Abstract:
A sensor mounted on the distal end of an intra-corporeal catheter detects pressure applied thereto. The sensor includes a chip that is deformable in accordance with pressure applied thereto, strain gauges mounted on the chip, a sensing plate, a projection, a cap and a tube. A cover covers the chip, the sensing plate, the projection and the cap and gives a smooth tapered shape to the catheter&#39;s distal end. The projection and the cap transmit pressure applied to the catheter&#39;s distal end to the sensing plate and tilt the sensing plate in accordance with the pressure. The strain gauges issue detection signals according to the degree and the direction of the tilting. The pressure applied to the catheter&#39;s distal end is detected based on the issued signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sensor mounted on an intra-corporeal medical device. More particularly, the present invention relates to a sensor mounted on the distal end of a catheter for detecting pressure acting thereon. 
     2. Description of the Related Art 
     Catheters and endoscopes are medical devices that are inserted into passageways in the human body such as blood vessels to perform medical treatment. These devices typically include a tube, which is inserted into the body, and a manipulator for manipulating the tube from outside the patient&#39;s body. The tube is inserted into an intra-corporeal passageway, such as a blood vessel. Then, the doctor uses the manipulator to guide the distal end of the tube to a desirable point, where he or she performs measuring (e.g., measurement of the blood pressure) or medical treatment (e.g. vasodilation). 
     The intra-corporeal passageways are curved and branched and their diameters vary at different locations. In addition, obstacles such as a thrombus may narrow the passageways. The doctor therefore uses the manipulator to bend the tube in order to guide the tube&#39;s distal end through branches. 
     When operating a prior art catheter, the doctor senses an increase in the insertion resistance of the tube based on the tactile feeling from the catheter and thus senses curved parts, narrow parts and obstacles in the passageway. This allows the doctor to determine the advancing direction of the catheter, The doctor needs to be experienced and must sometime use his or her instincts in manipulating the catheter&#39;s distal end. Further, judging the direction of the insertion pressure is often difficult. 
     To solve the above drawback, it has been proposed that a sensor be mounted on the distal end of a tube for detecting the magnitude and direction of pressure applied to the sensor by contact of the sensor and the passage way&#39;s inner wall. 
     Japanese Unexamined Patent Publication No. 6-190050 discloses a medical device having a sensor mounted on the distal end of the tube. The sensor of this publication, which is shown in FIG. 17, includes a flexible tube  101  and a plurality of strain gauges provided on the periphery of the tube&#39;s distal end. The tube  101  has a plurality of beams  102  and slits  103  at its distal end. The strain gauges are provided mostly on the beams  102 . Contacting the sensor against the inner wall of an intra-corporeal passageway deforms the beams  102 . This causes the strain gauges to issue signals in accordance with impedance based on the deformation of the beams  102 . The magnitude and the direction of the pressure applied to the tube&#39;s distal end are measured based on the signal. The doctor determines the advancing direction of the tube by monitoring the measurement results. Determining the advancing direction of the tube is thus facilitated. 
     However, since the above described strain gauge type sensor has minuscule beams and slits, the manufacturing of the sensor is complicated. This structure is also disadvantageously large. 
     Further, having projections and recesses as shown in FIG. 17, the sensor&#39;s distal end often gets snagged even when advancing in a straight passageway. This may develop a thrombus in the passageway. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a highly biocompatible sensor mounted on an intra-corporeal medical device for detecting the magnitude and the direction of pressure applied to the device&#39;s distal end. 
     A sensor according to the present invention is mounted on the distal end of a medical device. The sensor includes a chip that becomes deformed in accordance with pressure applied thereto and a piezoelectric element that issues detection signals in accordance with the chip&#39;s deformation. The pressure is detected based on the issued signals. The sensor also includes a pressure transmitting element mounted distal to the chip. The transmitting element is smaller then the chip and transmits pressure to the chip. The sensor has a cover covering the piezoelectric element and the pressure transmitting element. The cover impart a tapered shape to the device&#39;s distal end. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principals of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings. 
