indium solder is used as for the bonding pad  34 . In another alternative embodiment, a conductive polymer such as silver epoxy is used for the bonding pad  34 . 
     The sensor  24  is bonded to an application specific integrated circuit  36  by a method of low temperature thermal compression bonding known in the art. The sensor  24  is thereby directly electrically coupled to the circuit  36  with the bonding pads  34 . The circuit  36  is directly electrically coupled to the conductive member  22  carried by the catheter body  12 . In the preferred embodiment, the conductive member  22  is comprised of about two to six stranded copper wires bonded to the integrated circuit  36  that are soldered to the circuit  36 . In an alternative embodiment, the conductive member  22  is attached to the circuit  36  by press contacting. In the most preferred embodiment, the conductive member  22  has separate high and low power input wires, separate high and low signal output wires, and a clock wire used as a toggle to step through plural sensing sites  14 . 
     The sensor  24  is positioned within the catheter body  12  so that the trough  28  opens upwardly through the catheter recess  20 . In the preferred embodiment, the catheter  10  has up to about thirty-two sensing sites  14  positioned along the catheter axis  15 . Each sensing site  14  has a sensor  24  directly electrically coupled to a circuit  36  by at least one bonding pad  34 . In the preferred embodiment, catheter  10  is connected to a computer based data acquisition system (not shown) that supplies power to and receives vertebrate condition data from the sensing site  14  via the conductive member  22 . The circuit  36  is monolithically incorporated with the sensor  24  so the circuit  36  amplifies an electrical signal from the sensor  24  and multiplexes signals received from multiple sensors  24  by a simple count and select scheme known in the art so as to minimize the number of conductive members  22  affixed to the circuit  36  and, thereby, reduce the diameter of the catheter  10 . 
     The mechano-responsive element, i.e., a fluid  32  or a gel  32   a , is located in the sensor trough  28  so that it substantially fills the trough  28  and the catheter recess  20 . In the preferred embodiment, the fluid  32  is silicon oil, used to measure vertebrate pressures. The fluid  32  rises above the outer surface  38  of the catheter body  12  and is sealed with the film  16  so that the fluid  32  bulges above the surface  38  so that no air bubbles are trapped at the sensing site  14 , thereby substantially eliminating measurement error caused by trapped air bubbles. The catheter  1   0  with the fluid  32  is able to measure pressures in the range of about −50 to +300 mmHg exerted upon the sensing site  14 . Sensed pressure is transferred via the fluid  32  to the sensor floor  30 , which deforms in proportion to the pressure exerted. 
     In an alternative embodiment, as seen in FIG. 2A, the gel  32   a , e.g., a polyester gel, a polyamide gel, or a hydrogel, is responsive to non-pressure vertebrate conditions, e.g., temperature, water, glucose, pH. The gel  32   a  is placed in the sensor trough  28  and catheter recess  20 , and sealed by the catheter film  16 , so it does not bulge above the outer surface  38  of the catheter body  12 . The gel  32   a  generally swells or shrinks in volume when acted on by non-pressure conditions, thereby changing pressure the gel  32   a  exerts on the sensor floor  30  in proportion to the vertebrate condition. In this embodiment, the catheter film  16  covering the sensing site  14  is permeable to allow the vertebrate condition to interact with the gel  32   a , as shown by directional arrows C (FIG.  2 A). Each sensing site  14  on a catheter  10  having plural sensing sites  14  can measure a different vertebrate condition by multiplexing signals from different sensing sites  14  and using a simple count and select scheme. For example, temperature may be measured in conjunction with any of the other measurements by interrogation of the circuit elements employed in a temperature circuit. For example, diodes with a negative temperature coefficient are included as part of a constant voltage delivery circuit contained on the circuit  36 . These diodes deliver an increase in voltage with temperature to a bridge network. The diodes can be monitored to measure temperature. 
     Other variations of the structure will be apparent to a person of ordinary skill in the art in view of other patents such as U.S. Pat. Nos. 4,274,423 and 4,722,348, the disclosures of which are incorporated herein by reference. 
     Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific detail, representative apparatus and illustrative example shown and described. This has been a description as the present invention is currently known. However, the invention itself should only be defined by the appended claims.