for better understanding of the context in which a sensor chip according to the present invention is going to be used , a sensor and guide wire assembly 1 of a conventional design is illustrated in fig1 . the sensor guide 1 comprises a hollow tube 2 , a core wire 3 , a first coil 4 , a second coil 5 , a jacket or sleeve 6 , a dome - shaped tip 7 , a sensor element or chip 8 , and one or several electrical leads 9 . the proximal end of the first coil 4 is attached to the distal end of the hollow tube 2 , while the distal end of the first coil 4 is attached to the proximal end of the jacket 6 . the proximal end of the second coil 5 is connected to the distal end of the jacket 6 , and the dome - shaped tip 7 is attached to the distal end of the second coil 5 . the core wire 3 is at least partly disposed inside the hollow tube 2 such that the distal portion of the core wire 3 extends out of the hollow tube 2 and into the second coil 5 . the sensor element 8 is mounted on the core wire 3 at the position of the jacket 6 , and is through the electrical leads 9 connected to an electronic unit ( not shown in the figure ). the sensor element 8 comprises a pressure sensitive device in the form of a membrane 10 ( not visible in the figure ), which through an aperture 11 in the jacket 6 is in contact with a medium , such as blood , surrounding the distal portion of the sensor guide 1 . although not shown in the figure , the sensor element 8 further comprises an electrical circuitry , which in a wheatstone bridge - type of arrangement is connected to one or several piezoresistive elements provided on the membrane 10 . as is well known in the art , a certain pressure exerted on the membrane 10 from the surrounding medium will thereby correspond to a certain stretching of the membrane 10 and thereby to a certain resistance of the piezoresistive elements mounted thereon and , in turn , to a certain output from the sensor element 8 . it should therefore be clear that it is highly preferable that this output from the sensor element 8 does not change due to factors that are not related to a real change in the physical properties of the surrounding medium . as was mentioned above , one such factor is so - called bending artefacts , the source of which is that a bending of the sensor guide 1 is transferred to a deformation of the membrane 10 . here , the discussion above about piezoresistive elements coupled in wheatstone bridge - type of arrangement should only be seen as an illustrative exemplification ; in short , the basic problem is that a pressure sensitive device , such as a membrane , can be influenced by a bending of a sensor guide . to remedy the potentially adverse effects from bending artefacts , the present invention provides a new design of a sensor chip to be used in a sensor and guide wire assembly . fig2 shows a portion of a sensor and guide wire assembly 21 comprising a core wire 22 and a sensor element or chip 23 according to the present invention . the sensor chip 23 comprises essentially two portions , a mounting portion 24 , which is attached to the core wire 22 , and a pressure sensitive portion 25 , on the upper side of which a pressure sensitive device in the form of a membrane 26 is provided . a recess 27 , which is provided in the under side of the sensor chip 23 , separates partly the mounting portion 24 from the pressure sensitive portion 25 . the sensor element 23 is disposed inside a jacket or sleeve 28 , and is through at least one electrical lead 29 in contact with an electronic unit ( not shown in the figure ). the mounting portion 24 can be attached to the core wire 22 by means of gluing . for reasons to be discussed below , the term “ mounting portion ” should , however , not be interpreted exclusively ; in fact , also the pressure sensitive portion of a sensor chip can be attached to a core wire . fig3 is a schematic illustration of a sensor chip 33 according to the invention . the sensor chip 33 comprises a mounting portion 34 , which is attached to a core wire 32 by means of an adhesive 38 , and a pressure sensitive portion 35 , on the upper side of which a pressure sensitive device in the form of a membrane 36 is provided . a recess 37 , which is provided in the under side of the sensor chip 33 , constitutes a border between the mounting portion 34 and the pressure sensitive portion 35 . at the position of the recess 37 , a thin portion remains , such that the sensor chip 33 has a thinner cross section at this position than at the other positions of the sensor chip 33 . the bending resistance of this thin portion is less than the bending resistances of the other portions of the sensor chip 33 . during introduction of a sensor and guide wire assembly in small and tortuous vessels , the core wire is frequently bent , and in fig3 the core wire 32 is shown in such a bent state . from the simplified picture shown in fig3 , it should therefore be clear that when the core wire 32 is bent , the thin portion acts as a hinge between the mounting portion 34 and the pressure sensitive portion 35 . because of this articulated action of the sensor chip 33 , the pressure sensitive portion 35 is not forced to adapt to bending deformations of the core wire 32 , i . e . the pressure sensitive portion 35 does not have to follow the curvature of the core wire 32 . deformations caused by bending of the core wire 32 will thereby not be transferred to deformations of the pressure sensitive portion 35 and the membrane 36 , respectively . according to the invention , a pressure sensitive device , such as a membrane , should preferably be located rather close to an end of a sensor chip , and the longitudinal distance between the pressure sensitive device and a recess should be small , so that the pressure sensitive portion becomes rather short . in other words , by the provision of a recess , the effective length of a pressure sensitive portion has actually been reduced , which in itself makes the sensor chip less sensitive to bending artefacts . ( to understand this statement , one can imagine that if the length of the sensor chip would be infinitely small , the sensor chip , when attached to a core wire by a soft adhesive , would not at all conform to any deformations of the core wire , and would thereby be practically insensitive to bending artefacts .) this last feature is valid also when the pressure sensitive portion is attached to a core wire . it is therefore contemplated that the pressure sensitive portion as well as the mounting portion of a sensor chip can be attached to a core wire . preferably , a soft adhesive , such as silicone , should be used . as an example , the total length of a sensor chip according to the invention can be about 1 mm , the width could be about 150 μm , and the thickness of the chip could be about 150 μm . a suitable depth of a recess provided somewhere in the middle of the chip could then be about 50 μm . further , although not shown in the figures , the sensor chips described above can comprise piezoresistive elements coupled in a wheatstone bridge , with one part of the bridge being connected to a membrane and the other part of the bridge being connected to the chip surface outside the membrane . with such an arrangement , the sensor chip is a piezoresistive pressure transducer in that a certain pressure in the medium surrounding the sensor chip corresponds to a certain deformation of the membrane and , in turn , to a certain resistance of the wheatstone bridge . the output signals from the pressure transducer will thereby reflect the pressure in the medium surrounding the sensor . according to the invention a new design of a sensor chip is provided . the sensor chip is adapted to be mounted on a core wire , which is a part of a sensor and guide wire assembly . the invention relates thereby also to a new and improved design for a sensor and guide wire assembly , which has improved sensor chip characteristics especially regarding resistance against bending artefacts . preferably , the sensor chip is made from a single piece of silicon that is etched to the desired shape , including the aforementioned recess . with the new design of the sensor chip , the design of the core wire can be simplified , e . g . the core wire has not to be provided with mounting structures like recesses , which , in turn , lowers the total production costs for the sensor and guide wire assembly as a whole . although the present invention has been described with reference to specific embodiments , also shown in the appended drawings , it will be apparent for those skilled in the art that many variations and modifications can be done within the scope of the invention as described in the specification and defined with reference to the claims below . for example , features described above in this patent specification may be combined with features set forth in u . s . pat . nos . re . 35 , 648 ; 6 , 112 , 598 ; and 6 , 167 , 763 and the u . s . application entitled “ sensor and guide wire assembly ” ( cited above ). the entire contents of these four documents are thus incorporated herein by reference . it should in particular be noted that the improved characteristics of a sensor guide provided with a sensor chip according to the invention are not dependent on the design of the other parts of the sensor guide . therefore , a sensor and guide wire assembly may or may not include parts like jackets or sleeves , coils and tips with special shapes . furthermore , the core wire , on which the sensor chip is mounted , may extend along essentially all the length of the sensor guide , or the core wire may only be provided at the distal portion of the sensor guide . it is also contemplated that more than one recess is provided , and the recess ( es ) could be provided at the same side of the sensor chip as the pressure sensitive device .
the invention relates to a sensor and guide wire assembly for intravascular measurements of physiological variables in a living body , comprising a core wire and sensor element . the sensor element according to the invention comprises basically a mounting portion , which is mounted to the core wire , and a pressure sensitive end portion whose upper side is provided with a pressure sensitive device , such as a membrane . a recess , which is provided between the mounting portion and the pressure sensitive portion , provides the sensor element with an articulated action when the core wire is bent .