     FIG. 1 is a partial diagrammatic perspective view illustrating the distal end of a catheter according to a first embodiment of the present invention; 
     FIG. 2 is a plan view illustrating a layout of strain gauges; 
     FIG. 3A is a circuit diagram illustrating a set of strain gauges for detecting deformation in x-direction; 
     FIG. 3B is a circuit diagram illustrating a set of strain gauges for detecting deformation in y-direction; 
     FIG. 4A is a cross-sectional view illustrating a step for manufacturing sensor chips; 
     FIG. 4B is a plan view illustrating a step for manufacturing a sensor chip; 
     FIG. 5A is a cross-sectional view illustrating a step for manufacturing sensor chips; 
     FIG. 5B is a plan view illustrating a step for manufacturing a sensor chip; 
     FIG. 6A is a cross-sectional view illustrating a step for manufacturing sensor chips; 
     FIG. 6B is a plan view illustrating a step for manufacturing a sensor chip; 
     FIG. 7 is a cross-sectional view illustrating a step for manufacturing sensor chips; 
     FIG. 8 is a cross-sectional view illustrating a step for manufacturing sensor chips; 
     FIG. 9 is a cross-sectional view illustrating a step for manufacturing sensor chips; 
     FIG. 10 is a partial diagrammatic perspective view illustrating a step for assembling a sensor; 
     FIG. 11 is a partial diagrammatic perspective view illustrating a step for assembling a sensor; 
     FIG. 12 is a partial diagrammatic perspective view illustrating a step for assembling a sensor; 
     FIG. 13 is plan view of a sensor chip illustrating the layout of another set of strain gauges; 
     FIG. 14A is a circuit diagram illustrating the strain gauges of FIG. 13 for detecting deformation in x-direction; 
     FIG. 14B is a circuit diagram illustrating the strain gauges of FIG. 13 for detecting deformation in y-direction; 
     FIG. 15A is a circuit diagram illustrating yet another strain gauge for detecting deformation in x-direction; 
     FIG. 15B is a circuit diagram illustrating yet another strain gauge for detecting deformation in y-direction; 
     FIG. 16 is a diagrammatic view of a medical catheter; and 
     FIG. 17 is a partial diagrammatic side view illustrating a prior art sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A blood vessel catheter  11  having a sensor according to a first embodiment of the present invention will now be described with reference to FIGS. 1 to  12  and  16 . 
     As shown in FIG. 16, a catheter  11  includes a tube  12  and a manipulator  92  attached to the proximal end of the tube  12 . The tube  12  is inserted into a human body, and the manipulator  92  is used to manipulate the tube&#39;s movement from outside the body. 
     The tube  12  has a sensor  14  mounted on its distal end. The sensor  14  detects pressure applied to the distal end of the tube  12 . The manipulator  92  includes a plurality of wires (not shown) provided in the tube  12  and a wire controlling device (not shown). 
     An air compressor  94  is provided at the proximal end of the tube  12 . The compressor  94  sends air to an expandable balloon attached to the distal end of the tube  12 . An air pipe  93  is connected to the compressor  94  and extends through the tube  12 . Sending air into the balloon expands a narrowed blood vessel. 
     A display device  98  is also provided at the proximal end of the tube  12 . The display device  98  displays the results of the sensor&#39;s measuring. A cable  97  extends through the tube  12  and connects the sensor  14  and the display device  98 . 
     The doctor manipulates the wire controlling device to bend the distal end of the tube  12 , thereby guiding the tube  12  through branched blood vessels. When the distal end of the tube  12 , that is, the sensor  14 , contacts the vessel&#39;s inner wall, the sensor  14  issues signals indicative of the magnitude and the direction of the pressure and transmits the signals to the display device  98 . The device  98  displays the results of the sensor&#39;s detection. 
