MINIATURIZED PRESSURE SENSOR

Abstract: A miniaturized transducer for in vivo measurements of physiological pressure has a transducer unit (2, 3) and an inner tube (1) with a wall opening (6). A diaphragm (5) covers the wall opening. Distinguishing for the invention is that the diaphragm is protected by an outer tube (8) arranged outside the inner tube, and having a wall opening (9) situated radially opposite the wall opening of the inner tube. An end closure closes off the distal end of the tube and a seal (16) seals the outer tube against the inner tube. The lumen (4) of the inner tube is in communication with the atmospheric pressure.

Description: Miniaturised Pressure sensor The present invention relates to a miniaturised pressure transducer intended for measurements in vivo of physical pressure, e.g. bloodless measurement of the pressure in the coronary vessel by inserting the transducer into vena cava. Pressure transducers of the kind mentioned above are known from our WO 88/00023. Such a known transducer is illustrated in Figure 1, and includes an inner tube 1 in which a pressure transducer element in the form of a silicon wafer 2 and an light conductor 3 is inserted. The lumen 4 of the tube is in communication with atmospheric pressure. The distal part of the inner tube 1 is completely surrounded by a diaphragm 5. The inner tube has an opening 6 covered by the diaphragm. The diaphragm is mechanically affected by the physical pressure which is to be measured, and its movements are transferred to the light conductor 3. The amount of light reflected from the flat surface 7 of the silicon wafer 2 and picked up by the light conductor then is proportional to the physical pressure which is to be measured. A disadvantage with the known structure is that the diaphragm is unprotected and is subjected to the risk of damage. Such damage could mean that the diaphragm material is torn away from the tip of the transducer. The risk of damage to the diaphragm is particularly likely to occur in the case where the transducer, which has an outer diameter in the order of magnitude 0.5 mm is inserted in a catheter which perforates the tissue in which the pressure is to be measured. The present invention has the object of providing a solution to the above-mentioned problem. The diaphragm must meet certain mechanical requirements. It must be impermeable, flexible and not dependent on temperature. The more flexible the diaphragm material is, the better will be the resolution of the pressure signals. In the known pressure transducer, a compromise must be made between the mechanical flexibility and the mechanical tearing strenght of the diaphragm material. The pressure transducer itself is subjected to certain demands regarding its exterior dimensions. Its outside diameter should therefore not exceed about 0.46 mm, and its axial length should be short, hardly longer than about 4 mm. The above mentioned problem is solved in accordance with the invention by mechanically protecting the diaphragm by an outer tube, which is placed over the inner tube and which has a wall opening situated directly opposite the opening of the inner tube. The diaphragm, the thickness of which can be then reduced considerably, is fixed between the outer and inner tubes, and only the part of it which is exposed by the opening of the outer tube can be subjected to damage. This exposed diaphragm surface is considerably less than the exposed diaphragm surface in the embodiment in Figure 1. The arrangement of an outer tube having an outside diameter not greater than 0.46 mm has been enabled by the miniaturisa¬ tion of the silicon wafer 2 and a reduction of the diameter of the light conductor 3, compared with the pressure transducer described in the mentioned WO 88/00023. Since the exposed diaphragm surface is considerably reduced, the tearing strength requirement for the diaphragm material is also considerably reduced, in turn resulting in the avail¬ ability for use of considerably more tubes of diaphragm material than was previously the case. It will thus be possible to use diaphragm material which is soft and which gives rise to high-resolution signals. Different embodiments of the invention will now be described in more detail below, and in connection with the accompanying drawings, where Figure 1 is a longitudinal section of a known miniaturised pressure transducer, Figure 2 is a longitudinal section of a first embodiment of the pressure transducer in accordance with the invention, Figure 3 is a longitudinal section of a second embodiment of the pressure transducer in accordance with the invention, Figure 4 is a longitudinal section of a third embodiment of the invention, and Figure 5 is an end view of the tip of the embodiment in accordance with Figure 4. The pressure transducer in Figure 2 includes an inner tube 1, a silicon wafer 2, an optical fibre 3, the wafer and fibre moving together in the lumen 4 of the inner tube. The dia¬ phragm, which is here much smaller than the one in Figure 1, covers the opening 6 in the inner tube. An outer tube 8 with a wall opening 9 is arranged outside the inner tube and coaxial therewith. The openings 6 and 9 are radially opposite each other and the diaphragm is fixed between the outer and inner tubes 1 in the area round the openings. The outer tube 8 is arranged in the distal part 10 of the pressure transducer. A distal end cap 11 closes the distal ends of the inner and outer tubes pressure-tight. A helix 12 is anchored in the end closure 11 and extends forwards in the longitudinal direction of the tube. A safety thread 13 extends between the forward end of the helix 12 and the end cap 11. The object of the safety tread is to ensure that if the helix loosens from the end cap 11, the helix will accompany the transducer when it is pulled up out of the body, and also that the turns of the helix will not be extended to a greater extent than is per¬ mitted when the transducer is pulled backwards. A core filament 14 is anchored in the end closure 11 and preferably tapered. The helix 12 surrounds the core filament. The helix 12 can be bent backwards over itself, and the task of the core filament 14 is to increase the bending resistance of the helix for increasing amounts of such backward bending. The inner tube 1 is connected at its proximal end to a thin tube 15 which is in communication with atmospheric pressure. The joint between the thin tube 15 and the inner tube 1 is inside the proximal end of the outer tube 8. In order that the tube will not crumple when the pressure transducer is taken through veins and arteries it is wound with a dimension- stabilishing helix ribbon 17. The outer tube 8 has an annular joint 16 in which the thin tube 15 is fitted. This joint is arranged in the proximal end of the outer tube and is sealed by welding, soldering or glueing. The unit formed by the optical fibre 3 and silicon wafer 2 is glued to the interior wall of the inner tube 1 as illustrated at the glued joint 18. The inner tube 1 is in turn glued against the interior wall of the outer tube 8, as illustrated at the glued joint 19. An optional, unillustrated wall opening for glue can be made in the inner tube for glueing the unit comprising the optical fibre 3 and silicon wafer. The inner tube 1 and the thin tube 15 can be joined to each other using a welding technique. According to an alternative embodiment, the tube 1 which is wider than the tube 15, can be made integral with the narrower tube 15 by the units 1, 15 being made from a single piece of material using an etching technique. In accordance with another embodiment of the invention il¬ lustrated in Figure 3, the tubes 1 and 15 are implemented in one piece and have the same inside and outside diameters. The length of the distal part 10 is also only 4 mm here and the outside diameter D is 0.46 mm. In the embodiment shown in Figures 4 and 5, the flexible tip formed by the details 12-14 in the embodiments according to Figures 1-3, is replaced by a stiff facet-ground tip 20 for penetrating tissue in which pressure is to be measured. The tip is formed by facet-grinding the end closure 11 and the outer tube 8. An example of the edges 21 occurring during grinding of the tip is illustrated in Figure 5. In the embodiments described above, the diameter of the optical fibre is 0.125 mm or less. The wall opening 9 is substantially rectangular, seen in plan from above. The dimensions of the wall opening are in the order of magnitude about 0.5 x 0.2 mm. The outer tube 8 is preferably steel treated with teflon, i.e. a steel tube with an outer coating of teflon. The inner tube 1 is preferably also made from steel. The inside diameter of the outer tube 8 is about 0.35 mm and the outside diameter of the inner tube is about 0.3 mm, the thickness of the diaphragm 5 thus being in the order of magnitude 0.05 mm. Many different diaphragm materials are conceivable, among these being silicone rubber and latex. The diaphragm is retained between outer and inner tubes by being squeezed and/or by being glued between the tubes. The end closure 11 and joint 16 are made either by glueing or soldering. Silver or tin are used as solder. The embodiments described above can be modified within the scope of the inventive concept.