     As shown in FIG. 1, the sensor  14  is covered by the distal end of the silicon rubber tube  12 . A substantially cylindrical flexible base  13  is provided in the tube  12 . The sensor  14  is mounted on the distal end (upper end in FIG. 1) of the base  13 . 
     Passing the tube  12  through a bend or a branch in a blood vessel applies pressure to the tube  12 . The direction of the pressure is different at each bend and branch. Detection signals transmitted from the sensor  14  notify the doctor of the location of branches of vessels, thrombi and the like. Accordingly, the doctor leads the distal end or the tube  12  to the desirable point without relying on his or her instincts. 
     The sensor  14  includes a sensor chip  15 , a cylindrical cap  16  and a projection  17 . The axis of the sensor chip  15 , the cap  16  and the projection  17  coincide with the axis C 1  of the catheter  11 . The sensor chip  15  includes a substantially square support plate  18 , a cylindrical pole  19  and a disk-like sensing plate  20 . A cylindrical recess is formed in the center on the top surface of the plate  18 . The pole  19  is vertically provided in the recess. The sensing plate  20  is mounted on the top of the pole  19 . The shape of the plate  20  may be a polygon. As shown in FIG. 1, the z-direction is parallel to the axis C 1  of the catheter  11 . The support plate  18  is secured to the distal end of the base  13  with adhesive. The pole  19  is taller than the top surface  18   a  of the support plate  18 . 
     As shown in FIG. 2, eight strain gauges R 1  to R 8  are integrally formed with the plate  20  on its top surface  20   a . The strain gauges R 1  to R 8  are perpendicular to the axis C 1  of the catheter  11 . Specifically, the gauges R 1  to R 4  are arranged parallel to the x-direction and the gauges R 5  to R 8  are arranged parallel to the y-direction. A part of each strain gauge R 1  to R 8  overlaps the distal end of the pole  19 . Bending the sensing plate  20  gives great tensile and compression stress to the gauges R 1  to R 8 . 
     Traces and pads are formed on the top surface  20   a  of the sensing plate  20  to which the strain gauges R 1  to R 8  are connected. As shown in FIG. 1, a substrate  21  is attached on the side of the base  13 . Pads  21   a  and traces  21   b  are formed on the substrate  21 . Wires W electrically connect the pads on the plate  20  and the pads  21   a  on the substrate  21 . The substrate  21  is made of flexible material, such as polyimide. The traces  21   b  are connected to the cable  97  in the tube  12  (see FIG.  16 ). 
     The cap  16  covers the strain gauges R 1  to R 8  on the sensing plate  20 . A notch  16   b  is formed in the side wall of the cap  16 . The wires W enter into the cap  16  through the notch  16   b.    
     The projection  17  has a substantially mushroom-like shape and is provided on the top surface  16   a  of the cap  16 . The projection  17  is covered by a flexible cover  22  made of silicon rubber like the tube  12 . The tube  12  and the cover  22  may be made of any deformable material such as polyimide and polyurethane. 
     In the embodiment shown in FIG. 1, the cover  22  is formed by extending the tube  12  to seal the sensor  14 . The outer surface of the cover  22  is smoothly formed. Asperities on a catheter may cause thrombi. The catheter  11  of the present invention has no asperities. Therefore the catheter  11  is suitable for usage in blood vessels. 
     Contacting the distal end of the catheter  11  to the inner wall of a vessel applies pressure to the catheter&#39;s distal end. The pressure tilts the projection  17 , which is transmitted to the sensing plate  20  via the cap  16 . This bends the plate  20  away from the direction of the pressure. The inclination angle of the projection  17  depends on the magnitude of the pressure. Therefore the bending of the plate  20  also depends on the magnitude of the pressure. For example, pressure along the x axis bends the plate  20  in the x-direction and generates tensile or compression stress in the strain gauges R 1  to R 4 , which are arranged parallel to the x axis, in accordance with the pressure. Pressure along the y axis tilts the plate  20  in the y-direction and generates tensile or compression stress in the strain gauges R 5  to R 8 , which are arranged parallel to the y axis in accordance with the pressure. 