Claims: CLAIMS 1. Miniaturised pressure transducer for in vivo measurements of physiological pressure, including a pressure transducer unit (2, 3), an inner tube (1) with a distal part (10) in which the pressure transducer unit is arranged directly opposite a through opening (6) in the wall of the inner tube and also is in communication with atmospheric pressure, a diaphragm (5) covering the opening in the wall of the inner tube and arranged for being mechanically affected by the physiological pressure which is to be measured, such as to transmit a corresponding movement to the pressure transducer unit, characterized by an outer tube (8) arranged outside the distal part of the inner tube and provided with a through opening (9) in its wall, this opening being situated radially opposite the wall opening in the inner tube, the diphragm being secured between the outer and inner tubes, a distal end cap (11) for the outer and inner tubes, an annular joint (16) between the proximal end part of the outer tube, and alternatively the exterior wall of the inner tube, or the exterior wall of a tube (15) attached to the inner tube. 2. Transducer as claimed in claim 1, characterized in that a helix ribbon (17) is wound round the outer tube or alterna¬ tively the inner tube (1) or the thin tube (15) joined to it. 3. Transducer as claimed in one or more of the preceding claims, characterized in that the inner tube (1) is etched to form a distal part (10), the outside and inside diameters of which are grater than the outside and inside diameters of the tube (15) extending from the distal end to the proximal end of the inner tube. 4. Transducer as claimed in either of claims 1 and 2, characterized in that the thin tube (15) is joined to the inner tube (1), that the joint between the thin tube and the inner tube is situated inside the outer tube and that the outside and inside diameters of the inner tube are greater than the respective diameters of the thin tube. 5. Transducer as claimed in one or more of the preceding claims, characterized in that the transducer unit (2, 3) is glued (18) to the inner wall of the inner tube, and in that the outer wall of the inner tube is joined (19) to the inner wall of the outer tube (8) . 6. Transducer as claimed in one or more of the preceding claims, characterized in that the inner tube has a wall opening added for inserting glue. 7. Transducer as claimed in one or more of the preceding claims, characterized by a helix (12) arranged at the distal end closure, said helix projecting forwards in the longitudinal direction of the tubes, a safety thread (13) being anchored at one end in the distal end part of the helix and at its other end in the end cap (11), there also being a preferably tapering core filament (14) arranged inside the helix, the filament being anchored at its base in the distal end cap (11) with its tip cantilevering out from the end closure (11). 8. Transducer as claimed in one or more of claims 1-6, characterized in that the end cap (11) is a facet-ground tip (20).

Inventors: ENGSTROEM TOMAS, HAMMARSTROEM OLA, HOEK BERTIL, LONC ROMAN, TENERZ LARS, ENGSTROEM, TOMAS, HAMMARSTROEM, OLA, HOEK, BERTIL, LONC, ROMAN, TENERZ, LARS
Applicants: RADISENSOR AB
Application year: 1989
Main classification: A61B5
IPC codes: A61B5, G01L9