     Specifically, pressure in the positive x-direction tilts the plate  20  to generate tensile stress in the strain gauges R 1 , R 2  and compression stress in the strain gauge R 3 , R 4 . Contrarily, pressure in the negative x-direction generates compression stress in the strain gauges R 1 , R 2  and tensile stress in the strain gauges R 3 , R 4 . 
     Similarly, pressure in the positive y-direction tilts the plate  20  to generate tensile stress in the strain gauges R 7 , R 8  and compression stress in the strain gauge R 5 , R 6 . Contrarily, pressure in the negative y-direction generates compression stress in the strain gauges R 7 , R 8  and tensile stress in the strain gauges R 5 , R 6 . 
     The individual strain gauges R 1  to R 8  have the same resistance. Tensile stress in the gauges R 1  to R 8  increases their resistance and compression stress lowers their resistance. The resistance of the gauges R 1  to R 8  changes according to the magnitude of the external force. Therefore, measuring the amount of change in the strain gauges&#39; resistance allows the pressure applied to the catheter&#39;s distal end to be detected. 
     The resistance of the strain gauges R 1  to R 4  changes according to the x axis component of the pressure and the resistance of the strain gauges R 5  to R 8  changes according to the y axis component of the pressure. Therefore, measuring the changes in the resistance of the strain gauges R 1  to R 4  and the resistance of the strain gauges R 5  to R 8  allows the direction of the pressure applied to the catheter&#39;s distal end to be detected. 
     When using the catheter  11 , adjusting the catheter&#39;s advancing direction in such a way that the pressure detected by the sensor  14  decreases prevents the distal end of the catheter  11  from being pressed against the inner wall of the blood vessel. Guiding the catheter&#39;s distal end to the predetermined point is thus facilitated. 
     As shown in FIG. 3A, the strain gauges R 1  to R 4  are in a bridge connection. A power source Vcc is applied to the node between the gauges R 1  and R 3  and the node between the gauges R 2  and R 4  is grounded. A voltage Vx is outputted from the node between the gauges R 1  and R 4  and the node between the gauges R 2  and R 3 . 
     As shown in FIG. 3B, the strain gauges R 5  to R 8  are in a bridge connection. A power source Vcc is applied to the node between the gauges R 5  and R 7  and the node between the gauges R 6  and R 8  is grounded. A voltage Vy is outputted from the node between the gauges R 5  and R 8  and the node between the gauges R 6  and R 7 . 
     As described above, stress given to the strain gauges R 1  to R 4  changes the resistance of the gauges R 1  to R 4 . The value of the voltage Vx varies according to the changes in the resistance of the gauges R 1  to R 4 . Similarly, stress given to the strain gauges R 5  to R 8  changes the resistance of the gauges R 5  to R 8 . The value of the voltage Vy varies according to the changes in the resistance of the gauges R 5  to R 8 . 
     The manufacturing method of the sensor  14  will now be explained. First steps to manufacture the sensor chip  15  will be explained with reference to the FIGS. 4 to  9 . 
     Each side of the supporting plate  18  of the sensor chip  15  is approximately 1 mm long. A plurality of the sensor chips  15  are formed on a single wafer. FIGS. 4A,  5 A,  6 A and  7  to  9  are cross-sectional views illustrating a pair of adjacent sensor chips  15  and FIGS. 4B,  5 B and  6 B are plan views illustrating a single sensor chip  15 . 
     As shown in FIGS. 4A and 4B, a first epitaxial growth layer  32  made of N-type single crystal silicon is formed to cover the entire surface of a substrate  31  made of P-type single crystal silicon through vapor phase epitaxy. An oxide film (SiO 2  film, not shown) is formed to cover the entire surface of the epitaxial growth layer  32 . A doughnut-shaped opening is formed on the oxide film by photo-etching. Next, boron is implanted in the silicon substrate  31  by ion implantation through the doughnut-shaped opening. Thermal diffusion is performed to the implanted boron. This forms a doughnut-shaped first p +  type silicon layer  33  in the first epitaxial growth layer  32 . The inner diameter of the first p +  type silicon layer  33  (the diameter of the doughnut&#39;s center hole) is equal to the diameter of the pole  19 . The oxide film is removed by etching. 
     Next, as shown in FIG. 5A and 5B, a second epitaxial growth layer  34  made of N-type single crystal silicon is formed to cover the entire surface of the first epitaxial growth layer  32  by vapor phase epitaxy. An oxide film (SiO 2  film, not shown) is formed to cover the entire surface of the epitaxial growth layer  34 . The oxide film is removed by photo-etching leaving a portion thereof that corresponds to the polo  19 . Next, boron is implanted in the silicon substrate  31  by ion implantation. Thermal diffusion is performed to the implanted boron. This forms a second p +  type silicon layer  35  in the second epitaxial growth layer  34 . The oxide film is removed by etching. 
     Next, as shown in FIG. 6A and 6B, a third epitaxial growth layer  36  made of N-type single crystal silicon is formed to cover the entire surface of the second epitaxial growth layer  34  by vapor phase epitaxy. An oxide film (SiO 2  film, not shown) is formed to cover the entire surface of the epitaxial growth layer  36 . The oxide film is removed by photo-etching leaving a portion thereof that corresponds to the shape of the sensing plate  20 . Next, boron is implanted in the silicon substrate  31  by ion implantation. Thermal diffusion is performed to the implanted boron. This forms a third p +  type silicon layer  37  in the third epitaxial growth layer  37 . The oxide film is removed by etching. 
     The above described steps form first, second and third p +  type silicon layers  33 ,  35  and  37  that include the part corresponding to the space around the pole  19  and the sensing plate  20 . Etching the first, second and third p +  type silicon layers  33 ,  35  and  37  according to the method described below forms the pole  19  and the sensing board  20 . 
     A mask (not shown) is formed on the third epitaxial growth layer  36 . Openings are formed on predetermined region on the mask. Next, boron is implanted in the epitaxial growth layer  36  by ion implantation. Thermal diffusion is performed on the implanted boron. This forms the stain gauges R 1  to R 8  on a region corresponding to the surface of the sensing plate  20 . Then, the mask is removed. 
     After performing sputtering or vacuum deposition of aluminum on the third epitaxial growth layer  36 , photolithography is performed on the layer  36  to form traces  38 . The traces  38  are shown only in FIGS. 7 to  9  to avoid complexity in the other drawings. 
     A passivation film (not shown) is formed on the entire surface of the third epitaxial growth layer  36  by accumulating molecules of SiN or Si 3 N 4  through chemical-vapor deposition (CVD) or the like, 
     Etching resist  39  is formed to cover the entire surface of the silicon substrate  31 . Part of the resist  39  is removed by photolithography to expose the surface of the third p +  type silicon layer  37 . Next, anodic oxidation is performed to the silicon substrate  31 . Anodic oxidation is a process in which the substrate  31  is used as an anode in an electrolyte and current between the anode and a cathode forms porous Si, SiO 2  or porous Al 2 O 3 . Hydrofluoric acid aqueous solution is used as the electrolyte in this embodiment. The anodic oxidation selectively changes the first, second and third p +  silicon layer  33 ,  35  and  37  to a porous silicon layer  40 . 
     Then, alkali etching is performed with tetramethyl ammonium hydroxide (TMAH) to selectively etch the porous silicon layer  40 . The porous silicon layer  40  is more solvable with alkali than other parts on which anodic oxidation was not performed. The difference in etching speeds removes the porous silicon layer  40  leaving the pole  19  and the sensing plate  20 . 
     The remainder of the first epitaxial growth layer  32  around the pole  19  serves as a stopper to protect the pole  19  and the sensing plate  20 . An excessive pressure applied to the distal end of the catheter  11  tilts the plate  20  and the pole  19  and cause them to contact the stopper  41 . In this manner, the stopper  41  limits the tilting of the plate  20  and the pole  19  to protect them. 
     Finally, the etching resist in FIG. 8 is removed and the sensor chip  15  is cut away from the substrate  31  through dicing. An described above, the senor chip  15  is easily manufactured using typical steps to manufacture semiconductors. Further, forming a plurality of sensor chips  15  on a single silicon substrate  31  simultaneously reduces the manufacturing cost of the sensor chips  15 . 
     The sensor chip  15 , which is manufactured in the above manner, is adhered to the base  13 . Then, the substrate  21  held by a jig (not shown) is positioned close to the sensor chip  15 . The pads  21   a  formed on the substrate  21  and the chip  15  are bonded by the wires W. 
     Next, as shown in FIG. 11, the substrate  21  is adhered to the flat portion  13   a  of the base  13 . In the above method, bonding of the wires W is easier than in a method in which both the sensor chip  15  and the substrate  21  are fixed to the base  13  before bonding with the wires W. 
     As shown in FIG. 12, the cap  16  with the projection  17  previously attached to its top surface  16   a  is attached to the top surface  20   a  of the sensing plate  20 . Covering the assembled parts with the cover  22  shown in FIG. 1 completes the mounting of the sensor  14  on the distal end of the catheter  11 . 
     Instead of forming the strain gauges R 1  to R 8 , a chip incorporating strain gauges may be mounted on the top surface  20   a  of the sensing plate  20 . 
     The cap  16  and the projection  17  may be integrally formed. Further, a substantially dome-shaped cap may be used so that the protection  17  can be omitted. 
     In the above embodiment, boron is used to form the first, second and third p +  silicon layers  33 ,  35  and  37 . 
     However, other p-type impurities such as gallium (Ga) may be used for this purpose. 
     The present invention is embodied in the blood vessel catheter  11 . The sensor  14  may be further employed in other types of medical devices inserted in an intestinum crassum, an intestinum tenue, a duodenum, other digestive tubes, a urethra, a uterine tube, lymphoduct, bile tube, a vagina, an acoustic meatus, a cavum nasi, an esophagus and bronchia. The sensor  14  of the present invention may be employed not only in the externally controlled catheter  11  but also may be provided at the distal end of a mobile medical device employing micro machine technology. Further, the sensor  14  may be employed not only for devices for human bodies but also for devices for animal bodies. 
     A second embodiment according to the present invention will now be described with reference with FIGS. 13 to  15 . To avoid a redundant description, like or same reference numerals are given to those components that are the same as the corresponding components of the first embodiment. 
     In the second embodiment, the number of strain gauges is different from the number of the strain gauges in the first embodiment. As shown in FIG. 13, the sensing plate  20  has a pair of strain gauges R 11 , R 13  extending parallel to the axis X and a pair of strain gauges R 12 , R 13  extending parallel to the axis Y. 
     As shown in FIG. 14A, the strain gauges R 11 , R 13  are connected in series between the power supply Vcc and the ground GND. The voltage Vx between the node between the strain gauges R 11 , R 13  and the ground GND is outputted. As shown in FIG. 14B, the strain gauges R 12 , R 14  are connected in series between the power supply Vcc and the ground GND. The voltage Vy between the node between the strain gauges R 12 , R 14  and the ground GND is outputted. This structure allows, as in the first embodiment, pressure applied to the distal end of the catheter  11  to be detected as the voltage Vx corresponding to the x axis component of the pressure and the voltage Vy corresponding to y axis component of the pressure. 
     In an embodiment shown in FIGS. 15A and 15B, the strain gauges R 11 , R 12 , R 13  and R 14  are arranged as shown in FIG. 13. A predetermined current is constantly passed through the individual gauges R 11  to R 14 . When the resistance of each gauge changes in accordance with its deformation, measuring the voltages Vx, Vy between the ends of each gauge allows the pressure applied to the sensor  14  to be detected. 
     Although several embodiment of the present invention have been described herein, it should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.